Vibrational Relaxation and Redistribution Dynamics in Ruthenium(II) Polypyridyl-Based Charge-Transfer Excited States: A Combined Ultrafast Electronic and Infrared Absorption Study

Abstract

Ultrafast time-resolved electronic and infrared absorption measurements have been carried out on a series of Ru(II) polypyridyl complexes in an effort to delineate the dynamics of vibrational relaxation in this class of charge transfer chromophores. Time-dependent density functional theory calculations performed on compounds of the form [Ru(CN-Me-bpy) x (bpy) 3-x ] 2+ (x = 1-3 for compounds 1-3, respectively, where CN-Me-bpy is 4,4'-dicyano-5,5'-dimethyl-2,2'-bipyridine and bpy is 2,2'-bipyridine) reveal features in their charge-transfer absorption envelopes that allow for selective excitation of the Ru(II)-(CN-Me-bpy) moiety, the lowest-energy MLCT state(s) in each compound of the series. Changes in band shape and amplitude of the time-resolved differential electronic absorption data are ascribed to vibrational cooling in the CN-Me-bpy-localized 3 MLCT state with a time constant of 8 ± 3 ps in all three compounds. This conclusion was corroborated by picosecond time-resolved infrared absorption

Ultrafast Relaxations in Ruthenium Polypyridyl Chromophores Determined by Stochastic Kinetics Simulations

“…Rate coefficients for these optical transitions are assumed to be the same as for the absorptions in Equations (5) and (6). As shown in Figure 5, the experimental time traces of the GSB and ESAs decay from peak intensities within 300 fs before leveling off to an asymptotic value on the picosecond timescale.…”

“…The nature of energy relaxations and the exact timing of charge injection into the semiconductor relative to other processes occurring in the cells are not fully quantified, although recent studies have made significant progress toward this goal. [6][7][8][9][10] It is important to understand these fundamental processes 2 in greater detail. In this work, we focus specifically on quantifying the kinetics of excitations that precede charge injection, revealed by simulations of time-resolved spectroscopies in solution.…”

Wave-Function Symmetry Control of Electron-Transfer Pathways within a Charge-Transfer Chromophore

“…The presence of the GSIVCT bleach reveals that the excited hole lies in a chromophore-centered orbital involved in the ground-state mixed-valence interactions, that is, a dπ z orbital extending along the bimetallic core. Furthermore, this is a hot state, since it deactivates prior to, or concomitantly with, vibrational cooling, which usually takes ∼10 ps. − Thus, this state is labeled hot- 3 MLCTz .…”

Short- and Long-Range Solvation Effects on the Transient UV–Vis Absorption Spectra of a Ru(II)–Polypyridine Complex Disentangled by Nonequilibrium Molecular Dynamics

“…[7][8][9][10][11] Experimental research has been assisted by computational studies able to deliver atomistic insights into the solvent response dynamics and, in turn, on its impact on the electronic structure of the photo-excited solute. 6,[12][13][14][15][16][17][18][19][20][21][22] Within this framework, however, ultrafast experiments on solvated transition metal (TM) complexes are relatively few and recent, 18,19,[23][24][25][26][27][28][29] despite their ubiquitous employment in photovoltaics, optoelectronic devices and medical applications. [30][31][32][33][34][35] Most of the published work focuses on polypyridine complexes, 24,36 in particular on the prototype [Ru(bpy) 3 ] 2+ complex, where the electron localization/delocalization dynamics following the photoinduced CT has been widely studied and debated.…”

“…The solvent relaxation response to the solute out of equilibrium plays a crucial role in determining rates and energetics of any physicochemical process in solution. − In the past two decades, because of the rapid development of ultrafast spectroscopic techniques, an increasing interest has been devoted to the role played by the solvent in the ultrafast dynamics of photoexcited CT states, which are relevant in many photochemical and photophysical phenomena in different fields. − Experimental research has been assisted by computational studies able to deliver atomistic insights into the solvent response dynamics and, in turn, on its impact on the electronic structure of the photoexcited solute. ,− Within this framework, however, ultrafast experiments on solvated transition-metal (TM) complexes are relatively few and recent, ,,− despite their ubiquitous employment in photovoltaics, optoelectronic devices, and medical applications. − Most of the published work focuses on polypyridine complexes, , in particular on the prototype [Ru­(bpy) 3 ] 2+ complex, where the electron localization–delocalization dynamics following the photoinduced CT has been widely studied and debated. ,,,− In the past years, solvent-dependent analysis of the transient absorption spectra, ,,− supported by computational modeling, ,,− suggested the idea of a rather complex solvent response dynamics, with the solvent as a principal actor in the modification of the solute’s excited-state charge distribution, often through the formation or breaking of strongly interacting solute–solvent clusters.…”