Transient absorption of symmetrical carbocyanines

Transient absorption in several symmetrical cyanine and carbocyanine dyes is measured by time-resolved spectroscopy with subpicosecond white-light continuum in a wide spectral range (0.35-1 mu m). The results are compared with data available in the literature on ultrashort and long-lived transients. The linear shift law observed here for the S1Sn absorption bands in the wavelength scale allows one to predict the main induced absorption bands for members of several carbocyanine families (vinylogous series) which have not yet been studied. In the case of the shortest methine chains ( cyanines and monocarbocyanines) a few transient absorption bands are assigned to the formation of photoisomers. The absolute values of the excited-state or photoisomer cross sections of polymethine carbocyanines are evaluated by global spectral analysis, from the measured differential optical density spectra. Several cases of large values (higher than 10) of the absorption cross section ratio (excited/ground state) are found at new wavelengths, which allows applications of photo-induced absorption to photonics and laser technologies over a broad spectral range. (C) 1998 Elsevier Science S.A. All rights reserved.