Dye Stacking Goes Beyond Single-Molecule Photocatalysis: Modifying Twisted Segregated Stacking of Dyes in Coordination Polymers for Multiphoton-Driven Potent Reduction and Oxidation

Supreme photoreductive and photooxidative potentials
are important
for single-electron activation of inert bonds. A consecutive two-photon
excitation of a neutral dye and its radical cation can theoretically
achieve potent photoreductive and photooxidative driving forces. However,
the sluggish generation of key radical cationic intermediates and
their low concentrations do not favor second-photon excitation. Moreover,
the extremely short lifetime of excited-state radical cations and
the mutual interference between reductive and oxidative species raise
controversial kinetic requirements that are difficult to realize in
homogeneous mode. To circumvent these dilemmas, herein, we modified
the 10-phenylphenothiazine dye core into a coordination polymer through
a twisted donor–acceptor (D-A) segregated
stacking manner. This stacking mode retarded the recombination of D•+ and A•– after the first photoexcitation
and facilitated the long-range delocalization of separated carriers.
This system not only stabilized and accumulated the key radical cationic
intermediates to promote second-photon excitation but also circumvented
the mutual disturbance between photoreductive and photooxidative processes,
promoting the accessibility of extremely short-lived photooxidative
centers toward substrates during second photoexcitation. This protocol
went beyond the limitations of single-molecule photocatalysis, which
effectively exploited the potent photoreduction of cyanoarenes and
the potent photooxidation of alkylarenes/carboxylates in one pot,
directly achieving C(sp3)-C(sp2) radical cross-coupling
with added value and pharmaceutical interests.