Abstract
Efficient photomolecular motors will be critical elements in the design and development of molecular machines. Optimisation of the quantum yield for photoisomerisation requires a detailed understanding of molecular dynamics in the excited electronic state. Here we probe the primary photophysical processes in the archetypal first generation photomolecular motor, with sub‐50 fs time resolved fluorescence spectroscopy. A bimodal relaxation is observed with a 100 fs relaxation of the Franck‐Condon state to populate a red‐shifted state with a reduced transition moment, which then undergoes multi‐exponential decay on a picosecond timescale. Oscillations due to the excitation of vibrational coherences in the S1 state are seen to survive the ultrafast structural relaxation. The picosecond relaxation reveals a strong solvent friction effect which is thus ascribed to torsion about the C−C axle. This behaviour is contrasted with second generation photomolecular motors; the principal differences are explained by the existence of a barrier on the excited state surface in the case of the first‐generation motors which is absent in the second generation. These results will help to provide a basis for designing more efficient molecular motors in the future.
Highlights
Recent decades have seen rapid progress in the development of light driven nanomolecular machines.[1]
It is noteworthy that this excited state vibrational coherence survives structural relaxation from the FC state to the new emissive state
Such structural relaxation along an anharmonic potential energy surface would normally lead to dephasing of the initially excited coherence, suppressing its appearance in the final state
Summary
Recent decades have seen rapid progress in the development of light driven nanomolecular machines.[1]. Among the most promising power sources for such machines are the overcrowded-alkene based photomolecular motors.[1b,d–f,2] The first generation of such motors, which are based on an overcrowded alkene geometry with two stereogenic centres, undergo a unidirectional rotation via four steps, comprising of successive photochemical isomerisation and thermal helix inversion reactions (Scheme 1).[1d,3] These first generation motors showed high quantum yields for photochemical isomerisation and a favourable position of the photostationary state.[3b] the relatively high barrier in the rate determining thermal helix inversion step restricted the accessible rotation rate This prompted the development of second-generation motors, in [a] A.
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