Photoinitiator free energy changes

Each of the terms in Eq. (1) must be known in order to calculate the free energy change for a photoinitiated electron transfer process (Eq. (2)). 

Figure 35. Rate of polymerization as a function of free energy change (-AGet) for photoinitiated CQ-NPG systems. Polymerizing monomer mixture 2-ethyl-2-(hydroxymethyl)-1,3-propanediol triacrylate (TMPTA) and ethylated bisphenol-A dimethacrylate (bis-EMA) (1 1). Polymerization was initiated by an argon-ion laser beam. Data from Ref. [175].

The preexponential factor v is now the Franck-Condon term which includes the free-energy change and the reorganization energy (which can also depend on separation). When the donors and acceptors lack spherical symmetry, there will also be an orientation dependence. When the donor and acceptor moieties are linked by covalent bonds, there is a possibility that the electron transfer event involves the orbitals of the linkage bonds. Evidence for this effect has been noted in certain kinds of compounds [46]. The distance dependence of photoinitiated electron transfer has recently been reviewed [21]. 

Superieure de Chimie, Mulhouse, Fr. J. Photopolym. Sci. Technol. (2000), 13 (2), 237-242 (Eng.). The interactions of excited states of a coumarin or a ketocoumarin photosensitizer with a bisimi-dazole deriv., mercaptobenzoxazole photoinitiators, and titanocene were studied by laser absorption spectroscopy and by photocond. The redox potential of the compds. was measured and used in calcn. of the free enthalpy change for a possible electron transfer reaction. The coumarin forms radicals through an electron transfer reaction, while the ketocoumarin undergoes an energy transfer reaction with bisimidazole and a hydrogen abstraction reaction with a benzoxazole deriv. Thus, coumarin or ketocoumarin/free radical initiator combinations are suitable as initiators of radical polymn. reactions, esp. those applicable to laser imaging. [1707-68-2]. 

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