Temperature effect on the optical spectra of Iron(III) metal complexes exhibiting spin crossover and potential nonlinear optical properties

Abstract

Two complexes of formula [Fe$^{III}$(salten)(DEAS)](BPh$_{4}$) and [Fe$^{III}$(salten)(MPS)](BPh$_{4}$) are reported in which H$_{2}$ salten is the 4-azaheptamethylene-1,7-bis(salicylideneiminate) quinquadentate Schiff base ligand, DEAS is 4'-diethylamino-stilbazole, and MPS is 4'-[2-(methoxymethyl)pyrrolidinyl]-stilbazole. Both complexes exhibit a partial S=5/2 → 1/2 spin crossover behaviour on cooling down to low temperature. Additionally, DEAS exhibits potential nonlinear optical (NLO) properties in relation to an intense π-π* charge transfer transition between the amine and the pyridine moieties. The temperature dependence of the optical spectra of [Fe$^{III}$(salten)(DEAS)]$^{+}$ is compared to that of 4'-diethylamino-1-methylstilbazolium (DEAMS$^{+}$), a cation related to [Fe$^{III}$(salten)(DEAS)]$^{+}$, in which the NLO active DEAS ligand is linked to a methyl instead of an iron complex, and therefore exhibits no magnetic behaviour. The occurrence of a red shift upon cooling down is discussed in relation with the spin equilibrium, on the basis of the ZINDO calculated spectra. A similar shift is observed between [Fe$^{III}$(salten)(mepepy)]$^{+}$ complex (mepepy=1-(pyridin-4-yl)-2-(N-methylpyrrol-2-yl)-ethane) and Me-mepepy$^{+}$, N-methylated form of mepepy. The potential effect of these behaviours on the NLO properties is discussed, in a perspective of molecular materials with interacting electronic capabilities.