Near-Infrared Electroluminescence and Low Threshold Amplified

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Near infrared electroluminescence and low threshold amplified spontaneous emission above 800 nm from a thermally-activated delayed fluorescent emitter Hao Ye, Dae Hyeon Kim, Xiankai Chen, Atula S. D. Sandanayaka, Jong Uk Kim, Elena Zaborova, Gabriel Canard, Youichi Tsuchiya, Eun Young Choi, Jeong Weon Wu, Frédéric Fages, Jean-Luc Bredas, Anthony D’Aléo, Jean-Charles Ribierre, and Chihaya Adachi Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b02247 • Publication Date (Web): 29 Aug 2018 Downloaded from http://pubs.acs.org on August 30, 2018

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Chemistry of Materials

Near infrared electroluminescence and low threshold amplified spontaneous emission above 800 nm from a thermally-activated delayed fluorescent emitter Hao Ye,1,4 Dae Hyeon Kim,1,2 Xiankai Chen,3 Atula S. D. Sandanayaka,1,4 Jong Uk Kim,1,4 Elena Zaborova,5 Gabriel Canard,5 Youichi Tsuchiya,1,4 Eun Young Choi,6 Jeong Weon Wu,6,7

Frédéric

Fages,5

Jean-Luc

Bredas,3

Anthony

D’Aléo,2,5,7*

Jean-Charles

Ribierre,1,4,8* Chihaya Adachi1,4,9* 1

Center for Organic Photonics and Electronics Research (OPERA), Kyushu University,

744 Motooka, Nishi, Fukuoka 819-0395, Japan 2

Center for Quantum Nanoscience, Institute for Basic Science (IBS), Seoul 03760,

Republic of Korea 3

School of Chemistry & Biochemistry, Center for Organic Photonics and Electronics,

Georgia Institute of Technology, Atlanta, Georgia 30332-0400, USA 4

Japan Science and Technology Agency (JST), ERATO, Adachi Molecular Exciton

Engineering Project, c/o Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan 5

Aix Marseille Univ, CNRS, CINaM UMR 7325, Campus de Luminy, Case 913, 13288

Marseille, France 6

7

Department of Physics, Ewha Womans University, Seoul, Korea Building Blocks for Future Electronics Laboratory (2-B FUEL), The joint

CNRS-Ewha-Yonsei Laboratory, UMI 2002, Seoul, Republic of Korea

1

ACS Paragon Plus Environment

Chemistry of Materials 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

8

State-key laboratory of Modern optical instrumentation, College of Optical Science and

Engineering, Zhejiang University, 38 Zheda road, Hangzhou 310027, China 9

International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu

University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan

Abstract: Near infrared (NIR) organic light-emitting devices have aroused increasing interest because of their potential applications such as information-secured displays, photodynamic therapy and optical telecommunication. While thermally activated delayed fluorescent (TADF) emitters have been used in a variety of high-performance organic light-emitting diodes (OLEDs) emitting in the visible spectral range, efficient NIR TADF materials have been rarely reported. Herein, we designed and synthesized a novel solution-processable NIR TADF dimeric borondifluoride curcuminoid derivative with remarkable photophysical, electroluminescence and amplified spontaneous emission properties. This dye was specifically developed to shift the emission of borondifluoride curcuminoid moiety toward longer wavelengths in the NIR region while keeping a high photoluminescence quantum yield. The most efficient OLED fabricated in this study exhibits a maximum external quantum efficiency of 5.1% for a maximum emission wavelength of 758 nm, which ranks amongst the highest performance for NIR electroluminescence. In addition, this NIR TADF emitter in doped thin films displays amplified spontaneous emission above 800 nm with a threshold as low as 7.5 µJ/cm2, providing evidence that this material is suitable for the realization of high performance NIR organic semiconductor lasers. 2


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INTRODUCTION Near infrared (NIR) organic light-emitting diodes (OLEDs) have attracted increasing interest in recent years due to their potential applications in bioimaging, chemosensing, night-vision devices and information-secured displays.1, 2 Different kinds of emitters have been investigated for NIR electroluminescence (EL), which include conjugated

polymers,3,

4

lanthanide-metal complexes,8,

organic 9

π-conjugated

donor-acceptor

molecules,5-7

phosphorescent metal complexes10-12 and colloidal

inorganic quantum dots.13, 14 The external electroluminescence quantum efficiency (ηEQE) values reported for NIR fluorescent organic emitters including both solution-processed polymers and thermally-evaporated small molecules remain below 2.1%.5, 15 Lanthanide based NIR OLEDs with extremely sharp emission bands between 800 and 1600 nm have also been demonstrated but their ηEQE values remain still lower than 0.05%.8 While high-efficiency solution-processable inorganic quantum dots have been used in LEDs emitting in the visible region with ηEQE close to 20%,16,

17

their electroluminescence

performances in the NIR are not as good, with the highest ηEQE value of 8.6% obtained using silicon nanocrystals quantum dots emitting at about 850 nm.18 The best NIR OLED performance achieved so far have been obtained using phosphorescent heavy metal complexes. In particular, a new class of phosphorescent platinum(II) complexes was recently proposed and used in NIR OLEDs with an ηEQE value over 24%.10 However, due to the issues related to the shortage and high cost of heavy metals as well as the serious efficiency roll-off taking place generally at high current densities in phosphorescent OLEDs, the development of novel metal-free NIR fluorescent emitters that can provide 3



high photoluminescence quantum yield (PLQY) and overcome the spin statistics limit of conventional fluorescent dyes is of great importance. Over the last few years, thermally activated delayed fluorescence (TADF) emitters have been the subject of intensive studies and are now considered as the third generation of OLED materials.19 High-performance TADF OLEDs with 100% internal quantum efficiency have been already reported in the visible spectral range, providing evidence of the strong potential of these purely organic emitters for display and lighting applications.20 The molecular architecture of most efficient TADF OLED emitters developed so far is based on twisted donor-acceptor systems exhibiting lowest singlet and triplet excited states with a strong intramolecular charge-transfer character. This can lead to a small spatial overlap resulting in small energy gap ∆EST between the first singlet excited state (S1) and the first triplet excited state (T1). Such an excited-state electronic structure greatly facilitates the reverse intersystem crossing (RISC) process and the triplet harvesting in TADF OLEDs. It is important to mention that a fine molecular design is required to tune the conjugation between the acceptor and donor moieties in these twisted donor-acceptor TADF systems, which is usually achieved by controlling their dihedral angle. This is a critical aspect to obtain simultaneously an effective RISC process and a large fluorescence decay rate (kf). While this strategy has proven to be successful for TADF OLEDs in the visible spectral range, the development of such twisted donor-acceptor TADF emitters remains challenging for high efficiency far red and NIR EL.1 This is essentially due to the fact that PLQY tends to decrease as the emission is red-shifted according to the energy-gap law (which contributes to faster non-radiative recombination rates) and also because of 4


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stronger aggregation-induced quenching effects occurring in NIR emitting thin films. Recently, we reported on NIR TADF OLEDs with a maximum ηEQE value close to 10% for a maximum emission wavelength of 721 nm using a solution-processable donor-acceptor-donor borondifluoride curcuminoid derivative 2 (Fig. 1).21 The remarkable NIR TADF properties of this compound were explained by a nonadiabatic coupling effect21 taking place between its low lying excited states. The high PLQY values (up to 70 %), and large radiative decay rates measured in the NIR emitting films were associated with a substantial spatial overlap between the hole and electron wavefunctions characteristic of the optical transition. In addition to its strong potential for high performance NIR TADF OLEDs, this curcuminoid derivative was found to be also promising for organic NIR semiconductor lasers, as a direct consequence of the large PLQY and fast radiative decay rate of this emitter.21 This latter finding is extremely relevant for further works devoted to the realization of continuous wave organic solid-state lasers and organic laser diodes.9, 22-24 In this manuscript, we report on a new solution-processable dimeric borondifluoride curcuminoid derivative 1 (see its chemical structure in Fig. 1) containing triphenylamine donor groups and acetylacetonate borondifluoride acceptor units. This molecule 1 has an electron withdrawing group in meso position instead of the ester function in 2. While the efficiency of the electron withdrawing properties of the ester in 2 and the acetylacetone borondifluoride appended in meso position in 1 is similar, strong excitonic coupling effects taking place in 2 should result in a substantial shift of the emission toward longer wavelengths. Such a molecular structure is therefore expected to lead to a high-efficiency NIR TADF emission and a low threshold amplified spontaneous 5



emission (ASE) further in the NIR as compared to those of 2. The results will demonstrate that the original molecular design of 1 offers indeed the possibility to extend the emission wavelength of NIR TADF OLEDs and organic semiconductor lasers above 800 nm while keeping high EL and ASE/lasing performances. Importantly, the unprecedented demonstration of ASE at wavelengths longer than 800 nm in an organic semiconductor thin film should have a strong impact on future research devoted to organic solid state lasers and optical amplifiers operating at telecommunication wavelengths. EXPERIMENTAL SECTION Details on the synthesis, purification and characterization of the chemical properties of compound 1 are provided in the SI section (see Fig. S1-S4). For the characterization of its photophysical properties in solution, molecule 1 was dissolved in solvents having different polarities at a concentration around 10-6 M and the solutions were placed in quartz cuvettes. All solvents used for these photophysical measurements were of spectroscopic grade. For the characterization of the photophysical and ASE properties in thin films, the films were deposited onto precleaned fused silica substrates by spin-coating from chloroform solution. The thickness of these films were measured using a Dektak profilometer and found to be typically around 120 nm. Absorption spectra in both solution and film were recorded using a Varian Cary 5000 spectrometer and a UV-2550 spectrometer (Shimadzu), respectively. Steady-state PL spectra were obtained using a Fluoromax-4 spectrophotometer (Horiba Scientific) and corrected following the procedure reported by Parker et al.25 to take into account the 6


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response of the detector.26 The PLQY values in solution were measured using ruthenium trisbipyridine bischloride (PLQY of 0.021 in water) as a reference.27 For the thin films, PLQYs were determined using an integrating sphere system coupled with a photonic multichannel analyzer (Hamamatsu Photonics C9920-02, PMA-11) under a flowing nitrogen atmosphere.28 For the time-resolved PL measurements at room temperature, thin films were placed in vacuum (< 4 x 10-1 Pa). A nitrogen laser (Ken-X, Usho Optical systems) delivering 500 ps excitation pulses with a wavelength of 337 nm and a repetition rate of 20 Hz was used for photo-excitation. The transient PL emission was recorded using a streak camera (Hamamatsu Photonics C4334). Another experimental setup was used to properly investigate the prompt fluorescence process taking place at shorter time scale. For this purpose, a Noncolinear Optical Parametric Amplifier (NOPA) pumped by a fiber femtosecond laser (Amplitude Systems Tangerine) and a streak camera (Hamamatsu PLP system) with a time resolution around 10 ps were used. In those measurements, the pulse width and the repetition rate of the photo-excitation at 560 nm were 100 fs and 10 kHz, respectively. For the characterization of the ASE properties, the samples in nitrogen atmosphere were photo-excited by a nitrogen laser (excitation wavelength of 337 nm, pulse width of 800 ps and repetition rate of 8 Hz). A set of neutral density filters were used to vary the energy of the excitation pulses. The excitation beam was focused using a cylindrical lens into a stripe of dimension 0.5 cm x 0.08 cm. The emission was detected from the edge of the organic film using an optical fiber connected to a charge-coupled device spectrometer 7



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(PMA-11, Hamamatsu Photonics). For

the

fabrication

of

OLEDs,

poly(3,4-ethylenedioxythiophene):

poly(styrenesulfonate) (PEDOT:PSS) was purchased from Heraeus CleviosTM while 4,4’-bis(N-carbazolyl)–1,10-biphenyl

(CBP),

2,2’,2’’-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole)

(TPBi)

and

bis{2-[di(phenyl)phosphino]-phenyl ether oxide (DPEPO) were supplied by TCI chemicals. The PEDOT:PSS layers were spin-coated on top of cleaned prepatterned ITO glass substrates and then annealed at 180 °C for 30 min.. CBP:1 blends with different doping concentrations were spin-coated on top of the PEDOT:PSS layer from a chloroform solution. DPEPO and TPBi films were then thermally evaporated and cathodes consisting of 1 nm thick LiF and 100 nm thick Al were finally deposited by thermal evaporation to complete the devices. The active area of the devices was designed to be 4 mm2. Importantly, the devices were encapsulated in a glove box filled with nitrogen to avoid contamination by oxygen and moisture. For OLED characterization, current (J)-voltage (V)- luminance (L) characteristics were obtained under direct current (DC) driving using a source meter (Keithley 2400, Keithley Instruments Inc.) and external quantum efficiency measurement were performed using a C9920-12 Hamamatsu Photonics detector. The EL spectra were recorded using an optical fiber connected to a spectrometer (PMA-12, Hamamatsu Photonics). Similarly to what was done for the PL spectra, the spectroscopic sensitivity factor of the detector was taken into account to obtain the corrected emission EL spectra following the procedure reported by Parker et al.25 In addition, we have carried out quantum-chemical calculations to characterize the 8


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geometric and electronic structures of molecule 1. The initial geometry of the dye molecule was optimized with the range-separated functional ωB97XD, using the default range-separation parameter ω of 0.2 bohr-1 and the 6-31G(d,p) basis set.29 Then, an iteration procedure was employed to non-empirically tune the ω parameter with the implicit consideration of the dielectric environment via the polarizable continuum model (PCM); the dielectric constant ε was chosen to be 4.0, a representative value of nonpolar organic semiconductor materials. The Tamm-Dancoff approximation (TDA) in the framework of time-dependent density functional theory (TD-DFT) was employed to study the excited-state properties; all the excited-state properties of the molecule were examined at the TDA-tuned-ωB97XD/6-31G(d,p) level. All the quantum-chemical calculations were performed with the Gaussian 09 Rev D01 program.30 The spin-orbit couplings (SOC) were estimated by employing the Breit-Pauli spin-orbit Hamiltonian with an effective charge approximation implemented in the PySOC code.31 RESULTS AND DISCUSSION The ionization potential and electron affinity of 1 were estimated from cyclic voltammetry measurements. The results reported in Fig. S5 lead to values of +5.6 and +3.7 eV, respectively. This is consistent with the ionization potential of 5.6 eV obtained from photoelectron spectroscopy measurements in air and the electron affinity (LUMO energy) of +4 eV approximated using the optical bandgap. It is expected that the electron affinity obtained by electrochemistry is slightly different from the one estimated using the optical band gap, since the latter neglects any exciton binding energy and the dielectric media are different. We note that the ionization potential and electron affinity of 1 are nearly identical 9



to those previously reported for 2.21 The quantum-chemical calculations carried out earlier for molecule 2 allowed us to rationalize the mechanism of its TADF activity.21, 32 It was demonstrated that the TADF process in this molecular system occurs through nonadiabatic (vibronic) coupling effect between two low-lying excited states. Such an effect mediated by molecular vibrations has been found to play a major role in intersystem crossing upconversion in a class of donor-acceptor-donor molecules.32 In the case of 2, several low frequency vibration modes, related to the rotations of the terminal phenyl groups, were identified. These rotations induce a near degeneracy of the S1 and S2 states as well as the T1 and T2 states, which leads to: (i) a decrease in the ∆EST values; (ii) a strong vibronic coupling between the S1 and S2 states and between the T1 and T2 states; and (iii) the opening of T1 to S2 and T2 to S1 upconversion channels due to larger spin-orbit couplings. Here, the very large size of the dye 1 forced us to carry out quantum-chemical calculations only at the ground-state optimized geometry (since excited-state geometry optimizations were computationally prohibitive). The results are summarized in Fig. 2. The substantial overlap of the S1-state hole and electron wavefunctions on the borondifluoride curcuminoid backbones is consistent with what has been observed in 2. This feature is associated with a high S0-S1 transition dipole moment and thus a fast fluorescence radiative decay rate, which is necessary for efficient ASE/lasing activity. Given that we are able to consider here only the ground-state optimized geometry, the estimated ∆EST is high, on the order of 300 meV, while the spin-orbit coupling (SOC) between the S1 and T1 states is found to be 0.13 cm-1, which would lead to weak reverse intersystem crossing rate. However, considering the 10


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similarities in the chemical structures of the 1 and 2 dyes, the nonadiabatic coupling effects that have been evidenced in 2 should also take place in 1, since these effects are directly related to the vibrations of the triphenylamine groups that are present in both curcuminoid derivatives. The photophysical properties of the dimeric curcuminoid derivative were investigated at low concentration (

Near-Infrared Electroluminescence and Low Threshold Amplified

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