Excited State Structure Correlates with Efficient Photoconversion in Unidirectional Motors

Abstract

The design of unidirectional photomolecular motors demands a critical understanding of an ultrafast photochemical isomerization. An intermediate dark excited state mediates the reaction via a conical intersection (CI) with the ground state, but a correlation between molecular structure and photoisomerization efficiency has remained elusive. Here femtosecond stimulated Raman spectroscopy captures vibrational spectra of the dark state in a set of molecular motors bearing different substituents. A direct correlation between isomerization quantum yield, dark state lifetime, and excited state vibrational spectrum is found. Electron withdrawing substituents lead to activity in lower frequency modes, which we correlate with a pyramidalization distortion at the ethylenic axle occurring within 100 fs. This structure is not formed with an electron donating substituent, where the axle retains double bond character. Further structural reorganization is observed and assigned to excited state reorganization and charge redistribution on the sub-picosecond time scale. The correlation of the dark state structure with photoconversion performance suggests guidelines for developing new more efficient motor derivatives.

Effect of charge-transfer enhancement on the efficiency and rotary mechanism of an oxindole-based molecular motor

“…76 In particular, the graing of an electron-poor CN group on the rotor moiety improved the isomerisation QY up to 20% for a 2 nd generation Feringa-type motor also characterized by a longer-lived ($10 ps) S 1 dark state proposed to feature enhanced pyramidalization. 72,77 Here, we observe that the isomerisation QY of 1 increases four-fold compared to the originally reported oxindoles (QYs ca. 2-3%).…”

Boosting Second-Harmonic Generation in Monolayer Rhenium Disulfide by Reversible Laser Patterning

“…Such a unidirectional laser-induced crystal structure change can also be consistently explained by a photochemical process involving an asymmetric excited state, which is typical for photoisomerization of organic molecules. 47,48 Since several earlier studies mainly focused on thermal effects in multilayer MoTe 2 , 36−39 we verified the role of a substrate in the patterning process. Specifically, we used a Si/ SiO 2 substrate to increase the heat dissipation.…”

“…This observation implies that the hypothetical nonadiabatic thermal effect is only able to switch the crystal structure in the direction of 1T′ → 2H, but not in the reversed 2H → 1T′ direction. Such a unidirectional laser-induced crystal structure change can also be consistently explained by a photochemical process involving an asymmetric excited state, which is typical for photoisomerization of organic molecules. , …”

E/Z Molecular Photoswitches Activated by Two-Photon Absorption: Comparison between Different Families

“…Scientists have focused their efforts not only on discovering novel photochemical reactions, but also on taking advantage of the ones already known to build useful building blocks [ 1 , 2 , 3 , 4 , 5 ]. Photoactive molecular devices, such as molecular motors, rotors, or switches, are a clear example [ 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 ]. The relevance of this field in the last decade has been acknowledged by the Nobel Prize in Chemistry 2016 awarded to Jean-Pierre Sauvage, Sir J. Fraser Stoddart and Bernard L. Feringa “for the design and synthesis of molecular machines.” Among photoactive molecular devices, molecular switches are the most widely studied, due to their simpler photoactive mechanism and easier implementation in larger systems for applications [ 6 , 11 , 13 ].…”