A New Class of Nitroanilinic Dimer, the PNA O-Dimer: Electronic Structure and Emission Characteristics of O-Dimeric Aggregates

p-Nitroaniline (PNA) has been reported as a "J" aggregate species. In retrospect, this communication confirms a radically different "oblique" orientation of the PNA units in all three solid, liquid, and gas phases of the dimer, the O-dimer. The nonvanishing transition dipole moments (TDM) associated with the allowed electronic excitations of the O-dimer, computed using electron-hole pair density distribution (EDD and HDD) analyses ascertained the two monomers to be inclined at slippage (theta) and polarization (alpha) angles of 18.5 degrees and 55.4 degrees, respectively. A detailed structure-property relationship of the PNA O-dimeric aggregate was carried out using UV-vis absorption and matrix scan emission spectroscopy, supported by electronic structure calculations at DFT-M062X/6-31G+(d,p) level using integral equation formalism polarizable continuum model (IEFPCM). The computed potential energy surface (PES) implied the global minimum of the PNA O-dimer stabilized by 4.8 kcal.mol(-1), owing to bifurcated intermolecular hydrogen bonding. In the excited PNA O-dimeric aggregate, an exchange of excitation energy between the monomeric units resulted in two distinct electronic states separated by an interaction energy of -1644 cm(-1). The TD-DFT computed excited state equilibrium structures of the PNA O-dimer corroborated the experimentally observed pronounced Stoke's shift to internal conversion following vibrational relaxation of the allowed electronic excited states. On the basis of the detailed structural analysis of PNA O-dimer, the observed energy shifts in optical absorption spectroscopy were evident within the framework of exciton coupling model.