The Krohnke 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 approximate to -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.

Automated summary

Benzo[a]indolizines with ordered electron-withdrawing substituents were synthesized. This method allows for refined control of the photophysical properties of the compounds. The ability to substitute various groups on the heterocyclic skeleton offers unprecedented opportunities to study their fate in the excited state.