Excited state tracking during the relaxation of coordination compounds.

Juan Sanz García,Martial Boggio-Pasqua,Marco Campetella,Ilaria Ciofini

doi:10.1002/jcc.25800

Abstract

The ability to locate minima on electronic excited states (ESs) potential energy surfaces both in the case of bright and dark states is crucial for a full understanding of photochemical reactions. This task has become a standard practice for small‐ to medium‐sized organic chromophores thanks to the constant developments in the field of computational photochemistry. However, this remains a very challenging effort when it comes to the optimization of ESs of transition metal complexes (TMCs), not only due to the presence of several electronic ESs close in energy, but also due to the complex nature of the ESs involved. In this article, we present a simple yet powerful method to follow an ES of interest during a structural optimization in the case of TMCs, based on the use of a compact hole‐particle representation of the electronic transition, namely the natural transition orbitals (NTOs). State tracking using NTOs is unambiguously accomplished by computing the mono‐electronic wave function overlap between consecutive steps of the optimization. Here, we demonstrate that this simple but robust procedure works not only in the case of the cytosine but also in the case of the ES optimization of a ruthenium nitrosyl complex which is very problematic with standard approaches. © 2019 The Authors. Journal of Computational Chemistry published by Wiley Periodicals, Inc.

Highlights

Computational photochemistry has gained an ever growing interest among the scientific community over the last few decades
The preliminary ground state (GS) Density Functional Theory (DFT) and the excited states (ESs) time-dependent density functional theory (TD-DFT) optimizations were performed using the same hybrid functional PBE0[48] with the (6-31+G(d))[49] diffuse-augmented polarization valence-double-ζ basis set with one set of d polarization functions[50, 51] and a set of s and p diffuse functions[52, 53] for all atoms but hydrogens
Starting from a previously optimized structure in acetonitrile,[27] the GS geometry of the cis-(Cl,Cl)[RuCl2(NO)(tpy)]+ complex was reoptimized in vacuum using the standard hybrid functional B3LYP,[54, 55] as in ref. [27] with a double-ζ Ahlrichs-type basis set for the hydrogen atoms, a triple-ζ Ahlrichs-type basis set with one set of d polarization functions for the second- and third-row elements,[56] and a Stuttgart relativistic effective core potential[57] with its associated basis set[57] including two sets of f functions for the ruthenium,[58] this basis set will be denoted hereafter as “BS1”

Summary

Introduction

Computational photochemistry has gained an ever growing interest among the scientific community over the last few decades. Unlike the methods mentioned afore, this new formalism is based on the overlap between the NTOs describing the excited estates This new formalism proves to be very efficient in the problematic ES optimizations of TMCs. A detail computational study of the photochemical pathways involving TMCs is very challenging due to the high density of electronic states involved in these systems, and to their complex electronic structure. In order to gain a deeper understanding on the photochemical mechanisms, it would be necessary to understand the role of the higher excited states This lack of information has motivated the development of the new formalism presented in this paper to compare different ES of TMC (or any other chemical system).

Theoretical Background

Results and Discussion

Cytosine

Conclusions