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Cite This: Inorg. Chem. 2017, 56, 12978-12986

Excited State Properties of Heteroleptic Cu(I) 4H‑Imidazolate Complexes Martin Schulz,*,†,‡ Christian Reichardt,†,‡ Carolin Müller,† Kilian R. A. Schneider,‡ Jonas Holste,† and Benjamin Dietzek*,†,‡,§ †

Institute of Physical Chemistry, Friedrich Schiller University Jena, Helmholtzweg 4, 07743 Jena, Germany Leibniz Institute of Photonic Technology (IPHT), Department Functional Interfaces, Albert-Einstein-Straße 9, 07745 Jena, Germany § Center für Energy and Environmental Chemistry (CEEC), Friedrich Schiller University Jena, Philosophenweg 7a, 07743 Jena, Germany

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‡

S Supporting Information *

ABSTRACT: The excited state properties of three heteroleptic copper(I) xantphos 4H-imidazolate complexes are investigated by means of femtosecond and nanosecond time-resolved transient absorption spectroscopy in dichloromethane solution. The subpicosecond spectral changes observed after excitation into the MLCT absorption band are interpreted as intersystem crossing from the singlet to the triplet manifold. This interpretation is corroborated by DFT and TD-DFT results, indicating a comparable molecular geometry in the ground state (and hence the nonrelaxed singlet state) and the excited triplet state. Population of the triplet state is followed by planarization of the N-aryl rings of the 4H-imidazolate ligand on a 10 ps time scale. The planarization strongly depends on the substitution pattern of the N-aryls and correlates with the reduced moment of inertia for the planarization motion. The triplet state subsequently decays to the ground state in about 100 ns. These results demonstrate that the excited state processes of copper(I) complexes depend on the specific ligand(s) and their substitution pattern. Thus, the work presented points to a possibility to design copper(I) complexes with specific photophysical properties.

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INTRODUCTION Understanding the excited-state processes of chromophores and their dependence on the substitution pattern is a prerequisite for custom-tailoring the photophysical properties of chromophores. The excited state properties of homoleptic copper(I) diimine complexes are generally quite well understood,1−14 while the excited state properties of heteroleptic copper(I) phosphine diimine complexes15−22 have drawn much less attention (vide inf ra for a brief summary). So far, for both homoleptic and heteroleptic copper(I) complexes, comparable results have been obtained in terms of the nature of the excited state processes and the associated rate constants. However, it should be noted that for most of the investigated homoleptic and heteroleptic complexes, the diimine is a phenanthrolinetype ligand. Hence, it remains to be validated how generalizable the model that has been put forward in understanding the excited-state chemistry of the heteroleptic copper(I) complexes carrying a phenanthroline derived ligand is. We have recently reported on neutral, heteroleptic copper(I) phosphine 4H-imidazolate complexes with strong electronic absorption spanning almost the entire visible range (Figure 1).23 The 4H-imidazolate ligands are comparable to benchmark © 2017 American Chemical Society

phenanthroline ligands such as neocuproine in terms of steric demand in the vicinity of the copper center.24 However, the 4H-imidazolate ligand backbone is less rigid than phenanthroline and allows for rotational motion of the N-aryl rings, which has been shown to dominate the ultrafast photophysics in Ru(II) 4H-imidazolate complexes.25−28 Herein, we report on the dependence of the excited state processes on the N-aryl substituents of three Cu(I)-4H-imidazolate complexes by means of transient absorption spectroscopy with femtosecond and nanosecond temporal resolution. In the following, the excited state processes reported for homoleptic copper(I) diimine complexes7−9,12,29−33 as well as heteroleptic copper(I) phosphine diimine complexes16,17 will briefly be described (the description refers to complexes with a substituted 1,10-phenanthroline as the most commonly employed diimine ligand). In homoleptic copper(I) diimine complexes, excitation of the MLCT absorption band into the singlet manifold leads to the formation of a transient copper(II) center, which is followed by a pseudo-Jahn−Teller flattening Received: July 10, 2017 Published: October 24, 2017 12978

DOI: 10.1021/acs.inorgchem.7b01680 Inorg. Chem. 2017, 56, 12978−12986

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Inorganic Chemistry

Figure 1. Investigated copper(I) 4H-imidazolate complexes and the ground state absorption spectra in CH2Cl2 (r.t., ca. 2 × 10−5 M). The 4Himidazolate chromophore of the ligand is highlighted in violet.

with respect to the 4H-imidazolate plane, which takes place on a 10 ps time scale. This behavior will be derived and discussed in the following sections.

distortion from the (pseudo) tetrahedral toward a (distorted) square planar coordination geometry, usually on a subpicosecond time scale.9,29 Due to spin−orbit coupling of the copper ion, intersystem crossing (ISC) to the triplet manifold is possible and usually occurs on a 10 ps time scale.9,29,32 The system then decays radiatively to the singlet ground state on time scales up to the microsecond range.24 The formation of a square planar coordination environment allows interactions with donor molecules (e.g., solvent) via the apical coordination site, which results in nonradiative decay of the formed exciplex.9,24 Thus, hampering the flattening distortion by introduction of bulky substituents resulted in greatly enhanced emission time constants.7,34−36 A similar excited state behavior was reported for heteroleptic [Cu(I)(xantphos)(phenanthroline)]+ complexes.16,17 In dependence on the substitution pattern of the phenanthroline ligands, time constants for the flattening distortion and intersystem crossing of 0.7−1.4 ps and 6.8−7.4 ps have been observed, respectively.17 For the complexes reported in ref 17, the rate of the flattening distortion depends on the molecular weight of the phenanthroline ligands; larger distortion time constants were observed for heavier ligands. The bulkiness, however, seemed to play a minor role for the distortion time scale. The intersystem crossing time constant, on the other hand, was found to depend on the electronic properties of the phenanthroline ligands showing shorter ISC time constants for ligands with electron withdrawing substituents.16,17 In contrast, the copper(I) xantphos 4H-imidazolate complexes reported in this contribution undergo a fast subpicosecond ISC, followed by the planarization of the N-aryl rings

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RESULTS AND DISCUSSION

Synthesis. The synthesis of the copper(I)4H-imidazolate complexes CuN1P2 and CuN2P2 was previously described.23 CuN3P2 is described for the first time and was synthesized according to the published procedure.23 Briefly, stepwise mixing of [Cu(acetonitrile)4]PF6 with the xantphos ligand P2 and the respective 4H-imidazole in the presence of an anion exchange resin afforded deeply colored solutions. The pure product was obtained by recrystallization. CuN3P2 was characterized by one- and two-dimensional 1H, 13C, and 31P NMR spectroscopy; mass spectrometry; and elemental analysis (see Supporting Information for further details). Ground State Absorption Properties. All complexes feature an intense absorption band in the UV as well as a broad absorption band that spans almost the entire visible spectrum (Figure 1). In the UV, a strong narrow band between 250 and 350 nm is observed with a lower-energy shoulder at about 360 nm. In the visible range, a weaker but broader absorption band ranges from about 400 to 600 nm. The unusually broad and low-energy absorption is a feature of the polymethine character of the monoanionic 4H-imidazolate ligands, which are best described as (aza)-oxonoles (the neutral 4H-imidazole is best described as merocyanine).37−40 The chromophore properties of the 4H-imidazolate are retained upon complexation of the copper(I) xantphos fragment. However, in the copper(I) 12979


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390, 430, and 620 nm as well as positive differential absorption features above 700 nm. In CuN1P2 and CuN2P2, the 430 nm band is observed as a shoulder to the 390 nm band, while in CuN3P2 the situation is reversed and the 390 nm band appears as a shoulder to the 430 nm band. For all complexes, the ground state bleach is observed at around the 550 nm, i.e., in the long wavelength range of the respective steady state absorption spectra. Upon increasing the delay time, the 390, 430, and 620 nm bands decrease, while at 490 nm a new band is growing in, as are features above 700 nm. The decrease of the 390 nm band as well as the increase of the features above 700 nm continue during the first 5 ps, while the decrease of the 430 and 620 nm bands progresses within about the first 100 ps. The observed decrease/increase of the optical density difference at 430 nm/ 490 nm bands is most pronounced for CuN3P2. Finally, the bands at 490 nm/620 nm increase/decrease over about the first 100 ps for all three complexes. On longer time scales (>1 ns), the spectral features of CuN1P2 and CuN3P2 remain approximately unchanged. The spectral features of CuN2P2 slowly decay, after having reached a maximum at 490 nm within about 100 ps. About 50 ns after the pump pulse (see Supporting Information, Figures S7−S9), the absorption difference spectra of all three complexes are qualitatively the same as those obtained at about 1.8 ns, and the observed difference absorption decays monoexponentially to zero. Time Constants and Assignment of Underlying Molecular Processes. Global fitting of the data obtained between 100 fs and 1.8 ns (fs-time-resolved experiment) required two time constants and an infinite component. In order to temporally resolve the infinite component, a nanosecond time-resolved experiment was carried out and revealed a monoexponential decay (the initial delay time was 50 ns). The spectral signature of the infinite component (femtosecond-time-resolved experiment) is reflected in the initial spectrum of the nanosecond-time-resolved experiment. The three time constants obtained are summarized in Table 1. The shortest time constant τ1, i.e., 0.6 ps (CuN1P2), 0.8 ps (CuN2P2), and 0.8 ps (CuN3P2), is associated with the decrease of the 390 nm band and the rise of the features above 700 nm. This is reflected in the similar decay associated spectra, DAS(τ1), for the individual complexes. The second time constant τ2 obtained differs significantly among the three complexes and is quantified as 6 ps (CuN3P2), 9 ps (CuN1P2), and 18 ps (CuN2P2). Spectrally and irrespective of the complex, τ2 is associated with the continuous increase of the 490 nm band and the decrease of the 430 and 620 nm band. Subsequently, the system decays monoexponentially to the ground state with τ3: 73 ns (CuN2P2), 107 ns (CuN3P2), and 135 ns (CuN1P2). The DAS (Figures 3−5) associated with the first time constant reflects marked spectral changes over the entire visible spectrum up to 780 nm as detailed above. The photophysical process connected with the observed spectral changes is assigned to intersystem crossing (ISC) from the singlet to the triplet state. Similarly pronounced spectral changes were observed for [Cu(dmp)2]+12,31,41 and [Cu(dmp)(xantphos)]+17 and assigned with ISC, however, on a picosecond time scale. Flattening distortion might also take place on such short time scales; however, this process is generally associated with only very minor spectral changes in the UV/vis range.9,12,17,29 Thus, the spectral changes reflected in the DAS(τ1) are likely due to significant contributions of ISC to the ultrafast subpicosecond

complexes, the visible absorption band appears broadened, leading to the complexes’ dark violet color (see Figure 2).23

Figure 2. Normalized absorption spectra of the neutral ligand HN3, deprotonated ligand N3− (deprotonated with 1,8-diazabicyclo[5.4.0]undec-7-ene, DBU), and the complex CuN3P2 in dichloromethane (also see Supporting Information Figure S4 for the neutral ligand HN1, deprotonated ligand N1−, and CuN1P2 and Figure S5 for the neutral ligand HN2, deprotonated ligand N2−, and CuN2P2).

A tentative assignment of the observed electronic transitions can be made by comparison of the spectra of the complexes with the absorption spectra of the neutral ligands as well as the deprotonated ligands (see Figure 2). The absorption spectra of the neutral ligands HN1 and HN2 exhibit two bands in the UV region around 280 and 350 nm with nearly the same intensity ratio, while HN3 features a weak absorption band at 290 nm as well as a structured band at around 360 nm similar to HN1. In the spectra of the copper complexes, the absorption band between 250 and 300 nm is attributed to the xantphos-based transitions, while the 4H-imidazolate based transitions appear as shoulders around 310 nm (CuN2P2) as well as at 360 nm (CuN1P2, CuN3P2) and are allocated to intraligand transitions (πim → πim*). For the complexes, the absorption bands between 400 and 650 nm are significantly red-shifted with respect to the free ligands. CuN1P2 shows a maximum at 551 as well as shoulders at 520 and 580 nm, and CuN2P2 has a maximum at 517 nm with shoulders at about 480 and 550 nm. CuN3P2 exhibits a double peak at 531 and 454 nm as well as a shoulder at about 550 nm. Earlier TD-DFT calculations on CuN2P2 indicated the occurrence of MLCT transitions in the visible range.23 However, the similarity of the overall structure of the visible absorption bands of the complexes with respect to those of the deprotonated ligands suggests that also 4H-imidazolate-based intraligand transitions are involved. Excited State Absorption Properties. All complexes reported in this contribution were investigated in dichloromethane solution at room temperature by transient absorption spectroscopy upon excitation at 520 nm, i.e., in the MLCT absorption band. The transient absorption of the samples was probed with a white light continuum (generated in CaF2). The observed spectra are given in Figures 3−5 and in general show comparable transient absorption spectra as well as consistent time dependent changes. The spectra immediately observed after excitation show excited state absorption bands at about 12980


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Figure 3. (A) Selected femtosecond-time-resolved transient absorption spectra of CuN1P2 in dichloromethane at r.t. and the inverted ground state absorption spectrum shaded in gray. (B) Decay associated spectra (DAS). (C) Kinetic traces recorded at various probe wavelengths.

reorganization processes, namely, the planarization of the Naryl rings of the 4H-imidazolate ligand toward the imidazole plane (cf. Figure 6). Rotational motion of the N-aryl rings was already described for ruthenium(II) 4H-imidazolate complexes47 and can be understood as stabilization of the excited triplet state by the enlargement of the conjugated system. For CuN3P2, which bears a small para-methyl substituent, the observed time constant is the shortest, followed by CuN1P2 with the larger para-COOEt substituent and CuN2P2 with a meta-CF3 substituent. The dependence of the time constants on the substitution pattern can be rationalized when considering the impact of the mass and position of the substituents on the moment of inertia of the rotational motion, i.e., planarization of the N-aryl rings. In CuN3P2, rotational motion is comparatively fast, due to a small moment of inertia, since the small methyl group lies along the rotation axis. The COOEt group in CuN1P2 imposes a larger moment of inertia, leading to a slowdown of the rotational motion, with respect to CuN3P2, and consequently a larger planarization time constant is observed. The meta-CF3 group sits off the rotational axis and thus imposes the largest impact on the rotational motion leading to the largest planarization time constant among the studied complexes. Reduced moments of inertia were deduced from optimized (DFT) ground state structures of all three complexes using the method of Pitzer48,49 as implemented in

photoinduced kinetics. This is somehow unexpected, since for most Cu(I) phenanthroline-type complexes ISC time constants were reported to occur on a 10 ps time scale, as a consequence of weak spin−orbit coupling, induced by the faster flattening process.32,42 The attribution of the spectral changes associated with the subpicosecond time constants to ISC can be rationalized when considering strong spin−orbit coupling. The availability of large spin−orbit couplings was reported for copper(I) bis(2,9-dimethyl-1,10-phenanthroline) complexes ([Cu(dmp)2]+),32,43 and hence the general possibility of fast, subpicosecond intersystem crossing was proposed.32,43−45 Furthermore, the spin−orbit coupling is reduced upon structural rearrangement of the complex in the excited state, i.e., upon flattening of the initial tetrahedral geometry,7,32 which we assume takes place in parallel with ISC. Zou and co-workers theoretically (DFT, TD-DFT) found for copper(I)(phenanthroline)(diphosphine)-type complexes that the dihedral angle (DHA) between the P−Cu−P and N−Cu−N planes decreases in the relaxed S1 state but is very similar in the relaxed T1 with respect to S0.46 Similarity between the singlet state structure and the triplet state structure usually leads to increased ISC rates.46 The spectral changes associated with τ2 are represented by the decay of the 430 and 620 nm bands and the rise of the 490 nm band. These observations are interpreted as structural 12981


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the GPOP program.50 The exocyclic 4H-imidazolate nitrogen and the ipso-carbon of the N-aryl ring were set as pivot atoms for the rotation along the N−C bond. A correlation between the calculated moments of inertia and τ2 is given in Figure 6. Computational Results. We have carried out DFT and TD-DFT calculations with CuN2P2, in order to investigate the lowest energy singlet and lowest energy triplet state with respect to the ground state. All calculations were made with the PBE0 functional, which has been found suitable for copper(I) complexes (see Supporting Information for further details).20 A solvent sphere (dichloromethane) was taken into account. The obtained geometrical parameters for the calculated ground state are in good agreement with X-ray data, i.e., bond lengths, bite angles, as well as the DHA of the ground state (Table S1). The energies of the S1 as well as T1 state cannot directly be compared due to the different methodologies (DFT vs TDDFT). However, the relative differences with respect to the ground state are 1.3 eV (S1) and 1.4 eV (T1), respectively. Within the error of the calculation (ca. 0.2 eV) these energy differences are equal (we note that the planarization of the Naryl rings (vide inf ra) was not taken into account for the calculation of T1). The major contributions of the low energy singlet transitions (Table 2) involve charge transfer transitions from the HOMO, HOMO−1, HOMO−2, and HOMO−3 to the 4H-imidazolate based LUMO (Figure S12). The four

highest energy occupied orbitals are mainly of copper parentage with contributions from xantphos and the 4H-imidazolate (Table S2). The transitions are therefore characterized as MLCT transitions; however, some ILCT character is noted as well. The DFT calculations revealed that the dihedral angle between the P−Cu−P and N−Cu−N planes (see Figure 7) in the lowest energy triplet state (81.61°) is essentially the same as in the singlet ground state (81.84°) but heavily distorted in the relaxed S1 state (64.61°). The calculated DHA in the ground state is in agreement with X-ray data CuN2P2 (82.46°).23 The observation of a decreased DHA in the S1 state but essentially no change in the T1 state, with respect to S0, is in agreement with the calculated change of the DHAs in the S0, S1, and T1 states of [Cu(dmp)(PPh3)2]+, [Cu(dbp)(PPh3)2]+, and [Cu(dbp)(DPEPhos)]+, respectively (dmp = 2,9-dimethyl-(1,10phenanthroline); dbp = 2,9-di-n-butyl-(1,10-phenanthroline)).46 Contrarily, decreased DHAs in the T1 state with respect to S0 were theoretically found for copper(I) bisphenanthroline complexes7,10,32,44 as well as [Cu(I)(dmp)(xantphos)]+17 and [Cu(I)(dmp)(DPEphos)]+.20 For the latter, also the S1 geometry was calculated, revealing a similarly decreased DHA as in the T1 state with respect to S0.20,46 The computational results obtained by us as well as in the cited literature support the idea that fast intersystem crossing is 12982


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Table 1. Time Constants τ Determined by a Global Fit of the Ultrafast Transient Absorption Data As Well As Reduced Moments of Inertia Ired for the Rotation of the N-Aryl Moieties (vide inf ra) complex

τ1/ps

τ2/ps

τ3/ns

Ired/amuÅ2

CuN1P2 CuN2P2 CuN3P2

0.6 0.8 0.8

9 18 6

135 73 107

359 920 87

is in contrast to earlier reports on homo- and heteroleptic copper(I)−phenanthroline complexes. The fast ISC is rationalized on the basis of similar dihedral angles in the ground state (and hence the excited singlet state prior to flattening) presumably resulting in strong spin−orbit coupling. In this respect, the flattening distortion represents a quenching channel for the ISC. The spectral changes associated with time constants between 5 and 18 ps are interpreted as planarization of the N-aryl rings of the 4H-imidazolate ligand on about a 10 ps time scale. The planarization time constants vary markedly with the substitution pattern of the N-aryl rings. A positive correlation of the reduced moment of inertia of the rotational motion with the second time constant is obtained and reflects the impact of the substitution on the planarization of the N-aryl rings. All described processes, namely flattening, ISC, and planarization, can be expected to depend on the substitution pattern of N-aryl rings. In order to exploit the fascinating features of copper(I) 4H-imidazolate complexes, extended studies are necessary that include the investigation of the emission properties and calculation of spin−orbit couplings. Nonetheless, the reported observations demonstrate that the understanding of copper(I) excited state processes and hence the design of complexes with distinct properties is just at the beginning.

possible on the basis of similar geometries in the undistorted, i.e., not flattened, excited singlet and triplet states, as was earlier proposed.46,51

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CONCLUSION The excited state properties of three neutral, heteroleptic copper(I)4H-imidazolate complexes were investigated by means of femtosecond- and nanosecond-time-resolved pump−probe absorption spectroscopy as well as DFT calculations (cf. Figure 8). Excitation of the complexes at 520 nm (MLCT band) in dichloromethane furnished distinct excited state absorption patterns. The spectral changes associated with the fast subpicosecond processes are interpreted as ISC from the excited singlet to the triplet manifold, taking place on the same time scale as the flattening distortion, which 12983


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Figure 6. Correlation of the reduced moment of inertia for the rotation of the N-aryl rings with the second time constant τ2 (left). DFT optimized structure of CuN2P2. The dashed line marks the axis for the rotation of the N-aryl ring. For the sake of clarity, the xantphos ligand is given in light gray, and hydrogen atoms are omitted.

Table 2. Low Energy Transitions Calculated on the TD-DFT/PBE0 Level of Theory transition

energy/eV

wavelength/nm

oscillator strength

assignment

S1 S2 S3 S4

2.49 2.54 2.66 2.75

498 489 467 450

0.1888 0.2299 0.0329 0.0073

MLCT MLCT MLCT MLCT

H-1→LUMO H-1→LUMO H-3→LUMO H-3→LUMO

(29%), (68%), (20%), (70%),

HOMO→LUMO (59%) HOMO→LUMO (24%) H-2→LUMO (63%), HOMO→LUMO (14%) H-2→LUMO (21%)

repetition rate), was used to produce the 520 nm pump pulse with a TOPAS-C. A supercontinuum probe pulse generated in a CaF2 plate served as a broad-band probe. The polarizations of pump and probe were oriented at the magic angle. Probe and reference intensities were detected on a double stripe diode array and converted into differential absorption (DA) signals using a commercially available detection system (Pascher Instruments AB). The time resolution of the experiment was evaluated by the width of the coherent artifact,53,54 allowing an estimation of the cross correlation value between pump and probe to be on the order of 80 fs. The femtosecond time-resolved measurements were performed in quartz cells with a 1 mm optical path length with an approximate optical density of 0.4 at 520 nm. The integrity of the samples was ensured via absorption spectroscopy prior to and after each measurement. The DA signals recorded as a function of the delay time and the probe wavelength were chirp corrected and subsequently subjected to a global fitting routine using a sum of exponential functions for data analysis.55 The nanosecond time-resolved experimental setup was previously described.56 The pump pulse was delivered by a Continuum Surelite Laser, with repetition rate of 10 Hz and modified with a Continuum optical parametric oscillator to obtain the pump wavelengths of 520 nm. Probe light was delivered by a 75 W xenon arc lamp and dispersed by a ruled grating on to the sample using right angle geometry with respect to the pump light. The probe light was detected by a Hamamatsu R928 photomultiplier, and the signal was processed by a commercially available detection system (Pascher Instruments AB). The measurements were performed in a flow-through quartz cell (1 cm optical path length) at an approximate optical density of 0.4 at 520 nm. The integrity of the samples was ensured via absorption spectroscopy prior to and after each measurement. The sample was probed between 380 and 800 nm with 10 nm steps. The DA signals were recorded as a function of the delay time, and the probe wavelength was subjected to a global fitting routine using a sum of exponential functions for data analysis.57

Figure 7. P−Cu−P and N−Cu−N planes of DFT-optimized CuN2P2 in the ground state (GS), relaxed singlet excited state (S1), and triplet state (T1). The dihedral angles are in parentheses.

Figure 8. Overview of the investigated excited state processes (dark gray arrows).

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major contributions

EXPERIMENTAL SECTION

The setup for femtosecond-time-resolved transient absorption spectroscopy has been described previously.52 An 800 nm pulse, produced by an amplified Ti:sapphire oscillator (Libra, Coherent Inc., 1 kHz 12984


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Nuclear Interactions on the Excited-State Properties and Structural Dynamics of Copper(I) Diimine Complexes. J. Phys. Chem. B 2013, 117, 1921−1931. (9) Mara, M. W.; Fransted, K. A.; Chen, L. X. Interplays of Excited State Structures and Dynamics in Copper (I) Diimine Complexes: Implications and Perspectives. Coord. Chem. Rev. 2015, 282−283, 2− 18. (10) Chen, L. X.; Shaw, G. B.; Novozhilova, I.; Liu, T.; Jennings, G.; Attenkofer, K.; Meyer, G. J.; Coppens, P. MLCT State Structure and Dynamics of a Copper(I) Diimine Complex Characterized by Pump− Probe X-Ray and Laser Spectroscopies and DFT Calculations. J. Am. Chem. Soc. 2003, 125, 7022−7034. (11) Sandroni, M.; Pellegrin, Y.; Odobel, F. Heteroleptic Bis-Diimine copper(I) Complexes for Applications in Solar Energy Conversion. Comptes Rendus Chimie; Elsevier Ltd, 2016; pp 79−93. (12) Shaw, G. B.; Grant, C. D.; Shirota, H.; Castner, E. W.; Meyer, G. J.; Chen, L. X. Ultrafast Structural Rearrangements in the MLCT Excited State for Copper(I) Bis- Phenanthrolines in Solution. J. Am. Chem. Soc. 2007, 129, 2147−2160. (13) Chen, X. L.; Yu, R.; Zhang, Q. K.; Zhou, L. J.; Wu, X. Y.; Zhang, Q.; Lu, C. Z. Rational Design of Strongly Blue-Emitting Cuprous Complexes with Thermally Activated Delayed Fluorescence and Application in Solution-Processed OLEDS. Chem. Mater. 2013, 25, 3910−3920. (14) Garakyaraghi, S.; Danilov, E. O.; McCusker, C. E.; Castellano, F. N. Transient Absorption Dynamics of Sterically Congested Cu(I) MLCT Excited States. J. Phys. Chem. A 2015, 119, 3181−3193. (15) Vorontsov, I. I.; Graber, T.; Kovalevsky, A. Y.; Novozhilova, I. V.; Gembicky, M.; Chen, Y.; Coppens, P. Capturing and Analyzing the Excited-State Structure of a Cu(I) Phenanthroline Complex by TimeResolved Diffraction and Theoretical Calculations. J. Am. Chem. Soc. 2009, 131, 6566−6573. (16) Heberle, M.; Tschierlei, S.; Rockstroh, N.; Ringenberg, M.; Frey, W.; Junge, H.; Beller, M.; Lochbrunner, S.; Karnahl, M. Heteroleptic Copper Photosensitizers: Why an Extended π-System Does Not Automatically Lead to Enhanced Hydrogen Production. Chem. - Eur. J. 2017, 23, 312−319. (17) Tschierlei, S.; Karnahl, M.; Rockstroh, N.; Junge, H.; Beller, M.; Lochbrunner, S. Substitution-Controlled Excited State Processes in Heteroleptic Copper(I) Photosensitizers Used in Hydrogen Evolving Systems. ChemPhysChem 2014, 15, 3709−3713. (18) Czerwieniec, R.; Yersin, H. Diversity of Copper(I) Complexes Showing Thermally Activated Delayed Fluorescence: Basic Photophysical Analysis. Inorg. Chem. 2015, 54, 4322−4327. (19) Linfoot, C. L.; Leitl, M. J.; Richardson, P.; Rausch, A. F.; Chepelin, O.; White, F. J.; Yersin, H.; Robertson, N. Thermally Activated Delayed Fluorescence (TADF) and Enhancing Photoluminescence Quantum Yields of [Cu I (Diimine)(diphosphine)] + ComplexesPhotophysical, Structural, and Computational Studies. Inorg. Chem. 2014, 53, 10854−10861. (20) Kubiček, K.; Thekku Veedu, S.; Storozhuk, D.; Kia, R.; Techert, S. Geometric and Electronic Properties in a Series of Phosphorescent Heteroleptic Cu(I) Complexes: Crystallographic and Computational Studies. Polyhedron 2017, 124, 166−176. (21) Zhang, K.; Zhang, D. Synthesis and Study on a Series of Phosphorescent Cu(I) Complexes Having Sterically Blocking Ligands. Spectrochim. Acta, Part A 2014, 124, 341−348. (22) Yang, L.; Feng, J. K.; Ren, A. M.; Zhang, M.; Ma, Y. G.; Liu, X. D. Structures, Electronic States and Electroluminescent Properties of a Series of CuI Complexes. Eur. J. Inorg. Chem. 2005, 2005 (10), 1867− 1879. (23) Schulz, M.; Dröge, F.; Herrmann-Westendorf, F.; Schindler, J.; Görls, H.; Presselt, M. Neutral, Heteroleptic copper(I)-4H-Imidazolate Complexes: Synthesis and Characterization of Their Structural, Spectral and Redox Properties. Dalton Trans. 2016, 45, 4835−4842. (24) Lazorski, M. S.; Castellano, F. N. Advances in the Light Conversion Properties of Cu (I) -Based Photosensitizers. Polyhedron 2014, 82, 57−70.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01680. Synthesis and characterization details for CuN3P2, comparison of the UV−vis absorption spectra of the neutral and deprotonated ligands as well as the complexes, transient absorption data obtained between 50 and 1000 ns, and computational details (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Martin Schulz: 0000-0003-4989-5207 Benjamin Dietzek: 0000-0002-2842-3537 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS B.D. thanks the financial support by the FCI. C.R. is grateful for financial support from the Jena Graduate Academy for a Ph.D. scholarship. K.R.A.S. thanks the Carl-Zeiss-Foundation for a Ph.D. scholarship. The authors thank Prof. Rainer Beckert for putting the 4H-imidazoles at our disposal. Julia Preiss is gratefully acknowledged for helpful discussions. Finally, we would like to thank the COST Action CM1202.

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DEDICATION Dedicated to Prof. Rainer Beckert on the occasion of his retirement REFERENCES

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Excited State Properties of Heteroleptic Cu(I) 4H ... - ACS Publications

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