Yu Harabuchi

Photophysics of cytosine tautomers: new insights into the nonradiative decay mechanisms from MS-CASPT2 potential energy calculations and excited-state molecular dynamics simulations

A comprehensive picture of the ultrafast nonradiative decay mechanisms of three cytosine tautomers (amino-keto, imino-keto, and amino-enol forms) is revealed by high-level ab initio potential energy calculations using the multistate (MS) CASPT2 method and also by on-the-fly excited-state molecular dynamics simulations employing the CASSCF method. To obtain a reliable potential energy profile along the deactivation pathways, the MS-CASPT2 method is employed even for the optimization of minimum energy structures in the excited state and conical intersection (CI) structures between the ground and excited states. In the imino (imino-keto) form, we locate a new CI structure involving the twisting of the imino group, and the decay pathway leading to this CI is found to be barrierless, suggesting a remarkably efficient deactivation of imino cytosine. In the keto (amino-keto) form, the MS-CASPT2 calculations exhibit an efficient decay path to the ethylene-like CI involving the twisting of the C-C double bond in the six-membered ring, with a barrier of ∼0.08 eV from the minimum of the (1)ππ* state. In the enol (amino-enol) form, three types of CIs are identified for the first time. Among them, the ethylene-like CI with a similar molecular structure to the keto form provides the most preferred deactivation pathway in enol cytosine. This pathway exhibits a higher barrier of ∼0.22 eV and a higher energy of CI than those of keto cytosine. Nonadiabatic molecular dynamics simulations provide a time-dependent picture of the deactivation processes, including the excited-state lifetime of each tautomer. In particular, the decay time of the imino tautomer is predicted to be only ∼100 fs. Our computational results are in remarkably good agreement with the experimental findings of recent femtosecond pump-probe photoionization spectroscopy [J. Am. Chem. Soc., 2009, 131, 16939; J. Phys. Chem. A, 2011, 115, 8406], supporting the coexistence of more than one tautomer in the photophysics of isolated cytosine and that each tautomer exhibits a different excited-state lifetime.

Systematic exploration of minimum energy conical intersection structures near the Franck-Condon region.

Locating accessible conical intersections (CIs), especially minimum energy CI (MECI) structures, near the Franck-Condon (FC) region is one of the most important tasks in theoretical analyses of photoreactions. Many MECIs may exist around a FC point in molecules with many vibrational degrees of freedom. Usually, MECIs are optimized one by one starting from arbitrary chosen initial structures. In order to eliminate the arbitrariness, we have developed automated MECI search methods. In this paper, a new approach is described. It combines the seam model function approach with the recently proposed single-component artificial force induced reaction method. Starting from a FC point, the present method finds MECIs systematically. It requires neither a Hessian nor a derivative coupling vector. In an example of the automated search, the spin-flip TDDFT was employed as an efficient electronic structure calculation method, which, together with an automated algorithm to recognize proper electronic states, allowed for evaluation of energy and gradient in a black-box fashion. The present approach was tested with trans- and cis-1,3-butadiene, thymine, and coumarin molecules. The usefulness of the present approach was demonstrated by comparing obtained MECIs with those in the literature. It is hoped that the present technique will be useful in exploration of unknown photoreaction pathways.

Automated Search for Minimum Energy Conical Intersection Geometries between the Lowest Two Singlet States S0/S1-MECIs by the Spin-Flip TDDFT Method

Automated search for minimum energy conical intersection geometries between the lowest two singlet states (S0/S1-MECIs) was performed by combining the anharmonic downward distortion following (ADDF) method, the seam model function (SMF) approach, and the spin-flip (SF) TDDFT method. SMF/ADDF has been employed previously in automated searches for MECIs on potential energy surfaces (PESs) with expensive multireference methods. In this work, we adopt the SF-TDDFT method that enables efficient optimization of S0/S1-MECIs in the framework of TDDFT. To evaluate the performance of the present approach, it was applied to ethylene and 1,3-butadiene. The present method automatically gave unknown S0/S1-MECIs as well as all previously reported ones. Therefore, the present hybrid method of SMF/ADDF and SF-TDDFT is shown to be a promising approach to locate S0/S1-MECIs of large systems automatically with modest computational costs.

Dynamics Simulations with Spin-Flip Time-Dependent Density Functional Theory: Photoisomerization and Photocyclization Mechanisms of cis-Stilbene in ππ* States

On-the-fly dynamics simulations were carried out using spin-flip time dependent density functional theory (SF-TDDFT) to examine the photoisomerization and photocyclization mechanisms of cis-stilbene following excitation to the ππ* state. A state tracking method was devised to follow the target state among nearly degenerate electronic states during the dynamics simulations. The steepest descent path from the Franck-Condon structure of cis-stilbene in the ππ* state is shown to reach the S1-minimum of 4,4-dihydrophenanthrene (DHP) via a cis-stilbene-like structure (referred to as (S1)cis-min) on a very flat region of the S1-potential energy surface. From the dynamics simulations, the branching ratio of the photoisomerization is calculated as trans:DHP = 35:13, in very good agreement with the experimental data, trans:DHP = 35:10. The discrepancy between the steepest descent pathway and the significant trans-stilbene presence in the branching ratio observed experimentally and herein computationally is clarified from an analysis of geometrical features along the reaction pathway, as well as the low barrier of 0.1 eV for the pathway from (S1)cis-min to the twisted pyramidal structure on the S1-potential energy surface. It is concluded that ππ*-excited cis-stilbene propagates primarily toward the twisted structural region due to dynamic effects, with partial branching to the DHP structural region via the flat-surface region around (S1)cis-min.

Artificial Force Induced Reaction (AFIR) Method for Exploring Quantum Chemical Potential Energy Surfaces.

In this account, a technical overview of the artificial force induced reaction (AFIR) method is presented. The AFIR method is one of the automated reaction-path search methods developed by the authors, and has been applied extensively to a variety of chemical reactions, such as organocatalysis, organometallic catalysis, and photoreactions. There are two modes in the AFIR method, i.e., a multicomponent mode and a single-component mode. The former has been applied to bimolecular and multicomponent reactions and the latter to unimolecular isomerization and dissociation reactions. Five numerical examples are presented for an Aldol reaction, a Claisen rearrangement, a Co-catalyzed hydroformylation, a fullerene structure search, and a nonradiative decay path search in an electronically excited naphthalene molecule. Finally, possible applications of the AFIR method are discussed.

Exploring the Mechanism of Ultrafast Intersystem Crossing in Rhenium(I) Carbonyl Bipyridine Halide Complexes: Key Vibrational Modes and Spin–Vibronic Quantum Dynamics

The mechanism of ultrafast intersystem crossing in rhenium(I) carbonyl bipyridine halide complexes Re(X)(CO)3(bpy) (X = Cl, Br, I) is studied by exploring the structural deformations when going from Franck-Condon (FC) to critical geometries in the low-lying singlet and triplet excited states and by selecting the key vibrational modes. The luminescent decay observed in [Re(Br)(CO)3(bpy)] is investigated by means of wavepacket propagations based on the multiconfiguration time-dependent Hartree (MCTDH) method. The dominant coordinates underlying the nonradiative decay process are extracted from minima, minimum energy seam of crossing (MESX) and minimum energy conical intersection (MECI) geometries obtained by the seam model function (SMF)/single-component artificial force induced reaction (SC-AFIR) approach. By choosing the normal modes used in MCTDH from the MECI and MESX geometries, not only the degenerate energy points but also the low-energy-gap regions are included. For this purpose a careful vibrational analysis is performed at each critical geometry and analyzed under the light of the pertinent nonadiabatic coupling terms obtained from the linear vibronic coupling (LVC) model augmented by spin-orbit coupling (SOC) in the electronic diabatic representation.

A multireference perturbation study of the NN stretching frequency of trans-azobenzene in nπ* excitation and an implication for the photoisomerization mechanism

A multireference second-order perturbation theory is applied to calculate equilibrium structures and vibrational frequencies of trans-azobenzene in the ground and nπ* excited states, as well as the reaction pathways for rotation and inversion mechanism in the nπ* excited state. It is found that the NN stretching frequency exhibits a slight increase at the minimum energy structure in the nπ* state, which is explained by the mixing of the NN stretching mode with the CN symmetric stretching mode. We also calculate the NN stretching frequency at several selected structures along the rotation and inversion pathways in the nπ* state, and show that the frequency decreases gradually along the rotation pathway while it increases by ca. 300 cm(-1) along the inversion pathway. The frequencies and energy variations along the respective pathways indicate that the rotation pathway is more consistent with the experimental observation of the NN stretching frequency in nπ* excitation.

Time-Resolved Photoelectron Spectroscopy of Dissociating 1,2-Butadiene Molecules by High Harmonic Pulses.

Using 42 nm high harmonic pulses, the dissociation dynamics of 1,2-butadiene was investigated by time-resolved photoelectron spectroscopy (TRPES), enabling us to observe dynamical changes of multiple molecular orbitals (MOs) with higher temporal resolution than conventional light sources. Because each lower-lying occupied MO has particular spatial electron distribution, the structural dynamics of photochemical reaction can be revealed. On the femtosecond time scale, a short-lived excited state with a lifetime of 37 ± 15 fs and the coherent oscillation of the photoelectron yield stimulated by Hertzberg-Teller coupling were observed. Ab initio molecular dynamics simulations in the electronically excited state find three relaxation pathways from the vertically excited structure in S1 to the ground state, and one of them is the dominant relaxation pathway, observed as the short-lived excited state. On the picosecond time scale, the photoelectron yields related to the C-C bond decreased upon photoexcitation, indicating C-C bond cleavage.

Contrasting ring-opening propensities in UV-excited α-pyrone and coumarin

The photoisomerisation dynamics following excitation to the S1 electronic state of two structurally related heterocyclic molecules, α-pyrone and coumarin, in acetonitrile solution have been probed by time-resolved vibrational absorption spectroscopy. Following irradiation at 310 nm, α-pyrone relaxes rapidly from its initially excited state, with a quantum yield for parent molecule reformation of 68%. Probing the antisymmetric ketene stretch region between 2100 cm(-1) and 2150 cm(-1) confirms the presence of at least two isomeric ring-opened photoproducts, which are formed highly vibrationally excited and relax on a picosecond timescale. Following vibrational cooling, a secondary, thermally driven, isomerisation is observed with a 1.8(1) ns time constant. In contrast, coumarin reforms the parent molecule with essentially 100% efficiency following excitation at 330 nm. The conical intersections driving the non-radiative relaxation of α-pyrone have been investigated using an automated search algorithm. The two lowest energy conical intersections possess remarkably similar structures to the two energetically accessible conical intersections reported previously for coumarin, suggesting that the differing photochemistry is the result of dynamical effects occurring after passage through these intersections.

Implementation and performance of the artificial force induced reaction method in the GRRM17 program

This article reports implementation and performance of the artificial force induced reaction (AFIR) method in the upcoming 2017 version of GRRM program (GRRM17). The AFIR method, which is one of automated reaction path search methods, induces geometrical deformations in a system by pushing or pulling fragments defined in the system by an artificial force. In GRRM17, three different algorithms, that is, multicomponent algorithm (MC‐AFIR), single‐component algorithm (SC‐AFIR), and double‐sphere algorithm (DS‐AFIR), are available, where the MC‐AFIR was the only algorithm which has been available in the previous 2014 version. The MC‐AFIR does automated sampling of reaction pathways between two or more reactant molecules. The SC‐AFIR performs automated generation of global or semiglobal reaction path network. The DS‐AFIR finds a single path between given two structures. Exploration of minimum energy structures within the hypersurface in which two different electronic states degenerate, and an interface with the quantum mechanics/molecular mechanics method, are also described. A code termed SAFIRE will also be available, as a visualization software for complicated reaction path networks.