PAHs with Varying Peripheries

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]. 

Several all-benzenoid hydrocarbons 64-67 with cove -type instead of armchair periphery have been prepared (Fig. 3.15) [35, 63]. The resonances of the protons in the cove position are strongly shifted to low-field in H NMR spectra due to the deshielding of the aromatic surroundings. The UV-vis absorption spectra of these compounds in solution as well as in thin film show three types of bands (a, p and p) which are typical of polycyclic aromatic hydrocarbons. Similar to other all-benzenoid PAHs, the absorption maxima of these cove -types PAHs are progressively red shifted with increasing size of the 7i-system. 

The absorption maximum of 68a (Ama = 380 nm) shows a significant batho-chromic shift with respect to the corresponding band of HBC 10 (A ,ax = 359 nm) (0 0 band of the So transition). Reducing the symmetry increases the intensity of 0 0 band of the a (or I ) transition (486 nm) in the fluorescence spectrum 

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