Chiroptical activity of benzannulated N-heterocyclic carbene rhenium(I) tricarbonyl halide complexes: towards efficient circularly polarized luminescence emitters.

The design of enantiomerically pure circularly polarized luminescent (CPL) emitters would enormously benefit from the accurate and in-depth interpretation of the chiroptical properties by means of jointly (chiroptical) photophysical measurements and state-of-the-art theoretical investigation. Herein, computed and experimental (chiro-)optical properties of a series of eight enantiopure phosphorescent rhenium(I) tricarbonyl complexes are systematically compared in terms of electronic circular dichroism (ECD) and CPL. The compounds have general formula fac-[ReX(CO)3(N^CNHC)], where N^CNHC is a pyridyl benzannulated N-heterocyclic carbene deriving from a (substituted) 2-(pyridin-2-yl)imidazo[1,5-a]pyridin-2-ium proligand and X = Cl, Br and I, and display structured red phosphorescence with long-lived (τ = 7.0-19.1 µs) excited-state lifetime and dissymmetry factors |gLum| up to 4 × 10-3. The mixing of the character of the lowest-lying emitting triplet excited state is finely modulated between ligand centred (3LC), metal-to-ligand charge transfer (3MLCT) and halogen-to-ligand charge transfer (3XLCT) by the nature of the ancillary halogen and the chromophoric N^CNHC ligand. The study unravels the effect exerted by the nature of the excited state onto the ECD and CPL activity and will help to pave the way to construct efficient CPL emitters by chemical design.

Ultrafast Excited-State Nonadiabatic Dynamics in Pt(II) Donor-Bridge-Acceptor Assemblies: A Quantum Approach for Optical Control.

The ultrafast nonadiabatic excited state dynamics of (PTZ-N-benzyl-acetylide) (trans-bis-trimethylphosphine) Pt(II) (acetylide-NDI-bis-methyl) 1, representative of a series of Pt(II) donor-bridge-acceptor assemblies experimentally studied by the Weinstein group, University of Sheffield, is investigated by means of wavepacket propagations based on the multiconfiguration time-dependent Hartree (MCTDH) method. On the basis of electronic structure data obtained at the time-dependent density functional theory (TD-DFT) level, the subpicosecond decay is simulated by solving an 11 electronic states multimode problem, up to 18 vibrational normal modes, including both spin-orbit coupling (SOC) and vibronic coupling. A careful analysis of the results, within the diabatic representation, provides the key features of the spin-vibronic mechanism at work in this complex, distinguishing between the spin-orbit and vibronically activated ultrafast processes within the excited states manifold. The knowledge of the key active normal modes that promote selectively the population of specific electronic excited states opens a route toward optical control by selectively exciting these modes in order to drive the associated nonadiabatic processes. Relevant simulations, over 2 ps, are proposed to assess the impact of these selective vibrational excitations on the branching ratio between the primary photoproducts, namely, bridge-acceptor charge-transfer (CT) and donor-acceptor charge-separated (CS) electronic states. Whereas the excitation of the localized acetylide bridge C≡C bond stretching does not modify drastically the population of the low-lying electronic states within the first two ps, vibrational excitation of the out-of-plane twisting motion of the N-benzyl group linked to the donor entity favors the population of the 1,3CS states at the expense of the lowest 1,3CT states. This quantum study opens the route to IR optical control experiments based on the specific alteration of vibrational normal modes that activate vibronic couplings between key electronic excited states. However, the presence of critical crossings along the PES channels associated with these normal modes and the role of concurrent SOC driven ultrafast transfers of population should not be underestimated.

Spin-vibronic mechanism at work in a luminescent square-planar cyclometalated tridentate Pt(II) complex: absorption and ultrafast photophysics.

The absorption spectrum of [Pt(dpybMe)Cl] (dpyb = 2,6-di-(2-pyridyl)benzene), representative of luminescent halide-substituted tridentate cyclometalated square planar Pt(II) neutral complexes, has been revisited by means of non-adiabatic wavepacket quantum dynamics. The early photophysics has been investigated on the basis of four singlet and five triplet excited states, namely nineteen "spin-orbit states", coupled with both vibronic and spin-orbit couplings, and includes eighteen normal modes. It is shown that in-plane scissoring and rocking normal modes of the cyclometalated tridentate ligand are responsible for the vibronic structure observed at around 400 nm in the experimental spectrum of the complex. The ultrafast decay of [Pt(dpybMe)Cl], within 1 ps, follows a spin-vibronic mechanism governed by excited state electronic characters, spin-orbit, and active tuning mode interplay. Both spin-orbit coupling and Pt(II) coordination sphere stretching modes and in-plane scissoring/rocking of the cyclometalated ligand activate the ultrafast decay within 20 fs of absorption. At longer time-scales (>100 fs) an asynchronous stretching of the Pt-C and Pt-N bonds activates the depopulation of the upper "reservoir" electronic states to populate the two lowest luminescent T1 and T2 electronic states. The in-plane rocking motion of the ligand controls the T1/T2 population exchange which is equilibrated at about 1 ps. Stabilization of the upper non-radiative metal-centered (MC) states by out-of-plane ligand distortion of low frequency is not competitive with the ultrafast spin-vibronic mechanism discovered here for [Pt(dpybMe)Cl]. Modifying the Pt-C covalent bond position and rigidifying the cyclometalated ligand will have a dramatic influence on the spin-vibronic mechanism and consequently on the luminescence properties of this class of molecules.

Theoretical spectroscopy for unraveling the intensity mechanism of the optical and photoluminescent spectra of chiral Re(I) transition metal complexes.

In this work, we present a computational study that is able to predict the optical absorption and photoluminescent properties of the chiral Re(I) family of complexes [fac-ReX(CO)3L], where X is either Cl or I and L is N-heterocyclic carbene extended with π-conjugated [5]-helicenic unit. The computational strategy is based on carefully calibrated time dependent density functional theory calculations and operates in conjunction with an excited state dynamics approach to treat in addition to absorption (ABS) and photoluminescence (PL), electronic circular dichroism (ECD), and circularly polarized luminescence (CPL) spectroscopies, respectively. The employed computational approach provides, an addition, access to the computation of phosphorescence rates in terms of radiative and non-radiative relaxation processes. The chosen molecules consist of representative examples of non-helicenic (NHC) and helicenic diastereomers. The agreement between theoretical and experimental spectra, including absorption (ABS, ECD) and emission (PL, CPL), is excellent, validating a quantitative interpretation of the spectral features on the basis of natural transition orbitals and TheoDore analyses. It is demonstrated that across the set of studied Re(I) diastereomers, the emission process in the case of NHC diastereomers is metal to ligand charge transfer in nature and is dominated by the easy-axis anisotropy of the emissive excited multiplet. On the contrary, in the cases of the helicenic diastereomers, the emission process is intra ligand charge transfer in nature and is dominated by the respective easy-plane anisotropy of the emissive excited multiplet. This affects remarkably the photoluminescent properties of the molecules in terms of PL and CPL spectral band shapes, spin-vibronic coupling, relaxation times, and the respective quantum yields. Spin-vibronic coupling effects are investigated at the level of the state-average complete active space self-consistent field in conjunction with quasi-degenerate second order perturbation theory. It is in fact demonstrated that a spin-vibronic coupling mechanism controls the observed photophysics of this class of Re(I) complexes.

Are luminescent Ru2+ chelated complexes selective coordinative sensors for the detection of heavy cations?

The ability of [Ru(bpy)2(bpym)]2+ (bpy = 2,2'-bipyridine; bpym = 2,2'-bipyrimidine) to probe specifically heavy cations has been investigated by means of density functional theory for transition metals, group 12 elements and Pb2+. On the basis of the calculated Gibbs free energies of complexation in water it is shown that all reactions are favorable with negative enthalpies except for Hg2+, with the transition metal cations forming stable bi-metallic complexes by coordination to the bpym ligand. Comparison between the optical and photophysical properties of the Ru2+ probe and those of the coordination compounds does not demonstrate a high selectivity due to very similar characteristics of the absorption and emission spectra. Whereas by complexation the lowest metal-to-ligand-charge-transfer (MLCT) shoulder of [Ru(bpy)2(bpym)]2+ at 462 nm is more or less shifted to the red as a function of the cation, the second MLCT band at 415 nm, less sensitive to the complexation, gains in intensity and is slightly blue-shifted. The visible MLCT emission of [Ru(bpy)2(bpym)]2+ at 706 nm is altered by complexation leading to near IR (800-900 nm) emission in most of the coordination compounds. Complexation to some transition metal cations (Fe, Co, Rh and Pd) generates low-lying metal-centered (MC) excited states that quench luminescence. In contrast to the conclusion of experimental findings by Kumar et al. (Chem. Commun. 2014, 50, 8488-8490), [Ru(bpy)2(bpym)]2+ cannot be proposed as a fast and selective probe for monitoring Pd2+ in aqueous media. Indeed, it does not possess the optical and photophysical characteristics necessary to discriminate Pd2+ ions over a variety of other cations.

Substituent effects on the photophysical properties of 2,9-substituted phenanthroline copper(I) complexes: a theoretical investigation.

The electronic and nuclear structures of a series of [Cu(2,9-(X)2 -phen)2 ]+ copper(I) complexes (phen=1,10-phenanthroline; X=H, F, Cl, Br, I, Me, CN) in their ground and excited states are investigated by means of density functional theory (DFT) and time-dependent (TD-DFT) methods. Subsequent Born-Oppenheimer molecular dynamics is used for exploring the T1 potential energy surface (PES). The T1 and S1 energy profiles, which connect the degenerate minima induced by ligand flattening and Cu-N bond symmetry breaking when exciting the molecule are calculated as well as transition state (TS) structures and related energy barriers. Three nuclear motions drive the photophysics, namely the coordination sphere asymmetric breathing, the well-documented pseudo Jahn-Teller (PJT) distortion and the bending of the phen ligands. This theoretical study reveals the limit of the static picture based on potential energy surfaces minima and transition states for interpreting the luminescent and TADF properties of this class of molecules. Whereas minor asymmetric Cu-N bonds breathing accompanies the metal-to-ligand-charge-transfer re-localization over one or the other phen ligand, the three nuclear movements participate to the flattening of the electronically excited complexes. This leads to negligible energy barriers whatever the ligand X for the first process and significant ligand dependent energy barriers for the formation of the flattened conformers. Born-Oppenheimer (BO) dynamics simulation of the structural evolution on the T1 PES over 11 ps at 300 K confirms the fast backwards and forwards motion of the phenanthroline within 200-300 fs period and corroborates the presence of metastable C2 structures.

Ultrafast processes: coordination chemistry and quantum theory.

Coordination compounds, characterized by fascinating and tunable electronic properties, are capable of binding easily to proteins, polymers, wires and DNA. Upon irradiation, these molecular systems develop functions finding applications in solar cells, photocatalysis, luminescent and conformational probes, electron transfer triggers and diagnostic or therapeutic tools. The control of these functions is activated by the light wavelength, the metal/ligand cooperation and the environment within the first picoseconds (ps). After a brief summary of the theoretical background, this perspective reviews case studies, from 1st row to 3rd row transition metal complexes, that illustrate how spin-orbit, vibronic coupling and quantum effects drive the photophysics of this class of molecules at the early stage of the photoinduced elementary processes within the fs-ps time scale range.

Excited-state dynamics of [Mn(im)(CO)3(phen)]+: PhotoCORM, catalyst, luminescent probe?

Mn(I) α-diimine carbonyl complexes have shown promise in the development of luminescent CO release materials (photoCORMs) for diagnostic and medical applications due to their ability to balance the energy of the low-lying metal-to-ligand charge transfer (MLCT) and metal-centered (MC) states. In this work, the excited state dynamics of [Mn(im)(CO)3(phen)]+ (im = imidazole; phen = 1,10-phenanthroline) is investigated by means of wavepacket propagation on the potential energy surfaces associated with the 11 low-lying Sn singlet excited states within a vibronic coupling model in a (quasi)-diabatic representation including 16 nuclear degrees of freedom. The results show that the early time photophysics (<400 fs) is controlled by the interaction between two MC dissociative states, namely, S5 and S11, with the lowest S1-S3 MLCT bound states. In particular, the presence of S1/S5 and S2/S11 crossings within the diabatic picture along the Mn-COaxial dissociative coordinate (qMn-COaxial) favors a two-stepwise population of the dissociative states, at about 60-70 fs (S11) and 160-180 fs (S5), which reaches about 10% within 200 fs. The one-dimensional reduced densities associated with the dissociative states along qMn-COaxial as a function of time clearly point to concurrent primary processes, namely, CO release vs entrapping into the S1 and S2 potential wells of the lowest luminescent MLCT states within 400 fs, characteristics of luminescent photoCORM.

Phosphorescent Cationic Heterodinuclear IrIII /MI Complexes (M=CuI , AuI ) with a Hybrid Janus-Type N-Heterocyclic Carbene Bridge.

A novel class of phosphorescent cationic heterobimetallic IrIII /MI complexes, where MI =CuI (4) and AuI (5), is reported. The two metal centers are connected by the hybrid bridging 1,3-dimesityl-5-acetylimidazol-2-ylidene-4-olate (IMesAcac) ligand that combines both a chelating acetylacetonato-like and a monodentate N-heterocyclic carbene site coordinated onto an IrIII and a MI center, respectively. Complexes 4 and 5 have been prepared straightforwardly by a stepwise site-selective metalation with the zwitterionic [(IPr)MI (IMesAcac)] metalloproligand (IPr=1,3-(2,6-diisopropylphenyl)-2H-imidazol-2-ylidene) and they have been fully characterized by spectroscopic, electrochemical, and computational investigation. Complexes 4 and 5 display intense red emission arising from a low-energy excited state that is located onto the "Ir(C^N)" moiety featuring an admixed triplet ligand-centered/metal-to-ligand charge transfer (3 IL/1 MLCT) character. Comparison with the benchmark mononuclear complexes reveals negligible electronic coupling between the two distal metal centers at the electronic ground state. The bimetallic systems display enhanced photophysical properties in comparison with the parental congeners. Noteworthy, similar non-radiative rate constants have been determined along with a two-fold increase of radiative rate, yielding brightly red-emitting cyclometalating IrIII complexes. This finding is ascribed to the increased MLCT character of the emitting state in complexes 4 and 5 due to the smaller energy gap between the 3 IL and 1 MLCT manifolds, which mix via spin-orbit coupling.

Non-Symmetrical Sterically Challenged Phenanthroline Ligands and Their Homoleptic Copper(I) Complexes with Improved Excited-State Properties.

A strategy is presented to improve the excited state reactivity of homoleptic copper-bis(diimine) complexes CuL2 + by increasing the steric bulk around CuI whereas preserving their stability. Substituting the phenanthroline at the 2-position by a phenyl group allows the implementation of stabilizing intramolecular π stacking within the copper complex, whereas tethering a branched alkyl chain at the 9-position provides enough steric bulk to rise the excited state energy E00 . Two novel complexes are studied and compared to symmetrical models. The impact of breaking the symmetry of phenanthroline ligands on the photophysical properties of the complexes is analyzed and rationalized thanks to a combined theoretical and experimental study. The importance of fine-tuning the steric bulk of the N-N chelate in order to stabilize the coordination sphere is demonstrated. Importantly, the excited state reactivity of the newly developed complexes is improved as demonstrated in the frame of a reductive quenching step, evidencing the relevance of our strategy.

Spin-Vibronic Mechanism for Intersystem Crossing.

Intersystem crossing (ISC), formally forbidden within nonrelativistic quantum theory, is the mechanism by which a molecule changes its spin state. It plays an important role in the excited state decay dynamics of many molecular systems and not just those containing heavy elements. In the simplest case, ISC is driven by direct spin-orbit coupling between two states of different multiplicities. This coupling is usually assumed to remain unchanged by vibrational motion. It is also often presumed that spin-allowed radiationless transitions, i.e. internal conversion, and the nonadiabatic coupling that drives them, can be considered separately from ISC and spin-orbit coupling owing to the vastly different time scales upon which these processes are assumed to occur. However, these assumptions are too restrictive. Indeed, the strong mixing brought about by the simultaneous presence of nonadiabatic and spin-orbit coupling means that often the spin, electronic, and vibrational dynamics cannot be described independently. Instead of considering a simple ladder of states, as depicted in a Jablonski diagram, one must consider the more complicated spin-vibronic levels. Despite the basic ideas being outlined in the 1960s, it is only with the advent of high-level theory and femtosecond spectroscopy that the importance of the spin-vibronic mechanism for ISC in both fundamental as well as applied research fields has been revealed with significant impact across chemistry, physics, and biology. In this review article, we present the theory and fundamental principles of the spin-vibronic mechanism for ISC. This is followed by empirical rules to estimate the rate of ISC within this regime. The most recent developments in experimental techniques, theoretical methods, and models for the spin-vibronic mechanism are discussed. These concepts are subsequently illustrated with examples, including the ISC mechanisms in transition metal complexes, small organic molecules, and organic chromophores.

Excited-State Reactivity of [Mn(im)(CO)3 (phen)]+ : A Structural Exploration.

The electronic excited state reactivity of [Mn(im)(CO)3 (phen)]+ (phen = 1,10-phenanthroline; im = imidazole) ranging between 420 and 330 nm have been analyzed by means of relativistic spin-orbit time-dependent density functional theory and wavefunction approaches (state-average-complete-active-space self-consistent-field/multistate CAS second-order perturbation theory). Minimum energy conical intersection (MECI) structures and connecting pathways were explored using the artificial force induced reaction (AFIR) method. MECIs between the first and second singlet excited states (S1 /S2 -MECIs) were searched by the single-component AFIR (SC-AFIR) algorithm combined with the gradient projection type optimizer. The structural, electronic, and excited states properties of [Mn(im)(CO)3 (phen)]+ are compared to those of the Re(I) analogue [Re(im)(CO)3 (phen)]+ . The high density of excited states and the presence of low-lying metal-centered states that characterize the Mn complex add complexity to the photophysics and open various dissociative channels for both the CO and imidazole ligands. © 2018 Wiley Periodicals, Inc.

Charge-Transfer versus Charge-Separated Triplet Excited States of [ReI (dmp)(CO)3 (His124)(Trp122)]+ in Water and in Modified Pseudomonas aeruginosa Azurin Protein.

A computational investigation of the triplet excited states of a rhenium complex electronically coupled with a tryptophan side chain and bound to an azurin protein is presented. In particular, by using high-level molecular modeling, evidence is provided for how the electronic properties of the excited-state manifolds strongly depend on coupling with the environment. Indeed, only upon explicitly taking into account the protein environment can two stable triplet states of metal-to-ligand charge transfer or charge-separated nature be recovered. In addition, it is also demonstrated how the rhenium complex plus tryptophan system in an aqueous environment experiences too much flexibility, which prevents the two chromophores from being electronically coupled. This occurrence disables the formation of a charge-separated state. The successful strategy requires a multiscale approach of combining molecular dynamics and quantum chemistry. In this context, the strategy used to parameterize the force fields for the electronic triplet states of the metal complex is also presented.

Intriguing Effects of Halogen Substitution on the Photophysical Properties of 2,9-(Bis)halo-Substituted Phenanthrolinecopper(I) Complexes.

Three new copper(I) complexes [Cu(LX)2]+(PF6-) (where LX stands for 2,9-dihalo-1,10-phenanthroline and X = Cl, Br, and I) have been synthesized in order to study the impact of halogen substituents tethered in the α position of the chelating nitrogen atoms on their physical properties. The photophysical properties of these new complexes (hereafter named Cu-X) were characterized in both their ground and excited states. Femtosecond ultrafast spectroscopy revealed that early photoinduced processes are faster for Cu-I than for Cu-Cl or Cu-Br, both showing similar behaviors. Their electronic absorption and electrochemical properties are comparable to benchmark [Cu(dmp)2]+ (where dmp stands for 2,9-dimethyl-1,10-phenanthroline); furthermore, their optical features were fully reproduced by time-dependent density functional theory and ab initio molecular dynamics calculations. All three complexes are luminescent at room temperature, showing that halogen atoms bound to positions 2 and 9 of phenanthroline are sufficiently bulky to prevent strong interactions between the excited Cu complexes and solvent molecules in the coordination sphere. Their behavior in the excited state, more specifically the extent of the photoluminescence efficiency and its dependence on the temperature, is, however, strongly dependent on the nature of the halogen. A combination of ultrafast transient absorption spectroscopy, temperature-dependent steady-state fluorescence spectroscopy, and computational chemistry allows one to gain a deeper understanding of the behavior of all three complexes in their excited state.

Structural and Optical Properties of Metal-Nitrosyl Complexes.

The electronic, structural and optical properties (including Spin-Orbit Coupling) of metal nitrosyl complexes [M(CN)5(NO)]2- (M = Fe, Ru or Os) are investigated by means of Density Functional Theory, TD-DFT and MS-CASPT2 based on an RASSCF wavefunction. The energy profiles connecting the N-bound (η1-N), O-bound (η1-O) and side-on (η2-NO) conformations have been computed at DFT level for the closed shell singlet electronic state. For each structure, the lowest singlet and triplet states have been optimized in order to gain insight into the energy profiles describing the conformational isomerism in excited states. The energetics of the three complexes are similar-with the N-bound structure being the most stable-with one exception, namely the triplet ground state of the O-bound isomer for the iron complex. The conformation isomerism is highly unfavorable in the S0 electronic state with the occurrence of two energy barriers higher than 2 eV. The lowest bands of the spectra are assigned to MLCTNO/LLCTNO transitions, with an increasing MLCT character going from iron to osmium. Two low-lying triplet states, T1 (MLCTNO/LLCTNO) and T2 (MLCTNO/ILNO), seem to control the lowest energy profile of the excited-state conformational isomerism.

Absorption Spectroscopy and Photophysics of a ReI -dppz Probe for DNA-Mediated Charge Transport.

Optical properties of [Re(CO)3 (dppz)(py)]+ (dppz=dipyrido[3,2-a:2',3'-c]phenazine; py=pyridine) in acetonitrile, water and DNA have been investigated based on DFT, time-dependent-DFT (TD-DFT)/ conductor-like screening model, with and without explicit solvent molecules, and molecular dynamics. Whereas implicit solvent model is not appropriate to model optical properties of dppz-substituted metal complexes, adding explicit solvent molecules in interaction with dppz stabilizes the metal-to-ligand-charge-transfer (MLCT) transitions. Classical molecular dynamics simulations point to an important conformational flexibility, as evidenced by the coexistence of two conformers A and B. When considering the conformational sampling, the lowest band of the absorption spectrum is red-shifted and broadened up to 500 nm in agreement with the experimental spectra supporting important dynamical effects. The absorption spectra of [Re(CO)3 (dppz)(py-R)]+/ GC-DNA and [Re(CO)3 (dppz)(py-R)]+ /AT-DNA (R=CH2 -CH2 -COO- ) intercalated in both major or minor grooves exhibit a lowest energy charge separated (CS) band at about 600 nm and 500 nm, respectively, corresponding mainly to excitations from guanine and adenine to dppz. These states may play a central role into DNA-mediated charge transport processes. The over stabilization of the lowest 3 ILdppz state of [Re(CO)3 (dppz)(py)]+ in water as compared to acetonitrile could be responsible for the quenching of emission in water.

Electrochemical Generation and Spectroscopic Characterization of the Key Rhodium(III) Hydride Intermediates of Rhodium Poly(bipyridyl) H2-Evolving Catalysts.

We previously reported that the [RhIII(dmbpy)2Cl2]+ (dmbpy = 4,4'-dimethyl-2,2'-bipyridine) complex is an efficient H2-evolving catalyst in water when used in a molecular homogeneous photocatalytic system for hydrogen production with [RuII(bpy)3]2+ (bpy = 2,2'-bipyridine) as photosensitizer and ascorbic acid as sacrificial electron donor. The catalysis is believed to proceed via a two-electron reduction of the Rh(III) catalyst into the square-planar [RhI(dmbpy)2]+, which reacts with protons to form a Rh(III) hydride intermediate that can, in turn, release H2 following different pathways. To improve the current knowledge of these key intermediate species for H2 production, we performed herein a detailed electrochemical investigation of the [RhIII(dmbpy)2Cl2]+ and [RhIII(dtBubpy)2Cl2]+ (dtBubpy = 4,4'-di- tert-butyl-2,2'-bipyridine) complexes in CH3CN, which is a more appropriate medium than water to obtain reliable electrochemical data. The low-valent [RhI(Rbpy)2]+ and, more importantly, the hydride [RhIII(Rbpy)2(H)Cl]+ species (R = dm or dtBu) were successfully electrogenerated by bulk electrolysis and unambiguously spectroscopically characterized. The quantitative formation of the hydrides was achieved in the presence of weak proton sources (HCOOH or CF3CO3H), owing to the fast reaction of the electrogenerated [RhI(Rbpy)2]+ species with protons. Interestingly, the hydrides are more difficult to reduce than the initial Rh(III) bis-chloro complexes by ∼310-340 mV. Besides, 0.5 equiv of H2 is generated through their electrochemical reduction, showing that Rh(III) hydrides are the initial catalytic molecular species for hydrogen evolution. Density functional theory calculations were also performed for the dmbpy derivative. The optimized structures and the theoretical absorption spectra were calculated for the initial bis-chloro complex and for the various rhodium intermediates involved in the H2 evolution process.

Luminescent Dinuclear Copper(I) Complexes as Potential Thermally Activated Delayed Fluorescence (TADF) Emitters: A Theoretical Study.

The excited state properties of a series of binuclear NHetPHOS-Cu(I) complexes (NHetPHOS) have been investigated by means of density functional theory (DFT) and time-dependent 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 state 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 the solid point to the reduction of nuclear flexibility and consequently to a rather simple two-state model.

Interstate vibronic coupling constants between electronic excited states for complex molecules.

In the construction of diabatic vibronic Hamiltonians for quantum dynamics in the excited-state manifold of molecules, the coupling constants are often extracted solely from information on the excited-state energies. Here, a new protocol is applied to get access to the interstate vibronic coupling constants at the time-dependent density functional theory level through the overlap integrals between excited-state adiabatic auxiliary wavefunctions. We discuss the advantages of such method and its potential for future applications to address complex systems, in particular, those where multiple electronic states are energetically closely lying and interact. We apply the protocol to the study of prototype rhenium carbonyl complexes [Re(CO)3(N,N)(L)]n+ for which non-adiabatic quantum dynamics within the linear vibronic coupling model and including spin-orbit coupling have been reported recently.