Ultrafast back electron transfer processes in the photoexcited methylviologen-iodide charge transfer complexes

The ultrafast back electron transfer in the excited charge transfer complexes of the methylviologen with iodide ions has been investigated using femtosecond transient absorption spectroscopy. Methylviologen and iodide form two types of charge transfer complexes each characterized by a charge transfer band in the same spectral region. At low I^- concentrations mainly a 1:1 complex MV²⁺(I^-) is present while at high I^- concentrations both 1:1 and 1:2 complexes MV²⁺(I^-)₂ can be observed. Ultrashort laser pulses at 400 nm are used to excite both complexes in their charge transfer band. The observed transient absorption can be represented by a biexponential function with 1 ps and 20 ps time constants and attributed to the decay of the MV^+./I^. and MV^+./I₂ ^.- radical pair respectively. The excitation of the 1:1 complex leads to the formation of the MV^+./I^. radical pair while the excitation of the 1:2 complex leads to the formation of the MV^+./I^. and MV^+./I₂ ^.- radical pairs.