Photochemistry of arylacetylenyl-substituted stilbenes

A series of arylacetylenyl-substituted stilbenes 1–3 were synthesized as pure trans- and cis-isomers and their photochemistry was explored. The fluorescence spectra of trans-1–3 are highly red-shifted and much more intense (Φf = 0.66–0.81) compared to that of the parent trans-stilbene (Φf = 0.04). The fluorescence lifetime analysis and the low-temperature excitation spectra indicate that both planar and perpendicular conformations at the arylacetylene moiety should exist in the ground state because of free rotation around the single bonds, whereas in the excited singlet state, an immediate structural change to the stable planar conformation takes place that results in efficient fluorescence. Most remarkably, the fluorescence quantum yields are 0.44 and 0.35 for cis-2 and 3, respectively, which are unexpectedly high and more than 3000 times more efficient than that of the parent cis-stilbene (Φf = 10−5). All isomers exhibited mutual cis–trans photoisomerization. The trans/cis ratios at the photostationary state ([trans]/[cis])pss are 93/7, 96/4 and 100/0 for 1, 2 and 3, respectively, which indicates that 3 undergoes one-way cis-to-trans photoisomerization. The transient absorption spectra suggest that both 2 and 3 undergo intersystem crossing to the triplet state where the planar triplet state (3trans* and 3cis*) and the perpendicular triplet state (3p*) are in equilibrium, but the population of the planar triplet state is predominant. Thus, the photoisomerization should proceed partially from the excited triplet state.

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► A new class of arylacetylenyl-substituted stilbenes.
► Unusually efficient fluorescence from cis-isomers (Φf = 0.35–0.44) which is more than 3000 times more efficient than that of the parent cis-stilbene (Φf = 10–5).
► One-way cis-to-trans photoisomerization.