A case study of photo induced electron transfer between riboflavin and aliphatic amine: Deciphering different mechanisms of ET operating from femtosecond to microsecond time domain

• Electron transfer reaction dynamics of riboflavin and aliphatic amine.
• Spacious control over reaction mechanism in organized medium, β-CD.
• Presence of ultrafast time components in fluorescence decay profile.
• Generation of radical ions in triplet state of riboflavin and amine.
• Magnetic field effect proves the compartmentalization of both these ions.

Riboflavin, a redox active chromophore commonly present in biological systems, undergoes photoinduced electron transfer with an aliphatic amine, triethylamine (TEA), both in singlet and triplet spin states. The extent of electron transfer is quite high in water compared to that in organized assemblies of β-cyclodextrin (β-CD) where Rf and TEA are confined in different regions of such heterogeneous medium. The formation of ground state complex between RF and TEA is quite prominent in the results obtained from steady-state absorption and fluorescence and time-resolved fluorescence studies within nanosecond time region. However, the occurrence of direct electron transfer and formation of contact ion pair and solvent separated ion pair between singlet Rf and TEA are obtained from the time-resolved studies in the sub picosecond time scale. The PET between triplet Rf and TEA generates non-fluorescent radical ions which are characterized by transient absorption spectroscopy. The yield of the radical ions is higher in water compared to β-CD. However, it can be modulated by application of an external magnetic field of the order of less than 10 mT The semi reduced Rf generated through electron transfer undergoes H-abstraction. The signature of RfH* is much more prominent in bulk water rather than β-CD, where it becomes significant only in presence of magnetic field.

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