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

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Abstract
The lowest excited triplet (T1) states of diphenylacetylene and several α,ω-diphenylpolyynes (DPY) having two, three, four and six triple bonds were studied using continuous wave time-resolved electron paramagnetic resonance (CW-TREPR), pulsed EPR, and phosphorescence spectroscopy. Linear and planar molecular structures in the T1 states of DPY were suggested from the magnetophotoselection experiments and observation of a strong 0-0 band in the well-resolved phosphorescence spectra. The spin density distributions, which were obtained by electron spin echo envelope modulation measurements and semi-empirical molecular orbital calculations, and the phosphorescence polarization normal to the long axis of molecule for the 0-0 bands suggested that the T1 states of DPY have a B1u3(πxπx*) 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 T1 and Au3(πxπy*) states mainly due to their energy proximity which is probably characteristic of the excited states for the polyynes. The estimated energy gap between the πx3πy* and πx3πx* 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 T1 state and appearance of a ring twisting vibronic band with b1g symmetry in the phosphorescence spectra for DPY. Therefore, we concluded that the vibronic interaction between the πx3πy* and πx3πx* states promoted by the b1g vibration leads to a broadening of the potential surface of the T1 state which results in the fast radiationless decay to the ground state.