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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Heteroleptic Copper(I) Complexes Prepared from Phenanthroline and Bis-Phosphine Ligands: Rationalization of the Photophysical and Electrochemical Properties Enrico Leoni,§,# John Mohanraj,§,⊥ Michel Holler,† Meera Mohankumar,† Iwona Nierengarten,† Filippo Monti,*,§ Alix Sournia-Saquet,‡ Béatrice Delavaux-Nicot,*,‡ Jean-Franco̧is Nierengarten,*,† and Nicola Armaroli*,§ §

Istituto per la Sintesi Organica e la Fotoreattività, Consiglio Nazionale delle Ricerche, Via Gobetti 101, 40129 Bologna, Italy Laboratoire de Chimie des Matériaux Moléculaires, Université de Strasbourg et CNRS (LIMA - UMR 7042), 25 rue Becquerel, 67087 Strasbourg Cedex 2, France ‡ Laboratoire de Chimie de Coordination du CNRS (UPR 8241), Université de Toulouse (UPS, INPT), 205 Route de Narbonne, 31077 Toulouse Cedex 4, France # Laboratorio Tecnologie dei Materiali Faenza, ENEA, Via Ravegnana 186, 48018 Faenza (RA), Italy

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S Supporting Information *

ABSTRACT: The electronic and structural properties of ten heteroleptic [Cu(NN)(PP)]+ complexes have been investigated. NN indicates 1,10-phenanthroline (phen) or 4,7diphenyl-1,10-phenanthroline (Bphen); each of these ligands is combined with five PP bis-phosphine chelators, i.e., bis(diphenylphosphino)methane (dppm), 1,2-bis(diphenylphosphino)ethane (dppe), 1,3-bis(diphenylphosphino)propane (dppp), 1,2-bis(diphenylphosphino)benzene (dppb), and bis[(2diphenylphosphino)phenyl] ether (POP). All complexes are mononuclear, apart from those based on dppm, which are dinuclear. Experimental dataalso taken from the literature and including electrochemical properties, X-ray crystal structures, UV−vis absorption spectra in CH2Cl2, luminescence spectra and lifetimes in solution, in PMMA, and as powdershave been rationalized with the support of density functional theory calculations. Temperature dependent studies (78−358 K) have been performed for selected complexes to assess thermally activated delayed fluorescence. The main findings are (i) dependence of the ground-state geometry on the crystallization conditions, with the same complex often yielding different crystal structures; (ii) simple model compounds with imposed C2v symmetry ([Cu(phen)(PX3)2]+; X = H or CH3) are capable of modeling structural parameters as a function of the P−Cu−P bite angle, which plays a key role in dictating the overall structure of [Cu(NN)(PP)]+ complexes; (iii) as the P−Cu−P angle increases, the energy of the metal-to-ligand charge transfer absorption bands linearly increases; (iv) the former correlation does not hold for emission spectra, which are red-shifted for the weaker luminophores; (v) the larger the number of intramolecular π-interactions within the complex in the ground state, the higher the luminescence quantum yield, underpinning a geometry locking effect that limits the structural flattening of the excited state. This work provides a general framework to rationalize the structure−property relationships of [Cu(NN)(PP)]+, a class of compounds of increasing relevance for electroluminescent devices, photoredox catalysis, and solar-to-fuels conversion, which so far have been investigated in an unsystematic fashion, eluding a comprehensive understanding.

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INTRODUCTION

compounds are weak emitters both in solution and in the solid state, due to extensive excited state structural rearrangements13 that can be traced by ultrafast absorption14,15 and X-ray spectroscopy.16,17 Upon light irradiation, the ground-state quasi-tetrahedral geometry undergoes flattening distortions that promote nonradiative deactivations. When phenanthroline

Copper(I) complexes equipped with a suitable ligand environment may exhibit long-lived excited states and strong luminescence, which makes them increasingly important for electroluminescent devices,1,2 photoredox catalysis,3,4 solar-tofuels conversion,5−8 and sensing.9,10 A great deal of work has been devoted to the study of homoleptic [Cu(NN) 2]+ complexes (where NN denotes diimmine ligands, typically, substituted 1,10-phenanthrolines).11,12 In most cases, these © XXXX American Chemical Society

Received: October 10, 2018

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DOI: 10.1021/acs.inorgchem.8b02879 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry ligands are equipped with multiple bulky substituents (as a matter of fact, very limited cases), strong luminescence may be detected also for [Cu(NN)2]+ compounds.18−20 Seminal work by McMillin and co-workers demonstrated that heteroleptic complexes of general formula [Cu(NN)P2]+ (where P is a phosphine ligand such PPh3) can also show luminescence, but they proved to be relatively unstable.21,22 Eventually, substantial progress was achieved when the same group showed that chelating diphosphine ligands such as POP (bis[2-(diphenylphosphino)phenyl] ether)) may afford [Cu(NN)(PP)]+ complexes with good stability and photoluminescence quantum yields (PLQYs) above 15%,23,24 i.e., at least 2 orders of magnitude higher than typical [Cu(NN)2]+ counterparts.13 This finding prompted extensive research on [Cu(NN)(PP)]+ compounds, in an effort to understand their complex behavior and establish rational criteria for the design of copper-based materials exhibiting good chemical stability, high PLQY, long-lived and strongly reductant excited states.25−39 Very recently, this work has been comprehensively reviewed.40 In a previous paper, we carried out a systematic investigation of the thirty heteroleptic Cu(I) complexes which, in principle, are obtainable from the combination of five NN and six PP ligands.41 By means of 1H and 31P NMR spectroscopy, we demonstrated that, whatever the bis-phosphine moiety, highly stable heteroleptic [Cu(NN)(PP)]+ complexes are obtained only from 2,9-unsubstituted-1,10-phenantholines.41 When substituents at the 2,9 positions of the NN ligands provide limited steric hindrancee.g., 2,9-dimethyl-1,10-phenanthroline (dmp), and 2,9-diphenethyl-1,10-phenanthroline (dpep)the related complexes are stable in the solid state, but a dynamic ligand exchange is systematically observed in solution, with the homoleptic/heteroleptic ratio highly dependent on the bis-phosphine ligand. The outcome of the ligand exchange reactions is mainly influenced by the relative thermodynamic stability of the different possible complexes.41 Passing to bulky substituents on the “sentinel” 2,9-NN positions (e.g., 2,9-diphenyl-1,10-phenanthroline, dpp) only homoleptic [Cu(NN)2]+ and [Cu(PP)2]+ complexes are obtained, whatever the bis-phosphine ligand.41 These drawbacks have been recently overcome by embedding the NN chelator in a macrocyclic ring to afford pseudorotaxanes.39 In this work, we focus on ten [Cu(NN)(PP)]+ complexes exhibiting the highest possible stability in solution, in order to ease the rationalization of the photophysical and electrochemical properties, with the essential support of theoretical calculations. As NN ligands, we have selected 1,10-phenanthroline (phen) and 4,7-diphenyl-1,10-phenanthroline (Bphen), whereas PP ligands are bis(diphenylphosphino)methane (dppm), 1,2-bis(diphenylphosphino) ethane (dppe), 1,3-bis(diphenylphosphino)propane (dppp), 1,2-bis(diphenylphosphino)benzene (dppb), and bis[(2diphenylphosphino)phenyl] ether (POP) (Chart 1). The small natural bite angle of dppm (72°) enables only the formation of binuclear complexes (i.e., each ligand connecting two copper ions), whereas the other phosphine analogues (natural bite angle in the range 85−102°) yield mononuclear complexes.40 To the best of our knowledge, the emission properties of at least six compounds out of the ten systematically investigated here have been discussed, at least in part, earlier, namely, [Cu(phen)(μ-dppm)]22+,42−44 [Cu(phen)(dppe)]+,45,46 [Cu(phen)(dppb)]+,47,48 [Cu(phen)(POP)]+,23−25,27,35,47 [Cu-

Chart 1. Ligands Used for the Synthesis of the Heteroleptic [Cu(NN)(PP)]+ Complexes Investigated in the Present Work

(Bphen)(dppb)]+,49 and [Cu(Bphen)(POP)]+.25,50−52 The purpose of this paper is not just expanding the database of photoactive Cu(I) complexes available in the literature, but also providing a more comprehensive rationale of the structural, electronic, and luminescence properties of two selected classes of [Cu(NN)(PP)]+ systems, in order to consolidate the general understanding of the electronic properties of this important class of coordination compounds.

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RESULTS AND DISCUSSION The ten heteroleptic Cu(I) complexes were prepared from the [Cu(CH3CN)4][BF4] precursor by reaction with an equimolar amount of phen or Bphen and of the appropriate bisphosphine ligand (i.e., dppm, dppe, dppp, dppb, or POP). The synthesis and the structural characterization of these compounds were described in detail in our previous paper.41 Another PP ligand that was examined in that paper was dppFc = 1,1′-bis(diphenylphosphino)ferrocene (Figure S1), which also affords highly stable compounds in combination with the phen and Bphen. However, Cu(I) complexes with ferrocene-based ligands have limited photophysical interest because long-lived and potentially luminescent MLCT levels involving the copper ion and the π* orbitals centered on the NN ligand are quenched by intramolecular energy transfer to the ferrocene unit, which in turn deactivates not radiatively to the ground state.53 We confirm here this trend by density functional theory (DFT) calculations carried out on both the ground state and the lowest triplet excited state of the representative complex [Cu(phen)(dppFc)]+ (Figure S1). The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of this compound are very similar to those of typical [Cu(NN)(PP)]+ complexes (i.e., HOMO centered on the copper(I) d orbitals and LUMO on the phenanthroline π* orbitals), but, in this particular case, also the d orbitals of the iron(II) ion strongly contribute to the HOMO. On the other hand, the lowest triplet excited state is totally centered on the dppFc ligand (see triplet spin density, Figure S1), which corroborates the absence of luminescence from [Cu(phen)(dppFc)]+. Non-photoactive dppFc complexes have not been further examined in the present work and are discussed elsewhere.53 Ground-State Structural Analysis. The analysis of the ground-state geometry of [Cu(NN)(PP)]+ complexes is somewhat challenging because, as already evidenced by Kubiček et al.,35 different crystal structures may be found in the literature for the same compound. This suggests that the B


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Inorganic Chemistry molecular structures of Cu(I) complexes are strongly influenced by several factors (e.g., counterions, solvent used for crystallization, intramolecular and intermolecular πinteractions, etc.), and many virtually isoenergetic minima can be found by slightly changing the crystallization conditions. This unconventional behavior prompted us to investigate the five [Cu(phen)(PP)] + complexes (i.e., with bare phenanthroline ligand) by DFT calculations at the M06 level of theory54,55(see the Experimental Section for details). The PBE0 functional was also tested with the same aforementioned basis set, but the optimized geometries were unsatisfactory, when compared to the experimental X-ray structures. In fact, copper(I) complexes usually adopt very flexible coordination geometries since the metal center has a d10 electronic configuration with no net ligand-field stabilization energy due to d−d orbital splitting. Therefore, steric factors, noncovalent interactions, and also crystal packing strongly dictate the structure of this class of complexes. This is the reason why the M06 functional, characterized by higher accuracy for interactions dominated by medium-range correlation energy (e.g., van der Waals attractions and aromatic π-stacking), can provide more reliable results than PBE0 in describing the ground state structure of d10 Cu(I) complexes.54,56−58 In the top part of the panels of Figure 1 is reported a comparison between different X-ray structures found in the literature for [Cu(phen)(dppb)]+ (a, a′) and [Cu(phen)(POP)]+ (b, b′), as representative examples of the investigated series.23,24,41,47 In the bottom of the two panels of Figure 1 are reported the same comparison, but adding the DFT calculated geometries to the corresponding X-ray structures indicated above. By determining the root-mean-square deviation (RMSD) of all atomic positions, it is possible to estimate the quality of the theoretically evaluated geometries calculated by the M06 or PBE0 functionals. In fact, the RMSD between the M06 calculated structures and the X-ray geometries is comparable to that between the X-ray structures of identical compounds from different crystals (∼0.5 Å for both dppb- and POP-based complexes). Larger deviations are observed when the PBE0 structures are compared to the experimental ones (RMSD ≈ 0.8 Å). All these structural discrepancies are attributable to different π-stacking interactions or to different ways of modeling them. In the case of [Cu(phen)(dppe)]+, where no intramolecular π-interactions are observed, the RMSD between the X-ray, the PBE0-, and M06-calculated ones is only 0.53 Å in all the cases (Figure S2). Herein, the M06 functional is adopted to theoretically investigate both the structural and electronic properties of [Cu(phen)(PP)]+ compounds. We limit such investigations only to this series because, for the [Cu(Bphen)(PP)]+ family, intermolecular π-interactions of the phenyl groups in the 4,7positions of the phenanthroline ligand are so extensive that calculations on a single Cu(I)-complex unit could not adequately model the actual properties of such compounds, particularly in the solid state. The chelating PP ligands investigated in this work are characterized by a relatively wide range of P−Cu−P bite angles, from 87° to up to 135°, observed for the dinuclear complexes with the dppm ligand. In an effort to assess the effect of this parameter on the bond lengths and angles involving the metal center (and hence on the overall structure), we have theoretically investigated the two simplest possible [Cu(NN)(PP)]+ analogues of the investigated

Figure 1. Latin letters: superimposition of different X-ray crystal structures of [Cu(phen)(dppb)]+ (a, a′) and [Cu(phen)(POP)]+ (b, b′). Greek letters: comparison between experimental and theoretically calculated structures. All structures are superimposed by minimizing the root-mean-square deviation (RMSD) of all atomic positions (a, α and b, β) or by maximizing the superposition of the phenanthroline ligands alone (a′, α′ and b′, β′), which enables a more refined comparison between the molecular structures. All RMSD calculations, optimizations, and visualizations were performed by VMD 1.9.1.59 Hydrogen atoms are excluded from the statistics and omitted in the figure.

complexes, namely, [Cu(phen)(PX3)2]+, where X = H or CH3. The data reported in Figure S3 are obtained by gradually changing the PP bite angle of the [Cu(phen)(PX3)2]+ model complexes and by performing a geometry optimization of the C


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Inorganic Chemistry

the bite angles of the phenanthroline and bis-phosphine ligands are reproduced with great accuracy by the DFT optimization (Table S2). Also the interplanar angle between the phenanthroline and the plane defined by the P−Cu−P coordinates is in excellent agreement with the experimental one, since it is always within the interval defined by different conformers (with the only exception of [Cu(phen)(dppe)]+ (Figure S2) for which only one X-ray structure is available). All these findings further confirm the effectiveness of the adopted theoretical model. Ground-State Electronic Properties. With the help of the same model complexes mentioned above, we also tried to rationalize the ground-state electronic properties of the investigated [Cu(phen)(PP)]+ series. In Figure 3 the energy

other structural parameters (keeping the P−Cu−P bite angle frozen and imposing a C2v point group). Such a simple model reproduces well the trends of the corresponding structural parameters experimentally measured by X-ray diffraction analysis on a collection of around twenty [Cu(NN)(PP)]+ complexes available in the literature (Figure 2 and Table S1). It should be emphasized that the unrealistic

Figure 2. Key structural parameters of the model compounds [Cu(phen)(PH3)2]+ in C2v symmetry (full lines); data refer to a relaxed scan of the P−Cu−P angle at the M06 level of theory, in vacuum. Experimental data are taken from an X-ray structure reported in Table S1 (circles with relative errors).

behavior theoretically predicted for the Cu−P bond length at low P−Cu−P angles in the model compounds (i.e., a spurious elongation of the Cu−P distance) is basically due to the steric hindrance between the phosphine ligands that cannot be mitigated due to the overall C2v-symmetry constraint of the complexes (Figures 2 and S3). On the contrary, in real complexes, different substituents on the phosphorus atoms and additional degrees of freedom of the ligands (e.g., rocking, wagging) allow an almost constant Cu−P length of (2.25 ± 0.02) Å to be preserved, regardless of the PP bite angle, as it also happens for the model compounds, but only at larger P− Cu−P angles (Figures 2 and S3 and Table S1). On the contrary, the model compounds very well reproduce the trend of the Cu−N bonds as a function of the P−Cu−P bite angles. In fact, when the P−Cu−P angle increases, the Cu−N bond length also increases (Table S1 and Figure 2). Moreover, almost a perfect match is observed when comparing the experimental phenanthroline bite angles and theoretically predicted one of the model complexes (Figure 2). Notably, such N−Cu−N angle can be expressed as a function of the above-mentioned Cu−N bond length, assuming that the phenanthroline ligand is rigid (Figure S4); consequently, as the P−Cu−P angle increases, a reduction in the phen bite angle is observed. Taken in concert, the above findings unambiguously show the central role played by the P−Cu−P bite angle in dictating the overall structure of [Cu(NN)(PP)]+ complexes. DFT methods were also used to calculate the minimumenergy geometries of all the complexes belonging to the [Cu(phen)(PP)]+ series; the structures were fully optimized in vacuum without any symmetry constraint. A direct comparison between experimentally available key structural parameters for these complexes (data obtained from Table S1) and those theoretically computed at the M06 level of theory is reported in Table S2. A very good agreement is observed as far as the Cu−N and Cu−P bond lengths are concerned, and even

Figure 3. Energy diagram showing the frontier Kohn−Sham molecular orbitals of the model compound [Cu(phen)(PMe3)2]+ in a vacuum (C2v symmetry). Data refer to relaxed scans of the P−Cu−P angle. For each type of orbital reported in the graph, the associated symmetry and the corresponding isosurface is also displayed (isovalue = 0.04 e1/2 bohr−3/2).

diagram of the frontier molecular orbitals of the [Cu(phen)(PMe3)2]+ model complex is reported, again as a function of the P−Cu−P angle. At a low P−Cu−P angle (i.e., < 135°,) the HOMO is a b1 orbital strongly characterized by a σ* bond between the d(xz) orbital of the copper center and the lonepair on the phosphine ligands. Such a molecular orbital is more and more stabilized as the P−Cu−P angle increases. As a consequence, when P−Cu−P angles are larger than 135°, an orbital flipping occurs and the HOMO becomes an a1 orbital, which on the contrary is destabilized by an increase in the P− Cu−P angle. Such an orbital is essentially characterized by a nonbonding interaction between the copper d(x2−y2) orbital and the lone-pair orbitals on the phosphine ligands (Figure 3). On the contrary, the variation of the P−Cu−P angle has virtually no effects on the nature and energy of the LUMO and LUMO+1, since they are both localized on the π* orbitals of the phenanthroline ligand. As a consequence, mainly due to HOMO stabilization, the HOMO−LUMO energy gap is expected to increase by increasing the P−Cu−P angle up to an angle of 135−140°, when it reaches its a maximum value (i.e., ∼4.32 eV, see Figure 3). Then, due to orbital flipping, the HOMO−LUMO energy gap inverts its trend and slightly decreases, if the P−Cu−P angle is further increased beyond 140° (Figure 3). D


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Inorganic Chemistry

tion already at a scan rate of 0.1 V s−1. Such a one-electron process is only lightly affected by the type of diamine ligand (phen or Bphen). The highest potentials are observed for POP and dppm compounds, indicating stronger structural rigidity. The first reduction event is centered on the phen/Bphen ligands, and it is generally irreversible at a low scan rate. However, when the scan rate increases, especially in the case of phen compounds, it can become quasi-reversible. The extension of the electronic conjugation in the Bphen is reflected in a decrease of the reduction potential (40−70 mV with respect to the phen analogues, Table 1). Oxidation and reduction potentials show a good correlation with the energy of the frontier molecular orbitals, with the HOMO−LUMO energy gap being systematically higher than the ΔERedox of 1.2 ± 0.1 eV for all the reported complexes (Table S3). Only in the case of the binuclear compound [Cu(phen)(μ-dppm)]22+ an overstabilization of the HOMO is observed, if compared to electrochemical data; however, the overall trend is respected. Absorption Spectra and Vertical Excitations. The absorption spectra of [Cu(phen)(PP)]+ and [Cu(Bphen)(PP)]+ series at 298 K are depicted in Figure 4. Some of these compounds have been already reported in the literature (see Introduction), and our results essentially confirm the previous experimental findings. Upon dissolution in air-equilibrated CH 2 Cl 2 , spectral shapes and intensities do not vary significantly after over 10 days in the dark at 298 K, confirming the expected stability in the absence of 2,9 substituents on the

The scenario depicted by the simple model described above is anyhow extremely effective in explaining what is observed in the real case for the [Cu(phen)(PP)]+ series. In fact, as shown in Figure S5, the same findings already discussed for the model [Cu(phen)(PMe3)2]+ series are also qualitatively observed in real complexes. Actually, in the case of the [Cu(phen)(PP)]+ series, there is no complex displaying a PP bite angle well beyond the critical value of 135°. Notably, however, the dinuclear complex [Cu(phen)(μ-dppm)]22+ (with an experimental PP bite angle of 134° and a theoretically predicted one of 138°, Table S2) already undergoes the orbital flipping described in Figure 3. In fact, while all the other complexes of the series exhibit a b1 -type HOMO, in the case of [Cu(phen)(μ-dppm)]22+ such an orbital is the HOMO−1 and the HOMO has the same topology of the a1 orbital of the [Cu(phen)(PMe3)2]+ model compound (compare Figures 3 and S5). This orbital flipping explains the different absorption profile of [Cu(phen)(μ-dppm)]22+, with respect to the other complexes of the series (see below). In line with our model, also HOMO stabilization is observed upon increasing the PP bite angles within the phen series, as predicted for P−Cu−P angles within the range 80−140° (compare again Figures 3 and S5). In fact, the [Cu(phen)(PP)]+ complexes having smaller PP bite angles (i.e., those with PP = dppb, dppe, and dppp) display higher lying HOMOs, while such an orbital is more stabilized in [Cu(phen)(POP)] + and even more in [Cu(phen)(μdppm)]22+ (PP bite angles of 116° and 138°, respectively). As a result, as the PP bite angle of the [Cu(phen)(PP)]+ series increases, an increase in the HOMO−LUMO energy gap is also observed, according to the predictions and in very good agreement with electrochemical data (see below). Electrochemistry. The electrochemical properties of the ten Cu(I) complexes were investigated in CH2Cl2 at 298 K by both cyclic voltammetry (CV) and Osteryoung square wave voltammetry (OSWV). Experimental data are reported in Table 1 and Figure S6. All the complexes exhibit an oxidation process centered on the Cu(I) metal ion, which is always irreversible, except in complexes displaying small P−Cu−P angles (i.e., [Cu(phen)(dppb)]+, [Cu(phen)(dppp)]+, and [Cu(Bphen)(dppb)]+), which show a quasi-reversible oxidaTable 1. Selected Electrochemical Data of the Ten [Cu(NN)(PP)]+ Complexes in CH2Cl2 electrochemical dataa [V] [Cu(phen)(PP)]+ series

[Cu(Bphen)(PP)]+ series

type of PP ligand

Eox

Ered

ΔEredox

Eox

Ered

ΔEredox

dppb dppe dppp POP μ-dppm

+1.05b +1.04c +1.07b +1.29 +1.44

−1.50 −1.53 −1.55 −1.58d −1.35

2.55 2.57 2.62 2.87 2.79

+1.03b +1.03c +1.06 +1.28 +1.44f

−1.46 −1.46 −1.49 −1.53 −1.31

2.49 2.49 2.55 2.81 2.75

a

Data refer to OSWV or CV experiments; ferrocene is used as internal reference (Fc+/Fc is observed at 0.55 ± 0.01 V vs SCE). EOx = first oxidation potential, ERed = first reduction potential in V vs SCE; ΔERedox = Eox − Ered. bQuasi-reversible process at 0.1 V s−1. cShoulder at +0.93 V and peak at +1.44 V appear and increase during the experiment. dQuasi-reversible process at 1 V s−1. fMinor peak at +1.01 V attributed to the formation of some traces of [Cu(Bphen)]+, as already observed in similar cases (see e.g. ref 39).

Figure 4. Absorption spectra of the ten [Cu(NN)(PP)]+ complexes in CH2Cl2 solution at 298 K (top: phen series. bottom: Bphen series). For the sake of comparison, the molar absorption coefficients of the dinuclear complexes with PP = dppm is divided by two. E


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Inorganic Chemistry phenanthroline ligands (Figures S7 and S8).41 In the UV region, ligand-centered (LC) features of both the NN and PP ligands are predominant, while the bands observed in the visible spectral domain down to about 500 nm are attributed to metal-to-ligand-charge-transfer (MLCT) transitions involving the metal cation and the phenanthroline moieties.23,24,39,40 Interestingly, molar absorption coefficients of MLCT bands (λ > 350 nm) are generally higher for the Bphen complexes compared to the phen analogues. This enhancement is the highest for the dinuclear complex equipped with the dppm ligands. In both series, the spectra of the POP complexes are remarkably blue-shifted compared to the three mononuclear analogues, confirming a trend observed earlier for [Cu(NN)(PP)]+ systems, which typically show blue shift as a function of the bulkiness of the ligands.40 It must be also emphasized that the compounds with blue-shifted features (POP and dppm ligands) have the highest oxidation potentials, indicating a more rigid structure that disfavors flattening upon Cu(II) formation in the excited state.13 Notably, the energy of the MLCT bands well correlates with the bite angle of the bisphosphine ligands: as the PP bite angle increases, the energy of the MLCT absorption maximum increases (Figures S9 and S10). Time-dependent-DFT (TD-DFT) calculations, carried out on the fully optimized ground-state geometries of these complexes, further confirm the picture described above. In fact, a linear correlation is also observed by plotting the energy associated with the first singlet vertical excitation and the PP bite angle calculated by ground-state optimization (Figure S9, black dots and line). Such results are in line with the predicted increase in the HOMO−LUMO gap upon increasing the PP bite angle within this series of complexes (Table 1); a phenomenon that is mainly due to HOMO stabilization (Figure S5). With the aim of more deeply rationalizing the ground-state absorption properties of these complexes and to further confirm the correlation observed above (Figure S9), we theoretically investigated the model complex [Cu(Phen)(PMe3)2]+ by TD-DFT calculations, to evaluate the lowestlying singlet electronic transitions. The results are reported in Figure 5 for the lowest six excited states, which are named according to their symmetry and represented in terms of monoelectronic excitations, as in Figure 3. In the [Cu(phen)(PMe3)2]+ model compound, at low P− Cu−P angles (i.e., < 130°), the lowest energy transition corresponds to a b1 → b1 monoexcitation, leading to the population of an A1 excited state. Such a state is MLCT in nature and is mainly responsible of the unstructured absorption band experimentally observed in the [Cu(phen)(PP)]+ series at around 350−500 nm (Figure 4, top). Following the theoretical evidence proposed by the [Cu(phen)(PMe3)2]+ model, it should be emphasized that (i) the energy of the excitation to the lowest A1 excited state increases linearly with the P−Cu−P angle (see linear fitting in Figure 5); and (ii) the oscillator strength associated with such transition becomes weaker and weaker by increasing the P− Cu−P angle (Figure 5). Both these findings are in line with experimental data of the [Cu(phen)(PP)]+ series. In fact, a decrease in both the wavelength maximum and in the molar absorptivity of the lowest MLCT band is observed upon increasing the PP bite angle along the series (Figure 4, top− inset).

Figure 5. Lowest vertical excitations calculated for the model compound [Cu(phen)(PMe3)2]+ in vacuum (C2v symmetry), at different P−Cu−P angles. For each vertical excitation, the corresponding couple of orbitals involved in the transition is reported in the legend (top, according to the nomenclature used in Figure 3). Oscillator strength values are reported as small numbers inside the graph; transition to A2 excited states are symmetry forbidden (dashed). The most intense transition is to the lowest A1 excited state, and its transition energy increases linearly with an increase in the P−Cu−P angles (see linear fitting, red line).

Another important point that is well described by the relatively simple model herein adopted regards the excitedstate scenario at high P−Cu−P angles (i.e., >130°). At such large P−Cu−P angles, the transition to the A1 excited state is so destabilized that the excitation to B1 (a1 → b1 excitation) becomes the lowest in energy (Figure 5). This theoretical finding is able to justify the presence of a minor absorption tail at longer wavelength (λ > 400 nm) next to the main MLCT band (λmax ≈ 345 nm) of the experimental spectrum of the binuclear complex [Cu(phen)(μ-dppm)]22+ (Figure 4, top− inset). Luminescence Properties. The ten [Cu(phen)(PP)]+ and Cu(Bphen)(PP)]+ complexes were investigated as powders, in CH2Cl2 (298 and 77 K) and PMMA matrix (sample 1 wt %). The emission properties of some of these compounds have been already discussed, and our results confirm previous experimental findings,23−25,27,35,47,49−52 (excluding those which misinterpreted the second harmonic of the excitation light as the emission of the complex).42,49 All key data are gathered in Table S4 (solution and 77 K) and in Table 2 (solid state). In CH2Cl2 solution, (faint) luminescence is observable only for the dinuclear complexes with dppm and the bulky POP chelator (Figure S11). Photoluminescence quantum yields (PLQYs) are on the order of 10−3−10−4. For all the other compounds, no luminescence is detected in room temperature solution. This result is in line with what was reported elsewhere for some of these compounds and is typical for homoleptic and heteroleptic Cu(I) compounds lacking substituents in the 2,9 phenanthroline positions.13,23 NN ligands of this type are the least effective in preventing the photoinduced distortion of these Cu(I) tetrahedral complexes to square planar excitedstate geometries, which favor nonradiative deactivations.12 This phenomenon is extensively described in the literature and is virtually prevented in a rigid matrix at 77 K, where all of the compounds exhibit emission bands with maxima in the range F


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Inorganic Chemistry

Figure 6. Normalized emission spectra of [Cu(phen)(PP)]+ and [Cu(Bphen)(PP)]+ as powders at 298 K in quartz tubes; λexc = 380 nm.

Figure 7. Emission data for the [Cu(phen)(PP)]+ series in PMMA matrix at 298 K. No correlation can be observed between these emission parameters and the bis-phosphine bite angle. On the contrary, the emission maxima (black dots) and the PLQYs (red dots) appear to follow a similar trend and to correlate well.

of 560−650 nm and associated lifetimes of approximately 140−200 μs (Table S4 and Figure S12). Such bands are unstructured, which indicates a MLCT character; the observed blue-shift compared to 298 K (dppm and POP ligands) shows a limited geometric relaxation of the excited state.39 Bphen compounds tend to be red-shifted and longer-lived compared to phen analogues. A detailed temperature dependent study is discussed below. Luminescence spectra were recorded in PMMA matrix for all of the compounds (Figure S13). The PLQYs are below 5% (and as low as 0.2%) for the dppe, dppb, and dppp based Cu(I) complexes, regardless of the NN chelator; the dppm and POP systems show stronger emission, with PLQY in the range of 11−15%. PLQYs are typically stronger in powders, i.e., in the range of 35−45% for the dppm (dinuclear) and POP systems (Table 2 and Figure 7). We wish to emphasize that the emission intensity and the band position of powder samples can be substantially affected by the way the sample is selected or treated (e.g., grinding),31 as shown in Figures S14 and S15; therefore these measurements require particular care when comparisons among different compounds are made and rationalizations are attempted. This observation underpins the relevance of intermolecular interactions in the solid state, also affecting the excited state properties. It is also in line with the occurrence of non-negligible differences found in the X-ray

crystal structures of the same compounds, which was already highlighted above. The PLQY values along the two series appear to be scattered. The diagram in Figure 7 shows no linear correlations of emission energy (and PLQY) with the PP bite angle, differently from that observed for the absorption data (Figure S9, data from the [Cu(phen)(PP)]+ series in PMMA 1% matrix, Table 2). However, there is a consistent trend between emission energies and PLQYs: red-shifting of the emission bands implies lower quantum yields. This phenomenon can hardly be attributed only to the energy gap law,61−63 because variations of PLQY are very large (0.002−0.453, data from Table 2 for both phen and Bphen series), while spectral shifts are rather limited (530−620 nm, i.e., 2.34−2.00 eV). On the contrary, it can be supposed that excited-state relaxation processes are mainly responsible for these trends.12,35 It is well-known that Cu(I) complexes, upon excitation to their lowest-energy triplet state of MLCT nature, undergoes flattening distortion.12,64 The degree of such excited-state geometry relaxation strongly depends on the bulkiness and/or rigidity of both the ligands and the overall complex.35 In order to assess the nature and the geometry of the excited state, all the complexes belonging to the [Cu(phen)(PP)]+ series have been fully optimized in their lowest triplet state (T1) by a spin-unrestricted DFT approach. It should be

Table 2. Photophysical Data of the Complexes in 1 wt % PMMA and as Pure Powders at 298 K PMMA (1 wt %) λem [nm]

complex +

[Cu(Phen)(dppb)] [Cu(Phen)(dppe)]+ [Cu(Phen)(dppp)]+ [Cu(Phen)(POP)]+ [Cu(Phen)(μ-dppm)]22+ [Cu(Bphen)(dppb)]+ [Cu(Bphen)(dppe)]+ [Cu(Bphen)(dppp)]+ [Cu(Bphen)(POP)]+ [Cu(Bphen)(μ-dppm)]22+

588 612 598 565 550 598 618 603 577 560

powders τ [μs]

a

PLQY [%]

b

2.4 0.2 1.4 11.3 15.4 4.5 0.5 2.6 15.0 14.9

6.77 2.87b 7.75b 14.08 17.88 9.66 6.27b 13.34b 17.72 26.21

λem [nm]

PLQYa [%]

τ [μs]

583 600 590 566 533 600 598 600 581 575

24.1 5.1 8.1 36.6 45.3 20.8 1.2 21.5 35.3 4.0

9.95 4.30 7.27 12.75 33.76 8.65 0.52 14.38 11.72 6.58c

a Determined using an integrating sphere,60 λexc = 380 nm. bLuminescence lifetime reported as average of biexponential decay, λexc = 370 nm. cBest fit with biexponential equation results in τ1 = 2.76 μs (32%) and τ2 = 9.12 μs.

G


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or bulkiness of the bis-phosphine ligand itself;35 and (ii) to phenomena that are able to “lock” the complex geometry, i.e., inter- and intramolecular π-stacking interactions. By comparing the X-ray structures of the [Cu(phen)(PP)]+ series and their PLQYs in PMMA matrix (i.e., a solid-state sample in which the emitters are very diluted in the inert polymeric matrix, so that any intermolecular interaction can be reasonably neglected), we discovered a nice, yet simple, qualitative correlation: the higher the number of intermolecular π-interactions, the larger the PLQY (Table 3). These interactions exert a “locking” effect on a given complex, which is crucial to maximize the luminescence output because excited state distortions are minimized. Such a qualitative approach is able to qualitatively estimate the PLQY of a specific copper(I) complex, based solely on its ground-state structural parameters. Even though it is widely assumed that flattening is the main, if not the sole, mechanism responsible for the excited-state geometry relaxation in copper(I) complexes,12 it should be stressed that these molecules can undergo other types of excited-state distortion in order to relax their lowest 3MLCT state (T1). One of these modes is rocking. In the present study, we found “rocked” minima on the T1 potential energy surfaces of both [Cu(phen)(dppe)]+ and [Cu(phen)(dppp)]+. Such complexes are equipped with bis-phosphine ligands which are particularly flexible due to their aliphatic bridges. As a consequence, upon excitation, these complexes can undergo the formation of energetically favorable interligand πinteractions between the phenanthroline ligand and one of the phenyl substituents on the bis-phosphine, that can only be achieved by rocking distortions in the T1 excited state (Figure 8).

strongly emphasized that, as it occurs for the ground state, also the potential energy surface of T1 presents several minima, which are nearly isoenergetic even though they can display rather different structures. As an example, in Figure S16 are depicted some selected T1 minimum conformations adopted by [Cu(phen)(dppp)]+, which are all nearly isoenergetic (i.e., ΔE < 0.005 eV) despite their rather different geometries. The most stable T1 conformers generally adopt the most flattened geometry, which tends to assume a pseudo square-planar coordination around the metal center (e.g., in the case of [Cu(phen)(dppe)]+, an almost ideal square-planar complex is obtained, Figure S17). Key structural parameters of the most stable T1 flattened conformers are reported in Table S5. Compared to the groundstate, a Cu−N bond shortening is observed in the triplet excited state of all [Cu(phen)(PP)]+ complexes, as expected for an MLCT state characterized by a CuI → phen transition.35 As a consequence, the N−Cu−N angles vary from the typical (80 ± 1)° of the ground state to an average value of (84 ± 1)° in the lowest 3MLCT state. As far as the bis-phosphine ligand is concerned, upon T1 relaxation, an elongation of the Cu−P distance is observed for all the complexes, and a correlated decrease of the PP bite angle is registered with respect to S0. On the contrary, the degree of flattening distortions shows a much larger variability, which depends not only from one complex to another (i.e., depending on the type of bisphosphine used), but also on the different conformations that an identical complex can adopt on the T1 potential energy surface. In order to quantitatively evaluate the excited-state flattening distortions, we calculated the difference between the S0 and T1 dihedral angles defined by the phenanthroline and the P−Cu−P planes, respectively (Table S5). Such variation is very pronounced for the less bulky bis-phosphine ligands (i.e., dppe, dppp, and dppb), while it is reduced for the larger POP ligand, and almost negligible for the very rigid binuclear complex [Cu(phen)(μ-dppm)]22+ (Table S5). These structural theoretical data are compared to the photophysical experimental ones of the [Cu(phen)(PP)]+ series; the comparison is not extended to the Bphen series because intermolecular interactions involving the outbound phenyl groups could strongly bias comparisons with DFT data. The key results are reported in Table 3, where a very interesting trend can be found: the lower the excited-state flattening, the higher the PLQY. Since flattening distortions in the excited state are mainly due to the structural flexibility of the overall complex, it can be safely assumed that PLQYs are related (i) to the rigidity and/

Figure 8. Superimposition of the rocked (red) and flattened (blue) T1 minimum conformers for the [Cu(phen)(dppp)]+ complex, calculated at the M06 level of theory in vacuum. The structures are superimposed by minimizing the root-mean-square deviation of all the atoms of the phenanthroline ligand; hydrogen atoms are excluded from the statistics and omitted in the figure. The phenyl moiety responsible for the interligand π-interaction leading to rocking is highlighted in yellow.

Table 3. Excited-State Properties and Key Structural Parameters for the [Cu(phen)(PP)]+ Series

sample

flattening distortion between S0 and T1a [°]

PLQY in PMMA at 298 K [%]

number of intramolecular πinteractions in the ground state

[Cu(phen)(dppb)]+ [Cu(phen)(dppe)]+ [Cu(phen)(dppp)]+ [Cu(phen)(POP)]+ [Cu(phen)(μ-dppm)]22+

51.4 69.6 46.8 27.2 −4.1

2.4 0.2 1.4 11.3 15.4

1 0 0 2 4

Notably, in the case of [Cu(phen)(dppp)]+, both the rocked and the flattened T1 conformers are virtually isoenergetic (i.e., ΔE ≈ 0.001 eV), so that rocking seems to be as effective as the well-known flattening mode to stabilize the energy of the T1 state (Figure 8). On the contrary, in the case of the bis-phosphine ligands equipped with a more rigid bridge, the identification of rocked

a

This angle is determined as the difference between the S0 and T1 dihedral angles defined by the phenanthroline and the P−Cu−P planes. H


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Table 4. Energy Splitting ΔE(S1 − T1) between the Lowest Singlet and Triplet Excited States and Related Lifetimes in the Solid Statea ΔE(S1 − T1) [eV]

τ (S1) [ns]

τ (T1) [μs]

0.082 ± 0.004 0.094 ± 0.003 [0.115 ± 0.004] 0.113 ± 0.004 [0.114 ± 0.004]

158 ± 35 164 ± 27 [111 ± 21] 194 ± 36 [139 ± 29]

94 ± 2 139 ± 1 [179 ± 2] 130 ± 1 [123 ± 1]

sample +

[Cu(phen)(dppe)] [Cu(phen)(POP)]+ [Cu(phen)(μ-dppm)]22+

b

b

b

χ2 b

3.557 4.729 [2.085] 1.351 [2.149]

a

Data are calculated according to eq 1 and given at the 95% confidence interval. Experiments were carried out on solid-state samples as grinded powders (plain numbers) or dissolved in PMMA matrix (data in square brackets). bNot available data due to biexponential decays.

copper(I) complexes based on chelating bisimine and bisphosphine ligands.34,39,65 In the case of the ground-powder samples, all three complexes display monoexponential decays at all temperatures, confirming a fast equilibration between the emitting states of singlet and triplet spin multiplicity. However, it should be emphasized that the simple two-state approach (i.e., S1 and T1) adopted by eq 1 to model TADF phenomenon is probably oversimplified to take into account potentially important intermolecular interactions between the complexes in the powders; accordingly, in some cases, the χ2 of the fitting reaches values well above 2 (Table 4). This is particularly evident in the case of [Cu(phen)(dppe)]+ and [Cu(phen)(POP)]+ complexes, which display strong intermolecular interactions, as corroborated by their crystal packing obtained by X-ray diffraction.24,25,35,45,47 On the other hand, for the more rigid and “caged” binuclear complex [Cu(phen)(μdppm)]22+, eq 1 nicely fits the experimental results (χ2 ≈ 1, Table 4), proving that, in this case, a simple two-state model is perfectly suitable to take into account TADF. On the contrary, the use of PMMA matrix at 1 wt % doping concentration affords more uniform, transparent, and reproducible samples, where emission properties do not strongly depend on minimal variation on sample preparation as it happens for powders (see above). Moreover, since the complexes are very diluted in the inert polymeric matrix, intermolecular interactions can be safely ruled out. As a consequence, the adopted model is overall more reliable in fitting the experimental data (i.e., χ2 ≈ 2 in all investigated samples, see Table 4). Anyway, no dramatic differences are observed compared to powder data, especially in the case of the more rigid and “caged” [Cu(phen)(μ-dppm)]22+ dinuclear complex.

minima was not successful; all the attempted optimizations, starting from “rocked” guess, ended up into the well-known flattened T1 minima. Temperature-Dependent Luminescence Studies. Three complexes have been studied in detail regarding the temperature dependence of their luminescence and of their excited state lifetimes in the interval 78−358 K (both as ground powders and in diluted PMMA matrix). We have turned our attention to the phen series, in which intermolecular interactions are minimized. Our selection includes [Cu(phen)(μ-dppm)]22+ (binuclear complex and strong emitter) as well as [Cu(phen)(POP)]+ and [Cu(phen)(dppe)]+, as the strongest and weakest emitters of the mononuclear compounds. In Figures S18−S20 are collected the emission spectra of the three selected complexes (as ground powders) at different temperatures. For all the samples, a blue shift of the emission bands is detected when increasing the temperature, as typically found for emitters displaying thermally activated delayed fluorescence (TADF).34,65 The same phenomenon is also observed when these complexes are dissolved in a PMMA matrix (Figures S21−S23), although in some cases the sample preparation leads to a partial dissociation of the complexes and to a minor phosphorescence emission from the phenanthroline free-ligand, visible at a lower wavelength. To confirm the TADF hypothesis, we also investigated the variation in the excited-state lifetimes of the three complexes as a function of temperature. These data are gathered in Figures S24−S26 for the ground powders and in Figures S27 and S28 for the samples dissolved in PMMA matrix. The marked shortening of the excited-state lifetimes at higher temperatures (observed in both media) underpins the shifting from a pure triplet emission to TADF.39 On the basis of these observations, we have fitted the excited-state lifetimes as a function of temperature using eq 1:66 τexp =

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CONCLUSIONS By selecting two phenanthroline ligands without substituents in the 2,9 positions (phen, Bphen), we have systematically investigated the electronic and structural properties of ten highly stable and luminescent [Cu(NN)(PP)]+ complexes, where PP indicates five different bis-phosphine ligands. The elaboration of a large number of photophysical, electrochemical, and structural data, with the crucial support of DFT modeling, highlighted clear interconnections among the key properties. Theoretical tools and models to investigate [Cu(NN)(PP)]+ systems were successfully tested, demonstrating the central role played by the P−Cu−P bite angle and intramolecular π-stacking interactions in dictating the light absorption and emission features. These results provide a deeper insight on the correlation between structural and electronic properties in [Cu(NN)(PP)]+ complexes, opening the route to the optimization-by-design of a class of compounds of increasing relevance in the areas of photochemistry, catalysis and materials science.

3 + e−ΔE(S1−T1)/ kBT 3 τ(T1)

+

1 −ΔE(S − T )/ kBT 1 1 e τ(S1)

(1)

where τexp is the measured excited-state lifetime (see Experimental Section for details), ΔE(S1 − T1) is the energy gap between the lowest singlet and triplet excited states of the [Cu(phen)(PP)]+ complexes, τ(S1) and τ(T1) are the related excited-state lifetimes of singlet and triplet respectively, T is the temperature, and kB is the Boltzmann constant. The results of the fitting procedure are summarized in Table 4, while the quality of the fitting itself can be visually assessed from Figures S24−S28. The energy gap between the lowest singlet and triplet excited states (i.e., ΔE(S1 − T1) in Table 4) is around (0.10 ± 0.02) eV for all the complexes, regardless the type of solid-state sample under investigation. These results are in line with those reported in the literature for similar heteroleptic cationic I


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the multichannel scaling single-photon counting (MCS-SPC) technique with the use of the same luminescence spectrometer described above and equipped with a 60 W xenon flash lamp (Edinburgh μF920H) as the excitation source (100 Hz; 2-μs pulse width) and the aforementioned PMT as detector (minimum response 600 ps). The analysis of the luminescence decay profiles was accomplished with a homemade program based on the IgorPro software (WaveMetrics, Inc.), performing the numerical reconvolution of the instrumental response function. The quality of the fit was assessed with the same criteria previously described. The decays were fitted as monoexponential. Experimental uncertainties are estimated to be ±8% for τ determinations, ± 20% for PLQY, and ±2 nm and ±5 nm for absorption and emission peaks, respectively. Computational Details. Density functional calculations were carried out with the D.01 revision of the Gaussian 09 program package.69 Both the PBE0 hybrid functional70,71 and the M06 hybrid meta exchange-correlation functional54,55 were used to carry out the calculations. The ECP10MDF fully relativistic Stuttgart/Cologne energy-consistent pseudopotential with multielectron fit was used to replace the first 10 inner-core electrons of the copper metal center,72 and it was combined with the associated cc-pVTZ-PP triple-ζ basis set.73 On the other hand, Pople basis sets were adopted for nonmetal atoms: the 6-31G(d) basis set was used for C, H, N, O atoms,74,75 while the more flexible 6-311G(2d) basis for P.76 The geometry of all complexes was optimized in a vacuum (if not differently stated), starting from the X-ray molecular structure as an initial guess. The same level of theory was used to perform the calculation of Kohn− Sham energy levels. A frequency job always confirmed that the stationary point found by the geometry optimization was actually corresponding to a minimum on the potential energy surface (no imaginary frequencies). TD-DFT calculations,77−79 at the same level of theory used for geometrical optimization, were employed to calculate the first 24 singlet vertical excitations and the lowest 9 triplet vertical excitations were calculated within the linear-response formalism. For these calculations, the polarizable continuum model (PCM)80−82 was employed to take into account solvation effects. For having an estimation of both the geometry distortions and the energy of the first excited state, the lowest triplet excited state (T1) was optimized at the spin-unrestricted level of theory with a spin multiplicity of 3, using the above-mentioned density functionals. All the pictures showing molecular orbitals and spin-density surfaces were created using GaussView 5.83

EXPERIMENTAL SECTION

Electrochemistry. The electrochemical properties of all compounds were determined by cyclic voltammetry (CV) and Osteryoung square wave voltammetry (OSWV; see experimental selected curves in the Supporting Information). The cyclic voltammetric measurements were carried out with a potentiostat Autolab PGSTAT100. Experiments were performed at room temperature in a homemade, airtight three-electrode cell connected to a vacuum/argon line. The reference electrode consisted of a saturated calomel electrode (SCE) separated from the solution by a bridge compartment. The counter electrode was a platinum wire of about 1 cm2 apparent surface. The working electrode was a Pt microdisk (0.5 mm diameter). The supporting electrolyte [nBu4N][BF4] (Fluka, 99% electrochemical grade) was used as received and simply degassed under argon. Dichloromethane was freshly distilled over CaH2 prior to use. The solutions used during the electrochemical studies were typically 10−3 M in compound and 0.1 M in supporting electrolyte. Before each measurement, the solutions were degassed by bubbling Ar, and the working electrode was polished with a polishing machine (Presi P230). Under these experimental conditions, Fc+/Fc is observed at +0.55 ± 0.01 V vs SCE. Under standard conditions, OSWV parameters were obtained using an amplitude of 20 mV, a frequency of 20 Hz, and a step potential of 5 mV. Photophysical Measurements. Spectrofluorimetric-grade dichloromethane was used as solvent for spectroscopic investigations. The absorption spectra were recorded with a PerkinElmer Lambda 950 spectrophotometer. For the photoluminescence experiments, the samples were placed in fluorimetric Suprasil quartz cuvettes (1 cm) and deaerated by bubbling argon for at least 20 min. The uncorrected emission spectra were obtained with an Edinburgh Instruments FLS920 spectrometer equipped with a Peltier-cooled Hamamatsu R928 photomultiplier tube (PMT), having a spectral window of 185− 850 nm. An Osram XBO Xenon arc lamp (450 W) was used as the excitation light source. The corrected spectra were obtained via a calibration curve, determined using an Ocean Optics deuteriumhalogen calibrated lamp (DH-3plus-CAL- EXT). The luminescence quantum yields (PLQY) in solution were obtained from the corrected spectra on a wavelength scale (nm) and measured according to the approach described by Demas and Crosby,67 using an air-equilibrated water solution of [Ru(bpy)3]Cl2 (PLQY = 0.028)68 as reference. The emission lifetimes (τ) in the sub-microsecond time range were measured through the time-correlated single-photon counting (TCSPC) technique using a HORIBA Jobin Yvon IBH FluoroHub controlling a spectrometer equipped with a pulsed NanoLED (λexc = 373 nm; 200 ps time resolution after reconvolution) as the excitation source and a red-sensitive Hamamatsu R-3237−01 PMT as the detector (spectral window: 185−850 nm). The analysis of the luminescence decay profiles, taken on the emission maximum, was accomplished with the DAS6 decay analysis software provided by the manufacturer, and the quality of the fit was assessed with the χ2 value close to unity and with the residuals randomly scattered along the time axis. To record the 77 K luminescence spectra, sample solutions were put in quartz tubes (2 mm inner diameter) and inserted into a special quartz Dewar flask filled with liquid nitrogen, and SpectraLED (λexc = 370 nm; fwhm = 15 nm) was used as excitation source. Solid state measurements were carried out on (i) PMMA films containing 1 wt % of the complex drop-cast from dichloromethane solutions and (ii) powders, as obtained after the crystallization, placed in quartz tubes. The thickness of the films was not determined. Solidstate PLQY values were calculated by corrected emission spectra obtained from an Edinburgh FLS920 spectrometer equipped with a barium sulfate-coated integrating sphere (diameter of 3 in.) following the procedure described by Würth et al.60 For temperature-dependent measurements, the samples (PMMA films or powders) were placed inside an Oxford Optistat DN variabletemperature liquid-nitrogen cryostat (operating range: 77−500 K) equipped with an ITC5035 temperature controller and interfaced with the aforementioned Edinburgh FLS920 spectrometer. The temperature-dependent emission lifetimes were measured through

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02879. X-ray and DFT key structural parameters, OSWV voltammograms, electrochemical data, photophysical data, stability spectra in solution, solid state luminescence in various media and as a function of the temperature and variable temperature excited-state lifetimes (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(F.M.) E-mail: fi[email protected]. *(B.D.N.) E-mail: [email protected]. *(J.F.N.) E-mail: [email protected]. *(N.A.) E-mail: [email protected]. ORCID

Enrico Leoni: 0000-0001-6190-1708 Filippo Monti: 0000-0002-9806-1957 Jean-Franco̧is Nierengarten: 0000-0002-3333-9372 Nicola Armaroli: 0000-0001-8599-0901 J


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(16) Chen, L. X.; Zhang, X.; Shelby, M. L. Recent advances on ultrafast X-ray spectroscopy in the chemical sciences. Chem. Sci. 2014, 5 (11), 4136−4152. (17) Huang, J.; Mara, M. W.; Stickrath, A. B.; Kokhan, O.; Harpham, M. R.; Haldrup, K.; Shelby, M. L.; Zhang, X.; Ruppert, R.; Sauvage, J. P.; Chen, L. X. A strong steric hindrance effect on ground state, excited state, and charge separated state properties of a CuI-diimine complex captured by X-ray transient absorption spectroscopy. Dalton Trans 2014, 43 (47), 17615−17623. (18) McCusker, C. E.; Castellano, F. N. Design of a Long-Lifetime, Earth-Abundant, Aqueous Compatible Cu(I) Photosensitizer Using Cooperative Steric Effects. Inorg. Chem. 2013, 52 (14), 8114−8120. (19) Garakyaraghi, S.; Crapps, P. D.; McCusker, C. E.; Castellano, F. N. Cuprous Phenanthroline MLCT Chromophore Featuring Synthetically Tailored Photophysics. Inorg. Chem. 2016, 55 (20), 10628−10636. (20) Garakyaraghi, S.; McCusker, C. E.; Khan, S.; Koutnik, P.; Bui, A. T.; Castellano, F. N. Enhancing the Visible-Light Absorption and Excited-State Properties of Cu(I) MLCT Excited States. Inorg. Chem. 2018, 57 (4), 2296−2307. (21) Buckner, M. T.; Matthews, T. G.; Lytle, F. E.; McMillin, D. R. Simultaneous emissions including intraligand emission and chargetransfer emission from [Cu(PPh3)(phen)]+. J. Am. Chem. Soc. 1979, 101 (19), 5846−5848. (22) Blasse, G.; McMillin, D. R. On the luminescence of bis (triphenylphosphine) phenanthroline copper (I). Chem. Phys. Lett. 1980, 70 (1), 1−3. (23) Cuttell, D. G.; Kuang, S. M.; Fanwick, P. E.; McMillin, D. R.; Walton, R. A. Simple Cu(I) complexes with unprecedented excitedstate lifetimes. J. Am. Chem. Soc. 2002, 124 (1), 6−7. (24) Kuang, S. M.; Cuttell, D. G.; McMillin, D. R.; Fanwick, P. E.; Walton, R. A. Synthesis and structural characterization of Cu(I) and Ni(II) complexes that contain the bis 2-(diphenylphosphino)phenyl ether ligand. Novel emission properties for the Cu(I) species. Inorg. Chem. 2002, 41 (12), 3313−3322. (25) 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 Cu-I complexes. Eur. J. Inorg. Chem. 2005, 2005, 1867−1879. (26) Saito, K.; Arai, T.; Takahashi, N.; Tsukuda, T.; Tsubomura, T. A series of luminescent Cu(I) mixed-ligand complexes containing 2,9dimethyl-1,10-phenanthroline and simple diphosphine ligands. Dalton Trans 2006, 37, 4444−4448. (27) Zhang, L. M.; Li, B.; Su, Z. M. Phosphorescence Enhancement Triggered by π Stacking in Solid-State [Cu(N-N)(P-P)]BF 4 Complexes. Langmuir 2009, 25 (4), 2068−2074. (28) Armaroli, N.; Accorsi, G.; Holler, M.; Moudam, O.; Nierengarten, J. F.; Zhou, Z.; Wegh, R. T.; Welter, R. Highly luminescent CuI complexes for light-emitting electrochemical cells. Adv. Mater. 2006, 18 (10), 1313−1316. (29) Listorti, A.; Accorsi, G.; Rio, Y.; Armaroli, N.; Moudam, O.; Gegout, A.; Delavaux-Nicot, B.; Holler, M.; Nierengarten, J. F. Heteroleptic copper(I) complexes coupled with methano[60]fullerene: Synthesis, electrochemistry, and photophysics. Inorg. Chem. 2008, 47 (14), 6254−6261. (30) 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 (17), 3709−3713. (31) 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 [CuI(diimine)(diphosphine)]+ Complexes − Photophysical, Structural, and Computational Studies. Inorg. Chem. 2014, 53 (20), 10854−10861. (32) Tsubomura, T.; Kimura, K.; Nishikawa, M.; Tsukuda, T. Structures and photophysical properties of copper(I) complexes bearing diphenylphenanthroline and bis(diphenylphosphino)alkane: the effect of phenyl groups on the phenanthroline ligand. Dalton Trans 2015, 44 (16), 7554−7562.

⊥

(J.M.) Advanced Functional Polymers, Macromolecular Chemistry I, University of Bayreuth, 95447, Bayreuth, Germany. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the CNR (PHEEL; Progetto Bandiera N-CHEM; bilateral project CNR-CNRS Lebanon), the CNRS, the International Center for Frontier Research in Chemistry, and the LabEx “Chimie des Systèmes Complexes”. We thank Joanna M. Malicka and Gao Sheng for preliminary photophysical measurements and Alain Moreau (LCC) for electrochemical measurements.

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