EXCITED STATE ENGINEERING OF CONJUGATED ORGANIC MATERIALS: HETERO[8]CIRCULENES FOR OLED

EXCITED STATE ENGINEERING OF CONJUGATED ORGANIC MATERIALS: HETERO[8]CIRCULENES FOR OLED
In this report we present utilization of substituent-sensitive balance between fluorescence and non-radiative decay as a tool for optical tuning of promising materials for organic light emitting diode (OLED) applications. A series of recently synthesized N-butylated tetrabenzotetraaza[8]circulenes are studied computationally with CASPT2 method in order to explain the gradual decrease of fluorescence intensity with the rise of the number of substituents. The inter-system crossing (ISC) probability is found to increase upon the gradual side substitution of the circulene macrocycle as a result of a decrease of the S1–T1 energy gap due to deformation of the molecular plane and thereby due to distortion of the π-conjugation within the macrocycles. In contrast, the S1–T1 spin-orbit coupling matrix elements are quite insensitive to the number of outer substituents.
We also studied newly synthesized azatrioxa[8]circulenes in order to implement them into emitting layer of new OLEDs. Combinig the own blue emission of the azatrioxa[8]circulenes (ATOC) with the yellow-green emission of the exciplex between m-MTDATA and ATOC a broad visible electroluminescence (from 400 to 700 nm) is obtained. Thus constructed white emitting OLED device (WOLED) exhibits a luminance exceeding 23700 cd m2 and an external quantum efficiency about 3%.


During recent years organic photonics and optoelectronics is transferring into new field in materials science because of a number of advantages over the corresponding inorganic counterparts. [1-3] Really, organic materials provide flexible, sustainable and low-cost species for large-area, semi-conducting and semi-transparent devices with high conductivity, luminance and other superior radiation characteristics necessary, for example, for lighting wall-paper, displays, OLEDs, TV, solar cells, etc. [1] Special design of excited states by selection of donor-acceptor moiety and π-linker can produce a good dye with desired absorbance and luminescence. Furthermore, the versatility of the organic reactive path-ways and mechanistic routes in chemical synthesis provides an additional highly attractive advantage for photonic and optoelectronic engineering in manipulating organic luminescent materials properties [3-5]. In this way one can be able to adapt the photonic molecular material properties such as chain packing, charge-carriers conductivity, HOMO-LUMO gap and levels (where HOMO is the highest occupied molecular orbital, LU – lowest unoccupied), polarizability, frequency and oscillator strength of optical transitions. The most important is the engineering of photophysical parameters, in particular, the ISC rate and the luminescence efficiency. This also provides possibility to gather the excitons and charge transport parameters adaptation to the specific requirements of the device with a given desired application.
A particular intriguing strategy of manipulating organic optoelectronic materials properties can be realized with highly conjugated and symmetric molecules of hetero[8]circulenes [4-14]. These substances have attracted great attention during the last decade due to their promising luminescence properties together with their high stability and lowcost synthesis. In this report a series of N-butilated tetrabenzotetraaza[8]circulenes, recently synethsized by Osuka at el. [11] are studied in order to rationalize a gradual decrease of fluorescence quantum yield with the rise of the substituents. The CASPT2 calculations with spin-orbit coupling (SOC) account are used to explain the intersystem crossing rate in a series of circulenes which was related to a decrease of the S-T energy gap and SOC integral with the number of side-substituents in the hetero[8]circulene macrocycle.
Another important way of the excited state engineering for utilization of lightning ability of new chromophors based on hetero[8]circulenes represents a synthetic method of benzoannelation. We have shown recently that the -conjugation of the linear acene chromophore with the hetero[8]circulene residue provides an important inversion of the low-lying singlet excited states [10]. In total, an interesting synergetic effect of benzoannelation together with various alkyl- and O/NH-substitutions has been found for enhancing of absorption and emission intensity in a large series of newly designed benzoannelated hetero[8]circulenes [10]. This is summarized in a short diagram

Tetraoxa[8]circulene (TOC) Tetraazo[8]circulene (TAC)

Tetrabenzotetraoxa[8]circulene (TBTOC) Tetrabenzotetraaza[8]circulene (TBTAC)


All four important ancestor chromophors presented in Fig. 1 are highly symmetryc molecules of the D4h point group. They are planar and possess a specific -conjugation system with particular aromatic properties.






The azatrioxa[8]circulenes are more efficient fluorophors not only because of elimination of inversion symmetry in comparison with tetraoxa[8]circulenes which strongly reduces the electric dipole selection rule for the S1-S0 transition. The tert-butyl substitution and benzoannelation provide important structural and topological changes inside the hetero[8]circulene macrocycles. New efficient stable WOLED were obtained with these new azatrioxa[8]circulene fluorescent emitter layers based on exciplex with star-shaped molecule, 4,4‘4’’-tris[phenyl(m-tolyl)amino]triphenylamine (m-MTDATA) [13]. A number of azatrioxa[8]circulene moleculaes were synthesized [11].




The calculations were performed with computational resources provided by the Swedish National Infrastructure for Computing (SNIC) at the Parallel Computer Center (PDC) for High Performance Computing at the KTH Royal Institute of Technology, Sweden through the project ‘‘Multiphysics Modeling of Molecular Materials” SNIC 2016-34-43.

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