Transport and fluorescence emission properties of photoresponsive ligands

Abstract

New photoresponsive azobenzene systems L1–L10 have been synthesised and characterised. On irradiation at ∼330 nm, they undergo conversion fromE toZ form to a varying extent, depending on the nature and position of substitution on the azobenzene rings. They revert toE form in the dark. Photochemical equilibria have been studied in acetonitrile,o-dichlorobenzene and DMF.E forms are stabilized more ino-dichlorobenzene than in acetonitrile. Two of the molecules L1 and L5 show enhanced transport of Cu2+ ions across liquid membrane on irradiation. Compounds L2 and L3 undergoE→Z photoisomerisation on irradiation at ∼330 nm accompanied by fluorescence enhancement.Z→E isomerisation in the dark is accompanied by decrease in the fluorescence intensity. These are the first reported examples of fluorescence enhancement onE→Z isomerisation across N=N bond. Protonation of azo groups in L2 and L3 is accompanied by fluorescence enhancement. The origin of the fluorescence enhancement in thecis isomer is explained on the basis of the inhibition of photoinduced electron transfer due to the nonplanar geometry of this isomer which reduces the effective conjugation of the nitrogen lone pair electrons with theπ electrons of the fluorophore.