Positional Isomers of B₆C₆N₆ Nanorings: Stability, Reactivity, and Optical Properties from First Principles

The positional arrangement of BN and CC units in B₆C₆N₆ cyclic nanorings profoundly influences their stability, electronic structure, optical response, and reactivity. Here, we comparatively investigate eight positional isomers (C1-C8) using DFT and TD-DFT calculations. Among C1-C8, C1 is the most stable, and C8 is the most unstable in the range of 200-1000 K. Their relative stability is governed by B-N charge separation, homonuclear B-B and N-N defects (charge repulsion), and bond-angle distortion (ring tension). The HOMO-LUMO gaps range from 4.40 eV (C3) to 8.45 eV (C2), indicating distinct kinetic stability. Aromaticity analysis reveals that all isomers are nonaromatic. In the gas phase, the lowest-energy absorption bands of C1 and C3 are located at about 429 nm and 606 nm, respectively. Due to different transition mechanisms, namely locally excited (LE) for the former and charge-transfer (CT) for the latter, solvent polarity has dramatically different influence on these two absorption bands. Compared to their positions in the gas phase, these absorption bands are blue-shifted about 20 nm and 220 nm in water, respectively. Reactivity analysis identifies the B-B bond in C7 as the strongest electrophilic site (LEAE = -2.93 eV), with the surrounding framework serving as nucleophilic domains, endowing C7 with the strongest bifunctional reactivity. This work establishes a comprehensive structure-property map for B₆C₆N₆ isomers, providing guidance for designing BCN-based nanorings for catalysis, molecular recognition, and optoelectronics.