Orbital-Specific Energy Transfer

Abstract

The synthesis, structure, and photophysical properties of a new family of trinuclear CuRe(2) 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 CH(2)Cl(2) 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 Re(I)-based (3)MLCT excited state; the lifetimes are all significantly less than the corresponding BeRe(2) model complexes, which were also prepared and characterized. Electron transfer was found to be significantly endothermic for all five CuRe(2) complexes: this fact, coupled with the ca. 10 A donor-acceptor distance and favorable spectral overlap between the (3)MLCT emission profile and absorptions of the Cu(II) 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 Cu(II) 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 kappa(2) term from Forster theory. These results, supported by time-dependent DFT calculations on Cu(phacac)(2), indicate enhanced dipolar coupling to the d(xz) --> d(xy) transition of the Cu(II) center and thus represent an orbitally specific energy-transfer process occurring in this system.