Fluorinated Antimony(V) Tetraarylporphyrins as High-Valent Electron Acceptors with Unparalleled Reduction Potentials.

A series of fluorinated antimony(V) porphyrins, SbTPP(OMe)2·PF6, SbTPP(OTFE)2·PF6, SbT(4F)PP(OMe)2·PF6, SbT(35F)PP(OMe)2·PF6, SbT(345F)PP(OMe)2·PF6, SbT(4CF3)PP(OMe)2·PF6, SbT(35CF3)PP(OMe)2·PF6, and SbT(35CF3)PP(OTFE)2·PF6, have been synthesized with phenyl [P], 4-fluorophenyl [(4F)P], 3,5-difluorophenyl [(35F)P], 3,4,5-difluorophenyl [(345F)P], 4-trifluoromethylphenyl [(4CF3)P], and 3,5-bis(trifluoromethyl)phenyl [(35CF3)P], in the meso-positions. Additionally, the SbTPP(OTFE)2·PF6 and SbT(35CF3)PP(OTFE)2·PF6 carry trifluoroethoxy units in their axial-positions. The fluorination on the porphyrin peripherals ranges from zero fluorine atoms in SbTPP(OMe)2·PF6 to 30 fluorine atoms in SbT(35CF3)PP(OTFE)2·PF6. X-ray crystallography confirmed the structures of the investigated antimony(V) porphyrins. The absorption spectra depend on the number of fluorine atoms as it is blue-shifted with increasing fluorination. The series also exhibited rich redox chemistry with two reduction processes and one oxidation process. Remarkably, these porphyrins manifested the lowest reduction potentials reported among the main-group porphyrins, which are as low as -0.08 V vs SCE for SbT(35CF3)PP(OTFE)2·PF6. On the contrary, the oxidation potentials were found to be very large, that is equal to 2.20 V vs SCE or even higher for SbT(4CF3)PP(OMe)2·PF6 or SbT(35CF3)PP(OMe)2·PF6 and SbT(35CF3)PP(OTFE)2·PF6, respectively. These unprecedented potentials are due to a combination of two factors: (i) the +5-oxidation state of antimony in the porphyrin cavity and (ii) the presence of the strong electron-withdrawing fluorine atoms on the porphyrin peripherals. Density functional theory (DFT) calculations were used to support the experimental results. The systematic study of antimony(V) porphyrins, especially their high potentials, make them ideal for the construction of photoelectrodes and excellent electron acceptors for photoelectrochemical cells and artificial photosynthetic systems, respectively, for solar energy conversion and storage applications.

Correction: Solvent dependent triplet state delocalization in a co-facial porphyrin heterodimer.

Correction for 'Solvent dependent triplet state delocalization in a co-facial porphyrin heterodimer' by Susanna Ciuti et al., Phys. Chem. Chem. Phys., 2022, https://doi.org/10.1039/D2CP04291F.

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.

Solvent dependent triplet state delocalization in a co-facial porphyrin heterodimer.

The excited triplet state of a cofacial aluminum(III) porphyrin-phosphorus(V) porphyrin heterodimer is investigated using transient EPR spectroscopy and quantum chemical calculations. In the dimer, the two porphyrins are bound covalently to each other via a µ-oxo bond between the Al and P centres, which results in strong electronic interaction between the porphyrin rings. The spin polarized transient EPR spectrum of the dimer is narrower than the spectra of the constituent monomers and the magnitude of the zero-field splitting parameter D is solvent dependent, decreasing as the polarity of the solvent increases. The quantum chemical calculations show that the spin density of the triplet state is delocalized over both porphyrins, while magnetophotoselection measurements reveal that, in contrast to the value of D, the relative orientation of the ZFS axes and the excitation transition dipole moments are not solvent dependent. Together the results indicate that triplet state wavefunction is delocalized over both porphyrins and has a modest degree of charge-transfer character that increases with increasing solvent polarity. The sign of the spin polarization pattern of the dimer triplet state is opposite to that of the monomers. The positive sign of D predicted for the monomers and dimer by the quantum chemical calculations implies that the different signs of the spin polarization patterns is a result of a difference in the spin selectivity of the intersystem crossing.

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.

A charge transfer state induced by strong exciton coupling in a cofacial µ-oxo-bridged porphyrin heterodimer.

Photosensitizers with high energy, long lasting charge-transfer states are important components in systems designed for solar energy conversion by multistep electron transfer. Here, we show that in a push-pull type, µ-oxo-bridged porphyrin heterodimer composed of octaethylporphyrinatoaluminum(iii) and octaethylporphyrinatophosphorus(v), the strong excitonic coupling between the porphyrins and the different electron withdrawing abilities of Al(iii) and P(v) promote the formation of a high energy CT state. Using, an array of optical and magnetic resonance spectroscopic methods along with theoretical calculations, we demonstrate photodynamics of the heterodimer that involves the initial formation of a singlet CT which relaxes to a triplet CT state with a lifetime of ∼130 ps. The high-energy triplet CT state (3CT = 1.68 eV) lasts for nearly 105 µs prior to relaxing to the ground state.

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→AlPorFn-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→AlPorFn-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→AlPorFn-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 (AlPorFn).

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

A series of vertically assembled photoanodes, consisting of 5,10,15,20-tetrakis(3,4,5-trifluorophenyl)aluminum(iii) porphyrin (AlPorF3), a pyridine appended electron donor (PTZ-Py, PTZ = phenothiazine; TTF-Py, TTF = tetrathiafulvalene), and semiconductor TiO2, have been fabricated by exploiting the unique axial properties of AlPorF3. The new photoanodes were characterized by steady-state and transient spectroscopic techniques. Transient-absorption studies show that in the absence of a donor, both the photoanodes (AlPorF3-TiO2 and AlPorF3-Ph-TiO2) exhibit electron injection from AlPorF3 into the conduction band of TiO2 and the injection efficiencies are strongly dependent on the linker. Faster electron injection and recombination is revealed when AlPorF3 is directly bound to TiO2. Although a secondary electron donor is coordinated to AlPorF3 (viz., Donor-Py-AlPorF3-TiO2 and Donor-Py-AlPorF3-Ph-TiO2), the primary charge separation occurs in the form of electron injection from AlPorF3 to TiO2 followed by a secondary process involving photooxidation of the donor (PTZ and TTF) with AlPorF3˙+. The estimated electron injection lifetimes and the AlPorF3˙+ decay lifetimes strongly depend on the electron richness of the donor; the higher the electron density of the donor, the faster the electron injection and photooxidation witnessed. The photoanodes with TTF (TTF-Py-AlPorF3-TiO2 and TTF-Py-AlPorF3-Ph-TiO2) show faster injection and shorter decay lifetimes of AlPorF3˙+ over their PTZ counterparts (PTZ-Py-AlPorF3-TiO2 and PTZ-Py-AlPorF3-Ph-TiO2). The observed trends suggest that the strong secondary electron donor enhances the injection and the subsequent photooxidation processes in the investigated photoanodes. The successful mimicking of a sequential charge-separation process makes aluminum(iii) porphyrins potential sensitizers for the construction of photoanodes, especially for photocatalytic and dye-sensitized solar cells for conversion and storage of solar energy.

Electron spin polarization in an Al(III) porphyrin complex with an axially bound nitroxide radical.

The generation and transfer of electron spin polarization and coherence plays an important role in quantum information technologies and spintronics. In this context, the excited state spin dynamics of molecular systems in which a stable free radical is attached to a chromophore are of particular interest. In such complexes, the coupling between the electron spin on the free radical with those on the chromophore generates excited states referred to as sing-doublet, trip-doublet, and trip-quartet. Here, we study the light-induced electron spin polarization in an aluminum(iii) porphyrin (AlPor) complex in which the nitroxide (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) is covalently bound to the Al center via an ester linkage. In addition to the covalent bond to the TEMPO moiety, the Al center in AlPor also acts as a Lewis acid and can coordinate Lewis bases such as pyridine. Without pyridine bound, the spin polarized transient electron paramagnetic resonance spectrum of the complex at 80 K is unusual and displays a strong absorptive pattern with a Lorentzian lineshape. Coordination of pyridine to AlPor-TEMPO results in a dramatic change in the spin polarization pattern and a spectrum typical of the quartet state with broad wings from the ms = ±1/2 ↔ ±3/2 transitions with emissive/absorptive polarization and a narrow absorptive peak from the ms = ±1/2 ↔ ±1/2 transitions. At later times, the pattern evolves to a purely absorptive spectrum similar to that observed without pyridine. These changes are discussed in terms of a model in which back and forth transitions between the nearly degenerate lowest trip-doublet and trip-quartet states occur. It is argued that these transitions lead to the observed net polarization in AlPor-TEMPO and are fast enough that the outer lines are broadened. Density functional theory computations and the UV/Vis spectra suggest that the exchange interaction between TEMPO and the triplet state of AlPor increases when pyridine is bound, and the near degeneracy of the lowest excited states is lifted. It is argued that this slows the back and forth transitions which results in the strong change in the polarization pattern.

Reversible Solution π-Dimerization and Long Multicenter Bonding in a Stable Phenoxyl Radical.

Reversible solution π-dimerization is observed in the stable neutral phenoxyl radical 2,6-bis-(8-quinolylamino)-4-(tert-butyl)phenoxyl baqp and is spectroscopically characterized. This behavior, not previously observed for π-extended phenoxyl radicals, is relevant to the formation of long multicenter bonding in the π-dimer at low temperature akin to previously reported phenalenyl radicals. Our experimental data are supported in a quantitative manner by results from density functional theory (DFT) and ab initio molecular orbital theory calculations. Our theoretical results indicate that the solution dimer features strong bonding interactions between the two phenoxyl rings but that the stability of the dimer is also related to dispersion interactions between the flanking nearly parallel quinolyl rings.

Triplet electron transfer and spin polarization in a palladium porphyrin-fullerene conjugate.

Transient electron paramagnetic resonance (TREPR) spectroscopy is used to investigate the pathway and dynamics of electron transfer in a palladium porphyrin-fullerene donor-acceptor conjugate. The heavy Pd atom in the porphyrin greatly enhances the rate of intersystem crossing and as a result, electron transfer from the porphyrin to fullerene occurs via the porphyrin triplet state. The sign of the polarization pattern of the radical pair generated by the electron transfer is opposite in benzonitrile and the liquid crystal 5CB. This difference is the result of a change in sign of the spin-spin coupling, which allows the values of the dipolar and exchange couplings between the electrons in the charge-separated state to be estimated. In addition to the radical pair, signals from the fullerene triplet state are also observed. The polarization of the fullerene triplet state inverts with time, while the radical pair signal decays to a multiplet pattern that persists for times longer than the spin-lattice relaxation time. A kinetic model, developed to explain these effects, reveals that forward and reverse electron transfer between the charge-separated state and the fullerene takes place. This process, combined with singlet recombination of the radical pair accounts for the inversion of the fullerene triplet state polarization and the long-lived multiplet polarization of the radical pair.

Phosphorus(V) Porphyrin-Manganese(II) Terpyridine Conjugates: Synthesis, Spectroscopy, and Photo-Oxidation Studies on a SnO2 Surface.

A major challenge in designing artificial photosynthetic systems is to find a suitable mimic of the highly oxidizing photoactive species P680 in photosystem II. High-potential phosphorus(V) porphyrins have many attractive properties for such a mimic but have not been widely studied. Here, we report the synthesis and photophysical characterization of a novel phosphorus(V) octaethylporphyrin-oxyphenyl-terpyridine conjugate (PPor-OPh-tpy, 1) and its corresponding manganese(II) complex (PPor-OPh-Mn(tpy)Cl2, 2). The X-ray structure of 2 shows that the Mn(II) and P(V) centers are 11.783 Šapart and that the phenoxy linker is not fully conjugated with the terpyridine ligand. The porphyrin fluorescence in 1 and 2 is strongly quenched and has a shorter lifetime compared to a reference compound without the terpyridine ligand. This suggests that electron transfer from tpy or Mn(tpy) to the excited singlet state of the PPor may be occurring. However, femtosecond transient absorbance data show that the rate of relaxation to the ground state in 1 and 2 is comparable to the fluorescence lifetimes. Thus, if charge separation is occurring, its lifetime is short. Because both 1 and 2 are positively charged, they can be electrostatically deposited onto the surface of negatively charged SnO2 nanoparticles. Freeze-trapping EPR studies of 2 electrostatically bound to SnO2 suggest that excitation of the porphyrin results in electron injection from 1PPor* into the conduction band of SnO2 and that the resulting PPor•+ species acquires enough potential to photo-oxidize the axially bound Mn(II) (tpy) moiety to Mn(III) (tpy).

Modulation of Energy Transfer into Sequential Electron Transfer upon Axial Coordination of Tetrathiafulvalene in an Aluminum(III) Porphyrin-Free-Base Porphyrin Dyad.

Axially assembled aluminum(III) porphyrin based dyads and triads have been constructed to investigate the factors that govern the energy and electron transfer processes in a perpendicular direction to the porphyrin plane. In the aluminum(III) porphyrin-free-base porphyrin (AlPor-Ph-H2Por) dyad, the AlPor occupies the basal plane, while the free-base porphyrin (H2Por) with electron withdrawing groups resides in the axial position through a benzoate spacer. The NMR, UV-visible absorption, and steady-state fluorescence studies confirm that the coordination of pyridine appended tetrathiafulvalene (TTF) derivative (TTF-py or TTF-Ph-py) to the dyad in noncoordinating solvents afford vertically arranged supramolecular self-assembled triads (TTF-py→AlPor-Ph-H2Por and TTF-Ph-py→AlPor-Ph-H2Por). Time-resolved studies revealed that the AlPor in dyad and triads undergoes photoinduced energy and/or electron transfer processes. Interestingly, the energy and electron donating/accepting nature of AlPor can be modulated by changing the solvent polarity or by stimulating a new competing process using a TTF molecule. In modest polar solvents (dichloromethane and o-dichlorobenzene), excitation of AlPor leads singlet-singlet energy transfer from the excited singlet state of AlPor ((1)AlPor*) to H2Por with a moderate rate constant (k(EnT)) of 1.78 × 10(8) s(-1). In contrast, excitation of AlPor in the triad results in ultrafast electron transfer from TTF to (1)AlPor* with a rate constant (k(ET)) of 8.33 × 10(9)-1.25 × 10(10) s(-1), which outcompetes the energy transfer from (1)AlPor* to H2Por and yields the primary radical pair TTF(+•)-AlPor(-•)-H2Por. A subsequent electron shift to H2Por generates a spatially well-separated TTF(+•)-AlPor-H2Por(-•) radical pair.

Ultrafast charge separation and charge stabilization in axially linked 'tetrathiafulvalene-aluminum(III) porphyrin-gold(III) porphyrin' reaction center mimics.

The axial bonding ability of aluminum(III) porphyrin (AlPor) has been exploited to synthesize the vertically linked dyad 'aluminum(III) porphyrin-gold(III) porphyrin' (AlPor-Ph-AuPor(+)) and the two corresponding self-assembled triads 'tetrathiafulvalene-aluminum(III) porphyrin-gold(III) porphyrin' (TTF-py→AlPor-Ph-AuPor(+) and TTF-Ph-py→AlPor-Ph-AuPor(+)). The unique topology of these triads provides an excellent opportunity to investigate the sequential electron transfer in the perpendicular direction to the AlPor plane where the AlPor acts as a photosensitizer and primary electron donor while the AuPor and TTF serve as an electron acceptor and donor, respectively. The ground state properties of the dyad and triad suggest that there are no direct intramolecular interactions between the oppositely disposed AuPor and TTF units of the triad. However, the NMR and UV-visible absorption studies of the dyad reveal intermolecular interactions in non-coordinating solvents due to the coordination of counterion PF6(-) to the Al center of AlPor. Steady-state and femtosecond transient absorption studies of the dyad show that the lowest excited singlet state of AlPor ((1)AlPor*) is strongly quenched by ultrafast electron transfer to AuPor(+) with a time constant of 3.16 ps. The resulting charge separated state (AlPor(+)˙-AuPor˙) decays to ground state biexponentially with time constants of 27.26 and 2557 ps. Analogously, upon photo-excitation the triads also produce the same primary radical pair (AlPor(+)˙-AuPor˙). However, the formed radical pair is further involved in a rapid hole transfer from AlPor(+)˙ to TTF to form a stable final radical pair TTF(+)˙-AlPor-AuPor˙. The lifetime of the charge separated state exhibits an increase from 27.26 ps in AlPor-Ph-AuPor to 1393 ps in TTF-py→AlPor-Ph-AuPor(+) and 1484 ps in TTF-Ph-py→AlPor-Ph-AuPor(+). These results reveal successful charge stabilization in the self-assembled supramolecular reaction center mimics constructed via the axial linkage strategy.

Tuning Intramolecular Charge Transfer in Antimony(V) Porphyrin through Axial Fluorination.

Modulation of intramolecular charge transfer (ICT) has been tested in two antimony(V) porphyrins, SbT(DMP)P(OMe)2·PF6 and SbT(DMP)P(OTFE)2·PF6, where the meso-positions are occupied by 3,5-dimethoxyphenyl (DMP), and the axial positions are linked with either methoxy (OMe) or trifluoroethoxy (OTFE) units, respectively. The presence of the Sb(+5) ion makes the porphyrin center electron poor. Under this situation, placing electron-rich units in the meso-position creates a condition for push-pull type ICT in the SbT(DMP)P(OMe)2·PF6. Remarkably, it is shown that the ICT character can be further enhanced in SbT(DMP)P(OTFE)2·PF6 with the help of electron-withdrawing TFE units in the axial position, which makes the porphyrin center even more electron scarce. The steady-state and transient studies as well as solvatochromism studies establish the ICT in SbT(DMP)P(OMe)2·PF6 and SbT(DMP)P(OTFE)2·PF6, and the strength of the ICT can be modulated by exploiting the structural properties of antimony(V) porphyrin. The existence of ICT is further supported by density functional theory calculations. The transient studies show that upon excitation of these porphyrin, their charge-transfer states convert to a full charger-separated states with appreciable lifetimes.

Long-lived charge separation in novel axial donor-porphyrin-acceptor triads based on tetrathiafulvalene, aluminum(III) porphyrin and naphthalenediimide.

Two self-assembled supramolecular donor-acceptor triads consisting of Al(III) porphyrin (AlPor) with axially bound naphthalenediimide (NDI) as an acceptor and tetrathiafulvalene (TTF) as a secondary donor are reported. In the triads, the NDI and TTF units are attached to Al(III) on opposite faces of the porphyrin, through covalent and coordination bonds, respectively. Fluorescence studies show that the lowest excited singlet state of the porphyrin is quenched through electron transfer to NDI and hole transfer to TTF. In dichloromethane hole transfer to TTF dominates, whereas in benzonitrile (BN) electron transfer to NDI is the main quenching pathway. In the nematic phase of the liquid crystalline solvent 4-(n-pentyl)-4'-cyanobiphenyl (5CB), a spin-polarized transient EPR spectrum that is readily assigned to the weakly coupled radical pair TTF(.+)NDI(.-) is obtained. The initial polarization pattern indicates that the charge separation occurs through the singlet channel and that singlet-triplet mixing occurs in the primary radical pair. At later time the polarization pattern inverts as a result of depopulation of the states with singlet character by recombination to the ground state. The singlet lifetime of TTF(.+)NDI(.-) is estimated to be 200-300 ns, whereas the triplet lifetime in the approximately 350 mT magnetic field of the X-band EPR spectrometer is about 10 µs. In contrast, in dichloromethane and BN the lifetime of the charge separation is <10 ns.

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.

The electronic structure and dynamics of the excited triplet state of octaethylaluminum(III)-porphyrin investigated with advanced EPR methods.

The photoexcited triplet state of octaethylaluminum(III)-porphyrin (AlOEP) was investigated by time-resolved Electron Paramagnetic Resonance, Electron Nuclear Double Resonance and Electron Spin Echo Envelope Modulation in an organic glass at 10 and 80 K. This main group element porphyrin is unusual because the metal has a small ionic radius and is six-coordinate with axial covalent and coordination bonds. It is not known whether triplet state dynamics influence its magnetic resonance properties as has been observed for some transition metal porphyrins. Together with density functional theory modelling, the magnetic resonance data of AlOEP allow the temperature dependence of the zero-field splitting (ZFS) parameters, D and E, and the proton AZZ hyperfine coupling (hfc) tensor components of the methine protons, in the zero-field splitting frame to be determined. The results provide evidence that the ZFS, hfc and spin-lattice relaxation are indeed influenced by the presence of a dynamic process that is discussed in terms of Jahn-Teller dynamic effects. Thus, these effects should be taken into account when interpreting EPR data from larger complexes containing AlOEP.

Antimony(+5) ion induced tunable intramolecular charge transfer in hypervalent antimony(V) porphyrins.

The +5 oxidation state of antimony induced push-pull style intramolecular charge transfer in an elegantly designed axial dimethoxyantimony(V) porphyrin series: SbP(OMe)2·PF6, SbMP(OMe)2·PF6, SbDMP(OMe)2·PF6, SbTMP(OMe)2·PF6 with phenyl (P), 4-methoxyphenyl (MP), 3,5-dimethoxyphenyl (DMP), and 3,4,5-trimethoxyphenyl (TMP) units, respectively, in its meso positions. The Sb(+5) made the porphyrin ring electron-poor, whereas the methoxy groups on the phenyl unit produced electron-rich sites within the molecule. The presence of electron-poor and electron-rich parts in the same molecule resulted in a push-pull type intramolecular charge transfer (ICT). However, the ICT is strongly dependent on the position of the methoxy groups on the phenyl ring. The charge transfer character is more pronounced in meta-methoxy substituted antimony(V) derivatives (SbDMP(OMe)2·PF6, SbTMP(OMe)2·PF6) than the para-methoxy or no-methoxy substituted antimony(V) derivatives (SbP(OMe)2·PF6, SbMP(OMe)2·PF6). Steady-state and transient spectroscopic techniques, as well as solvatochromism techniques, were employed to establish the tunable ICT. Additionally, time-dependant density functional theory (TD-DFT) calculations were used to complement the experimental results. The systematic study of antimony(V) porphyrins, especially the tunable push-pull nature could play an important role in instigating high yield charge-separated states in multi-modular donor-acceptor systems for solar energy conversion and molecular electronic and photonic applications.

Sequential charge separation in two axially linked phenothiazine-aluminum(III) porphyrin-fullerene triads.

New supramolecular triads (PTZpy→AlPor-C(60), TPTZpy→AlPor-C(60)), containing aluminum(III) porphyrin (AlPor), fullerene (C(60)), and phenothiazine (phenothiazine = PTZ, 2-methylthiophenothaizine = TPTZ) have been constructed. In these triads the fullerene and phenothiazine units are bound axially to opposite faces of the porphyrin plane via covalent and coordination bonds, respectively. The ground- and excited-state properties of the triads and reference dyads are studied using steady-state and time-resolved spectroscopic techniques. The time-resolved data show that photoexcitation results in charge separation from the excited singlet state of the porphyrin to the C(60) unit, generating (Donor)py→AlPor(•+)-C(60)(•-), Donor = PTZ and TPTZ. A subsequent hole shift from the porphyrin to phenothiazine generates the charge-separated state (Donor)(•+)py→AlPor-C(60)(•-). The lifetime of the charge separation exhibits a modest increase from 39 ns in the absence of the donor to 100 ns in PTZpy→AlPor-C(60) and 83 ns in TPTZpy→AlPor-C(60). These lifetimes are discussed in terms of the electronic coupling between phenothiazine, the porphyrin, and C(60).