Impact of isoelectronic substitution on the excited state processes in polycyclic aromatic hydrocarbons: a joint experimental and theoretical study of 4a,8a-azaboranaphthalene

Substituting CC with the isoelectronic BN units is a promising approach to modify the optoelectronic properties of polycyclic aromatic hydrocarbons. While computational studies have already addressed trends in the electronic structure of the various isosteres, experimental data are still scarce. Here, the excited state spectroscopy and dynamics of 4a,8a-azaboranaphthalene were studied by picosecond time-resolved photoionization in a supersonic jet and analyzed with the aid of XMS-CASPT2 and time-dependent DFT calculations. A resonance-enhanced multiphoton ionization spectrum (REMPI) reveals the S1 origin at  = 33 830 ± 12 cm-1. Several vibrational bands were resolved and assigned by comparison with the computations. A [1+1] photoelectron spectrum via the S1 origin yielded an adiabatic ionization energy of 8.27 eV. Selected vibrational bands were subsequently investigated by pump-probe photoionization. While the origin as well as several low-lying vibronic states exhibit lifetimes in the ns-range, a monoexponential decay is observed at higher excitation energies, ranging from 400 ps at +1710 cm-1 to 13 ps at +3360 cm-1. The deactivation is attributed to an internal conversion of the optically excited S1 state via a barrier that gives access to a conical intersection (CI) to the S0 state. The doping significantly changes the energetic ordering of CIs and lowers the corresponding energy barrier for the associated deactivation pathway, as revealed by nudged elastic band (NEB) calculations.

The Electronic Structures of Azaphenanthrenes and Their Dimers

Insertion of a nitrogen atom modifies the electronic structures and photochemistry of polycyclic aromatic hydrocarbons by introducing nπ* states into the molecules. To better understand the electronic structures of isolated polycyclic aromatic nitrogen-containing hydrocarbons (PANHs) and their dimers as well as the influence of the position of the nitrogen atom in the molecule, we investigate three different azaphenanthrenes, benzo[f]quinoline, benzo[h]quinoline, and phenanthridine, in a joint experimental and computational study. Experimentally, resonance-enhanced multiphoton ionization (REMPI) spectroscopy is applied to characterize the excited electronic states. The REMPI spectra of the azaphenanthrene monomers have a rather similar appearance, with origins between 3.645 and 3.670 eV for the 1ππ* ← S0 transition. In contrast to the phenanthrene parent, 2ππ* ← S0 is broad and unstructured even at the band origin. The experiments are accompanied by density functional theory computation, and vibrationally resolved spectra are simulated using a time-independent approach. The differences between phenanthrene and the azaphenanthrenes are assigned to perturbations due to the low-lying 1(nπ*) state, which accelerates nonradiative deactivation. For the dimers, it is found that two π-stacked isomers with two electronic transitions each contribute to the electronic spectrum, leading to overlapping bands that are difficult to assign.

Time-resolved photoelectron spectroscopy of 4-(dimethylamino)benzethyne - an experimental and computational study

We investigated the excited-state dynamics of 4-(dimethylamino)benzethyne (4-DMABE) in a combined theoretical and experimental study using surface-hopping simulations and time-resolved ionisation experiments. The simulations predict a decay of the initially excited S₂ state into the S₁ state in only a few femtoseconds, inducing a subsequent partial twist of the dimethylamino group within ∼100 fs. This leads to drastically reduced Franck-Condon factors for the ionisation transition to the cationic ground state, thus inhibiting the effective ionisation of the molecule, which leads to a vanishing photoelectron signal on a similar timescale as observed in our time-resolved photoelectron spectra. From the phototoelectron spectra, an adiabatic ionisation energy of 7.17 ± 0.02 eV was determined. The experimental decays match the theoretical predictions very well and the combination of both reveals the electronic characteristics of the molecule, namely the role of intramolecular charge transfer (ICT) states in the deactivation pathway of electronically excited 4-DMABE.

IR/UV Double Resonance Study of the 2-Phenylallyl Radical and its Pyrolysis Products

Isolated 2-phenylallyl radicals (2-PA), generated by pyrolysis from a nitrite precursor, have been investigated by IR/UV ion dip spectroscopy using free electron laser radiation. 2-PA is a resonance-stabilized radical that is considered to be involved in the formation of polycyclic aromatic hydrocarbons (PAH) in combustion, but also in interstellar space. The radical is identified based on its gas-phase IR spectrum. Furthermore, a number of bimolecular reaction products are identified, showing that the self-reaction as well as reactions with unimolecular decomposition products of 2-PA form several PAH efficiently. Possible mechanisms are discussed and the chemistry of 2-PA is compared with the one of the related 2-methylallyl and phenylpropargyl radicals.

Stacking Is Favored over Hydrogen Bonding in Azaphenanthrene Dimers

N-Doped polycyclic aromatic hydrocarbons have recently emerged as potential organic electronic materials. The function of such materials is determined not only by the intrinsic electronic properties of individual molecules but also by their supramolecular interactions in the solid state. Therefore, a proper characterization of the interactions between the individual units is of interest to materials science since they ultimately govern properties such as excitons and charge transfer. Here, we report a joint experimental and computational study of two azaphenanthrene dimers to determine the structure and the nature of supramolecular interactions in the aggregates. IR/UV double-resonance experiments were carried out using far- and mid-infrared free-electron laser radiation. The experimental spectra are compared with quantum chemical calculations for the lowest-energy π-stacked and hydrogen-bonded structures. The data reveal a preference of the π-stacked structure for the benzo[f]quinoline and the phenanthridine dimer.

Imaging the Temporal Evolution of Molecular Orbitals during Ultrafast Dissociation

We investigate the temporal evolution of molecular frame angular distributions of Auger electrons emitted during ultrafast dissociation of HCl following a resonant single-photon excitation. The electron emission pattern changes its shape from that of a molecular σ orbital to that of an atomic p state as the system evolves from a molecule into two separated atoms.

Imaging polyatomic molecules in three dimensions using molecular frame photoelectron angular distributions

We demonstrate a method for determining the full three-dimensional molecular-frame photoelectron angular distribution in polyatomic molecules using methane as a prototype. Simultaneous double Auger decay and subsequent dissociation allow measurement of the initial momentum vectors of the ionic fragments and the photoelectron in coincidence, allowing full orientation by observing a three-ion decay pathway, (H+, H+, CH2(+)). We find the striking result that at low photoelectron energies the molecule is effectively imaged by the focusing of photoelectrons along bond directions.

Photo- and auger-electron recoil induced dynamics of interatomic Coulombic decay

At photon energies near the Ne K edge it is shown that for 1s ionization the Auger electron, and for 2s ionization the fast photoelectron, launch vibrational wave packets in a Ne dimer. These wave packets then decay by emission of a slow electron via interatomic Coulombic decay (ICD). The measured and computed ICD electron spectra are shown to be significantly modified by the recoil induced nuclear motion.

Free radical scavenging and skin penetration of troxerutin and vitamin derivatives

BACKGROUND

By its 'protective function', human skin is a potential target for the production of free radicals. The role played by topically applied antioxidants as inhibitors of oxidative stress damage was felt to be worth investigation.

OBJECTIVE

To investigate the free radical scavenging (superoxide, hydroxyl and peroxyl radicals) and skin penetration of troxerutin in association with ascorbyl palmitate and alpha-tocopheryl succinate, esters of two vitamins commonly used in skin care products.

METHODS

The compounds' scavenging activities, in a concentration-dependent manner, were as follows: hydroxyl radicals in a Fenton-based assay; superoxide radicals in a hypoxanthine/xanthine oxidase system; and lipid peroxidation inhibition of liver microsomes was induced by 2,2'-azobis-(2-amidinopropane) dihydrochloride (ABAP).

RESULTS

A synergic action was observed between alpha-tocopheryl succinate and troxerutin for hydroxyl radical scavenging, between the three compounds for superoxide scavenging and between troxerutin and ascorbyl palmitate in lipid peroxidation inhibition.

CONCLUSION

Using a stripping method, it was shown that the three substances, incorporated in a pharmaceutical preparation, permeated through human epidermis. Thus, this association can improve skin care products for preventing free radical-mediated damage.

The case-based Internet textbook ODITEB for multi-modal diagnosis of tumors--development, features and first experiences

Internet technologies offer the chance to build high-quality learning media for the education in medicine. In particular, the teaching of diagnostics with medical imaging can be supported by the excellent visualization and interaction capabilities. In cooperation with three radiological departments at German universities in Munich, Erlangen and Würzburg the distributed case-based Internet textbook ODITEB for tumor diagnosis of the GI-tract, liver, pancreas and thorax has been developed. It offers a growing collection of didactically prepared cases located on servers at the provider sites Munich, Erlangen and Würzburg, functionality similar to a real CT console, original DICOM data, X-rays and endoscopic and endosonographic videos, and expert-guided tours through the cases. In a first evaluation in summer 1998, 32 medical students graded the application with 1.9 ('good') on a scale from 1 ('very good') to 5 ('very bad').