Peng Cheng

CUNY Graduate Center, New York City, New York, United States

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Publications (4)31.8 Total impact

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    ABSTRACT: ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
    No preview · Article · May 2010 · ChemInform
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    ABSTRACT: As part of a continuing investigation of the topological control of intramolecular electron transfer (ET) in donor-acceptor systems, a symmetrical parachute-shaped octaethylporphyrin-fullerene dyad has been synthesized. A symmetrical strap, attached to ortho positions of phenyl groups at opposing meso positions of the porphyrin, was linked to [60]-fullerene in the final step of the synthesis. The dyad structures were confirmed by (1)H, (13)C, and (3)He NMR, and MALDI-TOF mass spectra. The free-base and Zn-containing dyads were subjected to extensive spectroscopic, electrochemical and photophysical studies. UV-vis spectra of the dyads are superimposable on the sum of the spectra of appropriate model systems, indicating that there is no significant ground-state electronic interaction between the component chromophores. Molecular modeling studies reveal that the lowest energy conformation of the dyad is not the C(2)(v)() symmetrical structure, but rather one in which the porphyrin moves over to the side of the fullerene sphere, bringing the two pi-systems into close proximity, which enhances van der Waals attractive forces. To account for the NMR data, it is proposed that the dyad is conformationally mobile at room temperature, with the porphyrin swinging back and forth from one side of the fullerene to the other. The extensive fluorescence quenching in both the free base and Zn dyads is associated with an extremely rapid photoinduced electron-transfer process, k(ET) approximately 10(11) s(-)(1), generating porphyrin radical cations and C(60) radical anions, detected by transient absorption spectroscopy. Back electron transfer (BET) is slower than charge separation by up to 2 orders of magnitude in these systems. The BET rate is slower in nonpolar than in polar solvents, indicating that BET occurs in the Marcus inverted region, where the rate decreases as the thermodynamic driving force for BET increases. Transient absorption and singlet molecular oxygen sensitization data show that fullerene triplets are formed only with the free base dyad in toluene, where triplet formation from the charge-separated state is competitive with decay to the ground state. The photophysical properties of the P-C(60) dyads with parachute topology are very similar to those of structurally related rigid pi-stacked P-C(60) dyads, with the exception that there is no detectable charge-transfer absorption in the parachute systems, attributed to their conformational flexibility. It is concluded that charge separation in these hybrid systems occurs through space in unsymmetrical conformations, where the center-to-center distance between the component pi-systems is minimized. Analysis of the BET data using Marcus theory gives reorganization energies for these systems between 0.6 and 0.8 eV and electronic coupling matrix elements between 4.8 and 5.6 cm(-)(1).
    Full-text · Article · Jul 2004 · Journal of the American Chemical Society
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    ABSTRACT: The pronounced ability of fullerene C60 to act as an electron and energy acceptor has led to the synthesis of a large number of compounds in which C60 is covalently linked to photoactivatable groups which can serve as potential donors. Such compounds are of interest as model systems for photosynthetic reaction centers and also have potential applications in photodynamic therapy because of the highly efficient photosensitization of singlet molecular oxygen formation by C60 and C60 derivatives. By far the largest number of such systems studied to date utilize porphyrins as antennas for efficient light capture in the visible region of the spectrum, and a variety of linkers. Photophysical studies as well as molecular modeling indicate that in conformationally flexible dyads the porphyrin (P) and C60 moieties are in close proximity, due to -stacking interactions, thus facilitating through-space interactions, as demonstrated by quenching of 1P* fluorescence and generation of fullerene-excited states (by energy transfer) or P+-C60- ion-pair states (by electron transfer).2,4a,b,f-h These ion-pair states can be relatively long-lived, due to the small reorganization energy and strong thermodynamic driving force for back-electron transfer, which places this process within the Marcus inverted region.4g, Recently attention has focused on rigidly linked systems in which the porphyrin (P) and (C60) moieties are in enforced close proximity or are forced apart by a saturated norbornylogous or steroid linker. As part of a program to understand the nature of the dialogue between P and C60 chromophores as the topology of P-C60 dyads is systematically varied, we now report photophysical data for the parachute-shaped dyad 1 and the corresponding zinc complex 1-Zn. We have reported previously the synthesis of 1 by Bingel-Hirsch addition of a strapped porphyrin malonate to C60.
    No preview · Article · Nov 1999 · Journal of the American Chemical Society
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    ABSTRACT: A novel covalently linked C60–porphyrin dyad has been prepared by cyclopropanation of C60 with a strapped porphyrin malonate; its fluorescence spectrum shows strong quenching of the porphyrin singlet excited state by the attached C60.
    No preview · Article · Jan 1999 · Chemical Communications