Green light triggered [2+2] cycloaddition of halochromic styrylquinoxaline—controlling photoreactivity by pH

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In our laboratory in Brisbane at QUT (www.macorarc.org), we work as a scientific collective in a highly project centred approach, putting together teams in such a way to achieve the best scientific outcomes. In our current study, our PhD student Kubra worked together with chemists spanning different career stages and expertise, which was key to a successful study. Vinh – a senior research fellow and biomaterials scientist in our team – always had a strong interest in applying photochemical concepts to address questions in cell biology, while Christopher (an ARC Laureate Fellow) and Hendrik (an ARC DECRA Fellow), are macromolecular photochemists with a physical chemistry and organic chemistry background, respectively. Christopher has always been fascinated by using light to probe chemical reaction mechanisms and the precision that monochromatic irradiation offers in terms of almost surgical precision like reaction control for soft matter materials design, while Hendrik’s interests are rooted in translating naturally observed concepts into synthetic macromolecular systems. Kubra’s strong dedication to the project and the team approach made the study possible, as she drives the [2+2] photocycloaddition red-shifting forward. Together we are joined by a passion for devising light-responsive materials concepts.

Our team has a long-standing interest in photochemical processes that function without a catalyst. We design such reactions for use in soft matter materials science, where they find applications ranging from functional photoresists for 3D laser lithography to dental material design and cell culture scaffolds. Over the past decade, we have focused on establishing an in-depth understanding of catalyst-free photochemical reaction systems with a technique that we developed in our laboratories: the so-called action plot (Fig. 1). In such an action plot, the conversion of a photochemical reaction is established at monochromatic wavelengths, with the exact same number of photons deposited into the system at each wavelength. We have found over the years that many reaction systems are strongly red-shifted in their reactivity compared to their absorption spectrum – an effect which is still not fully understood, but holds true for many photochemically allowed cycloadditions as well as Norrish-type I cleavage reactions. Thus, to achieve optimum conversions, having access to action plots is indispensable.

Fig. 1 Action plot and absorption spectrum of styrylquinoxaline moiety showing the red-shift in its reactivity compared to its absorption spectrum.

The current paper is driven by a similar rationale: Our quest to push [2+2] cycloadditions to ever higher wavelength is realized by a new substitution pattern of the double bond of a styrylquinoxaline moiety and we demonstrate that we can active the [2+2] cycloaddition at up to 550 nm (Fig. 1). Such long wavelengths are ideal for resist curing, and in biological applications, where green light is far more benign on cells compared to UV light. Thus, it was natural to assess our red-shifted system in the context of cell scaffolds. Additionally, the green light photoreactivity can be turned ‘on’ and ‘off’ through reversible protonation of the chromophore (Fig. 2). Such an inherent pH-switch of photoreactivity opens exciting avenues, especially in biologically system, where pH values are precisely controlled.

 

Fig. 2 The schematic representation of the general concept. Green-light induced [2+2] cycloaddition and controlling reactivity with a pH-switch.

Christopher Barner-Kowollik

Professor in Materials Science, Queensland University of Technology

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