Absorption photocyclization

Fig. 20. The photocycle of bacteriorhodopsia with its iatermediates including lifetimes and absorption wavelengths ia nm ia parentheses (158).

The silyloxy-substituted precursor polymers (88) can be photocyclized in a polymer-analogous fashion to yield polymeric intermediates (90) containing the 3,6-phenanthrylene unit. These intermediates can be converted thermally to yield conjugated polyarylenevinylenes, in this case poly(3,6-phenanthrylene-vinylene) (91), a polymer that displays a long-wavelength absorption maximum at about 360 nm 1114]. 

Figure 3.31. Simple diagram of the bacteriorhodopsin (BR) photocycle. The kinetics constants were based on data from time-resolved absorption or TR spectra given in references 137,143 and 144. (Reprinted with permission from reference [145]. Copyright (1991) American...

The application of picosecond TR spectroscopy by Doig el al. to study the J, K, and KL intermediates of the BR photocycle will be briefly described here. The 550 nm pump and 589 nm probe pnlses in the TR experiments were chosen to be near the absorption maxima of gronnd state BR (568 nm) and K (590nm) respec-tively. " Stokes TR spectra were obtained with time delays varying from Ops to 13 ns between the pnmp and probe pnlses in order to examine the structure and kinetics of the J —> K —> KL seqnence of the BR photocycle. ... 

The ortho substituents used were supposed to have no influcence on the absorption spectrum and to possess the same circular dichroism as the parent compound. The ortho substituted compound can only photocyclize in its exo-conformations, but equilibration by a rotation over the phenylethylene bond converts an M-exo into a P-endo form (or P-exo into M-endo) which cannot cyclize. Indeed, the optical yields of the 4-substituted [6]hclicenes from the ortho substituted (1+4) precursors are not lowered in comparison with the unsubstituted (2 + 3) precursor. On the other hand (4+1) precursors having a para substituent give again a low optical yield. The values found suggest that racemization in the excited state is slightly hindered, what may be due to steric hindrance. 

We have studied the initial steps in the PYP photocycle by detecting the absorption changes in the structure sensitive 1800-1100 cm 1 region, initiating the reaction with 60-fs 475-nm excitation. 

The Photoactive Yellow Protein (PYP) is the blue-light photoreceptor that presumably mediates negative phototaxis of the purple bacterium Halorhodospira halophila [1]. Its chromophore is the deprotonated trans-p-coumaric acid covalently linked, via a thioester bond, to the unique cystein residue of the protein. Like for rhodopsins, the trans to cis isomerization of the chromophore was shown to be the first overall step of the PYP photocycle, but the reaction path that leads to the formation of the cis isomer is not clear yet (for review see [2]). From time-resolved spectroscopy measurements on native PYP in solution, it came out that the excited-state deactivation involves a series of fast events on the subpicosecond and picosecond timescales correlated to the chromophore reconfiguration [3-7]. On the other hand, chromophore H-bonding to the nearest amino acids was shown to play a key role in the trans excited state decay kinetics [3,8]. In an attempt to evaluate further the role of the mesoscopic environment in the photophysics of PYP, we made a comparative study of the native and denatured PYP. The excited-state relaxation path and kinetics were monitored by subpicosecond time-resolved absorption and gain spectroscopy. 

Reversible photocyclization of diarylethene annealed to a bpy ligand in [Re(Cl) (CO)3(bpy-diarylethene)] follows the same general mechanism as isomerizations (Fig. 13) [128], Near-UV excitation into the Re—>bpy CT transition of the open form sensitizes cyclization of the diarylethene unit, which is accompanied by growth of an absorption band in the visible spectral region, between 500 and 600 nm. Energy transfer to the diarylethene unit and following cyclization occur with time... 

Since a fulgide, its E, Z isomers, and photocyclized form exhibit different absorption spectra, electronic absorption spectra are often used to investigate the photoreaction of fulgides. The UV absorption spectra data of some aryl lulgides are listed in Table 4.1. 

The transient absorption spectra show that besides the ring-closure product (C), a new transient species was found that could be quenched by oxygen. The authors assigned the new species to an excited triplet state. The time-resolved transient absorption spectra indicated that when the time delay was increased, the excited triplet state was decreased and the ring closure product was increased slightly. The decay of the excited triplet state can be fit to first-order kinetics. The lifetime ( ) of the transient is 0.8 ps in cyclohexane. The authors noted that the photocyclization process proceeds mainly via the excited singlet state, but the excited triplet state is... 

Absorption at 650 nm was found to increase in around 1 ps in accord with the decay of the absorption at 515 nm after irradiation with a 360-nm laser pulse (fiyhm 180 fs). The absorption bands at 515 and 650 nm were assigned to S i - Sn transitions of the open-ring form (or some intermediate produced from the excited state of the open-ring form) and the closed-ring form, respectively. By taking into account the pulse duration and the time constant of formation of 1.1 ps, this kinetic analysis reproduced the decay and the rise profile. This result clearly indicates that the photocyclization of the dithienylethene took place in 1 ps. 

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