Time-dependent density functional study of the electronic spectra of oligoacenes in the charge states -1, 0, +1, and +2

We present a systematic theoretical study of the five smallest oligoacenes (naphthalene, anthracene, tetracene, pentacene, and hexacene) in their anionic,neutral, cationic, and dicationic charge states. We used density functional theory (DFT) to obtain the ground-state optimised geometries, and time-dependent DFT (TD-DFT) to evaluate the electronic absorption spectra. Total-energy differences enabled us to evaluate the electron affinities and first and second ionisation energies, the quasiparticle correction to the HOMO-LUMO energy gap and an estimate of the excitonic effects in the neutral molecules. Electronic absorption spectra have been computed by combining two different implementations of TD-DFT: the frequency-space method to study general trends as a function of charge-state and molecular size for the lowest-lying in-plane long-polarised and short-polarised $\pi\to\pi^\star$ electronic transitions, and the real-time propagation scheme to obtain the whole photo-absorption cross-section up to the far-UV. Doubly-ionised PAHs are found to display strong electronic transitions of $\pi\to\pi^\star$ character in the near-IR, visible, and near-UV spectral ranges, like their singly-charged counterparts. While, as expected, the broad plasmon-like structure with its maximum at about 17-18 eV is relatively insensitive to the charge-state of the molecule, a systematic decrease with increasing positive charge of the absorption cross-section between about 6 and about 12 eV is observed for each member of the class.Comment: 38 pages, 11 figures, 7 tables, accepted for publication in Chemical Physic

Contribution to the analysis of the predissociated rovibronic structure of the symmetric isotopomers 16O3 and 18O3 of ozone near 10,400 cm-1: 3A2(A20)

The absorption spectrum of ozone was Recorded at low temperatures (down to −135 °C) by high resolution Fourier transform spectrometry and intra cavity laser absorption spectroscopy (ICLAS) near 10,400 cm−1. A preliminary analysis of the rotational structure of the absorption spectra of 16O3 and 18O3 shows that this spectral region corresponds to a superposition of two different electronic transitions, one with a very broad rotational structure, showing for the first time the asymmetric stretching frequency mode ν3 of the electronic state 3A2, the other formed by a completely diffuse band, probably the 201 band of a new transition due to the triplet electronic state 3B2. Predissociation effects induce large broadening of the rotational lines for the transition centered at 10,473 cm−1 identified as the 302 band of the Image electronic transition. The rotational structure cannot be analyzed directly but instead the band contour method was used to confirm the symmetry of the transition and to estimate the spectroscopic constants for the 16O isotopomer. The origin of the band is at 10,473±3 cm−1 and the value of the 16O3(3A2) antisymmetric stretching frequency mode is equal to 460±2 cm−1. We believe that the diffuse band is due to the 3B2 state and is located at about 10,363±3 cm−1 for 16O3 and 10,354±3 cm−1 for 18O3. The isotopic rules confirm the different results obtained for 18O3 and 16O3