Photorelaxation Induced by Water–Chromophore Electron Transfer

Relaxation of photoexcited chromophores is a key factor determining diverse molecular properties, from luminescence to photostability. Radiationless relaxation usually occurs through state intersections caused by distortions in the nuclear geometry of the chromophore. Using excited-state nonadiabatic dynamics simulations based on algebraic diagrammatic construction, it is shown that this is the case of 9H-adenine in water cluster, but not of 7H-adenine in water cluster. 7H-adenine in water cluster relaxes via a state intersection induced by electron transfer from water to the chromophore. This result reveals an unknown reaction pathway, with implications for the assignment of relaxation mechanisms of exciton relaxation in organic electronics. The observation of photorelaxation of 7H-adenine induced by water–chromophore electron transfer is a proof of principle calling for further computational and experimental investigations to determine how common this effect is

Nonadiabatic Deactivation of 9<i>H</i>-Adenine: A Comprehensive Picture Based on Mixed Quantum−Classical Dynamics

Mixed quantum−classical dynamics simulations at the multireference configuration interaction (MR-CIS) level were performed for 9H-adenine in order to understand its ultrafast nonradiative decay process. Dynamics simulations were also performed for the model system 6-aminopyrimidine. MR-CIS and complete active space perturbation theory (CASPT2) have been employed to characterize a large variety of qualitatively different conical intersections, the branches of the crossing seam connecting them, and the reaction paths from the Franck−Condon region for 9H-adenine. The results show a two-step process consisting of ultrashort deactivation from S3 to S1 and a longer exponential decay step corresponding to the conversion from S1 to S0

Can the Nonadiabatic Photodynamics of Aminopyrimidine Be a Model for the Ultrafast Deactivation of Adenine?

The reaction path for the ultrafast deactivation of 6-aminopyrimidine (6AP) has been investigated by means of ab initio surface-hopping dynamics. The electronic vertical excitation spectrum, excited-state S1 minima, and minima on the crossing seam of 6AP resemble well those found for adenine. The deactivation from the S1 to the S0 state takes place at the ultrafast time scale of 400 fs and involves the out-of-plane ring deformation of the C4 atom, a position that is sterically restricted in adenine by the imidazole ring. Mechanical restrictions have been used to simulate in a simple way the role of the imidazole group in adenine. As a result, deactivation via out-of-plane ring deformation of the C2 and N3 atoms are observed in good agreement with predictions for adenine. These dynamics results show that the previously suggested ring puckering deactivation paths really exist at a time scale, which is compatible with experimentally observed life times. The electronic structure of the crossing seam has been shown to have the same nature as those of simple biradicaloid systems, a feature which seems to be valid for any cyclic system

A Hessian-Free Method to Prevent Zero-Point Energy Leakage in Classical Trajectories

The problem associated with the zero-point energy (ZPE) leak in classical trajectory calculations is well known. Since ZPE is a manifestation of the quantum uncertainty principle, there are no restrictions on energy during the classical propagation of nuclei. This phenomenon can lead to unphysical results, such as forming products without the ZPE in the internal vibrational degrees of freedom (DOFs). The ZPE leakage also permits reactions below the quantum threshold for the reaction. We have developed a new Hessian-free method, inspired by the Lowe-Andersen thermostat model, to prevent energy dipping below a threshold in the local-pair (LP) vibrational DOFs. The idea is to pump the leaked energy to the corresponding local vibrational mode taken from the other vibrational DOFs. We have applied the new correction protocol on the ab-initio ground-state molecular dynamics simulation of the water dimer (H2O)2, which dissociates due to unphysical ZPE spilling from high-frequency OH modes. The LP-ZPE method has been able to prevent the ZPE spilling of the OH stretching modes by pumping back the leaked energy into the corresponding modes, while this energy is taken from the other modes of the dimer itself, keeping the system as a microcanonical ensemble

Photophysics and Deactivation Pathways of Thymine

Combined complete active space perturbation theory (CASPT2) and multireference configuration interaction calculations with single and double excitations (MR-CISD) were performed in order to explore possible deactivation pathways of thymine after photoexcitation. Equilibrium geometries are reported together with a total of eight extremes (minima or maxima) on the crossing seam (MXS), through which such radiationless transitions may occur. Furthermore, conformational analysis allows grouping these conical intersections in five distinct types. Reaction paths were calculated connecting the S1 1nπ* minimum with the lowest-energy MXS of each group. Two distinct types of paths were observed, both with features that should delay the internal conversion to the ground state. This is shown to provide a possible explanation for the relatively long excited-state lifetime of thymine

Nonadiabatic Deactivation of 9<i>H</i>-Adenine: A Comprehensive Picture Based on Mixed Quantum−Classical Dynamics

Mixed quantum−classical dynamics simulations at the multireference configuration interaction (MR-CIS) level were performed for 9H-adenine in order to understand its ultrafast nonradiative decay process. Dynamics simulations were also performed for the model system 6-aminopyrimidine. MR-CIS and complete active space perturbation theory (CASPT2) have been employed to characterize a large variety of qualitatively different conical intersections, the branches of the crossing seam connecting them, and the reaction paths from the Franck−Condon region for 9H-adenine. The results show a two-step process consisting of ultrashort deactivation from S3 to S1 and a longer exponential decay step corresponding to the conversion from S1 to S0

Is the Photoinduced Isomerization in Retinal Protonated Schiff Bases a Single- or Double-Torsional Process?

Nonadiabatic photodynamical simulations are presented for the all-trans and 5-cis isomers of the hepta-3,5,7-trieniminium cation (PSB4) with the goal of characterizing the types of torsional modes occurring in the cis-trans isomerization processes in retinal protonated Schiff base (RPSB), the rhodopsin and bacteriorhodopsin chropomhore. Steric hindrance of these processes due to environmental effects have been modeled by imposing different sets of mechanical restrictions on PSB4 and studying its response in the photodynamics. Both the mechanism toward the conical intersection and the initial phase of the hot ground state dynamics has been studied in detail. A total of 600 trajectories have been computed using a complete active space self-consistent field wave function. Careful comparison with higher level methods has been made in order to verify the accuracy of the results. The most important mechanism driving restricted PSB4 isomerization in the excited state is characterized by two concerted twist motions (bipedal and closely related to it nonrigid bipedal) from which only one torsion tends to be continued during the relaxation into the ground state. The one-bond-flip is found to be important for the trans isomer as well. The main isomerization trend is a torsion around C5C6 (equivalent to C11C12 in RPSB) in the case of the cis isomer and around C3C4 (C13C14 in RPSB) in the case of the trans isomer. The simulations show an initial 70 fs relaxation into twisted regions and give an average internal conversion time of 130−140 fs, timings that are fully compatible with the general picture described by femtosecond transient absorption spectroscopic studies

Steady and Time-Resolved Photoelectron Spectra Based on Nuclear Ensembles

Semiclassical methods to simulate both steady and time-resolved photoelectron spectra are presented. These approaches provide spectra with absolute band shapes and vibrational broadening beyond the Condon approximation, using an ensemble of nuclear configurations built either via distribution samplings or nonadiabatic dynamics simulations. Two models to account for the electron kinetic energy modulation due to vibrational overlaps between initial and final states are discussed. As illustrative examples, the steady photoelectron spectra of imidazole and adenine and the time- and kinetic-energy-resolved photoelectron spectrum of imidazole were simulated within the frame of time-dependent density functional theory. While for steady spectra only electrons ejected with maximum allowed kinetic energy need to be considered, it is shown that to properly describe time-resolved spectra, electrons ejected with low kinetic energies must be considered in the simulations as well. The results also show that simulations based either on full computation of photoelectron cross section or on simple Dyson orbital norms provide results of similar quality

Mechanism of Ultrafast Photodecay in Restricted Motions in Protonated Schiff Bases: The Pentadieniminium Cation

Ab initio surface-hopping dynamics simulations for the trans-penta-3,5-dieniminium cation (PSB3) are presented imposing different sets of mechanical restrictions in order to investigate the response of the molecular system to certain environmental degrees of hindrance. A general scheme for classification of photoisomerization mechanisms in conjugated chains based on the analysis of torsional angles is proposed allowing direct characterization of the different isomerization mechanisms proposed previously. On the basis of a statistical analysis of 300 trajectories a new photoisomerization mechanism−the Folding Table−was found. This mechanism and the One-Bond-Flip are almost entirely responsible for the photoisomerization process in PSB3