Ultrafast Intramolecular Charge and Proton Transfer coupled with Solvation Dynamics of DMSO

Abstract

Excited-state proton and charge transfer are one of the fundamental processes and play a key role in various chemical and biological systems. It has been received huge interest during the last decades and has been extensively studied by many time-resolved electronic and vibrational spectroscopic techniques. Solvation dynamics play important roles in charge transfer and proton transfer reactions and has been extensively studied by numerous experimental and theoretical methods. The reaction dynamics occurring in the solution phase are inherently dependent on the solvent properties. Solvation may facilitate chemical reactions by opening an energetically favored reaction channel and can stabilize the product state by dissipating the excess vibrational energy after the chemical reaction.

FSRS has been successfully applied to many ESIPT and ICT processes occurring in ultrafast time scales due to the high spectral and temporal resolutions. In this dissertation, we will discuss the excited-state intramolecular charge and proton transfer reaction of 1-amino anthraquinone (AAQ) and 1,2-dihydroxyanthraquinone (alizarin) with solvation dynamics of DMSO via femtosecond stimulated Raman spectroscopy (FSRS).

In chapter 3, we will discuss the ultrafast intramolecular proton transfers of 1,2-dihydroxyanthraquinone (alizarin-h2) and its deuterated product (alizarin-d2) in dimethyl sulfoxide (DMSO) by femtosecond stimulated Raman spectroscopy. The population dynamics in the solute vibrational mode of νC=O and the coherent oscillations observed in all of the skeletal vibrational modes νC=O and νC=C clearly show the ultrafast excited-state intramolecular proton transfer dynamics of 110 and 170 fs for alizarin-h2 and alizarin-d2, respectively. Interestingly, we have observed that the solvent vibrational modes νS=O and νCSC may also be applied to ultrafast structural dynamics at these frequencies for its “free” or “aggregated” species. From the kinetic analysis of the νS=O and νCSC modes of DMSO, the ultrafast changes in the solvation, or intermolecular interactions between DMSO molecules initiated by the structural changes of solute molecules, have been thoroughly investigated.

In chapter 4, we will discuss transient Raman spectra of AAQ in the dimethylsulfoxide (DMSO) solution by femtosecond stimulated Raman spectroscopy for the twisted ICT state of AAQ. An ultrafast (~110 fs) ICT dynamics of AAQ was observed from the major vibrational modes of AAQ including the νC-N + δCH and νC=O modes. The coherent oscillations in the vibrational bands of AAQ strongly coupled to the nuclear coordinate for the ICT process have been observed, which showed anharmonic coupling to the low frequency out of plane deformation modes. The vibrational mode of solvent DMSO, νS=O showed a decrease in intensity especially in the hydrogen-bonded species of DMSO, which clearly shows that solvation dynamics of DMSO including hydrogen bonding are crucial in understanding the reaction dynamics of AAQ with the ultrafast structural changes accompanying the ICT.