H-aggregation

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). 

FIGURE 8.5 Absorption spectra of zeaxanthin dissolved in pure ethanol (solid line), in ethanol/water mixture with 1 4 ratio (dotted line), and in 3 2 ethanol/water mixture. Minor bands of the H-aggregate are denoted by. ... 

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. 

Besides the main band, H-aggregates also exhibit weaker bands in the red part of the absorption spectrum (marked by in Figure 8.5). Although in some cases the position of these bands coincides with the vibrational bands of the monomeric carotenoid and can be therefore assigned to nonaggregated carotenoid molecules, certain spectral features do not match the vibrational bands... 

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). 

Kinetics recorded at the maxima of the Sj-S bands, monitoring dynamics of the lowest excited state, have revealed further differences, Figure 8.8. Although monoexponential decays have been observed for monomeric carotenoids (9ps for zeaxanthin and 24 ps for ACOA), aggregates exhibit more complicated decay patterns. The Sx decay of the H-aggregate requires at least four decay... 

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. 

Spano, F. C. 2009. Analysis of the UV/Vis and CD spectral line shapes of carotenoid assemblies Spectral signatures of chiral H-aggregates. J. Am. Soc. 131 4267-4278. 

Single crystals of H-aggregates were prepared from four compounds with the relation m-n=2. The crystallization of these compounds was very difficult and took several months to grow single crystals suitable for X-ray analysis. Various solvents were examined for crystallization. Among them, single crystals were obtained from the mixture of ethanol and... 

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