Ultrafast spectroscopic interrogation of structural dynamics and disorder of excited states in conjugated organic polymers
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The spectroscopy and dynamics of conjugated polymers have been widely studied in recent years, but there is limited empirical data that directly probes the role of structural dynamics and changes in conjugation length that occur during excited-state relaxation. An important challenge in the study of conjugated organic materials is to relate the properties of transient states that underlie macroscopic material responses directly with intra and intermolecular structure. We have studied the photo-induced relaxation dynamics of conjugated polymers (poly-(3-hexyl-thiophene) (P3HT) and poly-(3-cyclohexane-4-methyl-thiophene) (PCMT)) in solution using femtosecond broadband transient absorption spectroscopy (TAS), femtosecond stimulated-Raman spectroscopy (FSRS) and pump-dump-probe transient hole-burning spectroscopy. FSRS in particular provides a new perspective on dynamics of excited states according to the time-dependence of vibrational modes. Time-dependent studies with P3HT show that there are two different types of excited-state Raman signatures, which have been identified as modes along and peripheral to the exciton’s backbone according to the time dependence of these features and from comparisons to Raman spectra of other states of the polymer. Comparison of spectra collected at different Raman pump wavelengths shows that the C=C stretching feature peaks at lower energies when interrogated with lower energy Raman pump pulses, which is known as Raman dispersion and reflects variations in effective conjugation length probed. Studies of PCMT using FSRS show that there is time-dependent Raman dispersion: the C=C vibrational peak shifts to lower wavenumber on the time scales of 100-200 femtoseconds. This time-dependent Raman dispersion is consistent with an increase in ring-to-ring planarity and effective conjugation length, and offers a distinct spectroscopic signature of structural rearrangement in a polymer excited state. Transient hole-burning spectroscopy of RRa-P3HT in solution facilitated by population dumping through wavelength-selective stimulated emission exposes inhomogeneous broadening of the exciton absorption band in the near infrared. Dump-induced spectral diffusion of the exciton absorption band reflects structural fluctuations in locally excited polymer regions. This diffusion is observed to occur on timescales comparable to those observed in the non-equilibrium relaxation that follows direct excitation of the polymer (8-9 ps).