TripletâTriplet Annihilation in 9,10-Diphenylanthracene Derivatives

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Triplet – Triplet Annihilation in 9,10 – Diphenylanthracene Derivatives: The Role of Intersystem Crossing and Exciton Diffusion Tomas Serevi#ius, Regimantas Komskis, Povilas Adom#nas, Ona Adom#nien#, Gediminas Kreiza, Vygintas Jankauskas, Karolis Kazlauskas, Arunas Miasojedovas, Vygintas Jankus, Andrew P. Monkman, and Saulius Jursenas J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b01336 • Publication Date (Web): 29 Mar 2017 Downloaded from http://pubs.acs.org on March 30, 2017

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The Journal of Physical Chemistry

Triplet – Triplet Annihilation in 9,10 – Diphenylanthracene Derivatives: The Role of Intersystem Crossing and Exciton Diffusion Tomas Serevičiusa*, Regimantas Komskisa, Povilas Adomėnasa, Ona Adomėnienėa, Gediminas Kreizaa, Vygintas Jankauskasb, Karolis Kazlauskasa, Arūnas Miasojedovasa,c, Vygintas Jankusc, Andy Monkmanc and Saulius Juršėnasa

a

Institute of Applied Research, Vilnius University, Saulėtekio 3, 10257, Vilnius, Lithuania

b

Department of Solid State Electronics, Vilnius University, Saulėtekio 9, 10222, Vilnius, Lithuania c

Department of Physics, Durham University, South Road, Durham, DH1 3LE, UK

*

Corresponding author: Tel. +37052234051; e-mail address: [email protected].

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ABSTRACT Triplet – triplet annihilation (TTA) is an attractive way to boost the efficiency of conventional fluorescent OLEDs. TTA-active anthracene derivatives are often considered as state-of-the-art emitters due to the proper energy level alignment. In this work, TTA properties of a series of highly fluorescent non-symmetrical anthracene compounds bearing 9-(4-arylphenyl) moiety and 10-(4-hexylphenyl) fragments were assessed. Two different methods to enhance the TTA efficiency are demonstrated. Firstly, the intensity of TTA-based delayed fluorescence directly depended on the intersystem crossing (ISC) rate. This ISC rate can be significantly enhanced in more conjugated compounds due to the resonant alignment of S1 and T2 energy levels. While enhanced ISC rate slightly quenches the intensity of prompt fluorescence, the rise of the triplet population boosts the intensity of resultant delayed fluorescence. Secondly, the triplet annihilation rate can be significantly enhanced by optimization of triplet exciton diffusion regime in the films of anthracene derivatives. We show that the proper layer preparation technology has a crucial influence on uniformity and energetic disorder of the film. This enhances the non-dispersive triplet diffusion and increases the resulting delayed fluorescence intensity.


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INTRODUCTION Nowadays organic light emitting diodes (OLEDs) became a dominating technology in the small display market and begin to wade into the large diameter display world, constantly lowering the device price. Such a breakthrough was achieved due to the constant progress of molecular functional layers, enabling the enhancement of device performance and the reduction of processing costs. The most advanced OLEDs are capable to utilize triplet excitons, which are “dark states” in conventional fluorescent OLEDs. There are two ways to convert triplet excitons to emissive singlet states without involving complex phosphorescent molecules: by triplet – triplet annihilation1 and thermally activated delayed fluorescence2 (TADF). Though TADF has been extensively studied for the last several years, TTA emitters still are highly desirable. In contrast to TADF, which mostly can be observed in materials with charge – transfer character3, showing low energy gap between the lowest singlet and triplet states, TTA can be observed in simple fused polycyclic aromatic cores such as anthracene4 or rubrene5, where the double energy of the first triplet state (T1) is larger than of the first singlet state (S1), 2 × T1 > S1. Briefly, the mechanism of TTA involves the annihilation of two triplet excitons, when an intermediate triplet pair state is formed, having singlet, triplet or quintuplet character, resulting in singlet yield of 0.26. In a special case, when the double energy of T1 is lower than the second triplet state, triplet pair state cannot be formed, thus the total singlet yield is enhanced up to 0.57, boosting the internal quantum efficiency of OLED up to 62.5% and leading to the total possible external quantum efficiency (EQE) up to about 12.5%8. Anthracene and its derivatives are model materials for triplet annihilation, often used for TTA-enhanced OLEDs, especially in the blue spectral range due to a relatively small conjugation length1. Nearly maximal values of EQE, peaking at 12%7–9, were achieved in anthracene-based OLEDs. Additionally, TTA exhibiting anthracenes are also widely used in low-power light upconversion, where low energy photons are converted to high energy by TTA10–12. This phenomenon may help to enhance the efficiency of solar cells13. 3 ACS Paragon Plus Environment


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Highly efficient TTA in molecular films requires efficient intermolecular triplet diffusion14, which can be attained by increasing molecular concentration and by tuning the molecular architecture. However, in the case of anthracene, the modification of the molecular core is known to alter its fluorescence output15. Anthracene, despite a flat and rigid backbone, has low fluorescence quantum yield in solutions (ΦF = 0.3) due efficient intersystem crossing (ISC) involving higher energy triplet states14,16. The rate of ISC can be easily tuned by the modification of the molecular backbone at the C-9 and C-10 positions, which results in the rearrangement of energy levels, such that S1 is pushed bellow T2, resulting in the decrease of the ISC rate and ΦF enhancement up to the unity17. Unfortunately, small 9,10-aryl substituents usually cannot prevent crystallization in thin films, thus more bulky non-symmetric fragments, reducing intermolecular interactions must be used18,19. However, sometimes it is observed, that the introduction of bulkier fragments tends to decrease the fluorescence efficiency20–22, though no profound analysis of processes behind and their relation to the triplet diffusion processes and the resulting delayed fluorescence (DF) efficiency have been made. In this work we present two pathways to enhance the performance of triplet – triplet annihilation based delayed fluorescence in a series of 9,10-diphenylanthracene compounds modified at C-9 and C-10 para positions. The intensity of delayed fluorescence was related with the rate of intersystem crossing, which was enhanced in more conjugated compounds. Like the rate of intersystem crossing, thin-film preparation conditions were also shown to affect TTA properties through different triplet migration regime and diffusion rate. Lower energetic disorder and more efficient triplet migration were achieved in films with lower cristallinity.

EXPERIMENTAL METHODS Solvents were purified using standard procedure. Reactions were carried out under dry argon atmosphere using magnetic stirring. All reactions and the purity of the synthesized compounds were


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monitored by gas chromatography using a 7890A GC System with triple axis detector 5975C inert XL MSD (Agilent Technologies) and 6890N Network GC System (Agilent Technologies) together with thin layer chromatography using Silica gel 60 F254 aluminum plates (Merck). Purification of synthesized materials was performed by preparative column chromatography using silica gel Kieselgel 60 0.06 – 0.2 mm. 1H and 13C NMR spectra were recorded with a INOVA spectrometer (Varian Unity) (300 MHz) or ASCEND 400 (Brucker) (400 MHz) in chloroform-d3 or benzene-d6, using the residual solvent signal as an internal standard. HRMS spectra were obtained on a mass spectrometer Dual-ESI Q-TOF 6520 (Agilent Technologies). All melting points were determined using Digital Melting Point Apparatus IA9000 Series. Differential scanning calorimetry (DSC) measurements were carried out using DSC 8500 (Perkin Elmer) thermal analysis system at a heating/cooling rate of 10 C/min under nitrogen flow. Absorption and fluorescence spectra of the DPA derivatives were assessed in dilute 10-6 M tetrahydrofuran (THF) solutions and wet-casted thin films prepared from 5 × 10-3 M THF solutions. Absorption spectra were recorded on a UV−Vis−NIR spectrophotometer Lambda 950 (Perkin-Elmer). Fluorescence spectra of all materials were collected using excitation at 365 nm from a Xe lamp (FWHM 2×ET1 > ES149. In other words, the double energy of the excited triplet should be higher than of singlet, but lower than the second excited triplet, thus only singlet states can be formed with efficiency up to 50%7,49. When the second triplet state is below 2×ET1, the result of triplet fusion should be more complex6. In this case triplet fusion would form an intermediate triplet state, which


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would have 1 of 9 possible spin configurations: 1/9 singlet character, 3/9 triplet and 5/9 quintuplet character. Since quintuplets are energetically improbable in case of acenes7, the total singlet production from fusion should yield only 0.2.

Alteration of exciton diffusion regime by the adjustment of film morphology The efficiency of delayed fluorescence was found to greatly depend on the film preparation conditions. The intensity of prompt and delayed fluorescence was at least several times higher in 50 wt% Zeonex films prepared by drop – casting on a heated glass substrate rather than on a roomtemperature substrate. Additionally, substrate temperature had a direct impact on the triplet diffusion mechanism for more disordered compounds. The delayed fluorescence of compound 2 (see Fig. 7) prepared by drop – casting on the un-heated substrate decayed following -1 slope, showing the domination of a non-equilibrium triplet diffusion regime and high energetic disorder in thin film. In contrast, drop – casting on the hot substrate lowered energetic disorder and allowed the equilibrium triplet diffusion regime with -2 decay slope to be reached at late delays. For the rest of the compounds, only an increase of the delayed fluorescence was observed. Some insight in the differences in the thin film morphology can be seen in fluorescence microscopy images (see Fig. 7 b and c) of both kinds of thin films. In general, drop – casting on the hot substrate enhances the solvent evaporation speed and reduces the probability of molecular agglomeration, thus the molecules are more evenly distributed (see Fig. 7 b). In contrast, drop – casting at room temperature reduces the solvent evaporation speed, thus the molecules have more time for diffusion and agglomeration. This can be clearly seen from the fluorescence microscopy image (see Fig. 7 c), where molecular clusters of a larger size can be clearly seen. Further, the observed molecular aggregates may be composed of smaller aggregates (even of several molecules), which are way beyond of our resolution. Different thin film preparation conditions have a direct impact on the TTA efficiency and the delayed fluorescence intensity. TTA should be more efficient in large and



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uniform molecular aggregates, since two triplets can more easily annihilate there, however less efficient triplet diffusion between the aggregates results in lower total TTA output, being the limiting factor. More even molecular distribution in films, prepared by drop-casting on hot substrate, should reduce the number and size of molecular aggregates, though the enhanced triplet diffusion length boosts the probability of triplet encounters and leads to more efficient TTA. This finding clearly shows the importance of thin film preparation conditions on the TTA efficiency.

Fig. 7 a) Log – log scale decay transients of 50 wt% Zeonex films of DPA compound 2 prepared by drop-casting on glass at room temperature (circles) and 80°C (squares). b) and c) Fluorescence microscopy images of 50% wt Zeonex films of DPA compound 2 drop – casted at room temperature and 80°C, respectively. Electroluminescence properties Several non-symmetric DPA derivatives were utilized in OLED devices as blue light-emitting dopants and hole transport materials. The structure of device was: ITO (150 nm)/x (10 nm)/50 wt% doped x in TPBi (40 nm)/BCP (30 nm)/LiF (1 nm)/ Al (100 nm), where ITO was indium tin oxide, x was the DPA compound, TPBi was 2,2′,2"-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole), BCP was 2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline and LiF - lithium fluoride. The best performance of the non-optimized OLED was for the device with compound 3 in emitting and hole transport layers (OLED1, see Fig. 8). The OLED showed good I - V characteristics with the I-V on-


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set of about 2.6 V due to the good carrier balance. Electroluminescence (EL) peaked at about 440 nm and the spectrum was identical to its fluorescence spectra (see Fig. S8 in the Supporting Information). The estimated peak EQE reached 1.84% at low current densities (around 1 mA/cm2), however large roll-off was observed at higher current densities probably due to singlet-triplet annihilation52–54. The maximum estimated brightness reached 483 cd/m2 at the highest current densities. More pronounced EQE enhancement is expected after the optimization of the device

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The analysis of EL transient revealed the existence of delayed EL lasting for up to 1 ms and exhibiting spectrum similar to that of prompt EL (see Fig. S8 in the Supporting Information). Different time-scale of delayed fluorescence and electroluminescence decay can be attributed to different triplet exciton densities and film preparation conditions. Although there is no direct evidence showing that delayed EL is due to TTA and contribution from slower charge transport due 21 ACS Paragon Plus Environment


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to defect-trapped carriers can be expected55, influence of TTA is very likely in anthracene derivatives as reported by others56,57.

CONCLUSIONS In summary, we presented triplet – triplet annihilation analysis in a series of 9,10diphenylanthracene derivatives having different substituents at C-9 position. We showed how the alteration of molecular geometry and the conjugation length enhancement affects the TTA efficiency. TTA efficiency was found to be controlled by two different pathways – by adjusting the intersystem crossing rate and triplet migration regime. The most intense delayed fluorescence was observed in the most conjugated compound with the most rapid intersystem crossing. ISC was found to boost the triplet concentration thus the TTA intensity without the significant quenching of prompt fluorescence. The enhancement of ISC rate was achieved by the precise alignment of S1 ant T2 energy levels in anthracene compounds with slightly different conjugation length. Secondly, TTA efficiency was also shown to depend on the thin film morphology. Inefficient triplet migration due to the high energetic disorder and low triplet diffusion rate was found in thin films with large molecular aggregates resulting in lower output of delayed fluorescence. Conversely, TTA efficiency was remarkably enhanced in the films with enhanced homogeneity, showing efficient triplet diffusion due to the lower energetic disorder. The presence of TTA was also demonstrated in electroluminescence transients, when the TTA-based delayed electroluminescence was observed up to about 1 ms. Although the more rapid ISC enhances the intensity of delayed fluorescence, in order to achieve efficient delayed electroluminescence, low rate of ISC should be preserved. In case of electric pumping, the formation of triplet states is almost independent on the ISC rate, thus the triplet migration properties would have the crucial importance on the TTA efficiency. As we have shown, efficient triplet migration can be achieved in low cristallinity films that can be controlled by 22 ACS Paragon Plus Environment

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the precise selection of the C-9 and C-10 fragments of anthracene core. Further optimization of DPA core with small conjugated or non-conjugated (e.g. short branched aryl chains) substituents should help to reduce the crystallization and ensure efficient TTA.

ACKNOWLEDGEMENT Denisas Sokolas is acknowledged for assistance in synthesis, Sandra Mačiulytė is acknowledged for the DSC measurements. Arūnas Miasojedovas acknowledges support by project "Promotion of Student Scientific Activities" (VP1-3.1-ŠMM-01-V-02-003) from the Research Council of Lithuania. This project is funded by the Republic of Lithuania and European Social Fund under the 2007-2013 Human Resources Development Operational Programme’s priority 3.

ASSOCIATED CONTENT SUPPORTING INFORMATION Synthetic details, detailed quantum chemical calculations, differential scanning calorimetry data, delayed fluorescence transients in different ambients, XTOF transients, electroluminescence spectra, complete Gaussian 09 reference. This material is available free of charge via the Internet at http://pubs.acs.org.

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GRAPHICAL ABSTRACT


TripletâTriplet Annihilation in 9,10-Diphenylanthracene Derivatives

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TripletâTriplet Annihilation in 9,10-Diphenylanthracene Derivatives