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

Abstract

The fluorescence and phosphorescence spectra of N-benzyl-2-methyl-4-nitroaniline (BNA) orthorhombic crystal were measured between 5 and 200K. The fluorescence spectrum of BNA in a Shpol’skii matrix of n-heptane was also recorded at 5K. 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 270ms at 5K. It is presumed that the disappearance of vibronic structure above 30K 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.