Ultrafast motion in a third generation photomolecular motor

Abstract

Controlling molecular translation at the nanoscale is a key objective for development of synthetic molecular machines. Recently developed third generation photochemically driven molecular motors (3GMs), comprising pairs of overcrowded alkenes capable of cooperative unidirectional rotation offer the possibility of converting light energy into translational motion. Further development of 3GMs demands detailed understanding of their excited state dynamics. Here we use time-resolved absorption and emission to track population and coherence dynamics in a 3GM. Femtosecond stimulated Raman reveals real-time structural dynamics as the excited state evolves from a Franck-Condon bright-state through weakly-emissive dark-state to the metastable product, yielding new insight into the reaction coordinate. Solvent polarity modifies the photoconversion efficiency suggesting charge transfer character in the dark-state. The enhanced quantum yield correlates with suppression of a low-frequency flapping motion in the excited state. This detailed characterization facilitates development of 3GMs, suggesting exploitation of medium and substituent effects to modulate motor efficiency.

Fig. 1. Structures of successive generations of unidirectional motors and the unique function of 3GMs.

a Structures of three generations of unidirectional light-driven rotational motors; thicker lines indicate atoms above the plane of the page, light lines below. b Illustration of the potential of cooperative rotation in 3GMs to support translational motion following sequential photochemical (hν) and thermal (Δ) steps localized on axle-1 then axle-2. Colors are a guide to the eye for the motions of the otherwise symmetric rotors (structures from Kistemaker et al.). c Potential energy diagram showing photochemistry driven by light and heat.

Fig. 2. Electronic spectra and excited state dynamics of 3GMph.

a Steady-state absorption (black) and emission (red) spectra of 3GMph in ACN. Inset: chemical structure of 3GMph. b Time-resolved fluorescence measured at 560 nm after excitation at 410 nm. The red line shows the fit to a biexponential function and the bottom panel is the residual, which is oscillatory, indicating a role for coherently excited vibrational modes in excited state dynamics. The grey line shows the instrument response (width 43 fs). Inset: fluorescence dynamics at five emission wavelengths; c Transient vis-NIR absorption spectra at different time delays showing spectral evolution in excited 3GMph; spectra were measured in two spectral windows and stitched together at ca 800 nm. Inset: components of the transient absorption data recovered from global analysis assuming a simple sequential kinetic model with a single intermediate state to yield three evolution-associated difference spectra (EADS); d Population dynamics at four different wavelengths (circles) with quality of fit of the sequential global analysis model shown (solid line).

Fig. 3. Characterization of 3GMph excited state structural dynamics by transient Raman.

a Measured ground state Raman (GSR, pump off) and excited state Raman (ESR, pump on) spectra of 3GMph, the latter at two different Raman-pump wavelengths, 560 nm and 650 nm, both recorded 200 fs after excitation in ACN. The true ESR spectrum at 560 nm has been recovered from the complex lineshape by bleach filling (i.e. ESR_560nm + GSR, See text). Pump pulse at 440 nm; *indicates solvent artifact. b DFT calculated Raman spectra of the stable (black) and metastable (red) forms of the ground state. The computed frequencies were scaled by 0.98. c The bleach filled ESR_560nm spectra at different pump-probe delays (pump 440 nm, Raman-pump at 560 nm; the 650 nm Raman pump data and unfilled data sets are shown in Supplementary Fig. 10); *indicates solvent artifact. d Early time evolution of the bleach filled ESR_560nm in the bimodal ethylenic stretch region (1520–1750 cm⁻¹), here fit by two Gaussians (red at 1565 cm⁻¹ and blue at 1607 cm⁻¹). e Global fitting components of the FSRS data in (c) assuming the same three-state sequential model as for TA with the same fixed time constants.

Fig. 4. Solvent polarity dependence to the 3GMph dynamics.

a Evolution associated difference spectra of bright (top, EADS1) and dark (bottom, EADS2) states b nanosecond absorption spectra of metastable product, which decreases in amplitude between cyclohexane (CHX) and acetonitrile (ACN) solvents c femtosecond stimulated Raman spectra (bleach filled) of stable, metastable, bright and dark-states d impulsive Raman spectra obtained via Fourier transformation of the oscillatory residual from time-resolved fluorescence measurements.