Electron exchange in conformationally restricted donor-spacer-acceptor dyads: Angle dependence and involvement of upper-lying excited states

The rate constant for triplet energy transfer (k(TET)) has been measured in fluid solution for a series of mixed-metal Ru-Os bis(2,2':6',2"-terpyridine) complexes built around a tethered biphenyl-based spacer group. The length of the tether controls the central torsion angle for the spacer, which can be varied systematically from 37 to 130 degrees. At low temperature, but still in fluid solution, the spacer adopts the lowest-energy conformation and kTET shows a clear correlation with the torsion angle. A similar relationship holds for the inverse quantum yield for emission from the Ru-terpy donor. Triplet energy transfer is more strongly activated at higher temperature and the kinetic data require analysis in terms of two separate processes. The more weakly activated step involves electron exchange from the first-excited triplet state on the Ru-terpy donor and the size of the activation barrier matches well with that calculated from spectroscopic properties. The pre-exponential factor derived for this process correlates remarkably well with the torsion angle and there is a large disparity in electronic coupling through pi and sigma orbitals on the spacer. The more strongly activated step is attributed to electron exchange from an upper-lying triplet state localized on the Ru-terpy donor. Here, the pre-exponential factor is larger but shows the same dependence on the geometry of the spacer. Strangely, the difference in coupling through pi and sigma orbitals is much less pronounced. Despite internal flexibility around the spacer, k(TET) shows a marked dependence on the torsion angle computed for the lowest-energy conformation.