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The electrochemical properties of $\beta$-octaethylporphycene iridium complex (Ir-OEPo) were determined. Based on the electro-spectro measurement results, the reduction of Ir-OEPo did not occur at the central metal but at the ligand, while the reduction of $\beta$-octaethylporphyrin iridium complex (Ir-OEPor) occurred at the central iridium. A catalytic current was observed during the cyclic voltammetry (CV) measurements with trifluoroacetic acid (TFA) under a reductive condition, indicating the catalytic reactivity of Ir-OEPo for the hydrogen evolution reaction (HER). By constant potential electrolysis, hydrogen gas was detected by gas chromatography (GC) and the catalytic reactivity of Ir-OEPo was confirmed. The HER mechanism via ligand reduction of macrocyclic aromatic complexes could be one of the concepts for the development of new catalysts.

A new synthetic method for 2,7,12,17-tetraphenylporphycene (2e, TPPo) which avoids tetradecarboxylation by sublimation of the intermediate tetracarboxylic acid 8 is reported. Thus, the use of a pyrrol 13a bearing two orthogonal ester groups allows the synthesis of bipyrrol 12a, from which the benzyl ester groups are selectively removed to afford diester 11. The latter is transformed to dialdehyde 10 by using the McFayden–Stevens reaction, thus avoiding the unstable bipyrrol 9 formed during the tetradecarboxylation of 8.

The X-ray structure, electrochemistry, and optical spectroscopy of the porphycene analog of a benzochlorin, octaethylbenzochloracene (OEBzC) are reported. The OEBzC macrocycle is easier to oxidize and harder to reduce than octaethylporphycene (OEPc). The optical spectra of OEBzC and of the diacid salt are also reported. The optical spectrum for the one-electron oxidation of OEBzC is indicative of π-cation radical formation. In porphyrins, saturation of one pyrrole ring and addition of an exocyclic benzene ring progressively red-shifts the first absorption band. For porphycenes, saturation of one pyrrole ring results in a 35 nm blue shift in the first absorption relative to OEPc. Compared to 2,3-dihydroporphycene (DHPc), the first absorption band in OEBzC is red-shifted 24 nm. These studies further illustrate how the fused exocyclic benzene ring influences the macrocycle conformation, chemical and physical properties of porphycenes.

The redox behavior of iron porphycenes using the sodium mirror contact technique is reported. The resonance Raman spectra are obtained for each redox state, in order to explore the vibrational characteristics of these species in different redox states. The observed resonance Raman behavior of Fe^II porphycene anion radical and its dianion is interpreted using the vibrational analysis of free-base porphycene anion radical and dianion. For the first time, a species generated in the fourth reduction step, is confirmed by UV-vis spectroscopy and is assigned to Fe^I porphycene π dianion. The dependence of resonance enhancements of Raman bands for the Fe^I porphycene π dianion on excitation laser reveals structural distortion along the NC_aC_aN or C_bC_aC_aC_b segments of the pyrrole-pyrrole direct connection in the excited electronic state.

Unsubstituted porphycene was prepared in 65% yield from 2,7,12,17-tetra-tert-butylporphycene. Taking into account the yield of the substrate, this represents more than a five-fold improvement compared to the methodology used to date. Enhanced availability of the parent porphycene may be exploited in synthetic procedures based on derivatization of the unsubstituted compound.

Dual fluorescence is described and characterized for several 9-substituted porphycenes. Using both spectroscopic and computational studies, the phenomenon is related to the differential stabilization of the two trans tautomers in the excited state. The electronic nature of the 9-substituents affects both the energy and the lifetime of the two tautomers as well as their interconversion. The results are consistent with a two-step model for excited-state tautomerization that involves the population of an upper excited state, tentatively assigned to a cis species, which in turn provides an efficient non-radiative pathway to the ground state.

Porphycene having an imidazolium cation tag was synthesized and characterized by elemental analysis, UV-vis, NMR and ESI-mass spectroscopies. This porphycene derivative easily dissolves in various ionic liquids and produces singlet oxygen under irradiation by visible light (λ ≥ 460 nm). The photophysical parameters of the porphycene in ionic liquids were determined and the values were compared to those in acetonitrile. Photosensitizing reactions using this new porphycene for the oxidation of 1,5-dihydroxynaphthalene in ionic liquids were investigated and found to form 5-hydroxy-1,4-naphthoquinone. The recycled use of the porphycene was efficiently achieved in N,N,N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)amide ([TMPA][TFSA]) and N-methyl-N-propyl-piperidinium bis(trifluoromethanesulfonyl)amide ([P13][TFSA]).

A new and improved synthesis for porphycenes is presented. In particular, treating pyrroles with iodine monochloride results in quantitative yields and easier work up. This key step was employed for the synthesis of different 2,7,12,17-substituted porphycenes (TPrPc, TPPc, TtBPPc). The synthetic part of this project is followed by a global analysis of the physico-chemical properties of three exemplary porphycenes, including absorption, emission, and electrochemistry. In-situ UV-vis/nIR spectroelectrochemistry was used to determine the spectral signatures of the radical anion and cation species. Temperature-dependent fluorescence lifetimes were determined under air as well as under inert atmosphere.

A porphycene with a hydroxyethyl side chain was coupled to the 3′-terminus of an oligodeoxynucleotide via solid-phase synthesis. The resulting porphycene-DNA hybrid binds to a complementary region of a DNA target strand with greater affinity than the unmodified control oligonucleotide, resulting in an increase of the UV-melting point of 12.7 °C. Duplex formation is accompanied by an increase in porphycene fluorescence at 640 nm by 18%. When a tetrakis(p-hydroxyphenyl)porphyrin appended to the 5′-terminus of another DNA-strand is brought into close proximity of the porphycene by hybridizing it to the downstream-region of the template strand, 94% of the porphycene fluorescence is quenched. By quenching each others fluorescence to different degrees, porphycene and porphyrin, together, report on local DNA structure in a fashion reminiscent of that of molecular beacons. The introduction of porphycenes and the porphycene-porphyrin "two hybrid system'' to DNA-based structuring may open up new avenues to designed functional materials.

We describe attempts — not always successful — made over the years to improve the efficiency of porphycene synthesis and to produce novel compounds, custom-designed for specific purposes. New porphycenes are reported, some of them obtained rather unexpectedly as by-products of the planned reactions. Structure and energy computations of possible tautomeric forms in porphycenes substituted by one, two, three, and four tert-butyl groups lead to predictions regarding the kinetics and mechanisms of intramolecular double hydrogen transfer. The occurrence of tautomerization in single molecules of tert-butylsubstituted porphycenes is demonstrated by using fluorescence polarization techniques.

The butadiyne- and acetylene-linked 3,3′-porphycene dimers 1 and 2 were synthesized from the common intermediate, 3-ethynyl-2,7,12,17-tetrahexylporphycene 4. The butadiyne-linked dimer 1 was prepared from 4 by copper mediated Glaser-type homo-coupling. The acetylene-linked dimer 2 was synthesized by Sonogashira coupling of 4 and 2,7,12,17-tetrahexyl-3-iodoporphycene 5. These porphycene dimers were characterized by ¹H and ¹³C NMR spectroscopies, mass spectroscopy, X-ray diffraction analysis, UV-vis absorption and fluorescence spectra, cyclic voltammetry and differential pulse voltammetry. Crystal structure of 1showed a coplanar structure of two porphycene units. The absorption spectra in CH₂Cl₂ indicated the small interaction between the porphycene units through the linkage at 3,3′-positions. The electrochemical measurement showed two one-electron oxidation potentials and four one-electron reduction potentials indicating the electric interaction between the porphycene units.

Two ruthenium(II) carbonyl complexes of porphycene, (carbonyl)(pyridine)(2,7,12,17-tetra-n-propylporphycenato)ruthenium(II) (1) and (carbonyl)(pyridine)(2,3,6,7,12,13,16,17-octaethylpor-phycenato)ruthenium(II) (2), have been structurally characterized by single-crystal X-ray diffraction analysis. Cyclic voltammetry has revealed that the porphycene complexes undergo multiple oxidations and reductions in dichloromethane and the reduction potentials are highly positive compared to porphyrin analogs. UV-light irradiation (400 nm or shorter wavelength region) of a benzene solution of 1 and 2 containing external pyridine leads to dissociation of the carbonyl ligand from the ruthenium(II) centers to give the corresponding bis-pyridine complexes. The identical reaction has been also studied for a porphyrin derivative (carbonyl)(pyridine)(2,3,7,8,12,13,17,18-octaethylporphyriato)ruthenum(II) (3). The first-order kinetic analysis has revealed that the photosubstitution of all of the compounds occurs in the order of 10^-3 s^-1 at 298 K but proceeds faster for complexes of porphycene (1 and 2) than that of porphyrin (3).

Porphycene dimers connected by $m$-phenylene (DPc–mP) and 2,5-thienylene (DPc–T) linkages have been synthesized from 2,7,12,17-tetrahexyl-3-iodoporphycene by palladium-catalyzed cross-couplings. The crystal structures of porphycene dimers revealed that meta-phenylene linkage provides a highly twisted structure with the dihedral angel of 103° between two porphycene units, while thienylene linkage provides a nearly planar structure of two porphycene units. These porphycene dimers showed strong absorption thorough the visible to NIR regions and HOMO and LUMO energy levels of them are suitable as $n$-type materials of organic solar cells (OSCs) in combination with P3HT as a $p$-type material. The hole and electron mobilities of the blended films of the porphycene dimers and P3HT obtained by space-charge-limited-current (SCLC) method were 3.7 × 10${}^{-4}$ and 0.26 × 10${}^{-4}$ cm².V${}^{-1}$.s${}^{-1}$ for DPc–mP :P3HT, and 3.2 × 10${}^{-4}$ and 0.027 × 10${}^{-4}$ cm².V${}^{-1}$.s${}^{-1}$ for DPc–T :P3HT. Atomic force microscopy (AFM) and thin-film X-ray diffraction analysis (XRD) measurements indicated that the blended films of the porphycene dimers and P3HT formed amorphous films with smooth and low-roughness surfaces, whereas the blended film of porphycene monomer and P3HT created the highly crystalline film with huge domain structures. The OSCs composed of porphycene dimers and P3HT showed power conversion efficiencies of 0.08% was twice as high as that of monomer-based OSC.

A large diversity of porphyrin–diketopyrrolopyrrole conjugates and related structures formed by diketopyrrolopyrrole units and pyrrole-based moieties such as phthalocyanine, porphycene, calix[4]pyrrole or BODIPY have been reported since 2010. The new compounds, whether small molecules or polymeric materials, exhibit very interesting photophysical properties and have been tested in a range of technical or biological applications. This review summarizes the advances in the synthesis of such compounds. Their photophysical properties and potential applications are also briefly discussed.

We report syntheses of thiophene and dithiophene-substituted porphycenes (ThPc and DThPc) at 2,7,12,17-positions by McMurry coupling. The crystal structure of ThPc revealed that the porphycene plane shows a highly planar structure, and the dihedral angles between the porphycene core and thiophene are relatively small at 21$deg$ and 18$deg$. ThPc and DThPc exhibit red-shifted and broadened absorption because of the extension of $\pi$ conjugations through porphycene to the substituted thiophenes. We found that introduction of thiophene units onto porphycene results in decreasing the HOMO–LUMO differences effectively.

The cobalt complexes of meso-aryl substituted porphycenes were synthesized and characterized. The reduction potentials of the complexes were shifted to the positive side depending on the strength of the electron-withdrawing properties of the meso-substituents, while the optical properties, such as the absorption spectra of these complexes, were similar. This suggests that the energy levels of the molecular orbitals of the complexes were changed by the meso-substituents while the gaps of the orbitals were not significantly changed. The one-electron reduction of the complex did not afford the Co(I) species, but the ligand-reduced radical anion, which was characterized by electrospectrochemistry. The generated ligand-reduced species reacted with alkyl halides to form the Co(III)-alkyl complex. As a result, the reduction potential of the electrolytic reaction could be directly controlled by the substituents of the porphycene. The catalytic reaction with trichloromethylbenzene was also performed and it was found that the ratio of the obtained products was changed by the reduction potentials of the catalyst, $i.e$. the cobalt porphycenes.

Four novel $\beta$-dialkylated porphycenes, $i.e$. 2,12-(DEPoT and DPPoT) and 2,17-(DEPoC and DPPoC) were realized for the first time in porphycene chemistry. Unsymmetrically substituted monoalkylbipyrrole dialdehydes were subjected to McMurry coupling reactions to produce the isomeric $\beta$-dialkylated porphycenes. All these porphycenes exhibit unexpectedly good solubility in common organic solvents. The positional effects of alkyl groups could be controlled through diversification in structure of porphycene cores and, as a result, their photophysical properties. DPPoT presents significant fluorescence accompanied by reasonable singlet oxygen generation ability.

Mesoporous silica nanoparticles decorated with 2,7,12,17-tetraphenylporphycene have been prepared by reaction of aminated nanoparticles with 9-(glutaric $N$-succinimidylesteramide)-2,7,12,17-tetraphenylporphycene. The as-prepared nanoparticles were characterized by UV-vis spectroscopy, DLS and TEM.

The electrochemical properties of $\beta$-octaethylporphycene iridium complex (Ir-OEPo) were determined. Based on the electro-spectro measurement results, the reduction of Ir-OEPo did not occur at the central metal but at the ligand, while the reduction of $\beta$-octaethylporphyrin iridium complex (Ir-OEPor) occurred at the central iridium. A catalytic current was observed during the cyclic voltammetry (CV) measurements with trifluoroacetic acid (TFA) under a reductive condition, indicating the catalytic reactivity of Ir-OEPo for the hydrogen evolution reaction (HER). By constant potential electrolysis, hydrogen gas was detected by gas chromatography (GC) and the catalytic reactivity of Ir-OEPo was confirmed. The HER mechanism via ligand reduction of macrocyclic aromatic complexes could be one of the concepts for the development of new catalysts.

The aim of this review is to offer an update on the preparation and functionalization of porphycenes and expanded porphycenes covering the period of 2017 to 2022.