Elsevier

Chemical Physics

Volume 313, Issues 1–3, 27 June 2005, Pages 151-157
Chemical Physics

Bis-aryl substituted dioxaborines as electron-transport materials: a comparative density functional theory investigation with oxadiazoles and siloles

https://doi.org/10.1016/j.chemphys.2004.12.020Get rights and content

Abstract

We report on a detailed quantum-chemical comparison of the electronic structures, vertical electron affinities, and intramolecular reorganization energies for bis-aryl substituted dioxaborine, oxadiazole, and silole derivatives. The results indicate that the HOMO and LUMO energies of the substituted compounds can be tuned on the order of 2–3 eV via minor changes in the substitution patterns, with the HOMO and LUMO levels for the dioxaborine derivatives consistently the most energy stabilized. Additionally, large vertical electron affinities and comparable intramolecular reorganization energies confirm that dioxaborine systems are interesting candidates for electron transport materials.

Introduction

Organic, π-conjugated materials display semiconducting properties that allow for the low-cost fabrication of new generations of thin-film electronic, optoelectronic, and electrooptic devices such as light-emitting diodes, photovoltaic cells, field-effect transistors, and photorefractive cells [1], [2]. The successful implementation of such devices in commercial applications requires the design of materials with large carrier mobilities of both holes and electrons. Thus, it is of importance to consider the optimization of such physico-chemical properties as redox potentials, radical-ion stabilities, relaxation energies, and luminescence yields [3]. While hole-transport materials (e.g. triarylamine derivatives [4], [5]) have been relatively ubiquitous in the development and investigation of organic-based electronic devices, electron-transport materials have only recently undergone a substantial increase in consideration and design [6], [7], [8] due to a number of difficult key issues facing their development; these include electrochemical stability of the radical-anion versus molecular oxygen and water under ambient operating conditions [3], [8], and the difficulty of optimization of lowest-unoccupied molecular orbital (LUMO) energies to complement the Fermi energies of a variety of cathode materials in order to facilitate the injection of electrons [2]. Additionally, electron mobilities comparable to hole mobilities have yet to be attained [9].

Among the most effective and successful electron transport materials to date are those based upon oxadiazole [1,3,4-oxadiazole] and silole [1,1′-dimethylsilacyclopentadiene] derivatives. Several studies indicate that oxadiazole-based systems have both efficient electron-transport and hole-blocking properties in a variety of molecular architectures, including small molecule [10], polymer [11], and dendritic forms [12]. Silole derivatives display nondispersive and air-stable electron transport with mobilities two orders of magnitude greater than tris(8-hydroxyquinolinolato) aluminum (III) (Alq3) [9], another widely used electron transport material. These silole derivatives also have very high solid-state photoluminescence quantum yields in the absence of dopants [13].

The photoexcited state properties of dioxaborine [2,2-difluoro-1,3,2-oxaoxoniaboratine] derivatives have undergone investigation for a wide variety of functions, including: photocycloaddition and photoinduced electron-transfer reactions [14], [15] and two-photon absorption chromophores for the photodeposition of silver [16]. More recently, dioxaborines have also been envisioned as building blocks for new series of molecular electron transport materials in organic electronic devices due to their electronic (high electron affinities, reversible electrochemistry) and optical (absorption in the visible range, large fluorescence quantum yields) properties [17], [18]. Recent time of flight measurements have revealed electron mobilities two orders of magnitude larger than Alq3 for this class of materials [17].

In this paper, we report the results of a quantum-chemical assessment of bis-aryl substituted dioxaborines and their comparison to oxadiazole and silole model compounds, see Fig. 1. We use Density Functional Theory (DFT) to assess the electronic structure, vertical electron affinities, and intramolecular reorganization energies of these molecules. Our analysis also allows us to directly compare the one-particle molecular orbital properties that are commonly found in organic device literature (i.e. molecular orbital levels aligned relative to the Fermi energy of the electrodes) with (physically observable) properties such as electron affinities.

Section snippets

Theoretical methodology

The systems of interest include dioxaborine bis-aryl substituted at the 1- and 3-positions, and oxadiazole and 1,1-dimethylsilole substituted at the 2- and 5-positions; the aryl substituents are phenyl, p-N,N-dimethylaminophenyl, and p-nitrophenyl, see Fig. 1. Geometry optimizations of the neutral and radical-anion electronic configurations were performed by DFT calculations with the B3LYP functionals and a 6-31G* double-zeta plus polarization basis set. DFT methods have been proven to

Results and discussion

In the following Section, we present a qualitative molecular orbital analysis of the chemical structures shown in Fig. 1 as a means to compare their potential hole-blocking and electron-injection properties. Section 3.2 validates this comparison based on molecular orbitals by direct calculation of the vertical electron affinities. Finally, in Section 3.3, we evaluate the intramolecular reorganization energies, which are a key component of electron transport in the hopping regime [21].

Conclusions

In summary, the electronic structure, electron affinity, and intramolecular reorganization energy results presented here suggest that dioxaborine-based systems should compare favorably with the already successful electron transport systems based on oxadiazole and silole chemistry. The HOMO and LUMO levels for the bis-substituted dioxaborine species are consistently more strongly stabilized, indicating the possibility for both favorable hole-blocking and electron-injection properties. In

Acknowledgements

This work has been partly supported by the National Science Foundation (through the STC Program under Award DMR-0120967 and Grant CHE-0342321) and the Office of Naval Research.

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    Present address: Service de Chimie des Matériaux Nouveaux et Centre de Recherche en Electronique et Photonique Moléculaires, Université de Mons-Hainaut, Place du Parc 20, B-7000 Mons, Belgium.

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