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 Φf + 2ΦZE = 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 (ΦZE), the fluorescence quantum yield (Φf) 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, Φf, ΦZE, and τf. For the first time it has been shown that the well accepted equation Φf + 2ΦZE = 1 does not hold at all for these non-hydroxylic unconstrained meta analogues. Although ΦZE has been shown to be <10%, Φf 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.

Solvent H-bond accepting ability induced conformational change and its influence towards fluorescence enhancement and dual fluorescence of hydroxy meta-GFP chromophore analogue

“…20 The crystal structure of MOMIM has already been reported. 10 the methoxy group (in mOMe-MBDI) will remain in the same conformation as in the solid phase. In that scenario the proton ''H a '' of mOMe-MBDI will be spatially closer to the sp 2 nitrogen of the imidazolidinone ring (see Fig.…”

“…These molecules are not new and have been synthesised by different groups previously. 10,42 Some photophysical data have also been reported. For example Tolbert et al have synthesised mOMe-MBDI and measured the photophysical data in ethanol containing 1% DMSO solvent.…”

“…1 However, the same p-HBDI exhibits very low F fl (10 À3 or lower) and t fl (several hundred femtoseconds to several picoseconds) in solution. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] Differential optical behaviour of p-HBDI in a b-barrel and in solution has been attributed to structural t twist around the exocyclic double bond connecting benzene and imidazolidinone moieties. [1][2][3][4][5][6][7][8][9][10][11][12][13][14] Thus, the quest of enhancing fluorescence behaviours of the GFP chromophore in normal organic solvents is very active.…”

“…[15][16][17][18][19][20][21][22][23][24][25][26][27][28] There are a few reports describing how enhancement of solvent polarity or hydrogen bond donating ability of solvents reduces F fl . 10,11,29 However, there has been no such detailed investigation till date regarding solvent-property-controlled enhancement of F fl in the case of GFP chromophore analogues, especially enhancement of F fl in high polarity solvents. Hydrogen bond donating and accepting abilities of solvents have been shown to have significant effect on the photophysics of molecules exhibiting excited state proton transfer.…”

“…It has been reported in the literature that, para and meta derivatives of GFP chromophore analogue (m-HBDI and p-HBDI, Chart 1) spectroscopically behave differently in solution. 10,[34][35][36] It has also been shown that an electron donating group like an amine group in the meta orientation can get involved in the charge transfer process. [36][37][38][39][40] It has been reported in the literature that as solvent polarity increases fluorescence quantum yield and fluorescence lifetime decrease.…”