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Photochemical properties of the annelated 2 styrylquinoline (2SQ) derivatives 2 styryl benzo[g]quinoline (2SBQ) and 3 styrylbenzo[f]quinoline (3SBQ) and 3 styrylacridine (3SA) have been investigated by experimental and quantum chemical methods. It has been found that the trans isomer of 3SBQ undergoes the photoisomerization reaction with a quantum yield of ϕ tc = 0.19, which increases to 0.44 for the protonated form, with the fluorescence quantum yield decreasing from 0.74 to 0.09. In contrast, the trans isomers of 2SBQ and 3SA are photochemically inert in both neutral and protonated forms, ϕ tc < 10 -3 . Quantum chemical calculation by the PM3 method show that the difference in photochemical properties of 2SBQ and 3SBQ is due to the difference in the structure of frontier molecular orbitals, which, in turn, results in the difference of the shape of the potential energy surface of excited S 1 states. Comparison of the properties of 2SBQ and 3SBQ with those of 2SQ reveals size and topological effects in the photochemistry of aza dia rylethylenes.

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