Phosphorescent probe molecules

If we estimate the time required for S-T mixing and recombination for a radical pair to be 100 ns( and the lifetime of the plvaloyl radical at 31°C to be v6 ps(i5., we can estimate the rate constant for the exit of t-butyl/plvaloyl radicals from HDTCl micelles to be on the order of 10 -10 sec . This Is nicely In line with exit rates of small phosphorescent probe molecules from similar micelle systems. 

The original stabilizer (HBC) was modified as the rapid radiationless deactivation of the stabilizer is (at least partly) due to the intramolecular hydrogen bond, the H-atom was substituted by a methyl group (MBC). This "probe molecule" showed fluorescence and phosphorescence and enabled us to demonstrate the energy transfer to the stabilizer, simply by studying its sensitized luminescence. 

While we used the probe molecule to investigate the energy transfer by sensitized phosphorescence we now turn to the stabilizer itself (e.g. TIN with an intramolecular hydrogen bond) to study its deactivation in the excited states. 

There are three likely causes of the temperature dependence of phosphorescence of probe molecules K in polymeric and glassy matrices These are... 

Where the excited state solute or probe molecule has. In competition with phosphorescence, an efficient nonradlatlve decay path which Involves rotational motion of the carbon skeleton. It might be expected that there will exist an Intrinsic energy barrier to this rotation. Provision of sufficient thermal energy to overcome the barrier would thus enhance nonradlatlve decay at the expense of phosphorescence. This Is a purely Intramolecular property of the probe molecule and Is entirely Independent of the environment. 

The required accuracy for < ) depends, of course, on the purpose of the measurement.) Protein motions with < ) values greater than this will probably require that a longer-lived extrinsic fluorophore (e.g. pyrene) be attached to the molecule. Motions on the microsecond to second timescales can be monitored using phosphorescent probes (see Section 4.2). 

The rate constants of the phosphorescent probe s exit from cetyltrimethyl-ammonium chloride micelles were determined by Bolt and Turro [155]. The logarithm of the rate constant is a linear function of the number of the carbon atoms in the probe molecule. The apparent activation energy of the exit process is 9 kcal/mole. 

This facilitates the relative importance of radiationless decay by internal conversion or by quenching through collision with traces of impurities. Consequently, phosphorescence is rarely observable in fluid media. An important exception is in the case of ketones which have lowest energy - (mr ) triplet excited states (4). Here photon emission occurs at rates of 10 to 10 sec , fast enough to compete with solvent or impurity quenching if care is taken to deoxygenate the samples and purify the solvents. For molecules such as acetone, acetophenone, benzo-phenone, biacetyl and benzil, phosphorescence is readily observed in fluid solution at ordinary temperatures with (1/e) lifetimes of 50-500 ys. Heavy atoms promote phosphorescence rates. Dibromoacetonaph-thone (5), with a lowest (TnT ) triplet state is a useful phosphorescence probe of micellar systems. There is a whole literature on heavy-atom induced room-temperature phosphorescence applications in analytical chemistry (6),... 

The relationship between segmental and group motion in solid polymers and the rates of diffusion of small molecules suggests that diffusion measurements might be used to detect the transitions associated with the occurrence of this type of motion, and indeed this turns out to be the case. One of the most sensitive measurements of the occurrence of solid-phase transitions is obtained by observing the quenching of phosphorescent probes in solid polymers by the diffusion of oxygen. 

Based on steady-state and time-resolved emission studies, Scaiano and coworkers have concluded that silicalite (a pentasil zeolite) provides at least two types of sites for guest molecules [234-236], The triplet states of several arylalkyl ketones and diaryl ketones (benzophenone, xanthone, and benzil) have been used as probes. Phosphorescence from each molecule included in silicalite was observed. With the help of time-resolved diffuse reflectance spectroscopy, it has been possible to show that these triplet decays follow complex kinetics and extend over long periods of time. Experiments with benzophenone and arylalkyl ketones demonstrate that some sites are more easily accessed by the small quencher molecule oxygen. Also, diffuse reflectance studies in Na + -X showed that diphenylmethyl radicals in various sites decay over time periods differing by seven orders of magnitude (t varies between 20/is and 30 min) [237]. 

Luminescence decay curves are also often used to verify that samples do not contain impurities. The absence of impurities can be established if the luminescence decay curve is exponential and if the spectrum does not change with time after pulsed excitation. However, in some cases, the luminescence decay curve can be nonexponential even if all of the luminescing solutes are chemically identical. This occurs for molecules with luminescence lifetimes that depend upon the local environment. In an amorphous matrix, there is a variation in solute luminescence lifetimes. Therefore, the luminescence decay curve can be used as a measure of the interaction of the solute with the solvent and as a probe of the micro-environment. Nag-Chaudhuri and Augenstein (10) used this technique in their studies of the phosphorescence of amino acids and proteins, and we have used it to study the effects of polymer matrices on the phosphorescence of aromatic hydrocarbons (ll). 

It would be desirable to insert a probe into the polymer to ascertain the local environmental conditions. In addition to having microscopic dimensions, the probe must act as a timing device which specifies the time-scale of the observation. Such a probe is a fluorescent molecule. Its dimensions are about the size of a monomer residue, namely of the order of 10 A, and the lifetime of fluorescence, r, varies between about 10-9— 10"7 sec., depending on the fluorescent compound and the medium (9). Still longer time-scales, namely, 10"4—10 sec., are achieved with organic molecules in the phosphorescent state (21). 

The electronic excited state is inherently unstable and can decay back to the ground state in various ways, some of which involve (re-)emission of a photon, which leads to luminescence phenomena (fluorescence, phosphorescence, and chemiluminescence) (22). Some biologic molecules are naturally fluorescent, and phosphorescence is a common property of many marine and other organisms. (Fluorescence is photon emission caused by an electronic transition to ground state from an excited singlet state and is usually quite rapid. Phosphorescence is a much longer-lived process that involves formally forbidden transitions from electronic triplet states of a molecule.) Fluorescence measurement techniques can be extremely sensitive, and the use of fluorescent probes or dyes is now widespread in biomolecular analysis. For example, the large increase in fluorescence... 

Figure 4.9 Schematic drawing of near-field optical probes. By pulling out an optical fiber and coating with aluminum, an optical probe with a very small aperture (<100 nm) is created. The probe can detect fluorescence or phosphorescence from objects in the near field close to the aperture, such as a thin film or flat surface (a) or from a labeled molecule (b). The probe is relatively insensitive to light in the medium in the far field further from the tip. The excitation light source can be transmitted through the probe or externally.

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