Experimental and Computational Exploration of Ground and Excited State Properties of Highly Strained Ruthenium Terpyridine Complexes

Abstract

Dissociative electron transfer reactions are prevalent in one-electron reduced aryl halides; however, calculations applied to charge-transfer excited states of metal complexes suggest that this reaction would be strongly endergonic unless attention is paid to specific structural details. In this current study, we explore the effect of introducing intramolecular strain into a series of halogenated ruthenium(II) polypyridyls. Parent [Ru(tpy)2](2+) (1) (tpy = 2,2':6',2″-terpyridine) is compared with two complexes, [Ru(6,6″-Br2-tpy)(tpy)](2+) (2) and [Ru(6,6″-Br2-tpy)2](2+) (3) (6,6″-Br2-tpy = 6,6″-dibromo-tpy) that incorporate interligand van der Waals strain derived from the large halogen substituents. DFT calculations and the crystal structure of 3 show how this strain distorts the geometry of 3 as compared to 1. Time-dependent DFT calculations are used to explain the effect of this strain on electronic absorption spectra where, in particular, a transition observed in 3 is attenuated in 2 and absent in 1 and heralds interligand charge transfer mediated by the halogen substituent. Ultrafast transient absorption spectroscopy reveals coherent vibrational dynamics particularly in 3 but also in 2 that is interpreted as reflecting heavy-atom motion. Surprisingly, in spite of the additional strain, the excited-state lifetime of 3 is observed to be approximately a factor of 6 longer than 2. Constrained-DFT calculations show that while the excited behavior of 2 is similar to 1, the strain-induced geometric distortions in 3 cause a nesting of excited state triplet surfaces resulting in a longer excited state lifetime. This may afford the additional time needed to engage in photochemistry, and kinetic evidence is observed for the breaking of a C-Br bond in 3 and formation of a contact ion pair state.

Theoretical illumination of highly original photoreactive3MC states and the mechanism of the photochemistry of Ru(ii) tris(bidentate) complexes

“…A common route to promote ligand ejection reactivity in ruthenium(II) tris(bidentate) complexes involves the incorporation of steric congestion to weaken Ru-N bonds and stabilise 3 MC states thereby promoting their population, provided the 3 MLCT and 3 MC PES are not nested. 83 The results presented here, involving the incorporation of sterically unencumbered triazole-containing ligands with poor -acceptor character displaying original 3 photoproducts. This work has focused on the case of two bis(btz) complexes which produce exclusively trans photoproducts.…”

Theoretical Investigation of Phosphinidene Oxide Polypyridine Ruthenium(II) Complexes: Toward the Design of a New Class of Photochromic Compounds

“…It is therefore a common practice to neglect the SOC in the computation of PES of second row transition metals. 36,37,38,60,61,62 Although spin-orbit effects are not included in our calculations, we do take into account these effects in deriving the nonadiabatic mechanistic picture presented in section IV.…”

Computational characterization of competing energy and electron transfer states in bimetallic donor-acceptor systems for photocatalytic conversion

“…Here we have chosen three well studied Ru(II)-light harvesters (naming and Chemdraw structures also in Table S1), [Ru(bpy) 2 (dimethyl-bpy)] 2+ where bpy=2,2′-bipyridine called BPY , 56 cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato) ruthenium(II) called N3 , 57 and [Ru(dqp) 2 ] 2+ where dqp=2,6-bis(8-quinolinyl)-pyridine called DQP 41–43 (Chart 1). Each of these is well tested experimentally and known to produce relatively long-lived excited states with enough energy to be transferred to other metal centers or the conduction band of TiO 2 .…”

“…31–35 Specifically, quantum chemical calculations of the electron transfer (ET) and excitation energy transfer (EET) potential energy surfaces beyond the initially excited Franck-Condon region provided a more comprehensive theoretical understanding of the possible structural evolution accompanying photoinduced electronic processes for prototype light-harvesting transition metal complexes. 36–41 Many model photoactive D-A dyad systems comprised of one light harvesting Ru-complex 37–38, 42–44 coupled to a model Co-center 45–50 have been investigated. Donor-acceptor interactions have been widely studied and range from very weak to very strong, as nicely illustrated in the series of unbound (Ru|Co), weakly bound (Ru–Co), and tightly bound (Ru=Co) supramolecular Ru(II)-Co(III) systems studied by us and others.…”