Excitonic interaction, spectral shifts

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. 

Absorption studies of the model compound in polar and nonpolar solvents support the finding that the dimer is present in a form which allows for close proximity between the two porphyrin rings. The absorption spectra of the dimer and the monomer are shown in Fig. 9. The spectra in methanol show a significant blue-shift of the Soret peak and a small red-shift in the visible bands for the diporphyrin, consistent with the spectral properties of previously synthesized cofacial diporphyrins(16,17) wherein exciton interactions can take place. ( 18) In methylene chloride, the Soret blue shift appears to be much less (<5 nm with reference to 4). 

Figure 26. Diagram for the origin of UV-visible spectral shifts and splittings due to exciton interaction of two chromophores ( ) dipoles ( - ). In two limiting orienta-...

The photophysics of these six metallacycles has been studied in chloroform [71]. As expected for weakly interacting systems, the absorption spectra of the homonuclear species 9,10 and 9Zn, lOZn are very similar to those of the parent free-base and zinc-porphyrin chromophores in the Q-band region (Fig. 22), except for minor spectral shifts. A prominent difference between the planar and the slipped cofacial macrocycles is found in the Soret band region, in which a clear exciton splitting (of ca. 500 cm ) is present only for the latter compoimds (10 and lOZn). This result is as expected on the basis of the relative center-to-center distance in the two types of metallacycles (10.1 A in the slipped cofacial geometry as compared to 14.1 A in the planar one). The photophysics of the homo-dimers is very similar to that of the corresponding monomeric species. In particular, 9 and 10 exhibit the typical fluorescence of the free-base or zinc-porphyrin units (9 A. ,ax = 655, 716 nm, T = 5.7 ns 10 Amax = 656, 716 nm) and 9Zn and lOZn that of Zn-porphyrins (9Zn Amax = 608, 651 nm, t = 1.1 ns lOZn Amax = 600, 651 nm). The fife-times (9 and 10, 5.5 ns 9Zn and lOZn, 1.04 ns) are somewhat shortened (by 30-40%) with respect to the porphyrin components, as a consequence of the heavy-atom effect of the external ruthenium centers (see above for a detailed account of this phenomenon). 

There are three general ideas about absorption spectral shifts in biological systems, (i) Charge induced energy shifts the visual protein rhodopsinOj. (ii) Exciton interaction shifts dimer of chlorophyll in the reaction center( 2j. (Hi) Dispersion interaction shifts exists in all systems irrespective of the polar character of the environment and it depends on the polarizability of the surrounding molecules. 

A strong intermolecular interaction among Jt-electron chromophores is observed as Davydov splitting in the absorption spectrum. Kasha proposed a molecular exciton theory for the strong n-n interaction in molecular aggregates, e.g., molecular crystals, and the spectral shift due to the Davydov splitting can be described as a function of chromophore orientation [35,36]. Due to the two-... 

The photophysical processes of semiconductor nanoclusters are discussed in this section. The absorption of a photon by a semiconductor cluster creates an electron-hole pair bounded by Coulomb interaction, generally referred to as an exciton. The peak of the exciton emission band should overlap with the peak of the absorption band, that is, the Franck-Condon shift should be small or absent. The exciton can decay either nonradiatively or radiative-ly. The excitation can also be trapped by various impurities states (Figure 10). If the impurity atom replaces one of the constituent atoms of the crystal and provides the crystal with additional electrons, then the impurity is a donor. If the impurity atom provides less electrons than the atom it replaces, it is an acceptor. When the impurity is lodged in an interstitial position, it acts as a donor. A missing atom in the crystal results in a vacancy which deprives the crystal of electrons and makes the vacancy an acceptor. In a nanocluster, there may be intrinsic surface states which can act as either donors or acceptors. Radiative transitions can occur from these impurity states, as shown in Figure 10. The spectral position of the defect-related emission band usually shows significant red-shift from the exciton absorption band. 

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