Synthesis of Fluorogenic Arylureas and Amides and Their Interaction with Amines: A Competition between Turn-on Fluorescence and Organic Radicals on the Way to a Smart Label for Fish Freshness

Abstract

We describe the synthesis of fluorogenic arylureas and amides and their interaction with primary or secondary amines under air and light in organic-aqueous mixtures to give rise to a new class of persistent organic radicals, described on the basis of their electron paramagnetic resonance (EPR), as well as UV-vis, fluorescence, NMR, and quantum mechanics calculations, and their prospective use as multi-signal reporters in a smart label for fish freshness.

Keywords: arylureas; organic radicals; smart label; turn-on fluorescence; volatile amines.

Scheme 1

Synthesis of the chemical probes.

Figure 1

Chemical structures of the studied chemical probes.

Figure 2

Electron paramagnetic resonance (EPR) spectral evolution with time for an equimolecular solution of 1 and pyrrolidine. Experimental details of the data collection: modulation amplitude 0.1 mT, time constant 40.96 ms, conversion time 327.68 ms, gain 6.32 × 10⁴, power 20 mW, and microwave frequencies 9.7410 (5 min), 9.7410 (11 min), 9.7410 (17 min), 9.7410 (24 min), 9.7460 (30 min), 9.7410 (36 min), 9.7410 (43 min), 9.7410 (16 h), and 9.7760 (40 h) GHz.

Figure 3

Experimental (blue line) and simulated (dashed red line) EPR spectra of radical [1]^• in dimethylsulfoxide (DMSO)/water. Experimental details: modulation amplitude 0.02 mT, time constant 40.96 ms, conversion time 327.68 ms, gain 6.32 × 10⁴, power 20 mW, and microwave frequency 9.7768 GHz.

Figure 4

EPR spectra for the interaction of an equimolecular solution (2 × 10⁻³ M in DMSO/water 4:1 v/v) of 1 and pyrrolidine or n-butylamine. The measurements started at 20 min (pyrrolidine) and 6 min (n-butylamine) after the addition of the amine. Experimental details of the data collection: modulation amplitude 0.01 mT, time constant 40.96 ms, conversion time 327.68 ms, gain 6.32 × 10⁴, and power 20 mW in all cases. Microwave frequencies were 9.7718 (pyrrolidine, three scans) and 9.7713 GHz (n-butylamine, four scans).

Figure 5

A comparison of the EPR spectra of 1–7 with pyrrolidine (2 × 10⁻³ M equimolecular solutions in DMSO/water 4:1 v/v). The measurements were carried out 11 min after the addition of the amine. Experimental details of the data collection: modulation amplitude 0.02 mT, time constant 40.96 ms, conversion time 327.68 ms, gain 6.32 × 10⁴, power 20 mW, and three scans in all cases. Microwave frequencies were 9.7732 (1), 9.7726 (2), 9.7724 (3), 9.7727 (4), 9.7716 (5), 9.7713 (6), and 9.7715 GHz (7).

Figure 6

Evolution of the ¹H NMR spectrum of equimolecular solutions (2.5 × 10⁻³ M) of 1 + pyrrolidine in DMSO-d₆.

Figure 7

Evolution of the ¹³C NMR spectrum of equimolecular solutions (2.5 × 10⁻³ M) of 1 + pyrrolidine in DMSO-d₆.

Figure 8

Singly occupied molecular orbital (SOMO), SOMO + 1, and SOMO − 1 for compound [1]^•.

Figure 9

(Top) A summary of the formation and evolution of [1]^•. (Bottom left) Fluorescence measured (black line) and fitting curve (red line) for the fluorescence intensity decay process of a 10:1 pyrrolidine/1 solution, concentration C₁= 5.0 × 10⁻⁵ M in 3:1:1 (v/v) DMSO/water/acetone. (Bottom right) EPR spectral evolution with time for a solution of 1 and pyrrolidine in DMSO/water/acetone under the conditions reported in Figure 2. Insets: Images of the solutions under white light (left) and under UV light (366 nm) (right).

Figure 10

Quantum yield of different combinations between 1 and amines in DMSO; comparison between 1 without amine, pyrrolidine, histamine, putrescine, and cadaverine.

Figure 11

Evolution of the absorbance (left) and fluorescence (right) signals detected with solutions of 1 after the decay of fish samples over several hours.

Figure 12

From left to right: (1) reference 1 (1.68 × 10⁻⁵ M in DMSO); (2) reference + air (bubbled); (3) reference + fish sample (bubbled) (3 days at 25 °C); (4) reference + fish sample (bubbled) (1 day at 25 °C); (5) reference + histamine (bubbled); (6) reference + putrescine (bubbled); (7) reference + cadaverine (bubbled); (8) reference + pyrrolidine (bubbled); (9) reference + all amines (bubbled). (Top) Under visible light; (Bottom) under UV light (366 nm).

Figure 13

Response of a solution of 1 (5 × 10⁻⁵ M) in 0.25 mL of DMSO with 2 mg of gellant after the addition of amines in the gas phase under UV light at 366 nm.

Figure 14

Fluorescence increase in presence of biogenic amines in the gas phase generated from rotten fish.