The Excited‐State Creutz–Taube Ion

Abstract

The excited-state version of the Creutz-Taube ion was prepared via visible light excitation of [(NH 3 ) 5 Ru II (μ-pz)Ru II (NH 3 ) 5 ] 4 + . The resulting excited state is a mixed valence {Ru III-δ (μ-pz *À )Ru II + δ } transient species, which was characterized using femtosecond transient absorption spectroscopy with vis-NIR detection. Very intense photoinduced intervalence charge transfers were observed at 7500 cm À 1 , revealing an excited-state electronic coupling element H DA = 3750 cm À 1 . DFT calculations confirm a strongly delocalized excited state. A notable consequence of strong electron delocalization is the nanosecond excited state lifetime, which was exploited in a proof-of-concept intermolecular electron transfer. The excited-state Creutz-Taube ion is established as a reference, and demonstrates that electron delocalization in the excited state can be leveraged for artificial photosynthesis or other photocatalytic schemes based on electron transfer chemistry.

Ru Monoimines with Extended Excited-State Lifetimes and Geometrical Modulation of Photoinduced Mixed-Valence Interactions

“…Inspection of the kinetic trace at 17 700 cm −1 (Figure 3) reveals two processes, and global analysis fittings result in two exponential decays with time constants of 19 ps and 1.6 ns (Table S7). Target analysis of the data, considering a parallel decay model essentially identical to that one employed for the related excited-state Creutz−Taube ion, 10 affords differential absorption spectra shown in Figure 3. Both states present a bleach that peaks at 18 100 cm −1 , mirroring ground-state MBCT absorptions.…”

“…These values are similar to those of the PIIVCT band in the Ru pentaammine analogue with ε(PIIVCT) = 10 500 M −1 cm −1 , an energy of 7500 cm −1 , and a FWHM of 2200 cm −1 . 10 Importantly, the PIIVCT band in Ru(μ-pz)Ru 2+ is higher in energy than the ground-state IVCT band at 5240 cm −1 . 35 This difference can only be originated in a stronger electronic coupling in the excited state, since both mixed-valence systems are symmetric with pz •− or pz bridges, respectively, and therefore, the diabatic energy difference between donor and acceptor states involved in the charge transfer is zero in both cases (see the EDDMs for the calculated PIIVCT in Figure S24).…”

“…Similar differences in electronic coupling have been observed for the Creutz−Taube ion in the ground and excited states that were interpreted in terms of the second-order perturbation theory arguments related to super-exchange interactions. 10 This means that in the excited-state system, there is a greater number of virtual states close in energy that can mediate donor−acceptor super-exchange interactions than there is for the ground-state system. The same argument applies here.…”

“…Hole delocalization in the excited state by means of photoinduced mixed-valence (PIMV) interactions , was shown to extend MLCT lifetimes by a factor of 2 in bimetallic Ru polypyridyl complexes in comparison to that of their monometallic analogues and by 2 orders of magnitude when moving from [Ru­(NH 3 ) 5 (L)] n + with L = pzH + to the excited-state Creutz–Taube ion with L = {(μ-pz)­Ru­(NH 3 ) 5 } (model compounds in Figure ). Delocalization of the MLCT-excited hole enables nesting excited-state potential energy surfaces with that of the ground state, resulting in poor Franck–Condon factors for excited-state decay. Thus, the best possible scenario involves high donor–acceptor electronic coupling and full delocalization.…”

“…PIMV systems are also models for excited-state electron transfer processes, and thus, they are key players in solar-to-chemical energy conversion and other photocatalytic processes that produce fine chemicals. Like in the ground state, photoinduced intervalence charge transfer (PIIVCT) bands allow direct measurements of excited-state electronic coupling. , However, PIMV systems are inherently different from their ground-state analogues. For example, donor-acceptor roles can be reversed in the excited state, and a switch on of donor-acceptor electronic coupling was observed upon photon absorption .…”