Formation and decay of an excited singlet charge-transfer complex (or a singlet ion pair) produced by excitation of a ground-state complex of 1,8-dichloroanthraquinone with 2,5-dimethylhexa-2,4-diene at room temperature

In toluene containing 1,8-dichloroanthraquinone (DCAQ, 1.2 × 10 −3 mol dm −3 ) and 2,5-dimethylhexa-2,4-diene (DMHD, 1 mol dm −3 ) at room temperature, an equilibrium exists between free DCAQ and a ground-state complex ( 1 (DCAQ—DMHD) G ) of DCAQ with DMHD. However, in DMHD, only 1 (DCAQ—DMHD) G exists. On selective excitation of this complex by a picosecond 435 or 527 nm light pulse, a new absorption band (band C with λ max = 610 nm) is observed. Since band C appears within the duration of the excitation light pulse irrespective of DMHD concentration and decays following a single-exponential function with a lifetime of 100 ps, it is assigned to the absorption of an excited singlet charge-transfer complex ( 1 (DCAQ δ− —DMHD δ+ )*) or a singlet ion pair ( 1 (DCAQ − —DMHD + )) produced by direct excitation of 1 (DCAQ—DMHD) G . The decrease in band C is accompanied by a rate matching absorption increase in the DCAQ semiquinone radical (DCAQH ). Hence, the decay processes of 1 (DCAQ δ− —DMHD δ+ )* (or 1 (DCAQ − —DMHD + )) are interpreted in terms of an intramolecular proton transfer (yielding DCAQH and the DMHD radical (probably 2,5-dimethylhexa-2,4-dienyl radical)) and an intramolecular charge recombination (or a back electron transfer yielding 1 (DCAQ—DMHD) G and/or free DCAQ plus DMHD).