Bithiophene exciton relaxes to first excited state within 100 fs

What is it about?

The vibronic structures and nuclear dynamics in the first five excited singlet electronic states of bithiophene (2T) are investigated here to understand the structureless and broad first two electronic absorption bands, and the ultrafast excited state relaxation mechanism and it's time-domain behaviors.

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Why is it important?

2T is chosen in the present study as the oligothiphenes and polythiophenes have emerged recently as potential organic materials for their applications in organic photovoltaics. Therefore, a detail knowledge of both radiative and nonradiative decay mechanism and the factors tuning these processes in the isolated monomer/oligomer should pave the way to improve the performance of the oligothiophene based electronic devices. In this study a model 5x5 diabatic vibronic coupling Hamiltonian is constructed including eighteen vibrational degrees of freedom and the respective schrodinger equation is solved via a wavepacket propagation approach. The theoretical results are in good agreement with the experimental observations. It is found that strong nonadiabatic coupling between S1-S4 and S1-S5 states along totally symmetric modes predominantly responsible for the structureless and broad first absorption band, and overlapping S2, S3, S4 and S5 states form the second absorption band. The photorelaxation from highly excited S5 to the lowest S1 state takes place through a cascade of diabatic population transfers amongst the S1-S4-S5 electronic manifold within first ~100 fs. Totally symmetric C=C stretching, C-S stretching, C-H wagging, ring puckering and inter-ring bending modes collectively drives such relaxation dynamics. Therefore, the detailed knowledge obtained from the present study may be helpful in designing new and better efficient thiophene based electronics.