Emerging applications of curved aromatic compounds

Abstract

In this forum article, we discuss selected recent contributions to the field of nonplanar aromatic molecules, focusing on properties that arise from molecular curvature. Ten reports are highlighted to demonstrate the diverse applications of curved aromatics in materials science that would not be achieved by their planar congeners. In this forum article, we discuss selected recent contributions to the field of nonplanar aromatic molecules, focusing on properties that arise from molecular curvature. Ten reports are highlighted to demonstrate the diverse applications of curved aromatics in materials science that would not be achieved by their planar congeners. Graphene is a carbon allotrope comprising a single, flat layer of hexagonally arranged sp2-hybridized carbon atoms. A similar preference for a planar structure is also found in large benzenoid hydrocarbons, such as hexa-peri-hexabenzocoronene, which can be viewed as molecular fragments of graphene (nanographenes). Notably, different classes of curved nanographenes have been synthesized using various strategies [1.Majewski M.A. Stępień M. Bowls, hoops, and saddles: synthetic approaches to curved aromatic molecules.Angew. Chem. Int. Ed. 2019; 58: 86-116Crossref PubMed Scopus (223) Google Scholar]. In particular, incorporation of a five-membered ring into a nanographene is commonly used to create bowl-like, positively curved structures [2.Saito M. et al.Figuration of bowl-shaped π-conjugated molecules: properties and functions.Mater. Chem. Front. 2018; 2: 635-661Crossref Scopus (162) Google Scholar], whereas a seven- or eight-membered ring typically produces negative curvature and saddle-shaped molecular geometries [3.Márquez I.R. et al.Synthesis of distorted nanographenes containing seven- and eight-membered carbocycles.Chem. Commun. 2018; 54: 6705-6718Crossref PubMed Google Scholar]. Helicene motifs, which bear a structural relationship to negatively curved nanocarbons, have been extensively used to construct chiral nanographenes with unusual chiroptical features [4.Mori T. Chiroptical properties of symmetric double, triple, and multiple helicenes.Chem. Rev. 2021; 121: 2373-2412Crossref PubMed Scopus (220) Google Scholar]. Formation of closed aromatic loops yields neutrally curved systems, such as cycloparaphenylenes (CPPs), nanobelts, and cyclophanes, many of which display unique optical properties and host–guest chemistry [5.Guo Q.-H. et al.Aromatic hydrocarbon belts.Nat. Chem. 2021; 13: 402-419Crossref PubMed Scopus (69) Google Scholar]. In recent years, these nonplanar conjugated π-systems have received considerable interest because of their unprecedented optical, redox, charge transport, and self-assembly properties that depend on the type of curvature present in the molecule. In this forum article, we highlight some of the recently reported applications of curved aromatics in the fields of functional dyes, chemical sensors, and electroactive materials. In each of the ten examples, molecular curvature was essential for achieving the desired function. In 2022, Li and coworkers [6.Li Q.-Q. et al.Diazapentabenzocorannulenium: a hydrophilic/biophilic cationic buckybowl.Angew. Chem. Int. Ed. 2022; 61e202112638Google Scholar] reported a fluorescent bioimaging probe that featured the bowl-shaped diazapentabenzocorannulenium cation [1]+ (Figure 1A ). The curved structure of [1]+ and the presence of positively charged heteroatoms were proposed to confer solubility in polar solvents. Thus, the cationic buckybowl [1]+ could be used as a biocompatible fluorescence probe despite the lack of hydrophilic substituents. The chloride salt [1][Cl] was highly dispersible in water, showing fluorescence at 511 nm with a photoluminescence quantum yield of 6%. Confocal fluorescence imaging showed that [1][Cl] could selectively localize in mitochondria after cellular uptake by various tumor cell types. Such a selective subcellular localization was proposed to reflect the negative membrane potential of mitochondria. A cyclooctatetraene derivative 2 bearing two phenazine units fused at two nonadjacent double bonds of the cyclooctatetraene ring was described in 2021 by Goto and colleagues [7.Goto Y. et al.Dynamic polymer free volume monitored by single-molecule spectroscopy of a dual fluorescent flapping dopant.J. Am. Chem. Soc. 2021; 143: 14306-14313Crossref PubMed Scopus (16) Google Scholar] (Figure 1B). Compound 2 has a bent geometry in the ground state (S0), and undergoes planarization when excited to the lowest singlet excited state (S1), to produce fluorescence emission with a large Stokes shift of 4800 cm–1 (λmaxem = 518 nm). In viscous media, the planarization of S1 is partially suppressed, yielding a different fluorescence signature (λmaxem = 460 nm), which can be used for ratiometric fluorescence analysis. The authors showed that 2 is a suitable dual fluorescent probe for noninvasive measurement of dynamic free volume in polymers, which is an important quantity for developing membrane materials and chromatographic media. The excited-state planarization of 2 in a polystyrene sample occurred only when the molecule was surrounded by a free volume of more than 280 Å3. [4]Cyclo-2,8-chrysenylene 3, reported by Sato and coworkers in 2017 [8.Sato S. et al.Chiral intertwined spirals and magnetic transition dipole moments dictated by cylinder helicity.Proc. Natl. Acad. Sci. U. S. A. 2017; 114: 13097-13101Crossref PubMed Scopus (152) Google Scholar], represents a segment of the (12,8) chiral single-walled carbon nanotube (Figure 1C). Pure enantiomers of 3 showed exceptionally large dissymmetry factors in their circular dichroism and circularly polarized luminescence spectra, with values as high as |gabs| = 0.167 and |glum| = 0.152, respectively, recorded for (P)-3 at 443 nm. The |glum| value, in particular, was one order of magnitude greater than those determined earlier for metal-free organic molecules. Theoretical calculations predicted that the electronic (μ) and magnetic (m) transition dipole moments of 3 are oppositely oriented along the cylinder axis, thus maximizing the rotatory strength R = |μ|·|m|·cosθ with θ = 180°. Besides the large |glum| value, a large photoluminescence quantum yield of 0.80 was attained by compound 3 in toluene, thereby fulfilling the criteria for an efficient circularly polarized luminescence-emitting material. An example of using nonplanar aromatics for emerging applications in photovoltaics was reported in 2019 by Conrad-Burton and colleagues [9.Conrad-Burton F.S. et al.Controlling singlet fission by molecular contortion.J. Am. Chem. Soc. 2019; 141: 13143-13147Crossref PubMed Scopus (40) Google Scholar]. They synthesized the contorted perylene diimide (PDI) derivatives 4a and 4b by introducing two terphenyl bridges that spanned the bay regions of the PDI core (Figure 1D) [9.Conrad-Burton F.S. et al.Controlling singlet fission by molecular contortion.J. Am. Chem. Soc. 2019; 141: 13143-13147Crossref PubMed Scopus (40) Google Scholar]. The molecule of 4a was shown by X-ray crystallography to possess a longitudinally contorted backbone. Density functional theory calculations predicted the singlet (S1) and triplet (T1) energies of 4 to meet the energetic requirement for singlet fission, that is, E(S1) ≥ 2 × E(T1). The S1 state of 4 was experimentally shown to undergo singlet fission with a time constant of τ1 ≈ 2.5 ps, which was much faster than that of planar PDI (τ1 = 180–3800 ps). Host systems containing curved π surfaces can be designed for binding appropriately shaped guest molecules. In particular, binding of spherical guests, such as fullerenes, can be accomplished using either positively or neutrally curved surfaces, providing access to novel charge-transfer systems. The use of positive curvature is exemplified by molecular tweezers 5a and 5b, each consisting of two azabuckybowls interconnected via a carbazole or a phenanthrene unit, as described in 2018 by Takeda and coworkers (Figure 1E) [10.Takeda M. et al.Azabuckybowl-based molecular tweezers as C60 and C70 receptors.J. Am. Chem. Soc. 2018; 140: 6336-6342Crossref PubMed Scopus (81) Google Scholar]. Each tweezer could bind one fullerene molecule (C60 or C70) via a concave–convex interaction with large association constants (107–108 M−1 in toluene). Both tweezers exhibited enhanced two-photon absorption cross-sections upon complexation with fullerenes. Such nonlinear optical properties were proposed to originate from the intermolecular charge transfer in the supramolecular donor–acceptor–donor assemblies. The porphyrin–[10]CPP conjugate 6, reported by Xu and coworkers [11.Xu Y. et al.A supramolecular [10]CPP junction enables efficient electron transfer in modular porphyrin–[10]CPP⊃fullerene complexes.Angew. Chem. Int. Ed. 2018; 57: 11549-11553Crossref PubMed Scopus (69) Google Scholar], enabled self-assembly of charge-transfer systems containing chemically intact fullerenes (Figure 1F). The neutrally curved cavity of the hoop-like [10]CPP unit served as a binding site for C60 with a large association constant of 1.6 × 106 M−1 in toluene. Upon photoexcitation, the complex 6⊃C60 was demonstrated to form a metastable charge-separated state 6•+⊃C60•−. This occurrence of intra(supra)molecular electron transfer from the porphyrin unit to the bound C60 was monitored by the time evolution of the absorbances at 670 and 1090 nm. In 2020, Schaub and coworkers [12.Schaub T.A. et al.Exploration of the solid-state sorption properties of shape-persistent macrocyclic nanocarbons as bulk materials and small aggregates.J. Am. Chem. Soc. 2020; 142: 8763-8775Crossref PubMed Scopus (57) Google Scholar] reported the spiro[n,n]CPPs 7a (n = 8) and 7b (n = 10), both adopting a lemniscate-like shape (Figure 2A ) [12.Schaub T.A. et al.Exploration of the solid-state sorption properties of shape-persistent macrocyclic nanocarbons as bulk materials and small aggregates.J. Am. Chem. Soc. 2020; 142: 8763-8775Crossref PubMed Scopus (57) Google Scholar]. Sorption of small-molecule vapors by solid-state samples of 7a and 7b was demonstrated using quartz-crystal microbalance measurements and explained in terms of host–guest interactions between the nanohoops and analyte molecules. In comparison with [9]CPP, affinities of both lemniscates toward benzene and its derivatives were increased by 70–100%. The superior affinity of 7a and 7b over [n]CPPs was attributed to the supramolecular arrangement of molecules in these materials, in which the lemniscates are packed loosely, facilitating access of analyte molecules. A different operating principle was used by Li and colleagues [13.Li C. et al.Functionalized π stacks of hexabenzoperylenes as a platform for chemical and biological sensing.Chem. 2018; 4: 1416-1426Abstract Full Text Full Text PDF Scopus (31) Google Scholar], who developed an organic field-effect transistor (OFET) sensor using functionalized hexabenzoperylenes (HBPs, 8a–c; Figure 2B). The twisted HBP core contains two [6]helicene subunits and forms one-molecule-thick nanosheets via π stacking. OFET devices based on the trimethylsilyloxy-functionalized compound 8a and biotin-functionalized compound 8b could detect fluoride (detection limit = 1 μM) and streptavidin (detection limit = 36 pM), respectively. The choice of the twisted HBP skeleton was crucial because the π–π interaction-mediated assembly of HBP molecules into nanosheets for OFET function is not obstructed by the sensing substituents, as proved by the X-ray structure of the HBP derivative 8c. In 2021, Furukawa and coworkers [14.Furukawa S. et al.Ferroelectric columnar assemblies from the bowl-to-bowl inversion of aromatic cores.Nat. Commun. 2021; 12: 768Crossref PubMed Scopus (33) Google Scholar] demonstrated the use of positively curved aromatic cores as switchable components in ferroelectric materials. Their work relied on hexaalkoxy-substituted trithiasumanene derivatives 9, which form columnar assemblies via concave–convex π–π stacking (Figure 2C). Trithiasumanene was chosen as the bowl-shaped aromatic core because it possesses a dipole moment of 1.03 D and a low inversion barrier (1.9 kcal mol−1). Self-assembled columnar stacks of 9 were shown to undergo polarization upon application of a voltage pulse, followed by relaxation of the macro dipole moment through bowl inversion in the bulk. Such dynamic changes of aromatic curvature provide a new principle for ferroelectric switching, which could be used for fabrication of high-density memory materials. Zhang and colleagues [15.Zhang Y. et al.Charging a negatively curved nanographene and its covalent network.J. Am. Chem. Soc. 2021; 143: 5231-5238Crossref PubMed Scopus (26) Google Scholar] reported a covalent organic network 10 based on octabenzo[8]circulene (OB8C) for application in lithium ion batteries (Figure 2D). The carbon-rich network of 10 proved functional as an anode material despite lacking long-range order. At a current density of 100 mA g−1, the maximum discharge capacity was 830 mAh g−1 (after 66 cycles) and the Coulombic efficiency exceeded 98% (after 200 cycles). The porosity of the material, originating from the negative curvature of OB8C, was believed to allow diffusion of lithium ions with low barriers. The saddle-shaped OB8C units were also considered responsible for strong binding of lithium ions. As illustrated by the aforementioned examples, curvature of π-conjugated systems can enable a range of useful functionalities that are not accessible in conventional, planar aromatics. The shape of the π system can be used to modulate optical properties, such as the fluorescence wavelength, optical rotatory strength, and excited-state energetics. The unique stacking preferences of curved aromatics and their porous nature in the solid state are essential for their application as sensing, responsive, and electroactive materials. The emergence of such highly promising areas of application provides an important stimulus for continued exploration of curved aromatics and will motivate further developments of synthetic methodologies required for scalable production of these materials. Financial support from the National Science Center of Poland (UMO-2018/29/B/ST5/01842 to M.S.) is gratefully acknowledged. No interests are declared.

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