Ultrafast Dynamics in Light-Driven Molecular Rotary Motors Probed by Femtosecond Stimulated Raman Spectroscopy

Abstract

Photochemical isomerization in sterically crowded chiral alkenes is the driving force for molecular rotary motors in nanoscale machines. Here the excited-state dynamics and structural evolution of the prototypical light-driven rotary motor are followed on the ultrafast time scale by femtosecond stimulated Raman spectroscopy (FSRS) and transient absorption (TA). TA reveals a sub-100-fs blue shift and decay of the Franck-Condon bright state arising from relaxation along the reactive potential energy surface. The decay is accompanied by coherently excited vibrational dynamics which survive the excited-state structural evolution. The ultrafast Franck-Condon bright state relaxes to a dark excited state, which FSRS reveals to have a rich spectrum compared to the electronic ground state, with the most intense Raman-active modes shifted to significantly lower wavenumber. This is discussed in terms of a reduced bond order of the central bridging bond and overall weakening of bonds in the dark state, which is supported by electronic structure calculations. The observed evolution in the FSRS spectrum is assigned to vibrational cooling accompanied by partitioning of the dark state between the product isomer and the original ground state. Formation of the product isomer is observed in real time by FSRS. It is formed vibrationally hot and cools over several picoseconds, completing the characterization of the light-driven half of the photocycle.

Mapping the Excited‐State Potential Energy Surface of a Photomolecular Motor

“…Excitation at 450 nm exclusively pumps the S 0 →S 1 transition of 1 b . Figure B shows the TA spectrum for 1 b recorded 100 fs after excitation at 450 nm by 100 fs pulses; it is contrasted with the same spectrum for 1 a excited at 405 nm (which matches that reported for 390 nm excitation). The time‐dependent TA spectra of 1 b are shown for completeness (Figure S1).…”

“…A much more remarkable difference becomes apparent from the comparison of the time dependences (Figure C). The dark state formed on 1 a excitation was previously shown to act as the precursor to formation of the (vibrationally hot) 1 b product with a lifetime of 1.6 ps . A dark state with a similar spectrum is formed following excitation of 1 b (Figure B), but its decay is markedly accelerated, by a factor of approximately 7 (Figure C) compared to that of 1 a .…”

“…Whether the dark states reached by excitation of the different ground states have different structures as well as decay rates is important, and can in principle be resolved by transient vibrational spectroscopy. We recently showed that FSRS reveals details of the relaxation in 1 a from the FC state to the dark state and onwards to 1 b . It is thus interesting to compare the structures of the dark states reached by the different excitation routes.…”

Modeling and analyzing a photo-driven molecular motor system: Ratchet dynamics and non-linear optical spectra

“…It should be noted that, while roles of conical intersections (CIs) in photoisomerization processes of the photo-driven molecular motor system attract much attention experimentally [9][10][11] and theoretically [14,15,17,19], we found the difference between the wavepacket dynamics in internal conversion processes via CI and those via avoided crossing (AC) is minor when the system is subject to a dissipative environment [48]. This fact allows us to study both photoisomerization and thermalization using a single coordinate model with AC.…”

“…Experimentally, circular dichroism spectroscopy has been performed to investigate thermalization processes [6][7][8], and various ultrafast nonlinear laser measurements have been carried out to investigate details of photoisomerization processes [9][10][11]. These experiments aim to analyze dynamics and yields of each isomerization processes, to understand the roles of their substituents and solvents, and to achieve high performance rotation e.g.…”