Extremely High RAHB Energies Quantified via a Molecular Tailoring Approach in Fused-Ring Tropolones and Their Effect on the Electronic Structure and Optical Properties

The OH···OC RAHB energies were quantified via a molecular tailoring approach for a series of tropolones with the fused aromatic and antiaromatic cycles. At 3,4 and 5,6 fusions of the tropone ring with aromatic cycles, the RAHB enhancement is observed while the RAHB weakening takes place at 4,5 and 6,7 fusions. An opposite trend appears at the fusion of the tropone ring with the antiaromatic cycle. A linear ratio was found between the HOMA structure-based aromaticity indices as well as NICS magnetic ones for the tropone ring and the RAHB energy. The total RAHB energy is divided into σ- and π-components. The σ- and π-components were established to change in the same directions, appearing the synergism. This synergism causes extremely high RAHB energies above 30 kcal/mol in some tropolones. It was shown that RAHB acquires the features of CAHB in these cases due to the excess charge on the oxygen of the CO group. The influence of the RAHB strength on the frontier molecular orbitals patterns, energy, the size of the gap between them, and the λ_max absorption wavelength in the UV/vis range was studied. It was revealed that the HOMO energy elevates, the LUMO energy lowers, the HOMO-LUMO gap narrows, and the λ_max wavelength undergoes a bathochromic shift with RAHB strengthening in the studied tropolones.