Exciton assignments

Not reported if acceptor- or donor-bound exciton. Assigned to donor or ionized donor-bound excitons. 

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... 

In this article we have reviewed the results of a joint spectroscopic and morphological investigation of -sexilhienyl (T(J. The lowest singlet electronic level, which is assigned to I B , splits in the single crystal into four crystalline levels. The structure of the exciton band is investigated by the combined absorption and... 

PA at l. 48 eV appeal s instantaneously, shows spectral relaxation to the red, and decays on the same timescale of SE, as shown in Figure 8-9. We assign the observed PA to singlet Bu exciton transitions towards higher lying even parity (A ) states. We can speculate on the nature of this state within the proposed model. A possible candidate for the final slate is the inirachain biexciton. However, its energy level is located below the two-exciton stale by an amount equal to the bind-... 

Ley and Schanze have also examined the luminescence properties of the polymers Pq, Pio> P25> and P50 in solution at 298 K, and in a 2-methyltetrahydro-furan solvent glass at 77 K. These spectroscopic studies reveal that fluorescence from the 71,71" exciton state is observed at Amax=443 nm, 2.80 eV in the polymers P0-P50 at 298 and 77 K, but the intensity and lifetime of the fluorescence is quenched as the mole fraction of Re in the polymers is increased. This indicates that the metal chromophore quenches the 71,71" state. The quenching is inefficient even when the mole fraction is large, suggesting that interchain diffusion of the 71,71" exciton is slow compared to its lifetime [70]. Phosphorescence from the 71,71" state of the conjugated polymer backbone is observed at > max=b43 nm, 1.93 eV in P10-P50 at 77 K, and emission at Amax=690 nm, 1.8 eV is assigned to the d7i(Re) 7i oiy MLCT transition. 

Recently, a method was developed which allows assignment of the configuration around a secondary carbinol center combining kinetic resolution and the exciton chirality CD method241 (see Section 4.3.4.1.4., p 458). 

The following discussion is divided into three subsections the ketone chromophore (Section 4.4,1.1.), for which configurational assignments are based on the effect of ring dissymmetry and substitution pattern on the rotatory power of the n-rt transition. For conjugated chro-mophores (Section 4.4.1.2.) it is both the helicity of the chromophore and the vicinal substituent effect that determines the rotatory power of the 71-71 transition. Finally, the versatile stereochemical method, exciton chirality method (Section 4.4.2.), is based on the chiral interaction between the electric dipoles of the allowed transitions in two or more chromophores. 

The dibenzoate chirality rule 155, 156 extends the application of the exciton chirality method to molecules containing no suitable chromophore, but, rather two hydroxy groups which can be converted to benzoates or cinnamates. For example, the dibenzoate 1, obtained by benzoy-lation of the ra-diol, from microbial oxidation of ethylbenzene, displays a negative exciton Cotton effect and is hence assigned the 1 S.2R configuration157. 

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. 

---