Soluble Synthetic Multiporphyrin Arrays. 3. Static Spectroscopic and Electrochemical Probes of Electronic Communication

Spectroscopic insights into BSA-mediated deaggregation of m-THPC

Meta-tetra(hydroxyphenyl)chlorin (m-THPC) is among the most potent photosensitizers, known for its high singlet oxygen generation efficiency. However, its clinical effectiveness in photodynamic therapy (PDT) is compromised by its propensity to aggregate in aqueous solutions, adversely affecting its photophysical properties and therapeutic potential. A series of spectroscopic techniques, including UV-Vis absorption, fluorescence spectroscopy, and laser flash photolysis, revealed that m-THPC exhibits significant aggregation, particularly in MeOH-PBS mixtures with MeOH content below 30%. This aggregation adversely affects its photophysical properties leading to reduced fluorescence quantum yield and most importantly reducing its singlet oxygen quantum yield. This study introduces the use of bovine serum albumin (BSA) to counteract the aggregation of m-THPC, aiming to enhance its solubility, stability, and efficacy in physiological settings. Through advanced spectroscopic analyses we demonstrated that the m-THPC@BSA complex exhibits restored photophysical properties characteristic for monomeric form. Notably, the complex showed a significant restoration of the singlet oxygen quantum yield (ΦΔ = 0.21) compared to aggregated m-THPC. These results underscore the potential of BSA to preserve the monomeric form of m-THPC, mitigating aggregation-induced losses in singlet oxygen production. Our findings suggest that BSA-mediated delivery systems could play a crucial role in optimizing the clinical utility of hydrophobic photosensitizers like m-THPC.

Photoredox Catalysis by 21-Thiaporphyrins: A Green and Efficient Approach for C-N Borylation and C-H Arylation

Photoredox catalysis provides a green and sustainable alternative for C-H activation of organic molecules that eludes harsh conditions and use of transition metals. The photocatalytic C-N borylation and C-H arylation mostly depend on the ruthenium and iridium complexes or eosin Y and the use of porphyrin catalysts is still in infancy. A series of novel 21-thiaporphyrins (A2B2 and A3B type) were synthesized having carbazole/phenothiazine moieties at their meso-positions and screened as catalysts for C-N borylation and C-H arylation. This paper demonstrates the 21-thiaporphyrin catalyzed C-N borylation and het-arylation of anilines under visible light. The method utilizes only 0.1 mol % of 21-thiaporphyrin catalyst under blue light for the direct C-N borylation and het-arylation reactions. A variety of substituted anilines were used as source for expensive and unstable aryl diazonium salts in the reactions. The heterobiaryls and aryl boronic esters were obtained in decent yields (up to 88 %). Versatility of the 21-thiaporphyrin catalyst was tested by thiolation and selenylation of anilines under similar conditions. Mechanistic insight was obtained from DFT studies, suggesting that 21-thiaporphyrin undergo an oxidative quenching pathway. The photoredox process catalyzed by 21-thiaporphyrins offers a mild, efficient and metal-free alternative for the formation of C-C, C-S, and C-Se bonds in aryl compounds; it can also be extended to borylation reaction.

Decorrelated singlet and triplet exciton delocalization in acetylene-bridged Zn-porphyrin dimers

The controlled delocalization of molecular excitons remains an important goal towards the application of organic chromophores in processes ranging from light-initiated chemical transformations to classical and quantum information processing. In this study, we present a methodology to couple optical and magnetic spectroscopic techniques and assess the delocalization of singlet and triplet excitons in model molecular chromophores. By comparing the steady-state and time-resolved optical spectra of Zn-porphyrin monomers and weakly coupled dimers, we show that we can use the identity of substituents bound at specific positions of the macromolecules' rings to control the inter-ring delocalization of singlet excitons stemming from their B states through acetylene bridges. While broadened steady-state absorption spectra suggest the presence of delocalized B state excitons in mesityl-substituted Zn-tetraphenyl porphyrin dimers (Zn2U-D), we confirm this conclusion by measuring an enhanced ultrafast non-radiative relaxation from these inter-ring excitonic states to lower lying electronic states relative to their monomer. In contrast to the delocalized nature of singlet excitons, we use time-resolved EPR and ENDOR spectroscopies to show that the triplet states of the Zn-porphyrin dimers remain localized on one of the two macrocyclic sub-units. We use the analysis of EPR and ENDOR measurements on unmetallated model porphyrin monomers and dimers to support this conclusion. The results of DFT calculations also support the interpretation of localized triplet states. These results demonstrate researchers cannot conclude triplet excitons delocalize in macromolecular based on the presence of spatially extended singlet excitons, which can help in the design of chromophores for application in spin conversion and information processing technologies.

Intramolecular electron transfer in porphyrin-anthraquinone donor–acceptor systems with varying molecular bridges

Photoinduced electron transfer has been investigated in porphyrin anthraquinone (ZnTTP-AQ) donor–acceptor dyads having either ester (ZnTTP-AQ1) or ether (ZnTTP-AQ2) linkages. Both dyads were characterized by spectroscopic and electrochemical methods. Absorption spectra show absence of any ground state interaction between the porphyrin and anthraquinone moieties. The quenched fluoresence and lifetime indicate electron transfer from the porphyrin to the anthraquinone moiety. The quenching is more pronounced in ZnTTP-AQ1 with ester linkage, suggesting efficient electronic coupling compared to the ether linkage in ZnTTP-AQ2. Computational analysis and frontier molecular orbitals confirmed the formation of charged separated state por[Formula: see text]AQ[Formula: see text]. The electron transfer rates ([Formula: see text] of these triads are found in the range 0.43 × 108 to 10.52 × 109 s[Formula: see text] and are found to be solvent polarity dependent.

Increasing the lifetimes of charge separated states in porphyrin-fullerene polyads

Two linear polyads were designed using zinc(ii)porphyrin, [ZnP], and N-methyl-2-phenyl-3,4-fullero-pyrrolidine (C₆₀) where C₆₀ is dangling either at the terminal position of [ZnP]-C₆H₄-[triple bond, length as m-dash]-C₆H₄-[ZnP]-C₆₀ (1) or at the central position of [ZnP]-C₆H₄-[triple bond, length as m-dash]-C₆H₄-[ZnP(C₆₀)]-C₆H₄-[triple bond, length as m-dash]-C₆H₄-[ZnP] (2) in order to test whether the fact of having one or two side electron donors influences the rate of electron transfer, k_et. These polyads were studied using cyclic voltammograms, DFT computations, steady state and time-resolved fluorescence spectroscopy, and femtosecond transient absorption spectroscopy (fs-TAS). Photo-induced electron transfer confirmed by the detection of the charge separated state [ZnP˙+]/C₆₀˙^- from fs-TAS occurs with rates (k_et) of 3-4 × 10¹⁰ s^-1 whereas the charge recombinations (CRs) are found to produce the [ZnP] ground state via two pathways (central [ZnP˙+]/C₆₀˙^- (ps) and terminal central [ZnP˙+]/C₆₀˙^- (ns) producing [1ZnP] (ground state) and [3ZnP*]). The formation of the T₁ species is more predominant for 2.

A Rotaxane Scaffold for the Construction of Multiporphyrinic Light-Harvesting Devices

A sophisticated photoactive molecular device has been prepared by combining recent concepts for the preparation of multifunctional nanomolecules (click chemistry on multifunctional scaffolds) with supramolecular chemistry (self-assembly to prepare rotaxanes). Specifically, a clickable [2]rotaxane scaffold incorporating a free-base porphyrin stopper has been prepared and functionalized with ten peripheral Zn(II)-porphyrin moieties. Electrochemical investigations of the final compound revealed a peculiar behavior resulting from the intramolecular coordination of the Zn(II) porphyrin moieties to 1,2,3-triazole units. Finally, steady state investigations of the compound combining Zn(II) and free-base porphyrin moieties have shown that this compound is a light-harvesting device capable of channeling the light energy from the peripheral Zn(II)-porphyrin subunits to the core by singlet-singlet energy transfer.

Flexible Metal-Porphyrin Dimers (M=MnIII Cl, CoII , NiII , CuII ): Synthesis, Spectroscopy, Electrochemistry, Spectroelectrochemistry, and Theoretical Calculations

Four metalloporphyrin dimers linked by bridging amide-bonded xanthene moieties and that contain either Mn^III , Co^II , Ni^II , or Cu^II metal centers were synthesized. Various spectroscopic, electrochemical, and spectroelectrochemical methods were used to study trends in their properties. Their electronic structure and optical properties were analyzed through a comparison of the electronic absorption and magnetic circular dichroism (MCD) spectral data with the results of time-dependent (TD)-DFT calculations.

Donor–acceptor type A2B2porphyrins: synthesis, energy transfer, computational and electrochemical studies

Synthesis, photophysical, electrochemical and DFT studies of donor–acceptor type A₂B₂porphyrins and their Zn(ii) and Pd(ii) complexes have been described.

Excitation wavelength-dependent EPR study on the influence of the conformation of multiporphyrin arrays on triplet state delocalization

The optoelectronic properties of conjugated porphyrin arrays render them excellent candidates for use in a variety of molecular electronic devices. Understanding the factors controlling the electron delocalization in these systems is important for further developments in this field. Here, we use transient EPR and ENDOR (Electron Nuclear Double Resonance) to study the extent of electronic delocalization in the photoexcited triplet states of a series of butadiyne-linked porphyrin oligomers. We are able to distinguish between planar and twisted arrangements of adjacent porphyrin units, as the different conformations are preferentially excited at different wavelengths in the visible range. We show that the extent of triplet state delocalization is modulated by the torsional angle between the porphyrins and therefore by the excitation wavelength. These results have implications for the design of supramolecular systems with fine-tuned excitonic interactions and for the control of charge transport.

Transient EPR Reveals Triplet State Delocalization in a Series of Cyclic and Linear π-Conjugated Porphyrin Oligomers

The photoexcited triplet states of a series of linear and cyclic butadiyne-linked porphyrin oligomers were investigated by transient Electron Paramagnetic Resonance (EPR) and Electron Nuclear DOuble Resonance (ENDOR). The spatial delocalization of the triplet state wave function in systems with different numbers of porphyrin units and different geometries was analyzed in terms of zero-field splitting parameters and proton hyperfine couplings. Even though no significant change in the zero-field splitting parameters (D and E) is observed for linear oligomers with two to six porphyrin units, the spin polarization of the transient EPR spectra is particularly sensitive to the number of porphyrin units, implying a change of the mechanism of intersystem crossing. Analysis of the proton hyperfine couplings in linear oligomers with more than two porphyrin units, in combination with density functional theory calculations, indicates that the spin density is localized mainly on two to three porphyrin units rather than being distributed evenly over the whole π-system. The sensitivity of the zero-field splitting parameters to changes in geometry was investigated by comparing free linear oligomers with oligomers bound to a hexapyridyl template. Significant changes in the zero-field splitting parameter D were observed, while the proton hyperfine couplings show no change in the extent of triplet state delocalization. The triplet state of the cyclic porphyrin hexamer has a much decreased zero-field splitting parameter D and much smaller proton hyperfine couplings with respect to the monomeric unit, indicating complete delocalization over six porphyrin units in this symmetric system. This surprising result provides the first evidence for extensive triplet state delocalization in an artificial supramolecular assembly of porphyrins.

Triplet state delocalization in a conjugated porphyrin dimer probed by transient electron paramagnetic resonance techniques

The delocalization of the photoexcited triplet state in a linear butadiyne-linked porphyrin dimer is investigated by time-resolved and pulse electron paramagnetic resonance (EPR) with laser excitation. The transient EPR spectra of the photoexcited triplet states of the porphyrin monomer and dimer are characterized by significantly different spin polarizations and an increase of the zero-field splitting parameter D from monomer to dimer. The proton and nitrogen hyperfine couplings, determined using electron nuclear double resonance (ENDOR) and X- and Q-band HYSCORE, are reduced to about half in the porphyrin dimer. These data unequivocally prove the delocalization of the triplet state over both porphyrin units, in contrast to the conclusions from previous studies on the triplet states of closely related porphyrin dimers. The results presented here demonstrate that the most accurate estimate of the extent of triplet state delocalization can be obtained from the hyperfine couplings, while interpretation of the zero-field splitting parameter D can lead to underestimation of the delocalization length, unless combined with quantum chemical calculations. Furthermore, orientation-selective ENDOR and HYSCORE results, in combination with the results of density functional theory (DFT) calculations, allowed determination of the orientations of the zero-field splitting tensors with respect to the molecular frame in both porphyrin monomer and dimer. The results provide evidence for a reorientation of the zero-field splitting tensor and a change in the sign of the zero-field splitting D value. The direction of maximum dipolar coupling shifts from the out-of-plane direction in the porphyrin monomer to the vector connecting the two porphyrin units in the dimer. This reorientation, leading to an alignment of the principal optical transition moment and the axis of maximum dipolar coupling, is also confirmed by magnetophotoselection experiments.

The influence of phenylethynyl linkers on the photo-physical properties of metal-free porphyrins

Series of 5,10,15,20-tetraarylporphyrins 1 and 5,10,15,20-tetrakis[4-(arylethynyl)phenyl]porphyrins 2 were prepared via condensation of pyrrole with the appropriate benzaldehyde or 4-(arylethynyl)benzaldehyde derivative (3). Condensation of meso-phenyldipyrromethane with mixtures of benzaldehyde and 4-(trimethylsilyl-ethynyl)benzaldehyde gave a separable mixture of mono- (6), bis- (both cis-7 and trans-8) and tris[4-(trimethylsilylethynyl)phenyl]porphyrin (9). Following removal of the trimethylsilyl groups of 6–9, the 4-ethynylphenyl groups of 11–14 were coupled to 1-iodo-3,5-di(trifluoromethyl)benzene with Pd ( OAc )2 to give 15–18 bearing one, two (both cis- and trans-) and three 4-[bis-3,5-(trifluoromethyl)phenylethynyl]phenyl groups respectively. Coupling of 11 and 1-iodo-4-nitrobenzene with Pd ( OAc )2 gave porphyrin 19 with one 4-(4-nitrophenylethynyl)phenyl group. Porphyrin 24 with a p-quinone linked to the porphyrin core via a phenylethynyl group was prepared via similar chemistry. The absorbance spectra, emission maxima, excited-state fluorescence lifetimes, quantum yields of fluorescence, rates of fluorescence and rates of non-radiative decay were measured for each of the porphyrins. Absorbance spectra and emission maxima were nearly identical for all the porphyrins of this study, which suggests that the aryl groups and 4-(arylethynyl)phenyl groups are not strongly coupled to the porphyrin core in these metal-free compounds. Fluorescence quantum yields and rates of radiative decay were larger for porphyrins bearing 4-(arylethynyl)phenyl groups, while excited-state fluorescence lifetimes were somewhat shorter. These effects were additive for each additional 4-(arylethynyl)phenyl group.

Spectroscopic properties of porphyrin dimers incorporating phenylenevinylene linkers

A series of free-base, zinc and mixed free-base-zinc porphyrin dimers were investigated using electrochemistry, electronic absorption, resonance Raman and emission studies. The spectroelectrochemistry of these compounds was also examined. The electronic absorption and resonance Raman data suggest that the two porphyrins in the dimer are behaving as independent chromophores with limited communication. However, emission studies show that energy transfer occurs between the two units. The linking unit is seen to have limited influence on the properties of chromophores.

Use of palladium catalysis in the synthesis of novel porphyrins and phthalocyanines

Palladium catalysts offer a rich and highly versatile chemistry for the synthesis of novel porphyrins and phthalocyanines. These mild and flexible reactions have been used extensively in the preparation of interesting porphyrins and phthalocyanines, either in the synthesis of substituted precursors or in the modification of pre-existing macrocycles. For these tetrapyrrolic compounds, metal-mediated reactions such as these offer extensive benefits, which have been taken advantage of in order to add novel substituents, synthesize naturally occurring molecules and prepare multi-macrocyclic arrays. This review gives an overview of the use of palladium catalysts in the synthesis of porphyrins and phthalocyanines along with the applications of some of the compounds prepared.

Ground and Excited State Electronic Properties of Halogenated Tetraarylporphyrins: Tuning the Building Blocks for Porphyrin-based Photonic Devices

The rational design of molecular photonic devices relies on the ability to select components with predictable electronic structure, excited state lifetimes and redox chemistry. Electronic communication in multiporphyrin arrays depends critically on the relative energies and electron density distributions of the frontier molecular orbitals, especially the energetically close highest occupied molecular orbitals (a2u and a1u). To explore how these ground and excited state properties can be modulated, we have synthesized and characterized 40 free base ( Fb ), magnesium and zinc tetraarylporphyrins. The porphyrins bear meso-substituents with the following substitution patterns: (1) four identical substituents (phenyl, o-chlorophenyl, p-chlorophenyl, o,o'-difluorophenyl, pentafluorophenyl, mesityl); (2) one, two, three or four o,o'-dichlorophenyl substituents; (3) one p-ethynylphenyl group and three mesityl or pentafluorophenyl groups; (4) one p-ethynyl-o,o″-dichlorophenyl or p-ethynyl-o,o″-dimethylphenyl and three phenyl groups. For each neutral complex the ground state electronic properties were investigated using electrochemical methods and optical absorption spectroscopy. Similarly the absorption, emission, and relaxation properties of the lowest singlet excited state were probed by time-resolved absorption and fluorescence methods. Each oxidized complex was investigated by static absorption and liquid and frozen solution EPR spectroscopy. The collective results of these investigations have provided insights into the direct (orbital overlap) and indirect (inductive/conjugative) mechanisms by which halogenated phenyl rings influence the static and dynamic electronic properties of neutral and oxidized porphyrinic chromophores. Three key findings are as follows. (1) The effective electron-withdrawing strength of halogenated phenyl rings required to reverse the ordering of the a2u and a1u HOMOs in Mg versus Zn tetraarylporphyrins has been elucidated. (2) Appropriate halogenation can significantly increase the excited state lifetime of a Zn porphyrin relative to the unsubstituted complex. (3) Halogenation can be used to modulate redox potentials in a manner that complements the enhancement of other electronic properties. The insights gained from study of this library of porphyrins provide a foundation for tuning the electronic properties of monomeric porphyrins as building blocks for multichromophoric assemblies in optoelectronics and other applications.

A computational study on heterodimerization of charged porphyrins

The structures of the charged porphyrins and their dimers have been investigated with computational methods. Dimers have been formed based on electrostatic attraction of the opposite charges on two different porphyrin monomers, tetra ammonium porphyrin (TAP) and tetra carboxy porphyrin (TCP). Semi-empirical quantum mechanical calculations have been employed to explore the most stable ground-state structures of TCP, TAP and their hetero-dimers. Dimeric structures analyzed are all in face-to-face fashion indicating the strong electrostatic attraction between the two porphyrin rings. Calculations have also predicted that the protons transfer from [Formula: see text]; groups to -COO- groups when the interplanar separation is shorter than 3.7 Å.

Electron Transfer in the Reduction of Cobalt Porphyrin Incorporated into Nafion and Poly(4-vinylpyridine-co-styrene) Films

Potential step chronoamperospectrometry (PSCAS) measurement was carried out to investigate the one-electron transfer in the reduction of tetraphenylporphyrin cobalt(III) ([ Co III TPP (−2)]+) incorporated into a Nafion or poly(4-vinylpyridine-co-styrene) (P(VP-St)) film coated on an ITO electrode. The electron transfer of the redox centers in the two systems occurred by a physical diffusion mechanism. The fraction of the electroactive complex (R ct ) was independent of the CoTPP concentration in P(VP-St) but decreased with increasing CoTPP concentration in Nafion, especially at high concentrations. In both systems the apparent diffusion coefficient Dapp decreased with increasing CoTPP concentration. The Dapp value in Nafion (1.6 × 10−8 cm 2 s −1 at 0.02 M) was much higher than that in P(VP-St) (1.1 × 10−10 cm 2 s −1 at 0.02 M). The difference in the electron transfer process in the two systems was ascribed to the interaction of the redox center with the polymer framework, the morphology of the polymer matrix, the localization of the redox species in the hydrophilic/hydrophobic region, and the counterion migration under the potential step.

Electron transfer in the redox reaction of cobalt tetraphenylporphyrin incorporated in a Nafion film

Potential step chronoamperospectrometry (PSCAS) was carried out to analyze electron transfer in the redox reaction processes of 5,10,15,20-tetraphenylporphyrinatocobalt(II) ( Co II TPP (-2)) incorporated in a Nafion film. The reactions of Co II TPP (-2) to [ Co III TPP (-2)]+ and of [ Co III TPP (-2)]+ to Co II TPP (-2) took place through a diffusion mechanism, as confirmed by the first-order initial reaction rate with respect to the complex concentration in the matrix. However, the reaction of [ Co III TPP (-2)]+ to [ Co III TPP (-1)]2+ occurred by an electron-hopping mechanism, as confirmed by the second-order initial reaction rate with respect to the complex concentration. The fraction of electroactive complex (Rct) increased with the sample time after the potential step until it reached saturation. In the reactions of Co II TPP (-2) to [ Co III TPP (-2)]+ and of [ Co III TPP (-2)]+ to Co II TPP (-2), Rct approached 1.0, while in the reaction of [ Co III TPP (-2)]+ to [ Co III TPP (-1)]2+, only about 0.3 was reached. The apparent diffusion coefficient (Dapp) decreased in the order of [ Co III TPP (-2)]+ to Co II TPP (-2) > Co II TPP (-2) to [ Co III TPP (-2)]+>[ Co III TPP (-2)]+ to [ Co III TPP (-1)]2+. The different behavior of these redox reactions was ascribed to the microenvironment of the redox species in the matrix, interaction of the redox centers, especially the product with the framework, and counter ion migration.

Syntheses and excitation transfer studies of near-orthogonal free-base porphyrin-ruthenium phthalocyanine dyads and pentad

A new series of molecular dyads and pentad featuring free-base porphyrin and ruthenium phthalocyanine have been synthesized and characterized. The synthetic strategy involved reacting free-base porphyrin functionalized with one or four entities of phenylimidazole at the meso position of the porphyrin ring with ruthenium carbonyl phthalocyanine followed by chromatographic separation and purification of the products. Excitation transfer in these donor-acceptor polyads (dyad and pentad) is investigated in nonpolar toluene and polar benzonitrile solvents using both steady-state and time-resolved emission techniques. Electrochemical and computational studies suggested that the photoinduced electron transfer is a thermodynamically unfavorable process in nonpolar media but may take place in a polar environment. Selective excitation of the donor, free-base porphyrin entity, resulted in efficient excitation transfer to the acceptor, ruthenium phthalocyanine, and the position of imidazole linkage on the free-base porphyrin could be used to tune the rates of excitation transfer. The singlet excited Ru phthalocyanine thus formed instantly relaxed to the triplet state via intersystem crossing prior to returning to the ground state. Kinetics of energy transfer (k(ENT)) was monitored by performing transient absorption and emission measurements using pump-probe and up-conversion techniques in toluene, respectively, and modeled using a Förster-type energy transfer mechanism. Such studies revealed the experimental k(ENT) values on the order of 10(10)-10(11) s(-1), which readily agreed with the theoretically estimated values. Interestingly, in polar benzonitrile solvent, additional charge transfer interactions in the case of dyads but not in the case of pentad, presumably due to the geometry/orientation consideration, were observed.

The progress on design and synthesis of photoactive porphyrins-based dyads, triads and polymers

Porphyrins are the most frequently employed building blocks as electron donors and sensitizers in artificial photosynthetic models for solar energy conversion. Recently, we have reported a series of covalently linked, donor-acceptor dyads, triads and copolymers containing porphyrin analogs aiming to improve light-harvesting capacity and charge-separation efficiency with a potential application in solar cells. In this review, we would like to summarize our recent studies on these photoactive, porphyrin-containing composite systems focusing on the designs and properties of these systems based on intermolecular electro- and energy transfer.

Photophysical properties of self-assembling porphinatozinc and photoinduced electron transfer with fullerenes

Photochemical and photophysical properties of self-assembling 5-(4-pyridyl)-10,15,20-triphenylporphinatozinc ( Znpyp 3) have been studied by steady-state and time-resolved absorption in addition to time-resolved fluorescence spectroscopy. Self-assembling oligomers ( Znpyp 3)n( n = 3 at 0.1 mM) were synthetically generated and assigned to a zigzag chain oligomeric structure, where n is dependant on the concentration and temperature. The lifetimes of the singlet and triplet excited states of ( Znpyp 3)n depend on n and the axial ligation. The rate-constant of ( Znpyp 3)n for the intermolecular T-T annihilation process was smaller than that of monomeric porphyrins. In the photoinduced electron-transfer to fullerenes ( C 60 and C 70), it was revealed that the rate-constants and efficiencies for ( Znpyp 3)n were essentially the same as those of the monomer. In the back electron transfer, the rate-constants of oligomers were smaller than that of the monomeric porphyrin, which suggests hole-delocalization along the porphyrin chain.

Synthesis and optical properties of novel covalent and non-covalent porphyrin dimers

A new series of porphyrin dimers in different geometry have been synthesized. The reductive amination of dicarbonyl compounds, especially 2,5-hexadione and isophthaldhyde, with porphyrin monomers, α,α,α,α-5-(p-aminophenyl)-10,15,20-tris-(4-tolyl)porphyrin, α,α,α,α- and α,α,β,β-5,10,15,20-tetrakis-(o-aminophenyl)porphyrin in the presence of excess of NaBH 3 CN , afforded novel dimeric single and double bridged dimers of free base porphyrins, covalently linked through aliphatic or aromatic spacers. All the synthesized compounds were characterized by NMR, IR, UV-vis, and MS spectroscopy. Significant spectral changes occurred in the absorption spectra of these covalent porphyrin dimers comparing with that of the reference monomers, indicating strong electronic interaction between the adjacent porphyrin units. The UV-vis spectra showed the presence of another kind of non-covalent aggregation, which was observed at pH = 2.5-3 for some of these synthesized dimers. A similar behavior of association was also detected when α,β,α,β-5,10,15,20-tetrakis-(o-aminophenyl)porphyrin was measured at pH = 3.5 in acetone.

Singlet and triplet energy transfers in tetra-(meso-truxene)zinc(II)- and tetra-(meso-tritruxene)zinc(II) porphyrin and porphyrin-free base dendrimers

The synthesis, optical properties, and energy transfer features of four dendrimers composed of meso-tetrasubstituted zinc(II) porphyrin (ZnP) or a free base (P) central core, where the substituents are four truxene (Tru) or four tritruxene dendrons (TriTru), TruP, TriTruP, TruZnP, and TriTruZnP, are reported. Selective excitation of the truxene donors results in a photoinduced singlet energy transfer from the truxenes to the porphyrin acceptor. The rates for singlet energy transfer (k(ET)), evaluated from the change in the fluorescence lifetime of the donors (Tru and TriTru) in the presence and absence of the acceptor (P or ZnP) for TruP, TruZnP, TriTruP, and TriTruZnP, are 5.9, 1.2, 0.87, and 0.74 (ns)(-1) at 298 K and 2.6, 2.6, 2.7, and 1.2 (ns)(-1) at 77 K, respectively. A slow triplet-triplet energy transfer from truxene to porphyrin cores in glassy 2MeTHF at 77 K is also reported with rates of 1.3 × 10(3) and 0.10 × 10(2) s(-1) for TruZnP and TriTruZnP, respectively. If the Dexter mechanism for the triplet energy transfers is considered, these slow rates are easily explained by a poor orbital overlap between the truxene and porphyrin π systems. The fluorescence quantum yields (Φ(F)) are 0.20 and 0.16 for TruP and TriTruP and 0.08 and 0.10 for TruZnP and TriTruZnP, respectively at 298 K. At 298 K, a phosphorescence from TruZnP at 795 nm was also observed and is associated with the ZnP chromophore.

Photoinduced electron transfer in Zn(II)porphyrin-bridge-Pt(II)acetylide complexes: variation in rate with anchoring group and position of the bridge

The synthesis and photophysical characterization of two sets of zinc porphyrin platinum acetylide complexes are reported. The two sets of molecules differ in the way the bridging phenyl-ethynyl unit is attached to the porphyrin ring. One set is attached via an ethynyl unit on the β position, while the other set is attached via a phenyl unit on the meso position of the porphyrin. These were compared with previously studied complexes where attachment was made via an ethynyl unit on the meso position. Femtosecond transient absorption measurements showed in all systems a rapid quenching of the porphyrin singlet state. Electron transfer is suggested as the quenching mechanism, followed by an even faster recombination to form both the porphyrin ground and triplet excited states. This is supported by the variation in quenching rate and porphyrin triplet yield with solvent polarity, and the observation of an intermediate state in the meso-phenyl linked systems. The different linking motifs between the dyads resulted in significant variations in electron transfer rates.

Probing the rate of hole transfer in oxidized synthetic chlorin dyads via site-specific (13)C-labeling

Understanding electronic communication among interacting constituents of multicomponent molecular architectures is important for rational design in diverse fields including artificial photosynthesis and molecular electronics. One strategy for examining ground-state hole/electron transfer in an oxidized tetrapyrrolic array relies on analysis of the hyperfine interactions observed in the EPR spectrum of the pi-cation radical. This strategy has been previously employed to probe the hole/electron-transfer process in oxidized multiporphyrin arrays of normal isotopic composition, wherein (1)H and (14)N serve as the hyperfine "clocks", and in arrays containing site-specific (13)C-labels, which serve as additional hyperfine clocks. Herein, the hyperfine-clock strategy is applied to dyads of dihydroporphyrins (chlorins). Chlorins are more closely related structurally to chlorophylls than are porphyrins. A de novo synthetic strategy has been employed to introduce a (13)C label at the 19-position of the chlorin macrocycle, which is a site of large electron/hole density and is accessible synthetically beginning with (13)C-nitromethane. The resulting singly (13)C-labeled chlorin was coupled with an unlabeled chlorin to give a dyad wherein a diphenylethyne linker spans the 10-positions of the two zinc chlorins. EPR studies of the monocations of both the natural abundance and (13)C-labeled zinc chlorin dyads and benchmark zinc chlorin monomers reveal that the time scale for hole/electron transfer is in the 4-7 ns range, which is 5-10-fold longer than that in analogous porphyrin arrays. The slower hole/electron transfer rate observed for the chlorin versus porphyrin dyads is attributed to the fact that the HOMO is a(1u)-like for the chlorins versus a(2u)-like for the porphyrins; the a(1u)-like orbital exhibits little (or no) electron/hole density at the site of linker attachment whereas the a(2u)-like orbital exhibits significant electron/hole density at this site. Collectively, the studies of the chlorin and porphyrin dyads provide insights into the structural features that influence the hole/electron-transfer process.