Distance, orientation, and number controlled porphyrin-peptoid conjugates (PPCs) were efficiently synthesized. Cofacial (1, 2, and 4), slipped-cofacial (3), and unstructured (5) arrangements of porphyrins provided distinct optical and electronic properties characterized by UV-vis and circular dichroism spectroscopy. In addition, ECCD spectra confirmed the handedness of peptoid helices.

The energy flow during natural photosynthesis is controlled by maintaining the spatial arrangement of pigments, employing helices as scaffolds. In this study, we have developed porphyrin-peptoid (pigment-helix) conjugates (PPCs) that can modulate the donor-acceptor energy transfer efficiency with exceptional precision by controlling the relative distance and orientation of the two pigments. Five donor-acceptor molecular dyads were constructed using zinc porphyrin and free base porphyrin (Zn(i + 2)–Zn(i + 6)), and highly efficient energy transfer was demonstrated with estimated efficiencies ranging from 92% to 96% measured by static fluorescence emission in CH2Cl2 and from 96.3% to 97.6% using femtosecond transient absorption measurements in toluene, depending on the relative spatial arrangement of the donor-acceptor pairs. Our results suggest that the remarkable precision and tunability exhibited by nature can be achieved by mimicking the design principles of natural photosynthetic proteins.

Nature utilizes optimally organized pigments in light-harvesting complexes. To mimic the natural photosynthetic proteins, effective control over inter-pigment interactions is necessary to attain the desired photophysical properties. Previously, we developed porphyrin-peptoid conjugates (PPC) and displayed two porphyrins at defined positions on an α-helical peptoid using a flexible n-butyl linker. Herein, we synthesized new porphyrin-peptoid conjugates (PPC), where porphyrins are conjugated through a rigid C-C linkage to the helical peptoid via the Suzuki-Miyaura cross-coupling reaction. With PPC, we studied the effects of backbone conformation, inter-porphyrin distance, and the linker flexibility on porphyrin interactions. When the rigid C-C linkage was used, conformational homogeneity of the PPC increased, providing more effective intramolecular excitonic couplings between the porphyrins; however, the intermolecular porphyrin J-aggregation decreased. In PPC with a nonameric peptoid backbone, the formation of a threaded loop conformation was observed, which could be switched back to a helical conformation by N-terminal acetylation or by the addition of a protic solvent. This threaded loop-to-helix conversion restored the intramolecular porphyrin interactions. Our results suggest that PPCs represent an excellent system for control over porphyrin interactions and therefore are useful as a model system to elucidate pigment interactions in nature or as a molecular construct with switchable photophysical properties.