Long-Range Charge Separation in a Ferrocene–(Zinc
Porphyrin)–Naphthalenediimide Triad. Asymmetric Role of 1,2,3-Triazole
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Abstract
New
dyad and triad systems based on a zinc porphyrin (ZnP), a naphthalenediimide
(NDI), and a ferrocene (Fc) as molecular components, linked by 1,2,3-triazole
bridges, ZnP-NDI (<b>3</b>) and Fc-ZnP-NDI (<b>4</b>),
have been synthesized. Their photophysical behavior has been investigated
by both visible excitation of the ZnP chromophore and UV excitation
of the NDI unit. Dyad <b>3</b> exhibits relatively inefficient
quenching of the ZnP singlet excited state, slow charge separation,
and fast charge recombination processes. Excitation of the NDI chromophore,
on the other hand, leads to charge separation by both singlet and
triplet quenching pathways, with the singlet charge-separated (CS)
state recombining in a subnanosecond time scale and the triplet CS
state decaying in ca. 90 ns. In the triad system <b>4</b>, primary
formation of the Fc-ZnP<sup>+</sup>-NDI<sup>–</sup> charge-separated
state is followed by a secondary hole shift process from ZnP to Fc.
The product of the stepwise charge separation, Fc<sup>+</sup>-ZnP-NDI<sup>–</sup>, undergoes recombination to the ground state in 1.9
μs. The charge-separated states are always formed more efficiently
upon NDI excitation than upon ZnP excitation. DFT calculations on
a bridge–acceptor fragment show that the bridge is expected
to mediate a fast donor-to-bridge-to-acceptor electron cascade following
excitation of the acceptor. More generally, triazole bridges may behave
asymmetrically with respect to photoinduced electron transfer in dyads,
kinetically favoring hole-transfer pathways triggered by excitation
of the acceptor over electron-transfer pathways promoted by excitation
of the donor