Probing ultrafast reaction mechanisms of photo-excited dithizone through transient absorption spectroscopy and computational CASSCF studies

Abstract

Dithizone, with its derivatives and complexes, is characterized by seven classes of chromic reactions, which make it a remarkable candidate for diverse sensor applications. We now present dithizone’s chromic response to experimental photoexcitation as studied by ultrafast laser spectroscopy, density functional theory (DFT), and complete active space self-consistent field (CASSCF) calculations. After vertical excitation to the first excited state, within 240 fs the reaction proceeds towards an intersystem crossing (ISC) that is coupled with a conical intercept (CI), while the second path proceeds via a second CI. The ISC leads to excited state tautomerization within 1.6 ps to form a triplet excited state product that takes a longer time to return back to reactant ( ∼ 5 n s ), while the first and second CIs both lead to the ground state within 1.6 ps and 12 ps, respectively. Additionally, dynamics through both CIs allow for efficient repopulation of the reactant ground state. Theoretical calculations using minimum energy path analysis and CASSCF calculation (including n-electron valence state perturbation theory multireference perturbation theory correction) gave additional insight into ground and excited state conformational changes that are observed by means of experimental femtosecond laser pulse excitations of dithizone.