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A new class of salen functionalized novel ligand N,N′-Bis(salicylidene)-9-(3,4-diaminophenyl)acridine, H2daasal (two potential intercalators, salen and acridine, which are linked together via C–C bond) and its metal complex [Ni(daasal)] have been synthesized and characterized by X-ray diffraction, elemental analysis, UV–Vis, IR, 1H NMR, MALDI-TOF mass spectrometry and cyclic voltammetry methods. From the crystal structure, the salphen moiety is not coplanar with the acridine ring, having a dihedral angle of 68° and 76° in both H2daasal and [Ni(daasal)], respectively. [Ni(daasal)] shows a reversible oxidation cyclic voltammetric response near 1.03 V versus SCE in CH3CN (0.1 M TBAP) assignable to the Ni(II)/Ni(III) couple. The interaction of these compounds with calf-thymus (CT) DNA was examined using spectroscopic and viscosity measurements. These studies reveal that both compounds bind to CT DNA via an intercalative mode. Molecular-modeling studies also support an intercalative mode of binding to the model duplex d(CGCAATTGCG)2 possibly from the major groove with a slight preference for GC rich region. Additionally, [Ni(daasal)] promotes the cleavage of plasmid pBR322 DNA upon irradiation under terminal oxidant Oxone.

In our search for new DNA intercalating ligands, a novel bifunctional intercalator 11-(9-acridinyl)dipyrido[3,2-a:2′,3′-c]phenazine, acdppz (has two potentially effective intercalators via dipyridophenazine(dppz) and acridine which are linked together via C–C bond) and its corresponding Ru(II) polypyridyl complex [Ru(phen)2(acdppz)]2+ (where phen = 1,10-phenanthroline) have been synthesized and characterized. The electrochemical behaviors of the ligand and its complex have been thoroughly examined. The structure of acdppz and [Ru(phen)2(acdppz)]2+ were determined by X-ray crystallography. From the crystal structure of the complex, we found that the dppz moiety is not coplanar with the acridine ring, having a dihedral angle of 64.79 in the acdppz. The selected bond lengths and angles for the crystal structure of [Ru(phen)2(acdppz)]2+ were compared to the geometry-optimized molecular structure of [Ru(phen)2(acdppz)]2+ derived by Gaussian. The interaction of [Ru(phen)2(acdppz)]2+ with calf-thymus (CT) DNA was investigated by absorption and viscometry titration, thermal denaturation studies. The above measurements indicated that the complex binds less strongly with the CT DNA due to the intercalation by the ruthenium bound acdppz with an intrinsic binding constant of 2.6 × 105 M−1. Molecular-modeling studies also support an intercalative mode of binding of the complex to the model duplex d(CGCAATTGCG)2 possibly from the major groove with a slight preference for GC rich region. Additionally, the title complex promotes the cleavage of plasmid pBR322 DNA upon irradiation under aerobic conditions.

A bis(terpyridine)ruthenium(II) complex ([Ru]2+) was covalently connected via a floppy - OCH 2 CH 2 O - spacer to the free-base porphyrin (H) or zinc(II) porphyrin (Zn) or both, to obtain dyads ([HRu]2+, [ZnRu]2+) and triads ([HRuH]2+, [ZnRuH]2+, [ZnRuZn]2+). These compounds have been fully characterized by MALDI, UV-vis, 1 H NMR (1D and 1 H -1 H COSY) spectroscopies, and also by the cyclic and differential pulse voltammetric techniques. Absorption spectroscopy of these newly synthesized compounds shows that significant exciton coupling exists in non-polar solvents (cyclohexane and toluene) between the porphyrin ring and the bis(terpyridine)ruthenium(II) complex. Upon excitation within the Soret band of [HRu]2+/[HRuH]2+, free-base porphyrin fluorescence was found to be strongly quenched in non-polar and weakly quenched in polar solvents, probably due to ‘singlet-triplet’ energy transfer from the free-base porphyrin to the [Ru]2+ complex. Whereas, in [ZnRu]2+/[ZnRuZn]2+, zinc(II) porphyrin fluorescence was quantitatively and reasonably quenched in non-polar and polar solvents, respectively by mainly electron transfer from the zinc(II) porphyrin to the [Ru]2+ complex. The solvent plays a crucial role in the photophysical properties of these compounds, since the energy of the triplet metal-to-ligand charge-transfer (3MLCT) excited state is influenced by the polarity of the medium. Finally, [ZnRuH]2+ exhibits the combined fluorescence properties of [HRu]2+ and [ZnRu]2+ but the observed additional quenching in non-polar solvents for the zinc(II) porphyrin component is explained by energy transfer from the zinc(II) porphyrin to the free-base porphyrin and/or the bis(terpyridine)ruthenium(II) complex.

Two-photon absorption (2PA) and excited state absorption (ESA) properties of axial-bonding type hybrid porphyrin arrays are investigated in the picosecond regime under single-photon resonant excitation condition. A crossover from reverse saturable absorption (RSA) to saturable absorption (SA) and back to RSA is observed with the increase of excitation intensity. In the corresponding intensity ranges, third- and fifth-order phase conjugate signals from degenerate four wave mixing are observed. These observations are explained using rate equations for population densities in a three-level energy scheme. At higher intensities, 2PA is found to be dominant contributor to nonlinear absorption compared to ESA. � 2005 Elsevier B.V. All rights reserved.

A series of nucleobase appended tri-cationic porphyrins ( AT 4, TT 4, GT 4 and CT 4) have been synthesized and their binding and cleaving ability of DNA were investigated in this present study. UV-vis, fluorescence, circular dichroism and thermal melting studies were carried out to investigate binding of the porphyrin with calf thymus DNA (CTDNA). The apparent binding constant ( K app ) and the intrinsic binding constant ( K b) values calculated from UV-vis and fluorescence titrations, respectively, were comparable to 5,10,15,20-tetra(N-methylpyridinium-4-yl) porphyrin ( H 2 T 4) or slightly more for few compounds. Circular dichroism spectra of these porphyrins with DNA show variation in their mode of binding. Among the nucleobase appended tri-cationic porphyrins, the porphyrin in GT4 and TT4 exhibit strong intercalating ability in the DNA duplex whereas the porphyrin in AT 4 and CT 4 exhibit very less ability for intercalation. The intercalating efficiency of the porphyrin in GT 4 is even stronger than that of the H 2 T 4. The steric strain exists between the tri-cationic porphyrin moiety and the DNA may decrease depending on (a) the ease to exhibit keto-enol tautomerism due to the more acidic protons in guanine and thymine, (b) extended conjugation and (c) hydrogen bonding capability of the nucleobase moieties. Photocleavage proclivities with pBR322 DNA in the presence of light reveal that AT 4, TT 4 and CT 4 show same amount of cleavage but GT 4 shows more cleaving efficiency than H 2 T 4 due to the stacked guanine moieties those are more reducing than a single guanine residue. The singlet oxygen mechanism that is responsible for the photocleavage was confirmed with various inhibitors and interestingly with Tiron (disodium salt of 1,2-dihydroxy-3,5-benzenedisulfonic acid) the photocleaving efficiency increases due to the inhibition of superoxide formation.

The nonlinear absorption properties of ‘axial-bonding’ type hybrid porphyrin arrays based on a tin(IV) tetratolylporphyrin (SnTTP) scaffold are studied with picosecond and nanosecond pulses. The effect of different central metal atoms substituted adjacent to the tin(IV) porphyrin in the oligomer structure is discussed. In the picosecond regime the lifetimes of the excited singlet states and two-photon absorption (TPA) processes dominate leading to interesting switching of nonlinear absorption behaviour. The TPA cross-section ( σ TPA ) is found to be as high as 396 × 10 −46 cm 4 s photon −1 molecule −1 , for an oligomer with Sn and Ni porphyrin macrocycles. However, in the nanosecond regime the optical limiting performance has increased considerably with increasing number of porphyrins in the array and excited state absorption is found to play a major role.

2-(9-Anthryl)-1H-imidazo[4,5-f][1,10]phenanthroline (aip) and [2-(1-pyrenyl)-1H-imidazo[4,5-f][1,10]phenanthroline (pyip) and their complexes [Ru(phen)2(aip)]2+ (1) and [Ru(phen)2(pyip)]2+ (2) (phen = 1,10-phenanthroline) have been synthesized and characterized by elemental analysis, UV/Vis, IR and 1H NMR spectroscopy, FAB-MS and cyclic voltammetry methods. Both 1 and 2 are redox-active and undergo oxidation at 1.43 and 1.47 V (vs. SCE), respectively. Both 1 and 2 show only the 3MLCT emission typical of any polypyridylruthenium(II) complex. These new complexes are seen to be excellent binders of calf-thymus (CT) DNA, as revealed by the results of absorption, luminescence, viscometric titrations, and thermal denaturation studies. These results also indicate that while ruthenium-bound aip and pyip intercalate within the base pairs of the duplex, the phen moieties act only as spectator ligands in each case. Steady-state emission studies carried out in aqueous media with and without DNA reveal that [Ru(phen)2(pyip)]2+ is a better “molecular light switch” for DNA than [Ru(phen)2(aip)]2+. Under an identical set of experimental conditions of light and drug dose, the DNA photocleavage ability of [Ru(phen)2(pyip)]2+ is also higher than that of [Ru(phen)2(aip)]2+. “Inhibitor studies” suggest that the oxygen species are mainly responsible for the DNA photocleavage by these complexes. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Free-base (H2L2), copper(II) (CuL2) and zinc(II) (ZnL2) derivatives of a porphyrin-anthraquinone conjugate with an azomethine group separating the two photoactive subunits have been synthesized and characterized by mass (FAB), IR, UV-visible, 1H NMR and ESR spectroscopic techniques and also by cyclic and differential pulse voltammetric methods. Analysis of the data reveals that the spectral and electrochemical properties of the individual chromophoric entities are retained and that there is no specific π-π interaction between the porphyrin and anthraquinone subunits. H2L2 and ZnL2 are shown to exhibit substantial quenching (88–97%) of the porphyrin fluorescence compared to their corresponding monomeric analogues. An intramolecular electron-transfer mechanism is proposed for the substantial decrease in fluorescence in both derivatives. The fluorescence decays of porphyrin-anthraquinone conjugates are fit to 2/3 exponentials and indicate that multiple orientations of the porphyrin and anthraquinone groups contribute to the electron-transfer event. These results are in good agreement with steady-state fluorescence results. From the time-resolved fluorescence data, the electron-transfer rate constants are calculated, indicating kET values in the range of 1·1 × 109 to 9·9 × 1010 s−1 that are dependent upon the solvent.

Two dipyrrolyl derivatives, 2,3-di(1H-2-pyrrolyl)-7,12-dihydronaphtho[2,3-f]quinoxaline-7,12-dione (1) and 5,6-di(1H-2-pyrrolyl)-2,3-pyrazine-dicarbonitrile (2), bearing electron-withdrawing quinone or dicyano subunits, have been synthesized and fully characterized by various spectroscopic and electrochemical methods. Both 1 and 2 are specific binders of F- in organic solvents and show dramatic, binding-induced changes in their color (observable in the naked-eye experiments) and also optical and electrochemical signatures. These F--induced color changes remain the same even in the presence of a large excess of Cl-, Br-, I-, or ClO4-, thus rendering 1 and 2 to be efficient fluoride ion sensors. While Ka for F- binding by receptor 1 is ∼1.6 × 104 M-1, that for receptor 2 is an order of magnitude higher. 1H NMR titrations were carried out to monitor the binding of 1/2 with F-. These experiments not only provide evidence for the hydrogen-bonding interaction between the pyrrolic NH groups of these receptors an...

The reactions of OH, O −, SO4 −, N3 and eaq− with the ligands-1,10-phenanthroline (phen) and 6,7-dicyanodipyrido [2,2-d:2′,3′-f] quinoxaline (dicnq) and a series of their Ru(II) complexes ([Ru(phen)3]2+, [Ru(phen)2(dicnq)]2+, [Ru(phen)(dicnq)2]2+ and [Ru(dicnq)3]2+) have been studied by pulse radiolysis with optical detection. The OH and hydrated electron react with the ligands via adduct formation with diffusion controlled rates. The transient spectra obtained in the reactions of OH with phen has two peaks (350 and 430 nm) having nearly equal intensities and a broad maximum at 380 nm with dicnq. In the case of complexes, the rate constants for the reactions of OH and eaq− are in the range (7.5–11.8)×109 and (1.6–3.1)×1010 dm3 mol−1 s−1, respectively. The rate for the OH reaction increased with the dicnq content in the complex, whereas a reverse trend was seen in the hydrated electron reaction. Neither the ligand nor the Ru(II) complex except [Ru(phen)2(dicnq)]2+ is reactive toward the azide radical. In the reaction of OH with Ru(II) complexes, the transients have absorption maxima in the range of 380–435 nm whose intensity decreased with the dicnq content. The spectra measured in the reactions of OH and one electron oxidants (Cl2 −, SO4 − or N3 ) with the complexes are not identical unlike with ligands. The reaction mechanism involves the addition of OH and eaq− to the ligands of the complexes and the conversion of Ru(II)–Ru(III) by the one electron oxidant.

This article reviews recent developments that have taken place in research related to new porphyrin architectures and their host-guest chemistry, as described in a microsymposium bearing the same name held during the 3rd International Conference on Porphyrins and Phthalocyanines (ICPP-3). The usefulness of representative arrays, aggregates, donor-acceptor systems, and receptor motifs based on porphyrin architectures is highlighted as it applies to the fabrication of molecular devices, understanding the biomimetic electron transfer processes, and recognition of bioorganic substrates.

Owing to the use of porphyrins in photodynamic therapy for the treatment of malignant tumors, and the preferential interaction of lectins with tumor cells, studies on lectin–porphyrin interaction are of significant interest. In this study, the interaction of several free-base and metalloporphyrins with Momordica charantia (bitter gourd) lectin (MCL) was investigated by absorption spectroscopy. Difference absorp-tion spectra revealed that significant changes occur in the Soret band region of the porphyrins on binding to MCL. These changes were monitored to obtain association constants (Ka) and stoichiometry of binding. The tetrameric MCL binds four porphyrin molecules, and the stoichiometry was unaffected by the presence of the specific sugar, lactose. In addition, the agglutination activity of MCL was unaffected by the presence of the porphyrins used in this study, clearly indicating that porphyrin and carbohydrate ligands bind at different sites. Both cationic and anionic porphyrins bind to the lectin with comparable affinity (Ka =103−105 m−1). The thermodynamic parameters associated with the interaction of several porphyrins, obtained from the temperature dependence of the Ka values, were found to be in the range: ΔH° = −98.1 to −54.4 kJ·mol−1 and ΔS° =−243.9 to −90.8 J·mol−1·K−1. These results indicate that porphyrin binding to MCL is governed by enthalpic forces and that the contribution from binding entropy is negative. Enthalpy–entropy compensation was observed in the interaction of different porphyrins with MCL, underscoring the role of water structure in the overall binding process. Analysis of CD spectra of MCL indicates that this protein contains about 13%α-helix, 36%β-sheet, 21%β-turn, and the rest unordered structures. Binding of porphyrins does not significantly alter the secondary and tertiary structures of MCL.

Intramolecular electronic energy-transfer (intra-EET) dynamics has been investigated in 2-(9-anthryl)-1H-imidazo [4,5-f] [1,10]-phenanthroline (AIP), a newly synthesized bichromophoric molecule, us...

The crystal structure of the complex of meso-tetrasulfonatophenylporphyrin (H(2)TPPS) with jack fruit (Artocarpus integriflora) agglutinin (jacalin) has been determined at 1.8 A resolution. A porphyrin pair is sandwiched between two symmetry-related jacalin monomers in the crystal, leading to a cross-linking network of protein molecules. Apart from the stacking interactions, H(2)TPPS also forms hydrogen bonds, some involving water bridges, with jacalin at the carbohydrate-binding site. The residues that are involved in rendering galactopyranoside specificity to jacalin undergo conformational adjustments in order to accommodate the H(2)TPPS molecule. The water molecules at the carbohydrate-binding site of jacalin cement the jacalin-porphyrin interactions, optimizing their complementarity. Interactions of porphyrin with jacalin are relatively weak compared with those observed between galactopyranoside and jacalin, perhaps because the former largely involves water-mediated hydrogen bonds. While H(2)TPPS binds to jacalin at the carbohydrate-binding site as in the case of ConA, its mode of interaction with jacalin is very different. H(2)TPPS does not enter the carbohydrate-binding cavity of jacalin. Instead, it sits over the binding site. While the porphyrin binding is mediated by replicating the hydrogen-bonding network of mannopyranoside through the sulfonate atoms in the case of ConA, the plasticity associated with the carbohydrate-binding site accommodates the pluripotent porphyrin molecule in the case of jacalin through an entirely different set of interactions.

Two new, easy-to-prepare dipyrrolyl derivatives endowed with electron-withdrawing quinone or dicyano functionalities in their architecture permit the detection of fluoride ions under visual (naked-eye) as well as optical (absorption and fluorescence) and electrochemical conditions in organic solvents.

A series of photochemically active, “axial-bonding” type phosphorus(V) porphyrin−azoarene (P−A) conjugates, viz., [(TTP)PV(-O-AZM)2]+, [(TTP)PV(-O-AZT)2]+, [(TTP)PV(-O-AZB)2]+, and [(TTP)PV(-O-AZN)2]+ (TTP is 5,10,15,20-tetra(4-methylphenyl)porphyrinato dianion and O-AZM, O-AZT, O-AZB, and O-AZN are the axially coordinated 4-methoxy azophenoxo, 4-methyl azophenoxo, azophenoxo, and 4-nitro azophenoxo ligands, respectively) have been synthesized and fully characterized by mass (FAB), infrared, UV−visible, proton nuclear magnetic resonance (1D and 1H-1H COSY), and electrochemical methods. The spectroscopic data suggest that the two azoarene subunits in these hexa-coordinated systems exist in their trans isomeric forms and that there is minimal ground-state interaction between the axial ligands and the basal porphyrin π plane in each case. Continuous irradiation of CH3CN solutions containing these new P−A conjugates at 345±5 nm results in isomerization of the phosphorus-bound azoarene subunits to produce the ...

The well-known oxophilicity of aluminum(III) ion and the well-defined redox and photochemical properties of aluminum(III) porphyrins have been exploited to synthesize the functionally active, 'axial-bonding’-type free base–aluminum(III) porphyrin dimer (H2–Al) and free base–[aluminum(III) porphyrin]2 trimer (H2–Al2), as well as the corresponding copper(II) (Cu–Al, Cu–Al2) and zinc(II) (Zn–Al, Zn–Al2) derivatives. These dimeric and trimeric species have been fully characterized by mass (FAB), UV/VIS, proton nuclear magnetic resonance (1D and 1H-1H COSY) and electron spin resonance spectroscopies and also by the differential pulse voltammetric method. Comparison of their spectroscopic and electrochemical data with those of the corresponding monomeric porphyrins reveals that there is no apparent ring-to-ring interactions in these 'vertically’ linked dimers and trimers. The fluorescence quantum yields and singlet state life times were found to be lower for H2–Al and H2–Al2 in comparison with those of the monomeric chromophores. Electronic energy transfer from the aluminum(III) porphyrin to the free base subunit is detected in both H2–Al and H2–Al2. Finally, a comparison is made between the presently reported aluminum(III) porphyrin based arrays and the previously reported analogous arrays based on tin(IV), germanium(IV) and phosphorous(V) porphyrins with regard to their architectural features, spectroscopic properties and photochemical activities.

Enhancement in the third-order nonlinearity v ð3Þ , nonlinear absorption and optical limiting threshold with the introduction of charge transfer (CT) states are observed in a new class of azoarene phosphorus (V) porphyrins. Introduction of axial azoarene groups into the phosphorus porphyrin structure led to a rise in the measured hci values. Faster intersystem crossing from CT states to triplet and excited state absorption from the triplet states contribute to the enhancement of nonlinear absorption thereby reducing the optical limiting threshold by a factor of 2. 2002 Elsevier Science B.V. All rights reserved.

A new family of covalently linked ‘Sn(IV) porphyrin-anthracene’ diad (1), triad (2) and tetrad (3) donor-acceptor (D-A) systems have been designed and synthesized in good-to-moderate yields. While diad 1 possesses one anthracene subunit at the peripheral (meso) position of the tin(IV) porphyrin scaffold, triad 2 possesses twotrans axial anthracene subunits at the tin(IV) centre. On the other hand, tetrad 3 is endowed with both the peripheral and axial anthracene subunits in its architecture. These D-A systems have been fully characterised by elemental analysis, FAB-MS, UV-Vis,1H and13C NMR and electrochemical methods. UV-Vis,NMR and redox data suggest the absence of intramolecular π-π interaction between the porphyrin and the anthracene/s in 1–3. Fluorescence from the anthracene subunit in 1 and 3 is found to be quenched in comparison with the fluorescence of free anthracene in four different solvents. This is not the case with compound 2. Excitation spectral data provides evidence for an intramolecular excitation energy transfer (EET) from the singlet anthracene to the porphyrin in 1 and 3. The energy transfer efficiency is in the order: 2 (almost negligible) < 3 (~30%) < 1 (nearly quantitative), with the peripheral anthracene → porphyrin pathway being largely favoured. This orientation dependence of EET could be analysed using Forster's dipole dipole mechanism.

The hexafluorophosphate and chloride salts of two ruthenium(II) complexes, viz. [Ru(phen)(ptzo)2]2 and [Ru(ptzo)3]2+, where ptzo = 1,10-phenanthrolino[5,6-e]1,2,4-triazine-3-one (ptzo) — a new modified phenanthroline (phen) ligand, have been synthesised. These complexes have been characterised by infrared, UV-Vis, steady-state emission and1H NMR spectroscopic methods. Results of absorption and fluorescence titration as well as thermal denaturation studies reveal that both thebis- and tris-complexes of ptzo show moderately strong affinity for binding with calf thymus (CT) DNA with the binding constants being close to 105M-1 in each case. An intercalative mode of DNA binding has been suggested for both the complexes. Emission studies carried out in non-aqueous solvents and in aqueous media without DNA reveal that both [Ru(phen)(ptzo)2]2+ and [Ru(ptzo)3]2+ are weakly luminescent under these solution conditions. Successive addition of CT DNA to buffered aqueous solutions containing [Ru(phen)(ptzo)2]2+results in an enhancement of the emission. These results have been discussed in the light of the dependence of the structure-specific deactivation processes of the MLCT state of the metallo-intercalator with the characteristic features of its DNA interaction. In doing so, attempts have been made to compare and contrast its properties with those of the analogous phenanthroline-based complexes including the ones reported by us previously.

Optical limiting performance and third-order nonlinearity χ (3) are investigated in 5,10,15,20-(tetratolyl) porphyrinato phosphorus (V) dichloride ([(TTP)P V Cl 2 ] + ). Heavy atom effect exerted by the two axial chlorides is found to result in low fluorescence yield (∅ fl ), shorter lifetimes of S 1 ,T 1 and faster intersystem crossing (ISC), leading to larger excited state absorption (ESA) from T 1 →T n . This phenomenon is found to lower the limiting threshold by 30 times and increase the second hyperpolarizability by one and three orders in ps and ns time regimes, respectively.

Spectroscopic, electrochemical and fluorescence properties of five octahedral tin(IV) porphyrins ([( TpTP ) Sn ( O -m,p- C 6 H 3( CH 3)2)2] (1), [( TpTP ) Sn ( O -p- C 6 H 4( CH 3))2] (2), [( TpTP ) Sn ( O - C 6 H 5)2] (3), [( TpTP ) Sn ( O -p- C 6 H 4 O -o,p- C 6 H 3( NO 2)2)2] (4) and [( TpTP ) Sn ( O -p- C 6 H 4( NO 2))2] (5)) bearing electron donating/electron withdrawing aryloxo subunits as the axial ligands are investigated in detail. These complexes have been synthesized by the reaction of 5,10,15,20-tetra(4-methylphenyl)porphyrinato tin(IV) dihydroxide ([( TpTP ) Sn ( OH )2]) and either 3, 4-dimethylphenol, p-cresol, phenol, 4-(2,4-dinitrophenoxy)phenol or p-nitrophenol and were isolated in good-to-moderate yields. Analysis of the spectral data (FAB mass, IR, UV-vis and 1H NMR) of 1 - 5 suggests that the two trans axial aryloxo ligands are strongly bound in a symmetric manner at the tin center in these complexes. Specifically, the general symmetry observed in the 1H NMR spectra and the ring current effect experienced by the protons present on the axial ligands are revealing in this regard. Each porphyrin ring undergoes two successive, reversible/quasi-reversible, one-electron reductions in CH 2 Cl 2, 0.1 M TBAP. The bound nitroaromatic axial ligands of complexes 4 and 5 could also be reduced under the same set of experimental conditions. The wavelengths of maximum emission, the singlet energies and the Stokes’ shifts observed in the fluorescence spectra of 1 - 5 are close to the corresponding parameters of the reference porphyrin, [( TpTP ) Sn ( OH )2]. On the other hand, fluorescence intensities of complexes 1 - 4 are quenched in comparison with that of [( TpTP ) Sn ( OH )2] in three different solvents. A detailed analysis of the emission and redox potential data indicates that a photoinduced electron transfer from the axial aromatic subunit to singlet state of the basal tin(IV) porphyrin can, in principle, explain the fluorescence quenching observed in these donor-acceptor systems. The spectroscopic and redox features of these tin(IV) porphyrins are compared with those of the analogous phosphorus(V) porphyrins reported by us earlier.

Two new porphyrin arrays-a hexamer and a nonamer-have been synthesized and characterized by elemental analysis as well as mass, (1)H NMR, and UV-vis spectroscopic methods. The scheme of construction of these arrays employs a synthetic protocol involving sequential "organic" and "inorganic" reactions conducted, respectively, at the peripheral meso-phenyl ring and the central tin(IV) ion of the porphyrin scaffold. The architecture of the hexamer is such that it is based on a covalently linked tin(IV) porphyrin dimer, with each of the two tin(IV) centers trans-axially ligated to two free-base porphyrins, while the higher homologue features a tin(IV) porphyrin trimer as the basal unit, with its central metalloid ions having two free-base porphyrins as axial ligands. This extended, "axial-bonding"-type architecture of the new arrays has been investigated by the (1)H NMR method, which reveals characteristic ring-current-induced shifts and coupling patterns for the resonances due to protons of the axial free-base porphyrin subunits. The presence of any ring-ring (basal-basal, basal-axial, or axial-axial) interaction in these arrays is not obvious from their UV-vis and redox potential data, which are close to those of the corresponding constituent monomeric species. On the other hand, their singlet-state activities are quite different from those of the precursor reference compounds as probed by steady-state fluorescence. The results of the detailed investigations carried out on these hybrid, "bichromophoric" arrays have been interpreted in terms of the occurrence of intraarray, interchromophore energy- and electron-transfer reactions.

The interaction of several metallo-porphyrins with the galactose-specific lectin from Trichosanthes cucumeirna (TCSL) has been investigated. Difference absorption spectroscopy revealed that significant changes occur in the Soret band region of the porphyrins upon binding to TCSL and these changes have been monitored to obtain association constants (Ka) and stoichiometry of binding (n ). The dimeric lectin binds two porphyrin molecules and the presence of the specific saccharide lactose did not affect porphyrin binding significantly, indicating that the sugar and the porphyrin bind at different sites. The Ka values obtained for the binding of different porphyrins with TCSL at 25 8C were in the range of 2 10 3 25 10 5 M 21 . Association constants for meso-tetra(4-sulphonatophenyl)porphyrinato copper(II) (CuTPPS), a porphyrin bearing four negative charges and meso-tetra(4-methylpyridinium)porphyrinato copper(II) (CuTMPyP), a porphyrin with four positive charges, were determined at several temperatures; from the temperature dependence of the association constants, the thermodynamic parameters change in enthalpy (DH8) and change in entropy (DS8) associated with the binding process were estimated. The thermodynamic data indicate that porphyrin binding to TCSL is driven largely by a favourable entropic contribution; the enthalpic contribution is very small, suggesting that the binding process is governed primarily by hydrophobic forces. Stopped-flow spectroscopic measurements show that binding of CuTMPyP to TCSL takes place by a single-step process and at 20 8C, the association and dissociation rate constants were 1.89 10 4 M 21:s 21 and 0.29 s 21 , respectively.

"Equatorial" energy transfer: Orientation control of intramolecular energy transfer (EnT) is found in the donor–acceptor system shown, light absorbed by the "peripheral" anthracene is efficiently transferred to the porphyrin, but that by the "axial" anthracene subunits is not.

The crystal structure ofmeso-tetrasulfonatophenylporphyrin complexed with concanavalin A (ConA) was determined at 1.9 A resolution. Comparison of this structure with that of ConA bound to methyl α-d-mannopyranoside provided direct structural evidence of molecular mimicry in the context of ligand receptor binding. The sulfonatophenyl group of meso-tetrasulfonatophenylporphyrin occupies the same binding site on ConA as that of methyl α-d-mannopyranoside, a natural ligand. A pair of stacked porphyrin molecules stabilizes the crystal structure by end-to-end cross-linking with ConA resulting in a network similar to that observed upon agglutination of cells by lectins. The porphyrin binds to ConA predominantly through hydrogen bonds and water-mediated interactions. The sandwiched water molecules in the complex play a cementing role, facilitating favorable binding of porphyrin. Seven of the eight hydrogen bonds observed between methyl α-d-mannopyranoside and ConA are mimicked by the sulfonatophenyl group of porphyrin after incorporating two water molecules. Thus, the similarity in chemical interactions was manifested in terms of functional mimicry despite the obvious structural dissimilarity between the sugar and the porphyrin.

We present our experimental results on the excited state dynamics in several tetra tolyl porphyrins (with different central metal-ions) studied using degenerate four wave mixing with nanosecond incoherent light (DFWM-IL) and picosecond time-resolved four wave mixing (DFWM-PS). Incoherent light studies indicate three lifetimes, which have been attributed to different excited states. The shortest component with a decay constant of S n ( T 2 ) , 3–6 ps component is due to the vibrational relaxation of the Franck-Condon states in S 1 ( τ vib ) , and the long component of 20–100 ps is due to the population relaxation ( T 1 ) from S 1 to S 0 . The population relaxation times obtained from time-resolved DFWM studies using a picosecond laser substantiates these results.

A new porphyrin trimer has been self-assembled by employing the mutually non-interfering coordination properties of the ruthenium(II) and tin(IV) centers to form a multi-metal array. The photo- and electro-chemical properties of this array are also reported.

The synthesis, spectral characterization, and electrochemical properties of [Ru(phen)2(qdppz)]2+, which incorporates a quinone-fused dipyridophenazine ligand (naphtho[2,3-a]dipyrido[3,2-h:2‘,3‘-f]phenazine-5,18-dione, qdppz), are described in detail. Chemical or electrochemical reduction of [Ru(phen)2(qdppz)]2+ leads to the generation of [Ru(phen)2(hqdppz)]2+a complex containing the hydroquinone form (hqdppz = 5,18-dihydroxynaphtho[2,3-a]dipyrido[3,2-h:2‘,3‘-f]phenazine) of qdppz. Absorption and viscometric titration, thermal denaturation, topoisomerase assay, and differential-pulse voltammetric studies reveal that [Ru(phen)2(qdppz)]2+ is an avid binder of calf-thymus DNA due to a strong intercalation by the ruthenium-bound qdppz, while [Ru(phen)2(hqdppz)]2+ binds to DNA less strongly than the parent “quinone”-containing complex. DNA-photocleavage efficiencies of these complexes also follow a similar trend in that the MLCT-excited state of [Ru(phen)2(qdppz)]2+ is more effective than that of [Ru(phen)2(hqd...

We present our experimental results on the measurement of third-order optical nonlinearity in the ns and ps domain, in several Tetra Tolyl Porphyrin molecules (TTP), using degenerate four wave mixing (DFWM) and Z-Scan techniques. Our results indicate a very high value of nonlinearity for these molecules in the ns domain and reasonably high values in the ps domain. They are found to exhibit a strong nonlinear absorption at both 532 nm and 600 nm. The high value of nonlinearity for ns pulses is attributed to higher excited singlet and triplet states. Time-resolved studies indicate an ultra-fast temporal evolution of the nonlinearity in these molecules.

Jacalin (Artocarpus integrifolia agglutinin) specifically recognizes the tumor-associated T-antigenic disaccharide structure, Gal beta13GalNAc. Porphyrins and their derivatives are currently used as photosensitizers in photodynamic therapy to treat malignant tumors. In this study, the interaction of several free base porphyrins and their metal derivatives with jacalin is investigated by absorption and fluorescence spectroscopy. Each lectin subunit was found to bind one porphyrin molecule and the association constants were estimated to be in the range of 2.4 x 10(3) M(-1) to 1.3 x 10(5) M(-1) at room temperature for the interaction of different porphyrins with jacalin. These values are in the same range as those obtained for the interaction of monosaccharides to jacalin. Both free lectin and lectin saturated with the specific saccharide were found to bind different porphyrins with comparable binding strength indicating that porphyrin binding takes place at a site different from the sugar binding site. Further, both anionic and cationic porphyrins were found to interact with the lectin with comparable affinity, clearly indicating that the charge on the porphyrin does not play any role in the binding process and that most likely the interaction is mediated by hydrophobic forces. These results suggest that jacalin and other lectins may potentially be useful for targeted delivery of porphyrins to tumor tissues in photodynamic therapy.

The ‘marriage’ between porphyrin chemistry and the newly emerging molecular recognition science has opened up novel research opportunities leading to many biological and abiological applications. An overview of this fascinating research area is presented here.

Halodecarboxylation reaction of ferrocenylacrylic acid 1 and ferrocenyldienoic acid 3d with N-bromo- and N-iodosuccinimide in the presence of catalytic tetrabutylammonium trifluoroacetate at −40 °C and −78 °C affords the corresponding β-halovinylferrocenes 2a, 2b and δ-haloferrocenyldiene 4 in 37−72% yields. Heck reaction of β-iodovinylferrocene 2a with vinyl substrates (CH2CH−Z where Z = CO2Me, CO2Et, COMe, CO2H, CONH2, 4‘-NO2C6H4) in the presence of tri(4-tolyl)arsine/palladium acetate/lithium chloride/triethylamine in acetonitrile at 35−80 °C affords the corresponding ferrocenyldienes 3a−3f in 50−81% isolated yields. Similar reaction of δ-iodoferrocenyldiene 4 with vinyl substrates (CH2CH−Z where Z = CO2Me, CO2Et, CO2H, 4‘-NO2C6H4) affords the corresponding ferrocenyltrienes 5a−5d in 55−87% isolated yields. The ferrocene-capped conjugated dienes and trienes show excellent all-E stereoselectivity (vide NMR). The electronic, redox, and nonlinear optical properties of ferrocenylpolyenes have been evaluate...

The interaction of several free-base porphyrins and their corresponding copper(II) and zinc(II) derivatives with the galactose-specific lectin from snake gourd (Trichosanthes anguina) seeds has been investigated by absorption and fluorescence spectroscopic techniques. The lectin dimer contains two apparently equivalent binding sites for the porphyrins. Association constants obtained for the interaction of various porphyrins with the lectin are in the range 1.7 x 10(4)-6.2 x 10(5) M(-1), with the metalloporphyrins being seen to have higher affinity for the lectin compared with the free-base analogues. Both positively charged and negatively charged porphyrins bind to snake gourd seed lectin (SGSL) with comparable affinities, suggesting that binding occurs primarily via hydrophobic interactions. Further, binding of porphyrins is found to be largely unaffected by the presence of the sugar ligand, lactose, indicating that the binding sites for the carbohydrate and porphyrin are different. This study thus suggests that the lectin may serve as a receptor for some endogenous non-carbohydrate, hydrophobic ligand in vivo, in addition to the saccharide ligands. It also opens up the possibility of employing the T. anguina lectin in applications such as photodynamic therapy, which involve the use of porphyrins.

DNA binding and photocleavage characteristics of a series of mixed-ligand complexes of the type [M(phen)2LL]n+ (where M = Co(III), Ni(II) or Ru(II), LL = 1,10-phenanthroline (phen), phenanthroline-dione (phen-dione) or dipyridophenazine (dppz) andn = 3 or 2) have been investigated in detail. Various physico-chemical and biochemical techniques including UV/Visible, fluorescence and viscometric titration, thermal denaturation, and differential pulse voltammetry have been employed to probe the details of DNA binding by these complexes; intrinsic binding constants (K b) have been estimated under a similar set of experimental conditions. Analysis of the results suggests that intercalative ability of the coordinated ligands varies as dppz>phen>phen-dione in this series of complexes. While the Co(II) and Ru(II) complexes investigated in this study effect photocleavage of the supercoiled pBR 322 DNA, the corresponding Ni(II) complexes are found to be inactive under similar experimental conditions. Results of detailed investigations carried out inquiring into the mechanistic aspects of DNA photocleavage by [Co(phen)2(dppz)]3+ have also been reported.

It is proposed that singlet dioxygen reacting with guanosine or deoxyguanosine part of nucleotides does not, by itself, cause DNA cleavage. The strand break originates at the endoperoxide stage whenever this link evolves into a O-centered radical. The O-centered radical is then in a good spatial position to abstract an hydrogen intramolecularly from the ribose or desoxyribose part of the nucleotide. The carbon centered radical thus formed on the sugar part may lead to strand break either by a p-scission mechanism or by an homolytically induced solvolysis. High pH could also induce cleavage after the endoperoxide stage via a base catalyzed ring chain protomerism.

(H 2 )P-An 2 , a molecular triad in which protoporphyrin(IX) has been covalently linked, through ester bridges at the two propionate side chains, to two anthracene donor subunits has been synthesized and fully characterized by FAB-MS, UV-Visible and 1 H NMR spectroscopic methods. A comparison of the UV-Visible and 1 H NMR features of (H 2 )P-An 2 with those of the corresponding individual model compounds (i.e. protoporphyrin(IX) dimethyl ester, H 2 P and anthracene, An) reveal the presence of weak intramolecular ground-state interaction between the porphyrin and anthracene πplanes. Quenching of fluorescence due to anthracene (but not porphyrin) part of the molecule has been observed in three different solvents. Excitation spectral data provides evidence for an intramolecular excitation energy transfer (EET) from the singlet anthracene to the porphyrin. Detailed analysis of the data suggests that Forster's dipole-dipole mechanism does not adequately explain this energy transfer but, an electron exchange mediated mechanism can, in principle, contribute to the intramolecular EET. The bi-exponential fluorescence decay observed in the time-resolved studies has been interpreted in terms of the co-existence of "folded" and "extended" conformers of the molecule in solution. Finally, a comparison is made of the EET reactions of (H 2 )P-An 2 with those observed for the previously reported porphyrin-based energy/electron transfer systems which include (tetraaryl)porphyrin-based isomeric porphyrin-anthracene dyads and mesoporphyrin-based porphyrin-nitrobenzyl diester triads.

Covalently linked porphyrin–adenine (meso-5(4-(9-(2-oxyethyl)adenine)phenyl)-10,15,20-tritolylporphyrin, 1) and anthracene–thymine (1-(9-methylanthracene)thymine, 2) conjugates have been synthesized and fully characterized by elemental analysis, FAB mass, UV-vis, 1 H NMR, fluorescence and cyclic voltammetric methods. Detailed 1 H NMR studies reveal that 1 and 2 self-assemble in CDCl 3 solutions at 293 ± 3 K to form a two-point hydrogen-bonded, bichromophoric, supramolecular system 3 with a binding constant of 47 ± 5 M−1 and that both Hoogsteen- and Watson–Crick-type A–T assemblies exist in solution under these experimental conditions. Spectral and electrochemical data point out the possibility of occurrence of both energy and electron transfer reactions from the singlet excited state of 2 to 1 in the ensemble 3. The singlet state activity of the ensemble 3 has been probed mainly by the time-resolved fluorescence method and the results have been discussed in the light of those obtained earlier on similar ‘non-covalently’ or covalently bound bichromophoric systems.

Photodynamic induced cytotoxicity by porphyrin-DNA cross linker/intercalator hybrid diads and triads has been studied on the human leukemic cell line TF-1. Cells were incubated for 1 to 4 h with these new photosensitizers and irradiated with white light. Cell survival was assessed by the propidium iodide staining, using flow cytometry analysis. A comparison of the dark and light cell survival factor values suggests that irradiation has a significant effect on the toxicity at low concentrations for the porphyrin-chlorambucil diad and to a lesser extent at high concentrations for the porphyrin-acridone diad, the porphyrin-acridine diad and the porphyrin-cholic acid-chlorambucil triad. While the intrinsic antileukemic (via DNA cross-linking) activity of the chlorambucil moiety and the structural details may be responsible for the photoenhancement of the toxicity, the presence of acridine or acridone which are avid intercalators of DNA, is responsible for a similar effect seen for diads.

Covalently-linked, bichromophoric systems comprising of a porphyrin donor and an anthraquinone acceptor are shown to exhibit wavelength dependent photonuclease activity.

Photolysis of simple, readily available arene diazo compounds leads to the cleavage of DNA.

The new metallointercalator1, a mixed-ligand ruthenium(II) complex incorporating a quinone-fused dipyridophenazine ligand, effects an efficient cleavage of the supercoiled plasmid DNA upon activation with visible light. In addition, the quinone/hydroquinone redox couple1/2 represents a molecular light-switching device displaying interconversion between non-luminescent state1 and luminescent state2.

Isomeric donor-acceptor (DA) dyads in which an anthracene donor moiety is covalently linked, via a short ether bridge, to either orhto, metha or para position of one of the aryl groups of 5, 10, 15, 20-tetraphenylporphyrin have been synthesized and characterized by spectral and electrochemical methods. UV-visible and 1H nuclear magnetic resonance data of these DA systems suggest the presence of weak intramolecular π-π interaction between the porphyrin and the anthracene. Fluorescence from the anthracene subunit in each dyad in found to be quenched in comparison with the flourescence of free anthracene. Excitation spectral data provide evidence for an intramolecular excitation energy transfer (EET) from the singlet anthracene to the porphyrin and the energy transfer efficiency is found to be dependent on the site of attachment (i.e. ortho > meta > para) of the donor to the acceptor. Detailed analysis of the data suggests that Forster's dipole-dipole mechanism does not adequately explain this energy transfer and that an electron-exchange-mediated mechanism can, in principle, contribute to the intramolecular EET in these short ether bridged dyads. Furthermore, arguments based on the thermodynamic considerations and also the solvent-dependent fluorescencedata reveal that, while quenching of the fluorescence in the ortho isomer could be explained solely on the basis of EET, invoking an intramolecular electron transfer can rationalize the observed quenching in the meta and para isomers. Finally, a comparison is made of the EET reactions in these isomeric dyads with those observed for the previously reported porphyrin-based energy transfer systems which include a supramolecular, D4-A pentad porphyrin bearing four anthracene donor subunits.

Aryloxo derivatives of phosphorus(V) porphyrins of the type [(TpTP)P(OR)(2)](+)OH(-) where TpTP is the dianion of tetra-p-tolylporphyrin and OR is an axial aryloxo (2,4-dimethylphenoxo, 4-methylphenoxo, phenoxo, 4-nitrophenoxo, 4-(4-nitrophenoxy)phenoxo, or 4-(2,4-dinitrophenoxy)phenoxo) ligand have been synthesized and fully characterized by FAB-mass, UV-vis, fluorescence, infrared, and nuclear magnetic resonance ((1)H and (31)P) spectroscopies and cyclic voltammetric methods. Each new porphyrin shows a typical "normal UV-vis absorption spectrum" indicating the presence of a P(V) ion in the porphyrin cavity. The proton-decoupled (31)P NMR signal observed for these compounds, between -194 and -200 ppm, suggests that there exists an octahedral coordination around the phosphorus atom, and this supposition is further substantiated by the porphyrin ring-current-induced upfield shifts observed for protons on the two axial aryloxo ligands in the (1)H NMR spectra. Cyclic voltammetric studies reveal that each porphyrin undergoes two successive, one-electron reductions with the site of electron transfer being the porphyrin ring. The fluorescence quantum yield values of these porphyrins are found to be sensitive to the nature of the aryloxo ligand and also to the solvent polarity. The singlet state properties of these systems have been discussed in light of both the fluorescence and the redox potential data.

Results of absorption titration, thermal denaturation, and differential pulse voltammetric experiments suggest that [Co(phen)2DPPZ]3+ and [Ni(phen)2DPPZ]2+ (phen = 1, 10-phenanthroline and DPPZ = dipyrido[3,2-a:2‘,3‘-c]phenazine) are both avid binders of calf thymus DNA. On the other hand, while the cobalt(III) complex has been found to effect the photocleavage of the supercoiled pBR 322 DNA, the nickel(II) complex is ineffective under similar experimental conditions.

A tetraphenylporhyrin bearing four anthracene donor moieties (5,10,15,20-tetra(3-(9-methyloxyanthracenyl)phenyl)porphyrin, I) was synthesized and fully characterized. Spectroscopic and electrochemical data and results of metallation and intermolecular fluorescence quenching experiments suggest that there is no appreciable interaction between the porphyrin and anthracene moieties in I. Although intermolecular quenching of anthracene fluorescence by 5,10,15,20-(tetraphenyl)porphyrin is not obvious, the fluorescence of the anthracene donors in I is quenched by more than 90%. On the basis of the fluorescence excitation pectral data, the quenching observed is attributed to intramolecular singlet-singlet energy transfer from anthracene to the porphyrin. However, neither Forster's dipole-dipole formalism nor Dexter's electron exchange mechanism can explain adequately this intramolecular energy transfer. Thermodynamic considerations and solvent-dependent fluorescence data indicate that photoinduced electron transfer from singlet anthracene to the porphyrin can compete with energy transfer in this supramolecular system.

Time-resolved fluorescence studies were carried out on a series of free-base and zinc(II) derivatives of meso-tetraphenylporphyrins covalently linked to either 1,3-dinitrobenzene (DNB) or 1,3,5-trinitrobenzene (TNB) acceptor units. These acceptor units were linked at different sites (at the ortho, meta or para positions of one of the phenyl groups of meso-tetraphenylporphyrin) to the donor porphyrins such that the resulting isomeric intramolecular donor-acceptor complexes exhibit different centre-to-centre (ctc) distances and relative orientations. Biexponential fluorescence decay profiles observed for several of these covalently linked complexes were rationalized in terms of the presence of ''closed'' and ''extended'' conformers. Detailed analyses of the fluorescence decay data have provided a comprehensive understanding of the photoinduced electron transfer (PET) reactions occurring in systems containing zinc(II) porphyrin donors. It is observed that although DNB-linked zinc(II) complexes follow the trends predicted for the efficiency of PET with respect to donor-acceptor distance, the TNB-linked zinc(II) porphyrins exhibit a behaviour which is dictated by steric effects. Similarly, although the thermodynamic criteria predict a greater efficiency of charge separation in TNB-linked complexes compared with DNB-linked complexes, the reverse trend observed has been attributed to orientational effects. In the complexes containing free-base porphyrin donors, PET is expected to be less efficient from a thermodynamic viewpoint. In a few of these cases, fluorescence quenching seems to occur by parallel mechanisms other than PET.

Free-base, copper(II) and zinc(II) derivatives of 2-nitro-substituted 5,10,15,20-(tetra-aryl)porphyrins (aryl = phenyl, 2-naphthyl or diphenylpyrazolyl) have been synthesized and investigated by optical absorption and emission, 1H NMR, ESR and cyclic voltammetric methods. Red shifts and broadening of the optical absorption bands, downfield shifts of both the β-pyrrole and imino proton NMR peaks and decrements in the reduction potentials have been observed for these nitroporphyrins, in comparison with the corresponding spectral or electrochemical parameters for the unsubstituted parent porphyrins. Solvent-dependent fluorescence spectral data of the free-base and zinc(II) derivatives of the nitroporphyrins have been analysed in terms of twisted intramolecular charge transfer (TICT) in these complexes. However, the ESR spectral data of copper(II) derivatives of the nitroporphyrins are indistinguishable from the data for the unsubstituted analogues. All these features of the nitroporphyrins have been interpreted in terms of a perturbation of the frontier orbitals of the porphyrin π-ring system by the directly attached nitro group. Based on the results obtained, possible applications of the nitroporphyrins have been suggested.