Sterically hindered phthalocyanines: solid-phase interactions with carbon monoxide in matrix-entrapped thin functional films M. Alarjah Æ L. Paniwnyk Æ I. R. Peterson Æ J. P. Lorimer Æ D. J. Walton Received: 26 March 2009 / Accepted: 10 August 2009 / Published online: 26 August 2009 Ó Springer Science+Business Media, LLC 2009 Abstract A thin film comprising highly sterically hin- dered iron phthalocyanine derivatives incorporated into sol–gel networks shows effective sensitivity to carbon monoxide gas, when monitored by UV–visible absorption spectroscopy at 670 nm. The absorbance of a sol–gel thin film incorporating octaphenyl-di-tert-butylphthalocyanine iron(II) was particularly sensitive to carbon monoxide gas and showed a change in absorption after exposure to 1000 ppm CO gas for 1 min, while repetitive exposures produced only a limited loss of response. The optical response of the thin film gradually decreases with increasing temperature, and the binding energy for this sensing phenomenon was calculated to be 0.401 eV. The spectrum recovers at room temperature some 5 min after exposure to the gas. Introduction Sensing for gases in the ambient environment is an important research objective, and any new sensor system should be chemically selective, reversible, fast, highly sensitive, durable and insensitive to contamination or ‘poisoning’. It ought to be simple in operation, of a small size, simple to fabricate, relatively insensitive to tempera- ture, with low noise, and low manufacturing costs [1]. An important gas for sensing, because of its prevalence in the ambient environment, especially in the home, is the toxic gas carbon monoxide. There are a number of different electronic, electrochemical and redox sensor systems employed for this target, each offers different benefits for effective commercialisation, and suffers from different drawbacks [2–5]. Optical sensors are highly demanded as they can be used for chemical and biological molecules [6]. This report concerns an optical sensing methodology for CO sensing. The toxicity of this gas is due to its interaction with the iron porphyrin unit of haemoglobin, which is accompanied by a change in visible absorption spectros- copy. However, it has yet to prove practical to use this phenomenon for a commercial sensor, due to poor solu- bility and intractability of the phthalocyanine, and in par- ticular the prevalence of aggregation effects, in which the disc-like complexes stack together so as to limit access of target gas to molecules at the centre of the assembly, which diminishes the spectroscopic changes that are observed. Iron(II) phthalocyanine sensitivity to gases such as CO and NO 2 has been improved by solubilised phthalocyanine with N-donor ligand and incorporated into nanoparticulate metal oxide matrix, optical sensor could be develop as N-donor ligand permit ligand exchange with gas molecule easily [7]. Our aim in this work is to minimise these negative solubility and aggregation phenomena, and to enhance sensitivity of the gas interaction process by modifying the electron-density on the macrocyclic ring [8]. To this end we earlier reported the synthesis of highly sterically hin- dered iron(II) phthalocyanine derivatives, and their response to carbon monoxide in solution phase studies [8]. However, for a practical commercial sensor solid-phase operation is desirable, and we now report the entrapment of substituted iron phthalocyanine derivatives in thin solid film matrices and the consequences monitored by absorp- tion spectroscopy upon their exposure to carbon monoxide gas. The advantage of such a thin film system operating by optical change is that in principle a straightforward and M. Alarjah (&)  L. Paniwnyk  I. R. Peterson  J. P. Lorimer  D. J. Walton Faculty of Health and Life Science, Coventry University, Priory Street, Coventry CV1 5FB, UK e-mail: aa4780@coventry.ac.uk 123 J Mater Sci (2009) 44:5737–5742 DOI 10.1007/s10853-009-3803-2