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A near-IR-emitting sensitizer, boron-chelated tetraarylazadipyrromethane, has been utilized as an electron acceptor to synthesize a series of dyads and triads linked with a well-known electron donor, ferrocene. The structural integrity of the newly synthesized dyads and triads was established by spectroscopic, electrochemical, and computational methods. The DFT calculations revealed a 'molecular clip'-type structure for the triads wherein the donor and acceptor entities were separated by about 14 Å. Differential pulse voltammetry combined with spectroelectrochemical studies have revealed the redox states and estimated the energies of the charge-separated states. Free-energy calculations revealed the charge separation from the covalently linked ferrocene to the singlet excited ADP to yield Fc(+)-ADP(•-) to be energetically favorable. Consequently, the steady-state emission studies revealed quantitative quenching of the ADP fluorescence in all of the investigated dyads and triads. Femtosecond laser flash photolysis studies provided concrete evidence for the occurrence of photoinduced electron transfer in these donor-acceptor systems by providing spectral proof for formation of ADP radical anion (ADP(•-)) which exhibits a diagnostic absorption band in the near-IR region. The kinetics of charge separation and charge recombination measured by monitoring the rise and decay of the ADP(•-) band revealed ultrafast charge separation in these molecular systems. The charge-separation performance of the triads with two ferrocenes and a fluorophenyl-modified ADP macrocycle was found to be superior. Nanosecond transient absorption studies revealed the charge-recombination process to populate the triplet ADP as well as the ground state.

Stable with defined distance and orientation, self-assembled supramolecular triads composed of N,Ndimethylaminophenylfullerene-pyridine bound to zinc porphyrins by a newly developed “two-point” binding strategy are reported. For this, zinc porphyrin was derivatized to bear “hydrogen-bonding” functionalities, carboxylic acid or amide groups, and C60 was functionalized to bear a ligating group, pyridine, and a second electron donor, N,N-dimethylaminophenyl group. The supramolecular triads formed by self-assembly of the zinc porphyrin and fullerene derivatives via the two-point binding method were characterized by spectroscopic and electrochemical techniques and were modeled by using ab initio computational methods. Evidence for axial coordination of the pyridine entity to the zinc metal center and H-bonding between the pyrrolidine ring nitrogen and the pendant carboxylic acid or amide groups was obtained. In the supramolecular triads, the second electron donor, N,N-dimethylaminophenyl, promotes efficient charge separation upon excitation of the zinc porphyrin to yield the radical ion pairs. The radical ion pairs thus generated undergo slow charge recombination to yield relatively long-lived (30-40 ns) charge-separated states.

The effect of axial ligation on the photoinduced charge separation and charge recombination of a series of covalently linked porphyrin−C60 dyads is investigated. Toward this, meso-tetraphenylporphyrin and its zinc(II) derivative are functionalized at the ortho or para positions of one of the aryl groups to bear a fulleropyrrolidine entity through a flexible ethylene dioxide bridge to probe the effect of intramolecular electron transfer phenomena. In o-dichlorobenzene, 0.1 M (TBA)ClO4, the synthesized dyads exhibit seven one electron reversible redox reactions within in the potential window of the solvent and the measured redox potentials and UV−visible absorption spectra reveal charge−transfer interactions between the electron donor, porphyrin, and the electron acceptor, fullerene entities. The geometric and electronic structures of the dyads probed by ab initio B3LYP/3-21G(*) methods also revealed the existence of charge−transfer interactions. The excited state photochemical events are monitored by both ...

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. ...

Synthesis and physicochemical characterization of a molecular triad comprised of ferrocene, C60, and dinitrobenzene entities is reported. Electrochemical studies revealed multiple redox processes involving all three redox active, ferrocene, C60, and dinitrobenzene entities. A total of eight reversible redox couples within the accessible potential window of o-dichlorobenzene containing 0.1 M (TBA)ClO4 are observed. A comparison between the measured redox potentials with those of the starting compounds revealed absence of any significant electronic interactions between the different redox entities. The geometric and electronic structure of the triad is elucidated by using ab initio B3LYP/3-21G(*) methods. In the energy optimized structure, as predicted by electrochemical studies, the first HOMO orbitals are found to be located on the ferrocene entity, whereas the first LUMO orbitals are mainly on the C60 entity with small orbital coefficients on the dinitrobenzene entity. The subsequent LUMO's track the obs...

The acid−base equilibria of 2-(n-alkyl)fulleropyrrolidines and N-methyl-2-(n-alkyl)fulleropyrrolidines in aqueous micellar media of a sodium dodecyl sulfate surfactant are examined experimentally and modeled theoretically by using ab initio 3-21G(*) methods. The pKa values, for 2-(n-alkyl)fulleropyrrolidines and N-methyl-2-(n-alkyl)fulleropyrrolidines, determined both by HCl and NaOH titration with potentiometric detection of the end point, are equal to 6.3 ± 0.1 and 7.5 ± 0.1, respectively, and are independent of the alkyl chain length within a range of C1 to C8. The obtained results indicate that the fullerene cage fused to a pyrrolidine ring increases the acidity of the protonated pyrrolidine nitrogen mainly due to the induced electronic effects and, to a smaller extent, by structural effects. The 3-21G(*) estimated free energy difference, ΔE (with solvation corrections incorporated based on the Cramer-Truhlar SM5.4 solvation model), between the protonated and unprotonated fulleropyrrolidines, and the ...