Picosecond absorption and resonance Raman investigation of the dynamics of the photoreduction of 4,4′-bipyridine by aliphatic amines in acetonitrile solution

The photoreduction of 4,4 ' -bipyridine (44BPY) by 1,4-diazabicyclo [2.2.2] octane (DABCO) and triethylamine (TEA) has been examined in acetonitrile solution by picosecond transient absorption and time-resolved resonance Raman spectroscopy. Both the 44BPY S-1 and T-1 states are quenched by electron transfer via the formation of radical ion pairs. The S-1-state reaction is essentially unproductive as the singlet ion pair undergoes ultrafast intrapair back electron transfer. The T-1-state reaction leads to the anion radical upon reduction by DABCO and to a mixture of anion radical and N-hydro radical upon reduction by TEA. In the latter case, a contact ion pair of 70 ps lifetime is first produced and decays mainly by intrapair proton transfer (k(pt) = 1.2 x 10(10) s(-1)) and to a minor extent by dissociation into free ions (k(dis) = 2.3 x 10(9) s(-1)). The free and ion-paired 44BPY anion species have similar absorption spectra but are distinguishable from Raman spectroscopy by slightly different vibrational data. From the analysis of the Raman frequencies, it is suggested that the proton transfer is preceded by a rate-limiting intrapair reorientation step.