Add time:07/17/2019         Source:sciencedirect.com

Determination of primary/intrinsic electron transfer rate (ket = 1/τet, s−1) in bimolecular photoinduced electron transfer reactions (PET) in solution phase monitoring fluorescence quenching is a controversial issue, predominantly due to constraint imposed by diffusion, and measurement of rate of formation of transient intermediates, therefore primary electron transfer rate, was suggested (J. Phys. Chem. B 2015, 119, 11253 − 11261). Herein, we report electron transfer dynamic of excited 7-methylbenzo[a]pyrene (MBP) in acetonitrile (ACN) in presence of different aromatic amine donors from very low concentration to finally in neat donors to access intrinsic, diffusion free, rate constants of bimolecular PET by using steady state and femtosecond time resolved fluorescence and femtosecond transient absorption (fsTA) spectroscopy. It appears that in lower concentration of donors, diffusion control or stationary regime of ET, observed dynamics by time resolved fluorescence as well as transient absorption spectroscopy replicate to each other leading to determine the bimolecular quenching rates (kd, M−1 s−1) instead of primary or intrinsic ET rates. In very higher donor concentration or in neat donors, non-diffusive or intrinsic regime of ET, amalgamation of femtosecond time-resolved emission and fsTA studies along with global and target analysis determines the primary/intrinsic ET rates (ket) between excited MBP and studied donors which are one to two orders of magnitude larger than corresponding bimolecular quenching rate constants (kd) in ACN. Intrinsic electron transfer occurs with very high rate of ket (∼1011 s−1) and the process lies in Marcus normal region and back electron transfer occur very slow kbet (∼108 s−1), the process lies in Marcus inverted region.

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