Submitted Successfully!
To reward your contribution, here is a gift for you: A free trial for our video production service.
Thank you for your contribution! You can also upload a video entry or images related to this topic.
Version Summary Created by Modification Content Size Created at Operation
1
Wenzeng Duan
 -- 1602 2023-04-11 15:56:48 |
2 format correct
Conner Chen
Meta information modification 1602 2023-04-13 07:12:53 | |
3 format correct
Conner Chen
Meta information modification 1602 2023-04-14 02:23:08 |

Video Upload Options

Do you have a full video?
Send video materials Upload full video

Confirm

Are you sure to Delete?
Yes No
Cite
If you have any further questions, please contact Encyclopedia Editorial Office.
Liu, W.; Li, H.; Huo, Y.; Yao, Q.; Duan, W. Application of [2.2]Paracyclophane Skeleton in Modifying Dyes. Encyclopedia. Available online: https://encyclopedia.pub/entry/42949 (accessed on 03 May 2024).
Liu W, Li H, Huo Y, Yao Q, Duan W. Application of [2.2]Paracyclophane Skeleton in Modifying Dyes. Encyclopedia. Available at: https://encyclopedia.pub/entry/42949. Accessed May 03, 2024.
Liu, Wenjing, Huabin Li, Yanmin Huo, Qingxia Yao, Wenzeng Duan. "Application of [2.2]Paracyclophane Skeleton in Modifying Dyes" Encyclopedia, https://encyclopedia.pub/entry/42949 (accessed May 03, 2024).
Liu, W., Li, H., Huo, Y., Yao, Q., & Duan, W. (2023, April 11). Application of [2.2]Paracyclophane Skeleton in Modifying Dyes. In Encyclopedia. https://encyclopedia.pub/entry/42949
Liu, Wenjing, et al. "Application of [2.2]Paracyclophane Skeleton in Modifying Dyes." Encyclopedia. Web. 11 April, 2023.
Application of [2.2]Paracyclophane Skeleton in Modifying Dyes
Edit
This entry is adapted from the peer-reviewed paper 10.3390/molecules28072891

The [2.2]paracyclophane (PCP) ring has attracted extensive attention due to its features of providing not only chirality and electron-donating ability but also steric hindrance, which reduces intermolecular π–π stacking interactions and thereby improves the fluorescence properties of dyes.

fluorescent dye fluorescent probe AICPL

1. Introduction

[2.2]Paracyclophane (PCP) is a typical cyclophane that was first synthesized and isolated in 1949 as a pyrolysis product of para-xylene [1]. PCP includes two strongly interacting benzene unit “decks” with a distance of ~3.09 Å, which are connected by two ethylene “bridges” (~2.78 Å) (Figure 1) [2]. The two benzene rings stacked at such close proximity can lead to transannular π–π strain in the aromatic rings and cause deviation from the normal molecular structure. The strain, distorted structure and transannular effects alter the photophysical and optoelectronic properties of PCP scaffolds. Therefore, due to the intriguing structure and the unusual photophysical [3] and optoelectronic properties [4] of PCP scaffolds, they have been successfully used as privileged scaffolds in asymmetric synthesis [5], π-stacked polymers [6], energy materials [7], and organic fluorescent dyes [8] and have aroused wide interest in biological and materials science fields [9].
Molecules 28 02891 g001 550
Figure 1. Strain and structural parameters in ångstroms [Å] of PCP.
Conventional organic fluorescent dyes include cyanines [10], BODIPYs (4,4-difluoro-4-bora-3a,4a-diazas-indacene) [11], rhodamine analogues [12], squaraines [13], and porphyrins [14]. BODIPY dyes have a large π-conjugated skeleton and exhibit excellent photophysical properties in dilute solution. However, when in high concentration in solution or a state of aggregation, BODIPY dyes tend to adopt a face-to-face stacking mode (H-aggregation) and usually display an aggregation-caused emission quenching (ACQ) effect, which limits their application. Fortunately, the ACQ effect can be effectively overcome using an aggregation-induced emission (AIE) strategy, which was introduced by Tang et al. and has since been widely used [15]. Many AIE skeletons, such as 1,1,2,3,4,5-hexaphenylsilone and tetraphenylethylene, have been developed and applied in organic fluorescent dyes. These skeletons have twisted structures and can effectively inhibit the intermolecular π–π interactions occurring in the aggregation state. Taking advantage of PCP scaffolds, the introduction of a PCP group to BODIPY or BODIPY analogues can also result in elimination of the strong π–π interactions between the intermolecular indacene planes and achieve an AIE effect. Therefore, the introduction of the PCP skeleton to organic fluorescent dyes improves the photophysical properties of fluorescent dyes and has received increasing attention in recent decades.
Circularly polarized luminescence (CPL) has many potential applications in chirality sensing [16], optical displays [17][18][19], and chiroptical materials [20][21]. The two key factors to satisfy the requirements of the above applications during the development of CPL materials are high fluorescence quantum yield (Φf) and high luminescent dissymmetry factors (|glum|) [22]. In recent years, CPL-active small organic molecules (CPL-SOMs) have received wide attention due to their easy derivatization, excellent processability, tunable wavelengths, high fluorescence quantum yields, and low toxicity [23][24][25][26][27]. However, many CPL-SOMs suffer from the disadvantages of small |glum| values and aggregation-caused quenching (ACQ). An AIE strategy is also effective for achieving higher Φf and larger |glum| values, and has been applied to overcome ACQ in CPL-SOMs [28]. It is well known that AIE-active SOMs display no or weak fluorescence in dilute solution but become highly emissive as aggregates or in a solid state, which may increase Φf and amplify the |glum| values.

2. Application of PCP Skeleton in Modifying Dyes

It is well known that many reported organic fluorescent dyes suffer from small Stokes shifts, which affects their fluorescence emission efficiency. Therefore, it is necessary to develop a fluorescent dye with large Stokes shifts, superior optical properties, and good stability [29]. Due to the special electronic structure, [2.2]paracyclophanyl fluorescent dyes exhibit intrinsic fluorescence and can be applied to access new fluorescent dyes [30]. In 2019, Delcourt’s group reported a series of [2.2]paracyclophane-fused coumarin systems (Scheme 1) [31], which are compact heteroaromatic PCP dyes with unique three-dimensional (3D) structures. The introduction of the PCP skeleton allows the synthesis of 3D coumarin systems and improvement of the photophysical properties through large Stokes shifts and red-shifted absorption and emission bands. Furthermore, when planar chiral 4-formyl [2.2]paracyclophane was introduced to these [2.2]paracyclophane-fused coumarins, the resulting product exhibited promising chiroptical properties with |glum| up to 5 × 10−3.
Molecules 28 02891 sch001 550
Scheme 1. Chemical structures of PCP and PCP-related emitters (1−5).
In order to investigate the influence of the PCP motif and its “phane” interaction on the spectroscopic properties of fluorophores 1−5, the authors compared the different photophysical properties of these emitters (Table 1). Compared with [2.2]paracyclophane (Table 1, entry 1), the 3D coumarins (Table 1, entries 2−6) exhibited red-shifted absorption and emission bands. Moreover, when compared with the absorption and emission maxima of 4-amino [2.2]paracyclophane (Table 1, entry 7) and paracyclophane-deprived model compound 5a (Table 1, entry 9), the 3D coumarins also showed red-shifted absorption and emission bands. These results indicate that larger π-conjugation systems form between the PCP skeleton and the coumarin skeleton. Importantly, compared with commercially available coumarin 4, the wavelength absorption in the UV−vis spectra of fluorophores 1−3 was lower, and a remarkably large Stokes shift (230 nm) was obtained for coumarin 2d. In addition, in comparison with the behaviors of their unsubstituted analogues (±)-1b and (±)-1d, the 3D coumarins (±)-2a and (±)-2b with bromine atoms showed observed hypsochromic shifts of the emission bands, which indicated that the through-space interactions in the PCP skeleton could influence the photophysical properties of these 3D coumarins. Therefore, these results demonstrate that PCP can be introduced into coumarin fluorophores as a stable electron donor group and regulate the photophysical properties of these dyes.
Table 1. Structures and spectroscopic properties of PCP-related fluorescent dyes 1−14.
Entry Emitter Φf λabs (nm) λem in Solution (nm) λem in Other States (nm) References
1 PCP - 286, 302 356 - [31]
2 1a - 304, 320 445 - [31]
3 1d - 265, 300, 330 460 - [31]
4 1h - 270, 335 475 - [31]
5 1k - 330 470 - [31]
6 2b - 307, 325 450 - [31]
7 3 - 274, 324 386 - [31]
8 4 - 455 498 - [31]
9 5a - 294 418 - [31]
10 6d 0.87 589 619 636 ± 4 in amorphous
deposits		 [32]
11 7 0.95 543 556 636 ± 4 in amorphous
deposits		 [32]
12 8 0.064 722 795 1010 in aggregated state [33]
13 9 - 700 750 - [33]
14 10a - - 580 600 in the addition of Hg2+ [34]
15 11c 0.95 382 490 550 in solid state [35]
16 12a 0.39 325, 407 543 534 in aggregated state [36]
17 13 0.07 341, 423 534 531 in aggregated state [37]
18 14a 0.56 410, 470 641 686 in solid state [38]
BODIPY dyes tend to assemble into H-aggregates because of the large π-conjugated frameworks, and most BODIPY dyes usually show ACQ in the aggregation state. In order to solve this problem, BODIPY dyes can be employed as J-aggregation scaffolds. However, finding suitable building blocks for BODIPY J-aggregates remains a great challenge. In 2009, Meállet-Renault et al. [32] reported on a new type of hindered BODIPY dye prepared by modification of its pyrrole substituents. They introduced a PCP skeleton into the core of BODIPY to increase steric hindrance and prevent π–π stacking, which improved the photophysical properties of the BODIPY; these PCP-BODIPY dyes 6a−d showed red-shifted absorption and emission bands compared with Ph-BODIPY dye 7 (Table 1, entries 10,11, Scheme 2). Accordingly, the Stokes shifts of these PCP-BODIPY dyes 6a−d ranged between 881 and 1290 cm−1, which were higher values than for the Ph-BODIPY dye 7. These results may be due to the electron-donating nature and the steric hindrance of PCP, which enhance the conjugation effect and limit the possibilities for reorientation of the PCP group in the excited state. In particular, when their films were prepared by drop-casting, their emission bands were narrower than those in solution, and their fluorescence quantum yields increased dramatically and solid-state fluorescence were obtained, which may be attributed to the J-like aggregates in the film.
Molecules 28 02891 sch002 550
Scheme 2. Chemical structures of PCP-BODIPY and Ph-BODIPY (6−9).
In 2021, Liu and co-workers reported a new PCP-BODIPY dye 8 (Scheme 2) [33] as an example of a BODIPY dye with J-aggregation, which can induce second near-infrared fluorescence. First, the authors revealed that when the meso-position of BODIPY was changed from phenyl to [2.2]paracyclophane, compound PCP-BODIPY dye 8 showed red-shifted absorption and emission bands centered at 722 and 795 nm in comparison with its analog compound Ph-BODIPY 9 (λabs = 700 nm and λem = 750 nm) (Table 1, entries 12,13). This phenomenon can be attributed to the stronger electron-conjugation effect of the PCP group. In addition, they explored the J-aggregation behavior of PCP-BODIPY dye 8 by investigating the emission changes in tetrahydrofuran (THF)–water binary solvents with variations in water volumetric fraction. The results indicated that PCP-BODIPY dye 8 can exhibit J-aggregation and showed a weak NIR-II emission band around 1000 nm when the water volumetric fractions reached 80% and 90%. Moreover, the authors found that the photophysical properties of Ph-BODIPY 9 indicated H-aggregation-induced emission quenching in the aggregation state, which demonstrated that the introduction of the PCP group played a key role in the J-aggregation behavior of PCP-BODIPY dye 8.
Because of the NIR-II emission capability of PCP-BODIPY dye 8 in both THF–water and solid state, the authors also investigated the stability of the NIR-II emissive J-aggregates in NPs (Figure 2). The results indicated that the NIR-II emissive J-aggregates could be efficiently stabilized in a pluronic F-127 matrix. To verify the biological imaging capability of PCP-BODIPY 8, these authors prepared the PCP-BODIPY NPs by encapsulating the PCP- PCP-BODIPY 8 aggregates into a pluronic F-127 matrix and performed both in vitro and in vivo NIR-II imaging. The results of in vitro imaging revealed that the PCP-BODIPY 8 NPs had good NIR-II imaging penetration depth up to 8 mm, which was higher than the 6 mm of the clinically approved NIR-I dye indocyanine green (ICG). Moreover, the results of the in vivo imaging experiment showed that the brightness and clarity of PCP-BODIPY 8 NPs were higher than that of ICG under the same imaging conditions. Accordingly, the fluorescence of PCP-BODIPY 8 NPs decreased gradually with increase in time from 5 min to 24 h, in contrast to the undetectable fluorescence of ICG when the time was increased to 8 h. These results demonstrate that the PCP-BODIPY 8 NPs have higher resolution and longer-term NIR-II imaging ability than ICG. Furthermore, the potential application of PCP-BODIPY 8 NPs in mapping lymph nodes and image-guided cancer surgery was also investigated in nude mice. All the above results indicate that the steric and conjugation effect of the PCP skeleton plays a key role in manipulating the photophysical properties of BODIPY dyes and promoting their application in biological sensing and imaging.
Molecules 28 02891 g002
Figure 2. The NIR-II imaging of PCP-BODIPY in nanoparticles (NPs).

References

  1. Brown, C.J.; Farthing, A.C. Preparation and Structure of Di-p-Xylylene. Nature 1949, 164, 915–916.
  2. Hassan, Z.; Spuling, E.; Knoll, D.M.; Lahann, J.; Bräse, S. Planar chiral paracyclophanes: From synthetic curiosity to applications in asymmetric synthesis and materials. Chem. Soc. Rev. 2018, 47, 6947–6963.
  3. Gleiter, R.; Hopf, H. Modern Cyclophane Chemistry; Wiley-VCH: Weinheim, Germany, 2004.
  4. Morisaki, Y.; Chujo, Y. Through-Space Conjugated Polymers Based on Cyclophanes. Angew. Chem. Int. Ed. 2006, 45, 6430–6437.
  5. Liang, H.; Guo, W.C.; Li, J.X.; Jiang, J.J.; Wang, J. Chiral Arene Ligand as Stereocontroller for Asymmetric C−H Activation. Angew. Chem. Int. Ed. 2022, 61, e202204926.
  6. Morisaki, Y.; Chujo, Y. Synthesis of π-Stacked Polymers on the Basis of Paracyclophane. Bull. Chem. Soc. Jpn. 2009, 82, 1070–1082.
  7. Yu, H.; Arunagiri, L.; Zhang, L.; Huang, J.C.; Ma, W.; Zhang, J.Q.; Yan, H. Transannularly conjugated tetrameric perylene diimide acceptors containing paracyclophane for non-fullerene organic solar cells. J. Mater. Chem. A 2020, 8, 6501–6509.
  8. Zhang, M.Y.; Peng, Q.; Zhao, C.H. High-contrast mechanochromic fluorescence from a highly solid-state emissive 2-(dimesitylboryl)phenyl-substituted paracyclophane. J. Mater. Chem. C 2021, 9, 1740–1745.
  9. Morisaki, Y.; Chujo, Y. Planar chiral paracyclophanes: Optical resolution and transformation to optically active π-stacked molecules. Bull. Chem. Soc. Jpn. 2019, 92, 265–274.
  10. Peng, X.J.; Yang, Z.G.; Wang, J.Y.; Fan, J.L.; He, Y.X.; Song, F.L.; Wang, B.S.; Sun, S.G.; Qu, J.L.; Qi, J.; et al. Fluorescence ratiometry and fluorescence lifetime imaging: Using a single molecular sensor for dual mode imaging of cellular viscosity. J. Am. Chem. Soc. 2011, 133, 6626–6635.
  11. Liu, Z.P.; Jiang, Z.Y.; Yan, M.; Wang, X.Q. Recent Progress of BODIPY Dyes with Aggregation-Induced Emission. Front. Chem. 2019, 7, 712.
  12. Beija, M.; Afonso, C.A.M.; Martinho, J.M.G. Synthesis and applications of Rhodamine derivatives as fluorescent probes. Chem. Soc. Rev. 2009, 38, 2410–2433.
  13. Egorova, A.V.; Leonenko, I.I.; Aleksandrova, D.I.; Skripinets, Y.V.; Antonovich, V.P.; Obukhova, E.N.; Patsenker, L.D. New Sm (III) complexes as electronic-excitation donors of the Seta-632 squaraine dye. Opt. Spectrosc. 2015, 119, 59–65.
  14. Banala, S.; Fokong, S.; Brand, C.; Andreou, C.; Kräutler, B.; Rueping, M.; Kiessling, F. Quinone-fused porphyrins as contrast agents for photoacoustic imaging. Chem. Sci. 2017, 8, 6176–6181.
  15. Hong, Y.M.; Lam, J.W.Y.; Tang, B.Z. Aggregation-induced emission. Chem. Soc. Rev. 2011, 40, 5361–5388.
  16. Zhu, D.L.; Jiang, W.; Ma, Z.T.; Feng, J.J.; Zhan, X.Q.; Lu, C.; Liu, J.; Liu, J.; Hu, Y.Y.; Wang, D.; et al. Organic donor-acceptor heterojunctions for high performance circularly polarized light detection. Nat. Commun. 2022, 13, 3454.
  17. Li, M.; Li, S.H.; Zhang, D.D.; Cai, M.H.; Duan, L.; Fung, M.K.; Chen, C.F. Stable enantiomers displaying thermally activated delayed fluorescence: Efficient OLEDs with circularly polarized electroluminescence. Angew. Chem. Int. Ed. 2018, 57, 2889–2893.
  18. Wang, Y.F.; Li, M.; Teng, J.M.; Zhou, H.Y.; Chen, C.F. High-performance solution-processed nondoped circularly polarized OLEDs with chiral triptycene scaffold-based TADF emitters realizing over 20% external quantum efficiency. Adv. Funct. Mater. 2021, 31, 2106418.
  19. Wang, Y.F.; Li, M.; Teng, J.M.; Zhou, H.Y.; Zhao, W.L.; Chen, C.F. Chiral TADF-Active Polymers for High-Efficiency Circularly Polarized Organic Light-Emitting Diodes. Angew. Chem. Int. Ed. 2021, 60, 23619–23624.
  20. Yeom, J.; Yeom, B.; Chan, H.; Smith, K.W.; Dominguez-Medina, S.; Bahng, J.H.; Zhao, G.P.; Chang, W.S.; Chang, S.J.; Chuvilin, A.; et al. Chiral templating of self-assembling nanostructures by circularly polarized light. Nat. Mater. 2015, 14, 66–72.
  21. Han, X.N.; Han, Y.; Chen, C.F. Pagoda arene and i-Pagoda arene. J. Am. Chem. Soc. 2020, 142, 8262–8269.
  22. Wu, Z.G.; Han, H.B.; Yan, Z.P.; Luo, X.F.; Wang, Y.; Zheng, Y.X.; Zuo, J.L.; Pan, Y. Chiral Octahydro-Binaphthol Compound-Based Thermally Activated Delayed Fluorescence Materials for Circularly Polarized Electroluminescence with Superior EQE of 32.6% and Extremely Low Efficiency Roll-Off. Adv. Mater. 2019, 31, 1900524.
  23. Sánchez-Carnerero, E.M.; Agarrabeitia, A.R.; Moreno, F.; Maroto, B.L.; Muller, G.; Ortiz, M.J.; de la Moya, S. Circularly Polarized Luminescence from Simple Organic Molecules. Chem. Eur. J. 2015, 21, 13488–13500.
  24. Sawada, Y.; Furumi, S.; Takai, A.; Takeuchi, M.; Noguchi, K.; Tanaka, K. Rhodium-Catalyzed Enantioselective Synthesis, Crystal Structures, and Photophysical Properties of Helically Chiral 1,1′-Bitriphenylenes. J. Am. Chem. Soc. 2012, 134, 4080–4083.
  25. Murayama, K.; Oike, Y.; Furumi, S.; Takeuchi, M.; Noguchi, K.; Tanaka, K. Enantioselective Synthesis, Crystal Structure, and Photophysical Properties of a 1,1-Bitriphenylene-Based Sila -helicene. Eur. J. Org. Chem. 2015, 2015, 1409–1414.
  26. Tanaka, H.; Inoue, Y.; Mori, T. Circularly polarized luminescence and circular dichroisms in small organic molecules: Correlation between excitation and emission dissymmetry factors. ChemPhotoChem 2018, 2, 386–402.
  27. Li, X.N.; Xie, Y.J.; Li, Z. The progress of circularly polarized luminescence in chiral purely organic materials. Adv. Photonics Res. 2021, 2, 2000136.
  28. Mei, J.; Leung, N.L.C.; Kwok, R.T.K.; Lam, J.W.Y.; Tang, B.Z. Aggregation-induced emission: Together we shine, united we soar! Chem. Rev. 2015, 115, 11718–11940.
  29. Li, Y.Y.; Wang, M.H.; Tao, Y.T.; Zhang, R.L.; Zhou, M.; Tao, P.; Feng, P.C.; Huang, W.; Huang, H.; Miao, W.J. Highly stable and biocompatible nanocontrast agent encapsulating a novel organic fluorescent dye for enhanced cellular imaging. Powder Technol. 2019, 358, 110–119.
  30. Benedetti, E.; Delcourt, M.L.; Gatin-Fraudet, B.; Turcaud, S.; Micouin, L. Synthesis and photophysical studies of through-space conjugated paracyclophane-based naphthalene fluorophores. RSC Adv 2017, 7, 50472–50476.
  31. Delcourt, M.L.; Reynaud, C.; Turcaud, S.; Favereau, L.; Crassous, J.; Micouin, L.; Benedetti, E. 3D Coumarin systems based on paracyclophane: Synthesis, spectroscopic characterization, and chiroptical properties. J. Org. Chem. 2019, 84, 888–899.
  32. Vu, T.T.; Badré, S.; Dumas-Verdes, C.; Vachon, J.J.; Julien, C.; Audebert, P.; Senotrusova, E.Y.; Schmidt, E.Y.; Trofimov, B.A.; Pansu, R.B.; et al. New hindered BODIPY derivatives: Solution and amorphous state fluorescence properties. J. Phys. Chem. C. 2009, 113, 11844–11855.
  33. Li, K.; Duan, X.C.; Jiang, Z.Y.; Ding, D.; Chen, Y.C.; Zhang, G.Q.; Liu, Z.P. J-aggregates of meso-paracyclophanyl-BODIPY dye for NIR-II imaging. Nat. Commun 2021, 12, 2376.
  34. Duan, W.Z.; Han, Y.F.; Liu, Q.S.; Cui, J.C.; Gong, S.W.; Ma, Y.D.; Zhang, C.L.; Sun, Z.F. Design and synthesis of novel rhodamine-based chemodosimeters derived from paracyclophane and their application in detection of Hg2+ ion. Tetrahedron Lett. 2017, 58, 271–278.
  35. Ji, H.H.; Duan, W.Z.; Huo, Y.M.; Liu, W.J.; Huang, X.Q.; Wang, Y.L.; Gong, S.W. Highly sensitive fluorescence response of paracyclophane modified D−A type chromophores to trace water, pH, acidic gases and formaldehyde. Dyes Pigm. 2022, 205, 110491.
  36. Li, K.; Ji, H.H.; Yang, Z.R.; Duan, W.Z.; Ma, Y.D.; Liu, H.T.; Wang, H.W.; Gong, S.W. 3D boranil complexes with aggregation-amplified circularly polarized luminescence. J. Org. Chem 2021, 86, 16707–16715.
  37. Duan, W.Z.; Liu, W.J.; Liu, H.T.; Ji, H.H.; Huo, Y.M.; Wang, H.W.; Gong, S.W. AIE-active aurones for circularly polarized luminescence and trace water detection. Chem. Commun. 2022, 58, 13955–13958.
  38. Ji, H.H.; Liu, W.J.; Huo, Y.M.; Han, M.; Yao, Q.X.; Gong, S.W.; Duan, W.Z. Planar chiral AIEgens based on paracyclophane as efficient solid-state deep red circularly polarized luminescent emitters. Dyes Pigm. 2023, 209, 110915.
More
© Text is available under the terms and conditions of the Creative Commons Attribution (CC BY) license; additional terms may apply. By using this site, you agree to the Terms and Conditions and Privacy Policy.
Upload a video for this entry
Information
Subjects: Chemistry, Organic
Contributors MDPI registered users' name will be linked to their SciProfiles pages. To register with us, please refer to https://encyclopedia.pub/register :
Wenjing Liu
,
Huabin Li
,
Yanmin Huo
,
Qingxia Yao
,
Wenzeng Duan
View Times: 281
Revisions: 3 times (View History)
Update Date: 14 Apr 2023
Table of Contents
    1000/1000
    Hot Most Recent
    Feedback