The Origin of Time-Resolved Emission Spectra (TRES) Changes of 4-Aminophthalimide (4-AP) in SDS Micelles. The Role of the Hydrogen Bond between 4-AP and Water Present in Micelles
Journal of Physical Chemistry B, ISSN: 1520-6106, Vol: 107, Issue: 50, Page: 13986-13999
2003
- 54Citations
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Article Description
Hitherto, it has been commonly assumed that the changes observed in the time-resolved emission spectra (TRES) of the probe molecule placed in a micellar system have been a consequence of the process of solvation, related mainly to unspecific interactions. However, as follows from analysis of the shape and position of the TRES of 4-aminophthalimide (4-AP) dissolved in aqueous micellar solutions of SDS surfactant (0.15 M), the TRES changes especially in the first several tens of picoseconds cannot be explained only as a result of a nonspecific solvation process. The interpretation of the results obtained from the correlation function of spectral changes (C(t)) analyzed in terms of the process of solvation related to the relaxation of the environment would only be correct provided that there was only one emitting species in one excited state in a monocomponent solvent with no specific interactions. The excitation of the molecule of 4-AP leads to the intramolecular charge transfer (ICT) process, leading to a significant enhancement in the dipolar moment and the energy of the hydrogen bond formed by the carbonyl group of 4-AP (in the state S-ICT) and the water molecules present in the micelle. On the basis of TRES, the time-resolved area-normalized emission spectra (TRANES), and steady-state measurements, we have assigned the emission observed not only to the 4-AP in the S-ICT state but also to the S-exciplex (4-AP⋯HO)*. Thanks to the increase in the hydrogen-bond energy after excitation, the equilibrium between the uncomplexed (S-ICT) and complexed (S-exciplex) forms is shifted to the advantage of the latter. The duration of the isoemissive point determines the dynamics of formation of the hydrogen bond, which can provide interesting information on the properties of water molecules in the vicinity of 4-AP. The results permit the time separation of the processes of the S -exciplex formation (<50 ps) and solvation (τ ≅ 100 ps) and their assignment to respective TRES changes. Similar results to those obtained for 4-AP in SDS are expected for other micellar systems and other probes containing both donor and acceptor group forming sufficiently strong hydrogen bonds with the water molecules from the micellar system.
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