Ramesh C. Ahuja

Counterion specific interactions in dioctadecyldimethylammonium bromide monolayers at the monolayer/subphase interface

Abstract We have investigated the influence of halogen counterions on the molecular organization characteristics of dioctadecyldimethylammonium bromide (DOMA) monolayers at the air/water interface. Surface pressure- and surface potential-area isotherms and Brewster angle microscopy were used to characterize the head group-counterion interactions. The counterions used were F−, Cl−, Br− and I−. The presence of halogen ions in the subphase leads to compression of the π-A and a decrease in the surface potential. The effects of counterions on the microscopic features of molecular organization are, however, the most dramatic. DOMA at the air/water interface shows a homogeneous and featureless morphology during the whole compression process. The presence of F− and Cl− ions in the subphase does not lead to any significant changes. However, the introduction of Br− ions results in the appearance of dendritic structures in the liquid/solid coexistence region. The I− ions have an even greater effect on the morphology, in that solid domains are formed immediately after spreading. The influence of counterions on DOMA isotherms and morphology may be rationalized in terms of the degree of hydration of the various ions.

Pyrene-fatty acid-containing phospholipid analogues: Characterization of monolayers and Langmuir-Blodgett assemblies.

Abstract The following pyrene-containing phospholipid analogues have been studied: 1-palmitoyl-2(pyren-1-yl)decanoyl-sn-glycero-3-phosphatidylcholine§ (PPDPC), -glycerol (PPDPG), and -PE (PPDPE), 1-palmitoyl-2(pyren-1-yl)hexanoyl-sn-glycero-3-phosphatidylmethanol (PPHPC), 1-octacosanyl-2(pyren-1-yl)hexanoyl-sn-glycero-3-phosphatidylmethanol (C-28-O-PHPM) and 1-octacosanyl-2(pyren-1-yl)octanoyl-sn-glycero-3-phosphatidylmethanol (C-28-O-POPM). These compounds were first characterized by compression isotherms and surface potential measured at an air/water interface, after which conditions for their transfer onto Cd arachidate-coated quartz glass slides were sought. The transferred layers were investigated for their absorption in the range of 300–650 nm. All compounds revealed spectra characteristic for pyrene. It was further found that with the slide at an angle of 45° to the beam, the ratio of absorption of light polarized parallel to that polarized perpendicular to the layer plane, A∥/A⊥, was strongly dependent on the chemical structure of the lipid derivative; however, the surface pressure at which the film was maintained during the deposition had less effect. In fluorescence emission the excimer fluorescence intensity (Ie) exceeded by several times the intensity of emission from the monomeric pyrene species (Im). The most significant new findings of the present study came from preliminary fluorescence decay measurements of transferred layers of PPDPG. These were obtained using time-resolved photon counting with the sample held in vacuo and at a temperature of 10, 200, 250, 280, or 300 K. The observed rapid non-exponential decay of monomer fluorescence at 380 nm was rather insensitive to temperature. The decay of excimer emission at 380 K appeared to exhibit three components of approximate lifetimes of (a) below 0.1 ns, (b) 43 ns and (c) 83 ns. The relative proportions of the component lifetimes were sensitive to temperature. Notably, in the temperature range of 10–300 K the decay of excimer emission was observed immediately following the nanosecond flash, i.e. the kinetics of excimer formation from monomeric pyrene could not be resolved with the instrument used. Therefore, we conclude that the organization of pyrene in the phospholipid liquid crystal monolayer on quartz to be such that redistribution of electronic excitation to yield excimers is allowed within less than 0.1 ns following the formation of the excited monomeric species. This indicates the very close proximity of the pyrene moieties, yet without significant ground state interaction, as suggested by the similarity to the absorption spectra of pyrene in organic solvents.

Molecular cis-trans switching in amphiphilic monolayers containing azobenzene moieties

Abstract A novel principle of molecular architecture for monolayers containing azobenzene moieties is presented which allows the chromophores to be organized with a high density and with sufficient mobility for photochemical change. This monolayer represents a new medium for a data storage system; each single azobenzene group is in principle capable of storing the information of 1 bit. The physical properties of this monolayer have been investigated at the air-water interface. Data storage in the macrometer range is demonstrated with a single monolayer transferred on glass.

Langmuir films and Langmuir-Blodgett multilayers incorporating mechanically-threaded molecules-pseudorotaxanes

This paper describes the synthesis of a π-electron rich aromatic lipid containing a polyether thread intercepted by a 1,5-dioxynaphthalene moiety. The terminus of one end of the polyether is a methoxyl group and the other a glycerol unit linked via the 2-oxo position, while the 1,3-oxo functions are covalently attached to octadecyl chains. This compound, when cospread at the air-water interface with the tetracationic cyclophane, cyclobis(paraquat-p-phenylene), and the phospholipid, dimyristoylphosphatidic acid, as its monoanion, forms self-assembled pseudorotaxanes which are self-organized into a two-dimensional supramolecular array at the interface. The electrostatic interactions between the tetracationic cyclophane and the monoanions of the phospholipid anchor the cyclophane to the interface. This anchoring allows the naphthalene moiety to remain threaded through the cavity of the cyclophane by a combination of charge-transfer and hydrogen-bonding interactions. These Langmuir films can, subsequently, be transferred to a quartz support to fabricate Langmuir-Blodgett multilayers. In characterising these self-organized systems, surface pressure-area isotherms and surface potential measurements have been used in conjunction with fluorescence and UV-Vis spectroscopies. This research may have implications in sensor technology and molecular information storage devices.

Photoinduced electron transfer in Langmuir-Blodgett films.

Abstract Photoinduced electron transfer from an excited donor molecule (oxacyanine) to an acceptor molecule (viologen) located at the same interface in monolayer assemblies has been studied by measuring steady state fluorescence quenching and the excited state decay of the donor molecule. Donor and acceptor molecules are incorporated in matrix monolayers of arachidic acid and methyl arachidate, with a molar ratio 9:1. It is seen that the electron transfer efficiency increases with increasing donor density ( σ d =0.005−0.425 nm −2 ). The fluorescence decay functions of the donor are described by assuming at least two fluorescent species and the results show that the contribution of long-lived species increases with increasing donor density. The average rate constant for the excited state electron transfer depends on the donor density and was found to be 2×10 8 s −1 ( σ a = 0.025nm −2 ; σ d = 0.005 nm −2 ). The results are rationalized in terms of energy delocalization via incoherent exciton hopping.

Metal ion sensor based on dioctadecyldithiocarbamate-metal complex induced energy transfer

The complex formation of dioctadecyldithiocarbamate (DOTC) monolayers at the air-water interface by reaction with metal ions from the subphase was investigated using surface pressure/area (π/A) and surface potential/area (ΔV/A) isotherms and reflection spectroscopy. Metal ion complexation leads to metal specific changes in the π/A and ΔV/A isotherms and also to either a shift of the DOTC reflection spectrum or to the appearance of a new band in the UV-vis reflection spectrum. These properties were utilized for the construction of a Cu2+ -ion sensor based on the Langmuir-Blodgett (LB) technique. The sensor uses energy transfer (Forster type) from fluorescent oxacyanine dye layer to a DOTC layer with the complexing group in contact with the aqueous phase containing the Cu2+ ions. Copper complexes with the specific absorption band were formed instantaneously at the solid-liquid interface of the LB film. The quenching of the fluorescence of the oxacyanine layer increases with the surface density of the copper complexes in the top layer of the LB film.

Photoinduced electron transfer in monolayers at the monolayer-water interface.

We have investigated the photoinduced electron transfer from an excited donor (oxacyanine) incorporated in matrix monolayer at the air-water interface to an acceptor (methyl viologen) adsorbed at the monolayer-water interface from the aqueous subphase. The steady state surface density and the kinetics of adsorption of the acceptor at the monolayer-water interface were measured using surface pressure, surface potential, surface reflection and fluorescence techniques. The acceptor adsorption was found to be largely governed by the interfacial potential.

Generation and transmission of a surface pressure impulse in monolayers

Abstract Surface pressure impulse propagation through a monolayer at the air-water interface has been investigated. The pressure impulse confined to the monolayer plane is produced by the photoisomerization of an amphiphilic spiropyran. Arachidic acid, dimyristoylphosphatidylcholine and monomethyloctadecanedioate have been used as transmitting layers. The pressure impulse is detected with a microphone-type sensor at various distances. The longitudinal pulse velocity is in the 50–260 cm s −1 range. The results are interpreted in terms of a simple model in which a thin water layer under the monolayer moves along with it. The thickness of this layer is estimated to be about 100 μm, independent of the transmitting layer. It is also concluded that the dynamical compression modulus is up to four times larger than the stationary compression modulus.

Charge transfer interactions at the air-water interface and in Langmuir-Blodgett films

Abstract The charge transfer (CT) interactions between an amphiphilic electron donor dioctadecyl- p -phenylenediamine (L2) and an amphiphilic electron acceptor octadecyltetracyanoquinodimethane (C 18 TCNQ) have been investigated in monolayers at the air-water interface and in Langmuir-Blodgett (LB) films. Surface pressure-area π− A and surface potential-area Δ V − A isotherms together with surface-enhanced UV-visible reflection spectroscopy at the air-water interface have been used for the characterization of CT. The isotherms indicate that neither L2 nor C 18 TCNQ forms well-defined monolayers. However, for the L2-C 18 TCNQ mixed monolayers, CT interactions between the two components lead to the formation of stable monolayers as indicated by the π− A and Δ A − A isotherms. The absorption spectrum of L2 and C 18 TCNQ in CH 3 CN solution exhibits two radical-anion bands (750 nm and 850 nm) of TCNQ. At the air-water interface, the reflection spectrum of L2C 18 TCNQ mixed monolayer does not exhibit the radical-anion band of TCNQ − . However, the dimer band of TCNQ − 2 at 650 nm is observed. LB films on hydrophobized quartz plates were prepared in such a way that donor and acceptor were located either in the mixed monolayer or in adjacent monolayers. The LB exhibit the radical-anion bands of TCNQ as well as the radical-anion dimer band at 680 nm.

Photochemical investigation of pyrene-labelled monolayers

Abstract Mixed monolayers of dipalmitoylphosphatidic acid and two pyrene-labelled fluorescent analogues of dipalmitoylphosphatidylcholine were studied at the air-water interface. The pyrene moiety was covalently bound to the phospholipids in two different positions: at the end of one of the aliphatic chains and in the polar head group. Monolayers were characterized by measuring surface pressure- and surface potential-area isotherms, and the fluorescence and reflection spectra as a function of compression as well as monolayer composition. Information on the structural and dynamic properties of the monolayer were obtained from excimer-to-monomer intensity ratio, and from the change in the relative intensities of the vibronic peaks in the monomer emission bands. The photoinduced electron transfer between the fluorescent probe and the absorbed electron acceptor (methylviologen) depends on the pyrene location. When pyrene is located in the hydrophobic region of the monolayer, quenching of the excited state of pyrene occurs as a result of long-range photoinduced electron transfer between pyrene and methylviologen. However, when pyrene is located in the interfacial region of the monolayer, the observed decrease in fluorescence intensity is also due to the formation of a ground state complex between pyrene and methylviologen.