Electronic structures of unoccupied states in lithiumphthalocyanine thin films of different

polymorphs studied by IPES

Naoki Satoa,*, Hiroyuki Yoshidaa, Kiyohiko Tsutsumia,Michinori Sumimotob, Hitoshi Fujimotob, Shigeyoshi Sakakic,1

aInstitute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, JapanbDepartment of Chemistry, Kumamoto University, Kurokami, Kumamoto 860-8555, Japan

cDepartment of Applied Chemistry and Biochemistry, Kumamoto University, Kurokami, Kumamoto 860-8555, Japan

Abstract

Electronic structures of the unoccupied states in lithium phthalocyanine (LiPc) thin films of the x- and a-polymorphs were

directly observed with inverse photoemission spectroscopy (IPES). The energy position of the lowest energy band and the

lineshape of the second lowest energy band were different between the obtained IPE spectra for the two forms of the films. By

comparing with energy levels in a LiPc molecule and its dimers calculated using the density functional method, we interpreted

such differences as follows: the first band placed in lower energy for the a-form is due to more stabilized singly occupied

molecular orbital (SOMO) and the observed splitting of the second feature for the x-form is lead from the removed degeneracy of

6eg levels caused by a large intermolecular interaction along the molecular stacking axis.

# 2003 Elsevier Science B.V. All rights reserved.

Keywords: Unoccupied state; Electronic structure; Inverse photoemission; Lithium phthalocyanine; Thin film; Polymorphism

1. Introduction

Lithium phthalocyanine (LiPc) is a monovalent-

metal phthalocyanine (Pc) complex and a stable radi-

cal even in the air [1]; it shows polymorphism like

most of other Pc complexes. Then, physical properties

of its solids such as intrinsic semiconductivity and

magnetism in relation to the polymorphism have

attracted the attention of researchers in the field of

organic materials science [2–4]. Besides, LiPc thin

films of the x- and the a-forms demonstrate different

optical absorption spectra, while they have similar

electronic structures in the valence region as observed

with UPS [5,6]. In this work, electronic structures of

the unoccupied states in LiPc thin films of the x- and a-

forms were directly observed with inverse photoemis-

sion spectroscopy (IPES) in the BIS mode.

2. Experimental

Three kinds of LiPc thin films in the thickness of

5–30 nm were prepared as follows: (i) a film evaporated

Applied Surface Science 212–213 (2003) 438–440

* Corresponding author. Tel.: þ81-774-38-3080;

fax: þ81-774-38-3084.

E-mail address: naokis@tampopo.kuicr.kyoto-u.ac.jp (N. Sato).1 Present address: Department of Molecular Engineering, Gradu-

ate School of Engineering, Kyoto University, Kyoto 606-8501,

Japan.

0169-4332/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0169-4332(03)00128-4

on a room temperature gold substrate, (ii) a film

deposited on a gold one at about 200 8C, and (iii) a

film treated with liquid acetone after deposition at

room temperature. These procedures were employed

in consideration of the results reported on LiPc thin

films prepared on glass substrates [2,3]; the x- and the

a-form films are obtained on glass below 150 8C and

around 200 8C, respectively, and the x-form film is

transformed to the a-form one by the acetone treatment.

Characterization of the morphology for the thicker

films was carried out with optical and infrared reflec-

tion spectroscopies and X-ray diffraction. IPE spectra

were measured at room temperature using our home-

built apparatus [7]. Monoenergetic electron-beam

excitation of a sample film induced emission of

photons, which were focused by a concave mirror

on a bandpass detector with the maximum sensitivity

at 9.8 eV; the overall energy resolution of the IPE

spectra was about 0.8 eV.

Electronic structures of a LiPc monomer and its

dimers of the particular structures corresponding to

the x- and the a-forms were calculated using the

GAUSSIAN 98 program with the density functional

theory (DFT) method; the B3LYP functional was used

for exchange-correlation terms. A square planar struc-

ture of the D4h symmetry was assumed for the mole-

cular geometry optimization.

3. Results and discussion

As for the three kinds of the LiPc thin films

described as (i), (ii), and (iii) in the above section,

the optical absorption spectrum (obtained from the

Kramers–Kronig transformation of the reflection

spectrum) of (i) coincided with the reported spectrum

of the x-form film, and those of (ii) and (iii) were

rather similar to each other and in good agreement

with the reported one of the a-form film. However,

such a situation was slightly different in the case of the

infrared spectra and the X-ray diffraction patterns.

These observations showed that (i) and (iii) were

clearly assignable to the x- and the a-form films,

respectively, and that (ii) was not completely char-

acterized as the a-form film on the other hand. As a

result, (i) and (iii) were identified as the x- and the

a-form films, respectively, however, (ii) was insuffi-

ciently assigned to be the a-form film; an a-form film

is not always prepared on a heated substrate of gold in

contrast to glass. We will therefore examine the IPE

spectra observed for (i) and (iii) for the most part from

now on.

Fig. 1 depicts the comparison of the IPE spectra

between the a-form film and the x-form one. Spectral

lineshapes and relative intensities among spectral

features appear to be different with a small energy

shift between the two spectra. For a close look at such

differences, we tried to deconvolute the spectra using

Gaussian functions as shown also in Fig. 1. This

procedure has made it clear that the IPE spectra are

dependent on the thin film morphology as follows: the

energy positions of the lowest energy features (indi-

cated by the dashed arrows) are different between the

two spectra and as for the lineshape of the second

bands two Gaussian components are clearly resolved

for the x-form in contrast to the a-form where they

overlap (solid arrows).

The calculated electronic structures of a LiPc

monomer and two kinds of its dimers of a monoclinic

and a tetragonal structures corresponding to the a- and

the x-forms, respectively, are referred to consider the

differences noted above. The calculated results of

these dimers show that the triplet and the singlet states

are stabilized in the monoclinic and the tetragonal

Fig. 1. Comparison of the IPE spectra for the LiPc thin films

between (a) a-form and (b) x-form. The ordinate is the energy with

reference to the vacuum level of LiPc, which is assumed to align to

that of the gold substrate, and as for the abscissae spectral

intensities increase toward both sides. The spectral deconvolution

using Gaussian functions is shown for each spectrum. The arrows

point the energy positions of the spectral features to be noted; see

text for detail.

N. Sato et al. / Applied Surface Science 212–213 (2003) 438–440 439

structures, respectively; this is consistent with the

reported EPR results [4]. In Fig. 1, the spectral features

noticed by the Gaussian components are assigned

based on the molecular orbitals (MOs) calculated

for a LiPc molecule; the singly occupied molecular

orbital (SOMO) is the 2a1u orbital and the next

lowest unoccupied molecular orbital (NLUMO) is

the 6eg one.

The notable differences of the IPE spectra between

the a- and the x-forms come down to the lower energy

of the spectral feature derived from SOMO for the a-

form and the splitting of the second feature originated

from NLUMO for the x-form. While the lowest

unfilled levels calculated for the dimers of the mono-

clinic and the tetragonal structures are situated close in

energy, the energy difference observed for the first

features will be explained by the stabilization of extra

electrons introduced during the IPE process due to

their delocalization throughout the molecular arrange-

ment. On the other hand, the splitting of the second

feature in the case of the x-form can be interpreted by a

removal of the degeneracy of the 6eg levels caused by

a large overlap of these MO’s along the molecular

stacking axis. Such a result is regarded as the first

direct observation of a remarkable intermolecular

orbital interaction working among unoccupied states

in the organic solid state.

Acknowledgements

This work was supported in part by Grant-in-Aid

for Scientific Research No. 11304049 and 12CE2005

from the Ministry of Education, Culture, Sports,

Science and Technology of Japan.

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