Mirror-image rule

The emission spectrum of a fluorophore is the image of its absorption spectrum when the probability of Si — So transition is identical to that of So — Si transition. If, however, excitation of the fluorophore leads to an So — Sn transition, with n 1, internal relaxation will occur so that molecules reach the first excited singlet state before emission. 

This will induce an emission transition different from the absorption one. The mirror-image relationship is generally observed when the interaction of the fluorophore excited state with the solvent is weak. 

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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. 

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. 

Diastereomers include all stereoisomers that are not related as an object and its mirror image. Consider the four structures in Fig. 2.3. These structures represent fee four stereoisomers of 2,3,4-trihydroxybutanal. The configurations of C-2 and C-3 are indicated. Each stereogenic center is designated J or 5 by application of the sequence rule. Each of the four structures is stereoisomeric wife respect to any of fee others. The 2R R and 25,35 isomers are enantiomeric, as are fee 2R, iS and 25,3J pair. The 21 ,35 isomer is diastereomeric wife fee 25,35 and 2R,3R isomers because they are stereoisomers but not enantiomers. Any given structure can have only one enantiomer. All other stereoisomers of feat molecule are diastereomeric. The relative configuration of diastereomeric molecules is fiequently specified using fee terms syn and anti. The molecules are represented as extended chains. Diastereomers wife substituents on the same side of the extended chain are syn stereoisomers, whereas those wife substituents on opposite sides are anti stereoisomers. 

The enzyme-catalyzed interconversion of acetaldehyde and ethanol serves to illustrate a second important feature of prochiral relationships, that ofprochiral faces. Addition of a fourth ligand, different from the three already present, to the carbonyl carbon of acetaldehyde will produce a chiral molecule. The original molecule presents to the approaching reagent two faces which bear a mirror-image relationship to one another and are therefore enantiotopic. The two faces may be classified as re (from rectus) or si (from sinister), according to the sequence rule. If the substituents viewed from a particular face appear clockwise in order of decreasing priority, then that face is re if coimter-clockwise, then si. The re and si faces of acetaldehyde are shown below. 

The precision of the data is not such as to allow non-dipole interactions to be definitively ruled out, and more detailed study of this topic by careful measurement of the full angular distribution, as opposed to detection at a single angle, will be required to provide a complete probe. In the meantime a clear observation that enantiomer PECD curves have a mirror-image relationship... 

NMR data [95]. This new method requires two sets of dipolar couplings from two different protein orientations. Together with the backbone dipolar couplings that are typically used (i.e., amide NH, C N, CaC, CaHa and the two-bond HNC ), CaCp dipolar couplings are also needed. Provided that the orientation of one peptide plane is known independently, the dipolar coupling data give rise to two possible orientations for the subsequent peptide plane, where the conformations about the alpha carbon in these two orientations are mirror images. One of the conformations can be ruled out because of chirality. 

Reactant conversion into its mirror image, NARCISSISTIC REACTION REACTING BOND RULES REACTING ENZYME CENTRIFUGATION REACTION COORDINATE DIAGRAM POTENTIAL ENERGY DIAGRAM SADDLE POINT... 

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. 

Molecules that are not superimposable on their mirror images are termed dissymmetric. This term is used rather than asymmetric, since the latter means, literally, have no symmetry that is, it is applicable only to a molecule belonging to point group Cx. All asymmetric molecules are dissymmetric, but the converse is not true. Dissymmetric molecules can and often do possess some symmetry. It is possible to give a very simple, compact rule expressing the relation between molecular symmetry and dissymmetric character ... 

To show the validity of this rule, we first prove that if an Sn axis does exist the molecule cannot be dissymmetric that is, it must have a superimposable mirror image. 

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