Fluorescence from bisaryl-substituted maleimide derivatives

Article abstract of DOI:10.1039/c4pp00001c

A series of bisaryl-substituted fluorescent maleimides was synthesized via the Heck arylation. The compounds showed broad fluorescence emission bands in the visible region, a large Stokes shift in polar solvents and emission quantum yields varying from 0.

Full text of DOI:10.1039/c4pp00001c

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Fluorescence from bisaryl-substituted maleimide
derivatives†
Cite this: Photochem. Photobiol. Sci.,
2014, 13, 859
Milena Helmer Lauer,a Roberta Lopes Drekener,b Carlos Roque Duarte Correiab and
Marcelo Henrique Gehlen*a
A series of bisaryl-substituted fluorescent maleimides was synthesized via the Heck arylation. The
compounds showed broad fluorescence emission bands in the visible region, a large Stokes shift in polar
solvents and emission quantum yields varying from 0.04 to 0.71, depending on the structure and solvent
medium. The difference in dipole moments of ground and excited electronic states of about 12 Debye is
ascribed to a substantial charge shift and push–pull character of bisaryl-substituted maleimides. The
fluorescence decays of N-benzyl-3,4-bis(4-methoxyphenyl)-1H-pyrrole-2,5-dione (compound 5a) are
biexponential with short (1.3–7.6 ns) and long lived (11.5–13.6 ns) components in polar solvents, but in
1,4-dioxane and THF the decays become single exponential. On the other hand, N-benzyl-3-(4-methoxy-
phenyl)-4-(4-hydroxyphenyl)-1H-pyrrole-2,5-dione (compound 5b) exhibited a biexponential decay in
DMSO and in DMF with much shorter decay components, and such behavior indicated a charge shift
process combined with solvent assisted proton transfer in the excited state.
Received 1st January 2014,
Accepted 29th March 2014
DOI: 10.1039/c4pp00001c
www.rsc.org/pps
electron-donating and electron-accepting groups linked by a
1 Introduction
π-conjugated bridge.12,13 Upon electronic excitation an
Natural products with intrinsic or derived fluorescence have increase in charge separation may occur, and excited state
been used as molecular probes to study biological processes properties become dependent on the solvent polarity.14,15
like intracellular trafficking, membrane association and auto-
In this report, photophysical properties of bisarylated mal-
toxicity.1,2 For this purpose, some strategies such as synthesis eimides are determined by stationary and time resolved emis-
of fluorescent analogs have been successfully employed.1–3 sion spectroscopy. The compounds (see structures 5a–d in
Thus, the evaluation of the photochemical stability, quantum Fig. 1) were synthesized via the Heck arylation.16 Furthermore,
yield, and fluorescence lifetime of new fluorophores based on to investigate the effect of solvent polarity on energy levels and
natural products is an important issue for future applications to determine the difference in dipole moments, the Lippert–
as molecular probes.
Mataga approach was employed.17,18 Theoretical calculations
A class of compounds used for optical sensing is the using density functional theory (DFT) methods at the B3LYP/
aromatic imides. The photophysical properties of monoimides 6-311G++(2d,2p) level were used to evaluate the dipole moment
and polyimides are well-documented.4,5 However, the fluore- of the ground state of the compounds.
scent properties of bisaryl-substituted maleimide derivatives
have not been fully described, although their natural precur-
sors, polycitrin A and B, isolated from the marine ascidian Poly-
citor sp. by Kashman et al.,6 have been known for many years.
When the fluorophore presents a charge shift character in
the excited state, it can be used as a reporter of local polarity
in biological systems,7,8 polymers,9 and colloids.10,11 Molecules
with the so-called push–pull effect should have well positioned
2 Experimental
2.1 Synthesis
2.1.1 Materials and methods. The 1H NMR spectroscopic
data were recorded using a Bruker spectrometer at 250 MHz
and 500 MHz in CDCl3, (CD3)2CO, (CD3)2SO, and CD3CN solu-
tions. The chemical shifts are reported in ppm and referenced
to the residual solvent peak or tetramethylsilane (TMS). The
spectroscopic data are reported in the order of chemical shift (δ),
multiplicity, coupling constant (J) in hertz, and integrated
intensity. The 13C NMR spectroscopic data were recorded
using a Bruker spectrometer at 62.5 MHz and 125 MHz in
CDCl3, (CD3)2CO, (CD3)2SO, and CD3CN solutions. The chemi-
aInstitute of Chemistry of São Carlos, University of São Paulo, CEP 13560-590,
São Carlos, SP, Brazil. E-mail: marcelog@iqsc.usp.br; Tel: +55 16 33739928
bInstitute of Chemistry, State University of Campinas, CEP 13083-862, Campinas,
SP, Brazil
†Electronic supplementary information (ESI) available: Optimized geometry of
compounds, contribution of dipole moments in each direction, absorption
spectra and Lippert–Mataga plots. See DOI: 10.1039/c4pp00001c
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Fig. 1 Reaction scheme of the Heck arylation.
cal shifts are reported in ppm and referenced to the residual added POPd (4 mol%, 29.4 mg), sodium acetate (362 mg,
solvent peak. For the mass spectroscopy, the compounds were 4.41 mmol, 3 equiv.), and monoarylated maleic anhydride (3)
ionized by 70 eV EI and the exact masses were determined (300 mg, 1.47 mmol). Acetonitrile (15 mL) was added, followed
with a Waters GCT TOF mass analyzer. The microwave
(MW) reactions were conducted in a microwave synthesizer
(CEM Discover®) system with a power output of about ≤300 W.
These reactions were performed in glass vessels (capacity
10 mL) sealed with a septum. The temperature measurements
were conducted with an infrared temperature sensor mounted
under the reaction vessel. All of the experiments were per-
by the arenediazonium salt (5.88 mmol, 4 equiv.). The reaction
was heated at 80 °C for 2 h. After this time, the crude reaction
mixture was filtered through a plug of silica gel, and the fil-
trate was concentrated under reduced pressure. The product
was purified by flash chromatography (hexanes–ethyl acetate,
4 : 1) to provide the corresponding Heck adduct.
3,4-Bis-(4-methoxyphenyl)-furan-2,5-dione (4a). Obtained as a
formed using a stirring option, whereby the contents of the yellow solid, in 89% yield.
vessel were stirred by means of both a rotating magnetic plate
1H NMR (500 MHz, CDCl3): δ = 4.06 (s, 6H), 7.50 (d, J =
located below the floor of the microwave cavity and a Teflon-
coated magnetic stir bar inside the vessel. The experiments
were carried out by simultaneously cooling with compressed air
passed through the microwave cavity and heating. Column
9.5 Hz, 4H), 8.63 (d, J = 9.5 Hz, 4H).
13C NMR (125 MHz, CDCl3): δ = 55.5, 114.4, 119.9, 131.4,
135.7, 161.7, 165.4.
3-(4-Methoxyphenyl)-4-(4-hydroxyphenyl)-furan-2,5-dione (4b).
chromatography was performed with silica gel (230–400 mesh) Obtained as a yellow solid, in 86% yield.
following the methods described by Still.19 Thin layer chromato-
1H NMR (500 MHz, CDCl3): δ = 3.89 (s, 3H), 5.37 (s, 1H),
graphy (TLC) was performed using silica gel GF254 (0.25 mm 6.88 (d, J = 8.5 Hz, 2H), 6.95 (d, J = 8.5 Hz, 2H), 7.55 (d, J =
thickness). For visualization, the TLC plates were either placed 8.5 Hz, 2H), 7.98 (d, J = 8.5 Hz, 2H).
13C NMR (125 MHz, CDCl3): δ = 55.5, 114.5, 116.0, 119.8,
under ultraviolet light or developed with phosphomolybdic acid
followed by heating. Air and moisture-sensitive reactions were
conducted in flame or oven dried glassware equipped with
120.1, 131.5, 131.7, 135.6, 135.9, 157.9, 161.7, 165.53, 165.54.
3-(4-Methoxyphenyl)-4-(4-methylphenyl)-furan-2,5-dione (4c).
tightly fitted rubber septa under a positive atmosphere of dry Obtained as a yellow solid, in 83% yield.
nitrogen. The reagents and solvents were handled using stan-
1H NMR (250 MHz, CDCl3): δ = 2.39 (s, 3H), 3.84 (s, 3H),
dard syringe techniques. Temperatures above room temperature 6.89 (d, J = 9.0 Hz, 2H), 7.21 (d, J = 8.2 Hz, 2H), 7.46 (d, J =
were maintained using a mineral oil bath heated on a hotplate.
2.1.2 Procedure for the Heck arylation of fumaric acid (1). To
8.2 Hz, 2H), 7.57 (d, J = 9.0 Hz, 2H).
13C NMR (62.5 MHz, CDCl3): δ = 21.6, 55.4, 114.6, 119.7,
a solution of Pd(OAc)2 (5 mol%, 7 mg) in 5 mL of acetic acid were 124.8, 129.5, 129.6, 131.6, 135.9, 136.9, 141.4, 161.8, 165.2, 165.3.
3-(4-Chlorophenyl)-4-(4-methoxyphenyl)-furan-2,5-dione (4d).
Obtained as a yellow solid, in 55% yield.
added fumaric acid (1) (366 mg, 3.15 mmol) and the 4-methoxy-
phenyldiazonium tetrafluoroborate salt (140 mg, 0.63 mmol).
The mixture was stirred for 2 h at 60 °C and then heating was
carried out to 100 °C for 1 h. After that, the solvent was removed
under reduced pressure. The residue was purified by flash chrom-
atography (hexane–ethyl acetate 5 : 1) to provide 3-(4-methoxyphe-
nyl)furan-2,5-dione (3) as a yellow solid, in 89% yield.
m.p. 141–143 °C.
1H NMR (250 MHz, CDCl3): δ = 3.86 (s, 3H), 6.92 (d, J =
8.9 Hz, 2H), 7.40 (d, J = 8.5 Hz, 2H), 7.38–7.57 (m, 4H).
13C NMR (62.5 MHz, CDCl3): δ = 55.5, 114.6, 119.2, 126.1,
129.3, 130.9, 131.7, 134.3, 137.1, 138.1, 162.2, 164.8, 164.9.
2.1.4 Procedure to obtain bisarylated N-benzyl maleimides
(5a–d). To a microwave test tube were added diaryl maleic
anhydride 4a–d (0.32 mmol), phenol (60 mg, 0.64 mmol), ben-
zylamine (68.6 mg, 0.64 mmol), DIPEA (330.6 mg, 2.56 mmol)
and 300 mg of powdered molecular sieves 4 Å. The reaction
mixture was stirred in a microwave reactor at 300 W and
110 °C for 1 min, and then kept under these conditions for
2 min. After this time, the crude reaction mixture was diluted
1H NMR (250 MHz, CDCl3): δ = 3.89 (s, 3H), 6.83 (s, 1H),
7.00 (d, J = 9.0 Hz, 2H), 7.98 (d, J = 9.0 Hz, 2H).
13C NMR (62.5 MHz, CDCl3): δ = 55.6, 114.9, 119.5, 121.0,
131.1, 146.2, 163.3, 164.0, 165.0.
2.1.3 Procedure for the Heck arylation of monoarylated
maleic anhydride (3). To a 50 mL round bottomed flask were
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in ethyl acetate (9 mL) and washed with HCl 4% solution. The cooled PMT-MCP (Hamamatsu R3809U-50) as a photon detec-
organic layer was dried with MgSO4, filtered, and concentrated tor. Light pulses at 395 nm were provided by frequency dou-
under reduced pressure. The residue was purified by flash bling the 150 fs laser pulse of a Ti–Sapphire Mira 900 laser
chromatography (hexane–ethyl acetate, 1.5 : 1) to provide the pumped by Verdi 5 W (Coherent). The fluorescence decays were
corresponding bisarylated N-benzyl maleimides.
N-Benzyl-3,4-bis(4-methoxyphenyl)-1H-pyrrole-2,5-dione (5a).
Obtained as a yellow solid, in 81% yield.
collected in magic angle mode (54.7°) using Glan-Laser polari-
zers (Newport), with 5 × 103 peak counts and a time increment
of 20–100 ps per channel. The typical instrument response func-
tion (irf) was 40 ps at FWHM.11 All analyzed samples had an
optical density below 0.5. Decays were recorded with the TC900
counting board and software from Edinburgh Instruments. Life-
times were evaluated by the reconvolution procedure with multi-
exponential decay models, according to eqn (1).
1H NMR (500 MHz, CDCl3): δ = 3.82 (s, 6H), 4.78 (s, 2H);
6.86 (d, J = 8.8 Hz, 4H), 7.26–7.36 (m, 3H), 7.44–7.50 (m, 6H).
13C NMR (125 MHz, CDCl3): δ = 41.8, 55.3, 114.1, 121.3,
127.7, 128.6, 128.8, 131.4, 134.1, 136.6, 160.7, 171.0.
N-Benzyl-3-(4-methoxyphenyl)-4-(4-hydroxyphenyl)-1H-pyrrole-
2,5-dione (5b). Obtained as a yellow solid, in 65% yield.
1H NMR (250 MHz, DMSO-d6): δ = 3.79 (s, 3H), 4.71 (s, 2H),
6.78 (d, J = 8.8 Hz, 2H), 6.97 (d, J = 8.8 Hz, 2H), 7.27–7.37 (m,
7H), 7.40 (d, J = 8.8 Hz, 2H).
X
IðtÞ ¼
bi expðꢀt=τiÞ
ð1Þ
i
In eqn (1), τi and bi are the decay time and the pre-exponen-
tial factor of the ith component, respectively. Decays from com-
pound 5a were also analyzed by the global analysis method.14
Global analysis was employed for five wavelengths distributed
over the emission band every 15 nm from the emission
maximum, using linked-decay times. The decay surfaces were
analyzed using the FAST software from Edinburgh Instruments.
13C NMR (62.5 MHz, DMSO-d6): δ = 41.6, 55.7, 114.65,
115.9, 119.8, 121.6, 129.1, 131.6, 131.8, 133.7, 134.8, 137.4,
137.4, 159.4, 160.6, 171.14, 171.12.
HRMS (ESI+): calcd for C24H19NO4/Na+ 408.1206; found
408.1201.
N-Benzyl-3-(4-methoxyphenyl)-4-(4-methylphenyl)-1H-pyrrole-
2,5-dione (5c). Obtained as a yellow solid, in 80% yield.
1H NMR (250 MHz, CDCl3): δ = 2.36 (s, 3H), 3.82 (s, 3H),
4.79 (s, 2H), 6.85 (d, J = 8.9 Hz, 2H), 7.15 (d, J = 8.0 Hz, 2H),
7.27–7.44 (m, 9H).
2.4 Theoretical calculations
All the structures were optimized before calculations. The
Osanger radius, that is the radius of the cavity in which the
fluorophore resides,14 as well as the ground state dipole
moment of the compounds were calculated by the density
functional theory (DFT) method using the Gaussian 03 soft-
ware20 at the B3LYP/6-311G++(2d,2p) level of theory.
13C NMR (62.5 MHz, CDCl3): δ = 21.5, 41.9, 55.3, 114.0,
121.1, 126.1, 127.8, 128.6, 128.8, 129.3, 129.7, 131.5, 134.4,
135.1, 136.6, 139.9, 160.8, 170.8, 170.9.
HRMS (ESI+): calcd for C25H21NO3/Na+ 406.1414; found
406.1422.
N-Benzyl-3-(4-chlorophenyl)-4-(4-methoxyphenyl)-1H-pyrrole-
2,5-dione (5d). Obtained as a yellow solid, in 78% yield.
1H NMR (250 MHz, CDCl3): δ = 3.83 (s, 3H), 4.79 (s, 2H),
6.87 (d, J = 8.9 Hz, 2H), 7.31–7.48 (m, 11H).
3 Results and discussion
3.1 Synthesis of bisarylated maleimides
13C NMR (62.5 MHz, CDCl3): δ = 42.0, 55.3, 114.2, 120.6,
127.5, 127.9, 128.7, 128.8, 128.9, 131.1, 131.6, 132.9, 135.7,
136.2, 136.4, 161.1, 170.4, 170.5.
Maleimides are described in the literature as compounds
exhibiting fluorescence,21,22 and their synthesis is
well-documented.22–24 In 2006, Correia and coworkers16 deve-
loped a direct methodology to obtain bisarylated maleic an-
hydrides using the Heck arylation with arenediazonium
tetrafluoroborates providing symmetrical and unsymmetrical
products with better yields. However, to obtain monoarylated
maleic anhydrides, an excess of maleic anhydride was
necessary.
An improved procedure with fumaric acid (1) as the sub-
strate in the Heck–Matsuda reaction, followed by a one-pot
cyclization was recently applied (see the reaction scheme of
Fig. 1). The use of Pd(OAc)2 in acetic anhydride at 60 °C pro-
vides the monoarylated acyclic compound. After the arylation
step the reaction system was heated to 100 °C to give the inter-
mediate compound 3 in 89% yield. The bisarylated maleic
anhydrides 4a–d were prepared by a second Heck–Matsuda
reaction, using dihydrogen dichlorobis(di-t-butylphosphinito-kP)-
palladate(II) (POPd) as a pre-catalyst and sodium acetate, in
HRMS (ESI+): calcd for C24H18ClNO3/Na+ 426.0867; found
426.0873.
2.2 Steady-state absorption and emission measurements
Steady-state experiments were employed to characterize the
new fluorophores and to determine their fluorescence
quantum yields. Electronic absorption and fluorescence emis-
sion measurements were carried out on Jasco V-630 and
Hitachi F-4500 spectrometers, respectively. Fluorescence
quantum yields (ΦF) of samples were calculated using 9,10-
diphenylanthracene (DPA) (ΦF = 0.9, cyclohexane) as a stan-
dard corrected by refractive index differences.14 All solvents
were of HPLC grade and were dried with 4 Å molecular sieves
prior to use. 1,4-Dioxane was previously distilled.
2.3 Time-resolved fluorescence measurements
Fluorescence decays were measured by a time-correlated acetonitrile solution for 1 hour, providing good yields. Finally,
single-photon counting technique (TCSPC) using a Peltier maleimide compounds were obtained in 65–81% yield with
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benzylamine, Hünig’s base, and phenol at 110 °C, in a micro- a typical positive solvatochromic shift of compound 5a is illus-
wave (300 W) for 1 minute. trated in Fig. 2 and its spectral parameters with increasing
In contrast to the results reported by Mazzanti and co- solvent polarity are summarized in Table 1. These results can
workers,25 our diarylmaleimides do not form atropisomers be ascribed to a push–pull effect, in which a significant charge
mainly because compounds 5a–d lack ortho substituents. The shift occurs.12 The behavior of typical push–pull chromophors,
absence of ortho substituents causes an easy and fast equili- like merocyanines, cyanostilbenes12 and p-dimethylamino-
bration between the isomers. This aspect of our fluorophores 4-nitrostilbene (DMANS),13 is in good agreement with the data
is also confirmed from the 1H and 13C NMR spectra of the reported here.
compounds, which show no signals of rotamers but only a
In general, the largest Stokes shift was observed in aceto-
sharp signal appearing for the methoxy/methyl groups. The nitrile and the lowest in THF. The fluorescence quantum yields
conformation (optimized geometry) and the dipole moments are higher in nonpolar solvents like 1,4-dioxane and THF
for the biarylmaleimides 5a–d can be found in the ESI.†
(values between 0.24 and 0.71), indicating that solvent de-
activation of the excited state is less effective in nonpolar media.
The same trends were found for triarylamine compounds.29
The increase in solvent polarity from 1,4-dioxane to DMSO
resulted in a red shift of the emission of compound 5a at about
757 cm−1, because of the solvent stabilization of the excited
state.30,31 Moreover, compounds 5b, 5c, and 5d followed the
same trend in emission as that observed for compound 5a.
The lowest Stokes shift was observed in 5b that has a
4-hydroxyphenyl group. In such a compound a solvent assisted
proton transfer process may take place. This process should
decrease the fluorescence quantum yield because the excited
state phenolate formed may have low emission efficiency, and
this behavior was barely observed in steady-state emission
measurements (quantum yield values between 0.04 and 0.38).
On the other hand, 5d exhibited the highest emission
quantum yield (0.71) compared to 5b (0.24) in 1,4-dioxane.
This fact may be related to the effect of substitution of OH by
Cl in position 4 of the phenyl substituent, precluding further
the solvent assisted proton transfer process.
3.2 Photophysics of bisarylated maleimides
3.2.1 Steady-state
experiments. Electronic
absorption
spectra of the compounds exhibited single non-structured
bands centered from 390 nm with no relevant effects below
300 nm. However, for solvents like diethyl ether and di-n-
propyl ether, we can observe an absorption band around
230 nm. Similar absorption band profiles were found for bis-
indolylmaleimides.26,27 The absorption spectra showed negli-
gible changes with solvent polarity. These spectra can be
found in the ESI.† All maleimide derivatives presented broad
fluorescence emission bands in the visible range, with emis-
sion peaks between 550 and 568 nm.
It is known that upon adding electron donating groups to
aromatic compounds, an increase in the maximum wave-
lengths of absorption and emission is expected. This behavior
was observed for these maleimides and is in good agreement
with the results of Xie et al.,28 in which semi-empirical
quantum chemical calculations were used to predict the
maximum wavelength of absorption and emission spectra of
some similar compounds.
The bisarylmaleimides presented here are in good agree-
ment with bisindolylmaleimide systems. In both cases, a sub-
stantial, but not complete, charge shift in the excited state
takes place.26,27,32–35
The issues raised here about the photophysics of the bisary-
lated maleimides from the analysis of the steady-state emis-
sion properties will in part be supported by the time-resolved
fluorescence behavior found in different solvents.
The fluorescence emissions had a notable solvatochromic
effect at about 960 cm−1, and the calculated Stokes shifts are
within the range of 6075–7292 cm−1, a strong indicator that
the dipole moment of these dyes in the excited state is higher
than the corresponding value in the ground state. For instance,
3.2.2 Time-resolved experiments. Time-resolved experi-
ments are crucial to characterize new fluorophores and to
investigate dynamic processes in the singlet excited state. The
global analysis of the fluorescence decay surface of compound
5a reveals short lived components in the range of 1.34–7.55 ns
and long lived components in the narrow range of 11.52–
13.55 ns with a biexponential fitting in polar solvents. However,
in nonpolar solvents like 1,4-dioxane and THF the decays
are single exponential and lifetimes are very close to values of
long lived components observed in the polar solvents.
The maleimides present two dipolar Lewis structures,
which contribute to a single excited species. The combination
of similar spectral behavior in different solvents and linear
Lippert–Mataga plots (see below) suggests that the emission is
due to the same dipolar species in all solvents. Therefore, we
can infer that the long decay corresponds to emission from the
charge shift or the push–pull state. The time-resolved para-
Fig. 2 Solvatochromic shift in emission spectra of 5a.
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Table 1 Steady-state data of maleimides
Δνd (cm−1
)
ΦF
e
Compound
Solvent
λabs a (nm)
λem b (nm)
E0–0 c (eV)
5a
1,4-Dioxane
THF
DMSO
DMF
Acetonitrile
402
406
406
404
400
540
544
563
556
558
2.60
2.59
2.52
2.54
2.57
6954
6248
6869
6767
7079
0.39
0.43
0.24
0.27
0.24
5b
5c
5d
1,4-Dioxane
THF
DMSO
DMF
Acetonitrile
404
410
413
411
401
540
546
568
564
558
2.59
2.57
2.49
2.51
2.56
6234
6075
6607
6600
7017
0.24
0.38
0.04
0.11
0.23
1,4-Dioxane
THF
DMSO
DMF
Acetonitrile
393
394
396
394
390
526
526
554
545
545
2.68
2.66
2.59
2.61
2.66
6434
6369
7202
7032
7292
0.32
0.53
0.34
0.36
0.35
1,4-Dioxane
THF
DMSO
DMF
Acetonitrile
392
394
394
392
389
526
529
550
548
543
2.68
2.67
2.60
2.62
2.66
6499
6477
7199
7262
7291
0.71
0.35
0.24
0.36
0.39
a Maximum absorption wavelength. b Maximum emission wavelength. c Energy of transition 0–0. d Stokes shift. e Fluorescence quantum yield.
meters are summarized in Table 2 and typical fluorescence
decays are depicted in Fig. 3. The values of E0–0 presented in
Table 1 support partially this assumption and a small decrease
of energy level with solvent polarity is observed.
The fluorescence of compound 5c followed a similar trend to
that observed for compound 5a, but only for 1,4-dioxane a single
exponential behavior was found. In the other solvents the fluo-
rescence decays were of biexponential character. This biexponen-
tial behavior may be ascribed to a specific interaction of the
Table 2 Time-resolved data of maleimides
χ2 b
a
Compound
Solvent
τ1 (ns)
τ2 (ns)
b1
b2
5a
1,4-Dioxane
THF
DMSO
—
—
2.18
7.55
1.34
—
13.79
13.52
11.52
13.45
13.55
12.86
12.01
2.65
—
—
0.17
0.24
0.08
—
1.00
1.00
0.83
0.76
0.92
1.00
1.00
0.89
0.89
1.00
1.00
0.92
0.76
0.93
0.95
1.00
1.00
0.75
1.00
0.94
1.05
1.07
1.09
1.07
1.07
1.05
1.05
1.12
1.14
1.12
1.11
1.00
1.17
1.18
1.12
1.05
1.07
1.23
1.19
1.11
△
Fig. 3 Fluorescence decays of compound 5a in: 1,4-dioxane ( ) λem
=
=
□
◊
540 nm; THF (☆) λem = 544 nm; DMSO ( ) λem = 563 nm; DMF ( ) λem
DMF
○
■
556 nm; acetonitrile ( ) λem = 558 nm; ( ) irf. λexc = 395 nm.
Acetonitrile
1,4-Dioxane
THF
DMSO
DMF
Acetonitrile
1,4-Dioxane
THF
DMSO
DMF
Acetonitrile
1,4-Dioxane
THF
DMSO
DMF
5b
5c
5d
—
—
0.83
0.99
—
0.11
0.11
—
3.28
excited state with the charge shift or push–pull character with
the electron donating solvents, and the short component in a
low weight percent would correspond to decay of a weak solvent
complex, as observed by Chandross and coworkers for intra-
molecular exciplex systems.36 For compound 5d, the presence of
the electron withdrawing 4-chlorophenyl group gives no differ-
ence in dynamic and spectral fluorescence properties when
compared with compounds 5a and 5c. It seems to indicate that
a single branch charge shift from the 4-methoxy donor group to
the carbonyl acceptor is operating in the charge shift process,
and the presence of the second aryl substituted branch with the
4-chlorophenyl group takes no effect in the whole charge shift.
11.48
13.88
13.66
11.97
12.67
13.75
12.39
11.55
8.62
—
—
3.77
7.20
5.58
1.44
—
—
5.22
—
0.08
0.24
0.07
0.05
—
—
0.25
—
10.51
12.00
Acetonitrile
1.77
0.06
a bi = Bi/∑Bi, Bi is the pre-exponential factor in the model function for
exponential component analysis. b χ2 = the chi-square value of the
decay fitting. For 5a, it corresponds to χ2g, the global decay fitting.
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Photochemical & Photobiological Sciences
ε ꢀ 1
n2 ꢀ 1
Δf ¼
ꢀ
ð4Þ
2ε þ 1 2n2 þ 1
In eqn (3), νA–νF is the Stokes shift (cm−1), a is the radius of
the Osanger cavity, h is Planck’s constant, c is the speed of
light, C is a constant, and μE and μG are the excited and
ground state dipole moments, respectively. In eqn (4), Δf is a
parameter called orientation polarizability, which is calculated
from the dielectric constant ε and the refractive index n of the
solvent.
A typical Lippert–Mataga plot for compound 5a is depicted
in Fig. 5. The plot was obtained based on the emission
maxima. Using eqn (3) a positive slope of 6681 cm−1 is
obtained. For the other compounds, reasonable linear fits
were obtained indicating no existence of strong specific
solvent effects (see in ESI†). From the calculated dipole
moment difference, according to eqn (3), and using the values
of the dipole moment in the ground state and Osanger cavities
evaluated from DFT quantum chemical calculations, the
excited state dipole moments of the maleimides were obtained
and the values found are reported in Table 3.
In general, the differences in dipole moment in ground and
excited states of the compounds are within a narrow range of
11–13 D, indicating a similar contribution of a charge shift
process in the excited states.41,42 In the case of compound 5b
no good correlation in the Lippert–Mataga plot was observed,
△
Fig. 4 Fluorescence decays of compound 5b in: 1,4-dioxane ( ) λem
=
=
□
◊
540 nm; THF (☆) λem = 549 nm; DMSO ( ) λem = 568 nm; DMF ( ) λem
○
■
564 nm; acetonitrile ( ) λem = 558 nm; ( ) irf. λexc = 395 nm.
Most notable is the excited state dynamics of 5b in DMSO
and in DMF. It exhibited biexponential decays with a very short
component of less than 1 ns as illustrated in Fig. 4.
This hydroxyl-substituted derivative may deprotonate in
polar solvents with the proton accepting character like DMF
and DMSO, thus emission from the corresponding phenolate
ion may occur. The value of the second decay component of
compound 5b in DMF and DMSO is in agreement with the
fluorescence lifetime of phenolate anions.37 For compound 5b
in 1,4-dioxane, THF, and acetonitrile the decays are of single
exponential character, which indicates that an excited state
charge transfer process does not occur, precluding further
proton transfer mediated by solvent interactions.
Considering that a solvent assisted excited state proton
transfer may play a role for this compound, if one uses 5a as a
model compound, the comparison of it with 5b allows the rate
constant of proton transfer (kpt) to be evaluated. In the
approximation of the very fast excited state intramolecular
charge shift process:
kpt ¼ ð1=τ1Þ5b ꢀ ð1=τ1Þ5a
ð2Þ
Using eqn (2), the values of kpt in DMF and DMSO are 0.9 ×
109 and 1.3 × 109 s−1, respectively. This result indicates that
the deactivation rate of the excited state via deprotonation
Fig. 5 Lippert–Mataga plot of compound 5a according to eqn (3). 1:
di-n-propyl ether, 2: diethyl ether, 3: THF, 4: CH2Cl2, 5: DMSO, 6: DMF,
occurs at a rate similar to that observed in solvent assisted 7: acetonitrile.
proton transfer processes studied by Kasha and coworkers.38
3.3 Dipole moment analysis
Table 3 Theoretical parameters and the calculated excited state dipole
moment (μE)
Fluorophores exhibiting the solvatochromic shift are valuable
probes to monitor the micropolarity in their surrounding
environment.39,40 This effect may be studied by the Lippert–
Mataga approach17,18 that correlates the effect of solvent
polarity on the Stokes shift with the difference in dipole
moments14,15 by
Compound
a0 (Å)a
μG (D)b
Δμ (D)c
μE (D)d
5a
5c
5d
6.0
5.9
5.7
4.3
4.2
3.0
11.9
12.5
12.6
16.2
16.7
15.6
a a0 = the radius of the Onsager cavity. b μG = the ground state dipole
moment. c Δμ = variation of dipole moments. d μE = the excited state
dipole moment.
2
hca3
2
νA ꢀ νF ¼
Δf ðμE ꢀ μGÞ þ C
ð3Þ
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Paper
probably due to the deprotonation process in polar solvents as
previously discussed.
7 Y. Suzuki and K. Yokoyama, Design and synthesis of intra-
molecular charge transfer-based fluorescent reagents for
the highly-sensitive detection of proteins, J. Am. Chem. Soc.,
2005, 127, 17799–17802.
4 Conclusions
8 K. Kudo, A. Momotake, J. K. Tanaka, Y. Miwa and T. Arai,
Environmental polarity estimation in living cells by use of
quinoxaline-based full-colored solvatochromic fluorophore
PQX and its derivatives, Photochem. Photobiol. Sci., 2012,
11, 674–678.
9 R. V. Pereira and M. H. Gehlen, Polymerization and confor-
mational transition of poly(methacrylic acid) probed by
electronic spectroscopy of aminoacridines, Macromolecules,
2007, 40, 2219–2223.
In summary, four maleimides based on natural products were
synthesized by the Heck reaction. The fluorescence properties
of these new bisarylated maleimide derivatives are modeled by
solvent and substituent effects. The compounds showed a
positive solvatochromic shift and a substantial charge shift
process. The difference in dipole moment between ground and
excited states of about 12 D corroborates this conclusion.
10 B. M. Sheikhali and R. G. Weiss, Effects of aggregation and
mesomorphic order on the photophysical properties of
4-alkyl-N-(p-cyanophenyl)piperidines. Molecules capable of
forming intramolecular charge-transfer states, J. Am. Chem.
Soc., 1994, 116, 6111–6120.
11 A. P. Garcia Ferreira, R. Frederice, K. P. F. Janssen and
M. H. Gehlen, Dually fluorescent silica nanoparticles,
J. Lumin., 2011, 131, 888–893.
Acknowledgements
The authors thank G. R. Nagurniak and A. B. F. da Silva, from
the Quantum Chemistry Group of IQSC – USP, for providing
quantum chemical data. Financial support was provided by
FAPESP (grants 2011/18215-8 and 2011/23832-6), CNPq, and
CAPES Brazilian research funds. M. H. Lauer thanks FAPESP
for the doctoral scholarship 2011/08727-1. R. L. Drekener
thanks FAPESP for the post-doctoral fellowship 2010/12193-0.
12 P. D. Zoon and A. M. Brouwer, A push-pull aromatic
chromophore with a touch of merocyanine, Photochem.
Photobiol. Sci., 2009, 8, 345–353.
13 I. D. Petsalakis, D. G. Georgiadou, M. Vasilopoulou,
G. Pistolis, D. Dimotikali, P. Argitis and G. Theodorakopoulos,
Theoretical Investigation on the Effect of Protonation on
the Absorption and Emission Spectra of Two Amine-Group-
Bearing, Red “Push-Pull” Emitters, 4-Dimethylamino-
4′-nitrostilbene and 4-(dicyanomethylene)-2-methyl-6-p-
(dimethylamino)styryl-4H-pyran, by DFT and TDDFT Calcu-
lations, J. Phys. Chem. A, 2010, 114, 5580–5587.
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