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Doublet-Triplet Energy Transfer Dominated Photon Upconversion Jianlei Han, Yuqian Jiang, Ablikim Obolda, Pengfei Duan, Feng Li, and Ming-hua Liu J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b02677 • Publication Date (Web): 16 Nov 2017 Downloaded from http://pubs.acs.org on November 17, 2017

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The Journal of Physical Chemistry Letters

Doublet-triplet energy transfer dominated photon upconversion Jianlei Han,†,# Yuqian Jiang,†,# Ablikim Obolda,‡,# Pengfei Duan,*,† Feng Li*,‡ and Minghua Liu*,†,§ †

Division of Nanophotonics, CAS Center for Excellence in Nanoscience, CAS Key Laboratory

of Nanosystem and Hierarchical Fabrication, National Center for Nanoscience and Technology (NCNST), No. 11 ZhongGuanCun BeiYiTiao, 100190 Beijing, P.R. China. ‡

State Key Laboratory of Supramolecular Structure and Materials，College of Chemistry, Jilin

University, Qianjin Avenue 2699, Changchun, 130012, 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, 100190, Beijing, P. R. China. Beijing, P.R. China. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (P.D.). *E-mail: [email protected] (F.L.). *E-mail: [email protected] (M.L.).

ACS Paragon Plus Environment

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Stable luminescent π-radicals with doublet emission have aroused a growing interest for functional molecular materials. We have demonstrated a neutral π-radical dye (4-N-carbazolyl2,6-dichlorophenyl)bis(2,4,6-trichlorophenyl)-methyl (TTM-1Cz) with remarkable doublet emission, which could be used as triplet sensitizer to initiate the photophysical process of triplettriplet annihilation photon upconversion (TTA-UC). Dexter-like excited doublet-triplet energy transfer (DTET) was confirmed by theoretical calculation. With the same sensitizer, a mixed solution of TTM-1Cz and aromatic emitters could upconvert red light (λ = 635 nm) to blue or cyan light. An anti-Stokes energy shift as large as 0.92 eV was observed from red to blue light upconversion. This finding of DTET phenomena offers a new kind of triplet sensitizer for TTAUC.

TOC GRAPHICS

KEYWORDS Photon upconversion, triplet-triplet annihilation, doublet exciton, energy transfer, raical dye


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Organic radicals have abundant applications such as spintronics,1 polarizing agents,2 organic magnetism3 and accelerating chemical reactions4 because their unpaired electron can easily take part in physical processes and chemical reactions. Generally, neutral radicals are quite unstable. However, through the molecular design, stable neutral radicals can be obtained and they are able to withstand oxygen and light for an extremely long time at room temperatures.5 Recently luminescent neural π-radicals6-9 have aroused a growing interest due to their special emission from doublet exciton, which is different from the emission of closed-shell molecules (from either singlet or triplet exciton, Figure S1). Thus they can be used to realize the particular functions which cannot be achieved by the closed-shell luminescent molecules. One example is that a luminescent neural π-radical was successfully used as the emitter of an organic light-emitting diode (OLED). Because the emission of the OLED comes from the double exciton of the πradical whose transition to the ground state is spin-allowed, the transition problem of triplet exciton of closed-shell emitters is thus circumvented10. Photo upconversion (UC) through annihilation between two-long-lifetime excited triplets (TTA) has recently come into the spotlight because of its wide applications that range from renewable energy productions to bioimaging and phototherapy. It attracted much attention due to its occurrence with low-intensity and non-coherent incident light.11-25 Figure S2 shows a typical scheme for TTA-based UC, in which a molecular sensitizer (triplet donor)-emitter (triplet acceptor) pair shares roles. A triplet excited state (T1) of the donor is formed via intersystem crossing (ISC) from the singlet excited state (S1) of the donor. Donor-to-acceptor (D-A) triplettriplet energy transfer (TTET) populates the T1 state of acceptor, and TTA between two acceptor triples produces a higher energy acceptor S1 state that consequently emits upconverted delayed fluorescence. Generally, metal organic complexes were commonly used as the triplet donor due


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to the high triplet quantum yield (ΦISC ≈ 100%). However, the energy loss ∆EST during ISC, typically hundreds of meV, is contradict to the efficient low energy photon upconversion (Figure S2).26 On the other hand, developing pure organic sensitizer to replace the metal organic complex is also an important issue in TTA-based photon upconversion research field.27-29 Here, we report a solution for overcoming the energy loss during ISC and replacing the metal organic complex sensitizer with pure organic π-radical dye in TTA-UC. This is based on excited doublet-triplet energy transfer (DTET) by directly exciting an organic π-radical, (4-N-carbazolyl2,6-dichlorophenyl)bis(2,4,6-trichlorophenyl)methyl (TTM-1Cz).6,7 We provide definitive experimental evidences that doublet energy transfer proceeds rapidly and efficiently from excited radical dye TTM-1Cz to the acceptor. Dexter-like doublet-triplet energy transfer could be observed in TTA photon upconversion system. In this work, we tried to use radical dye as an energy donor to initiate a TTA-based photon upconversion photophysical process. As shown in Figure 1a, through a direct excitation of a radical dye, doublet exciton (2D*) could sensitize an acceptor accompanying with doublet-triplet energy transfer. Different from the typical triplet exciton sensitized TTA-UC, the excited doublet exciton could directly sensitize the acceptor without energy loss resulted from intersystem crossing. This approach will open up a new research field about triplet sensitizer designing.


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Figure 1. (a) Illustration of TTA-based UC sensitized by doublet exciton. (b) Mechanism of electron-exchange-based Dexter-like doublet-triplet energy transfer. It is well known that triplet-triplet energy transfer follows the electron exchange mechanism (Dexter) with the characteristic short triplet interaction distance (ca. 1nm).30 Here, electronexchange-based mechanism also could be afforded to doublet-triplet energy transfer (Figure 1b): 2

D* + 1 A → 2 D + 3 A*

(1)

The energy transfer integral of such energy transfer process can be expressed as: VDA = ψ 2 D*ψ 1 A Hˆ ψ 2 D ψ 3 A* = ψ r Hˆ ψ p

(2)


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Ψr and Ψp are the spin-localized wave functions before and after energy transfer, respectively. To simplify the problem, we assume that Ψr and Ψp are composed of the same set of core orbitals, and the differences are only in the four highest occupied spin-orbitals, as shown in Figure 1b. Thus, we can express Ψr and Ψp in Slater determinant:

ψ r = ψ coreφDα,SUMOφAα, HOMOφAβ,HOMO ψ p = ψ coreφDβ ,SOMOφ Aα, HOMOφ Aα, LUMO

(3)

(4)

So, the coupling can be derived as V DA = φ Dα , SUMOφ Aβ, HOMO φ Dβ , SOMOφ Aα, LUMO  = φ Dα , SUMOφ Aβ, HOMO φ Dβ , SOMOφ Aα, LUMO  − φ Dα , SUMOφ Aα, LUMO φ Dβ , SOMOφ Aβ, HOMO  = − φ Dα ,SUMOφ Aα, LUMO φ Dβ , SOMOφ Aβ, HOMO 

where

φ α（β） is

(5)

the occupied spin-orbital for donor or acceptor as shown in Figure 1b. It shows the

same expression as triplet-triplet energy transfer,31 which is completely the exchange electronic integral. Therefore, when a stable radical dye is used as an energy donor, the corresponding excited doublet exciton can transfer energy to an acceptor through electron exchange by producing an excited triplet exciton and a ground state of donor. In this work, we have used a neutral π-radical dye TTM-1Cz as a triplet sensitizer to initiate TTA-UC by sensitizing two different acceptors 9,10-diphenylanthracene (DPA) and bis(phenylethynyl)anthracene (BPEA) (Figure 2a). DPA and BPEA have been widely reported in TTA-UC systems as two kinds of typical triplet annihilators sensitized by metal complexes. Here, we firstly tried to sensitize these


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two annihilators by using a radical dye TTM-1Cz and an anti-Stokes energy shift of 0.92 eV was observed from the mixture of TTM-1Cz and DPA excited by 635 nm light.

Figure 2. (a) Molecular structures of all the compounds used in this work. (b) Normalized absorption and emission spectra of DPA (0.1 mM; λex = 375 nm) and TTM-1Cz (0.1 mM; λex = 530 nm) in toluene. It has been reported that TTM-1Cz exhibited strong broad doublet exciton emission in the nearinfrared (NIR) region (600-800 nm) which could be used as brand-new emitter in organic lightemitting devices.10 A toluene solution of TTM-1Cz exhibited a broad absorption band centered at approximately 600 nm which is assigned to the electronic transition from SOMO to SUMO


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(Figure 2b). The PL spectrum (centered at 680 nm) demonstrated that the emission originated from the transition of SUMO to SOMO, that is, the radiative decay from the doublet excitons. Selective excitation of TTM-1Cz with the wavelength in the broad absorption band range would lead to the generation of doublet exctated state at 680 nm (1.82 eV), energetically suitable to sensitize both DPA and BPEA to their triplet state (triplet energy of DPA ET ≈ 1.77 eV13 and BPEA ET ≈ 1.74 eV32). The acceptor DPA shows intense π-π* absorption bands at 356, 375 and 395 nm with corresponding fluorescence ranging from 380 to 600 nm. It should be noted that, due to the broad absoprtion (500-650 nm) of TTM-1Cz, DPA can be sensitized by doublet excitons of TTM-1Cz which can be populated from the excitation of both 635 nm and 532 nm, and an anti-Stokes energy shift of 0.92 eV will be achieved from the excitation of 635 nm and a smaller one of 0.55 eV from 532 nm. Compared the absorption and emission between BPEA and TTM-1Cz, BPEA is also an candidate for doublet annihilator and these dye pairs have the potential to achieve UC (Figure S3a and S4a). In this work, we will carefully demonstrate the UC properties of TTM-1Cz/DPA pairs while TTM-1Cz/DPA excited by 532 nm and TTM1Cz/BPEA excited by 635 nm will be posted in the supporting informations.


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Figure 3. (a) Upconversion emission spectra of TTM-1Cz/DPA with different incident power density of 635 nm laser in deaerated toluene. (b) Dependence of UC emission intensity at 432 nm on the incident power density. The dashed lines are fitting results with slopes of 2.0 (blue) and 1.2 (red) in the low and high-power regions, respectively. (c) Time resolved doublet emission at 680 nm of TTM-1Cz with and without DPA. (d) Time resolved upconverted emission at 440 nm of the TTM-1Cz/DPA pair (λex = 635 nm). ([TTM-1Cz] = 0.1 mM, [DPA] = 5 mM, λex = 635 nm). As shown in Figure 3a, by sensitizing the DPA triplet with TTM-1Cz, the incident red light (λex = 635 nm) is successfully upconverted to the blue light (λex = 432 nm) in solution (Fig. 3a). Steady-state luminescence spectra at varied incident laser power clearly showed the upconverted emission of DPA at 432 nm. Moreover, due to the broad absorption band from 500-650 nm of TTM-1Cz, it also could sensitize DPA triplet with the incident green light (λex = 532 nm, Figure


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S3a) and another acceptor BPEA can be sensitized to the upconverted cyan light from red light (λex = 635 nm, Figure S4a). Figure 3b shows the excitation intensity dependent UC emission intensity from the TTM-1Cz/DPA mixed solution. In the low excitation regime, the slopes in the log-log plot were close to 2, which is consistent with the quadratic dependence of the annihilation process. By increasing the excitation intensity, the slope changed to 1, which indicated that the upconversion process superseded the thermal deactivation of acceptor triplets.[14] Remarkably, the crossover points between these two regions, Ith, were found to be a comparable value of 190 mW cm-2. Above Ith, the UC quantum efficiency turns to be saturated while the main deactivation channel for the acceptor triplets change to TTA process.33 To evaluate the efficiency of DTET, we compared the emission lifetime of TTM-1Cz with (τD′) and without (τD) the acceptor DPA (Figure 3c). The energy transfer efficiency ΦDTET could be calculated by the equation ΦDTET = (τD - τD′)/ τD × % to give a value 69%. This indicates that the energy transfer from doublet exciton to acceptor triplet is not that efficient which might be due to the short lifetime of doublet exciton of TTM-1Cz (τTTM-1Cz = 27 ns). The TTA-UC quantum yield (ΦUC) of the TTM-1Cz/DPA pairs in deaerated toluene could be quantified by using methylene blue (MB) as a reference. It has been demonstrated that the quantum yield could be defined as the ratio of emitted photon numbers to absorbed photon numbers. Thus, by considering that the absorption of two photons is required for generating one upconverted photon, the theoretical maximum efficiency for TTA-UC should be defined as 50%.34 In this work, we would set the maximum efficiency at 100% while the UC quantum yield is written as ΦUC′ (= 2ΦUC). With increasing the excitation intensity, the ΦUC′ value of TTM-1Cz/DPA system reached to saturated value of 0.25% (Figure S5). The low UC quantum yield, originating from the low TTM1Cz/DPA DTET efficiency, should be due to the short doublet lifetime of TTM-1Cz. TTA-UC


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mechanism could also be further confirmed by checking the upconversion lifetime. As shown in Figure 3d, the acceptor triplet lifetime could be calculated by the tail-fitting of the UC decay at 440 nm. The TTM-1Cz/DPA mixture solution showed a microsecond-scale lifetime, which supported the observed UC emission being produced from long-lifetime triplet species. The triplet lifetime of acceptor DPA (τA,T) was estimated as 8.2 µs according to the reported relationship of IUC(t) ∝ exp(-2t/τA,T).35,36 The blue UC emission showed excellent photophysical stability, as confirmed by the good maintenance of UC emission intensity after continuous excitation over 3000 s (laser intensity = 1031 mW cm-2) (Figure 4a), even under very high laser intensity (2025 mW cm-2). We have carefully retested the electron paramagnetic resonance (EPR) spectrum of TTM-1Cz/DPA mixture after long time laser irradiation (Figure 4b). The unpaired electron could be clearly observed which indicated that the radical sensitizer possesses remarkable stability under strong laser irradiation. The UC pairs TTM-1Cz/BPEA also exhibited excellent stability in UC process (Figure S6a and 6b). We also have measured the emission of pure TTM-1Cz in deaerated toluene under laser irradiation, in order to completely confirm the authenticity of doublet sensitized TTA-UC (Figure S7). By applying strong laser irradiation (635 nm) in pure TTM-1Cz deaerated toluene solution, no UC emission could be observed while only doublet exciton emission of TTM-1Cz around 680 nm could be detected. Similar phenomenon could be obtained by shining the green laser 532 nm. These results further confirmed that TTM-1Cz sensitizer TTA-UC should definitely occur.


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Figure 4. (a) Time dependence of upconverted emission intensity at 440 nm of TTM-1Cz/DPA in toluene solution (λex = 635 nm). (b) The EPR spectrum of TTM-1Cz/DPA in toluene solution after TTA-UC measurement. For illustrating the rationality of experiments, we apply quantum mechanics and molecular dynamics to study the doublet-triplet energy transfer process in TTM-1Cz/DPA system as well as TTM-1Cz/BPEA (SI part III). The Franck-Condon integrals for D1→D0 emission for TTM1Cz and S0→T1 absorption for DPA (Fig. S9) exhibited partial overlap which ensures the occurrence of energy transfer. Considering the electron exchange is short distance interaction, doublet-triplet energy transfer can only happen when donor and acceptor are nearby. Here, we simply selected one snapshot with the shortest centroid distance as example to study the energy transfer property, since the calculation of statistic averages during the equilibrium process which can represent the experimental observed values is too expensive and unnecessary. By using direct coupling method, the energy transfer integral for TTM-1Cz/DPA dimer with the shortest centroid distance was achieved as only 0.4 meV, which accords with the magnitude of electronic exchange integral obtained in triplet-triplet energy transfer.31 Then, with Fermi’s Golden Rule, the doublet-triplet energy transfer rate in such TTM-1Cz/DPA dimer is 5.26×108 s-1 which is


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larger than the observed value (4.51×107 s-1). Since the electronic exchange integral is exponentially decrease with the increase of distance, the relatively larger energy transfer rate in the dimer with the shorted distance ensure the occurrence of DTET between TTM-1Cz and DPA. The upconversion pair TTM-1Cz/BPEA also exhibited the same behavior under the same calculation (SI part III). In conclusion, we have demonstrated the first example of excited doublet exciton sensitized TTA-UC which enables upconverting red to cyan light and blue light. π-radical dye TTM-1Cz working as triplet sensitizer would provide a new perspective in TTA-UC because it not only offers a useful methodology for reducing the energy loss during triplet sensitization but also a new kind of pure organic triplet sensitizer. In addition, it reveals a new application for excited doublet exciton emission. This work underpins the important concept of doublet emission and stimulates the exploration of doublet-triplet energy transfer toward highly efficient TTA-UC, which would find a number of applications in many disciplines. ASSOCIATED CONTENT Supporting Information. Additional details of the supplementary figures and tables, and other information. (PDF) AUTHOR INFORMATION Notes #

These authors contribute equally.

The authors declare no competing financial interests. ACKNOWLEDGMENT


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This work was supported by National Key Basic Research and Development Program of China (Grant No. 2016YFA0203400, 2016YFB0401001, 2017YFA0206600) founded by MOST. The National Natural Science Foundation of China (Nos 51673050, 51673080, 21603043), “Strategic Priority Research Program” of the Chinese Academy of Sciences (XDB12020200), and “New Hundred-Talent Program“ research fund of the Chinese Academy of Sciences. P.D. thanks for the supporting of “New Hundred-Talent Program” research fund from the Chinese Academy of Sciences. REFERENCES (1) Colvin, M. T.; Giacobbe, E. M.; Cohen, B.; Miura, T.; Scott, A. M.; Wasielewski, M. R. Competitive Electron Transfer and Enhanced Intersystem Crossing in Photoexcited Covalent TEMPO-Perylene-3,4:9,10-bis(dicarboximide) Dyads: Unusual Spin Polarization Resulting from the Radical-Triplet Interaction. J. Phys. Chem. A 2010, 114, 1741-1748. (2) Dane, E. L.; Maly, T.; Debelouchina, G. T.; Griffin, R. G.; Swager, T. M. Synthesis of a BDPA-TEMPO Biradical. Org. Lett. 2009, 11, 1871-1874. (3) Tatiana, M.; Fernando, P. Carbon-based magnetism : An overview of the magnetism of. Elsevier Science Ltd 2013. (4) Jiao, Y.; Li, W.-L.; Xu, J.-F.; Wang, G.; Li, J.; Wang, Z.; Zhang, X. A Supramolecularly Activated Radical Cation for Accelerated Catalytic Oxidation. Angew. Chem. Int. Ed. 2016, 55, 8933-8937. (5) Muellegger, S.; Rashidi, M.; Fattinger, M.; Koch, R. Interactions and Self-Assembly of Stable Hydrocarbon Radicals on a Metal Support. J. Phys. Chem. C 2012, 116, 22587-22594. (6) Gamero, V.; Velasco, D.; Latorre, S.; Lopez-Calahorra, F.; Brillas, E.; Julia, L. 4-(Ncarbazolyl)-2,6-dichlorophenyl bis(2,4,6-trichlorophenyl)methyl radical an efficient red lightemitting paramagnetic molecule. Tetrahedron Lett. 2006, 47, 2305-2309. (7) Velasco, D.; Castellanos, S.; Lopez, M.; Lopez-Calahorra, F.; Brillas, E.; Julia, L. Red organic light-emitting radical adducts of carbazole and tris(2,4,6-trichlorotriphenyl)methyl radical that exhibit high thermal stability and electrochemical amphotericity. J. Org. Chem. 2007, 72, 7523-7532. (8) Heckmann, A.; Lambert, C.; Goebel, M.; Wortmann, R. Synthesis and photophysics of a neutral organic mixed-valence compound. Angew. Chem. Int. Ed. 2004, 43, 5851-5856. (9) Hattori, Y.; Kusamoto, T.; Nishihara, H. Luminescence, Stability, and Proton Response of an Open-Shell (3,5-Dichloro-4-pyridyl)bis(2,4,6-trichlorophenyl)methyl Radical. Angew. Chem. Int. Ed. 2014, 53, 11845-11848. (10) Peng, Q.; Obolda, A.; Zhang, M.; Li, F. Organic Light-Emitting Diodes Using a Neutral pi Radical as Emitter: The Emission from a Doublet. Angew. Chem. Int. Ed. 2015, 54, 70917095.


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