Unique aggregate structure of fluoroperylene diimide thin film

Article abstract of DOI:10.1021/jp0362382

A perfluorinated perylene diimide, N,N′-diperfluorophenyl-3,4,9,10- perylenetetracarboxylic diimide (DFPP), and its nonfluorinated analogue, N,N′-diphenyl-3,4,9,10-perylenetetracarboxylic diimide (DPP), showed similar UV-vis absorption and fluorescence spectra when they existed as monomeric dyes in solutions, whereas in solid films they exhibited great differences. Absorption band splitting was observed for DPP film; however, for DFPP film, the fine structure of absorptions similar to that in solution still remained in addition to the absorption peaks becoming broader and shifting to longer wavelength regions. Furthermore, it was found that DFPP molecules aggregated to form a crystalline film when the temperature of the substrate was elevated to 200°C. Through the simulation of the molecular conformation, the theoretical examinations of molecular interactions, the measurement of electron affinity, the observation of the film morphology by AFM and SEM, and the crystalline structure characterization by XRD, the unique aggregate structure of the crystalline DFPP thin film was suggested: DFPP molecules stacked with the perfluorinated phenyl groups straight over or below the perylene cores of the adjacent DFPP molecules.

Full text of DOI:10.1021/jp0362382

J. Phys. Chem. B 2004, 108, 5901-5904
5901
Unique Aggregate Structure of Fluoroperylene Diimide Thin Film
Min-Min Shi, Hong-Zheng Chen,* Yue-Wen Shi, Jing-Zhi Sun, and Mang Wang*
Department of Polymer Science and Engineering, State Key Lab of Silicon Materials, Zhejiang UniVersity,
Hangzhou 310027, People’s Republic of China
ReceiVed: July 31, 2003; In Final Form: March 5, 2004
A perfluorinated perylene diimide, N,N′-diperfluorophenyl-3,4,9,10-perylenetetracarboxylic diimide (DFPP),
and its nonfluorinated analogue, N,N′-diphenyl-3,4,9,10-perylenetetracarboxylic diimide (DPP), showed similar
UV-vis absorption and fluorescence spectra when they existed as monomeric dyes in solutions, whereas in
solid films they exhibited great differences. Absorption band splitting was observed for DPP film; however,
for DFPP film, the fine structure of absorptions similar to that in solution still remained in addition to the
absorption peaks becoming broader and shifting to longer wavelength regions. Furthermore, it was found
that DFPP molecules aggregated to form a crystalline film when the temperature of the substrate was elevated
to 200 °C. Through the simulation of the molecular conformation, the theoretical examinations of molecular
interactions, the measurement of electron affinity, the observation of the film morphology by AFM and SEM,
and the crystalline structure characterization by XRD, the unique aggregate structure of the crystalline DFPP
thin film was suggested: DFPP molecules stacked with the perfluorinated phenyl groups straight over or
below the perylene cores of the adjacent DFPP molecules.
1. Introduction
onstrated to be an organic air-stable electron acceptor with good
1
7
solubility in common solvents. In the present work, the UV-
vis absorption, fluorescence spectra, electron affinities, mor-
phologies, topographies, and XRD patterns of evaporated DFPP
films were examined and compared to those of DFPP’s
nonfluorinated analogue, N,N′-diphenyl-3,4,9,10-perylenetetra-
carboxylic diimide (DPP) film, aiming to clarify the effect of
fluorination on the aggregate structure of perylene diimide thin
films. On the basis of the results, possible DFPP molecular stack
in the film was proposed.
Perylene diimides, as organic n-type semiconductors with
high optical and thermal stability, have attracted much attention
1
-4
due to their potential applications in organic solar cells,
5,6
organic light-emitting diodes (OLEDs), and organic field-
effect transistors (OFETs), where an electron mobility as high
as 0.6 cm /(V‚s) for the vapor deposited perylene diimide film
7,8
2
was detected.
The substituents attached on the periphery of the perylene
chromophore in perylene diimides have a crucial effect on their
optical and electronic properties induced by the changes of the
molecular packing (aggregate structures) in the solid states via
the influences of intermolecular interactions. Many efforts have
been made to reveal the relationship between molecular stack
and optical spectra, ultimately, the electronic structure of
2
. Experimental Section
Materials. The molecular structures of DFPP and DPP were
depicted in Figure 1. DFPP was synthesized by condensation
of 3,4,9,10-perylenetetracarboxylic dianhydride with 2,3,4,5,6-
9-13
16
perylene diimide.
Graser et al. proposed that the degree of
pentafluoroaniline according to the reported procedure and was
“
area overlap” of successive π-conjugated perylene moieties
purified by temperature gradient sublimation. Anal. Calcd for
C36H8F10N2O4: C, 59.85; H, 1.12; N, 3.88. Found: C, 59.74;
9
,10
determined simply the shift of the absorption maximum.
1
Kazmaier et al. suggested that the perylene diimides, which
showed the larger valence and conduction band broadened by
aggregation (the bigger absorption widths), were also those with
the higher photosensitivities.11 Alternatively, Mizuguchi et al.
pointed out that the resonance interactions of transition dipoles
were responsible for the band splitting of the absorptions that
appeared in perylene diimide films.1
Organic semiconducting materials with strong electron-
withdrawing F substituents exhibited unique behaviors. For
example, both perfluorinated copper phthalocyanine and
H, 0.99; N, 3.82. H NMR (500 MHz, CDCl3, TMS): δ ) 8.82
(d, J ) 8 Hz, 4H), 8.76 (d, J ) 8 Hz, 4H). FTIR (KBr, pellet):
-
1
ν ) 1721, 1685 cm (CdO). DPP was prepared in a similar
way. Anal. Calcd for C36H18N2O4: C, 79.70; H, 3.34; N, 5.16.
Found: C, 78.88; H, 2.95; N, 5.12. FTIR (KBr, pellet): ν )
-
1
1703, 1665 cm (CdO). DFPP and DPP films were vacuum
2,13
-5
evaporated at a pressure below 1 × 10 Torr onto quartz, glass,
or ITO substrates with different temperatures, and the deposition
rate was 1-2 Å/s. All of these thin films were about 40 nm
thick, monitored with a quartz crystal oscillator.
1
4
15,16
perfluorinated naphthalene diimide
possessed high electron
Measurements. Elemental analyses data were determined on
a Perkin-Elmer 240C elemental analyzer. The NMR spectrum
was obtained on a Bruker AVANCE DMX500 nuclear reso-
nance spectroscope. FTIR spectra were obtained on a Vector
22 Fourier transform infrared spectroscope. UV-vis spectra
were taken on a Varian CARY 100 Bio spectrometer. Fluores-
cence spectra were recorded at an excitation wavelength of 470
nm on a Hitachi 4500 fluorescence spectrophotometer. Cyclic
2
mobilities of over 0.01 cm /(V‚s) and were stable in air because
of their lower LUMO energy levels caused by fluorination.
Similarly, we have recently designed and prepared a novel
perfluorinated perylene diimide, N,N′-diperfluorophenyl-3,4,9,-
1
0-perylenetetracarboxylic diimide (DFPP), which was dem-
*
Corresponding authors. E-mail: mwang@zju.edu.cn, hzchen@zju.edu.cn.
1
0.1021/jp0362382 CCC: $27.50 © 2004 American Chemical Society
Published on Web 04/17/2004

5902 J. Phys. Chem. B, Vol. 108, No. 19, 2004
Shi et al.
Figure 1. The molecular structures of (a) DFPP and (b) DPP.
Figure 3. UV-vis absorption (-) and photoluminescence (- ‚ - ‚ -)
spectra of perylene diimide thin films evaporated on various temperature
substrates: (a) DPP on the substrate at room temperature, (b) DFPP
on the substrate at room temperature, and (c) DFPP on the substrate at
2
00 °C. The excitation wavelength was 470 nm.
DPP film deposited on the substrate at room temperature (Figure
a), the fine structure of the DPP absorptions in solution
3
disappeared, and a new peak at 570 nm derived from π-π
interactions of the chromophores in the solid state appeared,18
indicating that a crystalline thin film was formed by overlapping
1
2
neighboring perylene moieties. For DFPP film evaporated on
the substrate at either room temperature (DFPP-1 film, Figure
3b) or 200 °C (DFPP-2 film, Figure 3c), the same three clearly
resolved absorption bands as those in solution still existed;
nevertheless, they became broader and red-shifted. The higher
the substrate temperature was, the greater the red-shift was. That
is to say, DFPP film showed molecular or monomeric absorp-
tions, implying the absence of π-π coupling. According to the
model of transition dipole interaction, the energy of an ordered
Figure 2. UV-vis absorption (-) and photoluminescence (- ‚ - ‚ -)
spectra of (a) DPP and (b) DFPP solutions in CHCl
3
. The excitation
wavelength was 470 nm.
voltammetry (CV) measurements of DFPP films vapor deposited
on ITO substrates were carried out on a CHI600A electrochemi-
cal workstation, with solutions in CH3CN containing 0.1 M
supporting electrolyte of tetrabutylammonium perchlorate, in a
three electrode cell, where ITO was the working electrode, Pt
wire was the counter electrode, and saturated calomel electrode
19
multilayer thin film is given by:
∆ECR ) ∆E + D - S + e
(1)
S
(SCE) was used as the reference electrode. The morphologies
of the films were observed on a Skeio SPI3800N atomic force
microscope (AFM) and a JEOL JSM5510 scanning electron
microscope (SEM). The crystalline structures of the DFPP films
deposited on glass substrates with different temperatures were
investigated with a Bede D1 system high-solution X-ray
diffractometer (XRD).
where ∆ECR is the transition energy, ∆ES is the energy of the
transition for the molecule in solution, D - S is the difference
between the van der Waals intermolecular energies in the excited
and the ground state as compared to that between the solid state
and the solution, and e is the sum of the coupling energies of
the transition dipoles in the film. For both the DFPP-1 and the
DFPP-2 films, the e value was zero because no band splitting
was observed in their UV-vis absorption spectra; thus D - S
of DFPP-1 film was obtained as ∆ECR (537 nm) - ∆ES (529
nm) ) 8 nm (-0.035 eV), and that of the DFPP-2 film was
3
. Results and Discussion
Shown in Figure 2 are the UV-vis absorption and fluores-
cence spectra of DFPP and DPP solutions in CHCl3, respec-
tively. It is found that the absorption and fluorescence spectra
in solutions were almost not affected by the fluorination of
perylene diimide. Both DFPP and DPP had absorption and
emission peaks identical to those of a progression of π-π*
transitions of the perylene ring, designated as 0-0, 0-1, and
∆
ECR (560 nm) - ∆ES (529 nm) ) 31 nm (-0.130 eV). Such
a great gap of van der Waals forces between DFPP-1 and
DFPP-2 films was interesting, but what caused it?
To find the answer to the mentioned question, the simulations
of the molecular conformations of DFPP and DPP molecules
were performed by AM1 semiempirical quantum mechanics
modeling. It was found that the angle between the perylene
moiety and the terminal substituent, the perfluorinated or
nonfluorinated phenyl group, is ∼70° for the DFPP molecule,
whereas it is ∼50° for the DPP molecule. The larger angle in
DFPP molecule can disrupt the planarity of the perylene diimide
molecule, and the strong electronegative F atoms in near end
groups are electrostatic repulsive from each other. As a result,
DFPP molecules were prevented from stacking through π-π
interactions of successive perylene rings. So, the reasonable
interpreting was that, on the substrate at room temperature,
0
(
-2, in addition to the absorption and emission peaks of DFPP
absorptions, 529, 493, and 461 nm; emissions, 541 and 580
nm) being red-shifted about 5 nm relative to those of DPP
absorptions, 524, 488, and 457 nm; emissions, 537 and 575
(
nm). These observations suggested that the solvation of DFPP
was stronger than that of DPP due to the introduction of strong
electronegative F atoms into DFPP.
In contrast, the absorption and fluorescence spectra of DFPP
and DPP films vapor deposited on various temperature quartz
substrates (Figure 3) showed obvious F substituent effects. For

Structure of Fluoroperylene Diimide Thin Film
J. Phys. Chem. B, Vol. 108, No. 19, 2004 5903
and DFPP films (Figure 3) and cyclic voltammograms of DFPP
films (Figure 4). It was disclosed that DPP film exhibited an
2
1
excimer emission peak at 618 nm (Figure 3a) because of the
short intermolecular distances, although π-π interactions
favored fluorescence quenching through photoinduced charge
transfer. Because DFPP molecules distributed randomly in
DFPP-1 film, the distance between a few of the perylene diimide
molecules must be close enough that DFPP-1 film also showed
an excimer emission at 626 nm (Figure 3b). However, in
DFPP-2 film, the distance between perylene chromophores was
remote so that DFPP-2 film only showed monomeric emission
peaks at 584 (0-0) and 646 nm (0-1), from which the gap of
the vibrational levels was found to be 0.204 eV, in agreement
with the wavenumber for the stretching vibration of the perylene
-
1
22
ring of 1594 cm (0.198 eV). It was also noticeable that the
fluorescence intensity of DFPP-2 film was about 5 or 20 times
higher, respectively, than that of DFPP-1 or DPP film because
there was no dimer formation. In Figure 4, the two reduction
peaks of DFPP-2 film were more positive than those of DFPP-1
film; that is, the electron affinity of DFPP-2 film was higher
than that of DFPP-1 film. It was believed that the electronic
characteristics of π-conjugated perylene rings were significantly
influenced by the inductive effects of near strong electronegative
perfluorinated phenyl groups in DFPP-2 film, leading to higher
electron affinity of DFPP-2 film.
Figure 4. The cyclic voltammograms of DFPP films formed on various
temperature substrates: (a) at room temperature and (b) at 200 °C.
The scanning rate was 50 mV/s.
DFPP molecules stacked randomly to give an amorphous film,
consistent with the N,N′-di(2,6-xylyl)-3,4,9,10-perylenetetra-
carboxylic diimide film, which also showed monomeric absorp-
tions because the large substituent, 2,6-xylyl group, was
perpendicular to the flat π-electron-system of perylene and was
sterically hindered to form crystals.20 Instead, on the substrate
at 200 °C, the energy of DFPP molecules was high enough to
overcome the intermolecular steric actions and the electrostatic
repulsions, so that DFPP molecules could arrange with the
perfluorinated phenyl group straight over or below the perylene
core of the adjacent molecule to achieve the most favorable
energetic state; that is, a crystalline DFPP film was obtained.
At the same time, π electrons of perylene received great van
der Waals forces.
The hypothesis can also be confirmed directly by SEM, AFM
(Figure 5), and XRD (Figure 6) characterizations. As AFM and
SEM photographs have shown, DPP film consisted of spherical
crystalline particles with diameters of about 250 nm, and
DFPP-1 film appeared very smooth with a typical root-mean-
square roughness (Rq) of 3.7 nm, indicating the characteristic
of an amorphous film. Interestingly, in DFPP-2 film, there were
many nanowires with diameters of 100-200 nm, implying that
an ordered crystalline film was obtained. As demonstrated in
The hypothesis for the aggregation of DFPP molecules was
supported by the results from the fluorescence spectra of DPP
Figure 5. AFM (upper) and SEM (bottom) images of perylene diimide thin films evaporated on various temperature substrates: DPP on the
substrate at room temperature ((a) AFM, (d) SEM), DFPP on the substrate at room temperature ((b) AFM, (e) SEM), and DFPP on the substrate
at 200 °C ((c) AFM, (f) SEM).

5904 J. Phys. Chem. B, Vol. 108, No. 19, 2004
Shi et al.
°
C. The film had a unique aggregate structure, where DFPP
molecules stacked with the perfluorinated phenyl groups straight
over or below the perylene cores of the adjacent DFPP
molecules. Knowing how the fluorination influences the ag-
gregate structure of perylene diimide thin films will definitely
help us to understand the unique behaviors of fluoroperylene
diimides in optoelectronic devices.
Acknowledgment. This work was supported by the National
Natural Science Foundation of China (Nos. 50173021, 50225312)
and by the fund for the century scholar-training project from
the Education Department of China.
References and Notes
(
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Figure 6, DFPP-1 film only exhibited a weak XRD peak at 2θ
(
)
5.34° (Figure 6a), implying low crystallinity. However,
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(
1
2
5.98° (Figure 6b), with some small peaks at 5.23°, 21.43°,
5.31°, 32.41°, 38.01°, and 43.75° (inset of Figure 6). From
(
these peaks, the d values were determined as 16.88, 8.34, 5.54,
.14, 3.51, 2.76, 2.36, and 2.07 Å. These d values could be
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2
002, 80, 2517.
4
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8
(
(
composed of 3.51 and 2.36 Å. It was easily observed that in
the first group the ratios of all d values to 16.88 Å were exactly
1
1
1
1
1
1
3
, /2, /3, /4, /6, and /8, and in the second group the ratio of
2
07.
3
.51 to 2.36 Å was /2. Therefore, it could be concluded that in
(
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DFPP-2 film most DFPP molecules aggregated to form a highly
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2
(
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1
(
4. Conclusions
(
In conclusion, a crystalline DFPP thin film, which showed
monomeric or molecular absorption and emission spectra, could
be prepared by elevating the temperature of the substrate to 200
1
(