Sitting-atop Research Articles

Exploring the stereochemistry of In(III)-A3B-type porphyrins axially ligated with p-PDA: synthesis, electronic properties and DFT calculations

In(III)-A3B-type porphyrins containing para-phenylenediamine (p-PDA) as an axial moiety have been efficiently synthesized and characterized spectroscopically to procure the structures of the synthesized complexes. The axial coordination of the amine moiety with In(III) porphyrins brings about a modest shift in the 1H NMR signals and the electronic absorptions in CH3OH, CH3COCH3, CHCl3, and CH3COOC2H5 reflect the hyperchromicity of p-PDA-In(III) porphyrins in comparison to non-ligated metal chelates. All the synthesized macrocyclic porphyrins have negative solvatochromism with the blue shift in the electronic absorption spectra in more polar solvents like CH3OH than those in the less polar CHCl3 as solvent. The photoluminescence spectra show maximum excitonic peaks at 623, 620, and 615 nm for p-phenylenediamine-In(III)-5,10,15-trikis(p-methylphenyl)-20-(p-nitrophenyl)porphyrin [p-PDA-In-tPP1], p-phenylenediamine-In(III)-5,10,15-trikis(p-methoxyphenyl)-20-(p-nitrophenyl)porphyrin [p-PDA-In-tPP2] and p-phenylenediamine-In(III)-5,10,15-trikis(p-dimethylaminophenyl)-20-(p-nitrophenyl)porphyrin [p-PDA-In-tPP3], respectively, which confirmed that the In(III) porphyrins are considered to be sitting atop (SAT) or out of the plane (OOP) type complexes. Further, the stability of the complexes was determined through thermogravimetric analysis, and the structural evolutions through different phases of the synthesized porphyrins were deduced from the powder X-ray diffraction. To get more insight into the molecular structures and their complexes, we carried out the quantum mechanical calculations using DFT-based dispersion which showed that from OH− to p-PDA substitution, the BDE energies drop considerably.

Push‐Pull Effect at Formation of Sitting‐Atop Metal‐Porphyrin Complex in Solvating Media: H‐Bonding and Electrostatic Repulsion

AbstractThis work presents the results of experimental and theoretical studies of the solvation effects on the kinetics of the porphyrins reaction with zinc and cadmium acetate, the structure and characteristics of the intermediate ‐ sitting‐atop (SAT) complex. Kinetic rate constants of metal complexes of tetra‐p‐substituted tetraphenylporphins formation in binary solutions with a regulated solvating ability were obtained. Extreme dependences of the rate constants on the solvent composition were found. The acceleration of the reaction was shown to be observed with an enhancement of the electron‐donating properties of the substituent. These data indicate the formation of the SAT complex as limiting stage. The structure and energy parameters of porphin‐zinc SAT complexes and its solvates were studied by DFT method using B3LYP and long range corrected CAM‐B3LYP hybrid functionals. A significant distortion of the porphin structure during the formation of an intermediate and its solvation was ascertained. The influence of these processes on structural and energy changes was founded to be the consequence of push‐pull effect due to the repulsion of NH hydrogen atoms by a metal ion (push) and stretching of these bonds (pull) during the formation of H‐bonds.

A hypervalent Bismuth(III) derivative of Tetrakis-4-methoxyphenyl porphyrin: Synthesis, spectroscopic characterisation and morphological investigations

In this study, we aimed to synthesize as well as spectroscopically characterize a sitting-atop (SAT) macro molecule of Tetrakis-4-methoxyphenylporphyrinato Bi(III). The bismuth porphyrin was prepared by the reaction of free base macrocycle, 5, 10, 15, 20-Tetrakis-4-methoxyphenylporphyrin H 2 TMP ( I ) with bismuth nitrate pentahydrate, Bi(NO 3 ) 3 ·5H 2 O in dry pyridine. The spectroscopic data demonstrates the existence of a binuclear moiety [Bi(TMP)NO 3 ] 2 (II) in the solid state and even in solution in which two bismuth porphyrins are interlinked by a bridging bidentate nitrate. Self dimerisation of bismuth porphyrin is expected to arise due to strong tendency of Bi 3+ ions to become hypervalent. The electronic absorption as well emission bands in complex ( II ) are bathochromically shifted relative to compound ( I ) with emission bands significantly quenched in the metal chelate. At pH ∼5, complex ( II ) is demetallized to form the dicationic analogs of I which tend to undergo prominent J-type supramolecular aggregation. Demetallation is also followed when a dichloromethane solution of metal chelate ( II ) is kept for a long period at room temperature. The macro rings were also studied by nano technology like SEM, TEM, EDX and powder XRD. Thermal analysis and invitro cytotoxicity screening of the macro rings against leukaemia K 562 strains have been further demonstrated. • A sitting-atop (SAT) macro molecule of Tetrakis-4-methoxyphenylporphyrinato Bi(III) has been synthesized. • Due to hypervalent nature of Bi 3+ centre, the bismuth porphyrin tends to exists as binuclear moiety. • TEM images reflect mutilayered nanosheets of synthesized complex. • Bismuth complex exhibit high antitumor activity against leukaemia K 562 strains relative to its free base ligand. • Due to out of plane coordination mode, the synthesized complex have low thermal stability.

Synthesis and spectroscopic properties of axial phenoxide and para amino phenoxide incorporated indium (III) porphyrins

Tetrakis-4-methoxyphenyl indium(III) porphyrins containing phenolic and para amino phenolic moieties in the apical sites have been successfully prepared and characterized spectroscopically. The phases and structure evolutions of prepared porphyrins were deduced from powder X-ray diffraction (PXRD). The axial coordination of phenolic moieties induces slight shift of the 1H NMR signals of indium porphyrin macrocycles. The measurements of UV–Visible absorptions of porphyrins in CH2Cl2 and CH3COOC2H5 reflects that axially ligated metal chelates at neutral pH display hyperchromicity of absorption bands compared to unligated metal porphyrin. The most striking feature was the structural stability of indium porphyrins under basic (pH = 10) as well as acidic media (pH = 4). On the other hand, the metal free ligand 5, 10, 15, 20-Tetrakis-(4-methoxyphenyl) porphyrin, H2TMP (I) undergoes J type aggregation in acidic media (pH = 4). The robustness of indium porphyrins especially in acidic media are in strong contrary to other sitting atop (SAT) or out of plane (OOP) complexes which are usually demetallized under acidic pH. All the prepared macro rings exhibit negative solvatochromism reflected by the blue shift of absorption bands in CH3COOC2H5 (more polar) than in CH2Cl2 (less polar). The photoluminescence emissions in indium(III) species are significantly quenched relative to metal free ligand (I) due to internal heavy atom effect and distortion of ligand plane. The quenching of fluorescence bands are more pronounced in axially ligated metal porphyrins. The lesser excited state lifetimes of indium(III) porphyrins relative to free base analog (I) may be ascribed to spin orbit coupling induced by indium(III) ion in the respective metal chelates.

Structure model of ferrochelatase from Salmonella Typhi elucidating metalation mechanism.

A homology model of ferrochelatase (HemH), the heme biosynthesis terminal step enzyme from Salmonella Typhi was generated to understand the mechanism of metal insertion into protoporphyrin IX for heme biosynthesis. The overall fold of membrane associated ferrochelatase (StFc) from S. Typhi is similar to human and Yeast ferrochelatase than Bacillus subtilis, and Bacillus anthracis. An insertion of 16 amino acid residues in helical switch having hydrophobic patch proposed to interact with membrane lipids and in opening and closing of heme binding cleft. The sequence analysis and the docking study revealed that the protoporphyrin binding site in StFc has a crucial replacement of Tyr/Met to Leu13 unique in comparison to other known structures, where Tyr13 observed in B. subtilis/B. anthracis while Met76 in human/yeast play important role in holding protoporphyrin in optimal orientation for metalation. A sitting-a-top (SAT) complex mechanism for metalation is proposed with His194 and Glu264 lie at the bottom and Leu13 on the top of the porphyrin ring. In addition, an entry and exit mechanism is also proposed for protoporphyrin binding into cavity by opening and closing of helical switch using molecular dynamics simulation studies of Apo and heme complexed model structure of S. Typhi HemH.

Synthesis, thermoanalytical and spectroscopic characterization of newly synthesized macrocyclic complexes of thallium(III) and tin(IV)

Spectroscopic, thermal and fluorescence properties of various thallium and tin complexes, X-Tl(III)t(4-Y)PP and X-Sn(IV)t(4-Y)PP where X = 5-SO3H-2-OHOC6H4,(5-SSA); 5-Cl-2-OHOC6H4,(5-CSA); and 5-NH2-2-OHOC6H4,(5-NSA) and Y = –Cl, –OCH3, are investigated in detail. Analysis of the spectral data [IR, UV–Vis and NMR (1H and 119Sn)] of complexes suggests that the two trans-axial ligands are strongly bound in a symmetric manner at the tin center in these complexes. Whereas the absorption and emission studies of thallium complexes characterized the sitting-Atop (SAT) feature of the complexes, the fluorescence lifetime of the tin complexes is found to be higher than thallium complexes. The structure and morphology of complexes were studied by X-ray powdered diffraction analysis. From the X-ray diffractograms, it could be revealed that the complexes are crystallized in mixed crystalline forms. Thermal studies of these porphyrins were carried out in an argon atmosphere from room temperature to 800 °C using thermal analyzer. Evaluation of the thermal analysis of the two metal complexes reveals that the tin(IV) porphyrin complexes are more stable than thallium(III) complexes. Some of the synthesized complexes of thallium and tin have been screened for biological activity against B. subtilis, M. luteus, S. aureus, P. florescens and E. coli by agar well diffusion methods that were found to be inactive against bacterial strains.

Human Ferrochelatase: Insights for the Mechanism of Ferrous Iron Approaching Protoporphyrin IX by QM/MM and QTCP Free Energy Studies.

Ferrochelatase catalyzes the insertion of ferrous iron into protoporphyrin IX, the terminal step in heme biosynthesis. Some disputes in its mechanism remain unsolved, especially for human ferrochelatase. In this paper, high-level quantum mechanical/molecular mechanics (QM/MM) and free-energy studies were performed to address these controversial issues including the iron-binding site, the optimal reaction path, the substrate porphyrin distortion, and the presence of the sitting-atop (SAT) complex. Our results reveal that the ferrous iron is probably at the binding site coordinating with Met76, and His263 plays the role of proton acceptor. The rate-determining step is either the first proton removed by His263 or the proton transition within the porphyrin with an energy barrier of 14.99 or 14.87 kcal/mol by the quantum mechanical thermodynamic cycle perturbation (QTCP) calculations, respectively. The fast deprotonation step with the conservative residues rather than porphyrin deformation found in solution provides the driving force for biochelation. The SAT complex is not a necessity for the catalysis though it induces a modest distortion on the porphyrin ring.

Synthesis, structure, and reactions of iridium(I) complexes with 5,10,15,20-tetraphenyl-21H,23H-porphine and 5,10,15,20-tetraphenyl-21H,23H-porphine dianion

One-step joint synthesis of two iridium porphyrin complexes, a donor-acceptor SAT (sitting a top) complex μ-(5,10,15,20-tetraphenylporphine)-bis-chloroiridium(I) and the covalent complex (5,10,15,20-tetraphenylporphinato)chloroiridium(III) by the reaction of free porphyrin and chloroiridic acid (H3O)2IrCl6 in boiling phenol was studied. The structure of complexes was confirmed by spectroscopy (UV/Vis, IR, 1H NMR) and TLC. The iridium(III) SAT complex with the hydride ligand in the first coordination sphere, (acetato)hydrido(5,10,15,20-tetraphenylporphine)iridium(III), was obtained by oxidative addition reaction, which is quite rare for porphyrin complexes. The thermodynamic stability of the complexes to oxidants (aerated acids) was studied by spectrophotometric titration.

Water-soluble silver(II) and gold(III) porphyrins: The effect of structural distortion on the photophysical and photochemical behavior

Abstract Equilibrium characterization, magnetic susceptibility and fluorescence measurements proved that the silver(II) porphyrin complex formed in the reaction of the Ag+ ion and the 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin ligand is of sitting-atop (SAT) type. The complex also exhibits typical SAT features as regards its photoredox degradation reactions, which might be utilized in photodynamic therapy. On the contrary, the corresponding gold(III) porphyrin is unambiguously of planar structure, which does not show any fluorescence and photoinduced degradation. The deviating structures, and hence, photoinduced behaviors of these complexes can be attributed to the effect of the different ionic radii (rion(Ag2+) = 94 pm; rion(Au3+) = 85 pm).

A valuable visual colorimetric and electrochemical biosensor for porphyrin

Porphyrin is able to specifically combine with phosphorus, thus a novel bifunctional sensing platform for determination of porphyrin by visual colorimetry and electrochemistry was demonstrated. A pretreated gold sheet (or electrode) with 2-mercatpoethanol (2-ME) was chemically modified by POCl 3 to obtain the surface phosphate active sites. The different stages of modified electrode were characterized by electrochemical impedance spectroscopy (EIS). The 1:1 cationic sitting-atop (SAT) complex P(V)–porphyrin was formed due to the high affinity of the modified gold sheet (or electrode) towards the porphyrin, resulting in electron transfer resistance increase of the electrode surface. Meanwhile, a dramatic color changing from burgundy to dark green of porphyrin solution was observed with the naked-eye within 3 s. What's more, this was reflected by the notable change of the Soret band of porphyrin when using UV–vis. Two sensing systems provide different sensitivity for porphyrin analysis. With visual colorimetry, porphyrin can be detected at a level of 1.0 × 10 −6 M, whereas the detection limit of porphyrin is 3.0 × 10 −8 M using the EIS method. The practical application of the sensor to determination of pheophytin which was obtained from fresh spinach leaves has been accomplished. The results demonstrate the facility and effectivity of our introduced bifunctional biosensor for quantitative analysis of porphyrin.

Synthesis, Characterisation and Spectroscopic Studies of Sitting-Atop Adducts of Dimethyl- and Diethyltin(IV) Dichlorides with meso-Tetraalkylporphyrins

The formation of the sitting-atop (SAT) adducts of meso-tetra( n-propyl)porphyrin and meso-tetra( tert-butyl)porphyrin with dimethyl- and diethyltin(IV) dichlorides has been investigated. These adducts have the general formula [(R2SnCl2)2H2T( n-propyl)P] and [(R2SnCl2)2H2T( tert-butyl)P]; [R = Me and Et] and have been characterised by means of 1H NMR, UV-Vis and elemental analysis methods.

Synthesis and Spectroscopic Characterization of the New Sitting-atop Complexes from Reaction of Zirconyl Nitrate and Free Base meso-Tetraarylporphyrins in Mild Conditions

A series of sitting-atop (SAT) complexes, [(ZrO)<TEX>$H_2t(X)pp(NO_3)_2$</TEX>], have been prepared via the reactions of free base meso-tetraarylporphyrins, <TEX>$H_2$</TEX>t(X)pp, with zirconyl nitrate hydrate, ZrO(N<TEX>$O_3)_2{\cdot}xH_2$</TEX>O. The products have been characterized by a variety of methods including <TEX>$^1H\;NMR,\;^{13}C$</TEX> NMR, IR and UV-Vis spectroscopies, elemental analysis and conductance measurements. The data indicate that the meso-tetraarylporphyrins coordinate with two pyrrolenine nitrogen atoms to the zirconyl cation located above the distorted porphyrin plane and two protons remain on the pyrrole nitrogens. Such half sandwich-type sitting-atop complexes may be considered as models for the initial steps of the metallation of the macrocycles.

Sitting-Atop Metallo-Porphyrin Complexes: Experimental and Theoretical Investigations on Such Elusive Species

The interaction between the sodium cation and two meso-aryl porphyrins (tetraphenylporphyrin, TPP, and tetra(4-methoxyphenyl)porphyrin, TMPP) leads to the formation of new species that have been identified as Sitting-Atop (SAT) complexes, where the metal ion interacts with the N atoms of the porphyrin core without the concomitant deprotonation of the N-H groups. These species have been attained in low polarity solvent through the interaction of the porphyrin free bases with sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (NaTFPB), and investigated in situ through a combination of spectroscopic techniques, such as UV/vis absorption and fluorescence (static and time-resolved), resonance light scattering, FT-IR, and (1)H NMR. All spectroscopic evidence points to the occurrence of a single equilibrium between each parent compound and its SAT complex, ruling out the presence of other metallo-, protonated, or aggregated porphyrins in solution. The 1:1 stoichiometry of the adducts has been determined via continuous variation method (Job's plot), and an estimate of the corresponding association constants in CH(2)Cl(2) at 298 K have been obtained by UV/vis titration (K(eq) = (9 +/- 4) x 10(5) L mol(-1) and (5 +/- 2) x 10(6) L mol(-1) for TPP and TMPP, respectively). Density-functional theory (DFT) calculations on SAT model complexes, [NaTPP(PF(6))] and [NaTMPP(PF(6))], have provided information on the molecular structure of these elusive species and on the nature and strength of the sodium-porphyrin interaction. It is found that the sodium ion is bound to the four nitrogen atoms of the porphyrin core. The involvement of the pyrrolic N atoms results in a modest but not negligible elongation of the N-H bonds, pyramidalization of the hydrogen atoms, and blue shift of the N-H stretching frequencies. Electronic structure and energy decomposition analysis reveal that covalent interactions, mainly consisting of porphyrin to sodium charge transfer interactions, are an important component of the sodium-porphyrin bond. Time-dependent DFT (TDDFT) calculations of the lowest excited states of the model systems provide an unambiguous interpretation of the absorption and emission properties of the experimentally identified SAT complexes.

Spectrophotometric studies of the thermodynamics of sitting-atop complexation between free base meso-tetraarylporphyrins and titanium(IV) chloride

Titanium(IV) chloride reacts with free base meso-tetraarylporphyrin and its ortho, meta and para-substituted derivatives (H 2T(X)PP; X: OCH 3, CH 3 and Cl) for formation of sitting-atop (SAT) complexes, [TiCl 4(H 2T(X)PP)]. The computer fitting of the variation of the absorbance versus mole ratio by KINFIT program was used for calculation of the formation constants of these complexes in chloroform. Thermodynamic parameters, Δ G°, Δ H°, Δ S°, have been determined and the influence of the temperature and the substituted aryl groups (electronic and steric effects) in the free base porphyrins on the stability of the SAT complexes was studied.

Thermodynamic studies of sitting-atop (SAT) complexation of uranyl and free base meso -tetraarylporphyrins

The interaction of uranyl with meso-tetraphenylporphyrin and its para-substituted derivatives (H2t(4-X)pp, X : H, Br, Cl, CH(CH3)2, OCH3, CH3) in chloroform produced 1 : 1 sitting-atop (SAT) complexes ((uranyl)H2t(4-X)pp). Formation constants were calculated by computer fitting of complex absorbance versus mole ratio data to appropriate equations and found to decrease with temperature increase. Thermodynamic parameters, ΔG 0, ΔH 0 and ΔS 0 were obtained. The formation constants vary with changing of the substituent on the aryl rings of H2t(4-X)pp in the following order: (uranyl)H2t(4-OCH3)pp > (uranyl)H2t(4-CH3)pp > (uranyl)H2t(4-CH(CH3)2)pp > (uranyl)H2tpp > (uranyl)H2t(4-Br)pp > (uranyl)H2t(4-Cl)pp.

Sitting-atop complex formation of free base meso-tetraarylporphyrins with zirconium(IV) chloride

We have succeeded in the preparation and spectroscopic characterization of sitting-atop (SAT) complexes of meso-tetraarylporphyrins (H 2t(X)pp) with zirconium(IV) chloride under mild conditions and at room temperature, where two pyrrolenine nitrogens in the SAT complexes, [(H 2t(X)pp)ZrCl 4], coordinate to a zirconium atom and two protons still remain on the pyrrole nitrogens. UV–Vis and NMR ( 1H and 13C) spectral data show that the porphyrin core of the SAT complexes is distorted and two pyrrolenine nitrogen atoms of the porphyrin act as electron donors to the zirconium atom of ZrCl 4.

Thermodynamic studies of sitting-atop complexation between free base meso-tetraarylporphyrins and antimony(III) chloride in chloroform

Interaction of para, meta and ortho-substituted meso-tetraarylporphyrins, (H2t(X)pp, X: OMe, Me, H and Cl) with SbCl3 in chloroform solution afforded 1 : 1 sitting-atop (SAT) complexes ([(SbCl3)(H2t(X)pp)]). The formation constants were calculated by KINFIT and found to decrease with increasing temperature. The thermodynamic parameters, ΔH°, ΔS° and ΔG°, were obtained. Formation constants of these complexes change with changing substituent (X) on the aryl rings of H2t(X)pp in the following order: (SbCl3)H2t(4-OMe)pp > (SbCl3)H2t(4-Me)pp > (SbCl3)H2tpp > (SbCl3)H2t(4-Cl)pp > (SbCl3)H2t(3-OMe)pp > (SbCl3)H2t(3-Me)pp> (SbCl3)H2t(2-OMe)pp > (SbCl3)H2t(2-Me)pp.

Formation, photophysics and photochemistry of thallium(III) 5,10,15,20-tetrakis(4-sulphonatophenyl)porphyrin: New supports of typical sitting-atop features

In aqueous solutions of thallium(III) and 5,10,15,20-tetrakis(4-sulphonatophenyl)porphyrin a kinetically labile metalloporphyrin of composition 1:1 (Tl IIIP 3−) is formed in a moderately fast bimolecular reaction ( k = 270 M −1 s −1). The formation constant of this sitting-atop (SAT) complex is rather high ( K′ = 9.5 × 10 6 M −1 at pH 6), due to the not too large size (compared to the core of the ligand) and the three-fold positive charge of Tl(III). The strong coordinative bond is manifested in the large shifts of the emission and absorption bands of the porphyrin. The directions of these shifts characteristically depend on the structure of the metalloporphyrin, making the identification of the sitting-atop type unambiguous. Compared to the corresponding values of the free-base porphyrin, the dramatically diminished fluorescence quantum efficiency of Tl IIIP 3− ( Q fl = 0.00079 versus 0.075) can be accounted for by the heavy-atom effect and the distortion to saddle shape. Both Soret- and Q-band irradiations of the Tl IIIP 3− complex lead to the degradation of the porphyrin with quantum yields of (1–3) × 10 −5 magnitude in both aerated and argon-saturated solutions. The primary photochemical step in this process is ligand-to-metal-charge-transfer reaction followed by the detachment of the reduced metal center. The latter step is favoured by the out-of-plane position of the metal in the case of SAT complexes. Subsequent to the demetallation, an irreversible ring-opening of the oxidized porphyrin takes place. The results on the photolysis of the Tl IIIP 3− complex confirm that the overall reaction is a four-electron process. Reversible dissociation of the Tl IIIP 3− complex also occurs upon irradiation.

Interpretation of EXAFS spectra for sitting-atop complexes with the help of computational methods

The metallation of tetrapyrroles is believed to proceed via a sitting-atop (SAT) complex, in which some of the pyrrole nitrogen atoms are protonated and the metal ion resides above the ring plane. No crystal structure of such a complex has been presented, but NMR and EXAFS (extended X-ray absorption fine structure) data has been reported for Cu 2+ in acetonitrile, which have been interpreted as the observation of a SAT complex. However, this interpretation has been challenged and other investigations have shown that there are many possible SAT structures. We have recently developed a method to combine quantum mechanical (QM) calculations and EXAFS fits (EXAFS/QM), which in principle is a standard EXAFS fit that employs all multiple-scattering information in an optimum and self-consistent way and uses the QM calculations to ensure that the obtained structures are chemically reasonable. By this approach, we show that out of the 15 putative SAT complexes, structures with the copper ion coordinating to two cis pyrrolenine nitrogen atoms and two or three acetonitrile molecules fit the experimental EXAFS spectrum best. However, an equally good fit can be obtained also by a mixture of the reactant and product complexes.

Photophysics and photochemistry of water-soluble, sitting-atop bis-thallium(I) 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin.

In aqueous solutions, thallium(I) ions and 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin form a kinetically labile metalloporphyrin of 2 : 1 composition (Tl(2)P(4-)). The formation constant of this sitting-atop (SAT) complex is relatively low (beta2/[H+]2= 3.55 x 10(3) M(-2) at pH = 7), due to the large size and rather small charge of Tl+. As a consequence of the considerably weak metal-ligand interaction in this system, the 1 : 1 species does not appear in detectable concentration. Both the absorption and the emission properties of the Tl(2)P(4-) complex are characteristic for the typical SAT metalloporphyrins. Compared to the corresponding values of the free-base porphyrin, the diminished fluorescence quantum efficiency (Qfl= 0.0131 vs. 0.056) of Tl(2)P(4-) can be accounted for by the heavy-atom effect, while the larger Stokes shift (442 vs. 282 cm(-1)) indicates a stronger distortion of the ligand plane. Both Soret- and Q-band irradiations of the Tl(2)P(4-) complex lead to the degradation of the porphyrin with quantum yields of magnitude 3 x 10(-4). The primary photochemical step in this process is ligand-to-metal charge transfer reaction, which is unusual for normal (coplanar) metalloporphyrins. In the case of SAT complexes, the kinetic lability facilitates the separation of the primary redox products, followed by an irreversible ring-opening of the oxidized porphyrin. Photoinduced electron ejection as a considerable step in the degradation mechanism could be ruled out.