The structure of octadecyl monolayers on the H-terminated Si(111) surface is investigated by molecular modeling simulations, using substitution percentages from 33.3% to 100% of the Si-H moieties by Sialkyl groups. In all calculations, two-dimensionally repeating boxes were used to mimic the modified surface. Calculations without this repeating box approach were shown to be unsuccessful. The results on the repeating boxes showed that only with a substitution percentage of ∼50% is there a good correlation between the structure of the monolayers as obtained from molecular modeling and the available experimental data. A variety of substitution patterns with a substitution percentage of 50% on the Si(111) surface were investigated, which showed that a zigzag-type pattern is most suitable to describe the structure of the layers. From the results of the investigations, an important conclusion for future experimental work is drawn. It is shown that the experimentally determined substitution percentage of 50-55% of the Si-H for Si-alkyl groups is close to the maximum value that can be reached on the H-terminated Si(111) surface.

The photopolymerization of spin-coated films of diacetylene derivatives containing phthalimido or naphthaldiimido moieties has been studied by monitoring the increase in the optical absorption in the visible region on irradiation at 308 nm. The phthalimido derivatives form blue polymers with absorption maxima at ca. 630 nm, corresponding to a highly conjugated, close to planar polydiacetylene (PDA) backbone configuration. The naphthaldiimido derivative yields a red polymer with an absorption maximum at 535 nm, indicating a backbone structure with a substantially reduced degree of π-bond conjugation. 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. Monomer conversions up to ca. 50% are found. The polymerized films are found to be photoconductive using the time-resolved microwave conductivity technique (TRMC). Mobile charge carriers are suggested to be formed via the triplet state of the arylimido moieties which undergoes long-distance charge transfer to polydiacetylene chains to form the arylimido radical anion and the mobile PDA radical cation or “hole”. A maximum value of 1.4 × 10-2 cm2/(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. This is attributed to a higher mobility of PDA holes in the former compound, resulting from the higher degree of backbone conjugation. The lifetime of the mobile carriers extends well into the microsecond region, which is considerably longer than previously found on direct ionization of PDA chains.