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Recent Advances in the Computational Characterization of π-Conjugated Organic Semiconductors
Published in John R. Reynolds, Barry C. Thompson, Terje A. Skotheim, Conjugated Polymers, 2019
Jean-Luc Bredas, Xiankai Chen, Thomas Körzdörfer, Hong Li, Chad Risko, Sean M. Ryno, Tonghui Wang
Not only are the acenes prototypical materials for organic semiconductor applications, their structures can be considered as platforms to which synthetic chemists can add functionality through substitution. Heteroatom substitution has been widely used to modify the molecular redox and optical characteristics; however, these substitutions also have great impact on the preferred molecular packing configurations and the resulting materials properties.128–130,134 For instance, in a series of azapentacenes,128 SAPT0 results for co-facial configurations at an intermolecular distance of 3.5 Å reveal that nitrogen substitution into the acene backbone reduces the exchange-repulsion. Arising from smaller wavefunction overlap (and, in turn, smaller electronic couplings) with increasing nitrogen content, the smaller exchange-repulsion energies are evidence for contracted electron density in the N-heteropentacenes. The dispersion and electrostatic energies are also generally reduced with nitrogen substitution. Hence, even though the molecules in these series are iso-electronic and the same intermolecular separations are considered, the nature and strength of the intermolecular interactions are found to vary in significant ways with such rather subtle changes in chemistry.
Fused Polycyclic Aromatic Compounds: [njAcenes, [njHelicenes, and Their Heterocyclic Analogues
Published in Atsushi Nagai, Koji Takagi, Conjugated Objects, 2017
Numerous studies on acenes have been conducted for understanding their electronic structures, stability, aromaticity as well as for developing their application to organic electronics. With an increase in the number of fused benzene rings, the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) energy gaps decrease.4 In addition, the aromaticity per ring and stability are also decrease with the number of fused benzene rings. This trend can be understood qualitatively by the aromatic sextet proposed by Clar.5 According to the Clar’s theory, [n]acenes possess only one aromatic sextet ring (Fig. 2.3). Thus, the aromaticity is shared among the rings, resulting in lower aromaticity per ring and lower stability for higher [n]acenes. In fact, pentacene has been known to be very sensitive to oxygen and light to give the endoperoxide 4 or the dimer 5 (Scheme 2.1). The lower stability as well as low solubility of hexacene, heptacene, and higher [n]acenes has prevented them from being isolated and well-characterized until recently. Accordingly, there have been a limited number of reports on them, and the synthesis of higher acenes is still a challenging goal.6 Although a huge number of synthetic methods have been developed for lower [n]acenes (n < 3), this chapter will focus on higher [n]acenes (n > 4).
Major Classes of Organic Small Molecules for Electronics and Optoelectronics
Published in Sam-Shajing Sun, Larry R. Dalton, Introduction to Organic Electronic and Optoelectronic Materials and Devices, 2016
Xianle Meng, Weihong Zhu, He Tian
Linear polycyclic aromatic hydrocarbons are composed of laterally fused benzene rings, so-called linear acenes. They are currently among the most widely studied organic π-functional materials due to their importance as organic semiconducting materials. Some typical representatives are anthracene, tetracene, and pentacene. Tetracene and pentacene are among the most promising molecular conductors for OFETs. Up to now, the chemistry, applications, and especially controlling the HOMO–LUMO gap of oligoacenes have been well reviewed by Wudl et al. [96].
Relative energies of increasingly large [n]helicenes by means of high-level quantum chemical methods
Published in Molecular Physics, 2023
The largest [n]helicene that has been synthesised includes 16 ortho-fused benzene rings ([16]helicene) [13,14]. Expanded helicenes (i.e. helicenes composed of alternating linear and ortho-fused rings) [15] of up to [23]helicene have been synthesised [16]. For comparison, linear acenes with only up to 7 benzene rings (heptacene) have been isolated in bulk [17], and the largest metastable linear acene that has been investigated experimentally includes 12 benzene rings (dodecacene) [18–20]. Thus, despite both helicenes and linear acenes being catacondensed PAHs, large helicenes are synthetically more accessible. Indeed, [n]acenes become more reactive with an increasing number of rings [21].