Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
Design and Synthesis of Monodisperse Macromolecular Starbursts Based on a Triazine Center with Multibranched Oligofluorenes as Efficient Gain Media for Organic Lasers Cheng-Fang Liu,† Ming Sang,† Wen-Yong Lai,*,† Ting−Ting Lu,† Xu Liu,† and Wei Huang†,‡ †
Key Laboratory for Organic Electronics and Information Displays (KLOEID) & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing University of Posts & Telecommunications, 9 Wenyuan Road, Nanjing 210023, China ‡ Shaanxi Institute of Flexible Electronics (SIFE), Northwestern Polytechnical University (NPU), 127 West Youyi Road, Xi’an 710072, Shaanxi, China S Supporting Information *
ABSTRACT: A set of monodisperse macromolecular starbursts which are composed of triazine center substituted by heterogeneous carbazole and diphenylamine units with multibranched oligofluorenes (TF1 and TF2) have been designed, synthesized, and explored as gain media for organic lasers. The thermal, morphological, photophysical, and electrochemical properties and optical gain characteristics of the resulting starbursts have been investigated in comparison with those of their linear counterparts (2FCz and 4FCz) to shed light on better understanding the structure−property relationships. The results manifest that the resulting starburst architectures based on a triazine center with multibranched oligofluorenes are beneficial for depressing the crystallization tendency, leading to enhanced amorphous morphologies, excellent thermal stabilities, and favorable facile solution processability. The novel heterogeneous donor−acceptor core structure based on triazine center plays a key role to dominate the electronic properties of the resulting multibranched starbursts and endows the molecules with low-lying LUMO energy levels. Promising amplified spontaneous emission (ASE) characteristics are recorded for the multibranched starbursts, in which TF1 and TF2 exhibit rather low ASE threshold (EthASE) of 4.3 and 10.3 μJ/cm2, respectively. Remarkably, TF1 and TF2 manifest enhanced ASE stability with no obvious spectral variations (within 2 nm for TF1 and 1 nm for TF2) and almost unchanged EthASE upon increasing the annealing temperature even up to 200 °C in air. In contrast, their linear counterparts 2FCz and 4FCz showed distinct EthASE variations with increasing the annealing temperature above 100 °C. The results suggest that the novel molecular design is beneficial for enhancing the thermal and optical stabilities as well as fine modulating the electrical properties, rendering the resulting multibranched triazine-centered starbursts advantageous as efficient gain media for electrically pumped organic lasers.
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INTRODUCTION
electrical pumping, a relatively high current injection is a prerequisite to achieve population inversion for OSLs. Although a variety of existing organic semiconductors exhibit good performance as light emitters for OLEDs, they generally fall short of carrier mobility and gain stability for attempting
Organic semiconductor materials, which combine novel optoelectronic properties with facile tunability of the chemical structures to achieve desired functions, have been intensely explored for organic light-emitting diodes (OLEDs),1,2 organic field-effect transistors (OFETs),3,4 organic photovoltaic cells (OPVs),5,6 etc. A particular challenge is as gain media for organic semiconductor lasers (OSLs).7 Especially, electrically pumped organic lasing has not been achieved yet.8 To attempt © XXXX American Chemical Society
Received: October 14, 2017 Revised: January 15, 2018
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DOI: 10.1021/acs.macromol.7b02204 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules Scheme 1. Synthetic Routes toward the Multibranched Starbursts TFx and the Linear Counterparts xFCza
Reagents and conditions: (a) n-BuLi, THF, 0 °C; (b) n-BuLi, THF, reflux; (c) 1,4-dioxane, 2 M K2CO3, Pd(PPh3)4, 100 °C; (d) toluene, 2 M K2CO3, Pd(PPh3)4, 100 °C.
a
fluorene arms.16−23 Good electroluminescence, superior optical gain, and excellent lasing performance with attractive optical gain properties have been demonstrated, showing great promise for organic lasing applications.19−23 For instance, a monodisperse six-armed macromolecular starburst based truxene core and oligofluorene arms showed promising optical gain characteristics with intriguing low lasing thresholds.19 Nevertheless, the electrical properties of this set of macromolecular starbursts have not been much improved, which is an essential prerequisite for attempting electrically pumped organic lasing. Further optimizing the molecular design is necessary. In this work, we report our attempt to explore efficient gain media with superior thermal, optical, and electrical properties to address the challenges for OSLs. A novel set of monodisperse macromolecular starbursts which are composed of triazine center substituted by heterogeneous carbazole and diphenylamine units with multibranched oligofluorenes (TFx) were designed, synthesized, and characterized. Triazine was selected as the center for its electron-withdrawing nature, excellent thermal stability, and favorable electron-transport abilities.27−30 Strong electron-donating carbazole and diphenylamine units were attached onto the triazine center to construct heterogeneous donor−acceptor core structure with the aim to fine modulate the electrical properties of the materials and induce enhanced amorphous morphological properties. Oligofluorenes with varying conjugation length were chosen as the branches owing to their high quantum efficiencies as well as excellent thermal and optical stability.31−33 The thermal, morphological, photophysical, and electrochemical properties and optical gain characteristics of the resulting multibranched
electrically pumped OSLs.9−11 The search for robust organic emitters with superior thermal, optical, and electrical properties as gain media for OSLs is highly demanded. Revealing the insight into the structure−function relationships is the key to shed light on the rational design and synthesis of highperformance organic gain media. From a fundamental viewpoint, the molecular design is vital to achieve efficient stimulated emission for organic emitters. Fluorene-based oligomers and polymers have been widely explored as promising gain media for OSLs due to their high quantum efficiencies, excellent thermal stability, good lasing properties, and facile chemical modification.12 Unfortunately, the relatively poor optical stability and the undesirable redshifted emission under thermal or electrical conditions have largely limited their further potential applications. Various chemical approaches have been developed to stabilize the emission, such as introducing spiro-configuration or bulky substitution,12,13 incorporating arylamine units (i.e., diphenylamine, triphenylamine, or carbazole),14,15 or constructing sterically demanding starburst architectures.16−19 Particularly, monodisperse macromolecular starbursts based on fluorene units have attracted much interest. This kind of material combines both the advantages of small molecules and conjugated polymers, possessing well-defined chemical structures, high purity, excellent reproducibility, good solution processability, and superior optoelectronic properties, which have been proven to be rather promising light emitters.16−26 In our previous contributions, we have explored a novel series of monodisperse starburst conjugated macromolecules based on truxene, triazatruxene, or pyrene cores with various oligoB
DOI: 10.1021/acs.macromol.7b02204 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
patterns, which showed weak and broad halos at ∼2θ = 20° as depicted in Figure 1 for TFx. The results manifested that
starbursts have been investigated in comparison with those of their linear counterparts (xFCz) to shed light on better understanding the structure−function relationships. As a result, rather low ASE thresholds of 4.3 and 10.3 μJ/cm2 in neat films were recorded for TF1 and TF2, respectively. Remarkably, TF1 and TF2 manifest enhanced ASE stability with no obvious spectral variations (within 2 nm for TF1 and 1 nm for TF2) and almost unchanged ASE thresholds upon increasing the annealing temperature even up to 200 °C, in sharp contrast to their linear counterparts (2FCz and 4FCz).
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RESULTS AND DISCUSSION Synthesis and Characterization. Synthesis of the triazinecentered multibranched starbursts is depicted in Scheme 1. To enhance the amorphous morphological properties of the resulting multibranched starbursts, asymmetric heterogeneous donor−acceptor core structure 2 based on triazine with one carbazole and two bis(bromophenyl)amine units was designed and synthesized. The synthesis of 2 was accomplished by a consecutive two-step synthetic procedure, in which sequent nucleophilic substitution of the cyanuric chlorine with 2,7dibromo-9H-carbazole and bis(4-bromophenyl)amine, respectively, was carried out in the presence of n-BuLi. The reaction was proceeded under precise control of the molar equivalents for the starting materials. In this case, the asymmetric molecule 2 substituted with one carbazole and two bis(bromophenyl)amines was isolated as the main product (≥85%), while the yield of the bicarbazole-substituted counterpart was quite low (≤10%) and the tricarbazole-substituted counterpart (
