Torsional and reorientational motion of a symmetric carbocyanine in alcohols and in aqueous micelle solutions: 3,3′-Diethylthiadicarbocyanine iodide

• Torsional photoisomerization of 3,3′-diethylthiadicarbocyanine iodide (DTDCI) in alcohols deviates from hydrodynamic Kramers theory.
• Solvent friction on torsion was well approximated by that acting on reorientational motion of the DTDCI molecule (Kramers–Hubbard model).
• In micelles, DTDCI photoisomerization is markedly decelerated, implying a high microviscosity similar to long-chain alcohols.
• DTDCI reorientation in micelles shows hindered rotor-like behavior, also implying a high degree of spatial and dynamic constraint.
• Results indicate that DTDCI molecules are confined near the hydrophilic surface regions of the micelles, rather than in the hydrophobic core.

The motion of a rod-like molecule in polar, monohydroxy alcohols and micelles was investigated by taking DTDCI (3,3′-diethylthiadicarbocyanine iodide) as probe fluorophore. The DTDCI molecule is capable of photoisomerization via a torsional motion across its cyanine bridge. While the photoisomerization was inhibited in more viscous alcohols, it followed an empirical viscosity dependence as ∝1/ηa (a ≪ 1), thus deviating substantially from the high-viscosity limit approximate form of the hydrodynamic Kramers’ theory. Solvent friction on torsional motion could be well represented by the rotational reorientation time-constants of DTDCI, as per the Kramers–Hubbard model. In micelles too, both photoisomerization and rotational reorientation rates diminished markedly, with the latter showing very definite features of a hindered-rotor system. Analysis of the data reveals that DTDCI molecules tend to reside at the hydrophilic surface region of the micelle, which is characterized by a very high degree of spatial and dynamical constraint, rather than the hydrophobic core composed mainly of the hydrocarbon tails of surfactant molecules. Thus, the molecular origin of solvent friction exerted on the motion of DTDCI appears to be different between aqueous micelle systems and long-chain alcohols.