Interchromophoric distances

Distance distributions can be determined when the interchromophoric distance does not change significantly during the donor lifetime. Otherwise, energy transfer is enhanced by translational diffusion of the donor and acceptor moieties towards each other. Then, information on relative diffusion coefficients of chain ends in oligopeptides (Katchalski-Katzir et al., 1981) or polymers can be obtained. 

For the design of complexing bifluorophores, much attention should be paid to the Forster critical radius of the donor-acceptor pair as compared to the interchromophoric distance (Figure 2.13). This critical radius depends on the donor quantum yield and on the spectral overlap between donor emission and acceptor emission. Complex-... 

For non-coplanar electric transition dipole moments pi0a and pj0a of the two chromophores at Rjj interchromophoric distance the exciton chirality is nonzero and defined by ... 

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 rate of EET between a pair of weakly coupled donor (D) and acceptor (A) molecules, according to Forster theory, [76] depends on the interchromophoric distance R, expressed in units of cm, their relative orientation (through the orientation factor k), and the spectral overlap I between donor emission and acceptor absorption spectra. The rate expression is ... 

With these issues in mind, dibenz[c,h]acridine was chosen as a spacer. As a result of its U-shape, chromophores attached at C-2 and C-12 could be held in parallel planes (Fig. 2). The C-2 to C-12 distance is known to be 7.24 A from the X-ray structure [23], however, the interchromophore distance in the molecular... 

They found that Forster s theory of resonance energy transfer was applicable to such systems if the Interchromophoric distance and orientation, the fluorescence efficiency of the donor, the extinction of the acceptor, and the overlap between the emission of the donor and the absorption of the acceptor are of magnitudes which produce a transfer rate constant of less than 10 s and a transfer efficiency which is not too close to 0 or 1. 

A full report of the c.d. of steroidal diol bis-(p-dimethylaminobenzoates) confirms the reliability of theoretical calculations of the coupled Cotton effects of the remote ester groups (exciton chirality method). The c.d. curves show two maxima of opposite signs, separated by some 27 nm, and with intensity inversely proportional to the square of the interchromophore distance. The profile of the observed c.d. curve results from the superimposition of two component curves, each of asymmetric shape.A review of the uses of chiroptical techniques for structural and conformational studies includes examples of the assignment of stereochemistry to steroids and terpenoids, among a wide variety of natural products. The Octant Rule for carbonyl compounds and the rules applicable to other chromophoric systems are discussed critically. 

The intensity of the exciton CD (/(-value) is inversely proportional to the square of the interchromophoric distance R,y provided the remaining angular part is the same.8... 

Most of the chromophores shown in Figure 16 are useful for exciton split CD analysis for short to medium interchromophoric distances of 13-15 A. For distances up to 50 A, only the porphyrins and metalloporphyrins can provide couplets sufficiently intense for configurational analysis. Therefore when the AC of remote stereogenic centers is sought, the tetraarylporphyrin (TPP) and metalloporphyrins make an excellent choice. 

In cases where the configurational analysis involves remote stereogenic centers with C-C distances of approximately 8-9 A and interchromophoric distances with RtJ of approximately 13-14 A, the observed CD couplet becomes very weak or even undetectable with chromophores with weak or even moderate absorption... 

Figure 17 UV-Vis and fluorescence data for TPP-COOH 19 and Zn-TPP-COOH 20. Bottom bisftetraarylporphyrin) derivative of 5a-cholestane-3a,17/3-diol 23 and 24 a positive helicity between the two effective transition moments defined in direction 5C/15C, the interchromophoric distance R, , UV-Vis and CD spectra of in CH2CI2 UV e, Revalues, and CD amplitudes A of other bischromophoric derivatives of 5a-cholestane-3a,17/3-diol 21 and 22.

The excimer of poly(2-napAla) showed an intense circular polarization, indicating that the excimer has a skewed chiral configuration. Therefore, the excimer in poly(2-napAla) may be formed not in the randomly coiled part (e.g., at the termini), where no particular excimer configuration is expected, but in the helix part of the polypeptide. Since the interchromophore distance in the helix part (6,0 A) may be too far to form an excimer, a small distortion of the side-chain and/or main-chain will be necessary to form the excimer. 

Ultrafast techniques have been used to observe energy transfer directly. For example, sub-picosecond time resolved intramolecular transfer examined in flexible bichromophoric coumarin molecules shows that exchange occurs within 1 to 20 ps depending polymethylene chain length . The distribution of interchromophoric distances in donor/acceptor coumarin supermolecules has been measured analysis of data ftom time resolved energy transfer . 

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