Aromaticity of Heterocirculenes

This review summarizes the results on the aromaticity of a series of synthesized and hypothetical neutral heterocirculene molecules and their double charged ions. The aromaticity of heterocirculenes is a direct reflection of their electronic structure responsible for the specific optoelectronic and photophysical properties. We show how the presence of a heteroatom in the outer macrocycle affects the aromaticity of hetero[8]circulenes. In addition, we also describe the change in aromaticity and strain energy for a series of the "lower" (n < 8) and "higher" (n > 8) hetero[n]circulenes. It was demonstrated that the loss of planarity with increased strain leads to an increased antiaromaticity of the lower hetero[n]circulenes, whereas higher hetero[n]circulenes demonstrate a more pronounced aromatic nature because of the small departure from planarity of each heteroarene ring in hetero[n]circulene molecule. Finally, we discuss the aromatic nature of the first examples of pi-extended hetero[8]circulenes

Impact of heteroatoms (S, Se, and Te) on the aromaticity of heterocirculenes

A series of thia[7]circulenes and novel Se-, Te-, S/Te-, and Se/Te-substituted [8]circulenes have been studied by calculations of nucleus-independent chemical shift indices and gauge including magnetically induced currents to interpret the impact of heteroatoms on the aromatic properties of these polyheterocyclic species. The calculations indicate that all the studied hetero[7]circulenes and hetero[8]circulenes consist of two concentric subsystems: an inner seven- or eight-membered core is antiaromatic because of the existence of a paratropic ring current, and an outer system of benzene and hetarene rings that exhibit aromatic behaviour due to the circulation of diatropic ring currents. Thus, most of the hetero[7]circulenes can be considered as slightly antiaromatic because of the slight domination of the paratropic ring currents over the diatropic ones, whereas hetero[8]circulenes represent aromatic species due to the prevailing contribution of the diatropic currents. The antiaromaticity gradually increases with more scattered arrangements of the thiophene and benzene rings in each series of di-, tri-, tetra-, and pentathia[7]circulenes because of the reduced conjugation effect between the neighboring thiophene and benzene rings. Loss of planarity with increased strain leads to an increased antiaromatic character of the lower representatives of the thia[n]circulenes, whereas higher thia[n]circulenes demonstrate a more pronounced aromatic nature because of the small deviation from planarity. The ring current topology is found to be quite insensitive to the heteroatom type, number of hetarene rings and the size of the inner ring; this clearly manifests the special electronic structure of hetero[n]circulenes containing two concentric cyclic subsystems

Structure, stability and electronic properties of one-dimensional tetrathia-and tetraselena [8] circulene-based materials: a comparative DFT study

Conjugated polymers gain much attention due to the promising applications in organic electronic device technology. In this work, we theoretically study the structures and electronic properties of a novel class of nanostructures, namely one-dimensional tetrathia[8]circulenes (TTC) and tetraselena[8]circulenes (TSC) predicted to be promising semiconducting soft materials. It is found that all nanoribbons are thermodynamically stable and that their electronic properties depend significantly on the type of fusing between the monomers. In particular, the band gap tends to decrease while moving from the directly fused TTC/TSC ribbons to the structures coupled via a benzene-core linker and then to the ribbons fused through a four-membered ring. Therefore, both coupling type and length of oligomers allow one to manipulate the electronic and optical properties of the studied ribbons. The band structure calculations of infinite nanoribbons reveal direct band gaps that decrease from 2.28 to 2.14 eV for the TTC ribbons of the first and second fusion types. The TSC structures demonstrate the same trend exhibiting band gap narrowing from 2.41 (type I) up to 2.11 eV (type II). The type III ribbons possess a lack of periodicity due to the close-lying energy minima for the possible twisting configurations of TTC and TSC moieties relative to the linking four-membered ring

Electronic Structure of Exciplexes and the Role of Local Triplet States on Efficiency of Thermally Activated Delayed Fluorescence

In this work, we present an investigation of the electronic states in a series of thermally activated delayed fluorescence (TADF) exciplexes formed with the popular electron-transport compound TpBpTa and hole-transporting TCTA, TAPC, TPD10, TPD, and NPB. We rationalize the photophysical behavior of exciplexes by using computational methods and demonstrate that the reason for the commonly observed temporal red shift in the time-resolved spectra is related to the distribution of molecular conformations, thus CT energy, in film. We also use spectrally resolved thermoluminescence (SRTL) measurements to give insight into the trapping phenomena in exciplex blends. The results demonstrate that trapped charge carriers in the majority of studied exciplexes recombine through the luminescent intermolecular CT state. In addition, we report OLED devices using the said exciplexes in the emissive layer. The best performance is obtained with the TCTA:TpBpTa and TAPC:TpBpTa exciplexes showing maximum external quantum efficiencies (EQEs) of 8.8% and 7.2%, respectively

Schiff Base Zinc(II) Complexes as Promising Emitters for Blue Organic Light-Emitting Diodes

Organometallic blue fluorescent Zn(II) Schiff base complexes are synthesized and explored computationally in order to use them in organic electroluminescent heterostructures. Characterization of these pyrazolone-based azomethine-zinc complexes was accomplished by various physicochemical techniques to get insight into their applicability as an active layer in light-emitting diodes. All the complexes demonstrate high thermal stability and remarkable photoluminescence both in solution and in the solid state with maximum in the blue region. Quantum chemical calculations of the first exited electronic state and vertical singlet-singlet electronic transitions by means of time-dependent density functional theory calculations and results show that the origin of the luminescence for the target complexes refers to the intraligand charge transfer within the Schiff bases. The constructed light-emitting diodes demonstrate low input voltage (3.2-4.0 V), brightness at a level of 4300-11,600 Cd m(-2), and external quantum efficiency of up to 3.2%, which is a good value for purely fluorescent organic light-emitting diodes.Funding Agencies|Ministry of Education and Science of the Russian FederationMinistry of Education and Science, Russian Federation [FZEG-2020-0030]; Ministry of Education and Science of Ukraine [0121 U107533, 0119 U100259, 0121 U109506]; Swedish Research CouncilSwedish Research CouncilEuropean Commission [2018-05973, 2020-04600]; Swedish National Infrastructure for Computing (SNIC) [2020-3-29]</p