On the photophysics of 1,6-diphenyl-1,3,5-hexatriene isomers and rotamers.

Herein, the low-lying electronic states of various isomers and rotamers of 1,6-diphenyl-1,3,5-hexatriene (DPH) are studied by a combination of density functional theory and multireference configuration interaction. Starting from the all-trans nuclear arrangement, trans-cis isomerization pathways in the electronic ground state and in the first excited triplet state were determined. Further, spin-orbit coupling calculations were carried out at selected points where singlet-triplet energy gaps are small. The calculations reveal that the primarily excited, optically bright 1(1)B(u) state undergoes a curve crossing with the optically dark multiconfigurational 2(1)A(g) state upon geometry relaxation in the excited state. The strong vibronic coupling of the two singlets in the neighborhood of the conical intersection provides a conclusive explanation for the experimentally observed fast equilibration of the states and the appearance of delayed fluorescence. With regard to the trans-cis isomerization of the central CC bond, the perpendicular conformation is found to represent a maximum on the energy profile not only of the electronic ground state, but also of the low-lying excited states. The lack of a strong driving force along the torsional coordinate explains the low tendency of DPH for isomerization. Finally, the results of our spin-orbit coupling calculations suggest that the intramolecular formation of DPH molecules in the T(1)(1(3)B(u)) state proceeds from the 1(1)B(u) state and involves intermediately the T(2)(1(3)A(g)) state.