Electron-transfer spectroscopy: donor–acceptor electronic coupling, reorganizational energies, reaction pathways and dynamics

Abstract

The emission spectra of several classes of transition metal complexes are examined for relationships between strong donor/acceptor (D/A) electronic coupling (expressed by the matrix element, H RP) and the Franck-Condon (FC) parameters in electron-transfer systems. The effects of large values of H RP on the FC parameters are expressed in terms of the configurational mixing of the diabatic electron-transfer states are interpreted in terms of the fraction of electron density that is delocalized ( α RP 2 ), and they are evaluated with respect to the limit that α RP 2 → 0 . Ion pair charge transfer spectra and outer-sphere electron-transfer systems are used to define this limit. One effect of values of α RP 2 ≥ 0.03 is the dramatic attenuation of electron-transfer reorganizational energies, and this effect is examined with respect to the variations in the vibronic contributions of the bpy ligand to the 77 K emission spectra of am(m)ine-polypyridine ruthenium(II) complexes. These contributions of high frequency vibrational modes are manifested in the carefully calibrated near infrared emission spectra as changes in the relative intensities of the vibronic sidebands or in the band shape on the low energy side of the spectrum. These vibronic contributions are most clearly exhibited in empirical reorganizational energy profiles (emreps) based on Λ x = hν vib(Δ I obsd/ I max(f)), where Δ I obsd is the intensity difference between the observed emission spectrum and the fundamental component. The emreps provide a useful approach for examining the experimental spectral data for contributions of high frequency vibrational modes to the excited state distortion. The respective emreps clearly display the attenuation of the bpy-centered vibronic contributions with the fraction of charge delocalized. The implications for the electron-transfer coordinate in these strongly coupled D/A complexes are considered. Strong electronic coupling between the donor and the acceptor (or between the initial and the product of electron-transfer states) to the ligand that links them can dramatically change the electron-transfer properties of the D/A complex. Work of the Taube group illustrates the variations in H RP that are induced by the bridging ligand. Other effects of strong coupling with the bridging ligand are: (1) reductions of the energy of the D/A electron-transfer absorption band that are roughly proportional to H RP; (2) bridging ligand distortions that alter H RP and also result in contributions of the bridging ligand vibrational modes to the electron-transfer reorganizational energy. The first of these effects is a characteristic of superexchange coupling and the substituted-dipyridyl ligand-bridged Ru(NH 3) 5 2+/Ru(NH 3) 5 3+ complexes (studied by Sutton and Taube) are considered as models. Cyanide-bridged metal complexes are models for the second effect. The entanglement of the nuclear coordinates of the bridging cyanide with H RP (i.e., vibronic coupling, with H RuCr = H RuCr 0 + b Q CN ; where Q CN is the CN bond length and b is a constant) results in anti-kinematic shifts of the ground state CN stretching frequencies of M(CN)Ru(NH 3) 5 complexes and in a failure of superexchange coupling in a series of [{Ru(NH 3) 5} 2M(MCL)(CN) 2] 6+ complexes (MCL = a tetraazamacrocyclic ligand). The electron-transfer emissions of these complexes, displayed as emreps indicate that the CN stretch contributes more than 100 cm −1 to the overall electron-transfer reorganizational energy in CN-bridged complexes, and implicate a dominant NH stretching mode-mediated nuclear tunneling pathway for back electron-transfer in these complexes. Excited state electron-transfer relaxation by means of NH mediated nuclear tunneling pathways appear to dominate for {Cr II(CN)Ru III} → {Cr III(CN)Ru II} at 77 K, but a more efficient relaxation pathway appears to be important for the am(m)ine-polypyridyl Ru II complexes.