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-dibiphenylenebutatriene (DBBT) have been measured and assigned with the aid of semiempirical INDO/S-SCI calculations. Geometries used for the INDO/S calculations were obtained by computing fully optimized structures for the arylbutatrienes with the AM1 Hamiltonian. The theoretical structure for TPBT is in good agreement with published X-ray data. Theory indicates that the nature of S-1 and S-2 depends upon the aryl substituent. For TPBT, S-1 and S-2 are both B-1 and show considerable CI mixing of pi-->pi* (HOMO-->LUMO) and pi'-->pi* (HOMO-1-->LUMO). For BXBT, S-1 (B-1u) is pi-->pi* and S-2 (B-1g) is pi'-->pi*, whereas for DEBT the description 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 theoretically. Inclusion of doubly excited configurations in the INDO/S calculations inverts the order of S-1 and S-2 for TPBT, but does not alter the description of these states for BXBT and DEBT. INDO/S-SCI and ab initio calculations have been carried out for unsubstituted butatriene and 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).