Linear polyacenes (LPAs) beyond pentacene are highly unstable and their application potential in the optoelectronics field is very limited. On the basis of theoretical studies at the M06L/6-311++G(d,p) level of DFT, we show that annulating dihydropyrazine units to LPA cores can yield large LPA mimics. This strategy enhances the aromaticity of the LPA core and also provides a way to modulate the HOMO-LUMO energy gap by choosing an appropriate LPA core and extending dihydropyrazine annulation. The study is conducted for LPA mimics containing up to six dihydropyrazine units annulated to benzene (pB1-pB6), naphthalene (pN1-pN6), anthracene (pA1-pA6) and tetracene (pT1-pT6) cores. The longest of them pT6 contains 34 linearly connected six-membered rings. The dehydrogenation energy (Edh) of the N-heterocycles of the LPA mimics showed endothermic character and indicated their higher stability than dehydrogenated N-heteroacenes. The total Edh (ΣEdh) is proportional to the increase in the number of heterocycles and the increase in the size of the LPA core. The aromaticity of individual rings of all the LPA mimics is assessed on the basis of the harmonic oscillator model of aromaticity (HOMA) and nucleus independent chemical shift (NICS) parameters. Both parameters showed strong linear correlation with ΣEdh, confirming the geometric, magnetic and energetic criteria of aromaticity. The electronic features of the LPA mimics assessed by analysing molecular electrostatic potential topography and molecular orbitals have shown that the LPA cores retain the reactivity of the parent LPA. Furthermore, significant mixing of the N-lone pairs of the heterocycle with carbon π-orbitals improves aromaticity and decreases the HOMO-LUMO energy gap.

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.