T1, T2 state energies and electron affinities of small α,ω-diphenylpolyenes investigated by anion photodetachment photoelectron spectroscopy and excited-state theory.

Photodetachment photoelectron spectroscopy (PD-PES) on molecular radical anions provides electron affinities and direct spectroscopic access to the lowest triplet states T(1) and T(2) of the corresponding neutral molecules. Herein, we apply this method to investigate trans-stilbene (SB), all-trans-1,4-diphenyl-1,3-butadiene (DPB), and all-trans-1,6-diphenyl-1,3,5-hexatriene (DPH). Our gas-phase T(1) energies agree with previous measurements on condensed crystals. The T(2) state energies are determined in this work for the first time. Experimental electron affinities, neutral excited-state transition energies and intensities are compared to theoretical data calculated by using DFT (ground states) or DFT/MRCI (excited states) methods. We give an explanation why intersystem crossing is of minor importance in all three compounds although the T(2) states are situated close below the S1 states.