Torsional Structural\nRelaxation Caused by Pt–Pt\nBond Formation in the Photoexcited Dimer of Pt(II) N C \nN Complex in Solution

[Pt(NCN)MeCN]<sup>+</sup> (NCN = 1,3-di(2-pyridyl)benzene,\nMeCN\n= acetonitrile) forms oligomers in the ground state due to metallophilic\ninteractions, and a Pt–Pt bond is formed with photoexcitation.\nUltrafast excited-state dynamics of the [Pt(NCN)MeCN]<sup>+</sup> dimer\nin acetonitrile is investigated by femtosecond time-resolved absorption\n(TA) and picosecond emission spectroscopy. The femtosecond TA signals\nexhibit 60 cm<sup>–1</sup> oscillations arising from the Pt–Pt\nstretching motion in the S<sub>1</sub> dimer. The excited-state absorption\nin the 500–700 nm region increases with time constants of 0.3,\n1.4, and 9.4 ps, which are assigned to contraction of the Pt–Pt\ndistance, structural change in the S<sub>1</sub> dimer, and S<sub>1</sub> → T<sub>1</sub> intersystem crossing, respectively.\nThe 1.4 ps structural change is attributed to torsional structural\nrelaxation proceeding in the S<sub>1</sub> dimer based on the computation,\nwhich indicates that a torsional angle around the Pt–Pt bond\nin the S<sub>0</sub> dimer is widely distributed around two potential\nminima, whereas that of the S<sub>1</sub> dimer has much narrower\ndistributions around noticeably different torsional angles.