Excited chromophore

In this chapter are summarized the photochemical reactions wherein the primary chemical event is inter- or intramolecular hydrogen transfer to the excited chromophor. In intermolecular reactions hydrogen abstraction usually implies reduction or hydrodimerization of the excited molecule intramolecular hydrogen abstraction is frequently followed by either ring closure of the diradical or fragmentation to afford unsaturated molecules. 

An interesting modification of such a reaction, wherein the reactivity of the excited chromophore is increased by protonation is outlined in the following example whereby a pyrrolenine derivative is photocyclized to a pyrrolidine derivative (4.75) 488). 

It is apparent from the quantity of material included in this chapter that there is an extensive body of work concerning the utilization of diene and polyene photochemistry in a synthetic setting. The unique behavior of the excited chromophores permits the application of powerful new methods for the construction of complex molecules. Unusual photochemical rearrangements and photocycloaddition pathways often lead to substantial increases in molecular complexity, allowing such processes to serve as key strategic steps in target oriented syntheses. 

For pairs of like chromophores at a fixed distance and with random and uncorrelated static orientations, the decay of emission anisotropy of the indirectly excited chromophore varies with time, tending to zero (Berberan-Santos and Valeur, 1991) in contradiction to earlier works where it was reported to be 4% of that of the directly excited chromophore. Therefore, because the probability that emission arises from the directly excited chromophore is 1/2, the decay of emission anisotropy of the latter levels off at r0/2. This can be generalized to an ensemble of n chromophores (with random and uncorrelated static orientations) the decay of emission anisotropy of the directly excited chromophore levels off at r0/n. 

Inhibition of the spontaneous emission from excited chromophores... 

The mechanism for radical initiation by chromophores such as q.2 is believed to begin with electron transfer from the excited chromophores to an acrylate group [21]. Electrochemical measurements suggest that this electron... 

Fig. 2. Simplified schematic image of the reaction coordinate diagram for excited chromophores of PYP and Rh undergoing the twisting and the coherent vibrations.

Therefore, the much faster reorganization of the excited chromophore - surrounding PNS environment interactions leading to the much faster dynamic Stokes shift of fluorescence than that observed for the PYP chromophore in bulk water environment takes place in this PYP analogue. 

In the case of the denatured PYP, the chromophore is surrounded by completely disordered environment of the water. The relaxations due to the interactions between the excited chromophore with a large dipole moment and surrounding disordered water environment produce a large extent dynamic Stokes shift of the fluorescence. Nevertheless, it is much slower compared with that taking place in the disordered PNS of the ferulic acid analogue as demonstrated in the analysis of the previous section (3.3). 

Femtosecond photoionization mass spectrometry might be useful in the study of the three-dimensional structure of large biomolecules. When a selectively excitable and ionizable chromophore is located on the outer (surface) part of large molecule, one can be detached in the picosecond time scale. However, when the excitable chromophore is located in the inner part of the big molecule, its detachment will require a much longer time, which is needed for spatial rearrangement of the molecule. So, even the simple mass spectrometry of bioorganic molecules with femtosecond laser ionization can reveal some details of their spatial structure. 

Figure B3.6.12 Depolarization of fluorescence indicates rotation of the chromophore. Monochromatic radiation from the source (S) has all but the vertically polarized electric vector removed by the polarizer (P). This is absorbed only by those molecules (see Fig. B3.6.5) in which the transition dipole of the chromophore is aligned vertically. In the case where these molecules do not rotate appreciably before they fluoresce ( no rotation"), the same molecules will fluoresce (indicated by shading) and their emitted radiation will be polarized parallel to the incident radiation. The intensity of radiation falling on the detector (D) will be zero when the analyzer (A) is oriented perpendicular to the polarizer. In the case where the molecules rotate significantly before fluorescence takes place, some of the excited chromophores will emit radiation with a horizontal polarization ( rotation ) and some with a vertical polarization. Finite intensities will be measured with both parallel and perpendicular orientations of the analyzer. The fluorescence from the remainder of the excited molecules will not be detected. The heavy arrows on the left of the diagram illustrate the case where there is rotation.

The triplet state of the unpaired electrons of oxygen play a key role in both the photon excitation and the potential relaxation mode of the excited chromophores of vision. The paramagnetic properties of oxygen provide a definitive method of determining whether oxygen is present in the chromophores of vision, a condition that would eliminate the Shiff-base theory of retinol reaction with opsin to form rhodopsin. The evaluation of the electron paramagnetic resonance of the chromophores of vision is discussed in Chapter 7. 

Where P is a polymer molecule containing a chromophore group, and A is an acceptor molecule with a suitable triplet energy level slightly below that of the excited chromophore in the polymer. 

Chromophores (Ch) are transformed after absorption of the actinic solar radiation in excited singlet ( Ch ) and triplet (3Ch ) states (Rabek, 1996) (Eq. 3-11). Excited chromophores sensitize the formation of macroalkyls from the matrix polymer (Eq. 3-12a) and singlet oxygen from the ground state oxygen (Eq. 3-12b) and accelerate homolysis of POOH via an exciplex (Eq. 3-12c), Reaction scheme 3-3. 

---