Photoexcited States in the Diimine Platinum(II) Metal Complex and the Stacking Interaction

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Emission behavior and polymorphism of Ipt(II)(dpphen)CN_2l (dpphen= 4,7-diphenyl-pheuanthroline) in the crystal has been studied. Two kinds of crystals (needle and plate types) of this compound were obtained by recrystallization from N,N-dimethylformamide (DMF), and show different emission behavior Needle crystal (Ipt(dpphen)(CN)_2l0.5H_2O). Shows triplet ligand centered (^3LC) emission around 500 nm, but plate crystal (Ipt(dpphen)(CN)_2l 0.5DMF) shows very broad red emission around 820 nm which is similar with excimer-like emission as observed in concentrated solution. Their X-ray crystal structures have been determined. In both crystal, the molecular planes are in stacking structure with a space-distance of 3.3-3.5A, which are close Pt-phen ring or phen-CN group overlapping sites, but there is no Pt-Pt interaction site. In the plate type crystal, there is no intermolecular phenanthrolines-overlapping site that seems to cause excimer interaction due to aromatic-ring overlap.In order t … More o clarify a main interaction factor that brings excimer-like association in solution and in crystal, .the electronic structure calculation of the compound by the density functional theory (DFT) method also performed. The optimized strtrcture obtained by calculation was is good agreement with the crystal structure. The lowest excited state of the eomplex is the mixed state of ^3MLCT and ^3LC, and is complicated, because there are so many states with different contribution in ^3LC nature within narrow energy. To consider how the state energy changes with an environment or association/stacking, quantum-chemical calculation was performed by changing the dielectric constant of the surroundings of the compound. The interesting result that a contribution of ^3MCT in the lowest. Excited state become remarkable is the case of surrounding with lower dielectric constant have been obtained. This suggests the increase of CT structure accompanied by association ar stacking in the crystal, and is considered that the clue explaining an excimer-like emission. However, slight possibility of an impurity cannot be excluded. Less