A DFT study on dihydropyrazine annulated linear polyacenes: aromaticity, stability and HOMO–LUMO energy modulation

Abstract

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.

A DFT study on 1,4‐dihydro‐1,4‐azaborinine annulated linear polyacenes: Absorption spectra, singlet‐triplet energy gap, aromaticity, and HOMO–LUMO energy modulation

“…These values are significantly less negative than the V min value 26.1 kcal/mol observed for the BAH bond of BH 3 which support the extended delocalization of nitrogen lone pair in LPA mimics. Unlike the previously reported dihydropyrazine annulated molecules, [15] V min of azaborinine annulated systems show only minor variation in the values, particularly for the interior LPA units.…”

“…This indicates that BH and NH heterocenter connection is more stabilizing for a larger LPA unit. In the case of dihydropyrazine annulated LPA mimics described in a previous paper, average E dh for a ring is found to be 38.8 kcal/mol whereas in the present azaborinine incorporated systems, the average E dh 81.2 kcal/mol is very high meaning that the 1,4‐dihydro‐1,4‐azaborinine‐based LPA mimics are more stable than the dihydropyrazine‐based systems. The total increase in the dehydrogenation energy (∑ E dh ) is directly related with the increase in the number and size of the LPA units.…”

“…[14] Very recently, a theoretical study on 1,4dihydropyrazine annulated LPA systems has been done by us showed the effective use of N centers for band gap tuning in LPA mimics. [15] Boron anion is effectively demonstrated as a substitute for carbon in the case of 5-membered cyclopentadienyl rings. [16] Boron incorporated thiophene-based conjugated polymers have been synthesized.…”

How to efficiently tune the biradicaloid nature of acenes by chemical doping with boron and nitrogen

“…Recent experimental studies [50][51][52] have suggested the introduction of nitrogen atoms into the acene framework (aza-acenes) as one possible route to stabilize longer oligomers. Concomitantly, it has been shown theoretically 53,54 that the hydrogenated forms of aza-acenes with inner N substitutions possess one more aromatic sextet ring compared to their pristine acene counterparts, which explain qualitatively the higher stability of these N-rich compounds according to Clar's rule. 55,56 According to the above discussion based on natural orbital occupations and unpaired electron distributions, our results clearly demonstrate the possibility of tuning the radical character of acenes by means of chemical doping.…”

Straightforward Design for Phenoxy-Imine Catalytic Activity in Ethylene Polymerization: Theoretical Prediction

“…In this work, we applied the M06L functional with 6-31G** for C, H, O, N, and Br, and LANL2DZ for Ti, Ni, and Zr. The selection of the M06L functional is based on previous benchmark studies [46][47][48]. Zhao and Truhlar [46] reported that M06L has reasonably good overall performance, and it is the best functional for transition metal energetics, and for the study of organometallic and inorganometallic thermochemistry.…”