Comparison of Magnetic Field-Modified Electronic Excitations in Ni(II) Compounds

NTDN (Ni[tn]$_2$[NO$_2$]$_2$) can be considered a paramagnetic analog material to the Haldane compounds NENP and NENB (Ni[en]$_2 $NO$_2$ClO$_4$ and Ni[en]$_2$NO$_2$BF$_4$; where en = C$_2$N$_2 $H$_{8}$ and tn = C$_2$N$_3$H$_10$). Except for the different bonding of one NO$_2$ group and the absence or presence of spin chains, NTDN and the Haldane compounds have nearly identical electronic coordination around the Ni$^{2+}$ ions. Here, we report and compare the magnetic field ($H$)-dependent polarized optical transmittance of the three materials in the range 9,000 to 22,000 cm$^{-1}$. The $H$ dependence is manifest in the varying intensities of certain electronic absorptions with applied field. Although all three materials possess similar $H$- sensitive excitations, the details of the $H$ dependence differ with the magnetic ground states. In NTDN, the intensity changes commence at $H$ = 0 and saturate at $\approx$ 10 T, whereas in the Haldane compounds, the onset of changes is at the gap- closing critical field, $H_C$, above which the intensity is linearly modified with field. The mechanism of the $H$- dependence is yet to be clarified and probably depends on the nature of the electronic excitation. Intensity variations with applied field are observed in both Ni$^{2+}$-to-NO$_2^-$ charge transfer transitions and Ni$^{2+}$ $d-d$ spin forbidden excitations.