ULTRAFAST PHOTOISOMERIZATION OF AZOBENZENE COMPOUNDS

The spectroscopy and dynamics of the azobenzene compound 4-(4'-aminoph enylazo) benzoic acid sodium salt (APE) is described and its applicabi lity as a reversible fast photoswitch for conformational control in sm all peptides is discussed. Addition of an electron withdrawing (carbox y-) and an electron donating (amino-) group on the phenyl rings of azo benzene shifts the absorption maximum of the pi-pi band of the result ing APE molecule from 320 to 420 nm. Reversible photoisomerization is detected by UV-vis, FTIR and NMR spectroscopy. Femtosecond time resolv ed ground and excited state dynamics were recorded, indicating that ex cited trans-APE reaches the electronic ground state within approximate to 1 ps. This ultrafast photoisomerization is followed by a wavelengt h dependent process in the 10 ps range which is interpreted as vibrati onal cooling of the product molecules. At delay times longer than 200 ps the absorption characteristics, as expected from the cw-difference spectrum, can be seen, indicating the completion of the photoprocess. (C) 1997 Elsevier Science S.A.