Redox-controlled fluorescence modulation (electrofluorochromism) in triphenylamine derivatives

Article abstract of DOI:10.1039/c4ra02675f

The study of the chemical and electrochemical fluorescence switching properties of a family of substituted triphenylamine derivatives is reported. First of all, the synthesis of a family of six compounds is described. They are characterized by electrochemistry, UV-vis and fluorescence spectroscopy and spectroelectrochemistry. Theoretical calculations were performed in order to corroborate the experimental results. While these compounds emit blue to green light under UV irradiation with a large quantum yield (37%) in the case of one molecule, the fluorescence intensity is quenched upon oxidation. The fluorescence behavior can be switched between the strong fluorescent (neutral) state and the non-fluorescent (oxidized) state with a high contrast (around 1500 for the fluorescence intensity for one of these molecules). Furthermore, the chromatic contrast of three of these molecules reaches 70% that can be important for further applications. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra02675f

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PAPER
Redox-controlled fluorescence modulation
(electrofluorochromism) in triphenylamine
derivatives†
Cite this: RSC Adv., 2014, 4, 34332
*
*
Cassandre Quinton, Valerie Alain-Rizzo, Cecile Dumas-Verdes, Fabien Miomandre,
Gilles Clavier and Pierre Audebert
´
´
The study of the chemical and electrochemical fluorescence switching properties of a family of substituted
triphenylamine derivatives is reported. First of all, the synthesis of a family of six compounds is described.
They are characterized by electrochemistry, UV-vis and fluorescence spectroscopy and
spectroelectrochemistry. Theoretical calculations were performed in order to corroborate the
experimental results. While these compounds emit blue to green light under UV irradiation with a large
quantum yield (37%) in the case of one molecule, the fluorescence intensity is quenched upon oxidation.
The fluorescence behavior can be switched between the strong fluorescent (neutral) state and the non-
fluorescent (oxidized) state with a high contrast (around 1500 for the fluorescence intensity for one of
these molecules). Furthermore, the chromatic contrast of three of these molecules reaches 70% that can
be important for further applications.
Received 26th March 2014
Accepted 11th July 2014
DOI: 10.1039/c4ra02675f
www.rsc.org/advances
Therefore triphenylamine derivatives constitute a family of very
promising candidates for applications in electrouorochromism.
Introduction
Indeed a study, which was made at the same time as ours, shows
that conjugated polymers based on triphenylamine are successful
materials for electrochemical uorescence switching.41
Reversible uorescent switching by electrochemistry or electro-
uorochromism1 is an attractive eld of research because of
several applications in biochemical elds,2–4 for the realization of
electrouorochromic windows5–8 or uorescent sensors.9 Elec-
trouorochromic materials display a reversible optical change in
the uorescence resulting from electrochemical oxidation or
reduction. Recently we showed interest in the study of tetrazine-
triphenylamine bichromophores for their dual uorescent
switching application.10,11 Such systems in which the uorophore
emission can be switched by changing the redox state of an active
moiety connected to it has been already described in the litera-
ture.12–18 However, a straightforward uorescence modulation
can also be envisaged with an organic dye acting both as the
light emitter and the redox switch.19–24 Triphenylamines are well
known derivatives used in electrochromism25–34 since they
display most of the time extremely stable cation-radicals35,36
which can be formed at moderate oxidizing potential. Further-
more, depending on their substituents, the absorption and
emission of triphenylamine can be tuned within all the visible
region.37–40
We have prepared several triphenylamine derivatives bearing
different groups on their para positions (Scheme 1) in order to
investigate the inuence of the nature of the substituent on
their photophysical and electrochemical properties. In addition
we performed theoretical quantum calculations to strengthen
the analysis of the experimental results. We also investigated
the photophysical behaviour (absorption and emission) of the
triphenylaminium corresponding cation-radicals using either
´
PPSM, CNRS UMR8531, ENS Cachan, UniverSud 61 Avenue du President Wilson,
94235 Cachan, France. E-mail: mioman@ppsm.ens-cachan.fr; alainv@ppsm.
ens-cachan.fr; Fax: +33-1-47402454
† Electronic supplementary information (ESI) available: Electrochemical data,
uorescence
switching
experiments,
theoretical
calculations,
NMR
Scheme 1 Two redox states system of triphenylamine derivatives 1–6
with different fluorescence properties.
characterizations. See DOI: 10.1039/c4ra02675f
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chemical or electrochemical oxidation depending on the
derivative. The aim of this work is not to develop highly
sophisticated triphenylamine derivatives but at a more modest
level to undertake a rst step in the analysis of the structure–
property relationships in various triphenylamine derivatives
with as long-term goal the denition of guidelines for the future
development of materials based on triphenylamine specically
designed for electrouorochromism.
Scheme 4 Synthesis of triphenylamine derivative 2.
Triphenylamine was rst monoformylated51,52 in 8 with a Vils-
meier–Haack reaction (yield of 77%). The synthesis of 2 was
achieved by the halogenation of 8 with iodine formed in situ
from potassium iodide and potassium iodate leading to a whole
yield of 70% from triphenylamine.53
The synthetic procedure to the substituted triphenylamine 3
is outlined in Scheme 5. First hydroxylamine hydrochloride and
sodium acetate action on aldehyde 1 led to the expected oxime 9
with a very good yield (97%).54 Then nitrile 3 was prepared
through an original Beckmann transposition of the interme-
diary oxime 9 adduct formed by nucleophilic aromatic substi-
tution of 3,6-dichloro-s-tetrazine. Conversion of aldoxime to
nitrile is well known,55–59 however it must be noted that the
utilization of 3,6-dichloro-s-tetrazine as an activator in a Beck-
mann transposition has never been reported.
Results and discussion
Synthesis
Design of substituted triphenylamines can be achieved employ-
ing two major strategies: either the synthesis of the triphenyl-
amine core stems from substituted diphenylamines (Kaneko
procedure) or primary phenylamines (Hartwig–Buchwald
coupling), either functionalization (such as halogenation, Vils-
meier–Haack reaction or pallado-catalysed cross-coupling reac-
tion) is performed on commercial triphenylamine derivatives. It
should be reminded that though triphenylamines are well known
to form stable cation-radicals, a free para position oen lets them
polymerize. Thus, to avoid polymerization during oxidation
process, triphenylamine derivatives bearing substituents on para
positions were targeted. In order to study the inuence of the
electrodonating or electrowithdrawing behaviors of the substit-
uents on photophysical properties, different groups were intro-
duced: halides, cyano, methoxy, carbonyl, aryl.
Thus targeted compounds 4 and 5 were obtained using a
classical one-pot Hartwig–Buchwald procedure10,42–45 (Scheme 2)
on anisidine with excess of corresponding bromo derivatives
(yields of 37 and 93% respectively).
The original Kaneko condensation46,47 of 1,4-cyclo-
hexanedione on bis(4-bromophenylamine) led to the dibromo-
triphenylamine 7 (yield of 65%). This latter underwent a
classical Vilsmeier–Haack reaction48–50 affording the trisubsti-
tuted triphenylamine 1 with a global yield of 44% (Scheme 3).
In order to enhance the global yield, another synthetic route
has been employed to obtain the compound 2 starting
from commercial triphenylamine as shown in Scheme 4.
Finally, the last compound 6 was synthesized by function-
alization of commercially available N,N,N-tri-(4-bromophenyl)-
amine through
a
Suzuki–Miyaura cross-coupling60 with
methoxyphenylboronic acid using Pd(PPh3)4 as the catalyst in a
mixture of toluene, methanol and aqueous sodium carbonate
with 83% yield as shown in Scheme 6.
Hence, the syntheses of six triphenylamine derivatives have
been successfully realized in few steps and relatively good
global yields.
Electrochemical investigations
Cyclic voltammetry (CV) was used to investigate the electro-
chemical behavior of the different triphenylamine compounds.
Scheme 5 Synthesis of triphenylamine derivative 3 through a Beck-
mann type rearrangement.
Scheme 2 Synthesis of triphenylamine derivatives 4 and 5.
Scheme 3 Synthesis of triphenylamine derivative 1.
Scheme 6 Synthesis of triphenylamine derivative 6.
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Photophysical studies
The UV-vis absorption and the uorescence spectra of 4 in
acetonitrile are shown in Fig. 2 and the photophysical data of
the triphenylamine derivatives are summarized in Table 2.
These compounds display one intense band in the UV region
in agreement with a p–p* transition localized on the phenyl
rings (see Theoretical calculations). It should be noted that in
the case of 6, the molar absorption coefficient is nearly twice the
one of the other triphenylamine derivatives. Indeed 6 contains
six phenyl rings whereas the others only three. Depending on
the substituent, the maximum of the absorption band is found
between 283 and 357 nm. Under UV irradiation, all these tri-
phenylamine derivatives show uorescence emission and the
maximum of this emission band varies between 382 and 532
nm depending on the substituent. The uorescence band shape
is a good mirror image of the absorption one. Quantum yields
are listed in Table 2 for compounds 1–6 by comparison against
quinine sulfate in H2SO4 (F ¼ 0.546).64 The quantum yields are
around 0.05 except for 6, for which the quantum yield is much
higher (0.37). The values of the lifetime is found in the same
range for all compounds, between 0.9 and 3.6 ns depending of
the substituent, as already reported.65
Fig. 1 Cyclic voltammetry of compounds 2 and 4 in dichloromethane
(10ꢀ5 mol Lꢀ1) + 0.1 M TBAPF6 on glassy carbon. Potentials referenced
to ferrocene. Scan rate: 100 mV sꢀ1
.
As expected, CV shows the one-electron fully reversible oxida-
tion of triphenylamine into its cation-radical (see Fig. 1 for 2
and 4 and ESI† for 1, 3, 5 and 6).
The effects of the substituent on the triphenylamine core on
the oxidation potential were examined and the data are gath-
ered in Table 1. The oxidation potential values are much lower
in the case of electrodonating substituents (4, 5 and 6) than in
the case of electrowithdrawing substituents (1, 2 and 3) as
expected. In fact, electrodonating groups like methyl, methoxy
and methoxyphenyl groups lead to an easier oxidation than
electrowithdrawing groups like aldehyde, nitrile or halide
groups. Therefore electrodonating substituents lead to a
cathodic shi in triphenylamine oxidation potential compared
to the one of the triphenylamine estimated at 0.47 V vs.
ferrocene.61–63 The extension of the conjugated link increases a
little the oxidation potential value as seen for molecules 6 (vs. 4)
because the methoxy group is farther. Conversely, the change of
a bromine atom for an iodine atom does not modify the
oxidation behavior (molecules 1 and 2).
Exchanging the bromine into an iodine atom does not
change the absorption and emission wavelengths (1 vs. 2).
However, the quantum yield and the lifetime are decreased
As a conclusion and as expected, the compounds 4, 5 and 6
can be easily oxidized with a mild chemical oxidant whereas the
compounds 1, 2 and 3 require electrochemical oxidation, as no
colorless one-electron chemical oxidant is available in this
potential range. Thus, in the case of 4, 5 and 6, the color can be
changed at lower potential (using less energy) than for reported
electrochromic triphenylamine based polymers.25
Fig. 2 Absorption (solid line) and fluorescence emission (dash line,
excitation wavelength: lexc ¼ 309 nm) spectra of 4.
Table 2 Photophysical properties for the triphenylamine derivatives
1–6 in acetonitrile: absorption wavelength [labs, nm], molar absorption
Table 1 Electrochemical data for compounds 1–6: standard potential coefficient [3, L molꢀ1 cmꢀ1], emission wavelength [lem, nm], fluo-
Eꢁ, anodic peak potential Epa, cathodic peak potential Epc and DEp
¼
rescence quantum yield [F], lifetime [s, ns]
Epa ꢀ Epc. Potentials are referenced to ferrocene. All measurements in
dichloromethane + TBAPF6 on glassy carbon
Compounds
labs
3
lem
F
s
1
2
3
4
5
6
353
355
311
283
301
357
22 500
24 100
22 700
22 500
25 000
54 300
531
532
447
384
382
418
0.06
0.04
0.09
0.06
0.03
0.37
2.7
0.9
1.5
3.6
1.8
1.7
Compounds
1
2
3
4
5
6
Eꢁ (V)
Epa (V)
Epc (V)
DEp (V)
0.79
0.84
0.75
0.09
0.78
0.83
0.72
0.11
0.85
0.93
0.77
0.16
0.17
0.26
0.08
0.18
0.22
0.34
0.09
0.25
0.25
0.30
0.20
0.10
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because of a stronger heavy atom effect (favouring spin–orbit
coupling to the non-emissive triplet state) with the iodine atom.
Furthermore, if we compare compounds 1 and 3, we can
observe a blue-shi of the absorption and emission bands from
1 to 3 because of a less important intramolecular charge
transfer from the nitrogen of the triphenylamine to the elec-
trowithdrawing groups (see Theoretical calculations).
Contrariwise, increasing the strength of the electrodonating
groups from 5 to 4 leads to a slight blue-shi of the absorption
band in agreement with a donor–donor built compound. In
addition, increasing the conjugation length of the link by one
phenyl ring in each branch from 4 to 6 leads to a red-shi of the
bands.
So all these triphenylamine derivatives are uorescent,
although sometimes with moderate yields, and are good
candidates for uorescent switching. Depending on the
substituents, absorption and emission wavelengths can be
tuned.
Fig. 3 Absorption spectra recorded upon oxidation of 6 by Cu(ClO4)2
in CH3CN as a function of R ¼ [CuII]/[6], [6] ¼ 4.8 ꢂ 10ꢀ6 mol Lꢀ1. Inset:
correlation of the absorbance at 1000 nm with the amount of
Cu(ClO4)2.
Chemical uorescence switching and electrouorochromism
As previously mentioned, these triphenylamine derivatives are
uorescent in the blue to the green region in their neutral state.
The generation of triphenylaminium cation can be achieved
either by electrochemical or chemical oxidation. The rst set of
Table 3 Chromatic contrast (DT%) for the triphenylamine derivatives
1–6 in acetonitrile
Compounds
l (nm)
DT%
experiments involved
a one-electron chemical oxidation:
Cu(ClO4)2 was used as a mild and clean oxidant and has been
recently reported to effectively generate arylaminium cation-
radicals.10,11,66–68 Another advantage of using Cu(ClO4)2 is that it
gives no absorption or emission in the UV-visible domain at low
concentrations and its reduction potential in acetonitrile is
0.7 V vs. Fc/Fc+, which matches well with the oxidation poten-
tials of 4, 5 and 6. The cation-radicals of the triphenylamine
derivatives are thus generated using Cu(ClO4)2 in acetonitrile,
this being the most suitable solvent to dissolve both the oxidant
and the triphenylamine derivatives, and are characterized by
absorption and emission spectroscopies.
3
4
5
6
719
364
759
360
706
369
980
447
17
16
73
44
69
49
71
59
donating triphenylamine 4, 5 and 6 display higher values
Upon titration with Cu(ClO4)2, the absorption band in the UV (around 70% at low energy and around 50% at high energy)
region decreases whereas two bands gradually appear at 360 than conjugated systems derived from thienyl-derivatized
and 759 nm for 4, 360 and 706 nm for 5 and 450 nm and 980 nm poly(triphenylamine)s30 and triphenylamine-based multi-
for 6 (Fig. 3; ESI† for 4 and 5). The new absorption bands are electrochromic material.31
attributed to the formation of the cation-radical. One can also
Fig. 4 displays the uorescence emission evolution of 6 upon
observe the appearance of two isosbestic points at 245 and 309 oxidation. The excitation wavelength was chosen at 307 nm, one
nm for 4, 263 and 323 nm for 5 and 307 and 374 nm for 6 in of the isosbestic points. Upon oxidation by Cu(ClO4)2, the
agreement with the existence of an equilibrium between the uorescence emission of 6 was gradually decreased and
triphenylamine derivative and its cation-radical. The correlation completely quenched for 1.05 equivalents of oxidant, which is
of the absorbance for the lower energy band with the amount of relevant to the formation of the non-uorescent cation-radical.
Cu(ClO4)2 reveals that oxidation is complete when 1.4 equiva- Thus, the uorescence intensity at the maximum is divided by
lents of Cu(ClO4)2 is added for 4, 1.5 for 5 and 1.0 for 6 (inset in 1425 for 6. The inset in Fig. 4 represents the correlation of the
Fig. 3; ESI† for 4 and 5). Conversely, even an excess of Cu(ClO4)2 uorescence quantum yield of 6 with the absorbance at 1000
does not oxidize 1, 2 or 3 whose standard potentials are too nm, which is characteristic of the cation-radical. One can
high, in accordance with the electrochemical studies.
observe an almost linear uorescence quenching upon
One important characteristic of electrochromic materials is increasing the cation-radical concentration. The deviation from
the chromatic contrast (DT%), which can be dened as the linearity can be attributed to a possible quenching through
transmittance difference between the redox states. These values energy transfer of the triphenylamine derivative uorescence by
are gathered in Table 3. If chromatic contrast is rather low in the its own cation-radical. One can in addition state that it is a static
case of 3 (17 and 16% at 719 and 364 nm respectively), the quenching because of the results in time-resolved emission
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Fig. 4 Fluorescence emission spectra recorded upon oxidation of 6
by Cu(ClO4)2 in CH3CN as a function of R ¼ [CuII]/[6], [6] ¼ 4.8 ꢂ 10ꢀ6
mol Lꢀ1, lexc ¼ 307 nm. Inset: correlation of the quantum yield with the
absorbance at 1000 nm.
experiments. The value of the lifetime is always the same
whatever the oxidation level ratio, only the intensity decreases.
For molecules 4 and 5, one can observe the same behavior
even if the uorescence emission is not totally quenched in
these two cases (see ESI†).
Fig. 5 UV-vis spectroelectrochemistry of 3 in acetonitrile (7 ꢂ 10ꢀ4 M)
+ TBAPF6 at various potentials: (a) from 1 to 1.5 V, (b) from 1.2 to 0.6 V.
The open circuit spectrum is used as the background and subtracted
from all the other subsequent spectra.
In order to test the reversibility of the systems and to check
the uorescence switching in the case of 1, 2 and 3, spec-
troelectrochemical studies were performed.
Absorption spectroelectrochemistry of 3 was performed in a
tailored thin layer spectroelectrochemical cell and the results
shown in Fig. 5. Upon oxidation (Fig. 5a), two main bands are
growing, located respectively at 370 and 710 nm, while the main
band of the neutral species at 310 nm (see ESI†) gradually
disappears. The long wavelength absorption band which
appears upon oxidation is characteristic of the cation-radical
form with a charge transfer character but less marked than
for other triphenylamine derivatives69 since no absorption in
the NIR region is observed. The presence of two isosbestic
points at 270 and 330 nm evidences the conversion of 3 into its
cation-radical form. Upon reduction (Fig. 5b), the variation in
opposite directions can be observed (except for the high energy
band at 230 nm that still grows), showing that the electro-
chemical reaction is actually reversible.
decreases at the beginning of the scan, whereas no change in
the redox state is induced by the potential in that range. A
photobleaching when exciting in the UV is likely to explain this
instability. This is also the reason why the recovered uores-
cence intensity at the end of the backward scan is signicantly
lower than the initial value, although there is also a contribu-
tion in this difference coming from the electrochemical reac-
tion (the nal current is also lower that the initial one, showing
remaining oxidized form at the end of the scan).
For comparison sake, uorescence spectroelectrochemistry
of 1 was also recorded. The variation of uorescence intensity at
530 nm when exciting at 350 nm upon a linear potential scan
from 1.0 to 1.6 V is shown in Fig. 7. A similar trend is observed
but this time the uorescence intensity remains constant when
there is no change in the redox state. Unfortunately, the elec-
trogenerated cation-radical of 1 is not stable enough at longer
times to record absorption spectroelectrochemistry. A progres-
sive consumption of the reactant is observed leading to a strong
decreasing signal both in uorescence and electrochemistry.
This is conrmed by UV-vis spectroelectrochemistry that shows
progressive disappearance of the bands of 1 upon electro-
chemical oxidation without any isosbestic points (see ESI†).
Finally these triphenylamine derivatives unambiguously
lead to a reversible switching of uorescence and thus behave as
on–off two-state systems.
Fluorescence
spectroelectrochemistry
(electrouoro-
chromism) for the same compound was performed in the same
cell with an excitation wavelength close to the isosbestic point at
330 nm. A broad emission band centered around 450 nm is
observed under open circuit potential (ESI†). When recording
the emitted light at this wavelength as a function of time when a
linear potential scan is applied to the electrode, the uores-
cence intensity shows a dramatic drop, followed by partial
recovery when the potential is swept back to the initial value
(Fig. 6). This behavior is expected when compared to other tri-
phenylamine compounds,69 which were shown to uoresce in
the neutral state and not in their rst oxidation state. However it
can be noticed that the recorded uorescence gradually
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calculations70 at the PBE0 level of theory with the 6-31+g(d)
basis set were subsequently performed.
For all these derivatives, the highest occupied molecular
orbital (HOMO) is always a p-orbital delocalized on the entire
molecule. The lowest unoccupied orbital (LUMO) is a p*-orbital
delocalized on one, two or three branches and onto the terminal
group in the case of electrowithdrawing CN and CHO groups,
only with a weak coefficient on the central nitrogen. All
compounds show one absorption band close to the UV-region.
This absorption band is due to two types of transition for the
triphenylamines 1, 2 and 3 substituted by CHO or CN attracting
groups. In the case of 1 and 3, one transition (lth ¼ 357 and 333
nm respectively) is from the HOMO to the LUMO localized on
the electron-withdrawing substituted branch and the other (lth
¼ 310 and 309 nm respectively) is from the HOMO to the p*-
orbital (L+2) localized on the two other branches (Fig. 8).
Absorption spectra constructed from these theoretical data gave
calculated absorption maxima at 357 and 328 nm for 1 and 3
respectively (Table 3). The charge transfer HOMO–LUMO tran-
sition is estimated at 357 nm for 1 and 333 nm for 3 in agree-
ment with the observed hypsochromic shi from 1 to 3. In the
case of 2, the absorption band is due to the HOMO–LUMO and
HOMO–L+4 transitions. In case of 5, two transitions are also
responsible for the absorption band: one from the HOMO to p*-
orbital (L+1) localized on two tolyl groups and the other from
the HOMO to the p*-orbital (L+2) localized on the methoxy-
phenyl group. Because of C3 symmetry, 4 and 6 have degen-
erated orbitals which increase the number of orbitals involved
in the transitions. In the case of 4, two transitions (with three
contributions for each transition, see the ESI†) can be noted
from the HOMO to the p*-orbitals L+1, L+2, L+3 and L+4. In the
case of 6, there are transitions from HOMO, Hꢀ1 and Hꢀ2 p-
orbitals to both LUMO and L+1 p*-orbitals. Representation of
the molecular orbitals involved in the electronic transitions of
compounds 1, 2, 4–6 are available in the ESI.†
Fig. 6 Variation of fluorescence intensity (red curve) of 3 with time
upon application of a linear potential scan from 0.9 to 1.6 V (10 mV sꢀ1).
The corresponding current vs. time is shown (dashed blue curve)
extracted from the CV.
We can notice that wavelength calculations t well with
experimental ones in the case of 1, 2 and 3 but are over-
estimated in the case of electrodonating substituted triphenyl-
amines 4, 5 and 6 (Table 4). Furthermore calculations conrm
that the molar absorption coefficient of 6 is nearly twice that of
the other triphenylamine derivatives, as mentioned above.
Quantum chemical calculations were also done on the
cation-radicals 1–6a of triphenylamine derivatives 1–6. All
oxidized triphenylamine derivatives show two absorption
bands: one is located in the near IR-region and the other one is
Fig. 7 Variation of fluorescence intensity (red curve) of 1 with time
upon application of a linear potential scan from 1.0 to 1.6 V (10 mV sꢀ1).
The corresponding current vs. time is shown (dashed blue curve)
extracted from the CV.
Theoretical calculations
Quantum chemical calculations were carried out on triphenyl-
amine derivatives 1–6. Geometry optimizations were rst per-
formed at the B3LYP level of theory and with the 3-21g basis
set. Time-dependent density functional theory (TD-DFT)
Fig. 8 Representation of the main molecular orbitals involved in the
electronic transitions of 3.
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Table 4 Experimental and calculated absorption wavelength (l1, nm),
molar absorption coefficient (31, L molꢀ1 cmꢀ1) for 1–6 (l1, 31) and their
respective cation-radicals (l2 and l3)
triphenylamine derivatives. At neutral state the solution should
emit the color of the two uorophores mixed together. Above
rst oxidation potential, only one triphenylamine compound
could be uorescent. If all triphenylamine derivatives are
oxidized, this system should be off. This could open the way
toward original multi-state electrouorochromic devices.
l1exp
l1cal
31exp
31cal
l2exp
l2cal
l3exp
l3cal
1
2
3
4
5
6
353
355
311
283
301
357
357
349
328
363
362
433
22 500
24 100
22 700
22 500
25 000
54 300
36 000
37 500
42 200
35 300
32 100
70 500
nda
nda
364b
360
360
450
365
344
361
352
347
420
715
777
719b
759
706
980
692
798
697
658
630
Experimental section
Spectroscopic measurements
1130
UV-visible absorption spectra were recorded on a Cary 5000
spectrophotometer in 1 cm optical length quartz cuvettes.
Corrected emission spectra were obtained on a Jobin-Yvon
Horiba Spex FluoroMax-3 spectrouorometer. Acetonitrile
(SDS, spectrometric grade) was employed as the solvent for
absorption and uorescence measurements. The uorescence
quantum yields were determined by using quinine sulfate in
H2SO4 (0.5 N) as a standard (FF ¼ 0.546). The estimated
experimental error is less than 10%. For the emission
measurements, a right-angle conguration was used. Fluores-
cence decay curves in solution were obtained using a time-
correlated single-photon counting method using a titanium–
sapphire laser pumped by an argon ion laser (Tsunami, by
Spectra-Physics, 82 MHz, 1 ps pulse width, repetition rate low-
ered to 4 MHz with a pulse-peaker, a doubling crystal was used
to reach 495 nm excitation). The Levenberg–Marquardt algo-
rithm was used for the non-linear least-squares t. Copper(II)
perchlorate hexahydrate was commercially available from
Aldrich.
a
Non-determined. b Estimated by spectroelectrochemistry.
Fig. 9 Representation of the main molecular orbitals involved in the
electronic transitions of the cation-radical of 5.
around 400 nm (see Fig. 3 for 6 and ESI† for 4 and 5). The weak
energy band is due to transitions whose contributions are from
lower levels to the single occupied molecular orbital (SOMO)
whereas the visible transition is due to transitions from SOMO
to higher orbitals (see Fig. 9 for 5 and ESI† for the other
compounds). In the case of 1 and 3, other orbitals are also
involved in the second band. In the case of 1 and 3, other
orbitals are also involved in the second band with transitions
from Sꢀ10 to SOMO and from Sꢀ2 to S+1 contributing.
Electrochemistry
Solvents (SDS, HPLC grade) and electrolyte salts (tetrabuty-
lammonium hexauorophosphate from Fluka, puriss.) were
used without further purication. Cyclic voltammetry was per-
formed in a three-electrode cell with a potentiostat (Versa-
STAT4, Princeton Applied Research) driven by a PC. Glassy
carbon electrode (1 mm diameter) was used as the working
Conclusions
Several triphenylamine derivatives have been synthesized and
their photophysical and electrochemical properties have been
investigated. The generation of triphenylaminium cations could
be achieved by either chemical oxidation or electrochemistry
and their photophysical properties have been investigated too.
As already known, triphenylamine derivatives could exhibit
moderate to high uorescence quantum yields in their neutral
state whereas their cation-radicals are not uorescent. In
conclusion, it has been possible to produce a reversible uo-
rescence switch by using triphenylamine derivatives. This
uorescent switching can be achieved for both electrodonating
electrode, whereas platinum wire and Ag+ (0.01
M in
acetonitrile)/Ag were used, respectively, as the counter and
reference electrodes. All the investigated solutions were dea-
erated by argon-bubbling for at least 2 min before performing
the electrochemical measurements. The reference electrode was
checked versus ferrocene as recommended by IUPAC.
Quantum chemical calculations
and electrowithdrawing substituted triphenylamine, but the use Calculations were performed with the Gaussian 03 at the MESO
of donor group is recommended for the fabrication of electro- calculation centre of the ENS Cachan (Nec TX7 with 32
uorochromic materials since the radical-cation is thus more processors of type Itanium 2). Molecules were drawn with the
stable. Furthermore,
a highly uorescent triphenylamine Gaussview 03 soware using included templates and their
derivative should be chosen in order to get an important uo- geometry optimized at the B3LYP/3-21+g(d) level of theory
rescent contrast. Finally, redox potential and uorescence (LANL2DZ for the iodine atoms). Infrared spectra were calcu-
wavelength and intensity can be tuned according to the nature lated on the nal geometry to ascertain that a minimum was
of the substituent on the triphenylamine core thanks to the obtained (no negative frequencies). Time-dependant density
considerable work which has already be done on triphenyl- functional theory (TD-DFT) calculations at the PBE0 level of
amine derivatives. A multi-color electrouorochromism could theory with the 6-31+g(d) basis set (LANL2DZ for the iodine
therefore be achieved using carefully chosen types of atoms) were subsequently performed. Reconstructed
34338 | RSC Adv., 2014, 4, 34332–34342
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4-[N,N-Di-(40-iodophenyl)amino]benzaldehyde 2. To a solu-
tion of 4-(N,N-diphenylamino)benzaldehyde 8 (5.01 g, 18.3
mmol, 1.0 eq.) in water (20 mL) were added potassium iodide
(3.97 g, 23.9 mmol, ꢁ1.3 eq.) and acetic acid (80 mL). The mixture
was warmed at 80 C and then potassium iodate (3.93 g, 18.4
mmol, 1.0 eq.) was added. The mixture was stirred at 80 ꢁC for 4
h. The reaction mixture was cooled to room temperature and
the solid product was ltrated and washed with ethanol. It was
recrystallized in ethanol/dichloromethane to give 2 as a white
solid (8.74 g, 91%). 1H NMR (400 MHz, CDCl3) d (ppm): 9.84 (s,
1H), 7.71 (d, J ¼ 8.7 Hz, 2H), 7.63 (d, J ¼ 8.7 Hz, 4H), 7.05 (d, J ¼
8.7 Hz, 2H), 6.89 (d, J ¼ 8.7 Hz, 4H); 13C NMR (100 MHz, CDCl3)
d (ppm): 190.6, 152.4, 145.8, 139.0, 131.5, 130.4, 127.8, 120.9,
88.9; IR (s in cmꢀ1): 1693, 1600, 1574, 1483, 1316, 1270, 1216,
1163, 1064, 817; m.p.: 144 ꢁC (lit.72 144); UV-vis (CH3CN): lmax(3)
¼ 313 (18 400 L cmꢀ1 molꢀ1), 355 (24 100 L cmꢀ1 molꢀ1); FF ¼
0.04 in CH3CN; s ¼ 1.16 ns; Eꢁ (C, CH2Cl2) vs. ferrocene: 0.78 V.
absorption spectra have been obtained by convolution of
Gaussian functions for each calculated transition with a full
width at half maximum (FWHM) of 3000 cmꢀ1 and a surface
equal to the oscillator strength as implemented in GaussSum.
The 3calc was thus obtained at the maximum of the band.
Electrouorochromism
A thin layer electrochemical cell71 (sodium chloride windows, Pt
minigrid working electrode, Ag pseudo-reference, Pt counter-
electrode, optical path ca. 200 mm) was lled with the solution
of the compound of interest. For uorescence spectroelec-
trochemistry, excitation and emission were focused on the
working electrode through optical bers connected to a Jobin-
Yvon Fluorolog 3 spectrophotometer. The uorescence near
the maximum emission wavelength was recorded vs. time
simultaneously to the current corresponding to low scan rate
cyclic voltammetry (10 mV sꢀ1). The electrochemical cell was
connected to a CHInstruments (CHI 600) potentiostat driven by
a PC. For UV-vis spectroelectrochemistry, the thin layer cell was
directly inserted in the spectrophotometer (Cary 5000, Varian).
The spectrum for open circuit potential is recorded and used as
the background for all other subsequent spectra.
4-[N,N-Bis-(40-bromophenyl)amino]benzonitrile 3. To
a
solution of 4-[N,N-di-(4-bromophenyl)amino]benzaldehyde
oxime 9 (200 mg, 0.45 mmol, 1.0 eq.) in anhydrous CH2Cl2 (0.05
M) were added 3,6-dichlorotetrazine (68 mg, 0.45 mmol, 1.0 eq.)
and 2,4,6-collidine (60 mL, 0.45 mmol, 1.0 eq.). The mixture was
stirred at room temperature for 3.5 hours then concentrated
under reduced pressure. The crude product was puried by a
silica gel ash chromatography [DCM/PE (5/5)] to give 9 (53 mg,
Synthesis
1
28%) as white solid. H NMR (400 MHz, CDCl3) d (ppm): 7.47–
7.39 (m, 6H); 6.94–7.01 (m, 6H); 13C NMR (100 MHz, CDCl3) d
(ppm): 150.8, 144.9, 133.5, 133.1, 127.4, 120.8, 119.3, 118.3,
104.2; IR (s in cmꢀ1): 3205–2922, 2214, 1604, 1579, 1503, 1482,
1313, 1287, 1268, 1175, 1068, 818; m.p.: 197 ꢁC (lit.40 197); UV-vis
(CH3CN): lmax(3) ¼ 311 (22 700 L cmꢀ1 molꢀ1); FF ¼ 0.09 in
CH3CN; s ¼ 10.38 ns; Eꢁ (C, CH2Cl2) vs. ferrocene: 0.85 V.
N,N,N-Tri-(4-methoxyphenyl)amine 4. In a Schlenk ask
under argon 4-bromoanisole (6.3 mL, 50 mmol, 3.1 eq.) was
added to a solution of tris(dibenzylideneacetone)dipalladium(0)
Pd2(dba)3 (857 mg, 0.936 mmol, 0.06 eq.) and 1,10-bis(diphe-
nylphosphino)ferrocene DPPF (801 mg, 1.47 mmol, 0.09 eq.) in
toluene (55 mL). The mixture was stirred at room temperature
for 10 minutes and then were added sodium tert-butoxyde (4.06
g, 42.2 mmol, 2.6 eq.) and p-anisidine (2.00 g, 16.2 mmol, 1.0
eq.). The mixture was heated at 90 ꢁC for 24 hours. Aer the
mixture was cooled down, dichloromethane (200 mL) and water
(200 mL) were added. Organic compounds were extracted with
dichloromethane (3 ꢂ 150 mL). The combinated organic phases
were dried over anhydrous sodium sulfate, ltrated and
concentrated under reduced pressure. The crude product was
puried by silica gel ash chromatography [DCM/PE (5/5) to
DCM] to give 4 (1.62 g, 30%) as white solid. 1H NMR (400 MHz,
CDCl3) d (ppm): 6.94 (d, J ¼ 8.7 Hz, 6H), 6.82 (d, J ¼ 8.7 Hz, 6H),
3.78 (s, 9H); 13C NMR (100 MHz, CDCl3) d (ppm): 154.3, 138.0,
119.6, 114.8, 55.8; IR (s in cmꢀ1): 2954, 1521, 1241, 1227, 1032,
830, 818; m.p.: 108 ꢁC (lit.73 102); UV-vis (CH3CN): lmax(3) ¼ 283
(25 500 L cmꢀ1 molꢀ1); FF ¼ 0.06 in CH3CN; s ¼ 3.64 ns; Eꢁ (C,
CH2Cl2) vs. ferrocene: 0.17 V
Reagents were commercially available from Aldrich and used
without further purication. Column chromatography was
performed with SDS 0.040–0.063 mm silica gel. All compounds
were characterized by the usual analytical methods: 1H, 13C
NMR spectra were recorded with a JEOL ECS (400 MHz) spec-
trometer. All chemical shis are referenced to the solvent peak
(J values are given in Hz). Melting points were measured with a
Koer melting-point apparatus. IR spectra were recorded with a
Nicolet Avatar 330 FT-IR spectrometer. This work has beneted
from the facilities and expertise of the Small Molecule Mass
Spectrometry platform of IMAGIF (Centre de Recherche de Gif –
http://www.imagif.cnrs.fr).
4-[N,N-Di-(40-bromophenyl)amino]benzaldehyde 1. To
a
solution of phosphorus oxychloride (1.5 mL, 16 mmol, 4.4 eq.)
in dimethylformamide (7.0 mL, 91 mmol, 24 eq.) at 0 ꢁC under
argon was added bis(4-bromophenyl)phenylamine (1.50 g, 3.72
ꢁ
mmol, 1.0 eq.). The mixture was stirred at 70 C for 60 h. The
mixture was then poured into the ice and neutralised with
saturated sodium carbonate solution (20 mL). The aqueous
layer was extracted with CH2Cl2 (3 ꢂ 20 mL). The organic layer
was dried over anhydrous sodium sulfate, ltered and concen-
trated under reduced pressure. The crude product was puried
by a silica gel ash chromatography [DCM/PE (5/5)] to give 1
(1.17 g, 67%) as yellow solid. 1H NMR (400 MHz, CDCl3) d
(ppm): 9.84 (s, 1H), 7.71 (d, J ¼ 8.7 Hz, 2H), 7.44 (d, J ¼ 9.2 Hz,
4H), 7.04 (d, J ¼ 8.7 Hz, 2H), 7.02 (d, J ¼ 8.7 Hz, 4H); 13C NMR
(100 MHz, CDCl3) d (ppm): 190.5, 152.4, 145.1, 133.0, 131.5,
130.2, 127.5, 120.5, 118.2; IR (s in cmꢀ1): 1486, 1073; m.p.:
154 ꢁC (lit.50 155–156); UV-vis (CH3CN): lmax (3) ¼ 353 (22 500 L
cmꢀ1 molꢀ1); FF ¼ 0.06 in CH3CN; s ¼ 2.67 ns; Eꢁ (C, CH2Cl2) vs.
ferrocene: 0.79 V.
N,N-Ditolyl-4-anisidine 5. To a solution of tris(dibenzylide-
neacetone)dipalladium(0) Pd2(dba)3 (120 mg, 0.131 mmol) and
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1,10-bis(diphenylphosphino)ferrocene DPPF (140 mg, 0.258 argon was added triphenylamine (5.00 g, 20 mmol, 1.0 eq.). The
mmol) in dried toluene (35 mL) under argon atmosphere was mixture was stirred at 70 ꢁC for 24 h. The mixture was then
added 4-bromotoluene (4 mL, 32.5 mmol) at room temperature. poured into the ice and neutralised with saturated sodium
The resulting mixture was stirred for 10 minutes. Then sodium carbonate solution (20 mL). The aqueous layer was extracted
tert-butoxide (2.070 g, 21.54 mmol) and 4-anisidine (0.99 g, 8.04 with CH2Cl2 (3 ꢂ 20 mL). The organic layer was dried over
mmol) were added. The resulting mixture was stirred at 110 ꢁC anhydrous sodium sulfate, ltered and concentrated under
for 19 hours then cooled to room temperature and concentrated reduced pressure. The crude product was puried by a silica gel
under reduced pressure. The crude product was puried by a ash chromatography [DCM] to give 8 (4.22 g, 77%) as yellow
silica gel column chromatography (petroleum ether/CH2Cl2 solid. 1H NMR (400 MHz, CDCl3) d (ppm): 9.80 (s, 1H), 7.67 (d, J
(8/2) until petroleum ether/CH2Cl2 (5/5)) to give 2.048 g of a grey ¼ 8.7 Hz, 2H), 7.34 (dd, J ¼ 7.8 and 7.8 Hz, 4H), 7.16 (m, 6H),
solid (93%). 1H NMR (400 MHz, CDCl3): d (ppm) ¼ 7.08–7.05 (m, 7.01 (d, J ¼ 8.7 Hz, 2H); 13C NMR (100 MHz, CDCl3) d (ppm):
6H), 6.97 (d, J ¼ 8.2 Hz, 4H), 6.84 (d, J ¼ 8.7 Hz, 2H), 3.82 (s, 3H), 190.6, 153.5, 146.3, 131.4, 129.9, 129.2, 126.4, 125.2, 119.5; IR
2.33 (s, 6H); 13C NMR (100 MHz, CDCl3): d (ppm) ¼ 155.7, 146.0, (s in cmꢀ1): 1687, 1583, 1504, 1488, 1330, 1304, 1289, 1220,
141.4, 131.3, 129.8, 126.5, 123.2, 114.7, 55.5, 20.8; IR (cmꢀ1): 1155, 824; m.p.: 132 C (lit.76 132).
ꢁ
3024–2830, 1606, 1499, 1273, 812; m.p.: 70 ꢁC (lit.74 70–73); UV-
4-[N,N-Di-(40-bromophenyl)amino]benzaldehyde oxime 9. To
vis (CH3CN): lmax(3) ¼ 301 (25 000 L cmꢀ1 molꢀ1); FF ¼ 0.03 in a solution of 4-[N,N-di-(40-bromophenyl)amino]benzaldehyde 1
CH3CN; s ¼ 5.22, 1.78 ns; Eꢁ (C, CH2Cl2) vs. ferrocene: 0.22 V.
(0.80 g, 1.8 mmol, 1 eq.) in dichloromethane (8 mL) was added a
N,N,N-Tri-(40-methoxy-1,10-biphenyl)amine 6. In a Schlenk solution of hydroxylamine hydrochloride (0.52 g, 7.4 mmol, 4
ask under argon tetrakistriphenylphosphine Pd(PPh3)4 (98 eq.) and sodium acetate (0.76 g, 9.3 mmol, 5 eq.) in ethanol (24
mg, 0.085 mmol, 0.1 eq.) was added to a solution of N,N,N-tri-(4- mL). The mixture was stirred at room temperature for 3 h and
bromophenyl)amine (413 mg, 0.857 mmol, 1.0 eq.) in toluene concentrated under reduced pressure. The crude product was
(20 mL). The mixture was stirred at room temperature for 15 puried by a silica gel ash chromatography [DCM/PE (1/9) to
1
minutes and then were added solutions of 4-methoxy- DCM/EtOH (9.7/0.3)] to give 9 (0.80 g, 97%) as white solid. H
phenylboronic acid (673 mg, 4.43 mmol, 5.2 eq.) in methanol NMR (400 MHz, CDCl3) d (ppm): 8.44 (s, 1H), 8.08 (s, 1H), 7.44
(6.5 mL), of sodium carbonate (561 mg, 5.29 mmol, 6.2 eq.) in (d, J ¼ 8.7 Hz, 2H), 7.37 (d, J ¼ 8.7 Hz, 4H), 7.01 (d, J ¼ 8.2 Hz,
distilled water (2.5 mL). The mixture was heated at 80 ꢁC for 84 2H), 6.96 (d, J ¼ 8.7 Hz, 4H); 13C NMR (100 MHz, CDCl3) d
hours. Aer the mixture was cooled down, dichloromethane (50 (ppm): 149.7, 148.6, 145.9, 132.7, 128.3, 126.5, 126.3, 123.1,
mL) and saturated ammonium chloride solution (50 mL) were 116.6; IR (s in cmꢀ1): 3289, 1605, 1581, 1513, 1486, 1313, 1289,
added. Organic compounds were extracted with dichloro- 1069, 926, 907, 871, 823; m.p.: 104 ꢁC; MS: calcd for
methane (3 ꢂ 50 mL). The combinated organic phases were
dried over anhydrous sodium sulfate, ltrated and concentrated
under reduced pressure. The crude product was puried by
silica gel ash chromatography [DCM/PE (5/5)] to give 6 (305
mg, 63%) as a white solid. 1H NMR (400 MHz, DMSO) d (ppm):
7.59 (d, J ¼ 8.7 Hz, 6H), 7.58 (d, J ¼ 8.7 Hz, 6H), 7.12 (d, J ¼ 8.7
Hz, 6H), 7.00 (d, J ¼ 9.2 Hz, 6H), 3.78 (s, 9H); 13C NMR (100
MHz, DMSO) d (ppm): 159.0, 146.5, 135.3, 133.4, 127.8, 127.5,
124.5, 114.3, 55.5; IR (s in cmꢀ1): 2991, 2965, 1391, 1252, 1061;
m.p.: 200 ꢁC (lit.75 237–240); UV-vis (CH3CN): lmax(3) ¼ 357
(54 300 L cmꢀ1 molꢀ1); FF ¼ 0.37 in CH3CN; s ¼ 9.95 ns; Eꢁ (C,
CH2Cl2) vs. ferrocene: 0.25 V.
Bis(4-bromophenyl)phenylamine 7. To a solution of bis(4-
bromophenyl)amine (7.57 g, 23.1 mmol, 1.0 eq.) in toluene
(50 mL) were added cyclohexane-1,4-dione (2.80 g, 25.0 mmol,
1.1 eq.) and p-toluene sulfonic acid (0.53 g, 0.31 mmol, 0.01 eq.).
The mixture was stirred at reux with a Dean–Stark apparatus
for 9 hours then concentrated under reduced pressure. The
crude product was puried by a silica gel ash chromatography
[DCM/PE (1/9)] to give 7 (5.77 g, 62%) as colorless oil. 1H NMR
(400 MHz, CDCl3) d (ppm): 7.33 (d, J ¼ 8.7 Hz, 4H), 7.26 (dd, J ¼
7.8 and 7.8 Hz, 2H), 7.06 (m, 3H), 6.23 (d, J ¼ 8.7 Hz, 4H); 13C
NMR (100 MHz, CDCl3) d (ppm): 147.0, 147.7, 132.5, 129.7,
125.5, 124.7, 123.9, 115.6; IR (s in cmꢀ1): 1580, 1483, 1311, 1286,
1272, 1069, 1009, 841, 823, 813.
C
19H1479Br2N2O [M + H]+ 444.9546, found 444.9555.
Acknowledgements
We thank the CNRS and the Ministry of French Research for
funding the project. We also thank Dr C. Allain for fruitfull
discussions.
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