Significant emission red-shift of BODIPY derivatives with strong electron-acceptor attached

Article abstract of DOI:10.1080/15421406.2017.1358018

A new red-fluorescent BODIPY derivative is designed and synthesized, which is configured by attaching an electron-withdrawing unit 2-dicyanomethylene-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran (TCF). By virtue of the strong electron-accepting nature of TCF unit, the compound BT exhibits a large red-shift both in absorption and emission. In contrast to parent BODIPY, the absorption shifted about 90?nm. And the emission maximum shifted to red region with the peak centered around 620?nm. The CIE coordinate and fluorescence decay data uncovered its emission character. Theoretical calculation of BT confirmed that the electron density was altered dramatically by TCF and lowered the energy gap between ground state. The character of HOMO/LUMO distribution is well in agreement with that of experimental value.

Full text of DOI:10.1080/15421406.2017.1358018

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Significant emission red-shift of BODIPY
derivatives with strong electron-acceptor attached
Xiaochuan Li, Saisai Sun, Ick Jin Kim & Young-A Son
To cite this article: Xiaochuan Li, Saisai Sun, Ick Jin Kim & Young-A Son (2017) Significant
emission red-shift of BODIPY derivatives with strong electron-acceptor attached, Molecular Crystals
and Liquid Crystals, 654:1, 139-145, DOI: 10.1080/15421406.2017.1358018
To link to this article: https://doi.org/10.1080/15421406.2017.1358018
Published online: 22 Nov 2017.
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Date: 28 November 2017, At: 01:55

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –

https://doi.org/./..
Significant emission red-shift of BODIPY derivatives with strong
electron-acceptor attached
a
a
b
b
Xiaochuan Li , Saisai Sun , Ick Jin Kim , and Young-A Son
aHenan Key Laboratory of Organic Functional Molecule and Drug Innovation, Collaborative Innovation Center of
Henan Province for Green Manufacturing of Fine Chemicals, Key Laboratory of Green Chemical Media and
Reactions, Ministry of Education, School of Chemistry and Chemical Engineering, Henan Normal University,
b
Xinxiang, Henan, P. R. China; BK, Department of Advanced Organic Materials Engineering, Chungnam
National University, Daejeon, South Korea.
ABSTRACT
KEYWORDS
A new red-fluorescent BODIPY derivative is designed and synthe-
sized, which is configured by attaching an electron-withdrawing unit
BODIPY; CIT coordinate;
fluorescence decay; red-shift;
TCF
2
-dicyanomethylene-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran (TCF). By
virtue of the strong electron-accepting nature of TCF unit, the com-
pound BT exhibits a large red-shift both in absorption and emission.
In contrast to parent BODIPY, the absorption shifted about 90 nm. And
the emission maximum shifted to red region with the peak centered
around 620 nm. The CIE coordinate and fluorescence decay data uncov-
ered its emission character. Theoretical calculation of BT confirmed that
the electron density was altered dramatically by TCF and lowered the
energy gap between ground state. The character of HOMO/LUMO dis-
tribution is well in agreement with that of experimental value.
Introduction
Fluorescence is an easy accessible signal in material science and technology, which now
confront stringent requirement both in biotechnology and in device configuration. Life
style becomes easier and easier owing to the technology development and imagination of
scientist. Complex logic operation in micro-scale, most in molecule scale, is wide spread
in natural world. How to transform it to artificial intelligence or smart materials requires
the fascinating fluorescent molecules with steerable emission wavelength and enhanced
photo-/thermal-stability. Fluorine-boron complex (FBC) is one the robust fluorescent dye
among the established typical fluorescent dye families, including coumarins, naphthalimide,
bisindolylmalimide, perylenediimides, and rhodamine [1–11]. Among these dye families, the
emission wavelength covered the whole visible region with deep-blue, blue, green, yellow, red,
and deep-red. Systematical engineer toward various purposes has been made. FBC steps into
ꢀ
the scientists’ attention and keeps sustainable attraction for decades [12–15]. 4,4 -difluoro-
4
-bora-3a,4a-diaza-s-indacene (BODIPY) is a typical member of FBC, which has receive
great interests owing to its high molar extinction coefficient, narrow emission full width
at half maxima (FWHM), outstanding emission quantum yields, thermal resistance, and
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.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/[414]
X. LI ET AL.
photostability. Multiple strategies were applied to engineer the molecular strcuture according
to requirement by chemical modification of the BODIPY core unit, involving alkynide
substitution, N-insertion, π-extension, etc [16–19]. Furthermore, new FBC compounds,
such as ditopic (N∼N, N∼C, C∼C, C∼O, O∼O) and tritopic borate complexes, joined and
enriched the F-B family members [20–27].
Since the aldehyde was introduced to the β-position of BODIPY by Jiao, alternative strat-
egy for chemical modification was opened by functionalization at this position [28]. In
this contribution, 2-dicyanomethylene-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran (TCF), a
strong electron-accepting group, was condensed with aldehyde, which may induce significant
change in electron distribution and thus lead to the change of optical properties that worthy
of investigation.
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. (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
H NMR. TLC was performed on aluminum-backed silica gel plates (Merck DC. Alufolien
1
Kieselgel 60 F254).
NMR spectroscopy and high resolution mass spectra (HRMS)
Nuclear magnetic resonance (NMR) spectra were recorded on a Brucker AM-400 spectrom-
eter operating at frequencies of 400 MHz for proton in CDCl . Proton chemical shifts (δ)
3
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. The mass spectra measured
on a LC-MS (Waters UPLC-TQD) mass spectrometer. High resolution mass spectra (HRMS)
were measured on a Brucker micrOTOF 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
−3
grade and were purified by distillation. The stock solution of compounds (2 × 10 M) was
prepared 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 bubbling nitrogen
through them.
Synthesis
Synthetic routes of target compound, β-[2-(3-cyano-5,5-dimethylfuran-2(5H)-ylidene)
malononitrile]-BODIPY (BT), are outlined in Scheme 1. The aldehyde could be intro-
duced to the β-position of BODIPY by standard Vilsmeier strategy [28]. Subsequently,

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
[415]/141
Scheme . The synthesis of BT.
condensation was proceeded between β-formyl-BODIPY and 2-dicyanomethylene-3-cyano-
,5,5-trimethyl-2,5-dihydrofuran (TCF) in the presence of ethanol, yielding the desired BT.
4
β-[-(-cyano-,-dimethylfuran-(H)-ylidene)malononitrile]-BODIPY (BT)
β-formyl-BODIPY (200 mg, 0.57 mmol) and 2-dicyanomethylene-3-cyano-4,5,5-trimethyl-
2
,5-dihydrofuran (136 mg, 0.68 mmol) were dissolved in anhydrous ethanol (80 mL). Several
drops of piperidine were added and the mixture was refluxed. After the starting materials
were well reacted, as confirmed by TLC, the mixture was cooled to room temperature and
poured into water (100 mL). Then it was extracted with dichloromethane (3 × 100 mL). The
separated organic phase was dried over anhydrous MgSO . Next, the solvent evaporated in
4
vacuum and the residue was loaded to the column. Column separation (silica gel 200–300
mesh, eluent: dichloromethane/methanol) produced 80 mg red liquid (26%).
1
H NMR(400 MHz, CDCl ): δ (ppm): 7.82 (1H, d, J = 16 Hz), 7.55 (3H, m), 7.30 (2H, m),
3
6
(
.52 (1H, d, J = 16 Hz), 6.20 (1H, s), 2.74 (3H, s), 2.63 (3H, s), 1.71 (6H, s), 1.53 (3H, s), 1.44
13
3H, s). C NMR(100 MHz, CDCl ): δ (ppm): 175.9, 174.7, 162.7, 155.0, 148.0, 142.6, 140.0,
3
1
1
5
33.9, 129.8, 129.6, 127.8, 124.6, 112.3, 112.2, 111.3, 111.1, 96.9, 95.6, 56.1, 26.6, 15.2, 15.0,
+
+
4.1, 13.1. HRMS (ESI): [M + Na] C H BF N NaO requires 556.3796; found [M + H]
31
26
2
5
56.3782.
Result and discussion
It has been well documented that π-extension can be proceeded by condensation between
methyl of BODIPY and aldehyde derivatives, such as anisaldehyde, p-dimethy-
laminobenzaldehyde, isonicotinaldehyde, etc, which could significantly red-shift the emis-
sion color from green to red [29–31]. Another strategy for this extension was established
by alkynide substitution at the β-position, such as phenylacetylene [32, 33]. By contrast,
the former is more effective in emission shift than the latter. However, finely adjustment
of the emission wavelength could be only realized by the latter strategy [33]. Owing to the
efficiently introduction of aldehyde to the β-position of BODIPY, various condensation with
aldehyde becomes practical and may also lead to significant influence to its optical properties.
According to the literature, only electron donor unit could be applied to condense with the
methyl of BODIPY. TCF, as a strong electron acceptor, attached to β-position of BODIPY
with a π-bridge is worthy of investigation as a supplementary strategy.
The absorption and emission behavior were investigated in various solvents. Due to the
introduction of TCF unit, it is hardly for BT to dissolve in weak polar solvents, such as hex-
ane and toluene. The solubility is poor but acceptable in dichloromethane, chloroform, THF,
acetonitrile, methanol, and dioxane, which are selected as the tested solvents. It is largely dif-
ferent from that of the parent BODIPY, which is a good solute in most organic media. The
photostability of BT inherits the characteristics of BODIPY. Long time exposure of BT solu-
tion to room light does not induce obvious decomposition, attesting its robust photostabil-
ity. The basic absorption/emission peaks, absorption coefficients, quantum yields, and Stokes

142/[416]
X. LI ET AL.
Table . Optical data of the BT in various solvents.
UV-Vis
Fluorescence
(nm)
Solvent
λ
(nm)
log εmax
λ
ꢀ
Stokes shift (nm)
τ (ns)
τ (ns)
abs
em
F
dichloromethane
chloroform
THF
Acetonitrile
Dioxane
//
//
//
/
//

.
.
.
.
.
.


a
—a
—
.
.
.a


a
.
.
.a
.
.
.
—a
—a
—a
a
—
.
—

—
.
—
Methanol

.
aThe data was not shown due to the weak emission.
shifts are collected in Table 1. Figure 1 shows the absorption curve of BT in selected organic
solvents, in which the solubility is better than non-polar solvents. All the absorption contour
were of similar shape with each other and the stronger absorption band sited from 570 to
5
90 nm with the character of double peak. The double absorption bands could be attributed
to S →S transition with the bands on the high- and low-energy side corresponding to 0-
0
1
1
and 0-0 vibrational transition, respectively. However, this feature disappeared with BT in
acetonitrile and methanol and the widths of the individual vibronic bands (0-1 and 0-0 vibra-
tional transition) are widened and merged into a single broad band. In contrast to that of
parent BODIPY (498 nm in methanol), the absorption band was shift around 70∼92 nm
toward longer wavelength, indicating a strong electron interfere between TCF and BODIPY
core [12]. According to the analysis of absorption variation in the tested solvents, there was
slightly negative solvatochromism. With the solvent polarity increasing, 20 nm blue-shift was
found. It indicates that the dipole moment of the molecule in the excited state (μ ) is expected
e
to be lower than that of the ground state (μg).
BT is emissive in organic solvents and the emission curve is shown in Figure 2a. Owing to
the strong electron-withdrawing nature of TCF, the emission intensity was significantly influ-
enced by solvent polarity. It is emissive in dichloromethane, chloroform, THF, and methanol.
However, the emission in dioxane and acetonitrile was very weak. The emission maximum
of BT in the tested solvents varied in the range of 617∼621 nm. In contrast to the 508 nm
(
[
green) emission of the parent BODIPY in methanol, around 110 nm red-shift was observed
12]. Therefore, the attachment of a strong electron-acceptor provides another strategy for
Figure . Absorption of BT ( µM) in the presence of various solvents.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
[417]/143
Figure . a) Emission spectra of BT in various solvents ( µM); b) CIE  chromaticity diagram of the emis-
sions of BT in solution.
shifting the emission of BODIPY to red region. The quantum yields were estimated and
ranged in 0.13∼0.18. Due to the emission energy was dramatically lowered and part of
the energy levels overlapped each other, leading to the non-radiation transition, thus loss
of energy. The emission color was indicated by CIE coordinates and shown in Figure 2b.
The CIE coordinate spot of BT were scattered in red region, which is in agreement of
their emission spectra in solution. The fluorescence life time were collected at their respec-
tive emission maximum in tested solvents and summarized in Table 1. All the decay data
shows a double exponential character, which indicates that there exists insufficient energy
transfer from TCF to BODIPY. Moreover, the lifetime of τ accounted to more than 85%
2
emission.
In order to obtain further insights into the nature of the geometrical, electronic, and optical
properties of BT, a comprehensive computational investigation was undertaken using Mate-
rial Studio. The first run for the ground-state energy-minimized structures were based on
DFT and LDA/DN basis set [34, 35]. Further refinement and optimization on structures were
undertaken using DND/B3LYP basis set. The calculated electrostatic potentials for BT are

144/[418]
X. LI ET AL.
Figure . Illustrations of the total electrostatic potential mapped onto the isosurface of BT. Note: the more
negative the value of the potential the more blue the color and vice versa.
Figure . Molecular orbital energy levels of BT.
shown in Figure 3. Not surprisingly, the electron density distribution on BF unit was signif-
icant lowered. Correspondingly, it is increased dramatically on the TCF unit. The size and
signs of frontier molecules orbitals are illustrated in Figure 4. Distinctive contributions of
each atomic orbital are discernible in the HOMO and LUMO diagrams, which dominated
the strong absorption band in UV-Visible region. As anticipated, the ground state HOMO
distribution was resided on both BODIPY and TCF unit with the latter outweighing a lit-
tle more of the former. Whereas, the LUMO orbital distribution vested mainly on BODIPY
moiety. There is little sign of intramolecular charge transfer between TCF and BODIPY moi-
ety. In addition, the energy gap was calculated to be 1.64 eV and corresponding to 624 nm
emission maximum in vacuum, which is in well agreement with the experimental value ∼
6
20 nm.
Conclusions
In summary, a strong electron acceptor, TCF, was attached to the β-position of BODIPY
with a π-bridge and fully characterized by NMR and MS technology. The new fluorescent
BT shows significant red shift in absorption (∼590 nm) in contrast to the parent BOD-
IPY (498 nm). Together with this absorption shift, emission also shifted and reached to
∼
620 nm. In addition, it inherited the photostability of BODIPY and exhibited excellent
resistance to room light. Due to the electron withdrawing nature of TCF, the electron
density distribution transfer to TCF part, which in agreement with the HOMO/LUMO
distribution. The energy gap also lowered to 1.64 eV in accordance to the 620 nm emission.
Further molecular configuration and application based on BT was current underway in
our lab.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
[419]/145
Funding
This work was supported by the National Natural Science Foundation of China (21272060), and
the Program for Innovative Research Team in University of Henan Province (15IRTSTHN003). This
study was supported by the Basic Science Research Program through the National Research Founda-
tion of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (Grant No. NRF-
2
014M3C1A9060739).
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