Letter pubs.acs.org/JPCL
Structural Relaxation of Photoexcited Quaterthiophenes Probed with Vibrational Specificity Jiawang Zhou, Wenjian Yu, and Arthur E. Bragg* Department of Chemistry, Johns Hopkins University, Baltimore, Maryland 21218, United States
Downloaded by MOUNT ROYAL UNIV on August 27, 2015 | http://pubs.acs.org Publication Date (Web): August 25, 2015 | doi: 10.1021/acs.jpclett.5b01472
S Supporting Information *
ABSTRACT: 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-ofphase (Z) and in-phase (Я) intraring CC 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. Excitedstate 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.
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subpicosecond to picosecond time scales.14−18 However, transient electronic spectroscopies provide limited explicit detail about evolving excited-state structures. In contrast, time-resolved spectroscopy at the vibrational level can serve as a highly sensitive probe of evolution in molecular conformation. Time-resolved Raman spectroscopy with resonant enhancement is a highly suitable method for interrogating the conformational dynamics of excited conjugated materials with vibrational sensitivity:13,3,19 not only does resonant enhancement increase Raman cross sections significantly, resonant Raman spectra are dominated by activity along nuclear coordinates that coincide with the primary structural differences between electronic states; for oligothiophenes, this involves differences between nominal benzoidal and quinoidal structures.20,21 Additionally, Raman frequencies of CC stretches in conjugated oligomers are known to be highly sensitive to the degree of electron delocalization along the conjugated backbone.20−22 Time-resolved resonance Raman spectroscopy (TR3) has been used previously to study dynamics of conjugated oligomeric systems19,23 but notably with coupled time and frequency resolution that obscures observation of critical structural changes on ultrafast (
