Benzenoids acenes

Figure 3. Peripheral topologies of large PAHs (A and B) all-benzenoid, (C) acene-like, (D) quinoidal.153154...

A catacondensed benzenoid with dihedral symmetry, viz. D2h is either a branched system or an (unbranched) linear acene. A centrosymmetrical (C2h) catacondensed benzenoid is either branched or unbranched. The D2h systems under consideration have been enumerated by the efficient algorithm invoking SCS s (cf. Sect. 6.4) [80], Table 20, in combination with Table 17, shows the known numbers for the branched catacondensed Dlh and C2h benzenoids. The numbers of unbranched catacondensed benzenoids with C2h symmetry are found under the designation d in Tables 14 and 15 for h < 20 and 21 < h < 30, respectively. 

Figure 2.8 shows how the experimentally observed wavelengths for two series of cata-condensed benzenoid hydrocarbons, the linearly annelated acenes and the angularly annelated phenes, change with increasing number of benzene rings. The shifts in the Lb and Bb positions are parallel to one another in both series, whereas the bathochromic shift of the Lj, band of acenes upon annelation is so pronounced that even in anthracene it masks the Lb band. 

Systematic studies of the absorption spectra of benzenoid aromatic hydrocarbons, mostly done by Clar in the 1930s and 1940s,279 showed that these compounds exhibit four types of UV VIS absorption bands, which are shifted in a regular way along a homologous series such as the linear acenes (benzene, naphthalene, anthracene,. ..) (Figure 4.16). 

Two other common subcategories of PAH are the acenes and the phenes. The acenes consist of benzoid rings fused in a linear arrangement, e.g., naphthalene, anthracene, naphthacene, etc. The phenes consist of benzenoid rings fused in an angular arrangement, e.g., phenanthrene, benzo[a]anthracene, etc. 

Cata-condensed polycyclic hydrocarbons contain benzenoid rings such that each fused carbon atom is common to no more than two rings. The benzenoid rings may be linearly annelated (acenes), as in the series naphthalene (19), anthracene (20), and naph-thacene (21), or they may be angularly annelated, as in phenanthrene (22), benz[a]anthracene (23), benzo[c]phenanthrene (24), chrysene (25), and tri-phenylene (26). Except for some isolated examples... 

Most benzenoid PHs actually can be characterized by a closed-shell electronic configuration accommodating their % electrons only in bonding orbitals. However, researchers faced difficulties for certain types of PHs due to their high reactivity. An important work by Bendikov et al. came into spotlight in 2004 when their computational study on oligoacenes supported that the longer acenes... 

The synthesis and study of open-shell PHs have become a rising hot topic nowadays, so we aim to provide a brief overview on recent advancements including theoretical studies and experimental characterizations of a series of benzenoid PH-based diradicaloids comprising higher order acenes, bis(phenalenyl)s. 

Besides the shape, another important factor that affects the electronic properties and chemical reactivity of PAHs is the nature of the periphery. According to Clar s classification, the graphitic molecules with armchair and cove peripheries shown in Fig. 3.14 (A and B) are all-benzenoid PAHs. In addition to these linear topologies, Stein and Brown considered two other peripheral structures, i.e. acene-like (C) and quinoidaT (D) structures, which lie in a higher energy state and thus show higher chemical reactivity [62]. 

For the acene and phene series, the UV-vis and fluorescence spectra shift dramatically when the number of the phenyl rings increases. In contrast, the shifts for all-benzenoid PAHs with either armchair or cove -type edges are small and they show a high chemical stability. Very recently, graphitic molecules with partial zig-zag periphery such as 68a, 68b and 73 (Scheme 3.19) were synthesized. It was found that the introduction of two or six extra re-centers onto the all-benzenoid graphitic molecules dramatically influences their electronic properties, chemical reactivity and two- and three-dimensional self-assembly [64]. 

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