Exciton-coupling band

Soret band. Exciton coupling in the "face-to-face" stacking results in an allowed state at higher energy than the monomer state and a forbidden state at lower energy (30,44). The y-oxo dimer emits both... 

The UV/Vis absorbance of 387 was blue-shifted from 413 nm in solution to 406 nm in the solid state, accompanied by a broadening and decrease in intensity of the n—n absorption bands. These spectral changes can be attributed to a strong exciton coupling between the phenylenethie-nylene moieties. 

The electronic spectra of Lu(III) 10 and In(III) 11 complexes of calix[4]arene ball-type Pcs in Fig. 3 are worthy of being cited, Fig. 15. It can be seen from the electronic absorption spectrum of 10 that Q band splits into two, with maxima 708 and 673 nm, as a result of exciton coupling between two Pc units. Coumpond 11 exhibits a broadened Q band indicating additional amount of aggregation of that compound, and a weak band around 630 nm due to exciton coupling [42,52-57]. 

A comparison of the UV-vVis spectra of the ball-type, its precursor, and mono Pc 28, 29, and 30 in (Fig. 8) in THF shows characteristic absorptions between 610 and 710 nm in the Q band region for metal-free Pcs. Because of the lower symmetry of the metal-free Pcs, the Q band splits into two intense bands in that region. An additional third band at 636 nm and a fourth band/shoulder at 610 nm are exciton coupling and charge transfer bands, respectively. The intensity of the third band of the compound 30 clearly indicates the presence of strong intramolecular interactions between two Pc macrocycles [45]. 

Figure 25. Diagrammatic representation for a system with two chromophores (A and B) held together by covalent bonding or weak intermolecular forces. Local excitations are shown (left and right) for the chromophores in their locally excited (A or B ) monomer states. In the composite molecule or system (center), excitation is delocalized between the two chromophores and the excited state (exciton) is split by resonance interaction of the local excitations. Exciton coupling may take place between identical chromophores (A=B) or non-identical chromophores (A B) but is less effective when the excitation energies are very different, i.e. when the relevant UV-visible bands do not overlap.

Although exciton coupling leads to shifted and broadened, if not split, UV-visible spectra of the composite molecule, when the chromophores are held in a chiral orientation, exciton coupling can be detected even more clearly in the CD spectrum as two oppositely-signed CE s typically corresponding to the relevant UV-visible absorption band(s). The signed order of the CD transitions correlates with the relative orientation of the relevant electric dipole transition moments, one from each chromophore, and hence the absolute configuration of the composite molecule (Exciton Chirality Rule.)[2] For the bis-anthra-... 

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