Delocalization of the Excited State and Emission Spectrum of the Platinum(II) Bipyridine Complex in Crystal: Periodic QM/MM Study

The emission spectrum of the platinum(II) bipyridine complex [Pt(CN) 2 (bpy)] (bpy = 2,2′-bipyridine) interestingly exhibits blue shift and peak broadening in crystal as temperature increases from 15 to 292 K. Periodic QM(DFT)/MM calculations here showed that the emission spectrum was assigned as metal–metal to ligand charge transfer triplet ( 3 MMLCT) excited state in which one-electron excitation occurred from d σ –d σ antibonding molecular orbital (MO) between several Pt atoms to the bpy π* orbital. Although the elongation of the Pt–Pt distance by the temperature rise was experimentally observed at the ground state, the QM/MM calculations indicated that the Pt–Pt distance at the 3 MMLCT excited state did not change so much by the temperature rise. The broadening of the emission spectrum by the temperature rise was reproduced by taking account of thermal population of vibrationally excited states of Pt–Pt stretching at the 3 MMLCT excited state. The blue shift of the emission peak by the temperature rise was explained by the thermal population of several possible structures at the 3 MMLCT excited state; in the most plausible structure, two Pt(II) complexes participate in the 3 MMLCT excited state (named 3 [Pt] 2 ), and in the second and the third ones, three and four Pt(II) complexes, respectively, participate in the excited state (named 3 [Pt] 3 and 3 [Pt] 4 ). Interestingly, 3 [Pt] 4 is more stable than 3 [Pt] 2 and 3 [Pt] 3 in the crystal structure at low temperature (LT = 10 K) but 3 [Pt] 3 is the most stable in the crystal structure at room temperature (RT = 293 K) because of the difference in the crystal structure. Considering thermal populations of these species, the emission peak was calculated at 1.95 eV in the LT crystal structure and 2.01 eV in the RT crystal structure, which agree well with the experimental values (1.87 and 1.94 eV at 15 and 292 K, respectively). These findings indicate that populations of the vibrationally excited state and several possible structures at the 3 MMLCT exited state are crucially important for correctly understanding the emission spectrum in crystal.