Acid-responsive behavior promoted by imine units in novel triphenylamine-based oligomers functionalized with chromophoric moieties

Article abstract of DOI:10.1016/j.jphotochem.2019.04.010

A series of oligomeric poly(ether-imine)s, named by us oligo(ether-imine)s were synthesized by polycondensation reaction of some di(ether-aldehyde)s with a CF₃-substituted and ortho-catenated triphenylamine (TPA) diamine. The obtained TPA-based

Full text of DOI:10.1016/j.jphotochem.2019.04.010

Accepted Manuscript
Title: ACID-RESPONSIVE BEHAVIOR PROMOTED BY
IMINE UNITS IN NOVEL TRIPHENYLAMINE-BASED
OLIGOMERS FUNCTIONALIZED WITH
CHROMOPHORIC MOIETIES
Authors: Andra-Elena Bejan, Mariana-Dana Damaceanu
PII:
S1010-6030(19)30281-3
DOI:
Reference:
https://doi.org/10.1016/j.jphotochem.2019.04.010
JPC 11799
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date:
Revised date:
Accepted date:
15 February 2019
29 March 2019
8 April 2019
Please cite this article as: Bejan A-Elena, Damaceanu M-Dana, ACID-RESPONSIVE
BEHAVIOR PROMOTED BY IMINE UNITS IN NOVEL TRIPHENYLAMINE-
BASED OLIGOMERS FUNCTIONALIZED WITH CHROMOPHORIC MOIETIES,
Journal of Photochemistry and amp; Photobiology, A: Chemistry (2019),
https://doi.org/10.1016/j.jphotochem.2019.04.010
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ACID-RESPONSIVE BEHAVIOR PROMOTED BY IMINE UNITS IN NOVEL
TRIPHENYLAMINE-BASED
CHROMOPHORIC MOIETIES
OLIGOMERS
FUNCTIONALIZED
WITH
Andra-Elena Bejan and Mariana-Dana Damaceanu*
"Petru Poni" Institute of Macromolecular Chemistry, Polycondensation and Thermostable
Polymers Department, Aleea Grigore Ghica Voda 41A, Iasi-700487, ROMANIA
* Corresponding author, email: damaceanu@icmpp.ro

Graphical abstract
Highlights


Oligo(ether-imine)s containing ortho-catenated triphenylamine units are developed
The excellent solubility enabled processing in films with granular/vesicular
morphology



The occurrence of a dual intramolecular charge transfer is suggested
Hipsochromically shifted UV-Vis absorption bands are obtained by acid protonation
In the acidic environment the quenched fluorescence of oligoimines is turned-on
ABSTRACT
A series of oligomeric poly(ether-imine)s, called by us oligo(ether-imine)s were synthesized
by polycondensation reaction of some di(ether-aldehyde)s with a CF3-substituted and ortho-
catenated triphenylamine (TPA) diamine. The obtained TPA-based oligomers with ortho-link
structural pattern, not encountered yet for this class of compounds, displayed excellent
organosolubility in the majority convenient and accessible solvents. This enabled their easy
processing in homogenous thin film with granular or vesicular morphology, another novel
feature highlighted for this class of compounds. The photo-optical activity of oligo(ether-
imine)s was investigated in detail by UV-Vis and fluorescence spectroscopy, which evidenced
the occurrence of a dual intramolecular charge transfer between TPA and both CF3 units and
imine-phenyl framework. The acid sensing properties were surveyed both in solution and

solid state, being found that upon protonation the planarity is lessen, the donor-acceptor
strength decreases and the ICT bands are hipsochromically shifted. Instead, due to pH-
induced conformational changes of the present molecules, the quenched fluorescence of the
neutral states was turned-on. The redox behavior measured by cyclic voltammetry before and
after protonation was contingent on the type of chromophoric unit incorporated into the
dialdehyde segment. Owing to their ability to promote favorable, multifunctional properties,
applications of these oligomers in optoelectronic devices or pH-sensors are foreseen.
Keywords: oligoimines; triphenylamine; intramolecular charge transfer; acid sensing;
turned-on fluorescence.
1. INTRODUCTION
Conjugated oligomers and polymers are used as viable alternatives to conventional
inorganic-based compounds [1] in various electronic or optoelectronic devices due to their
excellent optoelectronic behavior, light-weight, flexibility, large area processing or low
fabrication cost [2-4]. Imine-based compounds (also called azomethines or Schiff bases) are
among the large variety of conjugated compounds of increasing interest with great potential
for photonic and optoelectronic applications [5-7]. Unlike their isoelectronic vinylene
analogues, aromatic oligo- and polyimines can be obtained under environmentally friendly
conditions by simple synthesis which does not claim stringent reactions [8,9]. They exhibit
high thermal stability, semiconducting and optical properties, liquid crystalline potential,
complexation capability or biological activity, which enabled their use in light emitting
diodes, solar cells, catalysts, detectors or drugs [6,10,11].
Due to the basic character of the imine nitrogen, changes in optoelectronic features of
imine derivatives are possible by protonation with strong acids which endow these systems
with acid-sensory properties. For instance, the acid-base chemistry was applied to change the

absorption and photoluminescence properties of polyimines when bathochromic or
hypsochromic shift of absorption and/or emission bands were found after protonation in
correlation with the structural motif, kind of dopant or solvent polarity [12-17].
Unfortunately, the rigid rod like aromatic polyimine backbones induce low organo-
solubility and processing ability, consequently restricted availability and performance in the
above mentioned applications [18]. Various attempts have been made over the time to
improve the solubility of polyimines, the most used being the introduction of unsymmetrical
or symmetrical substitutions of the main-chain aromatic rings with flexible alkyl or alkoxy
side chains [19], polymerization in the cyclodextrin cavity [20] or insertion of flexible or
bulky, packing disruptive groups into the polymer backbones [21-23]. In this regard, the
incorporation of flexible ether linkages into polymer backbone is of particular concern due to
the breaking induction of linear progression and lower internal energy of rotation [24].
Besides, the ether linkage can promote other valuable properties, such as lower dielectric
constant, good optical transparency and better toughness compared to those polymers lacking
these units [25,26]. On the other hand, the introduction of bulky units in polymer chains can
also improve the solubility without compromising the thermal stability and other useful
properties. Thus, a successful approach proved to be the incorporation of the propeller shape,
three-dimensional triphenylamine (TPA) which can generates amorphous structures, while
increasing the solubility and film-forming capability by dramatically decreasing the
intermolecular aggregation [12,27]. Besides, TPA-based derivatives are characterized by high
thermal and morphological stability, the most important features being the excellent electron-
donating and optoelectronic properties that led to many valuable applications [28,29]. Hence,
TPA-based oligomers and polymers are successfully applied in light-emitting diodes [30],
electrochromic devices [31], dye-sensitized solar cells [32,33], organic field-effect transistors
[34], or photorefractive devices [35].

Nowadays, despite the significant importance of solution-processing TPA-based
polyimines for advanced applications, there are no reports in the literature on ortho-catenated
TPA-based polyimines, to the best of our knowledge. The connection of TPA group into
ortho position may provide a strong kink of the polymeric backbone, consequently leading to
the desired organo-solubility and interesting photo- and electroactivity responses. In this
context, the present research focusses on the design, synthesis and characterization of four
novel oligo(ether-imine)s in which the TPA unit is incorporated through the ortho position.
Moreover, we changed the substitution pattern of dialdehyde segments by introducing
different building blocks such as diphenyl, naphthyl, diphenylfluorene or phenolphthalein,
with the aim to modulate the conjugation degree, optical properties and solid state packing
ability. In addition to the general properties such as solubility, morphology and
thermostability, the influence of the molecular architecture onto electrochemical and photo-
optical behaviors was explored in detail. Furthermore, this article brings evidence for the acid-
responsive behavior of these oligomers promoted by the imine unit with a special emphasis on
the pH-induced conformational changes which turned-on the quenched fluorescence of the
neutral states.
2. EXPERIMENTAL
2.1. Starting materials
4-Fluorobenzaldehyde
(98%),
4,4′-dihydroxybiphenyl
(97%),
2,7-
dihydroxynaphthalene (97%), 4,4′-(9-fluorenylidene)diphenol (97%), phenolphthalein (98-
102%, calc. to the dried substance), sodium hydroxide (≥97.0%, NaOH), sodium hydride
(60% dispersion in mineral oil, NaH), dimethylsulfoxide (99.7%, DMSO), N-methyl-
pyrrolidone (HPLC grade, NMP), tetrahydrofuran (≥99.0%, THF), ethanol (99.5%),
petroleum ether (puriss, p.a., b.p. 40-60°C), ethyl acetate (anhydrous, 99.8%, EtOAc),

chloroform (anhydrous, 99%, CHCl3), N,N-dimethylformamide (anhydrous, 99.8%, DMF), p-
toluenesulfonic acid monohydrate (ACS reagent, ≥98.5%, p-TSA), methanol (anhydrous,
99.8%), acetonitrile (anhydrous, 99.8%, ACN), dichloromethane (HPLC grade, 99.9%,
DCM), tetrabuthylammonium perchlorate (for electrochemical analysis, ≥99.0%, TBAP),
potassium carbonate (anhydrous, 99.99%, K2CO3), indium tin oxide coated glass slide
(surface resistivity 30-60 Ω/sq, ITO) were purchased from Sigma-Aldrich. All reagents were
used without further purification.
2.2. Measurements
The NMR spectra were acquired on a Bruker Avance III 400 spectrometer operating at
1
1
400.1 and 100.6 MHz for H and 13C nuclei, respectively. H and 13C chemical shifts are
reported in δ units (ppm) relative to the residual peak of the solvent (ref. 1H: CDCl3 7.26 ppm;
13C: CDCl3 77.01 ppm, 1H: DMSO-d6: 2.51 ppm; 13C: DMSO-d6: 39.51 ppm).
The Fourier-transform infrared spectroscopy (FTIR) was recorded on a FT-IR Bruker
Vertex 70 Spectrophotometer in ATR mode, by using oligomer powders.
The average molecular weights were evaluated by gel permeation chromatography
(GPC) using a ParSEC Chromatography Ver. 5.67 Brookhaven Instruments Corp. apparatus
equipped with refraction and UV detectors and PL Mixed C Column. All measurements were
performed on oligomer solutions in chloroform. Calibration was carried out with polystyrene
standards (580-467,000 Da) having narrow molecular weight distribution.
The thermal stability was evaluated by thermogravimetric (TG) and differential
thermal analysis (DTA) measurements using a STA 449F1 Jupiter derivatograph (Netzsch),
operating at a heating rate of 10°C/min, in nitrogen atmosphere, from room temperature to
700°C.

UV-Vis absorption spectra were measured with a Shimadzu 1280 spectrophotometer
operating between 200 and 1100 nm, by using diluted polymer solutions (10-5
concentration) and thin polymer films deposited by spin-coating on quartz plates.
M
The photoluminescence spectra of oligoimines were recorded on a Perkin Elmer LS 55
apparatus using the samples in liquid or condensed phase.
The fluorescence quantum yield (QY) was measured with FLS 980 apparatus
(Edinburgh Instruments, England) equipped with aQuanta-phi integrating sphere accessory.
Oligomer solutions with absorbance lower than 0.1 were used for all experiments.
The morphology of the thin films as-deposited on glass slides was analyzed by
scanning electron microscopy (SEM) with a SEM/ESEM-EDAX- QUANTA 200 microscope
(Graz, Austria).
Cyclic voltammetry (CV) experiments were carried out on a Potentiostat-Galvanostat
(PG581, Uniscan Instruments) at a scan rate of 50 mV/s, with 0.1 M solutions of
tetrabuthylammonium perchlorate (TBAP) as electrolytes in acetonitrile (ACN) for oligomer
solution samples (110 − 3 M concentration) and dry methanol for oligomer films coated on
ITO disks. The redox potentials of oligomer films were measured using a Pt wire as counter
electrode and Ag/AgCl as reference electrode, while for oligomer solutions Pt electrode was
used as working electrode. The calibration was made with ferrocene/ferrocenium (Fc/Fc+)
redox couple (+0.42 vs. Ag/AgCl). The HOMO (Highest Occupied Molecular Orbital) and
LUMO (Lowest Unoccupied Molecular Orbital) energies were evaluated by assuming the
absolute energy level of Fc/Fc+ as -4.80 eV versus vacuum.
2.3. Synthesis of monomers
Ether-containing dialdehyde monomers, M1-M4 were synthesized as shown in
Scheme 1 and the detailed synthetic procedures are as follows.

4,4'-([1,1'-biphenyl]-4,4'-diylbis(oxy))dibenzaldehyde (M1) was obtained according
to the reference [36], following a slightly modified procedure. A mixture of 4,4′-
dihydroxybiphenyl (100 mg, 0.53 mmol) and sodium hydroxide (52 mg, 1.29 mmol) was
heated with stirring in 3 ml DMSO at 100°C, under strong nitrogen stream. After 6 h, 4-
fluorobenzaldehyde (149 mg, 1.2 mmol) was added dropwise to the reaction mixture and the
temperature was raised to 120°C. The reaction mixture was stirred for another 24 h at this
temperature, followed by cooling and pouring into water to precipitate the dialdehyde
monomer. The purification was carried out by recrystallization from a mixture of
tetrahydrofuran and ethanol, when 169 mg of M1 was obtained as yellow powder with 80%
yield.
1H NMR (CDCl3, ppm): 9.94 (s, 2H, Ha), 7.87 (d, 4H, 8.8 Hz, Hb), 7.62 (d, 4H, 8.4 Hz, He),
7.17 (d, 4H, 8.8 Hz, Hc), 7.12 (d, 4H, 8.4 Hz, Hd).
13C NMR (CDCl3, ppm): 190.76 (C1), 163.03 (C5), 154.70 (C6), 136.98 (C9), 131.99 (C3),
131.43 (C2), 128.66 (C8), 120.71 (C4), 117.74 (C7).
4,4'-(naphthalene-2,7-diylbis(oxy))dibenzaldehyde (M2) [37]
A solution of 2,7-dihydroxynaphthalene (500 mg, 3.12 mmol) in 2 mL NMP was
added dropwise over a solution of sodium hydride (60% dispersion in mineral oil, 312 mg,
7.81 mmol) in 2 ml NMP kept on an ice bath under nitrogen atmosphere. The reaction
mixture was stirred at 0°C for 2h, after that it was heated with stirring at 100°C for 5h. Then,
4-fluorobenzaldehyde (775 mg, 6.25 mmol) dissolved in 2 mL NMP was added dropwise, and
the temperature was increased to 120°C. The resulting reaction mixture was stirred for
another 24 h. The reaction was allowed to reach room temperature and quenched with water.
The organic phase was extracted with chloroform, and the combined organic phase was

washed with water and dried over anhydrous Na2SO4. After removing the solvent under
reduced pressure, the residue was purified by column chromatography using petroleum
ether:ethyl acetate = 10:1, to obtain the desired compound M2 as an orange powder with 26%
yield.
1H NMR (CDCl3, ppm): 9.94 (s, 2H, Ha), 7.91 (d, 2H, 8.8 Hz, He), 7.88 (d, 4H, 8.8 Hz, Hb),
7.38 (s, 2H, Hf), 7.24 (d, 2H, 8.8 Hz, Hd), 7.13 (d, 2H, 8.8 Hz, Hc).
13C NMR (CDCl3, ppm): 190.75 (C1), 162.76 (C5), 153.91 (C6), 135.35 (C10), 132.01 (C3),
131.62 (C2), 130.31 (C8), 128.20 (C9), 119.80 (C7), 118.09 (C4), 115.79 (C11).
4,4'-(((9H-fluorene-9,9-diyl)bis(4,1-phenylene))bis(oxy))dibenzaldehyde (M3)
The reaction was carried out in a similar way as described in [38], following a slightly
modified procedure for which 500 mg (1.428 mmol) 4,4′-(9-fluorenylidene)bisphenol, 68 mg
(1.713) mmol NaH 60% dispersion in mineral oil and 354 mg (2.856) mmol 4-
fluorobenzaldehyde were used. After quenching the reaction, the crude product was isolated
by filtration. The purification was made by recrystallization from a mixture of tetrahydrofuran
and ethanol to obtain M3 as a yellow powder with a 32% yield.
1H NMR (CDCl3, ppm): 9.92 (s, 2H, Ha), 7.83 (m, 4H, Hb+Hi), 7.44 (m, 4H, Hh+Hf), 7.34 (t,
2H, 7.4 Hz, Hg), 7.26 (m, overlapped with CDCl3 signal, 4H, He), 7.06 (d, 4H, 8.4 Hz, Hd),
6.96 (d, 4H, 8.8 Hz, Hc);

13C NMR (CDCl3, ppm): 190.75 (C1), 162.91 (C5), 153.96 (C6), 150.84 (C16), 142.28 (C11),
140.04 (C9), 131.91 (C3), 131.32 (C2), 129.75 (C8), 127.91 (C15), 127.81 (C14), 126.03
(C13), 120.39 (C12), 119.96 (C4), 117.71 (C7), 64.50 (C10).
4,4'-(((3-oxo-1,3-dihydroisobenzofuran-1,1-diyl)bis(4,1-phenylene))bis(oxy))-
dibenzaldehyde (M4)
The reaction was performed as described above for M1 by using 100 mg (0.31 mmol)
phenolphthalein, 30 mg (0.76 mmol) NaOH and 86 mg (0.69 mmol) 4-fluorobenzaldehyde.
The crude product, after isolation by precipitation into cold water, was purified by
recrystallization from ethanol. Yield: 35%.
1H NMR (CDCl3, ppm): 9.89 (s, 2H, Ha), 7.96 (d, 1H, 7.2 Hz, Hf), 7.83 (d, 4H, 8.4 Hz, Hb),
7.73 (t, 1H, 15.2 Hz, Hh), 7.59 (m, 2H, Hg+Hi), 7.38 (d, 4H, 8.8 Hz, He), 7.06 (m, 4H, Hc+Hd).
13C NMR (CDCl3, ppm): 190.65 (C1), 169.35 (C17), 162.25 (C5), 155.57 (C6), 151.58 (C11),
136.90 (C9), 134.40 (C13), 131.89 (C3), 131.63 (C2), 129.65 (C15), 128.97 (C8), 126.20
(C12), 125.37 (C16), 123.94 (C14), 90.78 (C10).
Synthesis of N,N-bis[2-amino-4-(trifluoromethyl)phenyl]-4-chloroaniline (M5)
was carried out in two steps, starting from 4-chloro-3-nitrobenzotrifluoride and p-
chloroaniline
following
a
reported
procedure
[39].
Briefly,
2-nitro-4-
(trifluoromethyl)chlorobenzene reacted with p-chloroaniline in the presence of K2CO3 when a
dinitro-derivative was obtained, which was further subjected to the reduction reaction with
Pd/C and hydrazine monohydrate in ethanol to obtain the diamine monomer.

2.4. Synthesis of TPA-based oligo(ether-imine)s
The oligomers reported here, O1-O4 were synthesized by solution polycondensation
reaction of equimolar amounts of triphenylamine (TPA)-based diamine, M5 with one of the
above described dialdehydes, M1-M4, in DMF (O1) or NMP (O2, O3, O4) as solvents, in the
presence of p-toluenesulfonic acid (p-TSA) as catalyst, except for O1 when no catalyst was
used (Scheme 2). In a typical experiment, oligo(ether-imine) O2 was prepared as follows:
diamine M5 (55 mg, 0.125 mmol), NMP (1.25 mL) and p-TSA (catalytic amounts) were
placed into a Schlenk tube and the resulting mixture was stirred under nitrogen until complete
dissolution. Then, the dialdehyde monomer M2 (70 mg, 0.125 mmol) was added and the
reaction mixture was stirred at 120ºC. The evolution of the reaction was monitored by thin
layer chromatography, and the reaction was stopped when no monomers were present
(approx. 48 h). The resulting O2 oligomer was precipitated by pouring into cold methanol,
followed by repeated washing with a mixture 1:4 v/v of water with methanol. After drying in
an oven at 100°C under vacuum, it was obtained in 60% yield.
GPC measurements of the investigated imines in CHCl3 solutions provided number
average molecular weight (Mn) values between 1030 and 2650 Dalton and polydispersity
index (Mw/Mn) between 1.50 and 1.89, demonstrating their oligomeric nature. This is the
reason why we called these compounds oligo(ether-imine)s and not poly(ether-imine)s.
However, it should be stated that these values may represent a crude estimation of molecular
weights and not an accurate one, since calibration was made with polystyrene standards with
polarity and backbone stiffness totally different of the studied compounds.
F3C
CF3
N
H
N
C
O
O
C
N
H
Cl

O1: Yield: 63%; FTIR (ATR): 3040 cm-1 (aromatic C-H stretching), 1701 cm-1 (CHOend
stretching), 1629 cm-1 (N=CH stretching), 1598, 1506, 1487 cm-1 (aromatic C=C stretching),
1325 cm-1 (C-NTPA stretching), 1233 cm-1 (C-O-C stretching), 1116 cm-1 (C-F stretch), 1076
cm-1 (C-Cl stretch), 823 cm-1 (p-substituted benzene).1H NMR (DMSO-d6, 25 °C), δ (ppm):
8.23 (-N=CH-), 9.95 (-CH=Oend).
F3C
CF3
N
H
N
C
O
O
C
N
H
Cl
O2: Yield: 35%; FTIR (ATR): 3059 cm-1 (aromatic C-H stretching), 1705 cm-1 (CHOend
stretching), 1628 cm-1 (N=CH stretching), 1596, 1500, 1488 cm-1 (aromatic C=C stretching),
1328 cm-1 (C-NTPA stretching), 1227 cm-1 (C-O-C stretching), 1118 cm-1 (C-F stretching),
1
1070 cm-1 (C-Cl stretching), 825 cm-1 (p-substituted benzene). H NMR (DMSO-d6, 25 °C),
δ (ppm): 8.32 (-N=CH-), 9.94 (-CH=Oend).
F3C
CF3
N
H
N
C
O
O
C
N
H
Cl
O3: Yield: 45%; FTIR (ATR): 3064 cm-1 (aromatic C-H stretching), 1706 cm-1 (CHOend
stretching), 1626 cm-1 (N=CH stretching), 1592, 1498, 1474 cm-1 (aromatic C=C stretching),
1331 cm-1 (C-NTPA stretching), 1239 cm-1 (C-O-C stretching), 1121 cm-1 (C-F stretching),
1
1077 cm-1 (C-Cl stretching), 829 cm-1 (p-substituted benzene). H NMR (DMSO-d6, 25 °C),
δ (ppm): 8.24 (-N=CH-), 9.92 (-CH=Oend).

F3C
CF3
N
H
N
C
O
O
C
N
H
Cl
O
O
O4: Yield: 34%; FTIR (ATR): 3063 cm-1 (aromatic C-H stretching), 1768 cm-1 (C=O
stretching), 1702 cm-1 (CHOend stretching), 1624 cm-1 (N=CH stretching), 1596, 1500, 1467
cm-1 (aromatic C=C stretching), 1334 cm-1 (C-NTPA stretching), 1241 cm-1 (C-O-C stretching),
1121 cm-1 (C-F stretching), 1084 cm-1 (C-Cl stretching), 834 cm-1 (p-substituted benzene).
1H NMR (DMSO-d6, 25 °C), δ (ppm): 8.32 (-N=CH-), 9.96 (-CH=Oend).
2.5. Preparation of polymer films
Very diluted oligomer solutions in tetrahydrofuran or chloroform, with concentration
of 1% were used to obtain nanoscale thin polymer films on various supports (glass, quartz or
indium tin oxide (ITO)-coated glass plates) by drop-casting or spin-coating technique, at a
speed rate of 1000 rpm. Prior spin-coating, the oligomer solutions were subjected to filtration
trough a 0.45 μm pore size of PTFE membrane syringe filter. These films, as deposited, were
gradually heated from room temperature up to 80°C and maintained at this temperature for 2h
to remove the residual solvent. The obtained films were used for further measurements.
3. RESULTS AND DISCUSSION
3.1. Design and structural characterization
The oligoimines reported in this work combine some structural features, e.g. ortho-
catenated triphenylamine core, conjugated imine bridges, a chromophoric moiety, such as a
biphenyl, naphthalene, 9,9-diphenylfluorene or phenolphthalein, and flexible ether linkages
which makes the well-known compromise structure/properties. The choice was to develop

processable compounds into deffectless thin films under environmentally friendly conditions,
without scarifying the inherent high thermal stability of imine derivatives, while preserving
their appealing optical properties and the expectable electroactivity due to the imine-
chromophore framework for future use in electro-optical applications. To this scope, four
aromatic dialdehydes incorporating ether linkages were successfully prepared by one step
procedure, as shown in Scheme 1. The synthesis of M1-M4 failed under nucleophilic
displacement reaction conditions of 4-fluorobenzaldehyde with various aromatic diols, like
4,4′-dihydroxybiphenyl, 2,7-dihydroxynaphthalene, 4,4′-(9-fluorenylidene)diphenol or
phenolphthalein, using a weak base such as K2CO3. Therefore, modified procedures were
applied by using a stronger base, e.g. NaOH or NaH and different solvents which gave
reasonable yields of dialdehyde monomers and less mono-substituted side product under
moderate temperature conditions. The resulting dialdehyde monomers were structurally
confirmed by 1H- and 13C-NMR spectroscopy as shown in Figure S1, Supporting Information
(SI).
Scheme 1. Synthetic pathway to dialdehyde monomers containing ether groups
Ortho-catenated triphenylamine (TPA)-based oligoimines O1-O4 were prepared by a
simple one step procedure involving high-temperature solution polycondensation reaction of
equimolar quantities of the triphenylamine diamine with one of the four synthetized

dialdehydes having different reactivity, as shown in Scheme 2. The reaction conditions were
optimized until satisfactory reaction yields. The oligoimine O1 has been prepared in DMF as
solvent without catalyst, while for O2, O3 and O4 the reactions were carried out in NMP, in
the presence of p-toluenesulfonic acid (p-TSA) as catalyst. No future attempts were made to
end-cap the oligomer chains. The oligoimines were obtained in light-yellow (O1), brown (O2,
O3) or dark-yellow (O4) powder-like forms. The reaction yield was found to be largely
dependent on the electron-donating potential of the amine and on the electron affinity of the
dialdehyde. Thus, values between 34 and 63% were obtained, being lower for the reaction
involving naphthalene-containing dialdehyde and higher when the biphenyl-containing
dialdehyde was used.
Scheme 2. Synthesis of TPA-based oligo(ether-imine)s

The O1-O4 structures were fully confirmed by 1H NMR and FTIR spectroscopy. The
peaks localized in the range of 8.32 - 8.22 ppm were associated with the proton resonance of
the imine group, confirming the formation of the -CH=N- bond. In addition, the NMR data
provided evidence for the total incorporation of the monomers into corresponding oligoimine
l
chains since no H-NMR signals were found due to the protons of the starting diamine and
dialdehyde monomers. The peaks from 9.92-9.96 ppm indicated the presence of end-capped
aldehyde protons which were slightly shifted compared to those found for monomer aldehyde
protons, suggesting the successful conversion into imine structure and the low reactivity of
the TPA-based diamine monomer. Figure S2, SI shows the 1H NMR spectrum of O1.
All oligomers exhibited characteristic absorption bands of the newly formed imine
moiety between 1629 and 1624 cm-1, at a shift depending on the conjugation length of each
oligomer (figure S3, SI). While the IR bands associated with NH2 group (3499‒3383 cm-1)
from the diamine monomer [39] totally disappeared, small shoulder like peaks were identified
in the FTIR spectra of all oligomers at 1706-1701 cm-1, being associated with end-chain
aldehyde groups. Aromatic C-H and C=C linkages in phenylene rings were identified by the
absorption bands at about 3064-3040 cm-1 and 1500-1498 cm-1, respectively. The strong
absorption band registered in FT-IR spectra of all oligomers in the range of 1241-1227 cm-1
were attributed to the aromatic ether stretching. The C–N stretching in the triphenylamine
core was found in the range of 1334–1325 cm-1. The presence of CF3 groups on benzene rings
was evidenced by the clear absorption peaks between 1121 and 1116 cm-1 due to the
stretching vibrations of C-F bonds [40]. Finally, the absorption bands from 1084-1070 cm-1
were linked with the C-Cl bond vibrations.
The solubility behavior of oligoimines O1-O4 was qualitatively evaluated in several
solvents, using 1 mg oligomer /1 ml solvent, and the results are summarized in Table S1, SI.
The incorporation of ortho-catenated TPA group into oligoimines led to easily soluble

compounds at room temperature in polar aprotic solvents, such as NMP, DMAc, DMF and
DMSO and even in less polar solvents, such as THF, DCM and CHCl3, which are convenient
and accessible solvents. The good organosolubility of these oligomers can be explained, on
one hand, by the molecular nonlinearity promoted by the ortho-connected phenylene rings
coming from the diamine segment and flexible ether bonds coming from the dialdehyde
segment and, on the other hand, by the presence of bulky, propeller-shaped triphenylamine
unit which hinders the chains packing into tight structures.
3.2. Thermal properties
The thermal features of oligo(ether-imine)s O1-O4 were investigated by
thermogravimetric (TG) analysis and differential thermal analysis (DTA), under nitrogen
atmosphere. Generally, the thermal stability is evaluated on the basis of the temperature at
which a specific weight loss takes place, typically 5 or 10%, or the temperature when the
compound starts to loss mass, known as onset temperature (IDT). Thus, the IDT and the
temperature of 10% weight loss (T10%) of O1-O4 were evaluated and listed in Table 1.
Table 1. Thermal properties of oligo(ether-imine)s O1-O4.
1st IDT
(oC)
237
2nd IDT
(oC)
Tmax1
(oC)
264
Tmax2
(oC)
380
T10%
(oC)
407
W670
(%)
68.55
Oligomer
357
O1
O2
O3
O4
286
340
337
466
492
467
337
406
411
541
534
489
328
412
402
51.01
62.79
61.76
IDT = temperature corresponding to the onset on the TG curve
T10% = temperature of 10% weight loss on the TG curve
Tmax= temperature corresponding to the maximum rate of decomposition on DTG
curve
W670 = char yield at 670°C
The TG curves indicated two-stage degradation processes: the first step begins at
temperatures between 237 and 340°C; the second one occurs between 357°C and 492°C. The

values of T10% were found in the range of 328-407°C, indicating a fairly high thermal
stability. The first step of decomposition is assumed to take place at the lateral substituents,
which are more sensitive at high temperatures. The second one is attributed to the degradation
of the oligoimine main chains. For each sample, the degradation process is not complete, the
remaining char yields being above 51.01% at 670°C, under nitrogen atmosphere. The TG
curves of O1-O4 are ilustrated in Figure 1.
Figure 1. TG curves of oligo(ether-imine)s O1-O4.
Based on the onset temperatures, the thermogravimetric analysis clearly illustrated that the
order of stability in nitrogen atmosphere is O3 > O4 > O2 > O1, depending on the stiffness
and molecular packing of the investigated oligoimines. O3 and O4 exhibited the highest
thermal stability in this series due to the presence of fluorene and phenolphthalein moieties
which restricts the rotational motion within the chains due to their bulky nature, thus
enhancing the rigidity and the degree of packing. The highest thermal stability of O2
compared to O1 can be judged in terms of the more rigid and planar naphthalene unit with
respect to the biphenyl moiety. Due to the lower molecular weights and decreased
conformational flexibility or free volume caused by the introduction of voluminous or planar

groups, the oligomers from this series are less thermally stable than the poly(ether-imide)
series based on the same diamine [41] or other polyimines based on triphenylamine [42].
DTA analysis of O1-O4 showed no clear evidence of a measurable glass transition or
melting point in the experimental temperature range.
3.3. Films quality and morphology
The quality and morphology of the thin oligoimine films spin-coated from THF
solutions on glass slides were analyzed by scanning electron microscopy (SEM), a powerful
technique for the investigation of solid state packing. The SEM micrographs of O1-O4 are
shown in Figure 2.
Figure 2. SEM images of oligo(ether-imine) films O1-O4.
O1
O2
O3
O4

The morphology of O1 and O2 consists of isolated grains which are homogeneously
dispersed into continuous films, which tend further to agglomerate in aggregates, especially in
the case of O1. The rigid aromatic biphenyl or naphthyl cores lead to a relatively high order
of molecular packing due to π-π intermolecular interactions of the neighboring molecules
which are reflected in the spherical entities observed in SEM images. Unlike O1 and O2, the
O3 and O4 films displayed a vesicular morphology, a feature not highlighted yet for this class
of compounds. It is well known that the hydrophobic blocks tend to associate together in a
polar environment to generate a vesicle membrane while the hydrophilic blocks are solvated
to form the vesicle corona which colloidally stabilizes the vesicle [43]. Having in mind the
oligoimines structural design, the diamine segment containing CF3 group possesses a more
hydrophobic character, whereas the dialdehyde segment containing ether groups is more
hydrophilic. The core-solvent interaction seems to be the factor that governs the self-
assembled morphology in our oligoimine systems. Starting from the simple spheres, oligomer
vesicles have evolved by collision or even hydrogen bonding that is able to regulate the
formation of vesicles.
3.4. Optical features
The photophysical properties of oligoimines O1-O4 in solution and solid state were
explored by using UV-Vis and photoluminescence (PL) spectral analyses. The optical data are
summarized in Table 2.
Table 2. Optical properties of the oligo(ether-imine)s O1-O4 in solution (10-5 M)
and solid state.
Compound
O1
λabs-sol.
(nm)
λabs-film
(nm)
λem-sol.
(ex. at 300 nm)
(nm)
λem-sol.
(ex. at 390 nm)
(nm)
295, 338s, 388s
300, 391s
357, 475
437, 504

285, 330s, 388 289, 331s, 395s
413, 528s
370, 464
436, 478
437, 480
O2
O3
275, 287s, 299,
310, 332s
276, 301, 310
277, 285s, 324s
330, 383s
-
428, 504
380
436, 492
-
O4
288, 285
M5
λabs - maximum absorption wavelength; λem - maximum PL emission wavelength; s - shoulder
In DMF solution, each oligoimine displayed a high energy absorption peak, at about
274-294 nm that can be associated with the π-π* benzenoid transitions which superpose with
π-π* transitions in naphthalene rings in the case of O2. The signature of fluorene unit in O3 is
well evidenced by the small peaks at 287, 297 and 310 nm, as shown in Figure 3a.
Figure 3. UV-Vis absorption spectra of oligoimines O1-O4: a) DMF solutions (10-5 M),
b) thin films (the inset represents the magnified image of the absorption edges)
(a)
0.5
TPA
O1
O2
O3
0.4
1.6
0.3
O4
0.2
 states
0.1
0.8
0.0
300
350
400
450
500
Wavelength (nm)
CT states
0.0
300
350
400
450
500
Wavelength (nm)
(b)

1.5
1.0
0.5
0.0
O1
O2
O3
O4
300
400
500
600
700
Wavelength (nm)
The scarce shoulder-like peak localized at about 330-333 nm is supposed to originate from
both localized π-π* states of the conjugated segments and the diamine segment since the
TPA-based monomer in DMF solution displayed an absorption band with two peaks, at 288
and 323 nm [41]. The well-resolved, low intensity absorption band detected at higher
wavelengths only for O1 and O2 can be attributed to the charge transfer (CT) transitions
formed between the electron-donor triphenylamine unit and electron-acceptor imine-phenyl
framework. This band is localized at about 390 nm and suggests that biphenyl and
naphthalene units promote a better molecular planarity compared to O3 and O4, enabling a
better charge transfer from the TPA to the imine unit. The absence of this band in the
electronic absorption spectra of O3 and O4 could be due to the presence of voluminous
diphenyl-fluorene or phenolphthalein cardo groups which lead to more twisted segments. The
solvent change did not reveal any particular differences in the UV-Vis absorption of all
oligomers, being observed only a small hypsochromic shift with polarity increase of both π-π*
benzenoid and CT bands (Figure S4, SI).
The maximum absorption peaks in solid state (film form) are red-shifted with respect
to those found in solution for O1, O2 and O4, probably due to intermolecular π-π interactions
and J type aggregate formation (Figure 3b) [44]. Unlike O4 for which a clear absorption

shoulder-like peak appeared at 380 nm, the O3 molecules appear to be less aggregated due to
the propeller structure of triphenylamine and voluminous diphenylfluorene moiety. It may
suggest that the molecules of O4 gain planarity in solid state and the CT states had come into
being.
The photoluminescence (PL) spectra of investigated oligomer series were firstly
measured in DMF solution. The excitation of O1 and O3 solutions with the wavelength
corresponding to the first absorption maximum (approx. 300 nm) led to a spectrum with two
emission bands, one narrow, in the UV region and another very large, spanning on the blue
and green spectral range, as shown in Figure 4a.
Figure 4. PL emission spectra of oligo(ether-imine)s O1-O4 in DMF solution (10-5 M) by
excitation at a) approx. 300 nm, b) approx. 390 nm, c) chromaticity diagram
(a)
900
O1
O2
O3
750
O4
600
450
300
150
300
400
500
600
Wavelength (nm)
(b)

500
400
300
200
100
0
O1
O2
O3
O4
400
500
600
700
Wavelength (nm)
(c)
The lowest wavelength band which is centered at 357 and 370 nm for O1 and O3,
respectively is probably due to the emission from the diamine segment since the TPA
monomer has a PL emission at about 380 nm [39]. This emission probably occurs in the case
of O2 and O4 too, but the PL bands are broad and not resolved. The highest wavelength band
may arise from two overlapped emissions which are pretty well resolved in the case of O4,
specifically the emissions from localized and CT states of TPA-imine-phenyl framework. To
prove this, the emission characteristics of O1-O4 were investigated by excitation at the
wavelength corresponding to the CT absorption band (Figure 4b). Changes in the excitation
energy influenced a little bit the PL spectra in terms of emission wavelength (small shifts) and
emission intensity. Since the CT states do not absorb directly at approx. 300 nm, probably a

fluorescence excitation energy transfer occurred. As shown by the chromaticity diagram in
Figure 4c, all oligomers displayed the PL emission in the blue-green spectral range, between
478 and 504 nm, being in good agreement with PL data reported for related polyimines [45].
To survey the influence of solvent polarity on the photoluminescence properties of the
understudy oligo(ether-imine)s, some measurements were made on O1 solutions in several
solvents varying in polarity: chloroform (CHCl3, ε = 4.81), ethyl acetate (EtOAc, ε = 6.02),
tetrahydrofuran (THF, ε = 7.58), N-methylpyrrolidone (NMP, ε = 33), dimethylformamide
(DMF, ε = 36.7) and dimethyl sulfoxide (DMSO, ε = 46.7). In a solvent with low polarity,
such as CHCl3, the PL emission maximum of O1 was registered at 488 nm, when excitation
was done with the wavelength corresponding to the CT band. The same band appeared at 485,
470 and 467 nm in EtOAc, THF and DMSO, respectively, denoting a negative
solvatofluorochromism (Figure S5, SI). However, this rule is not obeyed for NMP and DMF
solutions suggesting either an inverted solvatofluorochromic behavior [46], or the effect of
other factors such as hydrogen bonding that lead to the stabilization of the excited states. It is
mandatory to note that the PL intensity drastically increased in THF compared to the other
solvents. This feature indicates that the inherent fluorescence of O1 was not suppressed in this
environment via intramolecular charge transfer, neither by internal conversion upon bond
rotation, a common phenomenon encountered for polyimines [12, 15].
In the solid state, the fluorescence was totally quenched due to the non-radiative decay
by intermolecular energy transfer under molecular aggregation conditions. The deactivation of
the excited state by internal conversion involving imine bond rotation should also be
considered responsible for the suppressed fluorescence [12, 47].
3.5. Electrochemical properties

For the evaluation of the oxidation and reduction behavior, cyclic voltammetry (CV)
measurements were carried out on thin layers of O1-O4 coated from chloroform solutions
onto ITO as working electrodes. Methanol containing 0.1 M TBAP was used as electrolyte,
under ambient conditions. The methanol choice was motivated by the fast detachment of the
films during immersion in the electrolyte solution in any other appropriate solvent. The redox
cell was completed with Ag/Ag+ as reference electrode and Pt wire as counter electrode. The
cyclic voltammograms were recorded at a scan rate of 50 mV/s.
All investigated oligoimines contain the electron-donor triphenylamine moiety which
is able to release electrons and to render hole-transporting characteristics. To survey their
oxidation behavior in the anodic region, the potential was increased from 0 to 2 V (Figure 5a).
A small oxidation wave with the onset potential between 0.89 and 1.08 V vs. Ag/Ag+ could be
observed in the voltammograms of all oligoimines. It is considered to originate from the
triphenylamine core that upon oxidation generates radical cations. This band displays shifts in
correlation with the degree of packing induced by the aromatic chromophore from the
dialdehyde segment and less due to their electronic effects since these units are connected via
oxygene bridges.
Figure 5. CV diagrams of O1-O4 oligoimines: (a) the anodic region; (b) the cathodic region
(a)
(b)
1.2x10-4
9.0x10-5
6.0x10-5
0.0
9.0x10-6
6.0x10-6
O1
O2
O3
O4
ox
red
ox
-2.0x10-4
-4.0x10-4
-6.0x10-4
3.0x10-6
0.0
-C=O reduction
(phenolphthalein)
red
0.0
0.5
1.0
1.5
2.0
O1
O2
O3
O4
3.0x10-5
0.0
E (V)
-N=CH- reduction
0.0
0.5
1.0
1.5
2.0
-2.0
-1.5
-1.0
-0.5
0.0
E (V)
E (V)

Usually, a low oxidation potential indicates a strong ability of a compound to release
electrons and to behave as a p-type material. In the oligoimine series, it was found that the
first oxidation potential vary between 1.16 and 1.30 V vs. Ag/Ag+. The lowest onset potentials
were found for O3 and O4 with the highest rigidity and degree of packing promoted by
fluorene and phenolphthalein moieties. The highest onset potential was found for O1,
considered to be the less rigid molecule. The other quasireversible or irreversible oxidations
take place at higher oxidation potentials, between 1.50 and 1.65 V vs. Ag/Ag+. Since starting
with the second scan no electrochemical activity was registered in the anodic region, these
processes are assume to originate either at the free electron pair of the bridging ether group, or
from a following chemical reaction involving the formed radical cations.
In the cathodic region, all oligoimine films showed electrochemical activity in accord
with the electron deficient nature of the electron acceptor units that are imine and terminal
aldehyde units. The CV diagrams indicated the presence of two reduction peaks, except for
O4, for which three reduction peaks were registered (Figure 5b). The reduction potential that
ranges from -1.43 to -1.66 V vs. Ag/Ag+ is associated with the generation of radical anions at
the electron-acceptor imine units. The onset reduction potential value appears to be influenced
by the molecular rigidity induced by the chromophoric unit from the dialdehyde segment,
being shifted to more negative potentials for the most rigid oligoimines. The second reduction
peak appearing at more negative potentials can be assigned to the reduction of terminal
aldehyde groups. O4 displayed at -1.07 V vs. Ag/Ag+ a well-defined reduction peak due to the
electron acceptance of the C=O group from phenolphthalein. The electrochemical properties
of oligoimines O1-O4 are summarized in Table 3. The proposed mechanism for the
electrochemical oxidation and reduction of O1-O4 is illustrated in Scheme S1, SI.
Table 3. Redox potentials and energy levels of oligoimines O1-O4.
Reduction potential (V)
EHOMO ELUMO
Eg
Oligo- Oxidation potential

imine
(V)
(eV)
(eV)
(eV)
Eonset Eox1
Eox2
1.50
1.59
1.65
1.50
Eonset
-0.50
-0.54
-0.56
-0.57
Ered1
Ered2
-1.60
-1.48
-1.43
-1.66
Ered3
-1.85
-1.81
-1.97
-1.92
1.08
1.01
0.89
0.92
1.24
1.23
1.30
1.14
-
5.46
5.39
5.27
5.30
3.88
3.84
3.82
3.81
1.58
1.55
1.45
1.49
O1
O2
O3
O4
-
-
-1.07
Eonset - onset potential; Eox – anodic peak potential; Ered - cathodic peak potential
The electronic properties of O1-O4 can be evaluated on the basis of HOMO and
LUMO energies which were in turn calculated from the onset potentials. Assuming that the
ferrocene/ferrocenium is 4.8 eV below the vacuum level with Eonset = 0.42 V, the HOMO,
LUMO and electrochemical bandgap (Eg) can be estimated from the following equations:
ELUMO = -e(Eonsetred + 4.38), EHOMO =-e(Eonsetox + 4.38), Eg = EHOMO − ELUMO.
According to these relations, the LUMO was found between 3.81 and 3.88 eV, whilst
HOMO lies between 5.27 and 5.47 eV. The energy bandgap ranges between 1.45 and 1.58
eV. It is important to mention that HOMO, LUMO and Eg values fall within the energy
domain typically for this class of compounds, being comparable with those reported for other
triphenylamine-based poly(azomethine-ether)s [42]. It appears that the different linked
chromophore in the dialdehyde segment had a significant role in the modulation of the
electronic structure of the investigated oligo(ether-imine)s. The low energy level values found
for these systems suggest their suitability for different applications, such as solar cells.
3.5. pH-Responsive behavior
In general, the pH-responsiveness of a compound can be evaluated upon the
protonation and deprotonation cycle of a weak polybase and/or weak polyacid at different pH,
or by pH-induced conformation changes of the molecule [43]. To survey the effect of
protonation on the physical properties of understudy oligoimines, several techniques were
employed, namely FTIR, UV-Vis and PL spectroscopy, CV and SEM.

Imine-based oligomers and polymers are well known for their halochromic properties
associated with considerable spectral changes induced by protonation [13-15, 48]. Thus, the
effect of protonation on the optical properties of O1-O4 were studied with HCl acid in DMF
solution, and further on, due to the acid volatility, in thin films, by exposing the oligoimine
films to acid vapors. The changes induced in the absorption spectra of O1-O4 solutions by
adding increasing amounts of HCl acid 37% are illustrated in Figure 6(a-d). The addition of
only 10 μl HCl acid led to a new absorption band, hipsochromically shifted and centered at
about 362-365 nm, being accompanied by the disappearance of the weak neutral CT state
absorption bands. The hypsochromic shift of the absorption bands due to the protonation of
imine nitrogen atom with the methanesulfonic acid were previously reported on related
triphenylamine-based polyimines [12]. Neutral solutions of oligoimines O1-O4 were yellow-
brown colored and became less collored after HCl acid dropping (Figure 6b). The nitrogen
atom of the imine bond in contact with HCl acid undergoes protonation that seems to lead to
less planar molecules, as reflected by the blue shift with approx. 18 nm of the CT bands from
approx. 390 nm. As a consequence, the strength of π-electron delocalization is reduced.
Figure 6. UV-Vis spectral changes of O1-O4 in DMF (5×10-6 M) with the increasing amount
of the added HCl acid (a) and images of oligoimine solutions before and
after protonation with 10 l HCl acid (b)
(a)
O1
O2