Ethenyl indoles as neutral hydrophobic fluorescence probes

3‐(4‐Nitrophenylethenyl‐E)‐NH‐indole ( 1 ), 3‐(4‐nitrophenyl ethenyl‐E)‐N‐acetylindole ( 2 ), and 3‐(4‐nitrophenylethenyl‐E)‐N‐benzenesulfonylindole ( 3 ) are relatively less fluorescent in organic solvents, with fluorescence quantum yield (Φ_f) in the range of 0.002 to 0.066 depending on the solvent polarity. However, in bovine serum albumin (BSA)‐phosphate buffer, the fluorescence of these compounds gets drastically enhanced with Φ_f in the range of 0.21 to 0.67, depending on the substituent on the indolic nitrogen atom. Additionally, linear increase in the fluorescence intensity of 2 and 3 occurs on increasing the BSA concentration. These fluorescence properties together with the neutral, hydrophobic nature of these compounds make these fluorophores good fluorescence probe for studying the micropolarity of proteins like BSA and in general the ligand–protein interactions. Copyright © 2007 John Wiley & Sons, Ltd.