Low-melting dyes of tetra-alkyl azobenzene-3,3′,5,5′-tetracarboxylates

Article abstract of DOI:10.1016/j.dyepig.2019.02.006

A series of tetraalkyl azobenzene-m-tetracarboxylates was synthesized and characterized by polarized-light optical microscopy (POM), differential scanning calorimetry (DSC) and by UV-visible spectroscopy investigations. The dyes were readily obtained from

Full text of DOI:10.1016/j.dyepig.2019.02.006

Accepted Manuscript
Low-melting dyes of tetra-alkyl azobenzene-3,3′,5,5′-tetracarboxylates
Thamires dos Santos Moreira, Thalisson Firmo Dutra, Rodrigo Cristiano
PII:
S0143-7208(18)32304-0
https://doi.org/10.1016/j.dyepig.2019.02.006
DYPI 7340
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 16 October 2018
Revised Date: 30 January 2019
Accepted Date: 3 February 2019
Please cite this article as: Moreira TdS, Dutra TF, Cristiano R, Low-melting dyes of tetra-alkyl
azobenzene-3,3′,5,5′-tetracarboxylates, Dyes and Pigments (2019), doi: https://doi.org/10.1016/
j.dyepig.2019.02.006.
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ACCEPTED MANUSCRIPT
LOW-MELTING DYES OF TETRA-ALKYL AZOBENZENE-
3,3',5,5'-TETRACARBOXYLATES
Thamires dos Santos Moreira, Thalisson Firmo Dutra, Rodrigo Cristiano*
Departamento de Quimica, Universidade Federal da Paraíba, João Pessoa, PB,
58051-900, Brazil
Notes: the authors declare no competing financial interest
*
Corresponding author:
Rodrigo Cristiano
Email: rcristiano@quimica.ufpb.br
Phone: +55 83 3216 7433
ORCID: 0000-0002-5044-377X

ACCEPTED MANUSCRIPT
Abstract
A series of tetraalkyl azobenzene-m-tetracarboxylates was synthesized and
characterized by polarized-light optical microscopy (POM), differential scanning
calorimetry (DSC) and by UV-visible spectroscopy investigations. The dyes were
readily obtained from esterification reactions of azobenzene-m-tetracarboxylic acid and
alcohols of different carbon chains (methyl, ethyl, propyl, i-propyl, butyl, s-butyl, i-butyl,
pentyl, i-pentyl, hexyl, 2-ethylhexyl and octyl). All compounds showed good thermal
stability with temperatures of decomposition above 300 ºC. The increase and linearity
of alkyl chains were important to give low-melting products. Branched alkyl substituents
gave products with slightly higher melting points compared to their linear alkyl isomers.
The octyl and racemic 2-ethylhexyl substituents gave liquid azo dye products.
Photoisomerization of the azo group in the final compounds was investigated in
solution. Compounds showed a good resistance to photoisomerization and the
branched alkyl substituents gave products with considerable faster back relaxation,
from cis to trans configuration, when compared to the compounds with linear alkyl
substituents.
Keywords: azobenzene; low-melting dyes; photoisomerization; soft materials
2

ACCEPTED MANUSCRIPT
Highlights
•
•
Dyes based on tetra-alkyl azobenzene-m-tetracarboxylates were synthesized
Tetrakis-2-ethylhexyl and tetra-octyl azobenzene-3,3’,5,5’-tetracarboxylates (2k
and 2l) are liquid dyes at room temperature
•
•
The increasing in the size of the linear alkyl chains decreases melting points of
azobenzenetetracarboxylates to near and bellow room temperature
The photoisomerization process for final compounds were studied in solution
Graphical abstract
3

ACCEPTED MANUSCRIPT
1. Introduction
Azo dyes in a variety of chemical structures have been the focus of intense
scrutiny over the last decades, due to their thermal stability and optical properties. [1-3]
The photochromism of azo unit has been extensively investigated, from the pure
photochemistry and optomechanical energy conversion to visible-light switches, light
controllable nanocarriers, and nanosystems. [4-9] The azo group may act as an optical
switch in soft materials where the functional properties are tuning by the reversible
photoisomerization of N=N bond by light exposure. [10, 11] Such a property makes azo
dyes attractive linking groups in liquid crystals, able to bring molecular
ordering/disordering of the liquid crystalline phase. Successful exploitations were
achieved where the azo link is part of molecular structure of the mesogen [12, 13] or
used as a dopant in polymer-dispersed liquid crystals. [14] The E-isomer keeps an
anisotropic shape of the molecule that can be folded by incident UV light to its Z-
isomer, keeping or destroying the phase reversibly. Calamitic and discotic molecules
have been exploited under this strategy so far. Some interesting macro-structural
features of tetra-meta-substituted azobenzene were found to present stable hexagonal
phases over a large range temperature. [15, 16]
Most of the materials based on azobenzene dyes typically possess auxochrome
polar groups that may improve their intermolecular interactions and optical properties,
although are known to produce some solid compounds with considerable high melting
points. [17] Herein we show a series of simple tetraalkyl azobenzene-m-
tetracarboxylates (2a-l), where linear or branched aliphatic flexible chains may result in
enhanced processability properties in the material, better solubility in organic solvents
as well as improved thermal and chemical stability. This pattern of substitution also
affords a way of investigating possible columnar arrangements of the structure, since
they possess a disc-like shape. Moreover, films by photoactive dyes that can give non-
4

ACCEPTED MANUSCRIPT
crystalline domains after melting are interesting for applications in possibly low-cost
deposition techniques in photonic devices. Some molecules shown here are interesting
low-melting dyes of easy synthesis, two of them liquid at room temperature, with a
range from yellow to deep orange color. These liquid and low melting solid dyes are
found to represent an attractive and interesting alternative to other azo dyes reported
thus far.
2. Experimental
2.1 Instruments
IR spectra were recorded on a Shimadzu spectrometer model IRPrestige-21 in
1
13
ATR. H and C NMR spectra were recorded on a Varian Mercury 200 or on a Bruker
00 spectrometer. High-resolution mass spectra (HRMS) were obtained with a
4
MicrOTOF QII Bruker, with APPI ionization source, the samples being injected using a
Hamilton 500 µL syringe Model 1750 RN SYR. Thermal transitions and enthalpies were
determined by differential scanning calorimetry (DSC) using a DSC-60 Shimadzu
calorimeter. Thermogravimetric analysis (TGA) was carried out using a Shimadzu
analyzer with a TGA-50 module. UV-Vis analysis was performed on U3000 UV-3600
Shimadzu spectrometer. POM analysis was performed on Nikon Polarizing Microscope
Eclipse LV100POL equipped with a Mettler Toledo FP-90 hot stage.
2.2 Materials
Azobenzene-3,3’,5,5’-tetracarboxylic acid (1) was synthesized according to the
literature. [15] Tetra-methyl azobenzene-3,3',5,5'-tetracarboxylate (2a) has been
reported recently as key intermediate for a photoswitchable molecule able to binding
oxoanions in its Z form.[18] All reagents and solvents are used as received.
2.3 Synthesis of tetraalkyl azobenzene-3,3',5,5'-tetracarboxylates 2a-j
5

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A mixture of azobenzene-3,3’,5,5’-tetracarboxylic acid (1) (1 mmol), the
respective alcohol (50 mL) and concentrated sulfuric acid (0.2 mL) was stirred at reflux
for 24h. After this period the excess of alcohol was evaporated under reduced
pressure. Cold water (20 mL) was then carefully added, followed by extraction with
DCM (40 mL). The organic phase was washed with saturated solution of sodium
bicarbonate (20 mL), brine (20 mL) and dried over anhydrous sodium sulfate. The
solvent and any remaining alcohol were then removed by distillation under vacuum.
The crude product was purified by column chromatography in hexane/ethyl acetate
(
3:1) on silica and recrystallization from ethanol.
2
2
8
.3.1 Tetra-methyl azobenzene-3,3',5,5'-tetracarboxylate (2a). Yield: 63%. IR (ATR):
954, 2924, 2848, 1726 (C=O), 1431, 1334, 1315, 1251, 1203, 1176, 1105, 995, 920,
1
75, 790, 754, 723. H NMR (200 MHz, CDCl ) δ, ppm: 8.83 (t, J = 1.6 Hz, 2H), 8.78 (d,
3
13
J = 1.6 Hz, 4H), 4.01 (s, 12H). C NMR (50 MHz, CDCl ) δ, ppm: 165.7, 152.4, 133.2,
3
+
1
32.1, 128.2, 52.9. Q-TOF/MS: m/z calcd for C H N O [(M+H) ]: 415.1141; found:
20
18
2
8
415.1140. Mp.: 231.0 °C
2
2
8
1
.3.2 Tetra-ethyl azobenzene-3,3',5,5'-tetracarboxylate (2b). Yield: 72%. IR (ATR):
985, 1720 (C = O), 1446, 1365, 1311, 1240, 1188, 1097, 1022, 999, 966, 933, 921,
1
62, 752, 719. H NMR (500 MHz, CDCl ) δ, ppm: 8.83 (t, J = 1.6 Hz, 2H), 8.79 (d, J =
3
13
.6 Hz, 4H), 4.48 (q, J = 7.1 Hz, 8H), 1.47 (t, J = 7.1 Hz, 12H). C NMR (125 MHz,
CDCl ) δ, ppm: 165.2, 152.3, 132.9, 132.3, 127.8, 61.7, 14.3. Q-TOF/MS: m/z calcd for
3
+
C H N O [(M+H) ]: 471.1767; found: 471.1757. Mp.: 186.0 °C
24
26
2
8
2
2
7
4
.3.3 Tetra-propyl azobenzene-3,3',5,5'-tetracarboxylate (2c). Yield: 67%. IR (ATR):
962, 2929, 2875, 1724 (C = O), 1458, 1390, 1332, 1240, 1203, 1056, 1037, 991, 921,
1
54, 721. H NMR (500 MHz, CDCl ) δ, ppm: 8.83 – 8.79 (m, 2H), 8.76 (d, J = 1.6 Hz,
3
13
H), 4.36 (t, J = 6.7 Hz, 8H), 1.93 – 1.78 (m, 8H), 1.06 (t, J = 7.4 Hz, 12H). C NMR
6

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(
125 MHz, CDCl ) δ, ppm: 165.2, 152.3, 132.9, 132.3, 127.8, 67.3, 22.1, 10.4. Q-
3
+
TOF/MS: m/z calcd for C H N O [(M+H) ]: 527.2393; found: 527.2392. Mp.: 121.0 °C
28
34
2
8
2.3.4 Tetra-iso-propyl azobenzene-3,3',5,5'-tetracarboxylate (2d). Yield: 33%. IR
(
ATR): 2985, 2933, 1714 (C = O), 1456, 1369, 1309, 1238, 1199, 1143, 1107, 970,
1
9
23, 896, 831, 756, 717. H NMR (200 MHz, CDCl ) δ, ppm: 8.81 – 8.78 (m, 2H), 8.76
3
1
3
(
d, J = 1.5 Hz, 4H), 5.34 (hept, J = 6.1 Hz, 4H), 1.43 (d, J = 6.3 Hz, 24H). C NMR (50
MHz, CDCl ) δ,ppm: 164.9, 152.4, 133.0, 132.7, 127.9, 69.6, 22.1. Q-TOF/MS: m/z
3
+
calcd for C H N O [(M+H) ]: 527.2393; found: 527.2384. Mp.: 151.0 °C
28
34
2
8
2
2
7
.3.5 Tetra-butyl azobenzene-3,3',5,5'-tetracarboxylate (2e). Yield: 68%. IR (ATR):
955, 2870, 1720 (C = O), 1481, 1330, 1312, 1273, 1247, 1192, 1107, 1032, 970, 924,
1
58, 723. H NMR (200 MHz, CDCl ) δ, ppm: 8.79 (d, J = 7.9 Hz, 6H), 4.41 (t, J = 6.5
3
Hz, 8H), 1.94 – 1.69 (m, 8H), 1.51 (dq, J = 14.1, 7.2 Hz, 8H), 1.01 (t, J = 7.2 Hz,
13
1
2H). C NMR (50 MHz, CDCl ) δ, ppm: 165.4, 152.4, 133.1, 132.3, 127.9, 65.8, 30.8,
3
+
1
9.4, 13.9. Q-TOF/MS: m/z calcd for C H N O [(M+H) ]: 583.3019; found: 583.3012.
34
42
2
8
Mp.: 98.0 °C
2
2
1
.3.6 Tetra-sec-butyl azobenzene-3,3',5,5'-tetracarboxylate (2f). Yield: 30%. IR (ATR):
974, 2932, 1713 (C = O), 1450, 1377, 1327, 1312, 1242, 1194, 1178, 1113, 1098,
1
024, 995, 970, 941, 874, 758, 713. H NMR (400 MHz, CDCl ) δ, ppm: 8.82 (t, J = 1.6
3
Hz, 2H), 8.77 (d, J = 1.6 Hz, 4H), 5.23 – 5.13 (m, 4H), 1.89 – 1.67 (m, 8H), 1.40 (d, J =
13
6
.3 Hz, 12H), 1.01 (t, J = 7.5 Hz, 12H). C NMR (100 MHz, CDCl ) δ, ppm: 165.0,
3
1
52.5, 133.0, 132.8, 127.9, 74.1, 29.1, 19.7, 9.9. Q-TOF/MS: m/z calcd for C H N O
8
34
42
2
+
[
(M+H) ]: 583.3019; found: 583.3015. Mp.: 117.0 °C.
2.3.7 Tetra-iso-butyl azobenzene-3,3',5,5'-tetracarboxylate (2g). Yield: 63%. IR (ATR):
2957, 2874, 1722 (C = O), 1476, 1379, 1313, 1300, 1242, 1193, 1180, 1107, 993, 923,
7

ACCEPTED MANUSCRIPT
1
7
56, 718. H NMR (400 MHz, CDCl ) δ, ppm: 8.84 (t, J = 1.6 Hz, 2H), 8.79 (d, J = 1.6
3
Hz, 4H), 4.20 (d, J = 6.7 Hz, 8H), 2.16 (dp, J = 13.4, 6.7 Hz, 4H), 1.07 (d, J = 6.7 Hz,
13
2
4H). C NMR (100 MHz, CDCl ) δ, ppm: 165.3, 152.5, 133.1, 132.4, 128.0, 71.9,
3
+
2
8.0, 19.3. Q-TOF/MS: m/z calcd for C H N O [(M+H) ]: 583.3019; found: 583.3016.
34
42
2
8
Mp.: 139.0 °C.
2.3.8 Tetra-pentyl azobenzene-3,3',5,5'-tetracarboxylate (2h). Yield: 62%. IR (ATR):
2957, 2872, 1724 (C = O), 1477, 1335, 1240, 1192, 1105, 1062, 974, 923, 806, 756,
1
7
25. H NMR (200 MHz, CDCl ) δ, ppm: 8.85 – 8.80 (m, 2H), 8.78 (d, J = 1.4 Hz, 4H),
3
4.40 (t, J = 6.7 Hz, 8H), 1.95 – 1.71 (m, 8H), 1.57 – 1.28 (m, 16H), 0.95 (t, J = 6.9 Hz,
13
1
2H). C NMR (50 MHz, CDCl ) δ 165.2, 152.3, 132.9, 132.2, 127.9, 65.9, 28.4, 28.1,
3
+
2
2.4, 14.0. Q-TOF/MS: m/z calcd for C H N O [(M+H) ]: 639.3645; found: 639.3639.
36
50
2
8
Mp.: 63.0 °C
2
2
7
.3.9 Tetra-iso-pentyl azobenzene-3,3',5,5'-tetracarboxylate (2i). Yield: 57%. IR (ATR):
959, 2868, 1720 (C = O), 1475, 1310, 1238, 1226, 1188, 1173, 1109, 1101, 979, 922,
1
58, 723. H NMR (400 MHz, CDCl ) δ, ppm: 8.81 (t, J = 1.6 Hz, 2H), 8.77 (d, J = 1.6
3
Hz, 4H), 4.45 (t, J = 6.8 Hz, 8H), 1.83 (dq, J = 7.5, 6.5 Hz, 4H), 1.73 (q, J = 6.8 Hz,
1
3
8
1
6
H), 1.01 (d, J = 6.6 Hz, 24H). C NMR (100 MHz, CDCl ) δ 165.4, 152.5, 133.1,
3
+
32.4, 128.0, 64.6, 37.5, 25.4, 22.7. Q-TOF/MS: m/z calcd for C H N O [(M+H) ]:
36
50
2
8
39.3645; found: 639.3636. Mp.: 83.0 °C.
2
2
1
.3.10 Tetra-hexyl azobenzene-3,3',5,5'-tetracarboxylate (2j). Yield: 39%. IR (ATR):
954, 2929, 2858, 1724 (C = O), 1462, 1381, 1328, 1311, 1234, 1186, 1101, 1029,
1
008, 982, 903, 758, 723. H NMR (200 MHz, CDCl ) δ, ppm: 8.89 – 8.69 (m, 6H), 4.40
3
(
t, J = 6.6 Hz, 8H), 1.96 – 1.72 (m, 8H), 1.60 – 1.17 (m, 24H), 0.91 (t, J = 6.3 Hz, 12H).
13
C NMR (50 MHz, CDCl ) δ, ppm: 165.4, 152.4, 133.1, 132.4, 127.9, 66.1, 31.6, 28.8,
3
8

ACCEPTED MANUSCRIPT
+
2
5.8, 22.7, 14.2. Q-TOF/MS: m/z calcd for C H N O [(M+H) ]: 695.4271; found:
40 58 2 8
695.4271. Mp.: 36.0 °C.
2.4 Synthesis of tetraalkyl azobenzene-3,3',5,5'-tetracarboxylates 2k-l
A mixture of azobenzene-3,3’,5,5’-tetracarboxylic acid 1 (0.55 mmol), SOCl (5
2
mL) and DMF (1 drop) was stirred at reflux for 18h. Then, the excess of SOCl was
2
removed by distillation under vaccum. The obtained crude tetraacyl chloride was
reacted next without purification. To this product was added the respective alcohol (2.3
mmol) and pyridine (20 mL) and the resulting solution was stirred at reflux for 48h.
Then, the reaction mixture was cooled to room temperature and poured into ice/water
(
300 mL). Ethyl acetate (100 mL) was added and the organic phase was separated,
washed with water and dried over sodium sulphate. The solvents were removed by
distillation under vacuum affording a liquid crude product, that was purified by column
chromatography in hexane/ethyl acetate (3:1) on silica.
2.4.1 Tetrakis-(2-ethylhexyl) azobenzene-3,3',5,5'-tetracarboxylate (2k). Orange Oil.
1
Yield: 36%. H NMR (200 MHz, CDCl ) δ, ppm: 8.79 (dd, J = 6.0, 1.4 Hz, 6H), 4.33 (d,
3
13
J = 5.6 Hz, 8H), 1.78 (m, 4H), 1.60 – 1.15 (m, 32H), 1.09 – 0.76 (m, 24H). C NMR (50
MHz, CDCl ) δ, ppm: 165.2, 152.2, 133.2, 132.2, 127.8, 67.9, 38.8, 30.4, 28.9, 23.9,
3
+
2
2.9, 13.9, 10.9. Q-TOF/MS: m/z calcd for C H N O [(M+H) ]: 807.5523; found:
48
74
2
8
807.5519.
2
3
1
8
4
.3.12 Tetra-octyl azobenzene-3,3',5,5'-tetracarboxylate (2l). Dark Orange oil. Yield:
7%. IR (ATR): 2953, 2924, 2854, 1728 (C = O), 1467, 1381, 1328, 1313, 1232, 1186,
1
105, 983, 923, 758, 721. H NMR (500 MHz, CDCl ) δ, ppm: 8.83 (t, J = 1.6 Hz, 2H),
3
.78 (d, J = 1.6 Hz, 4H), 4.41 (t, J = 6.8 Hz, 8H), 1.89 – 1.79 (m, 8H), 1.52 – 1.21 (m,
13
0H), 0.88 (m, 12H). C NMR (125 MHz, CDCl ) δ, ppm: 165.4, 152.5, 133.06, 132.4,
3
9

ACCEPTED MANUSCRIPT
128.0, 66.1, 32.0, 29.4, 29.3, 28.8, 26.1, 22.8, 14.2. Q-TOF/MS: m/z calcd for
+
C H N O [(M+H) ]: 807.5523; found: 807.5519.
48
74
2
8
3. Results and Discussion
3.1 Synthesis
The tetraalkyl azobenzene-3,3’,5,5’-tetracarboxylates (2a-l) were synthesized
according to the Scheme 1. The azobenzene tetracarboxylic acid (1) was prepared
from the reaction of 5-nitro-isophthalic acid and dextrose in basic aqueous media,
using a literature protocol.[15] The final compounds were obtained via two methods
from the tetraacid 1: i) Fischer esterification, readily and suitable for small liquid
alcohols molecules and ii) via tetraacyl chloride freshly prepared from the reaction of
tetraacid 1 and SOCl , followed by esterification with the respective alcohol. The first
2
mentioned method was found to be better for the obtention of solid azo products,
compounds 2a-j. For compounds 2k and 2l, both liquid azo products, the second
method was more efficient.
Scheme 1. Synthesis of tetra-alkyl azobenzene-3,3’,5,5’-tetracarboxylates (2a-l).
Conditions: i) ROH, H SO4(cat.), reflux 24h; ii) 1. SOCl , DMF(cat.), reflux 18h; 2. ROH, Py,
2
2
reflux, 48h.
1
0

ACCEPTED MANUSCRIPT
3
.2 Thermal properties
The phase transitions, enthalpies and thermal stability of dyes 2a-l were
evaluated by differential scanning calorimetry (DSC), polarized optical microscopy
POM) and thermogravimetric analysis (TGA). The results are summarized in Table 1.
All compounds exhibited good thermal stability with decomposition temperatures above
00 °C, measured by TGA. Although the structure of molecules resembles a disc, they
(
3
did not show any liquid crystalline phases.
The melting points diminished almost linearly with increasing the size of the
alkyl chain, in tetra-n-alkyl azobenzenetetracarboxylates (2a, 2b, 2c, 2e, 2h, 2j and 2l).
2a with methyl substituents have the larger melting point at 232 °C and 2l, containing
n-octyl substituents, is a liquid at room temperature. Compounds from methyl to n-
pentyl substituents exhibited melting and crystallization points observed by repetitive
cycles of heating/cooling in DSC and POM analysis, with an increase in transition
-1
energies by expanding the size of the chains (from about 30 to 50 kJ.mol ). On the
other hand, compound 2j, with n-hexyl substituents melted slightly above room
temperature and did not show any crystallization on cooling. The DSC analysis for this
compound was performed in a range of temperature from -20 °C to 60 °C (Figure 1a).
The thermogram of 2j exhibited two well-defined peaks, a crystal-crystal transition at 18
°
C and the melting point at 32 ºC. Additional heating/cooling cycles by DSC showed no
transitions. After melting, this compound remains in a metastable liquid phase that did
not crystallize even after several weeks on lab bench.
1
1

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Table 1. Phase transition temperatures, thermodynamic data and thermal stability of studied
compounds.
a
a
b
Comp.
transitions – T [∆H] (∆S)
Tdec
2
2
2
2
2
2
2
2
2
2
ª
Cr 232 [29.7] (58.8) I
I 224 [29.4] (59.1) Cr
Cr 186 [39.8] (86.7) I
I 171 [40.4] (91.0) Cr
Cr 121 [36.7] (93.1) I
320
b
c
d
e
f
327
335
306
353
317
363
364
366
370
I 104 [36.3] (96.2) Cr
’
Cr 148 [14.5] (34.4) Cr 169 [32.2] (72.8) I
c
I 79 [3.9] (11.1) Cr
Cr 99 [45.3] (121.7) I
I 87 [46.0] (127.7) Cr
Cr 118 [28.6] (73.1) I
I 75 [20.7] (59.4) Cr
Cr 142 [41.7] (100.4) I
I 122 [43.8] (110.8) Cr
Cr 76 [48.5] (139.0) I
I 60 [50.3] (151.0) Cr
Cr 84 [23.3] (65.2) I
g
h
i
I 50 [21.1] (65.3) Cr
j
Cr 18 [8.41] (28.9) Cr’ 32 [14.5] (47.6) I
d
I…
e
2
2
k
l
I
360
368
e
I
a
-1
Note: Transition temperatures (°C), transition enthalpies (kJ. mol ) and transition entropies (J.
mol . K ) determined by DSC (peak temperatures) during heating (first line) and cooling
-1
-1
-1
b
(
second line) cycles (10 °C.min ). Cr = crystal; I = isotropic liquid. Temperatures (°C)
-1
c
determined by TGA, onset of decomposition under nitrogen atmosphere (20 °C.min ). Partially
crystallized. compound did not crystallize on cooling. liquid compound
d
e
1
2

ACCEPTED MANUSCRIPT
Figure 1. DSC curves of a) 2j, b) and c) 2d (first and second cycles, respectively) and d) 2l.
Compounds containing branched alkyl substituents 2d, 2g and 2i presented
larger clearing temperatures when compared to their corresponding linear alkyl chain
isomers. Large hysteresis in liquid-solid phase transition peaks were also observed for
these compounds. This may be due to more degrees of rotational freedom with the
increasing number of branching points. Interestingly, compound 2d, with i-propyl units,
gave a more crystalline profile on the first heating, with a crystal to crystal transition at
148 °C followed by the transition to the isotropic liquid at 169 °C (Figure 1b). On
cooling, however, this sample crystallized only partially, with a small energy transition
-1
(
3.9 kJ.mol ) peaking at 79 °C. On a second heating/cooling cycle, 2d started to
crystallize at 60 °C, with an exothermic peak, and then melted again at 169 °C (Figure
c). On second cooling, a very similar profile was obtained when compared to the first
cooling. POM analysis of compound 2d confirmed the same profile observed by DSC.
1
1
3

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The photomicrographs of the crystal textures under heating/cooling cycles are shown in
Figure 2. Figure 2a shows the appearance of the partial crystallization at a temperature
well below the melting point on cooling from the isotropic liquid. On heating the sample,
the crystallization progress with the growth of a larger area of crystal domains. Small
nucleation and growth of spherulites can also be seen in the texture (Figure 2b). On
further heating, this texture slowly starts to disappear. Just below the melting point
structures of growing crystalline needles takes place (Figure 2c). These needles begin
to fade at the temperature of phase transition to the isotropic melt (Figure 2d).
By comparison of the butyl substituent isomers 2e, 2f and 2g, the following
profile of melting point was found: (i-Bu > s-Bu > Bu). The increasing of one methylene
unit in 2i, with i-pentyl substituents, compared to the i-butyl in 2g, lowered the melting
point from 142°C to 84 °C and increased the hysteresis. On cooling from the isotropic
liquid, compound 2i keeps a supercooled liquid phase that crystallizes just at 50 °C.
Elongation and branching of alkyl chains were found to suppress crystallization
efficiently. Similar results were found by previous works, where racemic branching of
the alkyl groups in a variety of polycyclic tetraalkyl polyarene-tetracarboxylates
increases the mesophase stability, when compounds are liquid crystals, as well as
suppress crystallization of the materials.[19-21] Therefore, compound 2k, with 2-
ethylhexyl racemic substituents, is liquid at room temperature and, as well as its isomer
2l, did not crystallize even when stored in a refrigerator. Figure 1d shows a
representative DSC thermogram for a liquid azo dye, compound 2l. No transition was
observed from -20 °C to 40 °C.
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Figure 2. Photomicrographs of compound 2d obtained by POM (10 °C.min ): a) Partial
crystallization at 75 °C, on first cooling; b) crystallization at 66 °C during the second heating; c)
growth of crystalline needles at 162 °C, on second heating; and d) phase transition to isotropic
liquid at 169 °C, on second heating.
3.3 Optical properties
As expected, compounds 2a-l showed similar absorption spectra with a high
intensity band peaking at ca. 314 nm, corresponding to the π-π* transition of the trans-
isomer and a weak broad band in the visible region centered at 434 nm, associated
with the n-π* transition of the cis-isomer. Figure 3 depicts the absorption spectra of 2e
and 2g, as a representative case (for UV of other compounds see Supplementary file),
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before and after irradiation at 356 nm by a UV lamp. The samples in 10 M solution
were kept in dark for 48h before the analysis, to ensure that all compounds were in the
trans-configuration. Irradiation of trans-isomers in solution with UV light for time
increments of 5 minutes produced a decrease in the intensity of the 314 nm band,
accompanied by a slight increase in the 434 nm band (Figures 3a/c). This result is
characteristic of the trans to cis photoisomerization of the azo group. [12, 22] Though,
for compounds shown in this work, the photoisomerization is very slow. After 1500
seconds of irradiation, the reduction in the maximum absorption at 314 nm was only
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about 25 %. The reverse process is shown in Figures 3b/d. For this, the excited
solutions were maintained in dark and absorption spectra were registered at every 5
minutes.
Figure 3. Top (left): Absorption spectra of 2e in solution with different exposure time of UV light
(
trans to cis photoisomerization); Top (right): Back relaxation process for 2e from cis to trans.
Bottom (left): Absorption spectra of 2g in solution with different exposure time of UV light (trans
to cis photoisomerization); Bottom (right): Back relaxation process for 2g from cis to trans. No
UV corresponds spectra taken in the absence of UV light (0 min of UV light illumination). All
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spectra in dichloromethane solution (10 M).
Figure 4. Plots of molar absorptivity at the π-π* transition band as a function of exposure time
for excitation (trans to cis) and for back relaxation (cis to trans) of (a) 2e and 2g; and (b) 2j and
2k.
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Figure 4 shows the molar absorptivity of the π-π* transition band as a function of
exposure time, comparing the excitation and the back relaxation processes of isomers
2e and 2g with butyl and i-butyl substituents, respectively. In addition, the results for
compounds 2j and 2k with hexyl and ethylhexyl substituents, respectively are also
presented. The results showed that the reverse transformation from cis to trans is
slowed down by linear alkyl substituents compared to the branched alkyl substituents.
After 25 minutes of back relaxation, the absorbance of the π-π* transition band of 2e
with n-butyl substituents is only c.a. 90% of that of 2g with i-butyl substituents in the
same conditions. This effect is also increasing with the expansion of the alkyl chain.
Linear hexyl chains in 2j lowered the intensity of the π-π* transition band in about 80%
of that presented by branched racemic 2-ethylhexyl in 2k after the same period, as
shown in the plot in Figure 3b. A possible explanation for that may be due to a higher
steric hindrance by branched alkyl chains, which destabilize the cis configuration form
and in turn accelerate the reverse process to the trans configuration.
4. Conclusions
In summary, a series of tetraalkyl azobenzenetetracarboxylates in a meta
substitution pattern was synthesized and characterized. These dyes showed good
thermal stability with decomposition temperatures starting above 300 °C. The
increasing in the size of the linear alkyl chains brought their melting points down to
near and below room temperature with hexyl and octyl, respectively. The isomers with
substituents of 8 carbon atoms, linear as octyl and branched racemic as 2-ethylhexyl
gave liquid dyes at room temperature that did not crystallize even at lower
temperatures as 0 °C. Interesting, shorter chain in i-propyl substituents gave a product
able to stay in a metastable liquid in temperatures much below its melting point. The
study of photoisomerization of the azo group in final compounds was investigated in
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solution. These molecules exhibited trans to cis photoisomerization in a small
extension, even after 1500 seconds of irradiation with UV light. The return process to
trans-isomer was accelerated by the presence of branched substituents.
Acknowledgements
The authors would like to thank the Brazilian entities CNPq and CAPES for financial
support.
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LOW-MELTING DYES OF TETRA-ALKYL AZOBENZENE-
3,3',5,5'-TETRACARBOXYLATES
Thamires dos Santos Moreira, Thalisson Firmo Dutra, Rodrigo Cristiano*
Highlights
•
•
Dyes based on tetra-alkyl azobenzene-m-tetracarboxylates were synthesized
Tetrakis-2-ethylhexyl and tetra-octyl azobenzene-3,3’,5,5’-tetracarboxylates (2k
and 2l) are liquid dyes at room temperature
•
•
The increasing in the size of the linear alkyl chains decreases melting points of
azobenzenetetracarboxylates to near and bellow room temperature
The photoisomerization process for final compounds were studied in solution
Graphical abstract