Perylene molecules

Two typical dye molecules. The europium complex (a) transfers absorbed light to excited-state levels of the complexed Eu , from which lasing occurs. The perylene molecule (b) converts incident radiation into a triplet state, which decays slowly and so allows lasing to occur. 

In (perylene)2Pp6 the coexistence of neutral and charged perylene molecules has been proved by high-resolution NMR measurements (Fischer Dormann, 1998). 

The comparison of PTCDA with its parent perylene molecule is extremely interesting. For perylene MLs on Ag(l 11), electron diffraction suggests an orientational liquid, in which the molecules are positionally ordered in an incommensurate close-packed superlattice but orientationally disordered and mobile. The same activated Raman peaks as for PTCDA are observed but they are, however, orders of magnitude weaker, indicating that, while a molecular reaction centre may still exist in the perylene backbone, its residual activity would be too small for the molecule to recognize a preferred site. 

An example of exciplex formation in the solid state may be afforded by perylene doped crystals of pyrene which emit a green structureless fluorescence in addition to the blue and orange-red excimer bands of pyrene and perylene, respectively. Hochstrasser112 has shown that the energy of the emitting species is consistent with that of a charge transfer complex of pyrene and perylene molecules in a bimolecular unit of the pyrene lattice. 

The first indication that molecular compounds could exhibit interesting electrical properties apart from those of an insulator was given in 1954, when Akamatu et al. (1) reported a resistivity of p = 10 2 cm for a bromine salt of perylene. Normally perylene crystals themselves are insulating with p = 1014 1016 1 cm therefore, a dramatic change in electronic structure had occurred. The perylene molecule is shown in Fig. 1. 

Manifold self-organized and self-assembled mono- and multilayers of smaller molecules have been reported in literature, to name some examples multiple structured monolayers of iodobenzene [109], multilayers of perylene molecules [110], mulitilayer and monolayers of dialkylamino hydroxylated squaraincs [111], monolayers employing marker groups [112], and self-assembled monolayers of thiolate bound molecules [113, 114]. 

Finally, we stress the twofold role of the oxygen atoms for the substrate interaction. On the one hand, direct bonds with the substrate are formed. On the other hand, the electronegative anhydride groups increase the eleetron affinity of the 7i-electron system towards metal electrons, as the comparison with the perylene molecule (2 in Figure 12.2) shows perylene interaets much more weakly with Ag(lll) and at room temperature forms a orientationally disordered layer [48]. A third role of the oxygen atoms, namely their participation in intermolecular interactions, will be discussed briefly in Section 12.3. 

Perylene molecules (L ), having an extended ir-system, easily form n-7T interaction. Four peripheral C=C moieties of Le coordinate to... 

The energetics of this system Is of Interest at room temperature the first singlet level of the perylene molecule Is higher than that of the host exclmer. On the other at this temperature the exclplex level Is slightly lower than the exclmer level, and sensitized exclplex emission Is observed. At 77°K the host exclmer level Is appreciably lower than the exclplex level, yet on exciting the perylene there results only exclplex emission, with no exclmer emission. 

In 1991, Basche and Moerner included perylene in polyethylene [71]. The small and rigid perylene molecule has good emission and triplet properties, but absorbs in a difficult spectral region, around 445 nm. Because the spectral jumps we discuss here are mainly consequences of matrix dynamics, we must shortly discuss sample preparation. The samples of [71] were made from low-density polyethylene (crystallinity 25%), doped at low concentration with perylene, and were quickly quenched from the melt to liquid nitrogen temperature to reduce light scattering. The thin films thus obtained were 10 to 20 pm thick. The polymer structure is thus expected to be dominantly amorphous. 

FIGURE 18.4 A perylene molecule (a). The molecule on (b) is perylene diimide (PDI). The -NH group may be exchanged by other groups to obtain novel pigments with other hues and colors. 

The absorption spectrum of perylene is roughly the same as for molecules where substitutions have been made in the perylene molecule. As usual in planar systems, the polarization (transition moment) is in the xy-plane. The snbstituents are not directly involved in the transition and only slightly modify the energy of the transitions. This is enough to obtain pastel pigments with different hnes. 

Although far from presenting the lowest oxidation potential among many other known 7t-donors, the perylene molecule can be oxidised in several organic solvents, either by the direct action of chemical oxidants such as iodine or bromine, or electrochemi-cally. In dichloromethane solutions, at a potential of 0.9 V versus SCE (saturated calomel electrode), it... 

Figure 2.1. Schematic representation of the HOMO of the perylene molecule (a) and graphite-like overlap mode typical of the highly conducting perylene based conductors (b)...

Infrared spectroscopy of the 3 1 compound showed two types of perylene molecules, one pair of molecules associated with TCNQ and one free molecule [17]. The charge distribution was clarified by UV and near infrared spectroscopy [20]. For both perylene-TCNQ compounds a charge-transfer band was identified and also the monomer TCNQ spectrum. The typical bands of (TCNQ)J are not observed. For Per3TCNQ the UV spectrum clearly shows the existence of neutral perylene. Thus the suggestion from the crystalline structure that the flanking perylene molecules are neutral seems confirmed. Therefore, given the existence of the anion TCNQ, each of the two other perylene species share one positive charge. EPR lines are narrow for both compounds 0.61 G and 0.97 G for the 3 1 and 1 1 compounds respectively at room temperature, and 0.67 G and 0.84 G at 77 K [17]. 

Figure 2.5. View along a of the crystal structure of Per3(TCNQ), showing the stacking with the repetition motive one TCNQ sandwiched between two perylene species. This stack is flanked by extra perylene molecules. (Reproduced by permission of International Union of Crystallography, from ref. 19.)...

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