Visualizing Excited-State Dynamics of a Diaryl Thiophene: Femtosecond Stimulated Raman Scattering as a Probe of Conjugated Molecules

Abstract

Conjugated organic polymers based on substituted thiophene units are versatile building blocks of many photoactive materials, such as photochromic molecular switches or solar energy conversion devices. Unraveling the different processes underlying their photochemistry, such as the evolution on different electronic states and multidimensional structural relaxation, is a challenge critical to defining their function. Using femtosecond stimulated Raman scattering (FSRS) supported by quantum chemical calculations, we visualize the reaction pathway upon photoexcitation of the model compound 2-methyl-5-phenylthiophene. Specifically, we find that the initial wavepacket dynamics of the reaction coordinates occurs within the first ≈1.5 ps, followed by a ≈10 ps thermalization. Subsequent slow opening of the thiophene ring through a cleavage of the carbon-sulfur bond triggers an intersystem crossing to the triplet excited state. Our work demonstrates how a detailed mapping of the excited-state dynamics can be obtained, combining simultaneous structural sensitivity and ultrafast temporal resolution of FSRS with the chemical information provided by time-dependent density functional theory calculations.

Resonant broadband stimulated Raman scattering in myoglobin

“…Our results substantiate the potential of this technique as a competitive candidate for the vibrational characterization of complex macromolecules. Moreover, the extension of SRS to the domains of microscopy and time‐resolved studies boosts its significance as a state‐of‐the‐art spectroscopic tool for the biochemical and material sciences.…”

Two-Dimensional Impulsively Stimulated Resonant Raman Spectroscopy of Molecular Excited States

“…This goal requires the correct identification of the excited-state PESs involved in the photoinduced process as well as the mapping of their relative orientations and displacements. A number of different pulse schemes and strategies have been developed to meet these tasks, but the unambiguous identification of vibronic properties, such as quasiparticle couplings, mode-mixing and nonadiabatic effects, remains challenging [5,6]-in particular, on electronically excited states after the system has left the Franck-Condon (FC) regiondue to overlapping signal contributions arising from different physical processes. Linear vibrational techniques, such as infrared and spontaneous Raman spectroscopy, cannot monitor vibrational coherences on the excited states, whereas UV-visible absorption spectroscopy usually lacks the desired structural sensitivity.…”

Ultrafast Dynamics and Vibrational Relaxation in Six-Coordinate Heme Proteins Revealed by Femtosecond Stimulated Raman Spectroscopy

“…This recovery, quantified in Figure 3 , can be interpreted in two different ways: anharmonic mode coupling with a low frequency mode 4 or a picosecond geometrical reorganization able to accommodate the recombination of the distal ligand after its instantaneous photolysis. 39 Notably, the observation of the ν 4 pd hot band only for the Ngb case suggests ν 4 coupling that is different for the two 6-c systems, 42 which would imply different trends for the two 6-c proteins (not observed) in the presence of a ν 4 pd peak shift due to anharmonic coupling. Moreover, in striking contrast to the ν 4 pd dynamics measured in deoxy Mb 42 and in CO-bound Mb, 58 where the ν 4 anharmonic coupling induces an initial ∼4 cm –1 peak red shift followed by recovery, the ν 4 pd peak measured in both 6-c molecules shows a strong (>10 cm –1 ) and monotonic (in time) blue shift (a direct comparison is provided in Figure S2 of the SI ).…”