Design, synthesis and photoswitching of broad-spectrum fluorescent hexaarylbiimidazoles

A series of novel hexaarylbiimidazole (HABI)-containing fluorophores with broad-spectrum emissions ranging from blue, green and red emissions, have been designed and synthesized. Their photochromism and fluorescence switching properties are investigated by UV-Vis absorption and fluorescence spectra in solutions and PMMA films. By connecting carbazole (Cz), naphthalimide (NI) and perylene-mono-imide (PMI) to HABI with a spacer, three molecular photoswitches with broad-spectrum fluorescence varying from deep blue (Cz-O-HABI, lambda(em) = 390 nm, Phi(F) = 0.27), green (NI-O-HABI, lambda(em) = 490 nm, Phi(F) = 0.25) to far red (PMI-O-HABI, lambda(em) = 650 nm, Phi(F) = 0.25) have been obtained. The ON/OFF ratios are calculated to be about 8.9, 8.0 and 3.7 (calculated from FL spectra) for Cz-O-HABI, NI-O-HABI and PMI-O-HABI, respectively. Such a series of photoswitchable fluorophores constitute prototypes of a photochromic molecular library in which fluorescence resonance energy transfer can be reversibly modulated by an external stimulus, namely, UV light.