Benzo[a]imidazo[5,1,2-cd]indolizines – a new class of molecules displaying excited state intramolecular proton transfer

The new class of ESIPT-capable molecules, benzo[a]imidazo[5,1,2-cd]indolizines, bearing the 2-hydroxyphenyl substituent were prepared in a straightforward manner from imidazo[1,2-a]pyridines via a tandem [8+2]cycloaddition–[2+6+2]dehydrogenation reaction. The relationship between the structure and photophysical properties was thoroughly elucidated by comparison with simple analogues i.e. 2-(2′-hydroxyphenyl)imidazo[1,2-a]pyridines. Compared with parent 2-(2′-hydroxyphenyl)imidazo[1,2-a]pyridines the new chromophores strongly absorb blue light and emit in the yellow part of the spectrum. In contrast to 2-(2′-hydroxyphenyl)imidazo[1,2-a]pyridines, emission of π-expanded dyes in protic solvents is usually weaker than in aprotic ones, where they exhibit large Stokes shifts (5500–7000 cm−1). Unlike in solution, in the solid state luminescence for benzo[a]imidazo[5,1,2-cd]indolizines is shifted on average by 100 nm relative to the fluorescence of the corresponding imidazo[1,2-a]pyridines, which results in a relatively strong (Φfl = 0.18 to 0.27) emission of the red light. Benzo[a]imidazo[5,1,2-cd]indolizines possessing a methoxy substituent have much higher fluorescence quantum yields than analogues bearing fluorine and methyl substituents.