Ultrafast Relaxations in Ruthenium Polypyridyl Chromophores Determined by Stochastic Kinetics Simulations

Abstract

Maximizing the efficiency of solar energy conversion using dye assemblies rests on understanding where the energy goes following absorption. Transient spectroscopies in solution are useful for this purpose, and the time-resolved data are usually analyzed with a sum of exponentials. This treatment assumes that dynamic events are well separated in time, and that the resulting exponential pre-factors and phenomenological lifetimes are related directly to primary physical values. Such assumptions break down for coincident absorption, emission, and excited state relaxation that occur in transient absorption and photoluminescence of tris(2,2'-bipyridine)ruthenium(2+) derivatives, confounding the physical meaning of the reported lifetimes. In this work, we use inductive modeling and stochastic chemical kinetics to develop a detailed description of the primary ultrafast photophysics in transient spectroscopies of a series of Ru dyes, as an alternative to sums of exponentials analysis. Commonly invoked 3-level schemes involving absorption, intersystem crossing, and slow non-radiative relaxation and incoherent emission to the ground state cannot reproduce the experimentally measured spectra. The kinetics simulations reveal that ultrafast decay from the singlet excited state manifold to the ground state competes with intersystem crossing to the triplet excited state, whose efficiency as determined to be less than unity. The populations predicted by the simulations are used to estimate the magnitudes of transition dipoles for excited state excitations, and evaluate the influence of specific ligands. The mechanistic framework and methodology presented here are entirely general, applicable to other dye classes, and can be extended to include charge injection by molecules bound to semiconductor surfaces.

Spectroscopic Investigations Provide a Rationale for the Hydrogen-Evolving Activity of Dye-Sensitized Photocathodes Based on a Cobalt Tetraazamacrocyclic Catalyst

“…It must be stated that, due to similar time constants, the excited state relaxation of the dye as well as the hole injection and charge recombination processes overlapped temporally and could not be deconvoluted with the sum of exponentials analysis performed here. In the future, more advanced tools such as the recently reported stochastic kinetics simulations 101 could be applied to provide a better description of these early events. Second, thermally activated electron transfer from the reduced dye to the Co II center yielding the reduced (Co I ) catalyst predominantly occurred on a time window of 1.6 to 20 ns, with only a small contribution taking place on longer timescales (average time constant of 9 s).…”

Ruthenium Dye Excitations and Relaxations in Natural Sunlight

“…Our calculation of ϕ ' using the number of events is greater than the previously computed ϕ ISC though still significantly less than unity. 4…”

“…Analysis of our simulations has uncovered a relation between dye structure and the frequency of absorption and relaxation events under diffuse solar illumination, including rare events such as ESA and ESE that occur at the ppm-level, which was not accessible in our previous analysis of transient measurements. 4 Increasing the numbers of phosphonated ligands increases the frequency of relaxations within the singlet manifold, and decreases relaxations from the triplet state to the ground state. The influence of the ligands on intersystem crossing is not large.…”

“…With the validated scheme for LA in hand, the solar irradiation spectrum [19][20][21] in Figure 2B (solid black) can be used to calculate dye populations under natural sunlight. The product of the normalized signal components of the LA and TA spectra-fit as sums of Gaussians in ref 4 -and the AM 1.5 global tilt spectrum provide a foundation for capturing the response of set 6-Ru under 1-sun conditions (details in SI Section S8). The optical rate coefficients are then given by Equation Error: Reference source not found:…”

Adaptive response by an electrolyte: resilience to electron losses in a dye-sensitized porous photoanode

“…The I 2 À formed as a result of I À reactions with Dye + undergoes a disproportionation reaction to regenerate I À and I 3 types of reactions in specic locations. Recent examples of how this computational technique aids in analysis of the simulation results are presented elsewhere, 28,35,36 details pertinent to how it is implemented in this work are presented in several sections of the ESI. †…”