Anthracene, diffusion control reactions

Photosensitization of diaryliodonium salts by anthracene occurs by a photoredox reaction in which an electron is transferred from an excited singlet or triplet state of the anthracene to the diaryliodonium initiator.13"15,17 The lifetimes of the anthracene singlet and triplet states are on the order of nanoseconds and microseconds respectively, and the bimolecular electron transfer reactions between the anthracene and the initiator are limited by the rate of diffusion of reactants, which in turn depends upon the system viscosity. In this contribution, we have studied the effects of viscosity on the rate of the photosensitization reaction of diaryliodonium salts by anthracene. Using steady-state fluorescence spectroscopy, we have characterized the photosensitization rate in propanol/glycerol solutions of varying viscosities. The results were analyzed using numerical solutions of the photophysical kinetic equations in conjunction with the mathematical relationships provided by the Smoluchowski16 theory for the rate constants of the diffusion-controlled bimolecular reactions. 

The experimental and simulation results presented here indicate that the system viscosity has an important effect on the overall rate of the photosensitization of diary liodonium salts by anthracene. These studies reveal that as the viscosity of the solvent is increased from 1 to 1000 cP, the overall rate of the photosensitization reaction decreases by an order of magnitude. This decrease in reaction rate is qualitatively explained using the Smoluchowski-Stokes-Einstein model for the rate constants of the bimolecular, diffusion-controlled elementary reactions in the numerical solution of the kinetic photophysical equations. A more quantitative fit between the experimental data and the simulation results was obtained by scaling the bimolecular rate constants by rj"07 rather than the rf1 as suggested by the Smoluchowski-Stokes-Einstein analysis. These simulation results provide a semi-empirical correlation which may be used to estimate the effective photosensitization rate constant for viscosities ranging from 1 to 1000 cP. 

Perkey and Farhataziz [38] have measured the rate constants for the reaction of e am with some aromatic hydrocarbons (anthracene, nitrobenzene, phenan-threne, naphthalene, benzamide), with acetone and with dichloromethane, 1,2-dichloroethane, and 1-chloropropane. In the case of the aromatic molecules listed above, the rate constants are an order of magnitude larger than the corresponding values in water, reflecting the lower viscosity of liquid ammonia and indicating that the reactions are diffusion-controlled. In the case of biphenyl (B), the reaction of e am was found to be reversible [39] ... 

Absolute rate constants have been determined for aromatic triplet formation in acetone solutions of several aromatic compounds (5, 30). The formation curves were observed directly for anthracene and naphthalene triplet (5) and for diphenyl triplet. These rate curves were found to fit a first order rate law, and were interpreted as a bimolecular energy transfer process from a state of the solvent molecule which is probably the triplet, that is, by Reaction 11. These rate constants, as well as the triplet yields, are listed in Table VI. The rate constants for anthracene and naphthalene triplet formation appear to correspond to diffusion controlled rate constants. Two further points are of interest, which are in contrast with observations in other systems which will be discussed. In acetone, most of the yield of aromatic triplet (at concentrations of the aromatic compound of 5 X 10"3M or lower) is formed in diffusional processes such as collisional energy transfer. Any fast formation appears... 

A study of the photodimerisation of anthracene in supercritical CO2 at different densities has revealed that the reaction is about an order of magnitude more efficient in CO2 than in normal liquid solvents. This is rationalised in terms of a reaction whose mechanism is diffusion-controlled even for those rate constants which are of the order of lO M s. An investigation of the influence... 

For polymerization in the liquid state, the reaction kinetics is even more complicated due to the heat and the increase in viscosity associated with the reaction, the so-caUed Tromsdorff effects [47]. The reaction accelerates itself as the polymerization proceeds as a result of a positive feedback loop generated by the increase in heat as well as in viscosity and the diffusion-controlled termination of the polymerization process [48, 49]. An example is illustrated in Figure 6.5 for a mixture of a polystyrene doubly labeled with anthracene and fluorescein (PSAF) dissolved in MMA monomer [50]. A PSAF/MMA (5/95) mixture containing 2wt% of Lucirin TPO as a photoinitiator and 6 wt% of ethylene glycol dimethacrylate (EGDMA) as a... 

When the kinetics are diffusion controlled an important decrease of the reactivity is observed as the chain length increases. Thus, Horie and Mita, studying the extinction of the phosphorescence of benzyl groups (laser excited) by anthracene rings (triple-triplet absorption), observed that the rate constant k of the reaction between benzyl and anthracene becomes k/2 when anthracene molecules are attached to a polystyrenic chain and when both benzyl and anthracene are attached to... 

---