Langmuir-Blodgett films incorporating molecular wire candidates of ester-substituted oligo(phenylene-ethynylene) derivatives

Langmuir and Langmuir-Blodgett (LB) films of two “wire-like” oligo(phenylene-ethynylene) methyl ester derivatives, namely methyl-4-[(4″-(nonyloxyphenylethynyl)-4′-(phenylethynyl)]-benzoate and methyl-4-[(4″-hexadecyloxyphenylethynyl)-4′-(phenylethynyl]-benzoate (abbreviated as C9BPEB and C16BPEB), have been prepared and characterized. Surface pressure isotherms for both materials have been obtained, with C9BPEB showing more expanded monolayers. An analysis of the reflection spectra of monolayer films at the air-water interface suggests the formation of H-aggregates, and supports an organizational model in which tilt angles of C16BPEB and C9BPEB molecules with respect to the water surface are approximately 70° and 60°, respectively. The sequential transfer of monolayers of these BPEB ester derivatives onto solid substrates results in a Z-type deposition in the case of C9BPEB and Y-type for C16BPEB. The current-voltage (I-V) characteristics of LB monolayers are relatively symmetrical for both films, with the asymmetric contacts more evident in the I-V C16BPEB characteristics.