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 also highlighted.

The phenomenon of photon upconversion, in which a system absorbs two or more photons of lower energy and emits a photon of higher energy, has been used in numerous applications, including non-destructive bioimaging, deep-penetrating photodynamic therapy, catalysis, and photovoltaic devices. To date, photon upconversion has been observed typically in inorganic nanocrystals, nanoparticles, metal–organic frameworks, supra­molecular assemblies, and organic dyads. Herein, we demonstrate a new strategy for harnessing photon upconversionsupra­molecular upconversionbased on host–guest chemistry. We have identified a box-like fluorescent tetra­cationic host incorporating a thiazolo­thiazole emitter, which can accommodate a guest-sensitizer, 5,15-diphenyl­porphyrin, inside its cavity, and demonstrated that the host–guest inclusion complex displays triplet-fusion upconversion when the guest is excited with low-energy light. The strategy of supra­molecular upconversion has been employed successfully in two other host–guest complexeswith hosts comprised of anthracene emitters and a 5,15-diphenyl­porphyrin guestcorroborating the fact that this strategy is a general one and can be applied to the design of a new family of host–guest complexes for photon upconversion. More importantly, supra­molecular upconversion is accessible in solution under dilute conditions (μM) compared to most of the existing approaches that require significantly higher concentrations (mM) of emitters and/or sensitizers. Transient absorption spectroscopy and density functional theory have been employed in order to confirm a triplet-fusion upconversion mechanism. Host–guest complexation-mediated supra­molecular photon upconversion eliminates multiple issues in the existing systems related to high working concentrations, high incident laser power, and low optical penetration depths.

A cationic Pd(ii) complex containing syn-(Me,Me)bimane as a ligand was prepared and fully characterized. This complex represents the first well-defined case of a bimane scaffold coordinated to a metal center. The strongly-fluorescent syn-bimane chelates the Pd(ii) center via its carbonyl oxygen atoms, affording a non-fluorescent complex. The crystal structure of this complex shows that the coordinated bimane departs from planarity, with its bicyclic framework bent about the N-N bond. Spectroscopic evidence demonstrates that bimane coordination is reversible in solution.

Polar and polarizable π-conjugated organic molecules containing push−pull chromophores have been investigated extensively in the past. Identifying unique backbones and building blocks for fluorescent dyes is a timely exercise. Here, we report the synthesis and characterization of a series of fluorescent dyes containing quadrupolar A−D−A constitutions (where A = acceptor and D = donor), which exhibit fluorescence emission at a variety of different wavelengths. We have investigated the effects of different electron-withdrawing groups, located at both termini of a para-terphenylene backbone, by steady-state UV/vis and fluorescence spectroscopy. Pyridine and substituted pyridinium units are also introduced during the construction of the quadrupolar backbones. Depending on the quadrupolarity, fluorescence emission wavelengths cover from 380 to 557 nm. Time-resolved absorption and emission spectroscopy reveal that the photophysical properties of those quadrupolar dyes result from intramolecular charge transfer. One of the dyes we have investigated is a symmetrical box-like tetracationic cyclophane. Its water-soluble tetrachloride, which is non-cytotoxic to cells up to a loading concentration of 1 μM, has been employed in live-cell imaging. When taken up by cells, the tetrachloride emits a green fluorescence emission without any hint of photobleaching or disruption of normal cell behavior. We envision that our design strategy of modifying molecules through the functionalization of the quadrupolar building blocks as chromophores will lead to future generations of fluorescent dyes in which these A−D−A constitutional fragments are incorporated into more complex molecules and polymers for broader photophysical and biological applications.

The fluorescent dye syn-(Me,Me)bimane forms labile complexes with Na+, leading to fluorescence quenching in solution.