Low temperature emission spectra of optically nonlinear N-benzyl-2-methyl-4-nitroaniline crystal

The fluorescence and phosphorescence spectra of N-benzyl-2-methyl-4-nitroaniline (BNA) orthorhombic crystal were measured between 5 and 200 K. The fluorescence spectrum of BNA in a Shpol'skii matrix of n-heptane was also recorded at 5 K. The electronic absorption spectra parameters such as singlet and triplet state energies, dipole moments and oscillator strengths were calculated by semi-empirical and TD DFT methods. The calculated energies of singlet and triplet states and electronic transitions in the BNA molecule were compared with the experimental results. The phosphorescence decay time was estimated to be 270 ms at 5 K. It is presumed that the disappearance of vibronic structure above 30 K observed in the fluorescence spectra is caused by the nitro group vibrations while the structured phosphorescence originates from the trap states. The role of molecular shape towards emission processes in BNA crystal in terms of structure-property relationship is discussed. (C) 2012 Elsevier B.V. All rights reserved.