Low ASE Threshold and Efficient Electroluminescence from a

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Low ASE Threshold and Efficient Electroluminescence from a Carbazole Derivatized Excited State Intramolecular Proton Transfer (ESIPT) Dye Van T. N. Mai, Atul Shukla, Masashi Mamada, Satoshi Maedera, Paul E. Shaw, Jan Sobus, Ilene Allison, Chihaya Adachi, Ebinazar B. Namdas, and Shih-Chun Lo ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00907 • Publication Date (Web): 05 Oct 2018 Downloaded from http://pubs.acs.org on October 8, 2018

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Low ASE Threshold and Efficient Electroluminescence from a Carbazole Derivatized Excited State Intramolecular Proton Transfer (ESIPT) Dye Van T. N. Mai,1,2 Atul Shukla,1,3 Masashi Mamada,4,5 Satoshi Maedera,4,5 Paul E. Shaw,1,2 Jan Sobus,1,3 Ilene Allison,1,2 Chihaya Adachi,4,5,6,* Ebinazar B. Namdas,1,3,* Shih-Chun Lo.1,2,* 1

Centre for Organic Photonics & Electronics, the University of Queensland, Brisbane, QLD 4072, Australia

2

School of Chemistry and Molecular Biosciences, the University of Queensland, Brisbane, QLD 4072, Australia

3

School of Mathematics and Physics, the University of Queensland, Brisbane, QLD 4072, Australia 4

Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, Fukuoka 819-0395, Japan

5

JST, ERATO, Adachi Molecular Exciton Engineering Project c/o Center for Organic

Photonics and Electronics Research (OPERA), Kyushu University, Nishi, Fukuoka 819-0395, Japan 6

International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University, Nishi, Fukuoka 819-0395, Japan *Correspondence to: Shih-Chun Lo ([email protected]), Ebinazar B. Namdas ([email protected]), Chihaya Adachi ([email protected])

Keywords: OLEDs, organic laser dye, excited state intramolecular proton transfer, amplified spontaneous emission threshold, excited-state absorption

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ABSTRACT The search for new classes of efficient electroluminescent organic laser dyes with low amplified spontaneous emission (ASE) threshold (Eth) and low optical losses is crucial towards realisation of organic injection lasers. In this work, we report in-depth studies of an organic semiconducting laser dye, benzo[d]thiazol-2-yl)-5-(9H-carbazol-9-yl)phenol (HBTCz) and compared the photophysical study with its parent HBT dye. While HBT is not laseractive in solution, the carbazole derivatized dye (i.e., HBT-Cz) showed a low solution ASE threshold of 21 mJ/cm2, which is comparable to a common high-performing commercial laser dye, Rhodamine 6G (Rh-6G). HBT-Cz also exhibited a low film ASE threshold of 2.4 µJ/cm2 with a low optical loss coefficient of 1.5 cm-1, which is the lowest waveguide loss coefficient reported for solution-processed organic semiconductors. Organic light-emitting diodes based on this material showed a high maximum EQE of 1.9%, which is close to the theoretical EQE limit of the emitter, and high brightness of >2,300 cd/m2. Further insights into the lasing properties of the material were provided by steady state UV-visible spectroscopy, and transient absorption spectroscopy, shedding light on excited-state species absorption. To the best of our knowledge, this is the first report of a solution-processable organic small molecular dye, exhibiting a low ASE threshold and lowest optical loss coefficient, coupled with efficient electroluminescence.

The experimental demonstration of the first ruby laser in the early 1960s revolutionized science and technology. Material development has since played a key role in advancing new lasers. A notable material-based innovation was reported in the late 1960s and involved the use of organic materials as gain media for lasers. This was first demonstrated in the form of a fluorescent dye in solution,1 then later, a dye doped in an inert solid matrix.2 The advantages offered by using organic materials as gain media include: compact size, mechanical flexibility, 2 ACS Paragon Plus Environment

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wavelength tunability and ultra-short pulse. Moreover, low-cost manufacturing techniques such as spin-coating or ink-jet printing can be employed for device fabrication. So far, all organic lasers have been optically pumped by a secondary excitation source such as gas lasers, inorganic solid state lasers3 or light-emitting diodes (LEDs).4 Direct electrical pumping in organic materials is one of the major challenges faced in organic opto-electronics research and remains an intellectual quest for researchers.

Compared with optical excitation, electrical injection into organic materials is significantly more challenging for several reasons. First of all, it is very difficult to pass a high current density through organic materials due to their intrinsically low charge carrier mobilities as well as thermal degradation concerns.5-9 Furthermore, electrical excitation suffers from additional optical losses (e.g., excited-state absorption and singlet-triplet annihilation) caused by faster triplet state accumulation in accordance with spin statistics, where the singlet to triplet exciton ratio generated in a device is generally 1 to 3.10-12 As a result, the lasing threshold in an electrically pumped organic laser is significantly higher than that of optically pumped laser.13 Therefore, the search for an efficient electroluminescent organic laser material with low amplified spontaneous emission (ASE) threshold (Eth) and low optical losses is crucial for the quest toward organic laser diodes.

Recently, significant progress has been made on the development of organic dyes with low ASE thresholds.14-24 Among these, the bis-styrylbenzene14,15 and oligofluorene16,17 families of chromophores have been demonstrated to exhibit ASE thresholds as low as 0.32 µJ/cm2 for BSB-Cz,14 0.11 µJ/cm2 for spiro-SBCz,15 and lately, 0.09 µJ/cm2 for octafluorene.16 Further breakthroughs have been reported, and include the demonstration of an organic laser based on BSB-Cz operating in the quasi-continuous-wave regime.25 These results illustrate that organic


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laser dyes with low ASE thresholds are of importance for progress in the field and justify further development of organic laser dyes.

It is important to note that in addition to aforementioned dyes, a diverse class of chromophores, which exhibit excited-state intramolecular proton transfer (ESIPT), has also attracted extensive attention.26-30 One distinct characteristic of ESIPT dyes is their four-level photophysical scheme, which separates their absorption (enol form) from their emission (keto form).31,32 As a result, ESIPT chromophores typically have a large Stokes shift (≈150 nm) and a low triplet absorption cross section in the gain region.33 Therefore, losses due to selfabsorption and triplet pile-up can be minimized or eliminated, potentially leading to electrically pumped organic lasers. Examples of efficient ESIPT laser dye include poly(aryl ether) dendrimer QG2,26 and imidazole-based chromophore HPI-Ac27 with low ASE thresholds of 20 µJ/cm2 and 6 µJ/cm2, respectively. Another widely studied ESIPT chromophore is 2-hydroxyphenylbenzothiazole (HBT)28-30 exhibited low lasing thresholds in inert polymer matrix29 and single crystal nanowires.30 However, inert polymer matrix is not suitable for charge injection while single crystal nanowires are not compatible for common device fabrication techniques. Most of ESIPT materials also showed poor electrical properties, resulting in low external quantum efficiencies (EQEs) in organic light-emitting diodes (OLEDs).

N

HO

S

HBT

HO N S

N

(pCzOH) HBT-Cz

Figure 1. Chemical structures of HBT28-30 and HBT-Cz (pCzOH).34 In this paper, we showed remarkable improvement in both photophysical and lasing properties of a HBT derivative (as shown in Figure 1) in both solution and solid state by a simple 4 ACS Paragon Plus Environment

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chemical modification, namely attachment of a carbazole to a simple HBT, to form 2(benzo[d]thiazol-2-yl)-5-(9H-carbazol-9-yl)phenol (HBT-Cz or pCzOH).34 We found that while the parent HBT is not laser active in solution, HBT-Cz showed a low solution ASE threshold of 21 mJ/cm2, which is comparable to a common high-performing commercial laser dye, Rhodamine 6G (Rh-6G). A notably low ASE threshold of 2.4 µJ/cm2 was achieved from a solution processed HBT-Cz blend film, which is the lowest ASE threshold reported for ESIPT laser dyes processed from solution. We note that this threshold was achieved in a common 4,4'-bis(N-carbazolyl)-1,1'-biphenyl (CBP) host instead of an inert polymer matrix as reported for HBT.29 We also found that solution-processed HBT neat film did not exhibit ASE while HBT-Cz showed a low neat film ASE threshold of 8.8 µJ/cm2. OLED devices based on HBT-Cz in CBP had a high maximum EQE of 1.9% and brightness of >2,300 cd/m2. Further insights into lasing properties were provided by a transient absorption spectroscopy study, shedding light on excited-state species absorption that contribute to optical loss mechanisms in the gain medium. To the best of our knowledge, this is the first report of solution processable small molecular ESIPT dye demonstrating low ASE thresholds, a lowest optical loss coefficient and efficient electroluminescence, which is prerequisite for progressing toward organic laser diodes.

EXPERIMENTAL SECTION All commercial reagents and chemicals were used as received unless otherwise noted. Tetrahydrofuran was dried using a vacuum-argon solvent purification system before use. Xylene was stirred overnight with sodium metal (4% w/v), distilled and stored in activated 4 Å molecular sieves under argon. Dichloromethane was dried with calcium hydride overnight and freshly distilled prior to use. Petroleum with boiling points in the range of 40–60 °C and dichloromethane were distilled prior to use for column chromatography, using Merck LC60A 40–30 silica gel. Solvent ratio used for column chromatography is reported by volume. 5 ACS Paragon Plus Environment

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Synthetic route to HBT-Cz is shown in Scheme S1, Supplementary Information (SI) with the synthetic details in SI. Photoluminescence (PL) spectra were measured using Horiba Jobin Yvon FluoromMax. Solution and solid state PL quantum yields (PLQYs) were measured using Horiba Jobin Yvon FluoromMax and an absolute PLQY measurement system (Hamamatsu

Photonics

Quantaurus-QY

C11347-01),

respectively.

Transient

photoluminescence decay was recorded on a Hamamatsu Photonics Quantaurus-Tau C1136703. Ionization potential (i.e. highest occupied molecular orbital, HOMO, energy level) of HBT-Cz was obtained on a Riken Keiki AC-3 through photoelectron spectroscopy in air (PESA) measurements (Figure S1, as -5.95 eV versus vacuum level while the estimated lowest unoccupied molecular orbital, LUMO, energy level was obtain from the optical energy gap of 2.95 eV using the onset of neat film absorption). Film fabrication: same conditions were used to fabricate thin films for photophysical studies and solid state ASE measurements. Films were spin-coated from chloroform solution (freshly distilled) at 1,500 rpm on fused silica substrates. Concentration of HBT-Cz:CBP in the chloroform solution was optimized to 22–25 mg/ml to give thickness of 160–170 nm.

Glass substrates with a pre-patterned tin-doped indium oxide (ITO) coating (100 nm thick, 100 Ω/sq) as anodes were used. Substrates were washed by sequential ultrasonications in neutral detergent, distilled water, acetone, and isopropanol and then exposed to UV/ozone (Nippon Laser & Electronics Lab, NL-UV253) for 15 minutes to remove adsorbed organic species. After cleaning, the substrates were immediately transferred into the evaporation chamber. Organic layers were formed by thermal evaporation at a pressure lower than 4 × 10−4 Pa. The deposition rates of the organic layers were 1.0 Å/s while the rate for LiF was 0.05 Å/s. The current density−voltage−luminance (J-V-L) characteristics of the OLEDs were evaluated using a Keithley 2400 source meter and an absolute EQE measurement system


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(C9920-12, Hamamatsu Photonics, Japan). The reproducibility of the device performance of the presented devices was confirmed by measuring at least four different samples.

RESULTS & DISCUSSIONS Material synthesis. The synthesis of HBT-Cz was previously reported,34 involving serial protection and deprotection of an amine functional group as well as subsequent conversion of this deprotected amine into an iodo functional group in order to couple with the carbazole moiety (see Scheme S2, SI). Herein, we found HBT-Cz can be prepared via a shorter route as outlined in Scheme S1 by using a commercially available 4-iodo-2-hydroxybenzoic acid as the starting material instead of reported amine analogue. Our synthetic route eliminates not only the protection and deprotection steps of the amine starting material, but also the amineiodo functional group transformation (see Scheme S2), thereby reducing the total number of synthetic steps. Details of our new synthetic approach are described in Section S1 and 1H NMR of HBT-Cz is shown in Figure S2, SI.

Photophysical properties studies. Photophysical properties of HBT-Cz were probed in toluene, neat (spin-coated from chloroform solution) and blend films (5wt% HBT-Cz in CBP spin-coated from chloroform solution). Figure 2 shows the steady-state solution and film absorption and photoluminescence (PL) spectra, where the values are summarized in Table 1. The absorption maximum was found at 363 nm in the solution absorption spectrum of HBTCz and can be assigned as the S0 S1 (π π*) transition of its enol form (i.e., E E* as shown in Figure S3, SI). In toluene solution, HBT-Cz showed a small emission at around 425 nm along with a major PL peak at 512 nm (Figure 2), which can be attributed to emission from its enol and keto form, respectively.34 The fact that only a minor part of the emission is from the enol form indicates ESIPT is favourable in toluene. Additional computational data (DFT and TD-DFT with optimized structures in ground and excited-state and energy levels) 7 ACS Paragon Plus Environment

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and interpretation to the ESIPT process of HBT-Cz are shown in Section S2 (SI). This efficient ESIPT leads to a large Stokes shift (149 nm) and reduces the spectral overlap of the absorption and emission, which is desirable for lasing as optical losses arising from selfabsorption are minimized. In solid state, it was observed that the neat film absorption spectrum is similar to that obtained for solution. Interestingly, the PL of neat film is slightly narrower than that obtained in solution (Figure 2), which is contrary to the general observation of concentration-caused luminescence quenching. This can be explained by mechanical restriction of molecular rotation of HBT-Cz in solid state. For 5wt% HBTCz:CBP blend film, the absorption spectrum (Figure 2) was dominated by the host material due to the low blending ratio. However, the emission spectra and emission maxima (514 nm) were identical when compared to the neat film, indicating efficient energy transfer from CBP to HBT-Cz with negligible excimer emission from the blend film. Similarly to solution, both neat film and 5wt% blend film had large Stokes shifts of 145 nm and 144 nm, respectively, resulting from efficient ESIPT.


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1.0

Neat film 5wt% in CBP 1.0 Toluene

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0 300

Normalized PL (a.u.)

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Normalized absorbance (a.u.)

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0.0 400

500

600

Wavelength (nm)

Figure 2. Normalized absorption and photoluminescence (PL) spectra of HBT-Cz in toluene solution (blue dotted lines), neat film (green lines), and 5wt% in CBP (red lines). Excitation wavelength = 350 nm.


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λabs

λPL

Stokes shift

PLQY

Lifetime

kr

knr

(nm)

(nm)

(nm)

(%)a

(ns)

(x 108 s-1)a

(x 108 s-1)a

Solution

363

512

149

9

0.59a/0.58b

1.6

15.6

neat film

366

511

145

39

–a/3.25b

1.2

1.9

5wt% in CBP

380

510

134

41

–a/3.11b

1.3

1.9

a

under N2 for solution and Ar(g) for solid state. b in air

Table 1. Photophysical properties of HBT-Cz in toluene solution, spin-coated neat film and 5wt% blend film in CBP (from chloroform solution).

PLQY and TCSPC measurements. Solution and solid-state photoluminescence quantum yields (PLQYs) of HBT-Cz were probed by using a standard quinine sulfate solution (0.5 M in sulfuric acid with a PLQY of 55%)35 and an integration sphere,36 respectively. PLQYs of HBT-Cz neat and 5wt% HBT-Cz:CBP blend films were determined to be 39% and 41%, respectively, which are significantly higher than that of solution (9%). This indicated aggregate-induced luminescence enhancement. To gain insights into this, we performed timecorrelated single photon counting (TCSPC) measurements for both solution and films at room temperature (see Figure S4, SI). By combining the measured PLQYs and lifetime data, it is possible to calculate their radiative (kr) and non-radiative (knr) decay rates,37 which are summarized in Table 1. It was found that the knr (1.9 x 108 s-1) of the films (both neat and 5wt% in CBP) were lower than that (15.6 x 108 s-1) in solution, indicating restriction of bond rotation between the benzo[d]thiazole and 3-(9H-carbazol-9-yl)phenol moieties in solid state, leading to their higher PLQYs. Additional photophysical information of HBT-Cz can be found in Table S1 (solution), Table S2 (solid-state), SI. We note that the reported high solid-


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state PLQY of 91%34 would be most likely from single crystal HBT-Cz and is not achievable in amorphous thin films.

ASE properties in solution. With an understanding of the photophysical properties, ASE properties of HBT-Cz (10-2 M in toluene) were first characterized by optically pumping the samples in a 700 µL fused quartz cuvette with 1.9 mm path length, using a nitrogen laser emitting at 337 nm, delivering 3.5 ns pulses at a repetition rate of 1–20 Hz. It is noted that high concentrations of the chromophore are typically required to achieve sufficient population of excited-states for solution lasing,38,39 where no aggregate was observed for HBT-Cz (see PLQYs and lifetimes listed in Table S1, SI). The input laser beam was focused into a stripe with dimensions of 0.5 cm x 0.008 cm. A cylindrical lens and neutral density filters were used to control the output excitation energy from the nitrogen laser. Spectral output and output intensity were collected from the edge of the cuvette using a fibre-coupled CCD spectrometer and photodiode, respectively. The ASE threshold was determined by plotting output intensity emitted from the edge of the cuvette against the input excitation intensity. Figure 3a shows ASE threshold, identified by an abrupt change in the slope of the output intensity, followed by a linear increase in output signal as the pump energy increased beyond the threshold. For pump excitation below threshold, the spectra were broad and featureless, whereas for pump excitation above the lasing threshold, a beam peak at 532 nm was narrowed (Figure 3b) with full-width at half-maximum (FWHM) of less than 6.0 nm. In a 10-2 M solution (in toluene) of HBT-Cz, an ASE threshold of 21 mJ/cm2 was observed, which is comparable to that (6.7 mJ/cm2) of a common commercial laser dye, Rh-6G (7 x 10-3 M solution in ethylene glycol), measured under the same conditions. In comparison to Rh-6G with a nearly unity PLQY (98%),40 HBT-Cz had comparably strong ASE activity despite of its much lower solution PLQY (only 9% in toluene). This is because even with low PLQY, HBT-Cz solution has a radiative rate (kr) comparable to that of Rh-6G (kr = 2.2 x 108 s-1)40 and kr is inversely 11 ACS Paragon Plus Environment

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proportional to the ASE threshold.14,16 It is important to note that no lasing characteristic was observed for the same toluene solution of the parent dye HBT. This enhanced solution lasing of HBT-Cz compared to the parent HBT can be partly explained by the higher solution PLQY (9% for HBT-Cz versus 1.5% for HBT), resulted from the carbazole attachment.

a

1.0

Normalized intensity (a.u.)

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22.1 mJ/cm2 17.5 mJ/cm2 11.1 mJ/cm2

b

0.8

0.6

0.4

0.2

0.0 450

500

550

600

Wavelength (nm)


650

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Figure 3. (a) Input-output intensity (in logarithmic-linear scale) of 10-2 M solution of HBTCz in toluene (inset: optical image of ASE beam above the threshold), (b) Solution photoluminescence spectra of HBT-Cz (10-2 M in toluene) at excitation powers bellow and above ASE threshold.

Transient absorption spectroscopy (TAS) measurements. To understand the losses incurred due to excited-state absorption in HBT-Cz , we performed microsecond transient absorption spectroscopy (TAS) measurements. Solution TAS of HBT-Cz were recorded by exciting a toluene solution (2 x 10-3 M) at 360 nm. Excited state absorption of HBT-Cz was found at 400–500 nm (Figure S5a, SI) with stimulated emission between 500–600 nm and a similarly short-lived excited state absorption feature between 400–470 nm, which we assigned to absorption of the singlet exciton. Unfortunately, the data collected from toluene solution appeared to contain artefacts possibly caused by thermal effects, arising from partial absorption of the pump by the solvent. To avoid the absorption of the pump by the solvent, the excited-state absorption spectrum of HBT-Cz was then recorded using acetonitrile as the solvent. Similar excited-state absorption features were observed as those in toluene in the 400–500 nm region with an absorption maximum at 460 nm (Figure S5b, SI), although there was much less stimulated emission. Partially deoxygenating the acetonitrile solution (by purging with argon gas for 10 minutes) resulted in no observable changes in neither the shape of the absorption nor the magnitude of transient absorption (Figure S6, SI). The results suggested that the excited-state absorption within this absorption band is negligible. Together with steady-state photoluminescence spectrum recorded in acetonitrile (Figure S7, SI), the transient absorption maxima at 460 nm was assigned to excited state enol form. Because excited-state absorption in the photoluminescent region acts as an optical loss mechanism, the minimal overlap between the excited-state absorption band (seen in toluene and acetonitrile solution) and the photoluminescent region of the keto form is highly desirable for lasing 13 ACS Paragon Plus Environment

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application. These results suggest that the large Stokes shift (≈150 nm) and minimal overlap between the excited-state absorption and photoluminescence are the keys to achieve lasing for HBT-Cz.

ASE properties in solid state. Previously ASE study29 on HBT thin films showed an increase in ASE threshold (50 µJ/cm2) when blended in a hole-transporting polymer of polyN-vinylcarbazole (PVK) host compared to that (10 µJ/cm2) in an inert polymer matrix i.e., binder polymer of polycarbonate (PC-BisZ). As our ultimate goal is to progress toward organic laser diodes, we conducted our ASE studies in a common CBP host instead of inert polymer matrix in order to facilitate practical device fabrication. Consequently, blend films of HBT-Cz in CBP at various blending ratios (5wt%, 10wt%, 15wt% and 20wt%) and neat HBT-Cz were prepared from solution-processing using chloroform as the solvent. Solid state ASE properties were characterized using a similar setup as in solution, except spin-coated films on fused silica substrates were used, and samples were kept under vacuum (10-6 torr) to prevent degradation and oxygen quenching. Output spectrum and intensity were measured from the edge of the sample. Due to the optical slab waveguide structure forming between the fused silica substrate and air, spontaneously emitted photons were waveguided inside the organic thin film and amplified by the stimulated emission. Light amplification was seen by spectral narrowing and increased output power (Figure 5) while ASE thresholds were identified from the plot of output versus input intensity (Figures 4 & 5).


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5wt% in CBP

2.5

2.0

1.5

Normalized intensity (a.u.)

3.0

Output intensity (a.u)

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1.0 0.8 0.6 0.4 0.2 450

500

550

600

650

Wavelength (nm)

1.0

0.5

Eth = 2.4 µJ/cm2

0.0 1

10

Input pump intensity (µJ/cm2)

Figure 4. Input-output intensity (in logarithmic-linear scale) of solution processed HBT-Cz blend film (5wt% in CBP). Inset: Photoluminescence (blue line) and ASE spectra (red line) of the blend film at pump input of 6 µJ/cm2. A notably low ASE threshold of 2.4 ± 0.4 µJ/cm2 (Figure 4) was found in the 5wt% HBT-Cz in CBP blend films. This ASE threshold is the lowest value reported for solution-processed ESIPT films. Prior to our study, the lowest lasing threshold of 10 µJ/cm2 was reported for its parent HBT molecule.29 However, it was blended in an inert polymer matrix (film thickness = 300 nm), making the comparison of the two materials impossible. Therefore, we conducted identical ASE studies of HBT under the same experimental conditions, using 3wt% HBT in CBP, which has equal molar ratio (6.1molar%) to 5wt% of HBT-Cz in CBP. This gave an ASE threshold of 125 µJ/cm2 for HBT (Figure S8, SI), which is significantly higher than that (2.4 µJ/cm2) of HBT-Cz with the same molar ratio in CBP. This illustrates that the attachment of a carbazole to HBT also led to a remarkable improvement in solid state lasing properties. 15 ACS Paragon Plus Environment

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Furthermore, since literature29 highlighted that the optimal doping concentration of HBT was around 26wt%, we conducted similar ASE studies of 20wt% HBT blend film in CBP and this gave an ASE threshold of 55.8 µJ/cm2 (Figure S9, SI). The ASE threshold value is still significantly higher than that of our 5wt% HBT-Cz in CBP. In contrast to HBT, increasing the blend concentrations of HBT-Cz in CBP only gave rise to a slight increase in ASE thresholds. For 10wt%, 15wt% and 20wt% BHT-Cz blend ratios, all ASE thresholds were

Low ASE Threshold and Efficient Electroluminescence from a

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