Excellent CPL-emitters : Planar chiral cyclophane chemistry

[2.2]Paracyclophane is a unique aromatic compound consisting of two stacked benzene rings in proximity.  Planar chirality, which arises from the proximally fixed benzene rings, is one of the interesting structural characteristics of the [2.2]paracyclophane skeleton.  We reported practical methods for the optical resolution of planar chiral 4,12-disubstituted and 4,7,12,15-tetrasubstituted [2.2]paracyclophane isomers [1] below.

    

     

We have synthesized planar chiral [2.2]paracyclophane-based conjugated molecules.  We reported the first observation of chiral luminescence “circularly polarized luminescence = CPL” from the planar chiral molecules in the world [2,3].

CPL active molecules are the promising candidates for 3D light-emitting-devices, liquid crystals, dyes for forgery-prevention of documents, and so on.  The representative examples of CPL emitters based on the planar chiral [2.2]paracyclophane skeletons are as follows…

      

[1] (a) Pseudo-ortho-disubstituted [2.2]paracyclophane: Chem. Lett. 2012, 41(9), 990-992. DOI: 10.1246/cl.2012.990

(b) Pseudo-meta-disubstituted [2.2]paracyclophane: Chem. Commun. 2021, 57(73), 9256-9259. DOI: 10.1039/d1cc03320d

(c) Bis-(para)-pseudo-ortho-disubstituted [2.2]paracyclophane: Chem. Asian J. 2019, 14(10), 1681-1685. DOI: 10.1002/asia.201801741

(d) Bis-(para)-pseudo-meta-disubstituted [2.2]paracyclophane: Chirality 2018, 30(10), 1109-1114. DOI: 10.1002/chir.23010

(e) Bis-(para)-pseudo-ortho,meta-disubstituted [2.2]paracyclophane: J. Am. Chem. Soc. 2014, 136(9), 3350-3353. DOI: 10.1021/ja412197j

[2] Polym. Chem. 2012, 3(10), 2727-2730. DOI: 10.1039/c2py20530k

[3] Recent Accounts: (a) Bull. Chem. Soc. Jpn. 2019, 92(2), 265-274. DOI: 10.1246/bcsj.20180259

(b) In Circularly Polarized Luminescence of Isolated Small Organic Molecules, Springer, Singapore; 2020, Chapter 3, pp 31-52. DOI: 10.1007/978-981-15-2309-0_3

(c) In Progress in the Science of Functional Dyes, Springer, Singapore; 2021, Chapter 10, pp 343-374. DOI: 10.1007/978-981-33-4392-4_10

      

1.CPL-emitters consisting of planar chiral disubstituted [2.2]paracyclophanes

Optically active V-, N-, M-, triangle-shaped oligomers [4] and zig-zag-shaped polymers [2,5] were prepared using planar chiral disubstituted [2.2]paracyclophane building blocks.

      

         

All molecules emit bright CPL.  We found that the oligomers and polymers folded to form one-handed helical structures in the excited state.

          

[4] Chem. Eur. J. 2014, 20(27), 8386-8390. DOI: 10.1002/chem.201402930

[5] (a) Polym. J. 2015, 47(3), 278-281. DOI: 10.1038/pj.2014.118

(b) Polym. J. 2022, in press.

 

2.CPL-emitters consisting of planar chiral tetrasubstituted [2.2]paracyclophanes

We succeeded in the optical resolutions of planar chiral tetrasubstituted [2.2]paracyclophanes and in the syntheses of chiral molecules with unique optically active second-ordered structures, such as X-shaped molecules [6], dendrimers [7], one-handed double helix [8], cyclic oligomers [9], †-shaped molecules, and ‡-shaped molecules [10].  They also emit bright CPL.

      

     

[6] (a) Eur. J. Org. Chem. 2015, (35), 7756-7762. DOI: 10.1002/ejoc.201501181

(b) Chem. Eur. J. 2017, 23(26), 6323-6329. DOI: 10.1002/chem.201605598

(d) Macromolecules 2017, 50(5), 1790-1802. DOI: 10.1021/acs.macromol.6b02798

(d) Chem. Commun. 2017, 53(59), 8304-8307. DOI: 10.1039/C7CC03615A

[7] Chem. Eur. J. 2016, 22(7), 2291-2298. DOI: 10.1002/chem.201504270

[8] Chem. Asian J. 2016, 11(18), 2524-2527. DOI: 10.1002/asia.201601028

[9] Chem. Asian J. 2022, 17(2), e202101267(1-7). DOI: 10.1002/asia.202101267

[10] Bull. Chem. Soc. Jpn. 2020, 93(10), 1193-1199. DOI: 10.1246/bcsj.20200160

       

3.Cyclic molecules emitting CPL with high anisotropy factors

We synthesized optically active propeller-shaped cyclic molecules [1f,11] and one-handed helical molecules [12].  They emit intense CPL with high anisotropy factors (high |glum| values) of the order of 10–2.  We disclosed experimentally and theoretically that optically active cyclic and helical molecules with a single π-conjugated system emit excellent CPL with high |glum| values.

    

        

[11] (a) J. Mater. Chem. C 2015, 3(3), 521-529. DOI: 10.1039/c4tc02339k

(b) Asian J. Org. Chem. 2016, 5(3), 353-359. DOI: 10.1002/ajoc.201500468

[12] Bull. Chem. Soc. Jpn. 2022, 95(1), 110-115. DOI: 10.1246/bcsj.20210368

       

4.Control of axial chirality, helicity, and twist chirality by planar chirality

Axial chirality [13], helicity [1b,12,14], and twist chirality [14] were controlled by planar chirality of the [2.2]paracyclophane skeletons.  Intense CPL was observed from all molecules; in particular, dual emission was observed from molecules with the controlled axial chirality [13]. Helicity was induced to phenanthrene to be [3]helicene by the planar chiral [2.2]paracyclophane [1b]. Greater degree of anthracene twisting resulted in a higher CPL |glum| value [14].

        

        

[13] Chem. Eur. J. 2020, 26(65), 14871-14877. DOI: 10.1002/chem.202003188

[14] Chem. Asian J. 2022, 17(15), e202200418(1-6). DOI: 10.1002/asia.202200418

      

5.Systematic syntheses of optically active π-staked molecules: correlation between orientation of the stacked π-electron systems and CPL

π-Stacked molecules become planar chiral molecules depending on the orientation of the two stacked π-electron systems.  We have investigated correlation between orientation of the stacked π-electron systems and CPL by changing stacked angles (60° or 120°) and stacking positions (center or terminal) of the two π-electron systems [1d,15].

     

     

[14] (a) Mater. Chem. Front. 2018, 2(4), 791-795. DOI: 10.1039/C7QM00613F

(b) Bull. Chem. Soc. Jpn. 2021, 94(2), 451-453. DOI: 10.1246/bcsj.20200294

(c) ChemistrySelect 2021, 6(45), 12970-12974. DOI: 10.1002/slct.202103587

(d) Eur. J. Org. Chem. 2021, 5725-5731. DOI: 10.1002/ejoc.202101119

(e) Bull. Chem. Soc. Jpn. 2022, 95(4), 595-601. DOI: 10.1246/bcsj.20220018

(f) Bull. Chem. Soc. Jpn. 2022, 95, in press. DOI: 10.1246/bcsj.20220153


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