Synthesis and properties of arsenic(III)-reactive coumarin-appended benzothiazolines: A new approach for inorganic arsenic detection

Article abstract of DOI:10.1021/ic301730z

The EPA has established a maximum contaminant level (MCL) of 10 ppb for arsenic (As) in drinking water requiring sensitive and selective detection methodologies. To tackle this challenge, we have been active in constructing small molecules that react specifically with As³⁺ to furnish a new fluorescent species (termed a chemodosimeter). We report in this contribution, the synthesis and spectroscopy of two small-molecule fluorescent probes that we term ArsenoFluors (or AFs) as As-specific chemodosimeters. The AFs (AF1 and AF2) incorporate a coumarin fluorescent reporter coupled with an As-reactive benzothiazoline functional group. AFs react with As³⁺ to yield the highly fluorescent coumarin-6 dye (C6) resulting in a 20-25-fold fluorescence enhancement at λ_em ～ 500 nm with detection limits of 0.14-0.23 ppb in tetrahydrofuran (THF) at 298 K. The AFs also react with common environmental As³⁺ sources such as sodium arsenite in a THF/CHES (N-cyclohexyl-2-aminoethanesulfonic acid) (1:1, pH 9, 298 K) mixture resulting in a modest fluorescence turn-ON (1.5- to 3-fold) due to the quenched nature of coumarin-6 derivatives in high polarity solvents. Bulk analysis of the reaction of the AFs with As³⁺ revealed that the C6 derivatives and the Schiff-base disulfide of the AFs (SB1 and SB2) are the ultimate end-products of this chemistry with the formation of C6 being the principle photoproduct responsible for the As³⁺-specific turn-ON. It appears that a likely species that is traversed in the reaction path is an As-hydride-ligand complex that is a putative intermediate in the proposed reaction path.

Full text of DOI:10.1021/ic301730z

Supporting Information for:
Synthesis and Properties of Arsenic(III)-Reactive Coumarin-
Appended Benzothiazolines: A New Approach for Inorganic
Arsenic Detection
Vivian C. Ezeh and Todd C. Harrop*
Department of Chemistry, The University of Georgia, 1001 Cedar Street, Athens, Georgia
30602, United States
E-mail: tharrop@uga.edu
S1

Figure S1. (Top) UV-vis spectra of AF1 (black trace, 5.0 M), C6-CF3 (red trace, 5.7 M), and
SB1 (blue trace, 4.6 M). (Bottom) Fluorescence spectra of AF1 (black trace, 0.45 M, ex = 385
nm), C6-CF3 (red trace, 0.14 M, ex = 464 nm), and SB1 (blue trace, 0.57 M, ex = 469 nm).
Slit width = 5 nm. All spectra recorded in THF at 298 K. UV-vis and fluorescence spectra for
AF1 and C6-CF3 were originally reported in Inorg. Chem. 2012, 51, 1213 – we report AF1 and
all potential oxidized analog spectra on one figure for comparison purposes.
S2

Figure S2. UV-vis spectra of AF1 (black trace, 5.0 M), C6-CF3 (red trace, 4.8 M), and SB1
(blue trace, 4.1 M). Spectra recorded in THF/CHES (1:1, pH 9) at 298 K.
S3

Figure S3. Fluorescence response of 0.98 μM AF1 to 1 mol-equiv of AsI3 in THF/CHES (1:1,
pH 9) at 298 K. Initial trace (bold black) and final trace (bold red). ex = 471 nm, em = 496 nm,
slit width = 5 nm. Spectra obtained 45 min after mixing. Arrow displays direction of change
upon addition of As3+.
S4

Figure S4. UV-vis spectral monitor of AF1 (17.1 M) before (black trace) and after (colored
traces) the addition of 1 mol-equiv of NaAsO2 in THF/CHES (1:1, pH 9) at 298 K. Each trace
recorded at 30 min intervals, total time = 16 h. No significant change after ~ 5 h. Arrows depict
direction of change.
S5

Figure S5. Fluorescence response of 0.98 μM AF1 to 1 mol-equiv of NaAsO2 in THF/CHES
(1:1, pH 9) at 298 K. Scan intervals are 30 min for 3 h total time. Initial trace (bold black) and
final trace (bold red). ex = 471 nm, em = 496 nm, slit width = 5 nm. Arrow displays direction of
change upon addition of As3+.
S6

CH3 from Et2N
H
dHe
Hb, Hc
Hf
CH2 from Et2N
Hi
Hg
Ha
1
Figure S6. H NMR spectrum of SB1 in CDCl3 at 298 K. Top: Expansion of relevant regions.
Bottom: full spectrum. Peaks at 7.26 and 1.50 ppm are from residual CHCl3 and H2O in the NMR
solvent, respectively.
S7

13
Figure S7. C NMR spectrum of SB1 in CDCl3 at 298 K. Peak at 77.16 is from CDCl3. Inset
shows expansion of aromatic region.
S8

He
CH3 from Et2N
CH2 from Et2N
Hf
Hb,Hc,Hd
Hg
Hj
Ha
Hi
1
Figure S8. H NMR spectrum of SB2 in CDCl3 at 298 K. Top: Expansion of relevant regions.
Bottom: full spectrum. Peaks at 1.85 and 3.76 ppm are from THF workup. Peaks at 7.26 and 1.50
ppm are from residual CHCl3 and H2O in the NMR solvent.
S9

13
Figure S9. C NMR spectrum of SB2 in CDCl3 at 298 K. Peak at 77.16 ppm is from CDCl3.
Peaks at 68.13 and 25.77 ppm are residual THF from workup.
S10

1
Figure S10. H NMR spectra (aromatic region) of the reaction of AF2 and AsI3 (blue), AF2
(black), C6 (red) and SB2 (green trace) in CDCl3. See Figure S11 for full spectrum.
S11

1
Figure S11. H NMR spectra (full spectrum) of the reaction of AF2 and AsI3 (blue), AF2
(black), C6 (red) and SB2 (green trace) in CDCl3. Peaks from THF (1.85, 3.76 ppm), Et2O (1.21,
3.48 ppm), and Et3N•HI (1.49, 3.18 ppm) are also present from workup. Peaks at 7.26 and 1.50
ppm are from residual CHCl3 and H2O in the NMR solvent. See Figure S10 for aromatic region
expansion.
S12

1
Figure S12. H NMR spectra (full spectrum) of the reaction of AF1 and AsI3 (blue), AF1
(black), C6-CF3 (red) and SB1 (green trace) in CDCl3. Peaks from THF (1.85, 3.76 ppm), Et2O
(1.21, 3.48 ppm), and Et3N•HI (1.49, 3.18 ppm) are also present from workup. Peak at 7.26 ppm
is from residual CHCl3 in the NMR solvent. See Figure 5 (main text) for aromatic region
expansion.
S13

[As(dppe)]+
[SB2 + H]+
[AF2 + H]+
Figure S13. Full scan positive mode ESI mass spectrum of the reaction of AF2 and AsI3 under
sensing conditions after the addition of dppe (see the text). Selected peaks are labeled.
[I]-
Figure S14. Full scan negative mode ESI mass spectrum of the reaction of AF2 and AsI3 under
sensing conditions after the addition of dppe (see the text). Selected peak is labeled.
S14

Scheme S1. Synthetic scheme for the Schiff-base disulfide forms of the AFs, namely SB1 and
SB2. Step 1 involves oxidation of the aminothiophenol to its corresponding amine-disulfide form
(compound I). Condensation of I with 2 mol-equiv of the aldehyde furnishes the SB product
(step 2).
S15