Photopolymerization of a Diacetylene‐Phenazine Mixed Crystal

Photopolymerization of mixed crystals of 2,4‐heptadecadiynoic acid and phenazine was studied under optical excitation of either the phenazine at 420 nm or the diacetylene monomer at 280 nm. Polymer absorption at 514 nm or dark‐ and photoconductivity were used as a probe to continuously monitor product formation. The reaction velocity is constant up to about 10 percent conversion and decreases in a linear fashion for 0.15 ≲ c ≲ 0.4. Consequently, energy transfer from the primarily excited state to polymer molecules must be negligible for c < 0.1. This yields a upper limit of 10 −11 s for the lifetime of the optical precursor state which must be of singlet character. At 290 K the quantum efficiencies are E (420) = 0.1 and E (280) = 0.5 for phenazine and diacetylene excitation, respectively. The activation energies for photopolymerization are E (420) = 0.21 ± 0.02 eV and E (280) = 0.17 ± 0.02 eV. E (280) is attributed to chain growth whereas E (420) − E (280) ≈︁ 0.04 eV has to be associated with formation of a chain initiation species as a result of sensitizer excitation. Electron transfer between diacetylene and the excited phenazine is excluded as the rate‐determining step for the sensitized reaction.