Polarized UV-visible absorption, emission, and magnetic circular dichroism of a series of porphycenes are reported and interpreted in terms of the classical perimeter model as well as semiempirical CNDO/S, INDO/S, and PPP calculations. Differences relative to porphyrins follow readily from their different topology. Two results are particularly striking: (i) unlike the soft MCD chromophores, porphyrins, the porphycenes are negative-hard chromophores and provide a clear example of the competition between the µ+ contributions and the µ" contributions to the B terms of the Soret bands and (ii) fluorescence polarization results for free-base porphycene suggest the existence of a fast intramolecular proton-transfer process in the first singlet excited state, which calls for a closer examination by time-resolved methods.
Controlling the outcome of reactions is a central issue of chemical research. Physical tools can achieve this if they are able to precisely dissociate specific bonds of a molecule. However, to control synthesis, such tools must induce the formation of new bonds between two reactants to yield a more complex product. In the ideal case of an atom efficient synthesis, this product would contain all or at least most of the initial material. An electron beam is a physical tool that is capable of preparing molecules in reactive states or, at low electron energies, of initiating highly selective bond dissociation. The resulting fragments in turn can react with other molecules to yield stable products. This tutorial review focuses in particular on such low-energy electron-initiated molecular syntheses and their applications in the modification of surfaces. It thus emphasizes strategies towards the controlled and predictable formation of more complex products from small reactants initiated by interaction with low-energy electrons either through selective bond dissociation or formation of specific reactive molecular species. However, selective bond dissociation is not always desirable. This is briefly illustrated by the case of electron beam induced deposition where additional strategies may be required to control product formation.
Electron-beam-induced deposition (EBID), also referred to as focused electron-beam-induced processing (FEBIP), is a lowvacuum materials processing technique in which a focused electron beam is used to directly write nanometer-sized structures onto a substrate in a constant partial pressure of precursor molecules. 1À4 EBID has a unique and attractive combination of capabilities, including high spatial resolution and the flexibility to deposit self-supporting three-dimensional nanostructures on nonplanar surfaces. EBID offers a number of advantages compared to other vacuum-based nanofabrication strategies. EBID is capable of creating smaller features than ion-beaminduced deposition (IBID), with less amorphization and without ion implantation. 5À7 Although the resolution of EBID is comparable to that of electron beam lithography (EBL) and extreme ultraviolet lithography (EUVL), 8,9 it needs no resist layers or etching step for pattern transfer. The advantages of EBID have also been recently been combined with those of atomic layer deposition (ALD) to create purely metallic but geometrically well-defined nanostructures. 10 Current applications of EBID include repairing masks used in UV lithography, 11À14 creating line gratings on vertical cavity surface emitting lasers, 15 and fabricating tips for scanning probe microscopy. 16,17
The smallest catalyst: A new strategy to control chemical synthesis by exposure to low-energy electrons relies on the electrostatic attraction caused by the soft ionization of one of the reaction partners. This approach was used to induce a reaction between C(2)H(4) and NH(3) yielding aminoethane. The reaction resembles a hydroamination except that the electron beam replaces the catalyst used in the organic synthesis.
Valence and low-lying Rydberg states of acetylene (C 2 H 2 ) are reexamined in the singlet as well as in the triplet manifold. The major goal of this work is a better understanding of the valence states that contribute to the low-energy electron-energy-loss spectrum recorded under conditions where transitions to triplet states are enhanced. An appropriate theoretical treatment of these states has to include the low-lying Rydberg states because of their energetic proximity to some of the valence states. The CASSCF/CASPT2 method provides a suitable framework for such a task. For some important states the geometry was optimized at the CASPT2 level to allow a comparison with the results of other highly accurate methods that have been applied to acetylene in the past.
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