Structured Layer of Rhenium Dye on SiO₂ and TiO₂ Surfaces by Langmuir–Blodgett Technique

Abstract

We demonstrate the Langmuir-Blodgett assembly of two rhenium-bipyridine complexes containing a flexible or an aromatic bridge, and transfer of the monolayer to SiO2 and single crystal TiO2 substrates. Both of the complexes (ReEC and Re2TC) have a hydrophilic carboxylic acid group, which preferentially anchors into the water subphase, and forms stable monolayers at surface pressures up to 40 mN/m. The optimum conditions for the formation of complete monolayers of both ReEC and Re2TC were identified through characterization of the morphology by atomic force microscopy (AFM), the thickness by ellipsometry, and the surface coverage by X-ray photoelectron spectroscopy (XPS). X-ray reflectivity measurements (XRR) are consistent with the orientation of the molecules normal to the substrate, and their extension to close to their calculated maximum length. Parameters derived from XRR analysis show that there is a higher packing density for Re2TC monolayers than for ReEC monolayers, attributable to the more rigid bridge in the Re2TC molecule.

Driving Coordination Polymer Monolayer Formation by Competitive Reactions at the Air/Water Interface

“…When spread on Co­(CH 3 COO) 2 10 mM, DI formed a continuous monolayer interrupted by many pores (Figure a). These pores allowed us to determine a monolayer thickness of 2.26 ± 0.45 nm (see cross sections of Figure a–c), compatible with the formation of a monolayer in the assumption of a ligand conformation characterized by the aromatic rings standing perpendicularly to the water subphase . As expected, the coordination polymer monolayer showed remarkable stability over time, as confirmed by the morphology being almost unchanged after 1h of equilibration­(Figure b) .…”

Structural heterogeneity and dynamics of dyes on TiO2: implications for charge transfer across organic–inorganic interfaces

“…13 It also highlights the potentially important role of dye coverage and packing on charge injection efficiency. 35,62 This principle is also reflected in the recent results of Asbury et al 18 and Paoprasert et al 19 The former examined dyes with two anchoring groups, which are thus extremely rigid and cannot access conformations with the bipyridine ring close to the surface. As such, those authors observed an exponential decay of injection rate with increasing bridge length from Re2MeC (19 ps) to Re2PrC (240 ps).…”

“…It is clear that both chain length and flexibility must be considered in determining the effect of the bridge on charge injection, with the observed coupling strengths spanning an order of magnitude. If the bridge is rigid 18 or if a well-ordered, high-coverage monolayer is achieved, 62 then an exponential decay of injection rate as a function of insulating chain length may be expected. 7,18 In other cases, however, the flexibility of the chain adds to the heterogeneity of both the surface and the injection rates.…”

Photoisomerization Dynamics in a Densely Packed Optically Transformable Azobenzene Monolayer

“…The synthesis of ReAzoC and the ordered assembly of the molecular monolayer using the Langmuir–Blodgett (LB) method have been previously described . The use of the LB assembly allows the monolayer deposition conditions, particularly the surface pressure during assembly, to be selected to allow a closely packed single monolayer to be reproducibly deposited. , The LB assembly of ReAzoC exhibits surface pressure isotherms similar to methyl 4-[( E )-2-[4-(nonyloxy)­phenyl]­diazen-1-yl]­benzoate, an azobenzene-containing liquid crystal that also assembles in trans state under ambient conditions . For this study, monolayers were assembled at a surface pressure of 25 mN m –1 , corresponding to a surface coverage of 1.84 molecules nm –2 , a density consistent with a closely packed single monolayer .…”

“…XRR fitting procedures were used to construct a model of the thickness, electron density, and interface roughness of the molecular monolayer in its starting trans configuration. − The Re-bipyridine group of the ReAzoC molecule has a relatively high electron density in comparison with purely organic molecular layers, which simplifies the collection and analysis of the XRR data. ,, The structural parameters of the monolayer quantities depend mainly on the periodicity and amplitude of the reflectivity fringes apparent at high values of Q in Figure a. The parameters of the structural model are ρ sub , the electron density of the substrate, σ sub , the roughness of the substrate–monolayer interface, and L monolayer , ρ monolayer , and σ monolayer , which are the thickness of the monolayer, the average electron density of the monolayer, and roughness of the monolayer–air interface, respectively.…”