Synthesis of a doubly strapped light-harvesting porphyrin bearing energy donor molecules hanging on to the straps: An attempt toward macroscopic control over molecular conformation that affects the efficiency of fluorescence resonance energy transfer

We report here the synthesis of a light-harvesting molecule 1, in which 5,5'-diphenyl-2,2'-bithiophene units (energy donors) and a doubly strapped porphyrin (energy acceptor) are three-dimensionally connected through four alkyl chains to form a "universal joint"-like architecture. From the results of the optical properties of 1 in solution, fluorescence resonance energy transfer (FRET) takes place from the donor to the acceptor in 1 with the FRET efficiency of 99.7%. We prepared 1/polydimethylsiloxane (PDMS) elastomeric films in which 1 is connected to a polysiloxane network through covalent bonds. When we stretched the 1/PDMS film (elongation: up to 60%), the FRET efficiency decreased by 13.1%. The theoretical analysis suggests that the FRET efficiency of 1 (E) is virtually uninfluenced by any changes in the distance between the donor and acceptor (r). Therefore, the observed change in FRET efficiency should have been derived from the orientation factor (k²), which describes the relative orientation of the emission transition dipole of the donor and the absorption transition dipole of the acceptor. The anisotropic absorption spectral measurements support the notion that the transition dipoles of the fluorophores became orthogonally aligned upon stretching, as expected from the molecular design.