Quantum yields for polymers

Table II. Quantum yields for polymer degradation for polymers with different concentrations of metal-radical trap.

Thus the quantum yield for acid production from triphenylsulfonium salts is 0.8 in solution and about 0.3 in the polymer 2 matrix. The difference between acid generating efficiencies in solution and film may be due in part to the large component of resin absorption. Resin excited state energy may not be efficiently transferred to the sulfonium salt. Furthermore a reduction in quantum yield is generally expected for a radical process carried out in a polymer matrix due to cage effects which prevent the escape of initially formed radicals and result in recombination (IS). However there are cases where little or no difference in quantum efficiency is noted for radical reactions in various media. Photodissociation of diacylperoxides is nearly as efficient in polystyrene below the glass transition point as in fluid solution (12). This case is similar to that of the present study since the dissociation involves a small molecule dispersed in a glassy polymer. 

Table 2. Quantum yields for oolymer scission and crosslinking for some representative polysilanes (SiR R )n (a) Molecular weights were measured by GPC using polystyrene calibration standards (b) Ban, H Sukegawa, K. J. Polym. Sci., Polym. Chem. Ed., 1988, 26, 521...

In an attempt to sensitize the thiosulfate bond cleavage, benzophenone (10% by weight) was incorporated into the polymer film. Upon photolysis at 366 nm, the 639 cm 1 thiosulfate band was reduced (Figure 10) as in the case of direct photolysis at 254 nm and 280 nm. Since benzophenone is a known triplet sensitizer it is likely that the S-S bond cleavage in the thiosulfate group occurs from a triplet excited state in the sensitized reaction. Incidentally photolysis of a PATE film at 366 nm in the absence of benzophenone resulted in no loss of the 639 cm 1 IR peak. Unfortunately due to the film thickness, we were unable to obtain accurate quantum yields for either the direct or sensitized photolysis. Finally it should be noted that no chemical evidence has been presented to confirm disulfide formation. Results from the photolysis of a PATE-type model compound will be offered to substantiate the claim of disulfide formation as well as quantitate the primary photolysis step. But first, we consider photolysis of a PASE polymer film. 

The dependence of the fluorescence quantum yields and lifetimes of these stabilizers on the nature of the solvent suggests that the excited-state, non-radiative processes are affected by solvation. In polar, hydroxylic solvents, values of the fluorescence quantum yield for the non proton-transferred form are significantly lower, and the fluorescence lifetimes are shorter, than those calculated for aprotic solvents. This supports the proposal of the formation, in alcoholic solvents, of an excited-state encounter complex which facilitates ESIPT. The observed concentration dependence of the fluorescence lifetime and intensity of the blue emission from TIN in polymer films provides evidence for a non-radiative, self-quenching process, possibly due to aggregation of the stabilizer molecules. 

The ketone group is a useful model for other types of chromophores because it can be selectively excited in the presence of other groups in polymer chains such as the phenyl rings in polystyrene and so the locus of excitation is well defined. Furthermore there is a great deal known about the photochemistry of aromatic and aliphatic ketones and one can draw on this information in interpreting the results. A further advantage of the ketone chromophore is that it exhibits at least three photochemical processes from the same excited state and thus one has a probe of the effects of the polymer matrix on these different processes by determination of the quantum yields for the following photophysical or photochemical steps l) fluorescence,... 

The quantum yield for the chain-breaking process in a polymer molecule is given by... 

There is some contribution due to / -scission of the alkyl radical formed by the type I process, particularly in the MIPK and tBVK polymers. Loss of carbonyl occurs from photoreduction or the formation of cyclobutanol rings, and also from vaporization of the aldehyde formed by hydrogen abstraction by acyl radicals formed in the Norrish type I process. As demonstrated previously (2) the quantum yields for chain scission are lower in the solid phase than in solution. Rates of carbonyl loss are substantially different for the copolymers, being fastest for tBVK, slower for MIPK, and least efficient for MVK copolymers (Table I and Figure 1). 

A further possibility is that the signals arise from hydrated electrons or base radical ions produced by monophotonic ionization of the polymers. However, the quantum yield for photoionization of adenosine is reported to be approximately the same as that of poly(A) and poly(dA) [25], It is unlikely that photoionization of the polymers can account for the signals seen here since there is no detectable signal contribution from the photoionization of single bases [4], The most compelling argument that our pump-probe experiments monitor excited-state absorption by singlet states is the fact that ps and ns decay components have been observed in previous time-resolved emission experiments on adenine multimers [23,26-28]. 

Reactions analogous to these are observed with the polymers that contain metal-metal bonds along their backbones. Because the metal-metal bond chromophore absorbs in the visible region, the photochemical reactions of the polymers can be conveniently monitored by electronic absorption spectroscopy. The quantum yields for the reactions are in the range 0.1 to 0.6, depending on the specific polymer and the M-M bond.14 Sample reactions of the polymers are shown in equations 20-22. 

FIGURE 3. Plots of the quantum yields for disappearance of the Cp 2Mo2(CO)6 unit in polymer 5, Cp 2Mo2(CO)6 dispersed in PVC, and Cp 2Mo2(CO)6 in hexane/CCl4 (Cp = q5-C5H4CH3). 

---