A study of excited state complex formation.

A number of heterocyclic compounds have been used to quench the fluorescence from aromatic hydrocarbons in fluid solution and, in some cases, exciplex emission has been observed. A consideration of solvent effects and the redox potentials of the electron donor and acceptor molecules indicate that the quenching process involves excited state charge transfer interactions. The fluorescence spectra of several monofunctional and bifunctional compounds (aromatic hydrocarbons and amines) have been recorded and fluorescent excimer formation has been observed in some cases. Excimer formation is most efficient when 3 atoms connect the interacting groups. The fluorescence from several aromatic hydrocarbons has been found to be very efficiently quenched by bifunctional compounds such as a,w-diaminoalkanes. When the alkyl chain connecting the terminal amino groups contains less than 5 methylene groups then the quenching process involves both amino groups in the formation of a termolecular complex---in these cases, exciplex emission is relatively inefficient. Exterplex emission has been observed in some cases. The increased quenching efficiency obtained when diamines with 5 or more methylene groups in the connecting chain are used as quenchers is due to multi-collisional quenching rather than exterplex formation. The quenching of the fluorescence from dyes such as Rose Bengal by diaminoalkanes, a,w-dinaphthyl alkanes and related compounds appears to be due to the multi-collisional quenching process rather than exterplex formation. Molecules such as N-n-butyl-N,N-di-(2-naphthylmethyl)-amine and N-phenyl-(2,5-dimethyl) pyrrole, when excited, have more than one possible emitting species depending on the orientation of the interacting groups---one being more polar than the other. The effect of increasing the polarity of the solvent in solutions of these compounds is to stabilize the more polar excited state species and favour emission from this species.