Conformational Gating of Charge Separation in Porphyrin Oligomer-Fullerene Systems

Abstract

The rate of the photoinduced charge-separation in C 60 -terminated butadiyne-linked porphyrin oligomers P n (n = 4, 6) is strongly influenced by their molecular conformation. In these systems, the presence of the butadiyne linkers gives rise to a broad distribution of conformations in the ground state, due to an almost barrierless rotation of individual porphyrin units in the oligomer chain. The conformational states of these oligomers, either twisted or planar, could be selected by varying the excitation wavelength, thereby providing different initial excited states for charge separation. Charge separation in the different conformers was followed using both steady-state and 2D timeresolved emission using a streak camera system. Singular value decomposition (SVD) analysis applied on streak camera data provides here a powerful tool to study the conformational dependence of the charge separation in long P n C 60 systems. Both the kinetics and spectral changes accompanying charge separation could be analyzed for different populations of conformation. From this analysis we show that, for both systems studied, twisted conformations undergo faster charge separation than planar conformations. This disparity in charge separation rates was ascribed mainly to the difference in driving force for charge separation between twisted and planar conformations. Charge separation was also studied in oligomers P n C 60 coordinated to an octadentate ligand T8 that hinders the rotation of porphyrin subunits. The semicircular complexes P n C 60 -T8 show dramatic changes in their spectral properties, as well as slow excitation wavelength independent rate of charge separation and corresponding low efficiency compared to their linear counterparts. This slow charge separation rate was attributed to fast relaxation to the lowest excited vibronic state and lack of driving force for charge separation in these close to planar semicircular systems; i.e., the template systems behave like "normal" donor−acceptor systems without slow conformational relaxation. This work illustrates how control of conformation can be used to tune the rate of charge separation.

Cooperative Electronic and Structural Regulation in a Bioinspired Allosteric Photoredox Catalyst

“…To do so, the majority of allosteric receptors and catalysts in the literature have made use of structural-based regulation, wherein activity is controlled via changes in the shape or steric confinement around active moieties . On the other hand, regulation of catalysts, allosteric or otherwise, based on electron-transfer and light-harvesting processes has generally been possible by using control mechanisms that instead tailor the electronic state of the active site. − In natural systems, however, structural and electronic regulatory strategies often act synergistically to maximize the toggling between active states, with photosystem II (PSII) being a prime example (Figure A). − Photoactive systems are indeed good targets for coupled regulatory strategies because the interactions between their active components are not only dependent on the electronic environment , but also highly sensitive to structural cues given the fundamental distance dependence of through-space phenomena such as electron and energy transfer. , The challenge has thus been to develop regulatory strategies to control inorganic catalysts by constructively harnessing the electronic and structural perturbations that occur upon analyte binding to an allosteric framework.…”

“…10−13 Photoactive systems are indeed good targets for coupled regulatory strategies because the interactions between their active components are not only dependent on the electronic environment 14,15 but also highly sensitive to structural cues given the fundamental distance dependence of through-space phenomena such as electron and energy transfer. 16,17 The challenge has thus been to develop regulatory strategies to control inorganic catalysts by constructively harnessing the electronic and structural perturbations that occur upon analyte binding to an allosteric framework.…”

Photoinduced charge and energy transfer in molecular wires

“…12). 43 For simplicity, these calculations were performed assuming coplanar porphyrins for the entire series P n (n = 1-4, 6, 8). Also earlier computational calculations of the electronic spectra of the dimer by Beljonne et al 44 demonstrated the enhanced stabilization of the excited states.…”

Template-directed synthesis of linear porphyrin oligomers: classical, Vernier and mutual Vernier

“…These molecular wires are excellent candidates for charge- and energy-transport over distances of 5–20 nm. 2 – 4 , 9 However the synthesis of long monodisperse chains can be laborious. Here, we develop template-directed strategies for controlling the synthesis of linear porphyrin oligomers.…”