Long-lived, mobile charge carriers formed on photoexcitation of UV-polymerized, spin-coated films of arylimido-spacer-diacetylene derivatives

The photopolymerization of spin-coated films of diacetylene derivatives con taining phthalimido or naphthaldiimido moieties has been studied by monitor ing the increase in the optical absorption in the visible region on irradia tion at 308 nm. The phthalimido derivatives form blue polymers with absorpt ion maxima at ca. 630 nm, corresponding to a highly conjugated, close to pl anar polydiacetylene (PDA) backbone configuration. The naphthaldiimido deri vative yields a red polymer with an absorption maximum at 535 nm, indicatin g a backbone structure with a substantially reduced degree of pi-bond conju gation. The initial,"low-dose" quantum yields for monomer conversion, based on the total number of photons absorbed at 308 nm, range from 2 to 20. Mon omer conversions up to ca. 50% are found. The polymerized films are found t o be photoconductive using the time-resolved microwave conductivity techniq ue (TRMC). Mobile charge carriers are suggested to be formed via the triple t state of the arylimido moieties which undergoes long-distance charge tran sfer to polydiacetylene chains to form the arylimido radical anion and the mobile PDA radical cation or "hole". A maximum value of 1.4 x 10(-2) cm(2)/ (V s) for the product of the quantum yield for charge carrier formation and the hole mobility is found. The photoconductivity of the "blue" polymer is approximately an order of magnitude larger than for the "red" variety. Thi s is attributed to a higher mobility of PDA holes in the former compound, r esulting from the higher degree of backbone conjugation. The lifetime of th e mobile carriers extends well into the microsecond region, which is consid erably longer than previously found on direct ionization of PDA chains.