Proton-driven “one-way” photoisomerization due to energy transfer switching in styrylquinoline–merocyanine dyad

Photophysical and photochemical properties of the novel bichromophoric covalently linked dyad SQ3MC, where 2-styrylquinoline (SQ) and merocyanine (MC) chromophoric groups are connected by dioxytrimethylene bridge O-(CH2)3O, in comparison with model compounds 2-(4-methoxystyryl)quinoline MeSQ and 1-ethyl-2-(4-methoxystyryl)quinolinium perchlorate, which is O-methylated merocyanine MeMC, are investigated. In dyad SQ3MC, trans → cis photoisomerization of SQ moiety is quenched because of competing Forster resonance energy transfer (FRET) to MC fragment. However, reaction is switched on in protonated quasisymmetric dyad where positively charged SQ fragment obtains the same chromophoric π-system as in MC chromophore. After deprotonation, photoisomerization of SQ fragment in trans → cis direction is again switched off but goes in cis → trans direction. Effect of “one-way” photoisomerization in the dyad SQ3MC is kinetic in nature and based on the difference in rates of competing processes in the excited state, in contrast to well-known one-way reactivity of diarylethylenes with large aryl substituents, where effect is thermodynamic in nature and based on the difference in energies of the excited-state isomers and conformers.