Mechanistic Origin of the Vibrational Coherence Accompanying the Photoreaction of Biomimetic Molecular Switches

Abstract

The coherent photoisomerization of a chromophore in condensed phase is a rare process in which light energy is funneled into specific molecular vibrations during electronic relaxation from the excited to the ground state. In this work, we employed ultrafast spectroscopy and computational methods to investigate the molecular origin of the coherent motion accompanying the photoisomerization of indanylidene-pyrroline (IP) molecular switches. UV/Vis femtosecond transient absorption gave evidence for an excited- and ground-state vibrational wave packet, which appears as a general feature of the IP compounds investigated. In close resemblance to the coherent photoisomerization of rhodopsin, the sudden onset of a far-red-detuned and rapidly blue-shifting photoproduct signature indicated that the population arriving on the electronic ground state after nonadiabatic decay through the conical intersection (CI) is still very focused in the form of a vibrational wave packet. Semiclassical trajectories were employed to investigate the reaction mechanism. Their analysis showed that coupled double-bond twisting and ring inversions, already populated during the excited-state reactive motion, induced periodic changes in π-conjugation that modulate the ground-state absorption after the non-adiabatic decay. This prediction further supports that the observed ground-state oscillation results from the reactive motion, which is in line with a biomimetic, coherent photoisomerization scenario. The IP compounds thus appear as a model system to investigate the mechanism of mode-selective photomechanical energy transduction. The presented mechanism opens new perspectives for energy transduction at the molecular level, with applications to the design of efficient molecular devices.

Influence of the Chemical Design on the Coherent Photoisomerization of Biomimetic Molecular Switches

“…The vibrational coherence is further illustrated by the oscillations seen in the PA signature in the 430-nm trace at times >250 fs ( fig. 1C), as already reported for other IPs [3,4]. The crossover from the SE to the PA signals is in particular well observed at 520 nm (Fig.…”

“…For the previously reported IPs the fluorescence is shorter-lived than the Excited State Absorption (ESA) observed in UV transient absorption experiments [2][3][4]. A more accurate measurement of the residence time in S 1 resides in the crossover between SE and PA.…”

“…The impulsive appearance of the PA signal in the VIS region is a signature of vibrational coherence, which is preserved in NAIP, where the MeO group is removed and the photoreaction is faster, presumably due to an enhanced slope in the S 1 potential energy surface towards the CI. Instead no such signature is observed in dMe-MeO-NAIP, where the longer S 1 lifetime exceeds the typical vibrational coherence lifetime in the IPs [3,4]. Presumably, the nonplanar S 0 geometry of ZW-MeO-NAIP, imposed by the steric hindrance of the Me group at C 5 , accelerates the photoreaction as compared to the quasi-planar dMe-MeO-NAIP.…”

“…We perform transient absorption spectroscopy on two new, cationic members of the family, the NAIP and dMe-MeO-NAIP, which are respectively missing the MeO group on C 6' or the Me group on C 5 . We compare the dynamics to that of ZW-MeO-NAIP [3], itself very similar to MeO-NAIP [4]. In the VIS range, the successive signatures of Stimulated Emission (SE) and Photoproduct Absorption (PA) may be observed, allowing us to pinpoint accurately in time the passage of the system through the CI, as was also observed in Rho [5].…”

“…This is a remarkable biomimetic process in which a significant amount of mechanical energy is delivered into a few specific vibrational modes. The IP molecular family appears as a powerful model system, where combined transient absorption spectroscopy and state-of-the-art modeling (QM/MM) allows a detailed, mechanistic understanding of the coherent dynamics [4]. Several representatives of the IP molecules are now available with various substitutions (see Fig.…”