Isomerization and Properties of Isomers of Carbocyanine Dyes

Abstract

One of the important features of polymethine (cyanine) dyes is isomerization around one of C–C bonds of the polymethine chain. In this review, spectral properties of the isomers, photoisomerization and thermal back isomerization of carbocyanine dyes, mostly meso-substituted carbocyanine dyes, are considered. meso-Alkyl-substituted thiacarbocyanine dyes are present in polar solvents mainly as cis isomers and, hence, exhibit no photoisomerization, whereas in nonpolar solvents, in which the dyes are in the trans form, photoisomerization takes place. In contrast, the meso-substituted dyes 3,3′-dimethyl-9-phenylthiacarbocyanine and 3,3′-diethyl-9-(2-hydroxy-4-methoxyphenyl)thiacarbocyanine occur as trans isomers and exhibit photoisomerization in both polar and nonpolar solvents. The behavior of these dyes may be explained by the fact that the phenyl ring of the substituent in their molecules can be twisted at some angle, removing the substituent from the plane of the molecule and reducing its steric effect on the conformation of the trans isomer. In some cases, photoisomerization of cis isomers of meso-substituted carbocyanine dyes is also observed (for some meso-alkyl-substituted dyes complexed with DNA and chondroitin-4-sulfate; for 3,3′-diethyl-9-methoxythiacarbocyanine in moderate polarity solvents). The cycle photoisomerization–thermal back isomerization of cyanine dyes can be used in various systems of information storage and deserves further investigation using modern research methods.

Ultrafast Excited-State Dynamics of Tricarbocyanine Dyes Probed by Two-Dimensional Electronic Spectroscopy: Polar Solvation vs Photoisomerization

“…A schematic energy level diagram, depicting these interconversion dynamics involving A, B, and C isomers, is shown in Figure 6. IR-140 is mainly present as trans-isomer (thermodynamically most stable) and can exist as different isomers due to single C−C bond rotation in the polymethine chain due to the lack of rigidity which is observed in various cyanine dyes 6 (and further supported by DFT calculations in the following section, 4.3). Such photoisomerization facilitates the nonradiative channel which deactivated the dye molecules in the excited state and makes them decay faster.…”

“…The above-mentioned analysis of 2D spectra is indicative of the possible existence of different isomers and their interconversion upon photoexcitation. It was shown that most of the carbocyanine dyes undergo isomerization around the C–C bond of the polymethine chain . To identify various isomers, the DFT calculations are performed.…”

“…It was shown that most of the carbocyanine dyes undergo isomerization around the C−C bond of the polymethine chain. 6 To identify various isomers, the DFT calculations are performed. The stability of ground-state geometries can be compared by electronic stabilization energy after zero-point energy corrections.…”

“…Tricarbocyanine dyes (IR140, IR144, HITCI, HDITCP, IR125, etc.) are cationic members of the cyanine family where the two heterocyclic ends are connected with a heptamethine chain and the increase in conjugation gives rise to red-shifted (in the NIR region) absorption and fluorescence bands. , Due to these enriched photophysical properties, a large number of studies have been done on different tricarbocyanine dyes using a variety of spectroscopic techniques. The effect of solvent refractive index on steady-state spectra of cyanine dyes was investigated followed by solvent-dependent studies on steady-state fluorescence and fluorescence lifetime of tricarbocyanin dyes .…”

“…On the other hand, tuning the pulse spectrum (or equivalently, choosing a series of probes with different absorption spectra) would result in the excitation of higher vibrational manifold leading to concurrent vibrational relaxation, thus obscuring pure solvation dynamics at early time (<1 ps) during which a substantial part of solvation takes place (for example, >50% solvation in water completes within 200 fs − ). However, the presence of several ground-state isomers complicates the study of solvation dynamics. Also, the interconversion among these isomers upon photoexcitation was reported for various cyanine dyes − which renders the interpretation of results a daunting task.…”