Rotation correlation time as a measure of microviscosity of excited state isomerization reactions of three cyanine dyes in n-alcohol solutions

• Published in: Chemical physics letters 1994-02, Vol.218 (1), p.73-80
• Main Authors: Laitinen, E., Ruuskanen-Järvinen, P., Rempel, U., Helenius, V., Korppi-Tommola, J.E.I.
• Format: Article
• Language: English
• Subjects: Applied sciences; Chemical industry and chemicals; Dyes, pigments; Exact sciences and technology; Industrial chemicals
• ISSN: 0009-2614; 1873-4448
• DOI: 10.1016/0009-2614(93)E1465-S

Rotation correlation times of three chemically similar cyanine dyes of different sizes in n-alcohol solutions have been recorded at several temperatures by using polarized picosecond spectroscopy. For all three dyes the linear temperature dependencies of τ or on η/ T were observed to be independent of solvent up to viscosities of about 60 cP. The rotational motion of the dyes proceeds at much slower rates than the excited state isomerization in viscous solutions of the same fluidity. Isomerization seems to depend on special solvent-induced changes of the force field of the reactant and clearly proceeds faster, especially for the two larger dyes, than predicted by Kramers' theory at high values of friction. The Kramers-Hubbard model does improve agreement between observed and predicted rates but for only one of the three dyes.