Exciton splitting

The SHG/SFG technique is not restricted to interface spectroscopy of the delocalized electronic states of solids. It is also a powerful tool for spectroscopy of electronic transitions in molecules. Figure Bl.5.13 presents such an example for a monolayer of the R-enantiomer of the molecule 2,2 -dihydroxyl-l,l -binaphthyl, (R)-BN, at the air/water interface [ ]. The spectra reveal two-photon resonance features near wavelengths of 332 and 340 mu that are assigned to the two lowest exciton-split transitions in the naphtli-2-ol... 

Strong exciton splitting, e.g., 1000 cm-1, indicates extremely high rates of energy migration in a lattice. Even exciton splittings of 10cm 1... 

The energy difference I/ , -F2I=2 V12 is known as Davydov or exciton splitting, Figure 8.3. The shift of energy levels gives rise to new bands in the absorption spectrum denoted as the upper and lower Davydov (exciton) components. These components are the H- and J-bands observed in absorption spectra of molecular aggregates. 

A helical arrangement within columns was also found for other metal 3-diketonate complexes provided with chiral side chains (32) by Serrano and co-workers.35,36 These compounds form rectangular columnar mesophases with helical order within the columns. A spin-coated sample of 32 showed a positive exciton-splitted signal in the CD spectra, which was interpreted as a left-handed (M) helix. Annealing of the film resulted in much higher optical activities and a shift of the absorption maxima. The observed optical changes clearly point to a chiral organization of the columns in the mesophase. 

Lamola and Eisinger169 have shown that the oriented adjacent pairs of 1,3-dimethyl-thymine monomers formed by photosplitting of the appropriate dimer in frozen solution will photodimerize again with a quantum yield of close to unity, and that the pair of monomers shows no fluorescence, but an exciton splitting in the absorption spectrum. 

The signs of the exciton-split Cotton effect reflect the absolute configuration of the molecule if the direction of the transition moment in the chromophore is established. For example, chiral 9,9 -spirobifluorene derivative 1 has R configuration as indicated by a strong positive CD couplet (A = +1111.7) due to the LBb band of the anthracene chromophores, the electric transition moment of which is polarized along the long axis of the chromophore (Figure 14)100... 

The amplitude (A) of the exciton Cotton effect is inversely proportional to the square of the interchromophoric distance. Thus, weak exciton split Cotton effects are expected for remote dibenzoates. Nevertheless, exciton Cotton effects were used for the assignment of the configuration of dibenzoates in a steroidal skeleton separated by as many as seven or eight C—C bonds158. In one application, the absolute configuration of Wieland-Miescher ketone (—)-2 was established by the use of the dibenzoate chirality rule for the 4-bromobenzoylated derivatives of the epimeric 1,5-diols 3 and 4, obtained by reduction of (-)-2159. 

The CD spectrum of the 1,2-dibenzoate 6 in 9 1 MeOH/dioxane showed a pair of typical exciton-split Cotton effects with opposite signs centred upon the UV absorption (227 nm) of the benzoate chromophore AE235 5 -15.9 and A 221 5 +6.66. The negative longer wavelength Cotton effect clearly defines the negative chirality between the two electric transition dipoles of the benzoate chromophores... 

Figure 6.17 Exciton splitting and geometry of ordered array of planar molecules.

Spectral measurements suggesting exciton splitting were among early observations that led to the conclusion that the bacteriochlorophyll involved in the initial photochemical process exists as a dimer or special pair (Fig. 23-31),319/324 a conclusion verified by the structure determination. The special pair of BChl b lies in the center of the helical bundle that is embedded in the membrane. Nearly perpendicular to the rings of the special pair are two more molecules of BChl b. The central magnesium atoms of all four bacteriochlorophylls are held by imidazole groups of histidine side chains.319 325 Below the chlorophylls are... 

In summary, even though the implicit approximations used in the application of exciton CD to determine absolute configuration ignore important complications such as the exciton splitting order, d-orbital mixing, and vibronic couplings, it has been a remarkably successful method for determination of the absolute configuration. 

B. Mennucci, J. Tomasi and R. Cammi, Excitonic splitting in conjugated molecular materials A quantum mechanical model including interchain interactions and dielectric effects, Phys. Rev. B, 70 (2004) 205212. 

The perturbative IEFPCM-LR model has been applied to study EET between molecules in liquid solutions [26-28] and at liquid/gas interfaces [29], and to the exciton splitting in conjugated molecular materials [30],... 

The DECO formalism was first applied to a simple, hypothetical molecule in order to test the equations and the computational procedures for correctness, and to convey a feeling of signal magnitudes to be expected from these computations. In particular, the magnitude of the exciton splitting, the sign pattern in VCD and the distribution of IR intensities within the exciton manifold were obtained from these calculations. 

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