Picosecond Absorption Spectroscopy of Polymethine Cis‐Trans Isomerization

Abstract

Results are presented of time resolved fluorescence and absorption investigations of DDEOCI (3,3'-diethyl-6,6'-diphenyl-9-ethyl-oxacarbocyanine iodide) in solvents which differ in polarity and viscosity. The measured picosecond absorption and fluorescence kinetics probed at various wavelengths of the polymethine dye studied can be fitted to a double exponential decay with a minor fast component of lifetime τ1 and a major slower component of lifetime τ2. Furthermore, probing in the S1←S0 absorption region and in the fluorescence region results in a residual with a lifetime of some ns and belonging to the detected red-shifted photoisomer with a low quantum yield (φ < 10%). The lifetime and the preexponential factors of the two components are in methanolic solution uneffected by changes of the anion.The detected two picosecond lifetime components are supposed to belong to two isomeric species (all-trans and mono-cis isomer) with strongly overlapping absorption spectra.

Dispersion of Third Order Optical Nonlinearity of PseudoisocyanineJ‐Aggregates

“…J-aggregates of polymethine dyes have become a focus of interest in the interdisciplinary field of molecular nonlinear optics [1][2][3][4][5] and Langmuir-Blodgett engineering [6][7][8]. Polymethine dye J-aggregates were formed in LB layers, and storage of optical information in mono-*Author to whom correspondence should be addressed.…”

Dark states of symmetric polymethine dyes revealed by pump–dump–probe spectroscopy

“…All-trans isomers (usually considered the most stable ground state conformation of polymethine dyes) are excited vertically to the first excited state, which further relaxes via two competitive channels: fluorescence and rotation around the double bond. On the S 1 potential surface, the all-trans and 90° conformations are separated by an energy barrier of less than few tens of kJ/mol [38]. The molecule quickly overcomes the barrier and reaches the potential minimum of the 90° conformation.…”

“…Finally, cis isomers thermally revert to the initial trans form, which is the favoured direction of the ground state isomerization in cyanine dyes [25]. Alternatively, it has been suggested that in some cases moderately stable twisted species can exist in the ground state, corresponding to an extra shallow energy minimum on the S 0 surface [37,38]. The absorption spectra of cyanine cis or twisted isomers strongly overlap with those of trans isomers and usually are bathochromically shifted [39].…”