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Tunable luminescence from rare 2D Ga(III)/In(III) coordination polymers coexisting with three different conjugated system aromatic ligands† Xin Ming Wang,a Rui Qing Fan,*a Liang Sheng Qiang,a Wei Qi Li,b Ping Wang,a Hui Jie Zhanga and Yu Lin Yang*a
DOI: 10.1039/c3cc48867e www.rsc.org/chemcomm
Two rare 2D Ga/In-based coordination polymers in which one metal center coexists with three distinct aromatic ligands were synthesized. Helical channels along the 21 screw axis are exhibited to form a hcb net. The compounds exhibit tunable fluorescence from blue, green, white to yellow light by varying the temperature and solvents.
Since the first report of organic light-emitting diodes fabricated from tris(8-hydroxyquinolinolate)aluminum(III) (Alq3) in 1987,1 Group 13 complexes have attracted much attention due to their variety of structures, adequate electron transport, and luminescence properties.2 A binuclear gallium complex Ga2(saph)2q2 has been synthesized, which possesses a much higher luminescence efficiency and better thermal stability than the typical Alq3.3 Inspired by the prominent work of M. C. Hong, J. Zhang, O. M. Yaghi and their co-workers,4 we considered gallium and indium to be good candidates for construction of coordination polymers (Cps) due to the larger ion radii and more fascinating coordination properties than that of B and Al. However, the highest challenge from the synthesis process is the recognized bottleneck for the hydrolysis of Group 13 metal(III) salt5 and the smaller cation of Group 13 than the rare earth element, which enhances the difficulty of coordination. In this communication, we report two rare Ga(III)/In(III)-based twodimensional (2D) Cps {M[(2,20 -bpda)(1,4-bda)0.5(phen)]0.5H2O}n (M = Ga (Ga1), In (In1), 2,20 -H2bpda = 2,20 -biphenyldicarboxylic acid, 1, 4-H2bda = 1,4-benzenedicarboxylic acid, phen = 1,10phenanthroline) prepared from the reaction of mixing In(NO3)3, 2,20 -H2bpda, 1,4-H2bda and phen in a 4 : 1 : 1 : 1 molar ratio under hydrothermal synthesis at 160 1C for 5 days. For the hydrolysis reaction of Ga(III) and In(III) to occur, we finely regulated the range of the system pH. An exploration of the synthesis reveals that crystals of Ga1 and In1 are well formed only with pH in the range 3.4–3.5.
To the best of our knowledge, Ga1 and In1 are the first examples of the Group 13 Cps containing three different aromatic ligands from the single benzene ring (1,4-H2bda), biphenyl rings (2,2 0 -H2bpda) to three aromatic rings with the N-containing heterocyclic structure (phen). Ga1 and In1 are isomorphous, and therefore, only the structure of Ga1 is described in detail. A single crystal X-ray diffraction study unveiled the structure of Ga1 (Fig. 1). Every Ga3+ cation is seven coordinated with slightly distorted pentagonal bipyramid coordination geometry GaO5N2 by five O atoms (O1 and O2, O3, O5 and O6) from 2,20 -bpda2 adopting the m2 = Z1:Z1:Z1:Z0 bridging fashion, and 1,4-bda2 ligands adopting a chelating fashion, and two N atoms (N1 and N2) from phen ligands. It should be mentioned that seven coordinated mode of Ga/In(III) is rare and it is the first example among the reported Group 13 compounds, where the center metal Ga3+/In3+ coexists with three different aromatic ligands. The Ga–O bond lengths are 2.154(8)– 2.342(7) Å, then Ga–N bond lengths are 2.307(8) and 2.311(8) Å, respectively. These bond lengths are much longer than those observed in most other Ga(III) coordination polymers (average Ga–O/N bond lengths are 1.80–2.00 Å and 1.80–2.20 Å respectively).6
a
Department of Chemistry, Harbin Institute of Technology, Harbin 150001, P. R. China. E-mail: [email protected], [email protected]; Fax: +86-451-86418270 b Department of Physics, Harbin Institute of Technology, Harbin 150001, P. R. China † Electronic supplementary information (ESI) available. CCDC 953555 and 953556. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3cc48867e
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Fig. 1 (a) Asymmetric unit of Ga1; (b) perspective view of 1D chain; (c) the 2D layers containing single-stranded left- and right-handed helices linked; (d) schematic representation of the underlying network topology (color code: Ga, pink ball; 1,4-bda2, blue line; 2,2 0 -bpda2 turquoise line).
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The coexistence of three types of aromatic ligands give rise to a much crowded coordination sphere, thus the average bond distances extend to ease the steric hindrance.7 The phenyl rings of bpda2 in a nonplanar fashion (torsion angle is 56.331), lead to the formation of left- and right-handed helices, decorated by the phen ligands. The resulting handed helices with a pitch of 10.86 Å are alternately arranged in an equal ratio, extending into a 2D layered structure along the bc plane by bda2 anions. Each Ga(III) metal as the 3-connected nodes connects with two bpda2 and one bda2, to result in the generation of (6,3)-connected hcb topological net, with the grid size is 7.87(10) 20.55(20) Å (atom-to-atom distances). The 3D architecture of Ga1 or In1 is reinforced by the intermolecular hydrogen bonds interaction from OW1–HW1B O4, which is important for the molecular assembly8 (Fig. S7, ESI†). The emission spectrum of solid-state sample of Ga1 at 298 K is centered at 413 nm, producing blue emission (Commission Internacionale d’Eclairage (CIE) coordinates for Ga1 is 0.20, 0.19) (see Fig. 2). The spectrum of free 2,20 -H2bpda is centered at 412 nm, which is not perturbed upon its coordination to Ga(III) (Fig. S9, ESI†), thereby suggesting, that in Ga1, the observed emission is ligand based probably, which is assigned to the intraligand transitions of p* - p. In1 produces the selfsame blue emission as Ga1 (CIE coordinates are 0.20, 0.19). The emission bond of In1 appears around 401sh, 413, 436sh nm. Due to the existence of two shoulderpeaks in photoluminescence (PL) spectrum of phen ligand (lm = 365sh, 380, 400sh nm), and the maximum emission of In1 is just the same as that of free 2,20 -H2bpda ligand, therefore, the emission bond in In1 should be assignable to the intraligand charge transfer of 2,20 -bpda2 and phen. The difference observed in the fluorescence emission peaks of Ga1 and In1 likely have their origins including the metal ion in the unit cell and the steric proximity of ligands to each other. The solid-state PL spectra of Ga1 and In1 at liquid nitrogen temperature are researched. At 77 K, Ga1 emits intensely at 410 nm, giving light with CIE coordinates of 0.27, 0.30, which are at the blue end of the region generally considered white.
Fig. 2 Emission spectra of Ga1, In1 in the solid state at 298 K and 77 K (excited at 323 nm); CIE coordinates for Ga1 and In1 at 298 K (open symbols) and 77 K (closed symbols) (& = Ga1, n = In1), ‘‘‘‘denotes the pure white point (0.33, 0.33).
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Fig. 3 Molecular orbital amplitude plots of HOMO and LUMO energy levels of Ga1 and In1.
In1 has higher-energy (HE) emissions of two shoulder-peaks appearing around 400sh, 412, 428sh nm and the lower-energy (LE) emission bond centered at 542 nm. In1 generates light close to white which is at the green end with CIE coordinates of 0.28, 0.35. The HE maximum emission bond of Ga1 and In1 is nearly the same as that at 298 K, although the emission spectra exhibit more broader and unstructured bands. Meanwhile, the lifetimes of Ga1 and In1 increase conspicuously compared to those at 298 K (298 K: t = 7.24 ms for Ga1, t = 7.58 ms for In1; 77 K: t = 15.39 ms for Ga1, t = 11.62 ms for In1) (Fig. S11, ESI†), since cold conditions would be favorable for the rigidity of ligands with reducing the non-radiation decay and collisional quenching.9 To gain insights into the nature of photophysical properties of Ga1/In1, theoretical calculations on their energy levels were performed. The molecular orbital amplitude plots of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of Ga1/In1 are shown in Fig. 3. The electron densities of the HOMO singlet state are located in 2,2 0 -bpda2 units, whereas the densities for the LUMO are distributed mainly about the phen units. The transition from LUMO to HOMO involves interligand charge transfer between phen and 2,20 -bpda2. Namely, the Ga3+/In3+ and 1,4-bda2 contributions are small. The phen and 2,20 -bpda2 ligands which own a larger conjugate system than bda2 contribute most to the assignment in HOMO and LUMO energy levels. Therefore, this suggests that the emission observed in Ga1/In1 originates exclusively from ligand-to-ligand charge transfer (LLCT), namely, p* - p. Our TD-DFT calculations assign the energy level to HOMO - LUMO vertical transition (lcalc = 355 nm for Ga1 and lcalc = 330 nm for In1), in good correlation with the UV date and cyclic voltammetry (Table S7, ESI†). We have further examined the sensitivity of the emission response of Ga1 and In1 to the polar solvents (DMSO, CH3CN and CH3OH) at 298 K and 77 K (Fig. 4). The 1H NMR data indicate that Ga1/In1 still keeps a polymeric structure in solutions and does not decompose (Fig. S2 and S3, ESI†). At 298 K, Ga1 and In1 in different dilute solutions all generate stable royal blue emission with CIE coordinates of 0.15–0.16, 0.02–0.03, which are not affected by the solvent. It is notable that there is large
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Fig. 4 The emission spectra of Ga1 (a) and In1 (b) in different solvents at 298 K and 77 K (excited at 323 nm). CIE coordinates at 298 K (open symbols) and 77 K (closed symbols) (& = Ga1, n = In1). The inset shows how the coordinates vary: the coordinates based on the blue overlap closely at 298 K; the arrow shows the direction of change with the polarity of solvent increases from blue, cyan to white at 77 K. ‘‘‘‘denotes the pure white point (0.33, 0.33).
extent blue-shifted of PL in solution compared to that in solid state at 298 K, with the CIE color shift from light blue (0.20, 0.19) to royal blue (0.16, 0.03). It is due to that the intermolecular hydrogen bonds interaction in solid state are much more stronger than that in dilute solution,10 which can effectively decrease the HOMO–LUMO energy gap, so as to influence the p* - p transitions.11 The emissions of Ga1 and In1 are more obviously red shifted at 77 K than at 298 K and the largest shift is 107 nm for Ga1 and 175 nm for In1 in DMSO respectively. Notably, with the increasing of solution polarity (polarity: DMSO 4 CH3CN 4 CH3OH), the PL of Ga1 and In1 extend to the white region at 77 K. In1 exhibits a blue emission at 418 nm in CH3OH, a significantly red-shifted green emission centered at 490 nm in CH3CN, and a further redshifted to the green end emission at 409 and 550 nm in DMSO, with CIE coordinates of 0.31, 0.39, generally considered white emission. Ga1 exhibits a blue emission at 422 nm in CH3OH and CH3CN, a red-shifted white blue emission in DMSO, which is at the blue end of the region. The influence of the solvent on PL may be due to an increase of the dipole moment of excitation.12 An interesting phenomenon is observed, that is, the PL profile of In1 is more sensitive to the solvent than that of Ga1, namely, the solvent polarity could influence the relative intensity of HE and LE bands. At 298 K, the increase of the solution polarity gradually reduces the relative intensity of the HE peak but enhances the LE band for In1. In CH3OH solution, the HE emission band is much brighter than LE emission band; however, the LE emission band nearly dominates the total emission spectra in DMSO solution. In contrast, at 77 K, the effect of solventinduced aggregation on luminescence shows an increased relative intensity of the HE emission band with the increase of the solvent polarity, and slight changes in the vibronic components for the LE band, broadening the emission and gaining fine structure. These factors are the result of different intramolecular or intermolecular interactions among organic linkers and their energy transfer in solvent.13 Luminescence lifetimes for Ga1 and In1 in solutions at 77 K are mostly longer than that at 298 K (Fig. S12, ESI†). The most obvious observation is that the lifetime of In1 in CH3CN at 77 K (t = 10.77 ms) is 2.24-fold to that at 298 K (t = 4.81 ms). These features might be ascribed to an increased contribution from
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delayed fluorescence, which is caused by excimer formation at liquid nitrogen temperature.14 We investigate the quantum yields (F) of Ga1 and In1 in DMSO, CH3CN and CH3OH at 298 K (Table S5, ESI†). The fluorescence quantum yield (F) of Ga1 is found to be 0.273 in CH3OH, 0.223 in CH3CN and 0.182 in DMSO, which decreases with the increase of solution polarity. This effect is because of enhanced rate constants for the two nonradiative processes (internal conversion and intersystem crossing) in the Cps in polar solution.15 The corresponding F value of In1 is smaller than that of Ga1, which is 0.257, 0.199 and 0.120 in CH3OH, CH3CN and DMSO respectively, because of the heavy-atom quenching.16 To further clarify luminescence behavior in solution, the effect of deuterated solvents (DMSO-d6, CD3CN, CD3OD) on the luminescence of Ga1 and In1 have been investigated (Fig. S10, ESI†). Compared to those in normal solvents, the emission spectra in deuterated solvents exhibit similar characters, especially that Ga1 and In1 tend to give light from blue, green to yellow region at 77 K. The difference is that the luminescence lifetimes for Ga1 and In1 in deuterated solution are longer than that in the normal one (Fig. S14 and S15, ESI†). The lifetime of In1 in DMSO-d6 at 77 K (t = 13.27 ms) is even longer than that in the solid state (t = 11.62 ms). Therefore it is possible for the exchange of the hydrogen of free water molecules in the framework with the deuterium of deuterated solvent to occur. According to an early study,17 the luminescence quenching rate attributed to O–H bonds is proportional to the number of coordinating water molecules in the coordination sphere of Ga1 and In1, and this quenching rate displays such a strong isotope effect that the O–D bond essentially has no quenching effect.18 The isotope effect also contributes to the strong deuterium-contained H-bonds formed among D atoms from deuterated solvents and some D2O of the framework. The relatively shorter D O bonds compared to those in the non-deuterated system may enhance interaction of solute–solvent and stabilize the excited state of the electronic transitions.19 The thermogravimetric curves and variable-temperature X-ray powder diffraction showed that Ga1 and In1 network start to undergo phase transition above ca. 250 1C for Ga1, and 450 1C for In1, which induces the structure collapse (Fig. S5 and S6, ESI†). In summary, two novel 2D Group 13 Cps Ga1 and In1 have been synthesized from three different aromatic ligands, 1,4-H2bda, 2,2 0 -H2bpda, and phen. Such concordant coordination competition of three distinct conjugated ligands around one metal center is quite rare. Ga1 and In1 display stable blue fluorescence at 298 K. At 77 K, they exhibit a tunable emission from blue, green to yellow with the increasing polarity of the solvents, assuming solvent-dependent PL. The monochromaticity and stability of PL to Ga1 is better than In1, but the thermal stability of In1 is outstanding. Ga1 and In1 have potential applications in luminescent materials. This work was financially supported by the National Natural Science Foundation of China (Grant No. 21371040 and 21171044) and the National Key Basic Research Program of China (973 Program, No. 2013CB632900) and Supported by Program for Innovation Research of Science in Harbin Institute of Technology.
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Notes and references 1 C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett., 1987, 51, 913. 2 (a) S. T. Zheng, F. Zuo, T. Wu, B. Irfanoglu, C. Chou, R. A. Nieto, P. Y. Feng and X. H. Bu, Angew. Chem., Int. Ed., 2011, 50, 1849; (b) M. O’Keeffe and O. M. Yaghi, Chem. Rev., 2012, 112, 675; (c) H. B. Zhang, P. Lin, X. C. shan, F. L. Du, Q. P. Li and S. W. Du, Chem. Commun., 2013, 49, 2231; (d) X. T. Rao, T. Song, J. K. Gao, Y. J. Cui, Y. yang, C. D. Wu, B. L. Chen and G. D. Qian, J. Am. Chem. Soc., 2013, 135, 15559; (e) Y. J. Cui, H. Xu, Y. F. Yue, Z. Y. Guo, J. C. Yu, Z. X. Chen, J. K. Gao, Y. Yang, G. D. Qian and B. L. Chen, J. Am. Chem. Soc., 2012, 134, 3979. 3 J. Qiao, L. D. Wang, L. Duan, Y. Li, D. Q. Zhang and Y. Qiu, Inorg. Chem., 2004, 43, 5096. 4 (a) J. J. Qian, F. L. Jiang, D. Q. Yuan, M. Y. Wu, S. Q. Zhang, L. J. Zhang and M. C. Hong, Chem. Commun., 2012, 48, 9696; (b) H. Furukawa, K. E. Cordova, M. O’Keeffe and O. M. Yaghi, Science, 2013, 341, 974; (c) F. Haso, X. K. Fang, P. C. Yin, D. Li, J. L. Rossa and T. B. Liu, Chem. Commun., 2013, 49, 609; (d) X. K. Fang, L. Hansen, F. Haso, P. C. Yin, A. Pandey, L. Engelhardt, I. Slowing, T. Li, T. B. Liu, M. Luban and D. C. Johnston, Angew. Chem., Int. Ed., 2013, 52, 10500. 5 N. Stock and S. Biswas, Chem. Rev., 2012, 112, 933. 6 R. Hajjar, C. Volkringer, T. Loiseau, N. Guillou, J. Marrot, G. Ferey, I. Margiolaki, G. Fink, C. Morais and F. Taulelle, Chem. Mater., 2011, 23, 39.
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Cite this: Chem. Commun., 2014, 50, 5023 Received 20th November 2013, Accepted 25th March 2014
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Tunable luminescence from rare 2D Ga(III)/In(III) coordination polymers coexisting with three different conjugated system aromatic ligands† Xin Ming Wang,a Rui Qing Fan,*a Liang Sheng Qiang,a Wei Qi Li,b Ping Wang,a Hui Jie Zhanga and Yu Lin Yang*a
DOI: 10.1039/c3cc48867e www.rsc.org/chemcomm
Two rare 2D Ga/In-based coordination polymers in which one metal center coexists with three distinct aromatic ligands were synthesized. Helical channels along the 21 screw axis are exhibited to form a hcb net. The compounds exhibit tunable fluorescence from blue, green, white to yellow light by varying the temperature and solvents.
Since the first report of organic light-emitting diodes fabricated from tris(8-hydroxyquinolinolate)aluminum(III) (Alq3) in 1987,1 Group 13 complexes have attracted much attention due to their variety of structures, adequate electron transport, and luminescence properties.2 A binuclear gallium complex Ga2(saph)2q2 has been synthesized, which possesses a much higher luminescence efficiency and better thermal stability than the typical Alq3.3 Inspired by the prominent work of M. C. Hong, J. Zhang, O. M. Yaghi and their co-workers,4 we considered gallium and indium to be good candidates for construction of coordination polymers (Cps) due to the larger ion radii and more fascinating coordination properties than that of B and Al. However, the highest challenge from the synthesis process is the recognized bottleneck for the hydrolysis of Group 13 metal(III) salt5 and the smaller cation of Group 13 than the rare earth element, which enhances the difficulty of coordination. In this communication, we report two rare Ga(III)/In(III)-based twodimensional (2D) Cps {M[(2,20 -bpda)(1,4-bda)0.5(phen)]0.5H2O}n (M = Ga (Ga1), In (In1), 2,20 -H2bpda = 2,20 -biphenyldicarboxylic acid, 1, 4-H2bda = 1,4-benzenedicarboxylic acid, phen = 1,10phenanthroline) prepared from the reaction of mixing In(NO3)3, 2,20 -H2bpda, 1,4-H2bda and phen in a 4 : 1 : 1 : 1 molar ratio under hydrothermal synthesis at 160 1C for 5 days. For the hydrolysis reaction of Ga(III) and In(III) to occur, we finely regulated the range of the system pH. An exploration of the synthesis reveals that crystals of Ga1 and In1 are well formed only with pH in the range 3.4–3.5.
To the best of our knowledge, Ga1 and In1 are the first examples of the Group 13 Cps containing three different aromatic ligands from the single benzene ring (1,4-H2bda), biphenyl rings (2,2 0 -H2bpda) to three aromatic rings with the N-containing heterocyclic structure (phen). Ga1 and In1 are isomorphous, and therefore, only the structure of Ga1 is described in detail. A single crystal X-ray diffraction study unveiled the structure of Ga1 (Fig. 1). Every Ga3+ cation is seven coordinated with slightly distorted pentagonal bipyramid coordination geometry GaO5N2 by five O atoms (O1 and O2, O3, O5 and O6) from 2,20 -bpda2 adopting the m2 = Z1:Z1:Z1:Z0 bridging fashion, and 1,4-bda2 ligands adopting a chelating fashion, and two N atoms (N1 and N2) from phen ligands. It should be mentioned that seven coordinated mode of Ga/In(III) is rare and it is the first example among the reported Group 13 compounds, where the center metal Ga3+/In3+ coexists with three different aromatic ligands. The Ga–O bond lengths are 2.154(8)– 2.342(7) Å, then Ga–N bond lengths are 2.307(8) and 2.311(8) Å, respectively. These bond lengths are much longer than those observed in most other Ga(III) coordination polymers (average Ga–O/N bond lengths are 1.80–2.00 Å and 1.80–2.20 Å respectively).6
a
Department of Chemistry, Harbin Institute of Technology, Harbin 150001, P. R. China. E-mail: [email protected], [email protected]; Fax: +86-451-86418270 b Department of Physics, Harbin Institute of Technology, Harbin 150001, P. R. China † Electronic supplementary information (ESI) available. CCDC 953555 and 953556. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3cc48867e
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Fig. 1 (a) Asymmetric unit of Ga1; (b) perspective view of 1D chain; (c) the 2D layers containing single-stranded left- and right-handed helices linked; (d) schematic representation of the underlying network topology (color code: Ga, pink ball; 1,4-bda2, blue line; 2,2 0 -bpda2 turquoise line).
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The coexistence of three types of aromatic ligands give rise to a much crowded coordination sphere, thus the average bond distances extend to ease the steric hindrance.7 The phenyl rings of bpda2 in a nonplanar fashion (torsion angle is 56.331), lead to the formation of left- and right-handed helices, decorated by the phen ligands. The resulting handed helices with a pitch of 10.86 Å are alternately arranged in an equal ratio, extending into a 2D layered structure along the bc plane by bda2 anions. Each Ga(III) metal as the 3-connected nodes connects with two bpda2 and one bda2, to result in the generation of (6,3)-connected hcb topological net, with the grid size is 7.87(10) 20.55(20) Å (atom-to-atom distances). The 3D architecture of Ga1 or In1 is reinforced by the intermolecular hydrogen bonds interaction from OW1–HW1B O4, which is important for the molecular assembly8 (Fig. S7, ESI†). The emission spectrum of solid-state sample of Ga1 at 298 K is centered at 413 nm, producing blue emission (Commission Internacionale d’Eclairage (CIE) coordinates for Ga1 is 0.20, 0.19) (see Fig. 2). The spectrum of free 2,20 -H2bpda is centered at 412 nm, which is not perturbed upon its coordination to Ga(III) (Fig. S9, ESI†), thereby suggesting, that in Ga1, the observed emission is ligand based probably, which is assigned to the intraligand transitions of p* - p. In1 produces the selfsame blue emission as Ga1 (CIE coordinates are 0.20, 0.19). The emission bond of In1 appears around 401sh, 413, 436sh nm. Due to the existence of two shoulderpeaks in photoluminescence (PL) spectrum of phen ligand (lm = 365sh, 380, 400sh nm), and the maximum emission of In1 is just the same as that of free 2,20 -H2bpda ligand, therefore, the emission bond in In1 should be assignable to the intraligand charge transfer of 2,20 -bpda2 and phen. The difference observed in the fluorescence emission peaks of Ga1 and In1 likely have their origins including the metal ion in the unit cell and the steric proximity of ligands to each other. The solid-state PL spectra of Ga1 and In1 at liquid nitrogen temperature are researched. At 77 K, Ga1 emits intensely at 410 nm, giving light with CIE coordinates of 0.27, 0.30, which are at the blue end of the region generally considered white.
Fig. 2 Emission spectra of Ga1, In1 in the solid state at 298 K and 77 K (excited at 323 nm); CIE coordinates for Ga1 and In1 at 298 K (open symbols) and 77 K (closed symbols) (& = Ga1, n = In1), ‘‘‘‘denotes the pure white point (0.33, 0.33).
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Fig. 3 Molecular orbital amplitude plots of HOMO and LUMO energy levels of Ga1 and In1.
In1 has higher-energy (HE) emissions of two shoulder-peaks appearing around 400sh, 412, 428sh nm and the lower-energy (LE) emission bond centered at 542 nm. In1 generates light close to white which is at the green end with CIE coordinates of 0.28, 0.35. The HE maximum emission bond of Ga1 and In1 is nearly the same as that at 298 K, although the emission spectra exhibit more broader and unstructured bands. Meanwhile, the lifetimes of Ga1 and In1 increase conspicuously compared to those at 298 K (298 K: t = 7.24 ms for Ga1, t = 7.58 ms for In1; 77 K: t = 15.39 ms for Ga1, t = 11.62 ms for In1) (Fig. S11, ESI†), since cold conditions would be favorable for the rigidity of ligands with reducing the non-radiation decay and collisional quenching.9 To gain insights into the nature of photophysical properties of Ga1/In1, theoretical calculations on their energy levels were performed. The molecular orbital amplitude plots of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of Ga1/In1 are shown in Fig. 3. The electron densities of the HOMO singlet state are located in 2,2 0 -bpda2 units, whereas the densities for the LUMO are distributed mainly about the phen units. The transition from LUMO to HOMO involves interligand charge transfer between phen and 2,20 -bpda2. Namely, the Ga3+/In3+ and 1,4-bda2 contributions are small. The phen and 2,20 -bpda2 ligands which own a larger conjugate system than bda2 contribute most to the assignment in HOMO and LUMO energy levels. Therefore, this suggests that the emission observed in Ga1/In1 originates exclusively from ligand-to-ligand charge transfer (LLCT), namely, p* - p. Our TD-DFT calculations assign the energy level to HOMO - LUMO vertical transition (lcalc = 355 nm for Ga1 and lcalc = 330 nm for In1), in good correlation with the UV date and cyclic voltammetry (Table S7, ESI†). We have further examined the sensitivity of the emission response of Ga1 and In1 to the polar solvents (DMSO, CH3CN and CH3OH) at 298 K and 77 K (Fig. 4). The 1H NMR data indicate that Ga1/In1 still keeps a polymeric structure in solutions and does not decompose (Fig. S2 and S3, ESI†). At 298 K, Ga1 and In1 in different dilute solutions all generate stable royal blue emission with CIE coordinates of 0.15–0.16, 0.02–0.03, which are not affected by the solvent. It is notable that there is large
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Fig. 4 The emission spectra of Ga1 (a) and In1 (b) in different solvents at 298 K and 77 K (excited at 323 nm). CIE coordinates at 298 K (open symbols) and 77 K (closed symbols) (& = Ga1, n = In1). The inset shows how the coordinates vary: the coordinates based on the blue overlap closely at 298 K; the arrow shows the direction of change with the polarity of solvent increases from blue, cyan to white at 77 K. ‘‘‘‘denotes the pure white point (0.33, 0.33).
extent blue-shifted of PL in solution compared to that in solid state at 298 K, with the CIE color shift from light blue (0.20, 0.19) to royal blue (0.16, 0.03). It is due to that the intermolecular hydrogen bonds interaction in solid state are much more stronger than that in dilute solution,10 which can effectively decrease the HOMO–LUMO energy gap, so as to influence the p* - p transitions.11 The emissions of Ga1 and In1 are more obviously red shifted at 77 K than at 298 K and the largest shift is 107 nm for Ga1 and 175 nm for In1 in DMSO respectively. Notably, with the increasing of solution polarity (polarity: DMSO 4 CH3CN 4 CH3OH), the PL of Ga1 and In1 extend to the white region at 77 K. In1 exhibits a blue emission at 418 nm in CH3OH, a significantly red-shifted green emission centered at 490 nm in CH3CN, and a further redshifted to the green end emission at 409 and 550 nm in DMSO, with CIE coordinates of 0.31, 0.39, generally considered white emission. Ga1 exhibits a blue emission at 422 nm in CH3OH and CH3CN, a red-shifted white blue emission in DMSO, which is at the blue end of the region. The influence of the solvent on PL may be due to an increase of the dipole moment of excitation.12 An interesting phenomenon is observed, that is, the PL profile of In1 is more sensitive to the solvent than that of Ga1, namely, the solvent polarity could influence the relative intensity of HE and LE bands. At 298 K, the increase of the solution polarity gradually reduces the relative intensity of the HE peak but enhances the LE band for In1. In CH3OH solution, the HE emission band is much brighter than LE emission band; however, the LE emission band nearly dominates the total emission spectra in DMSO solution. In contrast, at 77 K, the effect of solventinduced aggregation on luminescence shows an increased relative intensity of the HE emission band with the increase of the solvent polarity, and slight changes in the vibronic components for the LE band, broadening the emission and gaining fine structure. These factors are the result of different intramolecular or intermolecular interactions among organic linkers and their energy transfer in solvent.13 Luminescence lifetimes for Ga1 and In1 in solutions at 77 K are mostly longer than that at 298 K (Fig. S12, ESI†). The most obvious observation is that the lifetime of In1 in CH3CN at 77 K (t = 10.77 ms) is 2.24-fold to that at 298 K (t = 4.81 ms). These features might be ascribed to an increased contribution from
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delayed fluorescence, which is caused by excimer formation at liquid nitrogen temperature.14 We investigate the quantum yields (F) of Ga1 and In1 in DMSO, CH3CN and CH3OH at 298 K (Table S5, ESI†). The fluorescence quantum yield (F) of Ga1 is found to be 0.273 in CH3OH, 0.223 in CH3CN and 0.182 in DMSO, which decreases with the increase of solution polarity. This effect is because of enhanced rate constants for the two nonradiative processes (internal conversion and intersystem crossing) in the Cps in polar solution.15 The corresponding F value of In1 is smaller than that of Ga1, which is 0.257, 0.199 and 0.120 in CH3OH, CH3CN and DMSO respectively, because of the heavy-atom quenching.16 To further clarify luminescence behavior in solution, the effect of deuterated solvents (DMSO-d6, CD3CN, CD3OD) on the luminescence of Ga1 and In1 have been investigated (Fig. S10, ESI†). Compared to those in normal solvents, the emission spectra in deuterated solvents exhibit similar characters, especially that Ga1 and In1 tend to give light from blue, green to yellow region at 77 K. The difference is that the luminescence lifetimes for Ga1 and In1 in deuterated solution are longer than that in the normal one (Fig. S14 and S15, ESI†). The lifetime of In1 in DMSO-d6 at 77 K (t = 13.27 ms) is even longer than that in the solid state (t = 11.62 ms). Therefore it is possible for the exchange of the hydrogen of free water molecules in the framework with the deuterium of deuterated solvent to occur. According to an early study,17 the luminescence quenching rate attributed to O–H bonds is proportional to the number of coordinating water molecules in the coordination sphere of Ga1 and In1, and this quenching rate displays such a strong isotope effect that the O–D bond essentially has no quenching effect.18 The isotope effect also contributes to the strong deuterium-contained H-bonds formed among D atoms from deuterated solvents and some D2O of the framework. The relatively shorter D O bonds compared to those in the non-deuterated system may enhance interaction of solute–solvent and stabilize the excited state of the electronic transitions.19 The thermogravimetric curves and variable-temperature X-ray powder diffraction showed that Ga1 and In1 network start to undergo phase transition above ca. 250 1C for Ga1, and 450 1C for In1, which induces the structure collapse (Fig. S5 and S6, ESI†). In summary, two novel 2D Group 13 Cps Ga1 and In1 have been synthesized from three different aromatic ligands, 1,4-H2bda, 2,2 0 -H2bpda, and phen. Such concordant coordination competition of three distinct conjugated ligands around one metal center is quite rare. Ga1 and In1 display stable blue fluorescence at 298 K. At 77 K, they exhibit a tunable emission from blue, green to yellow with the increasing polarity of the solvents, assuming solvent-dependent PL. The monochromaticity and stability of PL to Ga1 is better than In1, but the thermal stability of In1 is outstanding. Ga1 and In1 have potential applications in luminescent materials. This work was financially supported by the National Natural Science Foundation of China (Grant No. 21371040 and 21171044) and the National Key Basic Research Program of China (973 Program, No. 2013CB632900) and Supported by Program for Innovation Research of Science in Harbin Institute of Technology.
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