Unusual Bandgap Oscillations in Template-Directed {\pi}-Conjugated Porphyrin Nanotubes

TL;DR

This study reveals unusual size-dependent bandgap oscillations in porphyrin nanotubes, driven by aromaticity changes, leading to unique optoelectronic properties and potential for novel heterojunctions.

Contribution

First detailed DFT analysis showing size-dependent electronic oscillations in porphyrin nanotubes, contrasting typical quantum confinement effects.

Findings

Large bandgap oscillations with size in nanotubes

Oscillations caused by aromatic/non-aromatic alternation

Presence of direct, optically active bandgap states

Abstract

Using large-scale DFT calculations (up to 1,476 atoms and 18,432 orbitals), we present the first detailed analysis on the unusual electronic properties of recently synthesized porphyrin nanotubes. We surprisingly observe extremely large oscillations in the bandgap of these nanostructures as a function of size, in contradiction to typical quantum confinement effects (i.e., the bandgap increases with size in several of these nanotubes). In particular, we find that these intriguing electronic oscillations arise from a size-dependent alternation of aromatic/non-aromatic characteristics in these porphyrin nanotubes. Our analyses of band structures and orbital diagrams indicate that the electronic transitions in these nanostructures are direct-bandgap, optically active "bright" states that can be readily observed in photoelectron spectroscopic experiments. Most importantly due to their unusual bandgap oscillations, we find that both type I and type II donor-acceptor p-n heterojunctions are possible in these template-directed, "bottom-up synthesized" porphyrin nanotubes - a unique property that is not present in conventional carbon nanotubes.