Extension of time-resolved infrared (TRIR) measurements into the near-infrared region has allowed the first direct measurement of a mixed-valence band in the metal-to-ligand charge transfer (MLCT) excited state of a symmetrical ligand-bridged complex. Visible laser flash excitation of [(tpy)Ru(tppz)Ru(tpy)]4+ (tppz is 2,3,5,6-tetrakis(2-pyridyl)pyrazine; tpy is 2,2':6',6' '-terpyridine) produces the mixed-valence, MLCT excited state [(tpy)RuIII(tppz*-)RuII(tpy)]4+* with the excited electron localized on the bridging tppz ligand. A mixed-valence band appears at numax = 6300 cm-1 with a bandwidth-at-half- maximum, Deltanu1/2 = 1070 cm-1. In the analogous ground-state complex, [(tpy)Ru(tppz)Ru(tpy)]5+, a mixed-valence band appears at numax = 6550 cm-1 with Deltanu1/2 = 970 cm-1 which allows a comparison to be made of electronic coupling across tppz0 and tppz*- as bridging ligands.
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Dattelbaum et al. (2002) studied this question.
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