The interplay of antiaromaticity and diradical character in diarenoindacenes and diindenoarenes

Abstract

Over the past ∼15 years our group has performed multiple structure/properties relationship studies to assess how logical structural refinement can affect the antiaromaticity/diradicaloid continuum. Using precision organic synthesis, we can alter the chemical composition of both the pro-aromatic core and the outer fused arene groups. The rational design of antiaromatic diareno-fused s-indacene derivatives leads to pronounced variation of molecule paratropicity, i.e., the HOMO–LUMO energy gap, as determined experimentally (NMR, CV, UV-Vis, X-ray data) and computationally (NICS-XY scans, NICS2BC, bond current plots). Successive benzinterposition within the core motif affords diindenoarene structures where compound paratropicity is minimized yet diradicaloid character emerges. Using the same techniques of changing outer ring fusion with aromatic carbocycles and heterocycles created a series of structures where the diradical character and thus the singlet–triplet energy gap of the molecule could be varied in a controlled, predictable manner, as determined experimentally (NMR, CV, UV-Vis, X-ray, SQUID data) and computationally using high-level quantum chemical calculations. Arising from these fundamental studies, we have demonstrated that diarenoindacenes and diindenoarenes can act as the active layer in OFETs, often showing ambipolar charge characteristics with hole mobilities as high as 7 cm² V⁻¹ s⁻¹. We also established that our quinoidal/diradicaloid compounds possess large, atypical anti-ohmic conductance enhancement of their transport properties at longer molecular lengths, which suggests that this class of organic materials is a promising candidate for creating highly conductive and tunable nanoscale wires. Taken as a whole, our studies show that traditional physical organic chemistry concepts can be readily applied to modern organic materials research.