Emission behavior of naphthalimide-coumarin cassette

Article abstract of DOI:10.1080/15421406.2018.1466533

In this contribution, a dye (C₈-alkyl substituted Naphthalimide-Coumarin, CNC) with naphthalimide and coumarin incorporated together with -NHN = bridge unit. The compound was fully characterized by NMR and HRMS spectroscopic techniques. Highly emissive character was investigated in detail in various polar environments. The emission maximum varied from 430 to 470?nm. In polycarbonate film, CNC is also highly emissive with 560?nm emission peak. While in sold, the emission maximum was further shifted to 580?nm due to the more tightly packing mode than that in film. The C₈-alkyl substitution enhances the solubility of CNC and also contributes most to its solid emission.

Full text of DOI:10.1080/15421406.2018.1466533

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Emission behavior of naphthalimide-coumarin
cassette
Yun Liu, Xiaochuan Li, Kyeong Su Min & Young-A Son
To cite this article: Yun Liu, Xiaochuan Li, Kyeong Su Min & Young-A Son (2018) Emission
behavior of naphthalimide-coumarin cassette, Molecular Crystals and Liquid Crystals, 662:1,
139-146, DOI: 10.1080/15421406.2018.1466533
To link to this article: https://doi.org/10.1080/15421406.2018.1466533
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MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –
https://doi.org/./..
Emission behavior of naphthalimide-coumarin cassette
Yun Liua, Xiaochuan Li b, Kyeong Su Minc, and Young-A Sonc
aCollege of Material and Chemical Engineering, Zhengzhou University of Light Industry, Henan Provincial Key
Lab of Surface and Interface Science, Zhengzhou, P. R. China; bSchool of Chemistry and Chemical Engineering,
Henan Key Laboratory of Organic Functional Molecule and Drug Innovation, Henan Normal University, Xinxiang,
Henan, P. R. China; cBK, Department of Advanced Organic Materials Engineering, Chungnam National
University, South Korea
ABSTRACT
KEYWORDS
C-alkyl; coumarin; highly
emissive; naphthalimide;
solid emission
In this contribution, a dye (C8-alkyl substituted Naphthalimide-
Coumarin, CNC) with naphthalimide and coumarin incorporated
together with -NHN = bridge unit. The compound was fully charac-
terized by NMR and HRMS spectroscopic techniques. Highly emissive
character was investigated in detail in various polar environments.
The emission maximum varied from 430 to 470 nm. In polycarbonate
film, CNC is also highly emissive with 560 nm emission peak. While in
sold, the emission maximum was further shifted to 580 nm due to the
more tightly packing mode than that in film. The C8-alkyl substitution
enhances the solubility of CNC and also contributes most to its solid
emission.
1. Introduction
Nowadays, a fluorophore with larger Stoke’s shift is of great beneficial to the DNA sequencing
detection, in vivo optical imaging system, various sensors, smart logic devices, and artificial
light-harvesting apparatus, which has been attractive the interests of chemists, physicists, and
biochemists [1–11]. Configuring an energy transfer cassette is an effective way to enlarger
the gap of the excitation and emission wavelength. Self-absorption could be avoided effec-
tively by the enlarged Stoke’s shift. In addition, the fluorescent dyes are expected to have high
quantum yields in solution and/or in solid and structurally controllable emission wavelength.
Therefore, electronic excitation energy transfer (EET) is one of the key factors in configura-
tion of multichromophoric systems [12–16]. Distance between dye pairs is the key parame-
ter in EET mechanism; include Förster-type “Coulombic” interactions, Dexter-type exchange
interactions, and other contributions to short-range coupling due to orbital overlap effects
[17]. Fluorescence resonance energy transfer (FRET) is another typical EET models, which
has succeeded applied in bio-sensors. In FRET cassette, spectral overlap between the donor’s
emission and the acceptor’s absorptions is the key factor to efficiency of ET, which also set
a threshold for the selection of emitters [18]. However, another ET mechanism (through-
bond energy transfer, TBET) avoids the requirement of spectral overlap as that of the FRET.
CONTACT Prof. Young-A Son
National University,  Daehakro, Yuseong, Daejeon , South Korea; Prof. Xiaochuan Li
yason@cnu.ac.kr
Department of Advanced Organic Materials Engineering, Chungnam
lixiaochuan@htu.edu.cn
School of Chemistry and Chemical Engineering, Henan Normal University, East Jianshe Rd. , Xinxiang, Henan , China.
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gmcl.
©  Taylor & Francis Group, LLC

140
Y. LIU ET AL.
It affords the possibility of configuring larger Stoke’s shift by choosing suitable dye pairs
[19, 20].
Developing novel fluorescent molecules with larger Stoke’s shift are of important in mate-
rial and biological application. TBET and FRET are useful tools to reach this end. Herein,
two highly emissive fluorephore, coumarin and naphthalimide, are selected and incorporated
together with a bridge. It provides great potential in sensor application and optical device fab-
rication. The interaction occurred the bridge unit may influence the effect of ET, once the
electronic states of this bridge were altered between coumarin and naphthalimide. Uncover-
ing the principle behind them is the key issue to understanding the physical properties of the
energy transfer cassette.
2. Experiment
General procedures and materials
The solvents used in the reaction were carefully dried according to the standard procedure
˚
and stored over 4 A 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/13CNMR, 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 total synthesis of C8-alkyl substituted Naphthalimide-Coumarin (CNC) is shown
in Scheme 1. The synthesis is started from 4-Br-1,8-naphthalimide (1) and N, N-diethyl

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
141
Scheme . Synthetic routes of C-alkyl substituted naphthalimide-coumarin (CNC).
salicylaldehyde (4), which are commercially available. 1 can be easily iimidication when
treated with n-octyl amine in refluxed Ethanol [21, 22]. Subsequent substitution by nitrogen
atom of hydrazine hydrate yielded N-allkyl-4-hydrazinyl-1,8-naphthalimide (3) [22]. Then
it was condensed with 3-acetyl-7-diethylaminocoumarin (5), which was synthesized as
described by known procedure, yielding the target C8-alkyl substituted Naphthalimide-
Coumarin (CNC), a bright orange powder.
C8-alkyl substituted naphthalimide-coumarin (CNC)
N-allkyl-4-hydrazinyl-1,8-naphthalimide (3, 169.5 mg, 0.50 mmol) was charged into a dry
flask with anhydrous ethanol (80 mL). Then, 3-acetyl-7-diethylamino-coumarin (5, 129.5 mg,
0.50 mmol) was added. The mixture was heated to reflux for 10 h. 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 (50:1). Bright orange powder was obtained (246.5 mg, 85%).
1
Mp: 167–172 °C; H NMR (400 MHz, CDCl3): δ 8.68 (1H, b, –NH), 8.59 (1H, d,
J = 7.2 Hz, ArnapH), 8.53 (1H, d, J = 8.4 Hz, ArnapH), 8.21 (1H, d, J = 8.0 Hz, ArnapH), 8.07
(1H, s, ArcouH), 7.76 (1H, d, J = 8.4 Hz, ArnapH), 7.69 (1H, t, J = 7.8 Hz, ArnapH), 7.39 (1H, d,
J = 8.8 Hz, ArcouH), 6.64 (1H, d, J = 8.0 Hz, ArcouH), 6.49 (1H, s, ArcouH), 4.18–4.10
(2H, m, -CH2-), 3.45 (4H, q, J = 7.2 Hz, -(CH2)2-), 2.54 (3H, s, -CH3), 1.72 (2H, m, -CH2-),
1.41 (2H, m, -CH2-), 1.29–1.20 (14H, m, -(CH2)4-(CH3)2), 0.86 (3H, t, J = 6.8 Hz, -CH3). 13C
NMR (150 MHz, CDCl3): δ 164.4, 163.9, 161.3, 156.9, 144.9, 133.6, 131.1, 130.2, 130.1, 129.5,
125.6, 125.4, 123.2, 119.2, 113.5, 110.6, 109.7, 108.2, 96.7, 45.2, 40.3, 31.8, 29.4, 29.3, 28.2,
27.2, 22.7, 14.7, 14.1, 12.5. HRMS (ESI): [M+H]+ C35H41N4O4 requires 581.3128, found
[M+H]+ 581.3074.
3. Results and discussion
The absorption behavior of CNC was investigated in different organic solvents, includ-
ing hexane, toluene, chloroform, dioxane, tetrahydrofuran (THF), acetonitrile, methanol,
N,N-dimethylformamide (DMF), and dimethyl sulfoxide (DMSO) (Fig. 1). Due to the intro-
duction of C8-alkyl, CNC exhibit excellent solubility in the tested solvents. In addition, robust
stability of CNC toward light was observed with no color fading, even if several months’
exposure to room light. The fundamental absorption/emission maximum, coefficients,
quantum yields, etc are collected in Table 1. In medium polarity solvents (from toluene
to acetonitrile), the absorption contour indicates weak solvatochromism (Fig. 1). Similar
shapes were observed with two main absorption peaks around 430 and 530 nm. With the

142
Y. LIU ET AL.
Table . Optical data of CNC in several solvents.
UV-Vis
Fluorescence
Solvent
λabs (nm)
log εmax
λem (nm)
ꢀF
FWHM (nm)
Stokes shift (nm)
hexane
toluene
chloroform
dioxane
THF
acetonitrile
methanol
DMF
/
/
/
/
/
/
/
/
/
./.
./.
./.
./.
./.
./.
./.
./.
./.






—–
—–
—–
.
.
.
.
.
.
—–
—–
—–












—–
—–
—–
—–
—–
—–
DMSO
a Full width at half maximum is abbreviated as FWHM.
solvent polarity increasing further, the longest maximum absorption peaks was weakened.
Especially, the 530 nm absorption was significantly weakened in hexane. Medium solvent
polarity contributes more to the longest maximum absorption. S0→S1 and S0→S2 transition
could be assigned to the 530 and 430 nm, respectively. Obviously, the former are slightly
outweigh the latter. Absorption longer than 580 nm is almost optically transparent.
Notably, CNC is highly emissive in the studied organic media. Detailed emission param-
eters are listed in Table 1. The emission spectra in different solvents are presented in Fig. 2.
In different polarity solvents, the emission intensity of CNC is largely different. It was sig-
nificantly quenched in methanol, DMF, and DMSO. While in dioxane, chloroform, toluene,
and THF, CNC is highly emissive with the emission peaks ranging from 454 to 465 nm,
which leads to the blue fluorescence color. Mainly, the 0–0 transition of vibronic progress
contributes most to the blue emission, which suggests that the geometry of the ground state
and the excited are quite similar. Obviously, the emission band of coumarin is observed
(430 nm). Only in dioxane, a trace of 550 nm emission could be found. When the naph-
thalimide was excited, the coumarin emitted its typical fluorescence emission, indicating
no energy transfer from to coumarin naphthalimide. Even if there is perfect spectra overlap
between the donor emission (coumarin) and the acceptor absorption (naphthalimide), the
energy transfer is significantly influenced by the structure factor (C8-alkyl substituted) and
Hexane
Toluene
Chloroform
Dioxane
THF
Acetonitrile
Methanol
DMF
DMSO
0.5
0.4
0.3
0.2
0.1
0.0
400
500
600
Wavelength (nm)
Figure . Absorption spectra of CNC in several solvents (. × − M).

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
143
Acetonitrile
Acetonitrile
700
700
Chloroform
Chloroform
Dioxane
Dioxane
600
600
DMF
DMF
DMSO
DMSO
Hexane
Hexane
500
500
Methanol
Methanol
THF
THF
400
400
Toluene
Toluene
300
300
200
200
100
100
0
0
400
400
450
450
500
500
550
550
600
600
Wavelength/nm
Wavelength/nm
Figure . Emission spectra of CNC in various solvents (. × − M).
solvent polarity. Slightly positive solvatochromism showing in Fig. 1 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 environment. N, N-
diethylamino attaching to coumarin strongly donated electron to coumarin, but it is partially
inhibited by C8-alkyl substitution at naphthalimide. The electron density of naphthalimide
will be enhanced and it will be stabilized by polar environment in the excited state. Thus, the
emission of naphthalimide was quenched significantly. The bridge unit (−NHN = ) is also a
highly electron density part, which is easy influenced by C8-alkyl. Stabilization in the excited
state of naphthalimide by polar solvent could lead to very low emission quantum yield.
It should be noted that CNC is also highly emissive in film with 5 wt.% in polycarbon-
ate, as shown in Fig. 3. In comparison with the emission behavior in solution, the emission
maximum in the film state shows significant bathochromic shift toward longer wavelength
(560 nm). In comparison to the emission of naphthalimide, about 20 nm shifts could be cal-
culated due to the tighter packing in polymer matrix. According to the narrow emission band
(FWHM = 66 nm), it can be deduced that the monomers emission contributes mostly to the
film emission. In addition, polycarbonate could provide a polar matrix lattice, which effec-
tively isolates the dye molecules in polymer lattices. Similar to the emission in solution, the
excited dye molecules in polymer matrix dissipate part of the energy to polymer and to lower
energy state (S1). Both the polar environment and the irregular tight packing in polycarbonate
matrix contribute to the emission bathochromic shift. In the solid powder state, the emission
maximum was further red-shift to 580 nm, indicating tighter packing than that of film. Addi-
tionally, the emission in solid powder was broadened, with the full width at half maximum
reaching to 80 nm (broader than that of polycarbonate film 66 nm). The solid powder is com-
posed by micro crystals, in which the dye molecules were packed regularly than that it in poly-
mer matrix. Therefore, the excited dye molecules in solid state could dissipate more energy to
adjacent dye molecule easily, and then return to the ground state, emitting longer wavelength
fluorescence. Finally, the energy loss of excited dye molecules was enhanced in solid state in
contrast to that in polycarbonate film, leading to a further red-shift of fluorescence emission.
On top of that the FWHM of solid emission was broadened furtherly to 76 nm due to the more

144
Y. LIU ET AL.
Film
Solid
1.0
0.8
0.6
0.4
0.2
0.0
450 500 550 600 650 700 750
Wavelength (nm)
Figure . Emission spectra of CNC in film at  wt.% in polycarbonate. Emission and excitation slit widths
were set to be  nm.
complicated interactions between dye molecules in solid state than that in film between dye
molecules and amorphous polymer chains. More importantly, the highly emissive character
in polycarbonate film is attractive for CNC, which makes it a potential dopant emitter in host
materials. And the introduction of C8-alkyl in naphthalimide also avoids higher regular pack-
ing in solid, which contributes mainly to its solid emission. Generally, the solid emission will
be quenched in solid state due to the highly regular π-π stacking, and enhance nonradiation
energy transition. Without the long chain effect, quenched emission has been demonstrated,
for example C3-alkyl substitution [23].
Figure . CIE  chromaticity diagram of the emissions of CNC in solutions, film, and solid.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
145
The emission color changes in solution and film were characterized by Commission Inter-
nationale de L’Eclairage (CIE) coordinates (Fig. 4). In solution state, the emission color of
CNC scattered in blue region. In hexane and toluene, the CIE coordinates distributed in deep
blue region. With the polarity increasing (chloroform, THF, dioxane), the CIE coordinates
shifts slightly to light blue region, which is in agreement with their emission profiles in solu-
tion. Notably, the CIE coordinate of CNC in film state is observed and moves to green region.
In solid state, the emission color moved further to orange region due to the tight molecular
packing.
4. Conclusions
In conclusion, a new dye with two fluorophores was configured by naphthalimide and
coumarin with fully characterization. The absorption and emission behavior of CNC in var-
ious solvents, in film, and in solid state were studied. A long alkyl chain was introduced to
naphthalimide unit, enhancing the solubility in organic medium. CNC is highly emissive in
medium nonpolar solvents. Particularly, dioxane induces most emissive character with the
emission maximum at 460 nm. In polycarbonate film, CNC is also highly emissive with emis-
sion peak shifting to green region (560 nm). Due to the tightly molecular packing in solid, the
emission maximum was further shifted to the orange region (580 nm). The emission color of
CNC in solution/film/solid powder was characterized by CIE coordinates in detail. It indi-
cates that the excitation and excited state of CNC is easy to be interfered by the polar condi-
tions induced by the solvents or polymer matrix. Our efforts toward the application of TPN
in device fabrication and biological purpose are currently underway.
Funding
This work was supported by the National Natural Science Foundation of China (21272060, 21772034),
PCSIRT (grant no. IRT1061), and the Program for Innovative Research Team in University of Henan
Province (15IRTSTHN003). This study was also supported by the National Research Foundation of
Korea (NRF) (Grant No. NRF-2017R1E1A1A01074266).
ORCID
Xiaochuan Li http://orcid.org/0000-0002-6903-9574
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