Tuning of electron transfer reactions in pluronic-surfactant supramolecular assemblies

Synergistic dynamics of photoionization and photoinduced electron transfer probed by laser flash photolysis and ultrafast fluorescence spectroscopy

Photoionization (PI) and photoinduced electron transfer (PET) dynamics of coumarin 450 (C450) in micelles were investigated in the time domains of micro to femtoseconds using steady-state and time-resolved absorption and fluorescence spectroscopy. The PI of C450 occurs inside the micelles leads to the formation of C450 cation radical (CR) and hydrated electron, which is characterized by the respective transient absorption. The PI of C450 is monophotonic in nature and the yield is dependent on the charge of the micelles. The observation of amine CR in the transient absorption confirms the PET from amine to the excited state of C450 in micelles, which results in the quenching of both fluorescence intensity and lifetime. The decrease in femtosecond fluorescent decay of C450 and the absence of transient C450 radical anion in the presence of amine implies that the concerted ultrafast PET promoted PI and PET to the C450 CR-electron pair. The decrease in the time constant for the formation of relaxed state in the presence of amines is due to the ultrafast PET to the C450 CR-electron pair, which prevents the formation of a relaxed state through recombination at a longer time scale. In the present investigation, the recombination dynamics of the CR-electron pair is justified as one of the origins of the slow solvation in micelles. The influence of amine concentration on the decay of C450 CR indicates ET reaction between C450 CR and amine, which is further confirmed by the bleach recovery of C450 ground state in the presence of amine.

Kinetics and Energetics of Ultrafast Bimolecular Photoinduced Electron Transfer Reactions in Pluronic-Surfactant Supramolecular Assemblies

Understanding the kinetics and energetics of photoinduced electron transfer (PET) reactions in constrained media has attracted considerable research interest, as constrained media provide a handle to tune the microenvironments and consequently the mechanisms of PET reactions. In this study, PET reactions between excited 7-aminocoumarin acceptors and ground-state N,N-dimethylaniline (DMAN) donor have been investigated in mixed micellar media composed of triblock copolymer, P123, and anionic surfactant, sodium dodecyl sulfate (SDS), with varying SDS-to-P123 molar ratios (n values). The objective is to elucidate the role of the n values in the rates and energetics of PET reactions over the entire time range from the subpicosecond to the subnanosecond domain, especially in regard to the applicability of the two-dimensional ET (2DET) mechanism. It is observed that by changing the n values, there is a significant change in the hydration characteristics of the SDS-P123 mixed micelles, which in turn changes the kinetics to energetic correlations for the PET reactions. Fluorescence from the excited coumarin acceptors undergoes substantial quenching due to PET from DMAN donor in all of the studied micelles as evidenced from steady-state, subnanosecond time-resolved (TR) and ultrafast (subpicosecond/femtosecond) fluorescence up-conversion measurements. The quenching rate constants (k_q), estimated from subnanosecond TR fluorescence studies, and the individual component-wise decay rates (τ_i^-1), estimated from up-conversion measurements, increase gradually with increasing n value, corroborating well with the sequentially increased micropolarity of the mixed micelles. Interestingly, it is observed that the correlations of either k_q (from subnanosecond studies) or τ_i^-1 (from femtosecond studies) with the reaction exergonicity (-ΔG°) show the noteworthy Marcus inversion (MI) behavior in a very consistent and similar manner for the entire time window, from subpicoseconds to subnanoseconds. The onset of MI always appears at an exergonicity (-ΔG°_MI) much lower than solvent reorganization energy (λ_s), suggesting the involvement of 2DET mechanism throughout the subpicosecond to subnanosecond time domains. The present results thus provide a comprehensive picture of the kinetics and energetics of the PET reactions in constrained media for the whole time span and unequivocally establish the applicability of 2DET mechanism for the PET reactions in constrained media, eliminating any apprehensions about the effect of time resolution of the subnanosecond setup on the observed Marcus inversion behavior. This is indeed an important finding, providing valuable insights for PET reactions in constrained media, which has not been explored explicitly in any of the previous studies. Observation of MI behavior and the modulations in the PET reactions by simply changing the composition of SDS in the SDS-P123 mixed micelles are noteworthy findings of the present study and are expected to find suitable applications for better utilization and outcome of the PET reactions.

Altering intra- to inter-molecular hydrogen bonding by dimethylsulfoxide: A TDDFT study of charge transfer for coumarin 343

DFT and TDDFT methods were carried out to investigate the influences of intramolecular and intermolecular hydrogen bonding on excited state charge transfer for coumarin 343 (C343). Intramolecular hydrogen bonding is formed between carboxylic acid group and carbonyl group in C343 monomer. However, in dimethylsulfoxide (DMSO) solution, DMSO 'opens up' the intramolecular hydrogen bonding and forms solute-solvent intermolecular hydrogen bonded C343-DMSO complex. Analysis of frontier molecular orbitals reveals that intramolecular charge transfer (ICT) occurs in the first excited state both for C343 monomer and complex. The results of optimized geometric structures indicate that the intramolecular hydrogen bonding interaction is strengthened while the intermolecular hydrogen bonding is weakened in excited state, which is confirmed again by monitoring the shifts of characteristic peaks of infrared spectra. We demonstrated that DMSO solvent can not only break the intramolecular hydrogen bonding to form intermolecular hydrogen bonding with C343 but also alter the mechanism of excited state hydrogen bonding strengthening.