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Low-Threshold Light Amplification in Bifluorene Single Crystals: Role of the Trap States Paulius Baronas, Gediminas Kreiza, Povilas Adom#nas, Ona Adom#nien#, Karolis Kazlauskas, Jean-Charles Ribierre, Chihaya Adachi, and Saulius Jursenas ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14702 • Publication Date (Web): 26 Dec 2017 Downloaded from http://pubs.acs.org on December 26, 2017
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ACS Applied Materials & Interfaces
Low-Threshold Light Amplification in Bifluorene Single Crystals: Role of the Trap States Paulius Baronas, Gediminas Kreiza, Povilas Adomėnas, Ona Adomėnienė, Karolis Kazlauskas,* Jean-Charles Ribierre, Chihaya Adachi and Saulius Juršėnas P. Baronas, G. Kreiza, Dr. P. Adomėnas, Dr. O. Adomėnienė, Dr. K. Kazlauskas, Prof. S. Juršėnas Institute of Applied Research, Vilnius University, Saulėtekio 3, LT-10257 Vilnius, Lithuania E-mail:
[email protected] Prof. J.-C. Ribierre, Prof. C. Adachi Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan Keywords: single crystal, fluorene, amplified spontaneous emission, organic laser, fluorescence
ABSTRACT Organic single crystals (SCs) expressing long-range periodicity and dense molecular packing are an attractive amplifying medium for realization of electrically-driven organic lasers. However, amplified spontaneous emission (ASE) threshold (1-10 kW/cm2) of SCs is still significantly higher as compared to those of amorphous neat or doped films. The current study addresses this issue by investigating ASE properties of rigid bridging groups-containing bifluorene SCs. Introduction of the rigid bridges in bifluorenes enables considerable reduction of non-radiative decay, which along with enhanced fluorescence quantum yield (72-82%) and short excited state lifetime (1.5-2.5 ns) results in high radiative decay rates (~0.5 × 109 s-1) of the SCs making them highly attractive for lasing applications. The revealed ASE threshold of 400 W/cm2 in acetylenebridged bifluorene SCs is found to be among the lowest ever reported for organic crystals. Ultrafast transient absorption spectroscopy enabled to disclose pronounced differences in the excited state dynamics of the studied SCs pointing out essential role of radiative traps in 1 ACS Paragon Plus Environment
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achieving record low ASE threshold. Although the origin of the trap states was not completely unveiled, the obtained results clearly evidence that the crystal doping approach can be successful in achieving extremely low ASE thresholds required for electrically-pumped organic laser.
1.
Introduction
Organic semiconducting materials have been recognized as gain medium since the introduction of dye lasers in the late 1960s and have since been a subject of intense studies leading to applications in the fields of medicine, spectroscopy and data communication.1–5 Despite major advances in organic optoelectronic technologies, the operational electrically driven organic laser has not been demonstrated yet.4,6,7 To overcome multiple challenges originating from strong electrical pumping, organic solid-state lasers are being developed in terms of material and device design. While recent progress in designing lasers with distributed feedback (DFB) resonator structures and possessing light-emitting field effect transistor architectures allowed to significantly improve the device technology8–11, the critical issues related to low charge carrier mobility and high lasing threshold in the gain material still need to be addressed. On this point, organic single crystals (SCs) hold a great promise to be employed as a gain medium in the electrically-driven organic lasers.12,13 Superior performance of high-quality SCs is mainly caused by molecule alignment, which can ensure enhanced ambipolar carrier transport as well as attainment of critical carrier densities (up to several kA/cm2) for achieving population inversion.7,14 SCs featuring highly ordered molecular dipoles and being virtually free from light scattering facilitate light outcoupling and hold promise for reduced lasing threshold.15–17 Despite the availability of highly emissive SCs based on small molecules, the lowest attained amplified spontaneous emission (ASE) thresholds are still one order of magnitude higher as 2 ACS Paragon Plus Environment
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compared to those observed in the amorphous neat or doped films.4,18–22 Multiple factors limiting ASE threshold values in SCs are discussed in detail in recent reviews.4,12,13 The optical properties of excitons formed in molecular crystals mainly depend on the intermolecular coupling, which can be controlled via molecular packing, i.e. molecular size, separation, displacement and inclination. It was shown that strong excitonic coupling might be detrimental for ASE properties in both J- and H-type coupling cases: the former exhibiting enhanced reabsorption losses, the later resulting in low radiative decay rate. Therefore, the lowest ASE thresholds were achieved in the crystals with weak excitonic coupling, which is implemented by introduction of molecular spacers or slipped molecular arrangement.13 Weak coupling regime also inhibits exciton diffusion, and thus, reduces the impact of exciton annihilation at high exciton densities required for lasing action. Note that in high quality crystals exhibiting fluorescence quantum yield exceeding 80%, exciton annihilation can become a major non-radiative decay channel. Additionally, moderate electron-vibronic coupling (i.e. moderate Huang-Rhys factor) is desired, since it reduces the overlap between absorption and emission spectra, which translates to low reabsorption losses.23 One of the most critical factors limiting ASE performance is considered to be induced absorption to higher singlet, triplet or polaron states; hence the materials with virtually no overlap in the gain region are in demand.6,24 To overcome absorption losses doping strategy has been successfully implemented in thin films25–27 and even SCs, which have been investigated for lasing applications starting from early 1970. However, ASE threshold values are still above the lowest values recorded for undoped SCs.13,28,29 Furthermore, since the ASE threshold values are directly related to the radiative decay rate, molecules expressing short excited state lifetimes and high fluorescence quantum yield are preferred.13 In this regard, fluorene-based compounds were proved to be one of the most successful gain media for organic laser applications.4,19,20,22,27 Recently implemented rational design of bifluorene derivatives addressing all the above3 ACS Paragon Plus Environment
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mentioned issues have proved to be a successful strategy in achieving extremely low ASE thresholds (down to 700 W cm−2) in sublimation-grown SCs.17 Encouraged by the results of the previous research we designed a series of new bifluorene compounds featuring acetylene, ethylene, tolane and stilbene bridging groups of different rigidity. Thorough characterization of the optical properties of sublimation-grown SCs revealed the significant differences in their ASE performance. The influence of the bridging groups to the losses caused by excited state absorption was evaluated by transient absorption spectroscopy. Ultrafast time-resolved excited state dynamics revealed the presence of trap states in SCs, which played a key role in achieving record low ASE threshold.
2.
Experimental Section
Synthesis: Synthesis and identification of the bifluorene compounds BF-a, BF-t, BF-e and BF-s (Figure 1) is provided in the Supporting Information. Instruments and Methods: Proton and carbon nuclear magnetic resonance (1H and
13
C NMR)
spectra of the compounds in chloroform-d3 or benzene-d6 were recorded on Varian Unity Inova 300 (300 MHz) or Brucker ASCEND 400 (400 MHz) spectrometers using residual solvent signal as an internal standard. All reactions and purity of the synthesized compounds were monitored by gas chromatography using Agilent Technologies 7890A GC System with triple axis detector 5975C inert XL MSD and Agilent Technologies 6890N Network GC System, and TLC using Silica gel 60 F254 aluminum plates (Merck). Synthesized compounds were purified by preparative column chromatography using silica gel Kieselgel 60 0.06 - 0.2 mm.
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Differential scanning calorimetry (DSC) measurements were carried out using a Mettler Toledo DSC 1 thermal analyzer at a heating rate of 10 °C/min under nitrogen flow. DSC results are summarized in Table S1 in the Supporting Information. Absorption spectra of compounds in dilute solution were recorded on a UV−vis−NIR spectrophotometer Lambda 950 (Perkin-Elmer). Fluorescence spectra were measured using a back-thinned CCD spectrometer PMA-11 (Hamamatsu) and a xenon lamp coupled to a monochromator (FWHM
