Exciton energy transfer between optically forbidden states of molecular aggregates

3-ethyl-2-[3-(3-ethyl-2(3H)-benzoxazolylidene)-1-propenyl]benzoxazolium iodide (dye I) and pseudoisocyanine bromide are employed to form H aggregates as donors and J aggregates as acceptors. The energy of an H band of the H aggregates is higher than that of a J band of the J aggregates. It was confirmed that excitation of the H band does not emit fluorescence by comparison of excitation spectra of dye I H aggregates with that of dye I monomer. Absorption, fluorescence, and excitation spectra of spin-coated films of H aggregates mixed with various quantities of J aggregates have been observed. Excitation spectra probed at the J band are found to have a component of the H band. Fluorescence spectra originated from excitation of the H band are extracted and qualitatively analyzed. It is confirmed that excitation of the H band causes to emit fluorescence of a J band of the J aggregates. These phenomena show that exciton energy can transfer from the lowest energy in electronic states of the H aggregate, which state is optically forbidden, to electronic state of the J aggregate.     

Electrolyte-induced mesoscopic aggregation of thiacarbocyanine dye in aqueous solution: counterion size specificity

Countercation size specificity is presented for the electrolyte-induced aggregation of 3,3'-disulfopropyl-5,5'-dichloro-9-methyl thiacarbocyanine (TCC) dye in aqueous solution. Addition of electrolytes having a small monovalent cation (Na+, NH4+, or Cs+) induced pure H aggregates of TCC, whereas J aggregates were preferentially promoted by electrolytes with a large monovalent cation ([N(CH3)4]+ or [N(C2H5)4]+). The electrolyte-induced H aggregate (HS aggregate) differed spectroscopically from that spontaneously self-assembled in aqueous solution. Mesoscopic structure of the HS aggregates was revealed via polarized-light microscopy and atomic force microscopy; a rodlike morphology of 50-70 nm wide and tens to hundreds of micrometers long with very strong negative birefringence. A simple structural model based on semiempirical molecular orbital calculations can explain the aggregation behaviors: The anionic TCC monomer shows a considerable planar geometry between two benzothiazole end groups when it involves a sodium cation, which favors the H-type molecular arrangements in a face-to-face orientation. On the other hand, the TCC dye has a twisted conformation when it implicates a large tetramethylammonium cation, resulting in the formation of the J aggregates.     

In situ detection of birefringent mesoscopic H and J aggregates of thiacarbocyanine dye in solution

Polarized-light microscopy, fluorescence microscopy, atomic force microscopy as well as absorption and fluorescence spectroscopy were used to characterize mesoscopic structures of both supramolecular H and J aggregates of 3,3'-disulfopropyl-5,5'-dichloro-9-methyl thiacarbocyanine dye in aqueous solution. Polarized-light microscopy visualizes in situ the mesoscopic morphology of the H and J aggregates and distinguishes between them by their own colors. The H aggregate having a fibrous structure showed negative birefringence, namely, the refractive index along the fiber short axis was higher than that of the long axis, so that pi-electron chromophores of the dye molecule are likely to orient along the short axis of the elongated fibers. The degree of birefringence of the H aggregate fiber was approximately -0.3. Investigations on the concentration dependence of the absorption spectra showed that the amount of J aggregates increased at the expense of a decrease in the amount of H aggregates. With respect to the J aggregates, a small dot morphology was observed at a relatively low dye concentration of 3.0 mM. With an increase of the dye concentration up to 10 mM, the morphology changed into mesoscopic fibers. In contrast, fluorescence microscopy for the fibrous J aggregates reveals that the constituent molecules are approximately aligned along the long axis of the fibers.     

J-aggregation and its characterization in Langmuir-Blodgett films of merocyanine dyes

Langmuir-Blodgett (LB) films are constructed by successively transferring monomolecular layers formed at the air-water interface onto solid substrates. One of the advantages of the LB technique in fabricating molecular aggregates lies in the fact that it can employ various kinds of molecules by mixing them at the air-water interface. The mixed system may exhibit new properties that are not observed for individual components. This method would be useful, for example, in the studies of the formation and control of the J-aggregates of functional dyes that attract attention both in science and technology. In this paper, I review this subject mainly based on our recent results in merocyanines. LB films of merocyanine dyes, mixed with arachidic acid (C(20)), exhibit J-aggregate formation and have been serving as typical systems in revealing the physical and structural aspects of nanosized molecular aggregates constructed as monolayers. In the case of LB films of a merocyanine dye having benzothiazole as donor nucleus (abbreviated as DS), electron spin resonance (ESR) spectroscopy has been successful in determining the characteristic in-plane orientation of dye molecules with respect to the dipping direction, which led to the discovery of the flow orientation effect during the dipping process of LB films as the origin of optical dichroism often observed in LB films. In this article, after an introduction of ESR spectroscopy, three major topics on the merocyanine J-aggregation and its characterization in mixed films are discussed. The first topic is the observation and control of the size of J-aggregates in the dilution limit of dyes in arachidic acid matrix for a methyl-substituted DS (6-Me-DS). Dependence of atomic force microscopy (AFM) patterns on the molar ratio allows the identification of dye domains. J-band optical peak analysis based on the Kuhn's extended dipole model, supplemented by a novel application of femtosecond pump-probe optical spectroscopy, yields the size of the J-aggregates of 10(3). The second topic is the control of the J-band peak wavelengths by mixing two different kinds of dye molecules. The first case is the mixture of a J-forming 6-Me-DS and non-J-forming merocyanine analog, DO with benzo-oxazole instead of benzothiazole of DS. The second case is the mixture of both J-forming dyes but with different J-band peak positions, 6-Me-DS and another analog of 5-Cl-DS. The optical peak shifts depending on the molar mixing ratio will be presented. The last topic is related to the elucidation of electronic states of dye molecules in the J-aggregates. Light-induced ESR (LESR) of DS films with stable isotope ((15)N or (13)C)-substituted dyes provide clear evidence for the photoinduced charge transfer by the detection of hyperfine structures. Moreover, infrared (IR) spectroscopy of (13)C-enriched dye identifies the IR absorption peak of the relevant carbon in the chromophore. The results give evidence for the enhanced intramolecular charge transfer of dyes in the J-aggregates compared with an isolated merocyanine composed of donor and acceptor moiety. Lastly, the Cl attachment in 5-Cl-DS leads to a significant enhancement of the nitrogen hyperfine coupling in the LESR spectra. These examples and others demonstrate the potential of LB films of merocyanines in the studies of the nanosized molecular aggregates in monolayer assemblies.     

一价阳离子对卟啉二酸H_4~(2 )TSPP~(4-)J-聚集的影响

四— (对—磺酸基 )苯基卟啉 ( TSPP4-)作为超灵敏显色剂和电极修饰材料在光谱分析和电化学分析中有重要应用。它在酸性条件下以质子化二酸 H2 4TSPP4-的形式存在。七十年代 ,Paster-nack等人曾发现 H2 4TSPP4-的电子吸收谱随溶液 p H的改变产生复杂的变化 ,并推测与H2 4TSPP4-的分子缔合有关 [1] 。近年来 ,H2 4TSPP4-的分子缔合及其光谱性质重新引起人们的兴趣。基于光谱实验结果 [2 -6] ,一些作者认为 TSPP在强酸性条件下形成准一维的有序结构 ,类似于菁染料的 J-聚集体 ( Jelley- Scheibe聚集体 ) [3— 5 ] 。目前对 …     

Role reversal in a supramolecular assembly: a chiral cyanine dye controls the helicity of a peptide-nucleic acid duplex

A new dicarbocyanine dye bearing branched, chiral N-alkyl substituents was synthesized and its ability to form helical aggregates on peptide nucleic acid (PNA) double-helical templates was studied. The dye aggregates less effectively than an analogous dye bearing linear, achiral substituents, presumably due to steric problems with packing the branched substituents compared with the linear substituents. When the PNA duplex has a left-handed helicity, addition of the achiral dye leads to formation of a left-handed dye aggregate. However, when the chiral dye aggregates in the presence of this duplex, a right-handed structure is formed, suggesting that the dye alters the helicity of the underlying template. When a racemic PNA duplex (i.e., equal amounts of right- and left-handed helices) is used, no chirality is observed for the dye aggregate formed by the achiral dye but a right-handed helical aggregate is once again formed by the chiral dye. These results indicate that chirality is transferred from the dye to the PNA, as opposed to other examples of polymer-templated dye aggregation where chirality is transferred from the template to the dye.     

Photophysical Behavior of a New Gemini Surfactant in Neat Solvents and in Micellar Environments

A novel fluorescent gemini surfactant, 1,4-bis-(2'-(N-dodecyl pyridinio-4"-yl)ethenyl)benzene dibromide, abbreviated BDPEBB, has been synthesized and its photophysical properties have been studied in different environments. BDPEBB has a limited solubility in alcohols where it is found in aggregate form at concentrations>/=1 mM. In other solvents, e.g., water, it is only found in aggregate form, even at much lower concentrations. Solvent polarity has a small and insignificant solvatochromic effect but alcohols give a specific interaction with BDPEBB, causing a significant hypsochromic shift in absorption maxima and a large increase in relative fluorescence efficiency. Pyrene fluorescence is effectively quenched by BDPEBB. Pyrene also forms associative complexes with BDPEBB in water. These complexes are partly dissociated in the presence of surfactant micelles. Triton X-100 micelles provide a favorable environment for BDPEBB solubilization well distinguished from the behavior of ionic surfactants. Small quantities of BDPEBB have a large influence on the behavior of aqueous sodium dodecylsulfate (SDS) and sodium decylsulfate (SDeS) micelles, inducing the formation of large aggregates, visible by the naked eye. These large aggregates are most probably microcrystals of BDPEBB(2+)/2DS(-) or BDPEBB(2+)/2DeS(-). The aggregation number of SDS and SDeS micelles in the absence and in the presence of BDPEBB has been calculated by exploitation of the static luminescence quenching kinetics of Ru(bpy)(3)(2+) by 9-methylanthracene, both solubilized in the micellar phase. It has been observed that Ru(bpy)(3)(2+) inhibits the precipitation of SDeS micelles in the presence of BDPEBB. Our results suggest that double-chain surfactant chromophores should be employed with particular care if they are to be used as probes of the micellar phase. Copyright 2000 Academic Press.     

Counterion dependent dye aggregates: nanorods and nanorings of tetra(p-carboxyphenyl)porphyrin

Atomic force microscopy (AFM) of porphyrin aggregates formed on silica from acidic aqueous solution is used to investigate the basis for the previously reported counterion dependence of the optical spectra of aggregates of H(2)TCPP(2+), the diacid form of tetra(p-carboxyphenyl)porphyrin (TCPP). Resonance light scattering confirms the presence of excitonically coupled porphyrin aggregates in solutions of H(2)TCPP(2+) in both aqueous HCl and HNO(3). Aggregates formed in aqueous HNO(3) solutions show resonance light scattering (RLS) at wavelengths within both the H and J aggregate absorption bands and are imaged on the surface of silica as nanorods about 3 to 4 nm in height. H(2)TCPP(2+) aggregates in aqueous HCl solution exhibit RLS when excited within the blue-shifted Soret band (H band) and produce AFM images on silica of ring-shaped structures ranging from about 200 to 2000 nm in diameter. Fluorescence excitation and emission spectra reveal quenching of the Q-band emission in the aggregates at a pH less than 1 and confirm the existence of a single species, assigned to a dimer, at a pH just above 1. The morphology of the nanostructures as revealed by AFM provides insight into the structural basis for the counterion-dependent optical properties of H(2)TCPP(2+) aggregates.     

J Aggregation of meso-Ethylsubstituted Carbocyanine Dyes in Polymer Films

The J-aggregation capability of meso-ethylsubstituted thiacarbocyanine (Dye 1 and Dye 2) and oxacarbocyanine (Dye 3) dyes was studied in poly(vinyl alcohol) (PVA) and poly(vinylpyrrolidone) (PVP) films and aqueous solutions, as well as in water. In water, Dye 1 and Dye 3 form J aggregates and dimers, whereas Dye 2 occurs mainly in the dimer form. The addition of PVA to an aqueous solution leads to deaggregation of J aggregates of Dye 1 and Dye 3 and dimers of Dye 1, Dye 2, and Dye 3. Unlike PVA, PVP stimulates the J aggregation of Dye 1 in aqueous solution, with the J aggregate yield depending on the ratio between the PVP monomer unit and dye concentrations. The J aggregation of Dye 1 and Dye 3 takes place in PVA films, whereas the J aggregates of only Dye 1 are formed in PVP films. The photostability of Dye 1 J aggregates in aqueous solutions strongly depends on the spectral region of irradiation and the presence of atmospheric oxygen. The photostability is appreciably lower in aerated solutions and substantially increases in PVA films.     

Interaction of semiconductor colloids with J-aggregates of a squaraine dye and its role in sensitizing nanocrystalline semiconductor films

The role of bis(2,4,6-trihydroxyphenyl)squaraine, Sq, in sensitizing large bandgap semiconductors has been investigated in the present study. The dye in its aggregate form readily interacts with the TiO₂ colloids giving rise to a new charge transfer band in the red region. The apparent association constant for the dye aggregate and TiO₂ colloid as determined from a Benesi-Hildebrand plot is 1600 M^-1. Nanocrystalline semiconductor films prepared from TiO₂, ZnO, and SnO₂ colloids have been modified with Sq to probe the photosensitization effects. Both dye monomers and aggregates were found to participate in the charge injection process. An incident photon-to-photocurrent efficiency up to 0.7% has been observed.     

Interactions of cyanine dyes with nucleic acids. XXIV. Aggregation of monomethine cyanine dyes in presence of DNA and its manifestation in absorption and fluorescence spectra.

Absorption, fluorescence emission and excitation spectra of benzothiazole cyanine dyes--thiazole orange (TO) and 7-methyl-6-(3-methyl-2,3-dihydro-1,3-benzothiazol-2-ylidenmethyl) [1,3] dioxolo [4',5':4,5] benzo [d] [1,3] thiazolium methylmethosulfate (Cyan 13)--were investigated over a wide concentration range. The dyes form aggregates with a 'sandwich'-like structure in water solution. At low dye to DNA concentrations ratios, Cyan 13 and TO monomers appear to interact with the DNA. On increasing the dye to DNA concentrations ratio, free dye molecules aggregate with the DNA-bound ones. The spectra of the free dye aggregates and the aggregates formed on the DNA, are characterized by an anomalously large (more than 100 nm) Stokes shift. This suggests, that the pi-electron systems of the aggregates undergo substantial changes in excited state, compared to those of the monomers. The formation of aggregates consisting of the free and DNA-bound dye molecules can be explained using the half-intercalation model of the interaction of the cyanine dye monomers with the DNA.     

A structural model for cyanine dyes templated into the minor groove of DNA

3,3-Diethylthiadicarbocyanine (DiSC₂(5)) is a monocationic dye which forms cofacial dimers that insert into the minor groove of DNA [J. Seifert, R. Conner, S. Kushon, M. Wang, B. Armitage, J. Am. Chem. Soc. 121 (1999) 2987]. These dyes self-assemble into long helical aggregates in AT-rich regions with the dimers aligned in an end-to-end fashion. A model is presented that allows for the construction of large helical aggregates with continuously variable structural parameters. The spectra or excited states are computed using a direct intermediate neglect of differential overlap (INDO) single configuration interaction (SCI) method. Results are reported for both H- and J-type aggregates ranging in size from 2 to 6 dimers. A more approximate model based on transition charge densities enables calculations of larger aggregates. These models are used to derive structural parameters of both H- and J-type aggregates from the available spectral data, resulting in a new structural model for J-type aggregation in these systems.     

Interaction of cyanine dyes with nucleic acids. XVII. Towards an aggregation of cyanine dyes in solutions as a factor facilitating nucleic acid detection

Spectral properties of newly synthesized cyanine dyes, namely 1-[6-(4-[6-[2,6-dimethyl-4-(3-methyl-2,3-dihydro-1,3-benzothiazol- 2-ylidenmethyl)-1-pyridiniumyl]hexanoyl]piperazino)-6- oxohexyl]-2,6-dimethyl-4-(3-ethyl-2,3-dihydro-1,3-benzothiazol+ ++-2-ylidenmethyl)pyridinium (K-6) (bichromophoric dye) and 1-[5-di(3-[5-[2,6-dimethyl-4-(3-methyl-2,3-dihydro-1,3-benzothiazol++ +-2-ylidenmethyl)-1-pyridiniumyl]pentylcarboxamido]pro pyl) carbamoylpentyl]-2,6-dimethyl-4-(3-methyl-2,3-dihydro-1,3-benzo thiazol-2-ylidenmethyl) pyridinium (K-T) (trichromophoric dye) in solutions in the presence of and without deoxyribonucleic acid (DNA) were studied within a wide concentration range. It has been established that absorption, as well as fluorescence of investigated dye solutions, without DNA are mainly determined by H-aggregates of dye molecules. On the contrary, the fluorescence of dye solutions in the presence of DNA gives an intrinsic dye molecular fluorescence. H-aggregates are broken because of binding dye molecules with DNA. It has been suggested that both K-T and K-6 molecules bind mainly with DNA via the interaction of two chromophores. As the ratio of the number of dye molecules to that of DNA base pairs increases with an increase in dye concentration, a formation of dye molecule H-aggregates on DNA molecules are observed. Such aggregates have a different structure than those formed in the solutions without DNA. On the grounds of the data obtained, it is concluded that it is possible to use a dye aggregation capable of obtaining higher values for fluorescence enhancement of the DNA stains.     

Antiferromagnetic porous metal-organic framework containing mixed-valence [Mn(II)4Mn(III)2(μ4-O)2]10+ units with catecholase activity and selective gas adsorption

A multifunctional porous metal organic framework based on mixed-valence hexa-nuclear [Mn(III)(2)Mn(II)(4)O(2)(pyz)(2)(C(6)H(5)CH(2)COO)(10)] (pyz = pyrazine) units has been synthesized. The complex has been characterized by elemental analysis, IR spectroscopy, single-crystal X-ray diffraction analysis, and variable-temperature magnetic measurements. The structural analysis reveals that the bidentate pyz molecules connect each [Mn(6)] unit to its four [Mn(6)] neighbors through the peripheral Mn(II) centers, giving rise to a three-dimensional (3D) distorted diamond-like porous framework. Variable-temperature (2-300 K) magnetic susceptibility measurements show the presence of dominant antiferromagnetic interactions within the discrete [Mn(6)] cluster that have been fitted with a model containing three exchange constants developed for the complex (J(1) = -8.6 cm(-1), J(2) = -3.9 cm(-1), and J(3) = -100.0 cm(-1)). Using 3,5-di-tert-butyl catechol (3,5-DTBC) as the substrate, catecholase activity of the complex has been studied; the turn over number is determined to be of 2547 h(-1) in acetonitrile. This porous compound shows remarkable selectivity for adsorption of CO(2) over N(2) that may be correlated with the effect of window flexibility of the pore to the corresponding adsorbate molecules.     

一种多核镍取代砷钨酸盐对罗丹明-B染料光催化降解行为的影响

在前期研究中,我们在水热条件下合成了一种多核镍取代砷钨酸盐[化合物(1)][Ni(H2O)(en)2]2 [Ni(H2O)3(en)][Ni(H2O)(en)]{[(α-AsW6 O26) Ni6(OH)2 (en)2.5(B-α-AsW9O34)]2 H4[W4O16][Ni4(H2O)2(en)2]2 }·13H2...     

Modulating Dye Aggregation by Incorporation into 1D‐MgAPO Nanochannels

The fluorescing dye Pyronine Y has been incorporated by crystallization inclusion into three different one‐dimensional microporous aluminophosphate host materials. A computer‐aided rational choice of the framework of the host material made it possible to modulate the aggregation state of the guest dye molecules. Undesirable H‐type dimers of Pyronine Y are included within the large channels of the AFI structure, which allow the inclusion of any of the aggregated species of the dye. Density functional theory (DFT) calculations show that H‐type aggregate formation is suppressed within the ATS framework. Experimental results indicate that red‐emissive J‐type aggregates are formed instead, offering a one‐directional, organized, multicolour emission system that is interesting for energy transport. Complete suppression of aggregation is achieved by the inclusion of Pyronine Y within the AEL‐type structure, due to its particular topology and channel dimensions This results in a highly fluorescent hybrid system with extraordinarily preferential alignment of the chromophores. Here, we report experimental evidence and modelling insights for how the “cage effect” of the nanochannels can tune the optical properties of the hybrid composite material by influencing the aggregation state of the dye.     

Effect of conjugation path length on quadratic nonlinear optical properties of monomer and aggregates of zwitterionic merocyanine dyes

We present a quantum-chemical analysis of the conjugation path length effect on first hyperpolarizabilities of a series of zwitterionic merocyanine dyes whose synthesis has been reported earlier (J. Am. Chem. Soc.2002, 124, 9431, J. Am. Chem. Soc. 1997, 119, 3144). The effect of the conjugation path lengths is evaluated to demonstrate the engineering guidelines for enhancing molecular optical nonlinearity. The first hyperpolarizabilities are calculated for extended conjugated monomer and H and J type aggregates of merocyanine dyes, to provide insight into the intermolecular interactions and the relationship between structural and collective nonlinear optical properties. The molecular geometries for monomers are obtained via B3LYP/6-31G(d,p) level optimization including the SCRF/PCM approach, and the dynamic nonlinear optical (NLO) properties for monomer and aggregates are calculated with the ZINDO/CV method, including solvent effects. It is found that the chain length dependence of the first nonlinearity peaks at n = 6 and then it starts changing slowly for monomer and aggregates of zwitterionic merocyanine dyes. It is concluded that an excellent NLO response in solution might vanish when the active chromophore forms higher H aggregates. The importance of our results on the design of electrooptic materials has been discussed.     

A phosphorus supported multisite coordinating tris hydrazone P(S)[N(Me)N=CH-C6H4-o-OH]3 as an efficient ligand for the assembly of trinuclear metal complexes: synthesis, structure, and magnetism

A phosphorus supported multisite coordinating ligand P(S)[N(Me)N=CH-C(6)H(4)-o-OH](3) (2) was prepared by the condensation of the phosphorus tris hydrazide P(S)[N(Me)NH(2)](3) (1) with o-hydroxybenzaldehyde. The reaction of 2 with M(OAc)(2).xH(2)O (M = Mn, Co, Ni, x = 4; M = Zn, x = 2) afforded neutral trinuclear complexes [P(S)[N(Me)N=CH-C(6)H(4)-o-O](3)](2)M(3) [M = Mn (3), Co (4), Ni (5), and Zn (6)]. The X-ray crystal structures of compounds 2-6 have been determined. The structures of 3-6 reveal that the trinculear metal assemblies are nearly linear. The two terminal metal ions in a given assembly have an N(3)O(3) ligand environment in a distorted octahedral geometry while the central metal ion has an O(6) ligand environment also in a slightly distorted octahedral geometry. In all the complexes, ligand 2 coordinates to the metal ions through three imino nitrogens and three phenolate oxygens; the latter act as bridging ligands to connect the terminal and central metal ions. The compounds 2-6 also show intermolecular C-H...S=P contacts in the solid-state which lead to the formation of polymeric supramolecular architectures. The observed magnetic data for the (s = 5/2)3 L(2)(Mn(II))(3) derivative, 3, show an antiferromagnetic nearest- and next-nearest-neighbor exchange (J = -4.0 K and J' = -0.15 K; using the spin Hamiltonian H(HDvV) = -2J(S(1)S(2) + S(2)S(3)) - 2J'S(1)S(3)). In contrast, the (s = 1)(3) L(2)(Ni(II))(3) derivative, 5, displays ferromagnetic nearest-neighbor and antiferromagnetic next-nearest-neighbor exchange interactions (J = 4.43 K and J' = -0.28 K; H = H(HDvV)+ S(1)DS(1) + S(2)DS(2)+ S(3)DS(3)). The magnetic behavior of the L(2)(Co(II))(3) derivative, 4, reveals only antiferromagnetic exchange analogous to 3 (J = -4.5, J' = -1.4; same Hamiltonian as for 3).     

Unexpected aggregation of neutral, xylene-cored dinuclear GdIII chelates in aqueous solution

We have synthesized ditopic ligands L(1), L(2), and L(3) that contain two DO3A(3-) metal-chelating units with a xylene core as a noncoordinating linker (DO3A(3-) = 1,4,7,10-tetraazacyclododecane-1,4,7-triacetate; L(1) = 1,4-bis{[4,7,10-tris(carboxymethyl)-1,4,7,10-tetraazacyclododecane-1-yl]methyl}benzene; L(2) = 1,3-bis{[4,7,10-tris(carboxymethyl)-1,4,7,10-tetraazacyclododecane-1-yl]methyl}benzene; L(3) = 3,5-bis{[4,7,10-tris(carboxymethyl)-1,4,7,10-tetraazacyclododecane-1-yl]methyl}benzoic acid). Aqueous solutions of the dinuclear Gd(III) complexes formed with the three ligands have been investigated in a variable-temperature, multiple-field (17)O NMR and (1)H relaxivity study. The (17)O longitudinal relaxation rates measured for the [Gd(2)L(1-3)(H2O)(2)] complexes show strong field dependence (2.35-9.4 T), which unambiguously proves the presence of slowly tumbling entities in solution. The proton relaxivities of the complexes, which are unexpectedly high for their molecular weight, and in particular the relaxivity peaks observed at 40-50 MHz also constitute experimental evidences of slow rotational motion. This was explained in terms of self-aggregation related to hydrophobic interactions, pi stacking between the aromatic linkers, or possible hydrogen bonding between the chelates. The longitudinal (17)O relaxation rates of the [Gd(2)L(1-3)(H2O)(2)] complexes have been analysed with the Lipari-Szabo approach, leading to local rotational correlation times tau(1)(298) of 150-250 ps and global rotational correlation times tau(g)(298) of 1.6-3.4 ns (c(Gd): 20-50 mM), where tau(1)(298) is attributed to local motions of the Gd segments, while tau(g)(298) describes the overall motion of the aggregates. The aggregates can be partially disrupted by phosphate addition; however, at high concentrations phosphate interferes in the first coordination sphere by replacing the coordinated water. In contrast to the parent [Gd(DO3A)(H2O)(1.9)], which presents a hydration equilibrium between mono- and dihydrated species, a hydration number of q = 1 was established for the [Ln(2)L(1-3)(H2O)(2)] chelates by (17)O chemical shift measurements on Ln = Gd and UV/Vis spectrophotometry for Ln = Eu. The exchange rate of the coordinated water is higher for [Gd(2)L(1-3)(H2O)(2)] complexes k(ex)(298) = 7.5-12.0 x 10(6) s(-1)) than for [Gd(DOTA)(H2O)](-). The proton relaxivity of the [Gd(2)L(1-3)(H2O)(2)] complexes strongly decreases with increasing pH. This is related to the deprotonation of the inner-sphere water, which has also been characterized by pH potentiometry. The protonation constants determined for this process are logK(OH) = 9.50 and 10.37 for [Gd(2)L(1)(H2O)(2)] and [Gd(2)L(3)(H2O)(2)], respectively.     

Counterion-dependent excitonic spectra of tetra(p-carboxyphenyl)porphyrin aggregates in acidic aqueous solution

Aggregates of the diacid form of tetra(p-carboxyphenyl)porphyrin (TCPP) are found to be stabilized in aqueous solution at low pH in the presence of poly(vinyl alcohol). At pH values in the range from about 1 to 4, a split Soret band is observed which is independent of counterion and tentatively assigned to a dimer species. As the pH is made lower than 1, the spectra evolve to reveal the presence of porphyrin aggregates. As in the case of the well-known aggregates of the related tetra(p-sulfonatophenyl)porphyrin (TSPP) diacid, the concentration of spectroscopically distinguishable aggregates increases with increasing ionic strength or decreasing pH. Unlike aggregates of TSPP, however, TCPP aggregates below pH 1 have visible absorption spectra which depend on the counterion, which is Cl(-) or NO(3)(-) in this study. In this work, we present visible absorption, light-scattering, and resonance Raman spectra of TCPP diacid in its monomer, dimer, and aggregated forms and attempt to understand the structural basis for counterion-dependent structure and excitonic coupling in the aggregates. Evidence is presented for intercalation of inorganic counterions between porphyrin molecules in the aggregate, an effect which to our knowledge has not been previously reported.