Substituent Effect in the First Excited Singlet State of Monosubstituted Benzenes

Abstract

sEDA, pEDA, and cSAR descriptors of the substituent effect were determined for >30 monosubstituted benzenes in the first excited singlet S state at the LC-ωB97XD/aug-cc-pVTZ level. It was found that in the S state, the σ- and π-valence electrons are a bit less and a bit more affected, respectively, than in the S state, but basically, the effect in both states remains the same. In the S and S states, the d(C-X) distances to the substituent's first atom and the ring perimeter correlate with the sEDA and pEDA in the appropriate states, respectively. The energies and the gap of the frontier orbitals in the two states are linearly correlated and for the HOMO(S), LUMO(S), and HOMO(S)-LUMO(S) gap correlate also with the pEDA(S) and cSAR(S) descriptors. In all studied correlations, three similar groups of substituents can be distinguished, for which correlations (i) are very good, (ii) deviate slightly, and (iii) deviate significantly. Comparison of the shape of the HOMO(S) and HOMO(S) orbitals shows that for case (i) HOMO orbitals exhibit almost perfect antisymmetry against the benzene plane, for case (ii) the antisymmetry of HOMO in one of the states is either perturbed or changed, and for case (iii) one HOMO state has σ-character.

Unraveling substituent effects on frontier orbitals of conjugated molecules using an absolutely localized molecular orbital based analysis

“…Despite numerous efforts,16–24 Hammett constants still lack a rigorous theoretical basis, especially in the context of analyzing substituent effects on the photophysical properties of conjugated molecules. Also noteworthy are several other empirical constants, including analogs of the Hammett parameter25,26 and quantities defined based on charge population analysis,27,28 as descriptors exclusively for excited-state properties, while their general applicability and usefulness remain unclear.…”

Control of the Photo-Isomerization Mechanism in 3H-Naphthopyrans to Prevent Formation of Unwanted Long-Lived Photoproducts

“…The π-electron-donating character of the methoxy group has been studied recently in both, ground- [ 37 ] and excited state [ 38 ] of monosubstituted benzenes. The electron donating effect of the methoxy group is even greater in the excited S 1 state than in the S 0 state.…”

Ground‐ and Excited‐State Acidity of Analogs of Red Wine Pyranoanthocyanins,

“…In this context, Dobrowolski et al . have recently considered the general problem of developing substituent constants for excited states and provide a nice compilation of references to the use of Hammett constants and other descriptors to correlate substituent effects on p K a * values, rate constants for excited‐state deactivation and other excited‐state properties.…”