포르피린-플러렌 비공유 상호작용의 기체상 연구
- 포르피린-플러렌 비공유 상호작용의 기체상 연구
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fullerene complexs formed through π–
π interactions are of interest as a building block of functional supramolecular assemblies due to their novel photoelectric properties. Thus, their interaction has been the subject of studies in condensed phase. The contact distance and binding constant of porphyrin–
fullerene complex were measured in crystal structure and solution, respectively. However, the informations about intrinsic binding of porphyrin–
fullerene complex have never been obtained experimentally in the absence of solvent and counterion.
In this research, the interactions of monomeric porphyrin–
fullerene and dimeric porphyrin–
fullerene complexes as a function of porphyrin substituent were studied using mass spectrometry in the gas phase for the first time. The electrospray ionization (ESI) offers a reproducible means to bring the porphyrin–
fullerene complex ions in solution into the gas phase, and mass spectrometry enables the dissociation of the mass-selected complex ions in the absence of solvent. The complex is mass-selected and then characterized by collision-induced dissociation (CID) with Xe. The dissociation yield was measured as a function of collision energy, and the energy inducing 50% dissociation (E50%) was determined as a measure of binding energy between porphyrin and fullerene.
First, noncovalent interactions between protonated porphyrin and fullerenes (C60 and C70) were studied with five different meso-substituted porphyrins in the gas phase to understand the electronic effect of meso substituents. The protonated porphyrin–
fullerene complexes were generated by ESI of the porphyrin–
fullerene mixture in 3:1 dichloromethane/methanol containing formic acid. All singly protonated porphyrins formed the 1:1 complexes, whereas porphyrins doubly protonated on the porphine center yielded no complexes. Collisional activation exclusively led to a loss of neutral fullerene, indicating noncovalent binding of fullerene to protonated porphyrin. Experimental results show that C70 binds to the protonated porphyrins more strongly than C60, and electron-donating substituents at the meso positions increase the fullerene binding energy, whereas electron-withdrawing substituents decrease it. To gain insight into π–
π interactions between protonated porphyrin and fullerene, we calculated the proton affinity and HOMO and LUMO energies of porphyrin using Hartree–
Fock and configuration interaction singles theory and obtained the binding energy of the protonated porphyrin–
fullerene complex using density functional theory. Theory suggests that the protonated porphyrin–
fullerene complex is stabilized by π–
π interactions where the protonated porphyrin accepts π-electrons from fullerene, and porphyrins carrying bulky substituents prefer the end-on binding of C70 due to the steric hindrance, whereas those carrying less-bulky substituents favor the side-on binding of C70.
Secondly, noncovalent interactions of cationic porphyrin–
fullerene complexes were studied in to examine the effect of chargs on the binding energy. Tetracationic porphyrins having charged substituents at the meso positions and their complexes with C60 and C70 were produced by ESI. The total charge states of porphyrin ions were determined by the number of counter anions (p-toluenesulfonate) attached. Doubly-, triply-, and quadruply-charged porphyrin ions yielded 1:1 complexes with fullerenes. Collisional activation of 1:1 cationic porphyrin–
fullerene complexes resulted in the loss of fullerene as primary dissociation channel. Cationic porphyrin ions bind C70 more strongly than C60. For each charged porphyrin, the binding energy decreases with increasing number of counter anions, suggesting the ion-induced dipole interactions in addition to π–
π interactions. As the charges move close to the center of porphyrin, fullerenes more strongly bind to porphyrin. However, as the steric hindrance increases by bulky substituents, the binding energy decreases. In theory, orbital energies of the 4+ cationic porphyrins and their complexes with fullerenes as well as the binding energy of 4+ cationic porphyrin–
fullerene complex were calculated using density functional theory. Theoretical results show that the most stable conformers favor the less steric orientation of meso substituents with the localized charge closer to the porphyrin center.
Lastly, we studied the binding modes of fullerenes to bisporphyrin-based molecular tweezers and their dissociation energetics in the gas phase. Calixarene-linked bisporphyrins can form outside-bound π–
π complexes with fullerenes as well as inside-bound π-stacking complexes holding fullerenes like molecular tweezers. To differentiate the two binding modes, we studied CID of the mass-selected host–
guest complexes of calixarene-linked bisporphyrins with fullerenes (C60 and C70). The CID yield–
energy curves show that the inside-binding is almost twice as strong as the outside-binding and prefers C70 over C60. The inside-binding energy varies more significantly with meso substituents on porphyrins than with peripheral substituents on calixarene.
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