Hydrogen bonding interactions can decrease clar sextet character in acridone pigments.

Computed nucleus-independent chemical shifts (NICS), contour plots of isotropic magnetic shielding (IMS), and gauge-including magnetically induced current (GIMIC) plots suggest that polarization of the π-system of acridones may perturb the numbers and positions of Clar sextet rings. Decreasing numbers of Clar sextets are connected to experimental observations of a narrowing HOMO-LUMO gap and increased charge mobility in solid-state assemblies of quinacridone and epindolidione.

On the reciprocal relationship between σ-hole bonding and (anti)aromaticity gain in ketocyclopolyenes.

σ-Hole bonding interactions (e.g., tetrel, pnictogen, chalcogen, and halogen bonding) can polarize π-electrons to enhance cyclic [4n] π-electron delocalization (i.

How does excited-state antiaromaticity affect the acidity strengths of photoacids?

Photoacids like substituted naphthalenes (X = OH, NH3+, COOH) are aromatic in the S0 state and antiaromatic in the S1 state. Nucleus independent chemical shifts analyses reveal that deprotonation relieves antiaromaticity in the excited conjugate base, and that the degree of "antiaromaticity relief" explains why some photoacids are stronger than others.

Excited-state proton transfer relieves antiaromaticity in molecules.

Baird's rule explains why and when excited-state proton transfer (ESPT) reactions happen in organic compounds. Bifunctional compounds that are [4 + 2] π-aromatic in the ground state, become [4 + 2] π-antiaromatic in the first ππ* states, and proton transfer (either inter- or intramolecularly) helps relieve excited-state antiaromaticity. Computed nucleus-independent chemical shifts (NICS) for several ESPT examples (including excited-state intramolecular proton transfers (ESIPT), biprotonic transfers, dynamic catalyzed transfers, and proton relay transfers) document the important role of excited-state antiaromaticity.