Intramolecular charge transfer in reverse micelles studied by time-resolved electronic spectroscopy

Abstract

In this thesis research, intramolecular charge transfer dynamics of 4-dimethylamino-4'-nitrobiphenyl (DNBP) in reverse micelles has been investigated by steady-state and time-resolved electronic spectroscopy. Amphiphilic surfactant molecule, bis(2-ethylhexyl) sulfosuccinate sodium salt (Aerosol OT; AOT) forms water-in-oil reverse micelles with the presence of polar and nonpolar solvents. The size of the reverse micelles is controlled by the molar ratio of polar solvent to the surfactant molecules, and the reverse micelles of methanol/AOT/isooctane were stably formed with a wide range of hydrodynamic diameter. The photophysical properties of DNBP confined in the small cores of AOT reverse micelles have been explored by time-correlated single photon counting and femtosecond transient absorption spectroscopy. The absorption and emission maxima of DNBP in the AOT reverse micelles blue-shift compared to those in bulk methanol. Furthermore, the absorption maximum of DNBP shows slight red-shifts as the size of reverse micelles increases. The absorption bands of DNBP are strongly dependent on the solvent polarity. Thus the absorption bands of DNBP in the AOT reverse micelles can be understood as the change of the micropolarity in the micelle cores. Contrary to the little spectra changes in the emission bands of DNBP in the AOT reverse micelles upon the micelle size change, the emission intensities of DNBP show an enormous increase, especially in the small reverse micelles. The excited state dynamics of DNBP in various solvents must be considered for understanding this abnormal photophysical behavior of DNBP. It is well known that the excited state dynamics and emission quantum yields of DNBP are strongly correlated with the structural changes of DNBP in the excited state. The excited state of DNBP is strongly stabilized by the intramolecular charge transfer between electron donating dimethylamino group and the electron accepting dinitro group, which accompanies the torsional relaxation of biphenyl groups. In the reverse micelles, the torsional relaxation of biphenyl group in DNBP may be restricted or blocked due to the confinement effect of the small micelle cores or the sharp increase of the microviscosity of confined solvent molecules. The excited state dynamics of DNBP in the AOT reverse micelles have been investigated by several time-resolved spectroscopic techniques. The lifetime of DNBP in the reverse micelles obtained from time-correlated single photon counting measurements appears much longer than that in bulk methanol and becomes shorter as the size of the reverse micelles increases. The details of the excited state dynamics of DNBP have been studied by femtosecond transient absorption measurements. DNBP in bulk methanol clearly shows the transition from the locally excited state to the charge transfer state with the decay of the excited states within 10 ps. The formation of the charge transfer state in the excited state becomes much weaker and the excited state lifetime becomes much longer in less polar solvents. However, DNBP in nonpolar solvent shows an efficient intersystem crossing to the triplet state which decays with a much longer as 40 ns time constant. The excited state dynamics of DNBP in the AOT reverse micelles is somewhat similar to the bulk results in less polar solvents. The intramolecular charge transfer state is blocked, and the singlet excited state shows a slow dynamics similar to those in less polar solvents. Especially, we monitored the additional strong intersystem crossing to the triplet state only observed in nonpolar solvents of DNBP in the small AOT reverse micelles. The new excited state absorption band of DNBP in the small reverse micelles may be related to the single fission or aggregation-induced emission, which will be verified in the further explorations.