Tuning RAHB strength exciting from singlet to triplet state with different topology of PAHs chains

Abstract

Resonance assisted hydrogen bond (RAHB) are one of the strongest non-covalent interactions. Resonance assists the hydrogen bond in the π-electron system. Between –CHO group and –OH it can be given an RAHB between the hydrogen of the –OH group and the oxygen of the –CHO group, this system will be called quasi-ring. Aromaticity and unpaired electrons in the triplet state may suffer changes if the RAHB is integrated to an aromatic ring. In order to very this, the quasi-ring was placed in different polyclyclic aromatic hydrocarbons (PAHs) of different sizes and topologies. The structures analysed were linear and kink topologies; and also, they were classified by families (depending on the number of sixmembered rings (6-MR) that PAHs have), the bigger family calculated was integrated by five 6- MR plus a quasi-ring. The bond distances of O--H and C=C of the RAHB and the excitation energy were calculated for all the compounds. Spin density distribution and aromatic were analysed. For the linear compounds, spin density distribution and aromaticiy follow the same behaviour: as the chain lengthens the unpaired electrons locate in the middle part of the chain. The bond distances of the O--H and C=C for the S₀ are larger than the T₁; and as the chain gets longer the bond distances of T₁ increases until it gets a similar value as the S₀. If the quasi-ring is not in the linear trend but in the next possible placement, the behaviour is similar; but, the T₁ is larger than the S₀ and for the spin density distribution there is lightly spin density at the oxygen atoms. For kink compounds, spin density distribution and aromaticity follow a different behaviour than the linear compounds: the unpaired electrons locate in the quasi-ring, even if the chain lengthens. For the bond distances of O--H and C=C the S₀ and the T₁ states don’t follow any behaviour as the chain lengthens. To verify these statements, an analysis was made to a topology in which had a linear and kink stretch, and it was proved that for the linear stretch the behaviour was similar to the linear compounds; and for the kink stretch the behaviour was similar to the kink compounds. This is very interesting for future studies, as the bond distances can be tuned by changing the PAH topology