Regioselective Stepwise Bromination of [14]Triphyrins(2.1.1) and Their Effects on Structural, Spectral, and Redox Properties.

Regioselective stepwise bromination of meso-tetraaryl [14]triphyrins(2.1.1) was explored to investigate the effect of bromine substitution at the ß-pyrrole carbons of triphyrin(2.1.1) on the structural, spectral, photophysical, and redox properties. A series of ß-monobromo to ß-hexabromo triphyrins(2.1.1) 2-7 were synthesized by treating triphyrin(2.1.1) 1 with appropriate equivalents of N-bromosuccinimide at ambient temperature in decent yields. The regiochemistry of bromines in ß-brominated triphyrins(2.1.1) 3-5 and 7 was confirmed by X-ray crystallography, and the analysis revealed the effect of bromination of triphyrin(2.1.1) on the structural framework was significant in the case of hexabromotriphyrin(2.1.1) 7 compared to other macrocycles. Absorption spectroscopy showed that stepwise substitution of bromines at ß-pyrrole carbons of triphyrin(2.1.1) resulting in bathochromic shifts of absorption bands relative to triphyrin(2.1.1) 1 and hexabromotriphyrin(2.1.1) 7 exhibited absorption bands at longer wavelengths. The redox studies revealed that compounds 2-7 were easier to reduce than triphyrin(2.1.1) 1, and the first reduction potential wave shifted anodically with an increase in the number of bromine substituents at ß-pyrrole carbons of triphyrin(2.1.1) 1 from one to six. These structural, spectral, and electrochemical properties were also predicted by density functional theory calculations, and the analysis was consistent with the experimental observations.

Core-Modified Pentaphyrins(2.1.1.1.1) and Bis(difluoroborane) Complex: Synthesis, Structure, and Spectral and Redox Properties.

A series of hetero analogues of pentaphyrins(2.1.1.1.1) such as oxapentaphyrins(2.1.1.1.1) and thiapentaphyrins(2.1.1.1.1) were synthesized by 3 + 2 condensation of dipyrroethenedicarbinol with 16-oxatripyrrane/16-thiatripyrrane under mild acid-catalyzed reaction conditions. The stable macrocycles are freely soluble in organic solvents, and their identities were confirmed by a corresponding molecular-ion peak in high-resolution mass spectrometry, 1D and 2D NMR spectroscopy, and X-ray structure obtained for one of the oxapentaphyrin(2.1.1.1.1) macrocycles. The crystal structure and NMR studies indicated that the heterocyclic ring, such as furan in oxapentaphyrins(2.1.1.1.1) and thiophene in thiapentaphyrins(2.1.1.1.1), was inverted. In absorption spectra, the macrocycles showed one sharp band at ∼516 nm and one broad band at ∼744 nm. The spectral and X-ray studies supported the nonaromatic nature of these macrocycles. This is in contrast to the recently reported aza analogue of pentaphyrins(2.1.1.1.1), which showed antiaromatic behavior. Upon protonation, the core-modified pentaphyrins(2.1.1.1.1) macrocycles exhibited bathochromically shifted absorption bands with a distinct change in the color of the solution. The 1H NMR, nucleus-independent chemical shift, and anisotropy-induced current density studies indicated the presence of Mobius aromaticity in the protonated macrocycles. The core-modified pentaphyrins(2.1.1.1.1) can act as good coordinating ligands, as shown here by synthesizing a bis(difluoroborane) complex of one of the thiapentaphyrins(2.1.1.1.1).

Effects of Core Modification on Electronic Properties of para-Benziporphyrins.

In contrast to the nonaromatic meta-benziporphyrins, the para-benziporphyrins possess aromatic character depending on the type of five-membered ring present in the macrocyclic core. The effects of changing the para-benziporphyrinic core from C2N3 to C2NSN, C2NSeN, and C2NTeN by replacing the pyrrole with other five-membered heterocycles such as thiophene, selenophene, and tellurophene on aromatic properties of p-benziporphyrins are described here using spectral, electrochemical, X-ray, and density functional theory (DFT) studies. The missing core-modified p-benziporphyrins with C2NSeN and C2NTeN cores were synthesized by condensing 1 equiv of benzitripyrrane and 1,3-benzene-bis((4-phenyl)methanol with an appropriate diol such as 2,5-bis[(p-tolyl)hyroxymethyl]selenophene and 2,5-bis(hydroxymethyl)tellurophene under mild acid-catalyzed conditions at room temperature and characterized in detail by high-resolution mass spectrometry (HR-MS), one- & two-dimensional NMR, and X-ray crystallography of the one of the macrocycles, Selena p-benziporphyrin. The X-ray structure of Selena p-benziporphyrin revealed that the macrocycle was almost planar apart from the p-phenylene ring, which was deviated by 49.71° from the mean plane of the macrocycle defined by four meso carbons, unlike Selena m-benziporphyrin, which is relatively more distorted. NMR studies revealed that, as the core changes from C2N3 to C2NSN, C2NSeN, and C2NTeN, the diatropic ring current decreases, indicating that the aromatic character also decreases in the same order. X-ray structure and DFT studies also revealed that the distortion in the macrocycle increases as the pyrrole ring of p-benziporphyrin was replaced with other heterocycles such as furan, thiophene, selenophene, and tellurophene and that the tellura p-benziporphyrin was the most distorted macrocycle among core-modified p-benziporphyrins. Absorption and electrochemical properties were in agreement with these observations. Our repeated attempts on metalation of these p-benziporphyrins resulted in the successful synthesis of a Pd(II) complex of tellura p-benziporphyrin. The Pd(II) complex was characterized by HR-MS and NMR techniques, and the structure was optimized by DFT. The studies indicated that the Pd(II) ion was bonded to one of the pyrrolic nitrogens, tellurophene, tellurium, and two chloride ions in distorted square-planar geometry.

Synthesis and Studies of Strained Fluorenophyrins.

New examples of polyaromatic hydrocarbons embedded porphyrinoids named fluorenophyrins, which are fluorene-embedded porphyrins, have been readily synthesized starting with commercially available fluorene as a key precursor over a sequence of four simple synthetic steps. These new fluorenophyrin macrocycles were characterized and studied by high-resolution mass spectrometry, one-dimensional and two-dimensional nuclear magnetic resonance and absorption spectroscopies, electrochemical techniques, and density functional theory calculations. The studies indicated that the fluorenophyrins are nonaromatic, and π-conjugation in macrocycles was limited due to nonplanar arrangement of a fluorene moiety with the rest of the macrocycle.

Meso-Fused Carbatriphyrins(2.1.1) and Its Organo Phosphorus(V) Complex.

The first examples of phlorin analogues of meso-fused carbatriphyrins(2.1.1) and organo P(V) complex of one of the meso-fused carbatriphyrins were obtained by adopting a premodification strategy starting with fluorene as a fused polyaromatic precursor over a sequence of steps. The meso-fused carbatriphyrins(2.1.1) were obtained as their nonaromatic phlorin analogues owing to the unstable nature of the fully conjugated meso-fused carbatriphyrins(2.1.1). The organo P(V) complex of one of the meso-fused carbatriphyrins was prepared by treating the phlorin analogue of carbatriphyrin(2.1.1) with PCl3 in toluene/triethylamine at reflux for 1 h. The P(V) complex of the meso-fused carbatriphyrin(2.1.1) was found to be moderately aromatic and the resulting global conjugation pathways in the P(V) complex significantly alters the aromaticity of the fluorene unit. The nonaromatic nature of the phlorin analogues of the meso-fused carbatriphyrin(2.1.1) and the moderately aromatic nature of its P(V) complex were supported by X-ray crystallography, absorption spectroscopy, NMR studies together with nucleus-independent chemical shifts, anisotropy-induced current density, and harmonic oscillation stabilization energy calculations.

Covalently linked meso-tetraaryl triphyrin(2.1.1)-ferrocene(s) conjugates: synthesis and properties.

Functionalized meso-tetraaryl triphyrins(2.1.1) containing two meso-iodophenyl groups or a bromo group at the ß-pyrrole carbon were synthesized over a sequence of steps. The covalently linked conjugates such as a triphyrin(2.1.1)-(ferrocene)2 triad and a triphyrin(2.1.1)-ferrocene dyad were obtained in 47-51% yields by coupling the appropriate functionalized triphyrin with ethynylferrocene under mild Pd(0) coupling reaction conditions. The functionalized triphyrins(2.1.1) and their conjugates were thoroughly characterized by HR-mass spectrometry and 1D and 2D NMR spectroscopy. The absorption, electrochemical and DFT studies suggested weak interactions between the triphyrin(2.1.1) and ferrocenyl moieties in triads but relatively strong interactions in dyads.

Synthesis of Unsymmetrical Heterobenzisapphyrins.

Two examples of highly unsymmetrical heterobenzisapphyrins containing one benzene, two pyrroles, one furan, and one thiophene ring connected via four meso-carbons and one direct bond were prepared over a sequence of five steps in 8-9% yields. The X-ray structure revealed that the furan ring was inverted from the macrocyclic framework and the macrocycle was nearly planar. Spectral and electrochemical studies indicated that the macrocycle was nonaromatic in nature due to lack of conjugation.

Dibenzidecaphyrins (1.0.0.1.1.1.0.0.1.1) and Their Bis-BF2 Complexes.

The largest dibenziporphyrinoids discovered until date, dibenzidecaphyrins (1.0.0.1.1.1.0.0.1.1), were synthesized in 8-10% yields by condensing m-phenylene-based pentapyrrane with pentafluorobenzaldehyde under mild reaction conditions. The crystal structure obtained for one of the decaphyrin macrocycles revealed that the macrocycle adopts a figure eight conformation and showed nonaromatic features. The decaphyrin macrocycles were further characterized by various spectral, electrochemical techniques, and computational studies. The dibenzidecaphyrin macrocycle showed half the number of proton NMR resonances, indicating the high symmetry of the macrocycle. The decaphyrins showed a broad absorption in the 600-900 nm region along with a well-defined band at 448 nm, and the macrocycles were stable under redox conditions. The decaphyrins were treated with BF3·OEt2/triethylamine in CHCl3 at reflux temperature, followed by column chromatography, to afford bis-BF2 complexes of decaphyrin in 34-40% yields. The spectral and DFT studies supported a figure eight conformation for the bis-BF2 complexes, and electrochemical studies indicated that the bis-BF2 complexes were electron deficient compared to the free-base decaphyrin macrocycles.

Synthesis of meso-Tetraaryl Triphyrins(2.1.1).

A simple, straightforward [2+1] condensation of 5,6-diaryldipyrroethene dicarbinols with pyrrole under mild acid-catalyzed conditions resulted in the formation of highly desirable aromatic ß-free meso-tetraaryl [14]triphyrins(2.1.1) in 15-18% yields. The triphyrins(2.1.1) are very novel monoanionic tridentate ligands that form metal complexes readily as demonstrated here by preparing Re(I) complexes.

Synthesis of Nonaromatic and Aromatic Dithia Benzisapphyrins.

A series of [24π] dithia meta-benzisapphyrins and [22π] dithia para-benzisapphyrins were synthesized by 3 + 2 condensation of appropriate benzitripyrrane with bithiophene diol under mild acid catalyzed conditions. The dithia m-benzisapphyrins and dithia p-benzisapphyrins were thoroughly characterized by HR-MS, 1D and 2D NMR, absorption, and electrochemical techniques. Our studies showed that dithia m-benzisapphyrins are nonaromatic, whereas the dithia p-benzisapphyrins are aromatic in nature. Thus, we demonstrated here that the dithiabenzisapphyrins can be made aromatic by replacing the m-phenylene moiety of the benzisapphyrin macrocycle with a p-phenylene unit. Furthermore, the studies also indicated that the aromaticity of the dithia p-benzisapphyrins was relatively more compared to the reported heterosapphyrins. The structural and spectral characteristics including aromaticity of the m-benzisapphyrins and p-benzisapphyrins were also discussed with the help of DFT, NICS, and TD-DFT studies.

Benzofuran-/Benzothiophene-Incorporated NIR-Absorbing Triphyrins(2.1.1).

Novel benzofuran-/benzothiophene-incorporated triphyrins(2.1.1) were synthesized using readily available precursors under mild reaction conditions. The X-ray structure revealed that the benzofuran-incorporated triphyrin(2.1.1) macrocycle was slightly ruffled. The triphyrins(2.1.1) exhibit more absorption in the NIR region compared to previously reported dibenzofuran-/dibenzothiophene-incorporated hybrid macrocycles. The benzofuran-incorporated triphyrin shows significant red shift in absorption bands compared to that of benzothiophene-based triphyrin due to its more coplanar arrangement as supported by X-ray and DFT studies.

Synthesis of Phlorin Analogues of Dithiacorrphycene and Their Use as Specific Chemodosimetric Sensors for Fe3+ Ions.

The first examples of stable phlorin analogues of dithiacorrphycene were synthesized by treating appropriate tetrapyrranes with two equivalents of trifluoroacetic acid in CH2 Cl2 followed by oxidation with p-chloranil. The oxidative coupling of tetrapyrrane resulted in the formation of two unexpected phlorin analogues of dithiacorrphycenes that differed from each other at the meso-sp3 -carbon, and the two macrocycles were easily separated and isolated by column chromatography. The novel macrocycles were confirmed by X-ray crystallography and characterized by HRMS, 1D and 2D NMR spectroscopy, absorption spectroscopy, cyclic voltammetry, and time-dependent DFT studies. X-ray structure analysis revealed the highly strained and distorted nature of the macrocycles. The macrocycles showed one ill-defined absorption band at λ≈315 nm and a broad band at λ≈530 nm. Because both macrocycles possess two thiophene and two pyrrole rings with one ionizable inner NH, the cation-sensing properties of the phlorin analogues of dithiacorrphycenes have been investigated. The phlorin analogues of dithiacorrphycenes act as specific chemodosimetric sensors for Fe3+ ions by converting phlorin analogues of dithiacorrphycenes into dithiacorrphycene.

Synthesis, Structural, Spectral, and Electrochemical Studies of Selenabenziporphyrin and Its Pd(II) Complex.

A new nonaromatic selenabenziporphyrin was synthesized by (3 + 1) condensation of m-benzitripyrrane and 2,5-bis[( p-tolyl)hydroxymethyl] selenophene under mild trifluoroacetic acid-catalyzed reaction conditions. The selenabenziporphyrin was characterized by high-resolution mass spectrometry, one- and two-dimensional NMR spectroscopy, and X-ray crystallography. The crystal structure revealed that the macrocycle was planar with moderately tilted m-phenylene ring and that the phenylene ring completely blocks the macrocyclic π-delocalization. The selenabenziporphyrin exhibits one broad absorption band at 645 nm along with one sharp band at 415 nm, and electrochemical studies revealed that the macrocycle was electron-deficient. The selenabenziporphyrin readily forms organometallic Pd(II) complex when treated with PdCl2 in CH3CN/CHCl3 at reflux followed by recrystallization. The X-ray structure revealed that the Pd(II) ion was coordinated with two pyrrole "N"s, selenophene "Se", and m-phenylene ring "C" in square-planar fashion, and the complex retained its nonaromatic nature. The Pd(II) complex exhibits ill-defined absorption bands, and it was more electron-deficient than free-base selenabenziporphyrin macrocycle. Time-dependent density functional theory studies supported the experimental observations.

Synthesis and Properties of Covalently Linked AzaBODIPY-BODIPY Dyads and AzaBODIPY-(BODIPY)2 Triads.

The azaBODIPYs containing one and two formyl functional groups on the 1,7-aryl groups present at the azaBODIPY core were synthesized over sequence of steps and characterized by mass, NMR, absorption, and electrochemical techniques. The monoformylated and diformylated azaBODIPYs are very useful synthons to prepare a wide variety of new fluorescent compounds. The mono- and diformylated azaBODIPYs were treated with pyrrole under mild acidic conditions followed by column chromatographic purification to afford azaBODIPYs appended with one and two dipyrromethanyl groups. The dipyrramethanyl groups of azaBODIPYs were oxidized with DDQ and complexed with BF3·Et2O to obtain covalently linked azaBODIPY-BODIPY dyads and azaBODIPY-(BODIPY)2 triads. The dyads and triads were characterized in detail by HR-MS, 1D and 2D NMR, absorption, fluorescence, and electrochemical techniques and the structure of one of the triads was deduced by X-ray crystallography. The crystal structure of azaBODIPY-(BODIPY)2 triad revealed that the two BODIPY units were in perpendicular orientation with azaBODIPY unit. The absorption and electrochemical studies indicated a weak interaction among the BODIPY and azaBODIPY moieties and the moieties retain their independent characteristic features in dyads and triads. The preliminary fluorescence studies supported an efficient energy transfer from BODIPY unit(s) to azaBODIPY unit in dyads and triads.

Novel Rhodafluors: Synthesis, Photophysical, pH and TD-DFT Studies.

An efficient protocol for the synthesis of new rhodol derivatives has been developed. The synthesis involves condensation of 2-hydroxy benzophenone derivatives with 1, 3-dihydroxy benzene derivatives in solvents such as ionic liquid (N-methyl-2-pyrrolidonium hydrogen sulfate) and methane sulphonic acid. Both ionic liquid and methane sulphonic acid were found to be promising self-catalyzed solvents to bring out the conversion to form desired rhodols in excellent yields. In N-methyl-2-pyrrolidonium hydrogen sulfate reaction proceeds faster compared to methane sulphonic acid. The new fluorophores are investigated for their photophysical properties in various microenvironments and systematically characterized by means of density functional theory and time dependent density functional theory. Photophysical properties of the new rhodafluors found sensitive towards change in the pH of media and thus can be used as efficient pH sensors.

Novel pyrromethene dyes with N-ethyl carbazole at the meso position: a comprehensive photophysical, lasing, photostability and TD-DFT study.

Two novel BODIPY (pyrromethene, PM) dyes containing N-ethyl carbazole at the meso position are synthesized and their photophysical properties in different solvents and the photochemical stabilities and laser performances in n-heptane are investigated. The n-heptane solution of the dyes was used as a gain medium in a constructed narrow band dye laser, pumped by a Q-switched (10 Hz) frequency-doubled (532 nm) Nd:YAG laser and the results gave enhanced photo stabilities and similar peak efficiencies of the synthesized dyes as compared to parent dye PM567. When substituted at the meso position with N-alkyl carbazole, photostability is found to be increased in comparison to PM567, and also when substituted at 2- and 6-positions with a benzyl group instead of an ethyl group along with N-ethyl carbazole at the meso position, the photo stability is further increased in n-heptane. A comprehensive study on structural, photophysical and electronic properties of dyes by means of DFT and TD-DFT in the solvents of various polarities has revealed remarkable characteristics of the BODIPY chromophore.

Congeners of Pyrromethene-567 Dye: Perspectives from Synthesis, Photophysics, Photostability, Laser, and TD-DFT Theory.

In an attempt to develop photostable and efficient BODIPY (PM) dyes for use in liquid dye lasers, three new congeners of widely used laser dye, PM567, were synthesized and their photophysical properties in various organic solvents, laser performances, and photostabilities in a selected solvent, 1,4-dioxane, have been investigated using a frequency doubled Q-switched (10 Hz) Nd:YAG laser at 532 nm. The results of photostability study in nonpolar 1,4-dioxane revealed the remarkable enhancement in stability of the novel dyes compared to that of PM567 as well as improved laser performances. Cyclic voltammetry study strongly supports the observed enhancement in photostability of the novel dyes compared to that of PM567. The observed properties of the novel dyes in relation to those of PM567 have been rationalized by extensive use of DFT and TD-DFT using the B3LYP/6-31G(d) method of theory.

NIR-emitting boradiazaindacene fluorophores -TD-DFT studies on electronic structure and photophysical properties.

Density Functional Theory [B3LYP/6-31G(d)] and Time Dependent Density Functional Theory [TD-B3LYP/6-31G(d)] computations have been used to have more understanding of the structural, molecular, electronic and photophysical parameters of recently synthesized near IR-emitting acid switchable di-styryl BODIPY dyes. The structures have been optimized using function B3LYP and basis set used was 6-31G(d) for all the atoms and their geometries which are correlated with corresponding rotational isomers including rotational isomers of diprotonated forms in chloroform solvent. The observed energies of the optimized molecules suggest that there may be rotation about C-C single bond as the observed energy barrier is very low. The results of TD-DFT suggest that there is very good match between the observed and calculated absorptions diprotonated forms of one molecule. There is also good match between experimental and theoretical emission of neutral forms. More deviations are observed in the case emission of the diprotonated forms.