Photoinduced Electron and Energy Transfer in Aryldihydropyridines

Dimethyl 2,6-dimethyl-1,4-dihydropyridine-3,5-dicarboxylates (Hantzsch DHPs) fluoresce weakly in fluid solution. However, these compounds exhibit an efficient fluorescence both in a viscous medium (glycerin) at room temperature and in a glassy matrix at 77 K (but no phosphorescence, since ISC is negligible). DHPs bearing an aryl roup in position 4 have been synthesized. These contain two different pi systems separated by an sp(3) carbon (DHP-Ar dyads). The occurrence of energy and electron transfer processes between the chromophores is investigated through luminescence measurements. In particular, when Ar-3 emits at a slow rate (e.g., Ar = phenanthryl) or not at all (Ar = nitrophenyl) the intradyad forward/backward electron transfer sequence offers a path for arriving at the DHP-localized triplet and the corresponding phosphorescence is observed. When Ar-3 emits at a faster rate (Ar = acylphenyl), the phosphorescence from either of the two localized triplets, Ar-3 or (DHP)-D-3, can be observed depending on lambda(exc.) When the aryl group has a triplet energy lower than that of (DHP)-D-3, this functions as emitting (4-cyano-l-naphthyl) or nonemitting (MeO2CCH=CHC6H4) energy sink. The results document the possibility of building tailor-made Hantzsch aryldihydropyridines as versatile photoactivated dyads.