Fluorescence quenching by intermolecular pi¯pi interactions of 2,5-bis(N,Ndialkylamino)-3,6-dicyanopyrazines

Coumarin derivatives represent one of the most important chemical classes of organic fluorescent materials. The results of PPP molecular orbital calculations, after refinement by parameter optimisation, were found to provide a good account of the electronic absorption spectra for a series of substituted coumarins. The electronic structures of several of the derivatives and the nature of the electronic excitation process are discussed in terms of the calculated pi-electron charge densities.