Leveraging Torsional and Steric Strains: A Pre-macrocyclization Strategy Enables Conformation-Specific Fullerene Binding in m-Cyclophanes

Though the chemistry of resorcinarenes is half a century old, the conformationally-locked resorcinarene crowns are generally constructed using hydrogen bonds or covalent tethers. Often, covalent tethering involves extra post-macrocyclization steps involving upper-rim functionalities. We have leveraged the torsional and steric strains through α-substituents of the lower-rim C-alkyl chains and accomplished conformationally-rigid fluorescent m-cyclophane deep-crowns in a predetermined way. The strategy offers a pre-macrocyclization route conserving upper-rim functionalities, an aspect overlooked in resorcinarene chemistry. X-ray structural and computational analyses unveil the cause for conformational rigidity in m-cyclophanes due to α-branching in C-alkyls (linear vs. α-/β-branched). The conformationally-locked fluorescent deep-crown with a preorganized cavity captures hydrophobic spherical guest C60 in both solution and solid states specifically, when compared to conformationally-dynamic boats, enabling conformation-specific binding.

Fluorescent cyclophanes and their applications

With the serendipitous discovery of crown ethers by Pedersen more than half a century ago and the subsequent introduction of host-guest chemistry and supramolecular chemistry by Cram and Lehn, respectively, followed by the design and synthesis of wholly synthetic cyclophanes-in particular, fluorescent cyclophanes, having rich structural characteristics and functions-have been the focus of considerable research activity during the past few decades. Cyclophanes with remarkable emissive properties have been investigated continuously over the years and employed in numerous applications across the field of science and technology. In this Review, we feature the recent developments in the chemistry of fluorescent cyclophanes, along with their design and synthesis. Their host-guest chemistry and applications related to their structure and properties are highlighted.

Design, Synthesis, and Utility of Defined Molecular Scaffolds

A growing theme in chemistry is the joining of multiple organic molecular building blocks to create functional molecules. Diverse derivatizable structures—here termed “scaffolds” comprised of “hubs”—provide the foundation for systematic covalent organization of a rich variety of building blocks. This review encompasses 30 tri- or tetra-armed molecular hubs (e.g., triazine, lysine, arenes, dyes) that are used directly or in combination to give linear, cyclic, or branched scaffolds. Each scaffold is categorized by graph theory into one of 31 trees to express the molecular connectivity and overall architecture. Rational chemistry with exacting numbers of derivatizable sites is emphasized. The incorporation of water-solubilization motifs, robust or self-immolative linkers, enzymatically cleavable groups and functional appendages affords immense (and often late-stage) diversification of the scaffolds. Altogether, 107 target molecules are reviewed along with 19 syntheses to illustrate the distinctive chemistries for creating and derivatizing scaffolds. The review covers the history of the field up through 2020, briefly touching on statistically derivatized carriers employed in immunology as counterpoints to the rationally assembled and derivatized scaffolds here, although most citations are from the past two decades. The scaffolds are used widely in fields ranging from pure chemistry to artificial photosynthesis and biomedical sciences.

Recent Advances in Halogen Bonded Assemblies with Resorcin[4]arenes

Resorcinarenes are cavity-containing compounds when in the crown conformation, from the calixarene family of concave compounds. These easy to synthesize macrocycles can be decorated at the upper rim through the eight hydroxyl groups and/or the 2-position of the aromatic ring. They are good synthons in supramolecular chemistry leading to appealing assemblies such as open-inclusion complexes, capsules and tubes through multiple weak interactions with various guests. Halogen bonding (XB) is a highly directional non-covalent interaction by an electron-deficient halogen atom as a donor that interacts with a Lewis base, the XB acceptor. This tutorial review provides an overview of recent advances in halogen-bonded assemblies based on resorcinarenes and their derivatives, specifically focusing on discrete and capsular assemblies.

Macrocyclic Arenes Functionalized with BODIPY: Rising Stars among Chemosensors and Smart Materials

Macrocycles play a crucial role in supramolecular chemistry and the family of macrocyclic arenes represents one of the most important types of hosts. Among them, calixarenes, resorcinarenes and pillararenes are the most commonly encountered macrocyclic arenes, and they have received considerable attention. Boron-dipyrromethene (BODIPY) dyes are fascinating compounds with multiple functionalization sites and outstanding luminescence properties including high fluorescence quantum yields, large molar absorption coefficients and remarkable photo- and chemical stability. The combination of macrocyclic arenes and BODIPY dyes has been demonstrated to be an effective strategy to construct chemosensors for various guests and smart materials with tailored properties. Herein, we firstly summarize the recent advances made so far in macrocyclic arenes substituted with BODIPY. This review only focuses on the three macrocyclic arenes of calixarenes, resorcinarenes and pillararenes, as there are no other macrocyclic arenes substituted BODIPY units at the present time. Hopefully, this review will not only afford a guide and useful information for those who are interested in developing novel chemosensors and smart materials, but also inspire new opportunities in this field.

Is π-Stacking Prone To Accelerate Singlet-Singlet Energy Transfers?

π-Stacking is the most common structural feature that dictates the optical and electronic properties of chromophores in the solid state. Herein, a unidirectional singlet-singlet energy-transfer dyad has been designed to test the effect of π-stacking of zinc(II) porphyrin, [Zn₂], as a slipped dimer acceptor using a BODIPY unit, [bod], as the donor, bridged by the linker C₆H₄C≡CC₆H₄. The rate of singlet energy transfer, k_ET(S₁), at 298 K ( k_ET(S₁) = 4.5 × 10¹⁰ s^-1) extracted through the change in fluorescence lifetime, τ_F, of [bod] in the presence (27.1 ps) and the absence of [Zn₂] (4.61 ns) from Streak camera measurements, and the rise time of the acceptor signal in femtosecond transient absorption spectra (22.0 ps), is faster than most literature cases where no π-stacking effect exists (i.e., monoporphyrin units). At 77 K, the τ_F of [bod] increases to 45.3 ps, indicating that k_ET(S₁) decreases by 2-fold (2.2 × 10¹⁰ s^-1), a value similar to most values reported in the literature, thus suggesting that the higher value at 298 K is thermally promoted at a higher temperature.

Increasing the lifetimes of charge separated states in porphyrin-fullerene polyads

Two linear polyads were designed using zinc(ii)porphyrin, [ZnP], and N-methyl-2-phenyl-3,4-fullero-pyrrolidine (C₆₀) where C₆₀ is dangling either at the terminal position of [ZnP]-C₆H₄-[triple bond, length as m-dash]-C₆H₄-[ZnP]-C₆₀ (1) or at the central position of [ZnP]-C₆H₄-[triple bond, length as m-dash]-C₆H₄-[ZnP(C₆₀)]-C₆H₄-[triple bond, length as m-dash]-C₆H₄-[ZnP] (2) in order to test whether the fact of having one or two side electron donors influences the rate of electron transfer, k_et. These polyads were studied using cyclic voltammograms, DFT computations, steady state and time-resolved fluorescence spectroscopy, and femtosecond transient absorption spectroscopy (fs-TAS). Photo-induced electron transfer confirmed by the detection of the charge separated state [ZnP˙+]/C₆₀˙^- from fs-TAS occurs with rates (k_et) of 3-4 × 10¹⁰ s^-1 whereas the charge recombinations (CRs) are found to produce the [ZnP] ground state via two pathways (central [ZnP˙+]/C₆₀˙^- (ps) and terminal central [ZnP˙+]/C₆₀˙^- (ns) producing [1ZnP] (ground state) and [3ZnP*]). The formation of the T₁ species is more predominant for 2.

Controlling electron and energy transfer paths by selective excitation in a zinc porphyrin-BODIPY-C60 multi-modular triad

A multi-modular donor-acceptor triad composed of zinc porphyrin, BF₂-chelated dipyrromethene (BODIPY), and C₆₀ was newly synthesized, with the BODIPY entity at the central position. Using absorbance and emission spectral, electrochemical redox, and computational optimization results, energy level diagrams for the ZnP-BODIPY dyad and ZnP-BODIPY-C₆₀ triad were constructed to envision the different photochemical events upon selective excitation of the BODIPY and ZnP entities. By transient absorption spectral studies covering a wide femtosecond-to-millisecond time scale, evidence for the different photochemical events and their kinetic information was secured. Efficient singlet-singlet energy transfer from ¹BODIPY* to ZnP with a rate constant k_ENT = 1.7 × 10¹⁰ s^-1 in toluene was observed in the case of the ZnP-BODIPY dyad. Interestingly, in the case of the ZnP-BODIPY-C₆₀ triad, the selective excitation of ZnP resulted in electron transfer leading to the formation of the ZnP˙⁺-BODIPY-C₆₀˙^- charge-separated state. Owing to the distal separation of the radical cation and radical anion species (edge-to-edge distance of 18.7 Å), the radical ion-pair persisted for microseconds. By contrast, the selective excitation of BODIPY resulted in an ultrafast energy transfer to yield ZnP-BODIPY-¹C₆₀* as the major product. The ¹C₆₀* populated the low-lying ³C₆₀* via intersystem crossing prior to returning to the ground state. The present study successfully demonstrates the importance of supramolecular geometry and selection of excitation wavelength in regulating the different photoprocesses.

Application of the boron center for the design of a covalently bonded closely spaced triad of porphyrin-fullerene mediated by dipyrromethane

Charge separation stabilization is achieved by placing porphyrin and C₆₀ at the two ends of central BODIPY.

Experimental and computational study of BODIPY dye-labeled cavitand dynamics

Understanding the distance distribution and dynamics between moieties attached to the walls of a resorcin[4]arene cavitand, which is switchable between an expanded kite and a contracted vase form, might enable the use of this molecular system for the study of fundamental distance-dependent interactions. Toward this goal, a combined experimental and molecular dynamics (MD) simulation study on donor/acceptor borondipyrromethene (BODIPY) dye-labeled cavitands present in the vase and kite forms was performed. Direct comparison between anisotropy decays calculated from MD simulations with experimental fluorescence anisotropy data showed excellent agreement, indicating that the simulations provide an accurate representation of the dynamics of the system. Distance distributions between the BODIPY dyes were established by comparing time-resolved Förster resonance energy transfer experiments and MD simulations. Fluorescence intensity decay curves emulated on the basis of the MD trajectories showed good agreement with the experimental data, suggesting that the simulations present an accurate picture of the distance distributions and dynamics in this molecular system and provide an important tool for understanding the behavior of extended molecular systems and designing future applications.

Triplet photosensitizers: from molecular design to applications

Triplet photosensitizers (PSs) are compounds that can be efficiently excited to the triplet excited state which subsequently act as catalysts in photochemical reactions. The name is originally derived from compounds that were used to transfer the triplet energy to other compounds that have only a small intrinsic triplet state yield. Triplet PSs are not only used for triplet energy transfer, but also for photocatalytic organic reactions, photodynamic therapy (PDT), photoinduced hydrogen production from water and triplet-triplet annihilation (TTA) upconversion. A good PS should exhibit strong absorption of the excitation light, a high yield of intersystem crossing (ISC) for efficient production of the triplet state, and a long triplet lifetime to allow for the reaction with a reactant molecule. Most transition metal complexes show efficient ISC, but small molar absorption coefficients in the visible spectral region and short-lived triplet excited states, which make them unsuitable as triplet PSs. One obstacle to the development of new triplet PSs is the difficulty in predicting the ISC of chromophores, especially of organic compounds without any heavy atoms. This review article summarizes some molecular design rationales for triplet PSs, based on the molecular structural factors that facilitate ISC. The design of transition metal complexes with large molar absorption coefficients in the visible spectral region and long-lived triplet excited states is presented. A new method of using a spin converter to construct heavy atom-free organic triplet PSs is discussed, with which ISC becomes predictable, C60 being an example. To enhance the performance of triplet PSs, energy funneling based triplet PSs are proposed, which show broadband absorption in the visible region. Applications of triplet PSs in photocatalytic organic reactions, hydrogen production, triplet-triplet annihilation upconversion and luminescent oxygen sensing are briefly introduced.

A deep cavitand with a fluorescent wall functions as an ion sensor

The synthesis and characterization of a deep cavitand bearing a fluorescent benzoquinoxaline wall is reported. Noncovalent host-guest recognition events are exploited to sense small charged molecules including acetylcholine. The cavitand also exhibits an anion dependent change in fluorescence that is used to differentiate halide ions in solution.

Boron dipyrromethene chromophores: next generation triplet acceptors/annihilators for low power upconversion schemes

In the present study, the red-light absorbing platinum(II) tetraphenyltetrabenzoporphyrin (PtTPBP) was used as a triplet sensitizer in conjunction with two distinct iodophenyl-bearing BODIPY derivatives independently serving as triplet acceptors/annihilators poised for photon upconversion based on triplet-triplet annihilation. In deaerated benzene solutions, extremely stable and high quantum efficiency green (Phi(UC) = 0.0313 +/- 0.0005) and yellow (Phi(UC) = 0.0753 +/- 0.0036) upconverted emissions were observed from selective red excitation of the PtTPBP sensitizer at 635 +/- 5 nm. The current systems represent the first examples of photon upconversion where aromatic hydrocarbons do not serve the role of triplet acceptor/annihilator. Notably, the nature of the current chromophore compositions permitted highly reproducible upconversion quantum efficiency determinations while permitting the evaluation of the triplet-triplet annihilation quantum yields in both instances.