Electric Field Effects of Photogenerated Ion Pairs on Nearby Molecules: A Model for the Carotenoid Band Shift in Photosynthesis

Photoinduced charge separation within a zinc porphyrin electron donor—pyromellitimide electron acceptor pair possessing an 8.4 Å center-to-center distance and a linear orientation is shown to induce large electrochromic effects on the ground state absorption spectrum of a nearby carotenoid molecule. The orientation of the C—C backbone of the carotenoid is restricted relative to the direction of the electric field produced by the photogenerated ion pair. This is accomplished by covalently linking the electron donor–acceptor pair to the carotenoid with a calix[4]arene bridge. The bridge maintains its cone conformation resulting in an average dihedral angle of 35° between the donor–acceptor axis and the C—C backbone of the carotenoid. Using picosecond transient absorption spectroscopy, a 15 nm electrochromic red shift in the ground state absorption spectrum of the carotenoid was readily observed during the 3.7 ns lifetime of the photogenerated ion pair. The magnitude of this shift depends on the dielectric constant of the surrounding solvent. The data is used to calculate an electric field strength of about 5.5 MV/cm at the carotenoid in toluene. The magnitude of the electric field produced by the ion pair at the carotenoid is larger than the highest fields that can be applied externally. This approach can be used to study the influence of electric fields produced by ion pairs on the photophysics and photochemistry of nearby molecules.