The role of hydrogen bonding in the fluorescence quenching of 2,6-bis((E)-2-(benzoxazol-2-yl)vinyl)naphthalene (BBVN) in methanol.

The excited state hydrogen bonding dynamics of BBVN in hydrogen donating methanol solvent was explored at the TD-BMK/cc-pVDZ level of theory with accounting for the bulk environment effects at the polarizable continuum model (PCM). The heteroatoms of the BBVN laser dye form hydrogen bonds with four methanol molecules. In the formed BBVN-(MeOH) complex, the A-type hydrogen bond (N…HO), of an average strength of 25kJmol, is twofold stronger than the B-type (O…HO) one. Upon photon absorption, the total HB binding energy increases from 78.5kJmol in the ground state to 82.6kJmol in the first singlet (S) excited state. In consequence of the hydrogen bonding interaction, the absorption band maximum of the BBVN-(MeOH) complex, which was anticipated at 398nm (exp. 397), is redshifted by 5nm relative to that of the free dye in methanol. The spectral shift of the stretching vibrational mode for the hydrogen bonded hydroxyl groups (with a maximum shift of 285cm) from that of the free methanol indicated the elevated strengthening of hydrogen bonds in the excited state. The vibrational modes associated with hydrogen bonding provide effective accepting modes for the dissipation of the excitation energy, thus, decreasing the fluorescence quantum yield of BBVN in alcohols as compared to that in the polar aprotic solvents. Since there is no sign of photochemistry or phosphorescence, it seems reasonable in view of the outcomes of this study to assign the major decay process of the excited singlet (S) of BBVN in alcohols to vibronically induced internal conversion (IC) facilitated by hydrogen bonding.