In situ photoproduction of dichlorodibenzo-p-dioxin from non-ionic triclosan isolated in solid argon

Abstract

The infrared spectrum of triclosan [or 5-chloro-2-(2,4-dichlorophenoxy)phenol] isolated in a low temperature (∼15 K) argon matrix has been recorded and assigned with help of DFT claculations undertaken with the B3LYP functional and the 6-311++G(d,p) basis set. The obtained spectrum doubtlessly exhibits the characteristic vibrational signature of the neutral (phenol) form of the compound, which exists in two different conformations (forms I and II) in the matrix, in a I: II population ratio of ca. 0.75. Upon broadband UV irradiation of the matrix-isolated triclosan with unfiltered light provided by a xenon arc lamp, formation of 2,8-dichlorodibenzo-p-dioxin (2,8-DCDD) was observed, together with HCl. The reaction seems to occur through initial photoproduction of the triclosan phenol radical derivative, and involve in the initial step participation of dissociative (πσ^∗) excited states along the OH stretching coordinate, as observed previously for other phenol derivatives. The photochemically detached hydrogen atom derived from triclosan may then react with the closest located chlorine atom in the triclosan molecule to yield HCl and a biradical species, which can subsequently undergo a ring-closure reaction by intramolecular recombination, leading to the observed 2,8-DCDD.

Introduction

Triclosan [5-chloro-2-(2,4-dichlorophenoxy)phenol; Scheme 1] is a broad-spectrum antibacterial and antifungal agent found in a wide variety of consumer goods like detergents, dish-washing liquids, soaps, deodorants, cosmetics, lotions, anti-microbial creams and toothpastes [1], [2], [3], [4], [5]. It is also used as additive in various plastics and textiles [6]. Triclosan is also registered by the U.S. Environmental Protection Agency (EPA) for use as a pesticide [7]. Because of its widespread use, triclosan has been found in waste- and surface waters and streams [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], sediments [17], [18], [19], [20], fish [21], [22], [23], and even in human milk [24], [25].

It has been reported that triclosan can be photochemically converted to toxic 2,8-dichlorodibenzo-p-dioxin (2,8-DCDD; Scheme 2) in the environment [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], and although the potential health effects of triclosan on the general population are not well understood, there is a growing concern about the generalized use of this compound [39], [40], [41], [42], [43].

The first studies on the photochemistry of triclosan, performed for the compound in water solution or as a solid film, concluded that it can be photochemically converted into a series of photoproducts [27], [28], [29], [30], [31], [32]. The formation of at least five types of photoproducts has been postulated: monochloro- and dichlorohydroxydiphenyl ethers (of formula C₁₂H₉ClO₂ and C₁₂H₈Cl₂O₂, respectively) formed on reductive dechlorination involving replacement of chlorine atoms by hydrogen atoms; isomeric monochloro- and dichlorophenols, including a particularly toxic one (2,4-dichlorophenol), which can be generated by the photoinduced hydrolysis of either triclosan or the previously formed chlorohydroxydiphenyl ethers by loss of one of the aromatic rings; and 2,8-DCDD, which results from reductive photoinduced cyclization of triclosan. Also dichlorohydroxydibenzofuran was tentatively suggested as a possible photoproduct of triclosan. At high concentrations, triclosan was found to photopolymerize [37].

One of the general conclusions resulting from those studies was that 2,8-DCDD could only be obtained from the anionic (phenolate) form of triclosan, while the non-ionic molecule appeared to be unable to be transformed into the dioxin [27], [28], [29], [30], [31], [32]. However, later on, Cela and co-workers [33], [34], [35] demonstrated that non-ionic triclosan could also be photochemically converted into 2,8-DCDD, and that the triclosan→dioxin conversion occurs irrespective of pH and thus does not require basic media (although photodegradation at acidic pH seemed to be slower). Indeed, these authors observed the formation of 2,8-DCDD during photodegradation of triclosan using the photo solid phase micro extraction method (photo-SPME), where the analytes were irradiated after being extracted from water and retained in the polydimethylsiloxane coating of the SPME fibers. Under those conditions, the compound could be expected to be present in its molecular form. The same authors were also able to observe photoproduction of 2,8-DCDD in aqueous photodegradation experiments at pH 3 [34]. These results have been subsequently confirmed by other authors [36], [37], [38], [44].

In the present study we investigated the photochemistry of triclosan isolated in a low temperature solid argon matrix, in particular the possibility of photoproduction of 2,8-DCDD under these experimental conditions. The main advantage of this technique over the previously reported experiments is that once the reactant molecules were isolated in the inert rigid matrix media no diffusion is possible and, thus, only unimolecular reactions can be observed and putative products resulting from cross reactions involving recombination of species produced from different initial molecules of the reactant cannot be formed. Moreover, in a noble gas cryogenic matrix triclosan must exist in its neutral (phenol) form, whose vibrational signature can be precisely identified by infrared spectroscopy.

In this paper, the infrared spectrum of matrix-isolated monomeric triclosan is also reported and assigned. The harmonic vibrational frequencies (and infrared intensities) of the experimentally relevant conformers of the compound were evaluated at the DFT(B3LYP)/6-311++ G(d,p) level of theory and used to help assignment of the experimentally observed bands. While the room temperature Raman and infrared spectra of solid triclosan have already been briefly described [45], [46], no vibrational experimental data for the isolated mononomeric form of triclosan have been reported hitherto.