Linear polyacenes

Naphthalenedicarboxylic acid is useful in the synthesis of linear polyacenes,8 3-halo-2-naphthoic acids,4 and 3-amino-2-naphthoic acid.4... 

Derivatives of the linear polyacenes, naphthalene,46 anthracene,41,90,93 naphthacene,91,92 and pentacene,92 form stable photodimers M2 when irradiated in concentrated Oa-free solution or (exceptionally) in the crystalline state.94,95 Transannular a-bonding of the molecular dimer components results in a folding of the aromatic planes about the bonded atoms and a reduced -electron delocalization reflected in a shift of the absorption spectrum to much higher frequencies.46,92,96... 

Let G(, i = 1,... be a homologous series (such as linear polyacenes, phenes, etc.) of BHs. It is obvious that the following limits exist... 

Eq. (48) contains no empirical parameters, yet it has an impressive accuracy. The approximation for x is far worse. We believe that it could be vastly improved by using more sophisticated (such as the linear polyacenes) template. However, this would result in difficulties in inverting the function that describes the dependence of k on the index i. 

The Clar formula, the K(G) number, mnrs, and properties of various aromatic hydrocarbon molecules are given in Table 1. As deduced from the comparison of I and II and also of III and IV, in every member of linear polyacenes the mnrs remains to be unity regardless of the size of the molecule, while in zigzag polyacenes every kink contributes to increase the mnrs of the molecule. These different behaviors are in parallel with the relative stabilities of these two classes of benzenoid hydrocarbons... 

Naphthalene, Anthracene, and Linear Polyacenes Linear polyacenes possess two low-lying excited states, which in the Platt notation used in spectroscopy, are labeled as La and Lb the former is Biu and the latter is 1 B2u (7,19). The Biu state is a monoionic state, while B2U is a covalent excited state, which as in benzene, is dipole forbidden, but can be accessed by two photon spectroscopy. In naphthalene and anthracene, is the second excited state, but in higher acenes, becomes the first excited state (16,19). We will now show how to construct and conceptualize this excited state. Already at the outset, we encounter here a problem of choice, namely, that the set of covalent Rumer VB structures involves many structures (e.g., 42 in naphthalene), and one has to restrict the VB structure set in order to understand the constitution of these... 

Other structural invariants have also been considered as parameters influencing the E-value of benzenoid molecules (e.g. the length of linear polyacene fragments [82], the modified Hosoya index [65, 83-85], the number of bay regions [86, 87] etc.). In what follows we focus our attention only to approximate formulas of (n,m)- and of (n,m,K)-type. 

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... 

If the distance between and M is less than R, they exist as an ion pair, i.e., in a charge transfer state. If M and M are locked into original lattice states then the distance can have only certain values. From the energies for linear polyacenes, determined accurately by Silinsh, " it appears that, as the number of rings increased, charge separation in the... 

Bultinck, P Rafat, M Ponec, R., van Gheluwe, B Carbo-Dorca, R and Popelier, P. (2006) Electron delocalization and aromaticity in linear polyacenes atoms in molecules multicenter delocalization index. The Journal of Physical Chemistry A, 110, 7642—7648. 

In view of the obvious differences in the structure of the wave-functions for the linear and polycyclic conjugated polymers (linear polyacenes), let us analyze two representative members of both classes. In Table 3.5 values of the above described parameters are shown for polyene C50H52 and polyacene C5oH2g that consists of 12 benzene rings. The values of that describe the contributions to the correlation energy obtained from different Z-layers in the cue-CCSD method are also shown in the table. 

Table 3.5 Effectivity parameters for different I layers of the cue-CCSD wave function for polyene and linear polyacene... 

Fig. 3.14 Average specific (per electron) polarizabilities of linear polyacenes as a function of the number of the. r-electrons...

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-... 

The second noteworthy feature is the difference in the accuracy of the description of (a) and (y) in the HF and MP2 methods. In general, these methods are satisfactory in describing the values of the polarizabilities (except the case of linear polyacenes). At the same time, for the 2nd hyperpolarizabilities, there are significant deviations from the results obtained from the cue-CCSD calculations (especially for Y)benzene)-Therefore, a rough accounting for the electron correlation effects cannot guarantee a similarly accurate description of every optical property. As it will be shown below, this conclusion remains true for optical properties proportional to odd powers of the strength of the applied field. 

To be more specific, consider linear polyacenes for which a model geometry will be used here and throughout the paper the carbon backbone is formed by regular hexagons with the C-C bond length of 1.4 A and the C-H bond length of 1.09 A. 

Using the Gaussian program package [89], we computed S ) for the first ten linear polyacenes C4 +2H2n+4 at the UHF/6-31G level. The results are conveniently expressed via the linear regression... 

Quantum chemistry study of the van der Waals dimers of benzene, naphthalene, and anthracene Crossed (02 ) and parallel-displaced (C2 ) dimers of very similar energies in the linear polyacenes ... 

Ladder structures that are composed exclusively of aromatic carbon centers have been the focus of theoretical calculations and diverse synthetic efforts for a long time. Polyacenes are a major focus in this interest [40,41]. Polyacenes are band molecules composed exclusively of annelated six-member carbon rings. Linear polyacenes, poly(n)acenes, are, as polymers with low band gap energy, poorly accessible as a result of their high reactivity (oxidation, dimerization) [42-44]. Hitherto only oligo-( )acenes up to hexa- and heptacene [45,46] have been known and characterized. A synthetic entry to the corresponding polymer does not yet exist, although there are... 

Bultinck, R Ponec, R. Carb6-Dorca, R. A molecular quantum similarity and generalized population analysis of the aromaticity in linear polyacenes. J. Comput. Chem. 2007, 28, 152. 

---