CYCLIC CONJUGATION IN FLUORANTHENE AND ITS BENZO-DERIVATIVES. PART 1. CATACONDENSED SYSTEMS

Effect of Benzo-Annelation on Triplet State Energies in Polycyclic Conjugated Hydrocarbons

In a series of earlier studies, the effect of benzo-annelation was found to be a useful tool for tuning the aromaticity in polycyclic conjugated compounds to desired level. In this work we studied the (anti)aromaticity of benzo-annelated derivatives of three conjugated hydrocarbons (anthracene, fluoranthene and biphenylene) in their lowest lying singlet (S0) and triplet (T1) states by means of the energy effect (ef), harmonic oscillator model of aromaticity (HOMA), multicentre delocalization indices (MCI), magnetically induced current densities (MICDs) and nucleus independent chemical shifts (NICS). We showed that benzo-annelation is a topology-based effect which can be used to modify the T1 state excitation energies (E(T1)). A quantitative model was established being able to accurately predict the E(T1) based only on the numbers of angularly, linearly and geminally annelated benzene rings. In addition, it was demonstrated that the E(T1) can be directly related to the (anti)aromatic character of the central ring in the studied molecules in their S0 state.

Heteroatom effects on aromaticity of five-membered rings in acenaphthylene analogs

The pattern of cyclic conjugation was thoroughly studied in the series of N- and P-acenaphthylene derivatives using several different aromaticity indices: the energy effect (ef), multicenter delocalization index (MCI), harmonic oscillator model of aromaticity (HOMA) index, and nucleus independent chemical shifts (NICS). The Kekulé-structure-based reasoning predicts that there would be no cyclic conjugation in the "empty" five-membered heteroatom-containing rings in the studied molecules. It was found that according to the ef, MCI, and HOMA values, the extent of cyclic conjugation in the pentagonal rings is strongly influenced by the number and mutual arrangement of the hexagonal rings. In addition, it was revealed that in some of the examined molecules, the intensity of cyclic conjugation in the "empty" pentagons is even stronger than that of some hexagonal rings within the same molecule. The obtained results refute what one would expect based on "chemical intuition," which is usually strongly rooted to the Kekulé structures.Graphical abstract.

Effect of benzocyclobutadieno-annelation on cyclic conjugation in fluoranthene congeners

Earlier studies revealed that benzo-annelation has a peculiar effect on the intensity of cyclic conjugation in the five-membered ring of fluoranthene congeners. Now, the analogous effect of benzocyclobutadieno-annelation was examined and it was found show it is opposite to the effect of benzo-annelation: A benzocyclobutadiene fragment in angular (resp. linear) position with regard to the five-membered ring, decreases (resp. increases) the intensity of cyclic conjugation in the five-membered ring.

Cyclic conjugation in benzo-annelated triphenylenes

Cyclic conjugation in benzo-annelated triphenylenes was studied by means of the energy effect (ef) and the ?-electron content (EC) of the six-membered rings. A regularity that was earlier discovered in the case of acenaphthylene and fluoranthene congeners is now shown to hold also for benzo-annelated triphenylenes: Benzenoid rings that are annelated angularly with regard to the central six-membered ring Z0 of triphenylene increase the intensity of the cyclic conjugation in Z0, whereas linearly annelated benzenoid rings decrease the cyclic conjugation in Z0. The efand EC-values are strongly correlated, yet in a non-linear manner.

On the number of Kekulé structures of fluoranthene congeners

The Kekul? structure count K of fluoranthene congeners is studied. It is shown that for such polycyclic conjugated ?-electron systems, either K = 0 or K ? 3. Moreover, for every t ? 3, there are infinitely many fluoranthene congeners having exactly t Kekul? structures. Three classes of Kekul?an fluoranthenes are distinguished: (i) ?0 - fluoranthene congeners in which neither the male nor the female benzenoid fragment has Kekul? structures, (ii) ?m - fluoranthene congeners in which the male benzenoid fragment has Kekul? structures, but the female does not, and (iii) ?fm - fluoranthene congeners in which both the male and female benzenoid fragments have Kekul? structures. Necessary and sufficient conditions are established for each class, ? = ?0, ?m, ?fm, such that for a given positive integer t, there exist fluoranthene congeners in ? with the property K = t.

Effect of a ring on the cyclic conjugation in another ring: Applications to acenaphthylene-type polycyclic conjugated molecules

In a recent work, a method was developed for assessing the influence ief(G,Z0|Z1) of a ring Z1 on the energy effect of another ring Z0 in a polycyclic conjugated molecule G. Herein, a report is given of detailed numerical investigations of ief(G,Z0|Z1) aimed at the elucidation of the influence of various six-membered rings on the intensity of cyclic conjugation in the fivemembered ring of acenaphthylene-type molecules. The earlier discovered regularities for cyclic conjugation in acenaphthylene-type molecules (in particular, the PCP rule and the linear rule) could thus not only be rationalized, but also a number of hitherto concealed regularities could be envisaged.

Diradical character of some fluoranthenes

It is shown that some Kekul?an fluoranthenes are diradicals and that their ground state is a triplet. In the energetically less favorable singlet state, these hydrocarbons also exhibit pronounced diradical character. The diradical character y of the compounds under investigation was estimated using the unrestricted symmetry-broken (yPUHF) and complete active space (yNOON) methods. It was found that the yPUHF values better reproduce the diradical character of the investigated hydrocarbons. It was shown that the SOMO and SOMO-1 of a diradical structure occupy different parts of space with a small shared region, resulting in a spin density distribution over the entire molecule. The spatial diradical distribution in the singlet diradical structures was examined by inspecting the HOMOs and LUMOs for a and ? spin electrons. It was shown that the ?-HOMO and the ?-LUMO (as well as the ?-HOMO and the ?-LUMO) occupy practically the same part of space. In this way, there are no unpaired electrons in a singlet diradical structure, yet two of them occupy different parts of space, thus allowing the ?-electrons to delocalize.

On π-electron conjugation in the five-membered ring of fluoranthene-type benzenoid hydrocarbons

A fluoranthene-type benzenoid hydrocarbon (FTBH) is a polycyclic conjugated system obtained by joining two ordinary benzenoid hydrocarbons so as to form a five-membered ring. The main differences between the ?-electron properties of FTBHs and those of ordinary benzenoid hydrocarbons are caused by this five-membered ring. The most important structural factors influencing the ?-electron conjugation in the five-membered ring of FTBHs were analyzed and established.