A Light-Harvesting Antenna Resulting from the Self-Assembly of Five Luminescent Components: A Dendrimer, Two Clips, and Two Lanthanide Ions

We have investigated the self-assembly of three luminescent species in CH3CN/CH2Cl2, namely: 1) a polylysin dendrimer (D) composed of 21 aliphatic amide units and 24 green luminescent dansyl chromophores at the periphery, 2) a molecular clip (C) with two blue luminescent anthracene sidewalls and a benzene bridging unit that bears two sulfate groups in the para position, and 3) a near infrared (NIR)-emitting Nd3+ ion. For purposes of comparison, analogous systems have also been investigated in which Gd3+ replaced Nd3+. The dendrimer and the clip can bind Nd3+ ions with formation of [D center dot 2Nd(3+)] and [C center dot Nd3+] complexes, in which energy transfer from dansyl and, respectively, anthracene to Nd3+ ion takes place with 65 and 8% efficiency, in air-equilibrated solution. In the case of [C center dot Nd3+], the energy-transfer efficiency is quenched by dioxygen, thereby showing that the energy donor is the lowest triplet excited state of anthracene. In [D center dot 2Nd(3+)] the intrinsic emission efficiency of Nd3+ is much higher (ca. 5 times) than in [C center dot Nd3+] because of a better protection of the excited lanthanide ion towards nonradiative deactivation caused by interaction with solvent molecules. By mixing solutions of D, Nd3+, and C with proper concentrations, a supramolecular structure with five components of three different species, [D center dot 2 Nd3+center dot 2C], is formed. The excitation light absorbed by the clips is transferred with 100% efficiency to the dansyl units of the dendrimer and then to the Nd3+ ions with 65% efficiency either in the presence or absence of dioxygen. These results show that the [D center dot 2Nd(3+) center dot 2C] complex is able to efficiently harvest UV light by the 24 dansyl units of the dendrimer and the four anthracene chromophores of the two clips, and efficiently transfer it to the encapsulated Nd3+ ions that emit in the NIR spectral region.