Light-harvesting dendrimers.

Dendrimers are well-defined, tree-like macromolecules, with a high degree of order and the possibility to contain selected chemical units in predetermined sites of their structure. Dendrimers are currently attracting the interest of many scientists because of their unusual chemical and physical properties and the wide range of potential applications. It is possible to design and synthesize dendrimers containing a variety of chromophoric groups organized in the dimensions of time, energy and space so as to obtain efficient light-harvesting devices that can be useful for solar energy conversion and other purposes.

Luminescent dendrimers. Recent advances.

Luminescent dendrimers are currently attracting much attention since coupling luminescence and dendrimer research topics can lead to valuable new functions. Indeed, luminescence is a valuable tool to monitor both basic properties and possible applications (sensors, displays, lasers), and dendrimers are macromolecular compounds exhibiting a well-defined chemical structure with the possibility of containing selected chemical units in predetermined sites and of encapsulating ions or neutral molecules in their internal dynamic cavities. In this paper we will review recent advances in this field focusing our attention on their properties in fluid solution related to light harvesting, changing the "color" of light, sensing with signal amplification, quenching and sensitization processes, shielding effects, elucidation of dendritic structures and superstructures, and investigation of dendrimer rotation in solution.

Cyclam-cored dendrimers appended with four dendrons of two different types: intradendrimer energy transfer.

We have synthesized two cyclam-cored dendrimers appended with dendrons of two different types by proper protection/deprotection of the cyclam unit. The resulting dendrimers contain six naphthyl and two dansyl units (N6 D2) or two dansyl and six naphthyl units (N2 D6) at the periphery. Their photophysical properties have been compared to those of a dendrimer containing 8 dansyl units (D8) and a previously investigated dendrimer containing 8 naphthyl units (N8). The absorption spectra are those expected on the basis of the number of chromophores, demonstrating that no ground state interaction takes place. The emission spectra of N2 D6 and N6 D2 show naphthalene localized and naphthalene excimer emission similar to those observed in the case of N8, together with a much stronger dansyl emission with maximum at 525 nm. Addition of CF(3)SO(3)H to dendrimer solutions in CH(3)CN/CH(2)Cl(2) 1:1 (v/v) leads to protonation of the aliphatic amine units of the cyclam core at first and then of the aromatic amine of each dansyl chromophores. Cyclam can be diprotonated and this affects dansyl absorption and, most significantly, emission bands by a charge perturbation effect. Each dansyl unit is independently protonated in both dendrimers. The most interesting photophysical feature of these heterofunctionalized cyclam-cored dendrimers is the occurrence of an intradendrimer photoinduced energy transfer from naphthyl to dansyl chromophores of two different dendrons (interdendron mechanism). The efficiency of this process is 50 % for N6 D2 and it can be increased up to 75 % upon protonation of the cyclam core and formation of N6 D2(2H(+)). This arises from the fact that protonation of the amine units of the cyclam prevents formation of exciplexes upon naphthyl excitation, thus shutting down one of the deactivation processes of the fluorescent naphthyl excited state.

Phosphino-aminothiazoline platinum(II) and platinum(II)/gold(I) complexes: structural, chemical and vapoluminescent properties.

Phosphino-amino-thiazolines and -thiazoles can exist in solution in two tautomeric forms, in which the N-H proton involves the endo-cyclic or exo-cyclic nitrogen atom. The two tautomers show different reactivities toward alcoholysis; the imino form degrades more rapidly. Their bischelated platinum complexes were studied in the solid state by single crystal X-ray diffraction. Thus, the unique stereoelectronic features of the [Pt(PN(th))] (PN(th)=diphenylposphino-aminothiazoline) moiety were revealed. The complex cis-[Pt(PN(th))(2)] reacts with gold(I) salts to yield dimetallic compounds, the molecular structures of which have been determined by X-ray diffraction. Solid cis-[Pt(PN(th))(2)] shows vapoluminescent properties if exposed to alcohol vapors. A combined photophysical and crystallographic investigation has been carried out to clarify the unprecedented rigidochromic role of the alcohol in this phenomenon.

The chemistry of 6-hydroxyflavylium: zwitterionic base and p-quinoidal chalcones. A multiswitchable system operated by proton, electron and photon inputs.

The complex network of chemical reactions of the compound 6-hydroxyflavylium perchlorate was studied by different techniques including UV-vis absorption spectrophotometry, stopped-flow, electrochemistry, and photochemistry. The network is characterized by the formation of a very reactive zwitterionic base as well as by p-quinoidal chalcones. The trans-chalcone is metastable in very acidic solutions and exhibits reversible redox reactions, allowing to introduce for the first time in the flavylium network an electrochemical input.

First generation TREN dendrimers functionalized with naphthyl and/or dansyl units. Ground and excited state electronic interactions and protonation effects.

We report the photophysical properties (absorption and emission spectra, quantum yield, and lifetime) of five dendrimers of first generation based on a TREN (tris(2-aminoethyl)amine) skeleton functionalized at the periphery with naphthyl and/or 5-dimethylamino-1-naphthalenesulfonamide (hereafter called dansyl) chromophores. Each dendrimer comprises one tertiary amine unit in the core and three branches carrying a sulfonimido unit at the periphery, each one substituted by two identical or different moieties. In particular, TD6 and TN6 contain dansyl (D) or naphthyl (N) units, respectively, while TD3B3, TN3B3 and TN3D3 contain dansyl, naphthyl or benzyl (B) units at the periphery. The spectroscopic behaviour of these dendrimers has been investigated in acetonitrile solution and compared with that of reference compounds. For all dendrimers the absorption bands are red shifted compared to those of monomeric naphthyl and dansyl reference compounds. Moreover, the intense naphthyl and dansyl fluorescence is greatly quenched because of strong interactions between the two aromatic moieties linked by a sulfonimido unit. Protonation of the amine units of the dendrimers by addition of CF(3)SO(3)H (triflic) acid causes a decrease in intensity of the luminescence and a change in the shape of the emission bands. The shapes of the titration curves depend on the dendrimer, but in any case the effect of acid can be fully reversed by successive addition of base (tributylamine). The obtained results reveal that among the intradendrimer interactions the most important one is that taking place (via mesomeric interaction) between the various chromophores and a pair of sulfonimido groups.

A cyclam core dendrimer containing dansyl and oligoethylene glycol chains in the branches: protonation and metal coordination.

We have synthesized a dendrimer (1) consisting of a 1,4,8,11-tetraazacyclotetradecane (cyclam) core, appended with four benzyl substituents that carry, in the 3- and 5-positions, a dansyl amide derivative (of type 2), in which the amide hydrogen is replaced by a benzyl unit that carries an oligoethylene glycol chain in the 3- and 5-positions. All together, the dendrimer contains 16 potentially luminescent moieties (eight dansyl- and eight dimethoxybenzene-type units) and three distinct types of multivalent sites that, in principle, can be protonated or coordinated to metal ions (the cyclam nitrogen atoms, the amine moieties of the eight dansyl units, and the 16 oligoethylene glycol chains). We have studied the absorption and luminescence properties of 1, 2, and 3 in acetonitrile and the changes taking place upon titration with acid and a variety of divalent (Co2+, Ni2+, Cu2+, Zn2+), and trivalent (Nd3+, Eu3+, Gd3+) metal ions as triflate and/or nitrate salts. The results obtained show that: 1) double protonation of the cyclam ring takes place before protonation of the dansyl units; 2) the oligoethylene glycol chains do not interfere with protonation of the cyclam core and the dansyl units in the ground state, but affect the luminescence of the protonated dansyl units; 3) the first equivalent of metal ion is coordinated by the cyclam core; 4) the interaction of the resulting cyclam complex with the appended dansyl units depends on the nature of the metal ion; 5) coordination of metal ions by the dansyl units follows at high metal-ion concentrations; 6) the effect of the metal ion depends on the nature of the counterion. This example demonstrates that dendrimers may exhibit complete functionality resulting from the integration of the specific properties of their component units.

Luminescent ruthenium(II) bipyridine-calix[4]arene complexes as receptors for lanthanide cations.

The synthesis and photophysical properties of novel luminescent ruthenium(II) bipyridyl complexes containing one, two, or six lower rim acid-amide-modified calix[4]arene moieties covalently linked to the bipyridine groups are reported which are designed to coordinate and sense luminescent lanthanide ions. All the Ru-calixarene complexes synthesized in this work are able to coordinate Nd(3+), Eu(3+), and Tb(3+) ions with formation of adducts of variable stoichiometry. The absorbance changes allow the evaluation of association constants whose magnitudes depend on the nature of the complexes as well as on the nature of the lanthanide cation. Lanthanide cation complex formation affects the ruthenium luminescence which is strongly quenched by Nd(3+) ion, moderately quenched by the Eu(3+) ion, and poorly or moderately increased by the Tb(3+) ion. In the case of Nd(3+), the excitation spectra show that (i) the quenching of the Ru luminescence occurs via energy transfer and (ii) the electronic energy of the excited calixarene is not transferred to the Ru(bpy)(3) but to the neodymium cation. In the case of Tb(3+), the adduct's formation leads to an increase of the emission intensities and lifetimes. The reason for this behavior was ascribed to the electric field created around the Ru calix[4]arene complexes by the Tb(3+) ions by comparison with the Gd(3+) ion, which behaves identically and can affect ruthenium luminescence only by its charge. However, especially for compounds 1 and 3, it cannot be excluded that some contribution comes from the decrease of vibrational motions (and nonradiative processes) due to the rigidification of the structure upon Tb(3+) complexation. In the case of Eu(3+), compounds 1, 2, and 4 were quenched by the lanthanide addition but the quenching of the ruthenium luminescence is not accompanied by europium-sensitized emission which suggests that an electron-transfer mechanism is responsible for the quenching. On the contrary, compound 3 exhibits enhanced emission upon addition of Eu(3+) (as nitrate salt); it is suggested that the lack of quenching in the [3.2Eu(3+)] adduct is due to kinetic reasons because the electron-transfer quenching process is thermodynamically allowed.

Photochemical and photophysical properties of poly(propylene amine) dendrimers with peripheral naphthalene and azobenzene groups.

We report the preparation, the absorption spectra, and the photophysical and photochemical properties in dichloromethane solution of four dendrimers of the poly(propylene amine) family (indicated by POPAM or PPI) functionalised with naphthalene and trans-azobenzene units. Each dendrimer Gn, where n = 1 to 4 is the generation number, comprises 2(n+1)-2 (i.e., 30 for G4) tertiary amine units in the interior and 2(n1) (i.e., 32 for G4) naphthalene and trans-azobenzene units in the periphery. For comparison purposes, the photophysical and photochemical properties of model compounds of the peripheral units have also been investigated. We have found that the fluorescence of the naphthalene units is quenched by the tertiary amines (via electron transfer) as well as by the trans-azobenzene units (via energy transfer). The quantum yields of the trans-->cis and cis-->trans photoisomerisation of the azobenzene units have been measured at various excitation wavelengths. Quenching of the fluorescence of the excited naphthalene unit by the trans- and cis-azobenzene units is accompanied by the sensitisation of the cis-->trans (but not of the trans-->cis) isomerisation. The rate constant of the thermal cis-->trans isomerisation of the azobenzene units has also been measured. Comparison of the results obtained for model compounds and for the G4 dendrimer shows that the dendritic structure favours the trans configuration of the azobenzene units ("dendritic effect"), presumably because it is less demanding in terms of space and causes less crowding on the dendrimer surface.

Multistate/multifunctional systems. A thermodynamic, kinetic, and photochemical investigation of the 4'-dimethylaminoflavylium compound.

The 4'-dimethylaminoflavylium ion in aqueous solution undergoes an intricate network of chemical reactions controlled by pH and light excitation. It is shown that nine different forms are involved, including two species that are not present in previously investigated compounds of the flavylium family. The thermodynamic and kinetic constants of the equilibria and interconversion processes have been obtained by pH jump (included stopped-flow) experiments. The photochromic properties exhibited by the trans/cis chalcone forms have been investigated. The peculiar aspect of 4'-dimethylaminoflavylium, as compared to previously investigated compounds of the same family, is a close to planarity structure, as demonstrated by the X-ray analysis on the parent 4'-aminoflavylium compound (2.3 degrees torsion angle between the benzopyrylium and benzene ring). The results obtained show that the flavylium cation is strongly stabilized by the electron-donor character of the dimethylamino substituent on the benzene ring. The donor-acceptor interaction makes both the protonation of the amino group and the hydration of the flavylium cation difficult, with consequences on the tautomerization and cis/trans isomerization reactions. The multistate/multifunctional properties of 4'-dimethylaminoflavylium have been discussed in the frame of write-lock-read-unlock-erase cycles.

Luminescent lanthanide ions hosted in a fluorescent polylysin dendrimer. Antenna-like sensitization of visible and near-infrared emission.

We have investigated the complexation of the luminescent Nd(3+), Eu(3+), Gd(3+), Tb(3+), Er(3+), and Yb(3+) ions by a polylysin dendrimer containing 21 amide groups in the interior and, in the periphery, 24 chromophoric dansyl units which show an intense fluorescence band in the visible region. Most of the experiments were performed in 5:1 acetonitrile/dichloromethane solution at 298 K. On addition of the lanthanide ions to dendrimer solutions, the fluorescence of the dansyl units is quenched; in Nd(3+), Er(3+), and Yb(3+), a sensitized near-infrared emission of the lanthanide ion is observed. At low metal ion concentrations, each dendrimer hosts only one metal ion and when the hosted metal ion is Nd(3+) or Eu(3+), the fluorescence of all the 24 dansyl units of the dendrimer is quenched with unitary efficiency. Quantitative measurements were performed in a variety of experimental conditions, including protonation of the dansyl units and measurements in rigid matrix at 77 K where a sensitized Eu(3+) emission could also be observed. The results obtained have been interpreted on the basis of the energy levels and redox potentials of dendrimer and metal ions.

Forward (singlet-singlet) and backward (triplet-triplet) energy transfer in a dendrimer with peripheral naphthalene units and a benzophenone core.

The photochemical and photophysical behaviour of two dendrimers consisting of a benzophenone core and branches that contain four (4) and eight (5) naphthalene units at the periphery has been investigated in CH(2)Cl(2) solution (298 K) and in CH(2)Cl(2)/CHCl(3) 1:1 v/v rigid matrix (77 K). For comparison purposes, the photophysical properties of dimethoxybenzophenone (1), 2-methylnaphthalene (2) and of a dendron containing four naphthalene units (3) have also been studied. In both dendrimers 4 and 5, excitation of the peripheral naphthalene units is followed by fast (1.1 x 10(9) s(-1) at 298 K, > 2.5 x 10(9) s(-1) at 77 K for 5; 2.9 x 10(8) s(-1) at 298 K, 7 x 10(5) s(-1) at 77 K for 5) singlet-singlet energy transfer to the benzophenone core. On a longer time scale (>1 x 10(6) s(-1) at 298 K, >6 x 10(3) s(-1) at 77 K for 4; 3.1 x 10(7) s(-1) at 298 K, ca. 3 x 10(2) s(-1) at 77 K for 5) a back energy transfer process takes place from the triplet state of the benzophenone core to the triplet state of the peripheral naphthalene units. Selective excitation of the benzophenone unit is followed by intersystem crossing and triplet-triplet energy transfer to the peripheral naphthalene units. In hydrogen donating solvents, the benzophenone core is protected from degradation by the presence of the naphthalene units. In solutions containing Tb(CF(3)SO(3))(3), sensitization of the green Tb(3+) luminescence is observed on excitation of both the peripheral naphthalene units and the benzophenone core of 5. Upon excitation of the naphthalene absorption band (266 nm) with a laser source, intradendrimer triplet-triplet annihilation of naphthalene excited states leads to delayed naphthalene fluorescence (lambda(max)= 335 nm), that can also be obtained upon excitation at 355 nm (benzophenone absorption band). The results obtained show that preorganization of photoactive units in a dendritic structure can be exploited for a variety of useful functions, including photosensitized emission, protection from undesired photoreactions, and energy up-conversion.

Multistate/multifunctional behaviour of 4'-hydroxy-6-nitroflavylium: a write-lock/read/unlock/enable-erase/erase cycle driven by light and pH stimulation.

We have investigated the network of reactions observed for the photochromic 4'-hydroxy-6-nitroflavylium compound in aqueous solutions upon pH changes (including pH jump and stopped flow experiments) and light excitation. The changes observed in the NMR and UV/Vis spectra allowed identification of ten different forms in which this compound can be transformed depending on the experimental conditions. Equilibrium and kinetic constants have been determined. Compared with other members of the flavylium family, 4'-hydroxy-6-nitroflavylium is characterized by a large cis-->trans isomerization barrier, and a very efficient hydration reaction. These peculiar features allow writing, reading, storing and erasing photonic information on 4'-hydroxy-6-nitroflavylium by a novel cyclic process that involves the following steps: write-lock/read/unlock/enable-erase/erase.

Dendrimers as ligands. Formation of a 2:1 luminescent complex between a dendrimer with a 1,4,8,11-tetraazacyclotetradecane (cyclam) core and Zn2+.

We have investigated the formation of metal complexes between Zn2+ and two derivatives, 1 and 2, of the well-known 1,4,8,11-tetraazacyclotetradecane (cyclam) ligand. Compound 1 is 1,4,8,11-tetrakis(naphthylmethyl) cyclam, and compound 2 is a dendrimer consisting of a cyclam core with appended 12 dimethoxybenzene and 16 naphthyl units. Compound 1 exhibits an emission band with a maximum around 480 nm, assigned to the formation of exciplexes between amine and excited naphthyl units. Dendrimer 2 exhibits three types of weak emission bands, assigned to naphthyl localized excited states (lambdamax = 337 nm), naphthyl excimers (lambdamax ca. 390 nm), and naphthyl-amine exciplexes (lambdamax = 480 nm). In CH3CN-CH2Cl2 1:1 v/v, titration of ligand 1 with Zn2+ causes the disappearance of the exciplex emission and the appearance of a strong naphthyl localized fluorescence; the titration plot is linear and reaches a plateau for a 1:1 stoichiometry, showing that a highly stable [Zn(1)]2+ complex is formed. In the case of 2, titration with Zn2+ causes the disappearance of the exciplex band, with a concomitant increase in the excimer and naphthyl localized emissions; the titration plot is again linear, but in this case it reaches a plateau for a 2:1 stoichiometric ratio, showing the unexpected formation of a [Zn(2)2]2+ complex. Such an unexpected stoichiometry for the complex of the dendritic ligand has been fully confirmed by 1H NMR titrations. The results obtained show that the dendrimer branches not only do not hinder, but in fact favor coordination of cyclam to Zn2+.

Proton-driven self-assembled systems based on cyclam-cored dendrimers and [Ru(bpy)(CN)4]2-.

1,4,8,11-tetraazacyclotetradecane (cyclam), which is one of the most extensively investigated ligands in coordination chemistry, in its protonated forms, can play the role of host toward cyanide metal complexes. We have investigated the acid-driven adducts formed in acetonitrile-dichloromethane (1:1 v/v) solution by [Ru(bpy)(CN)4](2-) with 1,4,8,11-tetrakis(naphthylmethyl)cyclam (1) and a dendrimer consisting of a cyclam core appended with 12 dimethoxybenzene and 16 naphthyl units (2). [Ru(bpy)(CN)4](2-), 1, and 2 exhibit characteristic absorption and emission bands, in distinct spectral regions, that are strongly affected by addition of acid. When a solution containing equimolar amounts of [Ru(bpy)(CN)4](2-) and 1 or 2 is titrated by trifluoroacetic acid, or when [Ru(bpy)(CN)4](2-) is titrated with (1.2H)2+ or (2.2H)2+, [[Ru(bpy)(CN)4](2-).(2H+).1] or [[Ru(bpy)(CN)4](2-).(2H+).2] adducts are formed in which the fluorescence of the naphthyl units is strongly quenched by very efficient energy transfer to the metal complex, as shown by the sensitized luminescence of the latter. The [[Ru(bpy)(CN)4]2-.(2H+).1] and [[Ru(bpy)(CN)4](2-).(2H+).2] adducts can be disrupted (i) by addition of a base (1,4-diazabicyclo[2.2.2]octane), yielding the starting species [Ru(bpy)(CN)4](2-) and 1 or 2, or (ii) by further addition of triflic acid, with formation of (1.2H)2+ or (2.2H)2+ and protonated forms of [Ru(bpy)(CN)4](2-). It is shown that upon stimulation with two chemical inputs (acid and base) both [[Ru(bpy)(CN)4](2-).(2H+).1] and [[Ru(bpy)(CN)4](2-).(2H+).2] exhibit two distinct optical outputs (a naphthalene-based and a Ru(bpy)-based emission) that behave according to an XOR and an XNOR logic, respectively.

Cyclam-based dendrimers as ligands for lanthanide ions.

We have investigated the complexation of lanthanide ions (Nd3+, Eu3+, Gd3+, Tb3+, Dy3+) with three cyclam-based ligands (cyclam = 1,4,8,11-tetraazacyclotetradecane), namely 1,4,8,11-tetrakis(naphthylmethyl)cyclam (1), and two dendrimers consisting of a cyclam core appended with four dimethoxybenzene and eight naphthyl units (2) and twelve dimethoxybenzene and sixteen naphthyl units (3). In the free ligands the fluorescence of the naphthyl units is strongly quenched by exciplex formation with the cyclam nitrogens. Complexation with the metal ions prevents exciplex formation and revives the intense naphthyl fluorescence. Fluorescence and NMR titration experiments have revealed the formation of complexes with different metal/ligand stoichiometries in the case of 1, 2 and 3. Surprisingly, the large dendrimer 3 gives rise to a stable [M(3)3]3+ species. Energy transfer from the lowest singlet and triplet excited states of the peripheral naphthyl units to the lower lying excited states of Nd3+, Eu3+, Tb3+, Dy3+ coordinated to the cyclam core does not take place.

Ground and excited-state electronic interactions in poly(propylene amine) dendrimers functionalized with naphthyl units: effect of protonation and metal complexation.

We report the absorption spectra and the photophysical properties (fluorescence spectrum, quantum yield, and lifetime) of four dendrimers of the poly(propylene amine) family (POPAMs) functionalized at the periphery with naphthylsulfonamide (hereafter called naphthyl) units. Each dendrimer Gn, where n = 1 to 4 is the generation number, comprises 2n + 1 (i.e., 32 for G4) naphthyl functions in the periphery and 2n + 1--2 (i.e., 30 for G4) tertiary amine units in the branches. All the experiments have been carried out in acetonitrile solutions. Comparison with two reference compounds (N-methyl-naphthalene-2-sulfonamide, A, and N-(3-dimethylamino-propyl)-2-naphthalene-1-sulfonamide, B) has shown that the absorption spectra of the dendrimers are significantly different from those expected from the component units. Furthermore, the intense fluorescence band of the naphthyl unit (lambda max = 343 nm; phi = 0.15, tau = 8.5 ns) is strongly quenched in the dendrimers. The quenching effect increases with increasing generation and is accompanied by the appearance of a weak and broad emission tail at lower energy. Protonation of the amine units of the dendrimers by addition of CF3SO3H (triflic) acid causes a strong increase in the intensity of the naphthyl luminescence and a change in the form of the emission tail. The shapes of the titration curves depend on dendrimer generation, but in any case, the effect of the acid can be fully reversed by successive addition of a base (tributylamine). The results obtained show that in the dendrimers there are interactions (both in the ground and excited states) between naphthyl units as well as between naphthyl units and amine units of the branches; this gives rise to dimer/excimer and charge-transfer/exciplex excited states. Titration with Zn(CF3SO3)2 has the same effect as acid titration, as far as the final emission spectrum is concerned, but a much higher concentration of Zn(CF3SO3)2 has to be used and the shapes of the titration plots are very different. Titration with Co(NO3)2.6H2O causes a much smaller increase in the intensity of the naphthyl fluorescence compared with Zn(CF3SO3)2. The results obtained have shown that protonation and metal coordination can reveal the presence of ground and excited state electronic interactions in functionalized poly(propylene amine) dendrimers, and that the presence of photo-active units in the dendrimers can be useful to reveal some peculiar aspects of the protonation and metal coordination processes.

Photochemical and photophysical properties of a poly(propylene amine) dendrimer functionalized with E-stilbene units.

A second generation poly(propylene amine) dendrimer functionalized at the periphery with eight E-stilbene and eight 4-tert-butylbenzenesulfonyl units has been prepared. The absorption spectrum, fluorescence spectrum and decay, E<==>Z photoisomerization, and photocyclization of the Z-isomer of the stilbene units have been investigated in air equilibrated acetonitrile solutions. For comparison purposes, a reference compound of the peripheral dendrimer units, namely 4-tert-butyl-N-propyl-N-(4-styryl-benzyl)-benzenesulfonamide, has also been studied. The quantum yield of the E-->Z photoisomerization reaction (0.30) and the fluorescence quantum yield of the E isomer (0.014) are substantially smaller for the units appended to the dendrimer compared to those of the reference compound (0.50 and 0.046, respectively). The presence of a red tail and the biexponential decay of the emission band of the dendrimer indicate formation of excimers between the stilbene units appended at the poly(propylene amine) dendritic structure. Under the experimental conditions used (lambda(exc)= 313 nm), a Z/E photostationary state (around 9 : 1 for both reference compound and dendrimer ) is reached in the time scale of minutes. On continuing irradiation, other photoreactions take place in the time scale of hours: the stilbene moiety of compound undergoes photocyclization to phenanthrene (quantum yield 0.015), whereas in dendrimer photocyclization to phenanthrene is accompanied by other processes, including a photoreaction involving the internal amine groups.

Dendrimers as ligands: an investigation into the stability and kinetics of Zn2+ complexation by dendrimers with 1,4,8,11-tetraazacyclotetradecane (cyclam) cores.

We have investigated the complexation of Zn(2+) with 1,4,8,11-tetrakis(naphthylmethyl) cyclam (1; cyclam=1,4,8,11-tetraazacyclotetradecane) and with two dendrimers consisting of a cyclam core with four dimethoxybenzene and eight naphthyl appendages (2), and twelve dimethoxybenzene and sixteen naphthyl appendages (3). An important, common feature of model compound 1 and dendrimers 2 and 3 is that their potentially fluorescent naphthyl units are quenched by exciplex formation with the cyclam nitrogen atoms. Complexation with Zn(2+), however, prevents exciplex formation and results in the appearance of an intense naphthyl fluorescence signal that can be used for monitoring the complexation process. Luminescence titration, together with competition experiments and (1)H NMR titration, have shown that 1:1 and 1:2 (metal/ligand) complexes are formed in the cases of 2 and 3, whereas model compound 1 gives only a 1:1 complex. We have also investigated the 1:1 complexation kinetics by the stopped-flow technique. In the case of 1, a second-order process (k(1)=44x10(5) M(-1) s(-1)) is followed by two consecutive first-order steps (k(2)=0.53 s(-1) and k(3)=0.10 s(-1)). For 2, a slower second-order process (k(1)=4.9x10(5) M(-1) s(-1)) is followed by a slow first-order step (k(2)=0.40 s(-1)). In the case of 3, only a very slow second-order process was observed (k(1)=1.2x10(5) M(-1) s(-1)). The different metal-ion incorporation rates for model compound 1 and dendrimers 2 and 3 have been discussed in terms of conformational changes of the dendron subunits affecting the chelating properties of the cyclam core. This work reports the first kinetic study on metal-ion coordination by dendrimers with a well-defined coordination site.