Luminescent Dinuclear Copper(I) Complexes as Potential Thermally

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Luminescent Dinuclear Copper (I) Complexes as Potential TADF Emitters: A Theoretical Study Ange Stoianov, Christophe Gourlaouen, Sergi Vela, and Chantal Daniel J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b11793 • Publication Date (Web): 11 Jan 2018 Downloaded from http://pubs.acs.org on January 11, 2018

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The Journal of Physical Chemistry

Luminescent Dinuclear Copper (I) Complexes as Potential TADF Emitters: A Theoretical Study †

†

†

Ange Stoïanov, Christophe Gourlaouen, Sergi Vela, Chantal Daniel

†,*

†

Laboratoire de Chimie Quantique, Institut de Chimie Strasbourg, UMR-7177 CNRS/ Université de Strasbourg, 1 Rue Blaise Pascal BP 296/R8, F-67008 Strasbourg, France

ABSTRACT The excited states properties of a series of bi-nuclear NHetPHOS-Cu(I) complexes (NHetPHOS) have been investigated by means of density functional theory (DFT) and timedependent-DFT (TD-DFT). It is shown that experimental trends observed in powder, generally explored via S1 and T1 excited state energetics and S1 ⇔ T1 intersystem crossing (ISC) efficiency, are hardly analyzed on the basis of excited states properties calculated in solution. Indeed, several local minima corresponding to various structural deformations are evident on the lowest excited state potential energy surfaces (PES) when solvent correction is applied leading to a four-state thermally activated delayed fluorescence (TADF) mechanism. In contrast, preliminary simulations performed in solid point to the reduction of nuclear flexibility and consequently to a rather simple two-state model.

INTRODUCTION The efficiency of organic light emitting diodes (OLEDs) is mainly controlled by harvesting1 emission from singlet and triplet electronic states with two marginal cases. The OLED based on

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purely organic molecules that exhibit namely highly efficient fluorescent properties driven by i) fast and direct decay from the absorbing singlet state to the S0 electronic ground state with ns lifetimes2 and ii) the 3rd row transition metal complexes, seat of efficient intersystem crossing (ISC), which may display ∼100% T1 → S0 phosphorescence as exemplified by Ir(III) and Pt(II) complexes. 3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19

However the quantum yield of luminescence, its time-scale and the

covered domain of energy (color tuning) are not always up to the expectations despite the large amount of activity invested in the past decade in the design of new devices. Indeed, based on spin statistics, fluorescent and phosphorescent emitters are intrinsically unable to convert all excitons into light. Most of the time, optimization of luminescent molecules and materials is based on a realistic compromise between constraints, specifically wavelength of emission, time-scale and efficiency. In theory the appealing thermally activated delayed fluorescence (TADF) mechanism, known since the 70’s, has the potential to fully up-convert all triplet excitons via reverse ISC to the singlet state which is the source of fluorescence (Scheme 1). A number of qualitative analysis has pointed to basic criteria such as the HOMO-LUMO energy gap, electronic localization of the triplet state, singlettriplet energy gap and SOC. Whereas these simple rules are useful for a simple two-state S1/T1 model describing the luminescent properties after S0 → S1 absorption, they are inadequate in predicting fluorescent and phosphorescent properties in real molecules.


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Scheme 1. Thermally activated delayed fluorescence (TADF) mechanism principle (ISC: intersystem crossing; RISC: reverse ISC). In this respect, a detailed understanding of the interfering photophysical processes following the absorption and leading to the luminescent signal is mandatory. This aspect is crucial in transition metal complexes due to the high density of electronic excited states of different character and to the occurrence of efficient ISC that compete with internal conversion (IC) processes,20,21,22 both driven by spin-orbit and vibronic couplings. Moreover in the case where unsaturated metal atoms are present, low-lying metal-centered (MC) excited states appear as potential quenchers of luminescence.23 From this point of view the design of new materials based on copper (I) complexes with appropriate ligands is promising not only for cost-effectiveness but also because of the saturation of the d shells of the metal atom (d10 electronic configuration) that avoids the presence of MC states.24 Mono- and multi-nuclear copper complexes have been demonstrated to be on par with iridium complexes in terms of device efficiency and are currently being vetted. Molecular stability, high efficiency and color-tuning can be achieved by well designed ligands.25,26,27,28,29,30,31,32,33 However, even within this group of emitters, there is variation in the photophysical properties and


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device performance, which makes it difficult to rationalize.34 More specifically, correlation between structural relaxation in the lowest triplet excited states and efficiency of TADF is difficult to established calling for in-depth spectroscopic and theoretical investigations of the mechanism of luminescence in this class of Cu(I) complexes.27, 34,35,36,37,38,39,40,41 The aim of this article is to elucidate the excited state properties of a series of bi-nuclear NHetPHOSCu(I) complexes in order to rationalize the experimental trends observed in powder at room temperature and to investigate possible mechanisms for triplet-exciton harvesting. Both model and real systems have been scrutinized (see Scheme 2). H-m

H1

B-m

H2

B

Scheme 2. Experimental and model structures for the heteroleptic (H1), homoleptic (H2) and bridged (B) complexes (H-m and B-m: model structures; H1, H2 and B: experimental structures).


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PHOTOPHYSICAL MEASUREMENTS Photophysical measurements were performed on the heteroleptic and bridged Cu (I) complexes at 77K and 298K. Excitation and emission spectra were obtained from a FluoroMax-4 spectrometer equipped with a 150 W xenon-arc lamp and the luminescence quantum yields were measured using a Hamamatsu Photonics absolute PL quantum yield measurement system equipped with a 150 W xenon light source. The crystal structures obtained from a Bruker-Nonius Kappa CCD diffractometer were used for the starting point for the complete geometry optimisations. COMPUTATIONAL DETAILS All gas-phase calculations were performed with ADF 201342 at density functional theory DFT43,44,45 level using B3LYP functional.46 Polarised Triple ζ basis sets were used for all atoms.47 Scalar relativistic effects were taken into account through zero-order relativistic approximation ZORA Hamiltonian.

48

The solvent was modeled by the polarized continuum model (PCM) approximation

of dichloromethane (DCM).49,50,51 Full geometry optimizations were performed on all structures. Absorption spectra were computed on these optimized structures by mean of time-dependent-DFT (TD-DFT)52,53,54 at the same level of theory. Spin-orbit coupling (SOC) between the low-lying singlet and triplet states were computed through a perturbation of the TD-DFT states. Emission wavelengths of both excited singlet and triplet states were computed on the basis of optimized excited structures on which a TD-DFT was performed. This work was performed on the provided experimental crystal structures and on corresponding, simplified models (Scheme 2). All geometry optimizations in the solid state were performed using the Quantum Espresso package (QE) Version 5.1.1,55 the PBE+D2 scheme,56 Vanderbilt ultra-soft pseudopotentials,57 a 2x2x2 grid of k-points sampling the Brillouin zone and a kinetic energy cutoff of 35 Ry. The initial structure was


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taken from the existing crystallographic data, and cell parameters remained fixed during the optimization. RESULTS AND DISCUSSION

Structural properties The geometrical structures of the complexes in their S0 electronic ground state and lowest singlet and triplet-excited states have been fully optimized in gas phase. As illustrated by the results reported in Table 1, for the electronic ground state of H1 some discrepancy may appear between the optimized structures of the real complex H1, its model m-H1 and the experimental X-ray data. The calculated bond lengths are generally overestimated with respect to their experimental values, this effect being even more evident in the model system where methyl groups replace the phenyl groups.

Table 1. Important bond lengths (in Å) for a selection of structures depicted in Scheme 2 in their electronic ground states S0. For the H1 heteroleptic complex the X-ray data are given for comparison. Cu1-Cu2 Cu1-I1 Cu1-I2 Cu2-I1 Cu2-I2 Cu1-N Cu1-P3 Cu2-P1 Cu2-P2

H-m 2.96 2.89 2.91 2.91 2.89 2.11 2.25 2.26 2.26

H1 2.92 2.82 2.78 2.80 2.86 2.17 2.28 2.29 2.30

H1 X-ray 2.82 2.63 2.68 2.69 2.69 2.09 2.25 2.24 2.25

B-m 2.84 2.78 2.91 2.88 2.87 2.14 2.25 2.26 2.26

The results reported in Table 1 point to an important nuclear flexibility of the copper-iodide core of the complexes. Whereas the four copper-iodide bond lengths are comparable in the heteroleptic complex a distortion is observed for one of the Cu-I bond in the bridged complex B-m characterized by a shortened Cu1-I1 bond length of 2.78 Å and three very similar Cu-I bond lengths, which remain


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comparable to the H-m values. S1a

S0

S1b

Figure 1. Optimized structures of H-m in the S0 electronic ground state and lowest S1a and S1b excited states. Color code: Carbon (grey), Hidrogen (white), Nitrogen (blue), Iodine (magenta), Copper (pink), Phosphorous (orange). By optimizing the lowest singlet and triplet-excited states, several energy minima in the heteroleptic model complex (H-m) can be associated to spin densities that correspond essentially to HOMO → LUMO, HOMO-1 → LUMO and HOMO → LUMO+1 electronic excitations. Starting from the electronic ground state S0 structure (Figure 1) two important structures become evident in the singlet state manifold, namely S1a and S1b (Figure 1). Similar structures have been obtained as well for the corresponding T1a and T1b triplet states. In both cases, these structures correspond to minima on the lowest excited singlet and triplet potential energy surfaces. In the ground state, both iodides are equidistant to the two copper cations whereas in the excited state structures they move to either one or the other copper atom. This is illustrated by the optimized geometrical parameters reported in Table 2 for the Cu-I bond distances in the excited states.


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Table 2. Relevant bond distances describing the molecular deformation (in Å) of the computed H-m and H2 structures in different electronic excited states as depicted in Scheme 2. H-m

H2*

Cu-I Cu1-Cu2 Cu1-I1 Cu1-I2 Cu2-I1 Cu2-I2

S0 2.957 2.885 2.909 2.908 2.893

Cu1-Cu2 2.918 Cu1-I1 2.819 Cu1-I2 2.777 Cu2-I1 2.799 Cu2-I2 2.856 * Numbering according to scheme 2

S1a 2.641 2.837 2.847 2.912 2.884

S1b 3.503 3.852 3.818 2.801 2.806

T1a 3.081 2.740 2.748 3.168 3.197

T1b 3.221 3.419 3.580 2.724 2.709

2.714 2.752 2.800 2.856 2.869

3.503 3.152 4.509 2.730 2.717

2.628 2.658 2.946 2.902 2.733

3.063 2.863 3.319 2.692 2.661

Indeed, the shortening of two Cu-I bond distances corresponds to an increase of the remaining two distances in the excited states, which is observed for the model as well as for the real complexes (see Table 2 for H-m and H2). The only exception is the S1a state of H-m and H2 for which the structure remains similar to the one of the electronic ground state. The Cu-Cu bond lengths are particularly affected by an electronic transition to S1b with a calculated elongation greater than 0.5 Å both in H-m and H2. At higher energies other minima have been found corresponding to more symmetric structures with one iodide bound to each of the copper atoms (Figure 2).


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Figure 2. Optimized structures of H-m in upper S1c (left) and T1c (right) excited states. The electronic configuration of this class of molecules is well represented by the frontier Kohn-Sham orbitals of the H-m complex depicted in Scheme 3. The HOMOs are delocalized over the copper/iodide core and the LUMOs are localized on the acceptor NHetPHOS ligand. The excited states correspond to charge transfer states of mixed metal-to-ligand and halide-to-ligand characters, so-called MLCT/XLCT as discussed for the H2 complex.27

HOMO - 1

HOMO

LUMO

LUMO + 1

Scheme 3. Kohn-Sham frontier orbitals of the m-H complex.

The adiabatic potential energy surfaces associated to both the lowest singlet and lowest triplet are characterized by several minima, the origin of which is the high density of excited states of various characters that generate a number of critical geometries by nuclear relaxation. In this respect dinuclear and mono-nuclear copper complexes40,41 behave differently despite of similar luminescence properties and a direct comparison between the excited state geometries is not pertinent. Indeed the


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density of electronic excited states of various characters is less dramatic in the mono-nuclear halide complexes41 leading to much more simple low adiabatic potential energy surfaces. Moreover, the configurations generated by the back and forth of the iodide between the two copper atoms cannot be generated in the mono-nuclear complexes.

Absorption and photophysical data The experimental absorption and emission spectra of the bridged (B) and heteroleptic (H1) complexes are represented in Figure 3. The absorption starts at about 450 nm and is characterized by a continuous increase in intensity at room temperature beyond 300 nm. At 77K the absorption reaches a plateau between 350-300 nm.

Figure 3. Absorption/emission spectrum of the hetero- (H1), and bridged (B) complexes at room temperature (RT) and 77°K measured in powder. The theoretical absorption spectrum of complex B is depicted in black in Figure 4 for comparison. It is characterized by two main bands not observable experimentally, namely one band composed of four MLCT/XLCTNHetPHOS transitions corresponding to HOMO → LUMO, HOMO-1 → LUMO, HOMO-2 → LUMO and HOMO → LUMO+1 and calculated at 2.93, 3.17, 3.39 and 3.42 eV,


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respectively and leading to the shoulder between 420-360 nm and a second band starting at 350 nm with a peak at 325 nm. A high density of excited states, calculated between 3.54 - 3.78 eV, constitutes the second band, all of them corresponding to CT from the copper/iodide core either to the NHetPHOS ligand or to the substituted phosphine (Scheme 4). The transition energies are reported in Table S1 of the SI section.

Scheme 4. Charge transfer from the copper/iodide core to NHetPHOS (left) and to the substituted phosphine (right) in the B complex. The theoretical absorption spectra of the three complexes H1, H2 and B are very similar as illustrated by their shape reproduced in Figure 4 as are those from the experimental measurements. The transition energies, composition and oscillator strengths associated to the lowest singlet states of the complexes are reported in Table S1 of the SI section.


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Figure 4. TD-DFT absorption spectra of B (black), H1 (blue) and H2 (red) in DCM. Whereas the lowest excited states that compose the shoulder correspond to the same transitions in B, H1 and H2, namely MLCT/XLCTNHetPHOS, the upper excited states include charge transfer to the triphenyl phosphine and substituted biphenyl phosphine ligands in H1 and H2, respectively. Spinorbit coupling has a very small effect on the shape of the absorption spectra of this class of complexes as illustrated by the TD-DFT absorption spectra computed with SOC and represented in Figure S1 of the SI section for the B complex. The photo luminescence quantum yields (PLQY), emission wavelengths and time scales measured at room temperature and at 77K in powder are reported in Table 3 for the heteroleptic (H1), homoleptic (H2) and bridged (B) complexes. The maximum of emission is centered on 508 nm (Figure 3). The available data in solution (CH2Cl2) are reported for comparison for both heteroleptic and homoleptic compounds. The luminescence lifetime increases significantly when going from room temperature to 77K, in which the bridged complex (B) has the longest living excited state lifetimes of 7.3 to 36.7 µs.

Table 3. Photophysical data of heteroleptic (H1), homoleptic (H2) and


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bridged (B) complexes depicted in Scheme 2 (PLQY: photo luminescence quantum yield) in powder at room temperature and at 77K. PLQY for H2 is estimated based on values given in references. H1 PLQY em λmax

τ em

B

68% / 88% 510 / 525 nm

27

64% / 83% 513 / 518 nm

69% 508 / 511 nm

4.3 / 18.7 µs

3.1 / 22.4 µs

7.5 / 36.7 µs

In CH2Cl2 26,27 593 nm 608 nm (531)a

em λmax a

H2 26

-

Present work

The PLQY decreases when substituting phenyl groups in H1 by pyridine’s in H2 while that of the bridged complex remains similar to that of H1. All three complexes behave very similarly in terms of their exciton lifetime as well as to their peak emissions. The molecular structures of the three complexes in their lowest singlet and triplet excited states as well as the SOC interactions have been determined in order to interpret their photophysical properties. Eight excited states minima have been obtained for H1 and H2; two singlets and six triplets (Tables 4 and 5), whereas seven optimized structures were determined for B, two singlets and five triplets (Table 6). The SOC norms calculated between the low-lying singlet and triplet excited states of the three complexes are reported in Tables S2, S3 and S4 of the SI section whereas the S1/T1 SOC interactions are reported in Table 7 for all three complexes.

Table 4. Relevant bond distances describing the molecular deformation (in Å), emission energies Ed and Ee (in eV), and emission wavelengths λem (in nm) of H1. Cu1-Cu2 Cu1-I1 Cu1-I2 Cu2-I1 Cu2-I2

S1a 2.714 2.752 2.800 2.856 2.869

T1a 2.628 2.658 2.946 2.902 2.733

S1ba 3.503 3.152 4.509 2.730 2.717

T1b 3.063 2.863 3.319 2.692 2.661

T1c 2.643 2.634 4.552 4.602 2.814

T1c’ 2.554 4.196 2.652 2.770 4.351

T1d 2.681 2.898 2.655 2.868 2.896

T1f 3.567 2.729 4.382 2.686 2.676


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Ed Ee

0.39 2.03

615 λem a Not fully converged

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0.65 1.82

0.48 2.00

0.57 1.94

0.96 1.79

1.00 1.88

0.99 1.54

0.73 1.85

681

623

638

691

658

803

669

Table 5. Relevant bond distances describing the molecular deformation (in Å), emission energies Ed and Ee (in eV), and emission wavelengths λem (in nm) of H2. Cu1-Cu2 Cu1-I1 Cu1-I2 Cu2-I1 Cu2-I2

S1a 2.752 2.738 2.823 2.874 2.862

T1a 2.623 2.656 3.002 2.853 2.762

S1ba 3.485 3.941 2.896 2.713 2.743

T1b 3.541 2.678 4.394 2.708 2.686

T1c 2.644 2.645 4.560 4.560 2.812

T1f 2.383 4.041 2.720 2.652 2.701

T1g 2.688 2.694 2.999 2.891 2.798

T1h 3.047 2.777 3.727 2.704 2.678

Ed Ee

0.45 2.07

0.61 1.90

1.73 1.10

0.68 1.93

1.11 1.80

0.98 1.64

0.52 1.94

0.80 1.87

653

718

643

689

757

638

664

599 λem a Not fully converged

The S1a state has been fully converged for the three complexes and is very similar to the one reported for the model complex H-m (Table 1) whereas the state S1b has only been converged for B. Rotation of the phosphine groups in H1 and H2 lead to oscillatory convergence. T1a and T1b are the related triplet states similar to those obtained for the model complex H-m. The Cu-Cu and Cu-I excited state bond length deformations are very similar to the one observed in the model complex H-m in associated excited states. The triplet potential energy surface exhibits a number of minima, some of which are nearly degenerate but are all characterized by different geometrical distortions. This is a consequence of the high nuclear flexibility of this class of molecules, this property being only slightly affected by bulky ligands as illustrated by a comparison between the distortion energies Ed in H-m, H1 and H2.


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Table 6. Relevant bond distances describing the molecular deformation (in Å), emission energies Ed and Ee (in eV), and emission wavelengths λem (in nm) of B. Cu1-Cu2 Cu1-I1 Cu1-I2 Cu2-I1 Cu2-I2

S1a 3.226 2.777 2.884 3.898 2.765

T1a 3.239 2.686 2.796 3.868 2.754

S1b 3.158 2.992 3.804 2.817 2.747

T1b 3.147 2.993 3.801 2.815 2.742

T1c 2.653 2.653 4.418 4.391 2.855

T1c’ 2.552 4.352 2.702 2.765 4.453

T1f 3.231 3.685 3.079 2.712 2.704

Ed Ee

0.46 1.84

0.54 1.68

0.67 1.74

0.67 1.71

0.85 1.77

1.03 1.85

0.69 1.72

λem

674

738

712

725

700

669

721

The wavelengths of emission associated to the minima of the lowest excited states range between 803-615 nm for H1, 757-599 nm for H2 and 738-669 nm for B. The fluorescence wavelength induced by the S1a state is calculated at 615 nm, 599 nm and 674 nm for H1, H2 and B, respectively. The first two values compare rather well with the experimental data reported in CH2Cl2 of 593 nm for H1 and 608 nm for H2 (Table 3). The calculated wavelength of emission of complex B (674 nm) exhibits a significant red shift as compared to H1 and H2. However solid-state calculations could partially cancel this effect. Indeed, in the experimental conditions reported in the present work, namely powder, the nuclear distortions discussed above are partially inhibited leading to higher wavelengths of emission as exemplified by the experimental values reported in Table 3 for H1 and H2, namely 510 and 513 nm, respectively. To confirm this statement, solid-state geometry optimizations have been performed for both H1 and H2, including periodic boundary conditions in the three crystal dimensions. These computations allow us to optimize the ground- and excited- states minima accounting for constraints of the crystal packing. Due to computational limitations, only the singlet ground state and the first low-lying triplet were optimized using this approach. Moreover, a singlet calculation on the triplet minima for H1 and


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H2 was performed to evaluate the emission wavelength, yielding λem = 706 and 699 nm for H1 and H2, respectively (see also Table 7). These values cannot be directly compared with those discussed so far, since these solid-state calculations (i) employ a different quantum chemistry method (eg. functional, basis set, pseudopotential) (see Comp. Details) and (ii) incorporate the environment. To evaluate the relevance of the first point and facilitate discussion, the singlet-triplet energy difference was recomputed using the same level of computation used in previous sections. Under this condition, λem is calculated et 617 nm (H1) and 551 nm (H2). To evaluate the second point, calculations were performed on isolated molecules using the new computational scheme, yielding λem = 782 and 737 nm. The effect directly attributable to the environment is, thus, a blue-shift of 0.17 and 0.09 eV for H1 and H2, respectively. When this contribution is added to our best computational estimate of λem in isolated conditions, we obtain λem = 568 and 529 nm for H1 and H2, respectively, much closer to the experimental value of ca. 520 nm (Table 3). Although such agreement might be just a matter of chance, it becomes clear that in the solid state, H1 and H2 molecules cannot deform as much as in the gas phase to reach the triplet minima (compare Tables 4 and 5 with Table 7), thus leading to a penalty in the triplet stability and, hence, to a blue shift in the emission spectrum.

Table 7. Relevant bond distances describing the molecular deformation (in Å) in the computed spin states of H1 and H2 in the solid state (T1 is the lowest triplet state obtained from the calculation performed in solid state). H1 Cu1-Cu2 Cu1-I1 Cu1-I2 Cu2-I1 Cu2-I2

S0 2.891 2.725 2.708 2.701 2.670

H2 T1 2.617 2.683 2.674 2.600 2.705

S0 2.780 2.659 2.701 2.690 2.715

T1 2.605 2.656 2.627 2.663 2.648


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Table 8. Calculated SOC (n/m/m for n ± mi in cm-1) between S1 and T1 for complexes H1, H2 and B. H1

S1 0 5/12/12 41/7/7 S1 0 16/6/6 24/35/35 S1 0 0 9/13/13 14/6/6 26/39/39 19/20/20 11/8/8

S1a T1a T1b H2 S1a T1a T1b B S1a S1b T1a T1b T1c T1c’ T1f

T1 14/10/10 0 0 T1 16/9/9 0 0 T1 7/14/14 13/5/5 0 0 0 0 0

The analysis of SOC effects restricted to the lowest S1 and T1 states (Table 8) indicates modest contributions that do not exceed a few tens of cm-1 but more important than in the model complex Hm where they do not exceed 8 cm-1. However when considering the second singlet and triplet states S2 and T2 (Tables S2, S3, S4 of the SI section) the analysis shows a dramatic increase of the SOC strength with values reaching a few hundred of cm-1, large enough for inducing efficient ISC. Both energetics and SOC effects indicate that the oversimplified picture of the TADF mechanism as depicted in Scheme 1 is not valid for this class of complexes. Indeed at least two sets of excited states are at play, namely S1/T1 and S2/T2.

A four states TADF mechanism The standard mechanism in TADF (Scheme 1) indicates that the S1/T1 SOC must be significant for reverse ISC, but also that the energy gap between the singlet and the triplet states is small enough


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that thermal energy allows these states to approach each other for efficient back transfer.27 Exemplarily, and perhaps unexpectedly, the S1 state has a negligible coupling with the T1 state in the three investigated complexes. However, the couplings between S1 and T2 are significant enough for inducing efficient ISC. Likewise, the coupling between the T1 and S2 states is significant as well. As far as the energy gaps are concerned S1 and T1 are nearly degenerate at Franck Condon in the three complexes (∆T1-S1 < 0.03 eV or 240 cm-1), S2 and T2 being separated by about 0.10 eV or 800 cm-1. This energy gap remains on the same order of magnitude for the T1a and T1b structures. The S1/T2 and T1/S2 energy gaps at Franck-Condon are of the order of 0.30 eV whereas they increase drastically by distortion of the H complexes to T1a (~ 0.65 eV). This effect is even more important in the B complex in which distortions to both the T1a and T1b geometries leads to S1/T2 and T1/S2 energy gaps greater than 0.75 eV. However, this effect is compensated for by strong SOC. For instance, the S2/T1a and S2/T1b SOC values amount to 475-582 cm-1 in H1, 340-440 cm-1 in H2 and 320-670 cm-1 in B (Tables S2, S3, S4 of the SI section). Taking the SOC and the energetics effects into account, we conclude that the standard TADF mechanism is most likely a simplification of the processes that actually occur in these systems. We propose the following mechanism in solvent for this class of copper (I) complexes (Scheme 5): After absorption to the MLCT/XLCT band (360-420 nm), ultra-fast decay driven by spin-orbit and vibronic coupling populates the T1 state. Thermal activation induces important nuclear distortions allowing the system to explore other regions of the lowest triplet potential energy surface that may exchange population with S1 by vibronic coupling (path 1 in Scheme 5) and S2 by SOC (path 2 in Scheme 5). Both states may be responsible for the broad fluorescence that characterizes this class of complexes between 500-700 nm. As soon as S2 is populated T2 is potentially populated as well because of their small energy gap (path 3 in Scheme 5). The population of T2 will re-populate the S1


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(path 4 in Scheme 5) state due to a prominent SOC.

Scheme 5. Qualitative mechanism of TADF in binuclear NHetPHOS-Cu(I) complexes. Of course this schematic view may be altered by the participation of upper electronic states or environment effects (film or powder). Moreover, a recent study performed on organic emitters for TADF58, has shown that small singlet-triplet energy gap (0.01-0.24 eV) combined with very modest SOC (0.27 – 1.54 cm-1) facilitates TADF. In the case of the Cu(I) complexes investigated here albeit rather small SOC values (Table 8) could activate T1/S1 up-conversion. Nevertheless, the whole mechanism seems to be mainly controlled by the S1/T1 and S2/T2 energy gaps and the T1/S2 and T2/S1 SOC. This mechanism differs from the simple one proposed traditionally for this class of complexes that is based on one single set S1/T1 in equilibrium59 and allows for a rationalization of the measured PLQY of the three complexes. Indeed in the case of H1, characterized by a high quantum yield


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(68%/88%), the four paths are very favorable with small S1/T1 and S2/T2 energy gaps that control paths 1 and 3 (Scheme 5) and large T1a,T1b/S2 and T2/S1a SOC that govern paths 2 and 4 (Scheme 5). Path 2 is less favorable in H2 because of weaker T1/S2 SOC terms both at T1a and T1b geometries as compared to H1. In addition, geometrical distortions to S1a, T1a and T1b are more costly in H2. These two effects explain a reduced value of PLQY (64%/83%). The low quantum yield of 69% measured in powder for complex B is explained by the substantial increase of the T1/S2 energy gaps at T1a and T1b minima (> 0.75 eV) more important than in H1 and H2 complexes for similar SOC values. In addition and in contrast to the H complexes, significant Cu-Cu bond elongation is necessary to reach the S1a and T1a minima of complex B (see Table 6). A constrained environment could hamper this distortion leading to less efficient initial thermal activation. This multi-state mechanism is confirmed by ultrafast multi-exponential photophysical decay recently observed for a Cu(I)-NHetPHOS-tris-m-tolylphosphine, both in solution and in solid state.60

CONCLUSIONS The structural, optical and luminescence properties of three bi-nuclear NHetPHOS-Cu(I) complexes investigated as potential TADF emitters have been investigated on DFT and TD-DFT calculations performed in solvent and in solid state. It is shown that a simple two-state model TADF mechanism is not realistic in solution and that at least four states, two singlet and two triplet states, have to be taken into account for investigating the potential of this class of complexes characterized by a high nuclear flexibility. Indeed up to eight minima corresponding to various structures of the low-lying singlet and triplet states, have been calculated possibly leading to broad emission between 600-800 nm for the three investigated molecules. In contrast the distortion is minor in solid state, as


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expected, leading to blue-shifted emission spectra in better agreement with the experimental findings for both the heteroleptic and homoleptic complexes. Consequently whereas the typical two-state model usually invoked for determining the TADF potential of a molecule could be valid in solid it is questionable in solution. To confirm this hypothesis, further investigation, based on efficient quantum chemistry methods for solid phase, should determine the role of the upper excited states in the mechanism. Whereas structural relaxation effects in Cu complexes have been widely investigated within the context of magnetic properties,61,62,63 further theoretical and spectroscopic studies should help at interpreting the TADF mechanism in this class of molecules.

ACKNOWLEDEGMENTS CYNORA GmbH has supported this work, and a special thanks to Dr. D. Ambrosek for the fruitful discussions. The quantum chemical calculations have been performed on the computer nodes of the LCQS, Strasbourg and thanks to the computer facilities of the High Performance Computing (HPC) regional center of University of Strasbourg.

ASSOCIATED CONTENTS: Supplementary Informations Cartesian coordinates of H-m and B-m in the electronic ground state and lowest excited states Cartesian coordinates of H1, H2 and B in the electronic ground state and lowest excited states Table S1. TD-DFT vertical transition energies (in eV) to the low-lying singlet 1A excited of H1, H2 and B complexes, associated oscillator strengths and wavelengths of absorption (in nm). Table S2. Calculated SOC (n/m/m for n±mi in cm-1) between the lowest S1a, T1a and T1b states of H1 at their optimized geometries with the other upper excited states.


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Table S3. Calculated SOC (n/m/m for n±mi in cm-1) between the lowest S1a, T1a and T1b states of H2 at their optimized geometries with the other upper excited states. Table S4. Calculated SOC (n/m/m for n±mi in cm-1) between the lowest S1a, S1b, T1a and T1b states of B at their optimized geometries with the other upper excited states. Figure S1. Comparison between TD-DFT absorption spectra of complex B computed without (left) and with (right) SOC.

CAPTIONS OF FIGURES

Figure 1. Optimized structures of H-m in the S0 electronic ground state and lowest S1a and S1b excited states. Color code: Carbon (grey), Hidrogen (white), Nitrogen (blue), Iodine (magenta), Copper (pink), Phosphorous (orange).

Figure 2. Optimized structures of H-m in upper S1c (left) and T1c (right) excited states. Figure 3. Absorption/emission spectrum of the hetero- (H1), and bridged (B) complexes at room temperature (RT) and 77°K measured in powder.

Figure 4. TD-DFT absorption spectra of B (black), H1 (blue) and H2 (red) in DCM.

Scheme 1. Thermally activated delayed fluorescence (TADF) mechanism principle (ISC: intersystem crossing; RISC: reverse ISC).

Scheme 2. Experimental and model structures for the heteroleptic (H1), homoleptic (H2) and bridged (B) complexes (H-m and B-m: model structures; H1, H2 and B: experimental structures).

Scheme 3. Kohn-Sham frontier orbitals of the m-H complex.


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Scheme 4. Charge transfer from the copper/iodide core to NHetPHOS (left) and to the substituted phosphine (right) in the B complex.

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Ultra-fast decay

S2

S1 T2

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Luminescent Dinuclear Copper(I) Complexes as Potential Thermally

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