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A detailed insight behind the structure of absorption bands of the photochromic couple dimethyldihydropyrene (DHP)/metacyclophanediene (CPD) is studied employing vibronic coupling theory. Two separate model molecular Hamiltonians, including a maximum of four electronic states and 18 vibrational modes for DHP and five electronic states and 20 vibrational modes for CPD, are constructed in a diabatic electronic representation. The parameters of the Hamiltonians are estimated from the electronic energies obtained from extensive density functional theory (DFT) and time-dependent DFT calculations. Based on these Hamiltonians' parameters, a detailed analysis of potential energy curves is performed in conjunction with positional and energetic locations of several stationary points in multi-dimensional potential energy surfaces. Based on the results of electronic structure calculations, quantum nuclear dynamics studies on the electronic excited states of DHP and CPD are performed to understand the impact of non-adiabatic effects on the formation of vibronic structures of absorption bands of these photo-isomers. [ABSTRACT FROM AUTHOR]

Based on 280 reference vertical transition energies of various excited states (singlet, triplet, valence, Rydberg, n → π*, π → π*, and double excitations) extracted from the QUEST database, we assess the accuracy of complete-active-space third-order perturbation theory (CASPT3), in the context of molecular excited states. When one applies the disputable ionization-potential-electron-affinity (IPEA) shift, we show that CASPT3 provides a similar accuracy as its second-order counterpart, CASPT2, with the same mean absolute error of 0.11 eV. However, as already reported, we also observe that the accuracy of CASPT3 is almost insensitive to the IPEA shift, irrespective of the transition type and system size, with a small reduction in the mean absolute error to 0.09 eV when the IPEA shift is switched off. [ABSTRACT FROM AUTHOR]

A mechanism is proposed for the ultrafast nonradiative relaxation from the second excited state ([D.sub.2]) down to the ground ([D.sub.0]) of naphthalene radical cation [N.sup.} to explain its photostability. The results reveal an ultrafast, unimolecular, radiationless decay mechanism between the optically active second excited state and the ground state of the naphthalene radical cation by internal conversion through two consecutive sloped conical intersections.

A mechanism for the photochemical conversion of 2-vinyl-1,3-terphenyl to 8,9a-dihydrophenanthrene is presented based on ab initio restricted active space self-consistent field calculations and a molecular mechanics-valence bond dynamics simulation of a model system, the syn isomer of 2-vinylbiphenyl. An extended crossing seam between the ground and first excited electronic states was found to be largely responsible for the efficient photocyclization of the photochemically active syn isomer.

A phenanthrene unit has been functionalized by several methylthiophene units in order to bring it a photochromic behavior. These compounds were characterized by NMR, absorption and emission spectroscopies, theoretical calculations as well as cyclic voltammetry. The association of a phenanthrene group with a photochromic center could open the door to a new generation of organic field‐effect transistors. [ABSTRACT FROM AUTHOR]

A dimethyldihydropyrene (DHP) photochromic unit has been functionalized by donor (triphenylamine group) and acceptor (methylpyridinium) substituents. This compound was characterized by NMR, absorption and emission spectroscopies as well as cyclic voltammetry, and its properties were rationalized by theoretical calculations. The incorporation of both electron‐donor and ‐withdrawing groups at the photochromic center allows i) an efficient photo‐isomerization of the system when illuminated at low energy (quantum yield: Φc‐o=13.3 % at λex=660 nm), ii) the reversible and quantitative formation of two endoperoxyde isomers when illuminated under aerobic conditions at room temperature, and iii) the storage and production of singlet oxygen. The photo‐isomerization mechanism was also investigated by spin‐flip TD‐DFT (SF‐TD‐DFT) calculations. [ABSTRACT FROM AUTHOR]

We describe our efforts of the past few years to create a large set of more than 500 highly accurate vertical excitation energies of various natures (π → π*, n → π*, double excitation, Rydberg, singlet, doublet, triplet, etc.) in small‐ and medium‐sized molecules. These values have been obtained using an incremental strategy which consists in combining high‐order coupled cluster and selected configuration interaction calculations using increasingly large diffuse basis sets in order to reach high accuracy. One of the key aspects of the so‐called QUEST database of vertical excitations is that it does not rely on any experimental values, avoiding potential biases inherently linked to experiments and facilitating theoretical cross comparisons. Following this composite protocol, we have been able to produce theoretical best estimates (TBEs) with the aug‐cc‐pVTZ basis set for each of these transitions, as well as basis set corrected TBEs (i.e., near the complete basis set limit) for some of them. The TBEs/aug‐cc‐pVTZ have been employed to benchmark a large number of (lower‐order) wave function methods such as CIS(D), ADC(2), CC2, STEOM‐CCSD, CCSD, CCSDR(3), CCSDT‐3, ADC(3), CC3, NEVPT2, and so on (including spin‐scaled variants). In order to gather the huge amount of data produced during the QUEST project, we have created a website (https://lcpq.github.io/QUESTDB%5fwebsite) where one can easily test and compare the accuracy of a given method with respect to various variables such as the molecule size or its family, the nature of the excited states, the type of basis set, and so on. We hope that the present review will provide a useful summary of our effort so far and foster new developments around excited‐state methods. This article is categorized under:Electronic Structure Theory > Ab Initio Electronic Structure Methods [ABSTRACT FROM AUTHOR]

The design of photoswitchable transition metal complexes with tailored properties is one of the most important challenges in chemistry. Studies explaining the underlying mechanisms are, however, scarce. Herein, the early relaxation dynamics towards NO photoisomerization in trans‐[RuCl(NO)(py)4]2+ is elucidated by means of non‐adiabatic dynamics, which provided time‐resolved information and branching ratios. Three deactivation mechanisms (I, II, III) in the ratio 3:2:4 were identified. Pathways I and III involve ultrafast intersystem crossing and internal conversion, whereas pathway II involves only internal conversion. [ABSTRACT FROM AUTHOR]

The design of photoswitchable transition metal complexes with tailored properties is one of the most important challenges in chemistry. Studies explaining the underlying mechanisms are, however, scarce. Herein, the early relaxation dynamics towards NO photoisomerization in trans‐[RuCl(NO)(py)4]2+ is elucidated by means of non‐adiabatic dynamics, which provided time‐resolved information and branching ratios. Three deactivation mechanisms (I, II, III) in the ratio 3:2:4 were identified. Pathways I and III involve ultrafast intersystem crossing and internal conversion, whereas pathway II involves only internal conversion. [ABSTRACT FROM AUTHOR]

The photophysics of the perylene radical cation (Pe•+) was studied using the molecular mechanics-valence bond (MMVB) hybrid force field. Potential energy surfaces of the first three electronic states were investigated. Geometry optimizations of critical points—including conical intersections between the relevant electronic states—were performed using the MMVB analytical energy gradient for cations. No accessible planar conical intersection between the D0 and D1 states of Pe•+ was found; this is consistent with the experimentally observed D1 lifetimes and the observation of D1 emission from this cation in the condensed phase. Benchmark RASSCF and TD-DFT calculations support the reliability of the MMVB results. [ABSTRACT FROM AUTHOR]

We present in this article a detailed study of the photophysics of the Ru(bpy)32+ complex based on a formalism using density functional theory and an a posteriori treatment of the spin-orbit coupling. The absorption and emission spectra were computed and a very good agreement was obtained with the available experimental data. This work also reveals for the first time that the triplet metal-centered (MC) dd state cannot deactivate radiatively and corresponds to a transient structure for the nonradiative de-excitation of the triplet metal-to-ligand charge transfer (MLCT) state. Thus, a competition takes place between the luminescence of the MLCT state and nonradiative decay back to the ground state via population of the MC state. This radiationless return to the ground state occurs via a deactivation funnel which has been characterized for the first time by optimizing the minimum energy crossing point between the triplet MC and singlet ground state potential energy surfaces. Its low-lying energy position relative to the energy necessary to access the MC minimum suggests that this funnel will be readily accessible. [ABSTRACT FROM AUTHOR]

We present an extension of the molecular mechanics-valence bond (MMVB) hybrid method to study ground and excited states of planar conjugated hydrocarbon cations. Currently, accurate excited state calculations on these systems are limited to expensive ab initio studies of smaller systems: up to 15 active electrons in 16 π orbitals with complete active space self-consistent field (CASSCF) theory using high symmetry. The new MMVB extension provides a faster, cheaper treatment to investigate larger cation systems with more than 24 active orbitals. Extension requires both new matrix elements and new parameters: In this paper we present both, for the limited planar case. The scheme is tested for the planar radical cations of benzene, naphthalene, anthracene, and phenanthrene. Calculated MMVB relative energies are in good agreement with CASSCF results for equilibrium geometries on the ground and first excited states, and conical intersections. [ABSTRACT FROM AUTHOR]

We propose a computational strategy that enables ionic and covalent ππ* excited states to be described in a balanced way. This strategy depends upon (1) the restricted active space self-consistent field method, in which the dynamic correlation between core σ and valence π electrons can be described by adding single σ excitations to all π configurations and (2) the use of a new conventional one-electron basis set specifically designed for the description of valence ionic states. Together, these provide excitation energies comparable with more accurate and expensive ab initio methods—e.g., multiconfigurational second-order perturbation theory and multireference configuration interaction. Moreover, our strategy also allows full optimization of excited-state geometries—including conical intersections between ionic and covalent excited states—to be routinely carried out, thanks to the availability of analytical energy gradients. The prototype systems studied are the cis and trans isomers of butadiene and hexatriene, for which the ground 1A1/g, lower-lying dark (i.e., symmetry forbidden covalent) 2A1/g and spectroscopic 1B2/u (valence ionic) states were investigated. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Excited-state dynamics simulations are performed on a Photoactive Yellow Protein chromophore analogue in water. The results of the simulations have shown that in water the chromophore has undergone single-bond photoisomerization, rather than double-bond photoisomerization.

The ground and the lowest triplet excited states of the tris-(1,4,5,8-tetraazaphenanthrene) ruthenium complex [[Ru[(tap).sub.3]].sup.2+] are studied. The electronic structures of the metal-to-ligand charge-transfer states (MLCT) and metal-centered (MC) triplet states are discussed by using density functional theory (DFT) and the MLCT-MC equilibrium is examined.

A new series of excited-state dynamics simulations of the Arg52GIn (R52Q) mutant of PYP is performed to elucidate the role of the arginine in the activation process. The studies have shown that the arginine has reduced the excited-state lifetime, thus playing a major role in photoprotection.

The identification and characterization of the excited-state reaction pathway leading to the formation of the potentially mutagenic thymine dimer in the gas phase are presented. The results have shown that, while the thermally induced [2 + 2] cycloaddition of two stacked thymines proceeds through a highly activated stepwise mechanism on the ground state, the photoreaction has occurred via a barrierless concerted mechanism on a singlet excited state.

A mechanism accounting for the photochromic properties of dimethyldihydropyrene (DHP)/metacyclophanediene (CPD) is proposed based on high level CASSCF/CASPT2 ab initio calculations. The study found that intermediate does not correspond to the lowest excited state minimum is mainly responsible for the inefficiency of this system.

Several ab initio calculations and molecular dynamics simulations of a photoexcited cytosine-guanine pair in gas phase are employed to analyze the dynamics of the ultrafast radiationless deactivation mechanism. The analysis reveals that such a process involves the transfer of a proton from the guanine to cytosine, which is always followed by a radiationless decay of the excited state.

An extensive ab initio study of the ground- and excited-state potential energy surfaces of pyracylene is presented. Complete active space self-consistent field (CASSCF) calculations reveal that there is an accessible sloped S0/S1 conical intersection, which leads to ultrafast internal conversion and explains the observed photostability.

A detailed mechanism for the intramolecular charge-transfer of 4-(dimethylamino)benzonitrile and 4-aminobenzonitrile is presented. The two emitting S1 species are adiabatically linked along the amino torsion reaction coordinate.

The origin of the photochromic properties of the diarylethenes is a conical intersection which are located computationally but dynamic calculations is shown to be necessary to explain why the conical intersection is accessible because the excited-state reaction path is not contained in the branching space defining the intersection. The main experimental observations i.e. picosecond time domain, quantum yield, temperature dependence and fluorescence can be interpreted on the basis of the results.

Unrestricted DFT (UDFT), time‐dependent DFT (TDDFT) and spin‐flip TDDFT (SF‐TDDFT) were used to investigate the potential energy surfaces of the ground and first two electronic excited states of the naphthalene, pyrene and perylene radical cations. In particular, conical intersections (which play a central role in the photophysics of these cations) were located with these DFT‐based methods. The results are consistent with accurate multiconfigurational wavefunction‐based ab initio methods. These show that naphthalene and pyrene cations can quickly relax nonradiatively from their excited states back down to the original ground state species through easily accessible conical intersections, but the perylene cation cannot do so, due to the absence of any accessible funnels between the lowest excited state and the ground state, leaving radiative decay as the most probable photophysical pathway. This study paves the way for using computationally efficient density functional theory (DFT)‐based methods in future investigations of the photophysics of much larger polycyclic aromatic hydrocarbons, for which multiconfigurational wavefunction‐based methods become prohibitively expensive. [ABSTRACT FROM AUTHOR]