Syntheses and properties of core-substituted naphthalene bisimides with aryl ethynyl or cyano groups

Article abstract of DOI:10.1039/b704489e

This paper reports the synthesis, the absorption and the emission spectra and the electrochemical and spectroelectrochemical properties of new naphthalene bisimides substituted in positions 2 and 6 of the napthyl core by cyano, pyrrolidino, 4-phenylethyny

Full text of DOI:10.1039/b704489e

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Syntheses and properties of core-substituted naphthalene bisimides with
aryl ethynyl or cyano groups{
Ste´phanie Chopin, Fre´de´rique Chaignon, Errol Blart and Fabrice Odobel*
Received 26th March 2007, Accepted 5th July 2007
First published as an Advance Article on the web 23rd July 2007
DOI: 10.1039/b704489e
This paper reports the synthesis, the absorption and the emission spectra and the electrochemical
and spectroelectrochemical properties of new naphthalene bisimides substituted in positions 2 and
6 of the napthyl core by cyano, pyrrolidino, 4-phenylethynyl, 4-ethynyl-N,N-dihexylaniline or
4-cyanophenyl groups. The synthesis of these five new compounds shares the common precursor:
2,6-dibromonaphthalene tetracarboxylic dianhydride. We show that the aryl ethynyl substituent
red-shifts the absorbance and enhances the fluorescence quantum yield of the naphthalene
bisimide, making these new compounds potential useful dyes. The cyano groups directly
connected to the naphthyl core significantly decrease the two first reduction potentials of the
molecule. The large electron affinity and the stability of the radical anion of the latter compound
make it ideally suited to transport electrons or to act as an electron acceptor.
optical and electronic properties which offer new possibilities
1 Introduction
of utilization for this class of compounds.
Naphthalene bisimides (NBI) are an important class of
compounds because they have been used in many areas, such
2 Experimental
as molecular sensors,1 light harvesting pigments,2 organic
semiconductors,3,4 photomolecular switches5 and the forma-
General methods
tion of hydrogen-bonded supramolecular assemblies.6,7
1H and 13C NMR spectra were recorded on a Bruker ARX
Naphthalene bisimide has also been used extensively by
300 MHz or a Bruker AMX 400 MHz spectrometer. Chemical
Wasielewski et al.8 and by other groups9,10 as an electron
1
shifts for H NMR spectra are referenced relative to residual
acceptor in molecular arrays for photoinduced electron
protons in the deuterated solvent (CDCl3 d = 7.26 ppm). High
resolution electrospray mass spectra (HR-ESIMS) were
transfer owing to its low reduction potential, its high-lying
excited-state and the intense and well-defined spectroscopic
signature of the radical anion.11 It is known that there are a
collected in positive mode on a MAT95XL from Thermo-
finnigan. Matrix assisted laser desorption mass spectrometry
nodes on the HOMO and LUMO orbitals at the imide
analyses were performed on a Bruker Biflex III MALDI-TOF
nitrogen atoms.12 Accordingly, the electronic properties of
spectrometer using dithranol as matrix. The instrument was
naphthalene bisimide are weakly affected by the substituents at
equipped with a nitrogen laser emitting at 337 nm, a 2 GHz
the bisimide group. On the other hand, there are significant
sampling rate digitizer, a pulsed ion extraction source and a
spin densities on the core of naphthalene to tune the electronic
reflectron. Fourier transform infrared spectra were recorded in
properties of NBI with substituents,2,7,12–14 but until
pressed KBr pellets on a Bruker Vector 22 spectrometer.
recently the introduction of substituents on these positions
The electrochemical measurements were performed with a
required tedious and multi-step transformations.15 Very
potentiostat–galvanostat MacLab model ML160 controlled
recently, Wu¨rthner’s group14 and our group10 reported an
by resident software (Echem v1.5.2 for Windows) using a
efficient procedure to brominate in one step naphthalene
conventional single-compartment three-electrode cell. The
working electrode was a platinum disk of 4 mm2 area, the
tetracarboxylic dianhydride with dibromoisocyanuric acid.
This strategy opens the way to easy functionalization of
auxiliary was a Pt wire and the reference electrode was
naphthalene bisimide core positions by various substituents.
the saturated potassium chloride calomel electrode (SCE). The
We report herein the synthesis and the characterization of a
supported electrolyte was 0.15 N Bu4NPF6 in dichloromethane
series of new naphthalene bisimide derivatives with interesting
and the solutions were purged with argon before the
measurements. All potentials are quoted relative to SCE. In
all of the experiments the scan rate was 100 mV s21 for the
Universite´ de Nantes, Nantes Atlantique Universite´s, CNRS, Faculte´ des
Sciences et des Techniques, Laboratoire de Synthe`se Organique (LSO),
cyclic voltammetry and 15 Hz for pulse voltammetry.
Spectroelectrochemical spectra were recorded in a thin quartz
UMR CNRS 6513, 2, rue de la Houssinie`re – BP 92208 – 44322
NANTES Cedex 3, , France. E-mail: Fabrice.Odobel@univ-nantes.fr;
Tel: (33)-2-51-12-54-29.
UV–visible cell (0.5 mm) surmounted with
a reference
electrode (SCE) and a platinum wire as counter electrode.
The working electrode was a platinum grid inserted within the
quartz cell and the other conditions are the same as those for
the cyclic voltammetry. UV–visible absorption spectra were
{ Electronic supplementary information (ESI) available: 1H, 13C
NMR and FTIR spectra of compounds 2–7 and overlay of the
absorption and emission spectra of 4 and 6. See DOI: 10.1039/
b704489e
This journal is ß The Royal Society of Chemistry 2007
J. Mater. Chem., 2007, 17, 4139–4146 | 4139

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recorded on a UV-2401PC Shimadzu spectrophotometer.
Fluorescence spectra were recorded on a SPEX Fluoromax
fluorimeter and were corrected for the wavelength dependent
response of the detector system (Hamamatsu R928). The
fluorescence quantum yields were determined using 5,10,15,20-
tetra(3,5-di-tert-butylphenyl)porphyrinato zinc(II) in DMF as
standard and using wfluo = 0.045.16
(t, 3J = 7.6 Hz, 8H), 2.75 (sept., 3J = 6.9 Hz, 4H), 1.65–1.50 (m,
3
8H), 1.31 (broad s, 24H), 1.20 (d, J = 6.6 Hz, 12H), 1.19 (d,
3J = 6.9 Hz, 12H), 0.90 (t, 3J = 6.4 Hz, 12H). 13C NMR
(75 MHz, CDCl3): d 162.6, 161.6, 149.2, 145.8, 137.5, 134.7,
130.8, 129.6, 127.8, 127.0, 124.6, 124.1, 123.6, 111.1, 107.8,
107.4, 90.5, 50.9, 31.6, 29.3, 27.2, 26.7, 24.0, 23.9, 22.6, 14.0.
IR (cm21): 3074, 3032, 2958, 2926, 2867, 2172, 1710, 1672,
1603, 1568, 1520, 1218, 1190, 1158, 1132, 814. MALDI-TOF:
m/z: calcd for C78H96N4O4 = 1152.7; found 1153.1 [M]+.
Thin-layer chromatography (TLC) was performed on
aluminium sheets precoated with Merck 5735 Kieselgel
60F254. Column chromatography was carried out either with
Merck 5735 Kieselgel 60F (0.040–0.063 mm mesh) or with
SDS neutral alumina (0.05–0.2 mm mesh). Air sensitive
reactions were carried out under argon in dry solvents and
glassware. Chemicals were purchased from Aldrich or Acros
and used as received. The solvents for the reactions were
distilled prior to use following the standard purification
procedures. 2,6-Dibromonaphthalene-1,4,5,8-tetracarboxylic
acid bisanhydride10 and 4-ethynyl-N,N-dihexylaniline17 were
prepared according to previously reported procedures.
N,N9-Di-(29,69-diisopropylphenyl)-2,6-bis(2-phenylethylenyl)
naphthalene-1,4,5,8-tetracarboxylic acid bisimide (4)
Under
a nitrogen atmosphere N,N9-di-(29,69-diisopropyl-
phenyl)-2,6-dibromonaphthalene-1,4,5,8-tetracarboxylic acid
bisimide 2 (30.4 mg, 0.041 mmol), Pd(PPh3)4 (4.7 mg,
4.08 mmol) and CuI (0.6 mg, 3.25 mmol) were dissolved in
a previously degassed mixture of dry THF (4 mL) and
triethylamine (4 mL). Phenylacetylene (22 mL, 0.20 mmol) was
added and the solution was stirred at 35 uC for 1.5 h. The
solvents were removed under vacuum and the residue was
extracted with dichloromethane until the aqueous layer was
neutral. The crude product was purified by flash silica gel
column chromatography (CH2Cl2 : petroleum ether, 1 : 1) to
give 4 as an orange solid (28.8 mg, 90%).
N,N9-Di-(29,69-diisopropylphenyl)-2,6-dibromonaphthalene-
1,4,5,8-tetracarboxylic acid bisimide (2)
2,6-Dibromonaphthalene-1,4,5,8-tetracarboxylic acid bisanhy-
dride 1 (601.6 mg, 1.41 mmol) was suspended in acetic acid
(30 mL) and heated at reflux for 30 min. 2,6-Diisopropyl-
aniline (1.6 mL, 8.47 mmol) was added and the mixture was
kept at reflux at 120 uC for another 25 min. The solution was
concentrated under vacuum and methanol (20 mL) was added
to the remaining residue. The precipitated solid was filtered,
washed with methanol and then with dichloromethane. The
resulted solid was purified by silica gel column chromato-
graphy (CH2Cl2 : petroleum ether, 1 : 1) to give 2 as an orange
solid (315.3 mg, 30%).
1H NMR (300 MHz, CDCl3): d 9.01 (s, 2H), 7.68–7.65 (m,
4H), 7.56 (d, 3J = 8.1 Hz, 1H), 7.53 (d, 3J = 7.8 Hz, 1H), 7.40–
7.36 (m, 10H), 2.75 (sept., 3J = 6.9 Hz, 4H), 1.21 (d, 3J =
3
6.6 Hz, 12H), 1.19 (d, J = 6.9 Hz, 12H). 13C NMR (75 MHz,
CDCl3): d 162.2, 161.4, 145.7, 137.7, 132.7, 130.3, 129.9, 129.8,
128.5, 127.7, 127.4, 125.7, 125.4, 124.2, 122.3, 121.9, 103.6,
89.3, 29.4, 24.0, 23.9. IR (cm21): 3063, 2962, 2927, 2869, 2203,
1714, 1675, 1575, 1430, 1316, 1228, 1217, 753, 720, 687.
MALDI-TOF: m/z: calcd for C54H46N2O4 = 786.3; found
786.8 [M]+.
3
1H NMR (300 MHz, CDCl3): d 9.12 (s, 2H), 7.53 (d, J =
3
8.1 Hz, 1H), 7.51 (d, J = 7.5 Hz, 1H), 7.36 (d, J = 7.8 Hz,
3
3
3
4H), 2.65 (sept., J = 6.9 Hz, 4H), 1.17 (d, J = 6.9 Hz, 24H).
13C NMR (75 MHz, CDCl3): d 160.9, 160.8, 145.3, 139.5,
130.1, 129.8, 129.0, 128.6, 125.6, 124.3, 29.4, 23.9. IR (cm21):
3062, 2958, 2927, 2868, 1721, 1679, 1560, 1415, 1310, 1228,
719. MALDI-TOF: m/z: calcd C38H36Br2N2O4 = 742.1; found
743.5 [M + H]+.
N,N9-Di-(29,69-diisopropylphenyl)-2,6-bis(4-cyanophenyl)
naphthalene-1,4,5,8-tetracarboxylic acid bisimide (5)
Under
a nitrogen atmosphere N,N9-di-(29,69-diisopropyl-
phenyl)-2,6-dibromonaphthalene-1,4,5,8-tetracarboxylic acid
bisimide 2 (30.4 mg, 0.041 mmol), Pd(PPh3)4 (2.4 mg,
2.04 mmol), Na2CO3 (17.3 mg, 0.163 mmol) and 4-cyanophe-
nylboronic acid pinacol ester (37.4 mg, 0.163 mmol) were
N,N9-Di-(29,69-diisopropylphenyl)-2,6-bis{2[4-(dihexylamino)
phenyl]ethynyl}naphthalene-1,4,5,8-tetracarboxylic acid
bisimide (3)
dissolved in
a previously degassed mixture of ethanol
(0.04 mL), toluene (0.24 mL) and water (0.11 mL). The solu-
tion was stirred at 56 uC for 16 h and then at 65 uC for 2 h.
The solvents were removed under vacuum and the residue
was extracted with dichloromethane. The crude product was
purified by flash silica gel column chromatography (diethyl
ether then CH2Cl2) to give 5 as a yellow solid (13.4 mg, 42%).
Under
a nitrogen atmosphere N,N9-di-(29,69-diisopropyl-
phenyl)-2,6-dibromonaphthalene-1,4,5,8-tetracarboxylic acid
bisimide 2 (31.9 mg, 0.043 mmol), Pd(PPh3)4 (4.9 mg,
4.28 mmol) and CuI (0.6 mg, 3.42 mmol) were dissolved in
a previously degassed mixture of dry THF (4 ml) and
triethylamine (4 ml). 4-Ethynyl-N,N-dihexylaniline (37.8 mg,
0.132 mmol) was added and the solution was stirred at 37 uC
for 16 h. The solvents were removed under vacuum and the
crude product was purified by silica gel column chromato-
graphy (CH2Cl2 : petroleum ether, 1 : 1) to give 3 as a blue
solid (26.6 mg, 54%).
3
1H NMR (300 MHz, CDCl3): d 8.74 (s, 2H), 7.73 (d, J =
3
3
8.4 Hz, 4H), 7.49 (d, J = 8.4 Hz, 4H), 7.44 (d, J = 8.1 Hz,
2H), 7.29 (d, 3J = 7.8 Hz, 4H), 2.67 (sept., 3J = 6.9 Hz,
4H), 1.16 (d, 3J = 6.6 Hz, 12H), 1.10 (d, 3J = 6.9 Hz, 12H). 13
C
NMR (75 MHz, CDCl3): d 162.1, 146.4, 145.1, 144.6, 135.4,
132.2, 130.1, 129.6, 128.7, 128.3, 126.3, 124.3, 123.6, 118.4,
112.4, 29.4, 23.9, 23.8. IR (cm21): 3069, 2964, 2927, 2870,
2231, 1715, 1673, 1425, 1314, 1229, 717. MALDI-TOF: m/z:
calcd for C52H44N4O4 = 788.3; found 789.8 [M + H]+.
1H NMR (300 MHz, CDCl3): d 8.90 (s, 2H), 7.56–7.50 (m,
3
6H), 7.38 (d, J = 7.5 Hz, 4H), 6.56 (d, J = 9.0 Hz, 4H), 3.28
3
4140 | J. Mater. Chem., 2007, 17, 4139–4146
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N,N9-Di-(29,69-diisopropylphenyl)-2,6-bis(N-pyrrolidinyl)
naphthalene-1,4,5,8-tetracarboxylic acid bisimide (6)
substitution of the bromo group of 2 by pyrrolidine in 89%
yield. Finally, the dicyanonaphthalene bisimide 7 was prepared
in quantitative yield by palladium-catalyzed cyanation of the
dibromonaphthalene bisimide 2 using zinc dicyanide following
the method reported by Maligres and co-workers.18 These
conditions were previously applied successfully by Wasielewski
and coworkers for the preparation of cyano derivatives of
perylene imide.19 All of the compounds are very soluble in
dichloromethane and were characterized by 1H, 13C NMR and
IR spectroscopy and mass spectrometry.
N,N9-Di-(29,69-diisopropylphenyl)-2,6-dibromonaphthalene-
1,4,5,8-tetracarboxylic acid bisimide 2 (41.2 mg, 0.055 mmol)
and pyrrolidine (2 mL) were stirred at 50 uC under a nitrogen
atmosphere for 5 min. The excess of pyrrolidine was removed
under vacuum and the residue was purified by flash silica gel
column chromatography (CH2Cl2) to give 6 as a purple–blue
solid (35.5 mg, 89%).
1H NMR (300 MHz, CDCl3): d 8.44 (s, 2H), 7.50 (d, J =
3
3
8.4 Hz, 1H), 7.47 (d, J = 6.9 Hz, 1H), 7.33 (d, J = 7.5 Hz,
3
3
3
UV–visible electronic absorption spectra
4H), 3.49 (broad t, J = 5.8 Hz, 8H), 2.75 (sept., J = 6.9 Hz,
4H), 2.01 (broad s, 8H), 1.17 (d, 3J = 6.6 Hz, 12H), 1.16 (d, 3J =
6.9 Hz, 12H). 13C NMR (75 MHz, CDCl3): d 163.7, 161.8,
147.6, 145.8, 131.3, 129.4, 125.2, 123.9, 123.1, 121.9, 105.4,
52.7, 29.1, 25.8, 24.2, 23.7. IR (cm21): 3096, 3064, 3026, 2962,
2928, 2869, 1693, 1653, 1566, 1451, 1213. HRMS-ESI (m/z):
calcd for C46H52N4O4 = 725.4067; found 725.4057.
The ground-state optical absorption spectra of 2–7, recorded
in dichloromethane, are shown in Fig. 1 and the spectroscopic
data are gathered in Table 1.
As expected, the introduction of substituents on the
naphthalene core led to significant changes of the absorption
spectrum compared to the unsubstituted naphthalene bisimide
(Fig. 1). The presence of the bromo or cyano substituents on
the 2,6 positions of naphthalene bisimide does not shift
significantly the maximum absorption of the p–p* transitions,
but only affects the vibronic pattern of the S0 to S1 transition
between 350–400 nm (Fig. 1). The fine vibronic structure of the
unsubstituted naphthalene bisimide becomes more complex as
a result of new vibronic modes being introduced by the
substituents. The attachment of a phenyl ring by a carbon–
carbon s bond on the naphthalene core (compound 5) partly
restores the initial vibronic structure of the unsubstituted
naphthalene bisimide and induces a slight bathochromic
shift of the spectrum. This is certainly the consequence of a
limited, but noticeable p-conjugation between the naphthalene
and the phenyl.
N,N9-Di-(29,69-diisopropylphenyl)-2,6-dicyanonaphthalene-
1,4,5,8-tetracarboxylic acid bisimide (7)
N,N9-Di-(29,69-diisopropylphenyl)-2,6-dibromonaphthalene-
1,4,5,8-tetracarboxylic acid bisimide 2 (37.2 mg, 0.05 mmol),
zinc cyanide (46.9 mg, 0.40 mmol), 1,19-bis(diphenylphos-
phino)ferrocene (3.9 mg, 7.03 mmol) and tris(dibenzylidene-
acetone) dipalladium(0) (7.2 mg, 6.99 mmol) were combined in
dioxane (4 mL) and heated at reflux for 40 h under a nitrogen
atmosphere. The crude product was diluted with dichloro-
methane, filtered through Celite, and the solvent was removed
under vacuum. The crude product was purified by silica gel
column chromatography (CH2Cl2 : petroleum ether, 2 : 1) to
give 7 as an orange–yellow solid (31.8 mg, 100%).
1H NMR (300 MHz, CDCl3): d 9.18 (broad s, 2H), 7.56 (d,
3J = 8.1 Hz, 1H), 7.54 (d, 3J = 7.5 Hz, 1H), 7.37 (d, 3J = 8.1 Hz,
The introduction of a triple bond between the naphthyl
core and the phenyl ring (compound 4) alleviates the steric
constraints of the ortho protons of the phenyl with those on
the 3 and 7 positions and increases thereby the length of the
p-conjugation. As a result, the narrowing of the HOMO–
LUMO gap causes a red-shift of the p–p* transition by
roughly 15 nm (0.76 eV). The presence of electron-rich
substituents such as amino groups (compounds 3 and 6) shift
the transitions to even lower energy. With reference to other
works on perylene bisimide20 and naphthalene bisimide12,14
derivatives, we attribute the new absorption band in the visible
spectrum as an intramolecular charge transfer transition. This
assignment is in agreement with the disappearance of this
transition upon acidification with HPF6, but which is restored
upon addition of triethylamine (Figure S21, ESI{). The
absorption spectrum of the protonated 3 is relatively similar
to that of compound 4, indicating that once the amino groups
are protonated they behave as electron-withdrawing groups
and they do not significantly affect the UV–vis. spectrum
(Figure S21, ESI{). In the push–pull compounds 3 and 6, the
transition probably corresponds to a shift of electron density
initially localized on the amino groups to the oxygen atoms of
the bisimide groups. Interestingly, in compound 3 the charge
transfer band is more red-shifted and more intense than that of
compound 6 (Fig. 1). This is probably the consequence of a
more extended p-conjugation that increases the magnitude
3
3
4H), 2.57 (sept., J = 6.9 Hz, 4H), 1.18 (d, J = 7.1 Hz, 12H),
1.16 (d, 3J = 7.1 Hz, 12H). 13C NMR (75 MHz, CDCl3): d
159.8, 145.2, 136.6, 130.6, 128.7, 128.6, 128.4, 128.2, 127.5,
124.5, 117.5, 115.3, 29.7, 29.5, 24.0, 23.9. IR (cm21): 3070,
3031, 2964, 2927, 2869, 2231, 1724, 1684, 1442, 1314, 1231,
722. HRMS-ESI (m/z): calcd for C40H36N4O4 + Na+ 659.2634;
found 659.2639.
3 Results and discussion
Synthesis
The preparation of the new compounds 2–7 is outlined in
Scheme 1. The synthesis relies on the known 2,6-dibromo-
naphthalene tetracarboxylic dianhydride 110,14 that was first
reacted with 2,6-diisopropylaniline in acetic acid to afford the
soluble 2,6-dibromonaphthalene bisimide 2.
Then, Sonogashira cross-coupling reaction of
2 with
4-ethynyl-N,N-dihexylaniline or with phenylacetylene under
standard conditions gave respectively 3 and 4. Suzuki cross-
coupling was then applied to dibromo derivative 2 with
4-cyanophenylboronic pinacol diester, using Pd(Ph3)4 as
catalyst and Na2CO3 as a base, to give the expected bis-
coupled product 5 in 42% yield. The N,N-dipyrrolidino-
naphthalene bisimide 6 was obtained by nucleophilic aromatic
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Scheme 1 Synthetic route for the preparation of the new naphthalene bisimides 3–7. Reagents and conditions: (i) 2,6-diisopropylaniline, acetic
acid, reflux, 25 min, 30%; (ii) Zn(CN)2, Pd2(dba)3, 1,19-bis(diphenylphosphino)ferrocene, dioxane, reflux, 40 h, quant.; (iii) pyrrolidine, 50 uC,
5 min, 89%; (iv) Pd(PPh3)4, Et3N, CuI, THF, phenylacetylene, 35 uC, 1.5 h, 90%; (v) Pd(PPh3)4, Et3N, CuI, THF, 4-ethynyl-N,N-dihexylaniline,
37 uC, 16 h, 54%; (vi) Pd(PPh3)4, Na2CO3, 4-cyanophenylboronic acid pinacol ester, ethanol–toluene–water, 56 uC, 15 h, then 65 uC, 2 h, 42%.
of the dipolar transition owing to a longer charge transfer
distance.
groups, therefore it may be the consequence of the intra-
molecular rotation of the phenyl unit with the naphthalene
that lowers the radiative quantum yield. Interestingly, the
fluorescence quantum yield of 4 has increased relative to 6 and
to the unsubstituted NBI, and is higher than that of substituted
NBI with primary amine.12,14 This underscores the benefit
of the triple bond on the fluorescence quantum yield. It is
noteworthy that the energy level of the singlet excited-state of
these new compounds, although still high, has decreased
compared to the unsubstituted naphthalene bisimide (Table 2).
Emission spectroscopy study
The luminescence spectra of the new naphthalene bisimides
2–7 were recorded in dichloromethane. The emission quantum
yield, the emission maxima and the 0–0 energy of the excited-
states (E00) are collected in Table 1.
Derivatives 4 and 6 are the only fluorescent compounds of
the series (Figures S19 and S20, ESI{), whereas the naphtha-
lene bisimides 5 and 7 substituted with cyano groups do not
exhibit detectable fluorescence. The cyano derivative 7 behaves
differently to the corresponding perylene imide derivative
substituted with cyano groups, which shows a relatively
high fluorescent quantum yield.19 The direct attachment of
a phenyl ring to the naphthyl core significantly diminishes the
fluorescence quantum yield. We also observed this pheno-
menon with a NBI derivative substituted with 4-phenylethynyl
Electrochemistry
To investigate the effects of the substituents of NBIs on the
redox potentials, the new compounds were studied by cyclic
voltammetry and the results are collated in Table 2. The cyclic
voltammograms of compounds 2–7 are shown in Fig. 2.
All of the compounds exhibit two reversible reductions that
are separated by about 500 mV. The compounds substituted
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(ERed1 = +0.01 V vs. SCE). With such a low reduction
potential, this latter molecule will certainly be useful as a final
electron acceptor in molecular arrays for long-range electron
transfer or for the development of materials exhibiting n-type
conducting behaviour.4 Compared to the corresponding
dicyano perylene diimide,19 compound 7 exhibits the same
first reduction potential, whereas the second potential is
cathodically shifted by circa 240 mV; this probably arises
due to the smaller distance between the two negative charges
localized on the oxygens of the bisimide groups. Although
compound 5 also bears two cyano groups as in 7, the higher
proximity of its redox potentials with those of 2, or of the
unsubstituted NBI than those of 7, implies that there is limited
communication through the phenyl group directly connected
to the naphthalene core. In contrast, the presence of a triple
bond enhances the electronic p-conjugation and induces a
stabilization of the LUMO frontier molecular orbitals, thus
rendering the molecule more reducible than the unsubstituted
naphthalene bisimide. This is clearly observed in compound 3,
substituted with the ethynyl N,N-dihexylamine group, which
displays relatively accessible reduction potential in spite of the
presence of electron-rich amino substituents. In compound 3,
the high lying HOMO along with the stabilized LUMO explain
the appearance of a low energy transition observed in the
visible absorption spectrum.
Electron-releasing substituents such as the pyrrolidino
group in 6 and the N,N-dihexylamino group in 3 destabilize
the radical anion and shift the reduction processes to more
negative regions, whereas the oxidation processes become
more easily accessible (Table 2). For these two compounds,
two reversible oxidation waves could be resolved within the
electrochemical window of the solvent. Surprisingly, the first
and the second oxidation processes are very close in compound
3, suggesting that the two positive charges are further away, so
as to not interact with each other.
Fig. 1 Electronic absorption spectra of the naphthalene bisimides 2–7
along with that of N,N9-bis(octyl)-1,4,5,8-naphthalenetetracarboxylic
acid bisimide (black dashed line) recorded in dichloromethane.
with electron-withdrawing groups such as 5 and 7 display
anodically shifted reduction waves with an extremely
positive reduction potential for the dicyano derivative 7
Table 1 UV–vis. absorption and emission characteristics of compounds 2–7 recorded in dichloromethane. n.d. = not detected due to low emission
quantum yield (a) or emission wavelength outside the spectral window explored by our photodetector, e.g. lem . 850 nm (b)
Compound
l
abs/nm (e/mol21 cm21
)
lem/nm wem
E00/eVa
2
3
4
5
6
7
408 (1.65 6 104), 387 (1.46 6 104), 363 (2.45 6 104), 259 (6.79 6 104)
692 (4.94 6 104), 408 (6.25 6 104), 374 (6.47 6 104), 287(4.01 6 104)
494 (2.92 6 104), 386 (1.37 6 104), 366 (1.18 6 104), 325 (6.31 6 104)
379 (1.77 6 104), 255 (4.41 6 104)
n.d. (a) n.d. (a)
n.d. (b) n.d. (b)
—
—
2.35
—
1.99
—
3.23
528
n.d. (a) n.d. (a)
623 0.20%
0.80%
602 (2.21 6 104), 365 (1.30 6 104), 349 (1.05 6 104), 300 (3.89 6 104)
393 (1.12 6 104), 378 (1.24 6 104), 370 (1.25 6 104), 360 (1.31 6 104), 261 (5.22 6 104) n.d. (a) n.d. (a)
Unsubstituted NBIb 382 (1.45 6 104), 360 (1.15 6 104), 343 (6.7 6 103)
387
0.16%
a
b
E00 was determined from the wavelength at the intersection of the absorption and the emission spectra. Unsubstituted NBI =
N,N9-bis(octyl)-1,4,5,8-naphthalenetetracarboxylic acid bisimide.
Table 2 One-electron redox potential of the naphthalene bisimides 2–7 recorded in dichloromethane containing 0.15 M of Bu4NPF6 as supporting
electrolyte and referenced versus a saturated calomel electrode (SCE)
Compound
ERed2(NBI2/NBI22
)
ERed1(NBI/NBI2)
EOx(NBI+/NBI)
EOx(NBI2+/NBI+)
2
3
4
5
6
7
20.96 V
21.00 V
20.96 V
20.96 V
21.36 V
20.62 V
21.04 V
20.42 V
20.57 V
20.48 V
20.43 V
21.03 V
0.01 V
.1.6 V
0.91 V
.1.6 V
.1.6 V
0.98 V
.1.6 V
.1.6 V
—
0.98 V
—
—
1.34 V
—
—
Unsubstituted NBIa
20.64 V
a
Unsubstituted NBI = N,N9-bis(octyl)-1,4,5,8-naphthalenetetracarboxylic acid bisimide.
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Fig. 2 Cathodic region of the cyclic voltammograms of compounds 2–7 recorded in dichloromethane containing 0.15 M of Bu4NPF6 as
supporting electrolyte.
Spectro-electrochemical study
Summary and conclusions
Transient UV–vis. spectra of compounds were recorded during
electrolysis experiments and the maximum absorption wave-
lengths and the absorption coefficients of the new species that
were formed are compiled in Table 3.
In this work, we have showed that the easily prepared dibromo-
naphthalene bisimide 1 is a reactive substrate in palladium-
catalyzed cross-couplings such as cyanation, Sonogashira
and Suzuki reactions. We showed previously that the same
compound could react smoothly with stannane in a Stille
cross-coupling reaction according to classical conditions.10
The newly synthesized naphthalene bisimide derivatives
were characterized by UV–vis. spectroscopy and we observed
The stability of the radical anion of the compounds studied
is demonstrated by the reversible recovery of the absorption
spectrum of the initial neutral material by reversing the
potential of the electrode. All of the electrochemical trans-
formations gave rise to isosbestic points indicating the stepwise
interconversion of one species into another (Fig. 3). In
compounds 4, 6 and 7 during the one-electron reduction,
the initial strong absorption band in the visible decreases to
leave new transitions at lower energies (Fig. 3). The second
reduction of the dicyano derivative 7 is accompanied by new
absorption bands, that are sufficiently distinct to those of the
neutral and the radical monoanion to detect its formation by
transient absorption spectroscopy. The spectral changes
observed with reduction of 7 are in good agreement with
those reported for the unsubstituted NBI.11
that the ethynyl substituted compound
4 is a soluble
compound with a bathochromic absorption spectrum display-
ing an intense absorbance in the visible region and a relatively
improved quantum yield compared to known NBI derivatives.
A further shift to the near-infrared could be obtained by
introducing electron donating groups on the ethynylphenyl
substituents (compound 3). This strategy could be useful, as it
is envisioned to transform a poorly absorbing naphthalene
bisimide molecule into a good chromophore in the visible
spectrum. The coloured naphthalene bisimide dyes with
absorption spectra spanning the solar spectrum could be
Table 3 Electronic absorption characteristics of the radical anions of the naphthalene derivatives measured in dichloromethane. The absorption
coefficients were determined assuming complete conversion during electrolysis
Compound
l )
abs/nm (e/mol21 cm21
32
42
839 (1.23 6 104), 715 (1.88 6 104), 670 (2.00 6 104), 588 (1.58 6 104), 518 (sh), 492 (2.81 6 104),
431 (1.56 6 104), 363 (4.16 6 104)
832 (7.7 6 103), 743 (1.19 6 104), 634 (7.7 6 103), 582 (1.03 6 104), 540 (6.9 6 103), 470 (2.07 6 104),
324 (3.77 6 104)
62
72
687 (6.81 6 104), 537 (sh), 506 (1.43 6 104), 469 (8.2 6 103), 282 (2.59 6 104)
770 (7.9 6 103), 690 (4.1 6 103), 608 (6.5 6 103), 532 (1.2 6 104), 490 (2.59 6 104), 403 (6.9 6 103),
381 (4.6 6 103), 361 (3.7 6 103), 306 (2.68 6 104)
722
600 (1.26 6 104), 549 (9.0 6 103), 509 (4.8 6 103), 459 (4.23 6 104), 432 (2.15 6 104), 294 (2.38 6 104)
755 (3.6 6 103), 683 (2.1 6 103), 605 (6.4 6 103), 474 (2.30 6 104)
Unsubstituted NBIa
a
Unsubstituted NBI = N,N9-bis(octyl)-1,4,5,8-naphthalenetetracarboxylic acid bisimide from reference 11.
4144 | J. Mater. Chem., 2007, 17, 4139–4146
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Fig. 3 Time-resolved spectra of naphthalene bisimide derivatives 3, 4, 6 and 7 during reductive electrolysis recorded in dichloromethane
containing 0.15 M of Bu4NPF6 as supporting electrolyte. The black bold trace is the initial spectrum and the red bold trace is the final spectrum.
The arrows indicate the changes upon reduction.
valuable candidates for light harvesting systems. The electro-
chemical study showed that by varying the substituents on the
core of naphthalene bisimides, it was possible to tune their
reduction and oxidation potentials to a large extent.
compound 7 are the high lying excited-state which precludes
quenching by energy transfer and the marked and distinct
spectroscopic signature of the radical monoanion and dianion
that renders them easily detected by the classical transient
spectroscopic experiments.
A particular interest is the low redox potential of dicyano
derivative 7, having a stable radical anion and dianion, making
it a potential candidate as a mono and bi-electron acceptor
for photoinduced electron transfer study or for use in the
development of electron conductors. In view of its utilization
as an electron acceptor, other noteworthy features of
In conclusion, we have described the synthesis and the
characterization of new attractive naphthalene bisimide
derivatives with unusual optical and redox properties that
are simple and cheap to prepare and that represent valuable
molecules as dyes or electron acceptors.
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and N. Mataga, Chem. Commun., 1998, 1567–1568; N. Mataga,
Acknowledgements
H. Chosrowjan, Y. Shibata, N. Yoshida, A. Osuka, T. Kikuzawa
and T. Okada, J. Am. Chem. Soc., 2001, 123, 12422–12423;
K. Okamoto, Y. Mori, H. Yamada, H. Imahori and S. Fukuzumi,
Chem.–Eur. J., 2004, 10, 474–483; Y. Mori, Y. Sakaguchi and
H. Hayashi, J. Phys. Chem. A, 2002, 106, 4453–4467; O. Johansson,
M. Borgstro¨ m, R. Lomoth, M. Palmblad, J. Bergquist,
L. Hammarstro¨m, L. Sun and B. Kermark, Inorg. Chem., 2003,
42, 2908–2918; B. Abraham, S. McMasters, M. A. Mullan and
L. A. Kelly, J. Am. Chem. Soc., 2004, 126, 4293–4300; D. W. Dixon,
N. B. Thornton, V. Steullet and T. Netzel, Inorg. Chem., 1999, 38,
5526–5534.
The French Research Ministry is gratefully acknowledged for
the financial support of this research through the programs
‘‘ACI Jeune Chercheur’’ and the ANR blanc ‘‘PhotoCumElec’’.
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