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23 AUGUST 2013

PHYSICAL REVIEW LETTERS

PRL 111, 086602 (2013)

Substituent-Induced Intermolecular Interaction in Organic Crystals Revealedby Precise Band-Dispersion Measurements
Hiroyuki Yamane* and Nobuhiro Kosugi
Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan(Received 9 May 2013; published 21 August 2013)
We reveal quite small but different intermolecular valence band dispersions of sub-100-meV scale incrystalline films of Zn and Mn phthalocyanine (ZnPc and MnPc) and fluorinated ZnPc (F16 ZnPc). Theintermolecular transfer integrals are found to be reasonably dependent on the intermolecular distance with relation. Furthermore, the angle-resolved photoemission spectra show anomalous
the 75  5 meV=A
dispersive behaviors such as phase flips and local-dimerization-derived periodicities, which originate fromthe site-specific intermolecular interaction induced by substituents.
DOI: 10.1103/PhysRevLett.111.086602

PACS numbers: 72.80.Le, 71.38.k, 73.20.At, 79.60.i

Organic semiconductors have attracted considerable
interest for (opto)electronics based on the degree of
freedom of molecules. Solid-state functionalities of suchorganic semiconductors are dominated not only by individual molecular properties but also by their intermolecular interactions. This concerted interplay governs a key
process of the electric conduction in organic electronics,
which consists of the charge injection at interfaces near the
Fermi level (EF ) [1–3] and the charge transport in organic
solids [4]. The coherence of the charge transport is dominated by the intermolecular coupling of individual molecular orbitals, depending on various conformations. In thisLetter, we discuss the electronic band dispersion, i.e.,energy versus wave vector [EðkÞ], of organic solids withhigh experimental accuracy, which can assess charge transport properties and related phenomena.
The intermolecular EðkÞ relation, originating from theperiodicity of the molecular stacking structure, is in general difficult to observe precisely. This is because a verysmall bandwidth due to the weak intermolecular interaction is easily obscured by structural and thermal energy
broadenings. Therefore, the intermolecular EðkÞ relation
has been mainly reported for high hole-mobility (h )materials with relatively large bandwidths ( 0:2 eV)[5–10]. Even in these cases, however, the intermolecularEðkÞ relation is dominated by charge transport propertiesof the target molecules themselves. Precise and systematic
study on the intermolecular EðkÞ relation has not yet been
established as a universal role in organic semiconductors.
Thus, the fundamental knowledge on the intermolecularinteraction is still insufficient.
In the present work, by using angle-resolved photoemission spectroscopy (ARPES) with synchrotron radiation, we
shed light on quite small intermolecular EðkÞ relations of
sub-100-meV scale in crystalline films of metal phthalocyanine (MPc, M ¼ metal). Even though MPc molecules
are archetypal hole-transport materials in organic photovoltaics, their (opto)electronic properties have not yet
been fully understood in relation to the intermolecular
0031-9007=13=111(8)=086602(5)interaction because of the small EðkÞ relation, which iseasily buried by energetic disorders. Such a small EðkÞrelation has not yet attracted much attention due to the lackof credible experimental data; however, present preciseEðkÞ measurements on MPc with different terminal groups
and central metals are found to be sensitive and essential tocharacterize the intermolecular interaction in terms of theintermolecular distance, the conformational geometry, andthe orbital character. Such a systematic approach provides
deeper insights into the nature of the intermolecular interaction, which further represents the importance of the site
specificity in the intermolecular interaction as a possible
origin of unique molecular electronic properties.
A precise measurement of the small intermolecular EðkÞrelation was carried out for MPc crystalline films on
Au(111). Since the interaction between the physisorbed
molecule and Au(111) is relatively weak [11,12], the selfassembling behavior by the intermolecular interaction
plays a crucial role in the film crystallinity and its morphology from the initial stage of the film growth.
MPc crystalline films were prepared by vacuum deposi min onto the cleantion of purified MPc powders at 2 A=
Au(111) surface at 300 K. The Au(111) substrate wascleaned by repeated cycles of Arþ sputtering and annealing
at 700 K, as confirmed by a core-level and a Shockley-state
photoemission, and LEED. For molecular films, the photoemission peaks show the thickness-dependent energy shift,originating from the photohole relaxation by surrounding
mediums [13,14]. Thus, the MPc deposition was continued
until the MPc-derived peaks were stabilized in energy withrespect to the deposition amount. The deposited MPc films
were subsequently annealed at 350–360 K, which is the
optimized temperature for the fabrication of MPc-based
photovoltaic cells [15]. This annealing induces the enhancement of the Au 4f photoemission peak as the result of
the molecular aggregation by further crystallization. The
sample crystallinity was characterized by N K-edge
x-ray absorption spectroscopy (XAS), LEED, and ex situ
CuK x-ray diffraction (XRD) (see the Supplemental

086602-1

Ó 2013 American Physical Society

Material [16]). The ARPES and XAS experiments were
performed at a high-brilliant vacuum ultraviolet andsoft x-ray in-vacuum linear-polarized undulator beamline 6U of UVSOR-III at the Institute for Molecular
Science. The ARPES spectra were obtained by using a
micro-channel-plate detector of a hemispherical electronenergy analyzer (A1, MB Scientific). The total energy
resolution of 13 meV and the angular resolution of 0.2
were used in the present work. In order to avoid the radiation
damage of the sample, by incident photons during the
ARPES measurements, we chose a different fresh sample
position after every measurement.
The ZnPc molecules on Au(111) form a flat-lying crystalline film with the intermolecular distance along thesurface normal (a? ) of 3.326 Å in a Stranski-Krastanov
growth mode (see the Supplemental Material [16]). In
order to investigate the EðkÞ relation in the - stacking
direction, the normal-emission ARPES spectra as a function of the photon energy (h) were measured for the ZnPc
crystalline film at the specimen temperature (T) of 15 K
[Fig. 1(a)]. Because of the growth mode of the ZnPc
molecule on Au(111), the small remanent substrate EF
appears and is utilized for the energy calibration for the
precise Eðk? Þ measurement. The ZnPc-derived peaks,labeled highest occupied molecular orbital (HOMO) and, show a small but clear dispersive behavior with h.
The HOMO peak appears at the binding energy (Eb ) of
1.32 eV at h ¼ 46 eV. With increasing h, the HOMO
peak shifts to the higher Eb side, and turns back at(a) α-ZnPc/Au(111) @ T = 15 K
hvh ¼ 58 eV. Such an inflection point is also found aroundh ¼ 90 and 130 eV.
After converting to k? from h in the second derivativeof the ARPES spectra, the Eðk? Þ map for the ZnPc crystalline film at 15 K is obtained [Fig. 1(b)]. For the k?
conversion, we used an inner potential of 1:0 eV, basedon the low-energy electron transmission experiment on
-CuPc [17]. The weak shoulder structure in the ARPES
spectra around Eb ¼ 2:2 eV is resolved in the Eðk? Þ map
as labeled HOMO-1 and is derived from a Zn 3dx2 y2orbital with the b1g symmetry [18]. In the Eðk? Þ map,the HOMO, HOMO-1, and  peaks show the dispersion
with the same k? periodicity. The inflection points in the
periodic dispersion correspond to the  and Y points in the
Brillouin zone of the ZnPc crystalline film, as determined by XRD. This agreement proves thatfrom a? ¼ 3:326 A
the observed periodic dispersion is ascribed to the delocalized band formation due to the weak intermolecularvan der Waals (vdW) interaction. The one-dimensionaltight-binding fitting, Eðk? Þ ¼ E0b  2t? cosða? k? Þ, asshown by white curves in the Eðk? Þ map, to the HOMOband dispersion at 15 K gives the dispersion center (E0b ) of
1.388 eV, the transfer integral (t? ) of 23 meV, and theresultant hole effective mass (mh ) of 15:0m0 , where m0 is
the free-electron mass. The observed mh gives the ideal hof 26:6 cm2 V1 s1 for the HOMO of -ZnPc crystals at
15 K from h > 20ðm0 =mh Þð300=TÞ [5].
In -ZnPc crystals, two nonequivalent molecules in
the unit cell should induce the HOMO-band splitting, as(b) α-ZnPc/Au(111) @ T = 15 Ke–

hv = 45 eV

Intensity

EF
Peak width

N
N
N

N
N

Zn
N

N
N

HOMO

hv
/eV
50

Intensity / normalized to photon flux

0

Binding Energy Eb / eV

Intensity / normalized to photon flux

Eb / eV 5

1

HOMO
2
HOMO-1
3

70
90
110

4

130
3

2

1

(c) α-ZnPc/Au(111): HOMO

EF = 060°

E

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PHYSICAL REVIEW LETTERS

PRL 111, 086602 (2013)

0 (= EF)

Binding Energy Eb / eV

HOMO

Bandwidthhv = 60 eV ( )hv = 90 eV (Y)Peak width= 380 meV

T/K
300
180
120
80

Peak width= 435 meV15

Bandwidth = 92 meV

4

5

Wave vector k⊥ / Å-1

SUM of–d2I / dEb2

2

1

0 (= EF)

Binding Energy Eb / eV

FIG. 1 (color online). Intermolecular EðkÞ relation in the ZnPc crystalline film on Au(111). (a) h-dependent normal-emission
ARPES spectra at 15 K (h step ¼ 4 eV). The experimental geometry, wide valence-band spectrum at h ¼ 45 eV, and the linear
combination of atomic orbitals (LCAO) pattern of the HOMO of ZnPc are also shown. (b) Eðk? Þ map obtained from the second
derivative of the ARPES spectra ( d2 I=dE2b ), with the tight-binding fitting (white curve). Sum of the d2 I=dE2b spectra is shown in
the right side of the Eðk? Þ map. (c) T dependence of the HOMO band at the  and Y points, measured at h ¼ 60 and 90 eV,
respectively. The HOMO-band dispersion is illustrated in the inset.

086602-2

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PHYSICAL REVIEW LETTERS(a) βBL-F16ZnPc/Au(111) @ T = 15 K

(b)

e–
60°

0

hv = 45 eV
hv

Y

E

1
Eb / eV 5

0
N

N

N
Zn

N
N

N
N

N

HOMO

hv
/eV
50
70

Binding Energy Eb / eV

observed for pentacene and rubrene with the split energy of0:4 eV by relatively strong intermolecular interaction[7–9]. In the present case of ZnPc, however, the HOMOband splitting is not resolved by the weak intermolecularinteraction. It might be reflected in the T-dependent widthand energy of the HOMO peak according to the small
polaron theory based on the electron-phonon (e-ph) coupling [19,20]. Figure 1(c) shows the T-dependent HOMO
peak at the  and Y points, measured at h ¼ 60 and 90 eV,
respectively. The -point HOMO shifts to the lower Ebside with increasing T, while the Y-point HOMO does notshift, indicating the narrowing of the bandwidth at higher
T. Furthermore, only the -point HOMO shows a peakwidth narrowing with increasing T (435 meV at 15 K and
380 meV at 300 K), which is the opposite trend of thethermal broadening in the remanent substrate EF . Thisevidence indicates that the -point HOMO is weakly split
at 15 K. Here, the bandwidth (4t? ) of 92 meV at 15 K is of
the same order as the reorganization energy for local
phonons in CuPc [21]. It is considered that the
T-dependent transition between the incoherent hoppingtransport by the local e-ph coupling and the coherent bandlike transport by the nonlocal e-ph coupling exists within
the energy scale of 100 meV in -ZnPc crystals.
Since functionalities of MPc can be modified by thechemical substitution of terminal groups and central metals
in the molecule, the present precise EðkÞ observation for
ZnPc is considered to be a starting system in carrying out asystematic study on the dependence of the intermolecularinteraction. In this context, we prepared crystalline filmsof fluorinated ZnPc (F16 ZnPc) and MnPc on Au(111),
obtained by the same sample preparation procedure for
ZnPc. It is confirmed by XAS and XRD that the molecules
form the -bilayer (BL )-crystalline phase [22,23] with
 for F16 ZnPc and the -crystalline phasea? ¼ 3:236 A
 for MnPc (see the Supplemental
with a? ¼ 3:273 A
Material [16]).
As shown in the inset of Fig. 2(a), the HOMO of
F16 ZnPc is derived from the C 2p and F 2p orbitals witha1u symmetry. Different intermolecular interaction
between F16 ZnPc and ZnPc is expected due to the F 2p
orbital and the different a? . The h dependence of thenormal-emission ARPES spectra for the F16 ZnPc crystalline film on Au(111) at 15 K is shown in Fig. 2(a). The
HOMO peak shows a clear dispersive behavior with h.
Figure 2(b) shows the Eðk? Þ map in the HOMO-band
region of F16 ZnPc, in comparison with that of ZnPc. We
found that the HOMO bandwidth of F16 ZnPc (120 meV)is larger than that of ZnPc (92 meV), which is reasonably
explained by a shorter intermolecular distance a? ¼
 for ZnPc.
 for F16 ZnPc than a? ¼ 3:326 A
3:236 AA more important result in Fig. 2(b) is the difference inthe HOMO dispersion phase between ZnPc and F16 ZnPc
crystalline films. From the  to Y point, the HOMO band of
ZnPc disperses to the lower Eb side, in agreement with the

Intensity / normalized to photon flux

PRL 111, 086602 (2013)

2
F16ZnPc

4

5

6

5

6

Y

0

1

90
110
130
4

3

2

1

0 (= EF)

Binding Energy Eb / eV

2
ZnPc

4

Wave vector k⊥ / Å-1

FIG. 2 (color online). (a) h-dependent normal-emission
ARPES spectra (h step ¼ 4 eV) of the F16 ZnPc crystalline
film on Au(111) at 15 K. The experimental geometry, wide
valence-band spectrum at h ¼ 45 eV, and the LCAO patternof the HOMO of F16 ZnPc are also shown. (b) Eðk? Þ map of the
HOMO band for crystalline films of F16 ZnPc and ZnPc, obtained
from the second derivative of the ARPES spectra, with the tightbinding fitting (white curve).

calculated band structure of -CuPc [24]. On the other
hand, the HOMO dispersion direction of F16 ZnPc is flippedfrom that of ZnPc. The observed EðkÞ-phase flip may
originate from the modification of the intermolecular
HOMO overlap between ZnPc and F16 ZnPc due to thedifferent conformational structure and an intermolecularinteraction between Pc  and F 2p.
The next modification is on the central metal, MnPc. The
HOMO of MnPc is derived from the half filled Mn 3d
orbital with eg symmetry, and the HOMO-1 of MnPc
corresponds to the HOMO of ZnPc [25–27]. The half filled
HOMO of MnPc induces the crystallinity-dependent magnetic properties [28]. Figure 3(a) shows the h dependenceof the normal-emission ARPES spectra for the MnPc
crystalline film on Au(111) at 15 K. The relative intensity
between the HOMO (Mn 3d) and HOMO-1 (C 2p) peaks
changes with h due to different ionization cross sections.
The Mn 3d and C 2p peaks show a clear dispersive
behavior with h. Moreover, the Mn 3d peak consists of
two components, indicating the relatively strong interaction at the Mn site. Figure 3(b) shows the Eðk? Þ map for
the MnPc crystalline film on Au(111) at 15 K, wherein the (see the and Y points are estimated from a? ¼ 3:273 A
Supplemental Material [16]). The agreement between
the periodicity of the C 2p dispersion and the Brillouinzone indicates that the observed C 2p dispersion is
ascribed to the intermolecular vdW interaction. From thetight-binding fitting, we found that the bandwidth for the C

086602-3

hv

(b)

e–
60°

EF = 0Y2a Y Y2aEb / eV 50

C 2p Mn 3d

hv
/eV
50

Binding Energy Eb / eV

Intensity / normalized to photon flux

E

1

Mn 3d
C 2p

2

3

70
90
110

4

130
4

3

2

1

4

0 (= EF)

5

Wave vector k ⊥ / Å-1

Binding Energy Eb / eV

FIG. 3 (color online). (a) h-dependent normal-emission
ARPES spectra (h step ¼ 4 eV) of the MnPc crystalline film
on Au(111) at 15 K. The experimental geometry, wide valenceband spectrum at h ¼ 45 eV, and the LCAO pattern of the
HOMO of MnPc are also shown. (b) Eðk? Þ map obtained fromthe second derivative of the ARPES spectra, with the tightbinding fitting (white curve).

2p band of MnPc (104 meV) is between ZnPc (92 meV)and F16 ZnPc (120 meV), which is consistent with
 for MnPc between a? ¼ 3:326 A
 fora? ¼ 3:273 A

ZnPc and a? ¼ 3:236 A for F16 ZnPc.
For the Mn 3d peak, the peak splitting and its dispersion
are resolved in the Eðk? Þ map. The k? periodicity of theMn 3d dispersion is 1=2 times that of the C 2p dispersion
as labeled Y2a and is simply thought to originate from thelocal dimerization and the resultant double-a? -periodicity
modulation at the Mn site. Such evidence is not observablefor the full-filled Co 3d peak of the CoPc crystalline
film (see the Supplemental Material [16]), suggesting theimportance of the half filled orbital character. This is anindication of the strong electron correlation at the Mn site,which induces the smaller mh of 3:6m0 for the topmost Mn
3d band as well as the antiferromagnetism found for the
MnPc crystalline film [28].

As summarized in Table I, we have succeeded in observation of the intermolecular EðkÞ relations of sub-100-meV
scale in crystalline films of various MPc molecules. Here,F16 CuPc (similar to F16 ZnPc) solids show the optical absorption at 688 nm for the BL phase and at 780 nm for the phase [23]. On the other hand, CuPc solids show the
optical absorption at 687 nm for the  phase and at 714 nm
for the  phase [29]. Since the optical absorption on MPc
solids is sensitive to intermolecular transition dipole
moments, it is considered that the crystalline structure ofBL -F16 ZnPc resembles that of -ZnPc rather than
-ZnPc. Furthermore, the LEED pattern of the F16 ZnPc
film corresponds to that of other nonfluorinated MPc films(see the Supplemental Material [16]), supporting the similarity of the crystalline structure between BL -F16 ZnPc and
-ZnPc. Although a little contribution of the F 2p orbitalon the C 2p bandwidth of F16 ZnPc exists, the t? -versus-a?
relation for the C 2p band of MPc can be linearly fitted in as
the present a? range with t? =a? ¼ 75  5 meV=A,
shown in Fig. 4. This parameter can be a reference forthe refinement of theoretical calculations on the intermolecular vdW interaction, which sometimes over- or underestimate the bandwidth.
In addition to the systematic characterization of the EðkÞwidth, based on the sensitivity of the small EðkÞ relation tosurrounding environments, we observed changes in theEðkÞ phase and the k periodicity by the substitution ofterminal groups and central metals in MPc, e.g., (i) theEðkÞ-phase flip of the HOMO band between ZnPc and
F16 ZnPc, and (ii) the double-periodicity modulation forthe half filled Mn 3d band of MnPc. These anomalous
behaviors in the intermolecular EðkÞ relation can be
explained by the presence of the site-specific intermolecular interaction due to substituents in MPc, which induces
miscellaneous electronic and magnetic properties. The
present results clearly indicate that, even in singlecomponent molecular systems, strong local electron correlations can be introduced by the molecular design and its
local intermolecular interaction.

Transfer Integral t⊥ / meV(a) α-MnPc/Au(111) @ T = 15 K
hv = 45 eV

TABLE I. The energy of the dispersion center E0b (eV), thetransfer integral t? (meV), the hole effective mass mh , and thehole mobility h (cm2 V1 s1 ) for the C 2p derived band of
ZnPc, MnPc, and F16 ZnPc crystalline films at 15 K, togetherwith their intermolecular distance a? (A).

ZnPc
MnPc
F16 ZnPc

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PHYSICAL REVIEW LETTERS

PRL 111, 086602 (2013)

a?

E0b

t?

mh

h

3.326
3.273
3.236

1.388
1.274
1.251

23
26
30

15:0m013:7m012:2m0

26.6
29.2
32.9

30

BL-F16 ZnPc

Cryst. MPc on Au(111)
C 2p band @ T = 15 K

α-MnPc25

α-CoPc (Ref. [16])α-ZnPcα-H2Pc (Ref. [16])

20
3.2

3.3

Intermolecular Distance

3.4

/Å

FIG. 4 (color online). The t? -versus-a? relation for the C 2pband in MPc crystals at 15 K. Experimental data for CoPc andH2 Pc are given in the Supplemental Material [16].

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PHYSICAL REVIEW LETTERS

As demonstrated above, precise and systematic intermolecular EðkÞ measurements on various organometallic
complexes enable us to reveal different intermolecularinteractions quantitatively and to obtain a quantitative
guiding principle for the design of functional molecularsystems and their theoretical considerations from the viewpoint of the intermolecular interaction in addition to theindividual molecular property, as a critical tool for organic
electronics.
We thank the staff of the UVSOR facility for their kind
support, in particular, T. Horigome in the development of
the ARPES system. We also thank T. Hosokai (Iwate
University) for fruitful discussion on the crystalline
structure of F16 ZnPc. This work was partly supported by
Grants-in-Aid for Scientific Research (A) (No. 23245007)
and for Young Scientists (A) (No. 24685032) from JSPS.

*Corresponding author.
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