The mirror-image rule

The emission spectrum of a fluorophore is the image of its absorption spectrum when the probability of the Si So transition is identical to that of the Su Si transition. If 

The fluorescence emission spectrum of NBD-labeled pepdde shows a good image-minor witb respect to the cxeKatjon spccinitn. 

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In the contrary, quinine cxcHaiion and emis rm spectra do not follow the mirror-image rule (Figure 2.10). 

It is interesting to ask why perylene fdlows the mirror image rule, but quinine emission exhibits one peak instead of the two peaks seen in its excitati on spectrum at 310 and 335 nm (Figure 1.3). In the case of quinine, the shorter-wavelength absorption peak is due to excitation to the second excited state (S, which relaxes rapidly to Hence, emission occurs predominantly /torn the lowest singlet state (5,). The emission spectrum of quinine is die mirror image of the absorption of quinine, not of... 

A rigorous test of the mirror image rule requires that the absorption and emission spectra be presented in appropriate units. The closest symmetry should exist between the modified spectra and F(T)/T, where e(P) is the... 

In many instances the absorption and emission maxima correspond to the same (V1-V2) vibrational pair. For example. Figure 3A shows that the 0- 2 vibronic transition has the strongest intensity in both the absorption and the emission spectra. This is called the mirror image rule and is followed by lumino-phores whose excited state distortion is zero or small. However, the mirror image rule may not apply for cases where large excited state distortion exists. Examples of each case are provided in the section dealing with organic luminophores. 

For some aromatic hydrocarbons such as naphthalene, anthracene and pery-lene, the absorption and fluorescence spectra exhibit vibrational bands. The energy spacing between the vibrational levels and the Franck-Condon factors (see Chapter 2) that determine the relative intensities of the vibronic bands are similar in So and Si so that the emission spectrum often appears to be symmetrical to the absorption spectrum ( mirror image rule), as illustrated in Figure B3.1. 

In general, the differences between the vibrational levels are similar in the ground and excited states, so that the fluorescence spectrum often resembles the first absorption band ( mirror image rule). The gap (expressed in wavenumbers) between the maximum of the first absorption band and the maximum of fluorescence is called the Stokes shift. 

These discussions provide an explanation for the fact that fluorescence emission is normally observed from the zero vibrational level of the first excited state of a molecule (Kasha s rule). The photochemical behaviour of polyatomic molecules is almost always decided by the chemical properties of their first excited state. Azulenes and substituted azulenes are some important exceptions to this rule observed so far. The fluorescence from azulene originates from S2 state and is the mirror image of S2 S0 transition in absorption. It appears that in this molecule, S1 - S0 absorption energy is lost in a time less than the fluorescence lifetime, whereas certain restrictions are imposed for S2 -> S0 nonradiative transitions. In azulene, the energy gap AE, between S2 and St is large compared with that between S2 and S0. The small value of AE facilitates radiationless conversion from 5, but that from S2 cannot compete with fluorescence emission. Recently, more sensitive measurement techniques such as picosecond flash fluorimetry have led to the observation of S - - S0 fluorescence also. The emission is extremely weak. Higher energy states of some other molecules have been observed to emit very weak fluorescence. The effect is controlled by the relative rate constants of the photophysical processes. 

Adrafinil has a chirality centre since the sulfur atom of the sulfinyl group still possesses an unshared electron pair and consequently can exist with either an R or S configuration. From the sequence rules, the unshared electron pair, which in the S enantiomer shown below is pointing towards the observer, has the lowest priority. The priority order of the groups attached to the sulfur atom is O > C(C,C,H) > C(C,H,H) > e . The mirror image of this formula represents the R isomer. Note that sulfoxides with two different groups attached always have a chirality centre, since the sulfur atom cannot oscillate through the plane (i.e. there is no pyramidal inversion). 

In the extreme case of grazing incidence, a field component exists only normal to the surface. Therefore an interaction is possible exclusively with transition moments or components thereof, orientated perpendicular to the surface. This anisotropy of interaction can also be explained by selection rules, which are based on symmetry consideration and include the mirror image of the analyte produced by the metallic surface. 

This is true because the operation consists of two parts a rotation C and a reflection . Since a reflection creates the mirror image, the operation is equivalent to rotating in space the mirror image. By definition, a molecule containing a axis is brought into coincidence with itself by the operation S and hence its mirror image, after rotation, is superimposable. The reader is reminded that 8i = a and >=, so that a molecule with either a plane or a centre of sym-metry is also optically inactive. However, the most general rule is a molecmle with a axis is optically inactive. Ck>aversely, it can be shown that a molecule without a axis is, in principle, optically active. 

According to this rule the normalized spectra of absorption and fluorescence expressed as a function of the frequency are specularly symmetric with respect to a line normal to the frequency axis at the intersection of these two spectra. The mathematical expression for the mirror image relationship is... 

Unfortunately, the rule that above the definition plane of a particular conformer is the direction from which the carbon atom numbers increase round the ring has the effect of reversing conformational designations between enantiomers - the mirror image of a-D-glucopyranose in the Ci conformation is oc-L-glucopyranose in the conformation. 

When a molecule has a structure with identical atoms or groups on the top and bottom (one side can be perfectly reflected into the other side), that molecule has a plane of symmetry (see Figure 9.9). When such symmetry occurs, the mirror image of one stereoisomer is superimposable on itself. Such a stereoisomer is called a meso compound. Because 2,3-dibromobutane has two stereogenic centers, the 2" rule predicts a maximum of four stereoisomers. Symmetry in one stereoisomer means that 46 is a meso compound, so there are only three stereoisomers (the two enantiomers 43 and 44 and the meso compomid 46). It is important to point out that 46 and 43 are diastereomers. Likewise, 46 and 44 are diastereomers, but 43 and 44 are enantiomers. 

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