Thickness Dependence of the Optical Anisotropy for Porphyrin Octaester Langmuir−Schaefer Films † (original) (raw)
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Structure-dependent optical anisotropy of porphyrin Langmuir–Schaefer films
2002
We have studied the optical anisotropy of Langmuir-Schaefer layers of PdC 10 OAP porphyrin, deposited onto gold substrates with thickness in the range 0-16 monolayers (ML). Deposition has been carried out at two values of the surface pressure P, corresponding to different layer structures. In one case (P ¼ 30 mN/m), molecules are well ordered in stacks oriented edge-on with respect to the substrate. In the other (P ¼ 10 mN/m), a complex reorganization of the system happens several days after deposition, to form a mesoscopic two-dimensional lattice. The spectra measured by reflectance anisotropy spectroscopy (RAS) in two cases are clearly characterized. In the former, the line shape is dominated by a characteristic, large structure appearing in coincidence with the Soret band of the molecule, the development of which from a ''peak-like'' to a ''derivative-like'' appearance occurs at a well-defined critical thickness H c (8 ML). In the latter, the line shape is always ''peak-like''. We explain both line shapes in terms of morphological characteristics of the layer, occurring at different thickness values. The present results clearly show the potential of RAS to characterize efficiently the deposition of organic materials, and suggest that in short time it will be used as an in situ and real time spectroscopy, as already done in inorganic growth.
Optical anisotropy of porphyrin Langmuir–Blodgett films
Surface Science, 2002
Langmuir-Blodgett (LB) films of 5,10,15,20-tetrakis-[4-(1-Heptyloxy)phenyl]porphyrin (H 2 THOPP) have been deposited onto quartz and silver substrates. The deposited layers (20 monolayers thick) have been characterised by reflectance anisotropy spectroscopy, a technique that recently has been successfully used to study the optical anisotropies of organic materials. For porphyrin LB films, the experimental results show that a large anisotropy signal is measured in the Soret band energy region. We show that this anisotropy is clearly connected to the derivative of the dielectric function of the layer. We propose an interpretation of the derivative-like line shape as due to either strain or similar perturbations on the otherwise symmetric top-layer molecules induced by the lower-lying tilted molecules aligned along a direction related to the growth.
A Reflectance Anisotropy Spectroscopy Investigation of Porphyrin Langmuir-Blodgett Films
physica status solidi (a), 2001
Until now, with a few exceptions, Reflectance Anisotropy Spectroscopy (RAS) has been applied mostly to inorganic semiconductors and metals, proving to be a powerful tool to investigate their optical properties, via anisotropies introduced by the surface in otherwise isotropic bulk materials. Recently, we have shown that RAS can be successfully used to study the optical anisotropies of organic Langmuir-Blodgett films. In this paper we have extended our study to thick Langmuir-Blodgett films of porphyrins, a class of molecules widely studied for the importance of their physico-chemical properties.
Optical response of monolayer films of a metal-free sulfonamido-porphyrin
Thin Solid Films, 1998
The influence of gaseous chlorine on monolayers of metal-free meso-tetra(4-amino sulfonyl)phenylporphyrin is discussed. These films exhibit characteristic intense optical absorption bands in the region 400-700 nm. Upon exposure to chlorine vapour in the concentration range 0.1-10 ppm, the Soret and Q-bands decrease in intensity and new bands at ∼455 nm and ∼660 nm appear. The rate of response has been measured as a function of deposition surface pressure and gas concentration. Repeated dosing experiments have shown that reversible and reproducible sensing action occurs provided that a threshold concentration is not exceeded. The recovery of the original optical spectrum can be accelerated with moderate heat treatment (60°C for 10 min). Ageing experiments have shown that the basic response of the monolayer is not effected over a time period of at least 5 months.
Journal of Colloid and Interface Science, 2005
Polyamic acid (PAA) containing free-base porphyrin and zinc(II) porphyrin chromophores was synthesized by copolymerization of diphenylether-type tetracarboxylic dianhydride and diamines. The monolayer of the alkylamine salts of PAA (PAASs) at the air/water interface was deposited on solid substrates by the Langmuir-Blodgett (LB) technique. The PAAS LB films thus obtained were converted to polyimide (PI) LB films by chemical treatment. The fluorescence of porphyrin moieties in the PI LB film was observed, because of the weak electron-accepting properties of the diphenylether unit. Therefore, the photophysically important processes, such as photoinduced electron transfer, excitation energy transfer, and excitation energy migration could be investigated in relation to the layered nanostructures of the ultrathin PI films. The fluorescence spectrum suggested that the aggregation of porphyrin moieties in the PI LB films was effectively prevented by the use of polymeric films. The surface plasmon (SP) measurement showed that the thickness of the monolayers was 0.9-1.0 nm for PAAS films and 0.32-0.40 nm on average for PI LB films. The absorption dichroism of the Soret band of porphyrin indicated that porphyrin moieties in the PAAS and PI LB films are oriented in parallel with the substrate. These results showed that the orientation and the spatial distribution of porphyrin units can be efficiently regulated in the PI LB films in a nanometer dimension.
Control over the Optical and Morphological Properties of UV-Deposited Porphyrin Structures
Chemistry of Materials, 2006
UV irradiation of porphyrin solutions in halogenated solvents leads to various extents of protonation, aggregation, and formation of solid deposits depending on the nature of the specific porphyrin, the light source, the initial solution concentration, the halogenated solvent, and the nature of the supporting substrate. In order to evaluate the role of various factors in controlling the main characteristics of the deposited samples, we investigated the behavior of a series of meso-substituted aryl porphyrins bearing protonable pyridyl peripheral groups [i.e., tetrakis(pyridyl)porphyrin isomers (ortho TpyP(2), meta TpyP , and para TpyP )] or hydroxyl peripheral groups [i.e., tetrakis(4-hydroxyphenyl)porphyrin (THPP) and tetrakis(3,5-dihydroxyphenyl)porphyrin (OHPP) together with some metal derivatives (Zn(II)TpyP and Mn(III)TpyP ]. In pulsed irradiation experiments, the size and morphology of the resulting porphyrin particles strongly depend on the single pulse duration, the total irradiation time and the delay between pulses. Size and dispersity of the samples can be controlled through a careful choice of the light source (pulsed deuterium lamp or He-Cd laser). The photogenerated HX halogen acid plays an important role in modulating the optical properties (UV/vis absorption and fluorescence emission) of the porphyrin particles. The protonation and ion-pairing (for TpyPs porphyrins) and the hydrogen-bonding properties (for THPP and OHPP) together with electrostatics contribute to stabilize the growing particles at the interface between the solution and the supporting surface. The possibility of controlling the size of the deposited particles in the nanoscopic range (100-200 nm) has been demonstrated.
Journal of Colloid and Interface Science, 2001
We describe in this paper the preparation of a self-assembled monolayer (SAM) film of 5,10,15,20-tetra(N-10-carboxydecylpyridinium-4-yl) porphyrin on an Au substrate by chemisorption of the multi-COOH terminal groups onto the metal surface. A film thickness of 19Å is deduced from surface plasmon resonance measurement. A significant red shift of the Soret band in the UV-vis spectrum of the porphyrin SAM compared with that of the porphyrin in solution suggests the formation of "head-to-tail" type aggregates in the SAM. The absence of the N-H in-plane mode and the presence of the out-of-plane mode in the infrared reflectionabsorption spectrum (IRAS) and the surface-enhanced infrared absorption spectrum (SEIRAS) further suggest that the porphyrin macrocycle is oriented flatly with respect to the surface of the metal substrate. In addition, there may be one of the four acid groups that is unbound to the metal surface, as evidenced by the presence of a very weak band at 1707 cm −1 due to the C= =O stretching mode of the COOH group in the SEIRA spectrum of the porphyrin monolayer on the Au island film. C
J. Mater. Chem., 2006
The preparation and characterization of a self-assembled material showing a high nonlinear response and good photostability to ultrashort laser pulses is presented. The material is built by alternate deposition of tetrakis(4-sulfonatophenyl)porphyrin diacid (H 4 TPPS 22 ) and poly(diallyldimethylammonium chloride) (PDDA) forming electrostatically self-assembled multilayers (ESAMs). UV-visible absorption and emission experiments show that in this matrix H 4 TPPS 22 is present mainly in its J-aggregated form. Furthermore, linear dichroism experiments on a 3 bilayer film show a preferential alignment of the porphyrin aggregate with the J-band transition dipole moment parallel to the film surface. The two photon absorption (TPA) properties of these films are investigated with the Z-scan technique at 806 nm, employing 130 fs pulses. The samples exhibit strong nonlinearities with a very large two-photon absorption coefficient b TPA of 50 cm GW 21 . The origin of this large response is investigated. It has been already demonstrated that aggregation enhances the molecular TPA cross section of H 4 TPPS 22 from 30 to 1000 GM in water solution thanks to cooperative effects. In a 20 bilayer film a further increase by a factor of 1.7 is observed and explained in terms of preferential alignment of J-aggregates in the multilayers.
Molecules
Reflectance anisotropy spectroscopy (RAS) has been largely used to investigate organic compounds: Langmuir–Blodgett and Langmuir–Schaeffer layers, the organic molecular beam epitaxy growth in situ and in real time, thin and ultrathin organic films exposed to volatiles, in ultra-high vacuum (UHV), in controlled atmosphere and even in liquid. In all these cases, porphyrins and porphyrin-related compounds have often been used, taking advantage of the peculiar characteristics of RAS with respect to other techniques. The technical modification of a RAS spectrometer (CD-RAS: circular dichroism RAS) allows us to investigate the circular dichroism of samples instead of the normally studied linear dichroism: CD-RAS measures (in transmission mode) the anisotropy of the optical properties of a sample under right and left circularly polarized light. Although commercial spectrometers exist to measure the circular dichroism of substances, the “open structure” of this new spectrometer and its high...