Role of Triplet States in the Photodynamics of Aniline

Abstract

The dynamics of excited heteroaromatic molecules is a key to understanding the photoprotective properties of many biologically relevant chromophores that dissipate their excitation energy nonreactively and thereby prevent the detrimental effects of ultraviolet radiation. Despite their structural variability, most heteroaromatic compounds share a common feature of a repulsive 1 πσ* potential energy surface. This surface can lead to photoproducts, and it can also facilitate the population transfer back to the ground electronic state by means of a 1 πσ*/S 0 conical intersection. Here, we explore a hidden relaxation route involving the triplet electronic state of aniline, which has recently been discovered by means of time-selected photofragment translational spectroscopy [J. Chem. Phys. 2019, 151, 141101]. By using the recently available analytical gradients for multiconfiguration pair-density functional theory, it is now possible to locate the minimum energy crossing points between states of different spin and therefore compute the intersystem crossing rates with a multireference method, rather than with the less reliable single-reference methods. Using such calculations, we demonstrate that the population loss of aniline in the T 1 ( 3 ππ * ) state is dominated by C 6 H 5 NH 2 →C 6 H 5 NH⸱ + H⸱ dissociation, and we explain the long nonradiative lifetimes of the T 1 ( 3 ππ * ) state at the excitation wavelengths of 294-264 nm.

Spin Crossover of Thiophosgene via Multidimensional Heavy-Atom Quantum Tunneling

“…This explains the widely varying quality of results obtained with NA-TST/WC that can be found in the literature. 36,53,62,66,107 Instanton theory avoids these issues by locating the optimal reaction mechanism at each temperature/energy of interest in a fully ab initio manner. This has allowed us to unravel the substantial role of multidimensional quantum nuclear tunneling in this reaction, which proceeds by a corner-cutting pathway.…”

“…In some cases, there may be a huge error cancellation from the two approximations, one which broadens and one which narrows the barrier. This explains the widely varying quality of results obtained with NA-TST/WC that can be found in the literature. ,,,, …”

Electronic structure of strongly correlated systems: recent developments in multiconfiguration pair-density functional theory and multiconfiguration nonclassical-energy functional theory

“…MC-PDFT has also been used to calculate the SOC effect in Ce + and CeH + , and MC-PDFT and XMS-PDFT provide a spin-orbit energy closer to the experimental value than does SA-CASSCF. 138 In two recent articles, 139,140 we have used MC-PDFT to study the photodynamics of SOC-promoted intersystem crossing. A greater range of photochemical processes will soon be available to PDFT methods because our group has recently completed the coding for efficient CMS-PDFT analytic gradients, as discussed in the next section.…”

Dipole Moment Calculations Using Multiconfiguration Pair-Density Functional Theory and Hybrid Multiconfiguration Pair-Density Functional Theory

“…MC-PDFT is a multireference method that uses a multiconfigurational wave function as the reference function and computes the total energy from the kinetic energy, density, and on-top pair density of that wave function . While the computational cost of MC-PDFT is lower than that of multireference perturbation theories or multireference configuration interaction, it has been shown to provide good accuracy for calculating the excitation energies, bond energies, reaction barriers, and molecular geometries. − In this paper, we derive an analytical procedure for computing dipole moments by MC-PDFT, we present illustrative applications, and we compare MC-PDFT dipole moments to those computed by complete active space self-consistent field (CASSCF) calculations, complete active space second-order perturbation theory (CASPT2), and multireference configuration interaction with single and double excitations and a Davidson correction for quadrupole excitations (MRCISD+Q). − The applications consist of equilibrium dipole moments of 18 first-row transition-metal diatomics, six polyatomic molecules, and nonequilibrium dipole moments of HF, CO, NO, and AlO for bond-stretched geometries.…”