Interfacial Organization-Induced Supramolecular Chirality of the Langmuir−Schaefer Films of a Series of PPV Derivatives

Chiroptical Properties in Thin Films of π-Conjugated Systems

Chiral π-conjugated molecules provide new materials with outstanding features for current and perspective applications, especially in the field of optoelectronic devices. In thin films, processes such as charge conduction, light absorption, and emission are governed not only by the structure of the individual molecules but also by their supramolecular structures and intermolecular interactions to a large extent. Electronic circular dichroism, ECD, and its emission counterpart, circularly polarized luminescence, CPL, provide tools for studying aggregated states and the key properties to be sought for designing innovative devices. In this review, we shall present a comprehensive coverage of chiroptical properties measured on thin films of organic π-conjugated molecules. In the first part, we shall discuss some general concepts of ECD, CPL, and other chiroptical spectroscopies, with a focus on their applications to thin film samples. In the following, we will overview the existing literature on chiral π-conjugated systems whose thin films have been characterized by ECD and/or CPL, as well other chiroptical spectroscopies. Special emphasis will be put on systems with large dissymmetry factors (g_abs and g_lum) and on the application of ECD and CPL to derive structural information on aggregated states.

Evaporative assembly of MEH-PPV rings using mixed solvents at the air/water interface

Controlling the morphology of conjugated polymers has recently attracted considerable attention because of their applications in photovoltaic (PV) devices and organic light-emitting diodes (OLEDs). Here, we describe the self-assembly of a common conjugated polymer, poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), into ringlike structures via solvent evaporation on an air/water interface. The films are monitored using Brewster angle microscopy (BAM) and transferred onto a solid substrate by either the Langmuir-Blodgett (LB) or the Langmuir-Schaefer (LS) method and further characterized by atomic force microscopy (AFM). The morphology of the MEH-PPV thin film at the air/water interface can be controlled by the spreading solvent. By mixing solvents of varying spreading coefficients and evaporation rates, such as chloroform and chlorobenzene, MEH-PPV can be assembled into micrometer-sized ring structures. The optical properties of these MEH-PPV ring structures are also characterized. Lastly, MEH-PPV can be used as a soft template to organize microscale structures of nanoparticles.

Interfacial assembly of an achiral zinc phthalocyanine at the air/water interface: a surface pressure dependent aggregation and supramolecular chirality

The aggregation and supramolecular chirality of the interfacial assemblies of an achiral phthalcyanine derivative, zinc 2,3,9,10,16,17,23,24-octakis(octyloxy)-29 H,31 H-phthaloxyanine (ZnPc), were investigated, and a surface pressure dependent behavior was observed. It was found that ZnPc could be spread as a Langmuir film on water surface and transferred onto solid substrates by the horizontal lifting method. The compound formed mixed J- and H-aggregates in the transferred Langmuir-Blodgett (LB) films. Deconvolution of the broaden Q-band revealed three main components of the spectra, which corresponded to H- and J-aggregates and medium transition aggregates, whose relative contents could be modulated by the surface pressure at which the films were transferred. On the other hand, the transferred LB films composed of these aggregates showed Cotton effects in circular dichroism spectra when the floating film was compressed over a certain surface pressure although the compound itself was achiral. The cooperative arrangement of the macrocylic ring in a helical manner through the interfacial organization was suggested to be responsible for such optical activity in the LB films. A possible explanation based on the cooperative arrangement of the ZnPc building blocks in a helical sense stacking in the aggregates was proposed.