Difluorinated cyclopentadithiophene derivatives for green emitters in organic light-emitting diodes: A theoretical investigation

https://doi.org/10.1016/j.jpcs.2021.110170Get rights and content

Highlights

  • New materials based on cyclopentadithiophene derivatives were designed for green emitters in OLEDs.

  • DFT and TD-DFT calculations were performed to investigate the optoelectronic properties of the compounds.

  • The compounds exhibited excelled nonlinear optical properties.

Abstract

Two types of difluorinated conjugated systems incorporating bridged cyclopentadithiophene, alternating fumaronitrile and 1,3,4-oxadiazole were designed and theoretically studied by density functional theory and its time-dependent extension with the B3LYP hybrid functional and the 6–311g(d,p) and 6–311++g(d,p) basis sets in tetrahydrofuran. The designed materials are denoted FCFO-1 (>Cdouble bondO) and FCFO-2 (>Cdouble bondC–(Ctriple bondN)2. The electronic band gap ranges from 2.46 eV (FCFO-1) to 2.30 eV (FCFO-2). The difference in optical behaviors may originate from the effect of the bridging group on cyclopentadithiophene derivatives. The materials were examined as green emitters (519–564 nm). Organic light-emitting diodes based on these new fluorescent materials [indium tin oxide anode (100 nm)/N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (hole-injection layer) (150 nm)/FCFO-1 or FCFO-2 (50 nm)/Al cathode (100 nm)], were theoretically designed. The current-voltage characteristics were simulated, and the threshold voltages were estimated to be 3.25 V for FCFO-1 and 2.17 V for FCFO-2. Electric dipole moment calculations show the molecules possess large electric dipole moments (approximately 9.88 D for FCFO-1 and approximately 11 D for FCFO-2) and they thus exhibit excellent nonlinear optical properties.

Introduction

Small molecules based on 1,3,4-oxadiazole as an electron transport material are extensively studied because of their interesting properties. They may provide several benefits, such as good electron-withdrawing behavior, thermal and chemical stability, and tunable optical properties [[1], [2], [3], [4]]. Additionally, they have emerged as compounds with great utility in the field of medicinal and materials chemistry [5]. 1,3,4-Oxadiazole-based small molecules and polymers are used in organic applications as materials for organic light-emitting diodes (OLEDs) [6] and organic solar cells [[7], [8], [9]]. Besides, they are used as electron transport and hole-blocking materials in OLEDs [10,11]. Studies have revealed that the electron-deficient 1,3,4-oxadiazole may be incorporated with electron-rich conjugated segments acting as a donor to yield strong intramolecular charge transfer (ICT) [[12], [13], [14]]. Currently, donor-acceptor-type conjugated materials are of great interest because of their high molecular hyperpolarizability (β) and large dipole moment (μ), which can produce a third-order nonlinear optical (NLO) effect [15].

More recently, our research group has made a systematic effort to investigate theoretically two kinds of homodimer and co-dimer from cyclopentadithiophene (CPDT) derivatives [[16], [17], [18]], having rigid and planar structures added to their excellent electro-optical properties. Moreover, easier π→π* ICT is observed in various CPDT derivatives [19,20]. The bridge-head position was functionalized with electron-withdrawing groups, such as carbonyl (>Cdouble bondO) or dicyanomethylene (>Cdouble bondC–(CN)2). These groups can yield various donor-acceptor materials with a low band gap [21]. The factors influencing CPDT's semiconductor nature and charge carrier mobility were analyzed in detail, revealing the role of the bridging groups in CPDT-based materials. Meanwhile, carbonyl as a bridging group on CPDT induces ICT, resulting in photoluminescence quenching [16].

In view of these facts, we have designed new donor-acceptor conjugated molecules containing CPDT and 1,3,4-oxadiazole derivatives as donor and acceptor moieties, respectively (see Fig. 1).

The fluorine-substituted side chain of both donor-acceptor subunits will result in a significant increase in the rigidity of the backbone, as shown for fluoro anisole [22]. It is expected that the resulting molecules will exhibit better NLO properties.

In the present work, quantum theoretical calculations of the molecular structure of the two types of compounds were performed with density functional theory (DFT). The geometries of the compounds were optimized with the B3LYP hybrid functional (Becke's three-parameter hybrid exchange functional combined with the Lee-Yang-Parr correlation functional) in combination with the 6–311g(d,p) and 6–311++g(d,p) basis sets in tetrahydrofuran (THF). The estimated energy barriers of electron and hole injection into the compounds are based on calculations of the ionization potential (IP) and the electron affinity (EA) as well as the reorganization energies (λe and λh) for hole and electron transport processes.

The most important properties that have to be controlled for OLEDs are the IP, EA, efficiency of charge carrier injection, electron and hole mobility, and light absorption and emission. To give a more quantitative description that can be related to the structure of organic electronic materials, it is important to understand the significant influence of tuning the molecular structure on electronic properties and device performance.

The time-dependent DFT (TD-DFT) method was used to calculate the absorption and emission spectra. The radiative lifetimes of spontaneous fluorescence from the lowest singlet excited state (S1) of the studied compounds were computed with use of the calculated emission energies and the oscillator strengths. The simulated current-voltage (IV) characteristics and the spectral response of OLEDs were constructed with Silvaco TCAD software.

Section snippets

Computational methods

To understand the operation of OLEDs and predict theoretically their electric behavior, the electronic descriptors were introduced, computed, and compared. These descriptors include the energy levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) and the energy gap between the molecular orbitals of the two examples of CPDT derivatives. All calculations were performed with Gaussian 09 [23] by DFT at the B3LYP level of theory. Two basis sets,

Geometry and electronic structures

Firstly, the geometries of both molecules were optimized with the DFT/B3LYP and CIS methods in the S0 and S1 states, respectively, with use of the 6–311g(d,p) basis set. The results showed slight deviations from planarity for the two molecules. The dihedral angles ϕ1 between CPDT and fumaronitrile and ϕ2 between fumaronitrile and 1,3,4-oxadiazole units for the two compounds (see Fig. 2) in the S0 and S1 states are given in Table 1. The ground-state optimized structures proved that the two

Conclusion

The present work aimed to design theoretically two novel green fluorescent emitters with push-pull structures and high charge mobility to be effectively applied in OLEDs. Computational calculations using the DFT/B3LYP method with the 6–311g(d,p) and 6-311++g(d,p) basis sets were used in this investigation in THF solution with use of the conductor-like polarizable continuum model. The geometries of the ground state (S0) and the first excited state (S1) were optimized, and vertical excitation

Author contributions

Said HAJAJI: Conceptualization, Investigation, Methodology, Software, Validation, Visualization, and Writing - original draft.

Rania ZAIER: Methodology, Software, Validation and Visualization.

Sahbi AYACHI: Investigation, Methodology, Project administration, Supervision, Validation, Writing - review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

All calculations in this work were performed at the Center for Research on Microelectronics and Nanotechnology of Sousse, Tunisia.The authors are grateful to Sonia Ouada, English language teacher, National School of Engineers, for grammatical and language corrections.

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