The impact of hydrogen bonding on 100% photo-switching in solid-state nitro–nitrito linkage isomers

Abstract

Two crystal systems: [Pd(Et₄dien)(NO₂)]OTf [1] and [Pt(Et₄dien)(NO₂)]OTf [2] (Et₄dien = N,N,N′,N′-tetraethyldiethylene-triamine, OTf = trifluoromethanesulfonate) are investigated by steady-state photocrystallographic methods. Both structures contain intermolecular hydrogen bonds to the ground state nitro-(η¹-NO₂) isomer, which are previously shown to limit the achievable level of nitro → nitrito photo-conversion. Irradiation at 100 K induces a mixture of endo-ONO and exo-ONO isomers in 1 and 2, with overall incomplete photo-activation. In contrast, irradiation at higher temperatures leads to much higher conversion, with 100% excitation in 1 at 150 K. The results show that the detrimental effects of hydrogen bonding on the photo-reaction are overcome at higher temperature, adding a new dimension of control to the isomerisation process.