KINETIC-ANALYSIS OF A DOUBLE PHOTOISOMERIZATION OF THE DTDCI CYANINE DYE

Simultaneous laser absorption spectroscopy and laser induced fluoresce nce under steady-state conditions, together with analysis of fluoresce nce spectra at various excitation conditions, have been carried out to elucidate the photoisomerization processes of 3,3'-diethylthiadicarbo cyanine iodide (DTDCI) in methanol solution. The data were obtained by varying the laser fluence at several wavelengths, and fits were perfo rmed considering two kinetic models, involving two or three isomers. T he results point to the formation of a second photoisomer under high e xcitation fluences. Its absorption spectrum is shifted toward longer w avelengths with respect to the spectra corresponding to the normal spe cies and the first photoisomer, which are strongly overlapped. Changes in the fluorescence spectra with the excitation fluence, appearing in the DTDCI lasing region, are attributed to the second photoisomer emi ssion. Values are obtained for the photoisomerization and back-photois omerization quantum yields, as well as for the thermal ground-state ba ck-isomerization yield and ground state absorption cross sections at s everal wavelengths for both photoisomers.