Theoretical insight into the deep-blue amplified spontaneous emission of new organic semiconductor molecules

As for the lasing of organic semiconductor materials, novel semi-conducting materials N,N'-bis(3-methylphenyl)-N,N'-dipheny1[1,1':4',1 ''-terphenyl]-4,4 ''-diamine (BMT-TPD) and N,N'-bis(3-methylphenyl)-N,N'-diphenyl-[1,1':4',1 '':4 '',1'''-quaterphenyl]-4,4'''-diamine (BMQ-TPD) were synthesized. In contrast to the N,N'-diphenyl-N,N'-bis(3-methyl-phenyl)-(1,1'-biphenyl)-4,4'-diamine (TPD), the well known organic gain medium for the lasing, the two new materials have lower amplified spontaneous emission (ASE) thresholds and enhanced lasing performance. The molecular geometries in the electronic ground state and the first optically excited state as well as the vibrational modes of the three compounds have been studied by means of the dependent density functional theory (DFT). Theoretical analysis shows that, accompanying with the increased number of central phenyl rings, the uniform high-frequency modes (around at 1200, 1300 and 1650 cm(-1)) of the three compounds are enhanced, which is driven by the C-H bending and C-C stretching in the central phenyl rings. Correspondingly, the enhanced vibration strength of the high-frequency modes and reinforced vibronic peak intensity bring out the distinct "four-level" energy level scheme which benefits population inversion for lasing and leads to the decreased ASE threshold. These results indicate that the synthesized materials are good candidates to act as active laser materials and our work provides an insight into the lasing of small organic molecules. (C) 2014 Elsevier B.V. All rights reserved.