Solvent dependent energy transfer of N-bridged naphthalimide- bisindolymaleimide fluorescent dyes

Article abstract of DOI:10.1080/15421406.2013.844882

Double build-in chromophores, naphthalimide (NIM) and bisindolymaleimide (BIM), were synthesized and characterized. Absorption and emission properties of double chromophores dye were fully investigated in various solvents. The priority of through bond ene

Full text of DOI:10.1080/15421406.2013.844882

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Solvent Dependent Energy Transfer
of N-bridged Naphthalimide-
Bisindolymaleimide Fluorescent Dyes
Xiaochuan Li a , Kaijuan Zhu a , Yajuan Li a , Hyungjoo Kim b &
Young-A Son b
a College of Chemistry and Environment Science, Key Laboratory of
Green Chemical Media and Reactions, Ministry of Education , Henan
Normal University , Xinxiang , Henan, 453007 , China
b Department of Advanced Organic Materials Engineering ,
Chungnam National University , Daejeon , 305-764 , Republic of
Korea
Published online: 16 Dec 2013.
To cite this article: Xiaochuan Li , Kaijuan Zhu , Yajuan Li , Hyungjoo Kim & Young-A Son (2013)
Solvent Dependent Energy Transfer of N-bridged Naphthalimide-Bisindolymaleimide Fluorescent Dyes,
Molecular Crystals and Liquid Crystals, 584:1, 18-26, DOI: 10.1080/15421406.2013.844882
To link to this article: http://dx.doi.org/10.1080/15421406.2013.844882
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Mol. Cryst. Liq. Cryst., Vol. 584: pp. 18–26, 2013
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2013.844882
Solvent Dependent Energy Transfer of N-bridged
Naphthalimide-Bisindolymaleimide
Fluorescent Dyes
XIAOCHUAN LI,1,∗ KAIJUAN ZHU,1 YAJUAN LI,1
HYUNGJOO KIM,2 AND YOUNG-A SON2,∗
1College of Chemistry and Environment Science, Key Laboratory of Green
Chemical Media and Reactions, Ministry of Education, Henan Normal
University, Xinxiang, Henan, 453007, China
2Department of Advanced Organic Materials Engineering, Chungnam National
University, Daejeon 305-764, Republic of Korea
Double build-in chromophores, naphthalimide (NIM) and bisindolymaleimide (BIM),
were synthesized and characterized. Absorption and emission properties of double
chromophores dye were fully investigated in various solvents. The priority of through
bond energy transfer (TBET) to NTM or BIM is solvent dependent. There appears
large different emission wavelength change in CHCl3 and DMF with alkyl attached
to BIM. However, the product of cyclohexyl substitution leads to a different emission
behavior, which is non-sensitivity to solvent polarity. Quantum chemical calculation for
the molecular orbital reveals that intramolecular charge transfer (ICT) is the key factor
for their different opto-electronic properties.
Keywords Biindolymaleimide; ICT; naphthalimide; solvent dependent; TBET
Introduction
Research into multichromphoric energy transfer cassettes has been motivated by their
promising application in organic field effect transistors, photovoltaic devices, probe, and
single molecule spectroscopy [1–5]. One of the chromophore was excited by the absorption
of a photon of light and then the energy transfer occurred with the donor returning to its
ground state simultaneously with raising of the acceptor to its excited state. The types for
the excitation energy transfer (EET) have two pathways: (i) through bond and (ii) through
space [6–8]. The first pathway is a non-radiative energy transfer from a donor to an acceptor
with conjugated bond connecting two chromophores. The second pathway requires strong
overlap between the emission spectrum of the donor and absorption of the acceptor. These
requirements led us design systems with two fragments twisted to each other.
In this report, two different chromophores, NTM and BIM, were incorporated together.
NTM and BIM have similar maxima absorption peaks at 350–500 nm. NTM was a typical
∗Address correspondence to Young-A Son, Department of Advanced Organic Materials and
Textile System Engineering, Chungnam National University, 220 Gung-dong, Yuseong-gu, Daejeon
305-764, Korea (ROK). Tel.: (+82)42-821-6620; Fax: (+82)42-821-8870. E-mail: yason@cnu.ac.kr;
or lixiaochuan@henannu.edu.cn
[206]/18

Solvent Dependent Energy Transfer
[207]/19
465 498
425
560
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
400
500
600
700
Wavelength / nm
Figure 1. Spectra overlap between NTM emission and BIM absorption.
green fluorescent dye with its maximum emission at 490 nm in THF (Fig. 1) [9,10]. The
longest maximum emission of BIM located at 560 nm. The –N– linker between the two
fragments was selected, which prevent the whole molecular from becoming planar because
if they did the system would behave as a single conjugated dye. Absorption of the two
fragments overlaps with each other, both of which have an electron deficient heterocyclic,
imide. The electron donor,
N
linker, located in the middle of NTM and BIM. Different
energy transfer induced by the middle N atom depending on different environment.
Experimental
Materials
4-Bromo-1,8-naphthalic anhydride and dibromomaleimide were purchased from Aldrich.
2 (N-alkyl-4-bomo-1,8-naphthalimide) was synthesized following the published procedure
[10–12]. The bromo atom of 2 was substituted by hydrazine hydrate and gave 3 [13]. 5 and
6 were synthesized according to our previous studying [10]. Modified synthetic procedure
of 6 increased the synthetic yield greatly. All the solvents used in reaction were analytical
grade and were dried according to the standard procedures. Solvents used in photochemical
measurement were spectroscopic grade and purified by distillation. The stock solutions of
compounds (2×10−3M) were prepared, and a fixed amount of these concentrated solutions
were added to each experimental solution.
Synthesis
6 (Bisindolylmaleic anhydride) was synthesized following a modified procedure according
to our previous study [10,12]. 5 (0.71 g, 2 mmol) was added to 10% KOH (200 mL) and
refluxed for 30 min. The reaction mixture cooled and acidified with 2N HCl. The red deposit

20/[208]
X. Li et al.
was collected. Aqueous was extracted by ethyl acetate (2 × 100 mL). The organic phase
was dried and evaporated in vacuum. The crude product, together with the red deposit, was
purified by chromatography on silica gel (EtOAc/hexane = 3:1). 667 mg red powder was
obtained with 95% yield. 1H NMR data of 6 was identical to previously report.
Synthesis of NTM-BIMs was followed by a one step procedure as shown in Scheme
1. To a magnetically stirred solution of 3 (0.1 mmol) and 6 (0.1 mmol) in absolute ethanol
(10 mL) added K2CO3 (28 mg, 0.2 mmol). The mixture was refluxed for 12 h and monitored
by TLC. When the TLC analysis revealed complete condensation, the reaction mixture was
cooled to room temperature. Solvent was evaporated in vacuum. The crude product was
chromatographed on silica gel (eluent: CH2Cl2) giving deep red solid powder.
Scheme 1.
1
NTM-BIM1 Yield 63%; H NMR (400 MHz, DMSO-d6) δ 11.48 (s, 1H), 10.93 (s,
1H), 10.86 (s, 1H), 7.26 (d, J = 8.1 Hz, 2H), 7.08 (d, J = 8.0 Hz, 2H), 6.95 (dd, J = 7.5,
5.0 Hz, 3H), 6.90 (d, J = 7.6 Hz, 3H), 6.78 (dd, J = 16.2, 8.6 Hz, 3H), 4.03 (d, J = 7.1 Hz,
2H), 2.36 (t, 2H), 2.03 (s, 6H), 1.83 (m, 2H), 1.23 (m, 2H), 0.91 (t, J = 15.3, 8.3 Hz, 3H);
13C NMR (100 MHz, DMSO-d6) δ 167.7, 166.8, 162.8, 162.5, 158.6, 135.7, 135.1, 134.7,
131.9, 131.4, 130.8, 130.5, 130.1, 128.1, 127.8, 127.1, 126.6, 122.1, 122.3, 120.8, 120.5,
116.8, 115.7, 114.3, 113.6, 107.3, 106.8, 39.7, 30.1, 20.7, 16.1, 15.8, 14.0; HRMS Calcd
for C38H31N5O4: 621.2376, found: 621.2380.
NTM-BIM2 Yield 60%; 11.48 (s, 1H), 10.86 (s, 1H), 7.26 (d, J = 8Hz, 2H), 7.20
(t, J = 8Hz, 1H), 7.07 (d, J = 8Hz, 2H), 7.02 (d, J = 8Hz, 1H), 6.99–6.94 (m, 3H), 6.90 (d,
J = 8Hz, 1H), 6.79 (t, J = 8Hz, 2H), 6.73 (t, J = 8Hz, 1H), 4.03 (q, J = 8Hz, 1H), 3.60 (s,
1H), 3.55 (s, 2H), 3.51 (s, 2H), 2.36 (s, 3H), 1.83 (s, 3H); 13C NMR (100 MHz, DMSO-d6)
δ 168.0, 167.1, 162.5, 161.3, 159.6, 135.3, 135.0, 134.1, 133.8, 132.0, 131.6, 131.1, 130.8,
129.0, 128.1, 127.5, 126.1, 123.8, 122.0, 121.4, 120.4, 117.2, 116.5, 115.5, 112.6, 106.1,
105.9, 41.3, 16.3, 15.6; HRMS Calcd for C37H27N5O4: 605.2063, found: 621.2065.
NTM-BIM3 Yield 55%; 11.47 (s, 1H), 10.85 (s, 1H), 7.26 (d, J = 8Hz, 2H), 7.19
(t, J = 8Hz, 1H), 7.07 (d, J = 8Hz, 2H), 7.02 (d, J = 8Hz, 1H), 7.99–7.93 (m, 3H), 6.90
(d, J = 8Hz, 1H), 6.79 (t, J = 8Hz, 2H), 6.73 (t, J = 8Hz, 1H), 4.06–3.97 (m, 1H), 3.59 (s,
1H), 2.36 (s, 3H), 1.83 (s, 3H), 1.56 (m, 2H), 1.35 (m, 4H), 1.27 (s, 2H), 1.21 (t, J = 8Hz,
2H); 13C NMR (100 MHz, DMSO-d6) δ 167.9, 166.9, 162.5, 162.3, 161.6, 136.9, 136.0,
135.5, 135.1, 132.8, 132.0, 131.6, 130.8, 130.3, 129.7, 128.8, 128.1, 127.6, 126.8, 125.7,
124.8, 123.3, 122.5, 120.2, 119.1 115.3, 114.1, 113.6, 107.8, 106.4, 55.9, 33.7, 31.4, 262,
24.6, 16.8; HRMS Calcd for C40H33N5O4: 647.2533, found: 647.2535.
Measurements
The NMR spectra were recorded on Brucker Advance 400 spectrometer, operating at
400 MHz (1H) and 100 MHz (13C) in DMSO-d6. Mass and high resolution spectra were

Solvent Dependent Energy Transfer
[209]/21
measured on a LC-MS (Waters120 QTof) and Brucker microOTOF II Focus, respectively.
Absorption spectra measured with PERSEE TU-1900 spectrophotometer with an external
slit width of 2.5 mm used to collect absorption. Prior to the spectroscopic measurements
solutions were deoxygenated by bubbling nitrogen through them.
For the theoretical study of excited states of NTM-BIMs, the Material Studio package
was used. The ground state geometries and HOMO/LUMO frontier orbital of NTM-BIMs
were calculated by LDA (local density approximation) methods at the PWC (Perdew-Wang
functional) level.
Results and Discussion
The UV-vis absorption and emission spectra of NTM-BIMs are measured in various
solvents of different polarity. Figure 2 shows the typical absorption spectra of NTM-BIMs
in THF (1×10−5M). Three compounds exhibit very similar absorption contour outline. The
absorption bands cross over 350–600 nm with continuously decreased tendency. Only an
absorption platform appears around 340 nm. It indicates that there are significant electronic
coupling between naphthalimide and bisindolylmalimide entities in the ground state.
Emission spectra of NTM-BIMs in various solvent of different polarity exhibit dis-
tinctly different emission behavior. Because of poor solubility in hexane, the emission
behavior in hexane was not investigated. In polar DMF, exciting NTM-BIM3 at 365 nm
provides an emission of 455 nm. The emission from BIM moiety is almost quenched
completely. In contrast, the emission from NTM moiety is observed at 540 nm for NTM-
BIM1 and 520 nm for NTM-BIM2, respectively. Whereas the fluorescence spectra of a
1:1 mixture of NTM and BIM exhibit dual emission bands at 498 nm and 560 nm. It
should be noted that the emission of NTM-BIM3 in DMF blue-shifted 40 nm compared
with that of NTM. Figure 3(c) shows that variation of the solvent polarity does not in-
duce significant emission shift for NTM-BIM3. The maxima emission wavelength varied
from 444 nm to 455 nm. For NTM-BIM1 and NTM-BIM2, variation of solvent polarity
induces significant change of maxima emission wavelength. In CHCl3, maxima emission
NPM-BIM1
NPM-BIM2
NPM-BIM3
1.0
0.8
0.6
0.4
0.2
0.0
300
400
500
600
Wavelength / nm
Figure 2. UV-vis absorption spectra of compounds NTM-BIMs (1.0×10−5 M) in THF.

22/[210]
X. Li et al.
a
)
CHCl3
1.0
0.8
0.6
0.4
0.2
0.0
THF
CH3CN
DMF
450
500
550
600
650
700
750
Wavelength / nm
b)
CHCl3
THF
CH3CN
1.0
0.8
0.6
0.4
0.2
0.0
DMF
450
500
550
600
650
Wavelength / nm
c
)
1.0
0.8
0.6
0.4
0.2
0.0
CHCl3
THF
CH3CN
DMF
400
450
500
550
600
650
Wavelength / nm
Figure 3. Fluorescence emission spectra of NTM-BIM1 (a), NTM-BIM2 (b), and NTM-BIM3
(c) in CHCl3, THF, CH3CN, and DMF.

Solvent Dependent Energy Transfer
[211]/23
of NTM-BIM1 and NTM-BIM2 are centered at 496 nm and 493 nm, respectively. The
emission from BIM moiety is quenched completely. These results indicate that solvent
polarity changing can induce different TBET direction with alkyl substituting on the top
of BIM moiety. However, this solvent polarity induced TBET direction is suppressed with
cyclohexyl substitution on the top of BIM moiety, in which only the TBET from BIM
to NTM observed. The fluorescence quantum yield of each compound was determined in
CHCl3 with reference to quinine bisulfate in 0.05M H2SO4 aqueous solution (Ф = 0.51).
The fluorescence quantum yield of the three dyes in different solvent ranged from 0.18 to
0.26.
All the NTM-BIMs exhibit large stokes shift. In CHCl3, the Stokes shifts of NTM-
BIM1 and NTM-BIM2 were estimated to be 131 nm with 365 nm excitation. However, the
Stokes shift of NTM-BIM3 is only 84 nm (372 nm excitation). In DMF, the Stokes shift
for NTM-BIM1 and NTM-BIM2 also up to 114 nm and 96 nm, respectively. For NTM-
BIM3, the Stokes shift is 85 nm in DMF, which is almost identical to that in CHCl3. It has
been clear that red-shifted emission does not necessarily induce a large Stokes shift because
the two photophysical parameters are of entirely different origin [14,15]. In principle, the
emission wavelength of fluorophore is related to the size of π-conjugation or the ICT
feature of the fluorophore [16]. For the Stokes shift, the geometry relaxation at the S1 state
after vertical excitation from S0 state is crucial.
According to the mentioned above, NTM-BIM3 is not sensitive to solvent polarity.
Emission wavelength of it is independent of the solvent polarity, although the emission
intensity decreased in more polar solvents, such as DMF. The Stokes shift around 84 nm is
an intrinsic property, not a solvent-polarity induced phenomena, or any ICT related results.
The fluorescence of NTM-BIM1 and NTM-BIM2 in CHCl3 exhibited blue emission.
However, light green emission was observed in DMF. The emission changed from blue to
light green together with the large Stokes shift will be useful for the dyes in fluorescent
molecular probes or intracellular fluorescent bio-imaging.
Figure 4 shows the solid-state emission of NTM-BIMs. The solid powder of NTM-
BIM1 and NTM-BIM2 exhibits bright orange color with 365 nm excitation. NTM-BIM3
NTM-BIM1
NTM-BIM2
NTM-BIM2
800
600
400
200
0
500
550
600
650
700
Wavelength / nm
Figure 4. Fluorescence spectra in solid state of NTM-BIMs.

24/[212]
X. Li et al.
Figure 5. Representative HOMO-1 diagrams of NTM-BIMs obtained from Dmol3 calculations.
is a dark red powder with very weak emission in solid state. The full width at half maximum
for NTM-BIM1 and NTM-BIM2 are 59 nm and 58 nm, respectively. The narrow solid-state
emission bands of NTM-BIM1 and NTM-BIM2 are unusual and intriguing, as spectral
broadening is a very common phenomenon for solid emitting materials. The alkyl attached
on NTM, which to some extent, prevents the molecules from packing compactly and thus
avoids the spectral broadening. Few intermolecular interactions between molecules provide
favorable factors that eliminate self-quenching and enhance their solid fluorescence.
To gain insights into the nature of electronic transitions of these dyes, molecular orbital
(MO) and optimization of molecular structure were carried out on the density function
theory (DFT) level using LDA/PWC basis set as implement in the Dmol3 package [17,18].
Distinctive contributions of each atomic orbital are discernible in the MO diagrams. Figure 5
shows the representative HOMO-1 diagrams of NTM-BIMs. An obvious different electron
density distribution can be observed between NTM-BIM1/ NTM-BIM2 and NTM-BIM3.
For NTM-BIM1/NTM-BIM2, the electron density in the HOMO-1s is well-distributed
over the NTM ring. In contrast, the electron density distribution in HOMO-1 for NTM-
BIM3 is contributed mainly from the indole and maleimide moiety. HOMOs/LUMOs of
the three dyes are almost identical to each other, and a representative HOMO and LUMO
is showed in Fig. 6. There appears to be a regular trend in the HOMOs and LUMOs for
NTM-BIMs. In HOMOs, electron spread over the two indole rings and maleimide ring,
whereas the electron centralizes on maleimide moiety in LUMOs. It is clearly indicated
Figure 6. Typical representative HOMO and LUMO diagram of NTM-BIMs. R = substituted butyl,
allyl, and cyclohexyl group.

Solvent Dependent Energy Transfer
[213]/25
that effective charge transfer states in acceptor-donor-acceptor molecular systems of NTM-
BIM1/NTM-BIM2, which leads to the sensitivity to the solvent polarity. Without the
effective charge transfer in NTM-BIM3, very weak sensitivity to the solvent polarity was
observed.
Conclusions
Acceptor-donor-acceptor molecular system with different chromophores, NTM and BIM,
was designed and synthesized. Its opto-physical properties were fully investigated in dif-
ferent solvent polarity. The experimental results showed that different solvent could induce
different excited state energy transfer direction for NTM-BIM1/NTM-BIM2. This phe-
nomenon typically occurred in CHCl3 and DMF. In CHCl3, the maximum emission located
around 490 nm. However, the maximum emission wavelength shifted over to 520 nm
in DMF. Interestingly, this trend was suppressed for NTM-BIM3 without effective de-
localization of electrons like NTM-BIM1/NTM-BIM2. NTM-BIM1/NTM-BIM2 also
exhibit bright solid-state emission with narrow emission bands (fwhm: 58 nm and 59
nm). These properties, especially for NTM-BIM1/NTM-BIM2 qualify them as multi-
functional fluorophores with potential application in light-emitting or –sensing devices.
Further investigation, together with newly designed molecules, along those lines is in
progress.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (grant no.
21072048), the PIRTUHP (grant no. 2012IRTSTHN006), PCSIRT (grant no. IRT1061),
and by the Basic Science Research Program through the National Research Foundation
of Korea (NRF) funded by the Ministry of Education, Science and Technology (grant no.
20120008198).
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