Far-Field Enhancement by Silver Nanoparticles in Organic Light

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Far-field Enhancement by Silver Nanoparticles in Organic light Emitting Diodes Based on D-#-A Chromophore Jayaraman Jayabharathi, Elayaperumal Sarojpurani, Venugopal Thanikachalam, and Palanivel Jeeva Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 18 Apr 2017 Downloaded from http://pubs.acs.org on April 18, 2017

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Industrial & Engineering Chemistry Research

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Far-field Enhancement by Silver Nanoparticles in Organic light Emitting Diodes Based on D-π-A Chromophore Jayaraman Jayabharathi*, Elayaperumal Sarojpurani, Venugopal Thanikachalam, Palanivel Jeeva Department of Chemistry, Annamalai University, Annamalainagar 608 002, Tamilnadu, India

*[email protected] Abstract Enhancement in brightness and luminous efficiency have been harvested by the incorporation of silver nanoparticles (Ag NPs) at ITO: 4,4'-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB) interface,

using

4’-(1-(4-methoxyphenyl)-1H-phenanthro[9,10-d]imidazole-2-yl)-styryl-N,N-

diphenyl-[1,1’biphenyl]-4-amine (MPID-TPA) as emissive layer. The enhanced device performance was obtained with 35 nm distance between Ag NPs and emissive material (MPIDTPA). The Ag NPs deposited devices easily inject holes into NPB layer results stabilized vacuum level of anode and increase the current density of OLEDs. The combined effect of farfield plasmonic coupling and hole injection ability of Ag NPs at ITO: HTL interface is the probable reason for the enhancement. A turn-off emission of MPID by Ag NPs and turn-on emission of MPID with the Ag NPs etched by H2O2 formed from the enzymatic oxidation of glucose have been analysed.

Keywords: Far-field enhancement; Hole injection; HTL; Brightness; Current efficiency; OLEDs.

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1. Introduction The surface plasmons (SPs) of metal nanoparticles (NPs) was enhanced the fabricated device efficiencies [1-10] and the distance between metal NPs and emissive materials affect the emission intensity. To improve OLED efficiency, efforts have been made to lower the hole injection barrier (HIB) of ITO: HTL interface [6-11]. The emission enhancement due to localized surface plasmon resonance (LSPR) takesplace with the distance is 5-15 nm (between NPs and emitting materials) whereas far-field enhancement is possible at the distance around 60 nm [12] in which radiative rate is increased [13]. Metal nanoparticles coated OLEDs tend to enhance the device efficiency via the combined effect of plasmonic enhancement [14-18] with reducing HIB [19-22]. The formed double layer (electric) on ITO

stabilized the fermi energy resulting an

increase of hole injection ability [19]. Localized surface plasmons of nanometals enhanced the decay process to harvest increased number of photons in the fabricated OLEDs. The SPR absorption, extinction coefficient and quantum confinement effect of metal NPs was exploited for developing monosaccharide sensors [23-47]. Effective fluorescent phenanthroimidazoles have been showed as metal sensor by our research group [48-52]. In the present investigation, silver nanoparticles embedded anode have been used to analyze the device performances. The enhanced device performance is attributed to (i) hole injection abilities of silver nanoparticles and (ii) plasmonic interaction of MPID-TPA with silver NPs. We also report 4-((E)-2-(1-(4-methoxyphenyl)-1H-phenanthro[9,10-d]imidazol-2-yl)vinyl)N,N-dimethylbenzenamine (MPID) as a sensitive fluorescence glucose sensor for first time. The strong electrostatic interaction between MPID and Ag NPs resulting turn-off fluorescence of MPID. However, oxidation of glucose by glucose oxidase generates H2O2 which leads to etching


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Ag NPs results turn-on fluorescence of MPID which could be used for the quantification of glucose. 2. Experiment and Characterization Sigma-Aldrich supplied chemicals for synthesizing MPID, MPID-TPA (Scheme 1) and silver nanomaterials. NMR and mass spectroscopic measurements were obtained on Bruker spectrometer (400 MHz) and Agilent LCMS VL SD, respectively. The frontier energy levels are determined with oxidation potentials determined with CHI 630A potentiostat electrochemical analyzer (platinum electrode- working electrode; platinum wire- counter electrode; Ag/Ag+ electrode - reference electrode; scan rate -100 mV s-1; 0.1M tetrabutylammoniumperchlorate in CH2Cl2 - supporting electrolyte). The Perkin Elmer (Lambda 35) instrument was used to measure absorption wavelength and diffused reflectance spectra (DRS) measurements were carried out using Lambda 35 spectrophotometer with RSA-PE-20 integrating sphere. Solvatochromic emission shifts were measured using LS55 fluorescence spectrometer (Perkin Elmer). The PL quantum yield (QY) was calculated with 0.5 M H2SO4 solution of quinine (0.54) as reference using the following equation: φunk = φstd  I unk   I  std

   

 Astd  A  unk

   

 η unk  η  std

   

2

[φunk – QY of unknown material; φstd - QY of standard; Iunk - emission

intensity of unknown material ; Istd - emission intensity of standard; Aunk - absorbance of unknown sample; Astd - absorbance of standard; ηunk- refractive index of the unknown material;

ηstd - refractive index of standard solution]. The QY of film was measured with integrating sphere (quartz plate). Decomposition temperature was measured with Perkin Elmer thermal analysis system (10° C min-1 ; nitrogen flow rate - 100 mL min-1). Glass transition temperature was recorded with NETZSCH (DSC-204) (10° C min-1 under nitrogen atmosphere). The



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morphology and size of Ag NPs and MPID-Ag nanohybrid was recorded with JEOL JEM 2100 HR-TEM (200 kV - resolution 0.1 nm). The chemical composition of Ag NPs and MPID-Ag nanohybrid was recorded XPS (X-ray photoelectron spectra: ESCA-3 Mark II spectrometer-VG Al Kα (1486.6 eV) radiation). The TEM (transmission electron microscopy) images was recorded on Philips TEM with 200 kV electron beam and the SAED (selected area electron diffraction) pattern was obtained on Philips TEM with CCD camera (200 kV). The powder XRD (X-ray diffractograms) was obtained with Eqinox 1000 diffractometer using Cu Kα rays (1.5406 Å ; current - 30 mA ; 40 kV). IR spectra were recorded using Agllent carry 650 spectrometer. 2.1. Synthesis of 4-((E)-2-(1-(4-methoxyphenyl)-1H-phenanthro[9,10-d]imidazol-2-yl)vinyl)N,N-dimethylbenzenamine (MPID). The product MPID was prepared by refluxing 9,10-phenanthrenequinone (5 mmol), 4 - N, N-dimethylaminocinnamaldehyde (5 mmol), 4-methoxyaniline (6 mmol) and ammonium acetate (61 mmol) in alcohol (30 mL) with temperature 90 ºC. The solvent was distilled off and the phenanthrimidazole MPID was used after purification. Yield: 81%. M.P. 198 ºC. 1H NMR (400 MHz, CDCl3): δ 2.98 (s, 6H), 3.98 (s, 3H), 6.47 (d, J=15.6 Hz, 2H), 6.60 (d, J=8.4 Hz, 2H), 7.15 (d, J=8.8 Hz, 2H), 7.20 (t, J=14.8 Hz, 1H), 7.25 (d, J=13.2 Hz, 1H), 7.28-7.31 (m, 3H), 7.56-7.64 (m, 2H), 7.71 (t, J=14.8 Hz, 1H), 7.88 (d, J=16.0 Hz, 1H), 8.68 (t, J=8.4 Hz, 1H), 8.73 (d, J=8.4 Hz, 1H), 8.86 (d, J=7.6 Hz, 1H).

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C NMR (100 MHz, CDCl3): δ 40.31, 55.72, 109.35,

112.27, 115.27, 120.71, 122.9, 123.37, 124.39, 126.62, 127.27, 128.41, 129.85, 130.28, 131.84, 135.45, 137.69, 151.39, 160.37. MALDI-TOF MS: calcd. for C32H27N3O: 469.22, Found: 469.20 [M+]. Anal. calcd. for C32H27N3O (%): C, 81.85; H, 5.80; N, 8.95. Found: C, 81.83; H, 5.82; N, 8.96.


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2.2. 2-(4-bromostyryl)-1-(4-methoxyphenyl)-1H-phenanthro[9,10-d]imidazole (BMPI) An

appropriate

quantity

of

9,10–phenanthrenequinone

(5

mmol),

4-bromocinnamaldehyde (5 mmol), p-anisidine (6 mmol) and ammonium acetate (61 mmol) in alcohol (20 mL) was refluxed 12 h under N2 atmosphere. The solvent was distilled off and the pure phenanthrimidazole BMPI was used. Yield: 75%. M. P 246 °C. 1H NMR (400 MHz, CDCl3): δ 3.86 (s, 3H), 6.93 (s, 2H), 6.97 (d, J=16.0 Hz, 1H), 7.12 (d, J=8.8 Hz, 1H), 7.32–7.35 (m, 4H), 7.51 (d, 2H), 7.5-8.01 (m, 4H), 8.25 (d, J=16.2 Hz, 2H), 8.86 (d, J=15.3 Hz, 2H).

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C

NMR (100 MHz, CDCl3): δ 51.46, 115.91, 120.66, 121.09, 121.54, 122.01, 122.35, 122.67, 124.08, 124.35, 125.08, 125.25, 125.93, 126.12, 126.43, 126.49, 126.62, 126.91, 127.22, 127.47, 127.63, 127.77, 128.95, 130.95, 132.48, 144.91. MALDI-TOF MS: calcd. for C30H21BrN2O: 504.08, Found: 505.4 [M+]. Anal. calcd. for C30H21BrN2O (%): C, 71.29; H, 4.19; N, 5.54. Found: C, 71.29; H, 4.20; N, 5.52. 2.3.

4’-(1-(4-methoxyphenyl)-1H-phenanthro[9,10-d]imidazole-2-yl)-styryl-N,N-diphenyl-

[1,1’biphenyl]-4-amine (MPID-TPA) A

mixture

of

2-(4-bromostyryl)-1-(4-morpholinophenyl)-1H-phenanthro[9,10-d]imidazole

(BMPI) (4.5 mmol), 4-(diphenylamino)phenylboronic acid (7.5 mmol), Pd(PPh3)4 (0.25 mmol) and aqueous Na2CO3 (15 mL) in toluene:ethanol (20:15 mL) was refluxed in N2 atmosphere ( 18 h). The solvent was distilled off and the pure MPID-TPA was used for further analysis. M.P. 240 ºC. Yield: 81%. 1H NMR (400 MHz, CDCl3): δ 3.82 (s, 3H), 6.55-6.61 (m, 6H), 6.72 (s, 2H), 6.77 (d, J=8.2 Hz, 2H), 7.08 (d, J=16.2 Hz, 2H), 7.10-7.18 (m, 6H), 7.29-7.32 (m, 7H), 7.45 (s, 2H), 8.03 (d, J=17.2 Hz, 2H), 8.76 (d, J=15.5 Hz, 2H). 13C NMR (100 MHz, CDCl3): δ 55.64, 106.52, 115.24, 117.87, 120.34, 124.79, 125.31, 125.53, 126.35, 126.53, 126.79, 130.24, 130.69, 130.91, 134.65, 136.84, 138.76, 139.83, 142.26, 151.18. MALDI-TOF MS: calcd. for



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C48H35N3O: 669.2, found: 669.8 [M+]. Anal. calcd. for C48H35N3O (%): C, 86.07; H, 5.27; N, 6.27. Found: C, 86.05; H, 5.29; N, 6.29. 2.4. Preparation of Silver Nanoparticles (Ag NPs) Dried powder of pericarp of T. bellirica fruit was kept in oven at 50 oC for 30 min to remove the moisture. About 0.05g dried powder in 15 mL water was warmed at 90 oC for 30 min. The extract was filtered through cellulose nitrate membrane filter paper (0.22 µm) and the extract was mixed with silver nitrate (0.01 M, 10 mL). The solution was kept at 60 oC (1 h) and then the silver nanoparticle (Ag NPs) solution was stored at 5 oC before use.

2.5. Synthesis of MPID-Ag Nanohybrid Aqueous solution of 20 mL citrate buffer, 5 mL MPID and 5U/mL of glucose oxidase was added to a stirred 20 mL Ag NPs solution, the stirring was continued for 6 h. The brown precipitate was washed to remove excess inorganic salts. 3. Fabrication of OLED A series of devices with double-layer configuration of ITO/Ag NPs [with Ag NPs (6/µm2 (II); 40/µm2 (III)] or without Ag NPs (I) /NPB (4,4'-bis[N-(1-naphthyl)-N-phenylamino]biphenyl) (35 nm)/MPID-TPA (60 nm) /LiF (1 nm)/ Al (100 nm) have been fabricated by vacuum deposition ( 5 x 10-6 torr) onto ITO with resistance of 20 Ω/square. Deposition of organic materials onto ITO at a rate of 0.1 nm s-1 and LiF was thermally evaporated on organic layer surface. The thickness of all layers were measured using a quartz crystal thickness monitor. Measurement of current density (J)-voltage (V)-Luminescence (L) were made simultaneously with Keithley 2400 sourcemeter.


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4. Computational Details Geometry optimization of MPID and MPID-TPA at ground state and excited state was carried out by DFT/ B3LYP/6-31G (d, p) and TD-DFT/ B3LYP/6-31G (d, p) method, respectively [53]. The excited state electronic transitions are analyzed by natural transition orbitals (NTOs) with dominant particle-hole pair contributions and the associated weights [53]. The out file obtained from TD-DFT method has been used to analyze natural transition orbitals with the multifunctional wavefunction analyzer [54]. 5. Results & Discussion 5.1. Characterisation of 4-((E)-2-(1-(4-methoxyphenyl)-1H-phenanthro[9,10-d]imidazol-2yl)vinyl)-N,N-dimethylbenzenamine (MPID) Figure 1 represents the optimized geometry of MPID, the bond C40-C41 between dimethylaminostyryl and phenanthrimidazole unit was quite conjugated with bond length of 1.35 Å. The N, N-dimethylaminostyryl

ring at C(2) is nearly coplanar with the

phenanthrimidazole unit with the torsional angle of ca. 11.6º whereas the methoxyphenyl ring at nitrogen is twisted with a dihedral angle of ca. 60.82° about the phenanthrimidazole ring. The decomposition temperature (Td - 224°C) and glass temperature (Tg - 84°C) of MPID was found from TGA and DSC, respectively (Figure 2a). The two major absorption (254 and 395 nm ) have been observed for the bare MPID, the absorption at 395 nm is attributed to HOMO–LUMO transition of MPID and the other absorption at 254 nm is due to π-π* transition of phenanthrimidazole moiety (Figure 2b). The orbital features of MPID can be calculated using TD-DFT method to analyze the excited state and the HOMO-LUMO energies status analysed with fermi level Ag NPs. The electronic delocalization of highest occupied molecular orbital (HOMO) were located on N, N-dimethylaminostyryl donor unit while the electronic



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delocalization of lowest unoccupied molecular orbital (LUMO) were located on the phenanthrimidazole acceptor unit. This difference in electron distribution supports the electron transition between donor (HOMO) and electron acceptor (LUMO). The computational analysis also evidenced the above said transitions: (i) HOMO to LUMO (fHOMO–LUMO 0.2365) with λabs = 395 nm and (ii) HOMO−1 to LUMO ( f(HOMO−1)–LUMO - 0.5740) with λabs = 254 nm. Fluorescence spectrum of MPID shows a emission at 420 nm (Figure 2c) attributing to LUMO-HOMO transition of MPID [55]. 5.2. Donor–π–Acceptor Molecular Design The ground state (S0) and excited state (S1) geometries of MPID-TPA were optimized using DFT/B3LYP/6–31G (d, p) and TD–DFT/B3LYP/6–31G (d, p) methods (Figure 1). The excited state twist angle (θ1) of MPID–TPA is increased to 30.83° when compared with ground state twist angle (θ1) (25.99°). The bond length (R1) of MPID–TPA is elongated by 0.01 Å from S0 to S1. The observed smaller change of geometry from S0 to S1 in MPID–TPA may decrease the non radiative emission results enhancement of photoluminance efficiency. The HOMO and LUMO of MPID-TPA were localized on TPA and MPID moieties, respectively (Figure 3). This bipolar molecular design with the balanced change transport property is an additional benefit of MPID–TPA molecule as the light emitting layer in which TPA acts as hole–transporting group and MPID as electron transporting group. The nature of excited states have been explained by analysing the natural transition orbitals (NTO) [54] of MPID–TPA have been analyzed (Figure S1b; Table S3). For S1 and S2 states, NTO hole is delocalized over entire molecular backbone whereas particle is localized on MPID, for comparison NTOs of MPID have also been calculated. Both S1 and S2 states exhibit a character of hybridized local and charge transfer state (HLCT) in which higher CT character of S1 state enhance the exciton utilisation efficiency (ηS).


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A large energy gap occurs between T1 and T2 arising from the acceptor group [56, 57] and very small ∆EST is observed between S1 and T2 states facilitating RISC (T2→S1) process in MPID– TPA as a result of their HLCT state character (Figures 4a & 4c). Due to extended π–conjucation in MPID–TPA, the CT state is stabilised than LE state and the reduced energy gap leading to full hybridisation of LE and CT states which inturn improves the OLED efficiency (Figure 4b). The HOMO energies of -5.11 eV(MPID) and -5.41 eV (MPID-TPA) (EHOMO = Eox + 4.8 eV) have been calculated from electrochemistry measurements (oxidation potential of TPAMPID and MPID-TPA have been calculated from cyclic voltametry measurement) (Figure S1a) and the LUMO energies -2.45 eV (MPID) and -2.60 eV (MPID-TPA) have been deduced from the equation, ELUMO = EHOMO– 1239/λonset [55] (Figure 2d). The thermal properties have been analysed to understand the device stability. For MPID–TPA, the glass transition temperature (Tg) and thermal decomposition temperature (Td) were measured as 130 and 426 °C respectively (Figure 2a). The higher thermal stability is due to the stronger rigidity which will be in favour of OLED stability. UV–vis spectra of MPID-TPA show absorption at 320 and 346 nm and emission at 418 and 448 nm in dichloromethane (Figure 2b and 2c). Compared with parent compound MPID (254 nm, εmax =39370 cm-1 M-1), the MPID–TPA show red shifted absorption maxima at 346 nm (εmax =28901 cm-1 M-1) which is attributed to the CT transition from donor (TPA) to acceptor (phenanthrimidazole). A red-shifted emission was observed for MPID–TPA relative to MPID due to extended D-π-A conjugation. The dipole moment (µe) in excited state of MPID and MPID–TPA has been calculated from the Lippert-Mataga plot [Stokes shift (va–vf) versus orientation polarizability, f (ε, n)] (Figure 5a; Tables S1 and S2) [58-62].The MPID–TPA show two independent slopes of two section fitted lines which reveal the existence of two different



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characters of excited state [63]. The calculated µe of MPID is 11.3 D (R2 -0.98) which indicates a single emissive state (LE) whereas for MPID-TPA two sets of linearity was obtained (Figure 5a). This indicates the nature excited states depends upon the polarity of the solvents. The dipolemoment was calculated to be 20.9 (R2 - 0.86) and 7.8 D (R2 - 0.96) for high and low polar solvents, respectively. The dipole moment of 7.8 D in low-polar solvents reveals that the S1 state possessed CT character in addition to LE. The quantum yield decreases with high solvent polarity and relatively high quantum yield was obtained between hexane and butyl ether. These factors shows that locally excited (LE) character has been introduced thus, S1 state in low polar solvents consists of both CT and LE components. However, observation of mono-exponential emission decay (Figure 5b) in low polar solvents reveal a new excited state i.e., HLCT state exists in the D-π-A architecture rather than a mixture of LE and CT states [64, 65]. The lifetime measurement reveals that this intercrossed excited state in different polar solvents should be a HLCT instead of two species state through addition of LE and CT. 6. Characterization of AgNPs and MPID-Ag Nanohybrid Figure 6 shows the XRD pattern of Ag NPs synthesized from T. bellirica fruit extract along with MPID and MPID-Ag nanohybrid. The JCPDS pattern of this crystalline MPID and MPID-Ag nanohybrid has not been reported so far. The diffraction peaks at 2θ of 38.25o, 44.30o, 64.39o, 77.35o and 81.37o corresponds to (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) interplanar reflections of face centered cubic crystal structure, respectively (JCPDS. No. 01-087-0597) [57-67]. Using Debye-Scherrer formula, D = kλ/βCosθ [D - average crystal size, k - Scherer coefficient (0.891), λ- X-ray wave length, θ -Bragg’s angle, β -full width at half maximum intensity], average crystallite size of Ag NPs is determined as 23.68 nm and the surface area is 41.19 m2/g. The (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) interplanar reflections of face


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centered cubic crystal are observed in the recorded XRD of MPID-Ag nanohybrid. This clearly shows the presence of FCC nanocrystalline silver in the MPID-Ag nanohybrid. The mean crystallite size of the MPID-Ag nanohybrid is 6.5 nm and the calculated surface area is 162.62 m2/g. The XRD pattern of MPID and MPID–Ag nanohybrid exhibits several peaks with the diffraction angles in the range of 10-30° which reveal that there is orderly arrangement of MPID in both MPID and MPID-Ag nanohybrid. The reflection of crystalline MPID at 7.95 o, 18.37 o, 21.06

o

and 28.36 o are also observed in the synthesized MPID-Ag nanohybrid. This

shows the presence of crystalline MPID bound to nanocrystalline silver, hence the MPID-Ag nanohybrid is characterized as MPID bound to face centered cubic nanocrystalline silver. Morphology and shape of the synthesized Ag NPs and MPID-Ag nanohybrid were determined by SEM (Figure S2) and TEM analysis (Figure 7). The TEM image shows the lattice fringes of the nanocrystals and the observed d-spacing (2.346 Å) corresponds to the 111-plane of face centered cubic (FCC) silver is in good agreement with the XRD d- spacing (2.35 Å) between (111) plane of FCC silver crystal (JCPDS. No.01-087-0597). the diffraction rings with bright spots in SAED pattern are indexed to (111), (200), (220), (311) and (222) planes, respectively of FCC silver which is in agreement with XRD results. The DLS images show the average size of the Ag nanoparticles and MPID-Ag nanohybrid as 24 and 6.5 nm, respectively (Figure 8a). The DLS confirmed the particles size calculated from XRD and TEM experiments. The repulsive forces exist with the electrical charge of the particles on the surface of Ag nanoparticles and MPID-Ag nanohybrid accounts the negative ζ potential of Ag NPs (-26.3 mV) and MPID-Ag nanohybrid (-10.3 mV)], which in turn increase the stability. The composition of Ag NPs and MPID-Ag nanohybrid was identified by XPS (Figure 8b). The observed carbon peak (284.7 eV) is due to the residual carbon of the



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sample. The binding energy peaks observed at 368.3 and 371.5 eV due to Ag 3d5/2 and 3d3/2, respectively confirmed the presence of metallic silver [68]. The α and β symmetrical peaks at 527.8 and 532.2 eV, shows that O1s profile is asymmetric. The two peaks of Ag 3d5/2 and 3d3/2 were observed at 368.9 and 372.9 eV, respectively for MPID-Ag nanohybrid confirmed the presence of Ag+ ion [69]. Similar with Ag NPs, the two peaks observed at 528.0 eV (α) and 532.8 eV(β) reveal two different kinds of oxygen species present in MPID-Ag nanohybrid. The N1S peak appears at 403 eV, shifting of Ag 3d5/2 and 3d3/2 peaks and O1S peaks confirm the formation of MPID-Ag nanohybrid. The observed results reveal that silver is oxidized to Ag+ by glucose oxidase [93] and the Ag+ is likely to linked with the MPID. The slight shift of the binding energy indicates that this binding is more likely at phenanthroimidazole ring nitrogen. The probability of binding of Ag+ ion to N, N-dimethylaminostyryl nitrogen is less likely due to the steric hindrance offered by the styryl moiety. The binding of Ag+ to the phenanthroimidazole nitrogen is likely to decrease the electron density at the bound nitrogen. This favors flow of electrons through conjugation from the lone pair of electron of N, N-dimethylaminostyryl nitrogen through the styryl moiety and enhances the emission. The void space in face centered cubic silver crystal is ~ 1.44 Ǻ which permits seating of the styryl moiety of the MPID derivative comfortably as shown in Figure S1c. The diameter of the styryl moiety is 1.30 Ǻ which is less than the void space of FCC silver crystal. The π- and feedback π -coordination bonds between the Ag+ ion and MPID result in a greatly enhanced electron interaction in MPID-Ag nanohybrid, which increases the binding energy of Ag3d5/2 and O1s electrons [70]. Fluorescence decay measurements were carried out for MPID, MPID-TPA and MPID-Ag nanohybrid (Figure 5b; Table 1 ). The decay of MPID was found to be 0.92 ns and the biexponential decay reveal the presence of two kinds of fluorescent components; (i) coplanarity


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of styryl moiety with phenanthrimidazole fragment and (ii) the N, N-dimethylaminostyryl ring is perpendicular to phenanthrimidazole unit. The more stable perpendicular arrangement of of MPID is predicted by DFT and hence the observed longer life time is corresponds to the same [71]. The two absorption peaks also in accordance with the results (Figure 2). A short life time component with decay life time of 0.85 ns (96 %) and the long time component of 2.68 ns (4 %) was obtained. The average life time calculated for MPID-Ag nanohybrid was 3.59 ns which is significantly longer than that of fluorophore MPID (0.92 ns). The fluorescence of the MPID-Ag nanohybrid following tri-exponential decay. Comparison with that of MPID the relative amplitude (A) of a short life component (0.85 ns) is decreased to 40 % (from 90%) and a newly formed component with decay life time and amplitude was 7.90 ns and 35%, respectively was observed. These results confirmed the new fluorophore in the MPID-Ag nanohybrid exist. Emission of both MPID and MPID-Ag nanobybrid shows single peak and other bands may be overlap with the existing band. The life time is inversely related to decay rate (kt) and it is sum of the radiative decay rate (kr) and nonradiative decay rate (knr) [72]. When the fluorophore MDPI is in contact with Ag NPs, surface plasmons of Ag NPs change both radiative and nonradiative decay rate of MDPI. The longer life time indicate that both kr and knr was decreased after the coupling effect [73] and quenched MPID emission was observed. The longer decay lifetime of the MPID–Ag nanohybrid is due to a decreased kr rate. The life time 7.90 ns due to the new component reveal the existence of a new fluorophore with slower kr than the other two components responsible for the lower kr of the nanohybrid. 7. Electroluminescent Properties To evaluate the device performances, a series of devices with double-layer configuration of ITO/Ag NPs with [6.0/µm2 (II); 40/µm2 (III)]/ without Ag NPs (I) / NPB (35 nm)/MPID-TPA



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(60 nm) /LiF (1 nm)/ Al (100 nm) have been fabricated (Table 1). The electroluminescent spectra of devices I - III are shown in Figure 9, the devices exhibit blue emission at 448 nm which corresponds to the band gap of MPID-TPA. The surface morphologies of vacuum deposited thin film of devices II and III have been studied by atomic force microscopy (AFM) topography images (Figure 9c) which shows the uniform distribution of Ag NPs on ITO anode. For far- field plasmonic enhancement, uniform size distribution of nanoparticle is important. The root mean square values (0.41 nm-Ag coated ITO & Ag NPs- 0.31nm) calculated for Ag NPs coated ITO is higher, supports the higher coverage of Ag nanoparticles which results high efficiency [85]. Surface plasmonic absorption of silver nanopartiocles (~400 nm) depends the size of NPs and if it exceeds 100 nm the absorption will disappear from visible spectrum. In optoelectronics applications, especially in OLEDs, AgNPs or any other metal nanoparticles with surface plasmonic absorption in the visible region are desirable. Otherwise, realizing the plasmonic enhancement is not possible. Hence, highly homogenous or monodispersed Ag NPs exhibiting plasmon absorption in 400 nm is necessary to achieve plasmonic enhancement. If there is an agglomeration between the nanoparticles, absorption moves away from visible region make it unworthy. Figures 5b and 5c evidences the Ag NPs on ITO enhances the lifetime of devices II and III when compared with reference device. The emission due to the effective electron-hole recombination with the applied voltage may passed via the plasmons present in the Ag NPs boundary of the bottom-emissive device. The triexponential decay parameters are observed for devices I-III (Table 1) and the estimated lifetime for device I is τ1 = 0.83, τ2 = 2.62 and τ3 = 10.68 ns. Among them, τ2 and τ3 are due to the two geometrical isomers of MPID-TPA (coplanar and perpendicular) in film state. The coplanar isomer appears with 30 % relative


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amplitude whereas the perpendicular isomer appears with 60 % relative amplitude . The lifetime 0.83 ns (τ1) with amplitude of 0.83 % is attributed to the scattering of photons from the NPB or ITO layers. Therefore τ2 and τ3 is taken to analyse the luminescent character of MPID-TPA in the hetero junction pattern. The plasmon–exciton coupling (LSPR enhancement) is effective with the distance between the emissive layer and NPs is 5–15 nm [85] which reduces the lifetime with increasing the decay rate without affecting the charge transport property [33]. In the current investigation, the distance between emissive layer and NPs is tuned to 35nm (NPB thickness -35 nm), Thus, the Ag NPs on ITO helped to liberate photons from the hetero junction results far-field plasmon enhancement (FFPE). Therefore, the optically strengthened pathway (hetero junction) increase the lifetime with increasing emission intensity [58]. The increased lifetime is linked to the density of Ag NPs on ITO which is low when compared to the density needed for LSPR enhancement. The measured higher relative amplitude of perpendicular isomer (τ3) is responsible for the light generating property of emissive material on combined with the Ag NPS, thus supports the interaction of far-field plasmons with MPID-TPA emission. In general, plasmonic coupling can be explained by; (i) Increase of life time with relative amplitude: lifetime of the excitons generated in the emissive layers [75]. In the case of localized plasmonic coupling, energy transferred from metal nanoparticles to emissive layer which reduces the lifetime of the generated excitons, results increase of decay rate with the enhanced light emission. But in far-field plasmonic enhancement, the distance between emissive layer and metal nanoparticle is higher than the limit necessary for localized plasmons to react with excitons. So, the possibility of energy transfer between them is ruled out. However, the light generated in the system has to pass through the nanoparticles present on anode. Metal nanoparticles are well known to have high scattering cross section when compared to conventional dye molecules. So



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light generated from the emissive layer undergo retardation effect [74] with metal nanoparticles which ease the way for light emission from the device [75]. As a result, the lifetime of the excitons increases with enhanced light emission (higher relative amplitude) from the device (Table 1); (ii) Increase of Luminance and current efficiency:

The NPB thickness (35 nm) is

responsible for the increased Luminance and current efficiency [13, 76, 77]. The brightness and current efficiency increases by high Ag NPs density and brightness increased by 23% and 61% and the current efficiency enhanced by 28% and 52 % corresponding to Ag NPs density of 6/µm2 and 40/µm2, respectively: the brightness and current efficiency enhancement in our study is due to far-field effect enhancement and (iii) Increased quantum yield(QY) : The increase of QY is also due to the far-field effect [12, 13]. The device III with 40/µm2 density Ag NPs coverage show intense emission than the reference device I. In device III (40/µm2 Ag NPs coverage) the HIB is reduced by stabilizing the Fermi state of Ag NPs results reduction in turn on voltage when compared with the reference device. The J-V-L curve shows that that the brightness and current efficiency increases by high Ag NPs density and brightness increased by 23% and 61% and the current efficiency enhanced by 28% and 52 % corresponding to Ag NPs density of 6/µm2 and 40/µm2, respectively. Ag NPs play important role in electrical parameters, such as charge carrier injection and transport and dopant into semiconductor which may be due to low coverage density of Ag NPs [NPB layer thickness (35 nm) > Ag NPs size (24 nm)]. The anode with 40/µm2 Ag NPs coverage reduced HIB results reduction in turn on voltage compared with the reference device. The larger interface area between emissive layer and HTL layer enhanced the charge injection into the emissive layer make effective recombination of electron-hole recombination results enhanced device


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performances [78-80]. The anode with low coverage of Ag NPs (6.0/µm2) exhibit poor hole injection into HTL results poor device performances due to imbalance charge recombination. The thickness of NPB layer also affects device performances in addition to Ag NPs density [12]. The enhanced efficiency of the device III can be obtained with the NPB layer thickness of 35 nm in the presence of Ag NPs (40/µm2) [12, 81]. The distance dependence of metal enhanced fluorescence with far-field enhancement is shown in Figure 10. The uniformly dispersed Ag NPs on ITO form interface dipoles which stabilized the work function of ITO and reduced the HIB. The enhanced device efficiencies can be explained by the merging of metal enhancement region with light emitting profile (MPID-TPA) [82-84] which is nearer to cathode. The donor-spacer-acceptor (D-π-A) compound, MPID-TPA with hybridized local and charge transfer (HLCT) state character as emissive layer in OLEDs exhibited higher efficiency: the lowlying LE-dominated HLCT state provides a high radiative transition rate for high luminescence quantum yield, whereas the high-lying CT-dominated HLCT state is responsible for a high efficiency through the enhanced reverse intersystem crossing process (Figure 4). HLCT as the emissive state with increased LE and decreased CT which in turn increased the quantum efficiency. Therefore, the efficient OLEDs performance is not accounted by LSPR but from farfield enhancement. Jeganathan etal., reported the maximum luminance from the device: ITO/ (with and without Ag NPs)/ α-NPD (30 nm) / Alq3/ LiF/ Al as 8665 cd/m2 and with gold nanoparticles 9000 cd/m2 [75]. Ma etal., reported the maximum luminance of 15909 cd/m2 with 60/µm2 gold nanoparticles thickness [85]. The maximum luminance 15925 cd/m2 obtained in the present study with double-layer configuration of ITO/Ag NPs /with Ag NPs (40/µm2) /NPB (35 nm)/MPID-TPA (60 nm) /LiF (1 nm)/ Al (100 nm) is higher than those reported by literature [75, 85]. Hence it is possible to improve the efficiency of these materials through modification of



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thickness of the nanomaterials, hole transport layer and emissive layer Effort will be made to modify the thickness of the various hole transport layer, density of noble metal NPs and varying emissive layer (D-π-A) molecule with HLCT emissive state to enhance the device efficiency in future of our studies. 8. Turn-on and Turn-off Fluorescence The surface plasmon resonance (SPR) of the Ag NPs shows single band at 412 nm (Figure 2) and absence of absorption in the region 400 - 500 nm of fruit extract and AgNO3 solution confirmed the SPR due to the formation of metallic spherical Ag nanoparticles [77, 86, 87]. A peak at 412 nm confirmed Ag NPs formed is spherical in shape. After coupling of Ag NPs with MPID, the absorption peak of MPID at 395 nm was reduced and the peak at 254 nm is shifted to 245 nm for MPID-Ag nanohybrid which is due to the coupling of MPID with Ag NPs. The absorption band of the MPID-Ag nanohybrid at 342 nm is blue shifted by 70 nm53 nm from bare Ag NPs and bare MPID , respectively. The blue shift might arise from the interaction between the MPID with Ag NPs in situ. On coupling with Ag NPs the fluorescence

intensity of the MPID peak at 420 nm was efficiently reduced to small band and a new band at 392 nm is blue shifted by 28 nm (Figure S4). The emission quenching is mainly resulted from the charge transfer (electron0 from LUMO (MPID) to the Ef of Ag NPs and exciton is not generated when the electron transfers from LUMO of MPID to Ef of Ag NPs, thus, quenching results (Figure 10) [88].

Energy transfer efficiency (E) can be calculated from the following equations, E= 1-

=1 -

∞

= R06/(R06+r60) : R06= 8.8 x 1023[қ2n-4ΦD J(λ)] in Å6 : J(λ) = (λ) εA (λ) λ4 dλ

(τ - life time of donor in presence of acceptor; τ0 - life time in absence of acceptor molecule; F – emission intensity of donor in presence of acceptor; F0 - emission intensity in absence of


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acceptor ; E - efficiency of energy transfer between donor and acceptor; R0 - critical distance; FD (λ) - corrected fluorescence intensity of donor ; εA(λ) - molar extinction coefficient of acceptor]. The parameter J, R0, r0 and E is determined as 4.31x10-12 cm3 L mol-1, 0.74 nm, 1.04 nm and 0.46 respectively [90] which is in accordance with FRET condition (static quenching interaction between Ag NPs and MPID). The value of r0 is less than 8 nm which is higher than R0, this also supports static quenching mechanism [91]. The electron transfer rate constant (ket) is calculated as 8.1 x 10-10 s-1 ( ket = 1/τads - 1/τ). The electron transfer efficiency (37.5%) is obtained using the equation, % E = (1-τMPID-Ag/τMPID) x 100. The ~100 emission intensity of the MPID was recovered in the presence of glucose and glucose oxidase (Figure S4) and same trend was observed with H2O2. However, various glucose oxidase concentration in absence of glucose did not alter the intensity fluorescence, which suggest that glucose oxidase itself not responsible for fluorescence recovery: in situ H2O2 formed from the oxidation of glucose (enzyme) is likely to be the reason for emission intensity recovery (Scheme 2; Figure S4). The binding interaction of Ag NPs with MPID is due to the larger surface curvature of the Ag NPs which reduces the steric hindrance by providing more unsaturated dangling bonds on the surface of Ag NPs [92]. The observed results reveal that silver is oxidized to Ag+ by glucose oxidase [93] and the Ag+ is likely to linked with the MPID. This binding is more likely at phenanthroimidazole ring nitrogen. The probability of binding of Ag+ ion to N, Ndimethylaminostyryl nitrogen is less likely due to the steric hindrance offered by the styryl moiety. The strong binding of Ag+ to the phenanthroimidazole nitrogen is likely to decrease the electron density at the bound nitrogen. This favors flow of electrons through conjugation from the lone pair of electron of N, N-dimethylaminostyryl nitrogen through the styryl moiety (Figure S1C), results enhancement due to effective electron-hole recombination.



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The spherical shape of Ag NPs was confirmed by TEM images and reduced to the size of 6.5 nm, after adding glucose and glucose oxidase, increase of glucose concentration shows absence of Ag NPs in the solution. The observed results shows that the Ag NPs were oxidised to Ag+ ion with the addition of 100 mM glucose and 0.5 U mL-1 glucose oxidase to Ag NPs, the SPR band at 412 nm decreased significantly and completely disappeared with further increase of glucose concentration (500 mM) (Figure S3). The optical results are in agreement with the TEM observation [94].The emission intensity was increased with increasing glucose concentration (0– 500 mM) (Figure S4). The selectivity of the sensing with glucose over other carbohydrates namely fructose, maltose, lactose and sucrose was analyzed increased emission intensity of the MPID was observed only with glucose which shows the excellent selectivity for glucose (Figure S4). 9. Conclusion In conclusion, uniformly dispersed plasmonic Ag NPs on ITO anode stabilized the work function of anode and reduced the hole injection barrier. The distance between the emissive layer and silver nanoparticles influence the enhancement effect. The increased lifetime, quantum yield with increased radiative rate and brightness of the devices is

due to the far-field plasmonic

effect. The device performances show 40% increased brightness and 52% increased luminous efficiency with reduced turn-on voltage when compared to the reference device due to the farfield effect of Ag NPs in visible spectral range.

Efficient glucose sensor has also been

developed.

Supporting Information: Contents

1. Figures- S1-S6


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2. Tables- S1- S4 This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgments

One of the author Dr. J. Jayabharathi thank Department of Science and Technology (DST) (EMR/2014/000094), Defence Research and Development Organization (DRDO) (213/MAT/1011), Council of Scientific and Industrial Research (CSIR) [No. 01/ (2707)/13EMR-II], University Grant Commission (UGC) (36-21/2008) and DST-Nano Mission (SR/NM/NS-1001/2016) for financial support.



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Figure 1. Ground state and excited state geometries of MPID and MPID-TPA.



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Figure 2. (a) DSC and TGA graphs of MPID and MPID-TPA; (b) UV-vis spectra of MPID, Ag NPs, MPID-TPA and MPID-Ag; (c) Emission spectra of MPID, Ag NPs, MPID-TPA and MPID-Ag; (d) Cyclic voltammogram of MPID and MPID-TPA.


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Figure 3. (a) HOMO and LUMO contour maps and molecular electrostatic potential (ESP) surface of MDPI, MDPI-TPA and MDPI-Ag.



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Figure 4. (a) Reverse intersystem crossing process of MPID-TPA; (b) Efficient hybridization of MPID-TPA; (c) Scheme of exciton decay process after hole and electron recombination in OLEDs of twisting MPID-TPA.


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Figure 5. (a) Lippert–Mataga plot of MPID and MPID-TPA in different solvents; (b) Life time spectra of MDPI, MDPI-TPA and MDPI-Ag; c) Life time spectra of devices I-III.



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Figure 6. X-ray diffraction patterns of MDPI, Ag NPs and MDPI-Ag.


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Figure 7. HR-TEM images and SAED pattern of Ag NPs and MPID-Ag.



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Figure 8. (a) Zeta potential of (a) Ag NPs and (b) MPID-Ag; (b) XPS of (a) Ag NPs and (b) MPID-Ag.


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Figure 9. Performances of devices I, II and III: (a) Luminance-Voltage; (b) Current efficiencyCurrent density; (c) Electroluminescent spectra of the devices I- III; (d) AFM images of (i) Ag NPs, (ii) Ag nanoparticles dispersed ITO anodes, (iii) device II and (iv) device III.



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Figure 10. Overlapping of MPID-TPA light emitting profile with far-field region of Ag NPs on ITO; (b) Quenching mechanism.


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Scheme 1. Synthetic route for MPID and MPID-TPA.



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Scheme 2. Schematic representation of turn-off fluorescence of MPID with Ag NPs and turn-on glucose sensing MPID with Ag+ ion


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Table 1. Decay parameters of MPID, MPID–Ag, MPID-TPA and Devices I- III and Device performances. τ1: τ2: τ3

A1: A2: A3

τave / χ 2

(ns)

(%)

(ns)

MPID

0.85:2.68

96:4

0.92/1.14

MPID–Ag

0.77:2.05:7.90

40:25:35

MPID-TPA

3.01

Device I Device II

a

L

(V)

(cd/m2)

0.30

-

-

-

-

-

3.59/1.01

0.33

-

-

-

-

-

100

3.01/1.02

0.39

-

-

-

-

-

0.83:2.62:10.68

10:30:60

7.38/1.00

0.41

6.0

9910

-

8.2

-

0.65:3.25:11.40

9:26:65

8.60/1.00

0.48

4.0

12198 (23%)

1.23

10.5 (28% )

1.28

Device III 0.52:3.85:12.01

8:30:62

8.64/1.00

0.59

3.5

15925 (61 %)

1.60

12.5 (52 %)

1.52

a

ZB

ηc

b

V1000

ɸ

ZLE

(cd A-1)

ZB = Bd/B, Bd – maximum brightness with Ag NPs; B – maximum brightness without Ag NPs: bZLE = LEd/LE, LEd – maximum luminous efficiency with Ag NPs; L – maximum luminous efficiency without Ag NPs: L – Brightness; ηc - Luminous efficiency; τ – Life time; A – Amplitude.



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Table of content


Far-Field Enhancement by Silver Nanoparticles in Organic Light

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