T‐Shaped 8‐Aryl(ethynyl)‐Substituted Psoralens With Tunable Charge‐Transfer Absorption

Two series of novel T‐shaped 8‐substituted psoralen derivatives bearing (hetero)aryl and arylethynyl substituents were synthesized via Suzuki and Sonogashira coupling starting from 8‐methoxypsoralen (8‐MOP). Predominantly, electron‐donating substituents such as anisyl and phenothiazine were introduced at the 8‐position of the psoralen for increasing the electron density in comparison to 8‐MOP. Photophysical properties were studied by absorption and emission spectroscopy in solution and in the solid state. Increasing the donor strength of the substituent causes a bathochromic shift of both absorption and emission maxima. The absorption behavior can be correlated with Hammett substituent parameters of remote substituents at the p ‐aryl moieties that are directly or by ethynyl spacing positioned at the 8‐position of psoralen. In most cases, fluorescence in the solid state is more pronounced than in solution. One derivative also displays significant aggregation‐induced emission. For two amino‐substituted psoralen derivatives, positive emission solvatochromism was observed and associated with a considerable change of dipole moment upon photonic excitation according to Lippert–Mataga analysis. Furthermore, the p K a value of the T‐shaped dimethylaminophenyl psoralen derivative was determined from absorption spectra. In addition, emission quenching is caused by protonation on the nitrogen atom. TD‐DFT (time‐dependent density functional theory) calculations are in good agreement with experimental data and rationalize the electronic structure.