Fullerene Complexation in a Hydrogen-Bonded Porphyrin Receptor via Induced-Fit: Cooperative Action of Tautomerization and C–H···π Interactions

Abstract

A supramolecular chiral hydrogen-bonded tetrameric aggregate possessing a large cavity and tetraarylporphyrin substituents was assembled using alternating 4H-and 2H-bonds between ureidopyrimidinone and isocytosine units, respectively. The aggregation mode was rationally shifted from social to narcissistic self-sorting by changing urea substituent size only. The H-bonded tetramer forms a strong complex with C 60 guest, at the same time undergoing remarkable structural changes. Namely, the cavity adjusts to the guest via keto-to-enol tautomerization of the ureidopyrimidinone unit and as a result, porphyrin substituents move apart from each other in a scissor blade-like opening fashion. The rearrangement is accompanied by C−H•••π interaction between the alkyl solubilizing groups and the nearby placed porphyrin π-systems. The latter interaction was found to be crucial for the guest complexation event, providing energetic compensation for otherwise costly tautomerization. We showed that only the systems possessing sufficiently long alkyl chains capable of interacting with a porphyrin ring are able to form a complex with C 60 . The structural rearrangement of the tetramer was quantitatively characterized by electron paramagnetic resonance pulsed dipolar spectroscopy measurements using photogenerated triplets of porphyrin and C 60 as spin probes. Further exploring the C−H•••π interaction as a decisive element for the C 60 recognition, we investigated the guest-induced self-sorting phenomenon using scrambled tetramer assemblies composed of two types of monomers possessing alkyl chains of different lengths. The presence of the fullerene guest has enabled the selective scavenging of monomers capable of C−H•••π interaction to form homo-tetrameric aggregates.

Recent advances in supramolecular fullerene chemistry

“…In addition to using large π-conjugated molecules, some small molecules also form host–guest complexes with fullerenes due to hydrogen bonding. 377–383 For instance, Stefankiewicz and coworkers reported a hydrogen-bonded capsule, self-assembled by eight amino acid functionalized molecules 64 through 48 hydrogen bonds between the carboxyl and amine moieties of adjacent monomers (Fig. 23a), which was confirmed by SCXRD and multiple NMR methods in chlorinated solvents.…”

“…Orentas, Wärnmark and coworkers developed a series of ureidopyrmidinone (UPy)-based hydrogen bonding tubes that exhibit intriguing dynamic behavior, such as self-sorting and solvent/guest-induced rearrangement. 380,381,385,386 By adding C 60 , an octameric tube (65) 4 comprising multiple hydrogen bonds was rearranged to capsular complex C 60 ⊂ (66) 4 due to encapsulation of C 60 (Fig. 23b), which was investigated by multiple NMR measurements.…”

Quantum chemical investigations on a supramolecular porphyrin-fulleropyrrolidine dyads for photovoltaic devices

“…For the construction of potential molecular architectureas a result of EDA interactions, both porphyrins [12,13] and fullerenes [14][15][16] have been extensively utilized as e cient building blocks for their typical physicochemical [17] and photophysical properties [18]. The porphyrin-fullerene conjugates are of remarkable utility due to their electronic complementary [19][20][21] and spontaneous interaction between porphyrin and fullerene [22][23][24][25][26][27][28][29][30][31][32][33][34]. Due to the presence of conjugated π-system, porphyrin macrocycle can absorb light in the spectral range of wide variety and as a result of this porphyrins may be suitably employed in the preparation of light-harvesting antennas [35].…”

Direct Comparison between Förster Resonance Energy Transfer and Light-Induced Triplet–Triplet Electron Resonance Spectroscopy

“…In conclusion, we have reported the first LITTER experiments with optically orthogonal chromophores and 2-color photoexcitation, obtaining larger modulation depths corresponding to a 4-fold shortening of the acquisition time with respect to previous 1-color measurements. LITTER can also exploit nonporphyrin chromophores including some commonly used as biological labels; examples include EB and other fluorophores used in light induced PDS methods such as ATTO Thio 12, Rose Bengal, Eyosin Y, and C 60 . ,,,, This makes the technique directly transferable to structural biology applications. Our dual spin/fluorescent labels enabled the first parallel distance measurements by ESR and FRET on exactly the same molecules, allowing for direct comparison and exploiting the complementarity of the two techniques, with LITTER yielding distance distributions and FRET providing information in the room-temperature liquid state.…”