Full Length ArticleFormation of poly(Azure A)-C60 photoactive layer as a novel approach in the heterogeneous photogeneration of singlet oxygen
Graphical abstract
Introduction
Singlet oxygen 1O2, i.e. oxygen in the form without unpaired electrons has been under high research interest since 1963 [1], [2]. It has been shown, that this form of oxygen has stronger oxidation properties comparing to triplet oxygen state, which results in its higher reactivity and electrophilicity, but also in its lower stability [3]. Singlet oxygen can be formed in the photoinduction process, where the appropriate photoactive molecule is excited from ground state, S0, to form lowest excited singlet state S1, by light illumination generally in one-photon transition. In the next step, the photosensitizer’s triplet state with longer lifetime, T1, is formed in the intersystem crossing process. The photosensitizer being in the triplet state can react either via Type I mechanism, i.e. hydrogen-atom abstraction or electron-transfer with substrates, producing free radicals that in the reaction with oxygen give ROS like superoxide radical anion, or via Type II mechanism, i.e. an energy transfer occurring in the collision of the excited photosensitizer with 3O2 molecules [1]. Many groups of compounds able to generate singlet oxygen are known: dyes and aromatic hydrocarbons, porphirines and tetrapyrroles, transition metals complexes, semiconductors oxides and carbon-based nanostructures, i.e. fullerenes, nanotubes and graphene [3], [4], [5]. The photosensitizing abilities of those carbon allotropes have been studied mainly in the bulk, though it has been demonstrated that fullerenes and carbon nanotubes, when deposited from the solution with polystyrene used to increase the adhesion with the solid support are able to generate singlet oxygen molecule upon illumination [6], [7], [8]. The yield of singlet oxygen photogeneration by C60 molecule is very high, but its practical use is limited, since it absorbs mainly in UV region. This can be overcome by an introduction of organic moiety absorbing in the lower energy visible region, either in the form of polymeric matrix or covalently attached organic chain [9], [10], [11]. When conducting polymers are considered, fullerenes, graphene and carbon nanotubes can be non-covalently incorporated into polymer matrix by electro-co-deposition from the solution of monomer and carbon nanostructures, forming the layer of nanocomposite on the electrode’s surface. Such approach has been already reported for polypyrrole or polythiophenes [12], [13], [14], [15], [16].
The lifetime of singlet oxygen in the diluted gas phase is relatively long, but due to the molecular interactions it is shortened, for example to about 4 μs in water. Thus, 1O2 has to be generated in situ in the reaction mixture with photosensitizer present in the solution or on the solid support. Since the heterogeneous photocatalysis possesses several advantages over homogenous approach, like ease of operation or easier product separation and purification steps, high attention is paid nowadays on developing methods of photoactive molecules immobilization, so that new solid materials capable of singlet oxygen photogeneration are formed. It is important, that in many cases only thin photoactive layer deposited on the solid support is effective enough to ensure high yield of 1O2 generation in Photodynamic Therapy (PDT), fine chemicals’ synthesis or wastewater treatment [3], [17], [18], [19], [20]. Our group has recently shown that organic thin films of phenothiazine can be immobilized on the solid surface by electrochemical polymerization or electrochemical reduction of the diazonium salts. Such surfaces were able to generate the active singlet oxygen in the reaction mixture, leading to the oxidation of 1,3-diphenylisobenzofuran (DPBF) or phenol [21], [22], [23].
In this work, thin photoactive layer containing two types of photosensitizers, i.e. fullerene C60 and Azure A, was electro-co-deposited on ITO/glass substrate. The main aim of such approach was to combine the high photosensitizing properties of both C60 and AA in order to form heterogeneous photocatalyst that utilizes broader range of light wavelength to produce singlet oxygen molecule. The presence of fullerene and Azure A molecules in the deposited layer is confirmed by means of electrochemical and spectroscopic methods. The singlet oxygen photogeneration by poly(AA)_C60 film was investigated with the TPCPD – specific 1O2 – quencher and in the process of α–terpinene oxidation.
Section snippets
Materials
Azure A (AA) (purity >90%) and fullerene C60 (purity 99.99%) were purchased from Sigma Aldrich and Across Organics, respectively. Tetrabutylammonium tetrafluoroborate (TBABF4), with or without camphorsulfonic acid (both of purity 99%, Sigma Aldrich) in dichloromethane (HPLC grade, Sigma Aldrich) was used as an electrolyte for the electrochemical deposition and characterization of the photoactive layer. 2,3,4,5-tetraphenylcyclopentadienone (TPCPD) (Acros Organics) in dichloromethane was used as
Electrochemical deposition of PAA_C60 layer on ITO/glass substrate
The electrochemical polymerization of phenothiazine derivatives, resulting in the formation of the polymeric layer on the solid surface, have been already widely discussed in the literature. The amine-derivatives of phenothiazine, like Azure A (AA), are commonly electropolymerized from an aqueous solution and it has been shown that, similarly to the mechanism of aniline electropolymerization, the acidity of the solution has strong effect on the forming layer [29], [30]. In this work, poly(Azure
Conclusions
In the presented work, two photosensitizers – Azure A and C60, deposited onto ITO/glass substrates were applied as source of singlet oxygen in the photooxidation of α-terpinene. The proposed straightforward strategy of the electrochemical co-deposition of organic monomer and C60 on the ITO/glass surface results in the stable photoactive layer, which structure was confirmed with electrochemical and spectroscopic measurements. Obtained within presented procedures PAA_C60 layer can be activated by
Acknowledgements
This work was supported by the National Science Center, Poland (grand number: 2016/21/D/ST5/01641). Authors acknowledge ESPEFUM laboratory (at CSE) for access to XPS experimental setup. Authors are grateful to the networking action funded from the European Union’s Horizon 2020 research and innovation program under grant agreement No 691684.
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