J aggregates

Higgins D A and Barbara P F 1995 Excitonic transitions in J-aggregates probed by near-field scanning optical microscopy J. Chem. Phys. 99 3-7... 

J-aggregation Jahn-Teller distortion Jahn-Teller effect J ai Osh Jalaric acid Jameson cell Jams... 

There are two main kinds of dye aggregates, characterized by their typical spectral properties J-aggregates and H-aggregates. The absorption band maximum (f-band) of the J-aggregates is shifted bathochromicaHy with respect to that of an isolated molecule (M-band) the absorption maximum of the H-aggregates is shifted hypsochromicaHy (H-band). The dyes can also form dimers with a shorter absorption wavelength (D-band). 

Tetrachloro-l,l0dieth5l-3,30disulfobutylbenzimidazolocarbocyanine sodium salt [18462-64-1] shows a high tendency toward J-aggregation as dye... 

Fig. 6. Effect of added supersensitizers on a J-aggregated spectral sensitizing dye, l,l -dieth5i-2,2 -quinocyariine chloride [2402-42-8] (1), for which...

J-aggregate studied in the near-field. Chem. Phys. Lett., 381, 368-375. 

Figure 11.3 Hierarchical pattern of cyanine-dye J-aggregates. Dye molecules form strongly fluorescent nanoscale J-aggregates J-aggregates arrange at the rim of the micrometer-sized polymer droplet droplets arrange in a regular mm-sized two-dimensional array (adapted from Ref 39).

Karthaus, O., Okamoto, K., Chiba, R. and Kaga, K (2002) Size effect of cyanine dye J-aggregates in micrometer-sized polymer Domes . Int. J. Nanosci., 1, 461—464. 

Attwood, D. Gibson, J., Aggregation of antidepressant drugs in aqueous solution, J. Pharm. Pharmacol. 30, 176-180 (1978). 

If the transition dipoles are aligned in a head-to-tail formation, then a red shift is expected. This is the reported explanation for the sharp bands at 573 and 578 (J bands). The narrow half-bandwidths of the split J aggregate absorption suggest that the exciton states are not strongly coupled with external perturbations. The two distinct electronic transitions were proposed to arise from two structural modifications of the aggregates. 

Increasing the initial concentration of zeaxanthin to 10 4 M, Figure 8.6b, produces a different dependence on the ethanol/water ratio. Under these initial conditions, adding water to a final ethanol/water ratio of 3 2 leads to a distinctly different absorption spectrum than that observed at lower initial concentration. The vibrational structure of the S2 state is preserved and a new absorption band characteristic of J-aggregates appears at 530 nm. When the water content was increased... 

Regardless of the initial concentration of zeaxanthin, when ethanol is used as the primary solvent, a water content larger than 50% always promotes the formation of H-aggregates (Ruban et al. 1993, Billsten et al. 2005). Thus, it seems that the proper choice of solvent may shift the concentration window in which the J-aggregates are formed. 

A large set of results obtained in recent years for various carotenoids (see, e.g., Simonyi et al. (2003) for review) suggests that planarity of the carotenoid molecule is crucial for aggregation. This hypothesis is supported by the observation that zeaxanthin and astaxanthin, both fairly planar molecules, form aggregates more readily than other carotenoids. Moreover, zeaxanthin and astaxanthin are the only two carotenoids studied so far that can, depending on preparation conditions, form exclusively either H- or J-aggregates (Billsten et al. 2005, Kopsel et al. 2005, Avital... 

In some cases, a transition between J- and H-aggregates has also been observed. Mori et al. (1996) showed that astaxanthin H-aggregates transform into J-aggregates in a few hours. These authors studied the transformation in the 2°C-32°C temperature range and concluded that assemblies corresponding to H- and J-aggregates are separated by an energy barrier that allows the H to J... 

FIGURE 8.7 Transient absorption spectra recorded 3ps following excitation for zeaxanthin (a) and ACOA (b). The spectra were measured with excitation at 400 nm (H-aggregates), 485 nm (monomers), and 525 nm (J-aggregates). 

FIGURE 8.8 Kinetics at the maxima of the S1-SN bands of aggregated and monomeric forms of zeaxanthin (a) and ACOA (b). Probing wavelengths were 555 (zeaxanthin monomer), 560 (zeaxanthin H-aggregates), 605 (zeaxanthin J-aggregates), 520 (ACOA monomer), and 530 nm (ACOA H-aggregates). 

Much less is known about excited-state dynamics of carotenoid J-aggregates, as only zeaxanthin J-aggregates have been studied to date. Only two decay components of -5 and 30ps were needed to fit the kinetics recorded at the maximum of the Sj-S band, Figure 8.8. Since no annihilation studies were carried out, the origin of these components is not known. It is likely that the 5ps lifetime is due to annihilation whereas the 30 ps component corresponds to the. S, lifetime, which is even longer than that of the H-aggregates. 

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