International Symposium on Ultrafast Dynamics in Molecular and Material Sciences

Ultrafast near-IR stimulated Raman spectroscopy of photoexcited π-conjugate molecules in solution

Tomohisa Takaya

Department of Chemistry, Faculty of Science, Gakushuin University,

Tokyo 171-8588, Japan E-mail: tomohisa.takaya@gakushuin.ac.jp

Photoconductive polymer molecules in solutions are one of the

important models of isolated one-dimensional π-conjugate systems. The

dynamics of π-electrons in an isolated chain will be strongly affected by the

fluctuation of polymer conformation caused by solvent molecules. Recently, we

have developed an ultrafast stimulated Raman spectrometer for observing the

structural dynamics of neutral and charged excitations in π-conjugate systems

as well as their electronic dynamics 1,2). We applied the spectrometer to a

fundamental photoconductive polymer, poly(3-hexylthiophene) (P3HT),

dissolved in organic solvents and found that the obtained stimulated Raman

spectra are composed of two different transients, singlet excitons and charged

excitations. The singlet excitons are created immediately after the photo-

excitation of P3HT and then the charged excitations are formed from the singlet

excitons with a time constant of around 10 ps. The time constant is well

correlated with the viscosity of solvents, indicating that the formation of

charged excitations is controlled by a fast conformational change of the

polythiophene chain. The results suggest that the functions of photoconductive

polymers will be tunable if the conformation of polymer chains is appropriately

controlled.

Fig. 1 Time-resolved near-IR stimulated Raman spectra of poly(3-hexylthiophene)

(P3HT) in toluene with photoexcitation at 480 nm.

References

1. T. Takaya and K. Iwata, J. Phys. Chem. A 2014, 118, 4071.

2. T. Takaya and K. Iwata, Analyst 2016, 141, 4283.