Synthesis of boron-dipyrromethene-ferrocene conjugates

Synthesis, spectral, electrochemical and photophysical properties of four BODIPY-ferrocene conjugates in which one or two ferrocenyl groups were covalently connected either directly to boron-dipyrromethene framework or to meso-phenyl group of boron-dipyrromethene unit are described. The BODIPY-ferrocene conjugates were prepared by adopting different synthetic routes. The absorption studies indicated the presence of charge transfer band in BODIPY-ferrocene conjugates in which the ferrocenyl group(s) were directly connected to boron-dipyrromethene framework. The electrochemical studies on conjugates indicated that ferrocenyl group was difficult to oxidize whereas boron-dipyrromethene unit was easier to reduce. The conjugates were non-fluorescent due to electron transfer from ferrocene to boron-dipyrromethene unit. However, when ferrocene was oxidized to ferrocenium ion with an oxidizing agent, the conjugates exhibited fluorescence with decent quantum yields (0.17-0.31) and lifetimes (3.8-5.2 ns).     

Synthesis of functionalized core-modified sapphyrins and covalently linked porphyrin–sapphyrin dyads

We adopted simple synthetic strategy to synthesize mono-functionalized thiasapphyrins containing functionalized aryl group in the meso-position at thiophene side. The thiasapphyrin building block containing iodophenyl functional group was coupled with three different porphyrin building blocks with N₄, N₃S and N₂S₂ cores containing meso-ethynylphenyl functional group under mild Pd(0) coupling conditions to synthesize three covalently linked diphenyl ethyne bridged porphyrin–thiasapphyrin dyads. The porphyrin–thiasapphyrin dyads were characterized by mass, NMR, absorption, electrochemical and fluorescence techniques. The NMR, absorption and electrochemical studies indicated that the two components in dyads interact weakly and retain their individual identities. The steady state fluorescence studies indicated that the porphyrin fluorescence is reduced to a significant extent because of energy and/or electron transfer to the thiasapphyrin unit. The protonation studies indicated that N₄ porphyrin unit is more basic, whereas N₃S and N₂S₂ porphyrin units are less basic compared to thiasapphyrin unit in respective dyads. We explored the potential of dyads as fluorescent anion sensors and showed that two out of three dyads can be used as fluorescent anion sensors.     

Synthesis and studies of covalently linked meso-furyl boron-dipyrromethene-ferrocene conjugates

Four meso-furyl BODIPY-ferrocene conjugates 1–4 in which one or more ferrocene groups were connected directly to BODIPY core or meso-furyl group were synthesized by coupling of appropriate bromo meso-furyl BODIPYs with α-ethynylferrocene under mild Pd(0) coupling conditions. The compounds were characterized by HR-MS mass, NMR, absorption, electrochemistry and fluorescence techniques. The absorption studies of compounds 1–4 showed charge transfer band in addition to BODIPY absorption bands indicating that the BODIPY and ferrocene moieties interact within the conjugates. On the other hand, the charge transfer band is absent in meso-phenyl BODIPY-ferrocene conjugate due to the orthogonal arrangement of ferrocene appended meso-phenyl group with BODIPY core which prevents the interaction between the two moieties. The electrochemical studies showed strong oxidation due to ferrocene moiety and reduction due to meso-furyl BODIPY unit. The compounds 3 and 4 which contain two and three ferrocenyl groups respectively were oxidized at the same potential with two and three electrons involved in the redox process. The compounds 1–4 are weakly fluorescent due to electron transfer from ferrocene unit to BODIPY unit. However, the fluorescence can be restored by oxidizing the ferrocene to ferrocenium ion which prevents the electron transfer between the two moieties. The computational studies support the experimental results.     

Synthesis of covalently linked boron-dipyrromethene-chromophore conjugates using 3-bromo boron-dipyrromethene as a key precursor

3-Bromo boron dipyrromethene (3-bromo BODIPY) has been used as key synthon to prepare one ethynyl bridged and six ethynylphenyl bridged BODIPY-chromophore conjugates using mild Pd(0) coupling conditions. The chromophores possessing very distinct features, such as anthracene, BODIPY, terpyridine, porphyrin, Zn(II)porphyrin, 21,23-dithiaporphyrin and thiasapphyrin were connected at 3-position of boronboron-dipyrromethene dye by coupling of 3-bromo BODIPY with ethynyl or ethynylphenyl chromophore in toluene/triethylamine in the presence of catalytic amount of AsPh₃/Pd₂(dba)₃ at 40 °C followed by column chromatographic purification. The spectral studies indicated that the interaction is stronger in ethynyl bridged BODIPY-chromophore conjugate compared to ethynylphenyl bridged BODIPY-chromophore conjugates. The steady-state fluorescence indicated that in ethynyl bridged BODIPY-anthracene conjugate, the BODIPY unit act as energy acceptor and showed a possibility of energy transfer from donor anthracene unit to acceptor BODIPY unit on selective excitation of anthracene unit. However, in ethynylphenyl bridged BODIPY-porphyrin conjugates, the BODIPY unit act as energy donor and exhibited a possibility of singlet-singlet energy transfer from BODIPY unit to porphyrin unit.     

Synthesis and characterization of meso-ferrocenylethynyl 5,15-diphenylporphyrins

A series of four meso-ferrocenylethynyl (5,15-diphenylporphyrinato)nickel(II) derivatives have been synthesized by Sonogashira coupling reactions. Three of these compounds contain the electron-withdrawing groups including -CHO, -CHC(CN)₂, and -CCC₆H₄NO₂ at the remaining meso position, with a view to preparing push-pull chromophores, in which ferrocene serves as the electron donor. All the new compounds have been characterized spectroscopically and the molecular structure of one of these porphyrins (compound 11) has also been determined. The studies show that although the ferrocenylethynyl group can extend the π system of the central porphyrin core, the cyclopentadienyl rings of ferrocene are almost orthogonal to the porphyrin ring. This hinders ferrocene serving as a good electron donor in these systems.     

Synthesis and fluorescence properties of covalently linked homo- and hetero-porphyrin dyads containing meso-tolyl porphyrin and meso-furyl porphyrin sub-units

Three porphyrin building blocks with N₄, N₃S and N₂S₂ cores having three meso-furyl groups and one meso-iodophenyl group were synthesized and characterized. The porphyrin building blocks were used to synthesize six porphyrin dyads such as N₄-N₄, N₃S-N₃S, N₂S₂-N₂S₂, N₄-N₃S, N₄-N₂S₂ and N₃S-N₂S₂ containing meso-tolyl and meso-furyl porphyrin sub-units under mild Pd(0) mediated coupling conditions. Steady state fluorescence studies indicated an efficient energy transfer from the meso-tolyl porphyrin sub-unit to the meso-furyl porphyrin sub-unit in all six dyads. This study supported the argument that the meso-furyl porphyrins can be used as good energy acceptors when meso-aryl porphyrins act as energy donors in their metal free form.     

Synthesis of three-dimensionally arranged porphyrin arrays via intramolecular meso-meso coupling

The synthesis and photophysical properties of three-dimensionally arranged porphyrin arrays with through-space electronic communication are reported. 1,3,5-Trioxamethylphenylene bridged Zn(II) porphyrin trimer 3 was coupled by Ag(I)-promoted oxidative coupling reaction to give porphyrin cage 5 comprising three meso-meso linked diporphyrins, which was then transformed by oxidation with DDQ and Sc(OTf)₃ into porphyrin cage 7 comprising three fused diporphyrins. Intramolecular meso-meso coupling reaction was applied to porphyrin pentamer 11 to provide porphyrin array 12 consisting of a porphyrin core flanked by two meso-meso linked diporphyrins. Further oxidation of 12 with DDQ and Sc(OTf)₃ afforded triply stacked porphyrin array 13 that is comprised of a porphyrin core flanked by two porphyrin tapes. UV-vis-NIR absorption and fluorescence spectra of 5, 7, 12, and 13 showed their distorted conformations and electronic interaction within the stacked porphyrin arrays.     

Spectroscopic properties of meso-thienylporphyrins with different porphyrin cores

The absorption and fluorescence properties of a series of meso-thienylporphyrins with different porphyrin cores (N₄, N₃O, N₃S and N₂S₂ cores) were studied and compared with the corresponding meso-tetraarylporphyrins. The replacement of six-membered phenyl groups with five-membered thienyl groups at meso-positions resulted in red shifts and broadening of absorption and emission bands, low quantum yields and decreased S₁ state lifetimes and the maximum effects were observed for meso-tetrathienylporphyrin with N₂S₂ porphyrin core. Similar observations were noted for the dications of meso-thienylporphyrins compared to the dications of the corresponding meso-tetraarylporphyrins. These results suggest that the replacement of six-membered aryl group with five-membered thienyl groups at meso-positions, the electronic properties of the porphyrin were altered significantly.     

Photochemistry of covalently-linked multi-porphyrinic systems

Synthesis, structural characteristics, and optical and electrochemical properties of various covalently-linked porphyrin arrays are described. First, aromatic-spacer bridged diporphyrins were prepared in which the diporphyrin geometries were conformationally-restricted and thus suitable for detailed studies on the exciton coupling and the intramolecular energy and/or electron transfer reactions. Secondly, the Ag(I)-salt oxidation of 5,15-diaryl Zn(II) porphyrins provided meso–meso-linked Zn(II)-diporphyrins. This reaction is advantageous in light of its high regioselectivity and easy extension to longer porphyrin arrays. The doubling reaction was repeated up to the synthesis of a discrete 128-mer, which is, to the best of our knowledge, the longest man-made molecule. Finally, the oxidation of meso–meso-linked Zn(II) porphyrin arrays with a combination of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and Sc(III)(OTf)₃ produced fused porphyrin arrays with full π-conjugation, which displayed extremely small HOMO–LUMO gaps that reach into the infrared region.     

Donor–acceptor perylenediimide–ferrocene conjugates: synthesis,photophysical, and electrochemical properties

Donor–acceptor, perylenediimide–ferrocene conjugates have been synthesized by Suzuki, and Sonogashira coupling reactions. The photophysical and electrochemical properties of these conjugates are discussed. It has been shown that fluorescence as well as the electron affinity of the perylenediimide can be tuned by attaching the appropriate ferrocenyl derivatives.     

Synthesis,Structure and Photophysical Properties of Ferrocenyl or Mixed Sandwich Cobaltocenyl Ester Linked meso‐Tetratolylporphyrin Dyads

We report here the design and synthesis of porphyrin–metallocene dyads consisting of a metallocene [either ferrocene or mixed sandwich η⁵‐[C₅H₄(COOH)]Co(η⁴‐C₄Ph₄) connected via an ester linkage at meso phenyl position of either free‐base or zinc porphyrin. All these dyad systems were characterized by various spectroscopic and electrochemical methods. A dimeric form of this molecule was observed in the X‐ray crystal structure of Zn‐TTPCo. The absorption spectra of all four dyads indicated the absence of electronic interactions between porphyrin macrocycle and metallocene in the ground state. However, interestingly, in all four dyads, fluorescence emission of the porphyrin was quenched (19–55%) as compared to their monomeric units. The quenching was more pronounced in ferrocene derivatives rather than cobaltocenyl derivatives. The emission quenching can be attributed to the excited‐state intramolecular photoinduced electron transfer from metallocene to singlet excited state of porphyrin and the electron‐transfer rates (k_ET) were established in the range 1.51 × 10⁸ to 1.11 × 10⁹ s^?1. They were found to be solvent dependent.     

Synthesis and properties of covalently linked BF2–oxasmaragdyrin–porphyrin dyads and triad

Two covalently linked diphenyl ethyne bridged unsymmetrical dyads containing porphyrin and BF₂–oxasmaragdyrin and Zn(II)porphyrin and BF₂–oxasmaragdyrin units and one covalently linked triad containing Zn(II)porphyrin, porphyrin and BF₂–oxasmaragdyrin units were synthesized by coupling appropriate functionalized macrocycles under Pd(0) coupling reaction conditions. The dyads and triad were freely soluble in common organic solvents and confirmed by ES-MS spectra. 1D and 2D NMR techniques were used to characterize the dyads and triad. Absorption and electrochemical studies of dyads and triad showed the overlapping features of the constituted macrocycles indicating that the macrocycles retain their basic features in the dyads and triad. The BF₂–oxasmaragdyrin absorbs at lower energy and emits strongly in the visible region compared to porphyrin/Zn(II)porphyrin. Thus, BF₂–oxasmaragdyrin acts as energy acceptor and porphyrin/Zn(II) porphyrin act as energy donor in dyads and triad. The steady state and time-resolved fluorescence studies supported an efficient energy transfer from porphyrin/Zn(II)porphyrin to BF₂–oxasmaragdyrin unit in dyads and triad.     

Facile Synthesis,Characterization, and Fluorescence Studies of Novel Porphyrin Appended Thiazoles

A facile and high yielding synthesis of porphyrin appended thiazoles 5 from the reaction of 5‐(4‐thiocarboxamidophenyl)‐10,15,20‐triphenylporphyrin with α‐bromo ketones has been described. The fluorescence studies of synthesized porphyrin appended thiazoles 5 in chloroform indicate that porphyrin π system is not greatly perturbed by substitution of a thiazole moiety at meso‐phenyl ring even in the excited state.     

Azobenzene‐Bridged Porphyrin Nanorings: Syntheses,Structures, and Photophysical Properties

Azobenzene‐bridged β‐to‐β and meso‐to‐meso porphyrin nanorings were successfully synthesized by a palladium‐catalyzed Suzuki–Miyaura coupling reaction in a logical synthesis. The dimeric structure was confirmed by XRD analysis. The azo linkages in di‐ and tetramers are in the all‐trans conformation, whereas in the trimers one azo linkage can be interconverted between cis and trans under external stimulation. When trimeric isomers are heated to 333 K or higher, the azo linkages will be in the all‐trans configurations: the pure all‐trans trimer can be kept in the dark for several months. Fluorescence anisotropy and pump‐power‐dependent decay results revealed excitation energy transfer for azobenzene‐bridged zinc–porphyrin nanorings. The distances between porphyrin units of these azobenzene‐bridged porphyrin arrays are almost the same, but the exciton energy hopping (EEH) times for each wheel are markedly different. The dimer and meso‐to‐meso tetramer possess relatively short excitation energy transfer (EET) times (1.28 and 2.48 ps, respectively) due to their good planarity and rigidity. In contrast, the EET time for the trimeric zinc(II)–porphyrin array (6.9 ps) is relatively long due to its nonradiative decay pathway (i.e., cis/trans isomerization of azobenzene). Both di‐ and tetramers exhibit relatively high fluorescence quantum yields, whereas the trimers show weak emission because of structural differences.     

Synthesis of flexible dimeric meso-tetrakis-porphyrins

Monofunctionalisation of meso-tetrakis-porphyrins through introduction of a carboxylic group in the meso position of the phenyl group confers the necessary characteristics to anchor them through stable amide bonds to functionalised supports or to molecules. In this Letter we describe the synthesis, characterisation and photophysical evaluation of such a functionalised flexible dimeric porphyrin, bis-(meso-tetrakis-5,10,15-triphenyl-20-(p-carboxyphenyl)-porphyrinyl)-1,6-hexanediamide.     

Synthesis of phenylethyne-linked porphyrin dyads

Four new porphyrin dyads have been prepared for studies in artificial photosynthesis. The two porphyrins are joined at the meso positions via a phenylethyne linker and are present in zinc/zinc or zinc/free base metalation states. The porphyrin bearing the ethynyl unit incorporates zero, one, or two pentafluorophenyl groups at non-linking meso positions for tuning the porphyrin redox potentials. The synthetic approach entailed Pd-mediated coupling of porphyrin building blocks that bear a single ethynylphenyl or bromo/iodo substituent.     

Synthesis of porphyrin-diazacrown ether and porphyrin-cryptand conjugates for fluorescence detection of copper(II) ions

Palladium-catalyzed amination was used for the synthesis of the macrocycles containing trioxadiamine linker and the fragments of either 3,3′-disubstituted biphenyl or 2,7-disubstituted naphthalene. Catalytic macrocyclization gives diazacrown ether-based cryptand with trioxadiamine linker. The synthesized compounds were involved in Pd-catalyzed arylation of zinc meso-(bromophenyl)porphyrin to obtain polymacrocyclic conjugates bearing one or two porphyrin units. The synthesized conjugates were studied as fluorescence sensing material and molecular probes for 18 metal ions. It was found that two of them are very promising fluorescent chemosensors for copper(II) cations.     

Synthesis and properties of triazole bridged BODIPY-conjugates

A series of triazole bridged BODIPY-conjugates were synthesized under click reaction conditions. The 3-azido BODIPY was generated in situ by treating 3-bromo BODIPY with NaN₃ in CH₃CN at room temperature for 60 min and reacted with appropriate ethynyl containing chromophore/redox active unit, such as ferrocene, BODIPY, Zn(II) porphyrin, 21,23-dithiaporphyrin, BF₂-smaragdyrin in the presence of CuI/DIPEA in THF/CH₃CN solvent. The conjugates were purified by column chromatography and obtained pure compounds in 45–50% yields. The conjugates were characterized by HR-MS, 1D, 2D, ¹⁹F and ¹¹B NMR and X-ray crystallography for BODIPY-ferrocene conjugate. Absorption and electrochemical studies showed features of both the moieties present in the conjugates and also support interaction between the two moieties in the conjugates. The fluorescence studies supported an efficient energy transfer from BODIPY unit to BF₂-smaragdyrin unit in BODIPY–BF₂-smaragdyrin conjugate but the energy transfer was not efficient in BODIPY–Zn(II) porphyrin and BODIPY–21,23-dithiaporphyrin conjugates.     

Photocatalytic Hydrogen Evolution Based on Efficient Energy and Electron Transfers in Donor–Bridge–Acceptor Multibranched‐Porphyrin‐Functionalized Platinum Nanocomposites

Two donor–bridge–acceptor conjugates (5,10,15,20‐tetrakis[4‐(N,N‐diphenylaminobenzoate)phenyl] porphyrin (TPPZ) and 5,10,15,20‐tetrakis[4‐(N,N‐diphenylaminostyryl)phenyl] porphyrin (TPPX)) were covalently linked to triphenylamine (TPA) at the meso‐position of porphyrin ring. The triphenylamine entities were expected to act as energy donors and the porphyrins to act as an energy acceptor. In this paper, we report on the synthesis of these multibranched‐porphyrin‐functionalized Pt nanocomposites. The conjugates used here not only served as a stabilizer to prevent agglomeration of Pt nanoparticles, but also as a light‐harvesting photosensitizer. The occurrence of photoinduced electron‐transfer processes was confirmed by time‐resolved fluorescence and photoelectrochemical spectral measurements. The different efficiencies for energy and electron transfer in the two multibranched porphyrins and the functionalized Pt nanocomposites were attributed to diverse covalent linkages. Moreover, in the reduction of water to produce H₂, the photocatalytic activity of the Pt nanocomposite functionalized by TPPX, in which the triphenylamine and porphyrin moieties are bonded through an ethylene bridge, was much higher than that of the platinum nanocomposite functionalized by TPPZ, in which the two moieties are bonded through an ester. This investigation demonstrates the fundamental advantages of constructing donor–bridge–acceptor conjugates as highly efficient photosensitizers based on efficient energy and electron transfer.     

Synthesis of porphyrin-quinolone conjugates

meso-Tetrakis(pentafluorophenyl)porphyrin reacts with propargyl alcohol to afford porphyrins substituted with one, two, three or four prop-2-yn-1-yloxy groups in the 4-position of the meso-aryl groups. These new porphyrin derivatives react with a 6-azidoquinolone under ‘click-chemistry’ conditions to give porphyrin-quinolone conjugates linked by 1,2,3-triazole units.