Optoelectronic Properties of Polycyclic Benzenoid Hydrocarbons of Various Sizes and Shapes for Donor‐π‐Acceptor Systems: A DFT Study

The zigzag and armchair‐edged benzenoid polycyclic aromatic hydrocarbons (PAHs) possessing circular, parallelogram, rectangular and triangular shapes have been studied for a systematic evaluation of their optoelectronic properties for the design of donor‐π‐acceptor (D‐π‐A) systems using M06L/6‐31+G(d) level of density functional theory (DFT). Molecular electrostatic potential (MESP) analysis of the PAHs is done to characterize their electron distribution while the time‐dependent DFT (TD‐DFT) analysis was used for the absorption spectral analysis. MESP analysis showed Clar's sextet like electronic arrangement in armchair‐edged systems whereas zigzag‐edged ones showed significant electron localization towards the edges. The absorption spectra revealed a linear trend in absorption maximum (λmax) for most of the armchair‐edged systems with respect to the number of π‐electrons. MESP based assessment of the electron rich/deficient features of PAH systems led to the design of PAH‐π‐PAH type D‐π‐A systems wherein a conjugated diene moiety functions as the π‐spacer. The D‐π‐A behaviour of these systems significantly enhanced with the introduction of electron donating functional group NMe2 on donor PAH and electron withdrawing group COOH on the acceptor PAH. The MESP features, frontier molecular orbital (FMO) distribution, and absorption spectral features supported the strong D‐π‐A character of functionalized PAH‐π‐PAH. Among the different shapes studied, the rectangular PAH moiety showed the most efficient tuning of HOMO‐LUMO gap. The optical and electronic properties of PAH, PAH‐π‐PAH and functionalized PAH‐π‐PAH systems suggest the high tunable character of these properties with respect to the size and shape of the PAH. Benzenoid polycyclic aromatic hydrocarbons (PAH) with various sizes and shapes in armchair and zigzag configurations have been analysed for their molecular electrostatic potential (MESP) features. The analysis provided an accurate description of the electron rich/deficient features of them, with respect to their sizes and shapes and found to be useful for the design and tuning of the optoelectronic properties of (PAH)donor‐(π)spacer‐(PAH)acceptor systems.