Regioselectively Halogenated Expanded Porphyrinoids as Building Blocks for Constructing Porphyrin–Porphyrinoid Heterodyads with Tunable Energy Transfer

Expanded
porphyrins have been attracting increasing attention owing
to their unique optical and electrochemical properties as well as
switchable aromaticity. Toward material applications, regioselective
functionalization of the expanded porphyrins at their periphery is
indeed challenging due to the presence of multiple reactive sites.
Herein, a set of regioselective halogenated isomers (L5-Br-A/B/C) of neo-confused isosmaragdyrin (L5) are synthesized
by a combination of the halogenation reaction of L5 and
sequential macrocycle-to-macrocycle transformation reactions of its
halogenated isomers. On this basis, the regioselectively functionalized
isosmaragdyrins are utilized as building blocks for constructing multichromophoric
porphyrinoids, specifically, heterodyads L5-ZnP-A/B/C, in which a common zinc porphyrin is linked at three different pyrrolic
positions of isosmaragdyrins, respectively, by Sonogashira coupling
reactions. The highly efficient energy cascade from porphyrin to isosmaragdyrin
is elucidated using steady-state/time-resolved spectroscopies and
theoretical calculations. Notably, the energy transfer processes from
the porphyrin to the isosmaragdyrin moieties as well as the excitation
energy transfer rates in L5-ZnP-A/B/C are highly dependent
on the linking sites by through-bond and Förster-type resonance
energy transfer mechanisms. The site-selective functionalization and
subsequent construction of a set of heterodyads of the expanded porphyrinoid
would provide opportunities for developing new materials for optoelectronic
applications.