Jason Quenneville

Conical intersections and double excitations in time-dependent density functional theory

There is a clear need for computationally inexpensive electronic structure theory methods which can model excited state potential energy surfaces. Time-dependent density functional theory (TDDFT) has emerged as one of the most promising contenders in this context. Many previous tests have concentrated on vertical excitation energies, which can be compared to experimental absorption maxima. Here, we focus attention on more global aspects of the resulting potential energy surfaces, especially conical intersections which play a key role in photochemical mechanisms. We introduce a new method for minimal energy conical intersection (MECI) searches which does not require knowledge of the non-adiabatic coupling vector. Using this new method, we compute MECI geometries with multi-state complete active space perturbation theory (MS-CASPT2) and TDDFT. We show that TDDFT in the linear response and adiabatic approximations can predict MECI geometries and energetics quite accurately, but that there are a number of qualitative deficiencies which need to be addressed before TDDFT can be used routinely in photochemical problems. †Dedicated to Professor M. A. Robb on the occasion of his 60th birthday.

Photochemistry from first principles — advances and future prospects

Abstract Detailed simulation of photochemistry poses considerable challenges because quantum mechanical effects are important in determining both the electronic potential energy surfaces and the subsequent nuclear dynamics. We provide a brief overview of the ab initio multiple spawning (AIMS) method which addresses the problem by solving both the electronic and nuclear Schrodinger equations simultaneously. We discuss our recent AIMS simulations of cis–trans photoisoimerization in ethylene as an example application. The prospects of the method for modeling of photochemistry in large organic molecules and condensed phases are assessed.

Quantum Chemistry Studies of Electronically Excited Nitrobenzene, TNA, and TNT

The electronic excitation energies and excited-state potential energy surfaces of nitrobenzene, 2,4,6-trinitroaniline (TNA), and 2,4,6-trinitrotoluene (TNT) are calculated using time-dependent density functional theory and multiconfigurational ab initio methods. We describe the geometrical and energetic character of excited-state minima, reaction coordinates, and nonadiabatic regions in these systems. In addition, the potential energy surfaces for the lowest two singlet (S(0) and S(1)) and lowest two triplet (T(1) and T(2)) electronic states are investigated, with particular emphasis on the S(1) relaxation pathway and the nonadiabatic region leading to radiationless decay of S(1) population. In nitrobenzene, relaxation on S(1) occurs by out-of-plane rotation and pyramidalization of the nitro group. Radiationless decay can take place through a nonadiabatic region, which, at the TD-DFT level, is characterized by near-degeneracy of three electronic states, namely, S(1), S(0), and T(2). Moreover, spin-orbit coupling constants for the S(0)/T(2) and S(1)/T(2) electronic state pairs were calculated to be as high as 60 cm(-1) in this region. Our results suggest that the S(1) population should quench primarily to the T(2) state. This finding is in support of recent experimental results and sheds light on the photochemistry of heavier nitroarenes. In TNT and TNA, the dominant pathway for relaxation on S(1) is through geometric distortions, similar to that found for nitrobenzene, of a single ortho-substituted NO(2). The two singlet and lowest two triplet electronic states are qualitatively similar to those of nitrobenzene along a minimal S(1) energy pathway.

Chapter 8 - Azobenzene photoisomerization: Two states and two relaxation pathways explain the violation of Kasha's rule

Azobenzene is a photochromic molecule, which undergoes trans-cis isomerization upon irradiation in the near-UV. The photo-isomerization reaction is ultrafast, efficient, and clean and has attracted much attention for its possible application in molecular electronics, data storage and nonlinear optics. In violation of Kashas rule, the isomerization yields vary anomalously with the excitation wavelength. Two possible reasons have been proposed to explain the violation of Kashas rule. Based on the investigation of quantum yields in substituted azobenzenes, Rau suggested a large deformation of the molecular structure along the torsional coordinate in S 2 , which would quench the isomerization reaction proceeding along the inversion coordinate in S 1 . Fujino et al. could not find evidence for torsional motion in time-resolved fluorescence and resonance-Raman spectra but observed ultrafast S 2 → S 1 internal conversion. To explain the violation of Kashas rule, Fujino assumed that additional relaxation pathways for high vibrational levels in Si could quench the observed quantum yield after excitation of S2. The photoisomerization reaction was studied by time resolved photoelectron spectroscopy (TRPES). It proposes a new relaxation pathway, which can reconcile the two conflicting models. First results were recently published in a communication. This chapter presents an improved data analysis and a detailed discussion of the experimental data.