Redox Control of Photoinduced Electron Transfer in Axial Terpyridoxy Porphyrin Complexes

Advances and opportunities in Group 15 porphyrin chemistry

The chemistry of Group 15 porphyrins has been established relatively well among the main-group porphyrins. Thus far phosphorus(III), phosphorus(V), arsenic(III), arsenic(V), antimony(III), antimony(V), and bismuth(III) porphyrins have been reported. Their unique axial-bonding ability, rich redox, and optical properties offer an advantage over other main-group or transition metal porphyrins. They could be excellent candidates for a variety of applications such as solar energy harvesting, molecular electronics, molecular catalysis, and biomedical applications. Despite these unique properties, the Group 15 porphyrins are not exploited at their fullest capacity. Recently, there has been some interest, where the richness of Group 15 porphyrin chemistry was explored for some of the above applications. In this context, this article summarizes recent advances in Group 15 porphyrin chemistry and attempts to unravel the tremendous opportunities of these remarkable porphyrins.

Hypervalent Phosphorus(V) Porphyrins with meso-Methoxyphenyl Substituents: Significance of the Number and Position of Methoxy Groups in Promoting Intramolecular Charge Transfer

To study the photophysical and redox properties as a function of meso-aryl units, a series of hypervalent phosphorus(V) porphyrins, PP(OMe)₂·PF₆, PMP(OMe)₂·PF₆, PDMP(OMe)₂·PF₆, P345TMP(OMe)₂·PF₆, and P246TMP(OMe)₂·PF₆, with phenyl (P), 4-methoxyphenyl (MP), 3,5-dimethoxyphenyl (DMP), 3,4,5-trimethoxyphenyl (345TMP), and 2,4,6-trimethoxyphenyl (246TMP) units, respectively, have been synthesized. The P(+5) in the cavity makes the porphyrin ring electron-poor, whereas the methoxy groups make the meso-phenyl rings electron-rich. The presence of electron-rich and electron-poor portions within the porphyrin molecule promoted an intramolecular charge transfer (ICT). Also, the study suggests that the ICT depends on the number and position of the methoxy groups. The ICT is more prominent in m-methoxy-substituted phosphorus(V) porphyrins (PDMP(OMe)₂.PF₆, P345TMP(OMe)₂·PF₆) and almost no ICT was found in no-methoxy, o-methoxy, and/or p-methoxy phosphorus(V) porphyrins (PP(OMe)₂·PF₆, PMP(OMe)₂·PF₆, P246TMP(OMe)₂·PF₆). Transient absorption studies indicate that the ICT takes place on the picosecond time scale. The most striking results come from P246TMP(OMe)₂·PF₆, where each phenyl ring carries three methoxy units, like the P345TMP(OMe)₂·PF₆, but it failed to induce the ICT process. Electrochemical studies and time-dependent density functional theory (TD-DFT) calculations were used to support the experimental results. This study extensively explores why and how slight variations in meso-aryl substitutions lead to intricate changes in the photophysical and redox properties of phosphorus(V) porphyrins.

Rational Design and Synthesis of OEP and TPP Centered Phosphorus(V) Porphyrin-Naphthalene Conjugates: Triplet Formation via Rapid Charge Recombination

Six new "axial-bonding" type "phosphorus(V) porphyrin-naphthalene" conjugates have been prepared consisting of octaethylporphyrinatophosphorus(V) (POEP⁺)/tetraphenylporphyrinatophosphorus(V) (PTPP⁺) and naphthalene (NP). The distance between the porphyrin and NP was systematically varied using polyether bridges. The unique structural topology of the octaethylporphyrinatophosphorus(V) (POEP⁺) and tetraphenylporphyrinatophosphorus(V) (PTPP⁺) enabled construction of mono- and disubstituted phosphorus(V) porphyrin-naphthalene conjugates, respectively. The steady-state and transient spectral properties were investigated as a function of redox properties, distance, and molecular topology. Strong electronic interactions between the phosphorus(V) porphyrin and NP in directly bound conjugates were observed. The established energy diagrams predicted reductive electron transfer involving singlet excited phosphorus(V) porphyrin and NP to generate high-energy (∼1.83-2.11 eV) charge-separated states (POEP/PTPP)^•-(NP)^•+. Femtosecond transient absorption spectral studies revealed rapid deactivation of singlet excited phosphorus(V) porphyrin due to charge separation wherein the estimated forward rate constants were in the range of 10⁹-10¹⁰ s^-1 and were dependent on the distance between the NP and porphyrins units, as well as the redox potentials of the type of the phosphorus(V) porphyrin. Additionally, due to high exothermicity and low-lying triplet states, the charge recombination process was found to be rapid, leading to populating the triplet states of phosphorus(V) porphyrins.

Donor-acceptor conjugates derived from cobalt porphyrin and fullerene via metal-ligand axial coordination: Formation and excited state charge separation

Two types of cobalt porphyrins, viz., meso-tetrakis(tolylporphyrinato)cobalt(II), (TTP)Co (1), and meso-tetrakis(triphenylamino porphyrinato)cobalt(II), [(TPA)4P]Co, (2) were self-assembled via metal-ligand axial coordination of phenyl imidazole functionalized fulleropyrrolidine, ImC[Formula: see text] to form a new series of donor–acceptor constructs. A 1:2 complex formation with ImC[Formula: see text] was established in the case of (TTP)Co while for [(TPA)4P]Co only a 1:1 complex was possible to positively identify. The binding constants [Formula: see text] and [Formula: see text] for step-wise addition of ImC[Formula: see text] to (TTP)Co were found to be 1.07 × 105 and 3.20 × 104 M[Formula: see text], respectively. For [(TPA)4P]Co:ImC[Formula: see text], the measured [Formula: see text] values was found to be 6.48 × 104 M[Formula: see text], slightly smaller than that observed for (TTP)Co. Although both cobalt porphyrins were non-fluorescent, they were able to quench the fluorescence of ImC[Formula: see text] indicating occurrence of excited state events in the supramolecular donor-acceptor complexes. Electrochemistry coupled with spectroelectrochemistry, revealed the formation of cobalt(III) porphyrin cation instead of a cobalt(II) porphyrin radical cation, as the main product, during oxidation of phenyl imidazole coordinated cobalt porphyrin. With the help of computational and electrochemical results, an energy level diagram was constructed to witness excited state photo-events. Competitive energy and electron transfer from excited CoP to coordinated ImC[Formula: see text], and electron transfer from Im1C[Formula: see text]* to cobalt(II) porphyrin resulting into the formation of PCo[Formula: see text]:ImC[Formula: see text] charge separated state was possible to envision from the energy diagram. Finally, using femtosecond transient absorption spectroscopy and data analysis by Glotaran, it was possible to establish sequential occurrence of energy transfer and charge separation processes. The lifetime of the final charge separated state was [Formula: see text] 2 ns. A slightly better charge stabilization was observed in the case of [(TPA)4P]Co:ImC[Formula: see text] due to the presence of electron rich, peripheral triphenylamine substituents on the cobalt porphyrin.

Decelerating Charge Recombination Using Fluorinated Porphyrins in N,N-Bis(3,4,5-trimethoxyphenyl)aniline-Aluminum(III) Porphyrin-Fullerene Reaction Center Models

In supramolecular reaction center models, the lifetime of the charge-separated state depends on many factors. However, little attention has been paid to the redox potential of the species that lie between the donor and acceptor in the final charge separated state. Here, we report on a series of self-assembled aluminum porphyrin-based triads that provide a unique opportunity to study the influence of the porphyrin redox potential independently of other factors. The triads, BTMPA-Im→AlPorF_n-Ph-C₆₀ (n = 0, 3, 5), were constructed by linking the fullerene (C₆₀) and bis(3,4,5-trimethoxyphenyl)aniline (BTMPA) to the aluminum(III) porphyrin. The porphyrin (AlPor, AlPorF₃, or AlPorF₅) redox potentials are tuned by the substitution of phenyl (Ph), 3,4,5-trifluorophenyl (PhF₃), or 2,3,4,5,6-pentafluorophenyl (PhF₅) groups in its meso positions. The C₆₀ and BTMPA units are bound axially to opposite faces of the porphyrin plane via covalent and coordination bonds, respectively. Excitation of all of the triads results in sequential electron transfer that generates the identical final charge separated state, BTMPA^•+-Im→AlPorF_n-Ph-C₆₀^•-, which lies energetically 1.50 eV above the ground state. Despite the fact that the radical pair is identical in all of the triads, remarkably, the lifetime of the BTMPA^•+-Im→AlPorF_n-Ph-C₆₀^•- radical pair was found to be very different in each of them, that is, 1240, 740, and 56 ns for BTMPA-Im→AlPorF₅-Ph-C₆₀, BTMPA-Im→AlPorF₃-Ph-C₆₀, and BTMPA-Im→AlPor-Ph-C₆₀, respectively. These results clearly suggest that the charge recombination is an activated process that depends on the midpoint potential of the central aluminum(III) porphyrin (AlPorF_n).

Photodynamic activity of Sn(IV) meso-tetraacenaphthylporphyrin and its methyl-β-cyclodextrin inclusion complexes on MCF-7 breast cancer cells

A novel Sn(IV) meso-tetraacenaphthylporphyrin (SnTAcP) has been synthesized and characterized. SnTAcP was complexed with methyl-[Formula: see text]-cyclodextrin (m[Formula: see text]-CD), a nanocarrier that enhances water solubility, and the complexes were evaluated as PDT agents using MCF-7 breast cancer cells. A relatively low singlet oxygen quantum yield value of 0.36 was obtained in DMF, and the lowest energy Q band lies at 608 nm on the edge of the therapeutic window. SnTAcP was found to be non-toxic in the dark and phototoxic towards MCF-7 breast cancer cells with a half-maximal inhibitory concentration (IC[Formula: see text] value of 11 ± 1.1 [Formula: see text]g · mL[Formula: see text] after 30 min of irradiation with a 625 nm Thorlabs LED that provides a dose of 432 J · cm[Formula: see text]. A higher IC[Formula: see text] value of 21 ± 1.1 [Formula: see text]g · mL-1 was obtained for the m[Formula: see text]-CD inclusion complex of SnTAcP.

Sn(IV) Multiporphyrin Arrays as Tunable Photoactive Systems

A series of four arrays made of a central Sn(IV) porphyrin as scaffold axially connected, via carboxylate functions, to two free-base porphyrins has been prepared and fully characterized. Three arrays in the series feature the same free-base unit and alternative tin-porphyrin macrocycles, and one consists of a second type of free-base and one chosen metallo-porphyrin. A thorough photophysical investigation has been performed on all arrays by means of time-resolved emission and absorption techniques. Specific focus has been given at identifying how structural modifications of the free-base and tin-porphyrin partners and/or variation of the solvent polarity can effectively translate into distinct photophysical behaviors. In particular, for systems SnTPP(Fb)₂ (1) and SnOEP(Fb)₂ (2), an ultrafast energy transfer process from the excited Sn(IV) porphyrin to the free-base unit occurs with unitary efficiency. For derivative SnTPP(FbR)₂ (3), the change of solvent from dichloromethane to toluene is accompanied by a neat change in the intercomponent quenching mechanism, from photoinduced electron transfer to energy transfer, upon excitation of the Sn(IV) porphyrin unit. Finally, for array SnTpFP(Fb)₂ (4), an ultrafast electron transfer quenching of both chromophores is detected in all solvents. This work provides a general outline, accompanied by clear experimental support, on possible ways to achieve a systematic fine-tuning of the quenching mechanism (from energy to electron transfer) of Sn(IV) multiporphyrin arrays.

Surface anchored self-assembled reaction centre mimics as photoanodes consisting of a secondary electron donor, aluminium(iii) porphyrin and TiO2 semiconductor

Vertically assembled photoanodes, consisting of aluminum(iii) porphyrin, an electron donor, and semiconductor TiO₂, have been fabricated and their photophysical properties investigated.

Charge-separation in panchromatic, vertically positioned bis(donor styryl)BODIPY-aluminum(iii) porphyrin-fullerene supramolecular triads

Three, broad band capturing, vertically aligned supramolecular triads, R2-BDP-AlPorF3←Im-C60 [R = H, styryl (C2H2-Ph), C2H2-TPA (TPA = triphenylamine); ← = coordinate bond], have been constructed using BODIPY derivative (BDP, BDP-Ph2 or BDP-TPA2), 5,10,15,20-tetrakis(3,4,5-trifluorophenyl)aluminum(iii) porphyrin (AlPorF3) and fullerene (C60) entities. The C60 and BDP units are bound to the Al center on the opposite faces of the porphyrin: the BDP derivative through a covalent axial bond using a benzoate spacer and the C60 through a coordination bond via an appended imidazole. Owing to the bis-styryl functionality on BDP, the constructed dyads and triads exhibited panchromatic light capture. Due to the diverse absorption and redox properties of the selected entities, it was possible to demonstrate excitation wavelength dependent photochemical events. In the case of the BDP-AlPorF3 dyad, selective excitation of BDP resulted in singlet-singlet energy transfer to AlPorF3 (kEnT = 1.0 × 1010 s-1). On the other hand, excitation of the AlPorF3 entity in the BDP-AlPorF3←Im-C60 triad revealed charge separation leading to the BDP-(AlPorF3)˙+-(C60)˙- charge separated state (kCS = 2.43 × 109 s-1). In the case of the Ph2-BDP-AlPorF3 dyad, energy transfer from 1AlPorF3* to 1(Ph2-BDP)* was witnessed (kEnT = 1.0 × 1010 s-1); however, upon assembling the supramolecular triad, (Ph2-BDP)-AlPorF3←Im-C60, electron transfer from 1AlPorF3* to C60 (kCS = 3.35 × 109 s-1), followed by hole shift (kHS = 1.00 × 109 s-1) to Ph2-BDP, was witnessed. Finally, in the case of the TPA2-BDP-AlPorF3←Im-C60 triad, only electron transfer leading to the (TPA2-BDP)˙+-AlPorF3←Im-(C60)˙- charge separated state, and no energy transfer, was observed. The facile oxidation of Ph2-BDP and TPA2-BDP compared to AlPorF3 in the latter two triads facilitated charge separation through either an electron migration or hole transfer mechanism depending on the initial excitation. The charge-separated states in these triads persisted for about 20 ns.

Tuning photochemical properties of phosphorus(v) porphyrin photosensitizers

Photosensitizing and emission properties of P(v) porphyrins were studied. The nature of the axial ligands, occupying the apical position on the P centre adopting an octahedral coordination geometry, strongly influences singlet oxygen generation and charge transfer and allows switching between the two processes.

A Transient EPR Study of Electron Transfer in Tetrathiafulvalene-Aluminum(III) Porphyrin-Anthraquinone Supramolecular Triads

The stabilization of light-induced charge separation in two axially bound triads based on aluminum(III) porphyrin (AlPor) are investigated using the electron spin polarization patterns of the final radical pair state. In the triads, TTF-(Ph)n-py-AlPor-AQ, (n=0, 1) anthraquinone (AQ) is attached covalently to the Al(III) center, while the donor tetrathiafulvalene (TTF) coordinates to Al(III) on the opposite face of the porphyrin ring via the appended pyridine (py). The dyad AlPor-AQ has been studied previously (M. Kanematsu, P. Naumov, T. Kojima, S. Fukuzumi, Chem. Eur. J. 17 (2011) 12372.) and shown to undergo fast light-induced charge separation and triplet recombination. Here, it is shown that by coordinating pyridine-appended TTF to the porphyrin, the charge separation can be stabilized. The spin polarized transient EPR spectra of the state TTF·+AQ·− can be observed in both the glass phase and in liquid solution and show that the state is formed from a singlet precursor on a timescale of less than ~0.5 ns. Using structural models to fix the geometry of the radical pair and the strength of the dipolar coupling, it is possible to determine the sign and approximate magnitude of the exchange coupling between TTF·+ and AQ·−. In contrast, other similar triads, which display relatively large ferromagnetic coupling, the exchange coupling is found to be small and antiferromagnetic. This difference can be rationalized as a result of differences in the structure of the bridge between the porphyrin and the acceptor.

Axially assembled photosynthetic reaction center mimics composed of tetrathiafulvalene, aluminum(III) porphyrin and fullerene entities

The distance dependence of sequential electron transfer has been studied in six, vertical, linear supramolecular triads, (TTF-Ph(n)-py → AlPor-Ph(m)-C60, n = 0, 1 and m = 1, 2, 3), constructed using tetrathiafulvalene (TTF), aluminum(III) porphyrin (AlPor) and fullerene (C60) entities. The C60 and TTF units are bound to the Al center on opposite faces of the porphyrin; the C60 through a covalent axial bond using a benzoate spacer, and the TTF through a coordination bond via an appended pyridine. Time-resolved optical and EPR spectroscopic methods and computational studies are used to demonstrate that excitation of the porphyrin leads to step-wise, sequential electron transfer (ET) between TTF and C60, and to study the electron transfer rates and exchange coupling between the components of the triads as a function of the bridge lengths. Femtosecond transient absorption studies show that the rates of charge separation, k(CS) are in the range of 10(9)-10(11) s(-1), depending on the length of the bridges. The lifetimes of the charge-separated state TTF˙(+)-C₆₀˙⁻ obtained from transient absorbance experiments and the singlet lifetimes of the radical pairs obtained by time-resolved EPR are in good agreement with each other and range from 60-130 ns in the triads. The time-resolved EPR data also show that population of the triplet sublevels of the charge-separated state in the presence of a magnetic field leads to much longer lifetimes of >1 μs. The data show that a modest stabilization of the charge separation lifetime occurs in the triads. The attenuation factor β = 0.36 Å(-1) obtained from the exchange coupling values between TTF˙(+) and C₆₀˙⁻ is consistent with values reported in the literature for oligophenylene bridged TTF-C60 conjugates. The singlet charge recombination lifetime shows a much weaker dependence on the distance between the donor and acceptor, suggesting that a simple superexchange model is not sufficient to describe the back reaction.

Ultrafast charge separation and charge stabilization in axially linked ‘tetrathiafulvalene–aluminum(iii) porphyrin–gold(iii) porphyrin’ reaction center mimics

Sequential electron transfer leading to charge stabilization in newly synthesized vertically aligned ‘tetrathiafulvalene–aluminum(iii) porphyrin–gold(iii) porphyrin’ supramolecular triads is reported.

Photophysics of soret-excited tin(IV) porphyrins in solution

The photophysics of low-chlorin tin(IV) tetraphenylporphyrin dihydroxide, a core building block for axially substituted supramolecular tin porphyrin constructs, has been studied in a variety of hydrogen-bonding, nonpolar, and aprotic polar solvents using steady-state, nanosecond, and femtosecond time-resolved emission, and femtosecond time-resolved absorption methods. In hydrogen-bonding solvents the metalloporphyrin exists as solvated monomers, and its Soret-excited S2 state in these solvents exhibits the expected linear energy gap law relationship with first-order population decay times in the 0.8 to 1.7 ps range. Evidence is presented that this metalloporphyrin aggregates in other solvents at the concentrations typically used for these ultrafast measurements and yields species-averaged time-resolved data. Cw laser excitation in the Q-band under deaerated conditions produces weak S2-S0 fluorescence (photon upconversion) as a result of triplet-triplet annihilation.

Light-induced hole transfer in a hypervalent phosphorus(V) octaethylporphyrin bearing an axially linked bis(ethylenedithio)tetrathiafulvalene

A phosphorus(V) porphyrin bearing an axially linked bis(ethylenedithio)tetrathiafulvalene, dyad 1, and its radical cation phosphorus(V) porphyrin- O-CH2 -(bis(ethylenedithio)tetrathiafulvalene)+•, dyad 2, have been synthesized and studied as an electron hole donor-acceptor system. The absorption spectrum of dyad 1 does not show evidence for electronic coupling between the porphyrin and the bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) moieties. However, the steady-state fluorescence of the porphyrin chromophore is quantitatively quenched and its transient fluorescence lifetime is shortened compared to a reference compound in which the BEDT-TTF moiety is replaced by a methoxy group. Chemical oxidation of the BEDT-TTF moiety in dyad 1 to give dyad 2 results in recovery of the fluorescence intensity. This behavior suggests that the fluorescence quenching in dyad 1 is the result of intramolecular hole transfer from the the excited porphyrin to the BEDT-TTF moiety. The occurence of hole transfer in dyad 1 is confirmed by freeze-trapping and time-resolved electron paramagnetic resonance (EPR) measurements. The freeze-trapping EPR experiments show that steady-state irradiation of the complex leads to accumulation of its radical cation (dyad 2) while the transient EPR measurements at 5 °C show that flash irradiation of dyad 1 results in formation of a radical-ion pair with a lifetime of at least 300 ns. The triplet state of the porphyrin, which is formed by intersystem crossing and gives a strong transient EPR spectrum in the reference compound, is not observed for dyad 1. Together, the fluorescence quenching and the polarization pattern of the radical pair suggest that the hole transfer occurs from the excited singlet state of the porphyrin with high efficiency.

Formation of a hybrid compound composed of a saddle-distorted Tin(IV)-porphyrin and a Keggin-type heteropolyoxometalate to undergo intramolecular photoinduced electron transfer

Nonplanar Sn(IV)-porphyrin complexes, [Sn(TMPP(Ph)(8))-Cl(2)] (1) and [Sn(TMPP(Ph)(8))(OMe)(2)] (2) (TMPP(Ph)(8): 5,10,15,20-tetrakis(4-methoxyphenyl)-2,3,7,8,12,13,17,18-octaphenylporphyrinato), were prepared and characterized by spectroscopic and electrochemical methods together with X-ray crystallography. Variable-temperature (1)H NMR study revealed that the coordination of the methoxo ligand of 2 is weak enough in solution to enhance the axial ligand exchange with a Keggin-type phosphotungstate (α-[PW(12)O(40)](3-)) due to the steric stress between the axial methoxo ligand and the peripheral phenyl groups of the porphyrin ligand. The formation of a novel 1:1 donor-acceptor complex, [Sn(TMPP(Ph)(8))(OMe)(α-[PW(12)O(40)])](2-) (4) was confirmed by (1)H NMR and UV-vis spectral titrations, and also by MALDI-TOF-MS measurements. Electrochemical measurements for the donor-acceptor complex in PhCN revealed that the Sn(IV)-TMPP(Ph)(8) moiety acts as an electron donor and the α-[PW(12)O(40)](3-) moiety acts as an electron acceptor and that the energy level of the electron-transfer (ET) state of the 1:1 complex (1.17 eV) is lower than that of the triplet excited states of the SnTMPP(Ph)(8) complex (1.31 eV). Femtosecond and nanosecond laser flash photolysis measurements indicate that intersystem crossing from the singlet excited sate to the triplet excited state occurs followed by intramolecular photoinduced electron transfer from the triplet excited state of the Sn(IV)-TMPP(Ph)(8) moiety to the α-[PW(12)O(40)](3-) moiety in the 1:1 complex in benzonitrile.

Photoinduced electron transfer in ruthenium(II)/Tin(IV) multiporphyrin arrays

The photophysical behavior of a series of heterometallic arrays made of a central Sn(IV) porphyrin connected, respectively, to two (SnRu(2)), four (SnRu(4)), or six (SnRu(6)) ruthenium porphyrin units has been studied in dichloromethane. Two different motifs connect the ruthenium porphyrin units to central tin porphyrin core, axial coordination via ditopic bridging ligands and/or coordination to peripheral pyridyl groups of the central porphyrin ring. A remarkable number of electron transfer processes (photoinduced charge separation and recombination processes) have been time-resolved using a combination of emission spectroscopy and fast (nanosecond) and ultrafast (femtosecond) absorption techniques. In these systems both types of molecular components can be selectively populated by light absorption. In all the arrays, the local excited states of these units (the tin porphyrin singlet excited state and the ruthenium porphyrin triplet state) are quenched by electron transfer leading to a charge-separated state where the ruthenium porphyrin unit is oxidized and the tin porphyrin unit is reduced. For each array, the two forward electron transfer processes, as well as the charge recombination process leading back to the ground state, have been kinetically resolved. The rate constants obey standard free-energy correlations with the forward processes lying in the normal free-energy regime and the back reactions in the Marcus inverted region. The comparison between the trimeric (SnRu(2)) and pentameric (SnRu(4)) arrays shows that all the electron transfer processes are faster in the latter than in the former system. This can be rationalized in terms of differences in electronic factors (due to the different connecting motifs) and driving force. In less polar solvents, such as toluene, the energy of the charge-separated states is substantially lifted, leading to a switch (from electron transfer to triplet energy transfer) in the deactivation mechanism of the excited ruthenium triplet.