The Kröhnke synthesis of benzo[a]indolizines revisited: towards small, red light emitters

Benzo[a]indolizines with an ordered arrangement of various electron-withdrawing substituents (NO2, CF3, CN, CO2R and COPh) were prepared directly from pyridinium salts and chloronitroarenes, allowing for refined control of the photophysical properties. Facile entry into almost unknown isoindolo[1,2-a]isoquinolines is disclosed to demonstrate the potential of this method. The rational manipulation of the substituents makes it possible to obtain electron-deficient dyes (HOMO ≈ −5.6 eV) with yellow, orange to red emission, large Stokes shifts (up to 9000 cm−1) and fluorescence quantum yields reaching 0.58. Strong emission in spite of the presence of an NO2 group has been rationalized by a large singlet–triplet energy gap combined with small spin–orbit couplings, hence small intersystem crossing. The ability to substitute various electron-withdrawing groups at multiple positions on this heterocyclic skeleton offers an unprecedented opportunity to study their effect on the fate of the molecules in the excited state.