Dual fluorescent N-Aryl-2,3-naphthalimides: Applications in ratiometric DNA detection and white organic light-emitting devices

Article abstract of DOI:10.1021/ol101760m

A ten element matrix of 5- and 6-substituted-(2,3)-naphthalimides was prepared for the appropriate placement of substituents necessary to promote dual fluorescence (DF). As prescribed by our balanced seesaw photophysical model this matrix yielded nine new DF dyes out of a possible ten compounds. From this set of nine DF dyes, 4-fluoronaphthalic amide (37) was selected as a probe for ratiometric detection of DNA and demonstration of panchromatic emission.

Full text of DOI:10.1021/ol101760m

ORGANIC
LETTERS
2010
Vol. 12, No. 21
4796-4799
Dual Fluorescent N-Aryl-2,3-
naphthalimides: Applications in
Ratiometric DNA Detection and White
Organic Light-Emitting Devices
Premchendar Nandhikonda and Michael D. Heagy*
Department of Chemistry, New Mexico Institute of Mining and Technology, Socorro,
New Mexico 87801, United States
mheagy@nmt.edu
Received August 5, 2010
ABSTRACT
A ten element matrix of 5- and 6-substituted-(2,3)-naphthalimides was prepared for the appropriate placement of substituents necessary to
promote dual fluorescence (DF). As prescribed by our balanced seesaw photophysical model this matrix yielded nine new DF dyes out of a
possible ten compounds. From this set of nine DF dyes, 4-fluoronaphthalic amide (37) was selected as a probe for ratiometric detection of
DNA and demonstration of panchromatic emission.
Compared to the intensiometric response of common single-
band fluorescent probes, the two-color response of dual
fluorescent (DF) dyes1 imparts key advantages. The ratio-
metric response of certain DF dyes is independent of probe
concentration, reduces errors due to photobleaching, uneven
loading of probe, and/or fluctuation in excitation intensity.2
Such features are keenly sought in cellular studies where
the local concentrations of the dyes cannot be controlled.
The inherent nature of excited states makes the prediction
of fluorescence features a difficult endeavor and in most
cases, optimal fluorescence properties are discovered by
screening large synthetic libraries.3
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40-50.
10.1021/ol101760m  2010 American Chemical Society
Published on Web 09/30/2010

Among the various fluorophore core structures, the naph-
thalimide class of dyes hold particular interest because of
their exceptional brightness and potential for two-color
emission.4 Considering the dramatically different photophys-
ics between the C10H8 isomers azulene and naphthalene,5 a
key hypothesis we sought to examine with our so-called
“seesaw model” invokes the use of symmetry as a predictive
tool in generating new DF dyes.5b Specifically, we sought
to compare fluorescence properties of N-aryl-2,3-naphthalic
anhydride (2,3-NI) systems relative to isomeric 1,8-NI (which
the model was originally based upon). The 5-membered
imide ring of 2,3-NI imposes a different steric environment
and rotational dynamics than the 6-membered imide ring of
1,8-NI. Therefore, to test our excited state model on the basis
of the point group (C2V) common to both isomers of NI, we
designed a ten element marix of novel 5- and 6-
substituted-(2,3)-naphthalimides.
To generate 2,3-naphthalic anhydrides, the inherent an-
hydride functionality requires an electron-deficient dienophile
such as maleic anhydride or dialkyl maleate and therefore
restricts electron-deficient dienes from participating.6 On the
basis of the general synthetic approach for these systems with
maleic anhydride as the de facto dienophile, syntheses
involving only electron-rich or neutral dienes have been
reported, thereby severely limiting their synthetic scope.7,8
To further complicate matters, our “seesaw” model requires
electron-withdrawing groups at the naphthalic ring to effect
DF.
For the appropriate placement of substituents at the 5- and
6-positions of 2,3-naphthalic anhydride, we report a synthetic
strategy where the pivotal step involves the use of 1-hy-
droxyphthalans. The general route to these various hemiac-
etals is outlined below in Scheme 1. From a practical
perspective, our efforts focused on developing a general route
to substituted 2,3-naphthalic anhydrides from common
substituted phthalic acids. In this procedure, analogues of
phthalic dicarboxylic acid were dehydrated in acetic anhy-
dride to give the corresponding anhydrides. Reduction of
these phthalic anhydrides with zinc and acetic acid yielded
two isomers, R and ꢀ (80:20, respectively).
Scheme 1. Synthesis of Electron Deficient 1-Hydroxyphthalans
Supporting Information). In the case of 3-nitrophthalic
anhydride (compound 2 in the Supporting Information), the
carbonyl group of 3-nitrophthalic anhydride was reduced
selectively with NaBH4 at -23 °C, to yield 7-nitrophthalide
as the major product. In the case of 3-substituted phthalic
anhydride, reduction occurred at the ꢀ-carbonyl position and
the selective reduction could be due to the formation of
chelates.9 These chelates were formed by complexation of
an appropriate substituent (i.e., NO2, F, N) with zinc. The
resulting phthalides were reduced with DIBAL-H in dichlo-
romethane to give 1-hydroxyphthalans.
In Scheme 2, 1-hydroxyphthalans were used for these
unprecedented Diels-Alder reactions. The key step is
Diels-Alder addition of in situ generated isobenzofurans by
heating of 1-hydroxyphthalans in AcOH with maleic anhy-
dride for 12 h. Vacuum evaporation of the solvent afforded
a mixture of exo and endo isomers in essentially quantitative
yield (Table 2 in the Supporting Information). At refluxing
temperature, the exo product was highly favored and at low
temperature exo and endo products were formed in equal
amounts.
Scheme 2. Synthesis of 5- and 6-Substituted-2,3-NI
Reduction occurred preferentially at the ꢀ-carbonyl func-
tion to give the corresponding phthalides as the major product
in 3- and 4-substituted phthalic anhydrides, which was
1
confirmed by H NMR and TLC analysis (Table 1 in the
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Ba¨urle, P. Angew. Chem., Int. Ed. Engl. 2001, 40, 4680–4683.
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On the basis of these observations, the exo isomer is
assigned as the thermodynamic product and the endo product
as the kinetically favored product. Recognizing this isomer
assignment contradicts Alder’s endo rule, the exo isomer was
(6) (a) Saito, I. Pure Appl. Chem. 1992, 64, 1305. (b) Kirsheubaum,
M. R.; Chen, S. F.; Behrens, C. H.; Papp, L. M.; Stafford, M. M.; Sun,
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1
confirmed by H-1H NMR coupling. Similar results were
2002, 9, 500–501
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1999, 40, 1191–1194
.
.
(9) Kayser, M. M.; Morand, P. Can. J. Chem. 1980, 58, 2484–2490.
Org. Lett., Vol. 12, No. 21, 2010
4797

reported by Miller et al.10 The methine protons at the sites
of cycloaddition allowed for the differentiation between exo
and endo products. Specifically, the aliphatic methyl protons
in the exo isomer displayed two singlets rather than two
multiplets.
a
Table 1. Summary of Fluorescence Wavelengths and ΦF
excitation emission molar absorptivity
entry
(nm)
(nm)
(L·mol-1·cm-1
)
10-3ΦF
30
31
32
33
34
35
36
37
38
39
372
373
361
365
360
372
375
361
360
370
435, 527
442, 472
415, 495
411, 501
391, 416
369, 432
444, 533
407, 546
390, 435
425
2 × 105
1 × 104
425, 18
5.1, 6
11.6, 4.3
27.3, 10
20, 21
132, 54
7.0, 182
7.9, 4.3
20.2, 10
20.1
4.1 × 104
1.1 × 104
1.4 × 104
1 × 104
Scheme 3
.
Synthetic Scheme for 5 × 2 Matrix Elements of the
Ten Compounds and SW (Table 1) Emission
1 × 104
2.5 × 104
1.2 × 104
2.5 × 104
a Fluorescence quantum yields relative to quinine sulfate in water. Errors
are on the order of (5% for quantum yields. Value for this data set were
obtained from dicholormethane.
and 33 are shown in Figure 1, whereas the remaining
compounds are shown in the Supporting Information, Figure
5.
DNA Interaction Studies. Out of ten compounds, we
selected a fluoro derivative (37) with a methoxy group for
human DNA detection studies. Because of the interesting
behavior that the 5-fluoro naphthalic amide derivative
displayed such as well-separated two-color emission it was
selected as a ratiometric probe for DNA detection. To the
best of our knowlegdge, a dye featuring dual fluorescence
has not been reported for ratiometric DNA detection. Figure
2 shows the “OFF-ON” switching behavior at 428 and 506
nm on addition of DNA (0-250 µM) to a solution of 37
provides a two-color emission conducive to ratiometric
analysis. To the best of our knowledge, a molecular probe
with dual fluorescence has not been reported for ratiometric
DNA detection. This probe has an isoemissive point at 475
nm. The relative fluorescence intensities (I428/I506) at the
maxima of 37 are plotted against the concentration of DNA
from 1 to 250 µM on gradual addition (Figure 1 in the
Supporting Information). The ratiometric response in emis-
In Scheme 3, 25, 26, 27, 28, and 29 anhydrides were
coupled with electron-releasing aminoarenes that have either
4-methoxy or 4-amino groups. The synthesis of each element
of the matrix involved a 1:1 ratio of the 25, 26, 27, 28, and
29 naphthalimide with the respective aminoarenes in a
minimum solvent of pyridine.
Among the pyridine derivatives, compound 30 displayed
two-color emission in a number of solvents whereas 35
displayed DF only in water. Both nitro derivatives, 31 and
36, displayed dual emission in different solvents. For
trifluoromethyl derivatives 34 and 39, compound 39 dis-
played only a single wavelength emission. Compounds 37
Figure 1
.
DF spectra of 37 and 33 (1 × 10-5) in the solvents indicated. Fluorescence spectra are normalized to the maximum of each data
set. Excitation wavelengths are shown in Table 1.
4798
Org. Lett., Vol. 12, No. 21, 2010

emission wavelength is more intense and the perceived light
is blue (Figure 3). Finally, for excitation at wavelengths
greater than 360 nm, such as 380 nm, the long wavelength
emission is more intense, and the emitting light is green.
Figure 3
.
Photographs of 37 at 1 × 10-6 M in 0.1% DMSO (5
mM of phosphate buffer) (a) excitation at 340 nm, (b) excitation
at 360 nm (c), and excitation at 380 nm.
Figure 2
.
A 1 × 10-6 M 37 sample with 10-250 µM DNA.
Ratiometric response showing the emission band at 425 nm
increasing upon addition of DNA; the emission band at 525 nm
decreasing upon addition of DNA.
On the basis of the present dearth of substituted 2,3-
naphthalic anhydrides, this synthetic strategy offers several
of these interesting chromophores in a relatively economical
procedure. The high-yielding general route can be used to
synthesize electron-poor or electron-rich substituted 2,3-
naphthalimides from commercially available substituted
phthalic acids. As proof of principle, this predictive model
yielded nine (DF) dyes from a small synthetic matrix of ten
compounds. Our hypothesis that 2,3-NI dyes would exhibit
similar photophysics to 1,8-NI on the basis of symmetry
proved to be a correct one with the added advantage that
the 5-ring carboximide/6-ring N-aryl juncture permits broader
fluorescence emission than the more restrictive 6-ring car-
boximide/6-ring N-aryl juncture of 1,8 NI. In addition these
dyes exceeded our goals prescribed earlier such as water
solubility, reduced chromophore size, and molecular weight,
as well as possessing a synthetic path that allows synthetic
diversity of substituents within the 2,3-NI framework.
Immediate applications of these new dyes have been shown
in detection of DNA, as well as new white-light fluorophores.
sion followed a linear trend. As a control experiment,
compound 37 was also tested with helical polypeptide
R-polybenzyl[γ]glutamate (PBLG). In this case 37 gave no
ratiometric response (supp. Info. Figure 3). So, potentially
this compound could be used as a ratiometric probe for DNA
detection. Here, we used the following single stranded DNA
sequence: TCC TGT GGA GAA GTC TGC CGT TAC TG.
New Lighting Sources. Organic light emitting diodes
(OLEDs) have gained attention as one of the most appealing
solutions for low energy consumption in solid-state lighting
because they display bright colors, reproducibility, and
stability.11 To date, very few small molecule systems have
been reported to display white-light emission.2,12 4′-Amino-
N-phenyl-(5-fluoro)-2,3-naphthalic imide (37) shows white
light emission where two emission bands have similar
fluorescence intensity (excitation at 360 nm). Upon excitation
at wavelengths less than 360 nm, such as 340 nm, the short
Acknowledgment. This work was supported by a grant
from the National Institute of Health. P.N. thanks Dr.
Virginia Chang and Dr. Peng Zhang (NMT) for many helpful
discussions
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Supporting Information Available: Synthetic and titra-
tion experimental details. This material is available free of
charge via the Internet at http://pubs.acs.org.
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