Aggregate excited-state interaction

Salares VR, Young NM, Carey PR, and Bernstein HJ. 1977. Excited-state (exciton) interactions in polyene aggregates—Resonance Raman and absorption spectroscopic evidence. Journal of Raman Spectroscopy 6(6) 282-288. 

While excited-state properties of monomeric carotenoids in organic solvents have been the subject of numerous experimental and theoretical studies (Polfvka and Sundstrom 2004), considerably less is known about excited states of carotenoid aggregates. Most of the knowledge gathered so far stems from studies of aggregation-induced spectral shifts of absorption bands of carotenoid aggregates that are explained in terms of excitonic interaction between the molecules in the aggregate. 

Carotenoids are highly lipophilic an active area of research concerns how carotenoids interact with and affect membrane systems (see Chapters 2 and 10). Also, the lipid solubility of these compounds has important implications for carotenoid intestinal absorption (see Chapter 17) models such as the Caco-2 cell model are being used to conduct detailed studies of carotenoid absorption/ competition for absorption (Chapter 18). The lipid solubility of these carotenoids also leads to the aggregation of carotenoids (see Chapter 3). Carotenoids aggregate both in natural and artificial systems, with implications for carotenoid excited states (see Chapter 8). This has implications for a new indication for carotenoids, namely, serving as potential materials for harnessing solar energy. 

Small molecules are already at the limit of detectability as far as proton NMR is concerned, whereas they are easily detected through 2H NMR. If other factors concur favorably (interactions between radicals at high concentration, tendency to aggregate, availability of low-lying excited states), the proton NMR signals can also be detected for radicals. Nevertheless, it is, in general, a hard task to perform high resolution studies on radicals. 

Enhanced excimer emission was also observed from PBAC bound to a-ZrP [20], Excimer formation from pyrene is well known in aqueous solutions [54], As in the case with AMAC, excimer formation is increased with PBAC concentration (Fig. 16) due to increased local concentrations but with two significant differences. Hydrophobic interactions between the pyrene molecules favor the aggregation of PBAC even at moderate coverages and the PBAC singlet excited state is much longer lived ( 200 nsec) than that of AMAC ( 10 nsec) these factors, in turn, promote excimer formation even at low loadings. The broad, red-shifted fluorescence band with a peak centered around 470 nm, characteristic of the pyrene excimer emission, is evident in Fig. 16. Rapid formation of the excimer at low coverages is also evident from the plot of the ratio of emission intensities at... 

The effect of aggregation or interchain interaction on the triplet excited state in Pt-acetylide polymers was investigated by comparing the optical properties of 1 with the pentiptycene-bridged polymer 15.21 The interchain interaction is prevented by the sterically demanding pentiptycene moiety in 15, and its photophysics is being dominated by the intrachain triplet exciton. 

---