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 (N6D2) or two dansyl and six naphthyl units (N2D6) 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 N2D6 and N6D2 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 CF3SO3H to dendrimer solutions in CH3CN/CH2Cl2 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 cyclamcored 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 N6D2 and it can be increased up to 75% upon protonation of the cyclam core and formation of N6D2(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.