Jeroen van Heyst

Dendrimers based on a bis-cyclam core as fluorescence sensors for metal ions

We have synthesized a novel dendrimer (1) based on two covalently linked cyclam units as a core appended to six branches, each one consisting of one dimethoxybenzene and two naphthyl units (cyclam = 1,4,8,11-tetraazacyclotetradecane). Such a dendrimer shows three fluorescence bands which can be assigned to naphthyl localized excited states (λmax = 336 nm), naphthyl excimers (λmaxca. 390 nm), and naphthyl–amine exciplexes (λmax = 510 nm). Protonation or complexation of the bis-cyclam core with Zn2+ does not affect the absorption spectrum of the dendrimer, but causes noticeable changes in the fluorescence intensity of the three component bands. Complexation with Cu2+ not only causes changes in the relative intensities of the fluorescence bands, but also the appearance of a new absorption band in the near UV spectral region. Analysis of the titration curves has allowed us to obtain clear evidence for the formation of 1 : 1 (1(H+), [Zn(1)]2+, [Cu(1)]2+) and 2 : 1 (1(2H+), [Zn2(1)]4+, [Cu2(1)]4+) species. Comparison with the behaviour of a previously investigated parent monocyclam dendrimer (2) suggests that in the 1 : 1 species of 1 both the cyclam units are involved in the complexation with Zn2+ and Cu2+.

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.

Photochemical and photophysical properties of a poly(propylene amine) dendrimer functionalised 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.