New semiconducting naphthalene bisimides N-substituted with alkoxyphenyl groups: Spectroscopic, electrochemical, structural and electrical properties

Article abstract of DOI:10.1039/c4ra00052h

Three new naphthalene bisimides with alkoxyphenyl N-substituents (abbreviated as NBI-4-n-ORPhs, where R = butyl, hexyl or octyl group) were synthesized as well as spectroscopically, electrochemically and structurally characterized. DFT calculations predicted a high value of the electron affinity (EA) and a low lying LUMO level for these compounds. These findings were corroborated by cyclic voltammetry which yielded the experimental |EA| values of ca. of 4.11-4.12 eV and the ionization potential (IP) values in the range of 6.33-6.37 eV. When solution processed, the investigated semiconductors formed highly ordered and oriented layers, whose supramolecular organizations were derived from the structures determined for the corresponding single crystals. In particular π-stacked molecules formed molecular rows with stacking direction parallel to the substrate on which the films were deposited. As determined by X-ray diffraction studies, zone-casting technique turned out to be especially suitable for depositing layers of very good structural homogeneity, desired orientation of the molecules and high coherence length. High |EA| values of NBI-4-n-ORPhs together with their ordered supramolecular organization in zone-cast layers made these new semiconductors very good candidates for fabrication of n-channel field effect transistors (FETs). The devices with the Parylene C dielectric reached an electron mobility up to 0.05 cm² V^-1 s^-1 in air operating conditions.

Full text of DOI:10.1039/c4ra00052h

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New semiconducting naphthalene bisimides
N-substituted with alkoxyphenyl groups:
spectroscopic, electrochemical, structural and
electrical properties†
Cite this: RSC Adv., 2014, 4, 14089
Renata Rybakiewicz,a Izabela Tszydel,b Joanna Zapala,cd Lukasz Skorka,a
Damian Wamil,a David Djurado,d Jacques Pecaut,e Jacek Ulanski,b
´
a
a
*
*
Malgorzata Zagorska and Adam Pron
Three new naphthalene bisimides with alkoxyphenyl N-substituents (abbreviated as NBI-4-n-ORPhs, where
R ¼ butyl, hexyl or octyl group) were synthesized as well as spectroscopically, electrochemically and
structurally characterized. DFT calculations predicted a high value of the electron affinity (EA) and a low
lying LUMO level for these compounds. These findings were corroborated by cyclic voltammetry which
yielded the experimental |EA| values of ca. of 4.11–4.12 eV and the ionization potential (IP) values in the
range of 6.33–6.37 eV. When solution processed, the investigated semiconductors formed highly
ordered and oriented layers, whose supramolecular organizations were derived from the structures
determined for the corresponding single crystals. In particular p-stacked molecules formed molecular
rows with stacking direction parallel to the substrate on which the films were deposited. As determined
by X-ray diffraction studies, zone-casting technique turned out to be especially suitable for depositing
layers of very good structural homogeneity, desired orientation of the molecules and high coherence
length. High |EA| values of NBI-4-n-ORPhs together with their ordered supramolecular organization in
zone-cast layers made these new semiconductors very good candidates for fabrication of n-channel
field effect transistors (FETs). The devices with the Parylene C dielectric reached an electron mobility up
Received 3rd January 2014
Accepted 4th March 2014
DOI: 10.1039/c4ra00052h
www.rsc.org/advances
to 0.05 cm2 ꢀ1 sꢀ1 in air operating conditions.
V
ambipolar transistors.2,10–12 Both low molecular mass13 and
polymeric bisimides14 are being used for this purpose.
Introduction
Naphthalene and perylene bisimides N-substituted with
alkyl groups are solution processable which is a signicant
advantage. However, n-channel transistors fabricated from
these compounds fail to operate in air because their electron
affinity |EA| is too low. A popular method of increasing the |EA|
value in this family of compounds is their aromatic core func-
tionalization with electron withdrawing groups.13 An evident
drawback of this procedure is a complicated (in some cases)
reaction pathway, in addition involving reactions which are not
completely selective.13 Alternatively, imide nitrogen functional-
ization can be applied, which usually is simpler. For example,
changing alkyl N-substituent for their highly uorinated or
peruorinated analogues leads to an increase of |EA| by ca. 150
meV, but frequently on the expense of solution
processability.15–17
Arylene bisimide derivatives constitute an important class of
compounds which show, in addition to their diversied cova-
lent and supramolecular chemistry,1 an interesting reversible
electrochemical behavior.2,3 Appropriately functionalized bisi-
mides can serve as dyes.4 They also show semiconducting
properties and for this reason they have been most intensively
studied as components of active layers in organic electronic
devices, mainly n-channel eld effect transistors5–9 and
aFaculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00664
Warsaw, Poland. E-mail: zagorska@ch.pw.edu.pl; apron@ch.pw.edu.pl
bDepartment of Molecular Physics, Technical University of Lodz, Zeromskiego 116, 90-
924 Lodz, Poland
cInstitute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01224
Warsaw, Poland
dCEA Grenoble, INAC/SPrAM (UMR 5819, CEA-CNRS-Univ. J. Fourier-Grenoble 1),
LEMOH, 17 Rue des Martyrs, 38054 Grenoble Cedex 9, France
In one of our previous papers we have demonstrated that the
redox properties of naphthalene bisimides are much more
sensitive to aryl versus alkyl substitution than their perylene
analogues.18 For example alkylphenyl substituted napthalene
bisimides show |EA| values higher by 60 to 90 meV as compared
eCEA Grenoble, INAC/SCIB, 17 Rue des Martyrs, 38054 Grenoble Cedex 9, France
† Electronic supplementary information (ESI) available: Experimental and DFT
calculation details, polarized optical microscopy images, evolution of the
electron mobility with time. CCDC 972316–972318. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c4ra00052h
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to their alkyl substituted derivatives.18 This effect is not
observed in perylene bisimides, where uorination of the
phenyl ring is required to increase |EA|.19,20 Thus, using alkyl-
phenyl substituted naphthalene bisimides n-channel transis-
tors showing high charge carriers mobility were fabricated.21
In this paper we demonstrate that alkoxyphenyl N-
substituted naphthalene bisimides also constitute a promising
class of solution processable organic semiconductors. We also
describe a joint experimental and theoretical investigation of
their spectroscopic and electrochemical properties. Then we
discuss the effect of the N-substituent lengths on their supra-
molecular organization in single crystals and in thin layers
deposited on glass using the so called “zone-casting” technique.
Finally we test them as components of active layers in air
operating n-channel eld effect transistors easy to fabricate by
zone-casting. Such new materials, showing good charge carriers
mobility, are still highly demanded, since they are scarcer than
those suitable for the p-channel devices.13 This is especially
important for the fabrication of the so called “complementary
circuits” where comparable performance of n- and p-channel
transistors must be assured.22
General procedure of the synthesis of N,N0-bis(4-
alkoxyphenyl)-1,4,5,8-naphthalene tetracarboxylic-1,4:5,8-
bisimides
4.2 mmol of a given p-alkoxyaniline and 2.0 mmol (0.439 g) of
zinc acetate dihydrate were added to a stirred suspension of 2.0
mmol (0.536 g) of 1,4,5,8-naphthalenetetracarboxylic acid
bisanhydride in 25 mL of dry 1-methyl-2-pyrrolidinone (NMP).
ꢁ
The resulting mixture was stirred at 180 C for 4 hours under
argon atmosphere, and then poured into 100 mL of cold water.
The obtained precipitate was ltered off, washed with water and
acetone and then dried in a desiccator. The obtained bisimides
were puried using a chromatographic column (silica gel,
chloroform + 1 vol% triethylamine). The resulting material was
crystallized from chloroform.
N,N0-Bis(4-n-butoxyphenyl)-1,8:4,5-
naphthalenetetracarboxylic (NBI-4-n-OBuPh)
1
Orange solid reaction yield of 60%. H NMR (CDCl3, 400 MHz,
ppm): 8.83 (s, 4H); 7.23 (d, 4H, J ¼ 8.8 Hz); 7.08 (d, 4H, J ¼ 8.8
Hz); 4.03 (t, 4H, J ¼ 6.4 Hz); 1.85–1.78 (m, 4H); 1.54–1.48 (m,
4H); 1.01 (t, 6H, J ¼ 7.6 Hz). 13C NMR (CDCl3, 100 MHz): 163.17;
159.52; 131.38; 129.30; 127.15; 127.03; 126.64; 115.36; 67.95;
31.29; 19.29; 13.87. FTIR (KBr, cmꢀ1): 3465, 3417; 3083, 2923,
2869, 1717, 1669, 1507, 1443, 1345, 1243, 1185, 1167, 980, 872,
829, 752, 743. Elemental analysis: calcd for C34H30N2O6: C,
72.58, H, 5.37, N, 4.98. Found: C, 72.60, H, 5.41, N, 5.00%.
One of the investigated compounds, namely the bisimide
with butoxyphenyl substituents, was already reported in the
literature.23 It was studied as a model compound of polyimides
containing naphthalene moiety, however its electrochemical,
structural and semiconducting properties were not investi-
gated. Moreover, it was synthesized using a different and more
complicated pathway.
N,N0-Bis(4-n-hexyloxyphenyl)-1,8:4,5-
naphthalenetetracarboxylic (NBI-4-n-OHePh)
Experimental section
Synthesis
1
Yellow solid, reaction yield of 65%. H NMR (CDCl3, 400 MHz,
All three compounds studied were prepared via a condensation
reaction between naphthalene dianhydride and the corre-
sponding p-alkoxyphenylamine (commercially available) as
shown in Scheme 1.
ppm): 8.83 (s, 4H); 7.25–7.22 (d, 4H, J ¼ 8.8 Hz); 7.09–7.06 (d,
4H, J ¼ 8.8 Hz); 4.03 (t, 4H, J ¼ 6.4 Hz); 1.87–1.80 (m, 4H); 1.53–
1.46 (m, 4H); 1.39–1.35 (m, 8H); 0.93 (t, 6H, J ¼ 6.8 Hz). 13C NMR
(CDCl3, 100 MHz): 163.15; 159.51; 131.37; 129.29; 127.11;
127.00; 126.61; 115.34; 68.26; 31.58; 29.20; 25.74; 22.63; 14.06.
FTIR (KBr, cmꢀ1): 3465, 3417, 2944, 2872, 1711, 1657, 1507,
1468, 1351, 1237, 1134, 1023, 974, 866, 833, 769, 736. Elemental
analysis: calcd for C38H38N2O6: C, 73.77, H, 6.19, N, 4.53. Found:
C, 73.75, H, 6.24, N, 4.54%.
N,N0-Bis(4-n-octyloxyphenyl)-1,8:4,5-
naphthalenetetracarboxylic (NBI-4-n-OOcPh)
NBI-4-n-OOcPh was prepared in a form of a yellow solid, reac-
tion yield of 73%. 1H NMR (CDCl3, 400 MHz, ppm): 8.83 (s, 4H);
7.25–7.22 (d, 4H, J ¼ 8.8); 7.09–7.06 (m, 4H, J ¼ 9.2); 4.03 (t, 4H, J
¼ 6.4 Hz); 1.87–1.79 (m, 4H); 1.53–1.45 (m, 4H); 1.42–1.27 (m,
16H); 0.91 (t, 6H, J ¼ 6.8 Hz). 13C NMR (CDCl3, 100 MHz):
163.16; 159.52; 131.38; 129.30; 127.13; 127.03, 126.63; 115.36;
68.29; 31.84; 29.37; 29.26; 26.08; 22.68; 14.12. FTIR (KBr, cmꢀ1):
3465, 3411, 2927, 2851, 2363, 1711,1657, 1576, 1507, 1474, 1389,
1341, 1291, 1237, 1188, 1167, 1038, 980, 839, 763, 737.
Elemental analysis: calcd for C42H46N2O6: C, 74.75, H, 6.82, N,
4.15. Found: C, 74.59, H, 6.82, N, 4.14%.
Scheme 1 Synthesis of N,N0-bis(4-alkoxyphenyl)-1,4,5,8-naphthalene
tetracarboxylic-1,4:5,8-bisimides, abbreviated as NBI-4-n-OBuPh (1),
NBI-4-n-OHePh (2) and NBI-4-n-OOcPh (3). R ¼ n-C4H9 (NBI-4-n-
OBuPh); n-C6H13 (NBI-4-n-OHePh); n-C8H17 (NBI-4-n-OOcPh).
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Table 1 Experimental parameters used for determining the single
crystallographic cells of alkoxyphenyl-substituted naphthalene
bisimides
Solution processing
Thin lms of the studied semiconductors were deposited on
glass or on SiO2 covered Si substrate by spin-coating or drop-
casting. For some structural studies and in the process of
transistors fabrication the studied bisimides were deposited by
zone-casting, which allows to obtain highly oriented, thin
semiconductors layers on large at surface area.24–26 In this
Molecule
NBI-4n-OBuPh
NBI-4n-OHePh
NBI-4n-OOcPh
Nimag
nbatch
texp
434
6
220
493
3
20
423
6
80
method a solution of organic semiconductor is continuously Ntot.re
13871
4095
0.0784
0.1054
13926
6431
0.0818
0.1255
9191
5214
0.079
0.1207
Nindre
supplied onto a moving substrate from a stationary, at nozzle.
In the meniscus formed between the nozzle and the substrate,
R1
wR2
gradients of concentration and temperature are created due to
continuous evaporation of the solvent. These gradients are the
driving forces of unidirectional crystallization within the
divergence slit of 1/4ꢁ. The powder was deposited on a crystalline
Si substrate to minimize the background contribution of the
sample holder. Continuous scans were recorded between 2 and
60ꢁ in q during 3 h using a X'Celerator detector.
X-ray diffraction studies of thin layers of bisimide semi-
conductors deposited on glass or SiO2/Si substrates were carried
out in the same diffraction geometry on a Panalytical Empyrean
formed layer. The deposition of a uniform, highly ordered layer
requires optimization of several parameters for each individual
semiconductor being deposited; the main ones are concentra-
tion and temperature of the solution, its supply rate, height of
the meniscus, temperature of the substrate and its moving
speed.
˚
diffractometer (CoKa1 radiation, l ¼ 1.789 A), equipped with a
Spectroscopic characterization
mirror and a divergence slit of 1/32ꢁ on the incoming beam side,
Soller slits of 0.04ꢁ and a PixCel3D rapid detector used in the 1D
scanning mode, on the diffracted beam side. Continuous scans
were recorded from 2ꢁ to 60ꢁ in 2q for the total time from 3 to 7
hours, depending on the scattering power of the studied layers.
1H and 13C NMR spectra were registered using a Varian Mercury
(400 and 100 MHz, respectively) spectrometer. FTIR spectro-
scopic investigations were performed using a BIO-RAD FTS-165
spectrometer. Solution and solid state UV-vis-NIR spectra were
recorded on a Varian Cary 5000 spectrometer.
Polarized optical microscopic studies
Cycling voltammetry investigations
Bisimide layers zone-cast on glass or quartz substrates, were
examined using polarized optical microscopy (POM) either in
the light transmission or in the reection mode.
Cyclic voltammograms of the synthesized naphthalene bisi-
mides were registered using an Autolab potentiostat (Eco
Chemie). The electrolytic medium consisted of the studied
bisimide (10ꢀ3 M) dissolved in 0.1 M methylene chloride solu-
tion of Bu4NBF4. The measurements were performed in an inert
atmosphere, using a platinum working electrode of the surface
area of 3 mm2, a platinum wire counter electrode and an Ag/0.1
M Ag+ reference electrode.
Transistors fabrication and testing
Bottom contact, top gate thin lm transistors were prepared in
the following manner. Using an appropriate mask from the
mask set, a 4 nm chromium layer was rst evaporated on a glass
substrate, serving as an adhesive layer for gold electrodes. Then,
50 nm thick gold drain and source electrodes were deposited on
chromium, resulting in channels of 4 mm width and 30 to 100
mm length. Typically on one glass substrate 18 pairs of drain and
source electrodes were patterned. In the next step a solution of a
given bisimide in chlorobenzene was zone-cast with the casting
direction along the channel length. The deposited layers were
then annealed at 100 ꢁC for 4 hours with the goal to remove the
residual solvent. A 1 mm layer of Parylene C was deposited (see
ESI for details†) on the annealed semiconductor active layer
which served as a dielectric and a protective layer. In the last
step a silver gate electrode was deposited by evaporation in
vacuum through a matching shadow mask from the masks set.
Output and transfer characteristics of the fabricated transistors
were measured in air, using needle electrodes and circuits with
voltage/current sources and electrometers (Keithley 2410).
X-ray diffraction studies
In the case of the investigations of single crystals, the X-ray
crystallography diffraction data were collected at 150 K using an
Oxford-Diffraction X Calibur S Kappa geometry diffractometer
(MoKa radiation, graphite monochromator, l ¼ 0.71073 A). The
˚
cell parameters were obtained with intensities detected on
nbatch batches of 5 frames. For three settings of f and j, Nimag
narrow data were collected for 11 increments in u with an
exposure time texp. Unique intensities detected on all frames
using the Oxford-Diffraction Red program were used to rene
the values of cell parameters. The hydrogen atoms were all xed
in ideal positions. The parameters Nimag, nbatch, texp, the total
number of reections Ntot.re, the number of independent
reections Nindre and the nal indices R1 and wR2 are all
collected in Table 1.
DFT calculations
Powder diffractograms were recorded in the reection Bragg–
Brentano q/2q geometry using a Panalytical powder diffractom- DFT calculations were carried out using Gaussian09 Revision
eter equipped with a Cu Ka radiation source (l¼ 1.5418 A) and a D.01 (ref. 27) package and employing hybrid B3LYP28–30
˚
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exchange correlation potential combined with 6-31G(d) basis
Similarly as other functionalized naphthalene bisimides,
set. Ground-state geometries were fully optimized until a stable compounds of the NBI-4-n-ORPh family undergo a two-step
local minimum was found, which was conrmed by normal- quasi-reversible electrochemical reduction to a radical anion in
mode analysis (no imaginary frequencies were present). the rst step and to a spinless dianion in the second one.43 Their
Symmetry constrains were put wherever possible imposing voltammetric response in the negative (vs. Fc/Fc+) potentials
either C2h or C2 point group. In order to improve numerical range is very similar – a representative voltammogram, recorded
accuracy of the calculations two-electron integrals and their for NBI-4-n-OOcPh, is shown in Fig. 1 whereas all electro-
derivatives were calculated in a modied way employing the chemical reduction data are collected in Table 2.
pruned (99 590) integration grid consisting of 99 radial shells
In the positive potential range the studied compounds
and 590 angular points per shell. The ground-state geometries undergo an irreversible oxidation. Only in the case of NBI-4-n-
were then optimized in solution using polarizable continuum OOcPh a clear oxidation peak can be distinguished (see Fig. 2).
model (PCM)31 at the same level of theory with dichloromethane For NBI-4-n-OBuPh and NBI-4-n-OHePh the oxidation is man-
and chloroform as solvents. The oscillator strengths and ener- ifested by steeply increasing anodic peak without any clear
gies of the vertical singlet excitations were calculated employing maximum up to the potential of 1.7 V vs. Fc/Fc+ (not shown).
time-dependant version (TD) of DFT32–38 and again at the same The potentials of the oxidation onsets for all three bisimides are
level of theory (B3LYP/6-31G(d)) and moreover applying listed in Table 2.
Coulomb-attenuated hybrid exchange-correlation (CAM-B3LYP/
The formal potential of the rst redox couple, E01 ¼ (E1red
+
6-31G(d)) functional39 with chloroform as the solvent. The TD- E1ox)/2, is very close for all compounds studied (in the range
DFT results were retrieved from output les using GaussSum from ꢀ1.03 V to ꢀ1.01 V vs. Fc/Fc+). It is signicantly higher
2.2.40 The nature of the multicongurational transitions was (by 70 to 130 mV) than the corresponding potential in naph-
analyzed with natural transition orbitals.41 Molecular orbital thalene bisimides N-substituted with alkyl groups18 and very
plots were generated from cube les with Gabedit 2.4.6.42
close to the values measured for alkylphenyl substituted
naphthalene bisimides.44 This observation indicates that the
LUMO level and, by consequence, the electron affinity (EA) of
the molecules studied are not affected by the electron donating/
withdrawing properties of the solubilizing substituent. This
problem will be discussed later.
Results and discussion
Electrochemical and spectroscopic studies – experiment vs.
DFT calculations
The studied organic semiconductors are depicted in Scheme 1.
They differ only by the length of the solubilizing alkoxy
substituent. All three compounds studied are readily soluble in
such halo derivatives as dichloromethane, chloroform, chloro-
benzene and others.
To the contrary, the onset of the irreversible oxidation
process in alkoxyphenyl N-substituted naphthalene bisimides
starts at lower potentials as compared to the case of alkylphenyl-
substituted ones.44
Fig. 1 Cyclic voltammogram of NBI-4-n-OOcPh (10ꢀ3 M) registered Fig. 2 Cyclic voltammogram of NBI-4-n-OOcPh (10ꢀ3 M) registered
in the negative potentials range. Scan rate 50 mV sꢀ1, electrolyte 0.1 M in the positive potential range. Scan rate 50 mV sꢀ1, electrolyte 0.1 M
Bu4NBF4 in CH2Cl2; potentials vs. ferrocene redox couple.
Bu4NBF4 in CH2Cl2; potentials vs. ferrocene redox couple.
Table 2 Redox potentials (vs. Fc/Fc+) measured for the two-step reduction and irreversible oxidation together electrochemically determined
electron affinity (EA) and ionization potential (IP) values for alkoxyphenyl-substituted naphthalene bisimides
Compound
E1red/V
E1ox/V
E2red/V
E2ox/V
|EA|/eV
E3ox (onset)/V
IP/eV
NBI-4-n-OBuPh
NBI-4-n-OHePh
NBI-4-n-OOcPh
ꢀ1.07
ꢀ1.06
ꢀ1.06
ꢀ0.98
ꢀ0.97
ꢀ0.96
ꢀ1.55
ꢀ1.53
ꢀ1.52
ꢀ1.42
ꢀ1.41
ꢀ1.39
4.125
4.115
4.110
1.23
1.27
1.26
6.33
6.37
6.36
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Electrochemical investigations are of crucial importance for
RSC Advances
It seems from the electrochemical studies that the effect of
any new organic semiconductor since the measured potentials the alkoxy substituent on the bisimide reduction is purely
of its reduction to a radical anion and its oxidation to a radical inductive in nature since no differences in the rst reduction
cation give access to the estimation of its electron affinity (EA) potential are observed for p-R-Ph-44 and p-R-O-Ph- derivatives
and ionization potential (IP), respectively. The “electrochemical” described in this paper. This is not unexpected because both
EA value is calculated using the formal potential of the rst types of substituents show electron withdrawing character with
redox couple listed in Table 2, i.e. E01, and applying eqn (1):
similar values of the eld/inductive, F, parameters.48 Since the
oxidation potential is, in turn, strongly inuenced by the pres-
ence of the alkoxy group (vide supra) it seems that the HOMO
and LUMO orbitals must be separated in space, the former
being located on the substituent whereas the latter – on the
bisimide core. In this respect it is instructive to compare the
electrochemical data with the results of DTF calculations which
do not only provide the energies, shape and positions of the
frontier orbitals but also give access to the EA and IP values. We
have carried out such calculations for NBI-4-n-OBuPh.
EA (eV)¼ꢀ|e|(E01 + 5.1)
(1)
In the estimation of EA, the formal redox potential, E01, must
be expressed in the absolute potential scale i.e. with respect to
the vacuum level. Trasatti attributed a value of ꢀ5.1 V in the
absolute scale to the formal redox potential of the Fc/Fc+ couple
in non-aqueous electrolytes.45
By analogy, the formal redox potential of the couple corre-
sponding to the oxidation of a neutral molecule to a radical
cation should be taken for the calculations of the “electro-
chemical” IP values. As already mentioned, this process in
alkoxyphenyl-substituted bisimides is irreversible and yields a
rather poorly dened electrochemical answer. Therefore no
formal redox potential can be determined for this redox reac-
tion. In such cases frequently the potential of the oxidation peak
onset is taken for the calculations. This approach has however
to be considered as a rather rough approximation, although in
many reversible redox couples the potential of the onset of the
oxidation peak is close to the formal redox potential.46
Following this procedure we have calculated the “electro-
chemical” IP, using the data collected in Table 2 and applying
eqn (2):
In the calculations higher-accuracy integration grid was used
in order to avoid energy oscillations (possibly due to many sp3
carbons). Initially, its geometry was constrained to the C2h
symmetry point group and optimized, however the normal
mode analysis revealed
a second-order stationary point.
Therefore, it was repeated, imposing C2 symmetry. In this case
the equilibrium structure was found. In addition to the calcu-
lations carried out for vacuum, the geometries of the molecule
were also analyzed in chloroform with the goal to compare the
theoretical spectrum with the experimental one which was
recorded for NBI-4-n-OBuPh dissolved in this solvent. Theoret-
ical investigations were also carried out in dichloromethane –
the solvent used in the cyclic voltammetry investigations of NBI-
4-n-OBuPh. In this case they were extended to the calculations
of the ionization potential (IP) and the electron affinity (EA) as
the difference in energy between the radical cation or the radical
anion at their equilibrium geometries and the ground-state
energy in vacuum and solution.
The frontier orbitals are depicted in Fig. 3. Consistent with
the electrochemical ndings the HOMO orbitals are located at
the phenyloxy segment of the substituent whereas the lobes of
LUMO are spread along the central (bisimide) part of the
molecule and have an inter-ring bonding character.
The results of the calculations are collected in Table 3. The
values of IP and EA do not match the exact values of HOMO and
LUMO because they correspond to equilibrium processes of
withdrawing and adding electrons rather than to the simple
ground-state analysis. The differences are also clearly seen in
IP (eV) ¼ |e|(E3ox (onset) + 5.1)
(2)
The values of EA and IP derived from the electrochemical
data are collected in Table 2.
A short comment is needed here. In eqn (1) and (2) we used
the value of ꢀ5.1 V as recommended by Trasatti,45 although
in the majority of papers a different value (ꢀ4.8 V) is attributed
to the Fc/Fc+ couple. Detailed justication for the preferable use
of ꢀ5.1 V can be found in.47
The following predictive messages can be retrieved from the
above described electrochemical studies: (i) high values of |EA|,
in each case exceeding 4.0 eV, make alkoxyphenyl-substituted
naphthalene bisimides very promising candidates for the
application as active layers in n-channel transistors operating in
air, their solution processability constitutes their additional
advantage; (ii) their IP is also very high (>6 eV), thus they are very
stable against oxidation but cannot be used as components of
ambipolar transistors or bulk heterojunction photovoltaic cells.
Table 3 Key electronic parameters calculated for NBI-4-n-OBuPh in
vacuum and solution
HOMO/eV
LUMO/eV
Eg/eV
IP/eV
|EA|/eV
Fig. 3 Projections of HOMO and LUMO orbitals of NBI-4-n-OBuPh
(isovalue ¼ 0.03); green stands for positive and red for negative signs
of the wave function.
Vacuum
CH2Cl2
ꢀ6.04
ꢀ6.18
ꢀ3.23
ꢀ3.21
2.82
2.87
6.94
5.51
2.02
3.89
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the results of the calculations carried out for vacuum and part of the spectrum naphthalene bisimides N-substituted with
dichloromethane solution, especially in the case of IP and EA. It alkoxyphenyl groups do not differ from their alkyl-substituted
is therefore obvious that solvent effects must always be taken analogues.44 In the higher energetic part of their spectra an
into account when comparing the calculated and experimental additional peak of lower intensity, possibly showing a vibronic
data.
character can be distinguished at 275 nm. This peak is not
The LUMO level (both in vacuum and in solution, ca. present in the spectra of alkylphenyl substituted naphthalene
ꢀ3.20 eV) is very close to that calculated for n-butylphenyl bisimides44 and can tentatively be attributed to a transition in
substituted naphthalene bisimide (NBI-4-n-BuPh),44 which is the aromatic substituent modied by the presence of the
expected taking into account similar inductive effect of the strongly electrodonating alkoxy group since similar bands are
alkoxyphenyl and alkylphenyl substituents on the bisimide core present in low49 and high50 molecular weight compounds con-
reduction.48 To the contrary, consistent with the location of the taining alkoxy-substituted phenyl (phenylene) moieties.
HOMO orbitals, the presence of this group in the substituent
The theoretical analysis of the UV-Vis spectrum of NBI-4-n-
strongly inuences the position of the HOMO level which is OBuPh was based on the TD-DFT calculations using the mole-
being raised by ca. 0.60 eV as compared to the case of NBI-4-n- cule geometry optimized (B3LYP) for the chloroform solution
BuPh.44 Finally, it should be stated that the same trend is and additionally applying CAM-B3LYP on the same geometry
observed in the experimental IP and EA values measured for for comparison.2 In order to cover as much of the spectrum as
NBI-4-n-OBuPh in this research and in the data reported for possible, 50 vertical excitations were calculated. They are listed
NBI-4-n-BuPh.44
in Table S1 and S2 of ESI.† Transitions of the highest oscillator
UV-vis spectra of all three compounds studied are very strengths together with the HOMO–LUMO excitation are pre-
similar. In Fig. 4 the spectrum of NBI-4-n-OBuPh is shown as a sented in Table 4, while in Fig. 4 these transitions are compared
representative example. In its lower energy part it is dominated with the experimental UV-Vis spectrum.
by a characteristic strong band of vibronic character with a
According to B3LYP the excitation which gives rise to the
maximum at 379 nm for the 0–0 transition. This band is most intensive band in the spectrum of NBI-4-n-OBuPh does not
attributed to the p–p* transition in the aromatic bisimide core originate from the HOMO–LUMO transition, which is
(NBI) as evidenced from the DFT calculations (vide infra). In this forbidden. The calculated three transitions of high oscillator
Fig. 4 Experimental UV-Vis spectrum of NBI-4-n-OBuPh solution in chloroform together with calculated (TD-DFT) vertical transitions of the
highest oscillator strength: calculated using (a) B3LYP/6-31G(d); (b) CAM-B3LYP/6-31G(d).
Table 4 Key singlet vertical excitations in NBI-4-n-OBuPh
Exc. no.
l/nm
Osc. strength
Symm.
Contribution
lmax
B3LYP
1
3
521.25
382.81
0
S–A
S–B
H / L (99%)
0.99653
0.98903
0.1839
H-5 / L (17%)
H-4 / L (24%)
H-2 / L (58%)
H-5 / L (79%)
H-2 / L (16%)
H-4 / L (74%)
H-2 / L (24%)
H-9 / L (93%)
H-2 / L (96%)
H-5 / L (88%)
H-9 / L (84%)
4
6
380.04
370.38
0.0649
0.2811
S–B
S–B
0.97450
0.98981
14
1
4
301.50
340.40
312.67
251.96
0.1287
0.6484
0.0813
0.0947
S–B
S–B
S–B
S–B
0.94151
0.98504
0.89313
0.86518
CAM-B3LYP
12
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strength (transitions 3, 4 and 6 in Table 4) closely match in
energy the dominant band in the experimental spectrum of
pronounced vibrational structure (0–0 transition at 379 nm).
This characteristic band should predominantly originate from
the p–p* transition in the bisimide core. However, the NTO
analysis of the vertical excitations reveals some contribution
from phenylene moieties (note that due to multicongurational
character of the transitions, NTO signicantly simplies the
interpretation). The calculations also predict an additional
transition of signicant oscillator strength in the vicinity of
300 nm. In the experimental spectrum a clear band can be seen
in this spectral region. No such band exists in NBI-4-n-BuPh.44
TD-DFT with CAM-B3LYP using the same geometry provides on
the other hand with much clearer picture of the excitations. In
this case the pronounced vibronic structure originates exclu-
sively from the highly privileged p–p* transitions on the bisi-
mide core (as conrmed by NTO analysis). Within this XC
functional there also exist transitions that give rise to the band
in the vicinity of 300 nm. For this reason and what can be seen
for different bisimides of similar structure2 this approach
(CAM-B3LYP) is more robust. Finally, it should be noted that the
theoretical calculations are in good agreement with the exper-
imental data.
X-ray diffraction studies
The capability of forming ordered and, if possible, oriented
structures is of crucial importance for potential application of
bisimide-based semiconductors as active layers in organic
electronics devices. For this reason we have undertaken a
detailed study of their crystal structure both in single crystals
and in thin layers focusing on the relationship between the
processing technique and the resulting supramolecular
organization.
We have succeeded in growing single crystals of all three
compounds studied. In Fig. 5a–c the crystal packings of NBI-4n-
OBuPh, NBI-4n-OHePh and NBI-4n-OOcPh, projected in the
(b, c) plane, are compared.
A pronounced effect of the alkyl substituent length on the
molecular packing pattern can clearly be noticed. In the unit NBI-4n-OHePh, triclinic (space group P1), Z ¼ 2, a ¼ 8.4404(13) A˚, b ¼
cell of NBI-4n-OBuPh, molecules form a herring bone arrange-
ment with butyl groups regularly packed along the c axis. The
intermolecular distance between the nearest neighbors inside
each row is about 0.5 nm. All molecules exhibit a similar
conformation with two internal torsion angles between the
Fig. 5 Projections in the (b, c) plane of crystallographic structures of
(a) NBI-4n-OBuPh, monoclinic (space group P21/c), Z ¼ 2, a ¼
5.0423(4) A˚, b ¼ 33.7458(18) A˚, c ¼ 7.9966(4) A˚, b¼ 100.002(5)ꢁ, (b)
ꢀ
8.8932(14) A˚, c ¼ 24.019(3) A˚, a ¼ 89.090(11)ꢁ, b¼ 80.490(11)ꢁ, g ¼
ꢁ
ꢀ
62.786(16) , (c) NBI-4n-OOcPh, triclinic (space group P1), Z ¼ 1,
a ¼ 5.2267(5) A˚, b ¼ 7.6170(5) A˚, c ¼ 22.3732(18) A˚, a ¼ 96.434(6)ꢁ, b ¼
95.114(7)ꢁ, g ¼ 100.318(7)ꢁ.
conjugated core and the substituent phenyl rings of 91 and 70ꢁ. molecule per unit cell. In each molecule, the phenyl substitu-
With increasing length of the alkyl groups, their conformation ents are rotated by 97ꢁ with respect to the conjugated core. A
becomes more extended, favoring a regular stacking of the stacking mode in which the conjugated core and the phenyl
molecules in the unit cell. For NBI-4n-OHePh, the adjacent substituents alternatively face each other along the b axis is
bisimide cores are slightly shied relatively to each other, which obtained. The closest distance between the conjugated cores of
results in a shorter stacking distance of 0.48 nm. The two molecules is around 0.55 nm.
internal core/phenyl substituent torsion angles are in this case
In as prepared powders of NBI-4n-OHePh and NBI-4n-OOcPh
58 and 72ꢁ. They however alternate in the two consecutive the crystal structure turned out to be the same as in single
molecules along the b axis, still giving two independent mole- crystals since the powder diffractograms calculated from the
cules per unit cell. In NBI-4n-OOcPh, the inter-alkyl interactions single crystal data satisfactorily matched the experimental ones.
impose the same relatively extended conformation of the NBI-4n-OBuPh required, however, some additional purication
molecules as in the case of NBI-4n-OHePh but with only one because as prepared powders of this compound, i.e.
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precipitated from the reaction medium and then recrystallized, respect to the substrate. The FWHM value of the reection at
showed a small admixture of a phase of different crystallo- 1.94 nm (corresponding to the molecules tilted by 55–60ꢁ with
graphic parameters.
respect to the substrate) yields a coherence length of ca. 100 nm,
All three naphthalene bisimides are solution processable which corresponds to the average size of the nanocrystals lying
and can be deposited in a form of thin layers on a variety of of the substrate's surface.
substrates. The resulting supramolecular organization as well
The diffractograms of the lms obtained by zone-casting are
as the degree of the molecules orientation in the layer depend of special interest since this processing technique was used in
mainly on the selected processing technique and the type of the the fabrication of transistors. A typical diffractogram obtained
substrate used but also on the concentration of the solution, for a layer of NBI-4n-OBuPh zone-cast on glass is shown in
temperature of solvent evaporation and others.
Fig. 7. In principle, it resembles the diffractogram registered for
The effect of the substrate can be illustrated by comparing the drop-cast layer with two broad peaks in the range of 2.55–
Bragg–Brentano diffractograms of NBI-4n-OBuPh thin layers 2.30 nm and a very thin reection centered at 1.94 nm. The
deposited on SiO2 covered Si and on glass (see Fig. 6). On glass coherence length, calculated from the FWHM value of the
one broad peak centered at 2.5 nm is obtained whereas on SiO2/ 1.94 nm reection, is 93 nm which means that these oriented
Si an additional peak at 1.94 nm is present. It is instructive to crystals are similar in size as those obtained by drop-casting.
comment on the 2.5 nm interlayer distance derived from both Similarly as in drop-cast layers they co-exist with much thinner
diffractograms in the context of the single crystal structure. The layers (ca. 20 nm) of molecules whose orientation is different.
maximum length of a NBI-4n-OBuPh in its conformational state
As already mentioned, NBI-4n-OHePh and NBI-4n-OOcPh
present in the single crystal is ca. 2.5 nm (length measured adopt a regular stacking sequence in their 3D crystallographic
between the two terminal methyl groups). This may suggest that structure with alkyl substituent in their most extended confor-
there exist crystallites in the layer, in which the molecules stand mation (see Fig. 5). These structural features should therefore
almost perpendicular to the substrate. In the layer deposited on facilitate the formation of highly ordered and oriented layers by
glass this is the only crystalline phase. The period of 1.94 nm, solution processing.
registered for the layer deposited on SiO2 can be interpreted as
Representative diffractograms of NBI-4n-OHePh layers spin-
originating from crystallites in which the semiconductor coated on SiO2 and on glass are shown in Fig. 8. They are very
molecules form a tilt angle of 55–60ꢁ with respect to the similar and show one broad reection centered at 2.41 nm. In
substrate. This is also a typical tilt angle which is formed by one both cases uniform thin layers of the thickness of ca. 30 nm
of two independent molecules with respect to the b axis in the (evaluated from the FWHM of the peak and by applying the
single crystal structure. Thus, two types of differently oriented Scherrer formula) are obtained in which the molecules are
crystallite co-exist in this layer. The spin-coated layer are very uniformly stacked within this period.
thin (ca. 20 nm, as evaluated from the FWHM value of the peak
and applying the Scherrer formula).
The length of the molecule in its extended conformation
adopted in the single crystal (Fig. 5) is 3.229 nm. The molecule
Drop-casting on glass yields somehow different diffracto- is tilted with respect to the b axis by an angle of ca. 50ꢁ.
grams from those obtained for spin-coated lms (see Fig. 6). Projection of this molecule on a direction perpendicular to
Three co-existing Bragg reections (2.54 nm, 2.37 nm and the b axis yields 3.229 ꢂ sin 50ꢁ ¼ 2.47 nm. This value is in a
1.94 nm) can be distinguished in this case corresponding to very good agreement with the period found for the thin layer.
crystallites in which molecules are differently oriented with Upon deposition of the thin layer the substrate compels
individual molecules to be tilted and aligned in a row, without
Fig. 6 Main features of X-ray diffractograms obtained for NBI-4n-
OBuPh layers deposited on: SiO2 covered Si by spin-coating (blue);
glass by spin-coating (red); glass by drop-casting (black). Fringes on
the blue profile originate from the reflectivity of a thin SiO2 layer on Si Fig. 7 X-ray diffractogram of a NBI-4n-OBuPh layer, zone-cast on
crystal.
glass.
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reasoning as in the case of NBI-4n-OHePh a tilt angle of 47ꢁ can
be found which gives a projected distance of 2.69 nm in the
single crystal, very close to the stacking distance of 2.61 nm
found in the thin layers. Thus, in this case again the stacking
pattern of the molecules with respect to the substrate resembles
that determined for the single crystal.
The presence of the second peak in thin lms spin-coated
on SiO2 can be considered as a sign of the coexistence of the
zones of oriented tilted molecules with 3D crystals which
are surface contacted through the (a, b) plane since this peak
can be attributed to the 001 reection in the direction
perpendicular to the substrate. This co-existence of two
different phases can be clearly observed in optical micro-
scope images in polarized and nonpolarized light which
show alternation of large, structurally and optically homo-
geneous domains with domains of different optical features
originating from different packing patterns (see Fig. S1 and
S2 in ESI†).
A representative diffractogram of a NBI-4n-OOcPH deposited
on glass by zone-casting is shown in Fig. 11. A periodic stacking
of 2.6 nm is also observed in this case with a coherence length of
150 nm. The presence of higher order reections (up to 10th
order) clearly shows a high degree of the molecules alignment
in the deposited layer.
To summarize this part of the paper, all three compounds
studied adopt in thin layers regular stacking modes. This effect
is more pronounced for hexyloxy and octyloxy substituted bisi-
mides. Zone-cast layers are much thicker as compared to spin-
coated or drop-cast ones, however they show a very regular
supramolecular organization characterized by a very high
coherence length. In all cases, several types of oriented crystals
are found in the samples. However, in the zone-cast samples of
hexyloxy and octyloxy substituted bisimides the best structural
homogeneity is found.
Fig. 8 Main features of X-ray diffractograms obtained for NBI-4n-OHePh
layers spin-coated on: SiO2 (blue); glass (black). Fringes on the blue profile
originate from the reflectivity of thermal SiO2 layer on Si crystal.
any translational shi along the stacking directions. Accord-
ingly, we can consider that the resulting mutual orientation of
the molecules is very similar to that found in the single
crystal.
In the diffractogram of a zone-cast layer a slightly lower
period is found (2.354 nm) compared to that measured for the
spin-coated layer. However this layer is thicker and yields a
coherence length of 138 nm, calculated from the FWHM value
of the 001 reection. A high degree of the molecules orientation
is manifested by the presence of the reections of higher order,
up to 009 (see Fig. 9).
Spin-coated layers of NBI-4n-OOcPh yield two types of
diffractograms with either one or two reections (see Fig. 10).
One reection at 2.61 nm is observed for lms deposited on
glass whereas for lms deposited on SiO2 an additional peak
can be noticed at 2.24 nm, which shows much smaller
FWHM.
The length of a NBI-4n-OOcPh molecule in its conformation
adopted in the single crystal is 3.684 nm. Applying the same
Fig. 10 X-ray diffractograms of NBI-4n-OOcPH layers deposited
from a chloroform solution (c ¼ 10 mg mLꢀ1) by spin-coating on:
native SiO2 on Si wafer (red) and thermal SiO2 on Si wafer (blue).
Deposited from a chloroform solution (c ¼ 2 mg mLꢀ1) on thermal
Fig. 9 X-ray diffractogram obtained for a NBI-4n-OHePh layer zone- SiO2 on Si wafer (green) and from a chlorobenzene saturated solution
cast on glass. Stars indicate peaks which cannot be indexed in the 00l on thermal SiO2 layer on Si wafer (black). Fringes originate from the
series.
reflectivity of thermal SiO2 layer.
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zone-cast active layer.21 For these reasons we started to use
Parylene C.
In Fig. 12 representative transfer and output characteristics
are shown for a transistor with an active layer consisting of
zone-cast NBI-4n-OOcPh. The charge carrier mobility, m, the
threshold voltage, Vth and the ON/OFF ratio were calculated
from the saturation regime, using the following eqn (3)
and (4):52,53
pffiffiffiffi
ꢀ
ꢁ
2L
d
Id
m ¼
(3)
(4)
WC dVg
ON
Id max
¼
OFF Id min
where L is the channel length, W is the channel width, C is the
dielectric capacitance per unit area, Id is the drain source
current, and Vg is the gate voltage.
Fig. 11 X-ray diffractogram of a NBI-4n-OOcPh layer zone-cast on
glass. 00l lines indicated in blue originate from crystals whose (a, b)
plane is preferentially oriented parallel to the substrate.
Application in n-channel eld effect transistors (FETs)
Detailed studies of alkoxyphenyl substituted naphthalene bisi-
mides unequivocally show that these compounds combine
facile synthesis and solution processability with redox proper-
ties predicting their air operating in the n-channel eld effect
conguration. Moreover, the hexyloxy and octyloxy derivatives
(and to a lesser extent the butyloxy one) have a strong tendency
to form oriented layers via solution processing. This property is
even further enhanced if zone-casting is applied as a processing
technique. In this last case, rows of p-stacked molecules are
formed with the p-stacking direction parallel to the surface of
the substrate. Such orientation of molecules is favorable for
electrical charge transport in the bottom contact top gate
transistor conguration. We have demonstrated it in one of our
previous papers where we showed that the transistors fabricated
by zone-casting exhibited almost three orders of magnitude
higher electron mobility21 than those in which the active layer
was deposited by spin-coating.18
The selection of the used dielectric material (Parylene C)
requires a short comment. A large variety of organic, inorganic
and hybrid (organic/inorganic) dielectrics have been tested in
organic eld effect transistors (OFETs).51 The use of purely
inorganic dielectrics and some of hybrid ones is virtually
impossible or at least extremely difficult in the fabrication of
bottom contact-top gate transistors by solution processing.
Therefore, in our research we limited ourselves to the applica-
tion of solution processable, preferably polymeric dielectrics,
due to their lm forming properties. Commercially available
uorinated polymer, CYTOP, is a popular choice of a dielectric
for both n- and p-channel eld transistors and initially we also
used it.18,44 However in the process of optimizing the device
fabrication we found that n-channel transistors with a Parylene
Fig. 12 Transfer (a) and output (b) characteristics registered for an
organic field effect transistor (OFET) with a NBI-4-n-OOcPh active
layer, deposited by zone-casting. Forward sweep – solid line; back-
ward sweep – dashed line.
Table 5 Field-effect characteristics of transistors with alkoxyphenyl
N-substituted naphthalene bisimides as active layers. Measurements
performed in air
Compound
m/cm2 Vꢀ1 sꢀ1
Vth/V
ON/OFF ratio
3.2 ꢂ 102
2.8 ꢂ 103
4.7 ꢂ 103
C
dielectric layer exhibited better parameters (threshold
NBI-4-n-OBuPh
NBI-4-n-OHePh
NBI-4-n-OOcPh
0.0086
0.0040
0.049
5.45
14.1
14.9
voltage, charge carriers mobility) as compared to those in which
CYTOP served as a dielectric, especially for transistors with
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The determined parameters for transistors fabricated from for Networking and Supercomputing, WCSS, Wroclaw, Poland.
all three compounds studied are collected in Table 5. The best (http://www.wcss.wroc.pl), under calculational Grant no. 283.
charge carriers mobility and the highest ON/OFF ratio was
obtained for the transistor in which the active layer consisted of
NBI-4-n-OOcPh. This is not unexpected since zone-cast layers of
References
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Based on high values of |EA| air operating of the transistors
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order to examine the working stability of these devices, their
characteristics were measured every seven days during four
weeks. They were constantly kept at room temperature in air
with humidity higher than 40%. It was found that the charge
carrier mobility calculated from the transistor characteristics
decreased ca. 5 times during the rst week and then stabilized,
without further decrease. In ESI (Fig. S3†) the evolution of the
charge carriers mobility with the time of exposure is shown.
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To summarize, we have demonstrated that naphthalene bisi-
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support of the Foundation for Polish Science within the Commun., 2003, 1710.
framework of START 2013 Programme. I. T. wants to acknowl- 20 H. Z. Chen, M. M. Ling, X. Mo, M. M. Shi, M. Wang and
edge the Grant 2012/07/B/ST8/03789 of the Polish National Z. Bao, Chem. Mater., 2007, 19, 816.
Science Centre (NCN) and she also wants to thank Andrzej 21 I. Tszydel, M. Kucinska, T. Marszalek, R. Rybakiewicz,
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