Blue light emitting naphthalimides for organic light emitting diodes Hidayath Ulla, B. Garudachar, M. N. Satyanarayan, G. Umesh, and A. M. Isloor Citation: AIP Conf. Proc. 1512, 1300 (2013); doi: 10.1063/1.4791530 View online: http://dx.doi.org/10.1063/1.4791530 View Table of Contents: http://proceedings.aip.org/dbt/dbt.jsp?KEY=APCPCS&Volume=1512&Issue=1 Published by the American Institute of Physics.
Additional information on AIP Conf. Proc. Journal Homepage: http://proceedings.aip.org/ Journal Information: http://proceedings.aip.org/about/about_the_proceedings Top downloads: http://proceedings.aip.org/dbt/most_downloaded.jsp?KEY=APCPCS Information for Authors: http://proceedings.aip.org/authors/information_for_authors
Downloaded 24 May 2013 to 218.248.46.105. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://proceedings.aip.org/about/rights_permissions
Blue Light Emitting Naphthalimides for Organic Light Emitting Diodes Hidayath Ulla#1, B.Garudachar2, M.N. Satyanarayan1, G.Umesh1, A.M. Isloor2 1
Optoelectronics Laboratory, Department of Physics, 2Department of Chemistry, National Institute of Technology Karnataka, Mangalore, India-575025, #
[email protected] Abstract. The photophysical, electrochemical, surface morphology and thermal properties of two novel blue lightemitting materials were studied. Results indicate that the molecules offer potential as non-doping light-emitting materials with good electron injection capabilities for fabrication of blue organic light-emitting diodes. Keywords: OLED, Naphthalimide, Blue, Optical, Electrochemical, Thermal, Surface morphology PACS: 81.05.Fb
were carried out in AUTOLAB PGSTAT 30 electrochemical analyzer. Differential Scanning Calorimetry (DSC) was done using SHIMADZU DSC-6 & Thermo Gravimetric Analysis (TGA) was done using EXSTAR 6000, TG/DTA 6300. SEM images were captured using JOEL JSM-6380LA.
INTRODUCTION Organic light emitting devices (OLEDs) have attracted lot of interest due to their potential applications in full-color flat panel displays & solidstate lighting [1-3]. The key point of OLED development is to find stable materials emitting pure colors of red, green & blue [4]. While a large number of exceptional red & green emitters have been developed that satisfy the requirements for OLEDs, efficient and stable organic blue emitters are still rare because blue OLEDs display much poorer performance compared to red & green emitters [5,6]. As a result, it is important to improve blue lightemitting materials with high luminescent efficiency, excellent morphological & thermal stability [7,8]. In this study, two novel blue-light emitting materials were designed for OLED applications. The derivatives were proposed as good blue light emitting materials.
RESULTS AND DISCUSSION The photophysical properties of organic materials are fundamental in understanding the functioning in many applications. Basic optical properties such as absorption (λA) and fluorescence (λF) maxima, optical band gaps (Egopt), Stokes shift (νA-νF), oscillator strength (f) & fluorescent quantum (ΦF) yields were measured in dilute chloroform (CHCl3) solution (10-5 mol L-1) & thin solid film and shown in Figure 1. The photophysical data are summarized in Table 1. The maximum absorption peaks, which can be assigned to the π-π* electronic transitions are situated in the broad wavelength range of 354-372nm. The PL maxima in thin solid films (462 & 457 nm) with broadened full width at half maximum (FWHM) > 61nm were red shifted by ~40nm compared to that in solution (419 & 424 nm respectively) with much narrower FWHM of ~45nm due to the condensing aggregation in solid films. The Stokes shift & oscillator strength values were in accordance with the data for other investigations on blue-emitting 1,8-naphthalimide derivatives. ΦF are 0.45 and 0.39. Optical band gaps of the derivatives are 2.84 eV and 2.89 eV.
EXPERIMENTAL DETAILS 2-(2-hydroxyethyl) -6- (2 - methylphenoxy) - 1Hbenzo [de] isoquinoline- 1,3 (2H) -dione [HEMPBIQD] & 2-[6-(2-methyl phenoxy)-1,3-dioxo 1H-benzo [de] iso quinolin- 2(3H)-yl] ethyl acetate [MPDBIQEA] was synthesized. The detail of the synthesis will be reported elsewhere. Absorption spectra were obtained on a UV-visible Spectrophotometer (Version Optics Inc. SD 2000). Fluorescence spectra were recorded using a JASCO FP6200 spectrofluoremeter. Electrochemical studies
SOLID STATE PHYSICS: Proceedings of the 57th DAE Solid State Physics Symposium 2012 AIP Conf. Proc. 1512, 1300-1301 (2013); doi: 10.1063/1.4791530 © 2013 American Institute of Physics 978-0-7354-1133-3/$30.00
1300 Downloaded 24 May 2013 to 218.248.46.105. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://proceedings.aip.org/about/rights_permissions
TABLE 1. Photophysical data of the naphthalimide derivatives Compound In solution (nm) In film (nm) λA λF FWHM λA λF FWHM HEMPBIQD 354 424 49 365 462 62 MPDBIQEA 361 419 45 372 457 61
νA - νF (cm-1)
f
ΦF
E gopt (eV)
5752 4999
0.29 0.37
0.45 0.39
2.84 2.89
shows that the quality of the organic films is good without any irregularities and defects.
FIGURE 1. UV-vis absorption and PL spectra of the molecules in dilute CHCl3 solution & thin solid film
Cyclic voltammetry was studied in 1 mmol L-1 acetonitrile solutions on Pt containing 0.1 mol L-1 of Bu4NPF6 as supporting electrolyte & ferrocene/ferrocenium as calibrant, referenced with Ag/AgNO3 electrode [6]. Electron affinities (LUMO) estimated from the onset of the reduction wave are 3.43eV & 3.31eV (below vacuum). This reveals that the derivatives have low-lying LUMO energy levels & promising electron-transport properties (n-type). The estimated ionization potentials (HOMO) of the derivatives estimated from the onset of the oxidation wave are 6.36eV & 6.31eV. The HOMO-LUMO energy gaps of the derivatives are 2.93eV & 2.99eV. The electrochemical data are summarized in Table 2.
FIGURE 3. SEM images of the thin films of the derivatives on ITO substrates
DSC & TGA were carried out at the rate of 100C/min in Nitrogen atmosphere. The TGA plot in Figure 2 shows some weight loss around temperature 1500C, which might be due to some volatile impurities or moisture present in material but no weight loss in the derivatives. At temperature around 2250C the material degrade completely. Hence the material is stable up to 2250C. The melting point (Tm) and thermal stability temperature (TS) are given in Table 2.
1.
CONCLUSIONS The optical, electrochemical, surface morphology and thermal properties of two naphthalimide derivatives were explored. The results indicate that the molecules offer potential as non-doping light-emitting materials with good electron transporting or holeblocking properties for fabrication of blue organic light-emitting diodes.
REFERENCES
2. 3. 4. 5. 6. 7. 8.
W. Brütting, Physics of Organic Semiconductors, Wiley VCH, 2005, pp. 233-267. C.W. Tang and S.A. Van Slyke, Appl. Phys. Letters, 51 913-915 (1987). C. Y. Mei, Polymer, 47, 4976-84 (2006). A. Kukhta et al., Journal of Fluorescence, 16, 375-378 (2006). G.H. Ding et al., Research on Chemical Intermediates, 34 (2-3), 299-308 (2008). J. Liu et al., Journal of Materials Chemistry, 18(14), 1659-1666 (2008) A. Islam et al., Journal of Physical Chemistry B, 109, 5509-5517 (2005) Y. Wang et al., Dyes and Pigments, 86, 190-196 (2010).
ACKNOWLEDGMENT FIGURE 2. DSC and TGA curves of the derivatives
This work was supported by Department of Information Technology, India and National Institute of Technology Karnataka, India.
In order to study the quality of the deposited films, SEM images were taken for HEMPBIQD (100nm) & MPDBIQEA (98nm) on ITO substrates. Figure 3
TABLE 2. Electrochemical and thermal data of the naphthalimide derivatives Compound HEMPBIQD MPDBIQEA
E oxonset vs E fc (V) 1.56 1.51
HOMO (eV) -6.36 -6.31
Eredonset vs Efc (V) -1.37 -1.49
LUMO (eV) -3.43 -3.31
E gEC (eV)a 2.93 3.00
Tm ( 0C) 194 201
TS (0C) 225 225
1301 Downloaded 24 May 2013 to 218.248.46.105. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://proceedings.aip.org/about/rights_permissions
