Improved crystallinity of the asymmetrical diketopyrrolopyrrole derivatives by the adamantane substitution

Article abstract of DOI:10.1016/j.dyepig.2019.108141

Alkylation of diketopyrrolopyrroles (DPPs) is a powerful tool for increasing the solubility and processability of these pigments. Moreover, alkylation contributes to solid state packing and structural ordering of DPPs. In this study, the influence of the alkylation and solubilization side group engineering to DPPs on the thermal and optical properties was systematically investigated. Two series (each containing 3 derivatives: N,N'-, N,O′- and O,O′-substituted examples) of alkylated DPPs by 2-ethylhexyl and ethyladamantyl substituents were synthesized. Separation of all formed DPP derivatives was accomplished in order to perform an in-depth study of their physicochemical properties. DSC measurements revealed that the O-substitution caused a decrease in the thermal stability of DPP derivatives. On the contrary, an ethyladamantyl side chain, as a rigid alicyclic substituent, contributed very effectively to the increase of the melting point and thermal stability. The new results provide insights into the development of DPP-alkylated regioisomers and their thermal and optical properties.

Full text of DOI:10.1016/j.dyepig.2019.108141

Journal Pre-proof
Improved crystallinity of the asymmetrical diketopyrrolopyrrole derivatives by the
adamantane substitution
Martin Cigánek, Patricie Heinrichová, Alexander Kovalenko, Jiří Kučerík, Martin Vala,
Martin Weiter, Jozef Krajčovič
PII:
S0143-7208(19)32326-5
DOI:
https://doi.org/10.1016/j.dyepig.2019.108141
DYPI 108141
Reference:
To appear in: Dyes and Pigments
Received Date: 1 October 2019
Revised Date: 12 December 2019
Accepted Date: 12 December 2019
Please cite this article as: Cigánek M, Heinrichová P, Kovalenko A, Kučerík Jiří, Vala M, Weiter
M, Krajčovič J, Improved crystallinity of the asymmetrical diketopyrrolopyrrole derivatives by the
adamantane substitution, Dyes and Pigments (2020), doi: https://doi.org/10.1016/j.dyepig.2019.108141.
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Graphical abstract

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Improved crystallinity of the asymmetrical diketopyrrolopyrrole
derivatives by the adamantane substitution
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Martin Cigáneka, Patricie Heinrichováa, Alexander Kovalenkoa, Jiří Kučeríka, Martin Valaa, Martin Weitera and
Jozef Krajčoviča,*
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aBrno University of Technology, Faculty of Chemistry, Materials Research Centre,
Purkyňova 118, 612 00 Brno, Czech Republic.
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*Corresponding author. E-mail address: krajcovic@fch.vut.cz
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ABSTRACT
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Alkylation of diketopyrrolopyrroles (DPPs) is a powerful tool for increasing the solubility and
processability of these pigments. Moreover, alkylation contributes to solid state packing and
structural ordering of DPPs. In this study, the influence of the alkylation and solubilization
side group engineering to DPPs on the thermal and optical properties was systematically
investigated. Two series (each containing 3 derivatives: N,N'-, N,O'- and O,O'-substituted
examples) of alkylated DPPs by 2-ethylhexyl and ethyladamantyl substituents were
synthesized. Separation of all formed DPP derivatives was accomplished in order to perform
an in-depth study of their physicochemical properties. DSC measurements revealed that the
O-substitution caused a decrease in the thermal stability of DPP derivatives. On the contrary,
an ethyladamantyl side chain, as a rigid alicyclic substituent, contributed very effectively to
the increase of the melting point and thermal stability. The new results provide insights into
the development of DPP-alkylated regioisomers and their thermal and optical properties.
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1. INTRODUCTION
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In the past few decades, soluble organic semiconductors have found a number of applications
in various fields of organic electronics, mainly due to their convenient processability using
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common organic solvents and lower production costs compared to their conventional
inorganic counterparts [1]. Organic semiconducting materials, suitable for applications in
organic electronics should exhibit high absorption coefficients [2], high thermal and chemical
stability [2] and good charge transport properties [3]. One of the most attractive groups of
organic pigments are derivatives of 2,5-dihydropyrrolo[4,3-c]pyrrolo-1,4-dione (DPP) [4].
They have a considerable application potential for low-cost solution processable electronic
devices [5]. The fundamental DPP structure can be modified by conventional chemical
pathways, producing various derivatives exhibiting desired chemical and physical properties
[6]. Currently, there are a number of scientific reports showing examples of successful
application of DPP derivatives as functional high-performance materials in dye-sensitized [7]
and bulk heterojunction solar cells [8], organic field-effect transistors [9]–[13], organic light-
emitting diodes [14],[15], sensors [16] and biosensors [17], fluorescence imaging [18].
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In general, in order to achieve high performance in the above-mentioned applications, the
conjugated length of the molecule has to be extended, which is possible either by
polymerization via direct arylation [19][20] or by the cross-coupling reactions on the side
aromatic rings of the DPP core [21],[22]. Most commonly, DPP derivatives are substituted by
thiophene-2-yl [23] groups at the 3 and 6 positions. The N,N'-unsubstituted DPP derivatives
have good thermal stability and photostability [2],[25], but on the other hand they also are
poorly soluble in most common organic solvents due to the strong intermolecular hydrogen
bonding between neighboring carbonyl-oxygen and lactam-nitrogen atoms [2]. Therefore, to
achieve high solubility of DPP derivatives, the N,N'-positions can be substituted with linear or
branched alkyl chains, which interrupt intermolecular hydrogen-bonds [26]. N,N'-alkylation
ranks among the most important and common methods of DPP substitution, which has
resulted in broader applications in the previously-mentioned areas of organic electronics [6].
However, the disadvantage of alkylation of DPPs is poor regioselectivity, which results in
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isolation of O-alkylated side products in moderate yields [27]. Alkylation is performed
through base-catalyzed nucleophilic substitution reactions. The influence of the bases,
solvents and reaction conditions used on the alkylation progress and regioselectivity has been
studied only rarely [28],[29]. However, two competitive anions O– and N– on the lactam core
contribute to a drop in the regioselectivity of N,N'-substitutions. Stas et al. [28] reported that
substitution of alkyl chains on oxygen atoms is possible alongside with the nitrogen
heteroatoms due to the delocalization of the negative charge between the two atoms of a
lactam group. Frebort et al. [30] already described the formation of O-alkylated by-products,
but very few reports so far have investigated the properties of these O-alkylated derivatives
[31],[32].
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We recently reported [33] synthesis and study of a novel N,N'-substituted DPP derivative
contained bulky ethyladamantyl groups. Substitution had a significant impact on highly
ordered adamantane-induced molecular packing, which resulted in exceptional ambipolar
charge transport of this new chromophore. Nevertheless, the synthesis of the new aforesaid
DPP derivative exhibited very low regioselectivity of N,N'-alkylation and thus, formation of
competitive by-products was largely observed.
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Herein, to systematically study the effect of branching and bulky substituents on alkylation
regioselectivity and their binding position, the optical and thermal properties of all
symmetrical (N,N'-; O,O'-) and asymmetrical (N,O'-) DPP products were determined. The 2-
ethylhexyl chain was chosen as a branching substituent for comparison with the aforesaid
bulky ethyladamantyl chains. The key was to isolate all these products in quantities enabling
the systematic study of optical and thermal properties and to examine both the effect of the
alkyl chain character and the binding site in the DPP molecule on these properties. Systematic
studies showed that asymmetrical derivatives tend to exhibit large Stokes shift which reduces
the reabsorption of the emitted luminescence. Moreover, O-substitution causes the
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disappearance of carbonyl group and the substitution on the highly polar ketone group induces
electron cloud polarization, which is beneficial for e.g. the two-photon absorption [34].
However, O,O'- and N,O'-substituted DPP derivatives have their drawbacks: substitution on
keto-group disturbs the crystalline organization in the solid state, which causes these materials
to have a paste-like consistency at room temperature [31], and this makes their application in
the field of organic electronics rather doubtful.
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However, O,O'- and asymmetrical N,O'-substituted derivatives can be of interest not only
from a fundamental but also from a practical point of view. Therefore, the main challenge of
the present paper is to improve the crystallinity and thermal stability of O,O'- and N,O'-
ethyladamantyl substituted DPP derivatives and investigate in detail their yet unknown optical
properties.
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2. EXPERIMENTAL SECTION
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2.1. Materials
2-Thiophenecarbonitrile (99%), sodium, iron(III) chloride (>97%, anhydrous), tert-Amyl
alcohol (≥99%), N,N-dimethylformamide (DMF, 99.8%, anhydrous), potassium carbonate
(99.99%, anhydrous), 2-ethylhexyl bromide (95%) and deuterochloroform (99.96 atom % D)
were purchased from Sigma-Aldrich (now Merck) and were used as received. Diisopropyl
succinate (98%) was purchased from Synthesia, Inc. and also was used as received. 1-(2-
Bromoethyl)adamantane (98%) was purchased from Provisco CS Ltd. Acetic acid (99%),
isopropyl alcohol (p.a.), methanol (p.a.), toluene (p.a.) and chloroform (p.a.) were purchased
from PENTA Ltd. and were used as received. Separation by column chromatography was
carried out on a Silica Gel 60Å (230–400 mesh, Sigma-Aldrich). All reactions were
performed in oven-dried apparatus, under argon atmosphere while magnetically stirred.
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2.2. Characterization
1H NMR spectra were recorded on a FT-NMR spectrometer Bruker Avance III 300 MHz or
500 MHz in CDCl3. Chemical shifts (δ) are given in parts per million (ppm) relative to TMS
as an internal reference. The melting point was determined on a Kofler apparatus and the
temperature was not calibrated. Mass spectra were recorded on a GC–MS spectrometer
Thermo Fisher Scientific ITQ 700 (DEP). Elemental analysis was measured with an elemental
analyser Flash 2000 CHNS Thermo Fisher Scientific.
Thermogravimetry (TG) was conducted on a TA Instruments Q5000IR (New Castle,
Delaware, USA) to analyze the stability of the derivatives and changes in mass before
degradation. A sample was placed on the Pt crucible sample holder and heated at 10 °C/min
from room temperature to 600 °C under a stream of nitrogen (ultra-high-purity) flow rate
40 mL/min. TG was used to analyze the thermal stability of the investigated materials.
Briefly, the TG record (dependence of mass on temperature) was derived and onset of the
derivate was determined.
Differential Scanning Calorimetry (DSC) was conducted on a TA Instruments Q2500
(New Castle, Delaware, USA) equipped with an RCS cooler to analyze the physicochemical
properties of derivatives. The samples, typically 3–7 mg, were weighed to aluminum pans
(Tzero®), hermetically sealed and measured under a stream of nitrogen flow rate 50 mL/min.
The loading temperature was 30 °C. The following temperature program was applied: heating
10 °C/min to temperature 50 °C below the degradation temperature determined using TG to
delete the thermal history of the sample. The sample was quickly (15 °C/min) cooled to
-90 °C and heated again at 10 °C/min. This cycle was used to estimate the physicochemical
structure of the sample upon fast cooling. Then, the sample was cooled slowly (3 °C/min) to
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-90 °C and heated again at 10 °C/min. This cycle was used to estimate the physicochemical
structure of the sample upon slow cooling.
Both TG and DSC records were evaluated using TRIOS and Universal analysis software
provided by TA Instruments.
Solutions were prepared by diluting materials in anhydrous chloroform. The concentration of
materials was 10-5 to 10-6 mol · dm–3. Solutions were characterized in a quartz cuvette
(Herasil®, Heraeus Quarzglas Co.). Optical measurements of thin films were performed on
quartz glass slides (Herasil® 102, Heraeus Quarzglas Co.) which were pre-treated by
ultrasonic cleaning in the following baths: 1) Detergent Neodisher (Miele, Inc., NJ, USA)
10 min; 2) deionized (Milli-Q) water 20 min; 3) isopropyl alcohol (a. p.) 10 min. Films were
deposited by spin-coating. The spin rate was 2000 rpm and the deposed material was
dissolved in chloroform. The final thickness of layers was in the range 50–100 nm.
Absorption spectra of samples were measured with a Varian Cary Probe 50 UV-VIS
spectrometer (Agilent Technologies Inc.). The fluorescence spectra were recorded with a
Horiba Jobin Yvon Fluorolog. This apparatus equipped with integration sphere was also used
to determine the fluorescence quantum yield by absolute method. The fluorescence lifetime
was measured with a Horiba Jobin Yvon Fluorocube.
2.3. Synthesis
The targeted molecules were synthesized according to the known general procedure [35],[36].
The synthetic route and the molecular structures are illustrated in Scheme 1.
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H
N
S
O
O
O
1. K2CO3
2. R–Br
1. Na
2. FeCl3 (cat.)
S
S
O
O
+
t
-Amyl alcohol
DMF
105°C, 2 hod, Ar
CN
102°C, 24 hod, Ar
O
N
H
S
Th-DPP: 59%
R
S
R
N
R
S
N
N
O
O
O
+
+
O
N
R
O
O
N
N
R
S
S
S
R
Th-DPP-1a: R = 2-ethylhexyl; 41%
Th-DPP-1b: R = 2-ethylhexyl; 18%
Th-DPP-1c: R = 2-ethylhexyl; 3%
Th-DPP-2a: R = adamantylethyl; 36% Th-DPP-2b: R = adamantylethyl; 20% Th-DPP-2c: R = ethyladamantyl; 6%
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Scheme 1. Procedure for the synthesis of the N,N'-; N,O'- and O,O'-alkylated DPP derivatives
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2.3.1. Synthesis of 2,5-dihydropyrrolo[4,3-c]pyrrolo-1,4-dione molecules
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Sodium (~1.3 equiv., 7.2 g, 313.2 mmol) was dissolved in tert-amyl alcohol (400 mL) heated
to reflux and with the addition of catalytic amount of iron(III) chloride. After dissolution of
all sodium, there was added in one portion 1.0 equiv. of thiophene-2-carbonitrile (26.2 g,
240.0 mmol) and the reaction mixture was stirred for 30 min at reflux. Then, 0.65 equiv. of
diisopropyl succinate (31.6 g, 156.2 mmol) dissolved in tert-amyl alcohol (80 mL) was
gradually added dropwise for 4 h and the mixture was stirred at reflux for 18 h. After that,
protolysis was performed by addition of diluted acetic acid to the reaction mixture cooled to
laboratory temperature. The mixture was refluxed for 6 h and then the heterogenic mixture
was filtered while hot and the filter cake was washed with hot water and isopropyl alcohol.
The crude product was refluxed in methanol for 1 h and, after that, it was filtered while hot to
get a pure product.
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Th-DPP: Dark purple solid (27.5 g, yield 59%). Melting point >400 °C, 1H NMR (300 MHz,
DMSO-d6, ppm): δ = 11.21 (s, 2H), 8.20 (d, J = 3.01 Hz, 2H), 7.93 (d, J = 3.04 Hz, 2H),
7.31–7.27 (m, 2H), Anal. calcd. for C14H8N2O2S2: C 55.98%, H 2.68%, N 9.33%, Found: C
55.42%, H 2.37%, N 9.71%.
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2.3.2. Synthesis of the alkylated DPP derivatives
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Anhydrous potassium carbonate (~5.3 equiv., 2.4 g, 17.4 mmol) was added to a solution of
Th-DPP (1.0 equiv., 1.0 g, 3.3 mmol in 45 mL of anhydrous DMF) and the mixture was
heated to 60 °C and stirred for 1 h. Then, 3.50 equiv. (11.6 mmol) of alkyl bromide (R–Br,
2-ethylhexyl bromide: 2.23 g; 1-(2-bromoethyl)adamantane: 2.81 g) dissolved in anhydrous
DMF (20 mL) was gradually added dropwise for 30 min. After 20 min, the mixture was
heated to 105 °C and stirred for 2 h. Then, DMF was distilled off by vacuum distillation, the
solid material was suspended in methanol and filtered to get a crude product.
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Th-DPP-1a: Dark red crystal material (0.71 g, yield 41%) was obtained after purification of
the crude product by column chromatography on silica gel (toluene, RF = 0.26). Melting point
126 °C, 1H NMR (300 MHz, CDCl3, ppm, Fig. S1): δ = 8.88 (dd, J = 5.1, 2.7 Hz, 2H), 7.64
(dd, J = 6.3, 3.9 Hz, 2H), 7.29 (dd, J = 7.2, 1.2 Hz, 2H), 4.04 (m, 4H), 1.88 (m, 2H), 1.40–
1.24 (m, 16H), 0.92–0.85 (m, 12H); 13C NMR (125 MHz, CDCl3, ppm): δ = 161.79, 140.44,
135.23, 130.48, 129.86, 128.41, 107.98, 45.89, 39.13, 30.25, 28.39, 23.59, 23.05, 13.99,
10.50; Anal. calcd. for C30H40N2O2S2: C 68.66%, H 7.68%, N 5.34%, Found: C 68.12%, H
7.47%, N 5.53%.
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Th-DPP-1b: Purple waxy material (0.31 g, yield 18%) was obtained after purification of the
crude product by column chromatography on silica gel (toluene, RF = 0.36). Melting point –,
1H NMR (300 MHz, CDCl3, ppm, Fig. S2): δ = 8.48 (d, J = 3.6 Hz, 1H), 8.19 (d, J = 3.9 Hz,
1H), 7.68 (dd, J = 4.1, 1.6 Hz, 1H), 7.50 (d, J = 3.2 Hz, 1H), 7.26 (dd, J = 4.3, 1.2 Hz, 1H),
7.20 (d, J = 3.5 Hz, 1H), 4.52 (m, 2H), 3.94 (m, 2H), 1.79 (m, 2H), 1.38–1.23 (m, 16H), 0.91–
0.85 (m, 12H); 13C NMR (125 MHz, CDCl3, ppm): δ = 166.38, 161.73, 149.62, 142.46,
138.73, 134.82, 131.63, 131.49, 130.09, 128.96, 128.23, 127.96, 114.30, 111.15, 72.45, 45.54,
39.37, 39.13, 31.26, 30.76, 30.29, 29.70, 29.09, 28.40, 24.12, 23.62, 23.03, 14.01, 11.28,
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10.51; Anal. calcd. for C30H40N2O2S2: C 68.66%, H 7.68%, N 5.34%, Found: C 68.21%, H
7.54%, N 5.12%.
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Th-DPP-1c: Dark purple waxy material (0.06 g, yield 3%) was obtained after purification of
the crude product by column chromatography on silica gel (toluene, RF = 0.68). Melting point
–, 1H NMR (300 MHz, CDCl3, ppm, Fig. S3): δ = 8.04 (dd, J = 4.9, 2.4 Hz, 2H), 7.55 (dd, J =
6.3, 3.9 Hz, 2H), 7.18 (dd, J = 5.7, 3.3 Hz, 2H), 4.55 (m, 4H), 1.82 (m, 2H), 1.54–1.29 (m,
16H), 0.98–0.83 (m, 12H); 13C NMR (125 MHz, CDCl3, ppm): δ = 147.72, 147.10, 138.53,
129.01, 124.49, 123.99, 119.16, 39.51, 31.46, 30.23, 29.71, 29.08, 23.01, 14.08, 11.27; Anal.
calcd. for C30H40N2O2S2: C 68.66%, H 7.68%, N 5.34%, Found: C 68.34%, H 7.93%, N
5.60%.
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Th-DPP-2a: Violet crystal material (0.75 g, yield 36%) was obtained after purification of the
crude product by column chromatography on silica gel (toluene/chloroform 3/1, RF = 0.60)
and the following recrystallization in toluene with addition of n-heptane. Melting point
321 °C, 1H NMR (500 MHz, CDCl3, ppm, Fig. S4): δ = 8.91 (d, J = 3.8 Hz, 2H), 7.64 (d, J =
4.9 Hz, 2H), 7.27 (dd, J = 6.0, 5.0 Hz, 2H), 4.14–4.11 (m, 4H), 1.99–1.82 (m, 6H), 1.75–1.72
(m, 7H), 1.68–1.65 (m, 19H), 1.53–1.51 (m, 3H); 13C NMR (125 MHz, CDCl3, ppm): δ =
161.28, 140.13, 135.23, 130.57, 129.69, 128.57, 107.87, 43.23, 42.30, 37.72, 37.11, 32.22,
28.63; EI [m/z] 624.89, Found 624.97, Anal. calcd. for C38H44N2O2S2: C 73.04%, H 7.10%, N
4.48%, S 10.26%, Found: C 73.15%, H 7.08%, N 4.42%, S 10.38%.
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Th-DPP-2b: Dark violet solid material (0.41 g, yield 20%) was obtained after purification of
the crude product by column chromatography on silica gel (toluene/chloroform 3/1,
RF = 0.65). Melting point 210 °C, 1H NMR (300 MHz, CDCl3, ppm, Fig. S6): δ = 8.42 (d, J =
3.9 Hz, 1H), 8.25 (d, J = 3.8 Hz, 1H), 7.69 (dd, J = 5.9, 4.1 Hz, 1H), 7.49 (dd, J = 5.6, 4.1 Hz,
1H), 7.27 (d, J = 3.2 Hz, 1H), 7.24–7.17 (m, 1H), 4.67–4.63 (t, J = 7.8 Hz, 2H), 4.06–4.00 (m,
2H), 1.98 (m, 6H), 1.75–1.67 (m, 26H), 1.62–1.53 (m, 3H); 13C NMR (125 MHz, CDCl3,
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ppm): δ = 166.15, 161.35, 149.43, 142.41, 138.76, 135.29, 131.65, 131.48, 130.02, 129.78,
128.91, 128.23, 113.87, 111.44, 66.79, 43.34, 42.93, 42.72, 42.27, 37.43, 37.09, 37.05, 32.20,
32.01, 28.67, 28.61; Anal. calcd. for C38H44N2O2S2: C 73.04%, H 7.10%, N 4.48%, Found: C
72.81%, H 7.01%, N 4.57%.
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Th-DPP-2c: Dark violet solid material (0.12 g, yield 6%) was obtained after purification of
the crude product by column chromatography on silica gel (toluene/chloroform 3/1,
RF = 0.95). Melting point 221 °C, 1H NMR (300 MHz, CDCl3, ppm, Fig. S7): δ = 8.05 (d, J =
4.5 Hz, 2H), 7.55 (d, J = 5.5 Hz, 2H), 7.19 (dd, J = 8.1, 1.1 Hz, 2H), 4.69–4.65 (m, 4H), 1.98
(m, 4H), 1.70–1.64 (m, 24H), 1.54 (s, 4H), 1.34–1.26 (m, 2H); 13C NMR (125 MHz, CDCl3,
ppm): δ = 147.52, 146.96, 138.29, 129.09, 124.43, 123.98, 118.92, 42.50, 36.88, 31.89, 31.39,
30.05, 28.45; Anal. calcd. for C38H44N2O2S2: C 73.04%, H 7.10%, N 4.48%, Found: C
72.96%, H 6.99%, N 4.51%.
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3. RESULTS AND DISCUSSION
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3.1. Thermal properties
The TG record of the sample Th-DPP-1a (Fig. S8, Supplementary Information) resulted in a
slow mass loss (5%) up to 289 °C, where the degradation processes started accompanied by
an intensive mass loss. Consequently, upon both fast and slow cooling, the DSC record (Fig.
S8) showed two endothermal peaks, which can be attributed to the melting of crystalline
structures. The comparison of lower temperature peaks showed that they had similar onsets
around 53 °C, but they differed in the enthalpies, i.e. the slow-cooled sample gave 12 J/g,
which was higher than the 10 J/g of the fast-cooled sample. The second peaks were
comparable, their enthalpies were around 56 J/g and onsets at 118 °C. This revealed formation
of two distinct crystalline structures, while the crystallites melting at lower temperatures were
affected by the cooling regime. We assume that the crystallites in sample Th-DPP-1a are
formed from 2-ethylhexyl chains. Longer linear aliphatic chains have a high tendency to form
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packed structures, but their substitution may induce formation of several stable forms
energetically close to each other (i.e. crystalline structures with similar melting temperatures)
[37],[38]. An asymmetric analog to the sample Th-DPP-1a is the sample Th-DPP-1b which
showed slightly lower degradation temperature at 270 °C (Fig. S9). The DSC record showed
only step transition after fast and slow cooling around 23 °C which was attributed to the glass
transition. However, an endothermal peak with the enthalpy of 40 J/g was observed around
30 °C, which started simultaneously with the run (first heating cycle) that is normally used for
deleting the thermal history of the sample. The endothermal peak was not observed in the
subsequent run. The sample was stored at 20 °C and the experiment was repeated after 9 days,
and the peak was observed again, at the same temperature with enthalpy 34 J/g. An attempt to
induce the crystallization at 10 °C for 30 minutes failed (Fig. S9). This means that the
crystallization of sample Th-DPP-1b is a slow process with kinetic constants in order of
several days. Unlike the Th-DPP-1a sample, Th-DPP-1b contains 2-ethylhexyl chains
asymmetrically bound to N- and O-atoms, which apparently influence the kinetics of
recrystallization. Furthermore, the symmetrically N,N'- substituted sample Th-DPP-1a
indicates the presence of two types of crystallites, while the asymmetrically N,O'-substituted
Th-DPP-1b revealed only one type of crystals. This indicates that the crystallites are formed
predominantly from 2-ethylhexyl bound on the N-atom, while the O-substitution does not
support the formation of crystals. Accordingly, the high temperature melting in the sample
Th-DPP-1a would be caused by larger crystals of 2-ethylhexyl chains probably composed of
chains from adjacent molecules.
263
264
265
266
The TG record of the N,N'-ethyladamantyl symmetrically substituted sample Th-DPP-2a
showed the degradation around 368 °C, but it was preceded by a negligible mass loss of 0.3 %
(Fig. S10). The mass loss can be attributed to the residual mass loss of either moisture or the
residual content of some ethyladamantyl-based impurity. In the DSC records (Fig. S10), the
11

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289
290
291
fast cooling resulted in a glass transition around 123 °C, followed by cold crystallization at
163 °C with enthalpy 36 J/g and melting at 320 °C with enthalpy 76 J/g. This means that fast
cooling initiated only partial crystallization of the melted sample. The slow cooling resulted in
glass transition at 133 °C followed by cold crystallization at 186 °C with enthalpy 30 J/g
followed by melting. In this case, however, the melting occurred in two steps, one started at
274 °C with enthalpy 13 J/g and the second started at 300 °C with enthalpy 29 J/g. Between
these two endothermal peaks no exothermal was observed (recrystallization), therefore the
melting corresponded to two distinct crystalline domains in a sample. Hence, the sample
Th-DPP-2a responded to a difference in the cooling rate by shifts in the glass transitions and
crystallinity degree. The stability of the asymmetrically substituted sample Th-DPP-2b
showed degradation at 264 °C (Fig. S11). The DSC record (Fig. S11) showed a glass
transition at 113 °C upon fast cooling, which shifted to 110 °C upon slow cooling. The
heating segment applied for deleting the thermal history showed an exothermal peak with
onset 250 °C, probably corresponding to degradation, while the consequent heating runs did
not show any melting process in the temperature range used. We conclude that in comparison
to the sample Th-DPP-2a, asymmetrical substitution hampers the formation of the crystals,
and the structure has the tendency to remain in amorphous rather than crystalline form, which
is less stable. The TG record of the O,O'-symmetrically substituted sample Th-DPP-2c
showed a continuous decrease in mass from 100 °C to 250 °C (mass loss around 20%) while
above 250 °C the rate of mass decrease accelerated, i.e. the sample was degraded (Fig. S12).
For this reason, the correct temperature of the degradation could not be determined using TG.
The DSC showed an endothermal process, probably melting with onset at 221 °C overlaid by
an intensive exotherm at 246 °C attributed to degradation of the melt (Fig. S12). We conclude
that O,O'-substitution results in formation of significantly less stable derivatives in
comparison to N,N'-substitution.
12

292
Table 1: Summarized thermal properties of the materials under investigation.
melting
cold
crystallization
°C
Tend
TG
Tg
°C
enthalpy melting II enthalpy
crystallization
°C
I
°C
55
55
53
30*
20
DSC degradation
Compound
Th-DPP-1a
Th-DPP-1b
Th-DPP-2a
Th-DPP-2b
Th-DPP-2c
DSC
J/g
6
10
°C
J/g
56
56
56
°C
150
150
150
180
180
180
360
360
360
250
250
250
200**
200
200
°C
289
first heating
after fast cooling
after slow cooling
first heating
after fast cooling
after slow cooling
first heating
118
118
118
12
40*
0,04
0,02
94
76
13
270
368
264
n.d.
23
23
22
321
320
274
210
207
330
300
8
133
163
186
6
36
30
after fast cooling
123
after slow cooling 133
first heating
29
62
2
after fast cooling
113
157
1
after slow cooling 110
first heating
after fast cooling
after slow cooling
36
29
221**
**when extended to 300°C melting occurred at 221°C followed by degradation around 246°C
*started simultaneously with experiment
293
13

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295
296
297
3.2. Optical properties
The optical properties of DPP materials were studied in solution as well as in thin spin-casted
films to investigate the effects given by the position and the type of solubilization group
substitution.
298
299
Fig. 1. Absorption and fluorescence spectra of studied DPPs dissolved in chloroform.
300
301
Table 2: Optical parameters of solutions determined from fluorescence and absorption measurements.
Substitution
QY [%]
Eg opt [eV]
λABS max [nm] λABS edge [nm] λPL [nm]
τ [ns]
Th-DPP-1a
Th-DPP-1b
Th-DPP-1c
547
533
505
569
602
598
563
602
–
63 ± 6
0.8 ± 0.5
–
6.05 ± 0.01
0.63 ± 0.02
0
2.23
2.16
Not det.
Th-DPP-2a
Th-DPP-2b
Th-DPP-2c
549
533
505
572
602
598
562
598
731
65 ± 5
0.7 ± 0.5
Not det.
6.07 ± 0.01
0.65 ± 0.02
0.32 ± 0.05
2.23
2.16
2.05
302
303
Absorption and fluorescence emission spectra are summarized in Fig. 1. We observed that
the major influence on the optical properties had the position of the substitution (N,N'-; N,O'-
and O,O'-) rather than the type of the solubilization group. Ethyladamantyl and 2-ethylhexyl
solubilization groups provided similar spectra in solution. Quantitative parameters describing
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306
14

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309
310
311
312
the optical properties of solutions are summarized in Table 2. Absorption maximum is shifted
with the change of substitution position: while N,N'-substituted DPPs has a maximum at
approximately 548 nm, N,O'-substituted derivatives has a maximum at 533 nm and O,O'- at
505 nm (see spectra on Fig. 1). These shifts seem to be mainly the consequence of
suppression of electron transition to lower vibration modes in excited state in the case of
N,O'- and O,O'-derivatives (zero phonon transitions are suppressed).
313
314
315
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318
319
The position of the alkyl group also has a major influence on fluorescence emission spectra
(see Fig. 1). The emission maximum of N,N'-derivatives was at 563 nm. Emission maximum
of N,O'-derivatives was bathochromically shifted to 600 nm. Emission spectrum of N,O'-
derivative has resolved vibronic structure in contrast to absorption/excitation spectrum. This
is probably because the excited molecule is more planar than in the ground state. A clear
emission spectrum of O,O'-substituted derivatives was found only in the case of
ethyladamantyl-substituted DPP with a maximum at 730 nm.
320
321
322
323
324
325
The photoluminescence quantum yields (PLQY) drop with substitution in O-position
dramatically. While N,N'-derivatives have relatively high fluorescence quantum yields about
60 %, N,O'-derivatives showed PLQY only of about 1%. The PLQY of O,O'-derivatives was
found to be too small for quantitative evaluation. The fluorescence quantum yield relates to
the fluorescence lifetime, which was found for N,N'-derivatives to be about 6 ns, for N,O'-
derivatives 0.6 ns and for O,O'-derivatives only 0.3 ns.
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328
329
330
331
The intersection point of emission and absorption/excitation spectra can be used to provide
information about the HOMO-LUMO difference Eg opt. This quantity was found to be 2.23 eV
for N,N'-derivatives, 2.16 eV for N,O'-derivatives and 2.05 eV for O,O'-derivatives,
respectively. These values are very close and thus provide other evidence of the above-
mentioned hypothesis about suppressed electron transitions from the ground state to the
excited state with the lowest vibration energy in the case of O-substituted derivatives.
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339
Next, thin films from studied DPPs were characterized. Prepared films were annealed at
different temperatures, from 50 to 300 °C with increments of 50 °C. The changes in the
layers’ quality (morphological as well as possible degradation) were studied by optical spectra
measurements. Absorption and fluorescence spectra of as-cast and annealed films are
summarized in plots on Fig. 2. The determined optical parameters are summarized in
Table 3. In contrast to solutions, a strong impact of the alkyl substitution position as well as
the kind of substituents on the absorption and emission spectra was observed.
340
341
342
343
344
Fig. 2. Optical spectra of thin films annealed at different temperatures. The curves marked by 0 are
spectra of as-cast films.
345
Table 3: Optical parameters of thin films.
as cast film
annealed film
Substitution
Th-DPP-1a
Th-DPP-1b
Th-DPP-1c
λABS max [nm]
λABS edge [nm]
λPL [nm] λABS max [nm]
λABS edge [nm]
λPL [nm]
508
518
508
613
671
622
696
732
-
510
562
509
613
672
621
695
-
-
Th-DPP-2a
Th-DPP-2b
Th-DPP-2c
566
541
506
600
623
730
691
740
-
593
547
627
608
633
588
-
-
≈750
16

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347
One of the most significant differences between 2-ethylhexyl and ethyladamantyl
derivatives is the thermal stability of the layers. The films prepared from 2-ethylhexyl
derivatives annealed already at low temperatures lost integrity and over 150 °C vanished from
the substrate (evaporated). This is consistent with findings from thermal characterization,
which showed that the melting point of 2-ethylhexyl derivatives is up to 120 °C while
ethyladamantyl substituted materials were melted at temperatures over 200 °C. Thus films
from ethyladamantyl derivatives had better thermal stability. In optical spectra this process
was observed as a decrease of signal intensity (see Fig. 2). Layer quality changes were also
observed by optical microscopy and the temperature induced changes were documented by
photos (see the Supplementary Information: Fig. S13–Fig. S14).
348
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359
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362
The decrease in the fluorescence intensity of the Th-DPP-1a derivative and of both N,O'-
substituted derivatives (see spectra in Fig. 2) is predominantly a result of the films’ integrity
losses during annealing, but a contribution of molecular packing changes cannot be excluded.
The fluorescence intensity of a layer from the Th-DPP-2a derivative unequivocally increased
after annealing, thus the fluorescence quantum yield was increased. This can be evidence for a
longer excitons lifetime as result of higher molecular order.
363
364
365
366
Thin films from O,O'-derivatives were not fluorescent as follows from curves in the plot
on Fig. 2, which correspond to the background signal from the fluorimeter without the
fluorescence signal. This result was expected in the context of the weak fluorescence of
solutions.
367
368
369
370
Looking at the absorption and emission spectra as a whole, we can observe significant
differences between the studied materials’ behavior in reaction to thermal annealing. As cast
films from Th-DPP-1a shows spectra with significantly lower zero phonon transition
intensity resulting in maximum shifted hypsochromically comparing to the diluted solution.
17

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376
This shift is typically assigned to H-aggregation. Thermal annealing has little influence to the
shape of the spectrum. On the other hand, the absorption of as-cast Th-DPP-2a is similar to
the diluted solutions but changes dramatically upon annealing. The annealed films showed
significant bathochromic shift revealing strong intermolecular interactions induced by the
ethyladamantyl N-substitution. These interactions can be the reason for high charge carrier
mobilities observed in our previous work [33].
377
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383
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385
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387
388
389
390
391
392
In the case of both N,O'-substituted derivatives (Th-DPP-1b and Th-DPP-2b) the optical
spectra have a shape which at first sight suggests small evidence for J-aggregation. Their as-
cast films have a very similar position of absorption (≈ 530 nm) and emission (≈ 735 nm)
maxima and in the case of the absorption maxima it is practically the same as for the solutions
(533 nm). This implies that molecules in films have a rather individual character, without
strong interactions. However, the absorption spectrum of film from the Th-DPP-1b derivative
has an absorption edge at 671 nm, while the film from the Th-DPP-2b derivative has an
absorption edge at only 623 nm. The absorption onsets in solution were 600 nm for both
derivatives. The extending of the absorption band of films to longer wavelengths is evidence
of intermolecular interaction, which seems to be more significant in the case of the
2-ethylhexyl derivative Th-DPP-1b. Observation of the thin films’ structure in optical
microscopy (see Fig. 3) revealed that the fresh as-cast layer from the 2-ethylhexyl derivative
(Th-DPP-1b) is crystalline, while the layer from the ethyladamantyl derivative (Th-DPP-2b)
seems to be amorphous. Intermolecular interaction of molecules in films is also manifested by
the emission maxima position, because in the case of films, they are in comparison with the
solution bathochromically shifted by approximately 130 nm.
393
394
395
Thermal annealing had practically no effect on the optical spectra shape of films from
ethyladamantyl-substituted derivatives. The rather amorphous character of the Th-DPP-2b
material was also confirmed by thermal characterization. In the case of the Th-DPP-1b
18

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399
400
401
402
derivative, thermal annealing caused a relative increase of absorption peak intensity at longer
wavelengths, and thus the absorption spectrum after annealing at 150 °C has a more
J-aggregation-like character. The film during annealing at all temperatures was at least
particularly melted according to the picture from microscopy (see the Supplementary
Information: Fig. S13–Fig. S14). Thus, possible structural changes which can explain the
observed changes in spectra, took place in the melt and during solidification. At temperatures
higher than 150 °C the film was destroyed.
403
404
405
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407
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409
410
411
412
413
414
415
416
417
418
419
420
The absorption maxima positions of as-cast thin films prepared from O,O'-derivatives are
very near to solutions (≈ 505 nm). The presence of π−π intermolecular interactions in films
can be deduced only from the bathochromic shift of absorption edges. In the case of
Th-DPP-1c derivative, the shift of absorption edge is only weak, but in the case of
Th-DPP-2c the absorption edge is even 120 nm shifted in comparison with the solutions.
While the overall spectra shape of the 2-ethylhexyl as-cast film suggests that the molecules in
the films keep a considerably individual character, the film of the ethyladamantyl derivative
seems to be at least partially ordered. The photo from optical microscopy confirms the
crystalline structure of the layer from this material (see Fig. 3). It should be noted that the
2-ethylhexyl derivative has a waxy character, thus it is rather more liquid than solid (see
photos on Fig. 4 and thermal characterization). This explains the observed weak
intermolecular interaction in the spectra. The fluorescence emission was not found in both
cases as was already reported. Thermal annealing in the case of the 2-ethylhexyl derivative
caused only layer destruction which is in good agreement with TGA data. It is a consequence
of the degradation of the material and the low melting point (see pictures in the
Supplementary Information). In the case of the ethyladamantyl derivative Th-DPP-2c, the
thermal annealing above 150 °C caused the absorption spectrum to have the J-aggregation
character with a maximum at 627 nm. However, with respect to the results from thermal
19

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423
424
characterizations, which show that this derivative has low thermal stability, it cannot be
guaranteed that the new order of material after annealing is not the result of co-crystallization
of the original material and degradation products. These changes after annealing at
temperature above 200 °C are results of re-crystallization from more or less melted matter.
425
426
Fig. 3. Thin films directly after spin-casting deposition (as-cast).
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428
429
430
Fig. 4. Pictures of the studied materials from optical microscopy. While most of the materials are
crystalline, O,O'-ethylhexyl derivative Th-DPP-1c is waxy.
20

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4. CONCLUSION
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447
448
449
In summary, we have synthesized and subsequently investigated two series of alkylated DPP
derivatives: N,N'-, N,O'- and O,O'-substituted by 2-ethylhexyl and ethyladamantyl chains. It
can be summarized that according to the observed optical spectra and the films’ pictures from
optical microscopy, the 2-ethylhexyl group enhances the films’ crystallization, especially in
the case of asymmetrical substitution in N,O'-positions. On the other hand, derivatives
alkylated by ethyladamantyl chains showed better order if the substitution was symmetrical in
O,O'-positions. This observation is in good agreement with the found low glass transition
temperatures (20–40 °C) in both cases.
Thermal measurements also revealed that substitution in the O-position leads to worse
thermal stability of materials. Next, it can be noted that the ethyladamantyl side group gave
the materials a higher melting point and better thermal stability than 2-ethylhexyl, as was
confirmed by all presented methods (TG and DSC).
The used solubilization groups and substitution positions have a significant influence on
thin films’ consistency and stability. Results from this work can prove valuable especially in
the design and fabrication of organic electrical and optoelectrical devices, which requires high
molecular order of material as well as structural stability.
450
ACKNOWLEDGMENT
451
452
453
This work was supported by the Czech Ministry of Industry and Trade Grant TRIO FV20022
and by the Czech Science Foundation, project No. 17-21105S. MC thanks project No. FCH-S-
19-5834.
454
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SUPPLEMENTARY INFORMATION
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Improved crystallinity of the asymmetrical diketopyrrolopyrrole
derivatives by the adamantane substitution
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Martin Cigánek, Patricie Heinrichová, Alexander Kovalenko, Jiří Kučerík, Martin Vala, Martin Weiter and
Jozef Krajčovič
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Brno University of Technology, Faculty of Chemistry, Materials Research Centre,
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Purkyňova 118, 612 00 Brno, Czech Republic
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1. Synthesis part
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Fig. S1. 1H NMR spectrum of the derivative Th-DPP-1a.
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Fig. S2. 1H NMR spectrum of the derivative Th-DPP-1b.
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Fig. S3. 1H NMR spectrum of the derivative Th-DPP-1c.
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Fig. S4. 1H NMR spectrum of the derivative Th-DPP-2a.
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Fig. S5. 1H–15N HMBC correlation 2D NMR spectrum of the derivative Th-DPP-2a.
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Fig. S6. 1H NMR spectrum of the derivative Th-DPP-2b.
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Fig. S7. 1H NMR spectrum of the derivative Th-DPP-2c.
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