# Tuning the Energy Levels of Adamantane by Boron Substitution

## TL;DR

This paper explores how replacing carbon atoms in adamantane with boron affects its electronic and optical properties, showing significant changes in energy levels and absorption.

## Contribution

The study reveals that boron substitution at specific carbon sites in adamantane can significantly alter its electronic and optical behavior.

## Key findings

- C1 substitution narrows the HOMO–LUMO gap by 56% in tetra-bora derivatives compared to pristine adamantane.
- Optical absorption maxima shift from 146 nm in pristine adamantane to 423 nm in C1-substituted and 277 nm in C2-substituted tetra-bora-adamantane.
- C2 substitution maintains allowed optical transitions, while C1 substitution leads to symmetry-forbidden transitions.

## Abstract

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 Td symmetry with an average rC-C of 153.7 pm, which progressively transforms to C3v and C1 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.