Spectral and Photochemical Properties of a Supramolecular Dyad with Pyrenylethenylquinoline as a Photochrome: Effect of the Bridging Group Structure

The spectral and photochemical properties of a biphotochromic dyad in which two 2-[2-(pyren-1-yl)ethenyl]quinoline (PEQ) photochromes are linked to each other by the bridging ortho-xylylene group have been studied. An excimer has been detected in the dyad according to a bathochromic shift of the fluorescence spectrum. Under exposure to light, the dyad undergoes photoisomerization reactions of PEQ photochromes and intramolecular [2+2]-photocycloaddition (PCA) with the formation of dipyrenyl-substituted cyclobutane. It is assumed that the tendency of PEQ photochromes to π-stacking interaction has a positive effect at the first step of the PCA reaction, promoting the formation of preorganized dimeric structures in the ground S₀ state, which give an excimer in the excited S₁ state, and a negative effect at the second step, preventing the movement of photochromes relative to each other, which is necessary to achieve conformations favorable to the PCA reaction.