Chemically Optimizing Operational Efficiency of Molecular Rotary Motors

Abstract

Unidirectional molecular rotary motors that harness photoinduced cis-trans (E-Z) isomerization are promising tools for the conversion of light energy to mechanical motion in nanoscale molecular machines. Considerable progress has been made in optimizing the frequency of ground-state rotation, but less attention has been focused on excited-state processes. Here the excited-state dynamics of a molecular motor with electron donor and acceptor substituents located to modify the excited-state reaction coordinate, without altering its stereochemistry, are studied. The substituents are shown to modify the photochemical yield of the isomerization without altering the motor frequency. By combining 50 fs resolution time-resolved fluorescence with ultrafast transient absorption spectroscopy the underlying excited-state dynamics are characterized. The Franck-Condon excited state relaxes in a few hundred femtoseconds to populate a lower energy dark state by a pathway that utilizes a volume conserving structural change. This is assigned to pyramidalization at a carbon atom of the isomerizing bridging double bond. The structure and energy of the dark state thus reached are a function of the substituent, with electron-withdrawing groups yielding a lower energy longer lived dark state. The dark state is coupled to the Franck-Condon state and decays on a picosecond time scale via a coordinate that is sensitive to solvent friction, such as rotation about the bridging bond. Neither subpicosecond nor picosecond dynamics are sensitive to solvent polarity, suggesting that intramolecular charge transfer and solvation are not key driving forces for the rate of the reaction. Instead steric factors and medium friction determine the reaction pathway, with the sterically remote substitution primarily influencing the energetics. Thus, these data indicate a chemical method of optimizing the efficiency of operation of these molecular motors without modifying their overall rotational frequency.

Light‐Gated Rotation in a Molecular Motor Functionalized with a Dithienylethene Switch

“…A quantum yield of Φ so→uo =5.6 % was measured and the quantum yield for the reverse photochemical isomerization step ( Φ uo→so ) was then calculated using the PSS 455 ratio, giving Φ uo→so =3.3 %. These values are in a similar range as the quantum yields that have been measured for structurally related molecular motors with and without substituents in the same position 60. When the UV/Vis sample was allowed to warm to room temperature, the original spectrum was recovered, thus indicating that the THI had taken place.…”

Computational design of faster rotating second-generation light-driven molecular motors by control of steric effects

“…As a contribution to the ongoing development of more efficient synthetic light-driven overcrowded alkene-based rotary molecular motors, 11,25,31,33,34,40,41 we have reported a systematic computational study aimed at deciphering how optimization of steric interactions may accelerate the rate-limiting thermal isomerizations of these motors.…”

“…[17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] Several examples of such systems known as either first-generation or second-generation rotary molecular motors are shown in Scheme 1.…”

Cold Snapshot of a Molecular Rotary Motor Captured by High‐Resolution Rotational Spectroscopy

“…Rotaxane‐based systems4, 5 and unidirectional rotary molecular motors6, 7 are among the systems designed to perform translational and rotary motion, respectively. The functional performance of these nanomachines clearly emerges from their unique structural properties.…”