The influence of ligand localized excited states on the photophysics of second row and third row transition metal terpyridyl complexes: Recent examples and a case study

Abstract

The photophysical behavior of Ru(II) and Os(II) diimine complexes having complex aromatic hydrocarbon diimine ligands has received considerable attention as systems exhibiting intramolecular energy transfer to yield excited states with lifetimes much longer than the parent diimine complexes. Here we present a focused discussion of the photophysical behavior of transition metal complexes with modified terpyridyl ligands. The overview includes, as an example of approaches used to evaluate such systems, spectroscopic studies of a pair of Ru(II) mono-and bis-terpyridyl complexes modified with vinylpyrene (Pyr-v-tpy) to have ligand localized excited states that are equal to or lower than the energy of the known MLCT state of the parent complexes, [Ru(Mpt) 2 ] 2+ and [Ru(Mpt)(dien)] 2+ (Mpt = 4'-tolyl-2,2',6',2"-trpyridine, dien = diethylenetriamine). The common observation is that the presence of Pyr-v-tpy serves to lengthen the excited state lifetime of the complex through interaction of MLCT and ligand localized (IL) states. For [Ru(Pyr-v-tpy) 2 ] 2+ the excited state lifetime increases by a factor of more than 10 4 relative to [Ru(Mpt) 2 ] 2+. For [Ru(Pyr-v-tpy)(dien)] 2+ , the 3 IL state is close in energy to the MLCT state of the parent [Ru(Mpt)(dien)] 2+ and, while the transient absorption spectrum is significantly perturbed relative to [Ru(Mpt)(dien)] 2+ , the excited state decay rate changes by only a factor of four. The long-lived excited state is formed in less than a ps, indicating strong coupling of the MLCT and ligand localized manifolds.

Redox Active Ion-Paired Excited States Undergo Dynamic Electron Transfer

“…25 It is also reasonable to expect that the halide brings the amide moieties into greater planarity with the bipyridine π system, increasing delocalization of the excited electron along the ligand π system. 26,27 This hypothesis is supported by Density Functional Theory (DFT) calculations that predict a decrease in the angle between the amide moiety and the pyridyl rings upon chloride ion-pairing, Figure S31.…”

Ligand Control of Supramolecular Chloride Photorelease

“…Previous reports have attributed related excited-state interactions to halide-induced planarization of the two pyridyl rings. 30,45 For 2 2+ *, the excited state was localized on the btfmb ancillary ligand (as discussed below), and a red shift in the PL was observed upon chloride titration. The red shift was accompanied by a change in the PL intensity, but precipitation and potential ligand substitution at higher chloride concentrations precluded a more detailed analysis.…”

In-Depth Studies of Ground- and Excited-State Properties of Re(I) Carbonyl Complexes Bearing 2,2′:6′,2″-Terpyridine and 2,6-Bis(pyrazin-2-yl)pyridine Coupled with π-Conjugated Aryl Chromophores

“…To extend room-temperature triplet excited state lifetimes and improve the photophysical properties of transition-metal complexes, many other strategies have been reported. − Among them, there is the bichromophoric approach, − ,− based on the attachment of an organic chromophore with a nonemissive triplet state close in energy to an emissive 3 MLCT state. Between the 3 MLCT and 3 IL states sharing a similar energy, an excited-state equilibrium may be established.…”

“…Between the 3 MLCT and 3 IL states sharing a similar energy, an excited-state equilibrium may be established. In such a case, the organic chromophore repopulates the luminescent 3 MLCT excited state, playing the role of an energy “reservoir” or excited-state storage element. − ,− Extending luminescence lifetimes via the excited-state equilibration strategy, however, is not accompanied by a quantum yield increase . The most popular chromophores used for this method are π-conjugated aryl groups: e.g., anthryl and pyrenyl. − ,− …”