Femtosecond fluorescence dynamics of rotation-restricted azobenzenophanes: New evidence on the mechanism of trans → cis photoisomerization of azobenzene

Ying Chih Lu, Wei-Guang Diau*, Hermann Rau

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

86 Scopus citations

Abstract

The ultrafast relaxation dynamics of two rotation-restricted (azobenzeno-25-phane and azobenzeno-4S-phane) and one rotation-free (4,4-dimethylazobenzene) azobenzene derivatives were investigated using femtosecond fluorescence up-conversion on both S1(n,π*) and S2(π,π*) excitations. On S2 excitation, pulse-limited kinetics with a decay coefficient of ∼100 fs corresponding to ultrafast S2 → S1 relaxation is found to be common for all molecules under investigation regardless of the molecular structure. This indicates that a direct rotational relaxation on the S2 surface is unfavorable. On S1 excitation, we observed biphasic fluorescence decay with a femtosecond component attributed to the decay of the Franck-Condon state prepared by excitation and a picosecond component attributed to the deactivation of the relaxed molecule on the S1 surface. This picosecond component is slowed by at least a factor of 2 for the rotation-restricted 2S-bridged molecule compared to that of the rotation-free molecule; for the even stronger rotation-restricted azobenzeno-4S-phane, the decrease is by a factor of 10. These differences in deactivation suggest that the relaxed states and probably the trajectories for rotation-free and rotation-restricted molecules are different on the S1 surface, which should be important for the quantum yield of photoisomerization.

Original languageEnglish
Pages (from-to)2090-2099
Number of pages10
JournalJournal of Physical Chemistry A
Volume109
Issue number10
DOIs
StatePublished - 17 Mar 2005

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