title: Ultrafast photo-induced nuclear relaxation of a conformationally disordered conjugated polymer probed with transient absorption and femtosecond stimulated Raman spectroscopies.
abstract: A combination of transient absorption (TAS) and femtosecond stimulated Raman (FSRS) spectroscopies were used to interrogate the photo-induced nuclear relaxation dynamics of poly(3-cyclohexyl,4-methylthiophene) (PCMT). The large difference in inter-ring dihedral angles of ground and excited-state PCMT make it an ideal candidate for studying large-amplitude vibrational relaxation associated with exciton trapping. Spectral shifting in the S1 TA spectra on sub-ps timescales (110 ± 20 and 800 ± 100 fs) is similar to spectroscopic signatures of excited-state relaxation observed with related photoexcited conjugated polymers and which have been attributed to exciton localization and a combination of resonant energy transfer and torsional relaxation, respectively. Measurements made with both techniques reveal fast PCMT S1 decay and triplet formation (τS1 = 25-32 ps), which is similar to the excited-state dynamics of short oligothiophenes and highly twisted polyconjugated molecules. On ultrafast timescales FSRS of S1 PCMT offers a new perspective on the nuclear dynamics that underlie localization of excitons in photoexcited conjugated polymers: Spectral dynamics in the C=C stretching region (1400-1600 cm(-1)) include a red-shift of the in-phase C=C stretching frequency, as well as a change in the relative intensity of in-phase and out-of-phase stretch intensities on a timescale of ∼100 fs. Both changes indicate an ultrafast vibrational distortion that increases the conjugation length in the region of the localized excitation and are consistent with exciton self-localization or trapping. Wavelength-dependent excited-state FSRS measurements further demonstrate that the C=C stretching frequency provides a useful spectroscopic handle for interrogating the degree of delocalization in excited conjugated polymers given the selectivity achieved via resonance enhancement.
authors.affiliation: Department of Chemistry, Johns Hopkins University, 3400 N. Charles St., Baltimore, Maryland 21218, USA.
authors.forename: Wenjian
authors.initials: W
authors.lastname: Yu
authors.affiliation: Department of Chemistry, Johns Hopkins University, 3400 N. Charles St., Baltimore, Maryland 21218, USA.
authors.forename: Paul J
authors.initials: PJ
authors.lastname: Donohoo-Vallett
authors.affiliation: Department of Chemistry, Johns Hopkins University, 3400 N. Charles St., Baltimore, Maryland 21218, USA.
authors.forename: Jiawang
authors.initials: J
authors.lastname: Zhou
authors.affiliation: Department of Chemistry, Johns Hopkins University, 3400 N. Charles St., Baltimore, Maryland 21218, USA.
authors.forename: Arthur E
authors.initials: AE
authors.lastname: Bragg