Excited-State Electronic Structure in Polypyridyl Complexes Containing Unsymmetrical Ligands

Abstract

Step-scan Fourier transform infrared absorption difference time-resolved (S(2)FTIR DeltaA TRS) and time-resolved resonance Raman (TR(3)) spectroscopies have been applied to a series of questions related to excited-state structure in the metal-to-ligand charge transfer (MLCT) excited states of [Ru(bpy)(2)(4,4'-(CO(2)Et)(2)bpy)](2+), [Ru(bpy)(2)(4-CO(2)Et-4'-CH(3)bpy)](2+), [Ru(bpy)(4,4'-(CO(2)Et)(2)bpy)(2)](2+), [Ru(4,4'-(CO(2)Et)(2)bpy)(3)](2+), [Ru(bpy)(2)(4,4'-(CONEt(2))(2)bpy)](2+), [Ru(bpy)(2)(4-CONEt(2)-4'-CH(3)bpy)](2+), and [Ru(4-CONEt(2)-4'-CH(3)bpy)(3)](2+) (bpy is 2,2'-bipyridine). These complexes contain bpy ligands which are either symmetrically or unsymmetrically derivatized with electron-withdrawing ester or amide substituents. Analysis of the vibrational data, largely based on the magnitudes of the nu(CO) shifts of the amide and ester substituents (Deltanu(CO)), reveals that the ester- or amide-derivatized ligands are the ultimate acceptors and that the excited electron is localized on one acceptor ligand on the nanosecond time scale. In the unsymmetrically substituted acceptor ligands, the excited electron is largely polarized toward the ester- or amide-derivatized pyridine rings. In the MLCT excited states of [Ru(bpy)(2)(4,4'-(CO(2)Et)(2)bpy)](2+) and [Ru(bpy)(2)(4,4'-(CONEt(2))(2)bpy)](2+), Deltanu(CO) is only 60-70% of that observed upon complete ligand reduction due to a strong polarization interaction in the excited state between the dpi(5) Ru(III) core and the excited electron.

Probing excited state charge transfer dynamics in a heteroleptic ruthenium complex

“…[Ru (III) (bpy) À (bpy) 2 ] 2+ ) and thus charge is localized on one of three ligands and the charge localization occurs on a femtosecond timescale. [12][13][14][15][16][17][18][19][20][21] Symmetry breaking via vibrational trapping or solvent fluctuation has been proposed as the possible mechanism for charge localization. 22 Though ultrafast dynamics of the prototype [Ru(bpy) 3 ] 2+ have been extensively studied to discern ISC, vibrational relaxation and charge localization processes, real time dynamics studies of mixed ligand (heteroleptic) complexes have not been extensively undertaken and studies on such complexes has gained impetus in recent years.…”

Long‐Lived, Emissive Excited States in Direct and Amide‐Linked Thienyl‐Substituted RuII Complexes

“…6) is expected based on previously reported studies of Ru II complexes with similar secondary amide substituents. Meyer and co-workers, [62] as well as George et al [5] have reported an analogous shift in the emission due to the slight electron-withdrawing character of the amide linkage (in these examples, the amide carbonyl was linked to the pyridine ring). In the case of the sec-amL ligand, attachment of the amide linkage to the pyridine ring via the amide N atom should result in a slight increase in the energy level of the acceptor (bipyridine) HOMO; producing a narrowing of the HOMO-LUMO gap, and a corresponding bathochromic shift in the emission (and in the ground state absorption spectrum).…”

Synthesis of Unsymmetrical Bis(phosphine) Oxides and Their Phosphines via Secondary Phosphine Oxide Precursors

“…In contrast, bidentate ligands can have different groups bonded to the donor atoms, making them unsymmetrical. Many examples of unsymmetrical ligands exist, notably: salen ligands (N donor atoms) [11], bypyridine and other bidentate pyridine ligands [12,13], phosphine-carboxylate ligands [3], phosphine-carboxylate ligands [1], phosphine-phosphine oxide ligands [14], phosphine-phosphite ligands [15], phosphinephosphinite ligands [15], and NHC ligands [16]. However, it is uncommon to encounter unsymmetrical phosphine-phosphine ligands [17][18][19][20][21][22][23][24][25].…”