Photoinduced electron transfer from aliphatic amines to coumarin dyes

Abstract

Electron transfer (ET) interactions of a series of 7-aminocoumarin dyes with aliphatic amine donors have been investigated using steady-state (SS) and time-resolved (TR) fluorescence quenching, picosecond laser flash photolysis (LFP) and pulse radiolysis (PR) techniques. For different coumarin–amine pairs, the estimated quenching constants ${\mathrm{(k}}_q)$ from SS and TR fluorescence measurements are found to be similar within the experimental error. That ET from amine donors to excited ${\mathrm{(S}}_1)$ coumarin dyes takes place has been established from the LFP and the PR results. For different coumarin–amine pairs, the $k_q$ values are seen to correlate well with the free energy changes ${\mathrm{(\Delta G}}^0)$ for the ET reactions following Marcus’ outer-sphere ET theory. The total reorganization energy (λ) estimated from this correlation is seen to be just similar to the solvent reorganization energy ${\mathrm{(\lambda }}_s\mathrm{).}$ The leveled-off $k_q$ value under diffusion-controlled condition ${\mathrm{(k}}_q^{\mathrm{DC}})$ appears to be much lower (∼2.5 times) for the present systems compared to the corresponding value obtained for the ET reactions in coumarin–aromatic amine systems. The large difference in the $k_q^{\mathrm{DC}}$ values with aliphatic and aromatic amines as the electron donors has been rationalized on the basis of the shapes of the highest occupied molecular orbitals (HOMO) of the amine donors. For aliphatic amines, since their HOMOs are largely localized on the amino group, a large orientation factor is involved in the encounter complexes for the ET reaction to take place. With π like HOMOs, such orientational restriction is just nominal with the aromatic amines as the electron donors. Present ET results under diffusive conditions have also been discussed with a comparison to the ET rates observed under nondiffusive conditions, where the amines are directly used as the solvent donors.