β-Octamethoxyporphycenes

Synthesis of porphyrin(2.1.2.1) Pd(II) complexes embedded with various π units and their singlet oxygen generation capacity

Novel porphyrin(2.1.2.1) Pd(II) complexes with various aromatic π rings (benzo, naphthalene and thiophene) embedded between dipyrrin units have been synthesized. Their molecular structures and optical and electronic properties were confirmed and fully investigated. These Pd(II) complexes showed moderate to good capacity of singlet oxygen generation under light irradiation.

Porphyrin(2.1.2.1) organopalladium complexes as efficient singlet oxygen sensitizers

Four new non-planar and non-aromatic porphyrin organopalladium complexes were synthesized. Conformational structures and optical and electronic properties of the obtained organopalladium complexes containing meso-substituted phenyl, p-tert-butylphenyl, or pentafluorophenyl groups were fully investigated. These complexes showed potent capacity for singlet oxygen (1O2) generation under blue-light irradiation, and the 1O2 quantum yields were in the range of 41%-56%, which were comparable to that of Ru(bpy)3Cl2 (57%), and such potency made these organopalladium complexes potential 1O2 photo sensitizers for photodynamic therapy.

Effect of β-Substitution and β,β'-Fusion on the Formation of Boron Complexes of Porphycenes

Boron(III) complexation was investigated in a series of β-substituted porphycenes. Unlike meso-arylporphycenes, these macrocycles undergo a facile complexation reaction. Upon fusion of the β,β'-positions of the porphycene ligand, the complexation resulted in fast insertion of boron, forming the cisoid-B2OF2 complex. However, in the case of the other β-substituted porphycenes, only bis-BF2 complexes formed. The effect of these substituents on the core geometry and photophysical properties are elaborated here.

Conformationally rigid, π-extended annulated porphyrinoids derived from the naphthobipyrrole motif

π-Extension in porphyrinoids can be achieved by fusing additional aromatic rings onto the macrocycle's periphery and such porphyrinoids are referred to as annulated porphyrinoids. Annulated porphyrinoids display contrasting properties in comparison with their non-annulated congeners. While an annulation strategy can create π-extended systems, the simultaneous incorporation of conformational rigidity in such porphyrinoids can ensure that they adopt a planar structure, and the advantages associated with the extended π-network can be leveraged. Hence, while synthesizing such porphyrinoids, judicial selection of the precursor becomes important. The ease of synthesis and the presence of a β-β'-linked o-phenylene bridge qualify 3,8-1,10-dihydrobenzo[e]pyrrolo[3,2-g]indole, commonly known as naphthobipyrrole, to be one such precursor suitable for the synthesis of conformationally rigid annulated porphyrinoids. This field of study has started to bloom only in the last decade and the examples reported so far are confined to the naphtho-versions of porphycenes (isomeric porphyrin), a few members of the aromatic/antiaromatic expanded porphyrinoids, and calix[n]bipyrroles. In view of this, the current review article aims to summarize the up-to-date developments in this area and discusses the synthesis, structure, and properties of the reported naphthobipyrrole-derived annulated porphyrinoids.

In-Core N4-Coordination of Palladium(II) in Dinaphthoporphycene: Synthesis, Structure, and Photophysical Studies

Dinaphthoporphycene (DNP) has emerged as a versatile ligand undergoing large out-of-plane distortion to form a cis-bimetallic complex with Pd(II) using Pd(OAc)₂ and out-of-plane monometallic complexes with Pd(acac)₂ and PtCl₂(PhCN)₂. Herein, we are finally able to synthesize the in-core complex with Pd(II) using PdCl₂(PhCN)₂ or PdCl₂. The crystal structure shows the palladium ion resides slightly above the N4-core, with the Pd(II) dimensionally dissenting with the typical square planarity displayed by the reported in-core DNP complexes with Ni(II) and Cu(II) ions. The deformed complex displays a blue shift in the absorption spectra compared to DNP and its metallo-derivatives. PdDNP exhibits a moderate singlet oxygen generation ability (18%).

3,6,13,16-Tetraalkylporphycenes: Synthesis and Exploration of Effect of Alkyl Groups on Structure, Photophysical Properties, and Basicity

Two new 3,6,13,16-tetraalkylporphycenes were synthesized following rational approach. The reason behind lower yield of the desired β,β'-bipyrroles was unraveled. The σ-donating effect of alkyl-substituents was more profound than reported positional...

10,15-Bis(ethoxycarbonyl)-5-(4-methoxycarbonylphenyl) B(III)subchlorin: A photosensitizer with high singlet oxygen producing efficiency

A novel A2B-type B(III)subchlorin has been synthesized for the first time in two ways possessing two different ester moieties upon macrocyclic periphery from meso-diethoxycarbonyl tripyrrane. Its photophysical and electrochemical properties have been explored. Introduction of the third meso-substituent resulted in the synthesis of the B(III)subchlorin as the major product with the formation of minor oxidized B(III)subporphyrin analogue. This subchlorin derivative was found to generate singlet oxygen much efficiently with quantum yield ([Formula: see text] 0.88.

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).

Rectangular Holes in Porphyrin Isomers Act As Mono- and Binucleating Ligands: Stereochemistry of Mono- and Diboron Porphycenes and Their Protonation Behaviors

The first boron complexes of porphycenes, structural isomers of porphyrin, are reported. They are synthesized in good yields by reacting the free-base porphycene ligands with BF₃·Et₂O through a microwave-assisted method. Depending on the substituent group of porphycenes, two different coordination structures, mono- and diboron porphycenes, are obtained simultaneously. The single crystal structures and DFT calculations suggest that the boron atom of the monoboron porphycene is favorably coordinated on the dipyrroethene site, and the regioisomer of diboron porphycene is of cisoid stereochemistry, which is more stable than transoid. We also investigate the protonation behavior of boron porphycene complexes. Diboron porphycene does not undergo protonation, whereas monoboron porphycene undergoes protonation at the nonboron coordinating pyrroline site, resulting in a red shift in both absorption and emission spectra. Protonation and deprotonation of monoboron porphycene can be reversibly triggered using acids and bases.

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.

3,6,13,16-Tetrasubstituted Porphycene: The Missing Link in Porphycene Chemistry

We have introduced the first 3,6,13,16-tetrasubstituted porphycene as its tetramethoxy analogue. This substitution pattern is one of the most general patterns yet missing in this isomeric porphyrin chemistry. This porphycene exhibits intense fluorescence along with the ability to coordinate with divalent metal ions; in particular, it forms the first stable Zn(II) complex among the tetrasubstituted porphycenes. Notably, the molecular structure of Zn1•Py displays supramolecular chirality.

Chromatographically separable ruffled non-planar isomeric octaalkylporphycenes: consequences of unsymmetrical substitution upon structure and photophysical properties

Two symmetry reduction induced unsymmetrical porphyrins (2.0.2.0) were synthesized and characterized with distinct structural, optoelectronic and nonlinear optical properties.

Photooxidation Reactions of Cyclometalated Palladium(II) and Platinum(II) Complexes

New C,N,S-cyclometalated palladium(II) and platinum(II) complexes have been synthesized and their structural, electrochemical, and photochemical properties examined. The blue color of these complexes in solution changed to yellow under visible-light irradiation. By measurement of the absorption spectra for quantifying changes in color, isosbestic points for each complex clearly indicated the presence of only two species responsible for the change of color. X-ray analysis revealed that the visible-light-induced yellow species were S-oxygenated sulfinato complexes. Photosensitized generation of singlet oxygen (¹O₂) was confirmed by the direct detection of singlet oxygen luminescence at 1275 nm. The present cyclometalated palladium(II) and platinum(II) complexes are efficient photosensitizers of singlet oxygen, which rapidly reacts with coordinating sulfur atoms.

Convenient Synthesis of 6,7,12,13-Tetrahydro-5H-Cyclohepta[2,1-b:3,4-b']diindole Derivatives Mediated by Hypervalent Iodine (III) Reagent

Bisindolyl alkaloids represent a large family of natural and synthetic products that display various biological activities. Among the bisindole compounds, 6,7,12,13-tetrahydro-5H-cyclohepta[2,1-b:3,4-b']diindoles have received little attention. Only two methods have been developed for the construction of the 6,7,12,13-tetrahydro-5H-cyclohepta[2,1-b:3,4-b']diindole scaffold thus far, including the classical Fischer indole synthesis conducted by reacting indole-fused cycloheptanone and hydrazines, and the condensation reaction to build the seven-membered ring. Here, we report for the first time a new route to synthesize 6,7,12,13-tetrahydro-5H-cyclohepta[2,1-b:3,4-b']diindoles through intramolecular oxidative coupling of 1,3-di(1H-indol-3-yl)propanes in the presence of PIFA, DDQ and TMSCl with moderate to excellent yields.

Versatile Approach for Reliable Determination of Both High and Low Values of Luminescence Quantum Yields

The determination of luminescence quantum yields by means of relative methods requires setting identical experimental conditions for both sample and reference compounds. This requirement has a critical impact on the applicability of these protocols, as it does not allow for the precise determination of low quantum-yield values using well-characterized high-quantum-yield standards. We show that using the simultaneous absorption and fluorescence-emission measurement (SAFE) approach [ Nawara and Waluk. Anal. Chem. 2017 , 89 , 8650 ], the sample and reference compounds can be effectively measured with different excitation-slit spectral bandpass or integration times, separately optimized for each chromophore. This unique feature simplifies the determination of luminescence quantum yields, allowing measurements of low quantum-yield values using well-characterized high-quantum-yield standards.

Functionalized 2,2'-Bipyrroles: Building Blocks for Pyrrolic Macrocycles

Functionalized N-unsubstituted 2,2'-bipyrroles are basic building blocks for the preparation of pyrrolic macrocycles and natural products, such as prodigiosines. The aim of this review is to provide a description of the most important methodologies used to prepare 2,2'-bipyrroles and their central role as building blocks for the synthesis of porphyrinoids and property-defining structural elements therein.

Gram-scale synthesis of porphycenes through acid-catalyzed oxidative macrocyclizations of E/Z-mixed 5,6-diaryldipyrroethenes

A set of novel porphycenes was synthesized on a gram-scale in high overall yields by optimization of macrocyclization of E/Z-mixed 5,6-diaryldipyrroethenes.

Synthesis, Characterization and Protonation Behavior of Quinoxaline-Fused Porphycenes

9,10-Quinoxaline-fused porphycenes 1a-H₂ and 1b-H₂ were synthesized by intramolecular McMurry coupling. As a result of the annulation of the quinoxaline moiety on the porphycene skeleton, 1a-H₂ and 1b-H₂ display absorption and fluorescence in the near infra-red (NIR) region. Additionally, the quinoxaline moieties of 1a-H₂ and 1b-H₂ act as electron-withdrawing groups, introducing lower reduction potentials than for pristine porphycene. The protonation occurred at the nitrogen atoms in the cavity of freebase porphycenes and at the quinoxaline moieties for their nickel complexes to give diprotonic species.

β-Hexaalkylporphycenes: Positional Effect of Alkyl Groups toward Design and Control of Structural and Photophysical Properties in Isomeric Hexaethylporphycenes

Two novel β-hexaalkylated porphycenes, i.e., 2,3,7,12,13,17- (HOT) and 2,3,6,12,13,16-hexaethylporphycenes (HIT) were introduced for the first time in porphycene chemistry. These were synthesized through McMurry coupling reactions of new isomeric unsymmetrically substituted triethylbipyrrole dialdehydes. The positional effects of alkyl groups could be manifested through significant alteration in structure of porphycene cores and, as a consequence their photophysical properties, not noticed in β-octaethylporphycene. HOT displays significant fluorescence accompanied by reasonable singlet oxygen generation ability.

The first synthesis of meso-dicycloalkylporphycenes: ring strain effects on structural and optical properties of isomeric porphyrins

Two novel meso-dicycloalkylporphycenes were synthesized for the first time, which showed remarkable ring-strain-induced switching of their fluorescence behaviors in the solution phase.

Spectroscopy and Tautomerization Studies of Porphycenes

Tautomerization in porphycenes, constitutional isomers of porphyrins, is strongly entangled with spectral and photophysical parameters. The intramolecular double hydrogen transfer occurring in the ground and electronically excited states leads to uncommon spectroscopic characteristics, such as depolarized emission, viscosity-dependent radiationless depopulation, and vibrational-mode-specific tunneling splittings. This review starts with documentation of the electronic spectra of porphycenes: Absorption and magnetic circular dichroism are discussed, together with their analysis based on the perimeter model. Next, photophysical characteristics are presented, setting the stage for the final part, which discusses the developments in research on tautomerism. Porphycenes have been studied in different experimental regimes: molecules in condensed phases, isolated in supersonic jets and helium nanodroplets, and, recently also on the level of single molecules investigated by optical and scanning probe microscopies. Because of the rich and detailed information obtained from these diverse investigations, porphycenes emerge as very good models for studying the complex, multidimensional phenomena involved in the process of intramolecular double hydrogen transfer.

Iridium-Based High-Sensitivity Oxygen Sensors and Photosensitizers with Ultralong Triplet Lifetimes

The photophysics of a series of bichromophoric molecules featuring an intramolecular triplet energy transfer between a triscyclometalated iridium(III) complex and covalently linked organic group are studied. By systematically varying the energy gap (0.1-0.3 eV) between the donor (metal complex) and acceptor (pyrene unit), reversible triplet energy transfer processes with equilibrium constant K ranging from ca. 500 to 40 000 are established. Unique photophysical consequences of such large K values are observed. Because of the highly imbalanced forward and backward energy transfer rates, triplet excitons dominantly populate the acceptor moiety in the steady state, giving rise to ultralong luminescence lifetimes up to 1-4 ms. Because the triscyclometalated Ir and triplet pyrene groups both impart relatively small nonradiative energy loss, decent phosphorescence quantum yields (Φ = 0.1-0.6) are attained in spite of the exceptionally prolonged excited states. By virtue of such precious combination of long-lived triplet state and high Φ, these bichromophoric molecules can serve as highly sensitive luminescent sensors for detecting trace amount of O2 and as potent photosensitizers for producing singlet oxygen even under low-oxygen content conditions.

Crossed McMurry Coupling Reactions for Porphycenic Macrocycles: Non-Statistical Selectivity and Rationalisation

Crossed McMurry reactions of bifuran- or bithiophenedicarbaldehydes with bipyrroledicarbaldehydes have been studied for the first time. Only those porphycenic macrocycles derived from homocoupled McMurry products were formed. The results are explained by using both density functional theory and electron propagator computations to model the electron affinity of the dialdehyde starting materials. It was predicted that bifuran\bithiophene cross-coupling would indeed occur, and this was demonstrated by the first synthesis of a novel dioxa,dithio hetero-porphycenoid annulene. This approach will allow the prior identification of viable substrates for related crossed McMurry reactions.

β-Decamethoxysapphyrin and Its N-Benzyl Analogue

The synthesis of a highly electron-rich decamethoxysapphyrin and its 27-N-benzyl analogue is reported for the first time. The effects of β-methoxy and 27-N-benzyl substitution on structure, anion binding, absorption, and electrochemical properties were explored in detail. Upon 27-N-benzyl substitution, counteranion-induced structural deformation arises in the diprotonated state, which could be clearly noticed both in solution (1)H NMR study and solid-state structural analysis. This type of anion-induced structural deformation is noted for the first time in β-substituted sapphyrins. Further, the free base sapphyrins generate singlet oxygen with moderate efficiency (∼42%); hence, they may act as good photosensitizers.

β-Tetrachlorotetramethoxyporphycenes: positional effect of substituents on structure and photophysical properties

We report the regiospecific synthesis of the first chloro-substituted porphycenes as two positional isomers of β-tetrachlorotetramethoxyporphycene.

β-Octakis(methylthio)porphycenes: synthesis, characterisation and third order nonlinear optical studies

A novel electron deficient β-octakis(methylthio)porphycene, along with its Zn(ii) and Ni(ii) derivatives, was synthesized for the first time.