SYNTHESIS, PHOTOISOMERIZATION, AND MOLECULAR-DYNAMICS OF BIS(9-ANTHRYLOXY)METHANE - COMPARATIVE-STUDY IN SOLUTION AND IN THE GAS-PHASE USING THE SUPERSONIC FREE JET TECHNIQUE

The bichromophoric molecule, bis(9-anthryloxy)methane, in which two anthracene rings are connected by the highly flexible O-CH2-O link has been prepared and was shown to undergo very efficient photocyclomerization in the singlet state (PHI(R) = 0.36) compared to other bisanthracenes. The S0-S1 transition has been studied both in solution, using absorption and fluorescence spectroscopies, and in a supersonic free jet, using fluorescence excitation and dispersed emission spectroscopy. The kinetic data deduced from the temperature dependence of the fluorescence quantum yield in solution indicate a fast nonradiative process with an activation energy of 3.7 kcal/mol. The comparison of the fluorescence excitation with hole burning spectra obtained in a supersonic expansion shows that only one conformer is present in the jet and that the fast nonradiative process responsible for the quenching of fluorescence is taking place in the isolated gas-phase excited molecule with an energy threshold < 1 kcal/mol. This low barrier in the isolated molecule points to the role of the solvent which hinders the molecular motions necessary to reach the geometry leading to the photoproduct.