Journal
CHEMPHYSCHEM
Volume 21, Issue 7, Pages 594-599Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/cphc.201901179
Keywords
excited state; fluorescence; coherence; molecular motor; photochemistry; ultrafast dynamics
Funding
- EPSRC [EP/R042357/1, EP/J009148/1]
- Netherlands Organization for Scientific Research (NWO-CW)
- European Research Council (ERC) [694345]
- Ministry of Education, Culture and Science [024.001.035]
- Center for Information Technology of the University of Groningen
- EPSRC [EP/R042357/1, EP/J009148/1] Funding Source: UKRI
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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 S-1 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.
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