Intramolecular charge transfer dynamics of Coumarin dyes in Reverse Micelles

Abstract

In this thesis research, intramolecular charge transfer dynamics of coumarin dyes in reverse micelle has been investigated by steady-state and time-resolved fluorescence spectroscopy. Amphiphilic surfactant molecule, bis(2-ethylhexyl) sulfosuccinate sodium salt (Aerosol OT; AOT) are known to form water-in-oil or methanol-in-oil reverse micellar structures by self-assembly in the presence of polar and nonpolar solvents. The size and stability of reverse micelle is critical to the molar ratio of polar solvent to the surfactant molecules. However, whether methanol is effectively encapsulated by the surfactant molecules forming true reverse micelles remain unclear. We found that for polar solvents having comparatively high solubility in nonpolar phase (e.g. methanol), effective formation of methanol reverse micelles depends on AOT concentration. Therefore, AOT concentration dependent studies have been done using Coumarin 153. The formation of reverse micelles was explored by Raman spectroscopic measurements, steady-state absorption, fluorescence and anisotropy measurements. The sulfonate head group of AOT is strongly interacted by polar solvents inside the reverse micelles as the concentration of AOT is increased, which is represented in the frequency change (decrease due to the weakening of S=O bond) of the (S=O) upon the formation of reverse micelles. The steady-state absorption and emission results show red-shifts in wavelengths upon increase of AOT concentration. A decrease in emission intensity at higher concentration is observed, suggesting Coumarin 153 is partitioned into polar regions of reverse micelle. Similarly, restriction in rotational dynamics of small reverse micelles at high AOT concentration is observed through anisotropy measurements. Hence, it is suggested that for successful formation of methanol AOT reverse micelles, AOT concentration is critical which in our case is found to be 0.5M. In addition, the photophysical properties of Coumarin 153 (C153) and Coumarin 481 (C481) confined in the small cores of AOT reverse micelles have been explored by time-correlated single photon counting (TCSPC). C481 shows enhanced fluorescence quantum yield especially in small reverse micelles of AOT. The quantum yield for C481 decreases sharply with increasing ω size as compared to C153. Large increase in emission lifetimes of C481 in AOT reverse micelles as compared to C153 may represent the confinement effect in nanoscopic reverse micelles. In highly polar media, nonradiative decay pathway through Twisted Intramolceular Charge Transfer (TICT) state is proposed for C481. In reverse micelles, the decay of excited states to ground state via nonradiative decay pathways is strongly decreased due to the diminished solute-solvent interactions. As formation of TICT involves 90° rotation of 7-N, N-dialkylamino group with respect to the 1,2- benzopyrone moiety. Such rotation is not expected in C153 and thus nonradiative decay rates do not show sharp decrease when molecule is confined in reverse micelles. Hence, large changes in quantum yields and lifetimes of C481 dye represent micropolarity changes and confinement effects.