Second Hyperpolarizabilities of Singlet Polycyclic Diphenalenyl Radicals: Effects of the Nature of the Central Heterocyclic Ring and Substitution to Diphenalenyl Rings

Fine-tuning the diradical character of molecular systems via the heteroatom effect

Heteroatoms were introduced as a novel modification strategy for fine-tuning the diradical character of molecular systems. Both the diradical character and the singlet-triplet energy gaps of diketopyrrolopyrrole based phenoxyl diradicaloids decreased as the size of the substituted heteroatoms (from O, S to Se atom) increased.

A DFT Study of the Modulation of the Antiaromatic and Open-Shell Character of Dibenzo[a,f]pentalene by Employing Three Strategies: Additional Benzoannulation, BN/CC Isosterism, and Substitution

Dibenzo[a,f]pentalene ([a,f]DBP) is a highly antiaromatic molecule having appreciable open-shell singlet character in its ground state. In this work, DFT calculations at the B3LYP/6-311+G(d,p) level of theory were performed to explore the efficiency of three strategies, that is, BN/CC isosterism, substitution, and (di)benzoannulation of [a,f]DBP, in controlling its electronic state and (anti)aromaticity. To evaluate the type and extent of the latter, the harmonic oscillator model of aromaticity (HOMA) and aromatic fluctuation (FLU) indices were used, along with the nucleus-independent chemical shift NICS-XY-scan procedure. The results suggest that all three strategies could be employed to produce either the closed-shell system or open-shell species, which may be in the singlet or triplet ground state. Triplet states have been characterized as aromatic, which is in accordance with Baird's rule. All the singlet states were found to have weaker global paratropicity than [a,f]DBP. Additional (di)benzo fusion adds local aromatic subunit(s) and mainly retains the topology of the paratropic ring currents of the basic molecule. The substitution of two carbon atoms by the isoelectronic BN pair, or the introduction of substituents, results either in the same type and very similar topology of ring currents as in the parent compound, or leads to (anti)aromatic and nonaromatic subunits. The triplet states of all the examined compounds are also discussed.

Diradical Character-Based Design for Singlet Fission of Bisanthene Derivatives: Aromatic-Ring Attachment and π-Plane Twisting

We demonstrate a diradical character-based molecular design for singlet fission using polycyclic aromatic hydrocarbons, bisanthene derivatives. Two types of chemical modifications-aromatic-ring attachment and π-plane twisting-are examined in order to satisfy the energy level matching condition for singlet fission. Detailed analysis of the electronic structures of the model molecules using nucleus-independent chemical shift, molecular orbitals, and their energies has demonstrated the usefulness of the relationship between the resonance structure and aromaticity and that between nonplanarity of π-conjugated systems and reduction of orbital overlap for tuning the diradical character. This result provides a novel design guideline for polycyclic aromatic hydrocarbons toward efficient singlet fission.