Anthracene molecular surface

Recently, the surface tensor model has been used together with the dielectric continuum model to calculate the orientational order parameters of solutes in nematic solvents [8,9,27], Figure 2.32 shows the theoretical results for anthracene and anthraquinone in nematic solvents with different dielectric anisotropy. Considering only the surface tensor contribution, positive Szz and Sxx and negative are obtained, with Szz > Sxx > Syy. This corresponds to what could be expected on the basis of the molecular shape the long axis (z) is preferentially aligned with the director, and the normal to the... 

Photochemical host-guest systems on the basis of photoswitchable macrocycles have also been investigated most recently in a number of publications. Schafer et al.23 and Anselmetti et al.24 modified a bifunctional resorcin[4]arene with two anthracene units and proved its switching function in photochemical induction and heating cycles. Moreover, they modified the lower rim of the resorcin[4]arene with four 10-(decylthio)decyl linkers for an oriented surface immobilization on gold via molecular self-assembly (Fig. 15.7). The affinity modulation of this optical switch was... 

In this section we analyze the surface investigation of molecular crystals by the technique of UV spectroscopy, in the linear-response limit of Section I, which allows a selective and sharp definition of the surface excited states as 2D excitons confined in the first monolayer of intrinsic surfaces (surface and subsurfaces) of a molecular crystal of layered structure. The (001) face of the anthracene crystal is the typical sample investigated in this chapter. 

For a coherent interpretation of the reported experimental data, we need a model of surface excitons, the structures I, II, and III being attributed to excitons confined, respectively, in the first, the second, and the third surface monolayer (see Fig. 3.5). The rapid decay of the van der Waals forces along the c axis explains the very fast transition, in a few molecular layers, from surface to bulk spectroscopy (the other two faces of the anthracene crystal do not show surface-confined excitons). 

In the particular case of the (001) plane of the anthracene crystal, we assume that the surface structure is very little different from that of a bulk layer. Indeed, the creation of two surfaces by cleavage is very easy The energy cost is very low and needs no molecular displacements of large amplitude. Thus, we adopt, for the (001) layer, the simplest assumption a 2D layer with periodicity, parallel to the plane of vectors a and b, preserved on the surface. We assume further that the molecules are rigid and that the symmetry plane (a, c) persists. Under these conditions, the surface layer has the monoclinic structure of a bulk layer, and the only parameters susceptible to modification are ... 

To summarize, the various estimations tend to show that the reorientation of the surface molecules is of the order of 1° for the anthracene crystal. These reorientations, in conjunction with the strong anharmonicity of the inter-molecular modes in the molecular crystals, suffice to explain the observed strong attenuation of the frequency of the surface mode At of lowest energy, which passes, at 5K, from 49.4 to 45.1 cm-1. 

Another approach for the development of PET-based molecular indicators for detection of nerve agents utilizes phosphorylation of an amine functionality, which is integrated into the structure of a fluorophore. This principle is used in the case of highly substituted anthracene bisimide (Figure 16.17f) [51] and fluoresceinamine (Figure 16.17g) [52]. These sensory compounds can be easily incorporated into polymer matrices [51] or deposited onto the surface of silica beads [52], thus yielding solid state detectors. 

Homogeneous and Inhomogeneous Contributions. Two other contributions to UPS linewidths for molecular solids have been articulated in a study of isopropyl benzene films at low temperatures ( ). The shape and size of the isopropyl benzene molecule prohibited the explicit observation of the surface effect discussed for anthracene. Isopropyl benzene was of interest as a model molecule for polystyrene, however. The measurements were carried out on condensed molecular-solid films in the temperature range 15°K < T < 150°K. 

Figure 4 displays the electron density distribution in the molecular plane of anthracene and contrasts it with that of phenanthrene along with the associated gradient vector field of the latter. In this figure, one can see the curved bond path linking the nuclei of the two hydrogen atoms, H4 and H5, in phenanthrene as well as the zero-flux interatomic surface they share, features lacking in the map of anthracene. 

Figure 4. a and b) Displays of the electron density p(r) contours in the molecular plane of anthracene and phenanthrene, respectively. The density increases from the outermost 0.001 au contour in the order 2 x 10", 4 x 10 , and 8 x 10" au with n starting at —3 and increasing in steps of unity. The intersections of the interatomic zero-flux surfaces with the planes of the figures and the bond paths are superimposed on the plots, (c) A display of the gradient vector field of phenanthrene in the same orientation and plane as in b). The gradient vector Vp(r) field clearly shows the shape of each atomic basin and its bounding zero-flux surface. 

The excited electronic state of the outermost molecular monolayer in anthracene is clearly seen in emission at low temperature. The monolayer next to the surface is blue-shifted by 10 cm-1 and the following one by 2 cm-1. The nature of these blue-shifts is now well understood and is related with the absence of neighbors for molecules located near the surface from the vacuum side. Therefore, for these molecules the gas-condensed matter (G-CM) shift of the electronic transition frequency is smaller than the G-CM shift in the bulk (see Fig. 9.1 we assume that the surface corresponds to the (a, b) plane of the anthracene crystal). For temperatures low compared with the blue-shift, the... 

The emission spectra of non-crystalline tetracene films have been interpreted in terms of emission from a molecular pair in a sandwich configuration.32 Substitutional and surface fluorescence in pentacene-anthracene crystals33 and the changes in luminescence of single crystals of pure anthracene, pure tetracene, and pentacene-doped tetracene as a result of electrode-induced changes in electrical charge carriers 34>35 have been examined. Hot bands in the fluorescence and phosphorescence of coronene have been analysed.38... 

---