Saitow Ken-Ichi, Naitoh Yukito, Tominaga Keisuke, Yoshihara Keitaro
The Graduate University for Advanced Studies, Myodaiji, Okazaki 444-8585, Japan.
Chem Asian J. 2008 Apr 7;3(4):696-709. doi: 10.1002/asia.200700351.
We studied photoinduced reactions of diiodomethane (CH(2)I(2)) upon excitation at 268 nm in acetonitrile and hexane by subpicosecond-nanosecond transient absorption spectroscopy. The transient spectra involve two absorption bands centered at around 400 (intense) and 540 nm (weak). The transients probed over the range 340-740 nm show common time profiles consisting of a fast rise (<200 fs), a fast decay ( approximately 500 fs), and a slow rise. The two fast components were independent of solute concentration, whereas the slow rise became faster (7-50 ps) when the concentration in both solutions was increased. We assigned the fast components to the generation of a CH(2)I radical by direct dissociation of the photoexcited CH(2)I(2) and its disappearance by subsequent primary geminate recombination. The concentration-dependent slow rise produced the absorption bands centered at 400 and 540 nm. The former consists of different time-dependent bands at 385 and 430 nm. The band near 430 nm grew first and was assigned to a charge-transfer (CT) complex, CH(2)I(2) (delta+)I(delta-), formed by a photofragment I atom and the solute CH(2)I(2) molecule. The CT complex is followed by full electron transfer, which then develops the band of the ion pair CH(2)I(2) (+)I(-) at 385 nm on the picosecond timescale. On the nanosecond scale, I(3) (-) was generated after decay of the ion pair. The reaction scheme and kinetics were elucidated by the time-resolved absorption spectra and the reaction rate equations. We ascribed concentration-dependent dynamics to the CT-complex formation in pre-existing aggregates of CH(2)I(2) and analyzed how solutes are aggregated at a given bulk concentration by evaluating a relative local concentration. Whereas the local concentration in hexane monotonically increased as a function of the bulk concentration, that in acetonitrile gradually became saturated. The number of CH(2)I(2) molecules that can participate in CT-complex formation has an upper limit that depends on the size of aggregation or spatial restriction in the neighboring region of the initially photoexcited CH(2)I(2). Such conditions were achieved at lower concentrations in acetonitrile than in hexane.
我们通过亚皮秒 - 纳秒瞬态吸收光谱研究了二碘甲烷（CH₂I₂）在乙腈和己烷中于268 nm激发时的光致反应。瞬态光谱包含两个吸收带，中心分别位于约400 nm（强）和540 nm（弱）处。在340 - 740 nm范围内探测到的瞬态呈现出共同的时间轮廓，包括快速上升（<200 fs）、快速衰减（约500 fs）和缓慢上升。这两个快速成分与溶质浓度无关，而当两种溶液中的浓度增加时，缓慢上升变得更快（7 - 50 ps）。我们将快速成分归因于光激发的CH₂I₂直接解离产生CH₂I自由基以及随后的初级偕偶复合使其消失。浓度依赖性的缓慢上升产生了中心位于400和540 nm处的吸收带。前者由385和430 nm处不同时间依赖性的谱带组成。430 nm附近的谱带首先增长，被归因于由光碎片I原子和溶质CH₂I₂分子形成的电荷转移（CT）络合物CH₂I₂(δ⁺)I(δ⁻)。随后CT络合物发生完全电子转移，接着在皮秒时间尺度上形成385 nm处的离子对CH₂I₂(⁺)I(⁻)谱带。在纳秒尺度上，离子对衰减后生成I₃(⁻)。通过时间分辨吸收光谱和反应速率方程阐明了反应机理和动力学。我们将浓度依赖性动力学归因于CH₂I₂预先存在的聚集体中CT络合物的形成，并通过评估相对局部浓度分析了在给定本体浓度下溶质是如何聚集的。虽然己烷中的局部浓度随本体浓度单调增加，但乙腈中的局部浓度逐渐饱和。能够参与CT络合物形成的CH₂I₂分子数量有一个上限，该上限取决于初始光激发的CH₂I₂相邻区域的聚集大小或空间限制。在乙腈中比在己烷中在更低浓度下就能达到这样的条件。
