The influence of substituents on the adsorption and aggregation of the anionic 3,3′-disulfopropyl-5,5′-dichloro-9-ethylthiacarbocyanine (THIATS) and its cationic analogue 3,3′-diethyl-5,5′-dichloro-9-ethylthiacarbocyanine (TDC) was investigated in Langmuir films at the air-water interface. Dioctadecyldimethylammonium bromide (DODAB) and octadecylammonium chloride (ODACl) were used to adsorb THIATS. Arachidic acid (AA) and dihexadecyl phosphate (DHP) were used to adsorb TDC. J-aggregate formation has been observed for the four different combinations of dyes and surfactants. The absorption and emission spectra of THIATS at the air-water interface revealed one narrow intense J-band which was not influenced by the chemical structure of the amphiphiles or by the concentration of THIATS in the subphase. Fluorescence microscopy experiments suggest that, depending on the subphase dye concentration, aggregation of dye molecules leads either to a layer of J-aggregates distributed homogeneously on the scale of a few micrometers or to the observation of individual domains of J-aggregates. For TDC adsorbed to an AA film, the absorption and emission spectra revealed the formation of two J-bands that coexist at the interface. The absorption spectra of TDC adsorbed onto a DHP film showed only one J-band, whose maximum gradually shifted from an initial value of 646 nm to 637 nm and then to 652 nm. The flexible alkyl spacer chain between the localized negative charges of the sulfate groups and the nitrogen atoms of the benzthiazole units allows a packing of the THIATS molecules that is less dependent on the charge distribution in the oppositely charged lipid film than for TDC. Fluorescence micrographs of THIATS/DODAB and TDC/AA films reveal different spatial features. They confirm the predominant influence of the lipid on the TDC/AA film morphology, as concluded from the spectral data. The presented study offers substantial information that was still lacking in this field of research: the time-dependence adsorption of the dyes onto charged monolayers as well as an investigation of the influence of the substitution pattern and the role of the packing of the amphiphiles on the formation of a specific type of aggregate.

Fluorescence upconversion experiments were performed on monomer solutions of a number of structurally similar thiacarbocyanine dyes. The results from these time-resolved experiments combined with the data obtained from steady-state absorption and emission and 1 H NMR spectra allowed us to assign the components with different decay times to distinct isomers of the thiacarbocyanine dye molecules. Furthermore, it was possible to relate the substitution pattern of the dye molecules to their photophysical properties. For all mesosubstituted dyes, a short-lived component of either 3.5 or 5.7 ps could be assigned to a mono-cis conformation of the thiacarbocyanines. This component emits at longer wavelengths. A second component ranging from 15 to 71 ps and emitting at shorter wavelengths was attributed to the all-trans conformation of the mesosubstituted trimethine dyes. Time-resolved emission spectra revealed in a very clear and direct way the evolution of the contribution of the two conformations to the spectra. The size and/or solvation of the N,N′-substituents have a parallel influence on the decay time of the fast and the slow decaying component. Besides the N,N′-substituents also the meso-substituent has an influence on the decay time and amplitude of the longer-lived component.

Polarized fluorescence microscopy was used to investigate the organization of domains of J-aggregates formed upon adsorption of the thiacarbocyanine dye THIATS (3,3‘-disulfopropyl-5,5‘-dichloro-9-ethyl-thiacarbocyanine) onto a Langmuir film of the oppositely charged amphiphile dioctadecyldimethylammonium bromide at the air−water interface. When combined with the measurement of the spectral properties of this film at the air−water interface, it was possible to propose a model for the orientation and packing of the aggregated dye molecules in these domains. The aggregates of THIATS formed circular domains with an average diameter of approximately 35 μm. Linear polarization of the excitation light beam led to fluorescence from two opposing quadrants of these circular domains. Upon rotation of the polarizer by 90°, the two other opposing quadrants became fluorescent whereas the fluorescence from the former fluorescent quadrants was extinguished. The orientation of the overall transition moment of these aggregates was found to be radial with relation to the large circular domains. Therefore, we proposed a model of radial growth of aggregated THIATS molecules, from a nucleation site into circular domains where the dye molecules adopt a brickstone arrangement.

The fluorescence characteristics and the topography of domains of J-aggregates formed by the thiacarbocyanine dye THIATS (3,3‘-disulfopropyl-5,5‘-dichloro-9-ethylthiacarbocyanine) adsorbed onto a dioctadecyldimethylammonium bromide (DODAB) layer and deposited on a solid substrate were studied using confocal fluorescence microscopy and atomic force microscopy (AFM). The fluorescence and topography of the films reveal a one-to-one correlation for the domains formed by the adsorbed cyanine dyes. The packing of the dye molecules in the bulk of the domains remains unchanged after deposition. The thickness of the dye layer, determined by atomic force microscopy, confirms that only a single dye layer, wherein the dye molecules have an edge-on orientation, is deposited in these domains. The influence of the deposition process (Langmuir−Blodgett or Langmuir−Schaefer) is mostly limited to the amphiphile regions between the two-dimensional dye crystals and to the rim of the two-dimensional dye crystals. For the film deposited using the Langmuir−Blodgett technique, the aggregation was disturbed at the rim of the quasi-circular domains. Dye material was however deposited as indicated by the monomer fluorescence. Polarization experiments revealed a random orientation for the dye molecules at the rim, in contrast with the polarization effect observed for the bulk of the domains. The film deposited using the Langmuir−Schaefer technique seems to remain structured at the rim of the domains. For these films, deposited on a hydrophobic substrate, however, the packing of the DODAB material between the domains undergoes drastic changes as shown by the AFM experiments. Due to the decreased stability outside the domains, collapse of the DODAB film occurs. Fluorescence spectra could be collected at different positions of the domains, revealing whether the dye molecules were present as monomer or J-aggregates.