An Apparent Angle Dependence for the Nonradiative Deactivation of Excited Triplet States of Sterically Constrained, Binuclear Ruthenium(II) Bis(2,2‘:6‘,2‘ ‘-terpyridine) Complexes

Abstract

The photophysical properties are reported for a series of binuclear ruthenium(II) bis(2,2':6',2"-terpyridine) complexes built around a geometrically constrained, biphenyl-based bridge. The luminescence quantum yield and lifetime increase progressively with decreasing temperature, but the derived rate constant for nonradiative decay of the lowest-energy triplet state depends on the length of a tethering strap attached at the 2,2'-positions of the biphenyl unit. Since the length of the strap determines the dihedral angle for the central C-C bond, the rate of nonradiative decay shows a pronounced dependence on angle. The minimum rate of nonradiative decay occurs when the dihedral angle is 90 degrees, but there is a maximum in the rate when the dihedral angle is about 45 degrees. This effect does not appear to be related to the extent of electron delocalization at the triplet level but can be explained in terms of variable coupling with a low-frequency vibrational mode associated with the strapped biphenyl unit.

Electron Exchange in Conformationally Restricted Donor–Spacer–Acceptor Dyads: Angle Dependence and Involvement of Upper‐Lying Excited States

“…In addition, a new emission band becomes apparent at higher energy and is cantered at about 675 nm ( Figure 3). This latter emission can be assigned [17] to the Ru-terpy unit by comparison to the corresponding Ru-Ru binuclear complexes. The relative intensity of this emission is always much less than that of the Os-terpy unit, despite the more favourable absorption profile of the Ru-terpy unit.…”

“…around 760 nm, but shows a pronounced shoulder at about 695 nm (Figure 1). Comparison with appropriate reference compounds, [19] including the corresponding Ru-Ru complexes, [17] shows that this emission arises from the Os-terpy unit. The spectral profile is independent of excitation wavelength and there is good agreement between excitation and absorption spectra across the entire region.…”

“…Because of steric constraints, this strategy does not permit isolation of the planar geometry, but the number of compounds is increased on binding simple cations to the central void inherent to the crown ether variants. Details of the photophysical properties of the binuclear Ru-Ru complexes are available in the literature, [17] as is a separate study of cation binding. [18] The absorption spectra of the various mixed-metal complexes are essentially superimposable and the characteristic features of a metal-polyA C H T U N G T R E N N U N G (pyridine) complex are easily recognized.…”

“…[16] As such, luminescence from the Ru-terpy unit becomes more favourable as the temperature is lowered. It should also be noted that the photophysical properties of the Ru-Ru binuclear complexes [17] are strongly dependent on temperature, because of thermal population of a high-lying, metal-cantered triplet state that is coupled to the ground state. Thus, decreasing the temperature serves to both slow down triplet-energy transfer and raise the emission probability for the Ru-terpy donor.…”

“…Emission from the Ru-terpy donor is too weak for similar measurements to be made at ambient temperature and there is the additional problem that the inherent triplet lifetime of the donor, measured for the Ru-Ru binuclear complexes, [17] is much shorter under these conditions (Table 1). As such, triplet-energy transfer has to compete with fast nonradiative decay of the donor triplet.…”