Photoselection

Weitz E and Flynn G W 1981 Vibrational energy flow in the ground eleotronio states of polyatomio moleoules Advances in Chemical Physics Vol. XLVIl, Photoselective Chemistry part 2, ed J Jortner, R D Levine and S A Rise, pp 185-235... 

The photoaggregation technique has been extended to the Ag/Cr system (116), where the naked, bimetallic species AgCr and Ag Cr can be selectively photogenerated, and identified, in the presence of the uni-metallic, parent clusters, with photoselectivity reminiscent of that of the Cr/Mo system 115). A typical trace is shown in Fig. 24. 

Fig. 23. A schematic representation of the photoselectivity experiments involving (Cr -.Mo Kr = mixtures deposited at 10-12 K and then sequentially subjected...

The electronic properties and thus the absorption bands of a polymer depend on the conjugation length Z. The absorption energy decreases as Z increases reaching a limiting value of 16 000 cm l as Z approaches infinity. If the polymer solution consists of a distribution of conjugation lengths (i.e. chromophores), photoselection of the different chromophores within the... 

Shand M, Chance R, LePostollec M, Schott M (1982) Raman photoselection and conjugation-length dispersion in conjugated polymer-solutions. Phys Rev B 25 4431 1436... 

Thus, when a population of fluorophores is illuminated by a linearly polarized incident light, those whose transition moments are oriented in a direction close to that of the electric vector of the incident beam are preferentially excited. This is called photoselection. Because the distribution of excited fluorophores is anisotropic, the emitted fluorescence is also anisotropic. Any change in direction of the transition moment during the lifetime of the excited state will cause this anisotropy to decrease, i.e. will induce a partial (or total) depolarization of fluorescence. 

This equation shows that, at time t, each anisotropy term is weighted by a factor that depends on the relative contribution to the total fluorescence intensity at that time. This is surprising at first sight, but simply results from the definition used for the emission anisotropy, which is based on the practical measurement of the overall ly and I components. A noticeable consequence is that the emission anisotropy of a mixture may not decay monotonously, depending of the values of r, and Ti for each species. Thus, r(t) should be viewed as an apparent or a technical anisotropy because it does not reflect the overall orientation relaxation after photoselection, as in the case of a single population of fluorophores. 

If excited molecules can rotate during the excited-state lifetime, the emitted fluorescence is partially (or totally) depolarized (Figure 5.9). The preferred orientation of emitting molecules resulting from photoselection at time zero is indeed gradually affected as a function of time by the rotational Brownian motions. From the extent of fluorescence depolarization, we can obtain information on the molecular motions, which depend on the size and the shape of molecules, and on the fluidity of their microenvironment. 

Horwitz, J., Heller, J. Photoselection and linear dichroism of retinals. A method for identification and measurement of various geometrical isomers. J. Biol. Chem. 248, 1051-1055 (1973). 

Figure 2.8. Energy level diagram (top) and spectra (bottom) illustrating the site-photoselection model of >... 

Of considerable interest is the fact that not only the steady-state anisotropy but also its kinetics depend on the excitation wavelength. In this case another red-edge effect connected with site photoselection may be observed. Dipole-orientational relaxation may occur not only by rotation of the dipoles surrounding the fluorophore but also by rotation of the aromatic group itself. If red-edge excitation results in the photoselection of fluorophores whose energy of interaction with the environment already corresponds to that in the excited state, then the relaxation-associated rotation should not be observed and the rotation that occurs should be completely Brownian in character.(22)... 

Experiments involving anisotropy of phosphorescence or of the absorption of the triplet state rely upon the same principles as the measurement of fluorescence anisotropy. All are based upon the photoselection of molecules by polarized light and the randomization of polarization due to Brownian motion occurring on the time scale of the excited state. Anisotropy is defined as... 

Evidently, correlation functions for different spherical harmonic functions of two different vectors in the same molecule are also orthogonal under equilibrium averaging for an isotropic fluid. Thus, if the excitation process photoselects particular Im components of the (solid) angular distribution of absorption dipoles, then only those same Im components of the (solid) angular distribution of emission dipoles will contribute to observed signal, regardless of the other Im components that may in principle be detected, and vice versa. The result in this case is likewise independent of the index n = N. Equation (4.7) is just the special case of Eq. (4.9) when the two dipoles coincide. 

Fluorescence excitation spectroscopy is thus a powerful technique for obtaining molecular information about systems of cellular size. At present, the technique is restricted to single small objects because of the requirement of angular integration of the emitted fluorescence. As work progresses, similiar information will be obtainable from spectra taken at a particular angle with respect to the exciting beam. This will allow extension of the photoselection concept to suspensions of particles and perhaps to individual cells. 

L. M. Folan and S. Arnold, Determination of molecular orientation at the surface of an aerosol particle by morphology-dependent photoselection, Opt. Lett. 13, 1-3 (1988). 

The method of photoselection to study the polarization of triplet-triplet transitions has been applied by El-Sayed and Pavlopoulos 6) to several poly-acenes. Let us discuss some of the results obtained on napthalene, where the -axishas been chosen along the long molecular axis and they-axis along the short one. The first very weak Si- - 5o absorption band x-polarized) is... 

The polarization study on naphthalene was complemented by Lavalette 39) who determined the polarized excitation spectrum, again using photoselection. The polarization of the strong Tm - Ti band at 4170 A was monitored as a function of the wavelength of polarized excitation into the singlet bands. As expected, a minimum polarization value of —0.18 was obtained at 2900 A near the 0—0 band of the S2 5q Mg) transition. 

El-Sayed, M. A., Pavlopoulos, T. Polarization of the triplet-triplet spectrum of some polyacenes by the method of photoselection. J. Chem. Phys. 39, 834 (1963). 

Time-Resolved Absorption Spectroscopy Advantages, 232, 389 applications, 232, 387-388 detectors, 232, 387, 392-393, 399 hemoglobin data analysis, 232, 401-415 kinetic analyses, 232, 390 photoselection effects, 232, 390-391 kinetic intermediates and. 

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