Emission behavior of perimidine attached BODIPY and its response to acid/base

Article abstract of DOI:10.1080/15421406.2017.1358016

A BODIPY based with perimidine attached derivative was developed and characterized. It was efficiently synthesized by condensation between aldehyde sited at β-position of BODIPY and 1,8-diamino-naphthalene. Owing to the electron-donor effect of N in heterocycle, emission was quenched completely. Upon protonation of N, the solution exhibited dramatic emission enhancement with emission peak locating at 510?nm, which is estimated to be 275-fold of initial emission. With OH^? addition, it could be reverted to the emission to non-emisison state again and establish a fluorescence switch. The underlying mechanism of fluorescence “off-on” is based on the intramolecular PET, which was attested by quantum chemistry calculation approach.

Full text of DOI:10.1080/15421406.2017.1358016

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Emission behavior of perimidine attached BODIPY
and its response to acid/base
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) Emission
behavior of perimidine attached BODIPY and its response to acid/base, Molecular Crystals and
Liquid Crystals, 654:1, 131-138, DOI: 10.1080/15421406.2017.1358016
To link to this article: https://doi.org/10.1080/15421406.2017.1358016
Published online: 22 Nov 2017.
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Date: 23 December 2017, At: 07:44

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –
https://doi.org/./..
Emission behavior of perimidine attached BODIPY and its
response to acid/base
Xiaochuan Lia, SaiSai Suna, Ick Jin Kimb, and Young-A Sonb
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,
Xinxiang, Henan, P. R. China; bBK, Department of Advanced Organic Materials Engineering, Chungnam
National University, Daejeon, South Korea
ABSTRACT
KEYWORDS
BODIPY; emission
enhancement; fluorescence
“off-on”; perimidine; PET
A BODIPY based with perimidine attached derivative was developed
and characterized. It was efficiently synthesized by condensation
between aldehyde sited at β-position of BODIPY and 1,8-diamino-
naphthalene. Owing to the electron-donor effect of N in heterocycle,
emission was quenched completely. Upon protonation of N, the
solution exhibited dramatic emission enhancement with emission peak
locating at 510 nm, which is estimated to be 275-fold of initial emission.
With OH− addition, it could be reverted to the emission to non-emisison
state again and establish a fluorescence switch. The underlying mecha-
nism of fluorescence “off-on”is based on the intramolecular PET, which
was attested by quantum chemistry calculation approach.
Introduction
Technology development is closely related with all aspects of our life. Various new materials
have been conquest and step into military and civilian service. However, the step toward
the understanding of our world will never stop which gives further demanding on smart
materials. Among various new types of materials, emissive molecules fascinate the physicists
and chemists. Intensive effort and development have been made and the spectra of fluorescent
molecules across the whole visible region from blue to deep-red. Up to now, typically emis-
sive molecules, such as coumarins, naphthalimide, bisindolylmalimide, perylenediimides,
and rhodamine, have been systematically engineered toward various purposes [1–11].
A significant application is used as signal translation and logic input/output. Therefore,
robust fluorescent dyes are desired for chemists, physicists and biochemists. Fluorine-boron
(F-B) complexes are one of the dyes that interested scientists over the past decade [12–15].
4,4ꢀ-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) is a typical example, 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 photostability. Great efforts also have been made to engineer according to requirement by
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

132/[406]
X. LI ET AL.
chemical modification of the BODIPY core unit involving alkynide substitution, N-insertion,
π-extension, etc [16–19]. Moreover, the F-B family has been enriched by exploring varied
chelating modes between heteroatoms and boron, including ditopic (N∼N, N∼C, C∼C,
C∼O, O∼O) and tritopic toward borate, which has been well documented [20–27].
Based on the framework of BODIPY, aldehyde could be efficiently introduced and cou-
pled with amine. In this contribution, a larger π-conjugate system, 1,8-diaminonaphthalene,
is coupled with BODIPY and form a heterocycle between BODIPY and naphthalene. The
electron rightful nitrogen atoms could be an unstable electron bridge and the photo-physical
properties could be modulated by providing external stimuli to this bridge. A detailed
response of the target molecule toward acid is presented in the text.
Experimental
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. (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).
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 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. 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
grade and were purified by distillation. The stock solution of compounds (2 × 10−3 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-1H-perimidine-BODIPY (PB), are outlined in
Scheme 1. The aldehyde was introduced to the framework of BODIPY by standard Vilsmeier

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
[407]/133
Scheme . Synthesis of PB.
condition [28]. Subsequently, cyclization and dehydration reaction proceeded between
β-formyl-BODIPY and 1,8-diaminonaphthalene in the presence of ethanol, yielding PB.
β-1H-perimidine-BODIPY (PB)
β-formyl-BODIPY (100 mg, 0.28 mmol) and 1,8-diaminonaphthalene (75 mg, 0.34 mmol)
were dissolved in anhydrous ethanol (25 mL) under nitrogen atmosphere. The mixture was
stirred in the dark overnight. After the starting materials were well reacted, as confirmed by
TLC, the mixture was cooled to room temperature. Next, the solvent evaporated in vacuum
and the residue was loaded to the column. Column separation (silica gel 200–300 mesh, elu-
ent: hexane/dichloromethane = 1:2) produced 35 mg brown liquid (25%).
1H NMR(400 MHz, CDCl3): δ (ppm): 12.85 (1H, s), 8.23 (1H, s), 7.53 (1H, d, J = 8.0 Hz),
7.42 (3H, m), 7.39 (1H, s), 7.30 (3H, t, J = 8.0 Hz), 7.15 (1H, b), 7.09 (1H, b), 6.07 (1H, s),
2.86 (3H, s), 2.60 (3H, s), 1.61 (3H, s), 1.35 (3H, s). HRMS (ESI): [M+Na]+ C30H25BF2N4Na
requires 513.3546; found [M+H]+ 513.3580. 13C NMR data was not obtained due to the poor
solutility in CDCl3.
Result and discussion
Since the aldehyde could be introduced to the β-position of BODIPY, there is an alternative
strategy in modification of it. Actually, the formylation at the β-position could be effective
proceed via Vilsmeier-Hacck reaction, which significantly open a new door toward versatile
functionalization approaches. The BODIPY core is an electron-deficient system due to
the combination of N-B-N. Once an aldehyde was introduced and condensed with ortho-
diamine compounds, nitrogen-carrying heterocyclic compound could be obtained. Due to
the electron donating nature of nigrogen, an electron rich atom, the emission of BODIPY
may be quenched by efficient photo induced transfer (PET). Regulation the donating nature
of nigrogen could be an effective way of modification the emission behavior of BODIPY.
The absorption and emission behavior were investigated in various solvents, including
polar and nonpolar solvents. It was found that PB exhibit poor but acceptable solubility
in most organic media, which is obviously lower that its parent structure. However, its
photostability of PB inherits the characteristics BODIPY. Long time exposure to the room
light does not induce obvious decomposition, attesting its robust photostability. Figure 1
shows the absorption spectra of PB in various solvents. Obviously, the strongest absorption
band locates around 505 nm with the peak center ranging from 501 to 507 nm. In contrast
to the parent BODIPY (498 nm), only several nanometers shift toward longer wavelength. In
addition, the absorption contour is similar to that of parent BODIPY. The intense absorption
around 505 nm could be attributed to S0→S1 transition and a shoulder peak around 474 nm,
ascribed to 0–1 vibration transition. Due to the substitution of perimidine, a relatively weak
absorption band was observed around 330 nm, corresponding to S0→S2 transition. In all

134/[408]
X. LI ET AL.
Figure . Absorption of PB ( µM) in the presence of various solvents.
the tested solvents, the absorption of PB exhibits little solvent dependence, indicating its
insensitivity to external environment.
However, its emission behavior is largely different from that of parent BODIPY. Owing to
the stronger electron donating ability of nitrogen, the emission intensity is very weak in all
the tested solvents. Therefore, any influence on the electron donation of N may be lead to
the change of emission behavior. Thus, an acid titration investigation was given as following
description. Trifluoroacetic acid (TFA) was chosen as proton source to realize protonation of
N atom. Figure 2 shows absorption change of PB in acetonitrile upon addition of TFA. The
absorption around 501 nm decreased gradually with the protonation of N atom, forming two
clear isosbestic points 492 nm (ε = 6.9 × 104 M−1 cm−1) and 513 nm (ε = 3.6 × 104 M−1
cm−1). It indicates that a two components transferring occurred in the above solution. Cor-
respondingly, the non-emission character turns to highly emissive. Figure 3 shows emission
increasing with the addition of TFA. Before acid addition, very weak emission of PB around
Figure . Absorption change of PB in CHCN upon addition of TFA (, , , , , , , , , , , , 
eq).

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
[409]/135
Figure . Emission change of PB in CHCN upon addition of TFA (, , , , , , , , , , , ,  eq).
527 nm was ignited and shifted to 510 nm with the continuous addition of TFA. The solu-
tion color was turned from light brown to bright green, approaching to the emission wave-
length of parent BODIPY (498 nm) [12]. The fluorescent “off-on” ration was estimated to be
275, which is much higher than that of the structural related compound, benzimideazole sub-
stituted BODIPY derivative [29]. Conversely, the protonation of nitrogen could be restored
to original state by OH− addition. The highly emissive state was quenched to non-emissive
state. Before and after the addition of TFA to PB in CH3CN, the quantum yields were esti-
mated to be 0.02 and 0.46, respectively, with reference to fluorescein (ꢀ = 0.85, 0.1N NaOH)
as a standard. It is in agree with the emission “off-on” behavior of PB in CH3CN induced by
TFA.
The fluorescence “off-on” mechanism is based on the nitrogen atom triggered intramolecu-
lar PET process, configuring an “off-on” system as shown in Figure 4. Without the protonation
of nitrogen, there will be efficient PET from the electron-rich nigrogen to the BODIPY core,
which may lead to the fluorescence quenching significantly. Once a proton was harbored by
nitrogen atom, the electron-donating ability will be weakened significantly. And therefore,
the PET process was blocked and the fluorescence of BODIPY was ignited. With alternative
addition of TFA and OH−, a fluorescence “off-on” switch could be established.
To shed more light on the geometrical, electronic, and optical properties of PB, we
undertook a comprehensive computational investigation using Material Studio. First, the
ground-state geometries were optimized with DFT and LDA/DN basis set [30, 31] to reduce
the run times. Further refinement and optimization on structures were undertaken using
Figure . PET based fluorescence “off-on”mechanism.

136/[410]
X. LI ET AL.
Figure . Illustrations of the total electrostatic potential mapped onto the isosurface of PB. Note: the more
negative the value of the potential the more blue the color and vice versa.
DND/B3LYP basis set. The calculated electrostatic potentials for PB are shown in Figure 5.
Not surprisingly, once a proton was harbored by nitrogen atom, the electron density was sig-
nificantly decreased.
The size and signs of frontier molecules orbitals are illustrated in Figure 6. Distinctive con-
tributions of each atomic orbital are discernible in the HOMO and LUMO diagrams, which
dominated the strong absorption band in UV-Visible region. HOMO distribution of PB was
localized on perimidine unit completely, while LUMO was mainly contributed by the BOD-
IPY unit. However, once the nigrogen atom protonated, both HOMO and LUMO of PB were
mainly donated by the orbital from the BODIPY unit. Obviously, there exists an efficient
photo-induced electron transfer between BODIPY and perimidine without protonation of
nitrogen atom. Conversely, this transfer was blocked with the electron density decreasing. The
energy gap between HOMO and LUMO can also be estimated by calculation. Before proto-
nation, the energy gap was calculated to be 0.47 eV, which is lower than that of protonated
Figure . Molecular orbital energy levels of PB in neutral and protonated form.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
[411]/137
product (0.96 eV). The increase of energy gap was in well accordance to the slightly blue shift
of emission.
Conclusions
In summary, a newly designed BODIPY derivative was configured by attaching perimidine
unit, which are isolated by a heterocycle. Upon TFA addition, the N atom belonging to hete-
rocycle was protonated and the emission was dramatically increased to 275-fold of the initial
state. OH− addition can quench the emission again. The emission of PB was dominated by the
intramolecular PET. It was further support by the quantum chemistry calculation approach.
And on top of that, the perimidine substituent demonstrates more efficient PET than benz-
imidazole substituent.
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 Pro-gram 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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