Environment-responsive multicolor fluorescent dyes based on nitrophenyl or nitrophenylethynyl oligothiophene derivatives: correlation between fluorescence and π-conjugation length

We report the unique environment-responsive fluorescence of nitrophenyl or nitrophenylethynyl oligothiophene derivatives with different numbers of thiophene rings. With an increase in the number of thiophene rings, the maximum fluorescence quantum yield (ΦF) was obtained even in low-polarity solvents. For 5,5″-bis(4-nitrophenyl)-2,2′-bithiophene (BNP2T), a nitro-group-containing fluorescent dye without a strong donor moiety, the maximum ΦF of 0.43 was observed in CHCl3, which is a large value, and multicolor fluorescence with a moderate quantum yield was observed in various solvents. Thus, we can design functional nitrophenyl-based fluorescent dyes that would find applications as bioimaging probes, polarity sensors, etc.