Tuning the Energy Levels of Adamantane by Boron Substitution

Adamantane is known to have two different carbon environments, the C1-type (or bridgehead) and C2-type (or methylene bridge), serving as a foundation to explore the effects of boron substitution at these sites. Using DFT with B3LYP/6-31G(d), the structural, electronic, and optical properties of 37 boron-substituted isomers were investigated. The adamantane structure has rigid <i>T_d</i> symmetry with an average <i>r</i>C-C of 153.7 pm, which progressively transforms to <i>C</i>_3v and <i>C</i>₁ symmetry in heavily substituted isomers. Analysis of the neutral and ionic species reveals a critical transition from electron-donating to electron-accepting behaviour at tri-boron substitution, confirmed by both DFT and coupled cluster calculations (CCSD(T)/CC-pVDZ). C1 substitution narrows the HOMO-LUMO gap significantly, achieving a 56% reduction compared to 44.5% for C2 substitution in tetra-bora derivatives compared to adamantane. Optical properties [CAM-B3LYP/6-311G(d,p)] show systematic red shifting with increasing boron substitution, with absorption maxima moving from 146 nm in pristine adamantane to 423 nm (C1) and 277 nm (C2) in heavily boron-substituted derivatives (tetra-bora-adamantane). While C1 substitution leads to symmetry-forbidden transitions, C2 substitution maintains allowed transitions, offering more consistent optical behaviour. These findings provide important insight for the design of adamantane-based materials with tailored electronic and optical properties.