Ligand-to-Ligand Charge Transfer Induced Red-Shifted Room Temperature Phosphorescence in Metal–Organic Frameworks

Research on room temperature phosphorescence (RTP) of metal-organic frameworks (MOFs) has been rapidly developed in recent years. However, it is still challenging to realize long-wavelength RTP (>580 nm). In this article, a new strategy is proposed to achieve the red-shifted RTP through constructing dual-ligand MOFs. Different from the single-ligand MOF, which lacks intermolecular interaction, the dual-ligand MOF can build up a stable donor-acceptor (D-A) relationship between two suitable simple ligands. Therefore, the induced charge transfer (CT) process from the donor units to the acceptor units in the framework can decrease the energy gap between the frontier orbitals, reducing the excited state energy levels. Moreover, by modulating the electron density and conjugation of the acceptor ligand, the energy of the triplet states of MOFs can be further reduced. As a result, the RTP centered at 588 nm is successfully achieved in a dual-ligand MOF. Also, we clearly describe the electron transporting path among the ground, 1CT, and 3CT states, revealing the emitting mechanism of the long-wavelength RTP. This work not only extends the D-A structure from the molecular level to the periodic structure of MOFs but also solves the problem of achieving long-wavelength RTP in MOFs from a new perspective.