Recent advances in subphthalocyanines and related subporphyrinoids

Subporphyrinoids constitute a class of extremely versatile and attractive compounds. Herein, a comprehensive review of the most recent advances in the fundamentals and applications of these cone-shaped aromatic macrocycles is presented.

A2B- and A3-Type Boron(III)Subchlorins Derived from meso-Diethoxycarbonyltripyrrane: Synthesis and Photophysical Exploration

The first direct fabrication of A₂B- and A₃-type B(III)subchlorins from meso-ethoxycarbonyl-substituted tripyrrane has been realized by condensation with appropriate acid chlorides (benzoyl chloride, butyryl chloride, and ethyl chlorooxoacetate). The aliphatic acid chloride-based annulation reaction is new to subporphyrinoid chemistry. The phenyl (6a)- or n-propyl (6b)-substituted derivatives could be oxidized to the corresponding B(III)subporphyrins upon refluxing with DDQ, whereas the triethoxycarbonyl moiety (6c) was found to be resistant to oxidation and exhibits the most red-shifted absorption (587 nm) and emission (604 nm). The study indicates that absorption and emission behaviors of the B(III)subchlorin can be tuned by the introduction of electron-rich or electron-deficient substituents at the meso-position. B(III)subchlorins 6a and 6c generate singlet oxygen efficiently (44 and 40%, respectively) and, thus, may find application as potential photosensitizers in photodynamic therapy (PDT).

Meso-Free Boron(III)subchlorin and Its μ-Oxo Dimer with Interacting Chromophores

Meso-free B(III)subchlorin 1 has been realized exclusively for the first time from meso-ethoxycarbonyl-substituted tripyrrane along with the first subchlorin dimer 2 as its μ-oxo analogue via a facile one-pot approach. The subchlorin is highly stable toward oxidation; hence, it was not contaminated with the corresponding subporphyrin analogue 3. The subchlorin (56%) and its dimer (30%) exhibit singlet oxygen generation ability for the first time. The B-O-B dimer displays strong exciton coupling between the two macrocycles.

meso-Free BIII Subporphyrins with Electron-donating Groups

meso-Free B^III 5,10-bis(p-dimethylaminophenyl)subporphyrins were synthesized. They display red-shifted absorption and fluorescence spectra, bathochromic behaviors in polar solvents, a high fluorescence quantum yield (Φ_F =0.57), and a small HOMO-LUMO gap mainly due to destabilized HOMO as compared with meso-free B^III 5,10-diphenylsubporphyrin. This subporphyrin serves as a nice precursor of various meso-substituted B^III subporphyrins such as B^III meso-nitrosubporphyrin, B^III meso-aminosubporphyrin, and meso-meso' linked B^III azosubporphyrin dimer. Reactions of meso-free B^III subporphyrins with NBS or bis(2,4,6-trimethylpyridine)bromonium hexafluorophosphate gave meso-meso' linked subporphyrin dimers, often as a major product along with meso-bromosubporphyrins.

BIII 5-Arylsubporphyrins and BIII Subporphine

Boron(III) 5-arylsubporphyrins and B^III subporphine are promising precursors for functional B^III subporphyrins bearing asymmetric meso-substituents. Herein, we report the first synthesis of these molecules. Among many aryl acid chlorides examined, 4-nitrobenzoyl chloride gave B^III 5-(4-nitrophenyl)subporphyrin in 10 % yield in condensation with triethylamine-tri-N-tripyrromethene-borane. The nitro group of this B^III subporphyrin was reduced with NaBH₄ to prepare B^III 5-(aminophenyl)subporphyrin, which was converted into B^III 5-phenylsubporphyrin via the corresponding diazonium salt. B^III subporphine was synthesized by condensation of triethyl orthoformate with triethylamine-tri-N-tripyrromethene-borane. Progressive removal of meso-phenyl substituents leads to continuous changes in the optical properties, whereas the B^III subporphine deviates from this trend in some properties.

meso-Functionalization of Boron(III) Subporphyrin with Boron(III) meso-Lithiosubporphyrin

Boron(III) meso-lithiosubporphyrin was prepared by bromine-lithium exchange of B-tolyl B^III meso-bromosubporphyrin with n-butyllithium at -98 °C. The resulting subporphyrinyllithium was treated with various electrophiles such as benzophenone, N,N-dimethylformamide, CO₂ , chlorotrimethylsilane, N-fluorobenzenesulfonimide, and dimesitylboryl fluoride to give the corresponding meso-functionalized B^III subporphyrins. The nucleophile was also used to construct B^III subporphyrin dimers such as bis(B^III subporphyrinyl)ketone, bis(B^III subporphyrinyl)carbinol, and disilane-bridged B^III subporphyrin dimer. The structural, optical, and electrochemical properties of these meso-functionalized B^III subporphyrins were examined by UV/Vis absorption and fluorescence spectroscopy, electrochemical studies, and DFT calculations.

Responsive, Water-Soluble Europium(III) Luminescent Probes

The design principles, mechanism of action and performance of europium(III) complexes that serve as strongly emissive and responsive molecular probes in water are critically discussed. Examples of systems designed to assess pH, selected metal ions and anions, including chiral species, as well as selected small molecules and biopolymers are considered, and prospects evaluated for improved performance in more complex biological media such as in bio-fluids and within living cells. Modulation of the emission spectral form, lifetime and degree of circular polarisation can be used to quantify the spectral response and permit calibration.

Recent Advances in Subporphyrins and Triphyrin Analogues: Contracted Porphyrins Comprising Three Pyrrole Rings

Subporphyrinato boron (subporphyrin) was elusive until the syntheses of tribenzosubporphine in 2006 and meso-aryl-substituted subporphyrin in 2007. These novel contracted analogues possess a 14π-electron conjugated system embedded in a bowl-shaped structure. They exhibit absorption and fluorescence in the UV/vis region and nonlinear optical properties due to their octupolar structures. The unique coordination geometry around the central boron atom in the structure of subporphyrin enabled investigation of rare boron species, such as borenium cations, boron hydrides, and boron peroxides. Along with the burgeoning development of the chemistry of subporphyrins, analogous triphyrin systems have also emerged. Their rich coordination chemistry as a result of their free-base structures, which are different from the boron-coordinating structure of subporphyrins, has been intensively investigated. On the basis of the unique structures and reactivities of subporphyrins and their related triphyrin analogues, supramolecular architectures and covalently linked multicomponent systems have also been actively pursued. This Review provides an overview of the development of subporphyrin and triphyrin chemistry in the past decade and future prospects in this field, which may inspire molecular design toward applications based on their unique properties.