Exploitation of Baird Aromaticity and Clar’s Rule for Tuning the Triplet Energies of Polycyclic Aromatic Hydrocarbons

Felix Plasser

doi:10.3390/chemistry3020038

Abstract

Polycyclic aromatic hydrocarbons (PAH) are a prominent substance class with a variety of applications in molecular materials science. Their electronic properties crucially depend on the bond topology in ways that are often highly non-intuitive. Here, we study, using density functional theory, the triplet states of four biphenylene-derived PAHs finding dramatically different triplet excitation energies for closely related isomeric structures. These differences are rationalised using a qualitative description of Clar sextets and Baird quartets, quantified in terms of nucleus independent chemical shifts, and represented graphically through a recently developed method for visualising chemical shielding tensors (VIST). The results are further interpreted in terms of a 2D rigid rotor model of aromaticity and through an analysis of the natural transition orbitals involved in the triplet excited states showing good consistency between the different viewpoints. We believe that this work constitutes an important step in consolidating these varying viewpoints of electronically excited states.

Highlights

IntroductionIncluding singlet fission [3,4], charge transport [5], and field-effect transistors [6]
Polycyclic aromatic hydrocarbons (PAHs) are a fascinating class of molecules providing versatile organic semi-conductors [1] for a variety of use cases such as photovoltaics [2]including singlet fission [3,4], charge transport [5], and field-effect transistors [6]
(∆E(S0 /T1 )), we find that it almost vanishes for cyclobutadiene (CBD, 1) in line with CBD’s expected antiaromaticity

Summary

Introduction

Including singlet fission [3,4], charge transport [5], and field-effect transistors [6] Due to their high tunability, PAHs and their derivatives are suitable for more intricate applications such as logic gates [7], single molecule conductance [8,9], thermoelectrics [10], and multi-stage redox systems [11]. It is helpful to establish clear structure-property relationships bridging between the molecular structure and the observable properties As one option, this can be achieved by considering the biradical character and unpaired electrons [22,24,25,26,27]. Even more Baird’s rule [30] of excited state aromaticity allows to effectively predict triplet energies [31,32,33,34]

Results

Discussion

Conclusion