Thermally Activated Delayed Fluorescence (Green) in Undoped Film

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C: Energy Conversion and Storage; Energy and Charge Transport

Thermally Activated Delayed Fluorescence (Green) in Undoped Film and Exciplex Emission (Blue) in Acridone-Carbazole Derivatives for OLEDs Qamar Tabrez Siddiqui, Ankur A Awasthi, Prabhjyot Bhui, Mohammad Muneer, Chandrakumar R. S. Kuttay, Sangita Bose, and Neeraj Agarwal J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08357 • Publication Date (Web): 13 Dec 2018 Downloaded from http://pubs.acs.org on December 14, 2018

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The Journal of Physical Chemistry

Thermally Activated Delayed Fluorescence (Green) in Undoped Film and Exciplex Emission (Blue) in Acridone-Carbazole Derivatives for OLEDs Qamar T. Siddiqui,a,b Ankur A. Awasthi,a Prabhjyot Bhui,c Mohammad Muneer,b Kuttay R. S. Chandrakumar,d Sangita Bosec* and Neeraj Agarwala* aSchool

of Chemical Sciences, UM-DAE Centre for Excellence in Basic Sciences, University of Mumbai,

Kalina, Santacruz (E), Mumbai, 400098, India bDepartment

cSchool

of Chemistry, Aligarh Muslim University, Aligarh, India

of Physical Sciences, UM-DAE Centre for Excellence in Basic Sciences, University of Mumbai,

Kalina, Santacruz (E), Mumbai, 400098, India dTheoretical

Chemistry Section, Bhabha Atomic Research Centre, Mumbai, 400085, India.

Abstract: Donor-acceptor-donor (DAD) materials (1,2) having acridone as acceptor unit and carbazole as donor were synthesized for opto-electronic applications. Carbazole was substituted on 2,7 positions of acridone in 1, while, 3,6-trifluoromethylphenyl carbazole was substituted in 2. Steady state and time dependent emission properties of these compounds were studied in detail to get insight on their possible thermally activated delayed fluorescence (TADF) behaviour. The singlet-triplet energy gap (∆EST) was found to be as low as 0.17 eV (1) and 0.15 eV (2), favourable for TADF materials. Both these materials were found to be efficient green TADF emitters in organic light emitting diode (OLED) devices. 1

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Interestingly, the TADF properties were observed for the first time in undoped 1, 2 based devices i.e. without host matrix, unlike the most commonly reported TADF emitters. Furthermore, an exciplex emission at 465 nm was observed in the blends of 1, 2 with polyvinylcarbazole (PVK) in 1:7 (w/w) ratio. OLEDs with the blend of 1,2 with PVK as the active layer showed an intense electroluminescence at 465 nm matching well with the exciplex photoluminescence. Thus, we show that acridone-carbazole derivatives (1,2) offers variable electroluminescence; as undoped TADF green emitter and blue exciplex emitter when doped in PVK.

2


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INTRODUCTION Organic Light Emitting Diodes (OLEDs) have garnered a lot of attention due to their ecofriendly nature, low cost of production and superior performance.1-11 A simple architecture of OLED consist of two working electrodes sandwiching the emissive material (active layer). In OLEDs it is important to generate the exciton (electron – hole pair) in the emissive layer. To control the hole and electron mobilities for optimal operation other organic materials such as hole and electron transporters have been used.12-14 At room temperature, under electrical excitation, the fluorescent molecule typically generates 75% triplet and 25% singlet excitons, resulting in maximum 25% Internal Quantum Efficiency (IQE).13 Usually, triplet excitons are non-emissive at room temperature as they are deactivated by heat, prompting widescale research into how to improve efficiency of devices. Phosphorescent materials have been used to harvest singlet and triplet excitons to achieve 100% internal efficiency in OLED. However, such emissive, phosphorescent materials are synthesized using non-renewable, expensive and rare heavy metal atoms (e.g. Pt, Ir etc.), hampering their prospect for large scale application.1, 3, 15-17 Delayed fluorescence (DF), namely thermally activated delayed fluorescence (TADF) and triplet fusion delayed fluorescence (TFDF) in fluorescent molecules have been identified as other

ways

to

improve

the

OLED

efficiency.18-25

DF

significantly

improves

electroluminescence as it allows us to generate more singlet excitons. For TFDF, internal efficiency can reach ~62.5% as additional singlet excitons are generated via triplet-triplet annihilation (TTA).19-22 On the other hand, TADF materials possess very small energy gap between the singlet and triplet energy levels (ΔEST), which enable the conversion of triplet excitons into singlet via reverse inter system crossing (RISC), using thermal energy. Thus, 3



theoretically, TADF materials offers 100% internal quantum efficiency.23-26 Report by Adachi et al. opened up the OLED research field for detailed investigation of TADF emitters.25, 27 Design and engineering of materials possessing TADF properties is a major challenge, as the molecule should possess small ΔEST to promote RISC from lowest triplet to lowest singlet state. Simultaneously, for high fluorescence quantum yield, rate of internal conversion (kIC) should be considerably less than radiative decay constant (kr). Also, it is important to have short TADF lifetime for improved device efficiency as the chances of TTA gets reduced.28-30 It has been reported that small ∆EST can be achieved in push-pull molecules comprising of electron donor and acceptor units, yielding singlet and triplet states with strong charge transfer character.31,

32

Though, the charge transfer is not the prerequisite for TADF,

however, it lowers the ∆EST. Charge transfer state results in small HOMO-LUMO overlap and leads to decrease in electronic exchange energy, thus small ∆EST can be achieved. Dielectric nature of medium and molecular geometry also influence the photophysical properties of these molecules and in turn TADF efficiency, making it a challenge to design good TADF materials. Recently, push-pull molecules based on acridone, quinacridone and acridine have been reported for their applications in organic electronics.33-37 Acridone core acts as electron acceptor and can be substituted with various electron donating units easily, while maintaining its emissive nature. Substitution of secondary acyclic aromatic amines have been previously reported, however, substitution with cyclic amine have been rarely investigated. In this article, we designed and synthesized two acridone derivatives substituted with carbazole (1) or 3,6-trifluoromethylphenyl carbazole at 2,7 positions (2). Photophysical properties of these compounds were studied in detail to get insight on their 4


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TADF properties. Several studies on TADF properties in doped system have been reported recently,36-39 however, to the best of our knowledge, TADF property in undoped system are rare. Host matrix influences the several photophysical properties of emitters, thus, we report TADF properties of undoped 1,2 (without host matrix). Furthermore, effect of host polyvinylcarbazole (PVK) on emission of 1,2 was also studied in solution as well as thin film. Interestingly, both 1 and 2 showed blue shifted emission with very high fluorescence intensity (~ 10 fold) as compared to neat emitters, attributed to exciplex emission. Applications of 1,2 as emitter and their exciplex in OLEDs are also discussed.

RESULTS AND DISCUSSIONS Synthesis: Generally, amination is carried out with Buchwald-Hartwig coupling under palladium catalysed reaction.40 We have recently reported B-H coupling on 2,7dibromoacridone with secondary non-cyclic aryl amines.33 B-H reaction with cyclic amine such as carbazole did not work in our hands. Beside B-H amination, several other methods have been reported in literature.41-53 Ullman coupling, Cu catalyzed Chan-Lam Coupling, Pdcatalyzed C–N cross-coupling reaction of sulfonamides and aryl halides, cross-coupling reaction of aryl halides (iodide or bromide) with aromatic/heteroaromatic amines employing lanthanum(III) oxide are few of the many reports.54-58 For the amination of acridone with cyclic aryl amine, here carbazole, we adopted palladium free reaction using 1,10-phenanthroline and copper iodide as catalysing agents. Compounds 1,2 were synthesized as shown in Scheme 1. Aryl substituted carbazole (3), was synthesized by the Suzuki-Miyaura coupling of 3,6-dibromo carbazole and p-trifluoromethyl phenyl boronic acid. For amination reaction, 2,7-dibromoacridone was reacted with excess of carbazole in 5



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the presence of copper iodide and 1,10-phenathroline at 140 °C for 24 h. Mono- and di amination was observed, products were extracted and purified by column chromatography followed by recrystallization with ethyl acetate/hexane. Compounds 1,2 were characterised by multiple spectroscopic techniques like 1H-NMR,

13C-NMR,

FTIR, MALDI-TOF etc. (See

supporting Information). Solubility of these compounds were found to be good in organic polar solvents e.g 15 mg/mL in dichloromethane. R Br

O N

Br +

R

R

R

1,10-Phenanthroline N H

0

CuI, K2CO3, 140 C, 24 h

O

N

R

N

N

R

R = H, 1

CF3 2

Scheme 1. Synthesis of acridone-carbazole derivatives, 1-2

1H-NMR

of 2,7-bis(carbazolyl)-10-methylacridone (1) and 2,7-bis((3,6-p-trifloromethyl-

phenyl) carbazolyl)-10-methylacridone in CDCl3 are shown in Supporting Information (Figure S32). Singlet of two hydrogens at 1- and 8-positions of acridone appeared at 8.83 and 8.85 ppm, respectively in 1,2. Singlet nature of this signal shows the substitution of amine at 2and 7-positions of acridone. A doublet at 8.2 ppm for four protons in 1 is assigned to hydrogen at 4 & 5-positions of carbazole moiety. Similarly, signal at 8.46 ppm in 2 appears as singlet and is assigned for 4,5-positions of aryl substituted carbazole. Signal at ~7.8 ppm is assigned to 4,5-positions of acridone. N-methyl protons appeared at about 4.1 ppm in 1,2. A molecular ion peak was observed for compounds 1,2 in MALDI-TOF. 6


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Photophysical studies: Ground and excited state properties of acridone-carbazole derivatives 1,2 were studied using absorption and emission spectroscopy in solution and thin films. Absorption and emission spectra of 1,2 were recorded in 2-methyl tetrahydrofuran (Me-THF) and are shown in figure 1. A low intensity broad peak at 410 nm and emission was observed at about 490 nm. The absorption at 410 nm is assigned to charge transfer band investigated through solvatochromic studies (See Supporting Information). Absorption studies revealed negligible change in peak position (410 nm) on varying solvent polarity, however, steady-state peak maxima altered with solvent polarity. Emission maxima displayed significant red shift of up to 55 nm (~ 460 nm in hexane and ~515 nm in DMSO) and broadening in emission when going to higher polarity of solvents (See Supporting Information). In non-polar solvents, the spectrum is well resolved showing two peaks. The first peak belongs to the locally excited (LE) state, whereas the second peak arises from the charge transfer (CT) state. It is well reported that locally excited (LE) S1-S0 transition are independent of solvent polarity, therefore, emission maxima remains unaltered in solvents of varying polarity.59 The CT transitions are broad and their emission maxima depends on polarity of the medium. Our results indicate that the emission maxima is arising from 1CT transition. Furthermore, steady-state emission studies of both the molecules in Me-THF were studied at room temperature as well as at low temp (77 K). At room temperature, emission maxima was observed at 480 nm for 1 which is assigned to relaxation from charge transfer state. Whereas, 2 displays emission maxima at (450 nm) arising from locally excited singlet state (1LE) and charge transfer emission at 480 nm. At 77K, two new peaks in lower energy region at ~520 nm and ~550 nm is observed for both 1, 7



2 expected to arise from LE and CT triplet states. It is well known that sensitization by heavy atom (e.g. Br, I, Pt, Ir etc.) promotes spin orbit coupling and thus enhancement in emission from triplet state.16, 17, 60-62 Therefore, sensitization was carried out using ethyl iodide. On addition of ethyl iodide (1:10 v/v in N2 purged Me-THF) the intensity of both the low energy peaks at ~520 and ~550 nm increases many folds and hence is attributed to phosphorescence. The singlet and triplet energy level were estimated from the onset of fluorescence and phosphorescence peaks. The singlet triplet energy gap (∆EST) was calculated for both, locally excited (LE∆EST) as well as the charge transfer states (CT∆EST). The LE∆E

ST

was found to be 0.4 eV for 1 and 0.34 eV for 2, whereas the CT∆EST was found to be

0.17 eV (1) and 0.15 eV (2). It is worth mentioning that ∆EST of 1 has been previously reported to be 0.42 eV, which we believe is the difference between the singlet and triplet of the locally excited states.36 It is realized to have low ∆EST (0.1-0.3 eV) for thermally activated delayed fluorescence and the charge transfer states of the compounds 1,2 satisfy this condition. TADF was further studied using fluorescence transient measurements.

Figure 1: Normalised absorption, fluorescence and phosphorescence spectra of (A) 1 and (B) 2 in MeTHF. Phosphorescence peak is shown in circle.

8


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Fluorescence life time of 1,2 were determined in various solvents and traces are shown in Figure 2. It represents the prompt fluorescence lifetime of 1and 2, in solution, obtained on TCSPC setup which is described in the materials and methods section. The lifetimes were obtained in a 0-200 ns time window, to obtain only the prompt fluorescence lifetime. Fluorescence decay was expected to be multi-exponential due to prompt and delayed fluorescence components. In our case, transient measurements at room temperature yielded single exponential decay for 1,2 arising from prompt fluorescence only. The absence of any other component could be due to triplet state quenching by dissolved molecular oxygen. Thus, the lifetimes were recorded again on purging the solution with N2 gas, to enhance triplet utilisation for TADF. Interestingly, we observed the prompt fluorescence lifetime increase by ~6 ns for 1, and ~5 ns for 2 (Table 1) in various solvents (Supporting Information). Such an increase in lifetime on nitrogen purging is possible when triplet state is in close proximity of singlet. This result correlates well with the reported results of Adachi et al.26, 63

Figure 2: Transient Lifetime measurements (λex = 406 nm) of (A) 1 and (B) 2 in THF under air and nitrogen atmosphere. 9



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Table 1: Photophysical data of 1,2 Comp

1

a abs

a em

nm (log )

(nm)

242

(4.83),

292

471

a

F

0.44

b abs

b em

cτ PF

cτ DF

dτ DF

e

F

f∆E S

(nm)

(nm)

(ns)

(µs)

(µs)

425

501

11.5 (O2)

305.8 (O2)

362

0.60

0.17

17.6 (N2)

446.1 (N2)

12.1 (O2)

12.1 (O2)

16

0.48

0.15

17.2 (N2)

27.1 (N2)

(4.48),

T

321 (4.44), 416 (3.60) 2

260 (4.85), 299 (4.87)

472

0.40

424

502

324 (4.80), 415 (3.60) aIn

dichlomethane; bin thin film; cin acetonitrile; din PMMA doped film; ein nitrogen purged dichloromethane; fin Me-THF

TADF lifetime is dependent on triplet state of molecule and short TADF lifetime (below ms) indicates chance of shorter triplet lifetime. It has been proposed that a short TADF lifetime is important for prospective TADF materials which is beneficial to suppress triplet quenching processes like triplet-triplet annihilation, triplet-polaron annihilation etc., thereby improving singlet fluorescence efficiency via TADF.30 The delayed fluorescence lifetime in TADF molecules are known to be in microseconds-milloseconds scale.4, 32 Generally, TADF lifetime has been obtained by doping the emitter in a solid host matrix such as mCBP, Zeonex etc.).29,

36

We believe that the presence of dopant may slightly alter the

photophysical behaviour of the emitter. Here, we have attempted to evaluate the TADF lifetime of undoped pure 1,2 in acetonitrile. Efficiency of thermally activated delayed fluorescence depends on various factors associated with singlet and triplet states, such as, the presence of molecular oxygen (air) will affect TADF emission and life time at room 10


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temperature (See Scheme S2). Thus, we carried out extensive fluorimetric analysis to find out the TADF lifetime in air saturated and nitrogen purged atmosphere at room temperature. Figure 3 shows the TADF lifetimes of 1,2 (in solution) obtained from emission spectra taken at various delay times and then peak intensity was plotted against delay time and fit to the equation, I=Ioe-t/. The lifetimes of all the samples were obtained in air and nitrogen saturated acetonitrile solutions. Here, delay time is much longer than 100 ns therefore prompt fluorescence can be ruled out. Fluorescence spectra of 1,2 were collected at various delay times (100 s to 1 ms), in air and nitrogen saturated acetonitrile. Associated emission spectra of 1,2 corresponding to various delay times are shown in the supporting information (Figures S20-23). Interestingly, emission peak maxima obtained with delay time matches well with prompt fluorescence for both 1,2. It is worth mentioning that life time of prompt fluorescence is 0.5 ev) between LUMO of acceptor and donor is favourable.72 The charge transfer depends on the the energy levels of HOMO and LUMO of the donor and 15



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acceptor molecules, respectively. Also, it is required that both EHOMO and ELUMO of donor should be greater than that of acceptor. In case, above conditions do not met energy transfer may takes place rather than CT. In present study, we find that in PVK:1/2, this condition is satisfied, where we see emission by exciplex only. According to Weller, energy of true exciplex emission (Eex) can be expressed by equation (1),73, 74 Eex = EHOMO (donor) – ELUMO (acceptor)+Udest-Ustab-Hesol +0.32 ev

(1)

where, Udest-Ustab are the destabilization and stabilization effects during exciplex formation, which cancel out each other. Hesol is the energy gain due to solubilisation of electron transfer and can be considered nearly zero in solid state. Considering this, the energy level of exciplex (Eex) for PVK:1/2 films would be 2.72 eV which is matching well with the experimental value i.e 2.79 eV. This new emission was assigned to an exciplex formation between PVK and active emitters (1,2). Fluorescence lifetime measurement of PVK:1/2 blend, monitored at 470 nm was found to be multiexponential and we attribute it to varying degree of aggregates and exciplex. Decay lifetime for PVK (0.6 and 8.0 ns) has been reported as bi-exponential.75 Figure 5b shows the clear difference between the multiexponential decay profile for 1 and its blend with PVK in thin film, hence, confirming that this new emission arises from exciplex. The transient decay of 1 decays multi exponentially at 510 nm, with three time constants of 0.3, 1.3 and 7.8 ns due to varying degree of aggregation in solid film (See Table 2). On analysing the blend of PVK and 1, we again find the decay to be tri-exponential but with an increase in the lifetimes. Of the three time constants obtained, the longest of 18 ns is due to the formation of an exciplex between PVK and 1 in thin films.

16


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Figure 5: (A) Emission of spectra of 1 (dotted line), PVK (solid line) and their blend (dashed line) in thin films and (B) Transient lifetime measurement for thin film of 1 and PVK:1 blend

We find that the emission from the exciplex is maximum at a ratio of 7:1 w/w (PVK:1/2). It is also of interest to note that the emission from the exciplex is much more intense, roughly 20 folds, in comparison to pure films of PVK or 1,2 having same concentration as in the blend (see supporting information). A possible reason for this intense emission is that, with PVK being a polymer, excitonic migration takes place across the polymeric backbone, which is prevented in exciplex formation. Since all excitons are utilised in the formation of an exciplex, and no loss of excitons happens due to migration,76-79 the intensity of the exciplex emission is found to much greater in comparison to emission of pure films of compound and PVK having same concentration. Theoretically, TADF emitters can achieve 100% singlet exciton harvestation, making them prospective candidates for OLED fabrication. Exciplex formation in OLED is well reported.74, 80-82

This phenomenon has been extensively utilised to attain colour tunability ranging from

pure blue (Jankus et al) to green to white OLED devices as well.83-87 Such wide range of colour tunability could be achieved in exciplex systems as the emission wavelength is independent of optical gap of emitters, however depends on the HOMO-LUMO gap between the donor-acceptor species. 17



To study the prospects of these materials for OLEDs, devices were fabricated and studied. Initially devices were fabricated with 1 or 2 as green emitter. Polyvinyl-carbazole (PVK) which is well known hole transport layer (HTL) is used in these studies. For devices where the organic compound (1 and 2) was used as an emitter, the best results were obtained by taking a blend of PVK and N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPD) in 1:1 (w/w). The most optimized device structure used was ITO/(PVK+NPD) blend/1 or 2/BPhen/LiF/Al (See schematic of the device geometry in Figure 6(a)). Typically, HBLs are most commonly placed after the emissive layer closer to cathode to confine the carriers and excitons. 4,7-Diphenyl-1,10-phenanthroline (BPhen) was used as a hole blocking layer (HBL). Electroluminance spectra of the devices showed the peak emission at 490 nm for 1 and 505 for 2 (see figure 6(b)). Peak maxima of EL and PL in thin matches quite well. The turn on voltage, VON was lower for the OLED comprised of 1, also the current efficiency at high luminance (~1000 Cd/m2) was almost twice as that of 2 (see Figure 6(c) and (d)) indicating that the device geometry is more suited for 1 for exciton harvesting. Thus, 1 behaves as a good non-dopant TADF emitter for OLEDs possibly because of good donor properteis of carbazole than 3,6-trifluoromethylphenyl carbazole. Aslo, it correlates well with the lower EST of 1 than that of 2.

18


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Figure 6 : Device characteristics of 1 and 2 as an emitter. (a) 3D schematic of the device geometry,: ITO/(PVK+NPD) blend/1 or 2/Bphen/LiF/Al; (b) EL spectra for 1 and 2 at 20 mA/cm2 and 5 mA/cm2, respectively, (c) & (d) device characteristics of 1 and 2.

As observed from the photophysical characterisations, the emission wavelength and efficiency of the devices is expected to be tuned by the formation of an exciplex with PVK. Thus, devices with the following geomtery, ITO/PEDOT:PSS/(PVK+1 or 2)blend/Bphen/LiF/Al were fabricated (See figure 7(a)). In these devices blends of PVK with 1 or 2 were taken as active emitting materials. The ratio of PVK:1 or 2 was kept at 7:1 (w/w) which had shown the maximum quantum efficiency in thin films as found in the photophysical studies. For these

devices,

an

additional

layer

of

Poly(2,3-dihydrothieno-1,4-dioxin)-

poly(styrenesulfonate) (PEDOT:PSS) was used as a HTL. Both these devices emitted in blue with the maximum emission obtained at 464 nm as seen from the EL spectrum in Figure 7(b). This EL spectrum matches well with exciplex emission obseved in thin film. VON for 2 19



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was found to be 5.3 V which is lower than that of the device based on 1 (Figure 7(c)). Bright light emission was obtained from both devices (~ 1000 Cd/m2 @ 10 mA/cm2) with current efficiency of ~ 65 Cd/A at high luminance of 9800 Cd/m2 for the device based on 2 (Figures 7(d) & (e)). Thus, the overall performance of 1,2 as exciplex emitters were better than their use as neat emitters (See Table 2 for comparison of the devices).

Table 2: Summary of different Organic light emitting devices fabricated from 1,2. Comp Method of deposition

Device geometry

VON (V)

L (Cd/m2)

EL (nm)

Efficiency CIE (Cd/A) Co-ordinate @15mA/cm2

1

Evaporated

ITO/(PVK+NPD) /1/Bphen/LiF/Al

8.0

~1250 @25mA/cm2

505

~4

0.211, 0.435

2

Evaporated

ITO/(PVK+NPD) /2/Bphen/LiF/Al

11.0

~650 @25mA/cm2

490

~1

0.176, 0.411

1

Spin-coated

ITO/PEDOT:PSS/(P 8.0 VK+1)blend/Bphen /LiF/Al

~1400

465

~9

0.139, 0.179

ITO/PEDOT:PSS/(P 5.3 VK+2)blend/Bphen /LiF/Al

~9800

460

~65

0.157, 0.203

2

Spin-coated

@15mA/cm

@15mA/cm

2

2

20


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Figure 7: Device characteristics of exciplex systems formed using blends of PVK with 1 and 2 (7: 1 by weight). (a) 3D schematic of the device geometry, ITO/PEDOT:PSS/(PVK+1 or 2)blend/Bphen/LiF/Al (b) Electroluminance spectra for the two devices (with 1 or 2) at J values of 10 mA/cm2 and 5 mA/cm2 respectively. The inset shows the blue emission from the devices. (c) - (e) Device characteristics of 1 and 2.

Conclusions: Carbazole and 3, 6-trifluoromethylphenyl carbazole were substituted on acridone to get donor-acceptor-donor (DAD) materials for organic light emitting diodes. Photophysical (steady state and time dependent emission) properties of these compounds were studied which revealed thermally activated delayed fluorescence (TADF) in them. The ∆EST for both these materials were found to be as low as 0.15 eV, favourable for TADF 21



properties. Both these materials were employed as green emitters in OLEDs devices. Unlike the commonly reported doped TADF emitters, we showed the TADF properties of emitters in undoped devices i.e. without host matrix. OLEDs of 1 showed low turn on voltage, and high luminance (~1000 Cd/m2) which is roughly two times than that of 2. Thus, 1 behaves as a good non-dopant TADF emitter for OLEDs we attribute it to the lower EST and better donor properteis of carbazole than 3,6-trifluoromethylphenyl carbazole. Furthermore, an exciplex emission (roughly 10 times more than neat emitter) was observed in blends of 1 and 2 with PVK. Exciplex emission (at 465 nm) was also seen in the OLEDs having blend of 1 or 2 with PVK as the active layer. Bright blue light emission was obtained from both devices (~ 1000 Cd/m2 @ 10 mA/cm2) with current efficiency of ~ 65 Cd/A at high luminance of 9800 Cd/m2 for the device based on 2. We found that performance of 1,2 as exciplex emitters were better than their use as neat emitters. Thus, we show that acridone-carbazole derivatives (1,2) offers variable electroluminescence; as undoped TADF green emitter and blue exciplex emitter when doped in PVK.

Methods: All chemicals were purchased from Sigma Aldrich and used as received. Organic solvents were dried using standard procedures, wherever anhydrous solvents were required. 1H and 13C

NMR were recorded using Varian 600 MHz spectrometer. Tetramethylsilane (TMS) in

CDCl3 was used as an internal reference (residual proton;  = 7.26 ppm), for recording of 1HNMR spectra. Mass spectra were recorded using Bruker MALDI-TOF. Cyclic Voltametry was performed on CH Instrument 620D Electrochemical Analyser. Typically, three electrode cell was employed with a glassy carbon working electrode, Ag/AgCl (non-aqueous) reference 22


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electrode and Pt wire counter electrode. The measurements were performed at room temperature in anhydrous acetonitrile with tetrabutylammonium hexafluorophosphate solution (0.1 M) as supporting electrolyte at a scan rate of 100 mV/s. The oxidation potential of Fc/Fc+ was used as internal reference. The absorption and fluorescence data were acquired with ca. ~6x10-6 M solution. UV-visible spectra were recorded using Shimadzu 1800 and steady-state fluorescence spectra were recorded on a Horiba Fluoromax-4 spectrofluorometer. A diode laser based time-correlated single-photon counting (TCSPC) spectrometer (IBH, U.K.) was used to obtain the time-resolved fluorescence measurements and has been described in details elsewhere.88-90 Excitation of 1,2 was done using a 374 nm diode laser (1 MHz repetition rate). The fluorescence transients were collected at magic angle (54.7°) configuration which ensures that the observed transient decays are not influenced by the rotational relaxation of the probe molecule. To obtain instrument response function (IRF), scattered excitation light from the suspended TiO2 particles in water was monitored. The IRF obtained was ∼160 ps. TADF lifetime was obtained on a Agilent spectrofluorometer, with emission spectra taken at varying delay times (100 s to 1 ms) and then peak intensity was plotted against delay time and fitted to the equation, I=Ioe-t/ Doped films of 1/2 : PMMA (1:9 w/w in chloroform) were spin coated on quartz at 4000 rpm for 30 s. Steady state fluorescence and its lifetimes studies of PMMA doped films of 1,2 were carried out on Fluorolog 3 spectrofluorometer. Delay of 100 and 5 s for 1 and 2, respectively, were given and decay data was collected. For OLEDs fabrication, solid films of the compounds were prepared with a spin coater (Holmarc HO-TH-05). Spin coating was done at 2000 rpm for 40 seconds. Vacuum deposition was done at base vacuum of 2x10-6 mbar. ITO coated substrate (15-25Ω/sq, Sigma Aldrich) 23



was cut into 22 x 12 cm of dimension. It was etched into desired pattern to incorporate four active devices (See device geometry in Figure 6(a) and 7(a)) with the help of Zn powder and 10% HCl. Substrate cleaning was done in three simple steps. It was first cleaned with soap solution and rinsed with distilled water. Further, it was sonicated with distilled water and propanol. The sonication time was 10 min in each step. The last step included cleaning with Trichloroethylene (TCE) vapours. After UV treatment for 1h the substrates were ready for device fabrication. Organic layer of 1 and 2 was thermally evaporated in vacuum (base vacuum ~ 10-6 mbar). The thickness was ~ 20 nm. Bathophenanthroline (Bphen), a wellknown hole blocking layer (HBL) was also thermally evaporated over the active layer followed by 1 nm LiF and 160 nm of Al. The geometry of the most optimized device was ITO/(PVK+NPD/1 or 2/Bphen/LiF/Al. All the theoretical calculations have been performed using the ORCA electronic structure based programs.91 RHF-types of Kohn-Sham based density functional theory (DFT) methods have been applied and the def2-TZVP basis set has been employed for all atoms to obtain the optimized geometry as well as their energies using the B3LYP exchange-correlation functional.92, 93 The geometry optimization and time time-dependent DFT (TDDFT) based calculations for the evaluation of electronic excitation spectrum have been performed using the Becke’s three-parameter exchange functional and Lee-Yang-Parr correlation functional (B3LYP) and BHHLYP exchange-correlation functional respectively. The grid based DFT has been used which employs a typical grid quadrature for the computation of the integrals. Typically, the grid consists of 96 radial shells with 36 and 72 angular points during the SCF procedure. For blue emitting devices, exciplex formation was initiated by making a blend of PVK with the organic compound 1 and 2. 7:1 weight ration of PVK and organic compound was taken 24


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which was dissolved in Chloroform. This was spin coated at 5000 RPM for 30 s which resulted in ~100 nm thick films. In these devices, best results were obtained by using a 45 nm layer of PEDOT:PSS as a HTL. This was grown by spin coating at 8000 rpm for 40 sec. The geometry of the most optimized devices were ITO/PEDOT:PSS/(PVK+1 or 2)/Bphen/LiF/Al. The active area of the devices were around 9-12mm2. J-V measurements of the devices was carried out using a 2400 Keithley sourcemeter at the emitting wavelength and the luminance-voltage (L-V) spectra were simultaneously recorded using lock-in detection. The lock-in signal was converted to light units using the conversion factors which were estimated using a calibrated light source The electro-luminance spectroscopy of the devices were measured using a set up consisting of a Bausch and Lomb 350-750 nm monochromator and Hamamatsu R212 photomultiplier tube (PMT) as the detector.

Supporting information: Spectra related to NMR, mass, absorption, emission and transient lifetime studies, cyclic voltammograms and figures related to DFT studies.

AUTHOR INFORMATION Corresponding Author *([email protected] and [email protected]) Author Contributions All authors have given approval to the final version of the manuscript. Funding Sources Department of Science and Technology for partial financial support (EMR/2017/000805). 25



ACKNOWLEDGMENT We thank Swati Dixit for assistance in cyclic voltammeteric studies. We thank Tata Institute of Fundamental Research, Mumbai for NMR and MALDI-TOF. We also thank Radiation and Photochemistry Division, Bhabha Atomic Research Centre for TCSPC. NA and SB thank Department of Science and Technology for partial financial support (EMR/2017/000805).

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Thermally Activated Delayed Fluorescence (Green) in Undoped Film

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