The effect of the carboxylic acid substituent position of phenyleneethynylene systems on the 2D self-organization is revealed by scanning tunneling microscopy.

Two-dimensional self-organization of a series of phenyleneethynylenes was investigated, at ambient conditions, by varying the length of alkoxy chain and introducing functional groups at the terminal positions using highresolution scanning tunneling microscopy (STM). The model phenyleneethynylene molecule, which does not possess any functional groups, self-organizes into wire like structures on surface. High-resolution STM imaging revealed that molecules are arranged in a skewed 1D fashion. The spacing between the molecular wires was successfully modulated by replacing hexyloxy (C 6 ) chains with dodecyloxy (C 12 ) chains. The initial step of the formation of all the molecular assemblies involves the alkyl CH • • • acetylenic π interactions (CH • • • π) leading to the organization of molecules as two types of strips. These strips further interlock to two types of 2D organizations. The hydroxyl as well as aldehyde groups present at the terminal positions of the phenyleneethynylene molecules play an important role in the interlocking process. An end-to-end assembly was observed in the case of phenyleneethynylene molecule possessing hydroxyl groups at the terminal positions, which is attributed to the intermolecular hydrogen bonding between the strips. The adsorption of molecules with two faces results in enantiomeric 2D structures and these aspects were investigated using molecular modeling studies. † Part of the "Hiroshi Masuhara Festschrift".

A model phenyleneethynylene, which does not possess any functional groups, self-organizes into wire-like structures on 2D surfaces. High-resolution STM imaging revealed that the molecules are arranged in a skewed 1D fashion. Analysis of various domains indicated the existence of two types of molecular packing arising from different modes of alkyl CH···π interaction, which was further supported by theoretical calculations.

An oligophenyleneethynylene (OPE), 1,4-bis(phenyleneethynyl)-2,5-bis(hexyloxy)benzene (2), is coupled with pyrene to extend the conjugation and allow its use as a light-harvesting molecule [Py-OPE (1)]. The absorption and emission maxima of 1 are red-shifted compared to those of 2. Similar differences in the singlet and triplet excited-state properties are evident. The fluorescence yield of 2 in toluene is 0.53, which is slightly less than the value for the parent OPE (2) of 0.66. The excited singlet and triplet of 1 as characterized from transient absorption spectroscopy exhibit lifetimes of 1.07 ns and 4.0 micros, respectively, in toluene. When 1 was cast as a film on a glass electrode (OTE) and excited with a 387-nm laser pulse, we observed the formation of excitons that decayed within a few picoseconds. When 1 was cast as a film on a SnO2-modified conducting glass electrode (OTE/SnO2), a small fraction of excitons dissociated to produce a long-lived charge-separated state. The role of the SnO2 interface in promoting charge separation was inferred from the photoelectrochemical measurements. Under visible light excitation, the OTE/SnO2 electrode was capable of generating photocurrent (approximately 0.25 mA/cm2) with an incident photon conversion efficiency (IPCE) of approximately 6%.