Amplification of Circularly Polarized Luminescence through Triplet

Photo upconversion based on triplet–triplet annihilation is a promising ... 377 and 396 nm which would be the transition from S0 to S1 (Figure S1 an...
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Amplification of Circularly Polarized Luminescence through Triplet− Triplet Annihilation-Based Photon Upconversion Jianlei Han,†,‡ Pengfei Duan,*,‡ Xianggao Li,*,† and Minghua Liu*,‡,§ †

School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, P. R. China CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence in Nanoscience, Division of Nanophotonics, National Center for Nanoscience and Technology (NCNST), No. 11 ZhongGuanCun BeiYiTiao, Beijing 100190, P. R. China § Beijing National Laboratory for Molecular Science, CAS Key Laboratory of Colloid, Interface and Chemical Thermodynamics, Institute of Chemistry, Chinese Academy of Sciences, No. 2 ZhongGuanCun BeiYiJie, Beijing 100190, P. R. China ‡

S Supporting Information *

complexes.1b Various strategies have been proposed to increase the luminescence dissymmetry factors. For example, Kawai and co-workers have demonstrated that supramolecular selfassembly could remarkably enhance the glum in chiral perylene bisimide systems.9 The value of glum exhibited obvious increase from 0.003 in molecularly dispersed state to 0.008 of high concentration in chloroform, and then reached to 0.018 when helical fibers formed.9b Tang et al. had amplified glum by introducing the concept of aggregation-induced emission (AIE).10 They have observed that a chiral AIE-active molecule, upon forming the helical assemblies, emitted right-handed circularly polarized light with large CPL dissymmetry factors of 0.08−0.32. The largest dissymmetry factor of −0.32 for pure organic compounds was achieved so far from the sample formed in Teflon-based microfluidic channels.10 Very recently, we have found that Förster resonance energy transfer (FRET) in a hybrid system could significantly amplify the glum value, which might be due to the energy transfer dominated coassembly of donor and acceptor.11 Herein, we demonstrate a new approach for amplifying luminescence dissymmetry factor through the photon upconversion process. Photo upconversion based on triplet−triplet annihilation is a promising process for its high efficiency, noncoherent excitation source and tunable excitation and emission wavelengths.12 Triplet−triplet annihilation-based photon upconversion (TTAUC) has a wide variety of applications such as photocatalysis,13 solar cell energy conversion,14 photodynamics therapy15 and bioimaging.16 Generally, in the TTA-UC process, the triplet state of sensitizer is populated by intersystem crossing (ISC) after excited by lower energy photons, then triplet−triplet energy transfer from sensitizer to acceptor give the triplet state of acceptor, in the last, triplet−triplet annihilation of two acceptor triplets generate one ground state of acceptor and one excited singlet state acceptor, which will irradiate the upconverted fluorescence (Figure 1). Despite the recent advances in this field, no research of upconverted circularly polarized luminescence in the TTA-UC system has been reported so far. In this work, we demonstrate circularly polarized luminescence in photon upconversion system. The

ABSTRACT: Amplification of circularly polarized luminescence (CPL) is demonstrated in a triplet−triplet annihilation-based photon upconversion (TTA-UC) system. When chiral binaphthyldiamine acceptors are sensitized with an achiral Pt(II) octaethylporphine (PtOEP) in solution, upconverted circularly polarized luminescence (UC-CPL) were observed for the first time, in which the positive or negative circularly polarized emission could be obtained respectively, following the molecular chirality of the acceptors (R/S). More interestingly, one order of magnitude amplification of the dissymmetry factor glum in UC-CPL was obtained in comparison with the normal promoted CPL. The multistep photophysical process of TTA-UC including triplet−triplet energy transfer (TTET) and triplet−triplet annihilation (TTA) have been suggested to enhance the UC-CPL, which provided a new strategy to design CPL materials with a higher dissymmetry factor.

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hiroptical functional materials showing circularly polarized luminescence (CPL) have been attracting increasing attentions1 in recent years for their wide potential applications in 3D optical displays,2 encrypted transmission and storage of information,3 bioencoding,4 photoelectric devices5 and even catalysts for asymmetric photochemical synthesis.6 In developing CPL materials, a key issue is to obtain high luminescence dissymmetry factor (glum), which is used to quantify the level of CPL, glum = 2 × (IL − IR)/(IL + IR), where IL and IR are the intensity of the left- and right-handed circularly polarized emissions, respectively. The maximum value of |glum| is 2, which means completely left or right circularly polarized light.1b,7 Although the highest glum was achieved from chiral lanthanide complexes,8 due to electric dipole forbidden transition and the small emission efficiency of these complexes, an interest in developing CPL active organic molecules is growing. Particularly, chiral small organic molecules formed a wide library of valuable CPL candidates for their high quantum yields, tunable wavelengths, easy derivatization and excellent processability.1b,c,7b Unfortunately, the glum of organic molecules is within the range of 10−5−10−3, much smaller than the lanthanide © 2017 American Chemical Society

Received: May 5, 2017 Published: July 7, 2017 9783

DOI: 10.1021/jacs.7b04611 J. Am. Chem. Soc. 2017, 139, 9783−9786

Communication

Journal of the American Chemical Society

Figure 2. (a) CD spectra of R-BINDPA and S-BINDPA in toluene (c = 5.0 × 10−4 M). (b) CPL spectra of R-BINDPA and S-BINDPA in toluene (c = 5.0 × 10−4 M, λex = 360 nm).

CD curves. Both enantiomers showed positive and negative CPL signals respectively around 450 nm with a dissymmetry factor of |glum| = 2 × 10−4. The sign of the CPL signal was consistent with the first Cotton band observed in the CD spectra. That is to say a positive signal was observed for SBINDPA and negative for R-BINDPA. It is well-known that DPA is an excellent acceptor in TTAUC due to the stable emission efficiency and relatively low triplet energy.20 Here, the enantiopure molecules have the almost identical photophysical property with DPA but a chiral structure. We selected S-BINDPA as acceptor in TTA upconversion coupled with PtOEP as a sensitizer. First of all, upconversion spectra with different incident power density of 532 nm was investigated, as shown in Figure 3a. The

Figure 1. (a) Schematic representation CPL emission. Promoted CPL from R/S-BINDPA excited at 360 nm with |glum| = 2 × 10−4 and upconverted CPL from R/S-BINDPA-PtOEP mixture excited by 532 nm with |glum| = 4 × 10−3. (b) Schematic representation of upconverted circularly polarized luminescence (UC-CPL) based on triplet−triplet annihilation. The donor molecules PtOEP are excited by 532 nm, after a sequence of triplet−triplet energy transfer from the donor to acceptor, triplet energy migration among the acceptor molecules, and triplet−triplet annihilation, the enantiopure acceptors emit CPL light.

CPL active enantiomers based on binaphthyldiamine moiety with two 4-(10-phenylanthracen-9-yl)phenyl groups were used as acceptors. Coupling with an achiral sensitizer Pt(II) octaethylporphine (PtOEP) in toluene, upconverted circularly polarized luminescence were recorded for the first time (Figure 1). More importantly, an amplification of luminescence dissymmetry factor glum was observed through such process, providing new insight into the design of the organic CPL materials. Biaryls with axial chirality tethering achiral π-conjugated chromophores have been investigated extensively to exhibit CPL.9b,17 This strategy worked very well to produce pure enantiomers with high fluorescence quantum yield. The enantiopure molecules, R-BINDPA and S-BINDPA were synthesized directly from the reaction of the corresponding 1,1′-binaphthyl-2,2′-diamine and 9-(4-bromophenyl)-10-phenylanthracene by palladium-catalyzed coupling reactions. After recrystallized from dichloroform, a high yield of 92% was obtained (see SI). Photophysical studies were first conducted on S-BINDPA in toluene at a concentration of 1.0 × 10−5 M. Absorption spectrum showed intense bands corresponding to the anthracene core with characteristic vibronic bands at 357 nm, 377 and 396 nm which would be the transition from S0 to S1 (Figure S1 and Table S1).18 Because of the electrondenoting behavior of nitrogen atom, fluorescence spectra of BINDPA shows a broad structureless band with maximum wavelength at 447 nm and absolute fluorescence quantum yield is ∼50% with a short lifetime about 2.2 ns (Figure S1 and Table S1). As expected, with the axial chirality binaphthyl linker, the enantiomers R-BINDPA and S-BINDPA showed almost identical absorption and emission spectra but with mirrorimaged circular dichroism (CD) and CPL signals (Figure 2).19 S-BINDPA showed a positive Cotton effect at 377 nm originating from S0 to S1 transition, whereas the negative Cotton effect at 307 and 283 nm could be assigned to S0−S2 transition. As expected, R-BINDPA exhibited mirror-imaged

Figure 3. (a) Upconversion emission spectra of S-BINDPA/PtOEP with different incident power density of 532 nm laser in deaerated toluene. (b) Double-logarithmic plots of the UC emission intensity of S-BINDPA/PtOEP as a function of excitation intensity of the green laser (532 nm). (c) UC quantum yield ΦUC of S-BINDPA/PtOEP in toluene solution as a function of excitation intensity of the green laser (532 nm). (d) Time resolved upconverted emission at 450 nm of the S-BINDPA/PtOEP pair (black square), the tail fit (red circle) and instrument response function (green), ([S-BINDPA] = 2 mM, [PtOEP] = 0.01 mM, λex = 532 nm).

phosphorescence intensity at 650 nm of PtOEP is very weak, indicating that the triplet excited state of PtOEP transferred its energy to S-BINDPA efficiently. The dependence of upconverted emission intensity on the excitation power density was shown in Figure 3b. The blue and red lines are the fitting results with slopes of 2.0 and 1.2 in the low and high excitation intensity ranges, respectively. And it demonstrated the TTA process (Figure S2).21 A Ith of 52 mW cm−2 was obtained, 9784

DOI: 10.1021/jacs.7b04611 J. Am. Chem. Soc. 2017, 139, 9783−9786

Communication

Journal of the American Chemical Society

indirectly either by inter- or intramolecular radiationless energy transfer.7a The radiationless energy transfer process could significantly influence the μgn with a negative effect.7a This could well explain the amplification of glum in TTA-UC systems. On the other hand, according to eq 1, the perturbation of magnetic dipole transition moment (mgn) in the excited state should not be ignored in the TTA-UC process due to the significant change of electron spin polarization. It has been observed that electron spin polarization could be caused by triplet−triplet annihilation.24 This inherent property of the TTA process could significantly amplify the magnetic dipole transition moment, which would be a positive factor to increase the glum of circularly polarized emission. This would also lead to a relatively large UCPL dissymmetry factor. In summary, two enantiopure axially chiral molecules consisting of two diphenylanthracene groups were synthesized and used as acceptor in TTA photon upconversion system. Upconverted circularly polarized luminescence was demonstrated for the first time in the TTA-UC system. More importantly, photon upconversion process could amplify the luminescence dissymmetry factor compared with normal promoted fluorescence. The integration of TTA-UC with CPL thus provides a new frontier in chiral functional materials. The concept of energy transfer amplified CPL emission would be widely applicable to design functional molecular systems in many disciplines.

above which TTA becomes the main triplet deactivation channel for the acceptor, and the UC quantum yield becomes saturation.22 A saturation quantum yield of about 11% was obtained for S-BINDPA/PtOEP by using Rhodamine B in ethanol as a standard and with the theoretical maximum of 50% (Figure 3c and SI).20a,22 In addition, a long UC emission lifetime at 450 nm was observed (τ = 284 μs, Figure 3d), which is longer than the phosphorescence lifetime of PtOEP of 65 μs (see SI Figure S3). On the other hand, the lifetime at 650 nm for the S-BINDPA/PtOEP pairs was less than 1 μs (see SI Figure S3), which indicated the fast and efficient TTET process in this system. The most interesting result in this work is the upconverted circularly polarized luminescence. For the first time, photon upconversion based CPL was observed in the UC pairs of SBINDPA/PtOEP and R-BINDPA/PtOEP (Figure 4). As

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Figure 4. (a) UC-CPL emission spectra of S-BINDPA/PtOEP with different incident power density of 532 nm laser in deaerated toluene. A 532 nm notch filter was used. (b) UC-CPL dissymmetry factor glum versus wavelength. ▲, S-BINDPA/PtOEP excited by 532 nm laser; Δ, S-BINDPA excited by 360 nm laser; ▼, R-BINDPA/PtOEP excited by 532 nm laser; ▽, R-BINDPA/PtOEP excited by 360 nm laser. ([BINDPA] = 2 mM, [PtOEP] = 0.01 mM). For all the UC-CPL measurements, a 532 nm notch filter was used.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b04611. Experimental details, additional photophysical data, detailed upconversion of R-BINDPA/PtOEP and additional upconversion CPL emission spectra (PDF)

expected, the UC-CPL intensity is dependent on the excitation intensity (Figure 4a). By increasing the green laser intensity, the UC-CPL emission increased gradually. However, the dissymmetry factor glum was almost the same showing independence of the excitation intensity, which indicated the inherent property of molecular chirality (Figure S6). It should be noted that a dissymmetry factor of |glum| = 2 × 10−4 was obtained in downconversion emission (Figure 4b) when excited with pure R- or S-BINDPA solution by Xe lamp or 360 nm laser. However, the glum value for the UC-CPL was 20-fold larger (4 × 10−3) when the solution of BINDPA and PtOEP was excited by 532 nm laser. It has been demonstrated that the glum is determined by both the electric dipole transition moment (μgn) and the magnetic dipole transition moment (mgn).7a,c,23 Because the magnetic dipole transition moments are typically much smaller than the electric dipole moments in pure organic systems, the denominator in eq 1 will be dominated by the first term μgn. ⎡ μgn ·mgn ⎤ ⎡ μgn ·mgn ⎤ 4Re glum = 4Re⎢ gn 2 ≈ ⎢ gn 2 ⎥ ⎥ ⎣ |μ | ⎦ ⎣ |μ | + |mgn|2 ⎦

ASSOCIATED CONTENT

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] *[email protected] ORCID

Pengfei Duan: 0000-0002-5971-7546 Minghua Liu: 0000-0002-6603-1251 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Basic Research Development Program (2016YFA0203400 and 2013CB834504), the National Natural Science Foundation of China (51673050 and 91427302), “Strategic Priority Research Program” of the Chinese Academy of Sciences (XDB12020200). P.D. thanks for the supporting of “New Hundred-Talent Program” research fund from the Chinese Academy of Sciences.

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That is to say a large glum value could be obtained in these systems with relatively small electric dipole transition moments. On the other hand, luminescence efficiency will be a lower value due to the small electric dipole transition moments. That is the reason why a large glum value could be observed in some chiral lanthanide complexes while low luminescence efficiency was obtained.1b,7c Riehl et al. have demonstrated that CPL could be accomplished by populating the emitting state

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DOI: 10.1021/jacs.7b04611 J. Am. Chem. Soc. 2017, 139, 9783−9786