Fluorescent photoswitches with improved emission efficiency based on aggregation-induced emission luminogens by eliminating the heavy-atom effect

Abstract

Solid-state small-molecule fluorescent photoswitches are characterised by their unique luminescence properties in solid form, making them highly suitable for practical applications involving surfaces and interfaces. However, a common challenge is that most such photoswitches with thiophene units exhibit subdued fluorescence due to the heavy atom effect of sulfur. To overcome this, we present a molecular design strategy that significantly enhances the solid-state fluorescence efficiency of these photoswitches. Our approach involves the substitution of thiophene units with thiophene dioxide, thereby mitigating the heavy atom effect. We have synthesized four tetraphenylethene-based molecules containing thiophene units to demonstrate the effectiveness of this strategy. Comparative analysis of their photochromic and fluorescence switching performances revealed that those containing thiophene dioxide units outperformed others in terms of photochromic properties and solid-state luminescence efficiency. Theoretical analysis suggests that this enhancement is due to the minimal involvement of the sulfur atom in the S₀ to S₁ transition in the photoswitchable molecule containing a thiophene dioxide unit, making its fluorescence properties largely unaffected by the heavy atom effect of sulfur. Furthermore, we found that the incorporation of 2,5-dimethylthiophene units improved the anti-fatigue performance, probably because the methyl groups hinder the oxidative dehydrogenation side reactions in the closed-ring isomer. The addition of a 2,5-dimethylthiophene dioxide unit not only improved the photochromic and fluorescence emission efficiency, but also significantly improved the anti-fatigue performance. As a result, these photoswitchable molecules exhibit outstanding dual-mode photoswitching capabilities and sustainable recyclability, positioning them as excellent photosensitive materials for photoprinting of digital, textual and patterned content.