SYNTHESIS, PHOTOPHYSICAL PROPERTIES, AND VALENCE PHOTOISOMERIZATION OF NEW FLUORESCENT AROMATIC NORBORNADIENES

Four norbornadiene (NBD) aromatic derivatives including electron-donor and electron-acceptor substituents, potentially useful for light ener gy photochemical storage, were synthesized by a Diels-Alder reaction, using a phase-transfer catalysis method. Electronic absorption and the fluorescence excitation and emission spectra were determined at room temperature in solvents of different polarities. The photoisomerizatio n kinetics of NBD aromatic derivatives into quadricyclane (QC) derivat ives were measured in different solvents. It was found that photochemi cal reactivities were rather high, between 0.1 and 0.3 s(-1), and that quantum yields (Phi(p)) were relatively large, ranging from 0.18 to 0 .36. These kinetic parameters were markedly influenced by the aromatic substituents and the solvent. The photochemical results support the h ypothesis of a radical-cation mechanism for the photoisomerization rea ction. The fluorescence quenching efficiency of NBD aromatic derivativ es was investigated. Evidence was found for fluorescence quenching of the fluorophore aromatic cycle (in QC derivatives) by the NBD moiety.