Photoelectron spectra of polycyclic aromatic hydrocarbons. Pyrene and coronene☆

The newly implemented parallelized quantum–classical dynamical approach, namely, Time Dependent Discrete Variable Representation (TDDVR) method is applied to the spectroscopically important phenanthrene, acenaphthene and pyrene, where several conical intersections exist in their four lowest singlet excited electronic states (X, A, B and C), to illustrate their various dynamical aspects. This parallel version shows almost linear scalability with increasing number of computing processors. First principles dynamics is performed to the adopted Hamiltonians to obtain photoelectron (PE) spectra, zero electron kinetic energy (ZEKE) spectra, population dynamics and many other dynamical observables. The reduced densities of the wave packet (WP) in coupled electronic surfaces are used to elucidate the intrinsic dynamical features. The TDDVR calculated PE and assigned ZEKE spectra of all CPAHs are found to be good agreement with the previous experimental data and other available theoretical results.

Electron impact ionization near threshold has been studied for the polyaromatic hydrocarbons coronene and corannulene using a high resolution electron monochromator in conjunction with quadrupole mass spectrometry. The mass spectra determined at an electron energy of 100 eV show a rich fragmentation pattern (particularly for coronene) including the presence of multiply charged molecular cations for both compounds. For the singly and multiply charged parent ions of coronene and corannulene we present the ionization efficiency curves near threshold from which the corresponding appearance energies (AEs) are determined using a nonlinear least squares fitting routine. The AEs obtained here are compared where available with previous values reported for both compounds using electron impact ionization and photoionization as well as with theoretical predictions.

The chromophore of pyrene molecule has been studied in detail by using ab initio (HF, DFT and MP2) and the semi-empirical methods (AM1, PM3 and ZINDO). For its molecular structure, HOMO, LUMO, molecular orbital energies and electronic spectra, the theoretical calculations using AM1 (for optimized structure) with ZINDO (for excitation energy) showed results consistent with experimental finding. Further investigation on 1-, 2- and 4-mono-substituted derivatives of pyrene with the effect of the substitution position and the substituents showed: (1) LUMO is decreased when the substituent is an electron acceptor group; (2) HOMO is increased when the substituent is an electron donor group. However, both electron donor and electron acceptor substituents are effective in reducing the energy gap between HOMO and LUMO in the mono-substituted pyrene. In terms of electronic spectra, the transition π→π^∗ shows a larger red shift if the substitutions on the pyrene ring are in the 1- and 4-positions. The red shift is even larger if the substituents contain both electron donor and electron acceptor groups. Nevertheless, a slight blue shift would occur, since the substituents are large groups and distort the planar structure of the parent pyrene molecule. This decreases the overlap of the π orbital and hinders the electron transition from π to π^∗. According to the above conclusion, semi-empirical AM1 and ZINDO methods in this work predict several electron luminescence materials among pyrene derivatives, and the results agree very well with the experimental findings. Presumably, the procedures of theoretical calculation employed in this study can be successfully applied to investigation on the electroluminescence characteristics of other materials, and further, to design novel materials for organic light emitting diode.

The influence of molecular oxygen and a common slip agent, erucamide, on the selectivity of pyrene attachment to polyethylene chains has been investigated. Films with covalently attached pyrenyl groups are useful to follow microscopic changes that occur when the bulk material is perturbed. Selectivity increases when pyrene-doped low-density polyethylene (PE37) films are thoroughly degassed (<10⁻⁵ Torr) prior to irradiation. Surprisingly, an increase in selectivity also increases with erucamide content in pyrene-doped PE37 films. Films were characterized using UV/Vis spectroscopy, steady-state and time-resolved fluorescence techniques, differential scanning calorimetry, and X-ray diffraction.