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Luminescent copper(I) halide and pseudohalide phenanthroline complexes revisited: simple structures, complicated excited state behavior†‡ Jörn Nitsch,a Christian Kleeberg,b Roland Fröhlichc and Andreas Steffen*a We have synthesized a series of luminescent trigonal [CuX(dtbphen)] (X = I (1), Br (2), Cl (3), CN (4), dtbphen = 2,9-di-tert-butylphenanthroline) and tetrahedral [Cu2(μ-I)2(L)2] (L = phenanthroline (5), 2,9-dimethylphenanthroline (6)) copper diimine complexes. Bearing in mind the chemical simplicity of this class of long-known Cu(I) phenanthroline compounds, it is surprising that they exhibit non-trivial photophysical properties, which have not been fully recognized. They display broad XMLCT absorption between ca. 450–600 nm, and the broad emission between ca. 550–850 nm in the solid state occurring with lifetimes on the μs timescale indicates phosphorescence, although the energetic overlap between excitation and emission suggests thermally activated delayed fluorescence (TADF) from S1. In line with the latter assumption, low temperature measurements of 1–6 in the solid state show an energetic separation of emission and excitation. However, a counter-intuitive decrease of emission intensity and simultaneous increase of the emission lifetime at low temperatures are observed for 1, which indicates two triplet states also being involved. Our DFT and TD-DFT calculations show that emission from the lowest excited triplet state T1 is of LMXCT nature, separated by only ca. 0.16 eV from S1. Low temperature photophysical measurements at

3

77 K in a glassy matrix of 2 in 2-Me-THF and of 6 in the solid state are in agreement with the theoretical Received 3rd December 2014, Accepted 5th March 2015

results, revealing in addition that π-interactions in the solid state also greatly influence the photophysical properties, making a clear conclusion towards TADF ambiguous. This study suggests that other related

DOI: 10.1039/c4dt03706e

simple and long-known Cu(I) systems may exhibit a similar, more complex excited state behavior than previously appreciated, involving several emitting states and important intermolecular interactions.

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Introduction Luminescent transition metal complexes of metals such as ruthenium, rhenium, iridium and platinum have gained much attention as potential photoactive materials in optoelectronic devices.1–11 Despite some rare exceptions,12–38 late transition metal complexes are usually phosphorescent emitting from their excited triplet state T1 due to the strong spin orbit coupling (SOC) mediated by the metal center.1,2,39–42 This can be an advantage for the design of OLEDs, as the employment of

a Institut für Anorganische Chemie, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg, Germany. E-mail: andreas.steff[email protected] b Institut für Anorganische und Analytische Chemie, Technische Universität CaroloWilhelmina zu Braunschweig, Hagenring 30, 38106 Braunschweig, Germany c Organisch-Chemisches Institut, Westfälische Wilhelms-Universität Münster, Corrensstr. 40, 48149 Münster, Germany † Dedicated to Prof. Lothar Weber on the occasion of his 70th birthday. ‡ Electronic supplementary information (ESI) available: Further details of single crystal X-ray diffraction studies, photophysical data and results of DFT and TD-DFT calculations of compounds 1–6 can be found in the ESI. CCDC 1006445, 1006215 and 1006216. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt03706e

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efficient triplet emitters can, in principle, lead to much higher external quantum efficiencies than for singlet emitters, such as normal organic chromophores. This strategy is known as the triplet harvesting effect1,2,9,43,44 and especially iridium complexes such as [Ir( ppy)3] and its derivatives have been explored for high efficiency OLEDs,1–5,41,43 although Pt complexes have also been considered.3,43,45 However, iridium and platinum are extremely rare in the Earth’s crust and it would make sense to substitute luminophores of those metals for more abundant and cheaper alternatives. Besides this economic aspect, iridium(III) d6 and platinum(II) d8 complexes can suffer from the presence of metal-centered (MC) d–d* states, of which the population from the triplet excited state T1 can lead to nonradiative decay. This process can become a problem in particular for blue emitters, as at such a high energy of the T1 state the d–d* states need to be in the UV energy region in order not to quench the luminescence, and modification of the ligand field in this direction is not trivial. Bypassing this natural limitation of designing phosphorescent blue emitters based on transition metals is potentially possible by the use of d10 complexes, i.e. coinage metal compounds with the metal center in the oxidation state +1, in which d–d* transitions are

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absent.46 Indeed, many efficiently luminescing d10 M(I) complexes have been identified,7,43,46–73 and copper appears to be particularly interesting due to its SOC constant of ξCu = 851 cm−1, similar to Ru (ξRu = 1200 cm−1), which is known to facilitate ultrafast intersystem crossing (ISC) Sn→Tn, e.g. within 15 femtoseconds in [Ru(bpy)3]2+.74 One particularly impressive example of OLEDs based on copper has been reported by Osawa and co-workers, who employed very simple trigonal distorted [CuX(dtpb)] complexes (dtpb = 1,2-bis(di-o-tolylphosphino)benzene; X = I, Br, Cl), giving an impressive external quantum yield of Φext = 21.3% as a result.75 Furthermore, mainly due to the pioneering work of Breddels and McMillin, it is known that Cu(I) complexes can exhibit thermally activated delayed fluorescence (TADF), i.e. back-ISC T1→S1 and subsequent fluorescence from S1.76–79 A requirement for this process to occur is a small energy gap ΔEST between S1 and T1, i.e. a few hundred cm−1, while the gap in typical Ir and Pt compounds is much higher (several thousand cm−1), and thus this process is not observed in the latter complexes. The longknown concept of TADF has gained renewed attention now that it has been demonstrated that this singlet-harvesting process provides a design motif in order to achieve fast and efficient emission with copper luminophores in OLEDs.1,50–53 Since then, a good number of new Cu(I) complexes exhibiting TADF have been reported,52,63–66 and it should be mentioned that very recently Adachi and co-workers were able to realize very efficient TADF in OLEDs with purely organic chromophores, i.e. carbazolyl dicyanobenzenes.80 Although color tuning and structural diversity are still unsolved issues, it clearly shows the potential of TADF processes for organometallic and organic luminophores alike. However, no structure– property relationship has been established for Cu(I) complexes so far in order to identify the factors which distinguish compounds able to undergo TADF from those which are not. As mentioned above, much effort has been devoted to generate new Cu(I) complex structures for highly luminescent compounds. Being interested in the structural requirements of Cu(I) complexes for exhibiting delayed fluorescence ourselves, and bearing in mind the simplicity of the Cu(I) halide complexes employed in Osawa’s high performance OLED, we decided to reinvestigate a series of trigonal and tetrahedral halogen and pseudohalogen copper(I) phenanthroline complexes.81–83 Some of these compounds have been known for decades without clearly defining their luminescence pro-

Scheme 1

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perties.83,84 Such simple systems appear, very much to our surprise, to exhibit a complicated excited state behavior, which has not been fully recognized in the past and has wide implications for future investigations.

Results and discussion Synthesis and characterization Complexes 1–6 were easily prepared by adding the corresponding copper(I) halides or pseudo-halide to solutions of phenanthroline ( phen) or its derivatives in tetrahydrofuran (2), dichloromethane (381) or acetonitrile (1, 4, 583 and 682) (Scheme 1). Compounds 1–3, 5 and 6 precipitate rapidly from the reaction mixture at room temperature, while 4 was formed after heating the suspension to reflux for 3 h. The resulting products were characterized by elemental analysis, single crystal X-ray diffraction studies, 1H and 13C {1H} NMR spectroscopy for 2, 3 and 4 (see below) and MALDI spectrometry for 3. While phenanthrolines bearing small substituents in the 2,9-positions (R = H, Me) lead to the formation of the dinuclear species 5 and 6 with bridging halides, the steric demand of the tBu group favors monomeric structures (1–3). The polymeric arrangement of 4 in the solid state appears to be a result of a Lewis acid–base adduct between the {Cu(CN)(dtbphen)} (dtbphen = 2,9-di-tert-butylphenanthroline) fragment and CuCN. Interestingly, while 2 and 3 are soluble and stable in dichloromethane and tetrahydrofuran, as indicated by 1H NMR spectroscopy, [CuI(dtbphen)] (1) shows decomposition in deuterated chloroform giving the free ligand and, presumably, copper iodide (Fig. S3‡). The reaction can also be observed by eye: within minutes a white precipitate is formed at the bottom of the NMR tube. In contrast, complexes [Cu2(μ-I)2( phen)2] (5) and [Cu2(μ-I)2(dmphen)2] (6) (dmphen = 2,9-dimethylphenanthroline) are poorly soluble in common organic solvents. Due to the polymeric structure, [Cu(CN)(dtbphen)·Cu(CN)]∞ (4) is only poorly soluble in hot dimethylsulfoxide (ca. 80 °C). Solutions of 2 and 3 are sensitive to oxygen, while in the solid state, all compounds show only little (2, 3, 5 and 6) to no oxidation (1 and 4) sensitivity. Single crystals suitable for X-ray diffraction studies were obtained by slowly cooling hot acetonitrile solutions of 1–3, while 4 had to be crystallized from hot

Synthesis of complexes 1–6; conditions for 1, 5, 6: acetonitrile, RT; 2: THF, RT; 3: dichloromethane, RT; 4: acetonitrile, reflux.

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Table 1

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Selected crystallographic data for complexes 1, 2 and 4

Formula Fw [g mol−1] Space group a [Å] b [Å] c [Å] β [°] V [Å3] ρcalc [g cm−1] Z T [K] λ [Å] GOF Rint R1 (I > 2σ(I)) wR2 (all data) Flack parameter CCDC No.

1

2

4

C20H24N2CuI 482.82 Orthorhombic Pca21 20.4147(4) 9.7229(2) 19.5841(3)

C20H24N2CuBr 435.86 Orthorhombic Pbca 9.6421(3) 19.6933(5) 19.7231(5)

3887.25(13) 1.650 8 223 0.71073 1.090 0.0500 0.0554 0.1319 0.51(4) 1006445

3745.12(18) 1.546 8 100 0.71073 1.031 0.0703 0.0309 0.0652

C20H24N2CuCNCuCN 471.53 Monoclinic P21/c 8.5167(3) 12.2692(3) 20.6552(5) 94.754(3) 2150.90(11) 1.456 4 100 0.71073 1.045 0.0502 0.0305 0.0684

1006215

1006216

dimethylsulfoxide. Despite the poor solubility of the dimers 5 and 6, it was possible to form a microcrystalline red solid by refluxing a fine powder suspension of them for several days in acetonitrile. The molecular structures in the solid state of 1, 2 and 4 are described below, whereas the single crystal X-ray diffraction data obtained for 3 and 6 matched those previously reported (Table 1), and a powder XRD of 5·MeCN matched the simulation of the previously reported structure (Fig. S2‡), thus conforming their identity.81–83 It is noteworthy that the elemental analyses of various synthetic batches are reproducibly in excellent agreement with the sum formula obtained from our X-ray diffraction studies, even for the acetonitrile solvate of 5. The only exception is [CuCl(dtbphen)] (3), for which no satisfactory values could be obtained, presumably due to partial oxidation. However, a MALDI mass spectrum shows the correct mass peak at 390.084 m/z.

X-ray diffraction studies Selected data obtained from single crystal X-ray diffraction experiments of 1, 2 and 4 are given in Table 1 and 2. Compounds 1 and 2 crystallize in the orthorhombic space group types Pca21 and Pbca, respectively, with the copper atom in a distorted trigonal coordination environment (Fig. 1) with Cu–X

Table 2

distances of 2.4432(13) Å for 1 and 2.2975(3) Å for 2, which are longer than in 3 (2.147(4) Å)81 due to the larger atomic radii of iodide (1) and bromide (2) compared to chloride (3). The steric demand of the tBu groups in the 2,9-positions of the organic ligand leads to a displacement of the Cu–X fragment out of the phenanthroline plane in 1–3, resulting in different Cu–N bond distances in the same molecule. In contrast, in the iodo-bridged dimers [Cu2(μ-I)2( phen)2]·MeCN (5·MeCN)82 and [Cu2(μ-I)2(dmphen)2] (6),83 the copper atoms lie in the plane of the phenanthroline ligand and exhibit a slightly distorted tetrahedral coordination geometry, resulting in longer Cu–I bond distances (2.563(1)–2.673(1) Å) than found in 1. Distortions from the optimal tetrahedral coordination geometry resulting in different Cu–N bond distances has been found before in related Cu(phen) complexes, e.g. in [Cu(BF4)(dtbphen)] (Cu–N: 1.990(2), 2.102(2) Å) and [Cu(dtbphen)2][BPh4] (Cu–N: 2.096(3)– 2.130(1) Å).85 Interestingly, unlike in 6, cuprophilic interactions are present in 5·MeCN as the Cu–Cu distance of 2.609(2) Å (vs. 3.024(2) Å in 6) is well below the sum of the van der Waals radii (2.8 Å).73 It is important to note that all of our copper phenanthroline halides exhibit π-stacking of the organic ligands, as indicated by the intermolecular phenanthroline mean plane distances of 3.414–3.629 Å. This leads to the formation of

Selected structural parameters for complexes 1–6

Compound

d(Cu–X) [Å]

d(phen–phen) [Å]

d(Cu–Nphen) [Å]

d(Cu–Cu) [Å]

[CuI(tbphen)] (1) [CuBr(dtbphen)] (2) [CuCl(dtbphen)]81 (3) [CuCN(dtbphen)·CuCN]∞ (4) [Cu2(μ-I)2(phen)2]82 (5) [Cu2(μ-I)2(dmphen)2]83 (6)

2.4432(13) 2.2975(3) 2.147(4)c 1.8558(19) 2.589(1), 2.632(1)c 2.563(1), 2.673(1)

3.457a/3.423b 3.497 3.629c — 3.455c 3.414

2.064(8), 2.086(8) 2.0649(16), 2.0857(14) 2.077(10), 2.093(10)c 2.0297(14), 2.0542(14) 2.078(6)–2.103(6)c 2.088(6)–2.093(6)

— — — — 2.609(2)c 3.024(2)

a

Measured at 223 K. b Measured at 100 K. c Measured at 295 K.

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Fig. 1 Molecular structures in the solid state of 1 (left) and 4 (right) obtained by single crystal X-ray diffraction. Ellipsoids are drawn at 50% probability level, H atoms omitted for clarity.

Fig. 2 Molecular structure in the solid state of 683 obtained by single crystal X-ray diffraction. Ellipsoids are drawn at 50% probability level, H atoms omitted for clarity.

intermolecular dimers for 1–3, whereas 5·MeCN and 6 thus form chains of dimers due to their iodo-bridged nature (Fig. 1 and 2). Compound 5 has been repeatedly crystallized as an acetonitrile solvate, while all other compounds reproducibly give solvent free crystalline material. However, compound 4 crystallizes as a Lewis acid–base adduct of the {Cu(CN)(dtbphen)} unit and Cu(CN), leading to a polymeric structure in the solid state (Fig. 1), as has been found for [Cu(CN)( phen)·CuCN].86 Not surprisingly, a disorder of the cyanide carbon and nitrogen atoms (refined as 50/50) in the polymeric chain is found, while the ‘head-groups’ consisting of the {Cu(CN)(dtbphen)} fragment are formed exclusively by a carbon–copper bond with a distance of 1.8558(19) Å. Similar to 1–3, the Cu–CN unit is displaced out of the phenanthroline plane, but no π-interactions are present within or between the chains, which is in contrast to previously reported solid state structures of polymeric CN-bridged {Cu(CN)( phen)} fragments.84,86

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Photophysical studies Solid state measurements. Both the absorption and excitation, which coincide nicely for all compounds indicating the presence of only one emitting species with no significant impurities, show an unstructured high energy band from 310–420 nm for 1–3 (Fig. 3 and ESI‡). Most likely, this excitation is a result of a high energy MLCT or XMLCT transition, as previously assigned for [Cu( phen)2]+ and its derivatives, although IL(π–π*) transitions are also possible (vide infra).58,62,87–92 In addition, they exhibit a broad shoulder between 420–600 nm as the lowest energy excitation. The dimeric compounds 5 and 6 behave very similar, although the lowest energy excitation is even broader, tailing to ca. 640 nm. In contrast, 4 shows only a high energy excitation band and the broad shoulder of the other copper phenanthroline halides is absent. It is important to note that the intensity of the lowest energy excitation band decreases in the order 1 > 2 > 3 > 4. It is possible that the intensity of this band is a result of a

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Fig. 3 Normalized solid state excitation (dashed, recorded at the respective emission maximum) and emission (solid, λexc = 396 nm) spectra, left: 1 (black), 2 (red), 3 (blue) and 4 (orange); right: 5 (red), 6 (black).

XMLCT, thus depending on the polarizability of the halide (or pseudohalide). Alternatively, the strength of the π–π interactions between the aromatic ligands, which are absent in 4, might play a role (Table 2) as such an influence has been found for intermolecular stacking in [Cu(dmphen)2]+ or for intramolecular π-interactions in [Cu(bpy(Mes)2)(diimine)]+.89,93,94 Upon excitation at 396 nm, complexes 1–3 show a broad and unstructured solid state emission in the red region of the spectrum, tailing into the near-IR (Fig. 3, left). The bathochromic shift of the emission maxima λmax from 624 nm (1) via 658 nm (2) to 677 nm (3) seems to be halide dependent with the order Cl > Br > I. However, the emission of the iodo-bridged dimers 5 and 6 is even further bathochromically shifted (Fig. 3, right). In contrast, the emission spectrum of [CuCN(dtbphen)·CuCN]∞ (4) shows a broad and unstructured band between 490–790 nm, thus hypsochromically shifted compared to 1–3. Osawa and co-workers found a similar red shift for a series of copper(I) phosphine halides [CuX(dtpb)] (dtpb = 1,2-bis(o-ditolylphosphino)benzene; X = Cl, Br, I).75 The authors suggested that the emission maximum is affected by the ligand-field strength (I− < Br− < Cl−), which would destabilize the filled d orbitals in this d10–Cu(I) system and thus reduce the HOMO–LUMO gap, apparently overcoming the stabilizing influence of the halide’s electronegativity which would give an opposite trend. Therefore, one can conclude that the electronic nature of the emitting state is influenced to some extent by a XLCT.75 This observation of opposing effects of the halides has also been reported for dicopper complexes,66 and in Cu4X4L4 clusters.73,95 The XLCT character of the excited states should be more pronounced in 5 and 6, with the copper atoms being bridged by two halides in a tetrahedral coordination geometry, accounting for the bathochromic shift of the emission in comparison to their unbridged counterpart 1. The hypsochromic shift of the emission of 4 could be rationalized either with a stabilized ground state or a destabilized excited state,

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which can be correlated, in a qualitative sense, to a larger HOMO–LUMO gap for this complex and a much smaller XLCT contribution of the emission compared to 1–3 (see Computational studies below). Alternatively, the larger halides in our series of copper phenanthroline complexes might permit less structural relaxation in the excited state for steric reasons, which would also account for the observed counterintuitive trend of the emission energies (vide infra). The quantum yield (QY) Φ at room temperature is below 0.05 for 2–4 and 5 (Table 3). Only the monomeric iodo complex 1 and the iodo bridged dimer 6 show moderate QY of Φ = 0.15 and Φ = 0.18, respectively. The slightly higher quantum yield of [CuCN(dtbphen)·CuCN]∞ (4) in comparison to the chloro and bromo complexes 2 and 3 might be due to its polymeric rigid arrangement. What is surprising though, is the comparison of Φ of [Cu2(μ-I)2( phen)2] (5) with that of [Cu2(μ-I)2(dmphen)2] (6), indicating that the substitution in the 2,9-position of the phenanthroline ligand has a positive influence on the radiative decay pathway by increasing the steric hindrance for a flattening distortion even in the solid state, which is a well-known pathway for radiationless decay in tetrahedral Cu(I) complexes.46–49,89–91,93,96,97 The emission lifetimes measured at room temperature are all in the range of several hundred nanoseconds (3 and 5) to a few microseconds (1, 2, 4 and 6), giving an estimate for the intrinsic lifetimes τ0 of between 25 μs and >100 μs (Table 3), a typical range for phosphorescence from a triplet state in Cu(I) luminescent complexes. However, one particular detail that caught our attention was the significant overlap of the excitation and emission spectra of the monomeric and dimeric copper phenanthroline halides, which is most pronounced for the iodo derivatives 1, 5 and 6. Usually, this spectral feature is found for emission from a singlet excited state with fluorescence lifetimes of a few nanoseconds, which is in contrast to the measured long life-

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Table 3 Selected photophysical data of 1–6 measured under an argon atmosphere at room temperature in the solid state (λexc = 396 nm) unless otherwise stated

Compound

λmax [nm]

τ [µs] (%)

Φ [%]

τ0 [µs]

[CuI(dtbphen)] (1) [CuBr(dtbphen)] (2) [CuCl(dtbphen)] (3) [CuCN(dtbphen)·CuCN]∞ (4) [Cu2(μ-I)2(phen)2] (5) [Cu2(μ-I)2(dmphen)2] (6)

624 658 677 587 688 667

3.7 (100)/17 (100) @ 4K 1.1 (86), 0.6 (14) 0.9 (20), 0.4 (80) 1.7 (100) 0.9 (88), 0.2 (12) 6.4 (100)/25 (100) @ 22K

15 2 100 ca. 36

times (vide supra) indicating a more complicated excited state behavior. A feasible explanation would be thermally activated delayed fluorescence (TADF) from the S1 state after back-intersystem crossing from S1←T1, the latter being the rate determining step for the observed emission lifetimes.39,78,79 This phenomenon has been reported before for other Cu(I) complexes,46–48,50–52,64–68,76,77,88–90,97 but not for copper phenanthroline halides. We therefore carried out low temperature studies on selected compounds in the solid state as well as in an optical glass for 2. Low temperature studies Solid state. The emission spectra of complexes 1, 2 and 6 below 200 K in the solid state develop a vibrational fine structure, in which two maxima and a broad shoulder are separated by ca. 1200–1350 cm−1 (Fig. 4 and ESI‡). Furthermore, the onset of the emission is significantly bathochromically shifted upon cooling to 77 K in comparison to the spectra recorded at room temperature, i.e. by ca. 1000 cm−1 for 1 and 6, and 1500 cm−1 for 2. A hypsochromic shift is observed for the excitation spectra, leading to the spectral separation of excitation and emission expected for phosphorescence (see ESI‡). However, the emission intensities first decrease with decreasing temperature for the complexes [CuI(dtbphen)] (1) and [CuBr(dtbphen)] (2), until a minimum is found at T = 25 K, while at T = 4 K we see again an increase (Fig. 5). This behavior

Fig. 4

is completely counterintuitive, as one would expect higher emission intensities with decreasing temperature due to the reduced vibrational relaxation pathways in the emitting excited state. The complex [Cu2(μ-I)2(dmphen)2] (6) shows the expected temperature dependence of the emission intensity at T = 297–77 K, but at T = 25 K the emission intensity drastically decreases. In general, these results suggest that different emission mechanisms for 1 and 6 are operative. We have also recorded the emission lifetimes of 1 and 6 as a function of temperature (Fig. 6). Both display, at first, an expected linear increase with decreasing temperature, but at temperatures below 77 K, this linearity is lost and the measured lifetimes jump to 17 µs at 4 K for 1 and to 24 µs at 54 K for 6. Breddels and coworkers described a similar temperature dependent evolution of the emission lifetime for cationic [Cu(biq)(PPh3)2]+ and [Cu(bpy)(PPh3)2]+ (biq = biquinoline, bpy = 2,2′-bipyridine), which show a nearly exponential increase below 100 K.77 In addition, they found that the curve, starting at room temperature at 7 µs, reaches a maximum of 50 µs at 30 K and then decreases again. Breddels and McMillin rationalized this behavior with a physical model, with at least two emitting states being involved and in which one excited state is thermally activated (Scheme 2).77,98 Depending on the specific π chromophore ligand, the authors concluded that the thermally activated state is either a singlet state, or that a

Variable temperature emission spectra of [CuI(dtbphen)] (1) (left) and [Cu2(μ-I)2(dmphen)2] (6) (right) recorded in the solid state.

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Fig. 5 state.

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Temperature dependent integrated emission intensity of [CuI(dtbphen)] (1) (left) and [Cu2(μ-I)2(dmphen)2] (6) (right) recorded in the solid

Fig. 6 Temperature dependent emission lifetimes of [CuI(dtbphen)] (1) (black) and [Cu2(μ-I)2(dmphen)2] (6) (red) recorded in the solid state.

Scheme 2 Energy level diagram for a multiple-excited-state-model showing the possible emission processes upon photoexcitation.77

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doubly split triplet state gives rise to this unusual photophysical behavior. This two state model for copper(I) complexes was adopted by Yersin and co-workers, who further concluded that in many cases the emission at room temperature is thermally activated delayed fluorescence (TADF) originating from a singlet excited state.1 As the rate determining step is the back-intersystem crossing S1←T1, the recorded lifetimes are in the microsecond range although the emission occurs from the singlet excited state. This principle has now been employed in OLEDs for singlet harvesting, using Cu(I) compounds as the luminophores, although recently the group of Adachi demonstrated, for the first time, the applicability of this concept for organics.1,64 Of particular interest is the fact that although the emission lifetime of 1 increases with decreasing temperature, indicating more efficient luminescence, the emission intensity decreases, which indicates the opposite, i.e. less efficient luminescence. Thus, it is feasible that in our Cu(I) phenanthroline halides a similar mechanism as described by Breddels and McMillin is operative,76,77,88,98 involving several states with different radiative and non-radiative decay rates in the emission process, which are thermally accessible. However, it is very well possible that the macroscopic structure in the solid state, i.e. intermolecular π-stacking between the phenanthroline ligands, influences the excited state behavior as the inter-ligand distances should be prone to temperature effects. Furthermore, the close intermolecular contacts might also allow for exciton diffusion in the crystal as, of which the energetic barriers would at different temperatures allow for various relaxation paths by phonon coupling, radiative and non-radiative alike. Such phenomena have been discussed intensively in perylene derivatives.99–101 Another factor that might affect the different emission mechanisms, in particular the marked difference between 1 and the dimeric compound 6, are the short copper– copper distances in the tetrahedral dimers 5 and 6 (see Table 2), which should shorten even further in the excited

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state and thus greatly influence the emission properties (see Computational studies below). Optical glass. In order to gain further evidence for the hypothesis of multiple excited states being involved in the emission process of 1–6, we have chosen [CuBr(dtbphen)] (2), due to its chemical stability in solution (vide supra), for low temperature studies in a glassy matrix of 2-methyltetrahydrofuran. This should allow for photophysical measurements independent of intermolecular π–π interactions between the phenanthroline ligands or other solid state effects. It has to be mentioned that 2 does not display any observable luminescence in dichloromethane or tetrahydrofuran at room temperature. However, at 77 K we were able to record an emission spectrum a glassy matrix of 2-methyltetrahydrofuran (Fig. 7). The two emission maxima in the optical glass at 646 nm and 692 nm, a result of a vibrational progression, are slightly bathochromically shifted compared to the solid state emission at 77 K or 4 K (see ESI‡), showing two emission maxima at 637 nm and 687 nm. The excitation spectrum resembles an absorption spectrum recorded in 2-methyltetrahydrofuran at room temperature, except that two additional broad bands of apparently very low oscillator strength between 410 nm and 550 nm are found. It is important to note that the very intense excitation band between 400 nm and 550 nm in the solid state at room temperature as well as at 77 K are missing in solution, and are of very low intensity in the glassy matrix. Our concentration dependent emission spectra at 77 K in the frozen glass show no changes in the band shape with decreasing concentration, ruling out aggregation and confirming that the observed emission in solution is not influenced by π-stacking of the phenanthroline ligands in 2, but rather a molecular property. Furthermore, raising the temperature of the glassy matrix leads to a decrease in emission intensity, which is quite the opposite behavior than observed in the solid state (see

Fig. S7‡). Overall, our findings suggest that intermolecular interactions are highly important, making a clear assignment of TADF, i.e. back-intersystem crossing T1→S1 and subsequent emission S1→S0, in the solid state difficult. However, the comparison of the photophysical data recorded in the solid and in solution indicate several emitting states being present in the solid state of such simple copper(I) phenanthroline halides. Computational studies Ground state S0. The ground state structures of [CuI(dtbphen)] (1), [CuBr(dtbphen)] (2), [CuCl(dtbphen)] (3), the dimeric compounds [Cu2(μ-I)2( phen)2] (5) and [Cu2(μ-I)2(dmphen)2] (6) and of the intramolecular π-stacked dimer of 2, i.e. {CuBr(dtbphen)}2 (2π), were optimized starting from their molecular structures obtained from single crystal X-ray diffraction studies.102 For ease of calculation, compound [CuCN(dtbphen)·CuCN]∞ (4) was replaced by its monomeric fragment [CuCN(dtbphen)] (4m). The geometric parameters thus obtained agree well with those obtained from the respective single crystal X-ray diffraction experiments, beside some variations of the tilt angle of the Cu–X group due to packing effects for 3 and 4m (see ESI‡). We will further focus the following discussion on complexes 2 and 2π due to the availability of photophysical data in the glassy matrix at 77 K, but the conclusions also apply to the other Cu phenanthroline halides (see ESI‡). For comparison, we will also discuss the model dimer complex 5. The molecular orbital diagram of 2 is depicted in Fig. 8, showing the HOMO

Fig. 7 Absorption (black, 293 K), excitation (blue, 77 K, λem = 670 nm) and emission spectra (red, 77 and 100 K, λexc = 390 nm) of [CuBr(dtbphen)] (2) in 2-methyltetrahydrofuran (c = 0.14 mM).

Fig. 8 Molecular orbital diagram and isosurface plots of selected frontier molecular orbitals of [CuBr(dtbphen)] (2); isocontour values: 0.02 (e bohr−3)1/2.

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as an antibonding combination of an in-plane Br( p) orbital and a Cu(d)-phen σ* orbital. A very similar bonding situation has been found in type 1 (T1) copper centers in proteins, which have been thoroughly studied by Solomon and coworkers.103,104 The lower lying occupied molecular orbitals HOMO−1 to HOMO−4 are mainly combinations of Cu(d) and Br( p) orbitals. The LUMO is mainly a π* orbital located at the 2,9-di-tert-butyl-phenanthroline ligand. The general picture does not change upon dimerization via π-stacking of the phenanthroline ligands in 2π, except that HOMO, HOMO−1 and LUMO are multiply degenerate (see ESI‡). Due to the varying halide participation in the HOMO of 1–4m (1: 63%, 2: 44%, 3: 27%), the energies of the HOMOs are very similar and do not scale with the polarizability or electronegativity of the halides, with 4m experiencing the largest HOMO–LUMO gap (I (1) = 3.03 eV, Br (2) = 3.10 eV, Cl (3) = 3.03 eV, CN (4m): 3.46 eV). The optimized dimeric complex [Cu2(μ-I)2( phen)2] (5) exhibits the two copper atoms in a distorted tetrahedral coordination geometry with overall D2h symmetry, of which the molecular orbital diagram is shown in Fig. 9. The HOMO to HOMO−12 are all either combinations of Cu(d) orbitals of the two copper atoms, or combinations of Cu(d) and I( p) orbitals. The virtual orbitals LUMO to LUMO+2 are each doubly degenerate due to the high symmetry of the molecule, and are all phenanthroline-based π* orbitals. The HOMO–LUMO gap in 5 of 2.71 eV is smaller than that found for 1 (3.03 eV, vide supra).

Fig. 9 Molecular orbital diagram and isosurface plots of selected frontier molecular orbitals of [Cu2(μ-I)2( phen)2] (5); isocontour values: 0.035 (e bohr−3)1/2.

TD-DFT studies

tage contributions to the excitations of 2 and 5 are given in Tables 4 and 5, respectively. The lowest two excited singlet states of 2 are found at excitation energies of 2.76 and 3.30 eV, respectively, with very low oscillator strengths and they can be described as nearly pure 1 XMLCT states. The higher lying excited singlet states are also 1 XMLCT states, although the percentage of XLCT and MLCT

The lowest 50 vertical excitations for the Franck–Condon (FC) singlet and triplet excited states were determined by TD-DFT calculations, using CAM-B3LYP due to the expected pronounced charge-transfer character of the transitions, starting from the optimized S0 ground state structure.105 Selected excitation energies and main transitions including their percen-

Table 4 Vertical electronic excitation energies and main transitions describing selected Franck–Condon states of 2 obtained by TD-DFT calculations

Energy State

[eV]

[nm]

Oscillator strength f

Transitions (%)a

Classificationb

S1 S2 S3 S4

2.76 3.30 3.46 3.61

449 375 358 343

0.0002 0.0008 0.0116 0.0301

H → L (83), H−3 → L (11) H → L+1 (86), H−3 → L+1 (8) H−2 → L (88), H−9 → L+1 (9) H−1 → L (83), H−8 → L (7)

1

S5

3.94

315

0.0242

H−2 → L+1 (85), H−1 → L+1 (8)

S12

4.55

272

0.0183

H−4 → L+1 (44), H−7 → L (24), H−6 → L+1 (23)

S15

4.85

256

0.2865

H−7 → L (25), H−6 → L+1 (22), H → L+5 (8)

T1 T2

2.60 2.66

476 467

— —

H → L (54), H−3 → L (13), H−6 → L+1 (12) H → L+1 (25), H−6 → L+1 (29), H−7 → L (9), H−4 → L+1 (7), H−5 → L (7)

a

Only major contributions >5% are given; H = HOMO, L = LUMO. transition equally.

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b

XMLCT XMLCT XMLCT 1 XLCT (53%) 1 MLCT (38%) 1 XLCT (71%) 1 MLCT (26%) 1 MLCT (59%) 1 XLCT (29%) 1 MLCT (45%) 1 XLCT (33%) 1 1

3

XMLCT XMLCT

3

If no percentage is given, the halide and the metal contribute to the

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Table 5 Vertical electronic excitation energies and main transitions describing selected Franck–Condon states of 5 obtained by TD-DFT calculations

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Energy State

[eV]

[nm]

Oscillator strength f

Transitions (%)a

Classificationb

S1 S2 S3 S7 S11 S15

2.67 2.69 2.79 3.09 3.32 3.68

465 461 444 401 374 337

0.0000 0.0057 0.0000 0.1695 0.0191 0.0192

H → L+1 (91) H → L (91) H−4 → L+1 (49), H−2 → L (37) H−1 → L (88) H−1 → L+2 (89) H−3 → L+3 (92)

1

S25

4.16

298

0.0357

H−5 → L+1 (67)

T1 T2 T3

2.55 2.60 2.82

485 476 440

— — —

H−12 → L+3 (27), H−13 → L+2 (24) H → L+1 (86) H−1 → L+1 (72)

a

Only major contributions >10% are given; H = HOMO, L = LUMO. transition equally.

varies. The first excited singlet state with reasonable oscillator strength ( f = 0.0116), i.e. S3, is found at 3.46 eV as a result of a combined transition HOMO−2 → LUMO and HOMO−9 → LUMO+1. The FC states S4 (3.61 eV), S5 (3.94 eV) and S12 (4.55 eV), which also exhibit reasonable oscillator strengths, can be formally described as electronic transitions mainly between HOMO−1, HOMO−2 and HOMO−4, respectively, and LUMO or LUMO+1. Interestingly, the high energy excitations between 260 and 300 nm also seem to arise from excitations to 1 MLCT states with some XLCT admixture, and not from phen (π–π*) transitions, as one could assume from previous reports. The two energetically lowest excited triplet states T1 and T2 are both of 3XMLCT nature with associated energies of 2.60 and 2.66 eV, respectively (Table 4). We note that excitation to give the charge-transfer states described above contain electron density redistribution from filled orbitals, which are antibonding with respect to either the copper-halide or the Cu–N( phen) bonds, to π* orbitals of the phenanthroline ligands. In other words, the excited states S1 and T1 should undergo a large geometrical change involving shortening of those bonds, and a considerable Stokes shift should be the consequence. Indeed, the FC-T1 state and its relaxed geometry differ by 0.8 eV (vide infra). In addition, the low spatial overlap of the donating and accepting orbitals, typical for CT states, leads to a very low energy gap between S1 and T1. Although this is derived from an analysis of the FC states, we see no reason why the geometrical changes should have a much larger effect on the energy of the relaxed T1 than on the S1, which would lead to an increased energy gap between these two states. Despite the blue-shift of excitation energies upon using CAM-B3LYP as the functional of choice to describe the chargetransfer properly,105 the calculated absorption spectrum of [CuBr(dtbphen)] (2) agrees well with our experimental absorption spectrum at room temperature, but also with the excitation spectrum (λem = 670 nm) obtained at 77 K in the glass

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b

XMLCT XMLCT 1 XMLCT 1 XMLCT 1 XMLCT 1 MLCT (30%) 1 XLCT (70%) 1 MLCT (40%) 1 XLCT (50%) 1

3

XMLCT XMLCT MLCT (51%) 3 XLCT (32%) 3 3

If no percentage is given, the halide and the metal contribute to the

matrix (Fig. 10). In particular, the weak bands at ca. 420 nm and 500 nm as the lowest energy excitations coincide nicely with the calculated values of 375 nm (3.30 eV) and 449 nm (2.76 eV) for the vertical S0→S1 and S0→S2 transitions with very low oscillator strength. A detailed analysis of 2π shows that due to its dimeric nature and the consequently multiple degeneracy of the frontier orbitals, a much higher density of excited states results (see Fig. 10 and ESI‡). Furthermore, the π interaction between the phenanthroline ligands leads to the formation of additional FC transitions, which are absent in 2. These two factors provide an explanation for the observed differences between the excitation spectra in the optical glass matrix and in the solid state, as the higher density of states and the high absorbance in the solid state make those states of low oscillator strength visible. This also highlights the importance of intermolecular interactions for understanding the photophysical properties. Similar to the monomeric compounds, the complex [Cu2(μ-I)2( phen)2] (5) also exhibits the first 11 singlet excitations as 1XMLCT transitions, and only the energetically higher lying excitations show varying MLCT and XLCT character (Table 5). The lower lying excitations, i.e. from the ground state S0→S1 up to S6, show very low oscillator strength f, and S0→S7 with an energy of 3.09 eV (401 nm) is the first excitation with a reasonable f value of 0.1695. The lowest two triplet states, T1 and T2, are both of 3XMLCT nature and are found as nearly degenerate states at 2.55 (485 nm) and 2.60 eV (476 nm), respectively. The excitation S0→T3 is found at 2.82 eV (440 nm), with a more pronounced MLCT contribution. Interestingly, all three triplet excited states are energetically very close to S1, i.e. within an energy range of 0.15 eV. Despite the fact that we have not taken into account the intermolecular π-interactions for our calculations, which are present in the solid state as confirmed by single crystal X-ray

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Fig. 10 Left: experimental absorption (black, 293 K) and excitation spectra (blue, 77 K, λem = 670 nm) in 2-MeTHF and calculated transitions of 2. Right: experimental excitation spectrum (blue: 77 K, black: 293 K, λem = 670 nm) of 2 in the solid state and calculated transitions of 2π.

diffraction studies and which should give a higher density of transitions due to dimer-of-dimer formation (vide supra), the calculated absorption spectrum of 5 is in good agreement with the experimental results obtained at room temperature (Fig. 11). Triplet excited states and emission properties We performed a geometry optimization of the first excited triplet state T1 for 1, 2, 2π and 5. Selected geometrical data for comparison of the ground state S0 and the relaxed T1 state are given in Table 6. The copper phenanthroline halides undergo a serious distortion in the triplet excited state T1, which is of XMLCT nature like the singlet excited state S1, indicating a small energy gap between these two states. The major difference between the structures of S0 and T1 lies in a significant shortening of the Cu–N( phen) bonds (vide supra), as well as in a lengthening and shortening of the N–C11 and C11–C12 bonds, respectively. The steric congestion around the copper(I) center arising from the tBu substituents at the phenanthroline ligands inhibits the halide to move closer to the Cu atom in the T1 state, which would be expected from the orbitals involved in the FC-T1 transitions (Table 4 and ESI‡). For the iodo-bridged dimer 5, the copper–copper distance is signifi-

Table 6

Fig. 11 Absorption/reflectance (solid; blue), excitation (solid; black: λem = 690 nm) and emission (dashed; black: λex = 450 nm) spectra recorded at room temperature in the solid state, and calculated TD-DFT singlet transitions (red bars) of [Cu2(μ-I)2( phen)2] (5).

Selected bond lengths [Å] obtained from the calculated S0 and T1 optimized structures

d(Cu–X)

d(Cu–N)

d(N–C11)

d(C11–C12)

d(Cu–Cu)

Cpd.

S0

T1

S0

T1

S0

T1

S0

T1

S0

T1

1 2 2πa 5

2.476 2.318 2.292 2.633

2.486 2.344 2.287 2.588

2.102 2.099 2.140, 2.025 2.127

1.950 1.946

1.359 1.356 1.349 1.353

1.390 1.388 1.380 1.369

1.452 1.453 1.441 1.446

1.403 1.402 1.391 1.428

— — — 2.594

— — — 2.572

b

2.080

a

The major distortions are located on one {CuBr(dtbphen)} fragment, and only the data of that dimer part are given. b An asymmetric bond distance change is observed in the T1 state. For one fragment the Cu–N distances were found to be 1.932 and 1.923 Å, whereas the other fragment experiences distances of 2.177 and 2.006 Å.

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Fig. 12 Total electron density difference plot between the optimized triplet state |T1〉 structure and the singlet ground state |S0f〉 at the same geometry for 2 (left; 1.86 eV, 665 nm) and 5 (right; 1.78 eV, 695 nm).

cantly shortened from 2.594 to 2.572 Å, and the same is true for the Cu–I bonds (2.633 vs. 2.588 Å). These geometrical changes are more facile compared to the monomeric counterparts 1–3, as no substituents at the phenanthroline ligand inhibit this in 5. At the same time, this flexibility might explain the differences in the quantum yields between 5 and the dimethyl substituted analog 6, which experiences a higher degree of rigidity. f The electron density change N(r)dif = n(r)total|T1〉 − n(r)total|S0 〉 upon emission T1→S0 leading to associated emission energies of 1.86 eV (665 nm) and 1.78 eV (695 nm) for 2 and 5, respectively, is shown in Fig. 12. The phosphorescence process in both, [CuBr(dtbphen)] (2) and [Cu2(μ-I)2( phen)2] (5), apparently involves electron density redistribution from the 2,9-ditert-butylphenanthroline or phenanthroline ligand π orbital (with some participation of the metal center) to the copper, halide and nitrogen atoms. Thus, the emission arising from the lowest lying triplet state is best described as a mixture of ligand to metal/halogen charge transfer (3LMXCT) and ligand centered (3LC) transition. It is important to note that the values for the emission energies of 2 and the dimeric 5 are in excellent agreement with the experimental values for the respective emission maxima of 658 and 688 nm in the solid state (Fig. 3 and Table 3), as well as with the emission in the glass matrix at 77 K of 2 (Fig. 9). However, in the case of [CuBr(dtbphen)] (2), it is not fully clear whether the hypsochromically shifted onset and overlap of the emission with the absorption and excitation at room temperature stems from participation of another excited triplet state, i.e. FC-T2, which is then only weakly coupled to T1, or whether (delayed) fluorescence from S1 is responsible for this emission phenomenon. The latter process is indeed feasible, as some Cu(I) compounds are already known to undergo “back-intersystem-crossing” S1←T1, and FC-S1 is energetically separated by only ca. 0.15 eV from T1 in 2 (Table 4). However, it is also possible that intermolecular π-stacking via the phenanthroline ligands including phonon coupling induces the significant blue shift of the emission onset. For [Cu2(μ-I)2( phen)2] (5) it is also clear that several states are involved in the observed emission process, and participation of S1 in the form of TADF is indeed feasible as the FC-S1 state is energetically very close to

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the optimized T1 state, i.e. less than 0.15 eV. However, we cannot deduce from our experiments whether a doubly split triplet state or S1 is involved. We recognize that the emission energy in 1–4 is red-shifted in the order CN > I > Br > Cl (Fig. 3), a trend which has also been found in [(2-diphenylphosphino)pyridine)3Cu2X2] (X = Cl, Br, I).66 The authors concluded that the halide dependent stabilization of the HOMO, which strongly participates in the transition leading to the emissive triplet state, is responsible for this trend. In our case we do not observe such a HOMO stabilization, but rather a decreasing halide participation (I > Br > Cl) (vide supra). In addition, a second transition contributes to small extents of ca. 8% to the formation of T1, and its composition varies with the halide (Table 4 and ESI‡), being responsible for the observed bathochromic shift of the emission maxima.

Conclusion We have synthesized and structurally characterized a series of some new and some previously known monomeric and dimeric copper(I) halogen and pseudohalogen complexes, bearing phenanthroline or its derivatives as π-chromophoric ligands. The monomers [CuX(dtbphen)] (X = I (1), Br (2), Cl (3), CN (4)) exhibit a distorted trigonal geometry, whereas the iodobridged dimers [Cu2(μ-I)2( phen)2] (5) and [Cu2(μ-I)2(dmphen)2] (6) show a distorted tetrahedral coordination environment around the copper atoms. Although very simple in their structure, these systems display an interesting and complicated photophysical excited state behavior in the solid state, which involves at least two emitting states. The excitation occurs via a XMLCT, and the resulting emission band, which shows the typical properties of a phosphorescent state (e.g. μs lifetimes), overlaps strongly with the excitation band. However, excitation and emission separate more in energy at low temperatures, but the emission intensity and the emission lifetimes do not increase linearly with decreasing temperature, as one would expect for an emissive T1 state. Our low temperature emission measurements suggest that at least two states can undergo thermally activated interconversion, but these find-

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ings differ from the measurements of [CuBr(dtbphen)] (2) in 2-Me-THF at 77 K, indicating that the π-stacking in the solid state is important. It is not fully clear what the exact nature of the second, energetically higher lying emitting state is. The DFT and TD-DFT calculations are in excellent in agreement with our photophysical studies in the solid state and in the glassy matrix, suggesting that the lowest excited singlet state S1 and the lowest excited triplet state T1 are separated by less than 0.15 eV. This would allow for thermally activated delayed fluorescence (TADF), although we cannot rule out the participation of other low lying emissive triplet states. It is also feasible that the photophysical behavior in the crystal is caused by exciton–phonon coupling, as it has been discussed in perylene derivatives.99–101 However, this study shows that one has to be careful with the rush assignment of TADF when low temperature powder measurements are used as a proof, as the specific structure and environment in the solid might effect the photophysical properties, which can significantly differ in frozen solution. Furthermore, we were surprised that although this class of Cu(I) phenanthroline compounds has been known for almost 30 years and has previously been studied photophysically,83,84 the complicated excited state behavior has not been fully recognized. If this is true for such simple structures of d10 Cu(I) complexes, we believe that mechanisms such as TADF or, generally speaking, the participation of more than just one emitting state in the solid state may well be more common than it is usually appreciated for related systems. With this in mind, we also believe that it is worth reconsidering Cu(I) compounds which have been reported in the past to be luminescent, but have not been studied carefully with respect to their properties.

Experimental section General procedures Copper(I) chloride, copper(I) bromide, copper(I) cyanide, copper(I) iodide, anhydrous 1,10-phenanthroline and neocuproine (2,9-dimethyl-1,10-phenanthroline, dmphen) were purchased from Sigma-Aldrich and used as received. 2,9-Di-(tertbutyl)-1,10-phenanthroline (dtbphen), [CuCl(dtbphen)] (3), [Cu2(μ-I)2(dmphen)2] (5) and [Cu2(μ-I)2( phen)2] (6) were prepared as previously reported.81,106 Preparations were carried out with dried solvents under an inert atmosphere. Elemental analyses were performed with an Elementar Vario Micro Cube. NMR spectra were recorded either on a Bruker Avance 500, Bruker Avance 200 or Bruker DRX 300 spectrometer. 1H and 13 C NMR data are given in parts per million ( ppm) relative to tetramethylsilane (TMS). Matrix-assisted laser desorption/ ionization measurements (MALDI) were performed on a Bruker Daltonics microflex MALDI-TOF instrument and using a DCTB : CHCl3 (1 : 3) matrix. [CuI(dtbphen)] (1). Copper(I) iodide (372.6 mg, 2.0 mmol) was added to a stirred solution of dtbphen (577.0 mg, 2.0 mmol) in acetonitrile (25 mL). Immediately, a yelloworange precipitate was formed, which was collected by

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filtration and washed with diethyl ether (2 × 10 mL). Yelloworange needle-like crystals (670 mg, 1 mmol, 50%), suitable for X-ray diffraction, were obtained by slowly cooling a hot solution of 1 in acetonitrile. The NMR spectrum in degassed, dry CDCl3 indicates decomposition of the complex to free dtbphen and presumably copper(I) iodide. For 1: 1H NMR (300 MHz, CDCl3): δ/ppm = 8.32 (d, 3JHH = 8.6 Hz, 2H, C4-H, C7-H), 7.96 (d, 3JHH = 8.6 Hz, 2H, C3-H, C8-H), 7.80 (s, 2H, C5-H, C6-H), 1.86 (s, 18H, tBu-H). For dtbphen: 1H NMR (300 MHz, CDCl3): δ/ppm = 8.14 (d, 3JHH = 8.6 Hz, 2H, C4′-H, C7′-H), 7.71 (m, 4H), 1.58 (s, 18H, tBu-H). Anal. calc. for C20H24N4ICu: C, 49.75; H, 5.01; N, 5.80. Found: C, 49.72; H, 5.02; N 5.72. [CuBr(dtbphen)] (2). THF (6 mL) was added to copper(I) bromide (94.8 mg, 0.661 mmol) and dtbphen (193.2 mg, 0.661 mmol). The resulting orange suspension was stirred for 3 d at room temperature. An orange precipitate was collected by filtration and washed with toluene (3 × 1 mL). Orange single crystals, suitable for X-ray diffraction studies, were obtained by slowly cooling a hot solution of 2 in acetonitrile (156.6 mg, 57%). 1H NMR (500 MHz, CD2Cl2): δ/ppm = 8.38 (d, 3JHH = 8.6 Hz, 2H, C4-H, C7-H), 8.00 (d, 3JHH = 8.6 Hz, 2H, C3-H, C8-H), 7.83 (s, 2H, C5-H, C6-H), 1.83 (s, 18H, tert-Bu-H). 13 C NMR (126 MHz, CD2Cl2) δ/ppm = 170.24, 144.11, 138.43, 127.48, 126.11, 122.60, 39.13, 30.93. Anal. calc. for C20H24N2BrCu: C, 55.11; H, 5.55; N, 6.43. Found: C, 55.00; H, 5.59; N 6.33. [CuCN(dtbphen)·CuCN]∞ (4). Dtbphen (224.2 mg, 0.7667 mmol) was dissolved in acetonitrile (20 mL). Copper(I) cyanide (68.6 mg, 0.7660 mmol) was added to give a colorless suspension. The suspension was stirred and heated to reflux for 3 d, by which time a yellow precipitate had formed, which was collected by filtration and washed with ether (3 × 5 mL). Single crystals suitable for X-ray diffraction were obtained by slowly cooling a hot solution of 4 in degassed DMSO over a period of several days (213.5 mg, 0. 4528 mmol, 59%). 1H NMR (200 MHz, DMSO-d6) δ/ppm 8.55 (d, 2H, J = 8.55 Hz), 8.00 (m, 4H), 1.63 (s, 18H, tBu). Anal. calc. for C22H24N4Cu2: C, 56.04; H, 5.13; N, 11.88. Found: C, 55.87; H, 5.04; N 11.73. Photophysical measurements UV-visible absorption spectra were obtained on an Agilent 1100 Series Diode Array spectrophotometer using standard 1 cm path length quartz cells. Solid state absorption spectra of the samples were recorded against BaSO4 in an integrating sphere mounted in an Edinburgh Instrument FLSP920 spectrometer. Excitation and emission spectra were recorded on an Edinburgh Instrument FLSP920 spectrometer, equipped with a 450 W Xenon arc lamp, double monochromators for the excitation and emission pathways, and a red-sensitive photomultiplier (PMT-R928) as detector. The excitation and emission spectra were corrected using the standard corrections supplied by the manufacturer for the spectral power of the excitation source and the sensitivity of the detector. The quantum yields were measured by use of an integrating sphere with an Edinburgh Instrument FLSP920 spectrometer, following a method described in the literature.107 The luminescence lifetimes were

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measured using a μF900 pulsed 60 W Xenon microsecond flashlamp, with a repetition rate of 100 Hz, and a multichannel scaling module. The emission was collected at right angles to the excitation source with the emission wavelength selected using a double grated monochromator and detected by a R928-P PMT. The instrument response function (IRF) was measured using a scattering sample and setting the emission monochromator at the wavelength of the excitation beam. The resulting intensity decay is a convolution of the luminescence decay with the IRF and iterative reconvolution of the IRF with a decay function and non-linear least squares analysis was used to analyze the convoluted data. Low temperature measurements were performed in an Oxford Optistat cryostat.

Dalton Transactions

Conflict of interest The authors declare no competing financial interest.

Acknowledgements This work was supported by the Deutscher Akademischer Austauschdienst (DAAD), the Deutsche Forschungsgemeinschaft (DFG grant STE-1834/4-1), and the Bavarian State Ministry of Science, Research, and the Arts for the Collaborative Research Network “Solar Technologies go Hybrid”. A.S. is grateful to Prof. T. B. Marder for his generous support. C.K. thanks the Fonds der Chemischen Industrie for a Liebig-Stipendium.

Computational details DFT calculations were carried out using the Gaussian 09 program.108 The geometric structures were fully optimized without any symmetry constraints using the MPW1PW91 functional within the LANL2DZ ECP basis set, augmented by polarization functions for all atoms except hydrogens.109–111 The unrestricted MPW1PW91 method (UMPW1PW91) was used for the optimization of the lowest energy triplet state.112 Harmonic vibrational frequency calculations were performed to verify that the optimized geometries represent the global energy minima. TD-DFT calculations were performed at the groundstate geometry for the 50 lowest-lying excited singlet and triplet states using the CAM-B3LYP functional with the previously employed basis sets. The iso-surface spin-density representations were generated using the GaussView 5.0 program.113 Atomic orbital contributions to the molecular orbitals have been determined with the AOMix software package.114 The determination of percentage of single particle contributions to the vertical excited states was carried out as described in the literature.115 The π-stacked dimer compound 2π was optimized using the ORCA 3.0.2 program suite116 with the PBE0117–123 functional as implemented in ORCA. The def2TZVP124,125 basis set was used for all atoms together with the auxiliary basis set def2-TZVP/J in order to accelerate the computations within the framework of RI approximation. Relativistic effects have been considered by using the ZORA approximation126 and van der Waals interactions have been considered by an empirical dispersion correction (GrimmeD3).127,128 TD-DFT calculations were performed with the CAM-B3LYP functional. Representations of molecular orbitals and transition densities were produced with orca_plot as provided by ORCA 3.0.2 and with gOpenMol 3.00.129,130

Author contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

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Luminescent copper(I) halide and pseudohalide phenanthroline complexes revisited: simple structures, complicated excited state behavior.

We have synthesized a series of luminescent trigonal [CuX(dtbphen)] (X = I (), Br (), Cl (), CN (), dtbphen = 2,9-di-tert-butylphenanthroline) and tet...
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