EXCITED ELECTRONIC STATES OF ARYLBUTATRIENES

Electronic absorption spectra for 1,1,4,4-tetraphenylbutatriene (TPBT) , 1,4-bis(9-xanthylidene)ethene (BXBT), and 1,1,4,4-dibiphenylenebutat riene (DBBT) have been measured and assigned with the aid of semiempir ical INDO/S-SCI calculations. Geometries used for the INDO/S calculati ons were obtained by computing fully optimized structures for the aryl butatrienes with the AM1 Hamiltonian. The theoretical structure for TP BT is in good agreement with published X-ray data. Theory indicates th at the nature of S-1 and S-2 depends upon the aryl substituent. For TP BT, S-1 and S-2 are both B-1 and show considerable CI mixing of pi-->p i (HOMO-->LUMO) and pi'-->pi* (HOMO-1-->LUMO). For BXBT, S-1 (B-1u) i s pi-->pi and S-2 (B-1g) is pi'-->pi*, whereas for DEBT the descripti on of S-1 and S-2 is inverted from that of BXBT. These predictions are supported by several pieces of absorption and luminescence data. The effects of rotating the TPBT phenyl groups have been studied theoretic ally. Inclusion of doubly excited configurations in the INDO/S calcula tions inverts the order of S-1 and S-2 for TPBT, but does not alter th e description of these states for BXBT and DEBT. INDO/S-SCI and ab ini tio calculations have been carried out for unsubstituted butatriene an d are in reasonable agreement. Both methods agree that S-1 is B-1g and that the first allowed electric dipole transition is B-1(1u)<--A(g).