Rigidification of 7‐Aminocoumarins Bearing Electron‐Withdrawing Groups Increases Emission Intensity in Polar Environments

A set of structurally unique coumarins possessing an amino group at position 7 and lactam ring fused to the coumarin lactone ring were synthesized for the first time via Pechmann reaction. The photophysics of these new polarized donor–acceptor dyes were compared with those of structural analogs bearing an ester and amide groups at position 4, revealing significant differences in absorption/emission maxima, solvatofluorochromism and emission intensity. Emission of coumarin‐lactams is blue in cyclohexane and orange in MeOH, whereas coumarin‐esters emit cyan light in cyclohexane and red light in MeOH. Crucially, rigidification of coumarin‐lactams’ skeleton combined with replacement of ester with amide functionality has a profound effect on emission intensity decrease in polar solvents: even in acetonitrile, fluorescence quantum yields are still around 50%. Computational studies probed the nature of the key excited states of all dyes and enabled us to venture the hypothesis that blocking free rotation of electron‐withdrawing group at position 4 is responsible for controlling non‐radiative dissipation of energy in more polar solvents.