Aspects of organic chemistry of fullerenes

Successive additions to [60]fullerene show a remarkable degree of regioselectivity. Two characteristic examples of additions, namely nucleophilic cyclopropanations of 6-6 double bonds as well as formations of 5–6 bridged open azafulleroids, have been systematically investigated by analytical, spectroscopic as well as by computational methods. Regioselective additions to activated positions within the carbon framework of fullerene derivatives are the key steps for both controlled ring opening of the fullerene core and the synthesis of heterofullerenes.     

Electron-exciton complex formation in organic solids

Sharp structure in the doubly differentiated electron transmission spectra of thin (? 150 Å) benzene films is shown to result from electron-exciton complex formation. The large energy shift between the electronic energy levels of the molecule and the transmission maxima is related to the electron binding energy. This is due to the polarizability difference between the Frenkel exciton and the organic molecule. From a simple Wigner-Seitz model, we show that the captured electron is localized between the exciton and the sorrounding molecules.     

Superconducting fluctuations in one-dimensional organic solids

It is suggested that the superconducting fluctuations observed by Coleman et al. just above the Peierls soft mode instability in psuedo one-dimensional organic solids is a reflection of Fröhlich's one-dimensional model in which superconductivity arises from a coupling of the electrons with moving lattice waves rather than from a BCS type pairing.     

Diffuse scattering from fullerenes

Fullerene solids exhibit textbook examples of orientational freezing transitions, i.e. structural transitions from a high temperature phase, where the molecules undergo rapid reorientation, to a low temperature phase where the molecules are locked-in into discrete orientations. In general, even low temperature phases are not completely ordered because of frozen-in orientational disorder, partly also because of a high density of stacking faults. Diffuse scattering has been used extensively to characterize both dynamic and static disorder in fullerene solids. A review is given on the experimental results as well as on the various techniques employed to analyze the data.     

Injection of holes into organic molecular solids

A model is proposed for injection of carriers (holes) from a metal into organic molecular solids. It is shown that the total injection current is restricted by the process of tunneling between the levels of a molecule and states in the metal. The role played by phonons in the tunneling transition is studied in detail, and the contribution from these transitions to the total injection current is estimated over a wide temperature range.     

Spin-flip induced magnetoresistance in positionally disordered organic solids

A model for magnetoresistance in positionally disordered organic materials is presented and solved using percolation theory. The model describes the effects of spin dynamics on hopping transport by considering changes in the effective density of hopping sites, a key quantity determining the properties of percolative transport. Faster spin-flip transitions open up "spin-blocked" pathways to become viable conduction channels and hence produce magnetoresistance. Features of this percolative magnetoresistance can be found analytically in several regimes, and agree with previous measurements, including the sensitive dependence of the magnetic-field dependence of the magnetoresistance on the ratio of the carrier hopping time to the hyperfine-induced carrier spin precession time. Studies of magnetoresistance in known systems with controllable positional disorder would provide an additional stringent test of this theory.     

Formation of neutral clusters during sputtering of gold

Polycrystalline Au was bombarded with 15 keV Ar⁺, and the resulting secondary neutral cluster yield distribution was measured by laser postionisation mass spectrometry. Neutral Au_n clusters containing up to 20 atoms were observed. The yield of Au_n clusters, Y_n, was found to follow a power in n, Y_n ∝ n^−3.4, but the yield of individual clusters depended on whether n was even or odd. This odd-even yield variation was caused by fragmentation of the cluster photoions. Simulation of photoion trajectories within the TOF spectrometer shows that the fragmentation dominantly occurs before the photoions enter the reflectron part of the spectrometer.     

Measurement of spin-lattice relaxation times of C in organic solids

A transient nuclear Overhauser effect (NOE) makes measurements of the ¹³C spin-lattice relaxation times in organic solids complicated. Extended Solomon equations are applied in order to describe ¹³C spin-lattice relaxation with ¹H r.f. field irradiation. Spin-lattice relaxation under r.f. irradiation is shown to be generally a triple-exponential process, but it can be reduced to be single-exponential under stronger r.f. field irradiation as well as in the absence of ¹H initial magnetizations. Based on numerical calculations, the difference between spin-lattice relaxation curves obeying T₁^C < T₁^H and those obeying T₁^C < T₁^H is clearly indicated. The methyl group resonances in solid-state -valine are examined, and the experimental results agree well with the theoretical results.     

Comparative electroabsorption studies of organic and inorganic solids

Spectral changes induced by moderate electric fields provide detailed insight into the electronic states of organic and inorganic solids. Although the basic effects, Stark effect and Franz–Keldysh effect, are the same in both types of material, the electroabsorption spectra vary strongly in size and spectral lineshape due to competing interactions. The large variance of the effects is demonstrated by representative examples of high mobility semiconductors, quantum wells, π-conjugated polymers, and charge transfer transitions in single crystals, disordered films and a double-quantum well. It is shown that only high-quality samples reveal the quantum mechanics of field-induced effects which are very sensitive to disorder.     

Formation of GaAs nanowhisker arrays by magnetron sputtering deposition

The possibility is demonstrated of fabricating arrays of cone-shaped GaAs nanowhiskers with a surface number density of up to 10⁹ cm^-2, a characteristic height ranging from 300 to 10000 nm, and a transverse size of approximately 200 nm at the base and from 200 to 10 nm or smaller at the top. The characteristic height of GaAs nanowhiskers varies in direct proportion to the effective thickness of the deposited material layer and in inverse proportion to the transverse nanowhisker size at the top. The growth of GaAs nanowhiskers is studied as a function of the deposition rate, the temperature, and the crystallographic orientation of the substrate. From an analysis of the obtained dependences of the nanowhisker size on these parameters, it is concluded that GaAs nanowhiskers are formed through the diffusion mechanism.     

Investigation of the dipolar dephasing NMR on organic solids

We present a method based on integro-differential equations describing the¹³C?¹H dipolar dephasing behaviour of carbon magnetization which results from monoprotonated carbons, non-protonated carbons as well as rapidly rotating methyl groups. Good agreement with theoretical calculations and experiment is obtained in ammonium tartrate and durene. The frequently applied empirical methods for determination the ratio of protonated and non-protonated carbons are analyzed. The dipolar dephasing time constants of non-protonated carbons vary substantially as a result, of variation in their heteronuclear second moments and thus in structure. Two different methods are performed for determination heteronuclear second moments from dipolar dephasing data.     

Picosecond X-ray diffraction probed transient structural changes in organic solids

In this Letter, we report on the experimental characterization of the geometry of short-lived electronically excited states in organic solids by time-resolved x-ray diffraction. Here, the structure factor of the organic crystal is measured as a function of time. Since this technique gives complete structural information, it is a very useful tool for learning more about atom motions on the excited-state energy surface-"beyond" the broad band typical of conventional spectroscopy. Although we used molecular crystals rather than free molecules, the compounds show detectable transient structural changes on the ps to ns time scale in our study.     

Low-temperature spectroscopy of organic molecules in solids by photochemical hole burning

Characteristic features of photochemical hole-burning (PHB) in the impurity spectra of low-temperature solids and PHB applications in molecular spectroscopy are considered. The evolution of a no-phonon hole and a phonon sidehole in excitation and fluorescence spectra is analysed on the basis of model calculations. Some more complex models for PHB are considered, which take into account reverse reactions, the optical thickness of the sample, the inhomogeneous dispersion of both homogeneous linewidths and transition energies of the photoproduct and quasi-static impurity-impurity interactions. The effects of PHB on fluorescence line narrowing are discussed. By PHB homogeneous linewidths of purely electronic and vibronic no-phonon lines in the spectra of some porphine and phthalocyanine derivatives in various matrices are measured and their temperature dependence is studied. The latter is found to be essentially different in polycrystalline and glassy matrices. Line-broadening mechanisms are discussed. By PHB the existence of an inhomogeneous distribution of vibrational frequencies in molecular impurities is established. The applications of PHB in the studies of photochemical reactions in solid solutions of phthalocyanine derivatives and tetracene are regarded and the mechanisms of these reactions are discussed. The results obtained by PHB for chlorophyll and its derivatives are presented.