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

A seriesof meso-ferrocenyl-porphyrin dyads linkedby four different π-conjugated bridging units (directly bound,vinyl, ethynyl, and phenyl) have been synthesized to investigate theinfluence of the conjugated linker on both the electronic and photochemicalproperties of the porphyrin chromophore. The basic structure consistsof 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 moietiesat the meso-position of the porphyrin ring, Soretand Q-band electronic transitions of the resultant dyads are red-shiftedcompared with those of the nonferrocenyl reference porphyrin system15-(4-methylbenzoate)-10,20-diphenylporphyrin zinc(II). The electronicproperties of these systems have been investigated by electrochemical(cyclic voltammetry) and computational (DFT/TDDFT) methods, whileUV/vis absorption and fluorescence emission spectroscopic analysisis also presented. Collectively, electronic and photophysical analysisindicate a strong electronic communication between the porphyrin macrocycleand directly bound ferrocenyl, vinylferrocenyl, and ethynylferrocenyldyads. The presence of a phenyl spacer acts to inhibit such electroniccommunication due to the orthogonal geometry of the bridging phenylring at the meso-position of the porphyrin macrocycle.In addition to electronic factors, and in particular for the directlybound 5-(ferrocenyl)-15-(4-methylbenzoate)-10,20-diphenylporphyrinzinc(II) dyad, computational analysis suggests that a significantruffling of the porphyrin macrocyle from planarity is required tofacilitate the bulky ferrocene group directly at the meso-position. Of particular note for each of the meso-ferrocenyl-porphyrin dyads is how fluorescence emission derivedfrom the porphyrin S1(π–π*) excitedstate is quantitatively quenched due to photoinduced charge-transferfrom the ferrocene unit onto the excited state porphyrin. Spectroelectrochemicalstudies demonstrate redox off/on switching of the porphyrin fluorescenceemission via ferricenium/ferrocene redox cycling. Interestingly, itwas found that the S0← S1fluorescenceemission is also switched-on following titration with the metal ionsCe(IV), Cu(II), and Fe(III) in acetonitrile. [ABSTRACT FROM AUTHOR]