Electronic excitations in four structurally similar but magnetically different Nickel chain compounds

We report the low temperature zero-field and magnetic field- dependent optical spectra of four Ni$^{2+}$ chain compounds: NENP (Ni[$en$]$_2$NO$_2$ClO$_4$), NENB (Ni[en]$_2 $NO$_2$BF$_4$), NTNB (Ni[$tn$]$_2$NO$_2$BF$_4$), and NINO (Ni[tn] $_2$NO$_2$ClO$_4$). The four differ in the counterions isolating the chains and the rings coordinated to the Ni$^{2+}$ ions. Despite the structural similarities, only three of the compounds exhibit the Haldane gap typical of a spin-1 chain; the fourth, NTNB, behaves like spin glass, likely due to finite chain effects.$^1$ We focus on the near infrared spin-forbidden (SF) electronic $d-d$ transitions and the visible Ni$^{2+}$-to-NO$_2^- $ charge transfer (CT) band. The zero-field absorption spectra differ in the $en$ and $tn$ ring compounds but are nearly identical in compounds with identical rings. The SF and CT band absorption intensities depend on field in a way that reflects the magnetic ground state. In the Haldane compounds, the onset of intensity changes occurs above the crossover field, whereas in NTNB the field-dependent absorption intensities respond to any finite field.$^1$ R.D. Willett and C. J. Gomez-Garcia, EMRS2007, Strasbourg.