Tim H. Richardson

Detection of Volatile Organic Compounds Using Porphyrin Derivatives

Seven different porphyrin compounds have been investigated as colorimetric gas sensors for a wide range of volatile organic compounds. The porphyrins examined were the free base and Mg, Sn, Zn, Au, Co, and Mn derivatives of 5,10,15,20-tetrakis[3,4-bis(2-ethylhexyloxy)phenyl]-21H,23H-porphine. Chloroform solutions of these materials were prepared and changes in their absorption spectra induced by exposure to various organic compounds measured. The porphyrins that showed strong responses in solution were selected, and Langmuir-Blodgett films were prepared and exposed to the corresponding analytes. This was done to determine whether they are useful materials for solid state thin film colorimetric vapor sensors. Porphyrins that readily coordinate extra ligands are shown to be suitable materials for colorimetric volatile organic compound detectors. However, porphyrins that already have bound axial ligands when synthesized only show a sensor response to those analytes that can substitute these axial ligands. The Co porphyrin displays a considerably larger response than the other porphyrins investigated which is attributed to a switch between Co(II) and Co(III) resulting in a large spectral change.

Supramolecular order in new polymer electrolytes

The structure and conductivity of crystalline, semicrystalline and amorphous complexes of poly(ethylene oxide) with sodium and lithium salts is briefly reviewed and the rationale for synthesizing low-dimensional, organized polyether complexes is discussed. New polymer electrolytes derived from: formula chim. where R is a long alkyl side-chain (principally -C 16 H 33 and n = 5) forming both two-component complexes with lithium salts and three-component systems incorporating RX, where R is a long alkyl chain-C 16 H 33 or-C 12 H 25 and X is -C 2 H 5 , -OH, and several ethers. Conductivities increase steeply with alkyl melting (33-43°C) and at a transition occurring at 60-70°C. Conductivities 10 -3 -10 -2 S cm -1 over the range 60-100°C are observed in the most highly organized systems. Langmuir-Blodgett films of the complexes (20 molecular layers) are readily deposited. They show temperature-independence of conductivity over the range 10°C to about 90°C.

The optical gas-sensing properties of an asymmetrically substituted porphyrin

In this paper we have investigated the NO2 gas-sensing properties of LB film assemblies of 5,15-bis(4-aminophenyl)-10,20-bis[3,4-bis(2-ethylhexyloxy)phenyl]-21H,23H-porphine (CAH4). The optical absorbance spectrum of these films is dramatically affected when exposed to low concentrations of NO2 gas. LB films of CAH4 were prepared by using ultra-fast deposition and characterized by imaging ellipsometry. The high deposition rates employed (500 mm min−1) led to an inhomogeneous structure with high porosity. The LB film exposed to 4.6 ppm NO2 showed a sensitivity of 60% relative absorbance change at 439 nm. The response was found to be faster than that measured in similar systems. The fast response can be explained in terms of the molecular structure of the porphyrin as well as the enhanced surface area of the porous film. The optical response of the CAH4 film gradually decreases as its temperature is increased, a result of a shift in the adsorption–desorption equilibrium towards desorption. An activation energy of 0.48 eV is obtained. Full recovery of the original spectrum after exposure to NO2 is obtained and can be dramatically accelerated with gentle heating (353 K). The concentration dependence of the optical response over the range 0.46–4.6 ppm NO2 obeyed a Langmuir adsorption model. Ageing experiments have shown that the basic response of the CAH4 assemblies is not affected over a time period of at least 1 year.

Gas sensing properties of porphyrin assemblies prepared using ultra-fast LB deposition

The UV–vis absorbance spectrum of LB film assemblies of 5,10,15,20-tetrakis(3,4-bis[2-ethylhexyloxyphenyl])-21H,23H-porphine is very sensitive to low concentrations of NO2. LB films prepared at very high deposition rates (∼1000 mm min−1) yield t50 response and recovery times of 25 and 33 s, respectively, and show a sensitivity of 60% relative absorbance change (at 430 nm) for 4.4 ppm NO2. Atomic force microscopy shows that the morphology of these films is characterised by isolated micron-size domains, which are themselves composed of grains of several nm in diameter. This unconventional LB structure leads to a useful sensing material as a result of the molecular functionality of the porphyrin coupled to the enhanced surface area of the porous film assembly. The optical response of the EHO gradually decreases as its temperature is increased, resulting from the shift in the adsorption-desorption equilibrium towards desorption. An activation energy for adsorption of 0.68 eV is obtained. The spectrum recovers fully after exposure to NO2 and the rate of recovery can be accelerated dramatically with gentle heating (∼350 K) for a few seconds. The concentration dependence of the optical response over the range 0.8–4.4 ppm follows a Langmuir model.

Surface plasmon resonance properties and gas response in porphyrin Langmuir–Blodgett films

Abstract Surface plasmon resonance (SPR) properties were measured for porphyrin Langmuir–Blodgett (LB) films and the response to NO 2 gas was investigated. The porphyrin molecule was 5,10,15,20-tetrakis(3,4-bis[2-ethylhexyloxyphenyl])-21H,23H-porphine (EHO). The EHO porphyrin LB films were deposited onto the cover glass with evaporated Ag thin films by vertical dipping method with a very fast deposition rate of 1000 mm min −1 . The SPR measurements were carried out at wavelengths of 488 and 632.8 nm. The SPR properties measured at 488 nm were considered to be related to the dispersion properties due to the optical absorption band of the EHO LB films. The thickness and the complex dielectric constants of the EHO LB films evaluated from the SPR properties measured at 632.8 nm were thought to be related to the island structure of the EHO LB films. From the NO 2 gas response measurements, the SPR properties at 488 nm were found to be more sensitive to NO 2 gas than those at 632.8 nm. The response rate to NO 2 gas and the recovery properties were also examined.

An investigation of the optical properties of tetraphenylporphyrin derivatives in Langmuir and Langmuir-Blodgett films

Abstract The optical properties of two tetraphenylporphyrin derivatives based on tetra(4-aminosulphonyl) phenylporphyrin have been examined. These porphyrins can exist in both free-base and dication states with their Soret absorption band occurring near 420 and 440 nm respectively. Floating monolayers can show the occurence of a new state characterized by two additional bands near 490 and 700 nm. The formation of this unknown state is dependent on both the subphase pH and the area occupied per monomer unit at the air-water interface. The effect of the subphase pH on the production of this state has been investigated systematically for both porphyrins and comparisons between the two habe been made. The optical absorption of Langmuir-Blodgett films for one of the porphyrins has also been examined. This has shown that the unknown state can be transferred on to glass substrates, though it is found not to be stable if stored in air.

Chlorine sensing properties of porphyrin thin films

Abstract This work describes the influence of gaseous chlorine on thin porphyrin films. LB films of metal and free base porphyrins based on meso-tetra (4-amino sulfonyl) phenylporphyrin [1]exhibit intense optical absorption bands in the region 400–700 nm. The change in the molecular environment of the porphyrin which arises as a result of exposure to the sensor gas is reflected in substantial changes to the optical absorption spectrum. This effect is reversible in the absence of the sensor gas. In-situ optical measurements have been performed to quantify the response of the films in a stream of gas of specified concentration and their subsequent recovery in air. Factors affecting the response such as choice of porphyrin, orientation of porphyrin molecules within the LB film, concentration of sensor gas and temperature have been studied.

Selective Detection of Volatile Organic Compounds by Spectral Imaging of Porphyrin Derivatives Bound to TiO2 Porous Films

In this work, the carboxylic acid derivatives of a free-base porphyrin, 5,10,15,20-tetrakis(4-carboxyphenyl)-21H,23H-porphyrin, and 10 of its metal derivatives (TCPPs) have been used for optical gas sensing. For this purpose, microstructured columnar TiO(2) thin films prepared by GAPVD (glancing angle physical vapor deposition) have been used as host materials for the porphyrins as they are non-dispersive and porous, allowing their use for UV-visible spectroscopy and gas sensing. The chemical binding between the dye molecules and the TiO(2) has been studied through infrared spectroscopy, and the obtained spectral changes have been found to be compatible with chelating and/or bidentate binding modes of the carboxylate groups on the TiO(2) surface. When hosted in the film, the UV-visible spectra of the porphyrins featured a blue shift and broadening of the Soret band with respect to the solution, which has been attributed to the formation of π-π aggregates between porphyrin molecules. The composite porphyrin/TiO(2) films obtained from each of the 11 porphyrins have been exposed to 12 different volatile organic compounds (VOCs), and their respective gas-sensitive properties have been analyzed as a function of the spectral changes in their Soret band region in the presence of the analytes. The set of composite films has shown high selectivity to the analyzed volatile compounds. For each analyte, an innovative way of showing the different responses has been developed. By means of this procedure, an imagelike recognition pattern has been obtained, which allows an easy identification of every compound. The kinetics of the exposure to several analytes showed a fast, reversible and reproducible response, with response times of a few seconds, which has been attributed to both the sensitivity of the porphyrins and the high porosity of the TiO(2) films. Also, increasing concentrations of the analytes resulted in an increase in the magnitude of the response, indicating that the sensor behavior is also concentration-dependent.

In situ visible spectroscopy of a gadolinium bisphthalocyanine LB film exposed to chlorine gas

Abstract We report the preparation of Langmuir-Blodgett (LB) films of a gadolinium phthalocyanine, doubledecker, molecular sandwich-type complex [Gd(Pc) 2 ] and its ultraviolet-visible spectroscopic characterisation. We have studied the changes in the optical absorbance spectra that occur upon exposure to very low concentrations (~10 ppm) of chlorine gas. The main absorption band in the visible region occurs at ~690 nm. Upon exposure to chlorine gas, the intensity of this band decreases with the simultaneous introduction of a new peak at ~734 nm, an isosbestic point occurring at 721 nm. These spectral changes are indicative of oxidation of the complex by chlorine to form the [Gd(Pc) 2 ] + species. The kinetics of this response suggest that the adsorption of chlorine by the LB film is a complex process, possibly involving up to three independent mechanisms. The LB film recovers almost fully (> 95%) upon switching off the chlorine supply.

Taking advantage of optical and electrical properties of organic molecules for gas sensing applications

Abstract The discipline of molecular electronics has grown rapidly over the last 10 years and is driven by the promise of the enhanced applied physical properties of functionalised organic materials compared to their inorganic partners. The subject can be divided generally into two broad themes, namely active molecular-scale electronics (or photonics), in which the control or generation of charge (or photons) at the nanoscale is attempted, and passive supra-molecular electronics (or photonics), in which the specific functionality of the molecules is modified by some interaction or process. In this paper, an example of the latter approach to molecular electronics will be given and this will describe the gas sensing properties of a tetra-substituted porphyrin molecule. The optical absorbance spectrum of LB film assemblies of 5,10,15,20-tetrakis(3,4-bis[2-ethylhexyloxy]phenyl)-21H,23H-porphine (EHO) is highly sensitive to low concentrations of NO 2 . LB films prepared at much faster than conventional deposition rates (∼1000 mm min −1 ) yield t 50 response and recovery times of 25 and 33 s, respectively, and show a sensitivity of 60% relative absorbance change (at 430 nm) for 4.4 ppm NO 2 . The morphology of these films is revealed using atomic force microscopy to contain isolated micron-size domains which are composed of grains of several nm in diameter. This unconventional structure leads to a useful sensing material as a result of the molecular functionality of the porphyrin coupled to the enhanced surface area of the porous film assembly. The EHO film shows a gradually diminishing optical response as its temperature is increased, resulting from the shift in the adsorption–desorption equilibrium towards desorption. The spectrum recovers fully after exposure to NO 2 . The rate of recovery is slow at room temperature but can be accelerated dramatically with gentle heating (∼350 K) for a few seconds. The kinetics of the gas sensing process have been modelled and found to fit Elovichian surface adsorption for an initial fast surface adsorption process. This is followed by a much slower diffusive process in which the NO 2 molecules diffuse through the bulk of the assembly. The concentration dependence of the optical response over the range 0.8–4.4 ppm follows a Langmuir model.