Proceedings PaperTheoretical Study Of The Existence Of Metastable Semiconducting P-Nitroaniline Formed At High Shock Pressure
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The application of pressure is known to enormously enhance electrical conductivities of existing insulating and semiconducting organic polymers and organic salts. However, none of these compounds are metastable, so that their superior high-pressure characteristics are not retained when the pressure is removed. In order to meet SDI requirements for improved, highly-sensitive optical and infrared detectors, for example, any breakthrough metastable organic conductor must combine superior mechanical and electrical properties which are maintained over large ranges of pressure and temperature. Semiempirical and ab initio LCAO-SCF and CI "tight-binding" calculations of the ground and excited electronic states of p-nitroaniline (PNA) indicate that certain polymorphs may have the potential for becoming strong quasi-one-dimensional metastable semiconductors when processed with uniaxial shocks. These calculations were done for the head-to-tail orientation and at various intermolecular separations of the PNA monomers in (PNA)n, n=2 and 3. The results show that the excited states associated with the intense charge transfer band and a weaker band are strongly red-shifted from 4.4 ev at ambient pressure to optical gaps of 3.76 ev and 1.33 ev, respectively, at a pressure of roughly 400 kbar. The effect of these shifts is to greatly enhance intra-and inter-molecular charge transfer during excitation. The accompanying increase of the monomer dipole moment by 4.5D at 400 kbar may result in strong crystals due to the increased dipole-dipole interaction. The state at 1.33 ev may facilitate an insulator-to-semi-conductor transition. For this state, electronic charge hopping can occur between the NO2 and NH2 groups of nearest-neighbor monomers. The minimum activation energy for hopping is found to be 0.4 ev at 0°K.