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Reactive hydrocarbon species are important in a multitude of different scientific areas. In this thesis, the vibrational spectra of hydrocarbon radicals, biradicals and their reaction product have been studied in a gas-phase environment. The specific molecules investigated here, are of particular importance in the field of combustion and astrochemistry. They were produced from suitable precursors in a pyrolytically heated micro-reactor and subsequently seeded in an appropriate carrier gas. As methodology, IR/UV ion dip spectroscopy has been utilized, which delivers massselected gas-phase IR spectra of all ionizable species detectable in the molecular beam. These, with the help of DFT calculations, allow for determination of the fingerprint IR spectra, identification of mass carriers and formulation of potential reaction mechanisms. All studies have been conducted in collaboration with the group of Prof. Dr. Anouk M. Rjis and the necessary potent IR radiation has been provided by the free-electron laser FELIX. Thus, the IR/UV measurements have been executed at the FELIX Laboratory of the Radboud University in Nijmegen. The first study presented in this thesis is the investigation of ortho-benzyne in Chapter 3.1. This molecule is of particular interest due to its uncommon electronic structure and its role in high-temperature reactions. Although, the infrared spectrum of o-C6H4 was not accessible, a number of reaction products were identified via their fingerprint spectra. Masses in the range from 78 - 228 were assigned to their respective carrier. The identified species include typical PAHs like naphthalene, phenanthrene, up to triphenylene. The identified masses further suggest a PAH growth heavily influenced by diradical 1,4-cycloaddition followed by fragmentation, as well as by classical HACA- and PAC-like mechanisms. These results were augmented by threshold photoionization measurements from Engelbert Reusch, who identified lighter reaction products, which have insufficient IR absorption or unsuitable ionization characteristics to be identified in the IR/UV experiment. An interesting observation is the identification of m/z = 152. This carrier has been assigned differently by the IR and TPES experiments. Whereas the IR spectrum clearly identifies the species as 2-ethynylnaphthalene, the TPES evidently is in great agreement with biphenylene. This is a good example how different experimental methodologies can benefit from each other to gain a deeper insight into the actual science of a particular system. Probably, the prime example for an aromatically resonance stabilized radical is benzyl. This radical is of high importance for many combustion studies, as it represents the primary high-temperature decomposition product of toluene. The goal of the study was the identification of the benzyl self reaction products and the results are discussed in Section 3.2. The radical was pyrolytically produced by its respective nitrite precursor. The mass spectrum showed that the benzyl self reaction formed two products with C11 and three with C14 constitution. All mass peaks were evenly spaced by two mass units, respectively, which suggests a close relation in formation. Indeed, the C11 products were identified as diphenylmethane and fluorene, which are simply connected via cyclization. The heaviest product was identified as phenanthrene, which is formed via the cyclization of bibenzyl to 9,10-dihydrophenanthrene and subsequent elimination of hydrogen. This result was quiet interesting as the intermediate of this reaction was often assumed to be stilbene, which was not observed in the study. Hence, the reaction seems to undergo cyclization first before phenanthrene is finally formed via hydrogen elimination. Expanding the molecular frame of benzyl by an additional methyl group leads to the xylyl radicals and its decomposition product the xylylenes. Also important in combustion research, xylyl radicals represent the preferred decomposition products of xylene, a frequently used anti-knock agent in modern gasoline blends. After further hydrogen elimination the xylyl radicals can then form their respective xylylenes. The results of the xylyl experiments are discussed in Section 3.3. Here the gas-phase vibrational spectrum in the fingerprint region for all three isomers has been recorded for the first time in isolation. Although, all isomers have a very similar structure and symmetry, and consequently similar vibrational bands, the resolution of the experimental data was exceedingly sufficient for a clear assignment. Additionally, the dimerization products of meta- and para-xylyl could also be identified. A similar approach was taken to determine the fingerprint spectra for the xylylenes. Here, only para-xylylene could be unambiguously identified as the carrier of mass 104. For both ortho- and meta-xylylene precursors, only isomerization products were observed as the carriers of mass 104; benzocyclobutene and styrene, respectively. A possible explanation is elaborated upon in the troubleshooting Sec- tion 3.4.3.5. In the final experimental section a study on the decomposition of phthalide is presented. The objective of this experiment was mainly focused around the formation of C7 species, particularly the fulvenallenyl radical C7H5. In fact, the first experimental fingerprint spectrum of isolated C7H5 in the gas-phase was measured and is displayed in Fig. 3.45. Furthermore, the experiment demonstrates that the pyrolysis products of phthalide are excellent soot precursors, as many heavier reaction products have been identified. These include typical PAH species like naphthalene and phenanthrene as well as their methylated isomers. A large number of molecules with terminal ethynyl moieties indicate a strong influence of HACA growth in the experimental environment. However, many formation pathways of products have been discussed, which are formed involving experiment specific species, like C5H5 and C7H5, and often include expansion steps from 5- to 6-membered rings.
The present work presents investigations on energy and charge transport properties in organic crystals. Chapter 4 treats exciton transport in anthracene, which is an example for weakly coupled π-systems. The electronic coupling parameter is evaluated by the monomer transition density approach. With these and the reorganization energy hopping rates are calculated in the framework of the Marcus theory. Together with the knowledge of the crystal structure, these allow us to calculate the experimental accessible exciton diffusion lengths, whose isotropic part fits nicely within the scattering of experimental values found in the literature. Furthermore, the anisotropy of the exciton diffusion lengths is reproduced qualitatively and quantitatively correct. This chapter also contains studies about electron and hole transport in both polymorphs (α and β) of perylene. Reorganization energies as well as diffusion coefficients for both crystal structures and types of charge transport were calculated. The best transport is hole transport in β-perylene, but it is strongly isotropic. The preferred transport direction is along the b-axis of the unit cell with couplings of greater than 100 meV. However, there is no transport along the c-axis. The diffusion constant in b-direction is bigger by two orders of magnitude than in c-direction (62.7•10-6 m2/s vs. 0.4•10-6 m2/s). Charge transport is calculated to be strongly anisotropic for holes as well as electrons in both modifications. To verify these results experimental electron mobilities have been compared to the simulations. Good agreement was found with errors of less than 27%. As it was shown above, the calculation and measurement of transport properties between weakly coupled systems is possible. However, it is difficult to exactly determine the quality of the electronic coupling. For this reason a collaboration about strongly interacting π-systems was started between us and the research group of Prof. Ingo Fischer. There, [2.2]paracyclophanes and its derivates were investigated to show how hydroxyl substitution influences absorption properties. Overall, a combination of SCS-MP2 and SCS-CC2 performs best to address the description of geometric and electronic structures for both ground and excited states of these model systems as well as their parent compounds benzene and phenol. Only [2.2]paracyclophane shows a double minimum potential regarding a twist and shift motion between the benzene/phenol subunits towards each other. All other systems are less flexible due to their substitution pattern. Almost all [2.2]paracyclophanes display minor changes in their geometric structure upon excitation to the S1 state: The inter-ring distance shortens, but qualitatively they keep their shift and twist characteristics, although the extent of these deformations diminishes. The exception is p-DHPC, which turns from a shifted ground state structure into a twisted excited state structure. Consequently, the intensity of the 0-0 transition cannot be observed experimentally due to small Franck-Condon factors and impurities of o-DHPC. In the present thesis, the structures and their changes due to excitation are explained by electrostatic potentials as well as antibonding (bonding) HOMO (LUMO) orbitals. Adiabatic excitation energies have been corrected by ZPEs and result in accuracies with errors smaller than 0.1 eV. Note that corrections on the B3LYP level worsen the results and one has to apply SCS-CC2 to achieve this accuracy. These calculations allow an interpretation of the experimental [1+1]REMPI spectra. Band progressions of the twist, shift and breathing of the [2.2]paracyclophane skeleton vibrations have been identified and show good agreement to the experiment. This work shows that the substitution pattern in [2.2]paracyclophanes can have a significant impact on spectroscopic properties. Because these properties are directly linked to the transport properties of these materials, the hereby gained insight can be used to design materials with customized transport properties. It was shown that the SCS-CC2 method is very appropriate to predict the interaction between the π-systems