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 (lambda(emi) = 689 nm), yellow polymorph Y-DMSO (lambda(emi) = 641 nm), and red pseudopolymorph RBCHCl3 (lambda(emi) = 721 nm). Crystals of R-DMF in the DMF solution at room temperature showed SMPT to yellow crystals Y-DMF (lambda(emi) = 627 nm), in which R-DMF first dissolved partly into the solution, and then the dissolved complex crystallized as Y-DMF. RBCHCl3 exhibited crystal-to-crystal phase transitions to R-DMF and Y-DMF by being heated to 150 degrees 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 degrees for RBCHCl3, R-DMF, Y-DMSO, and Y-DMF, respectively. The planar complexes in R-DMF and RBCHCl3 constructed dieters through intermolecular Pt-Pt and pi-pi 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 RBCHCl3 induced red-shifts in the absorption and emission colors from those of Y-DMF and Y-DMSO; thus, the photophysical properties of RBCHCl3 and R-DMF originate from the MMLIqCT state in contrast with the MLIqCT/LbzqLIqCT 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.