Bicyclic-ammonium-incorporated ylidic nitrogen groups for strong π-electron donation in push–pull benzene π-conjugated systems

Dialkylamino groups are electrically neutral π-electron-donating groups (πEDGs) typically used in push–pull benzene π-conjugated systems to shift their absorption toward longer wavelengths. To push the upper limit of the π-electron donation imposed by the dialkylamino groups while maintaining the neutral net electric charge, we developed novel ylidic nitrogen-based πEDGs, (quinuclidinio)amidyl (QA) and (1-azanorbornio)amidyl (ANA) groups, which were introduced to push–pull benzenes through nucleophilic aromatic substitution. The bicyclic ammonium structures incorporated in these groups enhanced their thermal stability and π-electron-donating ability. The QA and ANA groups donate more π-electrons than the pyrrolidinyl group, as demonstrated by comparing their σ+DCM values and conducting a competitive electrophilic bromination experiment. Changing the pyrrolidinyl group to the QA or ANA group achieved a bathochromic shift of 45–100 nm in the maximum absorption wavelength, depending on the push–pull π-conjugated system investigated (p-nitrobenzene, 1,8-naphthalimide, and an azo dye).