Blue Light-Emitting Electrochemical Cells Based on Angularly Structured Phenanthroimidazole Derivatives

Subeesh, Madayanad Suresh,Nguyen, Thao P.,Choe, Youngson American Chemical Society 2017 The Journal of Physical Chemistry Part C Vol.121 No.27

<P>Photophysical properties of four new ionic blue emitting phenanthroimidazole derivatives were investigated. Rarely utilized meta and ortho positions of C-2 substituted benzene linker of phenanthroimidazole were functionalized herein to generate blue emitters for light-emitting electrochemical cell (LEC) applications. To this end, pyrene and anthracene units were coupled to phenanthroimidazole core through meta or ortho linkage. This molecular design strategy is anticipated to generate deeper blue emission than the corresponding para isomers due to lack of extended conjugation. Highly contrasting properties were observed between ortho- and meta-coupled fluorophores. Unexpectedly, solubility issues hinder.the application of ortho-linked molecules in LEC devices. However, bright blue emissions were achieved in LEC devices when meta derivatives were mixed in the active layer of LEC with poly(ethylene oxide) and lithium triflate. Notably, the molecule having pyrene and phenanthroimidazole coupled through meta linkage (mPy) presented the best performance. It was identified that a fundamental trade off exists between efficiency and brightness for these tricomponent devices when the ion concentration is increased. Efficient device (device 3, maximum efficiency = 0.43 cdA(-1)) was the one with row amount of ions, whereas the bright one (device 5, maximum brightness = 585 cdm(-2)) had high ion concentrations. However, the performances of all of the tricomponent devices were reasonably high in comparison to their single component devices. In addition, modulation of charge transfer band was found feasible for these investigated molecules by switching the linking position, which provides us with an imperative clue to design ideal blue emitters for LEC applications incorporating pyrene and phenanthroimidazole units.</P>

Host–Dopant System To Generate Bright Electroluminescence from Small Organic Molecule Functionalized Light-Emitting Electrochemical Cells

Subeesh, Madayanad Suresh,Shanmugasundaram, Kanagaraj,Sunesh, Chozhidakath Damodharan,Chitumalla, Ramesh Kumar,Jang, Joonkyung,Choe, Youngson American Chemical Society 2016 The Journal of Physical Chemistry Part C Vol.120 No.22

<P>Albeit their easy accessibility and low cost, small organic molecules are not known for their high electroluminescence in light-emitting electrochemical cells (LECs). To construct a bright low-cost LEC device, the functions of charge transport and charge recombination should be separated in the active layer of LEC devices. Herein, we demonstrate that the widely used host-dopant strategy in organic light-emitting diodes (OLEDs) can significantly improve the electroluminescence from small organic molecule fueled LEC devices, provided the host molecules are carefully selected. Furthermore, performance of host-dopant small-molecule LEC devices hugely relies on the properties of host materials rather than the emitting luminophores. Conversely to the high performance of intramolecular charge-transfer (ICT) molecular systems in OLEDs, doped ICT fluorophores having a low-lying charge-transfer state can behave like exciton loss channels in the high ionic environment of LEC-active layers. Similar to the behavior of previously reported ICT molecules in polar solvents, our synthesized D-pi-A-pi-D phenanthroimidazole derivative exhibited fluorescence quenching and a huge blue shift of emission in the doped thin film of the ionic host. However, even with a less efficient emitter, high electroluminescence was achieved from a host-dopant LEC system. Our best device exhibited a maximum brightness of 5016 cd/m(2) at a current efficiency of 0.73 cd/A. This device outplays our previously reported nondoped LEC (ihpypn-LEC) with a 7-fold increase in the maximum brightness and over a 3-fold increase in the current efficiency at peak brightness. To the best of our knowledge, these peak brightness values recorded here (device 2) are the best among those reported by small organic molecule LEC devices so far. This report reveals the potential of small organic molecules, especially phenanthroimidazole derivatives, in casting bright and efficient low-cost host-dopant LECs with minimum effort and appreciable sustainability.</P>

Utilization of a phenanthroimidazole based fluorophore in light-emitting electrochemical cells

Subeesh, Madayanad Suresh,Shanmugasundaram, Kanagaraj,Sunesh, Chozhidakath Damodharan,Won, Yong Sun,Choe, Youngson The Royal Society of Chemistry 2015 Journal of Materials Chemistry C Vol.3 No.18

<▼1><P>A phenanthroimidazole derivative is used as an emitter in light emitting electrochemical cells (LECs).</P></▼1><▼2><P>An easily accessible, highly soluble, small-molecule phenanthroimidazole derivative has been synthesized and characterized. The synthesized compound shows strong luminescence in solution and exhibits good thermal stability. Single crystal X-ray crystallography studies were carried out. Correlations between the X-ray structures, photophysical properties and the performance in light-emitting electrochemical cells (LECs) are described. A yellowish green emission was achieved by using the target compound in a LEC device configuration. The constructed prototype device performance was promising with a maximum brightness of 125 cd m<SUP>−2</SUP>. The results suggest that the phenanthroimidazole derivative can function as an active material in LEC devices.</P></▼2>

Phenanthroimidazole Derivative as an Easily Accessible Emitter for Non-Doped Light-Emitting Electrochemical Cells

Subeesh, Madayanad Suresh,Shanmugasundaram, Kanagaraj,Sunesh, Chozhidakath Damodharan,Nguyen, Thao P.,Choe, Youngson American Chemical Society 2015 The Journal of Physical Chemistry Part C Vol.119 No.41

<P>We report a versatile approach to harvest electroluminescence from a nondoped light-emitting electrochemical cell (LEC) using an easily accessible phenanthroimidazole derivative. The authors investigated two different types, (i) ionic and (ii) neutral phenanthroimidazole derivatives by modifying our previously reported LEC emitter. Sky-blue electroluminescence was achieved by applying these modified emitter in LEC devices. In comparison to the parent molecule, a highly contrasting performance was exhibited by all the modified emitters except the neutral butyl derivative (nbpypn). By employing an ionic molecule (ihpypn) in a fully solution-processed typical LEC device structure, a peak brightness of 711 cd/m<SUP>2</SUP> was observed at a current efficiency of 0.18 cd/A. Our champion device (ihpypn-LEC) presented a 5-fold increase in maximum brightness at a ten times higher current density than its parent molecule. These peak brightness values are among the best comparing to those reported for LECs with the corresponding emission colors. Even though the neutral molecules did not show any high electroluminescence, their current efficiency at maximum brightness has improved 20 times when compared to its parent molecule utilized device. The study reveals that substituents on imidazole nitrogen has a critical impact on its performance in the LEC devices. This result is even more encouraging, considering that our molecular design can be applied to the majority of the imidazole derivatives and may open-up a plausible way of enriching the library of emitters for LECs with efficient and easily obtainable small organic molecules.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jpccck/2015/jpccck.2015.119.issue-41/acs.jpcc.5b07871/production/images/medium/jp-2015-07871s_0004.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/jp5b07871'>ACS Electronic Supporting Info</A></P>

Green Electroluminescence from Charged Phenothiazine Derivative

Shanmugasundaram, Kanagaraj,Subeesh, Madayanad Suresh,Sunesh, Chozhidakath Damodharan,Chitumalla, Ramesh Kumar,Jang, Joonkyung,Choe, Youngson American Chemical Society 2016 The Journal of Physical Chemistry Part C Vol.120 No.36

<P>A novel charged green-emitting organic small molecule, PPP, was synthesized and characterized by thermal, photophysical, electrochemical, and electroluminescence investigations. The theoretical properties of PPP were confirmed by means of computational studies. PPP exhibits a good thermal decomposition temperature of 355 degrees C. The compound PPP shows positive solvatochromism upon increasing the solvent polarity due to the more polarized excited state arising from the intramolecular charge transfer in the excited state. Solid-state emission of PPP was slightly red shifted compared to that of its solution emission spectrum, showing the reduced intermolecular interaction in the solid state. Solution-processed LEC devices were fabricated using PPP as a neat light-emitting layer. The fabricated single-component light-emitting electrochemical cell devices exhibited green electroluminescence centered at 530 nm with the CIE coordinates of (0.32, 0.58). Electroluminescent devices operated at very low turn-on voltages reveal a maximum luminance of 499 cd/m(2). These promising results are highly desirable for the development of low-cost lighting devices.</P>

Synthesis and photophysical characterization of an ionic fluorene derivative for blue light-emitting electrochemical cells

Shanmugasundaram, K.,Subeesh, M.S.,Sunesh, C.D.,Chitumalla, R.K.,Jang, J.,Choe, Y. Elsevier Science 2015 ORGANIC ELECTRONICS Vol.24 No.-

A highly fluorescent an ionic fluorene derivative 1 was synthesized and its photophysical, electrochemical and electroluminescence characteristics were investigated. Deep blue emissions were observed for compound 1 in solid as well as in dilute solutions. The synthesized compound shows high fluorescence quantum yield around 77% indicates that compound 1 can perform its role as efficient ionic emitter in LEC devices. Light-emitting electrochemical cell (LEC) devices were fabricated incorporating compound 1 without (device I) and with (device II) ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM.PF<SUB>6</SUB>). Devices I and II exhibited blue electroluminescence maximum centered at 455 and 454nm with CIE coordinates of (0.15, 0.21) and (0.16, 0.22), respectively. Maximum luminance and current efficiency of 1105cdm<SUP>-2</SUP> and 0.14cdA<SUP>-1</SUP> respectively, has achieved for device I while that of device II resulted in 1247cdm<SUP>-2</SUP> and 0.14cdA<SUP>-1</SUP> respectively.

2P-289 Charged phenanthroimidazole derivatives for small organic molecular based Light-Emitting Electrochemical cells

이현지,최영선,( Madayanad Suresh Subeesh ),박주미 한국공업화학회 2017 한국공업화학회 연구논문 초록집 Vol.2017 No.1

Electroluminescent properties of host - dopant LEC devices based on small molecular ions have been investigated. In spite of the low luminescence from emitter, most of the EL originated from the emitting species. The device studied in this paper yield a maximum brightness of 4225 cdm^-2 at a current efficiency of 0.43 cdA^-1. Over a 5-fold increase in the maximum brightness and around a 2-fold increase in the current efficiency at peak brightness was exhibited by the present LEC device compared to a host-only device (ihpypn-LEC). This study emphasize the importance of phenanthroimidazole derivative (ihpypn) as host material in LEC devices. Further modification of host material may be aimed towards suppressing high luminescence from the host molecules.

Blue-light emitting electrochemical cells comprising pyrene-imidazole derivatives

Lee, Hyeonji,Sunesh, Chozhidakath Damodharan,Subeesh, Madayanad Suresh,Choe, Youngson Elsevier 2018 Optical materials Vol.78 No.-

<P><B>Abstract</B></P> <P>Light-emitting electrochemical cells (LECs), the next-generation lighting sources are the potential replacements for organic light-emitting diodes (OLEDs). In recent years, organic small molecules (SMs) have established the applicability in solid-state lighting, and considered as prospective active materials for LECs with higher device performance. Here, we describe the synthesis of pyrene-imidazole based SMs, <B>PYR1,</B> and <B>PYR2</B> that differ by one pyrene unit and their characterization by various spectroscopic methods. To investigate the thermal, photophysical, and electrochemical properties of the two synthesized compounds, we performed thermogravimetric, UV–visible, photoluminescence (PL), and voltammetric measurements. The photoluminescence (PL) emission spectra of <B>PYR1</B> and <B>PYR2</B> measured in the acetonitrile solution, where <B>PYR1</B> and <B>PYR2</B> emit in the blue spectral region with peaks aligned at 383 nm and 389 nm, respectively. The fabricated LEC devices exhibited broader electroluminescence (EL) spectra with a significant red shift of the emission maxima to 446 nm and 487 nm, with CIE coordinates of (0.17, 0.18) and (0.18, 0.25) for <B>PYR1</B> and <B>PYR2</B>, respectively. The LECs based on <B>PYR1</B> and <B>PYR2</B> produced maximum brightness values of 180 and 72 cd m<SUP>−2</SUP> and current densities of 55 and 27 mA cm<SUP>−2</SUP>, respectively.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Pyrene-imidazole based SMs, <B>PYR1</B> and <B>PYR2</B> were synthesized and characterized. </LI> <LI> PL emission spectra of <B>PYR1</B> and <B>PYR2</B> emit in the blue spectral region. </LI> <LI> LECs exhibited broader EL spectra with a significant red shift than PL spectra. </LI> <LI> The LECs based on <B>PYR1</B> produced maximum brightness than <B>PYR2.</B> </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Photophysical, electrochemical, and quantum chemical properties of cationic iridium complexes with tunable emission color

Sunesh, C.D.,Chitumalla, R.K.,Subeesh, M.S.,Shanmugasundaram, K.,Jang, J.,Choe, Y. Elsevier S.A 2016 Journal of Electroanalytical Chemistry Vol.780 No.-

<P>We report the synthesis and characterization of the cationic iridium complexes [Ir(ppy)(2)(mPoxd)]PF6 (1), [Ir(dfppy)(2)(mpoxd)]PF6 (2), [Ir(piq)(2)(mpoxd)]PF6 (3), and [Ir(pq)(2)(mpoxd)]PF6 (4) bearing 2-phenylpyridine (Hppy), 2-(2,4-difluorophenyl)pyridine (Hdfppy), 1-phenylisoquinoline (Hpiq), and 2-phenylquinoline (Hpq) as cyclometalating ligands and 5-methyl-3-(2-pyridyl)-1,2,4-oxadiazole (mpoxd) as an ancillary ligand. UV-visible absorption spectra, photoluminescence (PL) emission spectra, and cyclic voltammetric measurements were obtained to explore the photophysical and electrochemical properties of 1-4. Depending on the nature of the cyclometalating ligands, the complexes emit yellow-orange to blue light in acetonitrile solution at room temperature. The significant blue shift in the emission spectrum of 2 is due to the presence of electron-withdrawing fluorine atoms on Hdfppy, which stabilizes the highest occupied molecular orbital (HOMO) to a greater extent than in the other complexes. The electrochemical and photophysical properties of the complexes were also calculated using density functional theory (DFT) and time-dependent DFT simulations. The results indicate that the optical properties of the complexes can be effectively tuned by selective design of the cyclometalating and ancillary ligands. (C) 2016 Elsevier B.V. All rights reserved.</P>