Photothermal studies of the room temperature photo-induced spin state change in the Fe(III)(salten)(mepepy) complex

The complex [Fe(III)(salten)(mepepy)]BPh4 (salten = 4-azaheptamethylene-1,7-bis(salicylideneiminate; mepepy = 1-(pyridin-4-yl)-2-(N-methylpyrrol-2-yl)-ethene; BPh4 = tetraphenyl borate) has been investigated to determine the volume and enthalpy changes associated with the room temperature photo-induced spin crossover. Here we report the photophysical properties of the trans to cis isomerization of the mepepy ligand as well as the spin crossover of the Fe(III)(salten)(mepepy) complex in acetonitrile:water mixtures using photoacoustic calorimetry (PAC). The PAC studies indicate that the trans to cis transition of the mepepy ligand occurs faster than the ∼20 ns response time of the acoustic detector and gives rise to a negligible volume change (0.7 ± 0.3 mL mol−1) and an enthalpy change of 33 ± 10 kcal mol−1. These results are consistent with the loss of a charge assisted hydrogen bond between a water molecule and the pyridyl ring of the mepepy upon photoisomerization. In the case of Fe(III)(salten)(mepepy) photoexcitation, PAC results indicate that the high-spin to low-spin transition, also occurring in ≲ 20 ns, gives rise to small volume and enthalpy changes (0.9 mL mol−1 and 4 kcal mol−1). Analysis of the results indicate that the observed thermodynamics are related to a distortion of the Fe(II)(salten)(mepepy) complex associated with the cleavage of an Fe ⋯ N bond upon spin conversion.