Fluorescent Multiple-State Emitters Based on Benzonitrile-2-(2'-hydroxyphenyl)benzazoles (HBX)

We describe the synthesis, photophysical properties, and ab initio calculations on a series of 2-(2'-hydroxyphenyl)benzazoles (HBX), functionalized by a benzonitrile, prone to boost the fluorescent quantum yield in solution. HBX are well known to display the excited-state intramolecular proton transfer (ESIPT) process, but they typically lead to a quenched fluorescence in solution. The additional insertion of an ethynyl-triisopropylsilane substituent in the vicinity of the benzonitrile group acts as a cooperative moiety to obtain strong multiple-state emissive transitions with distinct emission wavelengths. Quantum yields of the ESIPT transition are up to 52% in solution, which are among the record values in the literature. The photophysical data can be further modulated by the nature of the heteroatom constitutive of HBX, paving the way for the development of ultrasensitive ratiometric probes.