Structural Chemistry-Assisted Strategy toward Fast Cis-Trans Photo/Thermal Isomerization Switch of Novel Azo-Naphthalene-Based Polyimides

pubs.acs.org/Macromolecules

Article

Structural Chemistry-Assisted Strategy toward Fast Cis−Trans

Photo/Thermal Isomerization Switch of Novel Azo-Naphthalene-

Based Polyimides

Catalin-Paul Constantin,* Ion Sava, and Mariana-Dana Damaceanu

Cite This: Macromolecules 2021, 54, 1517 −1538

Read Online

sı

*

Metrics & More

Article Recommendations

Supporting Information

ACCESS

ABSTRACT: Pursuing the research toward smart material

development, novel azopolyimides containing highly photo-

responsive azo-naphthalene moieties are synthesized and fully

characterized toward eﬃcient, fast, and stable cis−trans photo/

thermal isomerization switch. To this aim, a novel triphenyl-

methane diamine carrying a photochromic receptor is prepared

and used in polycondensation with aromatic dianhydrides. Insights

into molecular weights and thermal, optical, and electrochemical

characteristics are employed to accomplish basic characterizations.

To prove the switching characteristics insuﬃciently explored for

polyimides (PIs), trans−cis and cis−trans photo/thermal isomerizations are surveyed by UV−vis and ¹H NMR investigations. UV−

vis absorption studies reveal unprecedented fast trans−cis switches (47−130 s) through rotation mechanism in dimethylformamide,

with one of the highest eﬃciencies reported nowadays for PIs, up to 90.2%. Slower thermal cis−trans relaxation is experienced owing

to a nonpolarity-assisted inversion mechanism. After 15 repetitive trans−cis−trans switching cycles, an excellent stability up to

96.29% is recorded. The PI ﬁlms obtained without any dilution in solid state reach the photostationary state after longer irradiation

time but with excellent eﬃciency, up to 75.7%, and trans−cis−trans conversion stability, up to 92.68%, after 10 cycles. The

1

photoisomerization studies are completed by H NMR that evidences spectral proﬁles change during irradiation. Perspective

optoelectronic applications that demand photocontrollable functions are foreseen.

ultraviolet (UV) and/or visible spectral range. The properties

of azopolymers are dependent on the azo linkage itself and are

also controlled by the groups on both sides of the bridge. The

azo groups can be covalently bonded to benzene rings,

naphthalene units, aromatic heterocycles, and other aromatic

systems. For azopolymers, the strong absorptions appearing in

the abovementioned wavelength range are mainly related to

the electronic transitions of the aromatic azo core structures.

Because of light absorption, the electronic excitation can cause

energetic, and structural variations. One of the most important

variations at the molecular level of azopolymers is the trans−

cis photoisomerization of the azo core. Reversible trans−cis−

trans isomerization of the molecules causes structural changes,

resulting also in diﬀerent optical spectral proﬁles. During trans-

to-cis conversion, the intensity of the absorption band

corresponding to the n−π* transition increases, whereas that

of the π−π* band decreases.^5,6Structural changes of ABn are

1. INTRODUCTION

In pursuit of developing modern materials suitable to

accomplish the needs of the ever-growing modern society,

the scientiﬁc community was committed to implement new

materials with particular speciﬁcations. Polymers, as modern

materials, developed rapidly, being nowadays integrated in

technological ﬁelds, starting from microelectronics up to

biology. Also, they have many attractive properties, such as

biocompatibility, controlled composition and chain length, and

tunable physicochemical properties. From their classical and

general characters, polymers have evolved over time into more

and more versatile structures. Thus, smart polymeric materials

have emerged, encompassing a wide range of compounds with

unique potential in electronics, optoelectronics, and biological

applications. On the other hand, in the past decades, great

interest has been directed to the development of materials that

are optically responsive and have a promising potential for

many photonic technologies. Among them, materials based on

azobenzene (ABn)-containing polymers (azopolymers) have

attracted considerable interest because of their multiple

applications such as photoresists, materials for optical storage

data, cell culture matrices, optical polarizers, wavelength ﬁlters,

and many others.¹⁻⁴The aromatic azo cores endow these

compounds with strong light absorption capability in the

Received: September 22, 2020

Revised: December 11, 2020

Published: January 19, 2021

https://dx.doi.org/10.1021/acs.macromol.0c02182

Macromolecules 2021, 54, 1517−1538

1517

Macromolecules

pubs.acs.org/Macromolecules

Article

Scheme 1. Synthetic Pathway to Diamine M1

reﬂected through the generation of a stable dipole moment for

the cis isomer (3.0 D) and the geometry change caused by the

reduction of the distance between the carbon atoms in 4 and 4′

positions from 9.0 Å for the trans isomer to 5.5 Å for the cis

isomer.⁶The multiple trans−cis−trans isomerization cycles

can also induce a large-scale mass migration and formation of

surface relief gratings (SRGs) on the azopolymer ﬁlms. This

fascinating consequence of photoisomerization of an azopol-

ymer ﬁlm under exposure to irradiation with an interference

pattern was reported 25 years ago by two independent research

groups.^7,8The SRGs formation is inﬂuenced by several

parameters, such as irradiation intensity in terms of wave-

length, irradiation time, chemical structure, and molecular

weight of the polymer; content of photosensitive ABn groups;

and the way they are anchored to the polymer.^9,10

The trans−cis photoisomerization is one of the most

important properties of aromatic azo compounds, which has

been widely explored to develop azopolymers with various

photoresponsive functions. Owing to the simplicity, ABns and

its derivatives have been intensively studied as extreme systems

to understand the mechanism of photoisomerization. Un-

fortunately, the photoisomerization mechanism of aromatic

azo compounds has not been fully elucidated because most of

the experimental and theoretical studies showed contradictory

results, and their interpretations were often subjects of ardent

disputes.¹¹However, it was agreed that the photoisomerization

rate of ABn chromophores depends on factors such as free

volume, temperature, mobility, and polarity of the polymer

system.^6,12,13

and others.¹⁴⁻¹⁸One drawback of PIs is their poor solubility

and processability by wet methods. To overcome this

inconvenience, various structural modiﬁcation strategies have

been used. For example, bulky side substituents,¹⁹⁻²²ﬂexible

linkages,^23,24noncoplanar biphenyl moieties,²⁵as well as

unsymmetrical structures have been used to enhance the

solubility of aromatic polymers, making them easily process-

able. Another eﬃcient approach to soluble PIs is to introduce

the propeller-shaped triphenylmethane (TPM) structure. Its

bulky structure with the pendant phenyl ring suppresses the

interchain interactions, facilitating the solvent penetration

between polymeric chains. In addition, this structural unit is

expected to improve the chain ﬂexibility toward an enhanced

photoisomerization response.

Scientiﬁc research on the modiﬁcation and functionalization

of PIs in order to endow them with smart functions is of

particular challenge nowadays, one perspective route being the

incorporation of ABn structural unit in their macromolecular

chains. In this context, several synthetic routes have been

developed so far, most of them involving the general

condensation reaction of ABn-functionalized mono-

mers^21,26−30or the postfunctionalization strategy which allows

ABn chromophores to be introduced into reactive precursor

polymers.³¹⁻³³The ﬁrst method is the most used because of

the better control of sequence chromophore distribution.

The ABn-functionalized monomers can be diamines or

dianhydrides or both of them.^{21,26,34−36}Dianhydrides

containing azo units have been obtained from trimellitic

anhydride acid chloride with diamines or diols containing ABn

units. Meanwhile, various ABn-based diamines have been

prepared and used afterward for the synthesis and character-

ization of azopolyimides, usually containing substituted or

unsubstituted ABn side chains. In this way, a broad range of

azopolyimides, azo-poly(amide-imides), or azo-poly(ester-

On the other hand, one point of reference regarding modern

polymers is attributed to polyimides (PIs), which embraced

the combination of high-performance features, including high

thermal and chemical resistance, mechanical strength, thermal,

electrical, or sound insulation properties, long-term durability,

1518

https://dx.doi.org/10.1021/acs.macromol.0c02182

Macromolecules 2021, 54, 1517−1538

Macromolecules

pubs.acs.org/Macromolecules

Article

imides) have been synthesized.^{26,34,37−39}Despite the large

number of studies on azopolyimides, there is no clear

understanding of the trade-oﬀ between the structure and

their photoisomerization behavior, the majority of the studies

providing only a general overview regarding the isomerization

mechanism. This is of particular interest as the current

challenge is to develop materials that can face simultaneously

requirements such as high ﬂexibility, high glass transition, and

thermal stability in a deﬁned environment along with eﬃcient,

fast, and stable optical response over time in view of future

optoelectronic applications. In this respect, studies on

thermally stable PIs exhibiting highly eﬃcient photoisomeriza-

tion response have been rarely reported to date, to our

knowledge, as PIs are known for their rigid structures. Because

of this shortcoming, the objective of this study is to bring to

the fore the most important aspects that allow to clearly

understand the processes and factors that inﬂuence the trans−

cis and cis−trans isomerization of azo-based PIs.

resulted azo-based derivative M1″ was washed several times with

deionized water and dried at 80 °C in an oven for 12 h to obtain 6.2 g

of a dark-brownish powder (68.5% yield).

¹H NMR (DMSO-d₆, 400.1 MHz, 25 °C): δ (ppm) 10.38 (1H, s,

H-1), 8.85−8.83 (1H, d, 8.2 Hz, H-14), 8.10−8.08 (1H, d, 8.07 Hz,

H-9), 8.07−8.05 (1H, d, 8.07 Hz, H-11), 7.97−7.95 (2H, d, 8.8 Hz,

H-4), 7.75−7.62 (4H, m, H-7, H-8, H-12, H-13), 7.01−6.99 (2H, d,

8.8 Hz, H-3).

¹³C NMR (DMSO-d₆, 100.6 MHz, 25 °C): δ (ppm) 161.62 (C-2),

147.42 (C-6), 146.55 (C-5), 134.47 (C-10), 131.01 (C-9), 130.83 (C-

15), 128.54 (C-11), 127.48 (C-13), 127.12 (C-12), 126.35 (C-8),

125.73 (C-4), 123.41 (C-14), 116.56 (C-3), 111.91 (C-7).

For this purpose, a novel structural strategy toward azo-

modiﬁed PIs has been approached by starting from an azo-

functionalized TPM-based diamine toward an unprecedented

fast and stable cis−trans photo/thermal isomerization switch.

A deep structural conﬁrmation of the diamine and PIs was

carried out by FT-IR spectroscopy and NMR spectroscopy,

followed by an insight into the molecular weight investigation

through the MALDI-TOF technique. Thermal, optical, and

electrochemical measurements were also employed to

complete the basic characterization of these polymers and to

highlight their high-performance properties. As these PIs

resembled diﬀerent macromolecular architectures because of

the structural alteration in the diimide segment, the photo/

thermal isomerization was monitored for each polymer to

survey the structural inﬂuence on the process eﬃciency. The

trans−cis and cis−trans switching characteristics were surveyed

2.1.2. 1-(4-(3-Bromopropoxy)phenyl)-2-(naphthalen-1-yl)-

diazene (M1′). In a 500 mL three-necked round-bottom ﬂask

equipped with a magnetic stirring bar, nitrogen inlet and outlet

connections, and a condenser, potassium carbonate (15.8 g, 114.7

mmol) and potassium iodide (76 mg, 0.46 mmol) were added at

room temperature to a stirred solution of azo-based intermediate M″

(5.6 g, 22.92 mmol) and 1,3-dibromopropane (7 mL, 68.82 mmol) in

acetone (180 mL). The reaction mixture was heated at 60 °C for 24 h,

and the solution became dark red. The occurrence of the newly

formed azo-based derivative M1′ was monitored by thin layer

chromatography (TLC) using chloroform. After completion, the

reaction mixture was cooled to room temperature and poured into

water, followed by extraction with ethyl acetate. The organic layer was

washed three times with water and dried over anhydrous sodium

sulfate and then concentrated under reduced pressure. The azo-based

derivative M1′ was puriﬁed by column chromatography using

chloroform as the eluent to aﬀord 6.2 g of a red solid (42.3% yield).

1

by UV−vis and H NMR investigations which proved to be

relevant to highlight their fast, stable, and eﬃcient photo/

thermal controllable responses in relation with the surrounding

environment.

2. EXPERIMENTAL SECTION

Details about the starting materials and instrumentation used in this

study are provided in the Supporting Information. The structures of

the dianhydrides used in the synthesis of azo-based PIs are displayed

in Figure S1.

¹H NMR (DMSO-d₆, 400.1 MHz, 25 °C): δ (ppm) 8.87−8.85

(1H, d, 8.2 Hz, H-10), 8.13−8.11 (1H, d, 8.07 Hz, H-15), 8.08−8.04

(3H, m, H-6, H-13), 7.79−7.75 (1H, d, 7.4 Hz, H-17), 7.74−7.64

(3H, m, H-11, H-12, H-16), 7.22−7.20 (2H, d, 8.9 Hz, H-5), 4.25−

4.22 (2H, t, 6.0 Hz, H-3), 3.73−3.70 (2H, t, 6.5 Hz, H-1), 2.35−2.29

(2H, m, H-2).

2.1. Synthesis of Azo-Based Diamine. The synthetic pathway

to the azo-based diamine, namely 4,4′-((4-(3-(4-(naphthalen-1-

yldiazenyl)phenoxy)propoxy)phenyl)methylene)bis(2-methylaniline)

(M1), envisaged four steps, with the isolation of two azo-based

intermediates M1″ and M1′ and one hydroxyl-based diamine (M−

OH), according to Scheme 1. The details are described as follows.

2.1.1. 4-(Naphthalen-1-yldiazenyl) Phenol (M1″). In a 250 mL

three-necked round-bottom ﬂask equipped with a magnetic stirring

bar and nitrogen inlet and outlet connections, 1-naphthylamine (5.4 g,

37.66 mmol) was suspended in deionized water (20 mL) and cooled

down to 0−5 °C in an ice bath. To this suspension, HCl (15 mL,

37%) was progressively dropped for 10 min, keeping the temperature

below 5 °C. Separately, a solution of NaNO₂(2.6 g, 37.66 mmol) and

deionized water (6 mL) was prepared, cooled down to 0−5 °C and

then slowly dropped over the 1-naphthylamine solution for 10 min.

Next, the freshly obtained diazonium salt solution was slowly added

over a suspension previously prepared from 4-aminophenol (5.4 g, 50

mmol) and deionized water (30 mL). The resulting mixture was kept

at 0−5 °C with stirring for 1 h and then allowed to reach the room

temperature. Next, the solution pH was adjusted to 7.5−8 with

K₂CO₃, and a brown solid started to precipitate. After ﬁltration, the

¹³C NMR (DMSO-d₆, 100.6 MHz, 25 °C): δ (ppm) 161.73 (C-4),

147.52 (C-8), 147.33 (C-7), 134.43 (C-14), 131.41 (C-15), 130.87

(C-9), 128.54 (C-13), 127.57 (C-11), 127.13 (C-12), 126.33 (C-16),

125.43 (C-6), 123.33 (C-10), 115.70 (C-5), 112.04 (C-17), 66.39 (C-

3), 32.24 (C-2), 31.55 (C-1).

2.1.3. 4-[Bis-(4-amino-3-methylphenyl)-4′-hydroxy-phenylme-

thane (M−OH). This was obtained according to a reported

procedure.⁴⁰Brieﬂy, 4-hydroxy-benzaldehyde (6 g, 0.049 mol) was

allowed to react at reﬂux temperature with ortho-toluidine (27.1 g,

0.2476 mol) in the presence of HCl (2 mL, 37%) for 24 h. After

puriﬁcation by column chromatography, using ethyl acetate as an

eluent, a slight beige powder was obtained with a reaction yield of

78%.

¹H NMR (DMSO-d₆, 400.1 MHz, 25 °C): δ (ppm) 9.15 (s, 1H, H-

14), 6.82−6.80 (d, 2H, H-11), 6.66−6.64 (d, 4H, H-5, H-12), 6.59−

6.57 (dd, 2H, H-7), 6.47−6.45 (d, 2H, H-8), 5.02 (s, 1H, H-9), 4.64

(s, 4H, H-2), 1.95 (s, 6H, H-1).

1519

https://dx.doi.org/10.1021/acs.macromol.0c02182

Macromolecules 2021, 54, 1517−1538

Macromolecules

pubs.acs.org/Macromolecules

Article

(M2), 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (M3), or

4,4′-(hexaﬂuoroisopropylidene)diphthalic anhydride (M4), were used

in a stoichiometric molar ratio in relation to the azo-based diamine

monomer (M1).

In a typical synthetic procedure, P1 was prepared as follows: the

azo-based diamine M1 (0.6 g, 1 mmol) and dry DMAc (7.5 mL) were

added to a 50 mL three-necked round-bottom ﬂask equipped with a

magnetic stirrer, nitrogen inlet and outlet connections, and a water

condenser. After the complete dissolution of the diamine (10 min),

4,4′-oxydiphthalic anhydride M2 (0.31 g, 1 mmol) was added to

obtain a solution containing 12% total solids. Then, the resulting

reaction mixture was stirred at room temperature to allow the

monomers to react and to facilitate the formation of the intermediate

polyamidic acid (PAA). The reaction evolution was monitored by

TLC using chloroform/acetone (50:1; v/v) as the eluent. After 12 h,

the diamine was no longer present on the TLC plates, and no

monomer signals were identiﬁed in the ¹H NMR spectra. Then, to the

obtained solution of PAA, pyridine (0.321 mL, 4 mmol) and acetic

anhydride (0.661 mL, 7 mmol) were added, and the mixture was

heated at 100 °C to perform the cyclization of PAA to the imide

structure. The transformation of PAA into the corresponding PI was

assessed by ¹H NMR spectroscopy that indicated no signals

corresponding to the protons of the amide and carboxyl groups

after 3 h of chemical imidization. The resulting orange polymer

solution was precipitated in deionized water, ﬁltered, and washed

several times with water, followed by ethanol Soxhlet extraction for 1

day to remove the solvent. The obtained polymer was dried in an

oven at 100 °C for 24 h to give 0.71 g of P1 (80% yield). The other

polymers of the series were obtained with 80.4% yield (P2) and

76.7% yield (P3) as orange powders.

¹³C NMR (DMSO-d₆, 100.6 MHz, 25 °C): δ (ppm) 155.45 (C-

13), 144.7b(C-3), 136.35 (C-10), 133.51 (C-4), 130.72 (C-5), 130.05

(C-11), 127.18 (C-7), 121.19 (C-6), 115.24 (C-12), 114.08 (C-8),

54.33 (C-9), 18.12 (C-1).

2.1.4. 4,4′-((4-(3-(4-(Naphthalen-1-yldiazenyl)phenoxy)-

propoxy)phenyl)methylene)bis(2-methyl-aniline) (M1). In a 250

mL three-necked round-bottom ﬂask equipped with a magnetic

stirring bar, nitrogen inlet and outlet connections, and a condenser,

cesium ﬂuoride (4.7 g, 29.4 mmol) and potassium iodide (48 mg, 0.28

mmol) were added to a stirred solution of azo-based intermediate

M1′ (5.4 g, 14.7 mmol) and diamine M−OH (4.6 g, 14.7 mmol) in

acetone (60 mL). The reaction mixture was heated at 60 °C for 24 h,

and the solution became dark red. The occurrence of the newly

formed diamine (M1) was monitored by TLC using chloroform/

acetone (50:1; v/v). After completion, the reaction mixture was

poured into water and then extracted with ethyl acetate. The organic

layer was washed three times with water and dried over anhydrous

sodium sulfate, followed by solvent evaporation under reduced

pressure. The diamine was puriﬁed by column chromatography using

chloroform/acetone (50:1; v/v) as the eluent to give 2.8 g of a

reddish solid (31.4% yield). mp 78−80 °C.

P1: ¹H NMR (CDCl₃-d₆, 400.1 MHz, 25 °C): δ (ppm) 8.89−8.87

(1H, d, 8.2 Hz, H-7), 8.04−8.02 (2H, d, 8.5 Hz, H-8), 7.98−7.96

(2H, d, 8 Hz, H-22), 7.93−7.88 (2H, m, H-3, H-4), 7.8−7.78 (1H, d,

7.3 Hz, H-1), 7.63−7.51 (5H, m, H-2, H-5, H-6, H-21), 7.46−7.44

(2H, d, 7.7 Hz, H-23), 7.14−7.04 (10H, m, H-9, H−14, H-15, H-16,

H-18), 6.9−6.88 (2H, d, 8.4 Hz, H-13), 5.51 (1H, s, H-19), 4.27−

4.25 (2H, t, 5.8 Hz, H-12), 4.19−4.17 (2H, t, 5 Hz, H-10), 2.33−2.28

(2H, m, H-11), 2.16 (6H, s, H-17).

MALDI-TOF (m/z): M_n1= 2725 g/mol, PDI = 1.18, M_n2= 2135

g/mol, PDI = 1.11.

P2: ¹H NMR (CDCl₃-d₆, 400.1 MHz, 25 °C): δ (ppm) 8.88−8.86

(1H, d, 8.2 Hz, H-7), 8.26−8.22 (4H, m, H-21, H-23), 8.11−8.09

(2H, d, 7.4 Hz, H-22), 8.04−8.02 (2H, d, 8.7 Hz, H-8), 7.92−7.9

(2H, m, H-3, H-4), 7.80−7.78 (1H, d, 7.2 Hz, H-1), 7.62−7.50 (3H,

m, H-2, H-5, H-6), 7.16−7.03 (10H, m, H-9, H-14, H-15, H-16, H-

18), 6.91−6.89 (2H, d, 8.4 Hz, H-13), 5.52 (1H, s, H-19), 4.27−4.25

(2H, t, 5.8 Hz, H-12), 4.19−4.17 (2H, t, 5 Hz, H-10), 2.33−2.28

(2H, m, H-11), 2.17 (6H, s, H-17).

¹H NMR (DMSO-d₆, 400.1 MHz, 25 °C): δ (ppm) 8.87−8.85

(1H, d, 8.2 Hz, H-23), 8.12−8.10 (1H, d, 8.1 Hz, H-28), 8.07−8.03

(3H, m, H-19, H-26), 7.78−7.76 (1H, d, 7.5 Hz, H-30), 7.73−7.63

(3H, m, H-24, H-25, H-29), 7.21−7.19 (2H, d, 9.0 Hz, H-18), 6.96−

6.94 (2H, d, 8.6 Hz, H-11), 6.86−6.84 (2H, d, 8.6 Hz, H-12), 6.63

(2H, s, H-5), 6.58−6.55 (2H, dd, 8.1 Hz, 1.6 Hz, H-7), 6.50−6.48

(2H, d, 8.1 Hz, H-8), 5.08 (1H, s, H-9), 4.63 (4H, s, H-1), 4.29−4.26

(2H, t, 6.1 Hz, H-14), 4.13−4.10 (2H, t, 6.0 Hz, H-16), 2.24−2.18

(2H, m, H-15), 1.96 (6H, s, H-3).

MALDI-TOF (m/z): M_n1= 2507 g/mol, PDI = 1.19, M_n2= 2483

g/mol, PDI = 1.12.

P3: ¹H NMR (CDCl₃-d₆, 400.1 MHz, 25 °C): δ (ppm) 8.90−8.88

(1H, d, 8.3 Hz, H-7), 8.04−8.01 (4H, m, H-8, H-22), 7.94−7.86 (6H,

m, H-3, H-4, H-21, H-23), 7.79−7.77 (1H, d, 7.4 Hz, H-1), 7.63−

7.51 (3H, m, H-2, H-5, H-6), 7.16−7.05 (10H, m, H-9, H-14, H-15,

H-16, H-18), 6.91−6.89 (2H, d, 8.5 Hz, H-13), 5.52 (1H, s, H-19),

4.27−4.25 (2H, t, 5.8 Hz, Hv12), 4.19−4.17 (2H, f, 5, H-10), 2.33−

2.28 (2H, m, H-11), 2.17 (6H, m, H-17).

¹³C NMR (DMSO-d₆, 100.6 MHz, 25 °C): δ (ppm) 161.90 (C-

17), 156.84 (C-13), 147.41 (C-21), 147.32 (C-20), 144.80 (C-2),

138.40 (C-10), 134.43 (C-27), 133.13 (C-6), 131.37 (C-28), 130.92

(C-5), 130.88 (C-22), 130.27 (C-11), 128.54 (C-26), 127.57 (C-24),

127.30 (C-7), 127.12 (C-25), 126.33 (C-29), 125.42 (C-19), 123.34

(C-23), 121.15 (C-4), 115.67 (C-18), 114.40 (C-12), 114.21 (C-8),

112.02 (C-30), 65.39 (C-14), 64.48 (C-16), 54.37 (C-9), 29.11 (C-

15), 18.02 (C-3).

MALDI-TOF (m/z): M_n1= 3705 g/mol, PDI = 1.23, M_n2= 3904

g/mol, PDI = 1.17.

3. RESULTS AND DISCUSSION

3.1. Synthesis and Structural Identiﬁcation of

Diamine M1. A novel azo-based diamine (M1) was

successfully synthesized following a four-step procedure,

according to the pathway described in Scheme 1. The chemical

structure of this diamine combines some unique structural

elements, such as azo, naphthalene, TPM, and ﬂexible aliphatic

units, which are expected to strongly contribute to the high-

MALDI-TOF (m/z): calcd for C₄₀H₃₈N₄O₂, M 606.77; found,

607.93 [M + H]⁺.

2.2. Synthesis of Polymers. Three azo-based PIs, P1−P3,

containing naphthalene units were prepared following a standard two-

step polycondensation procedure under chemical imidization

conditions according to the general reaction pathway displayed in

Scheme 2. The dianhydride monomers, 4,4′-oxydiphthalic anhydride

1520

https://dx.doi.org/10.1021/acs.macromol.0c02182

Macromolecules 2021, 54, 1517−1538

Macromolecules

pubs.acs.org/Macromolecules

Article

Scheme 2. Synthetic Pathway to Azo-Based PIs, P1−P3

1

Figure 1. Numbering chart (a), H NMR (b), and ¹³C NMR (c) spectra of azo-based diamine (M1) in DMSO-d₆.

1521

https://dx.doi.org/10.1021/acs.macromol.0c02182

Macromolecules 2021, 54, 1517−1538

Macromolecules

pubs.acs.org/Macromolecules

Article

performance features of the envisaged polymers, such as the

photoisomerization capability.

M1, being at a very low ﬁeld compared to a typical aliphatic

proton. The overall aromatic protons of the naphthalene unit

were not aﬀected by the successive structural changes in

diamine M1, so that the associated signals were found at

approximately the same ppm values as in M1″ and M1′.

Continuing the discussion regarding the NMR features, the

¹³C NMR spectral assignments for diamine M1 can be

considered as follows. The carbon signals are scattered in two

main groups located in the aliphatic (65.39−18.02 ppm) and

aromatic regions (161.90−112.02 ppm) and correspond to

their adjacent protons. In turn, the aromatic protons are

grouped into two types of signals. The ones which are located

at a lower ﬁeld (C-17, C-13, C-21, C-20, and C-2) correspond

to the carbon atoms connected to oxygen and nitrogen

heteroatoms. Among these, signals arising from the azo group

(C-21 and C-20) can be identiﬁed. The amino groups were

evidenced by the carbon signal (C-2) located at 144.80 ppm.

To bring additional information regarding the structural

conﬁrmation of this novel diamine M1, the FT-IR spectrum

was recorded (Figure S6, Supporting Information) and

discussed. As it was expected, the characteristic absorption

bands of the functional −NH₂(amino) group were located at

3499−3383 cm⁻¹(N−H stretching vibration), 1301 cm⁻¹

(aromatic C−N stretching vibration), and 772 cm⁻¹(−NH₂

out-of-plane bending vibration). Besides, the azo group as a

distinctive structural element was highlighted through the

absorption band at 1584 cm⁻¹(NN stretching vibration).

The presence of the aromatic system was evidenced by the

characteristic absorption bands located at 3049−3007 cm⁻¹

(aromatic C−H stretching vibration), 1620 cm⁻¹(aromatic

conjugated system CC stretching vibration), 1598 cm⁻¹

(aromatic CC asymmetric stretching vibration), 1501 cm⁻¹

(aromatic CC symmetric stretching vibration), and 837

cm⁻¹(aromatic C−H out-of-plane bending vibration). The

absorption bands corresponding to the aliphatic groups were

identiﬁed at 2930−2853 cm⁻¹(aliphatic C−H stretching

vibration), 1461−1383 cm⁻¹(aliphatic C−H in-plane bending

vibration), and 1136 cm⁻¹(aliphatic C−H out-of-plane

bending vibration). Other structural elements, like ether

bridges, showed characteristic absorption bands at 1461−

1383 cm⁻¹(aliphatic C−O−C in-plane bending vibration) and

1242 cm⁻¹(aromatic C−O−C stretching vibration).

Brieﬂy, the azo-based derivative M1″ was synthesized via

diazotization reaction by coupling the in situ-generated

diazonium salt of 1-naphthylamine with 4-aminophenol at

low temperature. Then, 1,3-dibromopropane was anchored to

M1″ through nucleophilic substitution reaction, with the

generation of the azo-based derivative M1′. This was subjected

in the ﬁnal step to the coupling with the previously synthesized

diamine, M−OH from o-toluidine, and 4-hydroxy-benzalde-

hyde in the presence of HCl, so as to build the desired diamine

M1. The diamine M1 and the corresponding intermediates,

M1″, M1′, and M−OH, were characterized by spectral

methods, which provided evidence for the correct chemical

structures. Thus, Figure S2, Supporting Information, illustrates

the ¹H NMR spectrum corresponding to the azo-based

precursor M1″. The signals which correspond to the protons

of the naphthalene unit are highly inﬂuenced by the presence

of the azo group, which exerts a strong deshielding eﬀect,

especially for the protons connected in the ortho (H-4) and

para (H-7 and H-8) positions related to this unit. For this

reason, H-4 is located at a very low resonance ﬁeld (8.85−8.83

ppm), atypical for an aromatic proton. The proton of the

hydroxyl group (H-1) appeared at downﬁeld shift (10.38 ppm)

with respect to all the aromatic signals, in the form of a broad

singlet. This structural unit also determines an important

shielding eﬀect on the protons H-2 because of the electron-

pushing capability of the sp³-hybridized oxygen atom. With

regard to M−OH, the amine protons were identiﬁed in the ¹H

NMR spectrum at 4.64 ppm along with the aliphatic protons at

5.02 ppm (the single proton of TPM) and 1.95 ppm (protons

of the CH₃groups) and the hydroxyl group proton at 9.15

ppm (Figure S3, Supporting Information).

1

The assignments for the H and ¹³C chemical shifts of the

intermediate M1′ and diamine M1, which are unknown

compounds, were performed on the basis of 2D NMR homo-

and heteronuclear correlations, namely H,H-COSY (homo-

nuclear correlation spectroscopy), H,C-HSQC (heteronuclear

single quantum coherence), and H,C-HMBC (heteronuclear

multiple bond coherence). ¹H-, ¹³C-, H,H-COSY, H,C-HSQC,

and H,C-HMBC NMR spectra recorded for the azo-based

derivative M1′ are shown in Figure S4, Supporting

Information. The binding of the aliphatic group to the main

azo-based structure was evidenced in the ¹H NMR spectrum of

M1′ by the disappearance of the signal associated with the

hydroxyl group at 10.38 ppm and the occurrence of the

aliphatic signals at 4.25−4.22 ppm (H-3), 3.73−3.70 ppm (H-

1), and 2.35−2.29 ppm (H-2) in the form of two triplets and

one multiplet. H-3 and H-1 protons are located downﬁeld

compared to H-2 because of the bonding of their associated

carbon atoms to the more electronegative oxygen and bromine

atoms which induce a deshielding eﬀect. The NMR spectra of

the ﬁnal diamine M1 are displayed in Figures 1 and S5,

Supporting Information. The most important aspect evidenced

by these spectra is the direct proof for the presence of the

deﬁning structural element of this compoundthe amine

group. Thus, the signal associated with amine protons is

located at 4.63 ppm (H-1), being typical for an aromatic

diamine. The binding of M−OH to the main core of M1′ was

highlighted through the position change of the signals

associated with the aliphatic protons adjacent to the newly

formed ether bridge. Also, the typical signal of the TPM unit

(H-9) was shifted from 5.02 ppm in M−OH to 5.08 ppm in

3.2. Design, Synthesis, and Structural Identiﬁcation

of PIs. Three PIs, P1−P3, were smartly designed and prepared

by using the newly synthesized diamine M1 and various

commercial dianhydrides (M2−M4) whose structures are

shown in Figure S1, Supporting Information. The concept was

to combine in the same polymer chain some structural

elements which are expected to facilitate the pursued

properties. Among them, the azo group is responsible for the

trans−cis behavior, the aliphatic unit induces the fast trans−cis

switch, the naphthalene rigid core acts as a “counterweight”,

whereas the TPM assembly enhances the solubility and renders

the expected thermal stability of the polymers. Such PIs were

prepared following a conventional two-step polycondensation

procedure, with the generation of the intermediate PAAs,

which were then converted to their imidic structures by

chemical imidization, as shown in Scheme 2.

To synthesize the PIs, the diamine M1 was ﬁrst dissolved in

DMAc to which equal equivalents of the dianhydrides (M2−

M4) were added to follow a polyaddition mechanism and to

generate the intermediate PAAs, P1′−P3′. The success of this

1

reaction was assessed through H NMR spectroscopy that

1522

https://dx.doi.org/10.1021/acs.macromol.0c02182

Macromolecules 2021, 54, 1517−1538

Macromolecules

pubs.acs.org/Macromolecules

Article

1

Figure 2. H NMR spectrum of azo-based PI, P1 in CDCl₃.

Figure 3. FT-IR spectrum of the azo-based PI, P1.

evidenced the absence of any signal corresponding to the

amide (around 10.5 ppm) and carboxylic (broad signal around

13.5 ppm) groups. The polycondensation reaction proceeded

homogeneously throughout the reaction time and aﬀorded

clear orange, low-viscous polymer solutions. After precipitation

in water, Soxhlet extraction with ethanol, and drying, the

resulting azo-based PIs, P1−P3 were obtained as orange

powders. The as-obtained PIs were identiﬁed by ¹H NMR and

FT-IR spectroscopies. According to the ¹H NMR spectra

illustrated in Figure 2 and Figure S7, Supporting Information,

all newly synthesized azo-based PIs possess the anticipated

structures.

The NMR data conﬁrmed the total incorporation of the

monomers into the corresponding polymer chains because of

the absence of any signal corresponding to the protons of the

monomeric counterparts. In addition, the complete imidization

is proved as no signal corresponding to the amide or carboxylic

protons of the PAA precursor are present. All the recorded

proton signals are scattered in two main domains, correspond-

ing to the aromatic (8.90−6.88 ppm) and aliphatic (5.52−2.16

ppm) regions. As seen for all polymers, the aromatic proton

signals corresponding to the TPM assembly are shifted to a

lower resonance ﬁeld compared to those from the diamine M1

because of the deshielding eﬀect of the imide group. The

aliphatic segment interrupts the inﬂuence of the imide group,

so that the signal originating from the azo-naphthalene core

remained practically unshifted but with the most deshielded

protons. In the same window are found the signals

corresponding to the protons of the diimide segments (H-

21, H-22, and H-23). Their distribution and location vary

according to the nature of the dianhydride, so that the signals

from the BTDA segment (P2) are at the highest ppm values,

whereas those coming from ODPA (P1) are shifted to a higher

resonance ﬁeld. The protons of the propylene bridge provided

signals which are located approximately in the same intervals

1523

https://dx.doi.org/10.1021/acs.macromol.0c02182

Macromolecules 2021, 54, 1517−1538

Macromolecules

pubs.acs.org/Macromolecules

Article

Figure 4. MALDI-TOF mass spectrum of azo-based PI, P3.

Table 1. Structural Assignments of the Peaks Recorded in the MALDI-TOF Mass Spectrum of P3 According to Figure 4

as in the case of diamine, whereas the aliphatic signals of the

Supporting Information, along with the collected data

summarized in Table S1, Supporting Information.

According to the FT-IR spectra, no absorption bands were

identiﬁed for the amino groups of the diamine or the carbonyl

groups of the dianhydrides, proving the full incorporation of

the monomers into the PI backbones. Further, no character-

TPM structure appear at downshift ﬁelds.

The PI structures were also identiﬁed by FT-IR spectros-

copy, and the corresponding recorded spectra are shown in

Figure 3 for P1 with band assignment and Figure S8,

1524

https://dx.doi.org/10.1021/acs.macromol.0c02182

Macromolecules 2021, 54, 1517−1538

Macromolecules

pubs.acs.org/Macromolecules

Article

istic absorption bands corresponding to the amide and

carboxyl groups were observed, demonstrating that the

conversion of the intermediate PAAs into ﬁnal PIs was

successfully performed by chemical imidization. The newly

formed imide ring was evidenced through the characteristic

absorption bands at 1787−1778 cm⁻¹(asymmetrical stretching

vibration of the imide CO bond), 1724−1721 cm⁻¹

(symmetrical stretching vibration of the imide CO bond),

1374−1372 cm⁻¹(stretching vibration of the imide C−N

bond), and 746−725 cm⁻¹(out-of-plane bending vibration of

the imide ring). The distinctive structural element of these

polymers, namely the azo group, has not been accurately

located, most likely being overlapped with the band at 1601−

1598 cm⁻¹(asymmetrical stretching vibration of the aromatic

CC bond). The other aromatic entities were located at

3116−3047 cm⁻¹(stretching vibration of the aromatic C−H

bond), 1503−1500 cm⁻¹(symmetrical stretching vibration of

the aromatic CC bond), and 841−771 cm⁻¹(out-of-plane

bending vibration of the aromatic C−H bond). The aliphatic

linkages were identiﬁed through the absorption bands in the

range of 2982−2846 cm⁻¹(stretching vibration of the aliphatic

C−H bond), 1472−1406 cm⁻¹(in-plane bending vibration of

the aliphatic −CH₃, −CH₂−, and −CH₂−O− bonds), and

1099−1051 cm⁻¹(out-of-plane bending vibration of the Ar−

CH₃bond). Besides,P2 revealed the presence of the keto

group of the BTDA-derived segment at 1673 cm⁻¹(stretching

vibration of the keto CO bond), whereas the triﬂuoromethyl

group of P3 was evidenced by the absorption band at 1105

cm⁻¹(stretching vibration of the aliphatic C−F bond).

3.3. Solubility and Molecular Weight Evaluation.

Generally, because of the strong interchain interactions,

aromatic PIs display moderate to poor solubility in common

organic solvents. The thoughtful structural design of these

novel PIs has led to a signiﬁcant improvement of their

solubility. Thus, besides the excellent solubility at room

temperature in conventional polar amidic solvents such as

N,N-dimethylformamide (DMF), N,N-dimethylacetamide

(DMAc), or 1-methyl-2-pyrrolidinone (NMP), these PIs

were soluble in less polar, convenient, and easily accessible

solvents such as dichloromethane (DCM), chloroform

(CHCl₃), or tetrahydrofuran (THF), up to a concentration

of 15%. Moreover, under heating, these polymers were partially

soluble in acetonitrile. Surprisingly, however, the PIs were

partially soluble in dimethylsulfoxide (DMSO) at room

temperature and became soluble only by heating. Their

enhanced solubility is mostly because of the side aliphatic−

aromatic chains containing azo-naphthalene structure which

limit the charge-transfer complex formation and reduce the

intermolecular interactions.⁴¹This structural entanglement

hinders the free rotation of the bonds and enables the

penetration of the solvent molecules between the macro-

molecular chains. The excellent solubility of these PIs in a

variety of solvents, from polar to less polar, and even nonpolar,

facilitates their easy processability by wet methods. Thus, they

are suitable candidates for practical applications in spin-coating

and casting processes under environmentally friendly con-

ditions for the targeted high-tech applications, including

advanced optoelectronics.

Supporting Information, whereas the structural assignments

of the peaks recorded in these spectra are provided in Tables 1

(P3), S2 (P1), and S3 (P2), Supporting Information.

All spectra contain two major series of peaks corresponding

to the two species of PIs, approximatively separated by 880

(P1), 893 (P2), and 1015 (P3) Da, which are the expected

masses of the repeat units of these PIs. Mainly, these species

correspond to the polymeric chains that contain stoichiometric

ratios between the diamine (M1) and each dianhydride (M2−

M4), and polymeric chains possessing diamine sequences as

end-capped structural elements. In addition, because of the

chemical imidization side reactions, the end-capped diamine

groups displayed attached sequences of acetic anhydride, a

feature which varied from one PI to another. Besides, all

species in discussion were cationized in diﬀerent ways, either

by H⁺or Na⁺. In the particular case of P3 (Figure 4 and Table

1), the main type of polymer chains (P3_Mw1) is related to the

stoichiometric ratio between the diamine M1 and 6FDA, and

amino end groups cationized with H⁺. The masses of the

polymeric chains belonging to the second series (P3_Mw2) with

low propensity were shifted to lower m/z values by 318 Da and

correspond to the P3 chains end-capped with amino groups

transformed by the acetic anhydride and ionized with Na⁺. The

overall calculations indicated up to eight and seven repetitive

units for P3_Mw1and P3_Mw2, respectively. Similar polymer

chains and ionization processes were encountered for P1, but

the specimen propensity is reversed (Figure S9 and Table S2,

Supporting Information). The main sequence of P1 was

attributed to the polymer chains ending with acetic acid-

modiﬁed amino groups and ionized with Na⁺, whereas a lower

propensity was found for specimens with amino₂end groups

and ionized with H⁺having the mass shifted by 186 Da. Thus,

up to eight repeating units were found for P1_Mw1and up to ﬁve

repeating units for P1_Mw2. In the case of P2, a diﬀerent

composition has been found. Here, the main sequence is

related to the polymer chains that obey the stoichiometry

between monomers (P2_Mw1) (Figure S10 and Table S3,

Supporting Information). Instead, a second series of lower

propensity masses has been assigned to P2 chains that also

obey the monomer stoichiometry but with the chain ends

consisting of amino groups modiﬁed with the acetic anhydride

residue (P2_Mw2). Similar to the other two polymers of the

series, P1 and P3, species containing free amino groups

(P2_Mw1) were cationized by H⁺, whereas those modiﬁed by the

acetic anhydride residue (P2_Mw2) were ionized by Na⁺. In this

respect, up to six repeating units were identiﬁed for P2_Mw1

species, whereas only ﬁve repeating units were found for P2_Mw2

species.

The number average molecular weight (M_n) of PIs has been

evaluated on the basis of the following equation

∑ NM_i

i

M_n=

∑ N

(1)

i

According to the data collected from the MALDI-TOF mass

spectra, the M_nvalues of the two series of specimens found for

each polymer were estimated as follows: M_n1= 2725 g/mol

and M_n2= 2135 g/mol for P1; M_n1= 2507 g/mol and M_n2

=

The molecular weights of the azopolyimides under study

were evaluated by MALDI-TOF measurements, which

involved the dry droplet technique with dithranol as the

matrix and NaI as the ionizing agent. The obtained spectra are

displayed in Figures 4 (P3), S9 (P1), and S10 (P2),

2483 g/mol for P2; and M_n1= 3705 g/mol and M_n2= 3904 g/

mol for P3. These molecular weight values place these PIs into

the category of low-molecular-weight polymers, a feature

expected because of the bulky nature and low reactivity of the

azo-diamine M1. The diﬀerent types of polymer chains found

1525

https://dx.doi.org/10.1021/acs.macromol.0c02182

Macromolecules 2021, 54, 1517−1538

Macromolecules

pubs.acs.org/Macromolecules

Article

a

Table 2. Thermal Properties of the Azo-Based PIs, P1−P3

polymer

T_onset1(°C)

T_max1(°C)

T_onset2(°C)

T_max2(°C)

T_onset3(°C)

T_max3(°C)

W₈₀₀(%)

T_g(°C)

P1

P2

P3

323

338

342

338

349

358

420

406

411

449

430

444

545

530

542

615

590

556

40

62

42

185

200

204

a

T_onset= onset temperature of decomposition; T_max= temperature of the maximum rate of decomposition; W₈₀₀= residual weight percentage at

800 °C; T_g= glass transition temperature.

for P1−P3 are consistent with the nature of the used

dianhydrides and their reactivity toward diamine M1. For

instance, P2 derived from a reactive dianhydrideBTDA

mainly contains polymer chains with the stoichiometric

composition of the monomers. Instead, P1 obtained from a

less reactive dianhydrideODPA displayed as the main

sequence polymer chains containing amino-terminal groups.

3.4. Thermal Properties. The thermal behavior of the

azo-based PIs, P1−P3 was investigated by thermogravimetric

analysis (TGA) and diﬀerential scanning calorimetry (DSC)

under nitrogen atmosphere, the collected data being listed in

Table 2.

phase transitions. These ﬁndings support the amorphous

character of the present PIs. Generally, the PIs substituted with

ABn in the side chains display slightly lower T_gvalues

compared to typical PIs with no side chains.^43,44This was also

true for the P1−P3 series, for which T_gwas approx. 20 °C

lower than those reported by other PIs without azo units.⁴⁵T_g

diﬀerences observed for this PI series can be correlated with

the nature of the diimide segment. Even if all polymers are

ruled in high percentage by the same structural features, the T_g

variation seems to be mediated by the nature and strength of

intermolecular interactions. As previously discussed in the case

of P2, the keto unit is prone to hydrogen-bond interactions

with the surrounding structural features, leading to the T_g

increase by 15 °C compared to P1. This is slso corroborated

with the more rigid structure of the diimide segment of P2

compared to P1. Although P3 contains more ﬂexible chains

because of the presence of hexaﬂuoroisopropylidene (6F)

groups, it displays a T_gvalue comparable with that of P2, a

feature that can be explained only in terms of the polymer

chain interactions developed through dipole−dipole eﬀects.

Nevertheless, the bulky nature of the 6F group will additionally

restrict the free rotation of the polymer backbones, thus

contributing to the T_gincrease. Overall, the high T_g

corroborated with good thermal stability is advantageous

from the point of view of the possible applications in

optoelectronic devices, where the operating temperatures

may rise signiﬁcantly.

3.5. Photo-Optical Features. Since PIs, P1−P3, under

this study are based on azo-naphthalene, aliphatic chains, and

diﬀerent diimide building blocks, it was necessary to survey the

inﬂuence of each chemical structural element on their

electronic absorption properties. This is of particular interest

for the future assessment of the trans−cis photoisomerization

behavior. The electronic absorption features of these PIs were

explored in comparison with that of the azo-naphthalene-based

diamine monomer (M1) in similar experimental conditions

(Figure 5).

Both PIs and the corresponding diamine displayed three

structured absorption bands, which correspond to diﬀerent

types of electronic transitions, typically for azo-based

derivatives. The high-intensity absorption band which falls at

λ_max∼ 384 nm in the UV region arises from the symmetry-

allowed π → π* transitions (S₀→ S₂) in the trans isomer. The

much weaker band located at the lowest energy (λ_max∼ 472

nm, visible region) can be accurately ascribed to the symmetry-

forbidden n → π* transitions (S₀→ S₁) in the cis isomer.⁴⁶⁻⁴⁸

As seen in Figure 5, the variation in the diimide segment had

no inﬂuence on the main electronic transitions because of the

absence of any important shifts. Instead, by comparing with

other azo-based derivatives,⁴⁹⁻⁵¹the above mentioned

transitions occurred at a higher wavelength, their red shift

being induced by the substituents present in the 4 and 4′

positions with respect to the azo group. Historically, ABn

derivatives have been divided into three categories based on

The polymers containing covalently bonded ABn chromo-

phore are usually characterized by two or three steps of

decomposition.⁴²In the particular case when ABn units are

incorporated into the PI side chains, a signiﬁcantly lowered

thermal stability is obtained in comparison with that of a

typical aromatic PIs.^43,44P1−P3 displayed the ﬁrst decom-

position step associated with the decomposition of the azo

group in the range of 323−342 °C, with the maximum rate at

temperatures between 338 and 358 °C, as determined from the

derivative thermogravimetric curves (DTG) (Figure S11,

Supporting Information). The second step of degradation

starts at temperatures between 406 and 420 °C and is related

to the cleavage of the aliphatic groups from the side chains,

whereas the third step assigned to the main polymer backbone

scission occurred at 530−545 °C, with the maximum rate at

556−615 °C. The carbonized residue of the azo PIs in

discussion was found to be between 40 and 62% at 800 °C and

can be ascribed to their high aromatic content coming from the

benzene and naphthalene units. According to the onset

temperature of the degradation process, each PI displayed a

diﬀerent thermal behavior which is mostly consistent with the

type of intermolecular interactions promoted by the diimide

segment. Thus, P3 displayed the highest thermal stability (358

°C) because of the strong dipole−dipole interactions induced

by the electron-withdrawing CF₃groups, followed by P2 (349

°C) that easily develops hydrogen bonding because of the

presence of the keto bridge in the diimide segment, whereas

P1 appears to be less thermally stable (338 °C) because of the

weaker intermolecular interactions and less packed polymer

chains that do not stabilize the azo group. As the temperature

passes the second degradation step of scission of the aliphatic

sites, including the keto group in P2 and hexaﬂuoroisopropy-

lidene group in P3, the molecular interactions exert no more

inﬂuence on the thermal stability, explaining why in the third

step P3 exhibits the lowest temperature of the decomposition

rate, whereas P1 the highest.

The glass-transition temperature (T_g), one of the most

important thermal parameters of PIs, was assessed by DSC

experiments, in the second heating run. According to the

recorded DSC curves (Figure S12, Supporting Information),

the glass transition of the azo PIs under study occurred at

temperatures between 180 and 206 °C, without any melting or

1526

https://dx.doi.org/10.1021/acs.macromol.0c02182

Macromolecules 2021, 54, 1517−1538

Macromolecules

pubs.acs.org/Macromolecules

Article

the molecule becomes lower and the absorption moves to a

higher wavelength. This is also true for these azo-polymers in

high polar solvents such as DMF that better stabilizes the

dipolar species compared to those in less polar solvents, like

CHCl₃. Studies on the absorption spectra of P1−P3 in

solvents of diﬀerent polarities (Figure S14, Supporting

Information) showed that in a less polar solvent (CHCl₃),

the main absorption band registered a 4 nm hypsochromic

shift than in a polar DMF solvent, whereas in a more polar

solvent (DMSO), a bathochromic shift of 2 nm was obtained.

This is of particular interest as the polarity of the solvent

inﬂuences the mechanism of isomerization. Thereby, polar

solvents favor rotation mechanism, whereas nonpolar solvents

favor inversion mechanism. Also, polar solvents assist and

stabilize the π → π* transitions to the detriment of n → π*

transitions.^47,57,58Following these investigations, a suitable

solvent was selected for the kinetic study of trans−cis and cis−

trans isomerizations of the compounds under study, which will

be detailed in the next section.

Figure 5. UV−vis spectra of the synthesized PIs, P1−P3, and diamine

M1 in DMF.

The solvent evaporation led to thin ﬁlms with spectral

patterns similar to those obtained in solution in terms of band

distribution (Figure S14, Supporting Information, brown

curves). It should be mentioned that an obvious localization

of π → π* and n → π* transitions occurs corresponding to the

trans isomer, respectively, similar to that encountered in polar

solvents. However, this time, some π−π interactions between

the naphthalene units probably occurred during ﬁlm

preparation, leading to the planarization of the azo-

naphthalene-extended conjugated system and a slight bath-

ochromic shift of the main transitions corresponding to the

trans isomer.

Any attempt to survey the ﬂuorescence properties of these

azopolyimides was reduced to only very weak ﬂuorescence

(Figure S15, Supporting Information), mainly originating from

the S₂states associated with the trans isomer.⁵⁰However, these

features are in line with previous reports with regard to the

ﬂuorescence quenching by the azo group.^59,60The emission

maxima were found for all PIs in the blue region, at approx.

the relative energies of π → π* and n → π* transitions: ABns,

aminoazobenzenes (aABn), and pseudostilbenes (pSB)

(Figure S13, Supporting Information).^47,52Accordingly, these

PIs subscribe to the aABn category because of the presence of

alkoxy substituent in the para position relative to the azo

group. Consequently, the electron-donating alkoxy group will

cause the π → π* transitions of the trans form to shift to higher

wavelengths, leading to a partial overlap with the n → π*

transitions. On the other hand, the azo group is part of the

naphthalene condensed system, which in turn contributes to a

further shift of this band to a higher wavelength, according to

the bathochromic shifts promoted by an extended conjugation.

On the other hand, the structural entanglement of these PIs

is expected to be sensitive to the surrounding environment,

especially to the solvent polarity.⁵³⁻⁵⁶When the electrons of a

solvent can rearrange to stabilize the excited states of a

molecule, the energy diﬀerence between the electronic levels of

Figure 6. CV curves of azo-based PIs, P1−P3 recorded in the cathodic region, in the TBAP/DMF electrolyte system.

1527

https://dx.doi.org/10.1021/acs.macromol.0c02182

Macromolecules 2021, 54, 1517−1538

Macromolecules

pubs.acs.org/Macromolecules

Article

Figure 7. Evolution of UV−vis absorption spectra of the diamine M1 and PIs, P1−P3 in DMF during UV light irradiation (insets highlight the

photoisomerization kinetics).

470 nm. Among all polymers, P1 presented a quite well-

resolved spectral proﬁle with an important contribution at 665

nm in the form of a shoulder. This can be only ascribed to the

PL emission from the S₁states associated with the cis isomer,

as excitation at the maximum corresponding to the π → π*

transition of the trans isomer is expected to induce the

occurrence of its cis counterpart. The PL spectral features

indicated that the trans−cis isomerization of azo chromo-

phores occurs easily for this polymer during the experimental

timescale, allowing the excitation energy transfer from the trans

isomer to the cis isomer.

3.6. Redox Features. The electrochemical features of the

azo-based PIs, P1−P3 were assessed as solute analytes to gain

a complete insight with regard to their redox behavior, with a

special concern on the azo group eﬀect. To this aim, cyclic

voltammetry (CV) experiments were performed on polymers

in the tetrabutylammonium perchlorate (TBAP)/DMF elec-

trolyte system, at a scan rate of 50 mV/s. The attempt to

survey the electrochemical properties of P1−P3 in the anodic

region failed because of the lack of any polymer redox activity

in the potential window between 0 and 1.6 V. Therefore, it will

be mentioned in the following sections only to the electro-

chemical response triggered by P1−P3 in the cathodic region.

The recorded CV curves consisted in two discrete reduction

processes, as shown in Figure 6.

The irreversible processes located at potential values from

−0.87 to −1.17 V versus Ag/Ag⁺are associated with the

reduction of both the azo and imide groups. Although for P1

and P2 these reduction processes appeared overlapped, they

are well separated in the case of P3. As the reduction of the azo

moiety involves a +2H⁺/+2e⁻or +4H⁺/+4e⁻electrode

process, hydrazo or amino derivatives are envisaged to be

formed,⁶¹a feature that is supported by 10 repetitive CV

measurements (Figure S16, Supporting Information). Accord-

ing to these, after the ﬁrst cycle, the azo group is

electrochemically converted to its hydrazo or amino counter-

parts which are no longer active in the cathodic region,

whereas the imide ring remains to be the only redox active

center. Thus, the following 9 cycles are clearly dominated by

the stable electrochemical response of the imide cycle. This

process occurring between −0.97 and −1.13 V versus Ag/Ag⁺

could be associated with the generation of radical anions of the

phthalimide cycle. To further support the above mentioned

statements, 10 repetitive CV experiments were also performed

by reversing the potential to the anodic region after traversing

the n-doping region. As expected, the ﬁrst cycle diﬀered from

the following 9 cycles, suggesting that the generated amino

1528

https://dx.doi.org/10.1021/acs.macromol.0c02182

Macromolecules 2021, 54, 1517−1538

Macromolecules

pubs.acs.org/Macromolecules

Article

derivatives are oxidized. Meanwhile, a new anodic peak appears

to evolve, starting with the second scan at 1.22−1.24 V,

especially well seen for P2 and P3. It can be associated with

the electrodeposition of a polynaphthaleneamine ﬁlm that is

much prone to be formed from the generated amino-

naphthalene. However, as detailed investigations are necessary

for an accurate analysis and as these are out of the scope of this

work, this subject is not further discussed.

isomerization under polar conditions, by favoring or inhibiting

it. According to literature data, the predominant mechanism

involves the rotation around the azo bond, with the breaking of

π bonds under π−π* excitation.^48,64The outstanding fast

response under irradiation clearly evidenced that the

intermolecular interactions between the azo chromophore

and solvent molecules controlled the isomerization process

rate of these PIs and the corresponding diamines by virtue of

an advantageous molecular design. The development of

intermolecular hydrogen bonds between the nitrogen atom

of the azo group and the solvent molecules favors the fast

switch to cis species, which appears to be the key to the

obtained fast isomerization kinetics.^47,65On the other hand,

the solvent polarity increase can better assist the rotation

mechanism toward fast switch,⁶⁶which is also true for these

PIs according to the UV−vis absorption data obtained for

solvents with various polarities reported in the previous

section. In this context, the values of the photoisomerization

constant rate (R) should be considered. It can be estimated

from the ﬁrst-order derivative applied to the initial (A₀) and

photostationary state (A_∞) absorption curves by using the

following equation^67,68

3.7. Investigations on Trans−Cis−Trans Isomeriza-

tion. Pursuing the research toward the development of

unprecedented PI-based smart materials, azo-containing side

chains were anchored on rigid imide-type main chains,

according to Scheme 2, so as to obtain polymers highly

photoresponsive to light irradiation. To modulate the photo-

switching response, the ﬂexibility of PI macromolecules was

tuned through structural design by changing the substitution

pattern in the diimide segment. This structural concept was

exploited for trans−cis−trans photoisomerization studies by

1

means of UV−vis absorption and H NMR spectroscopies,

which led to signiﬁcant understanding of the mechanism

responsible for the smart stimuli-responsive behavior of these

PIs.

3.7.1. Evaluation of Trans−Cis Photoisomerization by

UV−Vis Spectroscopy. The photoswitching studies were ﬁrst

performed on both the diamine M1 and PIs, P1−P3 in DMF

solutions and solid state, by using UV−vis spectrometry.

Before irradiation with UV light, the solution and ﬁlms

deposited on glass substrates were kept in dark for 24/48 h. By

UV light irradiation at 365 nm, isomerization from the trans to

cis species of the azo chromophore in all the investigated

compounds took place, according to the spectral proﬁles

displayed in Figure 7.

A₀− A_∞

R (%) =

× 100

A₀

(2)

Accordingly, the trans−cis conversion eﬃciency was found

to be 95.3% for M1, 87% for P1, 90.2% for P2, and 88% for

P3. The variation of the degree of photoisomerization among

these PIs can be attributed to the interactions between the

solvent molecules and the main backbone of the polymer

chains. The clear isosbestic points around 325−326 and 450−

459 nm prove that the photoisomerization reaction is a single-

step process that involves only two species (trans and cis) and

no side reactions. These are by far among the highest values

reported to date for the photoisomerization eﬃciency of

azopolyimides,^21,51being comparable to those achieved by

other azo-based compounds,⁶⁸and bring up new opportunities

for their use nowadays in optoelectronics.

Further on, the photoisomerization kinetics of PIs, P1−P3

and of the corresponding diamine monomer M1 were assessed

by monitoring the evolution of the absorbance band

corresponding to the π → π* transitions associated with the

trans isomer. The experimental data were analyzed according

to the following equation⁶⁸

It is obvious that the intensity of the absorption bands

located at 384 nm (due to the π → π* transitions in the trans

isomer) decreased progressively as the irradiation time

advanced, being assisted by the increase of the absorption

band corresponding to the π → π* and n → π* transitions in

the cis isomer, located at approx. 290 and 472 nm, respectively.

All PIs and the diamine behaved almost similarly, the only

diﬀerence being the irradiation time necessary to reach the

photoequilibrium state. Thus, the conversion of the trans

isomer to the cis isomer was completed in 50 s for P1, 47 s for

P2, 130 for P3, and 130 s for M1. On further continuous

irradiation with UV light up to 300 s, no important changes in

their absorption spectra were observed. Considering that the

photoisomerization of PIs is in discussion, the obtained results

are impressive, being comparable to those related to low-

molecular-weight compounds.^46,47,62,63These PIs are so

sensitive to light irradiation that they can undergo photo-

isomerization even in the natural sunlight. Unfortunately,

because of the lack of a standardized sunlight simulator, these

investigations remained unnoticed. Another remark is related

to the fact that the photoisomerization process is independent

of the nature of the support which carries the azo

chromophore receptor, being similar for both molecules

(M1) and polymer chains (P1−P3). However, this is not

true when considering the time necessary to accomplish the

complete trans-to-cis conversion. Such a behavior is mainly

mediated by the highly ﬂexible aliphatic side chain that favors

the isomerization process during UV light irradiation.

A₀− A_∞

kt = ln

A_t− A_∞

(3)

where t is the irradiation time, A₀is the initial absorbance, A_tis

the absorbance at a certain irradiation time, and A_∞is the

absorbance at the photostationary state. Plotting eq 3 versus

the irradiation time enabled the estimation of the photo-

isomerization rate constant (k) from the slope of the linear ﬁts

(Figure 7, inset images). As expected, the photoisomerization

kinetics of the PIs and diamine followed a ﬁrst-order kinetics.

Thus, photoisomerization rate constant values of 5.22 × 10⁻²,

8.6 × 10⁻², 8.4 × 10⁻², and 2.94 × 10⁻²were obtained for M1,

P1, P2, and P3, respectively. Because of the non-assisted

substituent eﬀect on the isomerization process, the present PIs

display fast photoisomerization response to 365 nm UV light

irradiation in DMF solutions and can compete even with small

molecules containing an azo chromophore.⁶⁶The diﬀerent

photoisomerization kinetics among the PIs can be rationalized

On the other hand, both the solvent type and the

substitution pattern of the azo group exerted a high impact

on the main mechanism involved in the trans−cis photo-

1529

https://dx.doi.org/10.1021/acs.macromol.0c02182

Macromolecules 2021, 54, 1517−1538

Macromolecules

pubs.acs.org/Macromolecules

Article

Figure 8. Evolution of the UV−vis absorption spectra of PIs, P1−P3 ﬁlms during UV light irradiation (insets represent the photoisomerization

kinetics).

in terms of dipole-type interactions between the solvent

molecules and polymer chains, with P3 having the most

important contribution because of the presence of ﬂuorine

atoms. Despite the small molecular structure resembled by

diamine M1, the k value is not so small, as it may be expected,

to the detriment of its high degree of isomerization (95.3%).

The reason can be related to the strong hydrogen bonds

between the amino groups and solvent molecules which can

entrap the diamine in a solvent cage so that the rotation of the

azo group is considerably hindered.

As majority of optoelectronic applications require the

material in the form of ﬁlms or coatings, the photochromic

behavior of PIs, P1−P3 was also investigated in solid state.

The polymer ﬁlms were cast from the DMF solution and left in

dark for 48 h. Figure 8 displays the variation in their electronic

absorption spectra during irradiation with UV light of 365 nm.

The PI ﬁlms exhibited a photochromic behavior similar to

that encountered in DMF solution, with the progressive

decrease of the absorption maxima at ∼384 nm corresponding

to the π → π* transitions in the trans isomer, and the gradual

increase of the absorption bands at 316 and 478 nm associated

with the π → π* and n → π* transitions in the cis isomer,

when the UV light exposure time enhanced. As expected, the

photostationary state was reached after a longer irradiation

time, approx. 900 s, with the trans−cis conversion eﬃciency

being of 68% for P1, 64.7% for P2, and 75.7% for P3, and still

very high for the solid-state photoisomerization. These values

are the direct consequence of the solvent absence when the

polymer chain movement is considerably hindered by strong

intermolecular interactions. At the photostationary state, there

is some content of the trans isomer left, mostly explained by

the presence of the naphthalene unit, which may induce steric

hindrance because of its involvement in π−π interactions much

more allowed in solid state. The longer time necessary to reach

the photostationary state reﬂects the lack of solvent

contribution to the azo rotation mechanism.^46,47Beside this,

a signiﬁcant implication of the inversion mechanism should be

considered as it requires a smaller free volume to undergo

isomerization, 0.12 nm³compared to 0.28 nm³for the rotation

mechanism.⁶⁹Similar to liquid samples, two isosbestic points

evolved at approx. 324 and 464 nm during UV light irradiation.

In addition, the kinetics of the trans-to-cis conversion in solid

state corresponds to a ﬁrst-order process (Figure 8, insets) that

occurred with a photoisomerization rate constant (k) of 6.34 ×

10⁻³s⁻¹for P1, 7.38 × 10⁻³s⁻¹for P2, and 8.18 × 10⁻³s⁻¹for

P3. These values are of 1 order of magnitude smaller compared

to those recorded for solutions, demonstrating the importance

of the surroundings to the overall photoisomerization

1530

https://dx.doi.org/10.1021/acs.macromol.0c02182

Macromolecules 2021, 54, 1517−1538

Macromolecules

pubs.acs.org/Macromolecules

Article

Figure 9. Evolution of UV−vis absorption spectra of the cis isomers of diamine M1 and PIs, P1−P3 in DMF during progressive heating at 70 °C

(insets represent the thermal cis−trans isomerization kinetics).

eﬃciency. Note that these are outstanding results, considering

that the kinetics of the PIs photoisomerization was evaluated in

solid state without the addition of diluting materials, this being

one of the few studies performed directly on azo-PI ﬁlms.²¹

3.7.2. Evaluation of Cis−Trans Thermal Isomerization by

UV−Vis Spectroscopy. After reaching the photostationary

state, the reverse transformation of the cis isomer to trans

isomer can occur either in the dark, under the inﬂuence of

temperature or by irradiating with visible light of 436 nm. In

this context, it is proposed to investigate the thermally induced

cis−trans isomerization of the diamine M1 and PIs, P1−P3 by

heating the cis isomers at 70 °C both in DMF solution and in

the solid state. For this purpose, freshly prepared DMF

solutions were kept in the dark for 24 h and then allowed to

reach the photostationary state by irradiating with UV light of

365 nm for 300 s. Then, the samples were subjected to heating

at 70 °C, and the evolution of the UV−vis absorption spectra

was monitored during diﬀerent periods of time. The UV−vis

absorption spectra recorded during cis−trans relaxation are

illustrated in Figure 9.

progressive decrease of the bands at 316 nm (π → π*

transitions, cis isomer) and 472 nm (n → π* transitions, cis

isomer) during exposure at 70 °C for diﬀerent intervals of

time. As thermal relaxation implies n−π* transitions (S₁→

S₀), the cis−trans relaxation occurs predominantly via

inversion mechanism, when the π bond of the azo group

remains intact.⁴⁸Instead, ABn systems substituted with −OH

or electron-donating dialkylamino groups in the ortho or para

position with respect to the azo group experience fast

relaxation of the cis isomer to trans isomer via rotation

mechanism, being less dependent on solvent polarity. In

addition, azo groups, as part of a push−pull system, undergo a

similar mechanism, being strongly assisted by the solvent

polarity.⁴⁸Ortho- and para- alkoxy-substituted azo derivatives

show relaxation times from several hours to a few days, and

their kinetics are almost independent of the solvent used.⁷⁰As

these PIs are classiﬁed as aABns without implication of any

substituents in the relaxation process, their slower relaxation

time was expected.

The thermal relaxation kinetics of PIs, P1−P3 were

evaluated in comparison with that of the starting diamine

M1, by following the variation of the absorbance band

corresponding to π → π* transitions associated with the

The thermal relaxation induced the total recovery of the

trans isomer after 1200 s for M1, 2880 s for P1, and 2340 s for

P2 and P3, being reﬂected in the gradual increase of the bands

at 384 nm (π → π* transitions, trans isomer) and the

1531

https://dx.doi.org/10.1021/acs.macromol.0c02182

Macromolecules 2021, 54, 1517−1538

Macromolecules

pubs.acs.org/Macromolecules

Article

Figure 10. Evolution of UV−vis absorption spectra of PIs, P1−P3 ﬁlms in cis forms by heating under dark at 70 °C (insets show the thermal

relaxation kinetics).

trans isomer. The experimental data were processed by using

the equation

heating in dark at diﬀerent intervals of time, a clear evolution

of the UV−vis absorption spectra was obtained, as illustrated

in Figure 10.

A_∞− A₀

kt = ln

The PI ﬁlms showed a relaxation behavior similar to that

registered in DMF solution. During heating, an increase of the

absorption maxima at approx. 384 nm (π → π* transitions,

trans isomer) occurred along with the decrease of the bands at

316 nm (n → π* transitions, cis isomer) and 478 nm (n → π*

transitions, cis isomer). The total recovery of the trans isomer

was attained after 2700 s, following a ﬁrst-order process

(Figure 10, insets). The rate constant (k) values corresponding

to the cis−trans thermal-assisted relaxation of PIs in solid state

were evaluated to be 2.22 × 10⁻³s⁻¹for P1, 3.38 × 10⁻³s⁻¹

for P2, and 2.17 × 10⁻³s⁻¹for P3, respectively. They are

comparable with those obtained in DMF solution, strengthen-

ing the statement according to which the predominant

mechanism involved in the thermal relaxation is inversion,

being independent of the surrounding environment polarity.

However, a small variation of k still exists, which is ascribed to

the diﬀerent ﬂexibilities rendered by the main polymer chains

of the investigated PIs.

A_∞− A_t

(4)

where t is the thermal relaxation time, A₀is the absorbance at

photostationary state, A_tis the absorbance at a certain

relaxation time, and A_∞is the absorbance of the fully

recovered trans isomer. In the same manner applied for

photoisomerization studies, eq 4 was plotted versus thermal

relaxation time (Figure 9, inset images). The linear ﬁtting

demonstrated that the thermal relaxation kinetics of these

compounds follow ﬁrst-order kinetics, with k values of 5.32 ×

10⁻³s⁻¹(M1), 1.62 × 10⁻³s⁻¹(P1), 1.94 × 10⁻³s⁻¹(P2),

and 2.07 × 10⁻³s⁻¹(P3). The PIs exhibited homogeneous k

values, demonstrating that the variation in the chemical

structure of the diimide segment minimally aﬀected the

relaxation behavior. Instead, in the case of the corresponding

diamine with a smaller molecular size, this process was almost

twice faster.

The cis−trans thermal-assisted relaxation behavior of PIs,

P1−P3 was also investigated in solid state. The PI ﬁlms were

kept in dark for 48 h, followed by irradiating with UV light of

365 nm for 1200 s to induce the photostationary state. By

In an attempt to evaluate the cis−trans back-relaxation at

room temperature, the kinetics of this phenomenon was also

followed by UV−vis spectroscopy. In this regard, after keeping

1532

https://dx.doi.org/10.1021/acs.macromol.0c02182

Macromolecules 2021, 54, 1517−1538

Macromolecules

pubs.acs.org/Macromolecules

Article

Figure 11. Photoisomerization eﬃciency vs concentration dependence for PIs, P1−P3 at 10 s irradiation with UV light.

the ﬁlms in dark for 48 h and subsequently irradiating up to the

photostationary state, they were allowed to relax in dark at

room temperature. The UV−vis absorption spectra corre-

sponding to the cis−trans back-relaxation at room temperature

at diﬀerent intervals of time are provided in Figure S17,

Supporting Information. Although the mechanism was similar

to the one encountered during the cis−trans back-relaxation at

70 °C, the total recovery of the trans isomer was attained after

a maximum of 1260 min (21 h), with a high percentage of

trans−cis conversion eﬃciency. Meanwhile, the rate constant

(k) values are only slightly lower compared to those obtained

for the cis−trans thermal back-relaxation, being 3.73 × 10⁻³

s⁻¹for P1, 3.65 × 10⁻³s⁻¹for P2, and 3.23 × 10⁻³s⁻¹for P3.

This entitled us to consider that the cis−trans isomerization

process is eﬃcient in our polymers even at room temperature.

3.7.3. Studies on Concentration-Dependent Photoisome-

rization Eﬃciency and Trans−Cis−Trans Conversion Stabil-

ity. If the solvent polarity in dilute solutions favored the

photoisomerization process, at higher concentrations, the

eﬀect is expected to be low, mostly because of the solution

viscosity which mediates the photoisomerization eﬃciency.^47,71

To prove this, the variation of the photoizomerization

eﬃciency as a function of PI solution concentration has been

investigated, at the same irradiation time. For this purpose,

several solutions of known concentrations were prepared and

kept in the dark for 24 h before irradiation. Each solution was

irradiated for 10 s with UV light of 365 nm, and the evolution

of the trans isomer absorbance (at 386 nm) was monitored. By

applying the methodology described in Section 3.7.1, the

photoizomerization eﬃciency for each solution was calculated

(Figure 11).

As seen in Figure 11, the variation of the photoirradiation

eﬃciency with the concentration change is conﬁrmed; this

increases rapidly as the solutions become more diluted but

with some diﬀerences with respect to the PI structure. The

overall values ﬁt the curves that are described by the following

equation.

y = intercept + m × x + n × x²

(5)

1533

https://dx.doi.org/10.1021/acs.macromol.0c02182

Macromolecules 2021, 54, 1517−1538

Macromolecules

pubs.acs.org/Macromolecules

Article

Figure 12. Stability of the switching process from trans to cis and cis to trans for PIs, P1−P3 in DMF solution and solid state.

1

Figure 13. Evolution of H NMR spectra of P3 in DMSO-d₆during UV light irradiation.

where m and n are constants speciﬁc for each polymer. This is

a quadratic equation which suggests the exponential nature of

the photoisomerization eﬃciency evolution. Accordingly,

dilute solutions had less impact on the eﬃciency increase,

whereas concentrated ones signiﬁcantly contributed to the

drop of the photoisomerization eﬃciency. P1 and P2 follow a

more linear behavior, whereas P3 undergoes a more eﬃcient

photoisomerization as the system becomes more diluted. Such

behavior can be related to the preferential interaction of the

polymer main chains with the solvent molecules, either

through hydrogen bonds or dipole−dipole interactions,

whose strength is dependent on solution concentration. It is

also believed that for highly concentrated solutions, a low

number of solvent molecules assisted the photoisomerization

by the rotation mechanism, so that the slower inversion

mechanism prevailed. This was also the case of DMSO-d₆

solutions used in H NMR photoisomerization experiments

reported in Section 3.7.4, where longer times were demanded

to reach the photostationary states.

1

Starting from the above results, the stability of the trans−cis

and cis−trans conversions of the diamine and PIs was explored

in DMF solution and solid state. Thus, Figures 12 and S18,

1534

https://dx.doi.org/10.1021/acs.macromol.0c02182

Macromolecules 2021, 54, 1517−1538

Macromolecules

pubs.acs.org/Macromolecules

Article

Supporting Information, show the evolution of the absorbance

corresponding to the π → π* transitions associated with the

trans isomer during successive switching cycles from the trans

isomer to cis isomer by irradiation at 365 nm for speciﬁc

periods of time and from the cis isomer to trans isomer by

heating at 70 °C at a characteristic time for each polymer.

By UV irradiation of both DMF solutions and ﬁlms, the azo

group in P1−P3 isomerized from the trans to the cis

conformation, resulting in a decrease of the trans π → π*

transitions, simultaneous with the increase of the cis n → π*

transitions. Subsequent heating of PIs induced the reverse cis-

to-trans isomerization, with the initial state accordingly

recovered. Under UV irradiation at 356 nm, the maximum

content of the cis isomer was obtained after 130 s for M1, 50 s

for P1, 47 s for P2, and 130 s for P3 in DMF solution, and

after 900 s for the ﬁlms of P1−P3, according to the previous

discussed data. Further, by heating at 70 °C, the trans isomer

was recovered almost to its initial content in DMF solution

after 1200 s (M1), 2880 s (P1), 2340 s (P2 and P3), and in all

PI ﬁlms after 2700 s. According to eq 5, trans−cis and cis−

trans conversion stabilities (X%) were evaluated by considering

the linear ﬁt of the absorbance values corresponding to the

trans (A_trans) and cis (A_cis) isomers from the ﬁrst (ci) and the

last cycle (cf).

isomer, the conjugated system formed by the azo−phenylene−

naphthalene framework is disturbed, so that these three

structural elements are no longer found in the same plane but

rather in an angular geometry. This induces a decrease in the

electron-withdrawing inﬂuence of the azo group on the

electron distribution at the ortho and para positions. On the

other hand, the oxygen atom near protons H-2 assists the

proton shifting via the electron pushing eﬀect, providing

electron abundance on the azo group. As a consequence,

because of the shielding eﬀect, the signals corresponding to H-

2 and H-3 of the cis isomer appear shifted to a higher

resonance ﬁeld (6.48 and 6.81 ppm) compared to the signals

of the trans isomer (7.71 and 8.02 ppm). Also, the aliphatic

protons (H-1) appeared aﬀected by this trans−cis conversion.

The electron density of the oxygen atom near H-1 is no longer

disturbed by the azo group, so that these aliphatic protons are

also shifted to a higher resonance ﬁeld (3.99 ppm for the cis

isomer compared to 4.12 ppm for the trans isomer). The

changes in the ¹H NMR proﬁle were imperceptible after

photoisomerization for both the TPM skeleton and the

naphthalene unit, as demonstrated by the unchanged signals

associated with the central proton of TPM and H-4,

respectively. All these modiﬁcations also apply to P1 and P2

as they rely on the same azo framework as P3. However, under

UV irradiation, the PIs behaved diﬀerently, the photo-

equilibrium state being attained after 90 min for P1 and P3

and 60 min for P2. By further irradiation up to 180 min, no

important changes in the ¹H NMR spectra were observed. The

higher irradiation time required to reach the photostationary

state is due to the higher concentration of the polymers in

DMSO-d₆solutions compared to that in UV−vis absorption

studies, according to the data discussed in Section 3.7.3.

(A

− A

)

trans_ci

trans_cf

cis_ci

X% =

× 100

A

cis_cf

(6)

After 15 repeating cycles, both azodiamine and azopolyi-

mides showed an excellent stability in DMF solution, with

values of 99.8% for M1, 95.45% for P1, 96.29% for P2, and

94.73% for P3. Meanwhile, after 10 cycles, the ﬁlms prepared

from these PIs displayed a trans−cis conversion stability of

92.68% (P1), 89.81 (P2), and 91.2% (P3). Taking into

consideration that these measurements were directly com-

pleted on PI ﬁlms, without any additional dilution in solid

state, we are entitled to consider the obtained results as the

most remarkable ones obtained for this class of high-

performance polymers.

4. CONCLUSIONS

New photoactive PIs containing azo-naphthalene moieties

anchored on TPM skeleton as receptors for light and

temperature stimuli were prepared by chemical imidization

of a new TPM diamine with three commercial aromatic

dianhydrides. Because of the structural changes in the diimide

segment, PIs with diﬀerent molecular weights resulted,

contingent with the reactivity of the each dianhydride. All

PIs displayed unusual organosolubility, from polar to nonpolar

solvents, like DCM, which allowed their facile processing into

homogeneous thin ﬁlms. The side azo chains reduced the

thermal stability and the glass-transition temperature with

respect to the related PIs with no side chains. The azo-

naphthalene−TPM-imide framework promoted interesting

spectroscopic and electroactive properties. Thus, UV−vis

absorption spectra evidenced the signature of the trans form

of the polymers at slightly higher wavelengths than usual,

which exhibited almost quenched ﬂuorescence. The PIs were

electrochemically active only in the cathodic region because of

the reduction of the azo group and imide ring. To investigate

the cis−trans photo/thermal isomerization switching capa-

bility, an elaborate study was performed by using UV−vis and

¹H NMR spectroscopies. The obtained results indicated that

the trans isomer is converted to the cis isomer in DMF through

the polarity-assisted rotation mechanism, whereas in solid

state, the nonpolarity-assisted inversion mechanism prevailed.

The trans−cis photoisomerization followed a ﬁrst-order

kinetics in a short period of time in solution, from 47 to 130

s, and with an excellent eﬃciency, between 87 and 90.2%,

being among the most performant results reported to date for

1

3.7.4. Evaluation of Trans−Cis Photoisomerization by H

NMR Spectroscopy. ¹H NMR experiments have been

employed to provide additional insights into the photo-

isomerization behavior of these azopolyimides. The measure-

ments were carried out in DMSO-d₆solutions (concentration

of 3 × 10⁻²M) which were ﬁrst maintained in dark for 24 h

and then irradiated with UV light of 365 nm at diﬀerent

intervals of time. The obtained ¹H NMR spectra are displayed

in Figures S19, Supporting Information, and Figure 13. Note

that the attempt to evaluate the photoisomerization process in

CDCl₃in which the signals are better resolved proved to be

1

unsuccessful as no important changes in the H NMR signals

were observed after 90 min of UV light irradiation (Figure S20,

Supporting Information). This provides a clear proof for the

dependence of solvent polarity on the photoisomerization

mechanism of PIs, P1−P3, which is consistent with the

rotation mechanism around the azo bond.

According to Figure 13, P3 experienced some important

1

changes in the H NMR proﬁle during UV light exposure in

terms of peak intensities and their distribution, suggesting the

conﬁgurational transformation of trans isomer to its cis

counterpart. The signals in close proximity to the azo group

perceive the most important inﬂuence of the structural

changes. With the transition from the trans isomer to the cis

1535

https://dx.doi.org/10.1021/acs.macromol.0c02182

Macromolecules 2021, 54, 1517−1538

Macromolecules

pubs.acs.org/Macromolecules

Article

PIs. Meanwhile, the photoactivity of these PIs in solution

proved to be concentration-dependent, with the highest

eﬃciency at a low polymer concentration. The PIs displayed

similar photoisomerization response in solid state but after

longer irradiation times (900 s), and with lower eﬃciency, of

68−75.7%, because of the hindered rotation of the azo group.

However, these are by far remarkable results, considering that

no dilution in solid state was involved during ﬁlm preparation.

Nevertheless, the cis−trans thermal relaxation at 70 °C that

allowed almost full recovery of the trans isomer occurred

predominantly via inversion mechanism during 1200−2880 s,

both in solution and solid state. A highly stable photoswitching

behavior was demonstrated by these PIs, up to 96.29% in

solution and up to 92.68% in solid state. ¹H NMR

spectroscopy proved to be an eﬃcient technique to monitor

the trans−cis photoisomerization, highlighting the signal

changes associated with the protons in the vicinity of the azo

group. The nice photochemical features reported here may

provide a novel approach toward the development of smart

materials with response to light irradiation that can be

exploited in molecular-based electronics with photocontrol-

lable functions.

orcid.org/0000-0001-5074-3804; Phone: +40-232-

217454; Email: constantin.catalin@icmpp.ro; Fax: +40-

232-211299

Authors

Ion Sava − “Petru Poni” Institute of Macromolecular

Chemistry, 700487 Iasi, Romania

Mariana-Dana Damaceanu − “Petru Poni” Institute of

Macromolecular Chemistry, 700487 Iasi, Romania;

orcid.org/0000-0001-5513-7643

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.macromol.0c02182

Notes

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

■

This work was supported by Executive Agency for Higher

Education, Research, Development and Innovation Funding

(UEFISCDI)Romania through the project ID PN-III-P4-

ID-PCE-2016-0708.

ASSOCIATED CONTENT

* Supporting Information

The Supporting Information is available free of charge at

https://pubs.acs.org/doi/10.1021/acs.macromol.0c02182.

■

REFERENCES

■

sı

(1) Priimagi, A.; Shevchenko, A. Azopolymer-based Micro- and

Nanopatterning for Photonic Applications. J. Polym. Sci., Part B:

Polym. Phys. 2014, 52, 163−182.

(2) Loebner, S.; Jelken, J.; Yadavalli, N.; Sava, E.; Hurduc, N.;

Santer, S. Motion of Adsorbed Nano-Particles on Azobenzene

Containing Polymer Films. Molecules 2016, 21, 1663.

(3) Wang, X. Azo Polymers, Soft and Biological Matter; Springer:

Berlin, 2017.

(4) Jelken, J.; Henkel, C.; Santer, S. Polarization Controlled Fine

Structure of Diffraction Spots from an Optically Induced Grating.

Appl. Phys. Lett. 2020, 116, 051601.

(5) Wang, D.; Wang, X. Amphiphilic azo polymers: Molecular

Engineering, Self-Assembly and Photoresponsive Properties. Prog.

Polym. Sci. 2013, 38, 271−301.

(6) Kumar, G. S.; Neckers, D. C. Photochemistry of Azobenzene-

containing Polymers. Chem. Rev. 1989, 89, 1915−1925.

(7) Rochon, P.; Batalla, E.; Natansohn, A. Optically Induced Surface

Gratings on Azoaromatic Polymer-films. Appl. Phys. Lett. 1995, 66,

136−138.

(8) Kim, D. Y.; Tripathy, S. K.; Li, L.; Kumar, J. Laser-induced

Holographic Surface-relief Gratings on Nonlinear-optical Polymer

Films. Appl. Phys. Lett. 1995, 66, 1166−1168.

(9) Yadavalli, N. S.; Santer, S. In-situ Atomic Force Microscopy

Study of The Mechanism of Surface Relief Grating Formation in

Photosensitive Polymer Films. J. Appl. Phys. 2013, 113, 224304.

(10) Hurduc, N.; Donose, B. C.; Macovei, A.; Paius, C.; Ibanescu,

C.; Scutaru, D.; Hamel, M.; Branza-Nichita, N.; Rocha, L. Direct

Observation of Athermal Photofluidisation in Azo-polymer Films. Soft

Matter 2014, 10, 4640−4647.

Starting materials; structures of dianhydride monomers

used in the synthesis of PIs, P1−P3, instruments, and

1

characterization; H NMR spectra of azo-based deriva-

1

tive M1″ in DMSO-d₆; H NMR spectra of M−OH in

DMSO-d₆; numbering scheme, H NMR, ¹³C NMR,

1

COSY, HMQC, and HMBC spectra of azo-based

derivative M1′ in DMSO-d₆; COSY, HMQC, and

HMBC spectra of azo-based diamine (M1) in DMSO-

d₆; FT-IR spectrum of azo-based diamine M1; ¹H NMR

spectrum of azo-based PIs, P2 and P3 in CDCl₃; FT-IR

spectra of azo-based PIs, P1−P3; FT-IR shifts of the

synthesized PIs; MALDI-TOF mass spectrum of P1;

structural assignments of the peaks displayed in the

MALDI-TOF mass spectrum of P1; MALDI-TOF mass

spectrum of P2; structural assignments of the peaks

displayed in the MALDI−TOF mass spectrum of P2;

ATG and DTG curves (inset) of azo-based PIs, P1−P3;

DSC curves of the azo-based PIs, P1−P3; types of

substituted ABns; UV−vis absorption spectra of PIs,

P1−P3 in various solvents and in solid state; PL spectra

of PIs, P1−P3 in DMF; 10 repetitive cyclic voltammo-

grams of azo-based PIs, P1−P3 in the DMF/TBAP

electrolyte system; evolution of UV−vis absorption

spectra of PIs,P1−P3 ﬁlms during the cis−trans back-

relaxation at room temperature; stability of the switching

process from trans to cis and cis to trans for M1 in

(11) Shibaev, V.; Bobrovsky, A.; Boiko, N. Photoactive Liquid

Crystalline Polymer Systems with Light-Controllable Structure and

Optical Properties. Prog. Polym. Sci. 2003, 28, 729−836.

(12) Yesodha, S. K.; Sadashiva Pillai, C. K.; Tsutsumi, N. Stable

Polymeric Materials for Nonlinear Optics: A Review Based on

Azobenzene Systems. Prog. Polym. Sci. 2004, 29, 45−74.

(13) Natansohn, A.; Rochon, P. Photoinduced Motions in Azo-

containing Polymers. Chem. Rev. 2002, 102, 4139−4176.

(14) Hergenrother, P. M. The Use, Design, Synthesis, And

Properties of High Performance/High Temperature: An Overview.

High Perform. Polym. 2003, 15, 3−45.

1

DMF; evolution of H NMR spectra of P1 and P2 in

DMSO-d₆during UV light irradiation; and comparison

of ¹H NMR spectra of P2 in CDCl₃before and after UV

light irradiation for 90 min (PDF)

AUTHOR INFORMATION

Corresponding Author

Catalin-Paul Constantin − “Petru Poni” Institute of

Macromolecular Chemistry, 700487 Iasi, Romania;

■

(15) Fink, J. K. High Performance Polymers; William Andrew Inc.:

Norwich, 2008.

1536

https://dx.doi.org/10.1021/acs.macromol.0c02182

Macromolecules 2021, 54, 1517−1538

Macromolecules

pubs.acs.org/Macromolecules

Article

(16) Damaceanu, M.-D.; Sava, I.; Constantin, C.-P. The Chromic

and Electrochemical Response of CoCl₂− Filled Polyimide Materials

for Sensing Applications. Sens. Actuators, B 2016, 234, 549−561.

(17) Sava, I.; Damaceanu, M.-D.; Constantin, C.-P.; Asandulesa, M.;

(35) Schab-Balcerzak, E.; Sobolewska, A.; Miniewicz, A.; Jurusik, J.;

Jarzabek, B. Photoinduced Holographic Gratings in Azobenzene-

Functionalized Poly(amideimide)s. Polymer 2007, 39, 659−669.

(36) Schab-Balcerzak, E.; Siwy, M.; Jarzabek, B.; Kozanecka-

Szmigiel, A.; Switkowski, K.; Pura, B. Post and Prepolymerization

Strategies to Develop Novel Photochromic Poly(esterimide)s. J. Appl.

Polym. Sci. 2011, 120, 631−643.

(37) Konieczkowska, J.; Schab-Balcerzak, E.; Siwy, M.; Switkowski,

K.; Kozanecka-Szmigiel, A. Large and Highly Stable Photoinduced

Birefringence in Poly(amideimide)s with Two Azochromophores Per

Structural Unit. Opt. Mater. 2015, 39, 199−206.

́

Wolinska-Grabczyk, A.; Jankowski, A. Structure − Promoted High

Performance Properties of Triphenylmethane - Containing Poly-

imides and Copolyimides. Eur. Polym. J. 2018, 108, 554−569.

(18) Rusu, R.-D.; Constantin, C.-P.; Drobota, M.; Gradinaru, L.-M.;

Butnaru, M.; Pislaru, M. Polyimide films tailored by UV irradiation:

Surface evaluation and structure-properties relationship. Polym.

Degrad. Stab. 2020, 177, 109182.

(19) Jang, W.; Shin, D.; Choi, S.; Park, S.; Han, H. Effects of Internal

Linkage Groups of Fluorinated Diamine on The Optical and

Dielectric Properties of Polyimide Thin Films. Polymer 2007, 48,

2130−2143.

(38) Konieczkowska, J.; Janeczek, H.; Malecki, J. G.; Schab-

Balcerzak, E. The Comprehensive Approach Towards Study of

(Azo)polymers Fragility Parameter: Effect of Architecture, Intra- and

Intermolecular Interactions and Backbone Conformation. Eur. Polym.

J. 2018, 109, 489−498.

(39) Bujak, K.; Kozanecka-Szmigiel, A.; Schab-Balcerzak, E.;

Konieczkowska, J. Azobenzene Functionalized “T-Type ” Poly(amide

imide)s vs. Guest-host Systems A Comparative Study of Structure-

Property Relations. Materials 2020, 13, 1912.

(40) Chiriac, A.-P.; Butnaru, I.; Damaceanu, M.-D. Electrochemi-

cally Active Polyimides Containing Hydroxyl-Functionalized Triphe-

nylmethane as Molecular Sensors for Fluoride Anion Detection.

Electrochim. Acta 2020, 353, 136602.

(41) Yen, H.-j.; Guo, S.-m.; Yeh, J.-m.; Liou, G.-S. Triphenylamine-

Based Polyimides with Trimethyl Substituents for Gas Separation

Membrane and Electrochromic Applications. J. Polym. Sci., Part A:

Polym. Chem. 2011, 49, 3637−3646.

(42) Sava, I.; Sacarescu, L.; Stoica, I.; Apostol, I.; Damian, V.;

Hurduc, N. Photochromic Properties of Polyimide and Polysiloxane

Azopolymers. Polym. Int. 2009, 58, 163−170.

(43) Damaceanu, M.-D.; Constantin, C.-P.; Bruma, M.; Begunov, R.

S. The Photo-optical and Electrochemical Activity Promoted by

Trifluoromethyl-Substituted and Ortho-catenated Triphenylamine

Core in Poly(ether-imide)s. Polymer 2018, 151, 34−46.

(44) Constantin, C.-P.; Damaceanu, M.-D.; Bruma, M.; Begunov, R.

S. Ortho-catenation and Trifluoromethyl Graphting as Driving Forces

in Physical Properties Modulation of Novel Ethanol-soluble

Triphenylamine-based Polyimides. Dyes Pigm. 2019, 163, 126−137.

(45) Butnaru, I.; Sava, I.; Damaceanu, M.-D. Exploring the Impact of

Triphenylmethane Incorporation on Physical Properties of Polyimides

with Emphasis on Optical and Halochromic Behaviour. Polymer 2020,

200, 122621.

(20) Liaw, D.-J.; Wang, K.-L.; Huang, Y.-C.; Lee, K.-R.; Lai, J.-Y.;

Ha, C.-S. Advanced Polyimide Materials: Syntheses, Physical

Properties and Applications. Prog. Polym. Sci. 2012, 37, 907−974.

(21) Sava, I.; Resmerita, A.-M.; Lisa, G.; Damian, V.; Hurduc, N.

Synthesis and Photochromic Behavior of New Polyimides Containing

Azobenzene Side Groups. Polymer 2008, 49, 1475−1482.

(22) Schab-Balcerzak, E.; Grobelny, L.; Sobolewska, A.; Miniewicz,

A. Cycloaliphatic-Aromatic Polyimides Based on Diamines with

Azobenzene Unit. Eur. Polym. J. 2006, 42, 2859−2871.

(23) Hsiao, S.-H.; Lin, K.-H. Polyimides Derived from Novel

Asymmetric Ether Diamine. J. Polym. Sci., Part A: Polym. Chem. 2005,

43, 331−341.

(24) Song, N.; Gao, L.; Ding, M. Optically Active Poly(amide −

imide)s Containing Axially Dissymmetric 1,1 ′-Binaphthyl Moieties. J.

Polym. Sci., Part A: Polym. Chem. 1999, 37, 3147−3154.

(25) Xu, J. W.; Chng, M. L.; Chung, T. S.; He, C. B.; Wang, R.

Permeability of Polyimides Derived from Non-Coplanar Diamines

and 4, 4 ′-(Hexafluoroisopropylidene) Diphthalic Anhydride. Polymer

2003, 44, 4715−4721.

(26) Sava, I.; Burescu, A.; Stoica, I.; Musteata, V.; Cristea, M.;

Mihaila, I.; Pohoata, V.; Topala, I. Properties of Some Azo-

copolyimide Thin Films Used in Formation of Photoinduced Surface

Relief Gratings. RSC Adv. 2015, 5, 10125−10133.

(27) Sava, I.; Stoica, I.; Mihaila, I.; Pohoata, V.; Topala, I.; Stoian,

G.; Lupu, N. Nanoscale Analysis of Laser-Induced Surface Relief

Gratings on Azocopolyimide Films Before and After Gold Coating.

Polym. Test. 2018, 72, 407−415.

(28) Konieczkowska, J.; Schab-Balcerzak, E.; Libera, M.; Mihaila, I.;

Sava, I. Surface Relief Gratings in Azopolyimides Induced by Pulsed

Laser Irradiation. Eur. Polym. J. 2019, 110, 85−89.

(46) Merino, E.; Ribagorda, M. Control Over Molecular Motion

Using the Cis −Trans Photoisomerization of the Azo Group. Beilstein

J. Org. Chem. 2012, 8, 1071−1090.

(47) Chang, C. W.; Lu, Y. C.; Wang, T. T.; Diau, E. W. G.

Photoisomerization Dynamics of Azobenzene in Solution with S₁

Excitation: A Femtosecond Fluorescence Anisotropy Study. J. Am.

Chem. Soc. 2004, 126, 10109−10118.

́

(29) Bujak, K.; Nocon, K.; Jankowski, A.; Wolinska-Grabczyk, A.;

Schab-Balcerzak, E.; Janeczek, H.; Konieczkowska, J. Azopolymers

with Imide Structures as Light-Switchable Membranes in Controlled

Gas Separation. Eur. Polym. J. 2019, 118, 186−194.

(30) Bujak, K.; Sava, I.; Stoica, I.; Tiron, V.; Topala, I.; Węgłowski,

R.; Schab-Balcerzak, E.; Konieczkowska, J. Photoinduced Properties

of ″T-Type ″ Polyimides with Azobenzene or Azopyridine Moieties.

Eur. Polym. J. 2020, 126, 109563.

(31) Chen, T.-A.; Jen, A. K.-Y.; Cai, Y. Two-Step Synthesis of Side-

Chain Aromatic Polyimides for Second-Order Nonlinear Optics.

Macromolecules 1996, 29, 535−539.

(32) Schab-Balcerzak, E.; Siwy, M.; Kawalec, M.; Sobolewska, A.;

Chamera, A.; Miniewicz, A. Synthesis, Characterization, and Study of

Photoinduced Optical Anisotropy in Polyimides Containing Side

Azobenzene Units. J. Phys. Chem. A 2009, 113, 8765−8780.

(33) Konieczkowska, J.; Wojtowicz, M.; Sobolewska, A.; Noga, J.;

Jarczyk-Jedryka, A.; Kozanecka-Szmigiel, A.; Schab-Balcerzak, E.

Thermal, Optical and Photoinduced Properties of a Series of Homo

and Co-polyimides with Two Kinds of Covalently Bonded Azo-Dyes

and Their Supramolecular Counterparts. Opt. Mater. 2015, 48, 139−

149.

(48) Wang, X. Azo Polymers: Azosystems and Eﬃciency of

Chromophore; Springer: Berlin, 2017. Chang, C. W.; Lu, Y. C.;

Wang, T. T.; Diau, E. W. G. Photoisomerization Dynamics of

Azobenzene in Solution with S1 Excitation: A Femtosecond

Fluorescence Anisotropy Study. J. Am. Chem. Soc. 2004, 126,

10109−10118.

(49) Konieczkowska, J.; Kozanecka-Szmigiel, A.; Janeczek, H.;

́

Małecki, J.; Wojtowicz, M.; Schab-Balcerzak, E. No Effect of the

Hydrogen Bonds on the Physicochemical Properties of the Guest-

Host Poly(amide amide) Azosystems and Efficiency of Chromophore

Orientation. Dyes Pigm. 2018, 156, 250−259.

(50) Sava, I.; Damaceanu, M.-D.; Nitschke, P.; Jarząbek, B. The First

Evidence of Redox Activity of Polyimide Systems Modified with Azo

Groups with Photo-Induced Response. React. Funct. Polym. 2018, 129,

64−75.

(51) Gnida, P.; Bujak, K.; Janeczek, H.; Schab-Balcerzak, E.;

Konieczkowska, J. Poly(amic acid)s vs. Polyimides with Π -

Conjugated −NN- Units: Cis-Trans Isomerization Reaction and

Kinetics of Thermal Imidization. Opt. Mater. 2020, 104, 109931.

(34) Schab-Balcerzak, E.; Sapich, B.; Stumpe, J. Photoinduced

Optical Anisotropy in New Poly(amide imide)s with Azobenzene

Units. Polymer 2005, 46, 49−59.

1537

https://dx.doi.org/10.1021/acs.macromol.0c02182

Macromolecules 2021, 54, 1517−1538

Macromolecules

pubs.acs.org/Macromolecules

Article

(52) Rau, H. Photochromism: Molecules and Systems; Duerr, H.,

Bouas-Laurent, H., Eds.; Elsevier: Amsterdam, 2003.

(53) Adachi, M.; Murata, Y. Relationship between π -Conjugation

Size and Electronic Absorption Spectrum: Novel π -Conjugation Size

Dependence of Indoaniline Dyes. J. Phys. Chem. A 1998, 102, 841−

845.

(54) Anderson, R. J.; Bensell, D. J.; Groundwater, P. W. Organic

Spectroscopic Analysis; Abel, E. W., Ed.; Royal Society of Chemistry:

Cambridge, 2004.

(55) Constantin, C.-P.; Damaceanu, M.-D. In-depth Investigation of

The Optical Effects in Rationally Designed Phenoxazine-Based

Polyazomethines with Activated Quenched Fluorescence. J. Phys.

Chem. C 2017, 121, 6300−6313.

(56) Bejan, A.-E.; Constantin, C.-P.; Damaceanu, M.-D. n-Type

Polyimides with 1,3,4-Oxadiazole-Substituted Triphenylamine Units -

An Innovative Structural Approach. J. Phys. Chem. C 2019, 123,

15908−15923.

(57) Horspool, W. M.; Lenci, F. CRC Handbook of Organic

Photochemistry and Photobiology, 2nd ed.; CRC Press: U.K., 2003.

(58) Joshi, N. K.; Fuyuki, M.; Wada, A. Polarity Controlled Reaction

Path and Kinetics of Thermal Cis-to-Trans Isomerization of 4-

Aminoazobenzene. J. Phys. Chem. B 2014, 118, 1891−1899.

(59) Rau, H. Spectroscopic Properties of Organic Azo Compounds.

Angew. Chem. 1973, 12, 224−235.

(60) Cembran, A.; Bernardi, F.; Garavelli, M.; Gagliardi, L.; Orlandi,

G. On the Mechanism of the cis −trans Isomerization in the Lowest

Electronic States of Azobenzene: S0, S1, and T1. J. Am. Chem. Soc.

2004, 126, 3234−3243.

(61) Harisha, S.; Keshavayya, J.; Kumara Swamy, B. E.; Viswanath,

C. C. Synthesis, Characterization and Electrochemical Studies of Azo

Dyes Derived from Barbituric Acid. Dyes Pigm. 2017, 136, 742−753.

(62) Petroffe, G.; Wang, C.; Sallenave, X.; Sini, G.; Goubard, F.;

́

Peralta, S. Fast and Reversible Photo-Responsive Wettability on TiO₂

Based Hybrid Surfaces. J. Mater. Chem. A 2015, 3, 11533−11542.

(63) Bujak, K.; Orlikowska, H.; Małecki, J. G.; Schab-Balcerzak, E.;

Bartkiewicz, S.; Bogucki, J.; Sobolewska, A.; Konieczkowska, J. Fast

Dark Cis-Trans Isomerization of Azopyridine Derivatives in

Comparison to Their Azobenzene Analogues: Experimental and

Computational Study. Dyes Pigm. 2019, 160, 654−662.

(64) Urban, M. W. Handbook of Stimuli-Responsive Materials;

WILEY-VCH Verlag GmbH & Co. KGaA: Weinheim, 2011.

́

(65) Garcia-Amoros, J.; Sanchez-Ferrer, A.; Massad, W. A.; Nonell,

S.; Velasco, D. Kinetic Study of the Fast Thermal Cis-To-

Transisomerisation of Para-, Ortho- and Polyhydroxyazobenzenes.

Phys. Chem. Chem. Phys. 2010, 12, 13238−13242.

(66) Smith, S.; Bou-Abdallah, F. The Kinetics of the Cis-to-Trans

Thermal Isomerization of 4-Anilino-4 ’-Nitroazobenzene are Highly

Influenced by Solvent Polarity. J. Thermodyn. Catal. 2017, 8, 1000181.

(67) Georgiev, A.; Bubev, E.; Dimov, D.; Yancheva, D.; Zhivkov, I.;

̌

Krajcovic, J.; Vala, M.; Weiter, M.; Machkova, M. Synthesis, Structure,

Spectral Properties and DFT Quantum Chemical Calculations of 4-

Aminoazobenzene Dyes. Effect of Intramolecular Hydrogen Bonding

on Photoisomerization. Spectrochim. Acta, Part A 2017, 175, 76−91.

(68) BN, S.; Yam, W. S.; Hegde, G. Photoresponsive Behavior of

Hydrophilic/Hydrophobic-Based Novel Azobenzene Mesogens: Syn-

thesis, Characterization and Their Application in Optical Storage

Devices. RSC Adv. 2019, 9, 40588−40606.

(69) Tylkowski, B.; Giamberini, M.; Prieto, S. F. Microencapsulation,

2nd ed.; Walter de Gruyter GmbH: Berlin, 2020.

́

(70) Garcia-Amoros, J.; Sanchez-Ferrer, A.; Massad, W. A.; Nonell,

S.; Velasco, D. Kinetic Study of the Fast Thermal Cis-to-Trans

Isomerisation of Para-, Ortho- And Polyhydroxyazobenzenes. Phys.

Chem. Chem. Phys. 2010, 12, 13238−13242.

(71) Serra, F.; Terentjev, E. M. Effects of Solvent Viscosity and

Polarity on the Isomerization of Azobenzene. Macromolecules 2008,

41, 981−986.

1538

https://dx.doi.org/10.1021/acs.macromol.0c02182

Macromolecules 2021, 54, 1517−1538

Article Doi

Structural Chemistry-Assisted Strategy toward Fast Cis-Trans Photo/Thermal Isomerization Switch of Novel Azo-Naphthalene-Based Polyimides

DOI: 10.1021/acs.macromol.0c02182

Source and publish data:

Authors:

Article abstract of DOI:10.1021/acs.macromol.0c02182

Full text of DOI:10.1021/acs.macromol.0c02182

Products guided by the article

R&D Labs maybe for 6370-47-4

Relevant to this article

Hot Product