Noncovalent Binding between Fullerenes and Protonated Porphyrins in the Gas Phase

Noncovalent interactions between protonated porphyrin and fullerenes (C-60 and C-70) were studied with five different meso-substituted porphyrins in the gas phase. The protonated porphyrin fullerene complexes were generated by electrospray ionization 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. The complex ion was mass-selected and then characterized by collision-induced dissociation with Xe. Collisional activation exclusively led to a loss of neutral fullerene, indicating noncovalent binding of fullerene to protonated porphyrin. In addition, the dissociation yield was measured as a function of collision energy, and the energy inducing 50% dissociation was determined as a measure of binding energy. Experimental results show that C-70 binds to the protonated porphyrins more strongly than C-60, and electron-donating substituents at the meso positions increase the fullerene binding energy, whereas electron-withdrawing substituents decrease it. To gain insight into pi-pi 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 pi-pi interactions where the protonated porphyrin accepts pi-electrons from fullerene, and porphyrins carrying bulky substituents prefer the end-on binding of C-70 due to the steric hindrance, whereas those carrying less-bulky substituents favor the side-on binding of C-70.