Orientation and Aggregation Behavior of Rhodamine Dye in Insoluble Film at the Air−Water Interface under Compression. Second Harmonic Generation and Spectroscopic Studies

Abstract

The absorption and fluorescence spectra and second harmonic generation (SHG) of the insoluble monolayer of bis-(N-ethyl,N-octadecyl)rhodamine (RhC18) at the air−water interface have been measured. These spectra were affected significantly by compression, and the observed changes were ascribed to the formation and structural rearrangement of aggregated species on the water surface during compression. The spectroscopic behavior of the monolayer was explained in accordance with its rheological properties, and the transition from disordered monomers to dimers, from dimers to aggregates, and from aggregates to two-dimensional arrays was proposed. SHG studies revealed that the RhC18 molecules in the expanded film region are oriented with their C 2-axis tilted away from the surface normal on angle θ distributed in the range of 31−39°. The rotational distribution around the C 2-axis was assumed to be 45−60° according to preferable intermolecular interactions with the water subphase and surrounding molecules. The θ angle distribution became slightly narrow because of the increase of molecular ordering caused by two-dimensional external pressure. The sharp increase of SHG intensity and the phase shift observed at high compression were ascribed to the formation of blue-shifted aggregates with their electronic transition being in resonance with the incident laser frequency. The results of spectroscopic and SHG studies were jointly analyzed, and the structural rearrangement within the monolayer during compression was described.

Aggregation states of rhodamine 6G in electrospun nanofibrous films

“…536 nm to 540 nm), with no appearance of band splitting. Such a behavior is characteristic of J-type dimers [38,[40][41][42].…”

“…These results show the presence of serious aggregation (H-type) even at very low concentration in casting films while the formation of fluorescent J-type dimers is enhanced in electrospun films. Both red shifts of the emission maxima and relationship of fluorescence intensity with surface density of adsorbed chormophores have been explained as a consequence of the increase of the polarizability at the dye surrounding environment resulting from the close packing of xanthene moieties [37][38].…”

Spectroscopic properties of mixed Langmuir–Blodgett films of rhodamine dyes and poly(N,N-diallyl-N-octadecylamine-alt-maleic acid)

“…This indicates on aggregation of dye molecules leading to densely packed films. According to the exciton model of the molecular aggregates [7,8,29], the splitting of the S 1 -electronic excited level in dimers occurs on two sublevels, respectively with higher and lower energies. The short-wave transition band is related to "parallel" dimers ("sandwich" dimers), whereas the long-wave transition band is related to "plane" dimers.…”

“…Rhodamine dyes which have a high fluorescence quantum yields are good candidates for such purposes. The dyes can be chemically modified by long alkyl chains in order to convert them into amphiphiles and to make them suitable for spreading into insoluble Langmuir monolayers [6][7][8][9][10][11]. However the most monolayers made of pure amphiphilic rhodamine dyes have a poor stability and are hardly suited for the LB method.…”

Confocal Fluorescence Microscope Studies of the Adsorptive Behavior of Dioctadecyl-rhodamine B Molecules at a Cyclohexane-Water Interface

“…24 The maximum surface density (Nmax) cannot be deduced from the experimental data directly, because it is unable to determine the fluorescence signal corresponding to the full surface coverage due to self-quenching. From the surface-pressure isotherm (Π-A curve) of an Rh-C18 monolayer compressed by the Langmuir-Blodgett technique on the water surface, 25 the average area-per-molecule value is about 210 Å 2 , corresponding to a surface density of ∼8 × 10 -11 mol/cm 2 . It is known that a repulsive interaction might prevent adsorbed species from packing tightly at the liquid-liquid interface, resulting in occupying a larger average area per molecule.…”