Angular polyacenes

The molecular geometry of the whole molecule of seven linear and five angular polyacenes was used to calculate the MFC values, which are compared with the aromatic stabilization energy (ASE) calculated from the Benson and Cohen group additivity values (CAV) within the thermochemical procedure.Table 14 presents normalized ASE values (divided per one 7r-electron) depicted as E, MEC/n n... 

Let us now consider several possible isomers of the linear polyacene angular polyacenes and helicenes (see Fig. 3.13). In Table 3.12 the results obtained for the C22H14-C42H24 systems (containing 5-10 benzene rings) are presented for structures corresponding to systems A, B, and G for different numbers, n, of the benzene rings in the chain. 

Next, let us consider the optical properties of polymers presented in Fig. 3.13, which involve conjugated aromatic rings. In Table 3.13 the calculated polarizabilities and 2nd hyperpolarizabilities are shown for systems presented in Fig. 3.13 (A— linear polyacene B—angular polyacene C—polybenzocyclobu-... 

Conj ugated Ladder Polymers. Since the 1930s double-stranded, ladder-type polymers have been prepared in a multistep process with limited success of cyclization (191,192). Other routes have also been explored such as those for poly(acrylonitrile) (193,194), poly(l,2-butadiene), poly(3,4-isoprene) (195), or poly(butadiyne)s (196). These materials were found to be poorly soluble and unworkable, with a considerable number of defects in the structure (incomplete cyclization, cross-linking, radical sites). The first successful synthesis of a ladder polymer with a completely defined structure was accomplished in 1991 by Sherf and Mullen (197). The first step was the AA/BB-t5q)e polycondensation of an aromatic diboronic acid with a substituted 2,5-dibromo-l,4-dibenzoylbenzene to give a single-stranded precursor PPP-type polymer, followed by cyclization to the ladder structure (Fig. 8). Several other examples exist that have resulted in ladder-type structures. These include angular polyacene (198,199), Diels-Alder polyaddition of AB-type diene-dienophiles (200), AA/BB-type Diels-Alder polyaddition of a bisdiene and a bisdienophile (201), thienylene imits (202),... 

Cyranski MK, Stepien BT, Krygowski TM (2000) Global and local aromaticities of linear and angular polyacenes. Tetrahedron 56 9663-9667... 

C. Synthesis of an Angular Polyacene as the Second Example of the Successful Classical" Synthesis of a Ladder Polymer... 

The photoluminescence behavior of the new angular polyacenes is characterized by the appearance of a structured emission band (shortest wavelength emission... 

The absorption spectra of the synthesized ladder polymers (see Fig. 14.3) display two groups of signals in the visible part of the spectrum. One of these, with an absorption maximum Amax of 396-402 nm (with vibrational fine structure 356 and —375 nm) is very similar to the absorptions of the 7r-system of the angular polyacene structure of Scherf and Chmil described previously, although shifted somewhat (-30 nm) hypsochromically... 

The target structures in the final example are fully aromatic polymeric hydrocarbons, consisting of all-carbon six-membered rings - so-called angularly annulated polyacenes 91 [55]. The structural difference between those and the methylene-bridged poly(phenylene)s is the replacement of the benzylic methylene bridges by vinylene moieties. 

An important conclusion emerges from the results in Table I The par-atropicity of the angular polycyclic 4n tt electron dianions is much more pronounced than that of the linear polyacenes. For example, the naphtha-cene dianion (2) gave highly resolved XH NMR lines and no half-field lines in its ESR spectrum, whereas the chrysene dianion (4) and the benz-... 

When the whole-molecule aromaticity indices are concerned, HOMA, EN, GEO, MEC/n, and Cohen— Benson E, it is clear that the aromaticity of these systems varies monotonically with the increase of the number of rings. This tendency is observed in both series, however with different sensitivity, which is in line with an observation of the changes of the position of the p-band in UV spectra in arenes. The aromaticity of the linear polyacenes is smaller than that of the corresponding angular systems, and moreover, it decreases more rapidly with an increase in the size of the system. This tendency is clearly shown by E and HOMA and is roughly in line with a decrease in the chemical stability of polyacenes. Evidently the above observation is followed by the... 

In this section, we discuss various members of the family of polycyclic hydrocarbons (see Fig. 3.13). Among them there are linear (A) and angular (B) polyacenes. 

Scheme 8 Synthesis of an angularly annelated polyacene ribbon.

Table 14.3 Coupling Reaction of 2,5-Dibromo-l,4-dibenzoylbenzenes to Single-Stranded Precursors for an Angularly Annelated Polyacene...

Table 14.4 Absorption and Emission Properties of the Angularly Annelated Polyacene Structures Derived from Monomers A and B as Fully Aromatic Ladder Polymers...

Fig. 14.3 Absorption spectra of the polymer-analogous cyclization products to angularly annelated polyacenes according to ---) Chmil and Scherf [36], (-) Tour and Lamba [37,38], and (- - -) Goldfinger and Swager [39].

Scheme 9 Synthesis of a nitrogen-containing angularly annelated polyacene structure according to Tour and Lamba [37,38].

Figure 4.3. Phenylenes and topologically related PAHs (a) linear [/VJphenylenes and polyacenes (b) angular/zigzag [/VJphenylenes and polyphenanthrenes/helicenes (c) circular [6]phenylene sheet and graphite (d) archimedene (C120) and fullerene C o).

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