Structural Relaxation of Photoexcited Quaterthiophenes Probed with Vibrational Specificity
Link to Publication: ACS Publication
Contributing Authors: Jiawang Zhou, Wenjian Yu, Art Bragg
Date Published: August 19, 2015
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) intra-ring C=C stretching features. Frequency shifts occurred on timescales 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 timescales 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 towards 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 intra-ring vibrational modes on interring torsional dihedrals.