New fluorescent derivatives from papaverine: Two mechanisms to increase the quantum yield

A series of new emissive imidazo[5,1-a]isoquinolines were synthesized, and their electronic properties investigated, compared, and discussed in light of their chemical structure. The electronic spectra show absorption bands around 330–350 nm and intense fluorescence emissions between 440 nm and 460 nm, with corresponding huge Stokes shifts (6464-8611 cm− 1 ). Depending on the position and the electronic properties of the substituents on the imidazo[5,1-a]isoquinoline nucleus, the emission quantum yield in dichloromethane solution enhances from 9% to 37%. The collected data suggest that two distinct mechanisms are responsible for the enhanced emission: the steric hindering of the rotation of the substituent and its electronic effect, improved by the molecular planarization attainable exclusively with electron-withdrawing substituents in para position.