A Theoretical Study of the N to O Linkage Photoisomerization Efficiency in a Series of Ruthenium Mononitrosyl Complexes

Abstract

Ruthenium nitrosyl complexes are fascinating versatile photoactive molecules that can either undergo NO linkage photoisomerization or NO photorelease. The photochromic response of three ruthenium mononitrosyl complexes, trans-[RuCl(NO)(py)4]2+, trans-[RuBr(NO)(py)4]2+, and trans-(Cl,Cl)[RuCl2(NO)(tpy)]+, has been investigated using density functional theory and time-dependent density functional theory. The N to O photoisomerization pathways and absorption properties of the various stable and metastable species have been computed, providing a simple rationalization of the photoconversion trend in this series of complexes. The dramatic decrease of the N to O photoisomerization efficiency going from the first to the last complex is mainly attributed to an increase of the photoproduct absorption at the irradiation wavelength, rather than a change in the photoisomerization pathways.

Excited State Tracking During the Relaxation of Coordination Compounds

“…Thanks to these calculations, the rationalization of these photoinduced chemical reactions was possible. [26][27][28] The photoisomerization mechanism was also confirmed by UV-Vis absorption spectroscopy [75] and quite recently by MS-CASPT2 calculations. [31] This latter study shed some light on the importance of the higher excited states.…”

“…In the computational studies involving a ruthenium center, the main assumption is that, after light absorption, the singlet excited states, initially populated, rapidly deactivate to the lowest triplet state by non-radiative decays (ISC and IC). [22][23][24][25][26][27][28][29][30][31] This assumption is supported by theoretical [32,76] and experimental [77][78][79][80][81][82] studies.…”

“…Additionally, we have chosen a remarkably difficult case of TMC, the ruthenium-nitrosyl family of complexes, in which, so far, we have not been able to optimize the higher excited states of interest, namely the brightest state within the spectral range from 300 to 500 nm. [26][27][28] A modestlysized ruthenium-nitrosyl complex has been used for the study: the cis-(Cl,Cl)[RuCl 2 (NO)(tpy)] + complex from ref. [27] ( Figure 2).…”

“…A detail computational study of the photochemical pathways involving TMCs is very challenging due not only to the high density of electronic states involved in these systems, but also to their complex electronic structure. Unsurprisingly, not so many computational studies can be found regarding the mechanistic description of photochemical reactions involving directly the metal center [22][23][24][25][26][27][28][29][30][31][32] (compared to the available literature concerning organic systems, see references [9,11,[33][34][35][36][37][38][39] for relevant review articles). One of the main limitations of the TMC photochemical studies is the difficulty of optimizing an n th excited state.…”

CASPT2 Potential Energy Curves for NO Dissociation in a Ruthenium Nitrosyl Complex

“…Computational photochemistry has proven to be a highly efficient tool to understand photo-induced molecular processes [31]. Although a vast majority of computational studies are dealing with organic photochemistry [32][33][34][35] due to the difficulty of computing photochemical pathways in metal complexes [36], recent computational investigations of photoisomerizable metal complexes have been published with the aim of understanding their photoswitching mechanisms [37][38][39][40][41][42][43][44][45]. Regarding ruthenium nitrosyl complexes, the N→O linkage photoisomerization mechanism in the trans-[RuCl(NO)(py) 4 ] 2+ (where py denotes a pyridine ligand) complex was investigated using density functional theory (DFT) and multi-state complete active space second-order perturbation theory photorelease could be observed, suggesting that weakly bound linkage isomers of nitric oxide are likely intermediates in the photolytic release of NO • [19,22].…”