Spectral and electrooptical absorption and emission studies on internally hydrogen bonded benzoxazole 'double' derivatives: 2,5-bis(benzoxazolyl)hydroquinone (BBHQ) and 3,6-bis(benzoxazolyl)pyrocatechol (BBPC). Single versusdouble proton transfer in the excited BBPC revisited

Ground and excited state dipole moments and polarizabilities of 2,5-bis(ben zoxazolyl)hydroquinone (BBHQ) and 3,6-bis(benzoxazolyl)pyrocatechol (BBPC) are determined by means of electrooptical absorption and emission measureme nts. BBHQ is found to exhibit a small, while BBPC a large increase of the s tatic polarizability in the Franck-Condon (FC) excited singlet state. The c hange of the dipole moments upon excitation to the FC state is zero within experimental error. However, both molecules show dipole moments in the fluo rescent states of their phototautomers, of about 5 D, the major component b eing parallel to the long molecular axis. The experimental and theoretical results strongly suggest that the fluorescent species of BBHQ and BBPC are monoketo-tautomers created by single proton transfer in the course of the e xcited state relaxation. For BBPC this was confirmed by a comparative photo physical study with its monomethoxy-derivative (MeBBPC), in which one activ e proton is replaced by the CH, group, These results lead to the revision o f previous conclusions [Grabowska et al,, Chem. Phys, Lett. 177 (1991) 17] stating that BBPC undergoes a double proton transfer in the excited state. For BBHQ the previously found single proton transfer mechanism of tautomeri zation is fully confirmed. (C) 1999 Elsevier Science B.V. All rights reserv ed.