Ultrafast Wiggling and Jiggling: Ir2(1,8-diisocyanomenthane)4(2).
J Phys Chem A
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Binuclear complexes of d8 metals (PtII, IrI, RhI, ) exhibit diverse photonic behavior including dual emission from relatively long-lived singlet and triplet excited states, as well as photochemical energy-, electron-, and atom transfer. Time-resolved optical spectroscopic and X-ray studies have revealed the behavior of the dimetallic core, confirming that M-M bonding is strengthened upon d*p excitation. We report the bridging ligand dynamics of Ir2(1,8-diisocyanomenthane)42+ (Ir(dimen)), investigated by fs-ns time-resolved IR spectroscopy (TRIR) in the region of CN stretching vibrations, (CN), 2000-2300 cm-1. The (CN) IR band of the singlet and triplet d*p excited states is shifted by -22 and -16 cm-1 relative to the ground state due to delocalization of the p LUMO over the bridging ligands. Ultrafast relaxation dynamics of the 1d*p state depend on the initially excited Franck-Condon molecular geometry, whereby the same relaxed singlet excited state is populated by two different pathways depending on the starting point at the excited-state potential energy surface: Exciting the long/eclipsed isomer triggers two-stage structural relaxation: 0.5 ps large-scale Ir-Ir contraction and 5 ps Ir-Ir contraction/intramolecular rotation. Exciting the short/twisted isomer induces a ~5 ps bond shortening combined with vibrational cooling. Intersystem crossing (70 ps) follows, populating a 3d*p state that lives for hundreds on nanoseconds. During the first 2 ps, the (CN) IR bandwidth oscillates with the frequency of the (Ir-Ir) wave packet, ca. 80 cm-1, indicating that the dephasing time of the high-frequency (16 fs)-1 CN stretch responds to much slower (~400 fs) -1 Ir-Ir coherent oscillations. We conclude that the bonding and dynamics of bridging di-isocyanide ligands are coupled to the dynamics of the metal-metal unit; and that the coherent Ir-Ir motion induced by ultrafast excitation drives vibrational dephasing processes over the entire binuclear cation.
AuthorsPizl, M; Hunter, BM; Greetham, GM; Towrie, M; Zalis, S; Gray, HB; Vlcek, A
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