Charge transfer induced unexpected red-shift absorption of Zn and Cu porous coordination polymers based on electron-withdrawing ligand

In the research of wide-range light-absorbing coordination polymers, to enhance intermolecular or intramolecular charge transfer (CT) and decrease the electron transition excitation energy, a dedicatedly designed novel pyridine diketopyrrolopyrrole (DPP) ligand with strong electron-withdrawing unit were introduced into the Zn, Cu and 1,4-benzenedicarboxylic acid (BDC) based coordination frameworks and porous coordination polymers DPP-Zn(BDC) and DPP-Cu(BDC) were obtained, respectively. By comparing study with physically-mixed samples DPP + Zn(BDC) and DPP + Cu(BDC), a fresh light-absorbing peak around 720 nm is observed and makes the samples exhibit a wide-range absorption in the range of 200–1400 nm and gives another experimental evidence for the research of charge transfer effect in naphthalenediimide-based metal-organic framework Zn2(NDC)2(DPNI) (J. Phys. Chem. Lett. 2013, 4, 453–458). The tremendous red-shift of absorbing onset wavelength could be ascribed to the enhanced intermolecular or intramolecular CT due to electron-withdrawing DPP ligands. At the mean time slow-photon effect is also discussed. The materials have been also characterized through powder XRD, FT-IR, SEM, XPS, solution-state spectra techniques and etc. This study underscores the importance of analyzing the extended structure for interpreting photophysical data based on the potential factors: CT, slow photon, or J-coupling. Moreover the research work might inspire the construction of light-responsive coordination polymers or metal-organic frameworks (MOFs) for charge or electron transfer.