Pyrylium aromaticity comparison with

Tetralone 213 may be considered as the specific analog of 3-hydroxy-3,4-dihydroisoquinolines 137 (Section III,C,4,a,i). Its isolation is more evidence of the lower tendency toward aromatization in recyclization products of benzo[c]annelated pyrylium salts in comparison with monocyclic cations. 

The 13C-NMR spectra of l-arylbenzo[c]pyrylium salts have been investigated in comparison with those of 1-arylnaphthalenes, N-methyliso-quinolinium, and 1-arylisoquinoline derivatives 75ZN(B)943]. The correlation obtained is analogous to relationships found for a series of pyrylium and pyridinium salts, and pyridine derivatives [82AHC(Suppl)]. The, 3C shifts of 1 -aryl-benzo[c]pyrylium salts are also a valuable source of information for ptfa values, aromatic character, and conjugative effects of these compounds (760MR324). 

That the observed effect of benzoannelation is consistent with the aromatic character of the benzenonium ion is confirmed by comparison with corresponding effects on the stabilities of tropylium49,167 and pyrylium ions168,169 shown in Scheme 24. In both cases the stabilities of the ions are severely reduced by the additional benzene rings. Indeed, the effect may be compared with the effect of benzoannelation on the aromatic stabilization of benzene itself, which is characteristically decreased by conversion to naphthalene and phenanthrene or anthracene. The fact that, in contrast to pAR, the pAa of the benzenonium ion is increased by benzoannelation implies that benzoannelation does not have as large an effect on the aromaticity of the benzenonium ion as on benzene itself. 

Much more stable are the 2- or 4-oxides, i.e., a-pyrones (unsaturated lactones) 20 and y-pyrones, which could, in principle, have aromatic character owing to their zwitterionic resonance structures (20A-20C). However, although protonated pyrones are definitely aromatic, the neutral compounds appear to have very little aromaticity. Bird s aromaticity index I6 for pyrylium is only 65.8% in comparison with benzene, whereas for 4-pyrone it is 37.2% and for 2-pyrone it is only 32.9%, as seen in Table 4 [22], In agreement with Table 2, the ring 0-(C = O) bond in 20A is a type X-Z bond, whereas the ring O = (C-O ) bond in the two other resonance formulas is a Y-Y bond. 

The aromatic character of pyrylium salts is also suggested by their UV spectra. A comparison can be drawn with the pyridinium ion, since this system is iso-ir-electronic with pyrylium and belongs to the same symmetry point group (C2v) (7lPMH(3)67). Thus, the energies of their respective molecular orbitals will be comparable and this is reflected in the similarity of their UV maxima. 

The photophysical properties of polyacene molecules depend markedly on the number of rings and the fluorescence behaviour of hexacene has now been compared with that of earlier members of the series. A similar comparison has been made of the photophysical properties of catacondensed aromatic poly-cycles. Fluorescence from an upper-excited singlet state has been described for benz[a]azulene derivatives while the fluorescence properties of some antiaromatic molecules have been described in detail. Several thiopyrylium and pyrylium salts have been studied and the effects of various substituents attached to the heterocycle have been examined in terms of the triplet yield. A full evaluation of the photophysical properties of 4-aminonaphthalimide, and its... 

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