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) (-bis-trimethylphosphine) Pt(II) (acetylide-NDI-bis-methyl) , 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.

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 -[ReX(CO)(N^C)], where N^C is a pyridyl benzannulated N-heterocyclic carbene deriving from a (substituted) 2-(pyridin-2-yl)imidazo[1,5-]pyridin-2-ium proligand and X = Cl, Br and I, and display structured red phosphorescence with long-lived ( = 7.

Binuclear Biphenyl Organogold(III) Complexes: Synthesis, Photophysical and Theoretical Investigation, and Anticancer Activity.

A series of four binuclear complexes of general formula [(C^C)Au(Cl)(L^L)(Cl)Au(C^C)], where C^C is 4,4'-diterbutylbiphenyl and L^L is either a bridging diphosphine or 4,4'-bipyridine, are synthetized with 52 to 72 % yield and structurally characterized by X-ray diffraction. The use of the chelating 1,2-diphenylphosphinoethane ligand in a 1 : 2 (P^P):Au stoichiometry leads to the near quantitative formation of a gold double-complex salt of general formula [(C^C)Au(P^P)][(C^C^)AuCl ]. The compounds display long-lived yellow-green phosphorescence with λ in the range of 525 to 585 nm in the solid state with photoluminescence quantum yields (PLQY) up to 10 %.

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.

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.

Towards Optimized Photoluminescent Copper(I) Phenanthroline-Functionalized Complexes: Control of the Photophysics by Symmetry-Breaking and Spin-Orbit Coupling.

The electronic and structural alterations induced by the functionalization of the 1,10-phenanthroline (phen) ligand in [Cu(I) (phen-R)] complexes (R=H, CH, -butyl, alkyl-linkers) and their consequences on the luminescence properties and thermally activated delay fluorescence (TADF) activity are investigated using the density functional theory (DFT) and its time-dependent (TD) extension. It is shown that highly symmetric molecules with several potentially emissive nearly-degenerate conformers are not promising because of low S/S oscillator strengths together with limited or no S/T spin-orbit coupling (SOC). Furthermore, steric hindrance, which prevents the flattening of the complex upon irradiation, is a factor of instability.

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

The ability of [Ru(bpy)(bpym)] (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 Pb. 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 Hg, 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 Ru probe and those of the coordination compounds does not demonstrate a high selectivity due to very similar characteristics of the absorption and emission spectra.

Excited-state dynamics of [Mn(im)(CO)(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)(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 S 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, S and S, with the lowest S-S MLCT bound states.

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).

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) -phen) ] 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 T potential energy surface (PES). The T and S 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.

Phosphorescent Cationic Heterodinuclear Ir /M Complexes (M=Cu , Au ) with a Hybrid Janus-Type N-Heterocyclic Carbene Bridge.

A novel class of phosphorescent cationic heterobimetallic Ir /M complexes, where M =Cu (4) and Au (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 Ir and a M center, respectively. Complexes 4 and 5 have been prepared straightforwardly by a stepwise site-selective metalation with the zwitterionic [(IPr)M (IMesAcac)] metalloproligand (IPr=1,3-(2,6-diisopropylphenyl)-2H-imidazol-2-ylidene) and they have been fully characterized by spectroscopic, electrochemical, and computational investigation.

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 CuL by increasing the steric bulk around Cu 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 E . Two novel complexes are studied and compared to symmetrical models.

Structural and Optical Properties of Metal-Nitrosyl Complexes.

The electronic, structural and optical properties (including Spin-Orbit Coupling) of metal nitrosyl complexes [M(CN)(NO)] (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 (η-N), O-bound (η-O) and side-on (η-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.

Strong Influence of Decoherence Corrections and Momentum Rescaling in Surface Hopping Dynamics of Transition Metal Complexes.

The reliability of different parameters in the surface hopping method is assessed for a vibronic coupling model of a challenging transition metal complex, where a large number of electronic states of different multiplicities are met within a small energy range. In particular, the effect of two decoherence correction schemes and of various strategies for momentum rescaling and treating frustrating hops during the dynamics is investigated and compared against an accurate quantum dynamics simulation. The results show that surface hopping is generally able to reproduce the reference but also that small differences in the protocol used can strongly affect the results.

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

Three new copper(I) complexes [Cu(L)](PF) (where L 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.

Charge-Transfer versus Charge-Separated Triplet Excited States of [Re (dmp)(CO) (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.

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

The electronic excited state reactivity of [Mn(im)(CO) (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 (S /S -MECIs) were searched by the single-component AFIR (SC-AFIR) algorithm combined with the gradient projection type optimizer.

Ultrafast Intersystem Crossing vs Internal Conversion in α-Diimine Transition Metal Complexes: Quantum Evidence.

Whereas third row transition metal carbonyl α-diimine complexes display luminescent properties and possess low-lying triplet metal-to-ligand charge transfer (MLCT) states efficiently accessible by a spin-vibronic mechanism, first row analogues hold low-lying metal-centered (MC) excited states that could quench these properties. Upon visible irradiation, different functions are potentially stimulated, namely, luminescence, electron transfer, or photoinduced CO release, the branching ratio of which is governed by the energetics, the character, and the early time dynamics of the photoactive excited states. Simulations of ultrafast nonadiabatic quantum dynamics, including spin-vibronic effects, of [M(imidazole)(CO)(phenanthroline)] (M = Mn, Re) highlight the role of the metal atom.

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

We previously reported that the [Rh(dmbpy)Cl] (dmbpy = 4,4'-dimethyl-2,2'-bipyridine) complex is an efficient H-evolving catalyst in water when used in a molecular homogeneous photocatalytic system for hydrogen production with [Ru(bpy)] (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 [Rh(dmbpy)], which reacts with protons to form a Rh(III) hydride intermediate that can, in turn, release H following different pathways. To improve the current knowledge of these key intermediate species for H production, we performed herein a detailed electrochemical investigation of the [Rh(dmbpy)Cl] and [Rh(dtBubpy)Cl] (dtBubpy = 4,4'-di- tert-butyl-2,2'-bipyridine) complexes in CHCN, which is a more appropriate medium than water to obtain reliable electrochemical data.

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

Optical properties of [Re(CO) (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.

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.

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.

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 S and T excited state energetics and S ⇔ T 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.