UV-Raman and fluorescence spectroscopy of benzene adsorbed inside zeolite pores

UV-Raman spectroscopy was used to investigate the host-guest interactions of benzene molecules adsorbed within siliceous MFI. With use of 244 nm ultraviolet laser light as the excitation source, several new peaks appear in the spectra. These peaks are not expected from normal Raman scattering of liquid benzene and not seen in the FT-Raman spectra of benzene in silicalite, but appear in the UV-Raman spectra because of the Raman resonance effect. Further investigations diminish the possibility that the new peaks are due to laserinduced chemical reactions. The fluorescence spectra show the fluorescence bands of benzene with vibration progressions, which suggests that the adsorbed benzene molecules are not clustered. Considering that the dimensions of benzene molecules closely match the channel sizes of MFI, it is proposed that the symmetry of D_6h-benzene in me excited state degrades to lower symmetry upon adsorption due to compression of benzene molecules inside MFI pores. In support of this result, UV-Raman spectra of toluene in MFI and benzene adsorbed in zeolite Beta were also studied.