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Electron Exchange in Conformationally Restricted Donor–Spacer–Acceptor Dyads: Angle Dependence and Involvement of Upper‐Lying Excited States
Author(s) -
Benniston Andrew C.,
Harriman Anthony,
Li Peiyi,
Patel Pritesh V.,
Sams Craig A.
Publication year - 2008
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200701548
Subject(s) - chemistry , excited state , dihedral angle , atomic orbital , electron transfer , triplet state , crystallography , atomic physics , electron , photochemistry , molecule , physics , hydrogen bond , organic chemistry , quantum mechanics
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°. At low temperature, but still in fluid solution, the spacer adopts the lowest‐energy conformation and k TET 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 π and σ 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 π and σ 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.