Photophysics of Cyclic α-Diketone Aromatic Ring Bichromophoric Molecules. Structures, Spectra, and Intramolecular Electronic Energy Transfer

The structures and spectral properties of a number of bichromophoric molecules are presented. These bichromophoric molecules are composed of an aromatic ring connected by two methylene chains to an α-diketone moiety. Both absorption and emission spectra can be attributed to a superposition of the invididual spectra of the separate chromophores. The critical transfer radii for electronic energy transfer from the aromatic (donor) chromophore to the α-diketone (acceptor) chromophore were calculated from the spectral overlap between the fluorescence spectrum of the aromatic moiety and the absorption spectrum of the α-diketone chromophore. The results show that this series of molecules is well suited for a mechanistic study of short-range intramolecular electronic energy transfer (intra-EET). Results of singlet-singlet intra-EET in this series of bichromophoric molecules are reported. The temperature and molecular structure dependence of the intra-EET efficiency were measured and analyzed. The results show that the transfer efficiency is strongly temperature and structure dependent, indicating that exchange interaction is responsible for intra-EET between close chromophores in a bichromophoric molecule. The contributions of interchromophoric distance and of the relative orientation of the two chromophores to exchange interactions are discussed. Phosphorescence of the diketone moiety (including temperature dependence) was used to evaluate the efficiency of triplet energy transfer. Direct excitation of the diketone resulted in weak phosphorescence, indicating that intersystem crossing is inefficient. However, excitation of the aromatic moiety resulted in much more intense phosphorescence as a result of the formation of triplet diketone via triplet energy transfer. Complementarity between singlet and triplet energy transfer was evidenced by the fact that some compounds in which singlet energy transfer was more efficient exhibited weaker phosphorescence and vice versa. Quantitative correlations between triplet energy transfer efficiency and interchromophore distance were not possible.