Conformationally rigid molecular and polymeric naphthalene-diimides containing C6H6N2constitutional isomers

Article abstract of DOI:10.1039/d1tc00564b

Organic thin films based on naphthalenediimides (NDIs) bearing alkyl substituents have shown interesting properties for application in OLEDs, thermoelectrics, solar cells, sensors and organic electronics. However, the polymorphic versatility attributed to the flexibility of alkyl chains remains a challenging issue, with detrimental implications on the performances. Aryl analogues containing C6H6N2 constitutional isomers are herein investigated as one of the possible way-out strategies. The synthesis of molecular and polymeric species is described, starting from naphthaleneteracarboxyldianhydride with isomeric aromatic amines and hydrazine. The materials are fully characterized by spectroscopy, thermal and structural X-ray diffraction methods, both as bulk powders and thin films, revealing a rich structural landscape. Depending on the stereochemistry of the branching aryls, the compounds show a variety of parallel stacking of the NDI cores, and high structural stability upon heating, up to 560 °C in the polymeric form. Thin films prepared by spin coating from organic solvent solutions and studied by grazing-incidence X-ray diffraction exhibit a high degree of crystallinity indicating the intrinsic tendency of these molecules to self-assemble in an ordered fashion without the need for any post-processing technique. In line with other NDI-based diimides, UV-vis spectroscopy indicates the optical band gaps falling in the visible region (2.87-3.02 eV). DFT calculations reveal a significant lowering of the frontier orbital energies of the hydrazido derivative. Beyond solution processing, the high thermal stability and the absence of polymorphic forms of these materials suggest that sublimation-based routes for films and device preparation can also be followed. This journal is

Full text of DOI:10.1039/d1tc00564b

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Materials Chemistry C
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PAPER
Conformationally rigid molecular and polymeric
naphthalene-diimides containing C6H6N2
constitutional isomers†
Cite this: J. Mater. Chem. C, 2021,
9, 10875
a
a
b
Vincenzo Mirco Abbinante,
Gonzalo Garcıa-Espejo,
Gabriele Calabrese,
´
c
b
d
Silvia Milita,*b Luisa Barba,
Diego Marini,
Candida Pipitone,
d
a
Francesco Giannici,
Antonietta Guagliardi *e and Norberto Masciocchi
*
Organic thin films based on naphthalenediimides (NDIs) bearing alkyl substituents have shown
interesting properties for application in OLEDs, thermoelectrics, solar cells, sensors and organic
electronics. However, the polymorphic versatility attributed to the flexibility of alkyl chains remains a
challenging issue, with detrimental implications on the performances. Aryl analogues containing C6H6N2
constitutional isomers are herein investigated as one of the possible way-out strategies. The synthesis of
molecular and polymeric species is described, starting from naphthaleneteracarboxyldianhydride with
isomeric aromatic amines and hydrazine. The materials are fully characterized by spectroscopy, thermal
and structural X-ray diffraction methods, both as bulk powders and thin films, revealing a rich structural
landscape. Depending on the stereochemistry of the branching aryls, the compounds show a variety of
parallel stacking of the NDI cores, and high structural stability upon heating, up to 560 1C in the
polymeric form. Thin films prepared by spin coating from organic solvent solutions and studied by
grazing-incidence X-ray diffraction exhibit a high degree of crystallinity indicating the intrinsic tendency
of these molecules to self-assemble in an ordered fashion without the need for any post-processing
technique. In line with other NDI-based diimides, UV-vis spectroscopy indicates the optical band gaps
falling in the visible region (2.87–3.02 eV). DFT calculations reveal a significant lowering of the frontier
orbital energies of the hydrazido derivative. Beyond solution processing, the high thermal stability and
the absence of polymorphic forms of these materials suggest that sublimation-based routes for films
and device preparation can also be followed.
Received 4th February 2021,
Accepted 16th June 2021
DOI: 10.1039/d1tc00564b
rsc.li/materials-c
chemical functionalization, rapid solution processability of
pristine molecular entities, simple deposition of thin films and
1. Introduction
Organic molecular semiconductors are a class of compounds superb flexibility.3 After several years of fundamental studies, they
that have attracted a great deal of attention, in the form of thin have recently been incorporated into large commodities,4 and
films, for appealing applications in photovoltaics, optoelectronics, rapidly filled the market as flexible light panels, rollable solar
computing and sensing1 and, more recently, low-power thermo- cells, cell phone displays and battery-free artificial retina, to
electric generators.2 These compounds greatly benefit from easy mention a few.5
Among the most investigated species, a number of studies
focused on symmetrically disubstituted naphthalene diimide
a
(NDI)6 and perylene diimide (PDI)7 molecules, where the
presence of p–p stacked aromatic cores, in optimized
geometrical arrangements, enhances important functional
properties, such as electron conductivity above all.8 In particular,
NDI compounds (having a smaller aromatic core than the PDI
congeners) possess two strong electron-withdrawing imide
groups enabling several NDI-based systems to achieve relatively
low LUMO energies, excellent ambient stability, and promising
charge-transport properties. Nonetheless, very slight changes in
the nature or position of the side groups may result in a big
`
Dipartimento di Scienza e Alta Tecnologia & INSTM, Universita dell’Insubria,
via Valleggio 11, 22100 Como, Italy. E-mail: norberto.masciocchi@uninsubria.it
b Istituto per la Microelettronica e Microsistemi, Consiglio Nazionale delle Ricerche,
via Gobetti 101, 40129 Bologna, Italy. E-mail: milita@bo.imm.cnr.it
c Istituto di Cristallografia @ Elettra Synchrotron, Consiglio Nazionale delle
Ricerche, c/o Area Science Park, 34139 Basovizza, Trieste, Italy
d
`
Dipartimento di Fisica e Chimica, Universita di Palermo, viale delle Scienze, Ed.
17, 90128 Palermo, Italy
e Istituto di Cristallografia & To.Sca.Lab., Consiglio Nazionale delle Ricerche,
via Valleggio 11, 22100 Como, Italy
† Electronic supplementary information (ESI) available: Details of the 2D-GIXD,
structural and spectroscopic characterization. See DOI: 10.1039/d1tc00564b
This journal is © The Royal Society of Chemistry 2021
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difference in molecular packing and in material properties, as here, the constitutionally different Pigments Red 194 and Orange 43
observed, for example, in many of the NDI-based species molecules.21 It is noteworthy that an additional degree of freedom,
discussed in a previous paper.9 Indeed, once isolated as powders that is the occurrence of structurally induced mesomeric forms in
and films, alkyl chain derivatives show a remarkable structural organic semiconductors, has also been recently unveiled.22
flexibility, witnessed by a rich polymorphic behavior and
In the present work, the synthesis of a number of NDIs is
reversible crystal-to-crystal and crystal-to-mesophase transformations, presented, based on C6H6N2 isomers (see Chart 1). Their rich
with detrimental implications for the applications/devices.10 structural landscape is unveiled and deeply discussed based on
On the other hand, while the performances of these materials
are heavily dependent on the supramolecular assembly of the
constituent molecules in the solid state (mostly in thin films),11
predicting/devising the best performing geometry presently
remains a challenging task. Stimulated by these considerations,
we extended our interest to less flexible residues. Inspired by
the earlier reports on amino-bearing residues in fullerenes,
polyfluorenes and PDI derivatives,12 where the presence of the
amino group was not found prejudicial to materials properties,
here we focus our attention on NDI-based solids bearing
amino- (or hydrazido)-substituted benzenes, where the
presence of aromatic rings is expected to block, to some extent,
the mobility of the N-linked moieties, preventing ‘‘easy’’ poly-
morphic transformations under working conditions.
The choice of aromatic residues (here, in the form of amino-
substituted phenyls) is also encouraged by the increased
conjugation which can extend off the NDI core, enhancing
the electron mobility. To the best of our knowledge, studies
on pendant amino groups for NDI are scarce and mostly
limited to their use as fluorescent probes13 and in cancer
therapy.14 Although, admittedly, NH2 groups are prone to
oxidation and molecular degradation (more so in solution than
in the solid phase), our studies show that the species presented
here are stable in air up to (at least) 280 1C. Thus, amino-group
reactivity seems not to be a worrisome issue.
One appealing, and so far underexplored, complementary
aspect is the impact of the constitutional isomers considered in
the present work on the NDI structural landscape, i.e. that of
molecular entities possessing the same chemical formula but
with a slightly different atom connectivity in their periphery. It
is noteworthy that these NDI-materials sharing constitutional
isomers are not the result of non-stereo-/regio-selective
syntheses in which it is difficult to separate mixtures (as in the
commercially available PDI-Br215 and N140016 dyes); instead, by
using predefined aminoaromatic residues, they are prepared as
pure chemical entities with unique crystallographic phases.
While having the same chemical composition (as polymorphs
do), crystal forms determined by the presence of constitutional
isomers possess distinct physical and chemical properties from
those one would expect in (conformationally different)
polymorphic forms, with additional upstream control and
greater stability. Similar considerations would apply as well to
the case of configurational stereoisomers, pointing to the
importance of understanding and addressing the selective
formation of solid organic materials of isomeric species in organic
electronics,17 as much as it is considered in pharmaceutical and
pigment industries.18 Notable examples in these industrially
relevant fields are the polymorphism of rubrene for efficient
OLEDs,19 stereoisomerism of amphetamine20 and, for what matters
Chart 1 Molecular sketches and labeling of the species discussed in this
work.
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the combination of multiple structural characterization (300 mg, 1.118 mmol, 1 eq.), 1,3-phenylenediamine (1.209 g,
methods including X-ray powder diffraction, spectroscopic 11.18 mmol, 10 eq.) and 20 mL of anhydrous MeOH were added
and thermal analyses, DFT calculations, and variable under a N2 atmosphere. The reaction mixture was heated at the
temperature diffraction methods. UV-Vis reflectance spectra refluxing temperature for 20 h. The mixture was then cooled to
on powders and grazing-incidence X-ray diffraction analysis rt and another 15 mL of MeOH were added leaving the mixture
of spin-coated films are presented with a general discussion of under stirring for an additional one hour. The suspension was
the properties and the possible processability routes of these filtered and NmpDI was obtained as a light brownish solid in
materials.
83% yield (417 mg, 0.929 mmol). 1H-NMR (400 MHz, DMSO-d6,
25 1C): d 8.76 (s, 2H), 7.22 (t, J 7.6 Hz, 1H), 6.74 (d, J 8 Hz, 1H),
6.64 (s, 1H), 6.59 (d, J 7.6 Hz, 1H), 5.34 (s, 2H); 13C-NMR (100
MHz, DMSO-d6, 25 1C): d 162.77, 149.47, 136.14, 130.37, 129.16,
126.94, 126.65, 115.85, 114.17, 113.87. Calc. for C26H16N4O4
(%): C, 69.6; H 3.6, N 12.5; Exp. C 68.9, H 3.8, N 12.4. m/z (ESI†):
447.3 (M ꢀ H)ꢀ, calc. for C26H15N4O4 : 447.4.
2. Experimental section
(a) Materials
Reagents and solvents were purchased from Merck (p-phenyle-
nediamine, 498%; m-phenylenediamine, 99%; o-phenyle-
nediamine, 498%; phenylhydrazine, 497%; benzoic acid,
499%; methanol, 99.8%; dimethylformamide, 99.8%; toluene,
99,8%; dimethylacetamide, 499%; m-cresol, 99%), BLD
(1,4,5,8-naphthalentetracarboxylic acid monoanhydride, 98%)
and Riedel-de-Haen (triethylamine, 99%) and used without
further purification. All reactions were performed under a N2
atmosphere.
1,4,5,8-Naphthalenetetracarboxyl-o-aminophenylenedimide
(NopDI). To a round bottom flask equipped with a stirrer and
condenser, 1,4,5,8-naphthalenetetracarboxylic dianhydride
(300 mg, 1.118 mmol, 1 eq.); 1,2-phenylenediamine (1.209 g,
11.18 mmol, 10 eq.) and 20 mL of anhydrous MeOH were added
under a N2 atmosphere. The reaction mixture was heated at the
refluxing temperature for 16 h. The mixture was then cooled to
rt and another 15 mL of MeOH were added, leaving the
suspension under stirring for 1 h. The mixture was filtered
and impure NopDI was obtained as a brownish solid (325 mg).
¨
(b) Syntheses of the materials
1,4,5,8-Naphthalenetetracarboxylic dianhydride (NDA). NDA The procedure was also repeated by changing the reaction time
was prepared using the synthetic method reported in ref. 23. from 16 to 4 h but, also in this case, the formation of the by-
Into a 500 mL three-necked flask, 25 g (0.087 mol) of 1,4,5, products could not be avoided. Further shortening the reaction
8-naphthalenetetracarboxylic acid monoanhydride, 50 mL of time to 1 h only led to incomplete NDA conversion, together
DMA and 100 mL of dry toluene were added under a N2 with the formation of a mixture of the condensation products.
atmosphere. The mixture was kept at 111 1C and stirred at this
1,4,5,8-Naphthalenetetracarboxyl-phenylhydrazinedimide
temperature for 2 h. Then, the flask was cooled to 10 1C, and (NphDI). NphDI was prepared following an already published
stirred overnight. Filtration gave 23.5 g (0.087 mol, quantitative procedure.26 Phenylhydrazine (2.199 mL, 22.371 mmol, 3 eq.)
yield) of 1,4,5,8-naphthalenetetracarboxylic dianhydride as a was transferred in a 250 mL round bottom flask, equipped with a
1
yellow solid. H-NMR (400 MHz, DMSO-d6, 25 1C): d 8.71 (bs, stirrer and condenser, containing 1,4,5,8-naphthalene-tetra-
4H). Calc. for C14H4O6 (%): C, 62.7; H 1.5; Exp. C 62.5, H 1.8. m/z carboxylic dianhydride (2 g, 7.457 mmol, 1 eq.) in 100 mL of
(ESI†): 299.0 (M + CH3OH ꢀ H)ꢀ, calc. for C15H7O7 : 299.2.
anhydrous DMF under a N2 atmosphere. The reaction mixture
1,4,5,8-Naphthalenetetracarboxylic p-aminophenylenedimide was heated at 140 1C for 6 h and then cooled to rt. DMF was
(NppDI). NppDI was prepared following the already published removed under vacuum and the reaction crude was dissolved in
procedures.24,25 To a round bottom flask equipped with a stirrer 100 mL of EtOAc and washed with 150 mL of water. The organic
and condenser, 1,4,5,8-naphthalenetetracarboxylic dianhydride layer was washed with a brine solution (1 ꢁ 100 mL). EtOAc was
(1 g, 3.73 mmol, 1 eq.), 1,4-phenylenediamine (4.033 g, then evaporated under vacuum and the residue was suspended
37.3 mmol, 10 eq.) and then 67 mL of anhydrous MeOH were in water, filtered, and washed with water, acetone and then with
added under a N2 atmosphere. The reaction mixture was heated CH2Cl2. NphDI was obtained in 65% yield (2.18 g, 4.861 mmol)
at the refluxing temperature for 20 h. The mixture was then as a light brownish powder. 1H-NMR (400 MHz, DMSO-d6,
cooled to rt, filtered and the brown solid was suspended in 25 1C): d 8.83 (s, 2H), 8.81 (s, 1H), 7.24 (m, 2H), 6.89 (m, 3H);
50 mL of MeOH for 1 h. The suspension was filtered and dried 13C-NMR (100 MHz, DMSO-d6, 25 1C): d 162.16, 147.02, 131.03,
under vacuum. NppDI was obtained as a brownish solid in 78% 128.83, 126.98, 119.63, 112.62. Calc. for C26H16N4O4 (%): C, 69.6;
yield (1.310 g, 2.90 mmol). 1H-NMR (400 MHz, DMSO-d6, 25 1C): H 3.6, N 12.5; Exp. C 69.4, H 4.2, N 11.9. m/z (ESI†): 447.3 (M ꢀ H)ꢀ,
d 8.69 (s, 4H), 7.02 (d, J 8.4 Hz, 4H), 6.67 (d, J 8.4 Hz, 4H), 5.30 calc. for C26H15N4O4 :447.4.
(s, 4H); 13C-NMR (100 MHz, DMSO-d6, 25 1C): d 163.21, 148.77,
Poly-1,4,5,8-naphthalenetetracarboxyl-p-phenylenedimide
130.40, 129.09, 126.96, 126.56, 123.24, 113.68. Calc. for (poly-NppDI). p-Phenylenediamine (60.4 mg, 0.559 mmol, 1 eq.)
C26H16N4O4 (%): C, 69.6; H 3.6, N 12.5; Exp. C 68.9, H 3.8, N was placed in a two-neck round bottom flask equipped with a
12.1. m/z (ESI†): 447.3 (M ꢀ H)ꢀ, calc. for C26H15N4O4 : 447.4.
stirrer and condenser under a N2 atmosphere. Then, 2.5 mL of
1,4,5,8-Naphthalenetetracarboxyl-m-aminophenylenedimide m-cresol, and subsequently triethylamine (TEA, 120 mL,
(NmpDI). To a round bottom flask equipped with a stirrer and 0.866 mmol, 1.55 eq., dropwise) were added. The reaction
condenser, 1,4,5,8-naphthalenetetracarboxylic dianhydride mixture was kept at 160 1C for 5 min and then 1,4,5,8-
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naphthalenetetracarboxylic dianhydride (150 mg, 0.559 mmol, structure solution were collected in the 3–1051 2y range for all
1 eq.), benzoic acid (273.08 mg, 2.236 mmol, 4 eq.) and another samples (3–751 for poly-NppDI), sampling at 0.021, with scan
4.2 mL of m-cresol were added. The reaction was left under time lasting approximately 16 h.
stirring at 175 1C for 15 h and then at 195 1C for an additional 3
Variable temperature PXRD analysis. Thermodiffractometric
hours. The mixture was then cooled to room temperature, experiments were performed from 30 to 540 1C (580 1C for
filtered and washed with methanol, H2O, acetone, and CH2Cl2 poly-NppDI). Powdered batches were deposited in the hollow
and dried in air, obtaining 176 mg of a yellow powder. Due to space of an aluminium sample holder of a custom-made
the very limited solubility in DMSO, very weak NMR signals heating stage (Officina Elettrotecnica di Tenno, Ponte Arche,
were measured: 1H-NMR (400 MHz, DMSO-d6, 25 1C): d 9.19 Italy). Diffractograms were acquired in air, in the most significant
(s, 2H), 8.82 (s, 1H), 8.77 (s, 1H), 8.51 (s, 1H), 7.64 (d, 2H), 7.03 (low-angle) 3–301 2y range, under isothermal conditions in 20 or
(m, 2H), 6.69 (S, 1H), 6.56 (m, 1H). Calc. for C20H8N2O4 (%): C, 30 1C steps. Since in powder diffraction experiments the samples
70.6; H 2.4, N 8.2; Exp. C 66.3, H 2.9, N 6.9.
are in direct contact with air and some thermal drifts/gradients
are present, the accurate phase transition temperatures are
calibrated using TG measurements.
(c) Physico-chemical characterization
UV-Vis spectroscopy. The UV-Vis reflectance spectra of the
powders were acquired in the 200–800 nm range, using a
(d) Preparation of thin films
Shimadzu UV-2600 spectrophotometer. BaSO4 was used as the The NDI derivatives were used as obtained from the washing
reflectance standard and the measurements were performed on and purification processes after their respective synthesis. For
pelletized powders. The Kubelka–Munk function F[R] was each compound, the solvent and the concentration of the
calculated using the reflectance spectrum R(E), where E is the solution were chosen close to the saturation limit. The least
photon energy, using the F[R] = (1 ꢀ R)2/2R relationship. Taking soluble NppDI derivative was dissolved in N-methyl-2-
F[R] as a representative of the sample absorbance spectrum, pyrrolidinone (NMP) and acetone (volume ratio 3 : 7) at a
extrapolation of the linear portion of the (F[R]*E)2 vs. E on the E concentration of 1 mg mLꢀ1, while the NmpDI and NphDI
axis provided experimentally accessible optical direct band gap solids were dissolved in N,N-dimethylformamide (DMF), with
values.
concentrations of 17 and 30 mg mLꢀ1, respectively. Thin films
NMR characterization. Solution 1H and 13C (APT) NMR were prepared by spin-coating 20 mL of solution onto 1 ꢁ 1 cm2
spectra were recorded in d6-DMSO at 400 and 100 MHz, respec- silicon substrates covered with a 300 nm-thick thermal silicon
tively, using a Bruker Avance 400 spectrometer (Fig. S1–S4, ESI†). oxide layer. Before spin coating, the SiO2/Si substrates
1H and 13C NMR data are reported as chemical shifts (in ppm, were cleaned sequentially for 5 minutes in acetone, ethanol,
internal TMS standard), multiplicity (s = singlet, bs = broad isopropyl alcohol and deionized water, and blow-dried with
singlet, d = doublet, t = triplet, and m = multiplet), coupling nitrogen gas. Finally, the substrates were etched by Ar/O2
constants (in Hz) and integration. The poor solubility of our plasma for 15 minutes (flux ratio of 3 : 1), aiming at improving
materials (see Table S1, ESI†) prevented NMR data collection in their surface wettability.
other organic solvents, and, similarly, limited the choice of
experimental conditions for other solution studies, such as the deposition of thin organic films for device fabrication
voltammetric measurements. (chloroform and chlorobenzene being much more widely
Though NMP and DMF are solvents not commonly used for
MS characterization. Mass spectra (Fig. S5–S8, ESI†) were employed), reports on thin film processing with less volatile
recorded on a Thermo Scientific, Q-Exactive Plus System using solvents (surprisingly including also methanesulfonic acid)
1 mg mLꢀ1 of centrifuged CH3OH solutions.
have appeared.27 Additionally, the high thermal stability and
Thermal analyses. Thermogravimetric (TG) and differential the absence of polymorphic phases of these molecular solids,
scanning calorimetric (DSC) traces were acquired from 30 to later discussed, suggest that sublimation processing of thin
900 1C (with a scan rate of 10 1C minꢀ1) using a STA 409 PC films and devices is a further viable option.
Luxxs analyser (Netzsch) under a nitrogen flow or in air and
For each film, the spin coating conditions were tuned to
alumina sample holders equipped with a pierced lid (Fig. S9, optimize the film homogeneity and reproducibility. The NppDI
ESI†). solution was stirred and heated up to 170 1C for 48 h before
Electrochemical characterization. The samples were dissolved being centrifuged at 104 rpm for 5 minutes to separate the
in DMSO (typically 0.15 mg mLꢀ1) and 0.1 M NBut4PF6 was suspended powders from the solution. The NppDI films were
added as the electrolyte. Sweeps at 0.1 V sꢀ1 rate were performed spin-coated at 1000 rpm for 30 s followed by a treatment at
in the ꢀ1.5 to +1.5 V interval. Ag/AgCl was used as the reference 5500 rpm for 30 s. For variance, the NmpDI and NphDI
electrode, taken at 4.4 eV below the zero vacuum-level (Fig. S10, solutions were heated up to ca. 130 1C for 10 minutes until a
ESI†).
complete dissolution of the powders was achieved. The NmpDI
X-ray powder diffraction (PXRD). PXRD measurements were films were spin-coated at 1500 rpm for 30 s, while the NphDI
performed using a Bruker AXS D8 Advance diffractometer in films were spin-coated at 1000 rpm for 30 s, followed by
Bragg–Brentano y : y geometry, equipped with
a Lynxeye 2500 rpm for 30s. All films were left to dry at room temperature.
position sensitive detector. DS: 0.51; generator setting: 40 kV, Each sample has been investigated both as fabricated and after
40 mA; Ni-filtered Cu-Ka radiation, l = 1.5418 Å. PXRD data for thermal annealing in air at 180 1C for 10 minutes.
10878 | J. Mater. Chem. C, 2021, 9, 10875–10888
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(e) X-ray structural studies
Ab initio crystal structure solution from X-ray diffraction
data. PXRD structure solution of the NppDI, NmpDI, NphDI
and poly-NppDI phases was performed using the TOPAS-R28
software. Standard peak search methods followed by profile
fitting allowed the accurate estimation of the low-angle peak
position. These values, through the SVD indexing algorithm,29
provided primitive orthorhombic or monoclinic cells [a = 26.12,
b = 5.16, c = 7.83 Å, b = 101.41, and GOF(20) = 16.7 for NppDI; a =
31.96, b = 8.53, c = 7.44 Å, and GOF(23) = 19.9 for NmpDI; a =
20.94, b = 5.39, c = 9.28 Å, b = 96.11, GOF(29) = 32.1 for NphPDI;
a = 10.74, b = 12.64, c = 5.02 Å, and GOF(10) = 20.1 for poly-
NppDI]. Space group determination through the analysis of
systematic absences after cell reduction indicated, for the four
phases, P21/c, Pbca, P21/n and Pnnm, respectively, later con-
firmed by successful structure solution and refinement. Density
considerations enabled the determination of the molecular
symmetry, the Z0 value and the content of the asymmetric unit.
Structure solution of the four molecular crystals was performed
using the Monte Carlo/simulated annealing technique using a
rigid model (flexible at the imide link) described by the
Z-matrix formalism with standard geometrical parameters.
The paucity of the diffraction peaks in the poly-NppDI PXRD
trace, their breadth and severe overlap, did not enable (semi)-
automatic structure solution. A few competitive structural
models were built through sensible stereochemical considerations
and tested against the experimental pattern, eventually validating a
set of collinear alternate copolymers of NDI/C6H4 fragments as the
best model. The accuracy of the refined metrical and structural
descriptors is less than ideal. Nevertheless, the material
periodicity and its main stereochemical features are (reasonably)
well addressed by the present model.
The final refinements were eventually carried out by the
Rietveld method, maintaining the rigid bodies introduced at
the structure solution stage (see Fig. 1 and Fig. S11, ESI†)
The background was modelled by a polynomial function of
the Chebyshev type and the peak profiles were described by the
Fundamental Parameters Approach30 and a common (refinable)
isotropic thermal factor was attributed to all atoms. March–
Dollase correction for preferred orientation31 was applied in the
form of g(hkl) and quoted below. Fractional atomic coordinates
and crystal structure details were deposited with the CCDC
(CSD Codes 2059769–2059772).† Specific sample-dependent
issues are discussed in the Results section.
Fig. 1 Final Rietveld refinement plots, with difference plot and peak
markers at the bottom. The observed and calculated curves (blue and
red traces, respectively) have been vertically shifted for clarity.
b = 96.132(5)1, V = 1044.2(1) Å3, Z = 2, rcalc = 1.426 g cmꢀ3
,
m(Cu-Ka) = 8.2 cmꢀ1, g(100) = 1.13; Rp and Rwp, 0.048 and 0.067
respectively, 7–1051 2y range. RBragg = 0.033.
Crystal data for NppDI. C26H16N4O4, fw = 448.43 g molꢀ1
orthorhombic, P21/c, a = 7.8185(4), b = 5.1566(2), c = 25.727(2) Å,
,
Crystal data for poly-NppDI. [C20H8N2O4]n fw = n ꢁ 340.30 g molꢀ1
(n ca. 8), orthorhombic, Pnnm, a = 10.82(2), b = 5.04(1), c =
12.58(2) Å, V = 685(3) Å3, Z = 2, rcalc = 1.65 g cmꢀ3, m(Cu-Ka) =
9.8 cmꢀ1, Rp and Rwp, 0.062 and 0.072 respectively, 8–701 2y
range. RBragg = 0.025.
b = 95.918(3)1, V = 1031.7(1) Å3, Z = 2, rcalc = 1.444 g cmꢀ3
,
m(Cu-Ka) = 8.2 cmꢀ1, g(010) = 1.16; Rp and Rwp, 0.082 and 0.109
respectively, 6–1051 2y range. RBragg = 0.047.
Crystal data for NmpDI. C26H16N4O4, fw = 448.43 g molꢀ1
,
orthorhombic, Pbca, a = 32.012(9), b = 8.5412(8), c = 7.443(1) Å,
V = 2035.0(7) Å3, Z = 4, rcalc = 1.464 g cmꢀ3, m(Cu-Ka) = 8.4 cmꢀ1
,
(f) Grazing incidence X-ray diffraction (GIXD) studies
g(100) = 1.22; Rp and Rwp, 0.072 and 0.102 respectively, 5–1051
2y range. RBragg = 0.038.
GIXD measurements were performed at the XRD1 beamline of
the ELETTRA synchrotron radiation facility in Trieste (Italy). l =
1.00 Å; beam size: 200 ꢁ 200 mm2. 2D-GIXD patterns were
Crystal data for NphDI. C26H16N4O4, fw = 448.43 g molꢀ1
,
monoclinic, P21/n, a = 20.956(2), b = 5.3945(3), c = 9.2901(6) Å,
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collected by using a 2 M Pilatus silicon pixel X-ray detector in the reaction mixture (see Chart S1, ESI†). Not being possible to
(DECTRIS Ltd) positioned perpendicular to the incident beam, isolate a single (pure) crystal phase of NopDI, its full structural
350 mm away from the sample. Incidence angles of the X-ray and analytical characterization was abandoned. It is worth noting
beam with the film surface of ai = 0.051 and 0.11 were chosen to that such o-aminophenylene condensation has been used in the
probe the uppermost film layers (the X-ray penetration depths synthesis of BBL, or poly(benzimidazobenzophenanthroline), a
are ca. 4 and 8 nm, for ai = 0.051 and 0.11 respectively), as well soluble polymer somewhat reminiscent of both NopDI and
as ai = 0.21 to probe the full film thickness (penetration depth: poly-NppDI), which has recently attracted the attention of many
ca. 30 mm). During the measurements, the samples were slowly for its excellent performances in n-channel OFETs.27c
rotated around the substrate normal (1801 in 60 s), to average
Finally, aiming at enhancing the thermal stability and the
signals originating from laterally inhomogeneous film regions. conductive properties of this class of materials, the polymerization
Both as-prepared and annealed films were investigated to study capability of the NppDI molecule was explored through the
the effect of thermal annealing on film crystallinity and synthesis of a polymeric species, poly-NppDI. A one-pot reaction
mosaicity.
using a 1 : 0.8 NDA : p-diamino-benzene molar ratio proved to
The visualization, analysis of the 2D-GIXD patterns and be unsuccessful (a complex mixture of products was obtained).
intensity profile extraction were performed using GIDVis Co-polymerization with NDA was also tried by reacting NDA and
software.32
NppDI monomers overnight in 1 : 0.8 ratio in refluxing toluene, but
this attempt also failed.
(g) Quantum-chemical calculations
However, taking inspiration from polymerization reactions
of NDA with a series of aromatic diamines,39 suitable conditions
for the obtention of the sought copolymer were unearthed.
Employing m-cresol as the solvent (a liquid with a high boiling
point and solvent power) and with the crucial usage of two
additives (excess benzoic acid – 4 eq. – and TEA – 1.5 eq.),
poly-NppDI was synthesized with a 93% yield. The role of these
additives can be explained by benzoic acid coordination to the
NDA oxygen atoms, making carbonyl groups more susceptible to
the nucleophilic attack, and by TEA-induced avoidance of
proton transfer from the benzoic acid to p-diaminobenzene.
Quantitative yields were obtained when employing 1 eq. of
p-diaminobenzene as the unique nucleophile and heating at
175 1C for 15 h and then at 195 1C for an additional 3 hours. poly-
NppDI appeared to be a nanosized material by X-ray diffraction
analysis. Its structural analysis (later discussed) confirmed the
Ground state geometry optimization and HOMO–LUMO energy
calculations for NphDI, NmpDI and NppDI were performed
with the Gaussian16 package,33 using the nonlocal hybrid
Becke three-parameter Lee–Yang–Parr (B3LYP) functional34,35
and the 6-31G(p,d) basis set. The experimentally determined
crystal structure of each compound was used as a starting point
for geometry optimization. The method and the chosen basis
set were validated against the already published LUMO value of
NphDI and HOMO/LUMO values of fluorinated analogues,36
with absolute differences of less than 0.1 eV.
3. Results and discussion
(a) Synthesis
The syntheses of the NppDI24,25 and NphDI23 molecules were formation of the sought material, though the average chain
already reported, though their characterization was limited to length is limited to a few (alternating) NDA/p-diaminobenzene
standard spectroscopic and (for NphDI) electrochemical monomers.
methods. In the present work, we have adopted the synthetic
approach of ref. 24 for what concerns NppDI, whereas some
(b) Crystal structures
modifications of the procedure reported in ref. 26 were applied The crystal structures of the NppDI, NmpDI and NphDI
to NphDI, regarding changes in the work-up procedure molecular phases contain symmetrically disubstituted NDI
(see details in the Experimental section). In particular, cores, lying about inversion centers in monoclinic or orthor-
instead of dissolving the extracted crude material in THF hombic space groups, with Z0 = 0.5. Their molecular structures
and subsequently adding water to crystallize the product at and crystal packings are shown in Fig. 2, and the relevant
4 1C overnight, the pure product was obtained upon washing stereochemical features are shown in Table 1, where the
the extracted reaction crude material with water, acetone and structural analogues 1,4,5,8-naphthalenetetracarboxyl-p-CH3-
then CH2Cl2, thus improving the product yield from 43%26 benzyldimide (NhbDI) and -p-CF3-benzyldimide (NfbDI) are
to 65%.
also included.40 As detailed in the Experimental Section, the
The syntheses of the NDI cores branched with m- and structural models presented here make use of rigid body
o-substituted diaminobenzenes (NmpDI and NopDI), not present descriptions of the stiff portions of the molecules. Accordingly,
in the scientific literature, were performed in strict similarity reliable structural information is mostly confined in the con-
with the protocol used for the p-regioisomer. NmpDI was easily formational freedom (the orientation of the branching residuals),
recovered as a pure solid in high yields (480%). The synthesis of in the intermolecular effects (the stacking periodicity) and in the
the NopDI molecule, attempted in several ways, mostly modifying degree of overlap of parallel, adjacent, NDI cores. With reference
temperature solvents and reaction times, proved to be very to the stacking periodicity, while for aliphatic substituents the p–p
difficult. Indeed, two ring-closure adducts, known as Pigment stacking distances were typically around 3.4
Orange 4337 and Pigment Red 194,38 were systematically detected (very similar to the interlayer interaction in graphite, 3.35 Å),41
Å
or lower9
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Fig. 2 Schematic drawings of the molecular structure and crystal packings of (a and b) NppDI, (c and d) NmpDI and (e and f) NphDI, together with a note
of their orientations. In (g), the polymeric poly-NppDI structure is shown. Color codes: C in grey, N in blue, O in red and H in white. Graphics realized by
Schakal42 (a, c and e) and Mercury43 (b, d, f and g).
in the present case this value increases to 3.5 Å or above for phenylhydrazine derivative (NphDI) shows a short(er) NDI–core
NppDI monomeric and polymeric derivatives, whereas parallel interaction.
stacking does not occur in the monoclinic crystal structure of
As per the relative slip of the aromatic core, for all crystal
NmpDI. In the latter case, the aromatic cores, stretching out in phases studied here, the stereochemical descriptors (which in
the approximate [102] direction, stack in herringbone geometry, aliphatic systems and in NhbDI and NfbDI as well, have found
tilted away one from the other by ca. 481 (see Fig. S12, ESI†). to cluster into two specific portions of the w, c plot)9 are more
Further comparative geometrical descriptors of NppDI vs. widely spread here (see Fig. 3). These findings indicate that the
NmpDI are the tilt angles of the aromatic cores vs. the lamellar most relevant interactions driving the formation of the stacks
stacking (100) [63.21 vs. 29.71] and the thickness of the are not anymore assignable to the NDI cores; instead, they must
molecular slabs [o11.5 vs. 16.0 Å], witnessing the more be attributed to the interlocking of the rigid (aromatic)
sprawled disposition of the NDI cores in NppDI. Only the branches. The geometrical degree of overlap (not the orbital
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Table 1 Relevant geometrical features for the crystal phases studied in this work. t is the NDI-R torsional angle, d is the interplanar distance of parallelly
stacked NDI-cores, w and c are the stereochemical descriptors defined in ref. 9, and H-bonds are the shortest intermolecular H-bond contacts. Egap
corresponds to the optical band gap energies derived as described in the Experimental section
Species
t, 1
d, Å
w, 1
c, 1
H-Bonds, Å
DoO (eqn (1))
Egap, eV
NppDI
NmpDI
82.6
78.4
84.7
80.0
78.6
63.0
—
3.63
no p–p
3.38
3.54
3.40
3.50
3.27–3.42
46.7
—
49.2
67.1
73.7
90.0
68.4–80.5
55.9
—
64.9
46.2
39.8
44.0
42.2–53.4
3.11 (Nꢂ ꢂ ꢂN) 3.22 (Nꢂ ꢂ ꢂO)
0.46
—
0.63
0.40
0.58
0.69
0.62–0.72
2.87
3.01
2.94
n.a.
n.a.
2.90
None
NphDI
3.06 (Nꢂ ꢂ ꢂN)
NhbDI40
NfbDI40
None
None
None
—
Poly-NppDI
Aliphatic NDI
the growth of microstructured materials, and very broad peaks
are observed in the PXRD trace. Thanks to the knowledge of the
stereochemical feature of the NppDI molecular counterpart, it
was possible to define a sensible structural model using
symmetry restraints and geometrical considerations, later
refined against the PXRD data using the standard Rietveld
technique.
Our result revealed the occurrence of ca. 10 nm chains of
elliptical section (suggesting a sequence of 8 NDI/1,4-phenylenes
along the c axis and an average molecular weight of 2700 Dalton),
packed in the crystal in a herringbone motif and a pseudo-
hexagonal lattice (Fig. 2d), reminiscent of smectic E liquid
crystalline phases. The fact that these chains are not extremely
long is also witnessed by the less-than-ideal elemental analysis
values reported in the Experimental section, where the C/N
molar ratio is slightly above 11 (and should be 10 in an infinite
chain), as if the chains were terminated by NDA, and not by
p-phenylenediamine residues.
Fig. 3 Stereochemical descriptors w,c for the p–p stacking of the crystal
phases of this work (red dots, values from Table 2) and the aliphatic
analogues (black dots) reported in ref. 9.
one) defined by:
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
DoO ¼ 1 ꢀ cos2w ꢀ cos2c
(1)
(c) Thermal stability
measures the complete (DoO = 1.0) or partial (0 o DoO o 1.0)
superimposition of neighboring NDI cores along the direction
orthogonal to the molecular plane. The values in the range
0.63–0.72 are typical for aliphatic substitutions and are also
found for the molecular NphDI and polymeric poly-NppDI
crystals. In contrast, in the molecular NppDI and NhbDI
species (DoO o 0.46), the NDI cores overlap to a minimal
extent; these compounds must be considered definite outliers
in the set of symmetrically substituted aliphatic and
aromatic NDIs.
The thermal stabilities of the di- and poly-imide species were
studied using the TG plots (Fig. 4) and the structural variations
occurring upon heating were followed by variable-temperature
X-ray experiments (Fig. 5). For NppDI, NmpDI, NphDI and
poly-NppDI bulk samples, isolated in powdered form, the
PXRD patterns were acquired from room temperature up to
the decomposition temperature, measured by TG to occur
near 300 1C for NphDI and at much higher temperatures
for the other materials. Comparing TG and PXRD data, no
Two of these materials, NmpDI and poly-NppDI, require a
special set of comments. NmpDI crystallizes forming extended
slabs running in the bc plane (thus normal to a), weakly bound by
hydrogen bonds of the N–HꢂꢂꢂN type. Given that the m-positioned
ꢀNH2 residue was found to be crystallographically disordered in
two positions roughly with a 1 : 3 ratio, this brings about a lack of
ideal periodicity and the occurrence of a (possibly conditional)
stereochemical local preference, with localized defects of the 2D
type. This hypothesis is also partially confirmed by the less-than-
ideal Rietveld fit shown in Fig. 1, possibly manifesting the slight
misorientation of each molecule in the crystal from the average
one, i.e. smoothing out the structural misfits.
Fig. 4 TG traces collected under N2 at a scan rate of 101 minꢀ1: NppDI in
black; NmpDI in magenta; NphDI in green; poly-NppDI in blue.
Finally, poly-NppDI is a rare example of a crystalline polymer
of NDI-based materials. Its extreme insolubility probably limits
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and partial (and differential) decomposition (to CO or CO2) of
the carbonyl residues. Of much simpler description is the TG
curve of the polymeric poly-NppDI phase: the observed 41.2 w%
loss is easily reproduced by the elimination of two C2O2N
fragments (40.9%), leaving behind only condensed polyaromatic
hydrocarbons.
The coefficients of thermal expansion were determined
using the cell parameter variations derived from the structure-
less Le Bail fitting using the linear approximation:
p(T) = p(To)[1 + kp (T ꢀ To)]
(2)
where p = a, b, c, (b) or V; T and T0 are the actual and initial
temperatures and kp is the pertinent linear (or volumetric)
thermal expansion coefficient. The entire set of measured
datapoints could be reproduced by the mathematically
equivalent Dp/pRT = kpDT; and the resulting kp values and their
uncertainties are shown in Table 2.
The kp values, spatially visualized in Fig. 6, are highly
anisotropic, in line with the crystal symmetries of these materials.
This is particularly true for poly-NppDI, where two nearly null
coefficients are observed along the b and c axes, at the expenses of
a very large value measured along a.
Additionally, the very high decomposition temperature of
poly-NppDI powders (onset near 615 1C) closely matches that of
films of the 2,3,6,7 regioisomer44 (measured by Td5, the 5%
weight loss temperature – 590 1C in N2 and 580 1C in air),
confirming the extreme thermal stability of aromatic polyimide
(co)polymers.45 The oxidative stability of all species was also
established by performing TG/DSC measurements in air
(see Fig. S9, ESI†).
(d) Spectroscopic analysis
The powdered materials appear light brown in color and, as
expected, show ‘‘absorbance’’ spectra, spanning the wide 400–
600 nm interval (Fig. 7). Such behavior is in line with other
NDI-based diimides, though a wider band is observed here.
Since most of the studied species (NphDI excluded, for the
presence of an sp3 hybridized N atom) may manifest extended
charge delocalization onto the aromatic branching residues,
the occurrence of several accessible states with low-energy
separation is in agreement with the observed broad absorption
band falling in the visible region.
Fig. 5 Variable temperature XRD scans (2y in the 3–301 range), from RT
(bottom) to 420, 420, 300, and 460 1C, respectively.
polymorphic changes upon heating were observed. This is in
striking difference with many aliphatic congeners, where the
occurrence of several crystalline and liquid crystalline phases
was the rule more than the exception.9
The observed mass variations are not easy to interpret. In all
three molecular compounds a double decomposition step is
observed, accounting for a total of 52 to 61 w% loss. These
values are in line with the elimination of the imidic branches
The steep increase observed in proximity to 400 nm, on the
verge of the violet-UV range, has been interpreted by the
Table 2 TG inflection points and the corresponding mass loss, and the linear and volumetric thermal expansion coefficients (10ꢀ6
linear regression of the PXRD data acquired in the 30–300 1C range
K
ꢀ1), calculated by
Species
TG inflection, 1
Mass loss, %
ka
kb
kc
kb
kV
DLDa
NppDI
NmpDI
NphDI
520.0 (624.2b)
460.3 (594.2b)
334.8 (464.9b)
643.0
35.2 (60.9b)
22.0 (51.9b)
25.4 (57.4b)
41.2
51(4)
37(3)
26(2)
109(3)
ꢀ34(1)
19(1)
104(4)
ꢀ4(3)
51(2)
55(2)
11(2)
ꢀ1(3)
79(5)
—
88(7)
0.0126
0.0068
0.0123
0.0113
112(3)
150(2)
104(6)
ꢀ46(1)
Poly-NppDI
—
a
Note: DLD is the degree of lattice distortion, or the spontaneous strain, that is the square root of the sum of squared eigenvalues of strain tensor
b
calculated for the 30–3001 range, divided by 3. Second inflection point and the total mass change after the second event.
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(not manifesting p–p stacking in the solid) and NphDI (where
significant bond conjugation through the N–N bond can easily
be excluded). As reference values, for markedly unconjugated
residues, the typical optical bandgap falls near 3.0 eV.46
(e) Electronic structure
As reported in Fig. 8, the HOMO and LUMO, derived from DFT
calculations, are mostly localized on the side aromatic rings
and on the naphthalene-diimide aromatic core, respectively.
In NppDI and NmpDI, the HOMO levels are aligned (at ca.
ꢀ5.6 eV); since they differ only for the (remote enough) position
of the amino group on the side rings, this has negligible
influence on the HOMO energy. Since the polyaromatic core
is the same in both compounds (see the optimized geometries
in Fig. S15 (ESI†), coordinates are provided in Tables S2–S4,
ESI†), the energies of the LUMO differ by less than 0.1 eV
(ꢀ3.15 and ꢀ3.23 eV). This finding can likely be ascribed to the
difference in dihedral angles (reported in Table S5, ESI†),
affecting a partial, though minor, electronic conjugation. On
the other hand, in NphDI, the energies of both HOMO/LUMO
are lowered (ꢀ5.95/ꢀ3.63 eV, in line with those reported in ref.
26) and favor 1eꢀ reduction to the corresponding radical anion,
useful for sensing and for the generation of n-type organic
semiconductors. These results suggest that the HOMO/LUMO
Fig. 6 Visualization of the thermal expansion tensors, calculated by
comparing the lattice metrics at RT and at 300 1C. Positive lobes in green, energy is sensitive to nitrogen substituents only when they are
(tiny) negative ones in red.
directly linked to the core rings. Experimentally measured
optical band gaps (measured in the solid) deviate by ca. 0.4–
0.6 eV from the DFT-calculated molecular ones (known to vary
depending on the basis-set used47), an effect which was already
observed in a large variety of organic semiconductors,48 and
also in strictly related NDI-based molecules and congeners
(up to ca. 0.7 eV).49
To experimentally corroborate our DFT simulations, cyclic
voltammetric measurements of NppDI, NmpDI and NphDI
(in DMSO) were performed (see Fig. S10, ESI†) and the data
Fig. 7 Absorbance spectra (in the form of F[R(E)] through Kubelka–Munk
transformation of the diffuse reflectance spectra) of NppDI (black), NmpDI
(red), NphDI (green) and poly-NppDI (blue).
occurrence of a direct bandgap, the actual values of which,
falling in the narrow 2.87–3.01 eV range and derived as
illustrated in Fig. S13 and S14 (ESI†), are reported in Table 1.
The lowest values (2.87 and 2.90 eV) are found for the
p-diaminophenylene derivatives, the molecular (NppDI) and
polymeric (poly-NppDI) ones, respectively. Such observations,
which cannot be explained by the crystal packing features
discussed above and quantified in Table 1, speak for a slightly
higher extent of conjugated bond overlap in these molecules.
Fig. 8 Energy scheme and the HOMO and LUMO isosurfaces for NphDI,
Slightly larger direct band gap energies are found for NmpDI
NppDI, and NmpDI (left to right).
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were analyzed following the procedure used for similar molecular arrangement in the film remains unchanged and that no
compounds (n-dopable conjugated NDI-based polymers).50 distinct thin-film structure is formed.
Similarly to these polymers, our molecular compounds show
Fig. 9 shows the 2D-GIXD images for the annealed NmpDI
two separate reduction steps (ca. 0.5 eV apart, see Table S6, (a) and NphDI (b) thin films, together with the corresponding
ESI†), but with slightly lower electrochemical band gaps (DEgel, integrated intensity profiles along the qz (Fig. 9b and e) and qxy
ca. 1.4 eV vs. ca. 2.0 eV in ref. 50). Such lowering is mostly (Fig. 9c and f) directions. The 2D-GIXRD images of both films
attributed to the changes of the absolute energy values of the exhibit several intense Bragg spots having a perfect symmetry
HOMOs, while those of the LUMOs are still located near ꢀ4 eV. with respect to the central vertical line, overlapped with minor
Thus, LUMO-dependent reduction or partial n-doping is a arc-shaped intensity contributions, the latter being more
favored process also in our molecular species synthesis.
extended and pronounced for the NmpDI film than for NphDI
(Fig. 9a and d). These features indicate the coexistence of a
majority of crystallites/domains highly oriented along one
(f) Thin film analysis
For the three molecular NDI derivatives, both the as-deposited specific direction with respect to the substrate surface (sharper
and thermally annealed thin films have been investigated. h00 signals), and a minority of less, or even randomly, oriented
The thin film structures, in terms of crystal phase, degree of crystallites (arc-shaped signals). For both films, all the reflections
crystallinity and texture (mosaicity) have been determined by have been indexed by using the monoclinic crystallographic
the analysis of the 2D-GIXD measurements.51 As anticipated, structure of the RT-stable bulk phases and by assuming the
these images have been collected just below and slightly (100) texturing (Fig. S17 and S18a, ESI†), as confirmed by the qz
above the critical angle for total reflection, and their mutual integrated intensity profiles (Fig. 9b and e), where only h00
comparison enables the retrieval of information on the reflections are observed. The molecular orientations associated
variation of the film structure along its thickness.52
with this texturing are schematically reported in Fig. 9a and b.
Due to the extremely poor solubility of NppDI (providing Interestingly, though sharing the same texturing, dictated by the
approximately 10ꢁ more dilute solutions than NmpDI and NphDI), stacking of molecular layers (arranged in slabs), the molecular
the as-deposited (too thin) films did not provide measurable contacts on the substrate surface distinctly differ with the angle
diffracted signals, and only few and weak signals appeared in the between the aromatic cores and the surface being 83.7 and 40.81
2D-GIXD images after annealing (see Fig. S16, ESI†). Although the for NmpDI and NphDI, respectively. However, in addition to
paucity of information does not allow a complete description of peaks of the h00 class, the qxy integrated intensity profiles
the NppDI film structure, the observed signals can be ascribed to (Fig. 9c and f) show the presence of further peaks, witnessing
the strongest reflections of the bulk phase, suggesting that the the presence of largely misoriented crystallites.
Fig. 9 2D-GIXD images of annealed (a) NmpDI and (d) NphDI thin films recorded for ai = 0.11. The corresponding integrated intensity profiles along qz,
[(b and e)], and along qxy, [(c and f)] are reported. The dashed lines in b, c and e are Gaussian fits to the experimental data at the positions where the
detector cuts hamper collection of the diffracted intensity.
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Fig. 10 Sketch of the dominant molecular arrangement of the (a) NmpDI and (b) NphDI thin films on the substrate surface, as retrieved from GIXD
analysis.
A closer look into the 2D-GIXD images of the NphDI film crystals containing the NDI cores, substituted by different
shows that all reflections are azimuthally misplaced with isomeric, constitutionally different, N-containing aromatic residues.
respect to those expected for an exact h00 texturing (Figure The complete structural analysis demonstrated the similarity of the
S18a, ESI†), as already observed in the 2D GIXD images of other bulk and film structures, and the morphological texturing of
highly ordered organic films53 (Fig. 10).
the latter. In search for NDI-based organic solids not showing the
However, the experimental positions of the reflection maxima polymorphic variability of the alkyl-chains analogues, in the present
can be properly reproduced by tilting by ca. 31 the (100) crystal work we report species which exhibit high thermal stability and
planes with respect to the substrate surface (Fig. S18b, ESI1). absence of polymorphic transitions up to melting. Both properties
This tilting only marginally affects the angle between the make the sublimation-route as a viable option for films and
aromatic cores and the substrate surface (lowered to ca. 40.11).
device preparation, in addition to solution-based methods, thus
The mosaicity of the oriented crystallites/domains, i.e. their compensating the limited solubility properties of our systems. The
angular deviation with respect to the main orientation, is structural, spectroscopic and thermal analyses presented in this
estimated from the analysis of the experimental azimuthal work, together with the determination of the anisotropy of thermal
profile of the strong 200 reflection (see Fig. S19, ESI†). The expansion coefficients, add further knowledge on how NDI cores
very small value of the determined mosaicity, ca. ꢃ 21 and ꢃ approach, and overlap, in the search for optimized performances
3.51 for NmpDI and NphDI, respectively, is a direct measure of (e.g., electron and thermal conductivity, and Seebeck coefficient),
the strong orientation of the domains (i.e., of the associated which are at the heart of thermoelectric generators based on
molecules).
The 2D GIXD patterns collected below and above the critical
aromatic-polyimides.54
DFT calculations indicated a significant lowering of the
angle for total reflection (Fig. S20 and S21, ESI† for NmpDI and frontier orbital energies of the hydrazido derivative (NphDI),
NphDI, respectively), probing only the first 4 nm close to the air fostering its use as an electronic acceptor. Such an effect can be
interface or the entire film thickness, allowed us to establish further, and easily, enhanced by adding electron-withdrawing
the homogeneity of the molecular arrangement across the film groups (such F or CF3 residues) on the aromatic rings in the
thickness. Regardless of the probed depth, the films show the molecular periphery. Work can be anticipated in the direction
same texturing, as evidenced by consistence of the extracted qz of exploring the synthesis and structure of NphDI fluorinated
profiles, and the same degree of mosaicity, given by the almost congeners, in search for soluble, filmable and dopable systems
invariance of the width of azimuthal profiles. Interestingly, the with improved electronic performances, electron mobility and
as-deposited films exhibit a high degree of crystallinity (slightly rich(er) carrier concentrations. Indeed, the preliminary results
lowered upon annealing), indicating the extreme propensity of indicate that polyfluorinated NDIs possess significantly
these molecules to self-assemble in an ordered fashion on the enhanced solubilities in organic solvents commonly used for
substrate surface without the need for any postprocessing film deposition (acetonitrile, DMF and THF
–
but not
technique (Fig. S22 and S23, ESI†).
chlorinated ones) and energy levels and redox properties
suitable for n-doping.
Finally, we have also prepared and characterized an organic
nanosized material, stable well above 500 1C and containing a
100% ordered 1 : 1 copolymer (of admittedly short chain
length), together with its structure determination from the
severely broadened XRD trace.
4. Conclusions
In this paper we presented the isolation and the thorough
characterization of powders and thin films of new molecular
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Paper
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This project was partially supported by MIUR (PRIN-2017,
Project 2017L8WW48, HY-TEC). We thank G. Chita for
assistance during the synchrotron X-measurements at Elettra.
`
The courtesy of G.B. Giovenzana (Universita del Piemonte
¨
D. V. Seletskiy, H. Colfen, A. Leitenstorfer and S. Mecking,
Orientale) for providing MS data is also acknowledged.
Adv. Funct. Mater., 2018, 14, 2714–2719; (d) B. Russ,
M. J. Robb, B. C. Popere, E. E. Perry, C.-K. Mai,
S. L. Fronk, S. N. Patel, T. E. Mates, G. C. Bazan,
J. J. Urban, M. L. Chabinyc, C. J. Hawkerand and
R. A. Segalman, Chem. Sci., 2016, 7, 1914–1919.
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