Luminescence self-absorption

It is realized today that in order for a LSC device to be efficient, one of the major requirements is that a great part of the solar spectrum (Fig. 1) should be absorbed, and that a large Stokes shift is needed between the absorption and the emission in order to prevent appreciable self-absorption of the luminescence. Another requirement involves a thermodynamical limitation of flux concentration which may be expressed31 by the inequality... 

Steady-State Spectral Convolution. The steady state absorption and emission spectra of dilute dye samples can be measured using standard spectroscopic techniques. Once the extinction coefficient, e( ), and the normalized luminescence spectrum, f(v), are known for a particular dye, the self—absorption probability r over a pathlength L in the sample containing the dye at a concentration C is given by... 

This technique suggests what is probably the simplest way to determine the probability of self-absorption empirically for a particular plate. The peak position of the luminescence spectrum can be calculated for a variety of self—absorption rates using the above formalism. Thereafter, the self-absorption rate for a particular device can be found (crudely) by a moderate resolution measurement of the peak position of the luminescence spectrum. 

Figure 7 Self-atsorption probabilities for rhodamine-575 This shows a juxtaposition of predicted self-absorption probabilities for three measurement methods spectral overlap convolution (solid), emission depolarization (boxes), and time-resolved spectra (bars). The second two techniques are plotted assuming the quantum efficiency of luminescence is one.

Due to the high accuracy possible with photon counting measurements of the lifetime, this technique yields the most accurate measurement of any technique we have studied of the product nr, the quantum efficiency of luminescence and the self—absorption probability, for low rates of self-absorption. 

CODES for Efficiency and Performance Evaluation. It is very useful to give prospective luminescent materials a "figure of merit" which measures the material s self-absorption characteristics (hence Q. etc.) in an LSC. We have defined ( 5) the crit-... 

An aqueous sample may be added to the cocktail directly, after minor prior processing, or at the end of a radiochemical separation procedure. Direct addition is the equivalent of gross activity counting discussed in Section 7.2.4 except that some spectral analysis may be possible. Alpha particles can be differentiated from beta particles by deposited energy, pulse shape, and decay time. Self-absorption is of no concern. Quenching and luminescence, discussed in Section 8.3.2, often occur. Identification by maximum beta-particle energy is approximate, and requires comparison to radionuclide standards. 

One advantage of polymers which is especially useful for laser-oriented applications is the red shift in the luminescence spectrum in comparison with their absorption. This phenomenon gives the opportunity of light amplification without losses caused by self-absorption in active media. Many efforts are now being concentrated on the manufacture of electronically pumped polymer lasers, although they have not yet met with success. 

A supramolecular assembly of macromolecules bearing antenna dendron has been reported. Pyrazole-anchored PBE dendrons were synthesized to examine the coordination behavior to transition-metal cations (Cu, Au, Ag) [31]. Self-assembled metallacycles were found. The Cu-metallacycle further formed luminescent fibers about 1 pm in diameter. The luminescence (605 nm) occurred by the excitation of the dendron (280 nm) and the excitation spectrum was coincident with the absorption spectrum of the dendron, suggesting the antenna effect. Interestingly, the luminescence of the Cu-metallacycle fiber disappeared when the fiber was dissociated into the individual metallacycles in C2H2. 

Macher Absorption and Self Luminescence Spectra of Cf3+ (5f ). Report UCRL-9654 (Mai 1961). J.chem. Phys. 36, 189 (1962). Chemistry Division Annual Progress Report for 196I, Report UCRL-10023 (Jan. 1962), S. 179ff. 

Since the same dye molecules can serve as both donors and acceptors and the transfer efficiency depends on the spectral overlap between the emission spectrum of the donor and the absorption spectrum of the acceptor, this efficiency also depends on the Stokes shift [53]. Involvement of these effects depends strongly on the properties of the dye. Fluoresceins and rhodamines exhibit high homo-FRET efficiency and self-quenching pyrene and perylene derivatives, high homo-FRET but little self-quenching and luminescent metal complexes may not exhibit homo-FRET at all because of their very strong Stokes shifts. 

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