Ultrafast Raman Spectroscopy as a Probe of Local Structure and Dynamics in Photoexcited Conjugated Materials

Abstract

An important challenge in the study of conjugated organic materials is to relate the properties of transient states underlying macroscopic material responses directly with intra- and intermolecular structure. We discuss recent efforts using the vibrational sensitivity of time-resolved Raman spectroscopy to interrogate structural properties of transient excited and charge-separated states in conjugated oligomers and polymers in order to relate them to molecular conformations and material microstructures. We focus on recent work with excited-state Raman spectroscopy that provides mode-specific signatures of structural relaxation in oligo- and polythiophenes, examination of structural heterogeneities associated with exciton localization in different structural motifs of amorphous polymers, and interrogation of correlations between microstructure and properties and dynamics of charge-separated states within polymer aggregates. On the basis of these recent efforts, we provide an outlook for further applying this method to elucidate relationships between the structure and properties of transient states and the photoresponses of conjugated materials.

Femtosecond stimulated Raman line shapes: Dependence on resonance conditions of pump and probe pulses

“…In the past decade, femtosecond stimulated Raman spectroscopy (FSRS) has become a powerful spectroscopic methodology that can provide the equilibrium and non-equilibrium vibrational signatures and track excited-state molecular dynamics with simultaneously high spectral and temporal resolutions [1][2][3][4][5][6][7]. Over recent years, a great variety of chemically and biologically relevant systems have been studied by FSRS spanning from organic photoacids and chromophores [8][9][10][11][12][13][14][15], molecular rotors [16], fluorescent proteins [3,17], photoreceptor proteins [18][19][20][21][22][23][24][25][26], calcium biosensors [4,[27][28][29], metal complexes [30,31], materials [32][33][34], and engineered molecular systems [35,36]. The underlying photophysical and photochemical processes including excited-state proton transfer, charge transfer, vibrational cooling, internal conversion, isomerization, and bond dissociation have been successfully revealed and discussed in the larger context of effectively delineating the structure-energy-function relationships [6,7].…”

Conformational Dynamics Guides Coherent Exciton Migration in Conjugated Polymer Materials: First-Principles Quantum Dynamical Study

“…Vibrational quantum coherence was found to persist on time scales of several hundred femtoseconds [11]. Meanwhile, the role of exciton trapping, exciton-polaron formation, and the response of excitonic species to structural changes remain controversial [8,12].Theoretical analysis of exciton transport in single chains (i.e., quasi-1D systems) generally relies on a Frenkel-Holstein Hamiltonian [13] description for Jaggregates, i.e., head-to-tail aligned molecular aggregates [14]. Besides Förster rate theory [1], quantum-classical studies of Ehrenfest type [15] and Surface-Hopping type [16] have been conducted, all of which are restricted in the treatment of exciton-phonon correlations and may not correctly describe the intricate interplay of electronic delocalization, trapping, and exciton migration.…”

“…Vibrational quantum coherence was found to persist on time scales of several hundred femtoseconds [11]. Meanwhile, the role of exciton trapping, exciton-polaron formation, and the response of excitonic species to structural changes remain controversial [8,12].…”

Triple sum frequency pump-probe spectroscopy of transition metal dichalcogenides

“…Most merits of a pump-probe experiment, such as sensitivity and selectivity, are determined by the choice of a specific probe methodology, of which there are many. [1][2][3][4][5][6][7][8][9][10] The development of Coherent Multidimensional Spectroscopy (CMDS) offers promising possibilities for new probes because CMDS methods can have increased selectivity compared to conventional methods. [11][12][13][14][15][16][17] CMDS uses multiple optical interactions to create a multiple quantum coherence within the material whose optical emission is measured.…”