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

C. J. ROBLES; ERRA-BALSELLS, R.; M. L. SALUM; ERRA BALSELLS, ROSA

Mendoza

Conferencia; 21st I-APS, Mendoza, mayo 17-20, 2011; 2011

Molecular structure and medium effect on cinnamics and ionic liquid cinnamates photochemistry Maria L. Salum, Cecilia J. Robles, Rosa Erra-Balsells CIHIDECAR ¨C CONICET, Departamento de Qu¨ªmica Org¨¢nica, FCEN, UBA. Pabell¨®n II, 3er Piso, Ciudad Universitaria (1428), Buenos Aires, Argentina. erra@qo.fcen.uba.ar 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 ¦µf ~ 10-3 ¨C 10-4 (¦Óf < 10 ps) and ¦µf E > ¦µf 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 ¦Ëmax ~ 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 ¦Ëmax (MeOH) for E-CH and E-CHAm were quite similar while ¦Ëmax (MeOH) for Z-CH and Z-CHAm were quite different (Fig. 1). For our surprise, the absorption ¦Ëmax (MeOH) for Z-CHAm was similar to the ¦Ëmax (H2O) for Z-CH. Thus, a clear H2O effect on the molecular structure of substituted Z-CH should be operating similarly to that of the amine moiety (Am) on the Z-CHAm studied Fig. 1) Figure 1: Absorption spectra: (left) E-CH and Z-CH (MeOH and H2O); (right) Z-CH (MeOH and H2O), and Z-CHAm (MeOH). Ab initio geometry optimization and semi-empirical electronic structural calculations were used to estimate the excited state energy for both E and Z forms of all the cinnamics (CH) and IL cinnamates (CHAm). These calculations predict that the optimum geometry for Z-CH in H2O is formed as an adduct with two and/or three H2O molecules (bellow, left) with similar structure than the optimum obtained for IL cinamates (Z-CHAm) (bellow, right):                                                           Z-CH in H2O Z-CHAm; Am=HO(CH2)2NH2