Free Base Porphyrin Dimers Research Articles

Singlet oxygen generation properties of an inclusion complex of cyclic free-base porphyrin dimer and fullerene C60

We demonstrate that a cyclic free-base porphyrin dimer and its inclusion complex with fullerene C60 possess the ability to generate singlet oxygen (1O2) under visible light irradiation.

Dye-sensitized solar cell based on an inclusion complex of a cyclic porphyrin dimer bearing four 4-pyridyl groups and fullerene C60

Cyclic free-base porphyrin dimers linked by butadiyne or phenothiazine bearing four 4-pyridyl groups and their inclusion complexes with fullerene C60 have been applied to dye-sensitized solar cells as a new class of porphyrin dye sensitizers.

Long-Lived Photoinduced Charge Separation in Inclusion Complexes Composed of a Phenothiazine-Bridged Cyclic Porphyrin Dimer and Fullerenes

C60, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), lithium-cation-encapsulated C60 (Li+@C60), and [6,6]-diphenyl-C62-bis(butyric acid methyl ester) (bis-PCBM) were included into a phenothiazine-bridged cyclic free-base porphyrin dimer (H4-Ptz-CPDPy(TEO)) in a polar solvent (benzonitrile) with large association constants of 1.3 × 106, 6.4 × 105, 3.2 × 106, and 2.5 × 105 M–1, respectively. Based on the electrochemical data, the lowest energy levels of the charge-separated (CS) states for the inclusion complexes of H4-Ptz-CPDPy(TEO) with C60, PCBM, Li+@C60, and bis-PCBM (designated as C60⊂H4-Ptz-CPDPy(TEO), PCBM⊂H4-Ptz-CPDPy(TEO), Li+@C60⊂H4-Ptz-CPDPy(TEO), and bis-PCBM⊂H4-Ptz-CPDPy(TEO)) composed of the phenothiazine donor and fullerene acceptors were determined to be 1.30, 1.40, 0.66, and 1.51 eV, respectively. Both C60⊂H4-Ptz-CPDPy(TEO) and PCBM⊂H4-Ptz-CPDPy(TEO) underwent electron transfer upon photoexcitation of the porphyrin and fullerene chromophores, and the resultant photoinduced CS states comp...

Facile synthesis, spectroscopic and electrochemical properties, and theoretical calculations of porphyrin dimers with a bridging amide-bonded xanthene moiety

A free base porphyrin dimer bridged by a flexible amide-bonded xanthene moiety and its binuclear zinc(II) complex zinc(II) complex were synthesized and characterized. Structural characterization by MS and 1 H NMR spectroscopy confirmed the bridged porphyrin dimer structure. The properties of the dimers were characterized by IR, UV-visible absorption, fluorescence and magnetic circular dichroism (MCD) spectroscopy, and electrochemistry studies. Theoretical calculations were carried out to analyze the electronic structures of porphyrin dimers with a bridging amide-bonded xanthene moiety.

Synthesis of a Series of Zinc(II)/Freebase Porphyrin Dimers and Trimers with Programmable Sequences from a Common Key Molecule

We have developed a new methodology that enables the synthesis of any sequence of metal porphyrin arrays starting from a common key molecule. Using this method, we prepared porphyrin dimers and trimers with varying component unit sequences via consecutive Suzuki coupling reactions using the same key porphyrin compound under the same reaction conditions. The key porphyrin compound was synthesized on a gram scale in one batch, and the coupling reactions afforded the desired oligomers in good yields. Thus, the prepared porphyrin arrays showed unique physical properties depending on the sequence of the central metals. The reaction is potentially applicable for the automated synthesis of porphyrin arrays.

Enhanced intersystem crossing due to long-range exchange interaction in copper(II) porphyrin-free base porphyrin dimers: HOMO and spacer dependence

Photodynamics induced by long-range exchange interaction was studied in two series of copper(II) porphyrin – free base porphyrin dimers linked via an aromatic spacer: one has a2u orbital as HOMO in the porphyrin π-system and the other has a1u orbital. Dependence on the HOMO as well as that on the spacer is presented for enhanced intersystem crossing (EISC) occurring in the free base half due to long-range coupling with the copper unpaired electron. Semilogarithmic plots of EISC rates vs. number of bonds show a linear correlation in each series of dimers. It was found that the two correlation lines show the same slope but different intercepts. This clearly indicates that electronic communication within the linkage does not depend on the terminal chromophore, while the terminal π-system affects the magnitude of EISC rates. Separately, a general expression for EISC rate, involving coupling between the terminal and linkage, was derived theoretically.

Supramolecular Structures of Inclusion Complexes of Cyclic Porphyrin Dimers and [6,6]-Phenyl-C61-Butyric Acid Methyl Ester (PCBM)

Abstract Cyclic nickel and free-base porphyrin dimers (Ni2-CPDPy and H4-CPDPy) include [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) both in solution and in the crystals. In the crystal packing of the inclusion complex of Ni2-CPDPy and PCBM, a nanotubular structure is constructed through the self-assembly of the porphyrin moiety. The PCBM molecules are linearly arranged in the inner channel of the nanotube to give a supramolecular peapod. On the other hand, in the crystal of the inclusion complex of H4-CPDPy and PCBM, a dimeric structure of the PCBM molecules is formed through π–π interaction between their phenyl groups.

Supramolecular Structures of Inclusion Complexes of C70 and Cyclic Porphyrin Dimers

Abstract Cyclic nickel and free-base porphyrin dimers (Ni2-CPDPy and H4-CPDPy) include fullerene C70 both in solution and in the crystals. Based on the 13C NMR spectra in solution, the included C70 molecule inside the cavity of Ni2-CPDPy shows both end-on and side-on orientations, whereas the C70 molecule within H4-CPDPy has only a side-on orientation toward the porphyrin rings. X-ray crystallography revealed both “end-on” and “side-on” orientations of C70 in the crystal structure of the inclusion complex of Ni2-CPDPy and C70. This is the first example of an X-ray crystallographic determination for an end-on orientation of C70 cocrystallized with porphyrins. On the other hand, only a side-on orientation of C70 was observed in the crystal structure of the complex of H4-CPDPy and C70. Further, a zigzag array of C70 molecules through van der Waals contacts with each other is formed along the monoclinic b axis in the latter crystal.

Superexchange mediated energy transfer in zinc(ii) porphyrin–free base porphyrin dimers: comparison of m- and p-bis(phenylethynyl) phenylene linked dimers

The singlet-singlet energy transfer rate in a new zinc(II) porphyrin-free base porphyrin dimer, having a m-bis(phenylethynyl)phenylene bridge, was found to be slower than that in the corresponding p-bis(phenylethynyl)phenylene-bridged dimer, despite the shorter donor-acceptor distance and pathway. The slower rate is interpreted as evidence for a major contribution of the superexchange mechanism.

Supramolecular Structures and Photoelectronic Properties of the Inclusion Complex of a Cyclic Free‐Base Porphyrin Dimer and C60

A cyclic free-base porphyrin dimer H4-CPD(Py) (CPD = cyclic porphyrin dimer) linked by butadiyne moieties bearing 4-pyridyl groups self-assembles to form a novel porphyrin nanotube in the crystalline state. The cyclic molecules link together through nonclassical C-H⋅⋅⋅N hydrogen bonds and π–π interactions of the pyridyl groups along the crystallographic a axis. H4-CPD(Py) includes a C60 molecule in its cavity in solution. In the crystal structure of the inclusion complex (C60⊂H4-CPD(Py)), the dimer “bites” a C60 molecule by tilting the porphyrin rings with respect to each other, and there are strong π–π interactions between the porphyrin rings and C60. The included C60 molecules form a zigzag chain along the crystallographic b axis through van der Waals contacts with each other. Femtosecond laser flash photolysis of C60⊂H4-CPD(Py) in the solid state with photoexcitation at 420 nm shows the formation of a completely charge-separated state {H4-CPD(Py)·+ + C60·−}, which decays with a lifetime of 470 ps to the ground state. The charge-carrier mobility of the single crystal of C60⊂H4-CPD(Py) was determined by flash photolysis time-resolved microwave conductivity (FP-TRMC) measurements. C60⊂H4-CPD(Py) has an anisotropic charge mobility (Σμ = 0.16 and 0.13 cm2 V(−1) s(−1)) along the zigzag chain of C60 (which runs at 45° and parallel to the crystallographic b axis). To construct a photoelectrochemical cell, C60⊂H4-CPD(Py) was deposited onto nanostructured SnO2 films on a transparent electrode. The solar cell exhibited photovoltaic activity with an incident photon to current conversion efficiency of 17%.

Synthesis of oligo( p-phenylene)-linked dyads containing free base, zinc(II) or thallium(III) porphyrins for studies in artificial photosynthesis

A series of ( p-phenylene) n -linked meso-mesityl-substituted porphyrin dyads ( n=2–4) was prepared via Suzuki coupling of zinc(II) and free base porphyrin building blocks. The resulting zinc(II)/free base porphyrin dyads were demetalated. The series of free base porphyrin dimers ( n=1–4), four other porphyrin dimers (with p-phenylene, diphenylethyne or diphenylbutadiyne linkers; and aryl or tridec-7-yl meso substituents), and several benchmark monomers were converted to the thallium(III)chloride complexes under mild conditions. The collection of eight Tl(III)Cl/Tl(III)Cl dimers is designed for studies of ground-state hole-transfer processes and comparison with the excited-state energy- and hole-transfer processes of the corresponding Zn(II)/free base dyads. Altogether, 18 new porphyrin arrays and benchmark monomers have been prepared.

Synthesis of Stannyl Porphyrins and Porphyrin Dimers via Stille Coupling and Their 119Sn NMR and Fluorescence Properties

Free base stannyl porphyrins and free base porphyrin dimers have been successfully synthesized via copper-free Stille coupling in 21-67% yields. This approach provides an access to stannyl porphyrin synthons that were previously unavailable. Moreover, variation of the reaction conditions selectively provides access to either stannyl porphyrins or porphyrin dimers. Full (119/117)Sn NMR analysis was used for characterization of the stannyl porphyrins and detailed (119)Sn-(1)H-(13)C NMR analyses were carried out on a series of the starting tin reagents and the stannyl porphyrins. These investigations indicate that significant structural information can be gathered by use of commonly known NMR techniques. Photophysical properties of the novel porphyrins prepared including absorption, emission, and fluorescence lifetimes were investigated. The stannyl porphyrins emitted in the visible region, and in all cases large Stokes shifts were observed. The emission intensities of the stannyl porphyrins were 100-fold higher than those of the starting bromoporphyrins. Measured fluorescence lifetime (S(0)-->S(1)) of the stannyl and dimeric porphyrins were in the 7.7-12 ns region.

Fluorescence Properties of Porphyrin Dimers Incorporating an Anhydride Linker

A preliminary study on a novel free base porphyrin dimer and a zinc prophyrin dimer by comparing their fluorescence properties with those of the corresponding monomers is presented. The fluorescence intensity of the free base dimer is five times higher than that of the monomer, but the fluorescence intensity of the zinc porphyrin dimer is nearly similar to that of the zinc porphyrin monomer. The emission spectra show that energy transfer occurs between the two units.

Spin-multiplicity of a moderately coupled triplet–doublet spin pair in a biphenylene-linked porphyrin dimer

Electron spin transient nutation spectroscopy has been applied to a moderately coupled excited triplet and doublet spin-pair. In a biphenylene-linked copper(II)-free base porphyrin dimer, laser irradiation of either the free base half or the copper half, leads to excitation to the excited states and relaxation to the lowest excited state of the dimer. In this lowest excited state, the free base moiety in the lowest triplet state is coupled to the copper porphyrin ground doublet state, and thus forming a triplet–doublet spin pair. Time-resolved electron paramagnetic resonance (TREPR) spectra of the dimer in frozen solution at 40 K show much wider spectral width and a prominent band in the centre of the spectra than either of those for the free base monomer triplet or that of the copper(II) monomer porphyrin. These features are quite different from the strongly-coupled and weakly-coupled triplet–doublet spin pairs. Transient nutation method by pulsed EPR is used to examine the spin-multiplicity of the excited state in the dimer at three magnetic field positions. It was found that spin-multiplicity depends largely on the magnetic field. This is rationalized in terms of an intermediate size of coupling. In such intermediate coupling cases, the degree of mixing of the triplet and doublet basis wavefunctions depends greatly on the relative orientation between magnetic field and molecular axis because the ratio of the coupling size to an energy difference between the unperturbed species, triplet and doublet, varies with magnetic field direction. A splitting of TREPR spectra in partially oriented samples suggests that exchange coupling |J| is around 10 mT, which reasonably matches the results of the pulsed experiments.

Solvent and Base Dependence of Copper-Free Palladium-Catalyzed Cross-Couplings between Terminal Alkynes and Arylic Iodides: Development of Efficient Conditions for the Construction of Gold(III)/Free-Base Porphyrin Dimers

In this paper, our attempts to optimize the Heck alkynylation (copper-free Sonogashira) reaction are presented. An efficient copper-free coupling protocol was needed for the synthesis of gold/zinc porphyrin dimers because previous methods had failed. Previous studies have usually focused on ligands, whereas this work focuses on the choice of solvent and base. The catalytic system throughout the investigation was formed from the stable precursor [Pd2(dba)3.CHCl3] together with the ligand triphenylarsine, an easy-to-handle, air-stable ligand. A model study was conducted to examine the dependence of the Heck alkynylation on the solvent and base. The most successful modification proved to be the addition of methanol, as a cosolvent, in combination with a nucleophilic tertiary base. The success of the methanol additive is hypothesized to be caused by the presence of a rate-determining deprotonation step featuring a charge-separated transition state. Finally, the very high yielding and successful synthesis of a series of porphyrin systems using these new conditions is presented. For the first time, gold porphyrin substrates could efficiently be coupled in Heck alkynylation reactions.

Time-resolved EPR spectra of a photoexcited phenanthrene-linked copper(II)-free-base porphyrin dimer: An intermediate-coupling case in a triplet-doublet spin system

Time-resolved electron paramagnetic resonance (TREPR) spectra are presented for a phenanthrene-linked copper(II)-free-base porphyrin dimer. In the lowest electronic excited state, the free-base half is in the (π, π*) triplet state and the copper-porphyrin half is in the doublet ground state due to the copper(II) spin. Because of the interaction between the triplet and doublet spins, the excited state of the dimer is described as a coupled triplet-doublet pair state. By choosing the excitation wavelength, this coupled state is produced via either the intersystem crossing within the free base or the energy transfer from the excited state of the copper porphyrin. The observed TREPR spectra show very large spectral widths compared to that of the triplet state in the free-base monomer. In addition, there is a prominent absorptive band in the center of the spectra regardless of the generation pathway. These features can be interpreted as characteristic properties in an intermediate-coupling case, where the degree of the triplet and doublet mixing largely depends on the molecular orientation relative to the magnetic field.

Light-Induced Electron Spin Polarization of a Weakly Coupled Triplet−Doublet Spin Pair in a Covalently Linked Porphyrin Dimer

The spin-polarization patterns of a weakly coupled triplet−doublet spin pair are studied in a covalently linked copper (II)−free base porphyrin dimer partially oriented in a liquid crystalline solvent. The triplet−doublet pair is generated in the dimer by either spin−orbit intersystem crossing (ISC) within the free-base moiety or via energy transfer (EnT) from the copper porphyrin. The two pathways result in two different spin-polarization patterns, which can be observed separately by selectively exciting either the free base porphyrin at 640 nm or the copper porphyrin at 540 nm. The time development and orientation dependence of the two spin-polarization patterns is investigated at two microwave frequencies. An analysis of the EPR transients reveals that the polarization associated with the EnT pathway rises monoexponentially, while the ISC polarization rises rapidly and decays biexponentially. The rise of the EnT polarization pattern and the fast decay component of the polarization from ISC are governed by the same characteristic lifetime of 2 μs independent of the molecular orientation. However, the ratio of the two decay components of the ISC polarization is strongly orientation dependent. Separately, a general treatment of the spin-polarized transient EPR spectra of coupled triplet−doublet spin pairs is presented and two possible mechanisms for the kinetic behavior involving spin-selective depopulation or relaxation between the spin sublevels of the triplet−doublet pair are investigated. It is shown that all the observed features of the polarization patterns including orientation and excitation wavelength dependence as well as the kinetics can be explained in terms of the model and are consistent with either of the two mechanisms. In addition to the weakly coupled triplet−doublet pair, narrow signal components are observed near the center of the spectrum. The microwave frequency dependence of the spectra confirms the previous assignment that these components are the quartet and doublet states of a strongly coupled triplet−doublet pair generated in conformations of the dimer in which the two porphyrins are in close proximity.

Weak Exchange Interaction of Copper(II) Pairs in Rigidly Linked Porphyrin Dimers

In a series of gable-type di-copper(II) porphyrin dimers as well as the corresponding mono-copper(II)-free base porphyrin dimers, Electron Paramagnetic Resonance (EPR) spectra are measured in toluene at 77 K and 300 K, in order to clarify spin-spin coupling between the two metal spins. By comparison of the EPR data obtained in fluid solution and rigid media, we have separated contribution of exchange interaction from that of dipole-dipole coupling, which is averaged out under the fluid condition by the molecular rotational diffusion. On the basis of the line-shape analysis of the EPR spectra, the magnitudes of exchange interaction are evaluated as 0 m 3 2 10 m 3 cm m 1 , depending upon the spacer unit. The observed spacer dependence of exchange coupling provides a clear evidence of through-bond mechanism. electron paramagnetic resonance copper(II) porphyrin dimers spin-spin coupling through bond spacer dependence molecular tumbling

Enhanced Intersystem Crossing in Gable-Type Copper(II) Porphyrin−Free Base Porphyrin Dimers: Evidence of Through-Bond Exchange Interaction

Spacer dependence of intersystem crossing (ISC) affected by a remote metal unpaired electron was studied in a series of copper(II) porphyrin−free base porphyrin dimers. In these dimers, fluorescence of the free base moiety is remarkably quenched by the copper counterpart which is linked via a phenanthrene, a naphthalene, or a benzene. Fluorescence lifetimes and quantum yields of the free base moiety in the heterodimers are 1/10−1/35 of the free base monomer depending upon the spacer, whereas no change was observed in the free base homodimers. The changes in the copper(II)−free base dimers are attributed to an enhancement of ISC in the free base half, due to interaction with an unpaired electron in Cu(II) ion of the other half. Estimated ISC rates from the experimental results exhibit a strong correlation with the number of bonds of the linkage but not with the center-to-center distance of the two halves. An expression for the ISC rate in the presence of an unpaired electron is derived. Exchange interaction between the copper unpaired electron and free base π-electrons gives rise to mixing between the excited states of the dimer when the free base moiety is in singlet and triplet excited states, and enhances ISC in the free base part. The observed spacer dependence leads to the suggestion that the ISC rate is governed by through-bond exchange interaction.

Intramolecular Energy Transfer in a Covalently Linked Copper(II) Porphyrin−Free Base Porphyrin Dimer: Novel Spin Polarization in the Energy Acceptor

The excitation wavelength dependence of time-resolved EPR is used to demonstrate the pathway of intramolecular energy transfer in a covalently linked copper(II)−free base porphyrin dimer. Spin polarized spectra are presented for selective excitation of both the copper(II) porphyrin donor (at 540 nm) and the free base porphyrin acceptor (at 640 nm) at 50 and 80 K. In all cases the observed spectra are assigned to the triplet state of the free base which is coupled weakly to the copper ground state doublet. The polarization pattern generated by selective excitation of the free base half is indicative of intersystem crossing (ISC), whereas excitation of the copper(II) half gives an eaa/eea polarization pattern. The latter is rationalized in terms of energy transfer via the lowest excited trip−quartet state of the copper(II) moiety, followed by selective depopulation from the spin states with doublet character in the weakly coupled free base triplet-copper doublet system. This leads to a spectrum which resemb...