2-STATE MODEL FOR THE PHOTOPHYSICS OF 9,9'-BIANTHRYL - FLUORESCENCE, TRANSIENT-ABSORPTION, AND SEMIEMPIRICAL STUDIES

The photophysics of 9,9'-bianthryl (BA) were investigated by means of fluorescence spectroscopy, nanosecond transient-absorption spectroscop y, and semiempirical calculations. Fluorescence spectra and lifetimes were measured in more than 50 solvents in order to get a detailed pict ure of the solvent dependence. The results show that the fluorescence lifetime is constant in solvents of low polarity (D < 5) and increases with solvent polarity in more polar solvents. Departures from this tr end can be traced to specific solute-solvent interactions. Excited-sta te singlet-singlet absorption spectra were measured in the ultraviolet range and show a marked solvent dependence. In polar solvents, the sp ectrum (lambda(max) = 315 nm) is closely related to those of the radic al ions of both BA and anthracene. The decay rate constant of this ban d is identical with that of the fluorescence emission in a range of so lvents of varying polarity (D > 5), thus providing direct proof of the charge-separated character of the fluorescent state in polar solvents . The 315 nm band is absent in isooctane, indicating that the fluoresc ent state is not of charge-separated character in this case. Semiempir ical calculations were carried out in order to rationalize the experim ental data. Careful consideration of the symmetry character of the ele ctronic states involved and of the solvent effect on these states indi cates that two distinct transitions are responsible for the observed f luorescence emission; in nonpolar solvents, a nonpolar state with D-2 symmetry and a torsion angle that is markedly smaller than 90 degrees is the fluorescent state, whereas in polar solvents fluorescence origi nates from a charge-separated perpendicular state of D-2d symmetry. Th is latter state is responsible for the large solvent effects on fluore scence and singlet-singlet absorption. Triplet-triplet absorption and intersystem-crossing efficiency data were also measured in several sol vents. They too are solvent-dependent but do not show characteristics of charge separation; they rather are influenced by specific solute-so lvent interactions.