Effect of Processing Conditions on the Formation of Aggregates and Phase Domains in Monolayers of the Hemicyanine Dye, 4-(4-(Dihexadecylamino)styryl)-N-methylpyridinium Iodide

Interfacial organization of achiral porphyrins via unidirectional compression: a general method for chiroptical porphyrin assemblies of selected chirality

Porphyrins are considered to be important scaffolds bridging supramolecular chemistry and chiral chemistry, where chirality selection via physical effects such as directional stirring and spin-coating has aroused particular interest. Nevertheless, these protocols could only work on a limited number of achiral porphyrins. It still remains a formidable challenge to pave a general avenue for the construction of chiral assemblies using achiral porphyrins. By means of a unique Langmuir-Schaefer (LS) technique of a unidirectional compression configuration, we herein have demonstrated that a series of achiral porphyrins could be facilely organized to form chiral interfacial assemblies of controlled supramolecular chirality. It has been disclosed that such a fascinating chirality selection scenario is intimately related to the direction of the compression-generated vortex-like flow, while the compression speed, one of the most significant parameters of the Langmuir technique, contributes less to this issue. With regard to a surface-pressure-dependent chirality selection phenomenon, it is suggested that the directional vortex-like flow generated by lateral compression might play a role in promoting the preferential growth of chiral assemblies showing an enhanced yet controlled CD signal. Our protocol might be, to some extent, a general method for achieving chiral porphyrin assemblies of controlled chirality.

UV-Vis Reflection-Absorption Spectroscopy at air-liquid interfaces

UV-Visible Reflection-Absorption Spectroscopy (UVRAS) technique is reviewed with a general perspective on fundamental and applications. UVRAS is formally identical to IR Reflection-Absorption Spectroscopy (IRRAS), and therefore, the methodology developed for this IR technique can be applied in the UV-visible region. UVRAS can be applied to air-solid, air-liquid or liquid-liquid interfaces. This review focuses on the use of UVRAS for studying Langmuir monolayers. We introduce the theoretical framework for a successful understanding of the UVRAS data, and we illustrate the usage of this data treatment to a previous study from our group comprising an amphiphilic porphyrin. For ultrathin films with a thickness of few nm, UVRAS produces positive or negative bands when p-polarized radiation is used, depending on the incidence angle and the orientation of dipole absorption. UVRAS technique provides highly valuable information on tilt of chromophores at the air-liquid interface, and moreover allows the determination of optical parameters. We propose UVRAS as a powerful technique to investigate the in situ optical properties of Langmuir monolayers.

Synthesis, crystal structure, and spectroscopic properties of new stilbazolium salt with enlarged π-conjugated system

A new stilbazolium dye 4-{(E)-2-[4-(dimethylamino)naphthalen-1-yl]ethenyl}-1-methylquinolinium iodide monohydrate (DANSQI) having enlarged π-conjugated system was synthesized and characterized by X-ray diffraction, IR, Raman, UV-Vis, Fluorescence, (1)H- and (13)C NMR spectroscopy. Quantum chemical calculations were performed to obtain electronic structure and vibrational data, using DFT. The crystals are monoclinic, space group P21/n, with a=8.0751(14), b=25.839(4), c=10.9031(15) Å, V=2141.4(6) Å(3), and Z=4 (at 300(2) K). The unit cell contains four molecules of the dye, participating in weak intermolecular interactions. The cation is nearly flat with a deviation of the planarity of 5.08 (1)°. The dye investigated is the first stilbazolium iodide containing water molecule in the solid state therefore the N-dimethylamino group declines significantly from planarity, as indicated by C13N1C2C3 torsion angle of 16.08°. The dye studied shows solvatochromism of 84 nm in visible region and very large Stokes shift up to 253 nm. The intensity of fluorescence bands strongly depends on solvent polarity.

Enhanced optical and nonlinear optical responses in a polyelectrolyte templated Langmuir-Blodgett film

Optical and nonlinear optical properties like fluorescence and second harmonic generation (SHG) of molecular materials can be strongly influenced by the mode of assembly of the molecules. The Langmuir-Blodgett (LB) technique is an elegant route to the controlled assembly of molecules in ultrathin films, and complexation of ionic amphiphiles in the Langmuir film by polyions introduced in the aqueous subphase provides a simple and efficient access to further control, stabilization, and optimization. The monolayer LB film of the hemicyanine-based amphiphile, N-n-octadecyl-4-[2-(4-(N,N-ethyloctadecylamino)phenyl)ethenyl]pyridinium possessing a "tail-head-tail" structure, shows fluorescence as well as SHG response. The concomitant enhancement of both of these linear and nonlinear optical attributes is achieved through templating with the polyanion of carboxymethylcellulose. Brewster angle and atomic force microscopy reveal the influence of polyelectrolyte templating on the morphology of the Langmuir and LB films. Polarized absorption and fluorescence spectroscopy provide insight into the impact of complexation with the polyelectrolyte on the orientation and deaggregation of the hemicyanine headgroup leading to fluorescence and SHG enhancement in the LB film.

Fluorescence enhancement in Langmuir-Blodgett films: role of amphiphile structure, orientation, and assembly

Suppression of self-quenching of fluorescence emission in supramolecular systems is of fundamental interest in the design and development of novel optical materials. While a number of molecular design and assembly strategies have been formulated for the fabrication of molecular crystals with enhanced light emission, parallel explorations based on systematic approaches in the domain of molecular ultrathin films are rare. We have investigated Langmuir-Blodgett films of amphiphilic molecules bearing the same hemicyanine choromophore headgroup but with different possibilities of attachment of the octadecyl hydrocarbon chain, under different deposition conditions. Fluorescence spectroscopy studies indicate enhanced light emission in the derivative with a "tail-head-tail" structure. The observed trends are attributed to the amphiphile structure, nature of deposition, and the chromophore orientations revealed through polarized absorption spectra of the films. The study suggests a simple design strategy toward realization of molecular ultrathin films with enhanced light emission.