Ab initio theoretical study of the electronic absorption spectra of polycyclic aromatic hydrocarbon radical cations of naphthalene, anthracene and phenanthrene

Photon energy dependence of the photoelectron spectra of the anthracene anion: On the influence of autodetaching states

Vibrationally resolved photoelectron spectra of anthracene anions have been measured for photon energies between 1.13 and 4.96 eV. In this energy range, photoemission mostly occurs via autodetaching electronically excited states of the anion, which strongly modifies the vibrational excitation of the neutral molecule after electron emission. Based on the observed vibrational patterns, eight different excited states could be identified, seven of which are resonances known from absorption spectroscopy. Distinctly different photon energy dependencies of vibrational excitations have been obtained for different excited states, hinting at strongly different photoemission lifetimes. Unexpectedly, some resonances seem to exhibit bimodal distributions of emission lifetimes, possibly due to electronic relaxation processes induced by the excitation of specific vibrational modes.

Vibrational Relaxation of XUV-Induced Hot Ground State Cations of Naphthalene

Time-resolved XUV-IR photoion mass spectroscopy of naphthalene conducted with broadband as well as with wavelength-selected narrowband XUV pulses reveals a rising probability of fragmentation characterized by a lifetime of 92 ± 4 fs. This lifetime is independent of the XUV excitation wavelength and is the same for all low appearance energy fragments recorded in the experiment. Analysis of the experimental data in conjunction with a statistical multistate vibronic model suggests that the experimental signals track vibrational energy redistribution on the potential energy surface of the ground-state cation. In particular, populations of the out-of-plane ring twist and the out-of-plane wave bending modes could be responsible for opening new IR absorption channels, leading to enhanced fragmentation.

Calculating X-ray Absorption Spectra of Open-Shell Molecules with the Unrestricted Algebraic-Diagrammatic Construction Scheme for the Polarization Propagator

X-ray absorption spectroscopy (XAS) is a powerful tool that provides information about the electronic structure of molecules via excitation of electrons from the K-shell core region to the unoccupied molecular levels. These high-lying electronic core-excited states can be accurately calculated using the algebraic-diagrammatic construction scheme of second order ADC(2) by applying the core-valence separation (CVS) approximation to the ADC(2) working equations. For the first time, an efficient implementation of an unrestricted CVS-ADC(2) variant CVS-UADC(2) is presented for the calculation of open-shell molecules by treating α and β spins separately from each other. The potential of the CVS-UADC(2) method is demonstrated with a set of small organic radicals by comparison with standard TD-DFT/B3LYP values and experimental data. It turns out that the extended variant CVS-UADC(2)-x, in particular, provides the most accurate results with errors of only 0.1% compared to experimental values. This remarkable agreement justifies the prediction of yet nonrecorded experimental XAS spectra like the one of the anthracene cation. The cation exhibits additional peaks due to the half-filled single-occupied molecular orbital, which may help to distinguish cation from the neutral species.

Theoretical Study on Molecules of Interstellar Interest. II. Radical Cation of Compact Polycyclic Aromatic Hydrocarbons

Radical cations of polycyclic aromatic hydrocarbons have been postulated to be molecular carriers of diffuse spectroscopic features observed in the interstellar medium. Several important observations made by stellar and laboratory spectroscopists motivated us to undertake a detailed theoretical study attempting to validate the recorded data. In continuation of our work on this subject, we here focus on a detailed theoretical study of the doublet ground (X̃) and low-lying excited (Ã, B̃, and C̃) electronic states of the radical cation of phenanthrene, pyrene, and acenaphthene molecule. A multistate and multimode theoretical model in a diabatic electronic basis is developed here through extensive ab initio quantum chemistry calculations. Employing this model, first-principles nuclear dynamics calculations are carried out to unravel the spectral assignment, time-dependent dynamics, and photostability of the mentioned electronic states of the radical cations. The theoretical results compare well with the observed experimental data.

Incorporation and electron transfer of anthracene in pores of ZSM-5 zeolites. Effect of Brønsted acid site density

The sorption course of anthracene (ACENE-3) into Brønsted-acidic medium pore MFI zeolites was monitored by in situ EPR and diffuse reflectance UV-visible absorption over one year. Weighed amounts of solid ACENE-3 were merely exposed to H(n)ZSM-5 (H(n)(AlO(2))(n)(SiO(2))(96-n)), with the following Brønsted acid site (BAS) densities, n = 0.0, 0.17, 0.57, 0.95, 2.0, 3.4, 6.6, dehydrated at 623 K under argon. The weighed amounts correspond to 1 ACENE-3 per zeolite unit cell. ACENE-3 is found to be incorporated as intact molecules in purely siliceous MFI (silicalite-1). Monte Carlo simulations indicate that ACENE-3 lies in the intersection of straight and zigzag channels. In contrast, the presence of BASs on the inner surface of channels induces spontaneous ionization of ACENE-3 (ionization potential = 7.44 eV). The charge separation as ACENE-3*(+)@H(n)ZSM-5*(-) is caused by the strong Coulombic field gradient of Si-O(-)(H(+))-Al BAS in the absence of any Lewis acid site. The rate and yield of ionization are found to increase dramatically with BAS density increase. The stabilization of ACENE-3*(+)@H(n)ZSM-5*(-) is explained by the tight fit between the rod-shape ACENE-3 and the channel dimensions and especially by the compartmentalization of ejected electrons as AlO(4)H*(-) centers away from the initial site of ionization. The final charge recombination occurs after more than one year and leads to ACENE-3 occluded in the straight channel in close proximity to BAS without any protonation of ACENE-3 (pK(a) = -13.5).

Aromaticity of Giant Polycyclic Aromatic Hydrocarbons with Hollow Sites: Super Ring Currents in Super-Rings

We present a systematic theoretical study based on semi-empirical, Hartree-Fock (HF), and density functional theory (DFT) models of a series of polycyclic aromatic hydrocarbons (PAHs) that exhibit hollow sites. In this study we focus particularly on the magnetic criteria of aromaticity, namely (1)H NMR and nucleus-independent chemical shifts (NICS), and on their relationships with other electronic properties. The computed shifts and NICS indices indicate that an external magnetic field induces exceptionally strong ring currents in even-layered PAH doughnuts, in particular in the layer directly adjacent to the central hole of double-layered compounds. These exceptionally strong ring currents also correlate with particularly small HOMO-LUMO gaps and electronic excitation energies and to abnormally high polarizabilities, indicating in turn that these compounds have a more pronounced metallic character. Comparison is made with further depictions of aromaticity in these systems and in [18]-[66]annulene rings by employing topological, structural, and energetic criteria.

Intramolecular Energy Transfer Involving Heisenberg Spin-Coupled Dinuclear Iron−Oxo Complexes

The synthesis, structure, and physical properties of a series of oxo-bridged dinuclear Fe(III) complexes containing pendant naphthalene groups are described. The compounds [Fe(2)O(O(2)CCH(2)-C(10)H(7))(tren)(2)](BPh(4))(NO(3))(2) (8), [Fe(2)O(O(2)CCH(2)-C(10)H(7))(TPA)(2)](ClO(4))(3) (9), Fe(2)O(O(2)CCH(2)-C(10)H(7))(2)(Tp)(2) (10), and Fe(2)O((O(2)CCH(2)CH(2))(2)-C(10)H(6))(Tp)(2) (11) (where tren is tris(2-aminoethyl)amine, TPA is tris(2-pyridyl)amine, and Tp is hydrotrispyrazolylborate) have been characterized in terms of their structural, spectroscopic, magnetic, and photophysical properties. All four complexes exhibit moderately strong intramolecular antiferromagnetic exchange between the high-spin ferric ions (ca. -130 cm(-)(1) for H = -2JS(1).S(2)). Room-temperature steady-state emission spectra for compounds 8-11 in deoxygenated CH(3)CN solution reveal spectral profiles similar to methyl-2-naphthyl acetate and [Zn(2)(OH)(O(2)CCH(2)-C(10)H(7))(2)(TACN-Me(3))(2)](ClO(4)) (13, where TACN-Me(3) is N,N,N-1,4,7-trimethyltriazacyclononane) but are significantly weaker in intensity relative to these latter two compounds. Time-resolved emission data for the iron complexes following excitation at 280 nm can be fit to simple exponential decay models with tau(obs)(S)()1 = 36 +/- 2, 32 +/- 4, 30 +/- 5, and 39 +/- 3 ns for compounds 8-11, respectively. The decays are assigned to the S(1) --> S(0) fluorescence of naphthalene; all of the lifetimes are less than that of the zinc model complex (tau(obs)(S)()1 = 45 +/- 2 ns), indicating quenching of the S(1) state by the iron-oxo core. Nanosecond time-resolved absorption data on [Zn(2)(OH)(O(2)CCH(2)-C(10)H(7))(2)(TACN-Me(3))(2)](ClO(4)) reveal a feature at lambda(max) = 420 nm that can be assigned as the T(1) --> T(n) absorption of the naphthalene triplet; the rise time of 50 +/- 10 ns corresponds to an intersystem crossing rate of 2 x 10(7) s(-1). A similar feature (though much weaker in intensity) is also observed for compound 8. The order-of-magnitude reduction in the T(1) lifetime of the pendant naphthalene for all of the iron-oxo complexes (tau(obs)(T)1 = 5 +/- 2 micros vs 90 +/- 10 micros for [Zn(2)(OH)(O(2)CCH(2)-C(10)H(7))(2)(TACN-Me(3))(2)](ClO(4))) indicates quenching of the naphthalene triplet with an efficiency of >90%. Neither the naphthalene radical cation nor the reduced Fe(II)Fe(III) species were observed by transient absorption spectroscopy, implying that energy transfer is the most likely origin for the quenching of both the S(1) and T(1) states. Spectral overlap considerations strongly support a Förster (i.e., dipolar) mechanism for energy transfer from the S(1) state, whereas the lack of phosphorescence from either the free naphthyl ester or the Zn model complex suggests Dexter transfer to the diiron(III) core as the principal mechanism of triplet quenching. The notion of whether spin exchange within the diiron(III) core is in part responsible for the unusual ability of the iron-oxo core to engage in energy transfer from both the singlet and triplet manifolds of naphthalene is discussed.