Electronic structures and dynamics of the excited triplet states of α,ω-diphenylpolyynes

Abstract

The lowest excited triplet (T 1 ) states of diphenylacetylene and several α,ω-diphenylpolyynes (DPY) having two, three, four and six triple bonds were studied using continuous wavetime-resolvedelectron paramagnetic resonance (CW-TREPR), pulsed EPR, and phosphorescencespectroscopy. Linear and planar molecular structures in the T 1 states of DPY were suggested from the magnetophotoselection experiments and observation of a strong 0-0 band in the well-resolved phosphorescencespectra. The spin density distributions, which were obtained by electron spin echo envelope modulation measurements and semi-empirical molecular orbital calculations, and the phosphorescencepolarization normal to the long axis of molecule for the 0-0 bands suggested that the T 1 states of DPY have a 3 B 1u (π x π x * ) electronic configuration. DPY showed an abnormally large |D| value of the zero-field splitting parameters that increased with increasing molecular size. Such an unusual behavior of the D values is interpreted in terms of the spin-orbit interaction between the T 1 and 3 A u (π x π y * ) states mainly due to their energy proximity which is probably characteristic of the excited states for the polyynes. The estimated energy gap between the 3 π x π y * and 3 π x π x * states for DPY ranges from 2900 cm−1 to 1400 cm−1. There existed a good correlation between the acceleration of the nonradiative decay rate constant from the T 1 state and appearance of a ring twisting vibronic band with b 1g symmetry in the phosphorescencespectra for DPY. Therefore, we concluded that the vibronic interaction between the 3 π x π y * and 3 π x π x * states promoted by the b 1g vibration leads to a broadening of the potential surface of the T 1 state which results in the fast radiationless decay to the ground state.