Ramakrishna G, Jose D Amilan, Kumar D Krishna, Das Amitava, Palit Dipak K, Ghosh Hirendra N
Radiation & PhotoChemistry Division, Bhabha Atomic Research Centre, Mumbai, India.
J Phys Chem B. 2006 May 25;110(20):10197-203. doi: 10.1021/jp060906c.
Femtosecond transient absorption spectroscopy has been employed to understand the excited state dynamics of Ru(bpy)(2)Sq (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 dpi(Ru) --> pi(bpy)-based metal-to-ligand charge transfer (MLCT) transition bands at approximately 480 nm, this class of molecules generally shows another dpi(Ru) --> pi(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 approximately 220 fs has been ascribed to the S(1) to S(0) internal conversion process. In addition to the bleach, we have detected two transient species absorbing at 730 and 1000 nm with a formation time approximately 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 ((1)MLCT) and triplet ((3)MLCT) 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 S(1) to S(0) energy gap. The observed results are consistent with the energy gap law for nonradiative decay from S(1) to S(0).
飞秒瞬态吸收光谱已被用于研究[Ru(bpy)₂Sq]⁺（I；bpy为2,2'-联吡啶，Sq为1,2 - 二羟基苯半醌形式的去质子化物种）及其衍生物的激发态动力学，这是一类被广泛研究的近红外（NIR）活性电致变色染料。除了在约480 nm处明确的基于dπ(Ru)→π(bpy)的金属 - 配体电荷转移（MLCT）跃迁带外，这类分子通常在约900 nm处还显示出另一个基于dπ(Ru)→π(Sq)(SOMO)的强MLCT带，已知该带具有氧化还原活性，并且随着配位二氧戊环部分氧化态的变化而可逆地漂白。为了更深入了解与该MLCT跃迁相关的光诱导电子转移动力学，已对在800 nm处激发该MLCT带进行了详细研究。光激发后立即观察到900 nm处的漂白，发现其恢复遵循三指数函数，超快成分起主要作用。约220 fs的超快成分归因于S₁到S₀的内转换过程。除了漂白外，我们还检测到两个在730和1000 nm处吸收的瞬态物种，两种物种的形成时间约为220 fs。已测得这两个瞬态物种的激发态寿命分别为1.5和11 ps，分别归因于激发单重态((¹)MLCT)和三重态((³)MLCT)。对不同但类似的衍生物（II和III）进行的瞬态测量也显示出类似的恢复动力学，只是内转换过程的速率随着S₁到S₀能隙的减小而增加。观察结果与从S₁到S₀的非辐射衰变的能隙定律一致。
