Since 1996, a growing number of strained macrocycles, comprising only sp2‐ or sp‐hybridized carbon atoms within the ring, have become synthetically accessible, with the [n]cycloparaphenyleneacetylenes (CPPAs) and the [n]cycloparaphenylenes (CPPs) being the most prominent examples. Now that robust and relatively general synthetic routes toward a diverse range of nanohoop structures have become available, the research focus is beginning to shift towards the exploration of their properties and applications. From a supramolecular chemistry perspective, these macrocycles offer unique opportunities as a result of their near‐perfect circular shape, the unusually high degree of shape‐persistence, and the presence of both convex and concave π‐faces. In this Minireview, we give an overview on the use of strained carbon‐rich nanohoops in host–guest chemistry, the preparation of mechanically interlocked architectures, and crystal engineering.

Efficient photoinduced electron transfer was observed across a [10]cycloparaphenylene ([10]CPP) moiety that serves as a rigid non-covalent bridge between a zinc porphyrin and a range of fullerenes. The preparation of iodo-[10]CPP is the key to the synthesis of a porphyrin-[10]CPP conjugate, which binds C , C , (C ) , and other fullerenes (K >10 m ). Fluorescence and pump-probe spectroscopy revealed intramolecular energy transfer between CPP and porphyrin and also efficient charge separation between porphyrin and fullerenes, affording up to 0.5 μs lifetime charge-separated states. The advantage of this approach towards electron donor-acceptor dyads is evident in the case of dumbbell-shaped (C ) , which gave intricate charge-transfer behavior in 1:1 and 2:1 complexes. These results suggest that [10]CPP and its cross-coupled derivatives could act as supramolecular mediators of charge transport in organic electronic devices.

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Small π-conjugated nanohoops are difficult to prepare, but offer an excellent platform for studying the interplay between strain and optoelectronic properties and increasingly, these shape-persistent macrocycles find uses in host-guest chemistry and self-assembly. We report the synthesis of a new family of radially π-conjugated porphyrinylene/phenylene nanohoops. The strain energy in the smallest nanohoop [2]CPT is approximately 54 kcal mol -1 , which results in a narrowed HOMO-LUMO gap and a red shift in the visible part of the absorption spectrum. Due to its high degree of preorganization and a diameter of ca. 13 Å, [2]CPT was found to accommodate C60 with a binding affinity exceeding 10 8 M -1 despite the fullerene not fully entering the cavity of the host (X-Ray crystallography). Moreover, the π-extended nanohoops [2]CPTN , [3]CPTN and [3]CPTA (N for 1,4-naphthyl; A for 9,10anthracenyl) have been prepared using the same strategy, and [2]CPTN has been shown to bind C70 five times more strongly than [2]CPT. Our failed synthesis of [2]CPTA highlights a limitation of the experimental approach most commonly used to prepare strained nanohoops, because in this particular case the sum of aromatization energies no longer outweighs the buildup of ring strain in the final reaction step (DFT calculations). These results indicate that forcing ring strain onto organic semiconductors is a viable strategy to fundamentally influence both optoelectronic and supramolecular properties.

Phospholides are easily obtained by treatment of the open-chain acetylenic phosphines shown by an excess of lithium at room temperature in THF (12 examples).

[ n ]Cycloparaphenylenes ([ n ]CPPs) with n =5, 8, 10 and 12 and their noncovalent ring‐in‐ring and [ m ]fullerene‐in‐ring complexes with m =60, 70 and 84 have been studied by direct and matrix‐assisted laser desorption ionization ((MA)LDI) and density‐functional theory (DFT). LDI is introduced as a straightforward approach for the sensitive analysis of CPPs, free from unwanted decomposition and without the need of a matrix. The ring‐in‐ring system of [[10]CPP⊃[5]CPP] +. was studied in positive‐ion MALDI. Fragmentation and DFT indicate that the positive charge is exclusively located on the inner ring, while in [[10]CPP⊃C 60 ] +. it is located solely on the outer nanohoop. Positive‐ion MALDI is introduced as a new sensitive method for analysis of CPP⊃fullerene complexes, enabling the detection of novel complexes [[12]CPP⊃C 60, 70 and 84 ] +. and [[10]CPP⊃C 84 ] +. . Selective binding can be observed when mixing one fullerene with two CPPs or vice versa, reflecting ideal size requirements for efficient complex formation. Geometries, binding and fragmentation energies of CPP⊃fullerene complexes from DFT calculations explain the observed fragmentation behavior.

To develop cure monitoring techniques for bisphenol A dicyanate ester (BPADCY) resin, UV, fluorescence, phosphorescence, and IR spectra were investigated. UV−vis spectra during the cure reaction indicates that the cyclotrimerization of cyanate esters to form triazine rings is the principal reaction, as supported by IR spectroscopic results. During the cure reaction, very strong luminescence emission has been found. Fluorescence emission intensity around 420 nm first increases, followed by decrease with a small red shift as the cure reaction proceeds. The aromatically substituted cyanurates formed during the cure reaction and their inner filter effect are responsible for the observed emission and its trend in intensity. In-situ fluorescence characterization showed similar results for the catalyzed cure reaction with cobalt acetylacetonate/nonylphenol as well as bisphenol A catalyzed resin in comparison to purified BPADCY, except for the faster rate in fluorescence spectral change. Phosphorescence emission of polycyanate resins appears at a 440 nm, which is about 20 nm longer than that of fluorescence. The phosphorescence lifetime has been found to be about 20 ms at room temperature and remains relatively unchanged throughout the cure reaction. FTIR in situ cure studies of BPADCY monomer indicates a linear relationship between the consumption rate of cyanate ester groups and the formation rate of the substituted triazine rings. Fluorescence changes are correlated to the extent of cure by IR technique.

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