Fluorescence response of a 4-trifluoroacetylaminophthalimide to iodide ions upon 254 nm irradiation in MeCN

Article abstract of DOI:10.1021/ol801184x

(Chemical Equation Presented) The title trifluoroacetylaminophthalimide derivative produced a violet fluorescence (λ^FL_max 392 nm) in MeCN, and it displayed a green emission (λ^FL _max 506 nm) after irradiation at 254 nm in the presence of iodide ions. The corresponding amidate ion of the trifluoroacetamide was identified as the green fluorescence emitter. The deprotonation reaction may be caused by proton-abstracting solvated electrons generated by a photochemical charge-transfer-to-solvent process from I^- to MeCN.

Full text of DOI:10.1021/ol801184x

ORGANIC
LETTERS
2008
Vol. 10, No. 14
3125-3128
Fluorescence Response of a
4-Trifluoroacetylaminophthalimide to
Iodide Ions upon 254 nm Irradiation in
MeCN
Hideki Okamoto,* Hiroyuki Konishi, Mami Kohno, and Kyosuke Satake
DiVision of Chemistry and Biochemistry, The Graduate School of Natural Science and
Technology, Okayama UniVersity, Okayama 700-8530, Japan
hokamoto@cc.okayama-u.ac.jp
Received May 23, 2008
ABSTRACT
The title trifluoroacetylaminophthalimide derivative produced a violet fluorescence (λFLmax 392 nm) in MeCN, and it displayed a green emission
(λFL
506 nm) after irradiation at 254 nm in the presence of iodide ions. The corresponding amidate ion of the trifluoroacetamide was
identified as the green fluorescence emitter. The deprotonation reaction may be caused by proton-abstracting solvated electrons generated
max
by a photochemical charge-transfer-to-solvent process from I- to MeCN.
Fluorometric and colorimetric probes for anions have at-
tracted much attention in the field of chemosensors.1 Among
such anionic species, halide ions are important analytes from
biological and environmental aspects. Namely, iodide is of
particular significance in biological activities such as the
thyroid gland function.2 Thus, it would be of significance to
construct an iodide-specific probe. Although a number of
halide-ion sensing probes have been proposed,3 much less
effort has been addressed toward developing iodide-selective
sensors,4 namely, fluorescent ones.5 Jang5a and Kang,5b
respectively, reported a tripodal receptor and an imidazolium-
based sensor which displayed fluorescence quenching by the
addition of I-. Carbazole-containing polymers were reported
to show colorimetric and fluorimetric responses to iodide.6
Also, photoinduced electron-transfer fluorescence quenching
by iodide7 has been used for discriminating halide ions.8
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1646. (b) Kimura, E.; Koike, T. Chem. Commun. 1998, 149, 5–1500. (c)
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Gale, P. A. Coord. Chem. ReV. 2003, 240, 1, and articles therein. (e) Gale,
P. A. Coord. Chem. ReV. 2006, 250, 2917, and articles therein. (f) Davis,
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Wen, Y.-S.; Sun, S.-S. J. Org. Chem. 2008, 73, 900–911. (d) Winstanley,
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Chem. Soc. 2004, 126, 16507–16514.
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2006, 39, 8303–8310.
10.1021/ol801184x CCC: $40.75
Published on Web 06/25/2008
2008 American Chemical Society

During our fluorescence studies of aminonaphthalimides,
we have found that the trifluoroacetylation of the amino
group markedly modifies the fluorescence properties of the
naphthalimide derivatives by proton dissociation equilibrium
of the amide part.9 Therefore, it is also expected that the
trifluoroacetylation of amino-substituted aromatic carbonyl
compounds, e.g., aminophthalimide, which displays an
intramolecular charge-transfer (ICT) fluorescence, enhances
the acidity of the amide proton and, as a result, effectively
modulates the ICT fluorescence character through proton
dissociation (Scheme 1).9,10
Scheme 1. Modification of Electronic Properties of 1 and 2
Figure 1. (a) Electronic absorption and (b) fluorescence (λex 320
nm) spectra of 1 (3.50 × 10-5 M) in the presence of halide ions
(1.75 × 10-4 M, as tetrabutylammonium) in MeCN.
In the present study, we have investigated the fluorescence
behavior of the title trifluoroacetamide 1 in the presence of
halide ions in MeCN, namely, under photoirradiation condi-
tions at 254 nm, and now describe its fluorescence response
to halide ions which was caused by the enhanced acidity of
the amide part. The trifluoroacetamide 1 displayed an
unexpected response to I- to afford the amidate ion 1a due
to the 254 nm irradiation.
nm for 20 s, are shown. In the presence of Cl-, Br-, or F-,
no appreciable change in the spectra was observed by the
irradiation (cf. Figure 1). In contrast, for the photoreaction
mixture of a 1-I- system, it is remarkable that new
absorption peaks appeared at around 285 and 365 nm (Figure
Figure 1 shows the absorption and fluorescence spectra
of the trifluoroacetamide 1 in the presence of 5 equiv of
halide ions. Cl-, Br-, and I- displayed no appreciable change
in the absorption profile. These halide ions only slightly
affected the fluorescence properties of the trifluoroacetamide
1 (Figure 1b). These facts indicate that the halide ions, Cl-,
Br-, and I-, did not significantly interact with the trifluo-
roacetamide 1 in both the ground and excited states. In
contrast, upon the addition of F-, the trifluoroacetamide 1
displayed remarkably red-shifted new absorption (λABSmax 381
nm) and fluorescence emission (λFLmax 506 nm) bands. Since
it has been reported that the basic F- ion can effectively
abstract the acidic proton3e of a 4-benzoylamidophthalimide
derivative,10b the red-shifted absorption and fluorescence
bands of the trifluoroacetamide 1 were assigned to the
amidate ion 1a. Formation of the amidate ion 1a was further
confirmed by addition of DBU to 1 (Figure S2, Supporting
Information). The fluorescence quantum yield ΦF was
determined to be 0.20 for 1 and 0.41 for 1a, thus the amidate
ion 1a fluoresced more efficiently than the amide form 1.
In Figure 2, the absorption and fluorescence spectra of
the trifluoroacetamide 1, observed after irradiation at 254
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(8) Urbano, E.; Offenbacher, H.; Wolfbeis, O. S. Anal. Chem. 1984,
56, 427–429.
Figure 2. (a) Electronic absorption and (b) fluorescence (λex 320
nm) spectra of 1 (3.50 × 10-5 M) in the presence of halide ions
(1.75 × 10-4 M, as tetrabutylammonium) observed after irradiation
at 254 nm for 20 s in MeCN.
(9) Okamoto, H.; Satake, K.; Kimura, M. ARKIVOC 2007, 8, 112–123.
(10) (a) Liu, B.; Tian, H. J. Mater. Chem 2005, 15, 2681–2686. (b)
Sarkar, M.; Yellampalli, R.; Bhattacharya, B.; Kanaparthi, R.; Samanta, A.
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.
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Org. Lett., Vol. 10, No. 14, 2008

2a). In the fluorescence spectrum of the mixture, the emission
ion 1a, and I3- (Figure S5). On the basis of the experimen-
tally determined molar extinction coefficients of 1, 1a, and
I3-, the photoreaction mixture displaying the absorption curve
for I- in Figure 2a was determind to be an ca. 0.8:2.2:1
band of the trifluoroacetamide 1 (λFLmax 392 nm) decreased,
and a new long-wavelength fluorescence band (λFL 506
max
nm) appeared (Figure 2b). Thus, only for the 1-I- system,
the absorption and fluorescence properties were significantly
modified by the 254 nm photoirradiation. Upon irradiation,
the color of the photoreaction mixture turned from colorless
to yellow, and the fluorescence changed from violet to green.
To determine the green-fluorescence emitter in the pho-
toreaction mixture of the 1-I- system, the excitation
spectrum of the 506 nm emission band was observed. As
shown in Figure 3, the excitation profile of the 506 nm
-
mixture of 1:1a:I3 (Figure S5). Since the absorption
-
spectrum of a mixture of trifluoroacetamide 1 and I3 was
the simple sum of these components (data not shown), the
possibility that the absorption curve for I- in Figure 2a was
caused by an interaction between 1 and I3- can be excluded.
Therefore, we concluded that the products for the photolysis
of the 1-I- system were the amidate ion 1a and I3 .
-
To reveal the significance of the trifluoroacetamide moiety,
the photolysis of the acetyl analogue 2 was investigated in
the presence of halide ions. In contrast to the case of the
trifluoroacetamide 1, the irradiation at 254 nm displayed a
minimal effect on the absorption and fluorescence spectra
of the 2-halide mixtures, even in the presence of I- (Figures
S6 and S7). Thus, no evidence was obtained about the
formation of the corresponding amidate ion 2a. These results
are consistent with that reported for a related 4-benzoyla-
midophthalimide: I- did not affect its absorption and
fluorescence behavior.10b The acidity of the amide part is
thus considered to be important for the formation of the
amidate ion 1a in the photoreaction. The pKa value of the
trifluoroacetamide 1 in MeCN was determined to be 20 (
0.1 by titration with triethylamine (pKa of conjugate acid
18.82).13 (See Figure S8 for the titration results.) For the
acetamide 2, the addition of DBU (pKa of conjugate acid
24.34)13 up to 100 equiv did not result in deprotonation of
the acetamide moiety (data not shown). The pKa value of
acetamide 2 is thus several orders of magnitude greater than
that of the trifluoroacetamide 1. Therefore, the strongly
electron-withdrawing trifluoroacetyl group played a crucial
role in the enhancement of the acidity, thus the formation
of the amidate ion 1a.
Figure 3
.
Excitation spectrum of the emission band (λFL
506
max
nm) of the photoreaction mixture of trifluoroacetamide 1 and I-
after irradiation at 254 nm (cf. Figure 2b) and absorption spectrum
of the amidate ion 1a (normalized at 367 nm).
emission band of the photolysate was essentially identical
to that of the absorption of the amidate ion 1a. Moreover,
the fluorescence emission band of the photoreaction mixture
of the 1-I- system (λFLmax 506 nm) was identical to that of
the amidate ion 1a (Figures 1b, 2b, and S2). Therefore, the
506 nm emitting species in the photoreaction mixture of
the 1-I- system was assigned to the amidate ion 1a. The
formation of the amidate ion 1a was also investigated by 1H
NMR spectroscopy, and the results are shown in Figure S3.
The absorption spectrum of the photoreaction mixture of
the 1-I- system was different than that of the amidate ion
1a (Figure 2a), thus an additional photoproduct(s) must be
considered to reveal the absorption properties of the photo-
reaction mixture. I- ions did not contribute to the absorption
spectrum since they displayed no absorption band in the
wavelength region >300 nm (Figure S4). It has been reported
that I- underwent photooxidation in MeCN.11 Among the
The time course of the photoreaction of the 1-I- mixture
was investigated to understand the photoreaction profile
(Figure 4). Upon 254 nm irradiation of the 1-I-mixture,
the absorption bands around 285 and 365 nm appeared within
10 s. The product yields for the photolysis are displayed in
-
the inset of Figure 4. The ratio of the photoproducts 1a:I3
was close to 2:1 during the irradiation.
possible oxidized species of I-, the triiodide, I3 , has been
-
known to show absorption bands in the wavelength region
(λABS
292, 363 nm, Figure S4)12 similar to that of the
max
photoreaction mixture. It was found that the absorption
spectrum observed for the 1-I- photoreaction mixture
(Figure 2a, curve for I-) was well simulated by the sum of
the absorption spectra of the trifluoroacetamide 1, amidate
Figure 4. Absorption spectral change for a mixture of trifluoro-
acetamide 1 (3.50 × 10-5 M) and I- (1.75 × 10-4 M) during
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irradiation at 254 nm. The inset shows the time course of the
103, 2006–2015.
-
photolysis: 1 (O), 1a (9), I3 (b).
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Org. Lett., Vol. 10, No. 14, 2008
3127

A plausible mechanism for the formation of the amidate
ion 1a by irradiation of the 1-I- system is shown in Scheme
2. The photoexcitation of I- in MeCN causes the formation
In summary, the fluorescence properties of the trifluoro-
acetamide 1 can be modified by F- and the combination of
I- and the 254 nm photolysis; F- deprotonated the amide
proton of 1a resulting in the formation of the amidate ion
1a in the ground state, while the combination of I- and the
254 nm irradiation caused the formation of the amidate ion
1a. Cl- and Br- did not affect the optical properties of the
trifluoroacetamide 1. The trifluoroacetamide 1, thus served
as a potential halide ion recognition probe in MeCN.
Moreover, it is worth noting that upon the 254 nm photolysis
of the 1-I- system (for 30 s) the intensity of the 506 nm
fluorescence band increased depending on the concentration
of I- (Figure 5). The trifluoroacetamide 1 might be applicable
for a quantitative probe for I-.
-
of the MeCN-solvated electrons (MeCN)n and iodine
radicals by a charge-transfer-to-solvent (CTTS) process.14
-
It has been reported that the solvated electrons (MeCN)n
can extract acidic protons.15 Thus, the acidic amide proton
of the trifluoroacetamide 1 was considered to be effectively
extracted by the (MeCN)n- to afford the amidate ion 1a. The
iodine radical counterpart was trapped by I- to form I2
·-
which finally afforded I3- via a disproportionation process.16
Scheme 2
.
Plausible Mechanism for the Photolysis
-
The ratio of 1a:I3 is thus expected to be 2:1 according to
this mechanism. For the photolysis of the 1-I- system, the
ratio of the photoproducts 1a:I3 was experimentally ob-
-
Figure 5. Dependence of fluorescence intensity at 506 nm upon
concentration of I- after 254 nm irradiation (30 s) of the 1-I-
system ([1] 3.50 × 10-5 M, λex 380 nm).
served to be ∼2:1 (Figure 4, inset).
The formation of amidate ion 1a was also observed for
the 254 nm photolysis of the 1-I- system in THF for which
photoinduced CTTS has been reported.17 Details of the
solvent effects on the photolysis are underway.
Acknowledgment. Part of the present study was supported
by a Grant-in-Aide for Scientific Research (C), KAKENHI,
from the Japan Society for the Promotion of Science (No.
19550141). The authors are grateful to the SC-NMR Labora-
tory of Okayama University for the NMR spectral measure-
ments.
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Supporting Information Available: Experimental pro-
cedures, electronic spectra of 1 and 2, and 1H NMR spectra
of 1 (photolysate and titration results). This material is
available free of charge via the Internet at http://pubs.acs.org.
(16) Cheng, M.; Bakac, A. J. Am. Chem. Soc. 2008, 130, 5600–5605.
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