Photodissociation of CH2I2 and subsequent electron transfer in solution

We studied photoinduced reactions of diiodomethane (CH2I2) 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 (approximate to 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 CH2I radical by direct dissociation of the photoexcited CH2I2 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, CH2I2 delta+center dot center dot center dot I delta-, formed by a photofragment I atom and the solute CH2I2 molecule. The CT complex is followed by full electron transfer, which then develops the band of the ion pair CH2I2+center dot center dot center dot 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 CH2I2 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 CH2I2 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 CH2I2. Such conditions were achieved at lower concentrations in acetonitrile than in hexane.