Surfaces photodimerization

Figure 4.4. Decay of the surface specular reflection vs thermal disorder, static disorder, and surface annihilation caused by photodimerization. The surface reflection intensity of structure I is plotted vs broadening by temperature (full circles) and by photodimerization (hollow diamonds) which causes static disorder and annihilation of surface anthracene molecules. The solid line is deduced from theoretical calculations (2.126) in the adiabatic approximation. The cloud of hollow diamonds suggest that the density a of unperturbed surface molecules has been reduced below the critical value, with the consequent collapse of the specular reflection cf. (4.20). The inset shows the perfect surface structure (1), the temperature-broadened surface structure (2), and the structure of a photodimerized surface (3), which allowed us to plot the experimental curves.

D. Shenoy, L. Beresnev, D. Holt, and R. Shashidhar, Tuning polar anchoring energy through chemical modification of photodimerized surfaces. Applied Physics Letters 80, 1538 (2002). 

The first photochemical reactions to be correlated with PMO theory were the dimerizations of anthracene, tetracene, pentacene, and acenaphthylene. 36> More detailed energy surfaces for the photodimerization reactions of butadiene have also been calculated. 30> In the category of simplified calculations lie studies of the regiospecificity of Diels-Alder reactions 37>, and reactivity in oxetane-forming reactions. 38,39) jn these... 

Section IV is devoted to excitons in a disordered lattice. In the first subsection, restricted to the 2D radiant exciton, we study how the coherent emission is hampered by such disorder as thermal fluctuation, static disorder, or surface annihilation by surface-molecule photodimerization. A sharp transition is shown to take place between coherent emission at low temperature (or weak extended disorder) and incoherent emission of small excitonic coherence domains at high temperature (strong extended disorder). Whereas a mean-field theory correctly deals with the long-range forces involved in emission, these approximations are reviewed and tested on a simple model case the nondipolar triplet naphthalene exciton. The very strong disorder then makes the inclusion of aggregates in the theory compulsory. From all this study, our conclusion is that an effective-medium theory needs an effective interaction as well as an effective potential, as shown by the comparison of our theoretical results with exact numerical calculations, with very satisfactory agreement at all concentrations. Lastly, the 3D case of a dipolar exciton with disorder is discussed qualitatively. 

It was first reported that the topochemical photopolymerization of diolefin crystals gave rise to cracks and deformation [7]. An atomic force microscopic (AFM) study made possible the observation that the photodimerizations of trans-cinnamic acids and anthracenes in the crystalline state induced surface morphological changes at the tens and hundreds of nanometers level by the transportation and rebuilding of the surface molecules [8]. The appearance of a surface relief grating on the single crystal of 4-(dimethylamino)azobenzene was demonstrated by repeated irradiation with two coherent laser beams [9]. 

If, as will become evident with the dimerizations discussed in Section III.C, phase rebuildings are essential for the success of solid state photodimerizations, the nonreactivity of some topochemically allowed systems can be looked at in a different way. First, if there is no reaction, the crystal surface will stay unchanged upon irradiation and this can be probed with AFM very sensitively. Figure 2 shows a particularly highly structured natural surface of photostable tetraphenylethylene 1 prior and after 10 min irradiation in air with a Hg high-pressure lamp through Solidex from a distance... 

It is of some interest to compare these results with those of 9-methylan-thracene 7b, whose photodimerization has been termed topochemically allowed (d = 3.87 A [22])]. If the main surface of 7b (100) is irradiated, AFM scans floes initially and upon continuation of the irradiation these increase [8]. This shows that there are also long-range molecular transports and that phase rebuilding again governs the process. In any case, there is no special mechanism if a reaction may be termed topochemically allowed by previous convention (see Section C). 

Fig. 12.22. Energy surface depicting the 5] and Sq potential energy surfaces for singlet state photodimerization of ethene. Adapted from J. Photochem. Photo-bioL, 105, 365 (1997), by permission of Elsevier.

The host cavity of a resorcin[4]arene bearing two anthracene moieties at the wider rim can be switched by the known photodimerization of the two anthracene substituents resulting in a closed cavity. The process can be reversed by heating of the solution. Whereas the cavity of the open state is well suited to bind ammonium ions such as 2-mer-captoethylamine hydrochloride, the closed form does not bind ammonium ions. The affinity modulation has also been studied by single-molecule force spectroscopy (Figure 12.7). For this purpose, the host has been modified with four didecylsulfide linkers in order to immobilize the host on gold surfaces. ... 

Both previously published models assume the need for two geometrical parameters to determine [2+2] photodimerization yields [11,12]. To test this assumption, CASSCF calculations were carried out for ethylene-nethylene as a model [2+2] photodimerization system. Triplet mechanisms have been proposed for CPD formation [18,19], however recent experiments have shown that the singlet pathway is dominant [9,10]. This CASSCF study is therefore only done for singlet surfaces. Assuming initial excitation of the separated ethylenes to a singly excited n state, an eventual transition to a doubly excited n state will lead to the formation of cyclobutane. The topology of the ( r) ( r ) surface and the crossing between the ( r) ( r ) and ( r) ( r ) surfaces as a function of d and rj will elucidate the [2+2]... 

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