Fluorescence Redox Switches Based on the Opening and Closing of an Oxazabicyclic Ring

Abstract

A reversible donor-acceptor fluorescence redox switch connected by a rigid but redox-adjustable spacer that can be chemically turned ON and OFF through the ring opening and ring closing of a heterobicyclic moiety is demonstrated. A coumarin-based oxazabicyclic derivative was efficiently synthesized as an example for illustration. While the rigid ring-closed oxazabicycle emits moderate fluorescence in toluene, the sodium borohydride induced ring opening of the heterobicyclic moiety results in a distinct decrease in fluorescence. The resulting nonfluorescence ring-opened form can be reverted to the original fluorescence ring-closed form via DDQ or H₂O₂ oxidation.

Key words

oxazabicycle - fluorescence redox switch - redox-­adjustable spacer

References and Notes

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Procedure for the Preparation of 4 To a solution of p-anisidine (1.0 mmol), 4-(trifluoromethyl)-benzaldehyde (1.0 mmol), isobutyraldehyde (1.0 mmol), and a catalytic amount of Yb(OTf)₃ (0.4 mmol) in DCE (10 mL) was stirred at r.t. for 12 h. 4-Hydroxy-7-dimethylamino-coumarin (1.0 mmol) was added to the mixture, and the resulting solution was refluxed for 3 h. After being cooled to r.t., the reaction was quenched with H₂O, and the product was extracted twice with CH₂Cl₂. The combined organic extracts were dried over MgSO₄, filtered, and concentrated. The resulting crude product was purified by column chromatography (1:9, EtOAc-hexanes) to give a white solid in a 67% yield; mp 287-288 ˚C. ^¹H NMR (300 MHz, CDCl₃): δ = 7.90 (d, J = 8.1 Hz, 2 H, ArH), 7.73 (d, J = 8.1 Hz, 2 H, ArH), 7.59 (d, J = 9.0 Hz, 1 H, ArH), 7.09 (d, J = 3.0 Hz, 1 H, ArH), 6.63 (dd, J = 8.4, 3.0 Hz, 1 H, ArH), 6.58 (dd, J = 8.7, 2.4 Hz, 1 H, ArH), 6.54 (d, J = 8.4 Hz, 1 H, ArH), 6.47 (d, J = 2.4 Hz, 1 H, ArH), 4.91 (s, 1 H, NH), 3.80 (s, 1 H, CH), 3.78 (s, 3 H, OCH₃), 3.03 [s, 6 H, N(CH₃)₂], 0.99 (s, 3 H, CH₃), 0.94 (s, 3 H, CH₃). ^¹³C NMR (75 MHz, CDCl₃): δ = 163.0, 159.9, 154.3, 153.1, 152.9, 142.7, 132.7, 131.0 (q, J _CF = 32.0 Hz), 129.4, 129.2, 126.1, 124.8 (q, J _CF = 3.5 Hz), 123.9 (q, J _CF = 270.0 Hz), 113.8, 113.5, 113.4, 108.6, 104.2, 101.2, 97.9, 95.4, 55.7, 41.8, 40.1, 33.2, 22.9, 22.1. IR (KBr): ν = 3280, 2935, 1685, 1616, 1326, 1125 cm^-¹. HRMS (EI): m/z calcd for C₃₀H₂₇F₃N₂O₄: 536.1923; found: 536.1922 [M⁺].

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Crystallographic data (excluding structure factors) for 4 has been deposited with the Cambridge Crystallographic Data Centre as supplementary publication number CCDC-744666. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, by emailing [e-mail], or by contacting the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: [phone].