Intramolecular charge transfer dynamics of Michler's ketone and its analogues confined in reverse micelles

Abstract

In this thesis research, intramolecular charge transfer dynamics of 4,4-bis(dimethylamino)benzophenone (Michler’s ketone; MK) and its analogues, 4,4-diaminobenzophenone (AMK) and 4,4-bis(diethylamino)benzophenone confined in the reverse micelles have been investigated by steady-state and time-resolved electronic spectroscopy. Michler’s ketone has been known to show the intramolecular charger transfer (ICT) character in the excited states in polar solvents. Although MK and its analogues have widely been used as a sensitizer in numerous photonic applications including dye-sensitized solar cells, the details of the ICT mechanisms in the excited state, which shows strong dependence on the kind of organic solvents, are not yet fully understood. Bis(2-ethylhexyl) sulfosuccinate sodium salt (Aerosol OT; AOT) is an amphiphilic surfactant which forms self-assembled water-in-oil reverse micelles when dissolved in organic solvent in the presence of polar solvent. The shape and size of reverse micelles are strongly dependent on the molar ratio of polar solvent to surfactant. In this work, the AOT reverse micelles with methanol and acetonitrile cores were used and the hydrodynamic radii of the polar cores were estimated as 1.7 nm. The steady-state absorption spectra of AMK, MK, and EMK in bulk solutions of methanol and acetonitrile shows that these MK analogues forms hydrogen bonding in methanol solution. The absorption bands in methanol appear broadened and slightly red-shifted compared to those observed in acetonitrile solutions. The steady-state emission spectra of MK analogues show strong solvent dependence. Compared to the ICT emission bands in methanol solution, the spectra obtained in acetonitrile solutions show further blue-shifted representing the additional formation of the twisted ICT state. On the other hand, the absorption and emission spectra of MK analogues obtained in the reverse micelles show almost no solvent dependence between the methanol and acetonitrile cores. Since the polar solvent core of the small reverse micelles include only small number of solvent molecules and the solvent dynamics in the reverse micelles are known as abnormally slow compared to the bulk, the exited state dynamics of MK analogues in the polar cores of the small reverse micelles appear almost invariant between the methanol and acetonitrile cores. Time-resolved emission spectra of MK analogues in bulk solvents and the reverse micelles with methanol and acetonitrile cores have also been obtained by a time-correlated single-photon counting (TCSPC) technique. The ultrafast ICT and twisted ICT dynamics of MK analogues are not determined in TCSPC measurements with a time resolution of ~150 ps. However, the emission lifetimes of the MK analogues in bulk and reverse micelle samples represent the solvent-dependent excited state dynamics of MK analogues with the hindered twisting of amino (AMK), dimethylamino (MK), and diethylamino (EMK) groups in the confined cores of the small reverse micelles. The emission lifetimes of MK analogous in the reverse micelles shows a strong dependence on the electron donating group (amino vs. diethylamino vs. diethylamino), which clearly shows that twisted ICT processes occur in the polar apolar solution of acetonitrile. The quantum yield and transient absorption measurements with a faster (~50 fs) temporal resolution are further required to elucidate the detailed mechanisms of ICT and twisted ICT in the excited states, which is left for future explorations.