Schematic diagram of the wheel-shaped artificial antenna-reaction center complex reported in Ref. 17. We use the short notation, BPF complex, to denote this photosynthetic device. The antenna-reaction center complex contains six light-harvesting pigments: (i) two bis(phenylethynyl)anthracene chromophores, BPEA a and BPEA b , (ii) two borondipyrromethene chromophores, BDPY a and BDPY b , and (iii) two zinc tetraarylporphyrin chromophores, ZnPy a and ZnPy b . All the chromophores are attached to a rigid hexaphenyl benzene core. In addition to the antenna components, the photosystem contains a fullerene derivative (F) containing two pyridyl groups, acting as an electron acceptor. The fullerene derivative F is attached to the both ZnPy chromophores via the coordination of the pyridyl nitrogens with the zinc atoms. For structural details of the BPF complex, we refer to Refs. 17 and 28.
Site populations as a function of time for the parameter set I. The inset plots depict the features of site populations for short time, at two different temperatures: T = 300 K and 77 K. The site populations of the BPEA moieties oscillate with a considerably large amplitude, while the oscillations of the other site populations are hardly observable.
This figure presents site populations as a function of time for the parameter set II. The inset plots show the site populations for short time, at two different temperatures: T = 300 K and 77 K. The amplitudes of the site-population oscillations are much smaller and die out earlier, compared to Fig. 2. This figure indicates that even for Λ > V, the energy transfer between BPEA chromophores is dominated by wave-like coherent motion.
Site populations as a function of time for the parameter set I, when the ZnPy a chromophore is in the excited state and all the other chromophores are in the ground state at t = 0. The inset plots depict the site populations at short time for two temperatures: T = 300 K and 77 K. Lowering the temperature enhances the oscillations of the charge density on the fullerene moiety. Despite the huge energy difference between F and F−, the charge of the fullerene site exhibits oscillatory behavior for short time, specially at lower temperatures.
Time evolution of the site populations for the parameter set II, starting with an exciton on the chromophore ZnPy a at t = 0. The inset plots depict the features of the site populations for a shorter time regime and at two temperatures: T = 300 K and 77 K. Lowering the temperature enhances oscillations of the charge density on the fullerene derivative. These results indicate that the population of the site F oscillates for short time, even for Λ > V. These oscillations are more pronounced at lower temperatures.
Time evolution of the populations on the site F, for both sets of parameters, I and II, comparing the double-exciton case (the two ZnPy chromophores are excited) with the single-exciton case. (a) Time evolution of the populations on the site F for the parameter set I. (b) Time evolution of the populations on the site F for the parameter set II. Note that the double-excitation significantly enhances the amplitude of the charge oscillations at the fullerene site for both sets of parameters, either at low or at high temperatures.
Time evolution of the population on the site F for the parameter set II when both ZnPy chromophores are excited at t = 0. (a) Effects of the coupling Δ on the time evolution of the populations on the site F. (b) Effects of the energy gap between an excited state of a ZnPy chromophore and the charge-separated state, E ch, on the time evolution of populations on the site F. (c) Effects of the reorganization energy λ on the time evolution of populations on the site F. As can be seen from these plots, the contribution of wave-like coherent motion to electron-transfer dynamics is significantly enhanced when strengthening the coupling between fullerene and porphyrin, lowering the energy gap between the fullerene and porphyrin sites, and decreasing the reorganization energy.
Chosen values of the excitonic couplings (V) and reorganization energies for energy transfer (Λ) of the six antenna chromophores. We choose two sets of parameters, one set (denoted by I) corresponds to V > Λ and the other set (II) to the opposite limit V < Λ. The calculated values of the time constants using both sets of parameters agree with the experimental values.
Comparison between the calculated values of the time constants (using the parameter sets I and II) to the experimental values reported in Ref. 17.
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