A series
of meso-ferrocenyl-porphyrin dyads linked
by four different π-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 S1 (π–π*) 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 S0 ← S1 fluorescence
emission is also switched-on following titration with the metal ions
Ce(IV), Cu(II), and Fe(III) in acetonitrile.
Corrole–fullerene dyads were prepared by treating anthracene-functionalized corroles with fullerene. Photo physical study indicated that the excited corrole unit was quenched due to the introduction of fullerene.
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