Chromophore assemblies within well-defined porous coordination polymers, such as metal−organic frameworks (MOFs), can emulate the functionality of the antenna rings of chlorophylls in light-harvesting complexes (LHCs). The chemical, electronic, and structural diversities define MOFs as a promising platform where photogenerated excitons can be displaced to redox catalysts similar to the reaction center of the LHC. The precise positioning of the pigments and complementary redox units enables us to understand the charge/ energy-transfer process within these crystalline solid compositions. In this study, we postsynthetically anchored tetraphenylporphyrinato zinc(II) (TPPZn)-derived complementary pigment within the 1D pores of 1,3,6,8-tetrakis(p-benzoicacid)pyrene (H 4 TBAPy)-derived NU-1000 MOF to form a high-density donor−acceptor system. The ground-and excited-state redox potentials of the donor and acceptor were chosen to facilitate an energy transfer (EnT) from the excited MOF (i.e., NU-1000*) to TPPZn and a charge transfer (CT) from excited porphyrin (i.e., TPPZn*). Thus, the processes depend on the excitation wavelength. The energy transfer process was spectroscopically probed by excitation−emission mapping: MOF emission was completely quenched at 460 nm, where the pyrene-centered emission was expected. Instead, the excited MOF efficiently transfers the energy to manifest a TPPZn-centered emission at 670 nm (k EnT ≈ 4.7 × 10 11 s −1 ). The excited TPPZn pigment, with a neighboring TBAPy linker, forms an artificial "special-pair"-like system driving the charge-separation process (k CT = 1.2 × 10 10 s −1 ). The findings demonstrate a synthetic MOF-based artificial LHC system where their well-defined structure will open up new possibilities as the separated charge can hop along the 1D pore channel for further mechanistic understanding and future developments.
Crystalline
metal–organic frameworks (MOFs) can assemble
chromophoric molecules into a wide range of spatial arrangements,
which are controlled by the MOF topology. Like natural light-harvesting
complexes (LHCs), the precise arrangement modulates interchromophoric
interactions, in turn determining excitonic behavior and migration
dynamics. To unveil the key factors that control efficient exciton
displacements within MOFs, we first developed linkers with low electronic
symmetry (as defined by large transition dipoles) and then assembled
them into MOFs. These linkers possess extended conjugation along one
molecular axis, engendering low optical bandgaps and improved oscillator
strength for their lowest-energy transition (S0 →
S1). This enhances absorption–emission spectral
overlap and boosts the efficiency of Förster resonance energy
transfer, which was observed experimentally by a sizable decrease
in emission quantum yield (QY), accompanied by a faster population
decay profile. We find that MOFs that orient these elongated linkers
along their asymmetric pore channel, e.g., the hexagonal pores in
an xly network, manifested >50% decrease in their
emission QY with faster decay profiles relative to their corresponding
solution dissolved linkers. This is due to an efficient migration
of photogenerated excitons at the crystallite peripheral sites to
internal sites, which was facilitated by polarized absorption–emission
overlap among the parallelly aligned linkers. In contrast, symmetric
MOFs, such as those with sqc-a topological net, orient
elongated linkers along two perpendicular crystal axes, which hinders
efficient exciton migration. The present study underscores that MOFs
are promising to develop artificial LHCs, but that to achieve an efficient
exciton displacement, appropriate topology-guided assembly is required
to fully realize the true potential of linkers with low electronic
symmetry.
Metal‒organic frameworks (MOFs) are widely studied molecular assemblies that have demonstrated promise for a range of potential applications. Given the unique and well-established photophysical and electrochemical properties of porphyrins, porphyrin-based MOFs are emerging as promising candidates for energy harvesting and conversion applications. Here we discuss the physical properties of porphyrin-based MOFs, highlighting the evolution of various optical and electronic features as a function of their modular framework structures and compositional variations.
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