Synthesis, Electronic and Photophysical Characterization of π-Conjugated meso-Ferrocenyl-porphyrin Fluorescent Redox Switches

A series of meso-ferrocenyl-porphyrin dyads linked by four different pi-conjugated bridging units (directly bound, vinyl, ethynyl, and phenyl) have been synthesized to investigate the influence of the conjugated linker on both the electronic and photochemical properties of the porphyrin chromophore. The basic structure consists of 5-(Fc)-15-(4-methylbenzoate)-10,20-diphenylporphyrin zinc(II), where Fc = ferrocene, vinylferrocene, ethynylferrocene, or phenylferrocene. Upon introduction of the various electron-donating ferrocenyl moieties at the meso-position of the porphyrin ring, Soret and Q-band electronic transitions of the resultant dyads are red-shifted compared with those of the nonferrocenyl reference porphyrin system 15-(4-methylbenzoate)-10,20-diphenylporphyrin zinc(II). The electronic properties of these systems have been investigated by electrochemical (cyclic voltammetry) and computational (DFT/TDDFT) methods, while UV/vis absorption and fluorescence emission spectroscopic analysis is also presented. Collectively, electronic and photophysical analysis indicate a strong electronic communication between the porphyrin macrocycle and directly bound ferrocenyl, vinylferrocenyl, and ethynylferrocenyl dyads. The presence of a phenyl spacer acts to inhibit such electronic communication due to the orthogonal geometry of the bridging phenyl ring at the meso-position of the porphyrin macrocycle. In addition to electronic factors, and in particular for the directly bound 5-(ferrocenyl)-15-(4-methylbenzoate)-10,20-diphenylporphyrin zinc(II) dyad, computational analysis suggests that a significant ruffling of the porphyrin macrocyle from planarity is required to facilitate the bulky ferrocene group directly at the meso-position. Of particular note for each of the meso-ferrocenyl-porphyrin dyads is how fluorescence emission derived from the porphyrin S-1 (pi-pi*) excited state is quantitatively quenched due to photoinduced charge-transfer from the ferrocene unit onto the excited state porphyrin. Spectroelectrochemical studies demonstrate redox off/on switching of the porphyrin fluorescence emission via ferricenium/ferrocene redox cycling. Interestingly, it was found that the S-0 <- S-1 fluorescence emission is also switched-on following titration with the metal ions Ce(IV), Cu(II), and Fe(III) in acetonitrile.