Strong electron donation induced differential nonradiative decay pathways for para and meta GFP chromophore analogues

Abstract

Z–E Isomerisation because of rotation around the exocyclic double bond (known as the τ-twist) and not any other internal conversion has been reported to be the major nonradiative decay channel for non-hydroxylic unconstrained para and meta GFP chromophore analogues. The equation Φ_f + 2Φ_ZE = 1 has been shown to hold well for both para and meta GFP chromophore analogues. If the above equation holds true, then upon reducing the extent of Z–E isomerisation (Φ_ZE), the fluorescence quantum yield (Φ_f) should increase. To probe the above proposition two sets of non-hydroxylic unconstrained para and meta GFP chromophore analogues were synthesized. Quite interestingly by introducing the strongly electron donating –NEt₂ group to the benzenic moiety these para and meta GFP chromophore analogues were shown to exhibit differential optical behaviour w.r.t. the extent of the solvatochromic shift, Φ_f, Φ_ZE, and τ_f. For the first time it has been shown that the well accepted equation Φ_f + 2Φ_ZE = 1 does not hold at all for these non-hydroxylic unconstrained meta analogues. Although Φ_ZE has been shown to be <10%, Φ_f is much lower than the expected near unity value for these meta analogues. After detailed investigation into the nonradiative excited state decay channel, contrary to literature reports, energy gap law governed internal conversion and not Z–E isomerisation was shown to be the major nonradiative decay channel for these meta analogues. Two models are put forward to understand the differential optical behaviour of these para and meta GFP chromophore analogues. Support from X-ray crystal structures, NMR experiments, and computational calculations has also been provided.