Low-temperature photoluminescence of p-nitroaniline and o-methyl-p-nitroaniline crystals

Photoemission spectra of para-nitroaniline (p-NA) and ortho-methyl-para-nitroaniline (MNA) single crystals measured between 5 and 250 K revealed that the MNA crystals emit phosphorescence and fluorescence in contrast with the p-NA crystals which have only fluorescence. It is assumed that the fluorescence of p-NA crystals at 5 K originates from the doublet state of negatively charged polarons while the fluorescence of MNA crystals at 60 K is due to trap states. The Stokes shift between absorption and emission onsets, measured at 5 K, is much larger in the p-NA crystals than in the MNA and meta-nitroaniline (m-NA) crystals. This fact is rationalized by different geometrical changes caused by excitation.