The Role of Spin-Orbit Coupling on the Linear Absorption Spectrum and Intersystem Crossing Rate Coefficients of Ruthenium Polypyridyl Dyes

TL;DR

This study uses computational methods to analyze how spin-orbit coupling affects the absorption spectrum and intersystem crossing rates in ruthenium polypyridyl dyes, aiding the design of better solar energy dyes.

Contribution

It provides a detailed computational analysis of spin-orbit effects on absorption and intersystem crossing in ruthenium dyes, highlighting the importance of singlet-triplet mixing and ultrafast transitions.

Findings

Spin-orbit coupling influences the shape of absorption bands.

Intersystem crossing occurs at ultrafast rates (~10^{13} s^{-1}).

Singlet-triplet state mixing is crucial for spectral properties.

Abstract

The successful use of molecular dyes for solar energy conversion requires efficient charge injection, which in turn requires the formation of states with sufficiently long lifetimes (e.g. triplets). The molecular structure elements that confer this property can be found empirically, however computational predictions using $\textit{ab initio}$ electronic structure methods are invaluable to identify structure-property relations for dye sensitizers. The primary challenge for simulations to elucidate the electronic and nuclear origins of these properties is a spin-orbit interaction which drives transitions between electronic states. In this work, we present a computational analysis of the spin-orbit corrected linear absorption cross sections and intersystem crossing rate coefficients for a derivative set of phosphonated tris(2,2'-bipyridine)ruthenium(2+) dye molecules. After sampling the ground state vibrational distributions, the predicted linear absorption cross sections indicate that the mixture between singlet and triplet states plays a crucial role in defining the line shape of the metal-to-ligand charge transfer bands in these derivatives. Additionally, an analysis of the intersystem crossing rate coefficients suggests that transitions from the singlet into the triplet manifolds are ultrafast with rate coefficients on the order of $10^{13}$ s$^{-1}$ for each dye molecule.