Pure Organic Room Temperature Phosphorescence from Excited

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Pure Organic Room Temperature Phosphorescence from Excited Dimers in Self-Assembled Nanoparticles under Visible and Near-Infrared Irradiation in Water Xiao-Fang Wang, Hong-Yan Xiao, Peng-Zhong Chen, Qing-Zheng Yang, Bin Chen, Chen-Ho Tung, Yu-Zhe Chen, and Li-Zhu Wu J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b00859 • Publication Date (Web): 03 Mar 2019 Downloaded from http://pubs.acs.org on March 3, 2019

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Journal of the American Chemical Society

Pure Organic Room Temperature Phosphorescence from Excited Dimers in Self-Assembled Nanoparticles under Visible and Near-Infrared Irradiation in Water Xiao-Fang Wang,[a,b] Hongyan Xiao,[a,b] Peng-Zhong Chen,[c] Qing-Zheng Yang,[c] Bin Chen,[a,b] Chen-Ho Tung,[a,b] Yu-Zhe Chen,[a,b]* and Li-Zhu Wu[a,b]*. [a] Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences Beijing 100190, China. [b] School of Future Technology, University of Chinese Academy of Sciences Beijing 100049, P. R. China. [c] Key Laboratory of Radiopharmaceuticals, Ministry of Education, College of Chemistry, Beijing Normal University, Beijing 100875, China. Supporting Information Placeholder ABSTRACT: Pure organic room temperature phosphorescence (RTP) has unique advantages and various potential applications.

However, it is challengeable to achieve organic RTP under visible and near-infrared (NIR)-light excitation, especially in aqueous solution. Herein we assemble difluoroboron β-diketonate compounds to form organic nanoparticles (NPs) in water. The resulting NPs are able to show efficient RTP, effective uptake and bright imaging of HeLa cells under both visible and NIR-light excitation. More strikingly, spectroscopic study, X-ray diffraction and DFT calculation reveal the efficient RTP in organic NPs is originated from dimers in their excited states. The multiple interactions and intermolecular charge transfer in the dimer structures are of significance in promoting the production of dimer triplet excited states and suppressing the nonradiative decays to boost the RTP under visible and NIR-light irradiation in water.

INTRODUCTION Room temperature phosphorescence (RTP) from pure organic compounds has triggered tremendous research efforts owing to their unique generation processes, long lifetime and low cost.1 However, production of RTP from pure organic molecules is difficult because of the inefficient intersystem crossing (ISC) from singlet to triplet excited state, the spin-forbidden radiative transitions from triplet excited state to singlet ground state (phosphorescence), easy nonradiative relaxation from low-lying triplet excited states and collisional quenching by oxygen.2 Over the past several years, two approaches have been put forward to obtain RTP from organic molecules: (1) introduction of heavy atom,3 or aromatic carbonyl groups4 to facilitate ISC and therefore radiative phosphorescence; (2) use of host-guest doping,5 polymerization,6 metal-organic framework coordination,7 and rigid crystal formation8 to restrict the vibration of organic molecules and suppress the nonradiative decay. Following these approaches, most RTP are currently achieved in solid state under ultraviolet (UV) light excitation. It is realized that bulk solid organic materials cannot be dispersed in aqueous solution to penetrate in cell membrane.9 Meanwhile, UV light has much phototoxicity and lower penetrability for cell and tissues. Visible and nearinfrared (NIR) light are ideal alternative excitation light.

However, organic molecules with large π-conjugation that have the ability to absorb visible light are easy to decay nonradiatively through excited states and thereby resulting in poor radiative performance.10 Although molecular interactions such as strong coupling in H-aggregates, halogen bonding and intermolecular electronic coupling have been introduced to improve RTP in solid state,3a, 8b, 11, 12 it is yet to illustrate the emissive species of RTP in-depth, especially at supramolecular level for self-assembled organic compounds.

Scheme 1. The molecular structures of compounds HNpCzBF2, Br-NpCzBF2 and I-NpCzBF2; General Jablonski diagram illustrating the production of visible and NIR light triggered RTP from the exquisite dimer in NPs.

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hydrodynamic diameters of the NPs determined by dynamic light scattering (DLS) measurements (Figure 1a and S3). These NPs remained stable in water even after storage for 2 weeks at room temperature (Figure S4).

Herein, we present the first example of bright RTP under visible and NIR light irradiation in water. Our design is based on difluoroboron β-diketonate compounds (BF2bdk) (HNpCzBF2, Br-NpCzBF2 and I-NpCzBF2, Scheme 1), in which difluoroboron is expected to act as an electron-accepting group and carbazole as a strong electron-donating unit. The intramolecular/intermolecular charge transfer from the donor to the acceptor is anticipated to shift the absorption to visible light region. Moreover, BF2bdk derivatives possess large two-photon absorption (TPA) cross sections,13 it is therefore possible to achieve RTP via two-photon NIR light excitation. As mentioned above, the introduction of bromine (Br) and iodine (I) on naphthalene is capable of facilitating spin-orbital coupling to enhance the ISC rate, thereby populating the triplet excited states. To our delight, the assembled organic nanoparticles (NPs) of the three compounds show RTP in water under visible and NIR-light excitation at ambient conditions. More importantly, for the first time, we identify that the efficient RTP in these assemblies is derived from the dimer pair in excited state rather than from the monomer of BF2bdk itself. The results presented not only provide a new model for in-depth understanding of the RTP emissive species, but also extend the RTP from solid state under UV light excitation to aqueous solution under visible and NIR-light excitation.

Importantly, the NPs exhibited intense RTP under visible light excitation in aqueous solution. Phosphorescence spectra were recorded by gated measurement with a delay time of 0.1 ms after excitation. Taken BrNPs as an example, upon visible light irradiation at 470 nm, the emission spectra of BrNPs in aqueous solution display fluorescence centered at 620 nm (τF = 9.25 ns) and phosphorescence centered at 636 nm (τP = 27.6 μs) at ambient condition. The redshift emission and long lifetime further confirmed the phosphorescence characteristics. Similar phosphorescence spectra of HNPs (λP = 625 nm, τP = 29.0 μs) and INPs (λP = 628 nm, τP= 17.2 μs) with BrNPs under ambient conditions were observed (Figure 1c-d, and Table 1). Owing to the stronger heavy atom effect of iodine over bromine, less intense fluorescence and shorter lifetimes of INPs (τF = 3.03 ns) than those of BrNPs

RESULTS AND DISCUSSION Difluoroboron β-diketonate derivatives were readily synthesized by a two-step reaction from acetyl carbazole derivatives and naphthoic acid methyl esters (Scheme S1). 1H and 13C NMR spectroscopy, HR-MS and single-crystal X-ray diffraction revealed their structures with high purity (See Supporting Information for details). The compounds displayed visible absorption peak centered at ~455 nm (with ε over 70000 M-1cm-1) (Figure 1b). The bathochromic shifts of fluorescence over 80 nm with increasing solvent polarity from toluene to methanol suggest the intramolecular charge transfer (ICT) character of (X)-NpCzBF2 type molecules (Figure S1 and Table S1-3).14 Their self-assembled organic nanoparticles were prepared by nanoprecipitation method in solvent/antisolvent medium upon sonication.15 By adjusting the ratio of THF/water, well-dispersed assembled NPs of HNpCzBF2 (HNPs), Br-NpCzBF2 (BrNPs) and I-NpCzBF2 (INPs) in aqueous solution were obtained (See Supporting Information for details). The XRD of the NPs showed typical amorphous patterns (Figure S2). The average diameters of the HNPs, BrNPs and INPs observed by scanning electron microscopy (SEM) are approximately 105, 85 and 110 nm, respectively. They are slightly smaller than the average

Figure 1. (a) SEM image of BrNPs. (b) Normalized UV absorption spectra of (X)-NpCzBF2 in THF (black line) and HNPs, BrNPs, INPs (red line). (c) Phosphorescence decays of HNPs, BrNPs and INPs under ambient conditions. (d) Normalized fluorescence spectra of (X)-NpCzBF2 in THF at room temperature (black line, c = 10 μM), phosphorescence spectra of (X)-NpCzBF2 in 2-MTHF at 77 K (blue line) and RTP spectra of HNPs, BrNPs, INPs. (λex = 470 nm, delayed time: 0.1 ms, c = 1 mM) (red line).

Table 1. The lifetime and quantum yield of the three compounds in THF, as powder and nanoparticles. compound

THF λF a

λP b

(nm) H-NpCzBF2

Powder

NPs

(nm)

τFa (ns)

φc (%)

λFa (nm)

λPa (nm)

τFa (ns)

τPa (ms)

φc (%)

λFa (nm)

λPa (nm)

523

556

3.49

41.0

631

640

37.10

0.13

36.2

610

Br-NpCzBF2

533

570

2.56

26.0

558

647

1.21

0.58

25.9

I-NpCzBF2

530

574

2.29

24.0

556

655

0.83

0.56

7.4

τF a

τP d

(ns)

τPa (μs)

(μs)

φc (%)

625

25.54

29.0

51.6

23.1

620

636

9.25

27.6

34.5

6.5

622

628

3.03

17.2

25.9

3.6

[a] at rt under air; [b] at 77 K; [c] Total photoluminescence quantum yield at rt under air; [d] at rt under nitrogen.


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were observed. Not surprisingly, the phosphorescence intensity and lifetime of NPs were enhanced dramatically after purging with nitrogen (Figure S5 and Table 1). We also achieved RTP of HNPs, BrNPs and INPs under NIR excitation. The (X)-NpCzBF2 were found to reach maximal emission by femtosecond laser excitation among 780-820 nm (Figure S6). Upon excitation at 820 nm, these NPs in aqueous solution emit brightly with the similar band shape and position as the emissions excited by 470 nm. After purging with nitrogen, the emission intensity of RTP was greatly enhanced (Figure S7). The quadratic dependence between the laser powers and emission intensities demonstrate the two-photon excitation process (Figure 2a-b and S8). With the simple and water-dispersible NPs, cellular imaging experiment was performed to examine the potential applications of the NPs in bioimaging by a confocal laser scanning microscopy (CLSM). The confocal images of HeLa cells after incubation with BrNPs for 4 h are shown in Figure 2. Emissions in green (500-550 nm) and red (570-620 nm) channels upon excitation by a 488 nm laser and an 820 nm femtosecond pulsed laser light were observed respectively, indicating that the NPs passed across the cell membrane and entered in the cell cytoplasm. Clearly, the two-photon excitation allows for using the RTP of self-assembled nanoparticles in biological system by NIR light irradiation, which possesses lower phototoxicity and higher penetrability for cells and tissues.

Figure 2. (a) The emission spectra of BrNPs excited at 470 nm (black line) and 820 nm (red line). (b) The emission spectra and the logarithmic plots of the emission integral of BrNPs at different excitation intensities (mW) by an 820 nm femtosecond pulsed laser light. (c-h) Confocal luminescence images of HeLa cells incubated with BrNPs (c = 1 μM in water). The excitation wavelengths were 488 nm (c, d) and 820 nm (f, g), respectively. (e) Bright field image of HeLa cells corresponding to (c) and (d). (h) Bright field image of HeLa cells corresponding to (f) and (g). Scale bar: 10 μm. The observation of RTP in the NPs are in sharp contrast with the absence of phosphorescence in THF under ambient

condition. To shed more light on the RTP, we compared the fluorescence spectra of the compounds in NPs with those in THF and solid state at room temperature. Notably, HNPs, BrNPs and INPs exhibited largely red-shifted, broad and structureless fluorescence, which were totally different from those of the monomer in THF (523, 533 and 530 nm) (Figure 1d and S9). The emission color changed from THF to NPs were evidenced by eye when irradiated by hand-held 365 nm UV light (Figure S10). Moreover, their fluorescence lifetimes (τF = 25.54, 9.25, and 3.03 ns) were much longer than those in THF solutions (τF = 3.49, 2.26 and 2.29 ns) (Figure S11). The largely red-shifted fluorescence spectra and long lifetimes of the NPs suggest the possible formation of new emissive species.16, 17 The excitation spectra of the fluorescence in NPs were red-shifted greatly as compared with that of monomer in THF, indicating that the associated BF2bdk chromophores at the ground states might be responsible for the new emissive species (Figure S12a).18 Besides, the phosphorescence profiles of the NPs were different from those of three compounds in dilute solution of 2methyltetrahydrofuran (2-MTHF) at 77 K. The typical monomeric phosphorescence of BF2bdk (556, 570 and 574 nm for H-NpCzBF2, Br-NpCzBF2, I-NpCzBF2) in 2-MTHF shifted to lower energy by about 54-69 nm for NPs at room temperature (Figure 1d, S13 and Table 1), further suggesting that the RTP of the NPs is originated from the associated molecules rather than from the isolated molecules. Possibly due to the same reason, solid powder of these three compounds exhibited very similar RTP as those of NPs (HNpCzBF2: λP = 640 nm, τP = 0.13 ms; Br-NpCzBF2: λP = 647 nm, τP = 0.53 ms; I-NpCzBF2: λP = 655 nm, τP = 0.56 ms) (Figure S14). It is worth noting that this kind of emission could only be obtained in NPs and solid powder as no phosphorescence was detectable even at very high concentration of (X)-NpCzBF2 in THF (Figure S12b). Single-crystal structures of Br-NpCzBF2 and I-NpCzBF2 were obtained and revealed that the adjacent molecules are in close proximity. As shown in Figure 3, Br-NpCzBF2 and INpCzBF2 adopt compact head-to-tail anti-parallel arrangement and exhibit strong multiple interactions in the spatially isolated dimers in crystal structure. Strong interlayer interaction between naphthyl and carbazole group (3.365 and 3.346 Å) in the dimeric structures, taking BrNpCzBF2 as example, were evidenced by their short distances. The dihedral angle between two molecules in (X)NpCzBF2 dimers are close to 0°. In these dimers, NPs lock the molecular conformation, restrict the molecular vibration and decrease oxygen quenching, thus reducing the nonradiative transition of triplet excitons to boost RTP under ambient condition (Tables S4). The excited-state transition configuration calculation of the monomeric and dimeric structures of Br-NpCzBF2 and INpCzBF2 derived from single-crystal structures were carried out based on TD-DFT method with B3LYP functional and 631G* (LANL2DZ for I) basis set (See Supporting Information for details, Figure 3 and Table S5-8). 3a, 4d, 19, 20 For isolated (X)-NpCzBF2 molecule, obvious intramolecular charge transfer from carbazole to dioxaborine ring were observed in their



Figure 3. The intermolecular packing in Br-NpCzBF2 (a) and I-NpCzBF2 (b) crystal; the energy levels of monomer and dimer of Br-NpCzBF2 (a) and I-NpCzBF2 (b) based on TD-B3LYP/6-31G* calculations. transition configurations and isosurfaces of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). In the dimeric structures, however, a new intermolecular charge transfer from an occupied orbital of one to the vacant orbital of neighboring molecule was observed instead (Figure S15). The dipole moments for Br-NpCzBF2 and I-NpCzBF2 dimer were calculated to be 13.15 and 12.78 Debye respectively, which were enhanced dramatically compared with their isolated monomers (9.59 and 9.80 Debye), suggesting the presence of intermolecular charge transfer in the dimer structures. Significantly, the energy level of the singlet and triplet excited state in the dimer decreased dramatically as compared with isolated monomer, thus resulting in significant intermolecular ISC channels and enhanced ISC process. In the monomer structure of Br-NpCzBF2, e.g., there is merely one minor transition contribution from S1 to T4 (S1 → T4) for ISC transition, while there are two major ISC channels (S1 →T3, T5) observed in the dimer structures (Table S5-S6). Furthermore, the calculated energy gap between the lowest singlet and triplet excited state of dimer (Br-NpCzBF2: ΔEST = 0.32 eV; I-NpCzBF2: ΔEST = 0.31 eV) were considerably smaller than those of the monomers (Br-NpCzBF2: ΔEST = 0.67 eV; I-NpCzBF2: ΔEST = 0.62 eV). According to the perturbation theory, the decreased energy gap would favor the spin-orbit coupling between singlet-triplet states in the dimer structures, thus leading to increased radiative rate constant from T1 to S0.6a, 21, 22 Most recently, Shuai and Peng et al23 proposed a pair of molecular descriptors (γ and β), related to the portion of (n, π*) transition and (π, π*) transition of molecular orbitals in lowest-lying singlet and triplet excited state, to precisely characterize RTP efficiency and lifetime through computational calculations. Taking all aforementioned into account, excitation to the singlet excited state of (X)-NpCzBF2 can be followed by the formation of a stabilized dimer excited states with the nearby preorganized ground state molecule. These lower energy

dimers, which can act as energy trap and collect all the excited energy from excited molecules, would increase the ISC channels between S and T states, decrease the ΔEST, and enable multiple intermolecular interactions for the occurrence of efficient RTP in the NPs. No RTP was observed in the NPs prepared by carbazole derivative under similar condition, supporting the important role of multiple interactions and intermolecular charge transfer of (X)NpCzBF2 in dimer structures (Figure S16).

CONCLUSION In summary, we have developed self-assembled organic nanoparticles from difluoroboron β-diketonate compounds in water, which show bright RTP, effective uptake and bright imaging of HeLa cells under both visible and NIR-light excitation. Spectroscopic analysis, DFT calculation and single crystal structure analysis demonstrate the RTP in NPs is originated from the dimeric excited states of the compounds. The multiple interactions and intermolecular charge transfer in the dimeric structures are effective in facilitating the ISC and reducing nonradiative decays, thus boosting RTP of the NPs. This is the first study, to the best of our knowledge, to expand pure organic RTP to visible and NIR light excitation in aqueous solution. A simple dimer formation strategy demonstrated here not only opens a window for in-depth understanding the nature of RTP from self-assembled pure organic compounds, but also allows for the development of bright RTP from pure organic phosphors under visible and NIR light irradiation for practical applications in aqueous media.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website.


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Experimental details, synthetic and preparation details, cell culture, calculations, single crystal data and others (PDF)

AUTHOR INFORMATION Corresponding Author *[email protected]; *[email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT Financial support from the National Natural Science Foundation of China (21871280 and 21525206), the Ministry of Science and Technology of China (2017YFA0206903), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB17000000), the Key Research Program of Frontier Sciences of the Chinese Academy of Sciences (QYZDY-SSW-JSC029), K. C. Wong Education Foundation are gratefully acknowledged. The authors thank Dr. Ye Tian (TIPC) for providing HeLa cells, Prof. Yong Chen (TIPC) for his helpful discussions.

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