Exploring the photoswitching pathways and efficiency of NO isomerization in ethylenediamine ruthenium nitrosyl complexes

Photo- and thermoswitchable molecular switches are attractive functional blocks, since structural modification induced by external stimuli can change the optical or ferroic properties of a material. In this work we report a series of photoswitchable ethylenediamine ruthenium nitrosyl complexes, in which the ligand environment significantly influences the thermal stability and photoconversion (population) of photoinduced linkage isomers (PLI) of the NO ligand. In the studied complexes, the energetically stable ground state (GS) nitrosyl linkage Ru-NO can be reversibly switched to the metastable isonitrosyl linkage Ru-ON (MS1) or the side-on nitrosyl linkage Ru-η²-(NO) (MS2) under blue (405-420 nm) or subsequent infrared (940-980 nm) irradiation, respectively. The reverse transformation back to GS can be induced by 500-700 nm irradiation or by temperature. It was found that the highest known decay temperature (<i>T</i>_d = 215 K) of the side-on MS2 isomer can be achieved in [RuNO(en)₂(H₂O)](NO₃)₃ (2) having the H₂O ligand as <i>trans</i>-to-NO ligand. Replacing H₂O by OH^-, [RuNO(en)₂OH](NO₃)₂ (1), leads to a decrease of <i>T</i>_d down to 207 K, but at the same time to an increase in the population of the linkage isomers Ru-ON (MS1, 47%) and Ru-η²-(NO) (MS2, 17%). Photocrystallographic analysis allowed us to unambiguously assign IR- and UV-vis spectroscopic signatures to the corresponding structural linkage isomer and study the mechanism of isomerization using ns pulsed excitation. As a result, we found that 410 nm pulsed excitation of the ground state isomer (Ru-NO, GS) first generates MS2 (GS → MS2) and only then produces isomer MS1 (MS2 → MS1) reaching a photostationary equilibrium GS ⇄ MS2 ⇄ MS1. According to modelled effective rate constants (<i>k</i>_1-4) of all transformations under 410 nm irradiation, the population of MS2 stays very low (less than 1%) due to the high rate constants of the MS2 → GS and MS2 → MS1 processes.