INFLUENCE OF CYANO SUBSTITUENTS ON THE EFFECTIVE S1 TORSIONAL POTENTIALS OF SYMMETRICAL AND SYMMETRY-DISTURBED DERIVATIVES OF 9,9'-BIANTHRYL IN CONDENSED MEDIA

Optical fluorescence spectra of 9-cyanoanthracene (9CA), 10-cyano-9,9'-bianthryl (CBA), and 10, 10'-dicyano-9,9'-bianthryl (DCB) in 2-methylbutane (2MB) were measured at several temperatures between 120 and 298 K. The 9CA spectra were subjected to a Franck-Condon (FC) analysis based upon a two-state model with eleven normal modes. On the basis of these results a simultaneous FC and band-shape analysis of the temperature-dependent spectra was carried out for CBA and DCB to determine the effective S1 torsional potential. The S1 double-minimum torsional potential of the symmetrical derivative DCB in 2MB has a local barrier at phi = 90-degrees of DELTAV = 290 cm-1, very similar to 9,9'-bianthryl (BA). On the other hand, a strong increase of the local barrier (DELTAV= 680 cm-1) can be deduced for the symmetry-disturbed derivative CBA from the model calculations. The minimum angle phi(min) was found to be 63-degrees and 62-degrees for DCB and CBA, respectively. The fluorescence quantum yields and lifetimes were determined for BA, CBA, and DCB in apolar solvents. The comparison of the calculated radiative transition rates (estimated from absorption spectra) with the experimental values confirms the validity of the Condon approximation. No evidence for an emission from a charge-transfer (CT) state could be found for CBA and DCB in 2MB.