Characteristics and Properties of Metal-to-Ligand Charge-Transfer Excited States in 2,3-Bis(2-pyridyl)pyrazine and 2,2‘-Bypyridine Ruthenium Complexes. Perturbation-Theory-Based Correlations of Optical Absorption and Emission Parameters with Electrochemistry and Thermal Kinetics and Related Ab Initio Calculations

Abstract

The absorption, emission, and infrared spectra, metal (Ru) and ligand (PP) half-wave potentials, and ab initio calculations on the ligands (PP) are compared for several [L(n)()Ru(PP)](2+) and [[L(n)Ru]dpp[RuL'(n)]](4+) complexes, where L(n) and L'(n) = (bpy)(2) or (NH(3))(4) and PP = 2,2'-bipyridine (bpy), 2,3-bis(2-pyridyl)pyrazine (dpp), 2,3-bis(2-pyridyl)quinoxaline (dpq), or 2,3-bis(2pyridyl)benzoquinoxaline (dpb). The energy of the metal-to-ligand charge-transfer (MLCT) absorption maximum (hnu(max)) varies in nearly direct proportion to the difference between Ru(III)/Ru(II) and (PP)/(PP)(-) half-wave potentials, DeltaE(1/2), for the monometallic complexes but not for the bimetallic complexes. The MLCT spectra of [(NH(3))(4)Ru(dpp)](2+) exhibit three prominent visible-near-UV absorptions, compared to two for [(NH(3))(4)Ru(bpy)](2+), and are not easily reconciled with the MLCT spectra of [[(NH(3))(4)Ru]dpp[RuL(n)]](4+). The ab initio calculations indicate that the two lowest energy pi orbitals are not much different in energy in the PP ligands (they correlate with the degenerate pi orbitals of benzene) and that both contribute to the observed MLCT transitions. The LUMO energies calculated for the monometallic complexes correlate strongly with the observed hnu(max) (corrected for variations in metal contribution). The LUMO computed for dpp correlates with LUMO + 1 of pyrazine. This inversion of the order of the two lowest energy pi orbitals is unique to dpp in this series of ligands. Configurational mixing of the ground and MLCT excited states is treated as a small perturbation of the overall energies of the metal complexes, resulting in a contribution epsilon(s) to the ground-state energy. The fraction of charge delocalized, alpha(DA)(2), is expected to attenuate the reorganizational energy, chi(reorg), by a factor of approximately (1 - 4alpha(DA)(2) + alpha(DA)(4)), relative to the limit where there is no charge delocalization. This appears to be a substantial effect for these complexes (alpha(DA)(2) congruent with 0.1 for Ru(II)/bpy), and it leads to smaller reorganizational energies for emission than for absorption. Reorganizational energies are inferred from the bandwidths found in Gaussian analyses of the emission and/or absorption spectra. Exchange energies are estimated from the Stokes shifts combined with perturbation--theory-based relationship between the reorganizational energies for absorption and emission values. The results indicate that epsilon(s) is dominated by terms that contribute to electron delocalization between metal and PP ligand. This inference is supported by the large shifts in the N-H stretching frequency of coordinated NH(3) as the number of PP ligands is increased. The measured properties of the bpy and dpp ligands seem to be very similar, but electron delocalization appears to be slightly larger (10-40%) and the exchange energy contributions appear to be comparable (e.g., approximately 1.7 x 10(3) cm(-1) in [Ru(bpy)(2)dpp](2+) compared to approximately 1.3 x 10(3) cm(-1) in the bp...

Probing the Transition between the Localised (Class II) and Localised‐to‐Delocalised (Class II–III) Regimes by Using Intervalence Charge‐Transfer Solvatochromism in a Series of Mixed‐Valence Dinuclear Ruthenium Complexes

“…[28][29][30][31][32][33][34][35][36][37][38][54][55][56] The parameter DE ox defines the potential difference between the [6…”

“…[28][29][30][31][32][33][34][35][36][37][38] The subsequent four one-electron processes correspond to successive reduction of the terminal bpy ligands and their potentials remain relatively constant with bridging ligand variation. In comparison, a cathodic shift of E red1 and E red2 is observed for the series of complexes that is consistent with the stabilisation of the p*(BL) LUMOs in the order ppz, dpq', dpb' for the fused bridging ligands, and 2,3-dpp, dpq, dpb for the unfused ligands.…”

A Theoretical Investigation into the Photophysical Properties of Ruthenium Polypyridine‐Type Complexes

“…However, theoretical analysis of the electronic states of these complexes faces major difficulties, in the shape of the large size of the molecules and the presence of strong electronic correlations. Quantum-mechanical methods based on the Hartree-Fock theory need post-Hartree-Fock treatments to provide reliable descriptions of the excited states, [9] but such approaches rapidly become intractable as the size of the system increases. This explains why calculations have until recently been performed with semiempirical methods [10,11] or on small models of the studied complex.…”

Spectroscopic and Computational Study of Ligand Photodissociation from [Ru(dipyrido[3,2‐a:2′,3′‐c]phenazine)(4‐aminopyridine)4]2+

“…Short lifetimes have already been reported for ruthenium complexes of similar structures. [23,[27][28][29][30] …”