Dynamics of intermolecular electron transfer from amines to the excited states of 9-fluorenone

• Dynamics of photoinduced electron transfer (PET) reactions.
• Singlet and triplet excited states of 9-fluorenone as electron acceptor.
• Aromatic and aliphatic amines as electron donors.
• Nonexponential dynamics of the PET reactions are discussed using a simple solvent reorientational model.
• Hydrogen bond has no influence on the forward PET reaction in aniline.

Dynamics of photoinduced electron transfer (PET) reactions between the singlet (S1) and the triplet (T1) excited states of 9-fluorenone or simply fluorenone and a few aromatic and aliphatic amines have been investigated under both diffusive and non-diffusive conditions. Formation of the fluorenone anion radical confirms the electron transfer (ET) from the amines to the excited states of fluorenone. Rate constants for both the forward ET process, kCS, and the charge recombination (CR) process, kCR, have been determined in acetonitrile and benzene solutions. Sub-picosecond time-resolved transient absorption study reveals that quenching of the S1 state in acetonitrile is biexponential. Lifetime of one of these two components is independent of quencher concentration and its values determined for aniline (7.1 ps), dimethylaniline (8.5 ps) and diethylaniline (6.8 ps) are very similar. It represents the non-diffusive component of the PET reaction. But the lifetime of the other component decreases with increasing quencher concentration and the rate of this diffusive component of the PET reaction nearly agrees with the value determined using steady state fluorescence quenching method. To determine the intrinsic values of the rates of the PET reactions involving the S1 state of fluorenone, the PET dynamics have been studied in these three neat donor solvents. The forward ET process is biexponential and the lifetimes of these two components are very similar in these solvents and vary in the range 0.3–0.5 ps and 6–8 ps. Nonexponential dynamics of the PET reactions conducted in neat donor–solvents have been discussed using a simple solvent reorientational model.

Non-exponential dynamics of photoinduced electron transfer reaction between the excited singlet state of 9-fluorenone and dimethylaniline is the result of different orientations of the donor solvent molecules around the acceptor molecule.Figure optionsDownload as PowerPoint slide