Fast ON/OFF Emission Photoswitching of Turn-On Mode Fluorescent Diarylethenes

Fluorescent chromophores having reversible photoswitching ability are applicable for various types of analysis and imaging. 1,2-Bis(2-ethyl-6-phenyl-1-benzothiophene-1,1-dioxide-3-yl)perfluorocyclopentene (1) is known to undergo turn-on mode fluorescent photoswitching. One serious drawback of the chromophore is its very low photocycloreversion (switching off) quantum yield (<10–3). To improve the yield, derivatives bearing various alkyl substituents at 2- and 2′-positions were prepared. It was found that introduction of bulky substituents, such as isobutyl or neopentyl groups, is effective in increasing the yield. The yield increases by almost 3 orders of magnitude from 5.9 × 10–4 to 4.6 × 10–1 by replacing ethyl substituents with neopentyl groups. Theoretical analysis revealed that the energy barrier in the photocycloreversion reaction is inferred from the calculated potential energy difference between 2Ao and 2Ac local minima in the excited-state (S1) potential energy surface, and the decrease in the energy difference by introducing bulky primary alkyl substituents at 2- and 2′-position increases the photocycloreversion quantum yield. Although substitution of primary alkyl groups increases the yield, secondary alkyl substituents are ineffective. 1,2-Bis(2-neopentyl-6-phenyl-1-benzothiophene-1,1-dioxide-3-yl)perfluorocyclopentene (6) undergoes very fast ON/OFF emission photoswitching cycles over 2000 times even in the presence of air without any marked fatigue, and the colored closed-ring isomer is stable at 80 °C for 50 h.