Theoretical Understanding on the Facilitated Photoisomerization of a Carbonyl Supported Borane System

A Computational Study Revealing the Unexplored Mechanisms of the Photo-isomerization of 4H-imidazole N-oxides and the Reverse Thermal Isomerization of their Photo-Products

The current computational study explores the photo-isomerization mechanisms of 4H-imidazole N-oxide systems leading to the formations of their experimentally reported photoproducts. Additionally, the reverse thermal isomerization reactions are also investigated. Quantum mechanical studies on 5-phenyl-4,4-dimethyl-4H-imidazole 3-oxides (IMO) and 1,3-dioxides(IMDO) with substitutions at 2-position reveal their excited state decay pathways forming the oxaziridine and trans,cis-dioxaziridine photoproducts, respectively. The vertically excited S2 state of 2-methyl-substituted IMO undergoes S2/S1 and S0/S1 conical intersections(CIs) through an upward CNO twist to form the respective oxaziridine. On the other hand, in the 2-phenyl-substituted IMO, a pathway through three consecutive CIs (S3/S2,S2/S1,S0/S1) leads to a downward twisted oxaziridine. The methyl-substituted IMDO is found to form both upward and downward twisted oxaziridines through low-lying S0/S1 CIs. The S0-S2 photo-excitation of these oxaziridines gives rise to their respective trans-dioxaziridines. The energy barriers of the oxaziridine→parent nitrone conversion processes are much lower in the studied IMO systems(27-33 kcal/mol) than their 2H-imidazole N-oxide analogues(~38 kcal/mol) which supports the experimentally reported faster thermal isomerizations of the former. These barrier heights in 4H-imidazole 3-oxides decrease from 2-methyl to 2-phenyl-substituted systems, and in the latter, it decreases with the increasing electron donating ability of the groups on phenyl (order of barrier height:-NO2>-OMe>-NMe2).

Revisiting fulgide photochromism: Mechanistic decoding and electron transport from computational exploration

The photochromic behavior of the fulgide molecule relies on ring-closure and ring-opening processes involving conical intersections during excited state transformation between isomers. The precise location and topography of these conical intersections significantly shape the decay process and fluorescence phenomena inherent to the molecule. This work combines electronic structure theory calculations using the density functional theory and wavefunction methods, as well as surface hopping simulation to analyze the photochemical behavior of an experimentally synthesized fulgide molecule, (E)-p-methylacetophenylisopropylidenesuccinic anhydride (1E). Our study reveals the conical intersection between the first excited state (S1) and the ground electronic state (S0), which emerges beyond the S1 minimum of 1E to the ring-closing side. The distinctive topography of this conical intersection appears to be sloped. These findings suggest a reduced quantum yield for the formation of the closed isomer, indicating a higher likelihood of reformation of the open isomer(s). The surface hopping simulation further supports this observation, revealing a mere ∼8% quantum yield for the formation of the closed isomer. In addition, the photoisomerization of the fulgide molecule initiates a cascade of conduction switching and holds great potential for applications in molecular electronics. Delving into the realm of molecular electronics, we have further examined the electron transport properties, disclosing the higher conductivity of the closed isomer.