Ultrafast dynamics and excited state deactivation of [Ru(bpy)₂Sq]⁺ and its derivatives

Abstract

Femtosecond transient absorption spectroscopy has been employed to understand the excited state dynamics of [Ru(bpy)₂Sq]⁺ (I; bpy is 2,2'-bipyridyl, and Sq is the deprotonated species of the semiquinone form of 1,2-dihydroxy benzene) and its derivatives, a widely studied near-infrared (NIR) active electrochromic dye. Apart from the well-defined dπ_Ru→π^∗_bpy-based metal-to-ligand charge transfer (MLCT) transition bands at ~480 nm, this class of molecules generally shows another dπ_Ru→π^∗_Sq(SOMO)-based intense MLCT band at around 900 nm, which is known to be redox active and bleaches reversibly upon a change in the oxidation state of the coordinated dioxolene moiety. To have better insight into the photoinduced electron transfer dynamics associated with this MLCT transition, detailed investigations have been carried out on exciting this MLCT band at 800 nm. Immediately after photoexcitation, bleach at 900 nm has been observed, whose recovery is found to follow a triexponential function with major contribution from the ultrafast component. This ultrafast component of ~220 fs has been ascribed to the S₁ to S₀ internal conversion process. In addition to the bleach, we have detected two transient species absorbing at 730 and 1000 nm with a formation time ~220 fs for both species. The excited state lifetimes for these two transient species have been measured to be 1.5 and 11 ps and have been attributed to excited singlet (¹MLCT) and triplet (³MLCT) states, respectively. Transient measurements carried out on the different but analogous derivatives (II and III) have also shown similar recovery dynamics except that the rate for the internal conversion process has increased with the decrease in the S₁ to S₀ energy gap. The observed results are consistent with the energy gap law for nonradiative decay from S₁ to S₀.