2-Amino-4-nitroaniline, a Known Compound with Unexpected Properties

2-Amino-4-nitroaniline crystallizes in a noncentrosymmetric space group, which gives it significant potential
for second-order nonlinear optical properties (NLO) in the bulk. Crystallographic and linear-polarized IR-spectrocopic data in the solid state unambiguously confirm a quinoide-like structure in the ground state in
contrast to UV and theoretical data, which indicate an aromatic one for the excited state. UV−vis spectral
elucidations in solutions with different polarities indicate a significant charge-transfer band with shifts of up
to 100 nm, corresponding to a large value for the molecular first hyperpolarizability. Calculations of the UV-
and IR-spectroscopic properties confirm the stabilization of the quinoide-like structure in the ground state, as
well as the theoretically predicted NLO properties at the molecular level and provide a value of βtot, which
is some 3.6 times higher than the analogous parameter for p-nitroaniline, a classical compound with
experimentally confirmed NLO properties.