Near infrared organic light-emitting diodes based on acceptor–donor–acceptor (ADA) using novel conjugated isatin Schiff bases Mohammad Taghi Sharbati a,n , Mohammad Navid Soltani Rad b,n , Somayeh Behrouz b , Alireza Gharavi c , Farzin Emami a a Department of Electrical and Electronics Engineering, Shiraz University of Technology, Shiraz, Iran b Department of Chemistry, Shiraz University of Technology, Shiraz 71555-313, Iran c Photonics Lab, Department of Electrical and Computer Engineering, Shiraz University, Shiraz, Iran article info Article history: Received 7 April 2010 Received in revised form 13 October 2010 Accepted 18 October 2010 Available online 26 October 2010 Keywords: OLED Near infrared emission ADA Electroluminescence Isatin Schiff base abstract Fabrications of a single layer organic light emitting diodes (OLEDs) based on two conjugated acceptor– donor–acceptor (ADA) isatin Schiff bases are described. The electroluminescent spectra of these materials range from 630 to 700 nm and their band gaps were measured between 1.97 and 1.77 eV. The measured maximum external quantum efficiencies (EQE) for fabricated OLEDs are 0.0515% and 0.054% for two acceptor–donor–acceptor chromophores. The Commission International De L’Eclairage (CIE) (1931) coordinates of these two compounds were attained and found to be (0.4077, 0.4128) and (0.4411, 0.4126) for two used acceptor–donor–acceptor chromophores. The measured I–V curves demonstrated the apparent diode behavior of two ADA chromophores. The turn-on voltages in these OLEDs are directly dependent on the thickness. These results have demonstrated that ADA isatin Schiff bases could be considered as promising electroluminescence-emitting materials for fabrication of OLEDs. & 2010 Elsevier B.V. All rights reserved. 1. Introduction Organic light emitting diodes (OLED) have an interesting attraction for full colour flat panel displays during the last years [1]. Although majority of investigations have been focused on electroluminescent (EL) devices (OLED) having emission in the visible spectrum there is considerable potential to be exploited for emission in the near infrared and infrared spectral ranges. These kinds of NIR-OLEDs have significant applications in night-vision readable displays [2], optical communications, laser technology and optical sensors [3–7]. Much of the work achieved on NIR-OLEDs has been focused on organic complexes of lanthanides such as Er 3+ , Nd 3+ , Tm 3+ and Yb 3+ [8]. However, they are known to have several drawbacks including the inherent low external quantum efficiency (EQE), expenses and less availability. Since there are a few reports on known organic materials having EL in the IR region, there have been extensive efforts to find novel organic chromophores having the aforementioned property. p-Conjugated acceptor–donor– acceptor (ADA) as well as donor–acceptor–donor (DAD) com- pounds have been widely studied for enhancing the OLED technol- ogy [9,10]. They are also employed for controlled optoelectronic properties and also output wavelength tuning. The donor–acceptor concept has been largely used in designing the red chromophores; nevertheless, a few molecules have been synthesized to emit in near infrared region. Thus, in search for novel organic chromo- phores having near infrared EL property, hereby we have designed and synthesized two novel ADA isatin Schiff bases with donor and acceptor fragments as shown in Fig. 1. The optical as well as electrochemical properties of these new chromophores were described using their NIR-OLEDs. As it can be seen in Fig. 1, for having the low band gap chromophores, 3-iminoindolinyl-2-one residue at both sides of chromophore is selected as an acceptor head, while spacers including 4,4 0 -phenoxybenzene (for 2a) and 1,5-naphthyl (for 2b) are designated as donor segments. Thus, the conjugated backbone between donor and acceptor segments provides the appropriate delocalization and hence could be effective in band gap lowering. 2. Experimental 2.1. General procedure for synthesis of 3,3 0 -(4,4 0 -oxybis (4,1- phenylene) bis (azan-1-yl-1-ylidene)) diindolin-2-one (2a) and 3,3 0 - (naphthalene-1,5-diylbis (azan-1-yl-1-ylidene)) diindolin-2-one (2b) To a double-necked round bottom flask (100 mL) equipped with a condenser was added a mixture of isatin (0.02 mol) and Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jlumin Journal of Luminescence 0022-2313/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2010.10.016 n Corresponding authors. Tel.: + 98 911 1700836; fax: + 98 711 7354523. E-mail addresses: m.t.sharbati@sutech.ac.ir (M. Taghi Sharbati), soltani@sutech.ac.ir (M.N. Soltani Rad). Journal of Luminescence 131 (2011) 553–558