Torsional Structural Relaxation Caused by Pt-Pt Bond Formation in the Photoexcited Dimer of Pt(II) N̂ C ̂ N Complex in Solution

[Pt(NCN)MeCN]⁺ (NCN = 1,3-di(2-pyridyl)benzene, MeCN = acetonitrile) forms oligomers in the ground state due to metallophilic interactions, and a Pt-Pt bond is formed with photoexcitation. Ultrafast excited-state dynamics of the [Pt(NCN)MeCN]⁺ dimer in acetonitrile is investigated by femtosecond time-resolved absorption (TA) and picosecond emission spectroscopy. The femtosecond TA signals exhibit 60 cm^-1 oscillations arising from the Pt-Pt stretching motion in the S₁ dimer. The excited-state absorption in the 500-700 nm region increases with time constants of 0.3, 1.4, and 9.4 ps, which are assigned to contraction of the Pt-Pt distance, structural change in the S₁ dimer, and S₁ → T₁ intersystem crossing, respectively. The 1.4 ps structural change is attributed to torsional structural relaxation proceeding in the S₁ dimer based on the computation, which indicates that a torsional angle around the Pt-Pt bond in the S₀ dimer is widely distributed around two potential minima, whereas that of the S₁ dimer has much narrower distributions around noticeably different torsional angles.