A Photoactive Antimicrobial Triphenylmethane Derivative

Abstract

Owing to the ever growing interest for the human health against infectious bacteria, various substances that display antimicrobial activity have been developed. For instance, inorganic silver nanoparticles have been actively investigated as antibacterial agents although the mechanism for the activity is not clearly understood. Recently, photochemically triggerable organic antimicrobial substances have received significant attention since these materials can be formulated with fabrics to produce self-decontaminating clothes. In addition, the irradiation-induced antimicrobial activity is attractive because of the readily available light source (eg. Sun). Among various candidates, benzophenone derivatives have been most actively investigated as photoactive antimicrobial agents owing to the some salient features including facile synthesis and generation of reactive radicals via photosensitization. Thus, benzoquinone with diverse substituents have been prepared and applied to the antimicrobial screening. It has been well known that photoinduced oxidation of certain electron donating group-substituted triphenylmethane derivatives produces brilliantly colored ionic species. Owing to this property, triphenylmethane derivatives have been investigated as precursors of colorants. A mechanistic pathway to the photochemical genartion of an ionic iminium product from a triphenyl methane drivative, 4,4',4''-tris(dimethylaminophenyl) methane (TPM, 1) is presented in Scheme 1. The first step of this process involves excited state single electron transfer (SET) from the triphenylmethane donor to presumably molecular oxygen. The radical cation intermediate 2, produced in this fashion, subsequently undergoes hydrogen atom loss to yield the iminium ion 3. Since the iminium ion 3 is fully conjugated, formation of the ionic moiety can be readily monitored by visible absorption spectroscopy and most often with naked eyes. The generation of reactive ionic/radical species from the photochemical oxidation of TPM is very intriguing since these reactive species might be able to retard the growth of microbes or destroy them. If TPM displays the antimicrobial activity, a new class of photoactive antimicrobial agent can be developed. In order to test the feasibility of TPM as a photoactive antimicrobial agent, a thin (ca. 1 m) polystyrene (PS) film containing TPM was prepared by spin-coating of a chloroform solution containing PS (MW: 280,000, 5 wt%) and TPM (1.5 wt%, based on the weight of PS polymer powder) on a glass substrate. Irradiation of UV light (254 nm, 1 mW/cm) to the film resulted in the gradual increase of the absorption in the visible region, confirming the successful generation of iminium product 3 in the polymer film (Fig. 1). In addition, the colorless transparent TMPcontaining film became pale purple after UV irradiation. We next investigated antimicrobial properties of the TPM containing PS films against Staphylococcus aureus