Spectroscopic, redox, and photochemical behavior of self-assembled donor−acceptor dyads formed by axial coordination of zinc tetraphenylporphyrin, (TPP)Zn, and fulleropyrrolidine bearing either pyridine or imidazole coordinating ligands were investigated. The UV−vis, 1H NMR, and ESI-mass spectral studies, as well as computational studies, revealed supramolecular 1:1 dyad formation between the electron donor [(TPP)Zn] and the electron acceptor, fulleropyrrolidine entities. The determined formation constant K values followed the order o-pyridyl ≪ m-pyridyl ≃ p-pyridyl ≪ N-phenyl imidazole entities of the fulleropyrrolidine. The evaluated thermodynamic parameters revealed stable complexation with complex dissociation enthalpies ranging between 26 and 32 kJ mol-1. The 1H NMR studies revealed axial coordination of the pyridine or imidazole ligands to the central zinc of (TPP)Zn, while the ESI-Mass spectral studies performed in CH2Cl2 matrix revealed the expected molecular ion peak of the self-assembled dyads. The geometric and electronic structures of the dyads were probed using ab initio B3LYP/3-21G(*) methods. Such studies revealed stable complexation between (TPP)Zn and fulleropyrrolidine entities. The majority of the highest occupied frontier molecular orbital (HOMO) was found to be located on the (TPP)Zn entity, while the lowest unoccupied molecular orbital (LUMO) was found to be entirely on the fullerene entity. The redox behavior of the isolated self-assembled dyads was investigated in o-dichlorobenzene, 0.1 (TBA)ClO4. A total of seven one-electron redox processes corresponding to the oxidation and reduction of zinc porphyrin ring, and the reduction of fullerene entities were observed within the accessible potential window of the solvent. These electrochemical results suggest weak interactions between the constituents in the ground state. The excited-state electron-transfer reactions were monitored by both steady-state and time-resolved emission as well as transient absorption techniques. In o-dichlorobenzene, upon coordination of either the pyridine or imidazole entities of fulleropyrrolidine to (TPP)Zn, the main quenching pathway involved charge separation from the singlet excited (TPP)Zn to the C60 moiety. The calculated rate of charge separation was found to range between 107 and 1010 s-1 depending upon the axial ligand (pyridine or imidazole) of the fulleropyrrolidine. However, in a coordinating solvent like benzonitrile, intermolecular electron transfer predominantly takes place mainly from the triplet excited (TPP)Zn to the C60 moiety. The present studies also revealed little or no quenching of the singlet excited fulleropyrrolidine upon coordination of (TPP)Zn.
Electrospray ionization was used to generate gas phase complexes of Ag+ with selected alpha-amino acids. Following storage (isolation without collisional activation) in an ion trap mass spectrometer, the mass spectra produced from the complexes of Ag+ with alpha-amino acids such as alanine, valine and tert-leucine contained peaks consistent with the formation of water or methanol molecule adduct ions. The same adduct ions were not present, however, in the mass spectra generated from the Ag+ complexes with phenylalanine, tyrosine and tryptophan following isolation and storage under similar conditions. For those complexes that showed reactivity, the uptake of water and methanol increased with longer storage times in the ion trap. A preliminary molecular modeling study using phenylalanine demonstrated that the aromatic ring coordinates the Ag+ ion, and the interaction between the metal ion and pi-system, in part, is assumed to prohibit the binding of water or methanol during isolation in the gas phase. This conclusion is supported by a comparison of the adduct formation by the Ag+ complexes with phenylalanine, 4-fluorophenylalanine and alpha-aminocyclohexanepropionic acid. In addition, collision induced dissociation experiments involving the Ag+ complexes of phenylalanine, tyrosine and tryptophan suggest that limiting the coordination of the Ag ion by the complexing molecule (i.e. by loss of a coordinating functional group and/or change in structure due to dissociation) results in the binding of a water or methanol molecule during storage in the ion trap. Surprisingly, the bare Ag+ ion, when trapped and stored under identical experimental conditions, formed neither adduct species, suggesting that the attachment of water or methanol may be due to interactions with a molecular orbital within the Ag+/molecule complex.
During our ongoing investigation of the formation and reactivity of gas-phase complex ions composed of Ag(I) and various R-amino acids, we discovered that the mass-to-charge ratio for the major collision-induced dissociation (CID) product generated from a binary Ag + complex with phenylalanine was consistent with the formation of an Ag + complex with an aldehyde. In this study we investigated and compared the fragmentation pathways for complexes of Ag + with phenylalanine, phenylalanine with exchangeable protium replaced with deuterium, phenylalanine with the carboxylic acid group labeled with 13 C, and phenylalanine with the benzylic group labeled with deuterium. The reaction pathways were determined using multidimensional dissociation steps in an ion-trap mass spectrometer. The dissociation experiments provide clear evidence for the formation of several novel product species, including the Ag + complex with phenylacetaldehyde, as well as the formation of an Ag + complex with either a benzyl carbene or styrene. These dissociation products are markedly different from those observed following the fragmentation of other transition and alkali metal adducts of phenylalanine. On the basis of the dissociation of the various isotope-exchanged and -labeled versions of phenylalanine, we propose several reaction pathways that implicate the formation of an Ag + complex with an aziridinone (Rlactam), for which a peak at the correct mass-to-charge ratio was observed in the MS/MS spectrum of the (M + Ag) + ion. A comparison of the apparent reactivity toward water and methanol in the ion-trap mass spectrometer of the Ag + -containing product ions to Ag + complexes with various low-mass organic molecules provided further evidence to support the proposed formation of the aldehyde and styrene complexes with Ag + ions. For instance, the apparent reactivity of the Ag + /aldehyde product ion generated from the CID of the (M + Ag) + ion is identical to that observed for a complex produced by the electrospray ionization of a solution containing Ag + ions and neat phenylacetaldehyde. Similar results were obtained for a dissociation product ion assumed to be a complex composed of Ag + ions and styrene.
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