Emission Switching of4,6-Diphenylpyrimidones: Solventand Solid State Effects.

The photophysics of 1-ethyl-4,6-bis(4-methoxyphenyl)-2(1H)-pyrimidone (1) and 1-ethyl-4,6-bis(4-(dimethylamino)phenyl)-2(1H)-pyrimidone (2) were investigated to determinethe mechanisms of emission switching in response to protonation. UV–visand steady state emission spectroscopy of the protonated and unprotonatedforms across a range of solvents reveal the polarity dependence ofthe vertical excitation energies. Emission lifetimes and quantum yieldsshow the solvent dependency of the excited states. Emission enhancementswere observed in polyethylene glycol solutions and in the solid state(both thin film and single crystal), demonstrating the role of intramolecularrotation in thermal relaxation of the excited states. TD-DFT calculationsprovide insights into the excited state geometries and the role ofintramolecular charge transfer. The collected data show that emissionof diphenylpyrimidones can be modulated by four factors, includingthe identity of the electron-donating auxochrome, protonation state,solvent polarity, and viscosity. [ABSTRACT FROM AUTHOR]