Structural Relaxation of Photoexcited Quaterthiophenes Probed with Vibrational Specificity.

Ultrafast structural relaxation of photoexcited 2,2':5',2″:5″,2‴-quaterthiophene (4T) and 3,3‴-Dihexyl-2,2':5',2″:5″,2‴-quaterthiophene (DH4T) in solution were interrogated with femtosecond stimulated Raman spectroscopy (FSRS). Relaxation was observed through time-dependent evolution in frequencies and intensity ratios of out-of-phase (Z) and in-phase (Я) intraring C═C stretching features. Frequency shifts occurred on time scales of 0.4 and 0.86 ps, respectively, dominated by a blue shift in the Z mode (6.2 and 11.5 cm(-1) shifts for 4T and DH4T, respectively). Intensity ratios evolved on similar time scales due to correlated intensity decreases and increases of Z- and Я-mode features. Excited-state quantum-chemical calculations with bithiophene demonstrate that mode frequencies are coupled to the torsional dihedral, such that the spectral evolution observed reflects excited-state relaxation toward a planar conformation. This work demonstrates the power of ultrafast Raman spectroscopy for probing dynamics in photoexcited conjugated materials with structural detail given the parametric dependence of intraring vibrational modes on interring torsional dihedrals.