Excited-state non-radiative decay in stilbenoid compounds:an ab initio quantum-chemistry study on size and substituent effects

In the framework of optoelectronic luminescent materials, non-radiative decay mechanisms are relevant to interpret efficiency losses. These radiationless processes are herein studied theoretically for a series of stilbenoid derivatives, including distyrylbenzene (DSB) and cyano-substituted distyrylbenzene (DCS) molecules in vacuo. Given the difficulties of excited-state reaction path determinations, a simplified computational strategy is defined based on the exploration of the potential energy surfaces (PES) along the elongation, twisting, and pyramidalization of the vinyl bonds. For such exploration, density functional theory (DFT), time-dependent (TD)DFT, and complete-active-space self-consistent field/complete-active-space second-order perturbation theory (CASSCF/CASPT2) are combined. The strategy is firstly benchmarked for ethene, styrene, and stilbene; next it is applied to DSB and representative DCS molecules. Two energy descriptors are derived from the approximated PES, the Franck-Condon energy and the energy gap at the elongated, twisted, and pyramidalized structures. These energy descriptors correlate fairly well with the non-radiative decay rates, which validates our computational strategy. Ultimately, this strategy may be applied to predict the luminescence behavior in related compounds

Excited State Specific Multi-Slater Jastrow Wave Functions

We combine recent advances in excited state variational principles, fast multi-Slater Jastrow methods, and selective configuration interaction to create multi-Slater Jastrow wave function approximations that are optimized for individual excited states. In addition to the Jastrow variables and linear expansion coefficients, this optimization includes state-specific orbital relaxations in order to avoid the compromises necessary in state-averaged approaches. We demonstrate that, when combined with variance matching to help balance the quality of the approximation across different states, this approach delivers accurate excitation energies even when using very modest multi-Slater expansions. Intriguingly, this accuracy is maintained even when studying a difficult chlorine-anion-to-$\pi^{*}$ charge transfer in which traditional state-averaged multi-reference methods must contend with different states that require drastically different orbital relaxations.Comment: 16 pages, 6 figures, 2 table

On the photorelease of nitric oxide by nitrobenzene derivatives; a CASPT2//CASSCF model

Nitroaromatics compounds can photorelease NO after UV absorption. The efficiency of the photoreaction depends on the molecular structure and two features have been pointed out as particularly important for the yield of the process: the presence of methyl groups at the ortho position with respect to the nitro group and the degree of conjugation of the molecule. In the present contribution we provide a theoretical characterization at the CASPT2//CASSCF level of theory of the photorelease of NO for four molecules derived from nitrobenzene through the addition of ortho methyl groups and/or the elongation of the conjugation. Our previously described mechanism obtained for the photorelease of NO in nitrobenzene has been adopted as a model for the process. According to this model, the process proceeds through a reactive singlet-triplet crossing (STC) region that the system can reach from the triplet 3 (π O π*) minimum. The energy barrier that must be surmounted in order to populate the reactive STC can be associated with the efficiency of the photoreaction. The here obtained results display clear differences for the efficiency of the photoreaction in the studied systems, and can be correlated with experimental results. The model thus proves its ability to highlight differences in the photoreaction efficiency for the nitroaromatic compounds studied here

Analytic gradients for state-averaged multiconfiguration pair-density functional theory

Analytic gradients are important for efficient calculations of stationary points on potential energy surfaces, for interpreting spectroscopic observations, and for efficient direct dynamics simulations. For excited electronic states, as are involved in UV–Vis spectroscopy and photochemistry, analytic gradients are readily available and often affordable for calculations using a state-averaged complete active space self-consistent-field (SA-CASSCF) wave function. However, in most cases, a post-SA-CASSCF step is necessary for quantitative accuracy, and such calculations are often too expensive if carried out by perturbation theory or configuration interaction. In this work, we present the analytic gradients for multiconfiguration pair-density functional theory based on SA-CASSCF wave functions, which is a more affordable alternative. A test set of molecules has been studied with this method, and the stationary geometries and energetics are compared to values in the literature as obtained by other methods. Excited-state geometries computed with state-averaged pair-density functional theory have similar accuracy to those from complete active space perturbation theory at the second-order

Light-induced molecular processes in organic-based energy conversion and biomimetic synthesis of natural products

Processes initiated by sunlight are fundamental steps in photovoltaic devices as well as in biosyntheses. The present work investigates the photoinduced processes in organic-based energy conversion materials and biomimetic synthesis of natural products by quantum chemical calculations. The work is performed in close collaboration with experimental groups and enables a deeper understanding of the observations. The detailed knowledge allows to predict the optimal conditions to initiate the photochemical syntheses and the chemical substitution to achieve the desired properties. In the first and second part of the thesis, two classes of molecules commonly used in organic-based optoelectronic devices are considered and potential factors influencing the performance of the optical devices are revealed. In the third part, the photochemical and biomimetic syntheses of two natural products and the details of the complex reaction mechanisms are elucidated. In the first part of the present work the deactivation pathways from the first excited singlet state S1 of thiophene and of small oligothiophenes containing up to four rings are investigated by state-of-the-art quantum chemical methods. For thiophene a low-lying S1/S0 conical intersection seam is easily accessible and drives the fast internal conversion. In the oligothiophenes barriers in combination with fast intersystem crossing channels inhibit this passage. The calculated spin-orbit coupling strength together with the singlet-triplet energy gaps can explain the decreasing triplet and increasing fluorescence quantum yields for growing chain length. The present theoretical results allow a deeper understanding of the deactivation pathways of thiophene and small oligothiophenes and are of potential interest for the photophysics of longer oligothiophenes and polythiophenes used in optoelectronic devices. In the second part the photoinduced dynamics of perylene diimide dyads based on a donor-spacer-acceptor motif are considered. The dyads based on pyridine spacer undergo energy transfer from the donor to the acceptor with near-unity quantum efficiency. In contrast in the dyads with phenyl spacers the energy transfer decreases below 50%, suggesting the presence of a competing electron transfer from the spacer to the donor. However, the measurements indicate that the spacer itself mediates the energy transfer dynamics. Ab initio calculations reveal the existence of bright charge transfer states which enable the energy transfer. This new energy transfer represents a first example that show how electron transfer can be connected to energy transfer for the use in novel photovoltaic devices. Additional experiments and calculations of subsystems demonstrate that the solvation time and not the polarity of the solvent is surprisingly the crucial property of the solvent for the charge and energy transfer dynamics. In the last part the photochemical syntheses of the two natural products intricarene and aplydactone are studied. Intricarene was isolated from a Carribbean coral and according to its proposed biosynthesis it arises from an oxidopyrylium intermediate via an intramolecular 1,3-dipolar cycloaddition. By a combination of experiments and theory it is shown that oxidopyrylium indeed forms under biomimetic and photochemical conditions and that it represents the key intermediate in the complex reaction cascade leading to intricarene. Triplet states as well as conical intersections enable the formation of intricarene and of an intriguing by-product which may constitute a new natural product. In the second part of the last chapter a quantum chemical study of the [2+2] photocycloaddition of dactylone to aplydactone is performed. Both compounds were isolated from a Madagascan sea hare and especially aplydactone exhibits an unprecedented molecular structure. However, for both compounds no total syntheses have been reported yet. According to the proposed biosynthesis, aplydactone is formed by a photochemical [2+2] cycloaddition out of dactylone but attempts to synthesize aplydactone through irradiation of dactylone failed. In the present work quantum chemical calculations elucidate the optimal biomimetic conditions to initiate the photochemical reaction and the different reaction pathways on the excited state potential energy surface are revealed. Overall, the last chapter highlights the importance of weak absorption bands and long-lived triplet states for the photochemical synthesis of natural products

Theoretical characterization of the lowest-energy absorption band of pyrrole

The lowest-energy band of the electronic spectrum of pyrrole has been studied with vibrational resolution by using multiconfigurational second-order perturbation theory (CASPT2) and its multistate extension (MS–CASPT2) in conjunction with large atomic natural orbital-type basis sets including Rydberg functions. The obtained results provide a consistent picture of the recorded spectrum in the energy region 5.5–6.5 eV and confirm that the bulk of the intensity of the band arises from a ππ∗ intravalence transition, in contradiction to recent theoretical claims. Computed band origins for the 3s,3p Rydberg electronic transitions are in agreement with the available experimental data, although new assignments are suggested. As illustrated in the paper, the proper treatment of the valence–Rydberg mixing is particularly challenging for ab initio methodologies and can be seen as the main source of deviation among the recent theoretical results as regards the position of the low-lying valence excited states of [email protected] ; [email protected]

A hybrid approach to excited-state-specific variational Monte Carlo and doubly excited states

We extend our hybrid linear-method/accelerated-descent variational Monte Carlo optimization approach to excited states and investigate its efficacy in double excitations. In addition to showing a superior statistical efficiency when compared to the linear method, our tests on the carbon dimer and cyclopentadiene show good energetic agreement with benchmark methods and experiment, respectively. We also demonstrate the ability to treat double excitations in systems that are too large for a full treatment by selective configuration interaction methods via an application to 4-aminobenzonitrile. Finally, we investigate the stability of state-specific variance optimization against collapse to other states' variance minima and find that symmetry, ansatz quality, and sample size all have roles to play in achieving stability.Comment: 44 pages, 9 figures, 7 tables plus supplementary material, accepted by The Journal of Chemical Physic

Analytic Gradients for Complete Active Space Pair-Density Functional Theory

Analytic gradient routines are a desirable feature for quantum mechanical methods, allowing for efficient determination of equilibrium and transition state structures and several other molecular properties. In this work, we present analytical gradients for multiconfiguration pair-density functional theory (MC-PDFT) when used with a state-specific complete active space self-consistent field reference wave function. Our approach constructs a Lagrangian that is variational in all wave function parameters. We find that MC-PDFT locates equilibrium geometries for several small- to medium-sized organic molecules that are similar to those located by complete active space second-order perturbation theory but that are obtained with decreased computational cost