Change in Luminescence Induced by Solution-Mediated Phase-Transition of Cyclometalated Platinum(II) Complex with...

Changes in luminescence due to solution-mediated phase transition (SMPT) and crystal-to-crystal phase transitions of a novel cyclometalated platinum(II) complex with isoquinoline-1-carboxylate (Iq-1-COO^-) [Pt(bzq)(Iq-1-COO)] (bzq^-: benzoquinolinate) were studied experimentally and theoretically. Recrystallization of the complex allowed three crystal forms depending on the solvent: red polymorph R_DMF (λ_emi = 689 nm), yellow polymorph Y_DMSO (λ_emi = 641 nm), and red pseudopolymorph RB_CHCl (λ_emi = 721 nm). Crystals of R_DMF in the DMF solution at room temperature showed SMPT to yellow crystals Y_DMF (λ_emi = 627 nm), in which R_DMF first dissolved partly into the solution, and then the dissolved complex crystallized as Y_DMF. RB_CHCl exhibited crystal-to-crystal phase transitions to R_DMF and Y_DMF by being heated to 150 °C and stored in atmospheric conditions, respectively. Molecular structures of each of the four crystal forms, analyzed by X-ray crystallography, showed different planarities because of the dihedral angles between the bzq^- and Iq planes being 3.15, 4.80, 8.35, and 17.7° for RB_CHCl, R_DMF, Y_DMSO, and Y_DMF, respectively. The planar complexes in R_DMF and RB_CHCl constructed dimers through intermolecular Pt-Pt and π-π interactions, whereas the distorted complexes in Y_DMF and Y_DMSO remained as monomers. Density functional theory (DFT) calculations, which reveal the relation between the molecular structure and thermodynamic stability, suggest that the SMPT is triggered by thermodynamic transformation from the metastable planar structure to the stable distorted structure. The intradimer interactions in R_DMF and RB_CHCl induced red-shifts in the absorption and emission colors from those of Y_DMF and Y_DMSO; thus, the photophysical properties of RB_CHCl and R_DMF originate from the MML_IqCT state in contrast with the ML_IqCT/L_bzqL_IqCT character for Y_DMF and Y_DMSO. DFT and time-dependent DFT (TD-DFT) calculations in the ground and excited states provide insight into the structural, electronic, and optical properties.