6-Hydroxyindole-based borondipyrromethene: Synthesis and spectroscopic studies

Article abstract of DOI:10.1039/c1ob06200j

A 6-hydroxyindole-based BODIPY, named BODIPY-OH, with distinct spectroscopic characteristics is reported. Through a systematic study of the spectroscopic characteristics of BODIPY-OH and BODIPY-O^- in various solvents containing an organic base,

Full text of DOI:10.1039/c1ob06200j

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PAPER
6-Hydroxyindole-based borondipyrromethene: Synthesis and spectroscopic
studies†
Chunchang Zhao,* Peng Feng, Jian Cao, Yulin Zhang, Xuzhe Wang, Yang Yang, Yanfen Zhang and
Jinxin Zhang
Received 19th July 2011, Accepted 29th September 2011
DOI: 10.1039/c1ob06200j
A 6-hydroxyindole-based BODIPY, named BODIPY–OH, with distinct spectroscopic characteristics is
reported. Through a systematic study of the spectroscopic characteristics of BODIPY–OH and
BODIPY–O- in various solvents containing an organic base, we found that the light-color of the
fluorophore can be tuned over a wide range by changing the polarity of solvent/base combinations.
The absorption color of the solution can be tuned over a range of 100 nm and the emission color within
a wide range from 571 to 681 nm by simply converting the phenol form of BODIPY–OH to the
phenolate form. Fluorescence of BODIPY–O- with high quantum yield shows relatively large Stokes
shift in solvent/base combinations, which are ascribed to the excited state deprotonation from
(BODIPY–OH)* to (BODIPY–O-)*, followed by emission from the ion form.
biochemical production of excited states from ground-state re-
Introduction
actants. Fireflies and their analogous beetles, the most studied
Borondipyrromethene (BODIPY) fluorescent dyes are of high
bioluminescent reaction systems, generate light over a wide range
of colors from green to red (emission maxima l = 530–640 nm).5–7
interest as fluorophores due to their valuable photophysical prop-
erties, such as high photo- and chemostability, high fluorescence
To explain the multicolor bioluminescence of fireflies and their
quantum yields, relatively high absorption coefficients, and sharp
analogous beetles as well as the biochemical origin of the light
absorption and fluorescence emission spectra.1 BODIPYs are well
emission, a vast amount of experimental and model studies have
been developed.8–9 All these studies demonstrate that the phenol–
phenolate equilibrium of oxyluciferin, a common bioluminescence
center, plays a critical role in varying the emission color; the
emission color can be tuned within a wide spectral range from
541 nm to 640 nm by varying only the solvent polarity and
the nature of interaction between the phenolate ion and the
countercation.10–11 Motivated by these observations, the research
discussed here explores an approach that BODIPY–OH with
straightforward phenol/phenolate interconversion is judiciously
designed and synthesized by the fusion of 6-hydroxyindole moiety
to the parent structure of BODIPYs to extend the p-conjugation.
Note examples of BODIPYs bearing phenolic subunits at position
3, 5, or 8 are found in the literature.2a,3c,12 However, all show depro-
tonation dependent fluorescence due to charge transfer from the
phenolate to the excited-state BODIPY moiety. In sharp contrast,
the BODIPY–OH designed here displays a spectral shift and a
fine-tuning of the spectra wavelength achieved simply by con-
trolling phenol/phenolate interconversion of the chromophore.
More importantly, both BODIPY–OH and BODIPY–O- (de-
protonation state) display emissions with high quantum yield
in solvents. In addition, the fluorescence spectra show relatively
large Stokes shifts in solvents in the presence of organic base, ob-
tained by the excited state proton transfer, with (BODIPY–OH)*
conversion to (BODIPY–O-)*, followed by emission from the
ion.
known to show strong emission of wavelengths over 500 nm,
their fluorescence is unaffected by solvent polarity and pH,
and their fluorescence emission spectra display a slight Stokes
shift.2 However, to finely tune the wavelength of BODIPYs
over a wide range including the NIR region, chemical modi-
fication approaches should be attempted and the synthesis is
usually tedious and non-trivial.3 Moreover, a small Stokes shift
can cause self-quenching and measurement error by excitation
interference, which can decrease the detection sensitivity to a
great extent.4 Therefore, dyes with a larger Stokes shift are very
promising for fluorescence bioassays. To date, the strategy to
design BODIPYs with a relatively large Stokes shift is rarely
investigated.
Herein, we report a carefully designed BODIPY named
BODIPY–OH with straightforward phenol/phenolate intercon-
version to overcome these deficiencies. The basis of employing
BODIPY–OH to achieve tunable light-color and relatively large
Stokes shift characteristic comes from bioluminescence, a natural
phenomenon, which converts chemical energy into light through
Key Laboratory for Advanced Materials and Institute of Fine Chemicals,
East China University of Science & Technology, Shanghai, 200237, P. R.
China. E-mail: zhaocchang@ecust.edu.cn
† Electronic supplementary information (ESI) available: A detailed de-
scription of absorption and emission spectra, HRMS spectra and NMR
spectra. See DOI: 10.1039/c1ob06200j
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The maximum emission wavelength of BODIPY–OH is in 571–
583 nm range, while that of BODIPY–OMe centered between 567
and 573 nm.
Results and discussion
Synthesis
In DMSO, however, a red-shifted absorption band of phenolate
anion BODIPY–O- (see discussion below) at 644 nm accompanies
the original neutral one at 558 nm. A shoulder centered at
approximately 606 nm can also be observed, which is assigned
to the 0–1 vibrational band of S0–S1 transition of BODIPY–
O-. Similar, the phenol group of BODIPY–OH can be partially
deprotonated from the excited state upon excitation at 558 nm,
therefore two emission bands are generated at ca. 583 nm and
680 nm. The shorter emission band comes from the neutral phenol
form, while the longer one can be assigned as the emission of
BODIPY–O- with a large Stokes shift ~120 nm. Upon excitation
at 644 nm into the absorption band of BODIPY–O-, only
one emission from BODIPY–O- at 680 nm is observed. These
experimental observations imply that a spectral red shift could be
achieved by simply controlling the conversion of BODIPY–OH
from phenol form to phenolate form.
BODIPY–OH was obtained easily by routine BODIPY
formation, using
a
three-step procedure via condensa-
tion of 2,4-dimethyl-3-ethylpyrrole with 2-benzoyl-3-methyl-6-
methoxyindole followed by removal of the methoxy protecting
groups (BBr3, CH2Cl2) and boron insertion with BF3·OEt2
(Scheme 1). The overall yield is 53% for three steps. The methyl
ether derivative BODIPY–OMe was also obtained in a one-
pot, two-step procedure via condensation of 2,4-dimethyl-3-
ethylpyrrole with 2-benzoyl-3-methyl-6-methoxyindole followed
by treatment with BF3·OEt2 (Scheme 1). The structure was fully
characterized by 1H NMR, 13C NMR, and HRMS analysis.
The deprotonation process of BODIPY–OH can be observed
in UV-vis and fluorescence titration experiments with varying pH
(Fig. 1). Increasing pH value from acidic to basic condition, a
decrease in the absorption band at 541 nm and a concomitant
increase of a new band at 612 nm were observed, with a distinct
isosbestic point at 559 nm. In the same way as the absorption
spectra, when BODIPY–OH was excited at the wavelength of the
absorption isosbestic point, the fluorescence spectra of BODIPY–
OH exhibited phenol/phenolate dependant blue-red switching,
as shown in Fig. 1. Increasing the pH value from 7.8 to 10, the
spectrum showed a decrease of the emission band at 575 nm and a
simultaneous increase of the emission band at 644 nm. The likely
characteristic isoemission point at 621 nm makes BODIPY–OH
an excellent ratiometric pH indicator.
The Ka values of BODIPY–OH were determined by fluorimet-
ric titration as a function of pH using the fluorescence emission
spectra. Nonlinear curve fitting of eqn (1) to the fluorescence data
F recorded as a function of pH yielded values of Ka, where Fmin
and Fmax are the fluorescence intensity at minimal and maximal
[H+], respectively, and n is the number of H+ dissociating from one
BODIPY–OH dye. In a DMSO-H2O mixture (1 : 1), the pKa value
is determined to be 9.5.
Scheme 1 Synthesis of BODIPY–OH and BODIPY–OMe.
Spectroscopic Properties of BODIPY–OH and BODIPY–OMe
Spectroscopic evaluation of BODIPY–OH and its methyl ether
derivative BODIPY–OMe was performed in several solvents of
varying polarity (Table 1, Figure S1, Figure S2, Table S1 and S2 in
supporting information). The optical features are characteristics
of the BODIPY platform. As is evident from Table 1, Figure S1,
S2 and Table S1, S2, the main absorption band of BODIPY–OH,
attributed to the 0-0 vibrational band of a strong S0–S1 transition,
is centered between 545 and 563 nm in the pure solvents, while that
of BODIPY–OMe centered between 544 and 555 nm. The S0–S1
absorption band shows minor solvent-dependent variation and
the maximum shifts of the absorbance are only 18 nm and 11 nm
for BODIPY–OH and BODIPY–OMe respectively, indicating a
weak charge transfer character for the lowest electronic excitation
in the ground-state geometry. Similar to the absorption spectra,
their emission spectra also show minor solvent-dependent shift.
Fmax[H+ ]n + FminKa
(1)
F =
[H+ ]n + Ka
Table 1 Spectroscopic data for BODIPY–OH and BODIPY–O- in various solvents in the absence or presence of organic base (DBU)
Solvent
ET(N)
labs (nm)
lem (nm)[UF]
–OH
–OH
546
548
–OMe
551
544
–O-(–OH+DBN)
629[0.18]
642
[0.22]
664
–OMe
567
Water
Methanol
1.000
0.762
577
[0.25]
571
Acetonitrile
DMSO
0.460
0.444
0.309
0.099
542
540
546
546
555
567
[0.22]
583, 680
[0.34]
577
[0.30]
586
[0.17]
675
[0.18]
656
[0.24]
655
558
644
545
574
Dichloromethane
Toluene
571
563
573
[0.40]
[0.24]
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Fig. 1 Absorption spectra (top) of BODIPY–OH in buffer solution as a
function of pH. Corresponding emission spectra (bottom) (lex = 559 nm).
pH was adjusted by NaH2PO4 and Na2HPO4.
Fig. 2 Normalized absorption (top) and fluorescence emission (bottom)
spectra of BODIPY–OH in solutions with varying solvent/base combina-
tions. Base: 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU).
The acidity constant in the excited state, pKa*, can be obtained
from the “Fo¨rster cycle”13 according to
1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) are shown in Fig. 3–4
and Fig. S3–S6†.
EOH −E
−
O
(2)
pK − pK*=
In the presence of the weaker base BA, two kinds of spectra
characteristics can be inspected. In nearly nonpolar solvents,
such as benzene, addition of BA results in the appearance of
a small red-shifted (9 nm) absorption band (Fig. 3 and Fig.
S3†), indicating the weak hydrogen-bonded interaction between
BODIPY–OH and BA. Accordingly, the emission spectrum
exhibits weak solvatochromism, and the emission band comes
from (BODIPY–OH)* excited state with 10 nm bathochromic
shift. Inspection of the spectra characteristics in polar solvents
shows that BODIPY–OH exists as mixtures with varying ratios
of the phenol and phenolate, the exception is in water and DMSO
in the presence of BA (Fig. S5†); in water and DMSO, only
BODIPY–O- is generated. Fluorescence of BODIPY–O- together
with BODIPY–OH is observed upon excitation of BODIPY–OH
(Fig. 4). Excitation of the phenol form results in partial conversion
of (BODIPY–OH)* to (BODIPY–O-)*, followed by emission
from the ion, thus a relatively large Stokes shift (~70 nm in protic
solvents and ~90 nm in CH3CN and DMF) and a secondary, weak
emission is observed.
2.303RT
where EOH and EO- are the energies of the electronic transitions
for BODIPY–OH and BODIPY–O-, determined as the 0-0
transition energies. These can be estimated by determining the
wave number corresponding to the intersection point of the
normalized absorption and emission spectra. The pKa* thus
obtained is 5.3 in a DMSO-H2O mixture (1 : 1).
Spectroscopic properties of deprotonated BODIPY–OH
The optical properties of BODIPY–OH in base/solvent systems
(Fig. 2) indicate a significant spectral red-shift by varying only the
solvent polarity and base/solvent combinations from acetonitrile
to CH3CN/DBU (1,8-diazabicyclo[5.4.0]undec-7-ene) combina-
tion, emission spectral maxima range from 571 nm to 681 nm,
while the absorption maxima range from 542 nm to 645 nm.
To elucidate the light-color modulation of BODIPY–OH by
varying solvent/base combinations, measurements of the spec-
troscopic properties of the phenol/phenolate equilibrium were
carried out in organic solvents using organic bases for deprotona-
tion. The spectra of BODIPY–OH in different solvents containing
butylamine (BA), 1,5-diazabicyclo[4.3.0]non-5-ene (DBN), and
In contrast, the absorption profiles of BODIPY–OH on ad-
dition of stronger bases DBN or DBU change drastically, and
the absorption band of the neutral phenol form is replaced by
that of a hydrogen-bonded complex (HBC) of BODIPY–OH with
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Fig. 3 UV-vis absorption (top) and fluorescence spectra (bottom) of
BODIPY–OH (5.0 ¥ 10-6 M) in benzene containing no and organic bases
(0.010 M).
Fig. 4 Fluorescence spectra of BODIPY–OH (5.0 ¥ 10-6 M) (top) in
solutions containing BA base recorded at the first excitation wavelength.
(lex = 556 nm for DMF, 542 nm for acetonitrile, 554 nm for EtOH and
553 nm for MeOH). In the presence of DBN (0.010 M) in various solvents
(bottom).
the bases and/or that of BODIPY–O- with the conjugate acids
of organic bases (i.e., H-DBN+ and H-DBU+) which behave as
countercations for BODIPY–O-. In benzene or toluene, upon
addition of either of DBN and DBU, a red-shifted absorption
band of phenolate ion is observed accompanied by the absorption
band of HBC of BODIPY–OH with the bases (Fig. 3). In the two
cases, selective excitation of either BODIPY–OH in an HBC or
BODIPY–O- results in emission exclusively from (BODIPY–O-)*
at lem ~656 nm. The fluorescence maximum of BODIPY–O- from
HBC shows a relatively large Stokes shift (~80 nm). This result
indicates that (BODIPY–OH)* in an HBC gives the excited state of
(BODIPY–O-)* by proton transfer, a well-known mechanism for
fluorescence of a phenol derivative making an HBC with organic
bases in a less polar solvents.14 Although the Stokes shift is not
greater than 100 nm, it is still a significant improvement when
compared to a typical Stokes shift of BODIPYs, which is always
2 ~ 3 times smaller than that reported here. In solvents that are
more polar than dichloromethane (ET (N) = 0.309), BODIPY–OH
is completely ionized at the ground state, and the phenolate form
exists exclusively. As a consequence, the fluorescence spectrum
of BODIPY–O-, generated by direct excitation of the phenolate
form, shows a small Stokes shift like that of typical BODIPYs.
It is worth noting that both BODIPY–OH and BODIPY–O-
display emission with high quantum yield in different solvents
(Table 1 and Table S2†). In particular, the quantum yield of
the anion form is in the range 0.17 ~ 0.24, which is much
larger compared to that of reported BODIPYs bearing phenolic
subunits. The reported examples usually have very low quantum
yields.
Fig. 4 shows the solvent-dependent emissions of BODIPY–O-.
The lem values range from 629 ~ 681 nm, and are modulated only
by the polarity of the enviroment, while the emission maximum
is bathochromic shifted by 3 ~ 5 nm from solutions containing
DBN to DBU. The variation of lem values for BODIPY–OH in
studied solvents containing DBN is evaluated by correlating the
emission maxima (lem) with the normalized Dimroth–Reichardt’s
solvent parameter ET(N) (Fig. 5).15 The plot shows two distinct
regions, one is in the ET(N) range from 0.099 to 0.444 for less
polar solvents and the other in the ET(N) range from 0.444 to
1.000 for polar solvents. In the region for less polar solvents, the
emission maxima shows positive solvatochromism (increasing lem
with ET(N)), and the lowest energy emission is from BODIPY–
O- in DMSO. With increasing polarity of solvents into the polar
region, the emission maxima shows negative solvatochromism. It
is worth noting that the plot is a straight line with respect to ET(N)
in protic solvents, represented by the equation lem = -63.5 ET(N) +
691.6 (r = 0.996). The negative slope and the longest emission
wavelength of BODIPY–O- in DMSO indicate that increasing
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Conclusions
In this paper, we have described the synthesis and systemic
study of the spectroscopic characteristics of BODIPY–OH and
BODIPY–O- in various solvents containing an organic base.
The straightforward phenol/phenolate interconversion induces
dramatic changes in absorption and emission spectra. The spectra
of BODIPY–O- vary depending on the environment polarity with
the wavelength finely tunable over a wide range. lem values of
BODIPY–O- can be modulated in the range 629 to 681 nm by
the polarity of solvents, while lem values of BODIPY–OH are
in the range 571 to 586 nm. These results demonstrate that the
color of the solution can be tuned, over a range of 110 nm, by
converting the phenol form of BODIPY–OH to the phenolate
form, which is caused by changing the environmental polarity
in the vicinity of BODIPY–OH. Furthermore, the fluorescence
of BODIPY–O- with high quantum yield shows relatively large
Stokes shift in solvent/base combinations, accomplished by
excited state deprotonation from (BODIPY–OH)* to (BODIPY–
O-)*, suggesting it is more suitable for use as a fluorescent probe
in bioassays.
Experimental
General methods
All chemical reagents and solvents for synthesis were purchased
from commercial suppliers and were used without further purifi-
cation. All moisture-sensitive reactions were carried out under
1
an atmosphere of argon. H NMR and 13C NMR spectra were
recorded on a Bruker AV-400 spectrometer with chemical shifts
reported in ppm (in CDCl3, TMS as internal standard) at room
temperature. Mass spectra were measured on a HP 1100 LC-MS
spectrometer.
Fig. 5 Emission maxima (top) (lem) plotted as function of ET(N). Inset:
the linear correlation of lem with ET(N) in protic solvents. Lippert–Mataga
plot (bottom) of the solvent effects on the spectra of BODIPY–OH in the
absence and presence of base.
Synthesis
BODIPY–OH. To
a
solution of 2-benzoyl-3-methyl-6-
polarity is not favorable for tuning the color to the red region, and
the optimum condition for achieving the strongest red shift of the
emission is in DMSO with bases.
To get an insight into the solvatochromic behaviors of enol
and enolate forms, the Stokes shift as a function of orientational
polarizability (Df ) was studied using the Lippert–Mataga model.
methoxyindole (1 g, 3.76 mmol) in CH2Cl2 (60 mL) was
added POCl3 (1.05 mL, 11.28 mmol) at 0 ◦C, the resulting
solution was stirred for a further 5 min, followed by addition of
2,4-dimethyl-3-ethylpyrrole. The mixture was warmed to room
temperature and stirred for 3 days, cooled to 0 ◦C, neutralized
with saturated Na2CO3, and washed with H2O. The organic phase
was dried with Na2SO4, and the solvent was removed to obtain
a dark oil. To the resulting oil in anhydrous CH2Cl2 was added
BBr3 (3.48 mL, 37.6 mmol) at -15 ◦C. The reaction mixture
was stirred for a further 1 h at -15 ◦C, then warmed to room
temperature, quenched with H2O, extracted with CH2Cl2, and
washed with H2O. The combined organic extracts were dried
with Na2SO4, and Na2SO4 was removed by filtration. Then Et3N
(5.6 mL) was added to the solution at room temperature, and the
2Df
DV =
(me − mg )2 + constant
(3)
4pe0hca3
where
e −1
n2 −1
(4)
Df = (
−
)
2e −1 2n2 +1
In the equations, a represents the radius of the cavity in which
the fluorophore resides. mg and me denote the dipole moments in the
ground and excited states, respectively. Deviation from the linear-
ity is usually an indication of the presence of a specific interaction
other than a simple dipole–dipole interaction. In our system, an
analysis of the Stokes shifts with Lippert–Mataga equation gives
strong deviation from linear correlation, indicating significant
effects of specific interactions and, particularly, hydrogen bonding
on the energy of BODIPY–OH* (Fig. 5).16
◦
resulting mixture was stirred for 5 min, cooled to 0 C, followed
by addition of BF3·OEt2, and stirred for another 30 min. The
reaction mixture was extracted with CH2Cl2, washed with H2O,
dried over Na2SO4, and the solvent was removed in vacuo. The
crude product was purified by flash chromatography (silica gel,
eluent: CH2Cl2 then EtOAc/CH2Cl2 1 : 10) to obtain 800 mg
1
(53%) BODIPY–OH: H NMR (400 MHz, CDCl3) d 1.03 (t,
3H), 1.40 (s, 3H), 1.60 (s, 3H), 2.35–2.41 (q, 2H), 2.69 (s, 3H),
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6.61–6.65 (dd, 1H), 7.1 (m, 1H), 7.34–7.37 (m, 3H), 7.54–7.56 (m,
3H); 13C NMR (100 MHz, CDCl3) d 161.8, 157.7, 146.5, 141.6,
141.5, 136.8, 135.3, 133.3, 132.7, 129.3, 129.1, 128.3, 127.0, 122.7,
112.4, 98.7, 17.2, 14.2, 13.4, 12.2, 11.3; HRMS (ESI) Calcd for
C24H22BF2N2O: 403.1793. Found: 403.1825. [M - H]-.
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methoxyindole (1 g, 3.76 mmol) in CH2Cl2 (60 mL) was added
POCl3 (1.05 mL, 11.28 mmol) at 0 ◦C, the resulting solution was
stirred for a further 5 min, and was followed by the addition of
2,4-dimethyl-3-ethylpyrrole. The resulting mixture was warmed
to room temperature and stirred for 3 days, cooled to 0 ◦C,
neutralized with saturated Na2CO3, and washed with H2O. The
organic phase was dried with Na2SO4, and the solvent was removed
to obtain a dark oil. To the resulting oil in anhydrous CH2Cl2 was
added Et3N (5.6 mL) at room temperature, and the mixture was
stirred for 5 min, cooled to 0 ◦C, followed by addition of BF3·OEt2,
and stirred for another 30 min. The reaction mixture was extracted
with CH2Cl2, washed with H2O, dried over Na2SO4, and the
solvent was removed in vacuo. The crude product was purified
by flash chromatography (silica gel, eluent: hexane/CH2Cl2 1 : 1)
1
to obtain 1.03 g (66%) BODIPY–OMe: H NMR (400 MHz,
CDCl3) d 1.02 (t, 3H), 1.38 (s, 3H), 1.57 (s, 3H), 2.39–2.33 (q, 2H),
2.67 (s, 3H), 3.94 (s, 3H), 6.65–6.61 (dd, 1H), 7.11 (d, 1H), 7.30 (d,
1H), 7.35–7.32 (m, 2H), 7.64–7.52 (m, 3H); 13C NMR (100 MHz,
CDCl3) d 162.1, 161.0, 147.1, 141.6, 141.3, 136.5, 135.4, 133.2,
132.9, 129.3, 129.1, 128.3, 128.1, 126.8, 122.2, 113.9, 94.9, 55.6,
17.2, 14.3, 13.3, 12.1, 11.3; HRMS (ESI) Calcd for C25H24BF2N2O:
417.1950. Found: 417.1938. [M - H]-.
Acknowledgements
We gratefully acknowledge the financial support of the National
Science Foundation of China (Grant no.: 20902021) and the
Scientific Research Foundation for Returned Overseas Chinese
Scholars (State Education Ministry).
13 T. Z. Fo¨rster, Elektrochem. Ber. Bunsen-Ges. physik. Chem., 1950, 54,
42.
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272 | Org. Biomol. Chem., 2012, 10, 267–272
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