Description
Molecular motors are prototypical nanoscale systems capable of converting external energy into directed motion, offering unique opportunities for light-controlled functionality at the molecular level. Among them, hemithioindigo (HTI) motors have emerged as benchmark systems [1-3] due to their robust, reversible Z–E photoisomerization and their sensitivity to chemical substitution, which enables fine control over photochemical pathways and efficiencies [4-6]. Previous studies reveal that photoisomerization is the most critical step governing the overall performance of the HTI motors, acting as the true kinetic bottleneck of the operational cycle. Consequently, strategies aimed at controlling and accelerating this process, by enhancing the forward quantum yield, are key to achieving faster and more efficient unidirectional rotation [7]. In particular, previous theoretical and spectroscopic studies have shown that HTI photoisomerization is initiated by a sudden electronic polarization following π→π excitation at the Franck-Condon (FC) point, driving the system toward conical intersections (CoIn) that govern ultrafast nonadiabatic relaxation [8,9]. Building on static analyses of ground- and excited-state electron density distributions in HTI derivatives bearing different substituents, recent work has highlighted how subtle changes in electronic structure can strongly influence the photochemical outcome and directionality of the motor cycle. However, a direct, site-specific experimental probe of the evolving electronic structure during these earliest stages has remained largely unexplored.
To access the primary steps of photoisomerization of HTI in the gas phase, we performed time-resolved X-ray photoelectron spectroscopy (TR-XPS) at the sulfur L2,3 edge using the FERMI free-electron laser (FEL) as probe [10]. Following optical excitation with 50 fs pulses centered at 400 nm and photoionization with FEL pulses at 200 eV, we directly monitor transient shifts in core-level binding energies, which act as a sensitive fingerprint of sudden polarization and of evolving local electronic environments. The measurements revealed the population of different excited states, rising with different delays after the photobleaching of the ground state, in agreement with recent core-level spectroscopic studies of photoinduced charge redistribution [11]. The ultrafast relaxation pathways through the manifold of electronic states show pronounced superimposed oscillations with a period of ~200 fs. These oscillations can be attributed to coherent nuclear motion initiated by the π→π excitation, which periodically modulates core-level energies as the molecular structure evolves [11]. As a result, intensity oscillations arise when spectral features move within fixed detection windows, providing direct evidence of strong coupling between electronic and nuclear degrees of freedom.
This study demonstrates that TR-XPS is a powerful, chemically specific technique to probe the earliest electronic and structural dynamics of molecular motors. By directly linking substituent-dependent electronic structure to sudden polarization, nonadiabatic relaxation, and coherent nuclear motion, our results establish TR-XPS as a key tool for elucidating ultrafast photoisomerization mechanisms and guiding the optimization of light-driven molecular machines. Such insights are particularly relevant given the central role of hemithioindigo derivatives in functional applications. Their efficient and fatigue-resistant photoisomerization under visible light makes them suitable for optomechanical actuators, responsive materials, and photopharmacological protocols aimed at controlling biological activity under mild conditions.
References:
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