Orbital-Specific Energy Transfer

The synthesis, structure, and photophysical properties of a new family of trinuclear CuRe2 chromophore-quencher complexes having the general form [Cu(pyacac)2(Re(bpy′)(CO)3)2](OTf)2 (where pyacac = 3-(4-pyridyl)-acetylacetonate and bpy′ = 4,4′-5,5′-tetramethyl-2,2′-bipyridine (tmb, 1), 4,4′-dimethyl-2,2′-bipyridine (dmb, 2), 2,2′-bipyridine (bpy, 3), 4,4′-dichloro-2,2′-bipyridine (dclb, 4), and 4,4′-diethylester-2,2′-bipyridine (deeb, 5)) are reported. Time-resolved emission data acquired in room-temperature CH2Cl2 solutions revealed excited-state lifetimes of 14.9 ± 0.7, 8.1 ± 0.4, 8.2 ± 0.4, 5.6 ± 0.3, and 5.0 ± 0.3 ns for complexes 1−5, respectively. The emission in each case is assigned to decay of the ReI-based 3MLCT excited state; the lifetimes are all significantly less than the corresponding BeRe2 model complexes, which were also prepared and characterized. Electron transfer was found to be significantly endothermic for all five CuRe2 complexes: this fact, coupled with the ca. 10 Å donor−acceptor distance and favorable spectral overlap between the 3MLCT emission profile and absorptions of the CuII center, implicates dipolar energy transfer as the dominant quenching pathway in these compounds. Gaussian deconvolution of the ground-state absorption spectrum of Cu(phacac)2 (phacac = 3-phenylacetylacetonate) allowed for a differential analysis of the spectral overlap between the donor emission spectra and the two observed ligand-field transitions of the CuII ion. Energy transfer was found to occur preferentially to the lower energy ligand-field band due primarily to more favorable dipole orientation as measured by the κ2 term from Förster theory. These results, supported by time-dependent DFT calculations on Cu(phacac)2, indicate enhanced dipolar coupling to the dxz → dxy transition of the CuII center and thus represent an orbitally specific energy-transfer process occurring in this system.