New long-chain donor-acceptor-donor pyromellitic diimide (PMDI) derivatives. A combined theoretical and experimental study

Article abstract of DOI:10.1016/j.dyepig.2018.04.053

This study involves the electrochemical, thermal and photophysical profiling of new pyromellitic diimide (PMDI) derivatives containing alkyl chains of different sizes. The photophysical investigation shows that all compounds exhibited absorption in the UV-B region (～290 nm). The band-gaps were calculated by onset peak values of around 4.03 eV. The compounds are photoactive in the UV-A region (312–328 nm) with a small solvatochromic effect in the excited state (Δλ_em = 16 nm). The electrochemical studies revealed that the reduction of the bisimide moiety showed two waves due to the formation of both the radical anion and a dianion. On the other hand, oxidation showed two waves due to the formation of radical cations and dications. The thermal properties were measured by differential thermal analysis (DTA) and thermogravimetric analysis (TGA), and the materials showed high thermal stability (T_d > 300 °C). Theoretical calculations were also performed to study the geometry and charge distribution of these compounds in their ground and excited electronic states. No significant changes in the absorption and emission maxima were found by changing the solvent or substituents attached to the PMDI structure.

Full text of DOI:10.1016/j.dyepig.2018.04.053

Accepted Manuscript
New long-chain donor-acceptor-donor pyromellitic diimide (PMDI) derivatives. A
combined theoretical and experimental study
Alexandre Gonçalves Dal-Bó, Rodrigo da Costa Duarte, Rodrigo Cercená, Michael
Peterson, Jamal Rafique, Sumbal Saba, Eduardo Zapp, Eduarda Sangiogo Gil, Paulo
Fernando Bruno Gonçalves, Fabiano Severo Rodembusch, Tiago Elias Allievi Frizon
PII:
S0143-7208(18)30433-9
10.1016/j.dyepig.2018.04.053
DYPI 6714
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 1 March 2018
Revised Date: 24 April 2018
Accepted Date: 25 April 2018
Please cite this article as: Dal-Bó AlexandreGonç, da Costa Duarte R, Cercená R, Peterson M, Rafique
J, Saba S, Zapp E, Gil ES, Bruno Gonçalves PF, Rodembusch FS, Frizon TEA, New long-chain donor-
acceptor-donor pyromellitic diimide (PMDI) derivatives. A combined theoretical and experimental study,
Dyes and Pigments (2018), doi: 10.1016/j.dyepig.2018.04.053.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
New long-chain donor-acceptor-donor pyromellitic diimide
(PMDI) derivatives. A combined theoretical and experimental
study
a
a
a
Alexandre Gonçalves Dal-Bó, Rodrigo da Costa Duarte, Rodrigo Cercená,
a
b
b
c
Michael Peterson, Jamal Rafique, Sumbal Saba, Eduardo Zapp, Eduarda
d
d
Sangiogo Gil, Paulo Fernando Bruno Gonçalves,
Fabiano Severo
e
f,
Rodembusch, Tiago Elias Allievi Frizon *
a
Universidade do Extremo Sul Catarinense (UNESC). Av. Universitária, 1105.
CEP 88806-000. Criciúma-SC, Brazil
b
Universidade Federal de Santa Catarina (UFSC). Campus Reitor João David
Ferreira Lima, s/n. CEP 88040-900. Florianópolis, SC, Brazil
c
Universidade Federal de Santa Catarina (UFSC). Rua João Pessoa, 2750.
CEP 89036-256. Blumenau, SC, Brazil
d
Grupo de Química Teórica, Universidade Federal do Rio Grande do Sul,
Instituto de Química, Av. Bento Gonçalves, 9500, CEP 91501-970, Porto
Alegre, RS, Brazil
e
Grupo de Pesquisa em Fotoquímica Orgânica Aplicada. Instituto de Química
(UFRGS), Av. Bento Gonçalves, 9500, CEP 91501-970. Porto Alegre-RS,
Brazil.
f
Universidade Federal de Santa Catarina (UFSC). Campus Araranguá. Rodovia
Governador Jorge Lacerda, 3201 - Km 35,4. Jardim das Avenidas. CEP 88906-
0
72. Araranguá, SC, Brazil.
ABSTRACT
This study involves the electrochemical, thermal and photophysical profiling of
new pyromellitic diimide (PMDI) derivatives containing alkyl chains of different
sizes. The photophysical investigation shows that all compounds exhibited
absorption in the UV-B region (~290 nm). The band-gaps were calculated by
onset peak values of around 4.03 eV. The compounds are photoactive in the
UV-A region (312-328 nm) with a small solvatochromic effect in the excited
state (∆λ =16 nm). The electrochemical studies revealed that the reduction of
em
1

ACCEPTED MANUSCRIPT
the bisimide moiety showed two waves due to the formation of both the radical
anion and a dianion. On the other hand, oxidation showed two waves due to the
formation of radical cations and dications. The thermal properties were
measured by differential thermal analysis (DTA) and thermogravimetric analysis
(
TGA), and the materials showed high thermal stability (T > 300°C). Theoretical
d
calculations were also performed to study the geometry and charge distribution
of these compounds in their ground and excited electronic states. No significant
changes in the absorption and emission maxima were found by changing the
solvent or substituents attached to the PMDI structure.
∗
Corresponding author: Tiago Elias Allievi Frizon
Tel.: +55 (48) 3431 3637. E-mail address: E-mail: tiago.frizon@ufsc.br
Keywords: PMDI, Organic sensitizers, Fluorescence, Thermal stability.
1
. Introduction
Renewable energy (RE) technologies are becoming more and more relevant in
the modern society due to the increasing energy demands. Unlike fossil fuel-
based energy sources, RE sources help in decreasing environmental pollution
and greenhouse gas [1]. In addition, RE sources, such as photovoltaics (PVs),
wind, thermal, solar, hydropower, and biomass, are reliable and sustainable
sources to meet the rising energy demands [2]. In this context, PV is attractive
technology for the direct conversion of sunlight into electricity with an
approximate efficiency of up to 25%. However, its use is restricted due to their
high production and environmental costs since some solar cells are based on
silicon [3]. The development of new technologies, such as dye-sensitized solar
cells (DSSCs) [4], organic PVs [5], perovskite PVs [6], and inorganic quantum
dot solar cells [7] have the potential to enhance energy conversion efficiency at
lower costs [8]. Dye-sensitized solar cells (DSSCs) also called Grätzel cells [9],
are PV devices that uses less expensive materials than those used in
conventional silicon solar cells. Furthermore, the fabrication process for DSSCs
are simpler [10], thus generating cost-effective PV devices [1] that show
approximately 13% efficiency [11]. DSSCs are promising PV devices due to the
relative low cost of materials, their easy manufacture and advantages related to
2

ACCEPTED MANUSCRIPT
transparency, color and mechanical flexibility [12]. In this regard,
photosensitizers, a key component of DSSCs, play an crucial role in light
harvesting. To date, a wide variety of different dyes, e.g., metal-free organic
dyes, zinc porphyrin dyes and ruthenium-based dyes, have been developed
and widely investigated [13,14]. However, in view of cost and environmental
problems associated with metal-based dyes, metal-free dyes are strongly
preferred. In last few years, there is increasing research interest in the
development of new organic sensitizers and reasonable power conversion
efficiencies by them [15,16]. In this context, pyromellitic diimides (PMDI)
derivatives dyes have been drawing attention because of their excellent
photochemical, thermal and chemical stabilities and low cost production that
make them promising candidates for DSSC applications.
The structures of pyromellitic diimides (PMDI), which have the smallest
perylene core, have low-lying LUMO energies; however, they also have limited
molecular area for intermolecular π overlap [17]. This feature makes them
applicable as building blocks for insulating aromatic polyimides for advanced
applications ranging from communications to aerospace technologies [18-22].
Therefore, in this work we report the synthesis, thermal, electrochemical and
photophysical characterization of PMDI derivatives, which contains alkyl groups
with different chain lengths. In addition, to study the geometry and charge
distribution of these compounds theoretical calculations were performed in their
ground and excited electronic states.
2
2
EXPERIMENTAL
.1 Materials and methods
All reagents were purchased from commercial sources and utilized without any
further purification. The solvents were purified according to standard procedures
[
23]. Infrared spectra were recorded on a IRPrestige-21 Spectrophotometer
Shimadzu) using KBr pellets. Proton and Carbon nuclear magnetic resonance
(
1
13
spectra ( H and C NMR) were recorded in CDCl at 400 and 100 MHz,
3
respectively. Chemical shifts (δ) are reported in ppm referenced to the TMS,
and the coupling constants (J) in Hertz (Hz). In the photophysical studies,
spectroscopic grade solvents were used. The UV-Vis absorption spectra in
solution were recorded on a Shimadzu UV-2450 spectrophotometer, and the
3

ACCEPTED MANUSCRIPT
steady-state fluorescence spectra were acquired on a Shimadzu RF-5301PC
spectrofluorometer. For compounds 6a-c, UV-Vis absorption and fluorescence
-5
-5
emission spectra were recorded in THF (~10 M) and CH Cl (DCM) (~10 M).
2
2
The quantum yield of fluorescence (FFL) was obtained by applying the dilute
-1
optical method. Quinine sulfate (QS) in H SO (0.5 mol L ) (Φ =0.55) was
2
4
FL
used as a standard for the quantum yield [24]. The uncertainties associated to
the quantum yield of fluorescence (∆Φ ) measurements using QS are on the
FL
order of 6% [25,26]. The electrochemical measurements were accomplished
with a DropSens potentiostat (STAT 400). The cyclic voltammetry experiments
-1
were performed using 0.1 mg mL solutions of 6a-c in 0.1 M tetra-n-butyl
ammonium hexafluorophosphate (TBAPF ) in dry DCM as the supporting
6
+
electrolyte, and the ferrocene/ferricenium (Fc/Fc ) redox couple as the internal
reference. The experiments were conducted using a standard three-electrode
cell with a circular glassy carbon electrode, a Pt-wire counter electrode and
+
Ag/Ag as the reference electrode (AgNO3 0.01 M in CH CN). The
3
measurements were performed in an electrolyte solution that had been purged
with purified nitrogen gas. The spectra of bisimide derivatives 6a-c were
obtained at open circuit potential using
a
Shimadzu UV-1800
spectrofluorophotometer. The thermal properties were examined using
differential scanning calorimetry on DSC-50 and SDT Q600 TA instruments
-1
using a nitrogen atmosphere with a flow rate of 100 mL min and a heating rate
-1
of 10°C min . The DSC curves were acquired using two heating-cooling cycles,
in order to obtain the thermal transitions and the glass transition temperatures,
respectively. Elemental analysis was performed by using a PerkinElmer 2400
Series II CHNS/O Elemental Analyzer System.
2
.2. Synthesis
General procedure for the synthesis of 3a-c
-Acetamidophenol (1) (13 mmol; 2 g), 1-bromooctane (2a) (13 mmol; 3.27 g),
4
potassium carbonate (32 mmol; 4.5 g) and butanone (50 mL) were combined in
a two-necked round flask. The mixture was refluxed for 18 h at 70°C. The
formed precipitate was removed by vacuum filtration, washed with hot
butanone, and recrystallized from MeOH/H O. The same procedure was used
2
4

ACCEPTED MANUSCRIPT
for the synthesis of compounds 3b and 3c using 1-bromodecane and 1-
romododecane, respectively.
N-(4-(octyloxy)phenyl)acetamide (3a)
-1
White solid. Yield: 91%. mp.: 104°C. IR (KBr) cm : 3272, 2938, 2854, 1650,
1
1
219. H NMR (CDCl , 400 MHz) 7.35 (d, 2H, J=8.81 Hz); 6.83 (d, 2H, J=8.81
3
Hz); 3.92 (t, 2H, J=6.80 Hz); 2.14 (s, 3H); 1.78 (m, 2H); 1.46-1.28 (m, 10H);
1
3
0
1
.89 (t, 3H, J=6.54 Hz). C NMR (CDCl , 100 MHz) 168.6, 156.0, 130.9, 122.0,
3
14.8, 68.3, 31.8, 29.4, 26.1, 22.7, 14.1. Elemental Analyses Calcd. for
C H NO : C 72.97; H 9.57; N 5.32%. Found: C 73.01; H 9.59; N 5.34%.
16
25
2
N-(4-(decyloxy)phenyl)acetamide (3b)
-1
White solid. Yield: 88%. mp.: 89-93°C. IR (KBr) cm : 3220, 2926, 2842, 1640,
1
1
219. H NMR (CDCl , 400 MHz) 7.35 (d, 2H, J=8.81 Hz); 6.79 (d, 2H, J=8.81
3
Hz); 3.90 (t, 2H, J=6.80 Hz); 2.10 (s, 3H); 1.75 (m, 2H); 1.27 (t, 14H); 0.88 (t,
1
3
3
6
H, J=6.55 Hz). C NMR (CDCl , 100 MHz) 168.7, 156.0, 130.9, 122.0, 114.8,
3
8.3, 31.9, 29.6, 29.4, 26.1, 24.1, 22.7, 14.1. Elemental Analyses Calcd. for
C H NO : C 74.18; H 10.03; N 4.81%. Found: C 74.21; H 10.06; N 4.83%.
18
29
2
N-(4-(dodecyloxy)phenyl)acetamide (3c)
-1
White solid. Yield: 86%. mp.: 99°C. IR (KBr) cm : 3310, 2923, 2838, 1652,
1
1
228. H NMR (CDCl , 400 MHz) 7.35 (d, 4H, J=8.81 Hz); 6.80 (d, 4H, J=8.81
3
Hz); 3.90 (t, 2H, J=6.80 Hz); 2.11 (s, 3H); 1.75 (m, 2H); 1.26 (m, 18H); 0.88 (t,
1
3
3
6
H, J=6.55 Hz). C NMR (CDCl , 100 MHz) 168.5, 156.0, 130.9, 121.9, 114.7,
3
8.3, 31.9, 29.7, 29.4, 26.1, 22.7, 14.2. Elemental Analyses Calcd. for
C H NO : C 75.19; H 10.41; N 4.38%. Found: C 75.23; H 10.44; N 4.41%.
20
33
2
General procedure for the synthesis of 4a-c
N-(4-(Octyloxy)phenyl)acetamide (3a) (6.7 mmol; 2 g) was mixed with conc. HCl
(
37.4 mL) and H O (54.6 mL) in a two-necked round flask. The reaction mixture
2
was refluxed for 24 h at 100°C. The reaction was cooled and poured into a
solution of NaOH (1 M). The precipitate was removed by filtration, washed with
water, and dried under vacuum to afford desired aniline 4a. The same
procedure was used for the synthesis of compounds 4b and 4c using N-(4-
5

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(decyloxy)phenyl)acetamide
or
N-(4-(dodecyloxy)phenyl)acetamide
as
precursors, respectively.
4
-(octyloxy)aniline (4a)
-1
Rosy solid. Yield: 95%. mp.: 35-39°C. IR (KBr) cm : 3401, 2926, 2831, 1518,
1
1
228. H NMR (CDCl , 400 MHz) 10.25 (br., 2H); 7.40 (d, 2H, J=8.80 Hz); 6.84
3
(d, 2H, J=9.20 Hz); 3.91 (t, 2H, J=13.20 Hz); 1.77 (m, 2H); 1.26 (m, 18H); 0.88
1
3
(
t, 3H, J=13.60 Hz). C NMR (CDCl , 100 MHz) 159,3, 124.4, 122,2, 115.5,
3
6
7
8.4, 31.8, 29.4, 26.0, 22.7, 14.1. Elemental Analyses Calcd. for C H NO: C
14 23
5.97; H 10.47; N 6.33%. Found: C 75.99; H 10.50; N 6.35%.
4
-(decyloxy)aniline (4b)
-1
Rosy solid. Yield: 91%. mp.: 108°C. IR (KBr) cm : 3407, 2911, 2838, 1504,
1
1
252. H NMR (CDCl , 400 MHz) 7.14 (d, 2H, J=8.80 Hz); 6.95 (br, 2H); 6.76 (d,
3
2
H, J=8.80 Hz); 3.87 (t, 2H, J=13.20 Hz); 1.77 (m, 2H); 1.30 (m, 14H); 0.88 (t,
13
3
H, J=13.69 Hz). C NMR (CDCl , 100 MHz) 157.1, 127.8, 122.0, 115.5, 68.5,
3
3
1.9, 29.6, 26.1, 22.7, 14.1. Elemental Analyses Calcd. for C H NO: C 77.06;
16
27
H 10.91; N 5.62%. Found: C 77.10; H 10.94; N 5.65%.
4
-(dodecyloxy)aniline (4c)
-1
Rosy solid. Yield: 87%. mp.: 110°C. IR (KBr) cm : 3395, 2923, 2850, 1507,
1
1
252. H NMR (CDCl , 400 MHz) 6.75 (d, 2H, J=8.80 Hz); 6.66 (d, 2H, J=8.80
3
Hz); 3.87 (t, 2H, J=6.46 Hz); 1.77 (m, 2H); 1.27 (m, 14H); 0.88 (t, 3H, J=13.50
13
Hz). C NMR (CDCl , 100 MHz) 15.6, 139.3, 116.7, 115.7, 68.7, 31.9, 29.7,
3
2
5
6.1, 22.7, 14.2. Elemental Analyses Calcd. for C H NO: C 77.92; H 11.26; N
18 31
.05%. Found: C 77.59; H 11.28; N 5.09%.
General procedure for the synthesis of 6a-c
Pyromellitic dianhydride (PMDA) (5) (3.08 mmol; 0.672 g) and glacial acetic
acid (60 mL) were combined in a three-necked round bottom flask equipped
with condenser, and the reaction mixture was stirred under an argon
atmosphere for 2 h at 110°C. After the solid had completely dissolved, 4-
(octyloxy)aniline (4a) (6.78 mmol; 1.50 g) was slowly added to the reaction. The
6

ACCEPTED MANUSCRIPT
mixture was stirred and refluxed for 24 h at 100°C. After completion of the
reaction, the mixture was cooled to room temperature. Product 6a, which had
precipitated from the reaction mixture, was isolated by filtration, and the residual
liquid was extracted with dichloromethane/water (3x20 mL), sodium carbonate
solution (20 mL) and dried over anhydrous sodium sulfate. The same procedure
was used for the synthesis of compounds 6b and 6c using 4-(decyloxy)aniline
(4b) and 4-(dodecyloxy)aniline (4c) as precursors, respectively.
2
,6-bis(4-(octyloxy)phenyl)pyrrolo[3,4-f]isoindole-1,3,5,7(2H,6H)-tetraone (6a)
-1
Beige solid. Yield: 77%. mp.: 102°C. IR (KBr) cm : 3286, 2931, 2846, 1660,
1
1
4
6
6
7
246. HNMR (CDCl , 400 MHz) 7.98 (s, 2H); 7.43 (d, 4H, J=8.81 Hz), 6.82 (d,
3
H, J=9.06 Hz); 3.90 (t, 4H, J=13.09 Hz); 1.77 (m, 4H); 1.30 (m, 20H); 0.88 (t,
13
H, J=6.95 Hz). C NMR (CDCl , 100 MHz) 168.8, 155.9, 130.9, 122.2, 114.9,
3
8.3, 31.8, 29.3, 26.1, 22.7, 14.1. Elemental Analyses Calcd. for C H N O : C
38
44
2
6
3.05; H 7.10; N 4.48%. Found: C 73.09; H 7.12; N 4.51%.
2
,6-bis(4-(decyloxy)phenyl)pyrrolo[3,4-f]isoindole-1,3,5,7(2H,6H)-tetraone (6b)
-1
Beige solid. Yield: 68%. mp.: 101°C. IR (KBr) cm : 3279, 2925, 2846, 1647,
1
1
4
6
1
246. H NMR (CDCl , 400 MHz) 7.90 (s, 2H); 7.34 (d, 4H, J=8.81 Hz); 6.78 (d,
3
H, J=8.81 Hz), 3.8 (t, 4H, J=13.35 Hz); 1.76 (m, 4H); 1.26 (m, 24H); 0.87 (t,
13
H, J=13.60 Hz). C NMR (CDCl , 100 MHz) 168.7, 155.9, 130.9, 127.9, 121.9,
3
14.6, 68.2, 31.9, 29.6, 26.0, 24.1, 22.7, 14.1. Elemental Analyses Calcd. for
C H N O : C 74.09; H 7.70; N 4.11%. Found: C 74.12; H 7.77; N 4.16%.
42
52
2
6
2
,6-bis(4-(dodecyloxy)phenyl)pyrrolo[3,4-f]isoindole-1,3,5,7(2H,6H)-tetraone
(6c)
-1
Beige solid. Yield: 71%. mp.: 99°C. IR (KBr) cm :3286, 2919, 2840, 1660,
1
1
4
6
1
246. H NMR (CDCl , 400 MHz) 7.69 (s, 2H); 7.35 (d, 4H, J=8.81 Hz); 6.82 (d,
3
H, J=9.06 Hz); 3.90 (t, 4H, J=6.55 Hz); 1.75 (m, 4H); 1.26 (m, 36H); 0.88 (t,
13
H, J=13.60 Hz). C NMR (CDCl , 100 MHz) 168.6, 156.0, 141.2, 130.9, 122.0,
3
14.7, 68.3, 31.9, 29.7, 29.6, 26.1, 22.7, 14.2. Elemental Analyses Calcd. for
C H N O : C 74.97; H 8.21; N 3.80%. Found: C 74.99; H 8.23; N 3.84%.
46
60
2
6
7

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2
.3 Theoretical calculations
All the calculations for PDMI molecules presented were computed using the
Gaussian16 package [27]. These calculations were performed using a short
alkyl chain, for computational efforts economy. A test was made using the same
alkyl chain reported in the experimental section and no significant difference
was found in the absorption and emission wavelength. For geometry
optimizations in the ground and in the first excited state, density functional
theory (DFT) was used with the LC-ωHPBE functional, a hybrid functional with
exchange-correlation and long-range corrected [28]. The basis set used for
geometry optimization was cc-pVDZ. The vertical transitions calculations were
performed using Time Dependent Density Functional Theory (TD-DFT) with the
jun-cc-pVTZ basis set [29]. This basis set was used because it shows a lower
computational cost than an aug-basis set, once the jun- basis set is constructed
by reducing diffuse functions from the augmented (aug) basis set [30]. The
solvent effects were considered using Polarized Continuum Model (PCM) and
the solvents considered were dichloromethane (DMC) and tetrahydrofuran
(THF) [31]. The NTO (Natural Transition Orbital) were render using ChemCraft
program [32]. Natural transition orbitals dramatically simplify the situation by
providing a compact representation of the transition density matrix. This is
accomplished using the corresponding orbital transformation of Amos and Hall,
which renders the transition density matrix diagonal and provides a unique
correspondence between the excited ‘particle’ and empty ‘hole’ [33].
3
3
RESULTS AND DISCUSSION
.1 Synthesis and spectroscopic characterization
The synthetic methodology for the preparation of all compounds is
demonstrated in Scheme 1. The alkylation of 4-hydroxyacetanilide (1) was
performed via a Williamson reaction in the presence of K CO using three
2
3
different alkyl bromides with 8, 10 or 12 carbon atoms. The synthesis involved
the use of alkoxide ions with an alkyl halide, and the reaction was generally
carried out using organic solvents or phase transfer catalysts in the presence of
a base followed by reflux for several hours. Compounds 3a-c were obtained in
yields of 91, 88 and 86%, respectively. To obtain the respective amines (4a-c)
8

ACCEPTED MANUSCRIPT
from 3a-c, the acetyl group was removed by adding distilled water and
concentrated HCl and refluxing the mixture for 24 h. The reaction mixture was
then cooled to room temperature and was subsequently basified with NaOH (1
M). The obtained solids were filtered and washed with water to give the desired
products in yields of 95, 91 and 87%, respectively.
O
NH
O
O
NH
NH2
HCl, H O
2
+
C H2n+1Br
n
NaOH (1 M)
OH
(2a) n=8
OR
OR
(
(
2b) n=10
2c) n=12
(
(
(
3a) R=C H
(4a) R=C H
8 17
(4b) R=C H
10 21
(4c) R=C H
12 25
(1)
8
17
3b) R=C H
10
21
3c) R=C H
12
25
Scheme 1. Synthetic route for the synthesis of 4a-c.
The PMDI derivatives were prepared by the reaction of two equivalents of the
amines (4a-c) with pyromellitic dianhydride (5) as shown in Scheme 2. The
reaction was heated in acetic acid to 110°C for 24 h under an argon
atmosphere. The crude product was cooled to room temperature, filtered and
the residual liquid was extracted with dichloromethane/water, sodium carbonate
solution and dried over anhydrous sodium sulfate to give pure products 6a-c in
7
7, 68, and 71% yields, respectively.
Scheme 2. Reaction strategy for the synthesis of 6a-c.
1
13
All compounds were characterized by H and C NMR spectroscopy (Figures
SI1-18). For compounds 6a-c, the obtained NMR spectra in CDCl showed very
3
similar profiles (data not shown). For instance, for compound 6a, a singlet at
9

ACCEPTED MANUSCRIPT
7
.98 ppm for the two hydrogens of the pyromellitic system and two 4H doublets
with coupling constants of 9.08 Hz centered at approximately 7.38 and 6.79
ppm from the phenyl rings of what used to be the amines can be seen in the
aromatic region. Additionally, a 4H triplet located at approximately 4.0 ppm with
a coupling constant of 6.44 Hz for the methylene groups adjacent to the oxygen
atoms in the alkyl portion of the molecule can be observed. In the 1.77 to 0.86
ppm region, a set of signals for 30 hydrogens can be observed, and these
13
signals correspond to the alkyl moieties of compound 6a. In the C NMR
spectrum, six different aromatic carbons were found, and the signals at 168.8
and 130.9 ppm from the pyromellitic system were obvious.
3
.2 Thermal characterization
The thermal behavior of compounds 6a-c was investigated using differential
thermogravimetric analysis (TGA) (Figure 1). The degradation temperatures
were evaluated under an inert N atmosphere. To better visualize the thermal
2
events, the DTG curves (inset) for all the compounds were obtained from the
TGA curves.
Figure 1. Thermograms (TGA) and DTG curves of compounds 6a-c.
10

ACCEPTED MANUSCRIPT
The overall degradation process of the PMDI derivatives indicates a main
degradation event occurs with a mass loss of approximately 80-95% at an initial
decomposition temperature higher than 250°C, and the event has a high
maximum degradation rate at 350°C. A second thermal event can be observed
for all compounds between 400-500°C. The preliminary results indicate that the
alkyl chain plays an important role on the thermal behaviors of these
compounds, and compound 6c showed a significantly greater thermal stability
with an initial decomposition temperature of approximately 350°C. To better
understand the thermal behavior of these derivatives, differential thermal
analysis (DTA) was also applied. The presence of an endothermic peak with no
loss of mass can be observed for compound 6a at approximately 100°C due to
the fusion of the compounds. The presence of two peaks between 330-474°C
related to the decomposition of 6a can also be observed. The relevant data
obtained by DTA are summarized in Table 1. It is worth mentioning that the
thermograms of 6b-c are similar to that of 6a.
Table 1. Differential thermal analysis (DTA) results of compounds 6a-c, where
Td is the decomposition temperature.
*
Td (°C)
Compound
Melting Point (°C)
st
nd
1
event
330
2 event
6a
6b
6c
102
101
99
474
491
480
320
354
3
.3 Electrochemical characterization
The electrochemical properties of compounds 6a-c were investigated by cyclic
voltammetry (CV) (Figure 2). CV is a useful method for measuring
electrochemical behavior and evaluating the relative HOMO and LUMO energy
levels and the band gap of molecules. The oxidation and reduction processes
were directly correlated to electron transfer at the HOMO (ionization potential)
and LUMO (electron affinity), respectively.
11

ACCEPTED MANUSCRIPT
-1
Figure 2. Cyclic voltammograms of 6a-c in 0.1 TBAPF /CH Cl at 100 mV s .
6
2
2
+
The reduction process (potential window: 0.0 to -1.8 V vs. Ag/Ag ) shows two
cathodic waves. The cathodic waves showed two one-electron reduction
processes from the pyromellitic diimide group because the molecule can accept
one and two electrons to generate the radical anion and a dianion, respectively
+
[
34,35]. The oxidation process (potential window: 0.0 to 1.5 V vs. Ag/Ag )
exhibits four waves, two anodic and two cathodic, and since the currents show
different intensities, the processes can be considered quasi-reversible. These
processes correspond to the stepwise formation of radical cations and dications of
the compounds [36]. Increasing the length of the flexible alkoxy chains in the
evaluated bisimides did not cause a significant change in the energy values of
the HOMO and LUMO levels of in the band gap value. To calculate the absolute
energies, the redox data were standardized with the ferrocene–ferricenium
couple (Table 2). The HOMO energy levels of the bisimides were obtained from
the onset potentials of oxidation (E ) and reduction (E ) [37]. The
ox
red
el
electrochemical band gaps (Eg values) of the bisimides were estimated from
the difference between the HOMO and LUMO levels. The optical band gaps
opt
(
Eg values) of 6a-c were determined by the absorption edge of their solutions.
The optical and electrochemical band gaps are summarized in Table 2. The
12

ACCEPTED MANUSCRIPT
coulombic interaction generally makes the optical band gap smaller than the
electrochemical band gap.
Table 2. Electrochemical and optical properties of compounds 6a-c, where Eox
is the onset potential of oxidation, Ered is the onset potential of reduction, Ip
(
HOMO) is the ionization potential, E (LUMO) is the electron affinity, E is the
a
g
band gap and λonset is the absorption onset wavelength.
Compounds
Parameters
6
a
6b
6c
a
E (V vs. NHE)
1.091
-0.751
-5.531
-3.689
1.842
1.129
-0.759
-5.569
-3.681
1.888
737.55
1.060
-0.787
-5.799
-3.919
1.847
ox
a
Ered (V vs. NHE)
b
Ip (HOMO) (eV)
c
E (LUMO) (eV)
a
el
d
Eg (eV)
λonset (nm)
740.57
1.674
702.48
opt
e
Eg (eV)
1.681
1.765
a
+
b
c
Notes: E
(vs. NHE)=Eox/red (vs. Ag/Ag ) +0.266; I =-(E +4,44) eV; λonset:
ox/red
p
ox
d
absorption onset wavelength; Optical band gap calculated from the low
e
energetic edge of the absorption spectrum (E =1240/λonset); E =E - I .
g
g
a
p
3
.4 Photophysical characterization
Figure 3 presents the electronic spectra of PMDI derivatives 6a-c in
tetrahydrofuran and dichloromethane. The relevant data from the UV-Vis
absorption spectroscopy are summarized in Table 3. The Strickler-Berg
0
relationship allowed us to calculate the pure radiative lifetimes (τ ) and the
0
e
radiative rate constants (k ) [38]. In this relationship, the pure radiative lifetime
0
can be defined as 1/k , and the oscillator strength (f ) can be obtained [39].
e
e
13

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Figure 3. UV-Vis absorption (left) and fluorescence emission (right) spectra for
-5
6
a-c in THF (~10 M). The insets show the photophysical data in
-5
dichloromethane (~10 M).
The PMDI derivatives present absorption maxima in the UV-B region
(
below 300 nm) due to electronic transitions in the heteroaromatic portion
of the molecule. The molar absorptivity coefficient (ε) for each of the
0
e
absorption maxima and the calculated radiative rate constants (k values)
indicate that the spin- and symmetry-allowed electronic transitions were
1
related to the ππ* transitions. Despite the observed allowed electronic
transitions, low values of the oscillator strength could be calculated.
Notably, in both solvents, these compounds each presented a very
intense absorption band below 270 nm, probably related to the benzenoid
ring present in the structure. This assumption was corroborated by the
lack of emission observed using 250 nm as the excitation wavelength.
Changes in the alkyl chain length did not seem to affect the photophysical
behavior of these compounds in the ground state. In addition, the
absence of solvatochromism revealed that these compounds, despite the
presence of donor and acceptor moieties in their structure, do not present
0
ICT character in the ground state. The similar radiative lifetime values (
τ
)
indicate that after radiation absorption, the fluorophores populate the
same excited state.
14

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The fluorescence emission spectra of the PMDI derivatives in both solvents are
also shown in Figure 3. The emission curves were obtained by exciting the
compounds at their maximum absorption wavelengths. The relevant data from
the fluorescence emission spectra are listed in Table 3. These compounds
present very similar photophysical behaviors in THF, and a main fluorescence
emission located at 326 nm could be observed for all compounds, indicating
that in this solvent, the changes in the chemical structure of the fluorophores do
not have a substantial impact on the location of the fluorescence maxima.
Moreover, the lack of solvatochromism in the emission curves shows that no
charge-transfer occurs in the excited state. On the other hand, in the more polar
solvent, dichloromethane, the emission curves of 6a and 6b are quite different
from their curves in THF. Compound 6a, which has the smaller alkyl chain,
presents a locally excited emission band at 326 nm and an additional redshifted
band located at 400 nm probably due to ICT character. In addition, compound
6
b presents a broader emission band that what was seen in THF, which may
indicate an ICT mechanism, is active for this fluorophore. The low solubility of
these compounds prevented additional investigations in more polar solvents or
even fluorimetric titrations. In spite of the significant rigidity of the PMDI core,
low values were found for the fluorescence quantum yields (0.019-0.069). In
this case, based on the free bond rotation of the benzene ring, as well as the
presence of the alkoxy chains, different types of non-radiative processes could
be envisaged, such as internal conversion and vibrational relaxation [40-43],
which may contribute significantly to the electronic deexcitation, where the
molecule loses its acquired excess energy.
15

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Table 3. Relevant photophysical data of the UV-Vis spectra of 6a-c, where λabs is the absorption maxima (nm), ε is the molar
3
-1
-1
0
e
8
-1
0
absorptivity (×10 L mol cm ), f is the calculated oscillator strength, k (×10 ) is the calculated radiative rate constant (s ), τ is
e
-1
the inherent emission lifetime (ns), λem is the emission maxima (nm), ∆λST is the Stokes shift (cm ), and Φ is the fluorescence
FL
quantum yield.
0
0
opt
Solvent
λabs
284
290
284
290
284
290
λem
328
312
328
312
328
312
ε
fe
k
τ
Eg
∆λST
4723
2431
4723
2431
4723
2431
ΦFL
e
Dichloromethane
Tetrahydrofuran
Dichloromethane
Tetrahydrofuran
Dichloromethane
Tetrahydrofuran
8.72
8.04
6.26
5.96
7.25
7.17
0.12
0.12
0.09
0.09
0.10
0.10
1.49
1.41
1.08
1.07
1.24
1.21
6.72
7.10
9.26
9.33
8.04
8.26
4.38
4.27
4.38
4.27
4.38
4.27
0.019
0.044
0.035
0.069
0.032
0.050
6a
6b
6c
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3
.5 Theoretical calculations
The calculated geometry for the ground and first excited states of the PMDI's in
DCM and THF are summarized in Table 4. The molecules in both solvents
presented small geometry differences, mostly in the dihedral angle between
carbonyl carbon, nitrogen and the two carbons of benzene. This difference is
expected, since the ground state geometry is not planar.
Table 4. Minimum energy structures of the ground state (S ) and the first
0
excited state (S ) for the PDMI generic structure, obtained from the LC-
1
ωHPBE/cc-pVDZ calculations in DCM and THF.
O
O
3
4
2
7
25
23
24
18
17
19
20
21
1
0
2
H C
5
6
1
2
7
CH
3
3
3
2
O
31
22
28
N
N
O
3
0
11
8
29
1
9
26
O
O
DCM
THF
S0
φC25-C23-N11-C10=56.31°
φC7-N8-C17-C18=56.30°
φC25-C23-N11-C10=55.02°
φC7-N8-C17-C18=55.02°
S1
φC25-C23-N11-C10=50.53°
φC7-N8-C17-C18=19.95°
φC25-C23-N11-C10=50.52°
φC7-N8-C17-C18=20.24°
φ is the dihedral angle between selected atoms.
The Table 5 presents the Natural Transition Orbitals (NTOs) for S0 and S1. It is
possible to see that these orbitals are π, so we assume that a π→π * transition
is occurring. The molecular orbitals do not show a spatial separation in the
S0→S1 transition, therefore, we do not observe a charge transfer, but only local
excitations.
17

ACCEPTED MANUSCRIPT
Table 5. NTO of PDMI in the ground (S ) and first excited states (S ) in DCM
0
1
and THF. The calculations were performed with LC-ωHPBE/cc-pVDZ level.
DCM THF
S0
S1
Analyzing the Table 6, we can see that the calculated absorptions wavelengths
are very close to the experimental ones, mainly in the THF, where the
difference between the theoretical and the experimental results were only 5 nm.
The obtained values for emission wavelengths were not so close using the LC-
ωHPBE functional, since the difference between the calculated and
experimental values were around 100 nm. It was experimentally observed that
the absorption wavelengths in DCM were smaller than in THF, and this was
observed in the calculations.
Table 6. Photophysical data obtained from the LC-ωHPBE/jun-cc-pVTZ
calculation in DCM and THF for PDMI, where λabs is the absorption wavelength
(nm), λemis is the emission wavelength (nm), µS0 is the dipole moment (Debye)
for the ground state, µS1 is the dipole moment (Debye) for the first excited state,
ƒabs is the absorption oscillator strength and ƒemis is the emission oscillator
strength.
Solvent
DCM
λabs
µS0
ƒabs
λem
µs1
ƒem
293.66
294.66
1.8029
1.8388
0.0022
0.0025
451.79
451.77
12.1934
12.1922
0.0056
0.0056
THF
4
. Conclusions
A series of new pyromellitic diimide (PMDI) derivatives was synthesized and
characterized. The photophysical investigations showed that all compounds
presented absorption in the UV-B region (~290 nm). The band-gaps were
18

ACCEPTED MANUSCRIPT
calculated from the onset peak values to be approximately 4.03 eV. The
compounds are photoactive in the UV-A region (312-328 nm) with a small
solvatochromic effect in the excited state (∆λ =16 nm). The thermal properties
em
were measured, and all the synthesized compounds showed high thermal
stability. Spectroelectrochemical measurements showed two waves due to the
formation of the radical anion and a dianion. Theoretical calculations showed
the coefficients for the fundamental state and first excited state appeared to be
π-type orbital. Thus, it was assumed that transitions of type π→π* were taking
place.
5
. Acknowledgements
The authors would like to thank the National Council for Scientific and
Technological Development CNPq and CELESC P&D 5697-8712/2012 and
5
697-0717/2017 for their financial support. The authors are also thankful to
CEBIME-UFSC and LNLS (proposal 20170066).
6
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V New pyromellitic diimide (PMDI) derivatives presenting alkyl chains with different
sizes.
V The compounds showed high thermal stability (Td > 300°C).
V The electrochemical reduction showed the formation of the radical anion and a
dianion.
V The electrochemical oxidation showed formation of radical cations and dications.
V Transitions type π→π*.