Pyrene-Like HOMO Governs Polaron Delocalization in Model Graphitic Strips: A Combined Experimental and Computational Analysis

Abstract

Polycyclic aromatic hydrocarbons (PAHs) with large graphitic cores have attracted significant attention as charge-transfer materials for photovoltaic and molecular electronics applications. In this work, we probe the redox and optoelectronic properties of novel hexabenzo[a,c,fg,j,l,op]tetracene (HBT) and its methoxylated analogue MeOHBT to seek whether long HBT-based graphitic strips can be viable candidates as efficient charge-transfer material. Our data reveal that despite the presence of eight electron-rich methoxy groups in MeOHBT the redox/optoelectronic properties of these two HBTs are very similar, an unusual finding in comparison with other smaller PAHs and poly-p-phenylene wires. Precise crystal structures of neutral HBTs and their cation radicals in comparison with carefully benchmarked DFT calculations revealed that the polaron in both HBT+• and MeOHBT+• is mainly localized at the central pyrene, with a minor spillover to peripheral biphenyl moieties, and mirrors the distribution of pyrene-like HOMO of neutral HBTs. Finally, aided by the DFT calculations, we show that redox properties of long HBT-based graphitic strips are nearly invariant to both substitution and length, with the polaron delocalization limited largely to only one pyrene moiety. This finding suggests that this class of wires could be promising candidate materials for long-range charge transfer studies.