THE POLARIZED ABSORPTION-SPECTRUM OF 1,3-DIPHENYL-1,2-PROPADIENE - INTERACTION BETWEEN MUTUALLY PERPENDICULAR EQUIVALENT PI-ELECTRONIC SYSTEMS

The polarized UV absorption spectrum of 1,3-diphenyl-1,2-propadiene (D PA) was measured in a stretched polymer film at 101 K, and the polariz ation directions of the electronic absorption bands were determined. P ariser-Parr-Pople (PPP) calculations extended to three dimensions were performed, and the position. intensity, and polarization of each abso rption band of DPA were obtained from the calculations. The observed 2 89.8 and 265.2 nm bands polarized along the longer molecular axis were assigned to the calculated S1 <-- S0 and S3 <-- S0 transitions, respe ctively, and the 237.1 nm band along the shorter molecular axis was as signed to the S4 <-- S0 transition. The results of configuration analy sis indicate that the second transitions (at 260.5 nm) of two styrenes , whose pi-electronic systems are perpendicular to each other, contrib ute equally to the third and fourth transitions observed at 265.2 and 237.1 nm of DPA. In addition, it was clarified that the first absorpti on band of DPA corresponding to Parity's forbidden transition was inte nsified by an increase of the ionization energy of the sp hybridized c arbon atom in the allenic moiety.