Effect of cyclodextrininclusion on the photoisomerization of arylethylenethioamides

The inclusion in cyclodextrins (CDx) of phenylethylenethioamide (PETA) and naphthylethylenethioamide (NETA) was investigated. For PETA, the UV, induced circular dichroism and nuclear magnetic resonance (NMR) spectra show that the molecule forms an inclusion compound (1 : 1) with alpha-CDx. From the variation of the absorption spectrum with increasing CDx concentration, the constants of the inclusion equilibrium (450 M(-1) and 130 M(-1) respectively for the trans and cis isomers) were calculated. The spectroscopic data suggest that for the trans isomer the inclusion involves the double bond, whereas for the cis isomer only the aromatic ring is included. The photochemical results are consistent with this model. Indeed, on irradiation, trans-cis photoisomerization is retarded by the alpha-CDx cavity (phi(t-c)=0.13), whereas cis-trans photoisomerization is favoured slightly (phi(c-t)=0.31): the cavity alters the decay ratio of the twisted state to the ground trans and cis states. For NETA, only the trans isomer forms an inclusion compound with beta-CDx as shown by the dependence of the W spectra on the beta-CDx concentration and by the enhancement of fluorescence. The value of the inclusion constant is 270 M(-1) However, the trans-cis quantum yield is not influenced by inclusion: only the naphthyl ring is encapsulated and the rotation around the ethylene group probably occurs outside the cavity.

The inclusion in cyclodextrins (CDx) of phenylethylenethioamide (PETA) and naphthylethylenethioamide (NETA) was investigated. For PETA, the UV, induced circular dichroism and nuclear magnetic resonance (NMR) spectra show that the molecule forms an inclusion compound (1 : 1) with alpha-CDx. From the variation of the absorption spectrum with increasing CDx concentration, the constants of the inclusion equilibrium (450 M(-1) and 130 M(-1) respectively for the trans and cis isomers) were calculated. The spectroscopic data suggest that for the trans isomer the inclusion involves the double bond, whereas for the cis isomer only the aromatic ring is included. The photochemical results are consistent with this model. Indeed, on irradiation, trans-cis photoisomerization is retarded by the alpha-CDx cavity (phi(t-c)=0.13), whereas cis-trans photoisomerization is favoured slightly (phi(c-t)=0.31): the cavity alters the decay ratio of the twisted state to the ground trans and cis states. For NETA, only the trans isomer forms an inclusion compound with beta-CDx as shown by the dependence of the W spectra on the beta-CDx concentration and by the enhancement of fluorescence. The value of the inclusion constant is 270 M(-1) However, the trans-cis quantum yield is not influenced by inclusion: only the naphthyl ring is encapsulated and the rotation around the ethylene group probably occurs outside the cavity. RI Sortino, Salvatore/E-4684-2011; Chiacchio, Ugo/E-7565-2012