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

Charles W Stark,Aleksander Rebane,Merle Uudsemaa,Matt Rammo,Aleksander Trummal,Katrin Petritsenko,Juri Pahapill,Meelis-Mait Sildoja

doi:10.1038/s42004-019-0213-5

Abstract

Origin of the initial charge separation in optically-excited Ruthenium(II) tris(bidentate) complexes of intrinsic D3 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 1MLCT 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 solute-solvent 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.

Highlights

Origin of the initial charge separation in optically-excited Ruthenium(II) tris(bidentate) complexes of intrinsic D3 symmetry has remained a disputed issue for decades
Despite numerous experimental- and theoretical studies indicating that relaxation and solvent dynamics are responsible for separation of charge after photoexcitation in model complexes [Ru(bpy)3]2+ or [Ru(phen)3]2+ 6–12, no particular mechanism has been identified as responsible for the non-zero permanent dipole moment change in the initial photoexcitation
Our step is to estimate the permanent dipole moment change in the lowest-energy 1MLCT band using the relation between Δμ and two-photon cross section that follows from the two essential states (2ES) model of 2PA27,28: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi jΔ~μj 1⁄4

Summary

Introduction

Origin of the initial charge separation in optically-excited Ruthenium(II) tris(bidentate) complexes of intrinsic D3 symmetry has remained a disputed issue for decades. Even when embedded in a solvent or other condensed dielectric environment, due to the exemplary structural rigidity[16,17,18] and fully occupied t2g orbitals on the Ru(II) center[19,20], complexes largely retain this D3 symmetry in the ground state and structurally equivalent initially excited state. These inferences are at odds with solvatochromism and electrochromism measurements, which show conclusively that in solution or solid matrix, 1MLCTabsorption involves a substantial change of the permanent electric dipole, Δμ = 5–15 Debye (D)[21,22,23,24]. The resulting absorption spectra are readily comparable to theoretical calculations, which elucidate, for the first time, a possible mechanism of nonsymmetric charge separation in the initial excited state

Methods

Results

Conclusion