Ultrafast Dynamics of the Metal-to-Ligand Charge Transfer Excited States of Ir(III) Proteo and Deutero Dihydrides

Abstract

For decades, transition metal hydrides have been at the forefront of numerous photocatalytic reactions leveraging either photoacid or photohydride generation. Of upmost importance is the nature of the M-H bond itself, which is typically the major site of photochemical reactivity, particularly in Ir(III) hydrides featuring metal-to-ligand charge transfer (MLCT) excited states. As a departure point for understanding the fundamental spectroscopy and photophysics of the MLCT excited states of Ir(III) diimine hydrides, cis-[Ir(bpy)H] (bpy = 2,2'-bipyridine) and its deuterated analogue cis-[Ir(bpy)D] were prepared and investigated. The robust nature of these molecules enabled detailed solution-based photophysical studies using ultrafast transient absorption and infrared spectroscopy, executed without the generation of permanent photoproducts. Static Fourier transform infrared and Raman spectra (λ = 785 nm) of these two molecules revealed weak but measurable Ir-H and Ir-D stretching vibrations centered at 2120 and 1510 cm, respectively. Short-lived (τ = 25 ps) MLCT excited states were observed for both cis-[Ir(bpy)H] and cis-[Ir(bpy)D] following femtosecond pulsed laser excitation at 480 nm in visible and near-IR transient absorption experiments. A similar time constant was measured for the in-phase and out-of-phase Ir-H stretching modes of the triplet excited state between 1900 and 2200 cm using transient IR spectroscopy. The Ir-D stretching modes in the MLCT excited state were masked by bpy-localized vibrations rendering quantitative evaluation of these modes difficult. The time-resolved infrared data were consistent with density functional theory calculated mid-IR difference spectra in both of these molecules, yielding quantitative matches to the measured IR difference spectra. The information presented here provides valuable insight for understanding the primary photophysical events and transient absorption and IR spectroscopic signatures likely to be encountered throughout metal hydride photochemistry.

Photo‐Initiated Cobalt‐Catalyzed Radical Olefin Hydrogenation

“…For example, Fukuzumi and Guldi showed that the 3 MLCT excited state of Ziessel's photocatalyst [Cp*IrH(bipy)] [35] undergoes facile proton transfer to MeOH, [36] while Miller and co‐workers reported photochemical H 2 evolution in MeCN via a photo‐driven disproportionation mechanism [37] . In view of the rich photochemistry of hydride complexes, the scarcity of transient absorption studies is quite surprising [3k,10,36,38,39] …”

“…[37] In view of the rich photochemistry of hydride complexes, the scarcity of transient absorption studies is quite surprising. [ 3k , 10 , 36 , 38 , 39 ]…”

Estimating the Wavenumber of Terminal Metal-Hydride Stretching Vibrations of Octahedral d6 Transition Metal Complexes

“…In certain cases the mode was verified by preparing the metal deuteride. 7,20,26,36,45,47,52,69,85,88,89,91,104,111,112,121,122,126,132,157,184,201,203,215,217,223 When the hydride is trans to a hydride, usually the antisymmetric H−M−H mode is observed and reported in the region between 1580 and 1900 cm −1 depending on the metal and coligands. The symmetric H−M−H mode at higher wavenumber (1886 cm −1 ) has been located for trans-RuH 2 (C 6 H 5 P-(OC 2 H 5 ) 2 ) 4 by the use of Raman spectroscopy along with the antisymmetric IR-active mode at 1643 cm −1 .…”

Photoinitiated Charge Transfer in a Triangular Silver(I) Hydride Complex and Its Oxophilicity

“…This provides the basis for the further investigation of ultrafast photoinduced dynamics in other solvents as well as in the gas phase . Note, that reports on ultrafast processes in transition‐metal complexes, and especially on multinuclear metal hydrides, remain rather scarce . Moreover, we will pursue the subject of silver hydride luminescence further, because it plays a major role in the design and application of atomically precise nanoclusters …”

“…Our combined experimental and theoretical results hence provide a basis for further detailed spectroscopic and ultrafast dynamics investigations, specifically for studying metal–metal interactions under electronic excitation and identification of hydride charge‐transfer processes. The dynamics of the latter are largely unexplored, but present essential relaxation steps in electronically excited metal hydrides and are decisive for their photochemistry. Moreover, we believe that they could also be utilized to gain a mechanistic understanding and control of the growth of metal nanoclusters and hydrides.…”