Investigation of electronic energy transfer in a BODIPY-decorated calix[4]arene

Extremely fast triplet formation by charge recombination in a Nile Red/fullerene flexible dyad

“…Fluorescence quantum yields are high (E80%) in both the solvents, with a radiative rate constant amounting to 1.7 Â 10 8 s À1 quite independently of the solvent. 32 Absorption and emission bands of 4 retain the positive solvatochromism of Nile Red, suggesting that the presence of the calixarene scaffold does not perturb the electronic properties of the chromophore. 63 Similarly, compound 5 displays the main spectroscopic properties of C 60 .…”

“…2-(2-Aminoethoxy)-9-diethylamino-5H-benzo[a]phenoxazin-5one (NR-NH 2 ), 32 25,26,27,28-tetra-n-propoxycalix [4]arene-5,17dicarboxylic acid (1) 42 and compound 4 32 were synthesized according to a literature procedure.…”

“…A preliminary conformational search for dyad 3 was performed in vacuum with molecular mechanics. In order to restrict the number of possible structures, as suggested by a previous work, 32 we performed a scan along each dihedral angle of the two spacers and, for each point of the scan, we optimized the geometry with MMFF94 force field [53][54][55][56][57] (PCmodel software, v. 10.0). 58 Among the structures characterized by an open conformation of the calixarene macrocycle (suggested by NMR data), the most stable were then reoptimized with MMFF94 and were used as starting point for DFT calculations.…”

“…In previous works some of us have studied calix [4]arenes similar to 3, functionalized at the upper rim with pairs of different chromophores linked to the calixarene through an analogous amide linkage. 28,32 These compounds were found to adopt a closed structure thanks to the formation of an intramolecular hydrogen bond between the NH and CO groups of the amide moieties. In case of compound 3, most likely the steric hindrance of the fullerene prevents the formation of the intramolecular COÁ Á ÁHN hydrogen bond.…”

“…Calix [4]arenes are ideal scaffolds for the preparation of chromophoric dyads designed for the study of energy/charge transfer phenomena. [25][26][27][28][29] The orientation, mutual distance and conformational freedom of the dyes can be accurately tuned thanks to (i) the availability of four different configurations (cone, partial cone, 1,2-alternate and 1,3-alternate) of the calixarene scaffold, 30 which project the substituents in different directions; 31,32 (ii) the possibility of finely modulating the residual flexibility of the scaffold thanks to the lower rim functionalization; [31][32][33][34] (iii) the possibility of attaching the dyes directly to the aromatic rings of the macrocycle 35 or through linkers of different rigidity/flexibility. 31,32 In our system the two dyes are linked to a flexible tetraalkylated cailx [4]arene scaffold through short but flexible linkers in order to endow the dyad of enough conformational freedom to allow the chromophores to get in close contact.…”