Modulating Electron Transfer Dynamics Via Graphene Interactions in Donor-Acceptor Systems

Sequential electron transfer or electron transfer/hole transfer between energetically well-positioned entities of photosynthetic reaction center donor-acceptor models is one of the commonly employed mechanisms to generate long-lived charge separated states.  Wealth of information, applicable towards light energy harvesting and building optoelectronic devices, has been acquired from such studies.  In the present study, we report charge stabilization in multi-modular donor-acceptor pentads comprised of phenothiazine, subphthalocyanine and fullerene as molecular components.¹  In these pentads, the subphthalocyanine and fullerene serve as primary electron donor and acceptor, while the phenothiazine entities act as secondary hole transferring agents. The newly synthesized compounds were characterized by optical absorption and emission, computational and electrochemical methods.  The redox potentials measured using differential pulse voltammetry were used to estimate free-energy changes for charge separation, hole migration, and charge recombination processes.  Using femto- and nanosecond transient absorption techniques, evidence for charge separation, and kinetics of charge separation and recombination were obtained in polar benzonitrile and nonpolar toluene solvents.  The kinetic data obtained by analyzing the time profiles of the radical ions revealed occurrence of ultrafast charge separation and relatively slow charge recombination processes as a result of electron transfer-hole shift mechanism in these pentads.  The measured rate of charge separation, k_CS were in the range of  1.2 - 4.4 x 10¹⁰ s^-1 while the rate of charge recombination, k_CR were in the range of 1.3 - 5.4 x 10⁶ s^-1, depending upon the design and solvent polarity, revealing charge stabilization in the pentads.  Further, by the interaction of phenothiazine entities of the triad with graphene, modulation of electron transfer events has been accomplished.  Details of these investigations will be presented.

¹KC, C. B.; Lim, G. N.; Zandler, M. E.; D’Souza, F., Org. Letts. 2013, 15, 4612-4615.