J Mol Model (2015):3 DOI 10.1007/s00894-015-2857-0
ORIGINAL PAPER
Photophysical properties of copper(I) complexes containing pyrazine-fused phenanthroline ligands: a joint experimental and theoretical investigation Shengxian Xu 1 & Jinglan Wang 1 & Feng Zhao 1 & Hongying Xia 1 & Yibo Wang 2
Received: 9 June 2015 / Accepted: 2 November 2015 # Springer-Verlag Berlin Heidelberg 2015
Abstract Two copper(I) complexes [Cu(Pyz-Phen)2]PF6 (1) and [Cu(POP)(Pyz-Phen)]PF6 (2) (Pyz-Phen=pyrazino[2,3-f] [1, 10]phenanthroline, POP = bis[2-diphenylphosphino]phenyl]ether) have been synthesized and characterized. The photophysical properties of these complexes in solution have been studied. The electronic absorption spectrum of complexes 1 exhibit the lowest-lying MLCT absorption band at 459 nm and high-energy ligand-based transitions at 275 nm, while that of complex 2 exhibits the MLCT/LLCT band at 400 nm and ligand π-π* band at 262 nm. In addition, both 1 and 2 show similar phosphorescence 3MLCT/3LLCT emissions with maximum emission wavelengths of 569 and 572 nm, respectively. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) were employed to rationalize the photophysical properties of the complexes studied. The theoretical data confirm the assignment of the experimental absorption spectra and the nature of the emitting states.
Electronic supplementary material The online version of this article (doi:10.1007/s00894-015-2857-0) contains supplementary material, which is available to authorized users. * Feng Zhao [email protected] * Hongying Xia [email protected] 1
School of Chemistry and Chemical Engineering, Jiangxi Science and Technology Normal University, Fenglin Street Nanchang, Jiangxi 330013, People’s Republic of China
2
Key Laboratory of Guizhou High Performance Computational Chemistry, Department of Chemistry, Guizhou University, Guiyang 550025, People’s Republic of China
Keywords Copper(I) complex . Density functional theory . Emission . Photophysical . Pyrazino[2,3-f] [1, 10] phenanthroline
Introduction Copper(I) complexes have attracted considerable attention since the pioneering work of McMillin on the photoluminescence properties of copper diimine complexes in the 1970s [1]. There are some advantages for Cu(I) complexes, including their low cost, earth abundancy, and few toxic properties. Furthermore, Cu(I) complexes show some photophysical properties similar to those of ruthenium(II) complexes that are widely used for solar energy conversion, OLED, photocatalysis, and so on [2–5]. However, it is noteworthy that the square planar structures of the types of complexes in excited states are easily attacked by solvent molecules, which results in the solvent-induced exciplex quenching and sequent reduction of the luminescent performance [6, 7]. The photophysical properties of Cu(I) complexes can be improved through the judicious choice of ligands. Until now, the number of ligands with different structural and electronic properties and the corresponding Cu(I) complexes have been designed and synthesized. Among them, the most commonly used ligands are based on 1,10phenanthroline (Phen) [8–10]. Generally, 2,9-substituted Phen with the bulky groups is preferred to improve the photoluminescence of Cu(I) complexes [11–15]. In addition, one can furthermore adjust the photophysical properties of Cu(I) complexes by changing the substituents at other positions of the Phen rings [16]. Another approach to alter the emission properties of Cu(I) complexes is directly through modification of the skeleton of the Phen ligand by fusing heterocyclic rings. This strategy has been employed in the
31
J Mol Model31 : 2 )5102(
Page 2 of 10
purchased from Aladdin-reagent Co. and used without further purification. Concentrated sulfuric acid and nitric acid el at. were purchased from Sinopharm Chemical Reagent Co. Fig. 1 Structures of N-heterocyclic fuesed phenanthroline ligands
design of Cu(I) complexes which bear the imidazole fused Phen ligand capable of providing the expanded πconjugated system [17–20]. Following a similar strategy, as shown in Fig. 1, replacement of the five-membered imidazole ring by the sixmembered pyrazine can give a novel pyrazino[2,3-f][1, 10]phenanthroline ligand (Pyz-Phen). The six-membered pyrazine is generally known to be an electron-poor heterocycle in nature compared to the five-membered imidazole and should cause significant changes in the photophysical properties of the corresponding Cu(I) complexes. Herein we present the homoleptic [Cu(Pyz-Phen) 2 ] + (1) and the heteroleptic [Cu(POP)( Pyz-Phen)]+ (2) complexes (Fig. 2) and thoroughly study theirs photophysical properties. The POP ligand was employed not only due to its greater rigidity but also its ability to prevent solvent-induced exciplex quenching [15]. The present work is to provide a detailed analysis of the structural and spectral properties of the newly synthesized complexes. Meanwhile, the assignment of the electronic absorption and emission spectra were also investigated using density functional theory (DFT) and time-dependent density functional theory (TDDFT), and the results are compared with the corresponding experimental data.
Experimental details Materials 1,10-phenanthroline (Phen), 2,9-dimethyl-1,10-phenanthroline, Ethylenediamine, potassium bromide, [Cu(CH3CN)4]PF6 (Tetrakis(acetonitrile)copper(I) hexafluorophosphate), and (bis[2-(diphenylphosphino)phenyl]ether) (POP) were
Fig. 2 Molecular structures of the Cu(I) complexes studied in this study
Characterization 1
H NMR spectra were performed in a Bruker AV400 MHz spectrometer, using tetramethylsilane (TMS) as internal reference. DMSO-d6 was used as the solvents. UV-vis absorption spectra were measured using a Perkin Elmer Lambda-900 spectrophotometer. Fluorescence spectra were determined with a Hitachi F-4500 fluorescence spectrophotometer. Synthetic procedure 2,9-Dimethyl-1,10-phenanthroline-5,6-dione A roundbottom flask containing 2,9-dimethyl-1,10-phenanthroline (3.35 g, 16 mmol) and potassium bromide (19.0 g, 160 mmol) was cooled in an ice bath. Concentrated sulfuric acid (60 ml) and concentrated nitric acid (30 ml) were added dropwise. The reaction mixture was heated under reflux for 3 hours, then cooled to room temperature and the solution poured slowly into deionized water (800 mL), neutralized with sodium bicarbonate, and extracted with CH2Cl2. After washed with water and dried, the chloroform was removed under reduced pressure to leave a yellow residue which was recrystallized from dioxane. Yield: 78 % (3.5 g). 1H NMR (400 MHz, DMSOd6): 8.20–8.22 (dd, 2H), 8.59–8.61 (dd, 2H), 2.82 (s,6H). 7,10-Dimethyl-pyrazino[2,3-f][1, 10]phenanthroline (PyzPhen) To a solution of dione compound 3 (3.7 g, 14 mmol) in absolute ethanol (200 ml) was added ethylenediamine (1.0 g, 16 mmol). The reaction solution was heated gently for 16 hours at 50 °C, then concentrated under reduced pressure to 100 mL and allowed to stand at room temperature for 3– 4 hours. The orange precipitate which formed was filtered and recrystallized from ethanol. Yield: 17% (0.63 g).1H NMR (400 MHz, DMSO-d6): 9.30 (d,2H,J=8.3 Hz), 9.10 (d, 2H), 7.76 (d, 2H, J=8.3 Hz), 2.84(s, 6H).
J Mol Model (2015):3
Complex 1 Two equivalents of Pyz-Phen were dissolved in a nitrogen-saturated solution of 25 % CH2Cl2 in acetonitrile (20 mL). The acetonitrile solution was slowly added to 1 equiv of [Cu(CH3CN)4](PF6) under nitrogen. The solution immediately turned dark red, and the mixture was stirred for 30 min at room temperature. The solution was evaporated to dryness, and the resulting red solid was recrystallized with diethyl ether from CH2Cl2. Yield: 40 % (29 mg). 1H NMR (400 MHz, DMSO-d6): 9.68 (s, 4H), 9.18 (s, 4H), 8.01 (s, 4H), 2.57 (s, 12H). Complex 2 One hundred milliliter flask was charged with [Cu(CH 3CN) 4 ](PF6 ) (31 mg, 0.1 mmol), POP (54 mg, 0.1 mg) and nitrogen-saturated solution of CH2Cl2 (10 mL). The mixture was stirred for 2 h under nitrogen. Pyz-Phen (26 mg, 0.1 mmol) was added. The mixtures turned dark red instantaneously, and then stirred for 10 h at room temperature. After 10 h, the mixture was filtered with kieselguhr and the clear red filtrate was evaporated to dryness. Red solid was recrystallized with diethyl ether from CH2Cl2. Yield: 57 % (58 mg). 1H NMR (400 MHz, DMSO-d6): 9.49–9.51 (d, 2H, J=8.0 Hz), 9.0 (s, 2H), 7.77–7.79 (d, 2H, J=8.0 Hz), 7.33– 7.77 (t, 3 H), 7.16–7.26 (m, 8 H), 7.05–7.07 (d,8 H, 8.0 Hz), 6.96–7.03 (m, 10 H), 2.51(s, 6 H). DFT calculations All calculations were performed using the Gaussian 09 [21] program package. The B3LYP exchange-correlation function [22, 23] was used to optimize the ground state geometries of complexes 1–2 using the polarized continuum model (PCM) in CH2Cl2 media. The 6-31G* basis set [24] was used on C, H, N, O, and P atoms. LANL2DZ basis set [25, 26] was adopted on Cu atom. Recent calculations with the B3LYP/6-31G(d)/LANL2DZ level for the similar structure Cu(I) complexes have proved their
Scheme 1 The synthetic pathways of the ligands and Cu(I) complexes
Page 3 of 10 31
reliability and gave good agreement with the available experiments [27–30]. The unrestricted B3LYP method was used to optimize the lowest triplet state geometries (T1) in CH2Cl2 solvent. On the basis of the optimized ground state geometries, TDDFT method [31, 32] associated with PCM [33, 34] in CH2Cl2 media at the same level of theory used for geometrical optimization were used to simulate the absorption spectra of complexes 1–2. The first 100 singlet vertical excitations were obtained form TDDFT output file to construct the calculated absorption spectra. Calculated electronic density plots for the frontier molecular orbitals were prepared using Gauss View 4.1.2 software and the spin-density populations are calculated using Multiwfn analyzer soft [35].
Results and discussion Synthesis The synthetic pathways of the ligands and Cu(I) complexes are shown in Scheme 1. 1,10-Phenanthroline-5,6dione was synthesized according to a modified literature procedure [36]. Pyz-Phen ligand was prepared by a condensation reaction according to the literature procedure [37]. Complex 1 was prepared by the conventional reaction of [Cu(CH3CN)4]BF4 with two equivalent of PyzPhen ligand. Complex 2 was prepared by the reaction of [Cu(CH3CN)4]BF4 with one equivalent of Pyz-Phen ligand and one equivalent POP ligand. Photophysical properties Figure 3 shows the UV-vis spectra of complexes 1–2 in CH2Cl2 solution at room temperature. The high-lying absorption band of complex 1 localized at 275 nm is observed, which
31
Page 4 of 10
Fig. 3 Absorption spectra in dichloromethane at 298K of complexes 1 and 2. (inset) the region of 300–540 nm is magnified to allow easy comparison
can be intuitively described as the ligand-centered π→π* transitions of the Pyz-Phen ligand. At longer wavelengths (350–500 nm), the lowest-lying absorption band maxima at 459 nm is present, which is assigned to metal-to-ligand charge transfer transitions (d(Cu)→π*(Pyz-Phen)) (MLCT) . In the case of complex 2, the highest-lying absorption band localized at 262 nm should be assigned to π(Pyz-Phen)→π*(PyzPhen)) transition with comparable contribution of ligand-toligand charge transfer (LLCT) originated from π(POP)→ π*(Pyz-Phen), which is supported by TDDFT calculations (see below). A weak MLCT absorption band at 400 nm is observed (see inset Fig. 3). This band is blue-shifted by 59 nm compared to that of complex 1 due to the replacement of the Pyz-Phen ligand by POP ligand, resulting in the increase of HOMO-LUMO energy gap of complex 2. The normalized emission spectra of 1–2 in CH2Cl2 solution are shown in Fig. 4. Complexes 1 and 2 show similar emission properties with broad and structureless features, suggesting that the nature of such emission could be assigned to the lowest 3MLCT character with some 3LLCT character. The emission maxima at 572 nm of complexes 2 exhibits a red-shifting of 3 nm compared with complex 1 (λmax 569 nm).
J Mol Model31 : 2 )5102(
Fig. 4 Normalized emission spectra of complexes 1 and 2 in dichloromethane solution
noteworthy that the dihedral angle between the two Pyz-Phen ligands is 90.0°for complex 1, indicating a perfect D2d tetrahedron-like structure, while the dihedral angle between POP and Pyz-Phen ligands is 89.2° for complex 2, suggesting that a slight distortion occurs. The frontier molecular orbital compositions for complexes 1 and 2 have been analyzed using Multiwfn analyzer soft [35] and are listed in Tables 1 and 2. To conveniently discuss the orbital characters, complex 1 has been
Theoretical calculations Ground state geometries and molecular orbital properties The optimized ground state geometric structures for complexes 1 and 2 are shown in Fig. 5. Vibrational frequencies were calculated based on these optimized geometries to verify that the geometries represented a minimum on the potential energy surface. It is
Fig. 5 Optimized ground state and triplet excited geometric structures of complexes 1 and 2 in CH2Cl2 solvent. H-atoms are omitted for clarity
J Mol Model (2015):3 Table 1 Frontier molecular orbital compositions (%) for complex 1 the ground state in CH2Cl2 solution at the B3LYP/631G*/LANL2DZ level
Page 5 of 10 31
Orbital
Energy(ev)
Main bond type
Cu −2.19
1st Pyz-Phen
Cu component 2nd Pyz-Phen
π*(Pyz-Phen)
0.47
49.7
49.8
L+3 −2.32 π*(Pyz-Phen) L+2 −2.32 π*(Pyz-Phen) L+1 −2.44 π*(Pyz-Phen) L −2.44 π*(Pyz-Phen) HOMO-LUMO gap (3.54 ev) H −5.98 d(Cu) H-1 −5.98 d(Cu) H-2 −6.68 d(Cu) H-3 −6.81 d(Cu) H-7 −7.24 π(Pyz-Phen)
2.91 2.91 0.80 0.80
67.8 29.3 74.9 24.3
29.3 67.8 24.3 74.9
72.3 72.3 88.9 90.0 0.00
13.1 14.6 5.57 4.97 50.0
14.6 13.1 5.57 4.98 50.0
L+4
segmented into three fragments, namely Cu, 1st Pyz-Phen, and 2nd Pyz-Phen units, whereas complex 2 has been segmented into four fragments, namely Cu, Pyz-Phen, Ppop, and PhPOP units, where Ppop denotes P atom in POP ligand, and PhPOP denotes the phenyl groups in POP ligand. Figure 6 shows the energy levels and plots of the main molecular orbitals of interest in complexes 1 and 2. For complex 1, due to the structural symmetry, the two highest occupied molecular orbitals (HOMO and HOMO-1) are degenerate and they have the same energies, both being localized on the metal Cu 3d orbital (mostly of 3dxy and 3dxz nature, respectively). The HOMO-2, lying ca. 0.7 eV below the HOMO-1, is composed of the metal Cu 3dyz orbital. At lower energy, ca. 0.13eV below the HOMO-2, the HOMO-3 is composed of a combination of 3d x 2 - y 2 and 3d z 2 orbitals.
Table 2 Frontier molecular orbital compositions (%) for complex 2 the ground state in CH2Cl2 solution at the B3LYP/631G*/LANL2DZ level
Contribution (%)
Orbital
Energy (ev)
Additionally, the first two lowest unoccupied molecular orbitals (LUMO and LUMO+1) are a degenerate couple of π* character delocalized on the Pyz-Phen ligand. At higher energies two degenerate orbitals (LUMO+2 and LUMO+3) with the π* character are found. In the case of complex 2, the distortions in the tetrahedral configuration brought by the replacement of each of two PyzPhen ligands by POP ligand break the degeneracy of molecular orbitals. The HOMO and HOMO-1 are separated by 0.40 eV. Here, the HOMO is mainly composed of the Cu 3dxy and π(Ph) orbitals with comparable contributions from P atom, whereas the HOMO-1 is composed of a combination of the Cu 3dx2-y2 and 3dz2 and π(Ph) orbitals. The HOMO-2 is mainly composed of the metal Cu 3dyz orbital. The HOMO-3 has major contributions from phenyl group in POP ligand. The three
Main bond type
L+3 −1.36 π*(Ph) L+2 −2.11 π*(Pyz-Phen) L+1 −2.27 π*(Pyz-Phen) L −2.45 π*(Pyz-Phen) HOMO-LUMO gap (3.59 ev) H −6.04 d(Cu)+π(Ph) H-1 −6.44 d(Cu)+π(Ph) H-2 −6.52 d(Cu)+π(Ph) H-3 −6.75 π(Ph) H-5 −7.17 π(Pyz-Phen) H-6 −7.19 π(Ph) H-11 −7.33 d(Cu)+π(Ph) a
69.9dxy 69.9dxz 86.4dyz 67.5 dz2 +22.5dx2-y2
Contribution (%)
Cu component
Cu
Pyz-Phen
PPOPa
PhPOPb
1.51 0.37 3.08 0.26
2.14 98.8 93.6 98.8
14.3 0.19 1.98 0.23
81.7 0.59 1.35 0.75
36.2 48.5 68.6 9.69 6.23 8.26 41.7
3.91 8.99 4.69 2.69 83.8 4.09 4.12
25.6 14.4 0.18 1.14 0.29 0.12 0.43
32.8 26.4 25.3 67.1 9.54 87.4 51.9
Components for P atom in POP ligand. b Components for phenyl group in POP ligand
22.8dxy 16.6dz2 +17.7 dx2-y2 63.9dxz
25.6dyz +10.6dz2
31
J Mol Model31 : 2 )5102(
Page 6 of 10 Fig. 6 Energy levels and the energy values calculated for the molecular orbitals of complexes 1 and 2 with representative contour plots of the molecular orbitals
LUMOs (LUMO, LUMO+1 and LUMO+2) all have π* character delocalized on the Pyz-Phen ligand, and the LUMO+3 has π*(Ph) character. In addition, it is clearly seen that the HOMO is slightly higher in energy for complex 1 (-5.98 eV) compared to complex 2 (-6.04 eV), while the LUMO is almost the same in energy (-2.44 eV for 1 and -2.45 eV for 2). Hence, the HOMO-LUMO energy gap increases on going from complexes 1 to 2. Theoretical absorption spectra To gain insight into the transition character of absorption spectra of all the complexes, TDDFT calculations were undertaken to simulate the absorption spectra. Table 3 lists the dominant energy singlet-singlet vertical excitations, their oscillator strengths, assignment configurations, and excitations with
maximum coefficients for complexes 1 and 2. Absorption spectra were simulated by a Gaussian convolution. A comparison of calculated and experimental absorption spectra for complexes 1–2 is presented in Fig. 7. For complex 1, we calculate an intense feature at 268 nm, in good agreement with the 275 nm experimental value. The absorption peak at 268 nm mainly originates form the transitions of H-7→L+4 (34.5 %). The H-7 mainly consists of π(Pyz-Phen), whereas the L+4 are predominantly composed of π*(Pyz-Phen). Thus, this band can be assigned to intraligand charge transfer (ILCT) in character. At longer wavelengths, the calculated lowest-lying absorption band localized at 447 nm, corresponding to the absorption peak at 459 nm observed in experiment, should be assigned to the H/H-1→L/ L+1 transitions originated from [d(Cu)]→[π*(Pyz-Phen)] with the metal-to-ligand charge transfer (MLCT) character.
Table 3 Electronic absorptions of complexes 1 and 2 in CH2Cl2 based on TDDFT calculations at the (B3LYP)/6-31g*/LANL2DZ level, together with the experimental values Complex
Excited state
Transition
Coeff
E (eV)/(nm)
Oscillator strength
Assigna
Exptl/ nm
1
3
0.2126
4.62/268 3.05/407
0.9036 0.0610
4.71/263
0.2574
MLCT MLCT ILCT MLCT/LLCT MLCT/LLCT ILCT LLCT LLCT/MLCT
459
57 1
0.37263 (28.0%) 0.37411 (28.0%) 0.41562 (34.5%) 0.63491 (80.6%) −0.29104 (16.9%) 0.26428 (14.0%) 0.25422 (12.9%) −0.24623 (12.1%)
2.79/445
2
H→L H-1→L H-7→L+4 H→L H→L+1 H-5→L+2 H-6→L+2 H-11→L+2
56
a
MLCT: metal-to-ligand charge transfer; LLCT: ligand-to-ligand charge transfer; ILCT: intra-ligand charge transfer
275 400 262
J Mol Model (2015):3
Page 7 of 10 31
Fig. 7 Comparison of the calculated (red line) and experimental (black line) absorption spectra in CH2Cl2 solution for complex 1 (a); complex 2 (b). Red vertical lines correspond to oscillator strength of calculated singlet-singlet transitions. The full width at halfmaximum (fwhm) is set to 1210 and 686 cm−1 for complexes 1 and 2, respectively
The main spectral features in experiments are well reproduced by our TDDFT calculation. With respect to complex 2, the agreement between the calculated and experimental spectra is quite good, even though the calculated spectra appear to be slightly red-shifted. The highest-lying absorption band calculated at 263 nm, which corresponds well to the experimental value of 262 nm, appears to mainly originate from the mixed transitions of H-5 [π(PyzPhen)]→L+2 [π*(Pyz-Phen)] (14.0 %), H-6 [π(Ph)]→L+2 [π*(Pyz-Phen)] (12.9 %) and H-11 [d(Cu)+π(Ph)]→L+2 [π*(Pyz-Phen)] (12.1 %) with ILCT/LLCT character. Additionally, the calculated lowest-lying absorption band localized at 407 nm, corresponding to the weaker absorption peak at 400 nm observed in experiments, should be assigned to the degenerate H/H-1 [d(Cu)+π(Ph)]→L/L+1 [π*(Pyz-Phen)] transition with MLCT/LLCT character. Additionally, in order to improve the analysis of the nature of absorption transitions, the natural transition orbital (NTO) analysis [38] have been performed for complexes 1 and 2 based on the TDDFT calculations, as presented in Table 1S. The NTO results further supported the assignment of the absorption transition nature. For comparison purpose, a long-range separated hybrid functional CAM-B3LYP [39], which is now widely used to CT excitations, was employed to predict the vertical absorption energies for complexes 1 and 2. However, it is clearly seen that the CAM-B3LYP functional provides poor results, which may be due to the fact that the present
CT excitations are shorter ranged (see Fig. 1S, Supporting information). Triplet excited states and emission properties The lowest-lying triplet geometries (T1) were optimized at the spin-unrestricted UB3LYP/6-31G*/LANL2DZ level in CH2Cl2 solution. The optimized structures of the T1 state of complexes 1 and 2 have been provided in Fig. 5. The remarkable geometry difference between the S0 and T1 state is the dihedral angle between the two ligands. The dihedral angle between two Pyz-Phen ligands for complex 1 changes from the 90.0° in the S0 state to 65.2° in the T1 state, while the dihedral angle between POP and Pyz-Phen ligands for complex 2 changes from the 89.2 to 72.5°. The calculated relaxation energies are a little large (see Table 2S, Supporting information), indicating the presence of significant structural relaxation. To gain insight into the nature of the emission properties of complexes 1 and 2, the emission energies are calculated using TDDFT method at their respective optimized T1 geometry. It is very important to find a suitable functional that would allow for the reliable prediction of emission properties. Here, five functionals (B3LYP, PBE0, MPW1B95, BMK, M06-2X) were employed to calculate the emissive energies for all complexes including CH2Cl2 solvent. Relatively larger change in the T1 emission energies calculated form the B3LYP (20 % HF) to M06-2X (54 % HF) functionals is reasonable since the
Table 4 Calculated phosphorescent emission of the studied complexes in CH2Cl2 solution at TDDFT/MPW1B95 level, together with the experimental values Complex
Major configuration
λemcal/nm
Assignment
λemExptl/ nm
1 2
LUMO→HOMO (90.9%) LUMO→HOMO (82.4%)
664 549
MLCT/LLCT MLCT/LLCT
569 572
31
J Mol Model31 : 2 )5102(
Page 8 of 10
Fig. 8 Spin density distribution contours (isovalue=0.001) for the lowest triplet state T1 of complexes 1 and 2. The values of the unpaired-electron spindensity population are depicted together with the electronic nature of the states
absolute values obtained using these functionals strongly depend on the amount of Hartree-Fock exchange incorporated in these hybrid functionals [40, 41] (see Table 2S, Supporting information). Overall, the MPW1B95 functional gave more satisfactory emissive energies and was employed to further explore the nature of the emissive state. Additionally, ΔSCF approach was also employed to calculate the emission energies [42, 43] (Table 2S, Supporting information). This method is found to be slightly worse than the TDDFT/B3LYP combination in our cases. The results of the TDDFT calculations are listed in Table 4. The lowest-energy emission of complex 1 originates mainly from LUMO→HOMO transition assigned to [π*(Pyz-Phen)→d(Cu)] with the 3MLCT character. The emission of complex 2 at 549 nm is also mainly contributed by LUMO →HOMO transition originated from [π*(PyzPhen)→d(Cu)+π(Ph)] with 3MLCT/3LLCT character. The nature of the emitting excited state of each complex can furthermore be identified by the unpaired-electron spindensity distribution, as shown in Fig. 8. For complex 1, the unpaired electron spin-density distribution in the T1 state (Cu, 0.52e; 1st Pyz-Phen, 0.15e; 2nd Pyz-Phen, 1.16e) illustrates the higher contribution of the d(Cu) to the 2nd Pyz-Phen ligand with little contribution of 1st Pyz-Phen ligand to the 2nd PyzPhen ligand and confirms the emission behavior of 3MLCT with little contribution of 3LLCT. The spin-density distribution is observed for complex 2 in the T1 state (Cu, 0.51e; POP, 0.35e; Pyz-Phen, 1.14e) and the 3MLCT/3LLCT characters are responsible for the T1 state of complex 2.
in the 400–500 nm region of the absorption spectra of complex 1 is assigned to the 1MLCT transition, while complex 2 displays the 1MLCT/1LLCT absorption character. Complexes 1 and 2 exhibit luminescence bands centered at 569 and 572 nm, respectively, attributed to the 3MLCT/3LLCT characters. Theoretical calculations furthermore support the assignments of the nature of the emitting excited states for complexes studied. These experimental and theoretical insights should be expected to provide some guides for the design and synthesis of efficient luminescent copper(I) complexes. Acknowledgments The authors acknowledge financial support by the National Natural Science Foundation of China (No.21462020, 21443010, and 21563013) and Jiangxi Science and Technology Normal University Key Laboratory of Organic-inorganic Composite Materials (Key training base). They thank the Guizhou University High Performance Computation Chemistry Laboratory (GHPCC) for help with computational studies.
References 1.
2.
3.
Conclusions In this paper, the homoleptic [Cu(Pyz-Phen)2]+ and heterleptic [Cu(POP)(Pyz-Phen)]+ complexes were synthesized and characterized. The absorption and luminescence properties of the two complexes are investigated experimentally and are interpreted by means of DFT and TDDFT calculations. Both experiment and theory show that the absorption band appearing
4.
5.
McMillin DR, Buckner MT, Ahn BT (1977) A light-induced redox reaction of bis (2, 9-dimethyl-1, 10-phenanthroline) copper(I). Inorg Chem 16(4):943–945. doi:10.1021/ic50170a046 Funaki T, Funakoshi H, Kitao Onozawa-Komatsuzaki ON, Kasuga K, Sayama K, Sugihara H (2012) Cyclometalated ruthenium (II) complexes as near-IR sensitizers for high efficiency dye-sensitized solar cells. Angew Chem Int Ed 51(30):7528–7531. doi:10.1002/ anie.201108738 Bolink HJ, Cappelli L, Coronado E, Gavina P (2005) Observation of electroluminescence at room temperature from a ruthenium(II) bis-terpyridine complex and its use for preparing light-emitting electrochemical cells. Inorg Chem 44(17):5966–5968. doi:10. 1021/ic0507058 Wong KMC, Chan MMY, Yam VWW (2014) Supramolecular assembly of metal-ligand chromophores for sensing and phosphorescent OLED applications. Adv Mater 26(20):5558–5568. doi:10. 1002/adma.201306327 Kumar P, Bansiwal A, Labhsetwar N, Jain SL (2015) Visible light assisted photocatalytic reduction of CO2 using a graphene oxide supported heteroleptic ruthenium complex. Green Chem 17(3): 1605–1609. doi:10.1039/c4gc01400f
J Mol Model (2015):3 6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
McMillin DR, Kirchhoff JR, Goodwin KV (1985) Exciplex quenching of photo-excited copper complexes. Coord Chem Rev 64(4):83–92. doi:10.1016/0010-8545(85)80043-6 Kober EM, Caspar JV, Lumpkin RS, Meyer TJ (1986) Application of the energy gap law to excited-state decay of osmium(II)polypyridine complexes: calculation of relative nonradiative decay rates from emission spectral profiles. J Phys Chem 90(16):3722– 3734. doi:10.1021/j100107a046 Armaroli N, Accorsi G, Holler M, Moudam O, Nierengarten JF, Zhou Z, Wegh RT, Welter R (2006) Highly luminescent Cu(I) complexes for light-emitting electrochemical cells. Adv Mater 18(10): 1313–1316. doi:10.1002/adma.200502365 Zhang QS, Zhou QG, Cheng YX, Wang LX, Ma DG, Jing XB, Wang FS (2006) Highly efficient electroluminescent from greelight emitting electrochemical cells based on Cu complexes. Adv Funct Mater 16(22):1203–1208. doi:10.1002/adfm.200500691 Wei FX, Fang L, Huang Y (2010) Synthesis, characterization, crystal structures, and photophysical properties of a series of roomtemperature phosphorescent copper(I) complexes with oxadiazolederived diimine ligand. Inorg Chim Acta 363(11):2600–2605. doi: 10.1016/j.ica.2010.04.041 Ichinaga AK, Kirchhoff JR, McMillin DR, Dietrich-Buchecker CO, Marnot PA, Sauvage JP (1987) Charge-transfer absorption and emission of Cu(NN)2+ systems. Inorg Chem 26:4290–4292. doi: 10.1021/ic00272a030 Cunningham CT, Cunningham KLH, Michalec JF, McMillin DR (1999) Cooperative substituent effects on the excited states of copper phenanthrolines. Inorg Chem 38(20):4388–4392. doi:10.1021/ ic9906611 Vogtle F, Luer I, Balzani V, Armaroli N (1991) Endoreceptors with convergent phenanthroline units: a molecular cavity for six guest molecules. Angew Chem Int Ed 30(10):1333–1336. doi:10.1002/ anie.199113331 Kern JM, Sauvage JP, Weidmann JL, Armaroli N, Flamigni L, Ceroni P, Balzani V (1997) Complexes containing 2,9-bis(pbiphenylyl)-1,10-phenanthroline units incorporated into a 56membered ring. synthesis, electrochemistry, and photophysical properties. Inorg Chem 36(23):5329–5338. doi:10.1021/ic970707v Cuttell DG, Kuang SM, Fanwick PE, McMillin DR, Walton RA (2001) Simple Cu (I) complexes with unprecedented excited-state lifetimes. J Am Chem Soc 124(1):6–7. doi:10.1021/ja012247h McCusker CE, Castellano FN (2013) Design of a long-lifetime, earth-abundant, aqueous compatible Cu(I) photosensitizer using cooperative steric effects. Inorg Chem 52(14):8114–8120. doi:10. 1021/ic401213p Bozic-Weber B, Constable EC, Furer SO, Housecroft CE, Troxler LJ, Zampese JA (2013) Copper(I) dye-sensitized solar cells with [Co(bpy)3] 2+/3+ electrolyte. Chem Commun 49(65):7222–7224. doi:10.1039/c3cc44595j Sandroni M, Kayanuma M, Planchat A, Szuwarski N, Blart E, Pellegrin Y, Daniel C, Boujtita M, Odobel F (2013) First application of the HETPHEN concept to new heteroleptic bis(diimine) copper(I) complexes as sensitizers in dye sensitized solar cells. Dalton Trans 42(30):10818–10827. doi:10.1039/c3dt50852h Wen CH, Tao GQ, Xu XH, Feng XQ, Luo RC (2011) A phosphorescent copper(I) complex: synthesis, characterization, photophysical property, and oxygen-sensing behavior. Spectrochim Acta A 79(5):1345–1351. doi:10.1016/j.saa.2011.04.067 Li Z (2011) Synthesis, characterization and theoretical analysis on a oxygen-sensing phosphorescent copper(I) complex. 81(1) 475– 480. doi: 10.1016/j.saa.06.040 Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA, Cheeseman JR, Scalmani G, Barone V, Mennucci B, Petersson GA, Nakatsuji H, Caricato M, Li X, Hratchian HP, Izmaylov AF, Bloino J, Zheng G, Sonnenberg JL, Hada M, Ehara M, Toyota K, Fukuda R, Hasegawa J, Ishida M, Nakajima T, Honda Y, Kitao O,
Page 9 of 10 31 Nakai H, Vreven T, Montgomery JJA, Peralta JE, Ogliaro F, Bearpark M, Heyd JJ, Brothers E, Kudin KN, Staroverov VN, Kobayashi R, Normand J, Raghavachari K, Rendell A, Burant JC, Iyengar SS, Tomasi J, Cossi M, Rega N, Millam NJ, Klene M, Knox JE, Cross JB, Bakken V, Adamo C, Jaramillo J, Gomperts R, Stratmann RE, Yazyev O, Austin AJ, Cammi R, Pomelli C, Ochterski JW, Martin RL, Morokuma K, Zakrzewski VG, Voth GA, Salvador P, Dannenberg JJ, Dapprich S, Daniels AD, Farkas O, Foresman JB, Ortiz JV, Cioslowski J, Fox DJ (2009) Gaussian 09, revision A. Gaussian Inc, Wallingford 22. Runge E, Gross EK (1984) Density-functional theory for timedependent systems. Phys Rev Lett 52:997–1000. doi:10.1103/ physrevlett.52.997 23. Mayo SL, Olafson BD, Goddard WA (1990) DREIDING: a generic force field for molecular simulations. J Phys Chem 94(26):8897– 8909. doi:10.1021/j100389a010 24. Hariharan PC, Pople JA (1974) Accuracy of AHn equilibrium geometries by single determinant molecularorbital theory. Mol Phys 27(1):209–214. doi:10.1080/00268977400100171 25. Hay PJ, Wadt WR (1985) Ab initio effective core potentials for molecular calculations. potentials for the transition metal atoms Sc to Hg. J Chem Phys 82(8):270–283. doi:10.1063/1. 448799 26. Wadt WR, Hay PJ (1985) Ab initio effective core potentials for molecular calculations. potentials for main group elements Na to Bi. J Chem Phys 82(1):284–298. doi:10.1063/ 1.448800 27. Costa RD, Tordera D, Orti E, Bolink HJ, Schonle J, Graber S, Housecrof CE, Constable EC, Zampese JA (2011) Copper(I) complexes for sustainable light-emitting electrochemical cells. J Mater Chem 21(8):16108–16118. doi:10.1039/c1jm12607e 28. Fraser MG, van der Salm H, Cameron SA, Blackman AG, Gordon KC (2013) Heteroleptic Cu(I) bis-diimine complexes of 6,6′-dimesityl-2,2′-bipyridine: a structural, theoretical and spectroscopic study. Inorg Chem 52(6):2980–2992. doi:10. 1021/ic302393p 29. Hsu CW, Lin CC, Chung MW, Chi Y, Lee GH, Chou PT, Chang CH, Chen PY (2011) Systematic investigation of the metalstructure photophysics relationship of emissive d10-complexes of group 11 elements: the prospect of application in organic light emitting devices. J Am Chem Soc 133(9):12085–12099. doi:10.1021/ ja2026568 30. Bozic-Weber B, Chaurin V, Constable EC, Housecroft CE, Meuwly M, Neuburger M, Rudd JA, Schonhofer E, Siegfried L (2012) Exploring copper(I)-based dye-sensitized solar cells: a complementary experimental and TD-DFT investigation. Dalton Trans 41: 14157–14169. doi:10.1039/c2dt31159c 31. Casida ME, Jamorski C, Casida KC, Salahub DR (1998) Molecular excitation energies to high-lying bound states from time-dependent density-functional response theory: characterization and correction of the time-dependent local density approximation ionization threshold. J Chem Phys 108(11):4439–4449. doi:10.1063/1. 475855 32. Stratmann RE, Scuseria GE, Frisch MJ (1998) An efficient implementation of time-dependent density-functional theory for the calculation of excitation energies of large molecules. J Chem Phys 109(19):8218–8224. doi:10.1063/1.477483 33. Perdew JP, Burke K, Emzerhof M (1996) Generalized gradient approximation made simple. Phys Rev Lett 77(8):3865–3868. doi:10.1103/PhysRevLett.77.3865 34. Perdew JP, Burke K, Emzerho FM (1997) K+emission in symmetric heavy ion reactions at subthreshold energies. Phys Rev Lett 78(7):1396–1396. doi:10.1103/PhysRevLett.78.1396 35. Lu T, Chen FW (2012) Multiwfn: a multifunctional wavefunction analyzer. J Comput Chem 33(5):580–592. doi:10.1002/jcc.22885
31
Page 10 of 10
Hiort C, Lincoln P, Norden B (1993) DNA binding of Δ- and Δ[Ru(phen)2DPPZ]2+. J Am Chem Soc 115(9):3448–3454. doi:10. 1021/ja00062a007 37. Sun PP, Duan JP, Lih JJ, Cheng CH (2006) Synthesis of new europium complexes and their application in electroluminescent devices. Adv Funct Mater 13(9):683–691. doi:10.1002/adfm. 200304378 38. Martin RL (2003) Natural transition orbitals. J Chem Phys 118: 4775–4777. doi:10.1063/1.1558471 39. Yanai T, Tew DP, Handy NC (2004) A new hybrid exchange–correlation functional using the Coulomb-attenuating method (CAMB3LYP). Chem Phys Lett 393:51–57. doi:10.1016/j.cplett.2004.06.011 40. Huang SP, Zhang QS, Shiota Y, Nakagawa T, Kuwabara K, Yoshizawa K, Adachi C (2013) Computational prediction for 36.
J Mol Model31 : 2 )5102( singlet- and triplet-transition energies of charge-transfer compounds. J Chem Theory Comput 9:3872–3877. doi:10.1021/ ct400415r 41. Xu SX, Wang JL, Xia HY, Zhang F, Wang YB (2014) Computational prediction for emission energy of iridium (III) complexes based on TDDFT calculations using exchange-correlation functionals containing various HF exchange percentages. 21: 22– 24. doi: 10.1007/s00894-014-2557-1 42. Lowry MS, Hudson WR, Pascal RA Jr, Bernhard S (2004) Accelerated luminophore discovery through combinatorial synthesis. J Am Chem Soc 126:14129–14135. doi:10.1021/ja047156+ 43. Vlcek A, Zalis S (2007) Modeling of charge-transfer transitions and excited states in d6 transition metal complexes by DFT techniques . Coord Chem Rev 251:258–287. doi:10.1016/j.ccr.2006.05.021
ORIGINAL PAPER
Photophysical properties of copper(I) complexes containing pyrazine-fused phenanthroline ligands: a joint experimental and theoretical investigation Shengxian Xu 1 & Jinglan Wang 1 & Feng Zhao 1 & Hongying Xia 1 & Yibo Wang 2
Received: 9 June 2015 / Accepted: 2 November 2015 # Springer-Verlag Berlin Heidelberg 2015
Abstract Two copper(I) complexes [Cu(Pyz-Phen)2]PF6 (1) and [Cu(POP)(Pyz-Phen)]PF6 (2) (Pyz-Phen=pyrazino[2,3-f] [1, 10]phenanthroline, POP = bis[2-diphenylphosphino]phenyl]ether) have been synthesized and characterized. The photophysical properties of these complexes in solution have been studied. The electronic absorption spectrum of complexes 1 exhibit the lowest-lying MLCT absorption band at 459 nm and high-energy ligand-based transitions at 275 nm, while that of complex 2 exhibits the MLCT/LLCT band at 400 nm and ligand π-π* band at 262 nm. In addition, both 1 and 2 show similar phosphorescence 3MLCT/3LLCT emissions with maximum emission wavelengths of 569 and 572 nm, respectively. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) were employed to rationalize the photophysical properties of the complexes studied. The theoretical data confirm the assignment of the experimental absorption spectra and the nature of the emitting states.
Electronic supplementary material The online version of this article (doi:10.1007/s00894-015-2857-0) contains supplementary material, which is available to authorized users. * Feng Zhao [email protected] * Hongying Xia [email protected] 1
School of Chemistry and Chemical Engineering, Jiangxi Science and Technology Normal University, Fenglin Street Nanchang, Jiangxi 330013, People’s Republic of China
2
Key Laboratory of Guizhou High Performance Computational Chemistry, Department of Chemistry, Guizhou University, Guiyang 550025, People’s Republic of China
Keywords Copper(I) complex . Density functional theory . Emission . Photophysical . Pyrazino[2,3-f] [1, 10] phenanthroline
Introduction Copper(I) complexes have attracted considerable attention since the pioneering work of McMillin on the photoluminescence properties of copper diimine complexes in the 1970s [1]. There are some advantages for Cu(I) complexes, including their low cost, earth abundancy, and few toxic properties. Furthermore, Cu(I) complexes show some photophysical properties similar to those of ruthenium(II) complexes that are widely used for solar energy conversion, OLED, photocatalysis, and so on [2–5]. However, it is noteworthy that the square planar structures of the types of complexes in excited states are easily attacked by solvent molecules, which results in the solvent-induced exciplex quenching and sequent reduction of the luminescent performance [6, 7]. The photophysical properties of Cu(I) complexes can be improved through the judicious choice of ligands. Until now, the number of ligands with different structural and electronic properties and the corresponding Cu(I) complexes have been designed and synthesized. Among them, the most commonly used ligands are based on 1,10phenanthroline (Phen) [8–10]. Generally, 2,9-substituted Phen with the bulky groups is preferred to improve the photoluminescence of Cu(I) complexes [11–15]. In addition, one can furthermore adjust the photophysical properties of Cu(I) complexes by changing the substituents at other positions of the Phen rings [16]. Another approach to alter the emission properties of Cu(I) complexes is directly through modification of the skeleton of the Phen ligand by fusing heterocyclic rings. This strategy has been employed in the
31
J Mol Model31 : 2 )5102(
Page 2 of 10
purchased from Aladdin-reagent Co. and used without further purification. Concentrated sulfuric acid and nitric acid el at. were purchased from Sinopharm Chemical Reagent Co. Fig. 1 Structures of N-heterocyclic fuesed phenanthroline ligands
design of Cu(I) complexes which bear the imidazole fused Phen ligand capable of providing the expanded πconjugated system [17–20]. Following a similar strategy, as shown in Fig. 1, replacement of the five-membered imidazole ring by the sixmembered pyrazine can give a novel pyrazino[2,3-f][1, 10]phenanthroline ligand (Pyz-Phen). The six-membered pyrazine is generally known to be an electron-poor heterocycle in nature compared to the five-membered imidazole and should cause significant changes in the photophysical properties of the corresponding Cu(I) complexes. Herein we present the homoleptic [Cu(Pyz-Phen) 2 ] + (1) and the heteroleptic [Cu(POP)( Pyz-Phen)]+ (2) complexes (Fig. 2) and thoroughly study theirs photophysical properties. The POP ligand was employed not only due to its greater rigidity but also its ability to prevent solvent-induced exciplex quenching [15]. The present work is to provide a detailed analysis of the structural and spectral properties of the newly synthesized complexes. Meanwhile, the assignment of the electronic absorption and emission spectra were also investigated using density functional theory (DFT) and time-dependent density functional theory (TDDFT), and the results are compared with the corresponding experimental data.
Experimental details Materials 1,10-phenanthroline (Phen), 2,9-dimethyl-1,10-phenanthroline, Ethylenediamine, potassium bromide, [Cu(CH3CN)4]PF6 (Tetrakis(acetonitrile)copper(I) hexafluorophosphate), and (bis[2-(diphenylphosphino)phenyl]ether) (POP) were
Fig. 2 Molecular structures of the Cu(I) complexes studied in this study
Characterization 1
H NMR spectra were performed in a Bruker AV400 MHz spectrometer, using tetramethylsilane (TMS) as internal reference. DMSO-d6 was used as the solvents. UV-vis absorption spectra were measured using a Perkin Elmer Lambda-900 spectrophotometer. Fluorescence spectra were determined with a Hitachi F-4500 fluorescence spectrophotometer. Synthetic procedure 2,9-Dimethyl-1,10-phenanthroline-5,6-dione A roundbottom flask containing 2,9-dimethyl-1,10-phenanthroline (3.35 g, 16 mmol) and potassium bromide (19.0 g, 160 mmol) was cooled in an ice bath. Concentrated sulfuric acid (60 ml) and concentrated nitric acid (30 ml) were added dropwise. The reaction mixture was heated under reflux for 3 hours, then cooled to room temperature and the solution poured slowly into deionized water (800 mL), neutralized with sodium bicarbonate, and extracted with CH2Cl2. After washed with water and dried, the chloroform was removed under reduced pressure to leave a yellow residue which was recrystallized from dioxane. Yield: 78 % (3.5 g). 1H NMR (400 MHz, DMSOd6): 8.20–8.22 (dd, 2H), 8.59–8.61 (dd, 2H), 2.82 (s,6H). 7,10-Dimethyl-pyrazino[2,3-f][1, 10]phenanthroline (PyzPhen) To a solution of dione compound 3 (3.7 g, 14 mmol) in absolute ethanol (200 ml) was added ethylenediamine (1.0 g, 16 mmol). The reaction solution was heated gently for 16 hours at 50 °C, then concentrated under reduced pressure to 100 mL and allowed to stand at room temperature for 3– 4 hours. The orange precipitate which formed was filtered and recrystallized from ethanol. Yield: 17% (0.63 g).1H NMR (400 MHz, DMSO-d6): 9.30 (d,2H,J=8.3 Hz), 9.10 (d, 2H), 7.76 (d, 2H, J=8.3 Hz), 2.84(s, 6H).
J Mol Model (2015):3
Complex 1 Two equivalents of Pyz-Phen were dissolved in a nitrogen-saturated solution of 25 % CH2Cl2 in acetonitrile (20 mL). The acetonitrile solution was slowly added to 1 equiv of [Cu(CH3CN)4](PF6) under nitrogen. The solution immediately turned dark red, and the mixture was stirred for 30 min at room temperature. The solution was evaporated to dryness, and the resulting red solid was recrystallized with diethyl ether from CH2Cl2. Yield: 40 % (29 mg). 1H NMR (400 MHz, DMSO-d6): 9.68 (s, 4H), 9.18 (s, 4H), 8.01 (s, 4H), 2.57 (s, 12H). Complex 2 One hundred milliliter flask was charged with [Cu(CH 3CN) 4 ](PF6 ) (31 mg, 0.1 mmol), POP (54 mg, 0.1 mg) and nitrogen-saturated solution of CH2Cl2 (10 mL). The mixture was stirred for 2 h under nitrogen. Pyz-Phen (26 mg, 0.1 mmol) was added. The mixtures turned dark red instantaneously, and then stirred for 10 h at room temperature. After 10 h, the mixture was filtered with kieselguhr and the clear red filtrate was evaporated to dryness. Red solid was recrystallized with diethyl ether from CH2Cl2. Yield: 57 % (58 mg). 1H NMR (400 MHz, DMSO-d6): 9.49–9.51 (d, 2H, J=8.0 Hz), 9.0 (s, 2H), 7.77–7.79 (d, 2H, J=8.0 Hz), 7.33– 7.77 (t, 3 H), 7.16–7.26 (m, 8 H), 7.05–7.07 (d,8 H, 8.0 Hz), 6.96–7.03 (m, 10 H), 2.51(s, 6 H). DFT calculations All calculations were performed using the Gaussian 09 [21] program package. The B3LYP exchange-correlation function [22, 23] was used to optimize the ground state geometries of complexes 1–2 using the polarized continuum model (PCM) in CH2Cl2 media. The 6-31G* basis set [24] was used on C, H, N, O, and P atoms. LANL2DZ basis set [25, 26] was adopted on Cu atom. Recent calculations with the B3LYP/6-31G(d)/LANL2DZ level for the similar structure Cu(I) complexes have proved their
Scheme 1 The synthetic pathways of the ligands and Cu(I) complexes
Page 3 of 10 31
reliability and gave good agreement with the available experiments [27–30]. The unrestricted B3LYP method was used to optimize the lowest triplet state geometries (T1) in CH2Cl2 solvent. On the basis of the optimized ground state geometries, TDDFT method [31, 32] associated with PCM [33, 34] in CH2Cl2 media at the same level of theory used for geometrical optimization were used to simulate the absorption spectra of complexes 1–2. The first 100 singlet vertical excitations were obtained form TDDFT output file to construct the calculated absorption spectra. Calculated electronic density plots for the frontier molecular orbitals were prepared using Gauss View 4.1.2 software and the spin-density populations are calculated using Multiwfn analyzer soft [35].
Results and discussion Synthesis The synthetic pathways of the ligands and Cu(I) complexes are shown in Scheme 1. 1,10-Phenanthroline-5,6dione was synthesized according to a modified literature procedure [36]. Pyz-Phen ligand was prepared by a condensation reaction according to the literature procedure [37]. Complex 1 was prepared by the conventional reaction of [Cu(CH3CN)4]BF4 with two equivalent of PyzPhen ligand. Complex 2 was prepared by the reaction of [Cu(CH3CN)4]BF4 with one equivalent of Pyz-Phen ligand and one equivalent POP ligand. Photophysical properties Figure 3 shows the UV-vis spectra of complexes 1–2 in CH2Cl2 solution at room temperature. The high-lying absorption band of complex 1 localized at 275 nm is observed, which
31
Page 4 of 10
Fig. 3 Absorption spectra in dichloromethane at 298K of complexes 1 and 2. (inset) the region of 300–540 nm is magnified to allow easy comparison
can be intuitively described as the ligand-centered π→π* transitions of the Pyz-Phen ligand. At longer wavelengths (350–500 nm), the lowest-lying absorption band maxima at 459 nm is present, which is assigned to metal-to-ligand charge transfer transitions (d(Cu)→π*(Pyz-Phen)) (MLCT) . In the case of complex 2, the highest-lying absorption band localized at 262 nm should be assigned to π(Pyz-Phen)→π*(PyzPhen)) transition with comparable contribution of ligand-toligand charge transfer (LLCT) originated from π(POP)→ π*(Pyz-Phen), which is supported by TDDFT calculations (see below). A weak MLCT absorption band at 400 nm is observed (see inset Fig. 3). This band is blue-shifted by 59 nm compared to that of complex 1 due to the replacement of the Pyz-Phen ligand by POP ligand, resulting in the increase of HOMO-LUMO energy gap of complex 2. The normalized emission spectra of 1–2 in CH2Cl2 solution are shown in Fig. 4. Complexes 1 and 2 show similar emission properties with broad and structureless features, suggesting that the nature of such emission could be assigned to the lowest 3MLCT character with some 3LLCT character. The emission maxima at 572 nm of complexes 2 exhibits a red-shifting of 3 nm compared with complex 1 (λmax 569 nm).
J Mol Model31 : 2 )5102(
Fig. 4 Normalized emission spectra of complexes 1 and 2 in dichloromethane solution
noteworthy that the dihedral angle between the two Pyz-Phen ligands is 90.0°for complex 1, indicating a perfect D2d tetrahedron-like structure, while the dihedral angle between POP and Pyz-Phen ligands is 89.2° for complex 2, suggesting that a slight distortion occurs. The frontier molecular orbital compositions for complexes 1 and 2 have been analyzed using Multiwfn analyzer soft [35] and are listed in Tables 1 and 2. To conveniently discuss the orbital characters, complex 1 has been
Theoretical calculations Ground state geometries and molecular orbital properties The optimized ground state geometric structures for complexes 1 and 2 are shown in Fig. 5. Vibrational frequencies were calculated based on these optimized geometries to verify that the geometries represented a minimum on the potential energy surface. It is
Fig. 5 Optimized ground state and triplet excited geometric structures of complexes 1 and 2 in CH2Cl2 solvent. H-atoms are omitted for clarity
J Mol Model (2015):3 Table 1 Frontier molecular orbital compositions (%) for complex 1 the ground state in CH2Cl2 solution at the B3LYP/631G*/LANL2DZ level
Page 5 of 10 31
Orbital
Energy(ev)
Main bond type
Cu −2.19
1st Pyz-Phen
Cu component 2nd Pyz-Phen
π*(Pyz-Phen)
0.47
49.7
49.8
L+3 −2.32 π*(Pyz-Phen) L+2 −2.32 π*(Pyz-Phen) L+1 −2.44 π*(Pyz-Phen) L −2.44 π*(Pyz-Phen) HOMO-LUMO gap (3.54 ev) H −5.98 d(Cu) H-1 −5.98 d(Cu) H-2 −6.68 d(Cu) H-3 −6.81 d(Cu) H-7 −7.24 π(Pyz-Phen)
2.91 2.91 0.80 0.80
67.8 29.3 74.9 24.3
29.3 67.8 24.3 74.9
72.3 72.3 88.9 90.0 0.00
13.1 14.6 5.57 4.97 50.0
14.6 13.1 5.57 4.98 50.0
L+4
segmented into three fragments, namely Cu, 1st Pyz-Phen, and 2nd Pyz-Phen units, whereas complex 2 has been segmented into four fragments, namely Cu, Pyz-Phen, Ppop, and PhPOP units, where Ppop denotes P atom in POP ligand, and PhPOP denotes the phenyl groups in POP ligand. Figure 6 shows the energy levels and plots of the main molecular orbitals of interest in complexes 1 and 2. For complex 1, due to the structural symmetry, the two highest occupied molecular orbitals (HOMO and HOMO-1) are degenerate and they have the same energies, both being localized on the metal Cu 3d orbital (mostly of 3dxy and 3dxz nature, respectively). The HOMO-2, lying ca. 0.7 eV below the HOMO-1, is composed of the metal Cu 3dyz orbital. At lower energy, ca. 0.13eV below the HOMO-2, the HOMO-3 is composed of a combination of 3d x 2 - y 2 and 3d z 2 orbitals.
Table 2 Frontier molecular orbital compositions (%) for complex 2 the ground state in CH2Cl2 solution at the B3LYP/631G*/LANL2DZ level
Contribution (%)
Orbital
Energy (ev)
Additionally, the first two lowest unoccupied molecular orbitals (LUMO and LUMO+1) are a degenerate couple of π* character delocalized on the Pyz-Phen ligand. At higher energies two degenerate orbitals (LUMO+2 and LUMO+3) with the π* character are found. In the case of complex 2, the distortions in the tetrahedral configuration brought by the replacement of each of two PyzPhen ligands by POP ligand break the degeneracy of molecular orbitals. The HOMO and HOMO-1 are separated by 0.40 eV. Here, the HOMO is mainly composed of the Cu 3dxy and π(Ph) orbitals with comparable contributions from P atom, whereas the HOMO-1 is composed of a combination of the Cu 3dx2-y2 and 3dz2 and π(Ph) orbitals. The HOMO-2 is mainly composed of the metal Cu 3dyz orbital. The HOMO-3 has major contributions from phenyl group in POP ligand. The three
Main bond type
L+3 −1.36 π*(Ph) L+2 −2.11 π*(Pyz-Phen) L+1 −2.27 π*(Pyz-Phen) L −2.45 π*(Pyz-Phen) HOMO-LUMO gap (3.59 ev) H −6.04 d(Cu)+π(Ph) H-1 −6.44 d(Cu)+π(Ph) H-2 −6.52 d(Cu)+π(Ph) H-3 −6.75 π(Ph) H-5 −7.17 π(Pyz-Phen) H-6 −7.19 π(Ph) H-11 −7.33 d(Cu)+π(Ph) a
69.9dxy 69.9dxz 86.4dyz 67.5 dz2 +22.5dx2-y2
Contribution (%)
Cu component
Cu
Pyz-Phen
PPOPa
PhPOPb
1.51 0.37 3.08 0.26
2.14 98.8 93.6 98.8
14.3 0.19 1.98 0.23
81.7 0.59 1.35 0.75
36.2 48.5 68.6 9.69 6.23 8.26 41.7
3.91 8.99 4.69 2.69 83.8 4.09 4.12
25.6 14.4 0.18 1.14 0.29 0.12 0.43
32.8 26.4 25.3 67.1 9.54 87.4 51.9
Components for P atom in POP ligand. b Components for phenyl group in POP ligand
22.8dxy 16.6dz2 +17.7 dx2-y2 63.9dxz
25.6dyz +10.6dz2
31
J Mol Model31 : 2 )5102(
Page 6 of 10 Fig. 6 Energy levels and the energy values calculated for the molecular orbitals of complexes 1 and 2 with representative contour plots of the molecular orbitals
LUMOs (LUMO, LUMO+1 and LUMO+2) all have π* character delocalized on the Pyz-Phen ligand, and the LUMO+3 has π*(Ph) character. In addition, it is clearly seen that the HOMO is slightly higher in energy for complex 1 (-5.98 eV) compared to complex 2 (-6.04 eV), while the LUMO is almost the same in energy (-2.44 eV for 1 and -2.45 eV for 2). Hence, the HOMO-LUMO energy gap increases on going from complexes 1 to 2. Theoretical absorption spectra To gain insight into the transition character of absorption spectra of all the complexes, TDDFT calculations were undertaken to simulate the absorption spectra. Table 3 lists the dominant energy singlet-singlet vertical excitations, their oscillator strengths, assignment configurations, and excitations with
maximum coefficients for complexes 1 and 2. Absorption spectra were simulated by a Gaussian convolution. A comparison of calculated and experimental absorption spectra for complexes 1–2 is presented in Fig. 7. For complex 1, we calculate an intense feature at 268 nm, in good agreement with the 275 nm experimental value. The absorption peak at 268 nm mainly originates form the transitions of H-7→L+4 (34.5 %). The H-7 mainly consists of π(Pyz-Phen), whereas the L+4 are predominantly composed of π*(Pyz-Phen). Thus, this band can be assigned to intraligand charge transfer (ILCT) in character. At longer wavelengths, the calculated lowest-lying absorption band localized at 447 nm, corresponding to the absorption peak at 459 nm observed in experiment, should be assigned to the H/H-1→L/ L+1 transitions originated from [d(Cu)]→[π*(Pyz-Phen)] with the metal-to-ligand charge transfer (MLCT) character.
Table 3 Electronic absorptions of complexes 1 and 2 in CH2Cl2 based on TDDFT calculations at the (B3LYP)/6-31g*/LANL2DZ level, together with the experimental values Complex
Excited state
Transition
Coeff
E (eV)/(nm)
Oscillator strength
Assigna
Exptl/ nm
1
3
0.2126
4.62/268 3.05/407
0.9036 0.0610
4.71/263
0.2574
MLCT MLCT ILCT MLCT/LLCT MLCT/LLCT ILCT LLCT LLCT/MLCT
459
57 1
0.37263 (28.0%) 0.37411 (28.0%) 0.41562 (34.5%) 0.63491 (80.6%) −0.29104 (16.9%) 0.26428 (14.0%) 0.25422 (12.9%) −0.24623 (12.1%)
2.79/445
2
H→L H-1→L H-7→L+4 H→L H→L+1 H-5→L+2 H-6→L+2 H-11→L+2
56
a
MLCT: metal-to-ligand charge transfer; LLCT: ligand-to-ligand charge transfer; ILCT: intra-ligand charge transfer
275 400 262
J Mol Model (2015):3
Page 7 of 10 31
Fig. 7 Comparison of the calculated (red line) and experimental (black line) absorption spectra in CH2Cl2 solution for complex 1 (a); complex 2 (b). Red vertical lines correspond to oscillator strength of calculated singlet-singlet transitions. The full width at halfmaximum (fwhm) is set to 1210 and 686 cm−1 for complexes 1 and 2, respectively
The main spectral features in experiments are well reproduced by our TDDFT calculation. With respect to complex 2, the agreement between the calculated and experimental spectra is quite good, even though the calculated spectra appear to be slightly red-shifted. The highest-lying absorption band calculated at 263 nm, which corresponds well to the experimental value of 262 nm, appears to mainly originate from the mixed transitions of H-5 [π(PyzPhen)]→L+2 [π*(Pyz-Phen)] (14.0 %), H-6 [π(Ph)]→L+2 [π*(Pyz-Phen)] (12.9 %) and H-11 [d(Cu)+π(Ph)]→L+2 [π*(Pyz-Phen)] (12.1 %) with ILCT/LLCT character. Additionally, the calculated lowest-lying absorption band localized at 407 nm, corresponding to the weaker absorption peak at 400 nm observed in experiments, should be assigned to the degenerate H/H-1 [d(Cu)+π(Ph)]→L/L+1 [π*(Pyz-Phen)] transition with MLCT/LLCT character. Additionally, in order to improve the analysis of the nature of absorption transitions, the natural transition orbital (NTO) analysis [38] have been performed for complexes 1 and 2 based on the TDDFT calculations, as presented in Table 1S. The NTO results further supported the assignment of the absorption transition nature. For comparison purpose, a long-range separated hybrid functional CAM-B3LYP [39], which is now widely used to CT excitations, was employed to predict the vertical absorption energies for complexes 1 and 2. However, it is clearly seen that the CAM-B3LYP functional provides poor results, which may be due to the fact that the present
CT excitations are shorter ranged (see Fig. 1S, Supporting information). Triplet excited states and emission properties The lowest-lying triplet geometries (T1) were optimized at the spin-unrestricted UB3LYP/6-31G*/LANL2DZ level in CH2Cl2 solution. The optimized structures of the T1 state of complexes 1 and 2 have been provided in Fig. 5. The remarkable geometry difference between the S0 and T1 state is the dihedral angle between the two ligands. The dihedral angle between two Pyz-Phen ligands for complex 1 changes from the 90.0° in the S0 state to 65.2° in the T1 state, while the dihedral angle between POP and Pyz-Phen ligands for complex 2 changes from the 89.2 to 72.5°. The calculated relaxation energies are a little large (see Table 2S, Supporting information), indicating the presence of significant structural relaxation. To gain insight into the nature of the emission properties of complexes 1 and 2, the emission energies are calculated using TDDFT method at their respective optimized T1 geometry. It is very important to find a suitable functional that would allow for the reliable prediction of emission properties. Here, five functionals (B3LYP, PBE0, MPW1B95, BMK, M06-2X) were employed to calculate the emissive energies for all complexes including CH2Cl2 solvent. Relatively larger change in the T1 emission energies calculated form the B3LYP (20 % HF) to M06-2X (54 % HF) functionals is reasonable since the
Table 4 Calculated phosphorescent emission of the studied complexes in CH2Cl2 solution at TDDFT/MPW1B95 level, together with the experimental values Complex
Major configuration
λemcal/nm
Assignment
λemExptl/ nm
1 2
LUMO→HOMO (90.9%) LUMO→HOMO (82.4%)
664 549
MLCT/LLCT MLCT/LLCT
569 572
31
J Mol Model31 : 2 )5102(
Page 8 of 10
Fig. 8 Spin density distribution contours (isovalue=0.001) for the lowest triplet state T1 of complexes 1 and 2. The values of the unpaired-electron spindensity population are depicted together with the electronic nature of the states
absolute values obtained using these functionals strongly depend on the amount of Hartree-Fock exchange incorporated in these hybrid functionals [40, 41] (see Table 2S, Supporting information). Overall, the MPW1B95 functional gave more satisfactory emissive energies and was employed to further explore the nature of the emissive state. Additionally, ΔSCF approach was also employed to calculate the emission energies [42, 43] (Table 2S, Supporting information). This method is found to be slightly worse than the TDDFT/B3LYP combination in our cases. The results of the TDDFT calculations are listed in Table 4. The lowest-energy emission of complex 1 originates mainly from LUMO→HOMO transition assigned to [π*(Pyz-Phen)→d(Cu)] with the 3MLCT character. The emission of complex 2 at 549 nm is also mainly contributed by LUMO →HOMO transition originated from [π*(PyzPhen)→d(Cu)+π(Ph)] with 3MLCT/3LLCT character. The nature of the emitting excited state of each complex can furthermore be identified by the unpaired-electron spindensity distribution, as shown in Fig. 8. For complex 1, the unpaired electron spin-density distribution in the T1 state (Cu, 0.52e; 1st Pyz-Phen, 0.15e; 2nd Pyz-Phen, 1.16e) illustrates the higher contribution of the d(Cu) to the 2nd Pyz-Phen ligand with little contribution of 1st Pyz-Phen ligand to the 2nd PyzPhen ligand and confirms the emission behavior of 3MLCT with little contribution of 3LLCT. The spin-density distribution is observed for complex 2 in the T1 state (Cu, 0.51e; POP, 0.35e; Pyz-Phen, 1.14e) and the 3MLCT/3LLCT characters are responsible for the T1 state of complex 2.
in the 400–500 nm region of the absorption spectra of complex 1 is assigned to the 1MLCT transition, while complex 2 displays the 1MLCT/1LLCT absorption character. Complexes 1 and 2 exhibit luminescence bands centered at 569 and 572 nm, respectively, attributed to the 3MLCT/3LLCT characters. Theoretical calculations furthermore support the assignments of the nature of the emitting excited states for complexes studied. These experimental and theoretical insights should be expected to provide some guides for the design and synthesis of efficient luminescent copper(I) complexes. Acknowledgments The authors acknowledge financial support by the National Natural Science Foundation of China (No.21462020, 21443010, and 21563013) and Jiangxi Science and Technology Normal University Key Laboratory of Organic-inorganic Composite Materials (Key training base). They thank the Guizhou University High Performance Computation Chemistry Laboratory (GHPCC) for help with computational studies.
References 1.
2.
3.
Conclusions In this paper, the homoleptic [Cu(Pyz-Phen)2]+ and heterleptic [Cu(POP)(Pyz-Phen)]+ complexes were synthesized and characterized. The absorption and luminescence properties of the two complexes are investigated experimentally and are interpreted by means of DFT and TDDFT calculations. Both experiment and theory show that the absorption band appearing
4.
5.
McMillin DR, Buckner MT, Ahn BT (1977) A light-induced redox reaction of bis (2, 9-dimethyl-1, 10-phenanthroline) copper(I). Inorg Chem 16(4):943–945. doi:10.1021/ic50170a046 Funaki T, Funakoshi H, Kitao Onozawa-Komatsuzaki ON, Kasuga K, Sayama K, Sugihara H (2012) Cyclometalated ruthenium (II) complexes as near-IR sensitizers for high efficiency dye-sensitized solar cells. Angew Chem Int Ed 51(30):7528–7531. doi:10.1002/ anie.201108738 Bolink HJ, Cappelli L, Coronado E, Gavina P (2005) Observation of electroluminescence at room temperature from a ruthenium(II) bis-terpyridine complex and its use for preparing light-emitting electrochemical cells. Inorg Chem 44(17):5966–5968. doi:10. 1021/ic0507058 Wong KMC, Chan MMY, Yam VWW (2014) Supramolecular assembly of metal-ligand chromophores for sensing and phosphorescent OLED applications. Adv Mater 26(20):5558–5568. doi:10. 1002/adma.201306327 Kumar P, Bansiwal A, Labhsetwar N, Jain SL (2015) Visible light assisted photocatalytic reduction of CO2 using a graphene oxide supported heteroleptic ruthenium complex. Green Chem 17(3): 1605–1609. doi:10.1039/c4gc01400f
J Mol Model (2015):3 6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
McMillin DR, Kirchhoff JR, Goodwin KV (1985) Exciplex quenching of photo-excited copper complexes. Coord Chem Rev 64(4):83–92. doi:10.1016/0010-8545(85)80043-6 Kober EM, Caspar JV, Lumpkin RS, Meyer TJ (1986) Application of the energy gap law to excited-state decay of osmium(II)polypyridine complexes: calculation of relative nonradiative decay rates from emission spectral profiles. J Phys Chem 90(16):3722– 3734. doi:10.1021/j100107a046 Armaroli N, Accorsi G, Holler M, Moudam O, Nierengarten JF, Zhou Z, Wegh RT, Welter R (2006) Highly luminescent Cu(I) complexes for light-emitting electrochemical cells. Adv Mater 18(10): 1313–1316. doi:10.1002/adma.200502365 Zhang QS, Zhou QG, Cheng YX, Wang LX, Ma DG, Jing XB, Wang FS (2006) Highly efficient electroluminescent from greelight emitting electrochemical cells based on Cu complexes. Adv Funct Mater 16(22):1203–1208. doi:10.1002/adfm.200500691 Wei FX, Fang L, Huang Y (2010) Synthesis, characterization, crystal structures, and photophysical properties of a series of roomtemperature phosphorescent copper(I) complexes with oxadiazolederived diimine ligand. Inorg Chim Acta 363(11):2600–2605. doi: 10.1016/j.ica.2010.04.041 Ichinaga AK, Kirchhoff JR, McMillin DR, Dietrich-Buchecker CO, Marnot PA, Sauvage JP (1987) Charge-transfer absorption and emission of Cu(NN)2+ systems. Inorg Chem 26:4290–4292. doi: 10.1021/ic00272a030 Cunningham CT, Cunningham KLH, Michalec JF, McMillin DR (1999) Cooperative substituent effects on the excited states of copper phenanthrolines. Inorg Chem 38(20):4388–4392. doi:10.1021/ ic9906611 Vogtle F, Luer I, Balzani V, Armaroli N (1991) Endoreceptors with convergent phenanthroline units: a molecular cavity for six guest molecules. Angew Chem Int Ed 30(10):1333–1336. doi:10.1002/ anie.199113331 Kern JM, Sauvage JP, Weidmann JL, Armaroli N, Flamigni L, Ceroni P, Balzani V (1997) Complexes containing 2,9-bis(pbiphenylyl)-1,10-phenanthroline units incorporated into a 56membered ring. synthesis, electrochemistry, and photophysical properties. Inorg Chem 36(23):5329–5338. doi:10.1021/ic970707v Cuttell DG, Kuang SM, Fanwick PE, McMillin DR, Walton RA (2001) Simple Cu (I) complexes with unprecedented excited-state lifetimes. J Am Chem Soc 124(1):6–7. doi:10.1021/ja012247h McCusker CE, Castellano FN (2013) Design of a long-lifetime, earth-abundant, aqueous compatible Cu(I) photosensitizer using cooperative steric effects. Inorg Chem 52(14):8114–8120. doi:10. 1021/ic401213p Bozic-Weber B, Constable EC, Furer SO, Housecroft CE, Troxler LJ, Zampese JA (2013) Copper(I) dye-sensitized solar cells with [Co(bpy)3] 2+/3+ electrolyte. Chem Commun 49(65):7222–7224. doi:10.1039/c3cc44595j Sandroni M, Kayanuma M, Planchat A, Szuwarski N, Blart E, Pellegrin Y, Daniel C, Boujtita M, Odobel F (2013) First application of the HETPHEN concept to new heteroleptic bis(diimine) copper(I) complexes as sensitizers in dye sensitized solar cells. Dalton Trans 42(30):10818–10827. doi:10.1039/c3dt50852h Wen CH, Tao GQ, Xu XH, Feng XQ, Luo RC (2011) A phosphorescent copper(I) complex: synthesis, characterization, photophysical property, and oxygen-sensing behavior. Spectrochim Acta A 79(5):1345–1351. doi:10.1016/j.saa.2011.04.067 Li Z (2011) Synthesis, characterization and theoretical analysis on a oxygen-sensing phosphorescent copper(I) complex. 81(1) 475– 480. doi: 10.1016/j.saa.06.040 Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA, Cheeseman JR, Scalmani G, Barone V, Mennucci B, Petersson GA, Nakatsuji H, Caricato M, Li X, Hratchian HP, Izmaylov AF, Bloino J, Zheng G, Sonnenberg JL, Hada M, Ehara M, Toyota K, Fukuda R, Hasegawa J, Ishida M, Nakajima T, Honda Y, Kitao O,
Page 9 of 10 31 Nakai H, Vreven T, Montgomery JJA, Peralta JE, Ogliaro F, Bearpark M, Heyd JJ, Brothers E, Kudin KN, Staroverov VN, Kobayashi R, Normand J, Raghavachari K, Rendell A, Burant JC, Iyengar SS, Tomasi J, Cossi M, Rega N, Millam NJ, Klene M, Knox JE, Cross JB, Bakken V, Adamo C, Jaramillo J, Gomperts R, Stratmann RE, Yazyev O, Austin AJ, Cammi R, Pomelli C, Ochterski JW, Martin RL, Morokuma K, Zakrzewski VG, Voth GA, Salvador P, Dannenberg JJ, Dapprich S, Daniels AD, Farkas O, Foresman JB, Ortiz JV, Cioslowski J, Fox DJ (2009) Gaussian 09, revision A. Gaussian Inc, Wallingford 22. Runge E, Gross EK (1984) Density-functional theory for timedependent systems. Phys Rev Lett 52:997–1000. doi:10.1103/ physrevlett.52.997 23. Mayo SL, Olafson BD, Goddard WA (1990) DREIDING: a generic force field for molecular simulations. J Phys Chem 94(26):8897– 8909. doi:10.1021/j100389a010 24. Hariharan PC, Pople JA (1974) Accuracy of AHn equilibrium geometries by single determinant molecularorbital theory. Mol Phys 27(1):209–214. doi:10.1080/00268977400100171 25. Hay PJ, Wadt WR (1985) Ab initio effective core potentials for molecular calculations. potentials for the transition metal atoms Sc to Hg. J Chem Phys 82(8):270–283. doi:10.1063/1. 448799 26. Wadt WR, Hay PJ (1985) Ab initio effective core potentials for molecular calculations. potentials for main group elements Na to Bi. J Chem Phys 82(1):284–298. doi:10.1063/ 1.448800 27. Costa RD, Tordera D, Orti E, Bolink HJ, Schonle J, Graber S, Housecrof CE, Constable EC, Zampese JA (2011) Copper(I) complexes for sustainable light-emitting electrochemical cells. J Mater Chem 21(8):16108–16118. doi:10.1039/c1jm12607e 28. Fraser MG, van der Salm H, Cameron SA, Blackman AG, Gordon KC (2013) Heteroleptic Cu(I) bis-diimine complexes of 6,6′-dimesityl-2,2′-bipyridine: a structural, theoretical and spectroscopic study. Inorg Chem 52(6):2980–2992. doi:10. 1021/ic302393p 29. Hsu CW, Lin CC, Chung MW, Chi Y, Lee GH, Chou PT, Chang CH, Chen PY (2011) Systematic investigation of the metalstructure photophysics relationship of emissive d10-complexes of group 11 elements: the prospect of application in organic light emitting devices. J Am Chem Soc 133(9):12085–12099. doi:10.1021/ ja2026568 30. Bozic-Weber B, Chaurin V, Constable EC, Housecroft CE, Meuwly M, Neuburger M, Rudd JA, Schonhofer E, Siegfried L (2012) Exploring copper(I)-based dye-sensitized solar cells: a complementary experimental and TD-DFT investigation. Dalton Trans 41: 14157–14169. doi:10.1039/c2dt31159c 31. Casida ME, Jamorski C, Casida KC, Salahub DR (1998) Molecular excitation energies to high-lying bound states from time-dependent density-functional response theory: characterization and correction of the time-dependent local density approximation ionization threshold. J Chem Phys 108(11):4439–4449. doi:10.1063/1. 475855 32. Stratmann RE, Scuseria GE, Frisch MJ (1998) An efficient implementation of time-dependent density-functional theory for the calculation of excitation energies of large molecules. J Chem Phys 109(19):8218–8224. doi:10.1063/1.477483 33. Perdew JP, Burke K, Emzerhof M (1996) Generalized gradient approximation made simple. Phys Rev Lett 77(8):3865–3868. doi:10.1103/PhysRevLett.77.3865 34. Perdew JP, Burke K, Emzerho FM (1997) K+emission in symmetric heavy ion reactions at subthreshold energies. Phys Rev Lett 78(7):1396–1396. doi:10.1103/PhysRevLett.78.1396 35. Lu T, Chen FW (2012) Multiwfn: a multifunctional wavefunction analyzer. J Comput Chem 33(5):580–592. doi:10.1002/jcc.22885
31
Page 10 of 10
Hiort C, Lincoln P, Norden B (1993) DNA binding of Δ- and Δ[Ru(phen)2DPPZ]2+. J Am Chem Soc 115(9):3448–3454. doi:10. 1021/ja00062a007 37. Sun PP, Duan JP, Lih JJ, Cheng CH (2006) Synthesis of new europium complexes and their application in electroluminescent devices. Adv Funct Mater 13(9):683–691. doi:10.1002/adfm. 200304378 38. Martin RL (2003) Natural transition orbitals. J Chem Phys 118: 4775–4777. doi:10.1063/1.1558471 39. Yanai T, Tew DP, Handy NC (2004) A new hybrid exchange–correlation functional using the Coulomb-attenuating method (CAMB3LYP). Chem Phys Lett 393:51–57. doi:10.1016/j.cplett.2004.06.011 40. Huang SP, Zhang QS, Shiota Y, Nakagawa T, Kuwabara K, Yoshizawa K, Adachi C (2013) Computational prediction for 36.
J Mol Model31 : 2 )5102( singlet- and triplet-transition energies of charge-transfer compounds. J Chem Theory Comput 9:3872–3877. doi:10.1021/ ct400415r 41. Xu SX, Wang JL, Xia HY, Zhang F, Wang YB (2014) Computational prediction for emission energy of iridium (III) complexes based on TDDFT calculations using exchange-correlation functionals containing various HF exchange percentages. 21: 22– 24. doi: 10.1007/s00894-014-2557-1 42. Lowry MS, Hudson WR, Pascal RA Jr, Bernhard S (2004) Accelerated luminophore discovery through combinatorial synthesis. J Am Chem Soc 126:14129–14135. doi:10.1021/ja047156+ 43. Vlcek A, Zalis S (2007) Modeling of charge-transfer transitions and excited states in d6 transition metal complexes by DFT techniques . Coord Chem Rev 251:258–287. doi:10.1016/j.ccr.2006.05.021
