Solute-solvent electronic interaction is responsible for initial charge separation in ruthenium complexes [Ru(bpy)3]2+ and [Ru(phen)3]2+

Abstract

Origin of the initial charge separation in optically-excited Ruthenium(II) tris(bidentate) complexes of intrinsic D 3 symmetry has remained a disputed issue for decades. Here we measure the femtosecond two-photon absorption (2PA) cross section spectra of [Ru(2,2′-bipyridine) 3 ] 2 and [Ru(1,10-phenanthroline) 3 ] 2 in a series of solvents with varying polarity and show that for vertical transitions to the lower-energy 1 MLCT excited state, the permanent electric dipole moment change is nearly solvent-independent, Δμ = 5.1-6.3 D and 5.3-5.9 D, respectively. Comparison of experimental results with quantum-chemical calculations of complexes in the gas phase, in a polarizable dielectric continuum and in solutesolvent clusters containing up to 18 explicit solvent molecules indicate that the non-vanishing permanent dipole moment change in the nominally double-degenerate E-symmetry state is caused by the solute-solvent interaction twisting the two constituent dipoles out of their original opposite orientation, with average angles matching the experimental two-photon polarization ratio.

Electronic States of Tris(bipyridine) Ruthenium(II) Complexes in Neat Solid Films Investigated by Electroabsorption Spectroscopy

“…In principle, changes in the permanent dipole moment upon electron excitation can be induced by electronic interaction with a nonsymmetric environment. However, due to structural rigidity of the complex and robust character of degenerate E states, the relevant changes are difficult to observe [ 15 ].…”

“…The solvatochromism of a RBY complex in liquid solvents is rather weak, and the solvent shifts of the MLCT absorption band [ 16 ] are similar in size to the higher-energy ( 1 B 2u ) singlet state for the nonpolar anthracene molecule [ 17 ] but rather quite small when compared to typical molecular charge-transfer bands [ 2 ]. Although the observed solvent dependence of the MLCT absorption band by Kober et al [ 16 ] was ascribed to the polar excited state with an electron localized on a single ligand and a permanent dipole moment of 14 D, this interpretation was later criticized for several reasons [ 2 , 15 , 18 ]. The absorption spectra of a related symmetric complex, [Ru(dcebpy) 3 ] 2+ (where dcebpy = 4,4′-dicarboethoxy-2,2′-bipyridine) in solvents, covering a range of dielectric constants, displayed as opposed to ref.…”

“…Recently, the authors of ref. [ 15 ] repeated the original experiments for an RBY complex [ 16 ] and in terms of a more comprehensive Liptay’s description of solvatochromism in a polarizable continuous medium [ 21 ] (but neglecting dispersion and higher multipole terms) received a permanent dipole moment for an MLCT state a factor of 2 lower than Kober et al [ 16 ]. More importantly, TDDFT quantum–chemical calculations performed for solute–solvent clusters containing up to 18 explicit solvent molecules placed in a solvent continuum indicate at the non-vanishing permanent dipole moments of both the ground and excited MLCT state.…”

Bis(bipyridine)ruthenium(II) Ferrocenyl β‐Diketonate Complexes: Exhibiting Nanomolar Potency against Human Cancer Cell Lines

“…A significant amount of research has been conducted on Ru II ‐polypyridyl complexes, specifically those which incorporate functionalized ligands derived from 2,2’‐bipyridine (bpy) or 1,10‐phenanthroline (phen). Their potential as photo‐induced cytotoxic compounds (Figure 1 D), [27] DNA intercalators (Figure 1 E), [15, 16, 28] and use in photodynamic therapy (PDT) have been widely assessed [29, 30] . Where these compounds can intercalate with DNA, Barton et al.…”

Novel Lipophilic Fluorophores with Highly Acidity‐Dependent Two‐Photon Response

“…This low energy transition appears much more evident in the 2PA spectrum compared to the 1PA case, and also seems to be enhanced in n ‐octanol. It is also evident that for the latter case, a discrete‐continuum representation of solvent environment, which includes an explicit n ‐octanol molecule, substantially increases the lowest‐energy oscillator strength and provides notably better match for observed spectral features than pure continuum model (Figure 5, lower panel), consistent with effects of solvent‐specific HB patterns on spectral line shapes [29] . The solvent dependence is consistent with an ion‐pairing dimer description, as formation would be enhanced in nonpolar solvents, while polar solvents could stabilize individual charged species.…”