Photodynamics of a Constrained Parachute-Shaped Fullerene−Porphyrin Dyad

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.