Excited states dynamics of polydiacetylenes: An ab initio and femtosecond spectroscopic investigation of the change from the acetylenic to the butatrienic structure

The configuration change from the acetylenic form =(RC-C=C-CR' =)(x) (x = 1 ,2,3.5) to the butatrienic form -(RC=C=C=CR'-)(x) (x = 1,2,3), considered a s model systems for the two alternative structures of polydiacetylene chain s, has been investigated through complete active space self-consistent fiel d and second-order perturbation approach CASSCF/CASPT2 calculations. The ch aracter and energetics of the low-lying excited states of both structures a re reported. The excited states properties of the oligomers are compared to those of the three-membered ring forms -(C=C-CH=CH-)(3) and -(CH=C=C=CH-)( 3). A qualitative interpretation of the femtosecond time-resolved molecular dynamics of the polydiacetylene backbone is proposed on the basis of wave packet propagations on associated potential energy curves connecting the el ectronic ground and excited states of the two structures in the cyclic form . (C) 2000 American Institute of Physics. [S0021-9606(00)70423-5].