An Assessment of RASSCF and TDDFT Energies and Gradients on an Organic Donor–Acceptor Dye Assisted by Resonance Raman Spectroscopy

The excitation energies and gradients in the ground and the first excited state of a novel donor–(π-bridge)–acceptor 4-methoxy-1,3-thiazole-based chromophore were investigated by means of MS-RASPT2/RASSCF and TDDFT in solution. Within both methods, the excitation energies strongly depend on the employed equilibrium structures, whose differences can be rationalized in terms of bond length alternation indexes. It is shown that functionals with an increased amount of exact exchange provide the best estimation of the ground and excited state properties. While B3LYP fails to predict the excitation energies due to its intrinsic problems in describing charge transfer (CT) states, the long-range corrected CAM-B3LYP and M06-2X functionals deliver good agreement with the experimental UV/vis absorption spectrum. The calculation of resonance Raman intensity patterns is used to discern which ground and excited state gradients are best. The results clearly evidence that both CAM-B3LYP and RASSCF excited state gradients and energies in combination with CAM-B3LYP ground state gradients are appropriate to describe the CT state of this push–pull chromophore

Controlling the Photophysical Properties of a Series of Isostructural d⁶ Complexes Based on Cr⁰, Mn^I, and Fe^II

Development of first-row transition metal complexes with similar luminescence and photoredox properties as widely used RuII polypyridines is attractive because metals from the first transition series are comparatively abundant and inexpensive. The weaker ligand field experienced by the valence d-electrons of first-row transition metals challenges the installation of the same types of metal-to-ligand charge transfer (MLCT) excited states as in precious metal complexes, due to rapid population of energetically lower-lying metal-centered (MC) states. In a family of isostructural tris(diisocyanide) complexes of the 3d6 metals Cr0, MnI, and FeII, the increasing effective nuclear charge and ligand field strength allow us to control the energetic order between the 3MLCT and 3MC states, whereas pyrene decoration of the isocyanide ligand framework provides control over intraligand (ILPyr) states. The chromium(0) complex shows red 3MLCT phosphorescence because all other excited states are higher in energy. In the manganese(I) complex, a microsecond-lived dark 3ILPyr state, reminiscent of the types of electronic states encountered in many polyaromatic hydrocarbon compounds, is the lowest and becomes photoactive. In the iron(II) complex, the lowest MLCT state has shifted to so much higher energy that 1ILPyr fluorescence occurs, in parallel to other excited-state deactivation pathways. Our combined synthetic-spectroscopic-theoretical study provides unprecedented insights into how effective nuclear charge, ligand field strength, and ligand π-conjugation affect the energetic order between MLCT and ligand-based excited states, and under what circumstances these individual states become luminescent and exploitable in photochemistry. Such insights are the key to further developments of luminescent and photoredox-active first-row transition metal complexes

Theoretical Assessment of Excited State Gradients and Resonance Raman Intensities for the Azobenzene Molecule

The ground state geometries and vibrational frequencies as well as the excitation energies and excited state gradients of the S₁(nπ*) and S₂(ππ*) states of <i>trans</i>- and <i>cis</i>-azobenzene are investigated by several DFT methods, namely B3LYP, PBE, M06-2X, CAM-B3LYP, and ωB97X. Excited state properties and in particular gradients are also assessed using the wave function based methods EOM-CCSD and RASPT2/RASSCF. Comparison with experimental data shows that the B3LYP functional gives the most accurate results for the ground state geometry and vibrational frequencies. The analysis of the vertical excitation energies reveals that the RASPT2 approach provides the most accurate excitation energies with deviations of the order of 0.1 eV. Among the TDDFT methods, the CAM-B3LYP functional shows the best performance on the excitation energies. By assessing the excited state gradients with respect to the reference RASPT2 data, the most accurate gradients are obtained with B3LYP, whereas other functionals as well as the EOM-CCSD and RASSCF calculations give less consistent results. Overall, despite the tendency of B3LYP to underestimate the excitation energies, this functional provides the most balanced description of both ground and excited state properties for both isomers of azobenzene in the Franck–Condon region

Photochemistry and Electron Transfer Kinetics in a Photocatalyst Model Assessed by Marcus Theory and Quantum Dynamics

The present computational study aims at unraveling the competitive photoinduced electron transfer (ET) kinetics in a supramolecular photocatalyst model. Detailed understanding of the fundamental processes is essential for the design of novel photocatalysts in the scope of solar energy conversion that allows unidirectional ET from a light-harvesting photosensitizer to the catalytically active site. Thus, the photophysics and the photochemistry of the bimetallic complex <b>RuCo</b>, [(bpy)₂Ru^II(tpphz)­Co^III(bpy)₂]⁵⁺, where excitation of the ruthenium­(II) moiety leads to an ET to the cobalt­(III), were investigated by quantum chemical and quantum dynamical methods. Time-dependent density functional theory (TDDFT) allowed us to determine the bright singlet excitations as well as to identify the triplet states involved in the photoexcited relaxation cascades associated with charge-separation (CS) and charge-recombination (CR) processes. Diabatic potential energy surfaces were constructed for selected pairs of donor–acceptor states leading to CS and CR along linear interpolated Cartesian coordinates to study the intramolecular ET via Marcus theory, a semiempirical expression neglecting an explicit description of the potential couplings and quantum dynamics (QD). Both Marcus theory and QD predict very similar rate constants of 1.55 × 10¹² – 2.24 × 10¹³ s^–1 and 1.21 × 10¹³–7.59 × 10¹³ s^–1 for CS processes, respectively. ET rates obtained by the semiempirical expression are underestimated by several orders of magnitude; thus, an explicit consideration of electronic coupling is essential to describe intramolecular ET processes in <b>RuCo</b>

Fate of Photoexcited Molecular Antennae - Intermolecular Energy Transfer versus Photodegradation Assessed by Quantum Dynamics

The present computational study aims to unravel the competitive photoinduced intermolecular energy transfer and electron transfer phenomena in a light-harvesting antenna with potential applications in dye-sensitized solar cells and photocatalysis. A series of three thiazole dyes with hierarchically overlapping emission and absorption spectra, embedded in a methacrylate-based polymer backbone, is employed to absorb light over the entire visible region. Intermolecular energy transfer in such antenna proceeds via energy transfer from dye-to-dye and eventually to a photosensitizer. Initially, the ground and excited state properties of the three push–pull-chromophores (e.g., with respect to their absorption and emission spectra as well as their equilibrium structures) are thoroughly evaluated using state-of-the-art multiconfigurational methods and computationally less demanding DFT and TDDFT simulations. Subsequently, the potential energy landscape for the three dyads, formed by the π-stacked dyes as occurring in the polymer environment, is investigated along linear-interpolated internal coordinates to elucidate the photoinduced dynamics associated with intermolecular energy and electron transfer processes. While energy transfer among the dyes is highly desired in such antenna, electron transfer, or rather a light-induced redox chemistry, leading to the degradation of the chromophores, is disadvantageous. We performed quantum dynamical wavepacket calculations to investigate the excited state dynamics following initial light-excitation. Our calculations reveal for the two dyads with adjusted optical properties exclusively efficient intermolecular energy transfer within 200 fs, while in the case of the third dyad intermolecular electron transfer dynamics can be observed. Thus, this computational study reveals that statistical copolymerization of the individual dyes is disadvantageous with respect to the energy transfer efficiency as well as regarding the photostability of such antenna

Structural Control of Photoinduced Dynamics in 4<i>H</i>‑Imidazole-Ruthenium Dyes

The photoinduced dynamics of a series of terpyridine 4<i>H</i>-imidazole-ruthenium complexes, which constitute a new family of panchromatic dyes, is investigated. The dynamics involves two excited states localized within the 4<i>H</i>-imidazole sphere. Upon MLCT excitation, an excited state is populated, which is localized on the central part of the 4<i>H</i>-imidazole ligand caused by its nonplanar conformation. The population of the second excited state is connected to a planarization of the 4<i>H</i>-imidazole ligand as revealed by viscosity-dependent measurements, and the excess electronic charge on the ligand is shifted to the terminal rings. The impact on the photoinduced dynamics of the electronic character of the substituent at the terminal rings and the protonation state of the 4<i>H</i>-imidazole ligand is studied. Significant changes in the lifetime of the excitation and alterations of the decay mechanism are observed depending on the interplay of the electronic character of the substituent and ligand protonation. In a NMe₂ substituted complex, the character of the substituent is changed from electron donating to electron withdrawing upon stepwise protonation, resulting in pH switchable decay mechanism

A Novel Ru(II) Polypyridine Black Dye Investigated by Resonance Raman Spectroscopy and TDDFT Calculations

The optical properties of a new (bipyridine)₂Ru­(4<i>H</i>-imidazole) complex presenting a remarkable broad absorption in the visible range are investigated. The strong overlap of the absorption with the solar radiation spectrum renders the studied complex promising as a black absorber and hence as a starting structure for applications in the field of dye-sensitized solar cells. The correlations between structural and electronic features for the unprotonated and protonated forms are studied by means of UV–vis absorption and resonance Raman (RR) spectroscopy modeled with the help of time-dependent density functional theory (TDDFT) calculations. The absorption spectra show two bands in the visible region, which TDDFT assigns to a metal-to-ligand charge-transfer (MLCT) state and to a superposition of three excited states with MLCT and intraligand charge-transfer character, respectively. Additionally, the analysis of the molecular orbitals and RR spectra in resonance with the first MLCT band shows that the effects of protonation favor a charge-transfer photoexcitation to the 4<i>H</i>-imidazole ligand. The RR spectra simulated for several excitation wavelengths covering the visible region are in excellent agreement with experimental data. In particular, it is noteworthy that the calculations are able to reproduce the wavelength dependence of the RR spectra provided that corrected excitation energies are employed. Interference effects between the electronic states contributing to the RR scattering are small for the investigated complex

Mechanism of Plasmon-Induced Catalysis of Thiolates and the Impact of Reaction Conditions

The conversion of the thiols 4-aminothiophenol (ATP) and 4-nitrothiophenol (NTP) can be considered as one of the standard reactions of plasmon-induced catalysis and thus has already been the subject of numerous studies. Currently, two reaction pathways are discussed: one describes a dimerization of the starting material yielding 4,4′-dimercaptoazobenzene (DMAB), while in the second pathway, it is proposed that NTP is reduced to ATP in HCl solution. In this combined experimental and theoretical study, we disentangled the involved plasmon-mediated reaction mechanisms by carefully controlling the reaction conditions in acidic solutions and vapor. Motivated by the different surface-enhanced Raman scattering (SERS) spectra of NTP/ATP samples and band shifts in acidic solution, which are generally attributed to water, additional experiments under pure gaseous conditions were performed. Under such acidic vapor conditions, the Raman data strongly suggest the formation of a hitherto not experimentally identified stable compound. Computational modeling of the plasmonic hybrid systems, i.e., regarding the wavelength-dependent character of the involved electronic transitions of the detected key intermediates in both reaction pathways, confirmed the experimental finding of the new compound, namely, 4-nitrosothiophenol (TP*). Tracking the reaction dynamics via time-dependent SERS measurements allowed us to establish the link between the dimer- and monomer-based pathways and to suggest possible reaction routes under different environmental conditions. Thereby, insight at the molecular level was provided with respect to the thermodynamics of the underlying reaction mechanism, complementing the spectroscopic results

Mechanism of Plasmon-Induced Catalysis of Thiolates and the Impact of Reaction Conditions

The conversion of the thiols 4-aminothiophenol (ATP) and 4-nitrothiophenol (NTP) can be considered as one of the standard reactions of plasmon-induced catalysis and thus has already been the subject of numerous studies. Currently, two reaction pathways are discussed: one describes a dimerization of the starting material yielding 4,4′-dimercaptoazobenzene (DMAB), while in the second pathway, it is proposed that NTP is reduced to ATP in HCl solution. In this combined experimental and theoretical study, we disentangled the involved plasmon-mediated reaction mechanisms by carefully controlling the reaction conditions in acidic solutions and vapor. Motivated by the different surface-enhanced Raman scattering (SERS) spectra of NTP/ATP samples and band shifts in acidic solution, which are generally attributed to water, additional experiments under pure gaseous conditions were performed. Under such acidic vapor conditions, the Raman data strongly suggest the formation of a hitherto not experimentally identified stable compound. Computational modeling of the plasmonic hybrid systems, i.e., regarding the wavelength-dependent character of the involved electronic transitions of the detected key intermediates in both reaction pathways, confirmed the experimental finding of the new compound, namely, 4-nitrosothiophenol (TP*). Tracking the reaction dynamics via time-dependent SERS measurements allowed us to establish the link between the dimer- and monomer-based pathways and to suggest possible reaction routes under different environmental conditions. Thereby, insight at the molecular level was provided with respect to the thermodynamics of the underlying reaction mechanism, complementing the spectroscopic results

Controlling Excited State Localization in Bichromophoric Photosensitizers via the Bridging Group

A series of photosensitizers comprised of both an inorganic and an organic chromophore are investigated in a joint synthetic, spectroscopic, and theoretical study. This bichromophoric design strategy provides a means by which to significantly increase the excited state lifetime by isolating the excited state away from the metal center following intersystem crossing. A variable bridging group is incorporated between the donor and acceptor units of the organic chromophore, and its influence on the excited state properties is explored. The Franck–Condon (FC) photophysics and subsequent excited state relaxation pathways are investigated with a suite of steady-state and time-resolved spectroscopic techniques in combination with scalar-relativistic quantum chemical calculations. It is demonstrated that the presence of an electronically conducting bridge that facilitates donor–acceptor communication is vital to generate long-lived (32 to 45 μs), charge-separated states with organic character. In contrast, when an insulating 1,2,3-triazole bridge is used, the excited state properties are dominated by the inorganic chromophore, with a notably shorter lifetime of 60 ns. This method of extending the lifetime of a molecular photosensitizer is, therefore, of interest for a range of molecular electronic devices and photophysical applications