Wolfgang Wegener

Effect of substituents on the T1 energy of trans-stilbenes

Abstract Oxygen-induced singlet—triplet absorption and electrochemiluminescence quenching experiments with substituted stilbenes indicate a small influence of monosubstitution or donor—acceptor disubstitution on the triplet energy. This implies a decrease of the S 1 T 1 energy difference particularly in the case of donor—acceptor substituted stilbenes.

Substituentenabhängigkeit bei der direktentrans →cis-Photoisomerisierung von donator-substituierten 4′-Diphenylphosphinyl-trans-stilbenen

From the fluorescence quantum yield ϕf, the fluorescence decay time τfw and the quantum yield ϕtc of thetrans →cis photoisomerization results the fraction α of perpendicular singlet (perp-S1) decaying to ground statetrans-stilbenes1. The α values of the donor substituted 4′-diphenylphosphinyl-trans-stilbenes1 in the solvents toluene andn-propanol are dependent on the donor substituent in1 and the solvent. It is concluded for1 that increasing polarity of the first excited singlet state (tr-S1) promotes thetrans →cis photoisomerization.

Substituent effects on the electronic structure of some styryl methyl sulphones studied by ultraviolet photoelectron spectroscopy

A gas-phase u.v. photoelectron spectroscopic study is reported on some para-substituted styryl methylsulphones in which the para-substituent varies in electronegativity and conjugating ability. The main purpose was to observe the substituent effect on electronic structure. The spectral analysis and the trends in ionization energies relative to the highest MOs show that the substituent has both short- and long- range effects, influencing even the orbitals of the –SO2Me group. Satisfactory linear relationships were found between the spectral data for the three highest MOs and the Hammett σp values, the wavenumbers of the SO2 stretching vibrations, and the 13C n.m.r. shifts for C-1 of the styryl group.