Tetraphenylethylenepyrrolo[3,2-b]pyrrole Hybrids as Solid-State Emitters

Letter pubs.acs.org/OrgLett

Cite This: Org. Lett. XXXX, XXX, XXX−XXX

Tetraphenylethylenepyrrolo[3,2‑b]pyrrole Hybrids as Solid-State Emitters: The Role of Substitution Pattern Bartłomiej Sadowski,† Khaled Hassanein,‡ Barbara Ventura,*,‡ and Daniel T. Gryko*,† †

Institute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland Istituto ISOF-CNR, Via P. Gobetti 101, 40129 Bologna, Italy

‡

S Supporting Information *

ABSTRACT: Two hybrid dyes possessing tetraphenylethylene moieties weakly conjugated with a pyrrolo[3,2-b]pyrrole core have been synthesized. Both dyes display a weak emission in solution, however, in the solid state a ∼100-fold increase in the fluorescence quantum yield is observed. The position of the molecular rotors about the core greatly influences the photophysical characteristics. The variances in emission properties were assigned to entirely different changes in dihedral angles upon excitation, which in turn have substantial effects on radiative rate constants, allowed transitions, and HOMO/LUMO distribution.

P

hybrid dyes bearing linkages between pyrrolo[3,2-b]pyrrole and TPE at either the 1 and 4 or 2 and 5 positions. The regioisomeric TAPPs possessing TPE units were successfully synthesized from the respective TPE-containing amine16 or aldehyde3b using our multicomponent reaction17 (Scheme 1). In the meantime, an improved protocol for the

ropeller-shaped molecules have attracted considerable attention since the aggregation-induced emission (AIE) phenomenon was described for the first time.1 Over the years, such skeletons, i.e., hexaphenylsiloles,2 tetraphenylethylene (TPE) derivatives,1c,d,3 polyphenylenes,4 tetraphenylthieno[3,2-b]thiophene S,S-dioxide,5 and diphenylbenzo[b]thiophene S,S-dioxide6 have been widely used in the design of new AIEgens. The molecules exhibiting AIE character have been used as components in OLEDs7 and field effect transistors8 and as fluorescent probes.9 The restriction of intramolecular rotations (RIR) model has been proposed as an explanation for the enhancement of luminescence during aggregation; however, Tang’s group has recently published a more general mechanism behind the AIE phenomenon, i.e. the restriction of intramolecular motions (RIM),10 by considering both rotational and vibrational deactivation modes. The tetraarylpyrrolo[3,2-b]pyrroles (TAPPs) constitute molecular rotors by themselves; however, in most cases, these dyes are strongly emissive both in solution and in the solid state.11 Indeed, planarization of this chromophore combined with πexpansion typically results in the decrease of the fluorescence quantum yield.12−14 In the context of AIE, the groups of Dong and Tang simultaneously reported that if electron-withdrawing groups are placed at positions 1 and 4 of the TAPP core, AIE behavior can be observed.15 We hypothesized that by designing hybrid molecules comprising both tetraphenylethylene and tetraarylpyrrolo[3,2-b]pyrrole, rotor-within-rotor dyes with intriguing photophysical properties will be created. It has been shown numerous times that there is a significant electronic coupling between aryl substituents and the pyrrolo[3,2-b]pyrrole core both in the ground and in the excited states.11−14 This communication strongly depends on the exact position of biaryl linkage, which prompted us to focus on two weakly coupled © XXXX American Chemical Society

Scheme 1. Synthetic Route Leading to Pyrrolo[3,2-b]pyrroleBased Regioisomers 1 and 2

synthesis of TAPPs using NbCl5 as a catalyst with yields up to 98% has been published.18 However, in our case, this new procedure did not produce even traces of the expected products 1 and 2. We were able to obtain crystals of 1 suitable for X-ray crystallography. Compound 1 showed Ci symmetry with an Received: March 29, 2018

A

DOI: 10.1021/acs.orglett.8b01011 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters

support the experimentally obtained data (structures were simplified by replacing t-Bu groups with Me groups). In solution, compound 1 shows spectroscopic features similar to those of the previously reported 2,5-bis(4-((3,5-bis(trifluoromethyl)phenyl)ethynyl)phenyl)-1,4-di-p-tolyl-1,4dihydropyrrolo[3,2-b]pyrrole (Figure S1),19 with an absorption maximum at 398 nm (hypsochromically shifted by 23 nm) and a fluorescence peak at 586 nm (bathochromically shifted by 64 nm). Conversely, its absorption is ∼50 nm bathochromically shifted vs 1,2,4,5-tetraphenylpyrrolo[3,2-b]pyrrole (TPPP).17a This is not surprising since many studies have shown that electronic communication is particularly strong through the 2 and 5 positions of the pyrrolo[3,2-b]pyrrole core.11 On the other hand, the absorption of compound 2 (maximum at 337 nm) follows the general pattern for TAPPs lacking electronwithdrawing and electron-donating substituents.11,17a In contrast, its fluorescence is markedly red-shifted (maximum at 662 nm with a tail extending in the NIR region, Figure 2) with respect to 1,2,4,5-tetraphenyl-pyrrolo[3,2-b]pyrrole (∼410 nm),17a leading to an unprecedented Stokes shift of 14600 cm−1 (Table 1). The latter can be attributed to the higher distortion of the excited state in 2 compared with 1, as the dihedral angle β increased significantly (by ca. 20°) during S0 → S1 transition (see Table S2). To the best of our knowledge, there are a still limited number of reports on TPE-containing molecules with AIE activity and large Stokes shifts in solution.20 The absorption band of both compounds is bathochromically shifted for solid samples with respect to solution, thanks to better electronic delocalization over the entire molecule in the crystal lattice. Direct comparison of atomic geometries for derivative 1 in the crystal with the calculated ground state (Table S2), in fact, revealed a smaller dihedral angle α between aryl rings and the core as well as shorter lengths of some bonds in the crystal lattice, which is probably caused by significant geometrical restrictions. In contrast to the vast majority of TAPPs,11,17 dyes 1 and 2 display a weak emission in solution. The two compounds, in fact, show emission quantum yields in the 10−3 scale (Table 1). This feature can be ascribed to the presence of the appended TPE groups, which promote nonradiative deactivation pathways by rotational or vibrational motions. The role of RIM effects in governing the photophysics of the dyes 1 and 2 in solution is confirmed by the strong enhancement of emission quantum yield when moving to the solid state. A ca. 100-fold increase is observed for both hybrid molecules, reaching ϕfl values of 0.77 and 0.16 for 1 and 2, respectively (Table 1). Moreover, both compounds show markedly blue-shifted emission in the solid state with respect to solution. This fact can be associated with significant geometrical restrictions in the crystal lattice, thus smaller distortion of the excited state and hence smaller Stokes shift in the solid state.21 Compound 2, in particular, displays an impressive fluorescence hypsochromic shift (ca. 200 nm) when passing from solution to the solid state (Figure 2 and Table 1). Nonradiative rate constants decrease by 2 orders of magnitude for compound 1 and 1 order of magnitude for compound 2 when moving from solution to the solid state, confirming the role of the restricted environment in the inhibition of nonradiative pathways. We hypothesize that the larger knr observed for compound 1 (Table 1) can be attributed to a higher number of rotational/vibrational modes for nonradiative deactivation in this derivative with respect to 2. According to calculations, during excitation the dihedral angle α decreases and at the same time the dihedral angle β increases because of steric repulsions between two substituted aryls (Table S1). We note that the increase in β is

almost perfectly planar pyrrolopyrrole core (with deviations from the core plane no greater than 1.0°) (Figure 1). X-ray

Figure 1. (a) Top view and (b) side view of dye 1 in the crystal as determined from X-ray analysis (CCDC 1826513). All hydrogens are omitted for clarity. Relevant dihedral angles of aryl units relative to the pyrrolo[3,2-b]pyrrole core were calculated as α = 0.5(α1 + α2) and β = 0.5(β1 + β2).

analysis also revealed that the dihedral angle between the aryl rings connecting TPE to the pyrrolo[3,2-b]pyrrole core (α) is only 29.0° (Table S1), while the values reported previously for simpler TAPPs were found to be >35°.15 Conversely, the dihedral angle between N-aryl rings and the core (β) in 1 (53.7°) is greater or comparable with the values determined previously for TAPPs possessing smaller aryl substituents.15 It seems that the tetraphenylethylene moieties play a crucial role in the arrangement of 1 in the crystal lattice. The distance of intermolecular CH···π interactions between hydrogen and πsystem of the nearest orthogonal aryl ring within a TPE moiety have a length of only 2.751 Å (Figure S2a). Spectroscopic and photophysical properties of derivatives 1 and 2 have been evaluated both in CH2Cl2 solution and in the solid state by means of steady-state and time-resolved optical techniques. Absorption and emission spectra are compared in Figure 2, while the relevant parameters are summarized in Table 1. In parallel, we employed computational methods in order to

Figure 2. Normalized absorption (solid line) and emission (dashed line) spectra of compounds 1 (top) and 2 (bottom) in CH2Cl2 solution (blue) and in the solid state (red). B

DOI: 10.1021/acs.orglett.8b01011 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters Table 1. Optical Properties of 1 and 2 Measured Either in Solution or in the Solid State 1 compd λabs/nm ε × 10−4/M−1·cm−1 λema/nm ΔSS/cm−1 ϕfl τ/ns kr/s−1 knr/s−1

2

CH2Cl2 398 5.4 586 8100 0.0085b 0.063d 1.3 × 108 1.6 × 1010

solid

CH2Cl2

516 4700 0.77 ± 0.04c 1.5e,f 5.1 × 108 1.5 × 108

337 6.5 662 14 600 0.0013b 1.6e 8.1 × 105 6.2 × 108

415

solid 392 468 4100 0.16 ± 0.04c 19.4e,f 8.2 × 106 4.3 × 107

a Emission maxima from corrected spectra. bEmission quantum yields, measured with reference to Coumarin 153 in ethanol. cAbsolute emission quantum yields. dLifetime measured using a streak camera (resolution 1 ps), λexc = 350 nm. eLifetimes measured using a TCSPC (resolution 0.2 ns), λexc = 373 nm. fMain component (90% of the decay).

as the energy difference between these transitions is rather small (ΔE = 0.46 eV). The computational studies clearly show that the changes in the TPE geometry parameters during excitation are similar for both 1 and 2 (Table S2). On the other hand, the dihedral angles α were found to decrease their value in the excited state for compound 1. At the same time, however, the dihedral angles β (related to biaryl N−C linkage) do not change in the excited state. Yet, the calculated oscillator strength for the S1 → S0 transitions is large (Figure 3). The situation is entirely different for dye 2. Again, there is a change in dihedral angles α but additionally the second dihedral angle β increases in the excited state from ∼50° to ca. 70° (Table S2). At the same time, the LUMO is almost perpendicular to the HOMO, which translates to a decrease in the oscillator strength for the S1 → S0 transition. In the solid state, the rigidity imposed by the crystal lattice hampers this rotation, which results in stronger emission. The HOMO and the LUMO orbital shapes determined for 1 partially overlap, and this balanced distribution suggests good electronic communication between the PP core and TPE unit (a weak acceptor), which is typical for most TAPPs synthesized previously.11a Conversely, the HOMO of 2 is localized on the PP core and the LUMO level is localized on the TPE unit, which has been noted earlier for TAPPs bearing electron-acceptor groups15a or larger aromatic moieties at positions 1 and 4.14a To further investigate AIE characteristics of dyes 1 and 2, the fluorescence spectra were measured in THF/H2O mixtures with water fractions ( f w) from 0 to 90% (Figure S5). Addition of water above 50% caused the sharp increase in emission intensity for both compounds. Interestingly, in the case of dye 2 this trend was followed by a sharp decrease, which points out to the formation on various types of aggregates depending of the water fraction. In summary, combining pyrrolo[3,2-b]pyrrole and tetraphenylethylene moieties effectively extended the π-surface of the resulting dyes and, at the same time, led to enhanced AIE properties. Regardless of the linking position, the emission enhancement while moving from solution to the solid state was ∼100-fold, but there are significant differences in their optical properties. Linking both units at positions 1 and 4 led to weak emission in the solution but a very large Stokes shift (14600 cm−1), which has its origin in increasing the dihedral angles around the heterocyclic core in the excited state. The latter one is reflected in the perpendicular arrangement of HOMO to LUMO in allowed transitions.

greater in the case of 2 (ca. 20°) and at the same time steric repulsions intensify; thus, rotational freedom is more suppressed, resulting in reduction of rotational/vibrational deactivation pathways. Moreover, in contrast to compound 1, compound 2 shows a 10-fold increase in the radiative rate constant within the lattice. This can be ascribed to not only suppression of RIM caused by rigidification but also to a less distorted excited state, as inferred by the higher emission energy22 (Figure 2 and Table 1) compared to solution. Singlet oxygen luminescence measurements in CH2Cl2 revealed no signal in any of the samples, indicating that either singlet oxygen is not produced or its quantum yield is below our experimental detection limit (around 0.1). This gives an indication that the population of the triplet state is not the favored nonradiative deactivation process of S1 in solution and confirms the role of RIM effects in governing knr. Overall, compound 1 shows a higher emission quantum yield with respect to 2, both in solution and in the solid state (from ∼5 to 6.5 times), also related to a remarkable value of kr (of the order of 108 s−1). To rationalize this observation, we determined computationally vertical excitation energies for both molecules (Figure 3 and Table S3). According to calculations, the lowest

Figure 3. Schematic presentation of energy diagrams for absorption and fluorescence for compounds 1 (left) and 2 (right).

excited-singlet state (S1) for compound 1 has the highest oscillator strength (f = 1.3031), and above this state, there is a cascade of much weaker absorbing or completely dark states. In the case of derivative 2, the S1 state is only weakly allowed (f = 0.0629). Above this state, the closely lying third and fifth excitedsinglet states are strongly allowed (f = 0.9648 and f = 0.7291, respectively). The S0 → S1 transition is not observed experimentally, as it is much weaker than that of S0 → S3 and C

DOI: 10.1021/acs.orglett.8b01011 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters

■

(7) (a) Yuan, W. Z.; Lu, P.; Chen, S.; Lam, J. W. Y.; Wang, Z.; Liu, Y.; Kwok, H. S.; Ma, Y.; Tang, B. Z. Adv. Mater. 2010, 22, 2159. (b) Huang, J.; Sun, N.; Chen, P.; Tang, R.; Li, Q.; Ma, D.; Li, Z. Chem. Commun. 2014, 50, 2136. (c) Mei, J.; Wang, J.; Sun, J. Z.; Zhao, H.; Yuan, W.; Deng, C.; Chen, S.; Sung, H. H. Y.; Lu, P.; Qin, A.; Kwok, H. S.; Ma, Y.; Williams, I. D.; Tang, B. Z. Chem. Sci. 2012, 3, 549. (8) Zhang, X.; Zhen, Y.; Fu, X.; Liu, J.; Lu, X.; He, P.; Dong, H.; Zhang, H.; Zhao, G.; Jiang, L.; Hu, W. J. Mater. Chem. C 2014, 2, 8222. (9) (a) Ding, D.; Li, K.; Liu, B.; Tang, B. Z. Acc. Chem. Res. 2013, 46, 2441. (b) Zhang, X.; Zhang, X.; Tao, L.; Chi, Z.; Xu, J.; Wei, Y. J. Mater. Chem. B 2014, 2, 4398. (c) Shi, H.; Kwok, R. T. K.; Liu, J.; Xing, B.; Tang, B. Z.; Liu, B. J. Am. Chem. Soc. 2012, 134, 17972. (d) Li, X.; Jiang, M.; Lam, J. W. Y.; Tang, B. Z.; Qu, J. Y. J. Am. Chem. Soc. 2017, 139, 17022. (10) Leung, N. L. C.; Xie, N.; Yuan, W.; Liu, Y.; Wu, Q.; Peng, Q.; Miao, Q.; Lam, J. W. Y.; Tang, B. Z. Chem. - Eur. J. 2014, 20, 15349. (11) (a) Krzeszewski, M.; Gryko, D.; Gryko, D. T. Acc. Chem. Res. 2017, 50, 2334 and references cited therein. (b) Domínguez, R.; Montcada, N. F.; de la Cruz, P.; Palomares, E.; Langa, F. ChemPlusChem 2017, 82, 1096. (c) Friese, D. H.; Mikhaylov, A.; Krzeszewski, M.; Poronik, Y. M.; Rebane, A.; Ruud, K.; Gryko, D. T. Chem. - Eur. J. 2015, 21, 18364. (d) Canjeevaram Balasubramanyam, R. K.; Kandjani, A. E.; Harrison, C. J.; Haroon Rashid, S. S. A. A.; Sabri, Y. M.; Bhargava, S. K.; Narayan, R.; Basak, P.; Ippolito, S. J. ACS Appl. Mater. Interfaces 2017, 9, 27875. (e) Dereka, B.; Rosspeintner, A.; Krzeszewski, M.; Gryko, D. T.; Vauthey, E. Angew. Chem., Int. Ed. 2016, 55, 15624. (12) Stężycki, R.; Grzybowski, M.; Clermont, G.; Blanchard-Desce, M.; Gryko, D. T. Chem. - Eur. J. 2016, 22, 5198. (13) (a) Janiga, A.; Krzeszewski, M.; Gryko, D. T. Chem. - Asian J. 2015, 10, 212. (b) Tasior, M.; Gryko, D. T. J. Org. Chem. 2016, 81, 6580. (c) Tasior, M.; Czichy, M.; Łapkowski, M.; Gryko, D. T. Chem. - Asian J. 2018, 13, 449. (d) Łukasiewicz, Ł.; Ryu, H. G.; Mikhaylov, A.; Azarias, C.; Banasiewicz, M.; Kozankiewicz, B.; Ahn, K. H.; Jacquemin, D.; Rebane, A.; Gryko. Chem. - Asian J. 2017, 12, 1736. (e) Krzeszewski, M.; Sahara, K.; Poronik, Y. M.; Kubo, T.; Gryko, D. T. Org. Lett. 2018, 20, 1517. (14) (a) Krzeszewski, M.; Kodama, T.; Espinoza, E. M.; Vullev, V. I.; Kubo, T.; Gryko, D. T. Chem. - Eur. J. 2016, 22, 16478. (b) Mishra, S.; Krzeszewski, M.; Pignedoli, C. A.; Ruffieux, P.; Fasel, R.; Gryko, D. T. Nat. Commun. 2018, 9, 1714. (15) (a) Li, K.; Liu, Y.; Li, Y.; Feng, Q.; Hou, H.; Tang, B. Z. Chem. Sci. 2017, 8, 7258. (b) Peng, Z.; Ji, Y.; Tong, B.; Shi, J.; Dong, Y. Huaxue Xuebao 2016, 74, 942. (c) Ji, Y.; Peng, Z.; Tong, B.; Shi, J.; Zhi, J.; Dong, Y. Dyes Pigm. 2017, 139, 664. (16) Luo, M.; Zhou, X.; Chi, Z.; Liu, S.; Zhang, Y.; Xu, J. Dyes Pigm. 2014, 101, 74. (17) (a) Janiga, A.; Głodkowska-Mrówka, E.; Stokłosa, T.; Gryko, D. T. Asian J. Org. Chem. 2013, 2, 411. (b) Krzeszewski, M.; Thorsted, B.; Brewer, J.; Gryko, D. T. J. Org. Chem. 2014, 79, 3119. (18) Martins, L. M.; de Faria Vieira, S.; Baldacim, G. B.; Bregadiolli, B. A.; Caraschi, J. C.; Batagin-Neto, A.; da Silva-Filho, L. C. Dyes Pigm. 2018, 148, 81. (19) Janiga, A.; Bednarska, D.; Thorsted, B.; Brewer, J.; Gryko, D. T. Org. Biomol. Chem. 2014, 12, 2874. (20) (a) Chen, S.; Hong, Y.; Liu, Y.; Liu, J.; Leung, C. W. T.; Li, M.; Kwok, R. T. K.; Zhao, E.; Lam, J. W. Y.; Yu, Y.; Tang, B. Z. J. Am. Chem. Soc. 2013, 135, 4926. (b) Qin, W.; Ding, D.; Liu, J.; Yuan, W. Z.; Hu, Y.; Liu, B.; Tang, B. Z. Adv. Funct. Mater. 2012, 22, 771. (c) Qin, W.; Li, K.; Feng, G.; Li, M.; Yang, Z.; Liu, B.; Tang, B. Z. Adv. Funct. Mater. 2014, 24, 635. (21) (a) Wu, Q.; Zhang, T.; Peng, Q.; Wang, D.; Shuai, Z. Phys. Chem. Chem. Phys. 2014, 16, 5545. (b) He, B.; Nie, H.; Chen, L.; Lou, X.; Hu, R.; Qin, A.; Zhao, Z.; Tang, B. Z. Org. Lett. 2015, 17, 6174. (22) Tramer, A.; Jungen, C.; Lahmani, F. Energy Dissipation in Molecular Systems; Springer: Berlin, 2005; pp 116−117.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b01011. Experimental procedures, compound characterization data, 1H NMR and 13C NMR spectra, cyclic voltammograms, computational details, and crystallographic data (PDF) Accession Codes

CCDC 1826513 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Barbara Ventura: 0000-0002-8207-1659 Daniel T. Gryko: 0000-0002-2146-1282 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The work was financially supported by the Polish National Science Centre (Grant Nos. PRELUDIUM 2016/23/N/ST5/ 00054 and MAESTRO 2012/06/A/ST5/00216). D.T.G. and B.S. thank Dr. I. Deperasińska (Institute of Physics PAS) for insightful discussions and Dr. A. Ciesielski and Prof. M. K. Cyrański (University of Warsaw) for photographs of the crystals. The Italian CNR (Project “PHEEL”), MIUR-CNR project “Nanomax” N-CHEM, CNR-ASRT (Egypt), Bilateral Cooperation Project “FLUO-NanoFAB” are acknowledged for financial support.

■

REFERENCES

(1) (a) Luo, J.; Xie, Z.; Lam, J. W. Y.; Cheng, L.; Chen, H.; Qiu, C.; Kwok, H. S.; Zhan, X.; Liu, Y.; Zhu, D.; Tang, B. Z. Chem. Commun. 2001, 1740. (b) An, B.-K.; Kwon, S.-K.; Jung, S.-D.; Park, S. Y. J. Am. Chem. Soc. 2002, 124, 14410−14415. (c) Tang, B. Z.; Zhan, X.; Yu, G.; Sze Lee, P. P.; Liu, Y.; Zhu, D. J. Mater. Chem. 2001, 11, 2974. (d) Hong, Y.; Lam, J. W. Y.; Tang, B. Z. Chem. Soc. Rev. 2011, 40, 5361. (e) An, B.K.; Gierschner, J.; Park, S. Y. Acc. Chem. Res. 2012, 45, 544−554. (2) (a) Hong, Y.; Lam, J. W. Y.; Tang, B. Z. Chem. Commun. 2009, 4332. (b) Lin, G.; Chen, L.; Peng, H.; Chen, S.; Zhuang, Z.; Li, Y.; Wang, B.; Zhao, Z.; Tang, B. Z. J. Mater. Chem. C 2017, 5, 4867. (3) (a) Jadhav, T.; Choi, J. M.; Shinde, J.; Lee, J. Y.; Misra, R. J. Mater. Chem. C 2017, 5, 6014. (b) Liu, Y.; Shan, T.; Yao, L.; Bai, Q.; Guo, Y.; Li, J.; Han, X.; Li, W.; Wang, Z.; Yang, B.; Lu, P.; Ma, Y. Org. Lett. 2015, 17, 6138. (c) Huang, J.; Sun, N.; Yang, J.; Tang, R.; Li, Q.; Ma, D.; Qin, J.; Li, Z. J. Mater. Chem. 2012, 22, 12001. (4) Zeng, Q.; Li, Z.; Dong, Y.; Di, C.; Qin, A.; Hong, Y.; Ji, L.; Zhu, Z.; Jim, C. K. W.; Yu, G.; Li, Q.; Li, Z.; Liu, Y.; Qin, J.; Tang, B. Z. Chem. Commun. 2007, 70−72. (5) Chen, B.; Zhang, H.; Luo, W.; Nie, H.; Hu, R.; Qin, A.; Zhao, Z.; Tang, B. Z. J. Mater. Chem. C 2017, 5, 960. (6) Guo, J.; Hu, S.; Luo, W.; Hu, R.; Qin, A.; Zhao, Z.; Tang, B. Z. Chem. Commun. 2017, 53, 1463. D

DOI: 10.1021/acs.orglett.8b01011 Org. Lett. XXXX, XXX, XXX−XXX