DETAILED EFFECTS OF ELECTRONIC EXCITATION ON THE HYDROGEN BONDS OF THE BENZOIC ACID DIMER

Abstract

High resolution spectral studies of the benzoic acid dimer in host single crystals have been made to investigate the details of the changes in H-bonding which occur upon electronic excitation. The principal host crystal used was benzene. An increase of H-acceptor ability in the first singlet state is shown by an anomalous H-isotope shift (a red shift upon deuteration). The occurrence of odd parity dimer modes in fluorescence also supports this conclusion. No odd parity modes appear in phosphorescence, demonstrating that the triplet state is not like the singlet. An analysis similar to that of a Jahn-Teller distortion shows that the triplet H-bonding is not very different from that in the ground state while the singlet is locked into an unsymmetrical structure by the H-bonds.