Conjugated Nanohoops Incorporating Donor, Acceptor, Hetero- or Polycyclic Aromatics

Abstract

In the last 13 years several synthetic strategies were developed that provide access to [n]cycloparaphenylenes ([n]CPPs) and related conjugated nanohoops. A number of potential applications emerged, including optoelectronic devices, and their use as templates for carbon nanomaterials and in supramolecular chemistry. To tune the structural or optoelectronic properties of carbon nanohoops beyond the size-dependent effect known for [n]CPPs, a variety of aromatic rings other than benzene were introduced. In this Review, we provide an overview of the syntheses, properties, and applications of conjugated nanohoops beyond [n]CPPs with intrinsic donor/acceptor structure or such that contain acceptor, donor, heteroaromatic or polycyclic aromatic units within the hoop as well as conjugated nanobelts.

Keywords: cyclacenes; cycloparaphenylenes; macrocycles; nanohoops; optoelectronic properties.

Figure 1

a) [n]Cycloparaphenylenes as conjugated nanohoops, b) nanohoops incorporating π‐systems other than benzene (Sections 2–6), and c) nanobelts (Section 7), covered in this review.

Scheme 1

Synthetic strategies to conjugated nanohoops used for the compounds reviewed herein.

Figure 2

Donor–acceptor nanohoops 1 and 2, 3 and 4, 5, 6 a and b, 7 a/b, 8, 84, 85, and 86.

Figure 3

a) Optical spectra of 1 with photographs of emission colors, reprinted with permission from ref. ; copyright 2015 Wiley‐VCH. b) Frontier molecular orbitals of 30 and methylated 5, reproduced from ref.  with permission from The Royal Society of Chemistry. c) Frontier molecular orbitals of 6 c, reprinted with permission from ref. ; copyright 2017 Wiley‐VCH.

Figure 4

Acceptor‐containing nanohoops 9, 10, and 11.

Figure 5

Thiophene‐ and furan‐containing nanohoops [n]12, [n]13 and 14, 15, and 16 and [n]17.

Figure 6

a) Molecular structure of [4]12 in the solid state (thermal ellipsoids shown at 50 % probability level, with enclosed cyclohexane) and tubular packing structure, reprinted with permission from ref. ; copyright 2015 Wiley‐VCH. b) Optical spectra of 22 (A), electronic circular dichroism spectra (B), and circularly polarized luminescence spectra (C) of the (P) and (M) isomers, and frontier molecular orbitals, reprinted with permission from ref. ; copyright 2019 American Chemical Society.

Figure 7

Carbazole‐containing nanohoops 18 and 19, 20 a and b and 21, 20 c, and 22.

Figure 8

Nanohoops with N‐containing six‐ring heterocycles [n]23–[n]26, 27–29, 30, 32, 33, [n]31, and 34.

Figure 9

Molecular structures or packing in the solid state (thermal ellipsoids shown at 30 % probability levels, H atoms and solvent molecules omitted for clarity) of a) catenane 34, reprinted from ref. , licensed under CC BY 4.0, https://creativecommons.org/licenses/by/4.0/.; b) Pd²⁺ complex of 33, reprinted with permission from ref. ; copyright 2017 American Chemical Society; c) [4]35 (substituents not shown), reprinted with permission from ref. ; copyright 2015 American Chemical Society, and d) C₆₀ complex of [2]37, reprinted with permission from ref. ; copyright 2019 American Chemical Society.

Figure 10

Porphyrin‐containing nanohoops [n]35, [n]36, and [n]37–[3]39.

Figure 11

Nanohoops with five‐ring‐containing PAHs [n]40 a, [4]40 b, [4]40 c, [n]40 d and e, [n , n ]41, 87, [n]42, 88, 43, and 44. The color red is used to highlight the dibenzo[a,e]pentalene moieties due to their small band gap and red color.

Figure 12

a) Two possible diastereomers of [3]42, reprinted with permission from ref. ; copyright 2017 Wiley‐VCH; b) frontier molecular orbitals of 43 c (with R=Me) and optical spectra of 43 a, reprinted with permission from ref. ; copyright 2020 American Chemical Society; c) electronic circular dichroism spectra of (+)‐44 and (−)‐44 with spectrum simulated for enantiomer (+)‐44 and calculated equilibrium structure of 44⊃2 C₆₀, reprinted with permission from ref. , copyright 2020 Wiley‐VCH GmbH.

Figure 13

Naphthalene‐containing nanohoops [n]45,[ ⁹⁷ , ⁹⁸ ] [n]46,[ ¹⁰⁰ , ¹⁰¹ ] 47, [n]48,[ ¹⁰² , ¹⁰³ , ¹⁰⁴ ] 49, 50 a, 50 b, 51 and 52 and phenanthrene‐containing hoop 53.

Figure 14

a) Optical spectra of [n]48 (n=8, 10, 12, 16), reprinted from ref. , “Synthesis and properties of [8]‐, [10]‐, [12]‐, and [16]cyclo‐1,4‐naphthylenes”, licensed under CC BY 3.0, https://creativecommons.org/licenses/by/3.0/ published by The Royal Society of Chemistry. b) Molecular structure of 53 in the solid state as pair of enantiomers and CD spectra of isolated enantiomers, reprinted with permission from ref. , https://pubs.acs.org/doi/10.1021/acscentsci.6b00240 (further permissions related to the material excerpted should be directed to the ACS).

Figure 15

Chrysene‐ (54 and 55 ^[109] ) and anthanthrene‐containing nanohoops (56 and 57 ^[111] ).

Figure 16

a) Circularly polarized luminescence spectra of the two enantiomers of (12,8)‐54, reprinted with permission from ref. . b) Molecular packing of (16,0)‐55 in the solid state in pairs of enantiomers, reprinted with permission from ref. ; copyright 2012 American Chemical Society. c) Molecular structures in the solid state of the C₁₂₀ complexes of 56 b, reprinted with permission from ref. ; copyright 2017 Wiley‐VCH.

Figure 17

Anthracene‐containing nanohoops 58, 59, 60, [n]61, 62, and 63.

Figure 18

a) Calculated diastereomeric structures and energies of 63. b) Experimental and theoretical CD and c) circularly polarized luminescence spectra of 63, reprinted with permission from ref. ; copyright 2020 Wiley‐VCH. d) Optical spectra of 65, reprinted with permission from ref. ; copyright 2020 Wiley‐VCH.

Figure 19

Pyrene‐containing nanohoops 64 and 65.

Figure 20

Hexabenzocoronene‐containing nanohoops 66, 67, 68, and 69, tribenzo[fj,ij,rst]pentaphene‐based hoop 70, and benzotetraphene‐ (71 and 72) and dibenzopentaphene‐ (73) containing hoops.

Figure 21

a) STM images of 67 on Au(111) in ultrahigh vacuum, reproduced from ref.  with permission from The Royal Society of Chemistry. b) Optical spectra of 68 and HBC and photograph of emission color, reprinted with permission from ref. ; copyright 2017 Wiley‐VCH. c) Molecular structure of the C₆₀ complex of 70 in the solid state (thermal ellipsoids shown at 50 % probability level, hydrogen atoms omitted for clarity), reprinted with permission from ref. ; copyright 2019 Wiley‐VCH.

Figure 22

Conjugated nanobelts 74, 75 and 76,[ ¹³⁹ , ¹⁴⁰ ] and 77.[ ¹⁴¹ , ¹⁴² ]

Figure 23

Conjugated nanobelts consisting of annelated six‐membered rings 78,[ ¹⁴⁴ , ¹⁴⁵ ] 79, 80 and 81, 82 and 83, 87 and 88.

Figure 24

a) Packing of 77 in the solid state, reprinted with permission from ref. ; copyright 2009 Wiley‐VCH. b) Optical spectra of 78 a and 78 b, reprinted with permission from ref. ; copyright 2004 American Chemical Society. c) Optical spectra (with photograph of emission color) and molecular structures in the solid state (thermal ellipsoids shown at 50 % probability level, hydrogen atoms omitted for clarity) of 79–81, from ref.  and , reprinted with permission from AAAS. https://science.sciencemag.org/content/356/6334/172 and copyright 2018 American Chemical Society. d) Optical spectra of 82 and 83, reprinted from ref. , “Synthesis of Armchair and Chiral Carbon Nanobelts”, copyright 2019, with permission from Elsevier.