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

Boron(III) complexation was investigated in a series of ß-substituted porphycenes. Unlike meso-arylporphycenes, these macrocycles undergo a facile complexation reaction. Upon fusion of the ß,ß'-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 ß-substituted porphycenes, only bis-BF2 complexes formed. The effect of these substituents on the core geometry and photophysical properties are elaborated here.

Bis(naphthobipyrrolyl)methene-derived hexapyrrolic BODIPY as a single-molecule helicate with near-infrared emission.

Helically twisted bis(naphthobipyrrolyl)methene-derived open-chain hexapyrroles have been synthesized as HCl salts and the corresponding BODIPY. Their solid-state structures elucidated by single-crystal X-ray diffraction analysis clearly showed the presence of intramolecular hydrogen bonds, which were concluded to play a pivotal role in stabilizing the twisted conformation. Both molecules were observed to be NIR active, with the BODIPY moiety emission extending beyond 800 nm.

Tuning the Photophysical Properties and Photostability of Bis(naphthobipyrrolylmethene) Derived BODIPY via Functionalization.

A series of NIR absorbing and emitting, naphthobipyrrole derived BODIPY dyes are reported. These dyes possess different functional groups viz. -CHO, -CN, -NO2 , -Br and Ph at its α,α'-positions. All the derivatives were characterized thoroughly via various spectroscopic means as well as single-crystal X-ray structural analyses. Impacts of functionalization on its photophysical properties as well as photostability were evaluated. For instance, incorporation of the phenyl moieties at the α, α'-positions by Suzuki coupling of the dibromo-derivative 4 with phenyl boronic acid led to substantial bathochromic shift in absorption (762 vs 727 nm) and emission (778 vs 744 nm) relative to the unsubstituted parent dye VI. Theoretical calculations have been performed by ground state optimization to evaluate the electronic transitions which supports experimental observation.

Synthesis of Trifluoromethyl-Substituted [14]Triphyrin(2.1.1), Its Selective Reduction to Triphachlorin, and Stable Isomeric Triphabacteriochlorins via Direct Detrifluoromethylation.

Two novel classes of reduced triphyrin(2.1.1), namely, triphachlorin and triphabacteriochlorin, are realized via selective reduction of the newly synthesized 7,12-bis(trifluoromethyl)triphyrin(2.1.1) 1 using p-tosylhydrazide. Triphachlorin 2 displayed red-shifted absorption (580 nm). Harsher conditions, however, led to a unique direct detrifluoromethylation of one CF3 moiety to form two isomeric triphabacteriochlorins 3 and 4, which exhibit blue-shifted intense lowest-energy absorption bands and intense emission (ϕf = 0.52 and 0.36, respectively) while generating singlet oxygen very efficiently (Ï•Δ = 0.88 and 0.86, respectively). These reduced porphyrinoids show very high oxidative stability as they remain unchanged upon refluxing with 2,3-dichloro-5,6-dicyanobenzoquinone in toluene.

Monothia [22]pentaphyrin(2.0.1.1.0): a core-modified isomer of sapphyrin.

A novel 22π-aromatic sapphyrin isomer endowed with an acene moiety was designed and realised for the first time as a core-modified monothia analogue. This macrocycle exhibited absorption and emission in the near-infrared region. It was diprotonated under strongly acidic conditions and bound to anions like sapphyrin. It showed unusual coordination chemistry, acting as a neutral ligand and undergoing large out-of-plane deformation to bind Pd(II) ions.

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-Tetrapropylporphycene: Rational Synthesis, Complexation, and Halogenation.

We have designed and synthesized 3,6,13,16-tetrapropylporphycene for the first time as its alkyl analogue from ethyl 4-propyl-1H-pyrrole-2-carboxylate. The substituent effect was found to be more intense than reported positional isomeric tetrapropylporphycenes. The freebase porphycene exhibited moderate fluorescence and complexation ability with divalent metal ions, including Zn(II), which displayed an enhanced emission quantum yield (∼30%). The Pd(II) complex and freebase ß-tetrabromoporphycene generated singlet oxygen efficiently (75 and 51%, respectively) and, hence, may find application as potential photosensitizers in photodynamic therapy.

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).

1,2-Phenylene-Incorporated Smallest Expanded Calix[4]pyrrole via One-Step Synthesis of Tetrapyrrane: A Fluorescent Host for Fluoride Ion.

Synthesis of tetrapyrrane 8 from acetone and pyrrole via one-step condensation was achieved for the first time along with a much-improved yield of the tripyrrane 9. Diborylation of the tetrapyrrane and subsequent "1 + 1" cyclocoupling with 1,2-diiodobenzene following the Suzuki protocol generated novel o-phenylene incorporated macrocycle belonging to the smallest meso-expanded calix[4]pyrrole family. The latter macrocycle displays exclusive turn-on fluorescence sensing of fluoride ion upon complexation via a unique partial cone conformation supported by DFT analysis in acetonitrile solvent.

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.

Synthetic access to calix[3]pyrroles via meso-expansion: hosts with diverse guest chemistry.

Calix[3]pyrroles were synthesized for the first time. These macrocycles display versatile guest binding ability based on the bridges connecting the two α-positions of the tripyrrane moiety e.g. with an ethene bridge they showed colorimetric sensing of fluoride and cyanide ions, whereas with a phenylene spacer they exhibited fluorometric sensing of fluoride ions only, and the macrocycle with an ethylene unit acted as a host towards the water molecule with unique penta-coordinated oxygen in the solid state.

Revisiting the intense NIR active bronzaphyrin, a 26-π aromatic expanded porphyrin: synthesis and structural analysis.

Dithiabronzaphyrin, with intense absorption and fluorescence in the NIR region, was synthesized as its ß-octaethyl analogue via a thiophene bridged terpyrrole. Solid state structural characterization obtained for the first time revealed inversion of the thiophene rings. Studies showed that there is no effect from protonation or temperature on the structural rigidity of the macrocycle. Crystal packing revealed four open dimeric trifluoroacetate [(CF3COO)2H]- moieties binding to the macrocycle in the protonated form via H bonding.

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.

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

Two novel ß-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 ß-octaethylporphycene. HOT displays significant fluorescence accompanied by reasonable singlet oxygen generation ability.

Annulated Isomeric, Expanded, and Contracted Porphyrins.

Compared to porphyrin, its isomeric, expanded, and contracted analogues are less well explored. This contrast is found to be even more drastic in the case of their peripherally annulated counterparts. Nevertheless, the chemistry of annulated isomeric, expanded, and contracted porphyrins started flourishing recently with considerable efforts over the past few years, as evidenced by an increasing number of publications. While keeping the essence of porphyrins, these annulated versions exhibit quite unique properties that have no precedence in their nonannulated counterparts. An in-depth update of research carried out so far in this emerging area will be presented in this review.

Hexaethylsubporphyrins: ß-alkyl analogues in the subporphyrin family.

Two new subporphyrins were synthesized for the first time from a ß-substituted pyrrole i.e. 3,4-diethylpyrrole via pyridine-tri-N-(3,4-diethylpyrrolyl)borane as building blocks. These ß-hexaethylsubporphyrins are true contracted congeners of ß-octaethylporphyrin (OEP). While the meso-triphenyl derivative of hexaethylsubporphyrin could be synthesized by following the reported method, the meso-free analogue could only be synthesized by condensation with trioxane, in the presence of catalytic methanesulfonic acid. These contracted macrocycles display interesting absorption, and emission behaviour including substituent dependent S2 fluorescence owing to the presence of flexible electron donating ethyl groups at their ß-positions. The optical response and ultrafast S2 state dynamics of these systems suggest that it may be possible to tune the properties of the subporphyrin to develop efficient systems for solar energy capture and conversion processes.

ß,ß'-Bipyrrole Fusion-Driven cis-Bimetallic Complexation in Isomeric Porphyrin.

An unprecedented cis-bimetallic complex of dinaphthoporphycene (DNP), namely [Pd2(µ-DNP)(µ-OAc)2], is reported. The most striking feature of this complex is that two palladiums coordinate to the macrocycle on the same side and are closely held together (Pd-Pd: 2.67 Å) by two bridging acetate ligands exhibiting significant metal-metal bonding interaction (bond order 0.18 evaluated by NBO analysis). Interestingly, replacing acetate with acetylacetonate (acac) could stabilize an unusual mono-palladium complex of DNP, where Pd coordinates unsymmetrically to two ring Ns above the macrocyclic plane, as well as coordinating with two Os of the acac ligand. Remarkably, the rigid DNP core displays enhanced complexation induced aromaticity (as per NICS and HOMA analysis), despite undergoing severe core deformation during complexation with metal ion(s) as noticed from their solid-state structures.

Calix[4]pyrroles with Shortest Possible Strap: Exclusively Selective toward Fluoride Ion.

Four new calix[4]pyrroles with the shortest possible strap so far through ortho-linking of the aromatic unit have been synthesized, including a naphthalene-derived fluorescent receptor. They show exclusive selectivity toward the fluoride ion as confirmed by (1)H NMR, isothermal titration calorimetry, and fluorescence spectroscopic study. Anion affinity could also be modulated further via functionalization at the strap. Computational analysis displays calix[4]pyrroles binding to fluoride ion in a very unusual 1,3-alternate conformation where the anion resides on the opposite side of the strap.

ß-Octamethoxy-Substituted 22π and 26π Stretched Porphycenes: Synthesis, Characterization, Photodynamics, and Nonlinear Optical Studies.

Three meso-expanded tetrapyrrolic aromatic macrocycles, including 22π and 26π acetylene-cumulene bridged stretched octamethoxyporphycenes and octamethoxy[22]porphyrin-(2.2.2.2), are reported, for the first time, by modification of previously reported synthetic methods. This strategy led to an enhancement in the overall yield of their corresponding octaethyl analogues. The methoxy-substituted expanded porphycenes display slightly blueshifted absorption relative to their ethyl analogues, along with very weak fluorescence, probably due to efficient intramolecular charge transfer (ICT). Additionally, the two-photon absorption (TPA) cross sections of these macrocycles were evaluated; these are strongly related to core expansion of the porphyrin aromaticity through increased meso-bridging carbon atoms as well as conformational flexibility and substitution effects at the macrocyclic periphery. In particular, the octamethoxy stretched porphycenes display strong TPA compared with the octaethyl analogues due to the dominant ICT character of methoxy groups with a maximum TPA cross section of 830 GM at 1700 nm observed for 26π-octamethoxyacetylene-cumuleneporphycene.