Molecular structure and medium effect on cinnamics and ionic liquid cinnamates photochemistry

MARÍA L. SALUM; CECILIA J. ROBLES; ROSA ERRA-BALSELLS

Mendoza

Conferencia; 21 st I-APS CONFERENCE; 2011

Cinnamic acids exists in both E (trans) and Z (cis) forms in the nature. The interest in both cinnamics has increased because they were detected in plant derived products such as foods, herbs medicines, cosmetics etc. Z-Forms are not commercially available and it is quite difficult their isolation from the E/Z natural mixtures extracted from plant tissues. Recently we described a highly efficient one-pot preparation-isolation of Z-cinnamic acids (Z-CHA) by photoisomerization of the ionic liquid (IL) of E-cinnamic acids (E-CHAm; E-cinnamates). Thus we were able to conduct a comparative study of the photochemical and thermal stability of cinnamic Z/E pairs as well as that of the corresponding ionic liquids (cinnamates) Z/E pair. Although some years ago trans-cis photoisomerization and photodimmerization of naturally occurring cinnamics were the object of several investigations, few photophysical studies have been conducted on the E- isomers, and practically no studies on the corresponding Z-forms. In our hands, fluorescence could not be detected for the non substituted Z- and E-cinnamic acid pair. On the contrary cinnamics with electron donating groups at the para and meta position showed fluorescence with quantum yield ~ 10-3 ? 10-4 (< 10 ps) and quamtum yield E > Z, as well as a clear and selective effect of the presence of water in the medium on the absorption spectra of the Z-isomers (blue shift of wavelength max ~ 10 to 30 nm). This medium effect was minimum on Z cinnamic cid and maximum on hydroxy- and methoxy-Z-cinnamics. The ionic liquid (IL) pairs E-CHAm/Z-CHAm showed a peculiar behavior. The absorption wavelength max (MeOH) for E-CH and E-CHAm were quite similar while wavelength max (MeOH) for Z-CH and Z-CHAm were quite different. For our surprise, the absorption wavelength max (MeOH) for Z-CHAm was similar to the wavelength max (H2O) for Z-CH. Thus, a clear H2O effect on the molecular structure of substituted ZCH should be operating similarly to that of the amine moiety (Am) on the Z-CHAm studied