Naphthalimide-coumarin: Dependent energy transfer cassette and its response to F?

Article abstract of DOI:10.1080/15421406.2016.1277890

Naphthalimide and coumarin were bonded together with –NHN= being as the linkage. The emission behavior was investigated in various solvents. The energy transfer is independent in weak and medium polarity solvents. Efficient energy transfer occurred with the emission of it in polar solvent. In addition, it is highly emissive in polycarbonate and the energy transfer from coumarin to naphthalimide is more efficient. Another significant feature is its selective F^? response. Upon the F^? addition, the absorption around 444?nm shifted to 486?nm. And the new absorption band decreased and another new absorption band appeared around 574 nm.

Full text of DOI:10.1080/15421406.2016.1277890

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Naphthalimide-coumarin: Dependent energy
transfer cassette and its response to F−
Xiaochuan Li, Saisai Shun, Dandan Shan & Young-A Son
To cite this article: Xiaochuan Li, Saisai Shun, Dandan Shan & Young-A Son (2017)
Naphthalimide-coumarin: Dependent energy transfer cassette and its response to F−, Molecular
Crystals and Liquid Crystals, 644:1, 257-266, DOI: 10.1080/15421406.2016.1277890
To link to this article: http://dx.doi.org/10.1080/15421406.2016.1277890
Published online: 27 Apr 2017.
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Date: 28 April 2017, At: 08:01

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –
http://dx.doi.org/./..
Naphthalimide-coumarin: Dependent energy transfer cassette
and its response to F−
Xiaochuan Lia,b, Saisai Shuna, Dandan Shana, and Young-A Sonc
aCollaborative Innovation Center of Henan Province for Green Manufacturing of Fine Chemicals, Key Laboratory
of Green Chemical Media and Reactions, Ministry of Education, Henan Normal University, Xinxiang, Henan, P. R.
China; bHenan Key Laboratory of Green Chemical Media and Reactions, School of Chemistry and Chemical
Engineering, Henan Normal University, Xinxiang, Henan, P. R. China; cBK, Department of Advanced Organic
Materials Engineering, Chungnam National University, South Korea
ABSTRACT
KEYWORDS
Naphthalimide; coumarin;
energy transfer; highly
emissive; F− response
Naphthalimide and coumarin were bonded together with –NHN=
being as the linkage. The emission behavior was investigated in vari-
ous solvents. The energy transfer is independent in weak and medium
polarity solvents. Efficient energy transfer occurred with the emission
of it in polar solvent. In addition, it is highly emissive in polycarbon-
ate and the energy transfer from coumarin to naphthalimide is more
efficient. Another significant feature is its selective F− response. Upon
the F− addition, the absorption around 444 nm shifted to 486 nm. And
the new absorption band decreased and another new absorption band
appeared around 574 nm.
Introduction
Configuration of energy-transfer (ET) is of an effective approach in DNA sequencing,
bio-image techniques, artificial light-harvesting, ion/molecule sensors, OLEDs, and logic
devices, which has captivated numerous chemists, physicists, and biochemists [1-12]. Elec-
tronic excitation energy transfer (EET) is the ubiquitous mechanism for energy transfer in
multichromophoric systems [13-17]. Generally, the fluorescent molecules are expected to be
developed as highly emissive in the solid-state, enlarged Stokes shift to avoid self-absorption,
and conveniently tunable emission color. Distance dependent is the typical character of
EET mechanism; include Förster-type “Coulombic” interactions, Dexter-type exchange
interactions, and other contributions to short-range coupling due to orbital overlap effects
[18]. Fluorescence resonance energy transfer (FRET) is one of the typical EET models, which
has been used in configuring various probes for bio-systems. In FRET systems, sufficiently
large spectral between the donor emission and the acceptor absorption is necessary for effi-
cient ET, which also limit the wavelength difference of two fluorophores [19]. Whereas the
through-bond energy transfers (TBET) does not need spectral overlap as that of the FRET. It
could afford a large wavelength difference between the two fluorophores by choosing suitable
dye pairs [20, 21].
CONTACT Prof. Young-A Son
National University,  Gung-dong, Daejeon, -, South Korea; Prof. Xiaochuan Li
yason@cnu.ac.kr
Department of Advanced Organic Materials Engineering, Chungnam
lixiaochuan@htu.cn School of
Chemistry and Chemical Engineering, Henan Normal University, East Jianshe Rd. , Xinxiang, Henan, , China.
©  Taylor & Francis Group, LLC

258
X. LI ET AL.
Analysis and configuration of novel luminescent molecules, based on the intramolecular
energy transfer, are of significant in biological application due to the enlarged Stokes shift.
Herein, two highly emissive fluorephore (coumarin and naphthalimide) were incorporated
together with the –NHN= bridge, which could be potentially applied in sensor design or
optical device fabrication. Any interaction occurred to the bridging unit would disturb the
energy transfer between coumarin and naphthalimide. Thus, photo-physical properties would
be altered.
Experimental
General procedures and materials
The solvents used in the reaction were carefully dried according to the standard procedure
˚
and stored over 4A molecular sieve. All the reagent-grade chemicals were purchased from
Sigma-Aldrich CO. LLC./Alfa Aesar-A Johnson Matthey Company (South Korea) and used
without further purification. Melting points were determined on a Mel-Temp IA9200 digital
®
melting point apparatus in a glass capillary and were uncorrected. All synthesized compounds
were routinely characterized by TLC and 1H NMR. TLC was performed on aluminum-backed
silica gel plates (Merck DC. Alufolien Kieselgel 60 F254).
1H/13C NMR, IR, and high resolution mass spectroscopy
1H and 13C nuclear magnetic resonance (NMR) spectra were recorded on a Bruker AM-400
spectrometer operating at frequencies of 400 MHz for proton 100 MHz for carbon in CDCl3.
Proton chemical shifts (δ) are relative to tetramethylsilane (TMS, δ = 0) as internal standard
and expressed in parts per million. Spin multiplicities are given as s (singlet), d (doublet),
t (triplet), and m (multiplet) as well as b (broad). Coupling constants (J) are given in Hertz.
Infrared (IR) spectra were run in KBr on an FTS-175C IR spectrometer (υmax in cm−1).
High resolution mass spectra (HRMS) were measured on a Bruker microOTOF II Focus
instrument.
UV-Vis and emission spectra
The absorption spectra were measured with a PERSEE TU-1900 and an Agilent 8453 spec-
trophotometer. Emission spectra were measured with Shimadzu RF-5301PC fluorescence
spectrophotometer. The solvents used in photochemical measurement were spectroscopic
grade and were purified by distillation. The stock solution of compounds (2×10−3 M) was
prepared in CH3CN, and a fixed amount of these concentrated solutions were added to each
experimental solution. All the experiments were done repeatedly, and reproducible results
were obtained. Prior to the spectroscopic measurements, solutions were deoxygenated by bub-
bling nitrogen through them.
Synthesis
The synthesis of Naphthalimide-Coumarin (NC) is shown in Scheme 1. The synthesis is
started from 4-Br-1,8-naphthalimide (1), which is commercially available. It can be conve-
niently to get imidization products of N-allkyl-4-Br-1,8-naphthalimide (2) [8, 22]. Subse-
quent substitution by nitrogen atom of hydrazine hydrate yielded N-allkyl-4-hydrazinyl-1,8-
naphthalimide (3) [22]. Finally, 3 was condensed with 3-acetyl-7-diethylamino-coumarin (5),

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
259
Scheme . Synthetic routes of Naphthalimide-Coumarin (NC).
which was synthesized as described by known procedure, yielding the target Naphthalimide-
Coumarin (NC), a bright orange powder.
Naphthalimide-Coumarin (NC)
To a magnetically stirred solution of N-allkyl-4-hydrazinyl-1,8-naphthalimide (3, 58.8 mg,
0.22 mmol) in anhydrous ethanol (30 mL), 3-acetyl-7-diethylamino-coumarin (5, 57 mg,
0.22 mmol) was added. The mixture was refluxed for 8h. After the starting materials were
full reacted, as confirmed by TLC, the solvent was evaporated in vacuum. The residue was
loaded to the column (silica gel 200-300 mesh) and eluted by mixed dichloromethane and
ethyl acetate (40:1). Bright orange powder was obtained (90 mg, 88%).
Mp: 233-236°C; IR (KBr, υ, cm−1): 772, 1102, 1136, 1242, 1275, 1328, 1377, 1414, 1518,
1540, 1577, 1587, 1618, 1656, 1689, 1721, 2974. 1H NMR(400 MHz, CDCl3): δ (ppm): 8.69
(1H, b, –NH), 8.57 (1H, d, J = 8.0 Hz, ArnapH), 8.52 (1H, d, J = 8.0 Hz, ArnapH), 8.21 (1H,
d, J = 8.0 Hz, ArnapH), 8.04 (1H, s, ArcouH), 7.75 (1H, d, J = 8.0 Hz, ArnapH), 7.68 (1H, t, J =
8.0 Hz, ArnapH), 7.36 (1H, d, J = 8.0 Hz, ArcouH), 6.61(1H, d, J = 8.0 Hz, ArcouH), 6.45 (1H, s,
ArcouH), 6.04–5.95 (1H, m, CH2=CHCH2N–), 5.31 (1H, d, J = 12.0 Hz, CHaHb=CHCH2N–
), 5.19 (1H, d, J = 8.0 Hz, CHaHb=CHCH2N–), 4.78 (1H, d, J = 8.0 Hz, CHaHb=CHCH2N–),
3.43 (4H, d, J = 8.0 Hz, –N(CH2CH3)2), 2.53 (3H, s, –N=C(CH3)–Coumarin), 1.23 (6H, d, J
= 8.0 Hz, –N(CH2CH3)2). 13C NMR (100 MHz, CDCl3): δ (ppm): 164.3, 163.8, 161.4, 157.0,
145.2, 134.0, 132.6, 131.4, 130.3, 129.7, 125.9, 125.5, 123.2, 119.3, 117.4, 113.4, 108.4, 97.2,
15.3, 42.4, 25.0, 14.8, 12.6. HRMS: [M+H]+ C30H29N4O4 requires 509.2189; found [M+H]+
509.2200.
Result and discussion
The spectral property of NC was investigated in several solvents, including hexane, toluene,
chloroform, dioxane, THF, acetonitrile, methanol, DMF, and DMSO. And NC shows excellent
solubility in most of the organic solvents. No color fading was observed even if several days
of exposure to the room light. The fundamental absorption/emission maximum, coefficients,
quantum yields, etc. are collected in Table 1. In different solvents, the absorption maximum
exhibit weak solvatochromism with 46 nm variation. Figure 1a shows the absorption spec-
tra of NC in various solvents. The absorption spectra are of similar shape with each other.
The longest absorption maximum were observed around 440-486 nm. In weak or medium
polarity solvents (from hexane to acetonitrile), strong absorption bands around 440 nm and

260
X. LI ET AL.
Table . Optical data of NC in several solvents.
UV-Vis
Fluorescence
Solvent
λabs (nm)
log εmax
λem (nm)
ꢀF
FWHM (nm)
Stokes shift (nm)
hexane
toluen
chloroform
dioxane
THF
acetonitrile
methanol
DMF
/




/

/
/
./.
.
.
.
.
./.
.
./.
./.

/

/



.
—
















.
.
.
.
.
.
.
.


DMSO

a Full width at half maximum is abbreviated as FWHM.
shoulder bands around 400 nm were observed. The absorption spectra are only barely affected
by solvent polarity with the maximum absorption value being slightly shifted hypsochromi-
cally (∼20 nm). The maximum absorption at longer wavelength side could be attributed to
S0→S1 transition with the bands on the high- and low-energy side corresponding to 0-1 and
0-0 vibration transition, respectively. While in strong polarity solvent, the absorption maxi-
mum shifted toward longer wavelength fatherly around 486 nm. Especially in methanol, the
0-0 vibration transition is strengthened and the 0-1 vibration transition is weakened signifi-
cantly.
The emission behavior in different solvents is largely different due to the polarity’s differ-
ence. Double/single emission bands are observed in Figure 1b, the emission spectra of NC in
several solvents. It is obviously that independent emission bands of the two chromospheres,
naphthalimide and coumarin, are found at 460 nm (coumarin) and 548 nm (naphthalimide)
when excited in weak or medium polarity solvents. It seems that the two fluorophores are
simply mixed together. Especially in dioxane, the emission maximum of naphthalimide and
coumarin could be clearly differentiated and no energy transfer occurred between naphthal-
imide and coumarin. However, the emission of coumarin was inhibited in strong polarity sol-
vents, such as acetonitrile, methanol, DMF, and DMSO. Only the emission of naphthalimide
was exhibited. Additionally, no obviously emission enhancement of naphthalimide was found.
When the coumarin was excited, the naphthalimide emitted indicating an energy transfer
from coumarin to naphthalimide. Even if there is perfect spectra overlap between the donor
emission (coumarin) and the acceptor absorption (naphthalimide), the energy transfer is sig-
nificantly influenced by the solvent polarity. The positive solvatochromism mentioned above
indicates that the value of the dipole moment in the excited electronic state (μe) is larger than
that in the ground state (μg). Therefore, the dipole-dipole interaction of the excited molecules
in the polar environment is stronger than that of the excited molecules in non-polar environ-
ment. Due to the electron donor (N,N-diethyl) attached to the coumarin moiety, the electron
density of coumarin will be enhanced and this leads to better stabilization by polar solvent
in the excited state. Thus, significant emission quenching of coumarin was observed in polar
solvent. Upon excitation of coumarin moiety in polar environment, excitation energy transfer
from coumarin to naphthalimide through bridge unit (–NHN= ) by energy related coupling
or resonance. Stabilization in the excited state by the polar solvent also result very low emis-
sion quantum yield.
It should be noted that NC is highly emissive in film with 5 wt.% in polycarbonate, which
is shown in Figure 1c. In comparison with the emission behavior in solution, the emission
maximum in the film shows significant bathochromic shift towards longer wavelength (562

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
261
Figure . Absorption (a) and emission (b) spectra of NC in several solvents (. × − M); (c) Emission spec-
tra of NC in film at  wt.% in polycarbonate. Emission and excitation slit widths were set to be  nm.
nm). About 20 nm shift can be calculated with reference to the emission of naphthalimide,
due to the tighter packing in polymer matrix. The narrow emission band (FWHM = 66 nm)
indicates that the monomers emission contributes mostly to the film emission. In addition,
the emission of coumarin was completely inhibited. It indicates that an efficient ET occurred
from coumarin to naphthalimide. However, the solid emission is very weak and almost
undetectable. It is likely that the tightly regular π-π stacking responsible for the emission

262
X. LI ET AL.
Figure . CIE  chromaticity diagram of the emissions of NC in solutions and film.
quenching, which is observed for most aromatic fluorophores [23-25]. In comparison, NC
was scattered and fixed in the polymer matrix. This packing model is tighter than that in
solution, but looser than that in solid. It leads to the bright emission of NC in film, avoiding
the nonradiative transition induced by closed compact.
The emission color changes in solution and film were characterized by CIE coordinates
(Figure 2). In solution, the emission color of NC varied from light blue to green. In weak and
medium polarity solvent (from hexane to THF), the CIE coordinates are scattered in light blue
and light green region. With the polarity increasing (acetonitrile, methanol, DMF, DMSO),
the CIE coordinates shifts to green region, which is in agreement with their emission contour
in solution. Notable change in the CIE coordinates of NC is found in film state. In film, the
emission color moves to orange region.
Any effect applied to the unit –NHN= , as the connector of the two fluorophores, would
lead to change in some properties. Deprotonating of –NHN= was investigated using a 2.0 ×
10−5 M solution in dioxane by addition of Lewis base F−. The titration spectra of F−for NC are
shown in Figure 3. Upon addition of F−, the maximum absorption around 444 nm decreased
gradually and a new more absorptive band around 486 nm appeared, shifting 42 nm toward
longer wavelength. Two clear isobestic points could be observed at 312 and 359 nm, which
indicates only two species co-exists in the equilibrium. With the F− addition, the color of the
NC solutions was gradually changed from light yellow-green to brown. It is likely that F− inter-
acted with the hydrogen atom of bridge unit –NHN= and lower the electron-donor ability of
bridge unit toward naphthalimide. Therefore, the lowered electron density of naphthalim-
ide would be lower the energy gap between HOMO/LUMO and shifted longest absorption
to 486 nm. Upon addition of F− continuously to the above solution, the intensive absorp-
tion band around 486 nm decreased and shifted to longer wavelength fatherly, forming a flat-
tened band (486-495 nm). At the same time, a new absorption band appeared at 574 nm. It
is likely that the electron density was lowered fatherly induced by the F− interacted with the
bridge unit –NHN=, lowness of the energy gap of HOMO/LUMO, and lowness of the absorp-
tion energy band (574 nm). Two clear isobestic points were also observed around at 340 and

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
263
Figure . Changes in the UV-Vis absorption of NC with F− addition in dioxane. (a) – equiv; (b) – equiv.
Figure . Comparison of percent increase of absorption of NC in dioxane at  nm in the presence of 
equivalents of several of anions.

264
X. LI ET AL.
Figure . H NMR titration of NC.
515 nm, indicating equilibrium between another two species. In the process of F− addition,
no obviously fluorescence emission changes were observed. With or without the existence of
F−, the solution of NC shows bright blue-green emission. The brown color induced by F−
addition could be reversed to the original yellow-green state by methanol addition.
It is also exhibit remarkably selectivity towards F− over other anions. The absorption of N−C
in dioxane shows a very weak response to other anions such as Cl−, Br−, I−, SO42−, NO3
,
H3PO4−, CO32−, ClO4−, and AcO− (Figure 4). The highly selective binding property of NC
toward F− could be attributed to the strong Lewis base property of F−. It is worth noting that
strong base addition, such as OH−, could also deprotonation of nitrogen and results similar
absorption change as that of F− addition. The binding mechanism was investigated by H NMR
titration, as depicted in Figure 5. With the F− addition, the protons signal around 8.68 ppm
was weakened indicating the interaction between –NH and F−. With more F− addition, the
proton at 8.04 ppm (belong to pyrone) shifted to 7.98 ppm due to the adjacent interaction
with F−. The proposed binding mechanism between NC and F− is depicted in scheme 2.
The occurrence of interaction of –NH and F− resulted a significant increasing of the electron
density on the nitrogen. And the electron rich on nitrogen may delocalized to the oxygen atom
Scheme . Concept of binding mechanism by F− addition.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
265
of imide, which results in enhancement of PCT (photo-induced charge transfer) process and
a red-shift of the PCT absorption band.
Conclusions
In summary, an incorporated fluorophore was configured by naphthalimide and coumarin
with fully characterization. The absorption and emission behavior of NC in various sol-
vents, in polycarbonate, and in solid state were investigated. In weak and medium polar-
ity solvents, the emissions of two fluorophores in one molecule are independent to each
other. In strong polarity, the energy transfer from coumarin to naphthalimide when
coumarin excited. However, the emission in strong polarity is very weak (low quantum
yield). It shows highly emissive in polycarbonate film. An efficient energy transfer from
coumarin to naphthalimide occurred in film. While in solid state, the emission of NC
was completely quenched due to tightly regular π-π stacking. Another important fea-
ture was its selectivity response to F−. Upon the addition of F−, the most intense absorp-
tion around 444 nm decreased and a new absorptive band appeared at 486nm. Fur-
ther addition of F−, the absorption band shifted to longer wavelength again. Such film
emission and selectivity response to F− are remarkable and hold promise for potential
implementation in future application. Our progress in optoelectronic devices fabrication is
underway.
Funding
This work was supported by the National Natural Science Foundation of China (21272060), PCSIRT
(Grant no. IRT1061), and the Program for Innovative Research Team in Science and Technology in Uni-
versity of Henan Province (15IRTSTHN003). This study was supported by the Basic Science Research
Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science,
ICT and Future Planning (Grant No. 2016901050).
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