Dual Interface Exciplex Emission of Quinoline and Carbazole

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Dual Interface Exciplex Emission of Quinoline and Carbazole Derivatives for Simplified Non-Doped White OLEDs Galyna Sych, Dmytro Volyniuk, Oleksandr Bezvikonnyi, Roman Lytvyn, and Juozas Vidas Grazulevicius J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09908 • Publication Date (Web): 04 Jan 2019 Downloaded from http://pubs.acs.org on January 7, 2019

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The Journal of Physical Chemistry

Dual Interface Exciplex Emission of Quinoline and Carbazole Derivatives for Simplified NonDoped White OLEDs

Galyna Sych1, Dmytro Volyniuk1, Oleksandr Bezvikonnyi1,2, Roman Lytvyn1*, Juozas V. Grazulevicius1 1Department

of Polymer Chemistry and Technology, Kaunas University of Technology,

Radvilenu pl. 19, LT-50254, Kaunas, Lithuania; e-mail: [email protected] 2Experimental

Physics Department, Faculty of Physics, Taras Shevchenko National

University of Kyiv, pros. Akademika Glushkova 4b, Kyiv, Ukraine *Present

address: Faculty of Chemistry, Ivan Franko National University, Kyryla i Mefodia

St. 6, Lviv, 79000, Ukraine

Abstract

Isomeric quinoline and 9-phenylcarbazole derivatives with different linking topology were synthesized as versatile exciplex-forming materials. Their exciplex forming properties were examined. The studied compounds showed dual electron-donating or accepting nature and formed blue emitting excited complexes with acceptor 2,4,6-tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine and orange emitting exciplexes with the donor 4,4′,4′′-tris[3-methylphenyl(phenyl)amino]-

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triphenylamine. The solutions and solid films of both the synthesized compounds showed violet fluorescence. Compound with meta-substitution exhibited higher energy of triplet level (2.64 eV) and higher ionization potential value (6.21 eV) in comparison to para-substituted one. Higher 5 % weight loss and glass transition temperatures of 339 °C and 84 °C were observed for para-isomer. The studied compounds were characterized by bipolar charge-transport properties with hole- and electron mobilities exceeding 10-4 cm2V-1s-1 at electric fields exceeding 105 V/cm. White electroluminescence was achieved via combination of two interface exciplex emissions in three-layered sandwich-type OLEDs. The best fabricated and optimized non-doped device with the three-layer structure emitted white electroluminescence with a high colour rendering index of 76, colour temperature of 8400 K and maximum external quantum efficiency of 3.15 %.

Introduction Bipolar conjugated compounds with donor-acceptor architecture are of great interest due to their wide present and potential application in optoelectronic devices

1–3.

A significant interest is focused on

development of white organic light emitting diodes (WOLEDs), as possible low-cost, highly efficient alternatives for back-lights in flat panel displays

4–8.

The ideal electroluminescent spectrum of

WOLED must cover whole visible electromagnetic spectrum. To achieve this aim various techniques have been applied including utilization of excimer-exciplex systems 9 and combination of two (blue and orange) or three (red, blue, green) emission colours in a single emissive layer of organic electroluminescent device 10. Employment of exciplex-forming systems as emissive materials can also be a successful strategy for fabrication of WOLEDs

11–13.

Exciplexes, if to compare them with

intramolecular excited states, as bimolecular systems promote easier adjustment and screening of donor and acceptor molecules for fabrication of highly efficient devices with desirable characteristics 14.

Careful and accurate choice of appropriate electron-donating and electron-accepting molecules

affords to tune the emission of exciplexes for the generation of white light 15,16. Utilization of delayed fluorescence exciplex pairs afford to reach internal quantum efficiency up to 100% due to efficient RISC process from T1 to S117–20. Highly efficient TADF exciplex system of 4,4′,4′′-tris[3methylphenyl(phenyl)amino]triphenylamine

(MTDATA)

and


tris-[3-(3-pyridyl)mesityl]borane

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(3TPYMB) enabled to achieve external quantum efficiency (EQE) of OLED as high as 5.4% Twice

higher

EQE

value

of

10%

was

realized

by

utilizing

bis(diphenylphosphoryl)dibenzo-[b,d]thiophene) (PPT) exciplex system

22,

the

21.

m-MTDATA:(2,8-

which allowed to reveal

the influence of photoluminescence quantum efficiency of exciplex on the EQE of the device. High efficiency of 11.6% with CIE coordinates of (0.29, 0.35) was achieved by utilization of two exciplex delayed fluorescence systems in WOLED, by simple adjustment of electron-donating molecules (D) in pair with 2,4,6-tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine (PO-T2T) as acceptor (A) for blue and red exciplex emissions

23.

To achieve white emission from recombination of these exciplexes,

rather complicated tandem device was constructed. It consisted of two individual exciplex-forming blend layers separated by a charge generation layer

21.

To simplify the structure of exciplex-based

WOLED, we propose in this work the new approach for achieving white light by mixing two exciplex emissions, with the structures of D-A/D and D-A/A. Warm-white electroluminescence is obtained using a donor-acceptor (D-A) compound which forms interface orange exciplex of D-A/D structure and blue exciplex of D-A/A structure. To design such D-A molecules capable of formation of D-A/D and D-A/A exciplexes, quinoline, known for its electron-accepting properties as well as thermal stability was used as electron-accepting unit

24–26.

Carbazole moiety was used as electron-donating unit taking into account that many

carbazole-based derivatives were characterized by good hole-transporting abilities, high triplet levels, good thermal properties and acceptable photochemical stability

27–29.

Combination of donor and

acceptor fragments affords effective D-A bipolar materials, which allow tuning HOMO-LUMO levels and maintaining charge balance in organic devices 30. In this work we propose unique application of donor-acceptor compounds as versatile exciplexforming materials which can form two different types of exciplexes D-A/D and A/D-A at the same time. Quinoline/carbazole derivatives were synthetized and their exciplex forming properties were investigated. Simplified non-doped WOLEDs based on two blue and orange interface exciplexes which were formed between three non-doped layers (A/blue interface exciplex/D-A/orange interface exciplex/D) were fabricated. In the fabricated WOLEDs, non-doped layers of the newly synthesized quinoline and carbazole derivatives with the different thicknesses acted as sky-blue and orange



interface exiplex-forming emitters. Additionally, these layers acted as hole- and electron- transporting modulators resulting in different distribution of holes and electrons at the sky-blue and orange interface exciplexes. Therefore, different white colour was recorded for devices with the same structure but with different thicknesses of the functional layers.

Experimental Instrumentation Nuclear magnetic resonance (NMR) spectra were recorded with Bruker Avance III [400MHz (1H), 101MHz (13C)] spectrometer at room temperature. The data are presented as chemical shifts (δ) in ppm against Si(CH3)4 as a standard. Mass spectra were measured by the electrospray ionization mass spectrometry (ESI-MS) method on Esquire-LC 00084 mass spectrometer. Elemental analysis was done with an Exeter Analytical CE-440 Elemental Analyser. Differential scanning calorimetry (DSC) measurements were performed with Perkin Elmer DSC 8500 equipment at heating and cooling rates of 10oC/min in a nitrogen atmosphere. Thermogravimetric experiments (TGA) were done using Perkin Elmer TGA 4000 apparatus at a heating rate of 20oC/min in a nitrogen atmosphere. Melting points were measured by MEL-TEMP (Electrothermal) melting point apparatus. Absorption spectra of 10-5 M solutions and neat films of the compounds were recorded using Perkin Elmer Lambda 35 spectrometer. Photoluminescence (PL) spectra of 10-5 M toluene solutions and solid films of the compounds were recorded at room temperature and at 80° K using Edinburgh Instruments’ FLS980 Fluorescence Spectrometer. Photoluminescence quantum yields (PLQY) of the solutions were obtained using the integrated sphere. Photoluminescence decay curves of the solid film of compounds and accurately prepared exciplex blends (1:1 ratio) were recorded at room temperature with the Edinburgh Instruments FLS980 spectrometer using a PicoQuant LDH-D-C-375 laser (wavelength 374 nm) as the excitation source. Cyclic voltammetry (CV) measurements were done using a micro-Autolab III (Metrohm Autolab) potentiostat-galvanostat equipped with a standard three-electrode configuration. A three- electrode cell equipped with a glassy carbon working electrode, an Ag/Ag (0.01 M in anhydrous DMF)


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reference electrode and a Platinum wire counter electrode was used. CV measurements were performed for anhydrous dimethylformamide solutions with 0.1 M of tetrabutylammonium hexafluorophosphate as the supporting electrolyte under nitrogen atmosphere at a scan rate of 0.1 V/s. The measurements were calibrated using ferrocene/ferrocenium (Fc) system, as an internal standard. Ionization potentials (IPPE) of solid samples were estimated by electron photoemission spectrometry in air. The samples were vaccum-deposited onto fluorine doped tin oxide (FTO) coated glass slides as substrates, exploiting setup consisting of ASBN-D130-CM deep UV deuterium light source, CM110 1/8m monochromator and 6517B Keithley electrometer. Photoelectron emission spectra were recorded exciting the samples from low energy to high energy with the step of 1 nm and recording electron photoemission current at different excitation energies 31. Charge transporting properties of vacuum deposited layers were investigated by time-of-flight (TOF) measurements using EKSPLA NL300 laser (excitation wavelength of 355 nm), 6517B electrometer (Keithley), and TDS 3032C oscilloscope (Tektronix) as it was described before

32.

Taking transit

times (ttr) from the photocurrent transients at applied voltage (U) and thicknesses of the layers (d) measured by the charge extraction by linearly increasing voltage (CELIV) technique assuming dielectric constant ε=3 for the studied compounds 33, equation μ=d2/(U×ttr) were utilized to calculate charge mobilities. Indium tin oxide substrates were patterned and cleaned for fabrication of OLEDs using layer-by-layer thermal

deposition.

Current

density-voltage

and

brightness-voltage

characteristics

were

simultaneously recorded using a source meter Keithley 2400 and certificated photodiode PH100-SiHA-D0 together with the PC-Based Power and Energy Monitor 11S-LINK (from STANDA). An Aventes AvaSpec-2048XL spectrometer was exploited to take electroluminescence (EL) spectra of the devices at different voltages. Using brightness, current density, and EL spectra, the current, power and external quantum efficiencies were calculated. Commission Internationale de l’Eclairage (CIE 1931) chromaticity coordinates (x, y) and colour rendering index (CRI) were obtained using EL spectra and software of FLS980 spectrometer.

Materials



o-Aminobenzophenone, 3-iodoacetophenone, 4-iodoacetophenone, glacial acetic and sulphuric acids, carbazole, copper, 18-crown-6 and 1,2-dichlorobenzene were purchased from Sigma Aldrich and used as received. N,N-Dimethylformamide (DMF) was distilled and dried over molecular sieves (4A). Other reagents and solvents were commercially purchased and were used as supplied. 4,4',4"-Tris(carbazol-9-yl)triphenylamine

(TCTA),

N,N'-bis(3-methylphenyl)-N,N'-bis(phenyl)-

benzidine (TPD), N,N'-bis(naphthalen-1-yl)-N,N'-bis(phenyl)-benzidine (NPB), di-[4-(N,N -di-p tolyl-amino)-phenyl]cyclohexane (TAPC), 4,4′,4′′-tris[3-methylphenyl(phenyl)amino]-triphenylamine (m-MTDATA), molybdenum trioxide (MoO3), lithium fluoride (LiF), and 2,2’,2’’-(1,3,5benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) were purchased from Sigma-Aldrich and used as received.

General procedure of the acid-catalyzed Friedlander synthesis Compounds were synthesized similarly to the previously reported procedure 34. To the solution of oaminobenzophenone (2 mmol) and iodoacetophenone (2 mmol) in glacial acetic acid (15 ml), sulphuric acid (5 ml) was added and refluxed for 12 hours. Upon completion of the reaction, the mixture was poured into ice water with concentrated ammonium hydroxide. The aqueous mixture was extracted with ethyl acetate; concentrated and dried over anhydrous Na2SO4. The product was purified by column chromatography using the eluent mixture of hexane and acetone with the volume ratio of 5:1 and crystalized from methanol. 2-(4-Iodophenyl)-4-phenylquinoline (pIPPQ) was prepared using the general procedure and was isolated in 77 % yield (0.62 g) as a yellowish powder. 1H

NMR (400 MHz, CDCl3) δ 8.14 (d, J = 8.4 Hz, 1H), 8.01 (d, J = 8.5 Hz, 2H), 7.82 (d, J = 8.2 Hz,

1H), 7.70 (s, 1H), 7.66 (t, J = 7.2 Hz, 1H), 7.57 (d, J = 8.5 Hz, 2H), 7.49 – 7.43 (m, 5H), 7.43 – 7.38 (m, 1H) ppm. 13C NMR (101 MHz, CDCl3) δ 155.58, 149.45, 148.80, 138.51, 138.27, 131.99, 130.12, 129.73, 129.56, 129.13, 128.66, 128.53, 126.59, 125.88, 125.72, 123.98, 118.86 ppm. ESI-MS (m/z): calculated for C21H14IN 408.25 (M++H), found 408.12. 2-(3-Iodophenyl)-4-phenylquinoline (mIPPQ) was prepared using the general procedure and was isolated in 70 % yield (0.56 g) as a yellowish powder.


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1H

NMR (400 MHz, CDCl3) δ 8.48 (s, 1H), 8.32 (d, J = 8.3 Hz, 1H), 8.12 (d, J = 7.8 Hz, 1H), 7.87 (d,

J = 8.4 Hz, 1H), 7.76 – 7.68 (m, 3H), 7.52 – 7.43 (m, 6H), 7.23 – 7.17 (t, J = 7.8 Hz, 1H) ppm. 13C NMR (101 MHz, CDCl3) δ 155.01, 149.69, 148.57, 141.50, 138.31, 138.15, 136.58, 130.50, 130.05, 129.87, 129.57, 128.69, 128.61, 126.82, 126.79, 125.95, 125.74, 119.08, 94.93 ppm. ESI-MS (m/z): calculated for C21H14IN 408.25 (M++H), found 408.19. General procedure of the Ullmann reaction: A mixture of pIPPQ or mIPPQ (0.3 g, 0.7 mmol), carbazole (0.15 g, 0.8 mmol), K2CO3 (0.31 g, 2 mmol), Cu (0.005 g, 0.07 mmol) and 18-crown-6 (0.002 g, 0.007 mmol) in 1,2-dichlorobenzene (9 mL) was heated at 170 °C under an argon atmosphere overnight. After cooling to room temperature, the reaction mixture was filtered through Celite. The solvent was removed under reduced pressure and the product was purified by column chromatography on silica gel using the eluent mixture of hexane and ethyl acetate with the volume ratio of 10:1. Recrystallization from isopropanol/DMF mixture provided pure products. 9-(4-(4-Phenylquinolin-2-yl)phenyl)-9H-carbazole (pCzPPQ) was obtained according to the general procedure and was isolated in 48 % yield (0.32 g) as white crystals. Mp: 152-154 °C 1H

NMR (400 MHz, CDCl3) δ 8.36 (d, J = 8.3 Hz, 2H), 8.21 (d, J = 8.4 Hz, 1H), 8.08 (d, J = 7.7 Hz,

2H), 7.87 (d, J = 8.4 Hz, 1H), 7.84 (s, 1H), 7.73-7.63 (m, 3H), 7.56 – 7.40 (m, 8H), 7.36 (t, J = 7.6 Hz, 2H), 7.23 (t, J = 7.4 Hz, 2H) ppm. 13C NMR (101 MHz, CDCl3) δ 156.00, 149.52, 148.92, 140.75, 138.78, 138.71, 138.34, 130.19, 129.78, 129.61, 129.15, 128.70, 128.56, 127.31, 126.61, 126.07, 125.90, 125.77, 123.57, 120.38, 120.13, 119.26, 109.89 ppm. Elemental analysis cal. for C33H22N2: C, 88.76; H, 4.97; N, 6.27. Found (%): C, 88.02; H, 4.33; N, 6.15. ESI-MS (m/z): calculated for C33H22N2 447.54 (M++H), found 447.51.

9-(3-(4-Phenylquinolin-2-yl)phenyl)-9H-carbazole (mCzPPQ) was obtained according to the general procedure and was isolated in 53 % yield (0.36 g) as white powder. Mp: 140-142 °C 1H

NMR (400 MHz, CDCl3) δ 8.35 (s, 1H), 8.24 (d, J = 7.8 Hz, 1H), 8.14 (d, J = 8.5 Hz, 1H), 8.09 (d,

J = 7.7 Hz, 2H), 7.84 (d, J = 8.4 Hz, 1H), 7.76 (s, 1H), 7.72 – 7.63 (m, 2H), 7.59 (d, J = 7.9 Hz, 1H), 7.50 – 7.38 (m, 8H), 7.34 (t, J = 7.6 Hz, 2H), 7.22 (t, J = 7.4 Hz, 2H) ppm. 13C NMR (101 MHz,



Page 8 of 30

CDCl3) δ 155.76, 149.57, 148.85, 141.78, 140.99, 138.35, 138.22, 130.40, 130.24, 129.75, 129.57, 128.66, 128.53, 128.04, 126.69, 126.45, 126.02, 125.98, 125.73, 123.44, 120.35, 120.02, 119.98, 119.18, 109.88 ppm. Elemental analysis cal. for C33H22N2: C, 88.76; H, 4.97; N, 6.27. Found (%): C, 88.11; H, 4.42; N, 6.25. ESI-MS (m/z): calculated for C33H22N2 447.54 (M++H), found 447.34.

Results and Discussion Synthesis Friedlander condensation of o-aminobenzophenone with corresponding iodoacetophenone afforded quinoline based compounds in high yield. The target compounds were synthesized by Ullmann-type cross-coupling reaction. Synthetic route for pCzPPQ and mCzPPQ is depicted in Scheme 1. Structures of compounds were confirmed using 1H NMR, 13C NMR (Supporting information) and mass spectroscopy.

N

O

I

N

O Cu, 18-crown-6,

NH2

pCzPPQ

K2CO3, DMF

H2SO4, AcOH

N

R

pIPPQ R=4-I mIPPQ R=3-I N

N

mCzPPQ

Scheme 1. Synthesis of pCzPPQ and mCzPPQ. Photophysical properties Since pCzPPQ and mCzPPQ consist of electron-accepting quinoline and electron-donating carbazole units linked through phenyl spacer, they were found to be capable of forming two different types of exciplexes: D-A/D and D-A/A. For the formation of such two types of exciplexes with pCzPPQ and mCzPPQ, appropriate commercially available acceptor and donor were selected. In previous studies pCzPPQ was used as efficient host for red PhOLEDs 35, but the exciplex-forming properties of these derivatives have not yet been reported.


1.0

a)

pCzPPQ Absorption/Fluorescence: toluene

1.2 1.0

0.8 0.6

mCzPPQ Absorption/Fluorescence: toluene

film

0.8 0.6

0.4

0.4

0.2

0.2

0.0

0.0 280 320 360 400 440 480 520 560 600

Wavelength (nm)

PL intensity (a.u.)

film

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Normalized intensity (a. u.)

Page 9 of 30

b)

pCzPPQ (PLTHF 77K) pCzPPQ (PHTHF 77K)

1.0

mCzPPQ (PLTHF 77K) mCzPPQ (PHTHF 77K)

0.8 0.6 0.4 0.2 0.0

S1

T1 T1 S1

350

400

450

500

550

600

Wavelength (nm)

650

700

Figure 1. (a) UV-VIS and PL spectra of 10-5 M toluene solutions and solid films of pCzPPQ and mCzPPQ (300 K, λexc=330 nm). (b) Photoluminescence spectra of THF solutions of pCzPPQ and mCzPPQ (77 K, λexc=374 nm). Phosphorescence spectra were recorded using delay of > 50 ms after excitation. First singlet (S1) and triplet (T1) energy levels (marked by arrows) were taken from the onsets of fluoresce and phosphorescence spectra.

pCzPPQ and mCzPPQ exhibited similar photophysical properties. Absorption and PL spectra of the dilute solutions (10-5 M) in different solvents and of solid films of compounds pCzPPQ and mCzPPQ are shown in Figure 1a and Figure S1 a. The wavelengths of maxima of the low-energy absorption and PL bands are presented in Table 1 and S1. The low-energy absorption bands were observed at the similar wavelengths (at ca. 343 and 340 nm in case of pCzPPQ and mCzPPQ, respectively) for the solutions in the solvents of different polarities as well as for solid films. Meanwhile, low-energy edges of absorption spectra of the films of pCzPPQ and mCzPPQ were slightly red-shifted in comparison to those of the solutions (Table 1). Shifts of the edges of absorption spectra of the films can presumably be explained by molecular interactions in the solid state. PL spectra undergo significant red shifts on going from less polar toluene (ε = 2.38) to highly polar acetonitrile (ε = 37.5). This observation shows that both molecules exhibit positive solvatochromism (Figure S1 a, b). This behaviour is attributed to the intramolecular charge transfer (ICT) character of emissions which is commonly observed for donor-acceptor compounds 36. The different Stokes shifts of the solutions of pCzPPQ and mCzPPQ were observed due to ICT. The Lippert-Mataga equation was used to analyse Stokes shifts of the solutions of pCzPPQ and mCzPPQ in different solvents (Supporting information). The Stokes shifts (νabs-νem) versus orientation polarizability of solvents



(Δf) dependencies were plotted and were linearly fitted (Figure S1 b). The smaller slope of 13950 cm1

was observed for pCzPPQ in comparison to that of 14644 cm-1 recorded for mCzPPQ indicating

difference in dipole moments of the emissive excited states of these compounds (Figure S1 b). Because of the differences in dipole moments of pCzPPQ and mCzPPQ, the solution of mCzPPQ in toluene demonstrated considerably lower PLQY of 11 % compared to 69 % observed for para- isomer (pCzPPQ). The same tendency was observed for the solid films of these compounds. The film of para- isomer exhibit higher PLQY value of 22 % relative to that of meta-isomer (10 %) (Table 1). The lower photoluminescence quantum efficiency of mCzPPQ in solution and solid state can be explained by higher dipole moment of mCzPPQ in comparison to that of pCzPPQ. Additionally, it can be explained by confined electron delocalization through meta-substitution of phenyl spacer 37. To obtain the first triplet energy levels of pCzPPQ and mCzPPQ, PL spectra of the solutions of pCzPPQ and mCzPPQ in THF and of solid films were recorded at the temperature of liquid nitrogen (Figure 1b, S2). The values of the first singlet and triplet energy levels were taken from on-sets of fluorescence and phosphorescence spectra respectively (Table 1). Meta-isomer mCzPPQ showed marginally higher value of lowest triplet excited state (ET) of 2.61 eV in comparison with para-isomer (pCzPPQ) with the triplet energy value of 2.57 eV (Figure 1b). The energies of the first excited singlet state (ES) were estimated to be 3.35 and 3.57 eV for THF solutions of pCzPPQ and mCzPPQ, respectively (Table 1). In addition, red-shift of ca. 40 nm of the room-temperature fluorescence spectrum was observed in comparison to PL spectrum recorded at low temperature (77 K) for THF solutions of pCzPPQ and mCzPPQ (Figure S2). Such a bathochromic shift can be explained by stabilization of the excited state of the molecules by the polar solvent at room temperature while the same process does not occur at 77K due to the solidification of the solvent (Figure S2) 38.


Page 10 of 30

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Table 1. Photophysical data for pCzPPQ and mCzPPQ.

pCzPPQ

λabs, tol/

λabs, film

λ PL, tol

λ PL, film

ηtol,

ηfilm,

τ1 / τ2 (χ2)

ET,

ES,

λonset, tol

λonset, film

nm

nm

%

%

ns

eV

eV

nm

nm

343/380

343/398

395

415

69

22

2.1 / -

2.57

3.35

2.61

3.57

(1.106) mCzPPQ

340/351

340/368

403

413

11

10

9.1 / 22.9 (1.247)

λabs is maximum of the low-energy absorption band; λonset is the onset of the last absorption band; λ PL is the wavelength of emission maximum of toluene solution (10-5M) or solid films of compounds; ηtol, ηfilm are photoluminescence quantum yields of toluene solution (10-5M) and solid film; τ1 / τ2 are photoluminescence lifetimes of neat films of compounds. To estimate the singlet (ES) and triplet (ET) energy levels of both isomers, we used empiric formula ES=1239/λS and ES=1239/λT, where λS and λT are wavelengths of onsets of fluorescence and phosphorescence spectra. These wavelengths are marked by arrows in Figure 1b.

Exciplex formation between synthesized molecules and commercially available acceptor or donors was identified by appearance of the bathochromically-shifted emission bands of the blends as compared to the emission of separate donors and acceptor (Figure S3). During the donor-acceptor intermolecular interaction in excited state electron is transferred from HOMO of an electron-donating molecule to LUMO of an acceptor

39.

Bipolar molecules pCzPPQ and mCzPPQ formed excited

complexes and behave as donors in pair with acceptor or as acceptors in pair with donor. Thus pCzPPQ and mCzPPQ form blue exciplexes with the acceptor PO-T2T and yellow exciplex with the donor m-MTDATA and blue exciplexes with other donors such as TCTA, TPD, NPB, and TAPC. Photoluminescence spectra of the blends as well of neat films of donors are presented in Figure S3 and the data are collected in Table S2. All equimolar blends, compared with individual donors and acceptor (pCzPPQ or mCzPPQ), demonstrated red-shifted broadband emission spectra which can be assigned to the formation of exciplexes. pCzPPQ or mCzPPQ acted as donors in the blends with acceptor PO-T2T and showed orange exciplex fluorescence with the intensity maxima at 464 and 458 nm, respectively. With the aim to obtain white light emission, blue exciplex pairs of pCzPPQ/mCzPPQ with PO-T2T and pairs of pCzPPQ/mCzPPQ with m-MTDATA, which emit orange light, were selected for the deeper investigation (Figure 2).



Exciplex 1

A :

Page 12 of 30

Exciplex 2

D-A

: D

N P

O

N

pCzPPQ

N

N N

P

O

N

N N

P

PO-T2T

N

O

N

N

m-MTDATA

mCzPPQ

Figure 2. Molecular structures of compounds forming blue and orange exciplexes. Absorption and PL spectra of the solid samples of the separate molecules and exciplex emitters are presented at Figure 3a. The emission of exciplex forming molecular mixture mCzPPQ:PO-T2T with the intensity maximum observed at 458 nm was blue-shifted relative to that of pair of pCzPPQ:POT2T with emission peak at 464 nm. This observation can be explained by the slightly higher energy of HOMO for mCzPPQ (Table 4). The similar hypsochromic shift of 5 nm was observed for PL emission of mCzPPQ: m-MTDATA system compared to that of pCzPPQ:m-MTDATA, which can be assigned to the lower LUMO value of mCzPPQ. UV-VIS spectra of the studied exciplex-forming molecular mixtures and corresponding acceptor and donors are depicted at Figure 3a. Absorption spectra of the exciplex-forming systems pCzPPQ:PO-T2T and mCzPPQ:PO-T2T are very similar to the spectra of constituting molecules pCzPPQ and mCzPPQ. The shapes of sbsorption spectra of the films of exciplex-forming mixtures of the compounds with m-MTDATA donor resemble that of pure m-MTDATA film

40.

Thus, absorption spectra of the blends overlap with absorption spectra of the

donors and acceptors used. Nonetheless, direct exciplex CT absorption of the ground state of the studied solid mixtures mCzPPQ/PO-T2T, pCzPPQ/PO-T2T, mCzPPQ/m-MTDATA and pCzPPQ/mMTDATA was not clearly detected (Figure 3a). Apparently, more sensitive method is required to record intermolecular charge-transfer absorption of the studied exciplexes 41


0.8

pCzPPQ:PO-T2T

0.8

PO-T2T (toluene)

0.6

0.6 0.4

0.4

0.2

0.2

0.0

0.0

pCzPPQ (film)

1.0

m-MTDATA (film)

1.0

pCzPPQ:m-MTDATA

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0

0.0

250 300 350 400 450 500 550 600 650

mCzPPQ (film) PO-T2T (film) mCzPPQ:PO-T2T PO-T2T (toluene)

1.0

1.0

0.8 0.6

10

3

10

2

10

1

10

0

10

5

10

4

10

3

10

2

10

1

0

0.6 0.4

0.2

0.2

0.0

mCzPPQ (film) m-MTDATA (film) mCzPPQ:m-MTDATA

1.0

0.0 1.0

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0

0.0

250 300 350 400 450 500 550 600 650

Wavelength (nm)

a) pCzPPQ (film) instrument response fitting

40

80

Counts

4

0.8

0.4

Wavelength (nm)

10

1.0

Normalized PL intensity (a.u.)

Normalized absorbance (a.u.)

PO-T2T(film)

Normalized absorbance (a.u.)

pCzPPQ(film)

1.0

Counts

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Normalized PL intensity (a.u.)

Page 13 of 30

120 pCzPQ:PO-T2T pCzPQ:m-MTDATA instrument response fitting

5000

10000

Time (ns)

15000

20000

10

4

10

3

10

2

10

1

m CzPPQ (film) instrument response fitting

0

10 5 10 10

4

10

3

10

2

10

1

0

50

5000

100

10000

150 200 250 300 mCzPQ:PO-T2T(1:1) solid film mCzPQ:m-MTDATA(1:1) solid film instrument response fitting

15000

20000

Time (ns)

25000

30000

b) Figure 3. UV-vis, PL spectra (a) and PL decay curves (b) of the thin films of individual compounds and molecular mixtures of pCzPPQ and mCzPPQ with PO-T2T and m-MTDATA. Fluorescence decays of the solid films of compounds and degassed exciplex-forming blends are depicted in Figure 3b. Emission of the studied solid mixtures is ascribed to exciplex emission according to the following considerations. The energies of emission maxima are in very good agreement with the energies of emission maxima of exciplexes which can be described as (Tables S3S5)

42:

D A D A hvmax ex ≃ IP ― EA ― EC, where IP is the ionization potential of the donor, EA is the electron

affinity of the acceptor, and ECis the electron–hole Coulombic attraction energy. In addition, PL decays of mCzPPQ/POT2T, pCzPPQ/POT2T, mCzPPQ/m-MTDATA and pCzPPQ/m-MTDATA were observed in microsecond range which is much longer than the nanosecond range of fluorescence of the individual donor and acceptor (Figure 3b).” Measured under vacuum condition (at pressure

Dual Interface Exciplex Emission of Quinoline and Carbazole

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