The symmetric C═C stretching frequency (ν(Я)) of conjugated polymers and oligomers is a sensitive spectroscopic reporter of molecular structure and material morphologies; however, thorough understanding of how structure affects this frequency is lacking because computational investigations of this relationship have been undertaken with limited approaches. We present a comprehensive computational investigation of the structure-dependent Raman spectroscopy of oligothiophenes, oligofurans, and oligopyrroles in their ground electronic states using density functional theory. We assessed how various functionals (BLYP, B3LYP, BHLYP, and CAM-B3LYP) impact predictions of length-dependent trends in ν(Я). The amount of Hartree-Fock exchange in a functional is critical for accurately treating π-delocalization and polarizability and hence the structure-dependent Raman behavior. BLYP and B3LYP fail to accurately predict trends in ν(Я) with oligomer length because they over-represent delocalization; in contrast, the range-corrected CAM-B3LYP functional produces the same trends observed experimentally for oligomers in solution and in the solid phase. Through comparisons with a simple mechanical model, we demonstrate that the length- and conformation-dependent spectroscopy of oligothiophenes results from a delicate balance between delocalization-induced softening of ν(Я) and the coupling of oscillators that increase ν(Я). These findings are used to address how variations in inter- and intramolecular order impact the Raman spectroscopy of polythiophenes.
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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.