Intramolecular Energy Transfer in Molecular Dyads Comprising Free-base Porphyrin and Ruthenium(II) Bis(2,2‘:6‘,2‘ ‘-terpyridine) Termini

Abstract

A molecular dyad has been synthesized in which free-base porphyrin and ruthenium(II) bis(2,2':6',2-terpyridine) subunits are linked via a meso-phenylene group. The distal terpyridine ligand bears a single phenylethynylene group. Selective illumination into the metal complex is followed by rapid intramolecular triplet-triplet energy transfer from the metal-to-ligand charge-transfer (MLCT) triplet to the lowest-energy π,π* triplet state localized on the porphyrin. This process is characterized by a reorganization energy of 0.14 eV and an electronic coupling matrix element of 76 cm - 1 . Because of the alkynylene substituent, the initially produced MLCT state is centered on the distal terpyridine. Therefore, triplet energy transfer must cross the proximal terpyridine ligand. At low temperature, nuclear tunneling renders the rate of triplet energy transfer activationless. Upon selective illumination into the lowest-energy singlet (S 1 ) state localized on the porphyrin, fast singlet-triplet energy transfer occurs, to populate the MLCT triplet with high efficiency. This process happens by way of Dexter-type electron exchange at room temperature, and the MLCT triplet can be identified as a reaction intermediate at low temperature. The activation energy for singlet-triplet energy transfer is only 0.05 eV, because of the smaller energy gap, and the electronic coupling matrix element is decreased to 11 cm - 1 , because energy transfer is spin-forbidden. At low temperature, dipole-dipole energy transfer becomes the main mechanism for decay of the porphyrin S 1 state. Excitation into the Soret band of the porphyrin is followed by rapid internal conversion to S 1 without energy or electron transfer to the appended metal complex.