New donor-acceptor-donor molecules based on quinoline acceptor unit with Schiff base bridge: synthesis and characterization

Author’s Accepted Manuscript New donor-acceptor-donor molecules based on quinoline acceptor unit with Schiff base bridge: synthesis and characterization Sonia Kotowicz, Mariola Siwy, Michal Filapek, Jan G. Malecki, Karolina Smolarek, Justyna Grzelak, Sebastian Mackowski, Aneta Slodek, Ewa Schab-Balcerzak

PII: DOI: Reference:

www.elsevier.com/locate/jlumin

S0022-2313(16)31168-1 http://dx.doi.org/10.1016/j.jlumin.2016.11.058 LUMIN14396

To appear in: Journal of Luminescence Received date: 29 August 2016 Revised date: 9 November 2016 Accepted date: 21 November 2016 Cite this article as: Sonia Kotowicz, Mariola Siwy, Michal Filapek, Jan G. Malecki, Karolina Smolarek, Justyna Grzelak, Sebastian Mackowski, Aneta Slodek and Ewa Schab-Balcerzak, New donor-acceptor-donor molecules based on quinoline acceptor unit with Schiff base bridge: synthesis and characterization, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2016.11.058 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

New donor-acceptor-donor molecules based on quinoline acceptor unit with Schiff base bridge: synthesis and characterization Sonia Kotowicza, Mariola Siwyb, Michal Filapeka, Jan G. Maleckia, Karolina Smolarekc, Justyna Grzelakc, Sebastian Mackowskic, Aneta Slodeka*, Ewa Schab-Balcerzaka,b* a

Institute of Chemistry, University of Silesia, 9 Szkolna Str., 40-006 Katowice, Poland

b

Centre of Polymer and Carbon Materials, Polish Academy of Sciences, 34 M. Curie-

Sklodowska Str., 41-819 Zabrze, Poland c

Institute of Physics, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus

University, 5 Grudziadzka Str., 87-100 Torun, Poland [email protected] [email protected] [email protected] [email protected] *

Corresponding authors.

Abstract Three solution-processable small organic molecules bearing quinoline as electron-accepting moiety were synthesized via condensation reaction of novel 6-amino-2-(2,2’-bithiophen-5-yl)4-phenylquinoline

with

2,2’-bithiophene-5-carboxaldehyde,

9-ethyl-9H-carbazole-3-

carbaldehyde and 9-phenanthrenecarboxaldehyde. The presence of alternating electrondonating and accepting units results in a donor-acceptor-donor architecture of these molecular systems. Thermal, photophysical, and electrochemical properties of these small molecules were examined and the experimental results were supported by the density functional theory calculations. The obtained molecular systems exhibited high thermal stability with decomposition temperatures (5% weight loss) exceeding 330ºC in nitrogen atmosphere. It was 1

found, based on DSC measurements, that investigated Schiff bases form amorphous material with glass transition temperatures between 88 and 190ºC. They also showed a UV-Vis absorption in the range of 250 - 500 nm both in solution and in solid state as film and blend with PMMA and PVK. Photoluminescence measurements revealed moderately strong bluelight emission of the imines in solution as well as in PMMA blend with quantum yields in the range of 2 - 26%. In the case of imines dispersed in PVK matrix the emission of green light was mainly observed. In addition, when mixed with plasmonically active silver nanowires, the compounds exhibit relatively strong electroluminescence signal, associated with plasmonics enhancement, as evidenced by high-resolution photoluminescence imaging. The energy band gap estimated based on cyclic voltammetry was between 2.38 and 2.61 eV.

Keywords: quinolone derivatives; Schiff bases; bithiophene; donor-acceptor-donor; plasmonic enhancement.

1. Introduction Quinoline and its derivatives are very appealing family of compounds that can be applied in many versatile fields. In particular, high thermal and oxidative stability, great electron transport properties, high photoluminescence (PL) efficiency and easy film formation makes quinolines important for many applications in organic (opto)electronics as components of OLEDs, detectors, photovoltaic cells (PV), thin-film transistors, and electrochromic devices [1-4]. Moreover, they are widely used as electron-accepting components in donor-acceptor (D-A) architectures used in solar-cell technology [5-7], optical limiting materials [8] or even as

corrosion

inhibitors

[9].

Since

Tang

and

van

Slyke

have

applied

tris(8-

hydroxyqunolinato)aluminum acting as electron transport material and green emission layer in OLED device [10], the quinoline derivatives mainly forming the chelate complexes have 2

become the key interest of both synthetic and technological communities. Bipolar copolymers based on quinoline aluminum moieties and N-vinylcarbazole, where quinoline was additionally substituted with hole transporting carbazole groups, have been synthesized and characterized [11,12]. The copolymers have appeared to exhibit great thermal stability, solubility in organic solvents, excellent PL performance and improved hole-transporting ability. The continuing research on the fabrication of OLEDs has shown that devices based on a copolymer consisting of carbazole and quinoline aluminum exhibit optimal performance with low electrochemical band gap (Eg) of 2.20 eV and maximum luminance of 487 cd/m2 [12]. A simple modification of 2,4-diphenylquinoline by introducing both chloro and methoxy substituents has yielded intense blue-light photoluminescence in solution and blended thin films, and in addition, the compounds in the solid-state architecture have exhibited photoluminescence quantum yields significantly higher (by over 60%) than in solution [13]. Dichlorosubstituted quinoline motif incorporated into poly-(phenylenevinylene) (PPV) provided better electron transport, enhanced electron affinity, and – by simultaneously decreasing the band gap - resulted in excellent PL with bluish-green emission maximum at 477 nm in the case of oligomer [14]. Furthermore, the quinoline motif is often applied as light emitting material in polymeric light emitting diodes (PLEDs), because it possess high thermal and oxidative stability, and can thus lead to the improvement of device performance. Quinoline derivatives are often used as a component of polyfluorenes-based D-A system [3,5,6,9,15] or as a core to be further modified by adding different electron-donating fragments, such as carbazole [7,11] or triphenylamines [9]. They seem to be thermally stable, soluble in most organic solvents, and due to D-A type structure feature enhanced hole transport, high electron affinity, and good charge injection. A blue PLED consisting of polyfluorene, substituted with electron donating triphenylamine and electron-withdrawing phenylquinoline has exhibited reversible oxidation process, maximum brightness of 2367

3

cd/m2 and power efficiency of 0.42 lm/W [9]. The design of quinoline derivatives substituted by different electron-acceptor (A) and electron-donor (D) groups has strong influence on their optical, electrochemical, and electroluminescent properties, all of which eventually impact the efficiency of constructed OLED devices. A series of 5,7-disubstituted quinolines bearing various D and A groups were synthesized and used to form aluminum complexes. The complexes have exhibited emission in the broad spectral range from 503 to 567 nm, being strongly dependent on the character of substituents in quinoline ring, where strong electrondonating diphenylamine (NPh2) substituent in quinoline ligand dives the most red-shifted maximum of emission [16]. Furthermore, aza-boro-diquinomethene complexes with various D and A groups in quinoline ring have shown blue to green emission, with the complex containing NPh2 group having a red-shift of emission to 527 in solution and to 597 nm in solid state. Such complexes are robust and thermally stable with glass transition temperatures over 187ºC and possess high PL quantum yields (0.47-0.93) [17]. Imidazole-isoquinoline derivatives have emerged as very thermally and morphologically stable compounds with blue emission in solution and red-shifted emission in thin films, and high PL quantum yields (0.270.93) [18]. Recently, we reported the synthesis and extensive characteristics of D-A system based on quinoline-bithiophene derivatives [19]. On the other hand, the compounds bearing Schiff base linkers seem to be attractive for optoelectronics [20-22]. They are interesting alternatives to conventional semiconductors due to their isoelectronicity with vinylene analogues and certain advantages in the synthesis. They can be obtained using condensation between amines and aldehydes in mild reaction conditions without metal catalyst, with only water as a byproduct. Moreover, the synthesized compound can be relatively easily purified. To the best of our knowledge compounds bearing both imine linkages and quinolone have been described in article [20]. It was found that the variety of substitution patterns of azomethine ligands derived from 2,4-disubstituted-7-aminoquinoline allows to modulate

4

positions of maxima of absorption and emission, where maxima of PL bands of quinolylimines ligands are located between 476-524 nm, whereas they undergo a bathochromic shift to 503-532 nm in zinc complexes [20]. Inspired by the findings described above we have undertaken a preparation and investigation of a new phenylquinoline derivatives. In the first step 2-(2,2’-bithiophen-5-yl)6-nitro-4-phenylquinoline was transformed into amino derivative, which was then used for preparation of final compounds creating donor (D) – acceptor (A) – imine bridge – donor (D1) system. It was found that the D-A-D materials increase the efficiency of organic devices by enhancing exciton dissociation, light absorption, and charge transportation [23]. The compounds presented in this article differ in electron donor group (D1) N-ethylcarbazole, 2,2’-bithiophene and phenanthrene connected by Schiff base linker with phenylquinoline. In addition to a thorough characterization of these new compounds we also demonstrate the potential of applying plasmonically active nanostructures for enhancing the performance of the D-A-D molecular systems, both in electroluminescence and PL imaging experiments. 2. Experimental section 2.1.Materials All chemicals and starting materials were commercially available and were used without further purification. Solvents were distilled as per the standard methods and purged with nitrogen before use. All reactions were carried out under argon atmosphere unless otherwise indicated. Column chromatography was carried out on Merck silica gel. Thin layer chromatography (TLC) was performed on silica gel (Merck TLC Silica Gel 60). 2.2. Synthesis of 6-amino-2-(2,2’-bithiophen-5-yl)-4-phenylquinoline (ABPQ) 2-(2,2’-Bithiophen-5-yl)-6-nitro-4-phenylquinoline (0.15 mmol) was dissolved in 3 ml of 1,2-dimethoxyethane (DME) under stirring. The mixture was kept under reflux with activated charcoal (4 mg) within 1h and left overnight under stirring at room temperature. The 5

reaction mixture was flushed with argon and stirred for 30 min, then Pd/C (2 mg) was added and the mixture and heated to 40ºC. Hydrazine monohydrate (19 µL, 0.4 mmol) was added to the suspension. The obtained mixture was refluxed for 5h, cooled and filtered off. The filtrate was concentrated under reduced pressure to afford crude product. The product was purified by column chromatography on SiO2 (hexane/ethyl acetate = 5/1 to 2/1) to give final product with 90% yield. ABPQ: 1H NMR (400 MHz, CDCl3, δ, ppm): 7.96 (d, 1H, J = 8.8 Hz), 7.60 (s, 1H), 7.58 – 7.44 (m, 6H), 7.29 (dd, 1H, J = 3.6, 1.2 Hz), 7.25 – 7.22 (dd, 1H, J = 5.2, 1.2 Hz), 7.19 (d, 1H, J = 4.0 Hz), 7.15 (dd, 1H, J = 8.8, 2.4 Hz), 7.05 (dd, 1H, J = 5.2, 3.6 Hz,), 6.92 (d, 1H, J = 2.4 Hz), 3.88 (s, 2H).

13

C NMR (101 MHz, CDCl3) δ 148.33, 146.52, 144.61, 144.44,

143.61, 139.16, 138.70, 137.73, 130.88, 129.40, 128.58, 128.22, 127.96, 127.37, 125.24, 124.61, 124.42, 123.96, 121.51, 117.89, 106.06. FTIR (KBr, cm-1): 3451, 3349 (-NH2 stretching), 3063 (C-H aromatic), 2320 (S-H stretching), 1622 (-NH2 deformation). Elem. anal. calcd (%)(C23H16N2S2) (559.79): C, 71.86; H, 4.19; N, 7.29; Found: C, 71.05; H, 4.00; N, 6.96. EI-HRMS calcd for (C23H16N2S2) 384.0762 found 384.0755. 2.3. Synthesis of SB-1 0.55 mmol of ABPQ and 0.5 mmol of 2,2’-bithiophene-5-carboxaldehyde were dissolved in 6 ml of ethanol and heated to 70C under argon atmosphere. Afterwards the three drops of trifluoroacetic acid were added to the synthesis. The reaction mixture was maintained for 7h, after that time the product was filtered, washed several times with methanol and dried at 60C in a vacuum oven for 24h with 65 % yield. SB-1: 1H NMR (400 MHz, CDCl3, δ, ppm): 8.56 (s, 1H), 8.15 (d, 1H, J=8.9Hz), 7.70 (s, 1H), 7.66 (dd, 1H, J1=8.9Hz, J2=2.3Hz), 7.62 (t, 2H), 7.58 (s, 2H), 7.57 (d, 2H, J=2.7Hz), 7.38 (d, 1H, J=3.8Hz), 7.32 (d, 1H, J=3.6Hz), 7.31-7.28 (m, 3H), 7.23 (d, 1H, J=3.8Hz), 7.19 (d,1H, J=3.8Hz), 7.08-7.04 (m, 3H). 13C NMR (101 MHz, CDCl3, δ, ppm): 152.93, 151.04, 149.00, 148.88, 147.67, 144.01, 142.59, 141.21, 140.42, 138.25, 137.65, 137.02, 133.58, 130.74, 129.60, 128.90, 128.67, 128.30, 128.15, 126.63, 126.52, 126.00, 125.22, 125.05, 124.64, 124.37, 124.23, 124.15, 118.10, 117.23. FTIR (KBr, cm-1): 3063 (C-H aromatic), 1653, 1623 (C=N), 1366 (C-N stretching). Elem. anal. calcd (%) for (C32H20N2S4) (560.77): C, 68.54; H, 6

3.59; N, 5.00; Found: C, 68.80; H, 3.90; N, 4.26. MALDI-TOF-MS m/z calcd for (C32H20N2S4) 560.05 found 560.0. 2.4. Synthesis of SB-2 and SB-3 0.18

mmol

of

9-etyl-9H-carbazole-3-carbaldehyde

or

0.2

mmol

of

9-

phenanthrenecarboxaldehyde and ABPQ (0.2 mmol, 0.22 mmol, respectively) were dissolved in 4 ml of N,N-dimethylacetamide (DMA). The reaction mixture was stirred under argon atmosphere and heated to 140C with four drops of trifluoroacetic acid and maintained for 24h. After overnight stirring solvents were evaporated, the products were dissolved in small amount of chloroform and precipitated in methanol. The precipitates were filtered, washed several times with methanol and dried at 60C in a vacuum oven for 24h with 51 and 57 % yield, respectively. SB-2: 1H (400 MHz, CDCl3, δ, ppm): 8.95 (s, 1H), 8.63 (d, 1H, J=1.3Hz), 8.17 (d, 1H, J=7.8Hz), 8.02 (dd, 1H, J1=8.5Hz, J2=1.5Hz), 7.62-7.59 (m, 4H), 7.54-7.51 (m, 3H), 7.497.44 (m, 3H), 7.33-7.29 (m, 5H), 7.09-7.03 (m, 2H), 6.92 (d, 1H, J=2.4Hz), 4.48-4.40 (m, 2H), 1.25 (t, 3H). 13C NMR (101 MHz, CDCl3, δ, ppm): 13.82, 37.82, 106.01, 108.7, 109.15, 117.89, 120.33, 121.4, 120.81, 123.06, 123.19, 123.95, 124.4, 124.61, 125.44, 126.74, 127.23, 127.35, 128.01, 128.2, 128.57, 128.75, 129.4, 130.89, 137.61, 138.78, 139.16, 140.71, 143.59, 144.49, 144.61, 146.51, 148.33, 162.21. FTIR (KBr, cm-1): 3064 (C-H aromatic), 2973, 2926 (C-H aliphatic), 1689 (C=N), 1348 (C-N stretching). Elem. anal. calcd (%) for (C38H27N3S2) (589.77): C, 77.39; H, 4.61; N, 7.12; Found: C, 76.91; H, 4.36; N, 7.04. MALDI-TOF-MS m/z calcd for (C38H27N3S2) 589.77 found 589.6. SB-3: 1H NMR (400 MHz, CDCl3, δ, ppm): (400 MHz, CDCl3, δ, ppm): 9.21 (dd, 1H, J1=7.9Hz, J2=1.5Hz), 9.13 (s, 1H), 8.81-8.74 (m, 1H), 8.70 (d, 1H, J=8.3Hz), 8.33 (s, 1H), 8.22 (d, 1H, J=8.8Hz), 7.98 (d, 1H, J=7.5Hz), 7.77 (dd, 1H, J1=8.8Hz, J2=2.3Hz), 7.74-7.71 (m, 3H), 7.70 (d, 1H, J=7.4Hz), 7.65-7.60 (m, 4H), 7.58 (dd, 2H, J1=13.6Hz, J2=6.4Hz), 7.33 (dd, 1H, J1=3.5Hz, J2=1.1Hz), 7.27 (dd, 1H, J1=5.1Hz, J2=1.0Hz), 7.23 (d, 1H, J=3.8Hz), 7.07 (dd, 3H, J1=5.1Hz, J2=3.6 Hz).

13

C NMR (101 MHz, CDCl3, δ, ppm): 160.05, 117.86,

121.51, 122.76, 122.94, 123.95, 124.46, 124.61, 125.23, 126.0, 127.43, 127.3, 127.95, 127.65, 128.22, 128.25, 128.32, 128.8, 129.4, 130.17, 130.87, 130.24, 130.4, 130.54, 130.71, 133.05, 137.54, 138.69, 139.15, 141.24, 143.6, 144.43, 144.62, 146.5, 148.31, 161.11. FTIR (KBr, 7

cm-1): 3068 (C-H aromatic), 1691 (C=N), 1364 (C-N stretching). Elem. anal. calcd (%) for (C38H24N2S2) (572.74): C, 79.69; H, 4.42; N, 4.89; Found: C, 79.99; H, 4.47; N, 4.43. MALDI-TOF-MS m/z calcd for (C38H24N2S2) 572.14 found 572.1. 2.5. Synthesis of silver nanowires Silver nanowires (AgNWs) were synthesized using the polyol process, in which ethylene glycol served as the reducing and solvent reagent. The product was purified by centrifugation process and the mixture was diluted with isopropyl alcohol and centrifuged. The supernatant containing silver particles and unreacted substrates was removed. Finally the product was redispersed in 2 ml of pure water. Scanning electron microscopy yields diameters of the nanowires in the range from 50 to 150 nm, while their lengths range from 4 to even 30 microns. 2.6. Blend and film preparation Blends were fabricated by dissolving compounds and poly(methyl methacrylate) (PMMA) or poly(N-vinylcarbazole) (PVK) in chloroform to obtain homogeneous mixtures, the same method was used to prepare films. The resulting mixtures (1% (v/v) concentration of the material in PMMA, 15% (v/v) concentration in PVK) and films (0.02 g of compound in 1 ml of chloroform) were casted on glass (PMMA blends) or spin-coated (PVK blends and films) and dried 24 h in a vacuum oven at 60°C. 2.7. Characterization Methods Nuclear magnetic resonance (1H NMR) spectra were recorded on a Bruker AC400 spectrometer in chloroform (CDCl3) as solvent and TMS as the internal standard. Differential Scanning Calorimetry (DSC) were measured using a Du Pont 1090B apparatus with a heating rate of 20ºC/min under nitrogen. Thermogravimetric analysis (TGA) was done with a Q-1500 apparatus with heating rate 20C/min under nitrogen atmosphere. Fourier Transform Infrared (FTIR) spectra were measured on Thermo Scientific Nicolet iS5 in the range of 4000 - 400

8

cm-1 as KBr pressed pellets. Elemental analysis were measured by Vario EL III apparatus (Elementar, Germany). UV-Vis absorption spectra were registered using a Perkin Elmer Lambda Bio 40 UV/VIS spectrometer. Photoluminescence spectra (PL) in solutions were measured using Varian Carry Eclipse Spectrometer, blends and films using Hitachi F-2500 Spectrometer. Quantum yields (Φf) measurements were estimated using the integrating sphere Avantes AvaSphere-80 with FLS-980 Spectrophotometer (Edinburgh Instruments). Quantum yields were determined by absolute method using the excitation wavelength with the most intense luminescence. Electrochemical measurements were carried out using Eco Chemie Autolab PGSTAT128n potentiostat, glassy carbon electrode (diam. 1.5 mm), platinum coil and silver wire as working, auxiliary and reference electrode, respectively. Potentials are referenced with respect to ferrocene (Fc), which was used as the internal standard. Cyclic voltammetry experiments were conducted in a standard one-compartment cell, in CH2Cl2 (Carlo Erba, HPLC grade), under argon. 0.2 M Bu4NPF6 (Aldrich, 99%) was used as the supporting electrolyte. The concentration of compounds was equal to 1.0·10-6 mol/dm3. Deaeration of the solution was achieved by argon bubbling through the solution for about 10 min before measurement. All electrochemical experiments were carried out under ambient conditions. 2.8. Electroluminescence and PL imaging Fluorescence intensity maps were measured on a Nikon Eclipse Ti inverted wide-field microscope equipped with an oil immersion objective (Plan Apo, 100x, Nikon) and coupled with Andor iXon Du-888 EMCCD camera. For excitation we used 405 nm and 485 nm LED illuminators, at excitation power of 60 µWy. The sample was illuminated from the AgNW side, next exciting the molecules. In order to directly compare fluorescence intensities, images were acquired for the same area of the sample for both excitation wavelengths. For spectral selection we used a dichroic beam splitter (Chroma 505DCXR) coupled with a narrow band9

pass filter (Chroma HQ610-40). Typical gains and acquisition times were 50 and 1 sec, respectively. In order to collect electroluminescence (EL) spectra the voltage was applied using a precise voltage supply (Gw Instek PSP-405) and the sample was fixed to an XYZ stage. Light from the OLED device was collected through a 30 mm lens, focused on the entrance slit (50 μm) of a monochromator (Shamrock SR-303i) and detected using a CCD detector (Andor iDus 12305). Typical acquisition times were equal to 10 seconds. The prealignment of the setup was done using a 405 nm laser. 3. Result and discussion 3.1. Schiff bases characterization New Schiff bases compounds were synthesized from new amine, that is, 6-amino-2(2,2’-bithiophen-5-yl)-4-phenylquinoline by condensation process with 2,2’-bithiophene-5carboxaldehyde (gives Schiff base denoted as SB-1), 9-ethyl-9H-carbazole-3-carbaldehyde (results in imine SB-2) and 9-phenanthrenecarboxaldehyde (gives imine SB-3). The chemical structures of obtained compounds are presented in Fig. 1.

NO 2 S

NH2 S

N

S

N

S

ABPQ

H H

O

H

O

S S

O N CH3 N

N S

N

H

S

N

S S

S S

SB-3

SB-1 N S

H

N

S

N H3C

SB-2

10

H

Fig. 1. Synthetic route and chemical structure of synthesized Schiff bases.

Novel 6-amino-2-(2,2’-bithiophen-5-yl)-4-phenylquinoline (ABPQ) was prepared from 2-(2,2’-bithiophen-5-yl)-6-nitro-4-phenylquinoline [19], which was stirred with active coal in dimethoxyethane (DME) for 24 hours, and subsequently catalytically reduced using palladium and hydrazine. It is interesting to note that the addition of active coal was necessary to conduct the reduction similar to the quinoline containing in position 2 fluorene instead bithiophene unit. Carrying out the reaction with different reductants, such as tin (II) chloride (SnCl2), hydrazine with ethanol, iron(III) acetylacetonate (Fe(acac)3) or even palladium on carbon (Pd/C) with hydrazine, gave only the unreacted 2-(2,2’-bithiophen-5-yl)-6-nitro-4phenylquinoline. The amine was fully characterized and obtained as yellow solid with a remarkable yield of 90%. The chemical structures of synthesized final compounds (SB-1–SB3) were confirmed by 1H NMR and FTIR measurements. In 1H NMR spectra of Schiff bases the proton from HC=N- bond was observed as a singlet at 8.56, 8.95 and 9.21 ppm for SB-1, SB-2 and SB-3, respectively. In the case of SB-2 the signals from aliphatic protons at 1.25 ppm (CH3) and at 4.42 ppm (CH2) were seen. The lack of signals typical for protons from primary amine and aldehyde groups at 3.88 and at about 10 ppm, respectively, confirms the condensation reaction. The chemical structure of prepared compounds was also characterized based on FTIR spectra, in which absorption band of imine group was detected in the spectral range of 1623 - 1691 cm1 (cf. section 2). In FTIR spectrum of SB-2 the bands typical for vibrionic stretching of aliphatic group at 2926 and 2973 cm-1 were observed. Moreover, in FTIR spectra of imines absorption bands characteristic for the C-N and C-H aromatic stretching in the range of 1348 - 1366 cm-1 and at about 3065 cm-1, respectively, were observed. Additionally, the results of the elemental analysis, which show good agreement between experimental and calculated contents of carbon, nitrogen and hydrogen, confirm the chemical structure of the synthesized Schiff bases. 11

3.2. Thermal properties Thermal properties of synthesized Schiff bases were investigated using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) under nitrogen atmosphere. The results of both DSC and TGA experiments are collected in Table 1.

Table 1. Thermal properties of the investigated compounds. Code

a,b

TGA

DSC (II heating run)

T5

T10

Tmax Char yield

Tg

Tc

Tm

[°C]a

[°C]b

d [°C]c [%]

[°C]

[°C]

[°C]

SB-1 371

399

447

49

88

146

234

SB-2 339

379

417

46

190

nd

nd

SB-3 336

403

449

45

172

nd

nd

T5, T10 - temperature based on 5%, 10% weight loss. cTemperature of the maximum decomposition rate.

d

Residual weight at 800 °C in nitrogen.nd- not detected.

Based on DSC heating scan traces it was found that the compounds could form amorphous phase, also known as molecular glass, which is an interesting combination of advantages associated with small molecules with the potential to form glassy phases [24]. The ability of small molecules to create the amorphous phase is significant manufacturing advantage. DSC curve is measured both during first and second heating run of SB-2 and SB-3 show only a glass transition temperature (Tg) at 190 and 172°C, respectively. In contrast, the compound bearing two bithiophene moieties (SB-1) exhibited only a melting endothermal peak at 234°C during first heating scan. Its DSC thermogram from the second heating run after rapid cooling exhibited Tg at 88°C, followed by crystallization exothermal peak at 146°C and melting occurring at 234°C (cf. Fig. 2b). (a)

(b)

12

I run

Fig. 2. DSC thermograms of SB-1 (a) and SB-2 (b) at 20ºC/min under nitrogen atmosphere.

The absence of melting endothermal and crystallization exothermal peaks observed for SB-2 and SB-3 implies that tendencies of crystallization were successively suppressed. The replacement of one bithiophene unit by carbazole and phenantrene moieties significantly increased the Tg value. The results of thermal stability examination exhibited similar TGA pattern characteristics for a one-step decomposition process with the temperature of the maximum decomposition rate (Tmax), as evidenced by the differential thermogravimetric (DTG) curves above 400°C. The 5% weight loss temperature (T5) considered as the beginning of decomposition was above 300°C for all compounds, with the highest measured for the compound with two bithiophene moieties (SB-1). 3.3. Molecular geometry calculations Quantum calculations were carried out using the Gaussian09 [25] program. Molecular geometry of the singlet ground state of the compounds was optimized in the gas phase on the B3LYP/6-31g++ level of theory [26,27]. For the compounds a frequency calculation was carried out, verifying that the optimized molecular structure corresponds to energy minimum, thus only positive frequencies were expected. The electronic structures and electronic transitions were calculated with use of the Polarizable Continuum Model (PCM) [28] in

13

dichloromethane as solvent. Fig. 3 shows the density of states diagrams for the SB-1-SB-3 compounds calculated with use of Gausssum [29] program. The Schiff bases molecules were divided into three parts 4-phenylquinoline, bithiophene and –N=CH-D1, where D1 denotes bithiophene in SB-1, substituted carbazole in SB-2 and phenanthrene in the case of SB-3, respectively and diagrams in Fig. 3 present the contribution of a group to a molecular orbitals.

SB-1 -N=CH-D1

bithiophene

quinoline-Ph

52

21

27

L+2

1

43

55

L+1

52

12

36

LUMO

38

15

47

HOMO

31

51

18

H-1

63

32

5

H-2

28

16

56

H-3

1

98

1

SB2 L+3

-N=CH-D1

bithiophene

quinoline-Ph

100

0

0

L+2

7

51

42

L+1

59

2

39

LUMO

16

22

62

HOMO

10

71

19

H-1

84

12

4

H-2

54

12

35

H-3

74

5

21

SB-1 L+3

SB-2

SB-3

14

-N=CH-D1

bithiophene

quinoline-Ph

99

1

0

L+2

2

40

58

L+1

44

15

41

LUMO

50

12

39

HOMO

4

78

17

H-1

64

13

23

H-2

48

11

41

H-3

71

29

0

SB3 L+3

Fig. 3. Density of states diagrams (left) and composition of selected molecular orbitals (right) of SB-1-SB-3. As can be seen from the diagrams in Fig. 3, in HOMOs of the SB-2 and SB-3 Schiff bases the bithienyl substituent plays significant role. In the case of SB-1 both bithiophene parts of the molecule participate in HOMO, although bithienyl substituent directly attached to the quinoline ring has a larger share (51%) than -N=CH-bithiophene part (31%). Generally the share of azomethine -N=CH-D1 fragment in HOMO is reduced in number from SB-1 to SB-3. At the same time, the HOMO-1 energy, which is mainly composed of the orbitals of -N=CHD1 fragment, is also reduced. The composition of the selected molecular orbitals is summarized in Fig. 3. LUMO of SB-1 and SB-2 compounds is composed of the antibonding orbitals of 4-phenylquinoline moiety with a contribution of orbitals of -N=CH-D1 fragment. In the case of SB-3 this part of the molecule has up to 50% share in LUMO, whereas in LUMO+1 the azomethine -N=CH-D1 fragments play a dominant role. The consequence of changes in the electronic structure is to increase the quantum efficiency of the emissions which reaches a maximum for SB-3 (vide infra). 3.4. Spectroscopic properties The electronic absorption and emission properties of new Schiff bases (SB-1–SB-3) were studied in solution (chloroform and NMP) and in solid state as blends with poly(methyl 15

methacrylate) (PMMA), poly(N-vinylcarbazole) (PVK) and thin film on glass substrate. UVVis measurements were carried out in the range of solvents transparency and in solid state from 300 to 700 nm. Fig. 4 depicts the representative UV-Vis spectra of this group of compounds, whereas their spectroscopic parameters extracted from the data are listed in Table 2.

0.5

ex= 340 nm PVK

0.4

CHCl 0.3

100

3 -5

c = 10 mol/L 0.2

1.4

b

Film Blend PVK Solution

1.2

PL intensity [a.u.]

Absorbance [a.u.]

0.6

200

SB-1 SB-2 SB-3 ABPQ 150

a

50

Absorbance [a.u.]

0.7

SB-1

1.0 0.8 0.6 0.4

0.1 0.2

0.0 250

300

350

400

450

500

550

600

650

0 700

0.0 250

Wavelength [nm]

300

350

400

450

500

550

600

650

700

Wavelength [nm]

Fig. 4. (a) UV-Vis spectra of all imines (SB-1–SB-3) and 6-amino-2-(2,2’-bithiophen-5-yl)-4phenylquinoline (ABPQ) and (b) UV-Vis spectra of SB-1 in chloroform solution (c = 10-5 mol/L) and the solid state as film and blend with PVK.

Table 2. UV-Vis spectroscopic data of the Schiff bases. Code

ABPQ

SB-1

CHCl3a (ε =4.81) λmax ε [nm] [dm3·mol-1·cm-1] 265 26220 343 19000 398 19200

NMPa (ε =33.00) λmax ε [nm] [dm3·mol-1·cm-1] 265 3800 351 20400 420 19010

Blend PVKb

Filmc

λmax [nm]   

λmax [nm]   

261

326

331

330

29987

38685 16

326 418

26925 62914

420

65146

344 420

422

SB-2

263 295 350 392

16964 13971 15966 17962

264 351 397

24561 19556 21112

331 344 390

352 396

SB-3

253 326 397

47486 23261 31013

340 396

37423 53022

331 344 397

407

a c

c = 10-5 mol/L. b 15% (v/v) concentration of compound in PVK.

0.02g of compound in 1 ml of chloroform. ε - dielectric constant.

The absorption spectra of the imines in solutions are closely resemble that of the starting amine, that is, 6-amino-2-(2,2’-bithiophen-5-yl)-4-phenylquinoline, and three maxima (max) can be distinguished at about 260, 330 and above 390 nm (cf. Table 2). The absorption band with max in the range of 253 – 264 nm is responsible for n– transition in aromatic rings. Considering the influence of the chemical structure of prepared imines on the absorption properties no significant differences between compounds SB-2 and SB-3 were found, which suggests similar electron donor strengths of phenanthrene and carbazole substituents. The max at the highest wavelength of compound bearing two bithiophene moieties (SB-1) is shifted to the red and it shows that the bithiophene unit is a stronger electron donor as compared to phenanthrene and carbazole moieties. Moreover, the λmax position shows no dependence on the polarity of the solvent, thus no solvatochromism was observed. Generally, solvatochromism depends on the extent of charge separation in the ground state of a chromophore. The energies of HOMO, LUMO and HOMO-LUMO gap were calculated in both solvents i.e. chloroform and NMP (cf. Table 3). Table 3. Energies of HOMO, LUMO and Egap calculated for SB-1 –SB-3 compounds in CHCl3 and NMP solvents.

Solvent

EHOMO [eV]

ELUMO [eV] 17

Egap [eV]

SB-1 CHCl3 NMP

-5.55 -5.36

CHCl3 NMP

-5.54 -5.35

-2.63 -2.42

2.92 2.94

-2.46 -2.25

3.09 3.10

-2.59 -2.36

3.02 3.06

SB-2

SB-3 CHCl3 NMP

-5.61 -5.42

The results show that in strongly polar NMP solvent the HOMO–LUMO gaps are almost the same as calculated for CHCl3 as can be seen from the Table 3. Although the energies of HOMO and LUMO in NMP are higher compared with chloroform solvent, but the energies of both orbitals (HOMO and LUMO) are shifted by a similar amount, which does not affect the value of the energy gap. For the imines in solid state as film on glass substrate, the electronic spectra are dominated by the one band above 390 nm with λmax at the same position as for the molecules in dissolved in chloroform. UV-Vis spectra of the Schiff bases as blends with PVK consist of absorption band with max at 331 and 344 nm characteristic for matrix and second band that can be attributed to investigated imines (cf. Fig. 4b). Fluorescence measurements revealed that all the compounds are photoluminescent and emitted radiation in the blue region. The representative PL spectra and the photographs showing the emission of imines are displayed in Fig. 5, whereas maxima of the registered emission band (em) and Stokes shifts are collected in Table 4 and 5. Table 4. Photoluminescence data of the investigated Schiff bases. Code

Medium

SB-1

CHCl3a Stokes shiftb NMP a

PL λex[nm] λem [nm] 340 350 473 473 2668 2668 491 492

360 473 493 18

390 473 494

400 473 495

420 474 495

430 480 496

440 493 498

460 519 531

SB-2

Blend PMMAc Blend PVKd

440 433 540;

444 417; 438; 541

442 416; 437; 542

445 -

449 -

-

-

-

-

CHCl3a

470 484 450 495 -

471 482 451 495 446

471 485 447 496 444

470 481 451 466; 498 446

470 4234 483 453 466; 498 -

472 485 466; 498 -

473 489 467; 498 -

472; 499 492 500 -

474; 506 500 -

466 474 447 464; 492 463; 523

466 476 446 464; 491 462; 524

467 476 446 464; 497 464; -

467 3776 474 447 462 -

467 3776 474 452 464 -

468 477 -

471 480 -

472; 499 486 -

474 498 -

Stokes shiftb NMPa Blend PMMAc Blend PVKd Filme CHCl3a

SB-3

Stokes shiftb NMPa Blend PMMAc Blend PVKd Filme a

c = 10-5 mol/L. b

Stokes shifts calculated according to the equation Δν=(1/λabs-1/λem)·107 [cm-1] for chloroform solution. c

1 % (v/v) concentration of compound in PMMA. d 15% (v/v) concentration of compound in PVK. e

0.02g of compound in 1 ml of chloroform. Bold data indicates the most intense luminescence.

The PL intensity of SB-1–SB-3 measured for the molecules dissolved in chloroform was higher as compared to NMP solvent. The polarity of the solvent influenced the em position, which was found at lower energy in NMP solution. Bathochromic shift in the range of 7 - 18 nm confirms the weak solvatochromic effect. Regardless of the differences in the chemical structure, the studied Schiff bases exhibited the most intense emission at the same em (cf. Fig. 5a). The emission spectra were recorded under different excitation wavelengths (λex). The increase of λex wavelength practically did not affect the em position (except for SB1), which was observed in the range between 466 and 496 nm. SB-1 in solution under λex 460 nm emitted light with em at above 510 nm. However, solutions of SB-1–SB-3 under various λex differ in PL intensity (cf. Fig. 5d). The utilization of ex at 340 and 350 nm for SB-1, 400 nm for SB-2 and 390 and 400 nm for SB-3 results in the highest emission intensity. 19

Table 5. The calculated Stokes shift of investigated compounds (SB-1–SB-3). Code Medium PL λex[nm] Stokes shift [cm-1]a SB-1

CHCl3

340 350 360 390 400 420 430 440 460 2668 2668 2668 2668 2668 2712 2976 3526 4542

b

NMP b SB-2

CHCl3

3443 3484 3526 3567 3608 3608 3648 3729 4977 b

4234 4279 4279 4234 4234 4324 4369 4324 4413

b

NMP

4849 4763 4892 4720 4806 4892 5060 5185 5510

c

Film SB-3

-

CHCl3 NMP

b

3200 3119 3220 -

-

-

-

-

3730 3730 3776 3776 3776 3821 3957 4002 4092

b

4092 4181 4181 4092 4092 4225 4356 4613 5109

c

Film

3591 3544 3637 -

-

-

-

a

7

Stokes shifts calculated according to the equation Δν=(1/λabs-1/λem)·10 [cm ].

b

c = 10-5 mol/L. c 0.02g of compound in 1 ml of chloroform

-

-

-1

240 240

SB-1 ex= 340 nm SB-2 ex= 400 nm

200

200

u.] PL intensity [a.

SB-3 ex= 390 nm 160

CHCl3 -5

c =10 mol/L

120

80

160 120 80

0 350 Ex 400 cita tion wa vel e

40

0 350

SB-3 NMP -5 c = 10 mol/L

b

40

400

450

500

550

600

650

700

750

Wavelength [nm]

1.2

c

800 700

500 450 ngt h [n m]

SB-1 blend PVK ex= 350 nm

SB-2 blend PVK ex= 350 nm

SB-3 blend PVK ex= 350 nm PVK ex= 340 nm

0.8 0.6 0.4 0.2 0.0 350

400

450

500

550

Wavelength [nm]

20

600

650

] m [n th g l en

600

SB-1 blend PVK ex= 340 nm

1.0

PL intensity [a.u.]

PL Intensity [a.u.]

a

700

e av

400 500 300

W

Fig. 5. (a) PL spectra of the Schiff bases in chloroform measured for ex that gives the most intense emission, (b) 3D PL graph of SB-3 under various ex, (c) normalized emission of the imines as blend with PVK together with the PL of PVK, and (d) photographs of investigated compounds in chloroform solution and PMMA blends under ex =366 nm.

The quantum yields (f) of the compounds dissolved in chloroform were estimated for the excitation wavelengths λex that result in the strongest PL emission :at 340 and 350 nm for SB1, at 400 nm for SB-2, at 390 and 400 nm for SB-3. The largest f was found for the imine bearing the phenanthrene unit (SB-3) (cf. Fig. 6).

Fig. 6. Values of the quantum yield (f) measured for the investigated Schiff bases in CHCl3 solution and PMMA blends.

The maximum quantum yield was associated with the changes in the electronic structures of the compounds. From SB-1 to SB-3 the share of azomethine -N=CH-D1 fragment in HOMO is reduced while reducing the energy of HOMO-1. At the same time the share of azomethine fragment reaches the 50% share in the LUMO of SB-3 compound (cf. section 3.3). Thus, the quantum efficiency of the emission reaches a maximum for SB-3.

21

The calculated Stokes shifts, whose values are summarized in Table 5, give the information about changes in geometry of the excited and the ground states. They are found to be similar for imines being in the range of 4234 - 2668 cm-1 (72 - 86 nm) in chloroform solution, suggesting the presence of the intramolecular charge transfer (ICT). Additionally, the electronic spectra of these compounds in chloroform were calculated using TD-DFT method [30] . The obtained electronic transitions for the lowest energy bands occur between HOMO→L+1 (76%) in SB–1 and HOMO→LUMO (97%/98%) in the case of SB–2 and SB–3 compounds. Contours of the molecular orbitals involved in the electronic transitions are depicted in Fig. 7.

Code SB-1

LUMO

HOMO

SB-2

SB-3

22

Fig. 7. Contours of the selected molecular orbitals of SB-1-SB-3 compounds. However in SB-1 the excitation process can be characterized as bth(51%) + R-CH=N-(31%)→ R-CH=N-(52%) + quinPh(36%) and thus the excitation comprises bithienyl parts of the molecule. In the case of SB-2 and SB-3 the calculated transitions have bth(71%)→ quinPh(62%) and bth(78%)→ R-CH=N-(50%) + quinPh(39%) character, respectively. Thus, it confirms that for these two compounds the excitations are related to the intramolecular charge transfer processes to a larger extent than in the case of SB-1 compound. Considering the luminescence of imines in solid state as thin film it was found that in the case of the imine with two bithiophene units (SB-1) its emission was quenched. In contrast, for the thin films made of the two other compounds (SB-2 and SB-3) a weak and broad emission band with λem at 450 nm was observed with a shoulder at 520 nm. The investigated Schiff bases dispersed in inert PMMA matrix emitted light with λem hipsochromically shifted in comparison to solution with slightly lower quantum yields, except for SB-1 (cf. Fig. 6). In the case of imine end-capped with bithiophene moieties (SB-1) dispersed in PVK two emission bands are observed under λex at 350 and 360 nm: the first one with λem below 450 nm and the second one with weaker intensity at about 540 nm. Excitation at λex = 340 nm suppressed the band at higher energy and increased the green emission. On the other hand, excitation at longer wavelengths leads to gradual disappearance of PL signal. In the PL spectra of SB-2 and SB-3 one broad band ranging from 400 to 700 nm with shoulder was observed. For imines with carbazole and phenanthrene substituents (SB-2 and SB-3) a band originating from the emission of PVK is not observed, which may indicate that the energy transfer from matrix to luminophores is almost complete (cf. Fig. 5c). In contrast, considering the PL emission of the SB-1, the emergence of the band attributable to the matrix points towards the Förster energy transfer between the molecules and the PVK. Taking into account the PL spectrum of the PVK matrix and the UV-Vis spectra of investigated imines 23

presented in Fig. 4a, it is clear that the emission of the PVK host overlaps with the absorption spectrum of the guest-Schiff bases, which is a prerequisite for the Förster energy transfer to occur. 3.5. Electrochemical properties The electrochemical properties of the synthesized imines (SB-1–SB-3) were investigated in CH2Cl2 solution by means of cyclic voltammetry (CV). The cyclic voltammograms are presented in Fig. 8. The electrochemical oxidation and reduction onset potentials were used for estimating of the ionization potentials (IP) and electron affinities (EA) of the materials (assuming that IP of ferrocene equals -5.1 eV) [31]. The calculated IP and EA values together with electrochemical energy band gaps (Eg) are presented in Table 6.

12 10 8 6 4 2

I ( A)

0 -2 -4 -6

SB-1 SB-2 SB-3 ABPQ

-8 -10 -12 -14 -16 -18 -20 -3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

+

E (vs. Fc/Fc ) (V)

Fig. 8. Cyclic voltammograms of amine and synthesized Schiff bases; GC as working electrode; sweep rate ν = 100 mV/s, 0.1 M Bu4NPF6 in CH2Cl2. Table 6. Electrochemical data for investigated imines. Code ABPQ SB-1 SB-2

Eox [V] 1.02 0.59 0.3

Ered [V] -1.74 -2.02 -2.08

IP [eV]* -6.12 -5.69 -5.4

EA [eV] -3.36 -3.08 -3.02 24

Eg [eV] 2.76 2.61 2.38

HOMO* [eV] -5.55 -5.54

LUMO* [eV] -2.63 -2.46

0.63 SB-3 *estimated by DFT

-1.95

-5.73

-3.15

2.58

-5.61

-2.59

All compounds exhibit similar behavior during the reduction processes. The first reduction step (Ered) (in the range from -1.75 to -2.05 V) is irreversible from thermodynamical point of view. It is undoubtedly caused by the reduction of quinoline moieties, as expected from the results of DFT calculations. However, Ered values differ slightly depending on the donor properties. This could be seen in particular for ABPQ - i.e. the compound possessing -NH2 group, which is a strong electron-donating functional group (EDG) that donates some of its electron density into a conjugated π system of quinoline via resonance inductive effects. On the other hand, differences during the oxidation process are more significant. First oxidation step in each case is irreversible, as expected. In all the measured compounds (at least one) terminal bithienyl fragment is incorporated. The bithiophene as strongly π-excessing compound possesses tendency to undergo dimerization after being oxidized as we have reported previously [19,32]. For SB-1 (possessing two bithienyl moieties in conjugation) Eox = 0.59 (cf. Fig. 9a) and stable p-dopable polymer is created. Very similar Eox was determined in the case of ABPQ (0.32 V). For SB-2 the oxidation is the easiest (occurs at 0.3 V) – it is the consequence of strong donating character of the carbazole moiety. However, when the potential is above 0.8 V electropolymerization was also observed (cf. Fig. 9b). Slightly different behavior was observed in the case of SB-3, where polymerization process occurred through phenanthrene moiety (cf. Fig. 9c). Nevertheless, the obtained species is bad pconductor i.e. maximum peak associated with polymer oxidation is in each following scan shifted to higher potential (this feature implies high resistance of arising layer). It can be seen (Table 6) that energy gaps obtained from cyclic voltammetry and DFT calculations are consistent in tendency. However, in each case the band gap determined experimentally is approximately 500 mV lower than predicted theoretically mainly due to overestimating LUMO energy by DFT methods [33]. 25

10

160

9

a

80 60 40

1 3 5 7 9 11 13 15

7

1 3 5 7 9 11 13

100

I ( A)

b

8

120

6 5 4

I ( A)

140

3 2

20

1 0

0

-1

-20

-2 -40

-3

-60

-4 -0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

-0.6

1.0

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

+

+

E (vs. Fc/Fc ) (V)

E (vs. Fc/Fc ) (V) 22 20

c

18 16

1 3 5 7 9 11 13

14 12

I ( A)

10 8 6 4 2 0 -2 -4 -6 -8 -1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

+

E (vs. Fc/Fc ) (V)

Fig. 9. Cyclic voltammograms of (a) SB-1, (b) SB-2 and (c) SB-3 (inset: cycle number); GC as working electrode; sweep rate ν = 100 mV/s, 0.1 M Bu4NPF6 in CH2Cl2.

3.6. Effect of plasmon excitations In order to test the applicability of the obtained compounds as a building block of optoelectronic devices, an OLED structure was fabricated, which consisted of ITO/PEDOT:PSS+AgNWs(1:1)/SB-1(SB-2/SB-3). While these structures exhibited no electroluminescence under applied external voltage, a layer of silver nanowires was incorporated

during

the

preparation

of

the

devices.

In

such

a

configuration,

electroluminescence signal was found, as shown in Fig. 10 for the structure based on the SB-3 molecule.

26

Fig. 10. Electroluminescence spectra obtained for an OLED structure containing SB-3 molecules. The emission was observed for external voltages above 10 V, and the maximum of this EL signal appeared around 670 nm, which is considerably red shifted as compared to the PL spectra, both in solution and in the solid state. The results obtained for the other two compounds were qualitatively similar. It is also important to mention that the EL intensity decreased considerably for voltages higher than 16 V, perhaps due to degradation of the structure. This result shows that silver nanowires, which are plasmonically active can be used for enhancing the EL intensity in OLED devices [34].Confirmation that the emergence of the EL signal is indeed associated with the interaction between emitting molecules and silver nanowires can be proved by direct experiment based on fluorescence imaging. In this experiment a structure, consisting of a layer of molecules deposited onto a surface previously covered randomly with silver nanowires, is excited over an area of 100 x 100 microns. In this way a fluorescence image is obtained. An example of such an image is displayed in Fig. 11 for SB-1 molecules placed on silver nanowires.

27

Fig. 11. Photoluminescence image of SB-1 molecules deposited on silver nanowires. The excitation wavelength of 405 nm was used. While the emission is observed all over the sample, it is possible to distinguish elongated shapes of enhanced emission intensity. The positions of these shapes correlate with the positions of silver nanowires, analogously to previous work [35,36]. This observation implies that coupling with silver nanowires enhanced PL emission of the synthesized compounds studied in this work. Importantly, the increase of the PL emission is stronger at the ends of the nanowires, presumably due to stronger scattering of plasmon excitations. Careful analysis of the intensities for molecules in the vicinity of the silver nanowires yields the average enhancement factor of the order of 3 and 7, for molecules placed along and at the ends of the nanowires, respectively, which is substantially higher than observed previously for P3HT polymer [35]. It is therefore straightforward to attribute the emergence of the EL signal observed for the OLED devices to the enhancement effect due to coupling between the molecules and silver nanowires.

28

4. Conclusions Three new compounds (SB-1–SB-3) comprising substituted bithienyl phenylquinoline linked via Schiff base linkage with carbazole, bithiophene and phenanthrene derivatives were prepared and characterized. These molecules possess donor-acceptor-imine bridge-donor (DA-N=CH-D1) architecture differing in D1 structure, which allowed to investigate the effect of D1 on selected structural, thermal and optical properties. Only the Schiff base end-capped with bithiophene was obtained as crystalline substance, but with ability to transform to the glassy state with Tg of about 100ºC, which is the lowest compared to other compounds. Introduction of carbazole and phenanthrene units increases Tg but decreases about 40ºC the initial decomposition temperature which is still high, that is, above 300ºC. Significant influence of donor structure on emission intensity was observed. Photoluminescence quantum yield (f) changes from about 2 up to 26% for compound with phenanthrene moiety (SB-3). All imines emitted blue light with em in the range of 467 – 473 and 442 – 451 in chloroform solution and in PMMA matrix, respectively. However, in blend with PVK emission of green light was seen. In PL spectra of compounds with carbazole and phenanthrene substituents (SB-2 and SB-3) in PVK a band originating from the emission of matrix was not seen, suggesting complete the energy transfer from PVK to luminophore. Additionally, ability of the Schiff bases for electroluminescence was preliminary tested. The possibility of emission enhancement was demonstrated by incorporation of silver nanowires due to plasmonics enhancement. Based on DFT results, it was found that contribution in HOMOs of a –N=CHD1 group do not play significant role with share between 4-31 %. The lowest and the highest share of –N=CH-D1 in HOMO was calculated for imine with phenanthrene (SB-3) and with two bithienyl units (SB-1), respectively. Electron affinity was similar for all investigated compounds and it was in the range of -3.15  -3.02 eV. Compound with carbazole moiety

29

(SB-2) exhibited the highest ionization potential (-5.40 eV) and the lowest energy band gap (2.38 eV). Acknowledgement The research was co-financed by the National Research and Development Center (NCBiR) under Grant ORGANOMET No: PBS2/A5/40/2014. The calculations have been carried out in Wroclaw Centre for Networking and Supercomputing (http://www.wcss.wroc.pl).

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Highlights 

New Schiff bases possess donor-acceptor-imine-bridge-donor architecture were synthesized and examined.



Thorough characterization of optical and electrochemical properties of novel Schiff bases has been carried out.

 

Optical and electrochemical measurements were compared with DFT calculations. The emission enhancement of compounds was demonstrated by incorporation of silver nanowires due to plasmonics enhancement.

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