Title Molecular Aggregates

Author(s)

Citation ICR annual report (2004), 10: 10-11

Issue Date 2004-03

URL http://hdl.handle.net/2433/65410

Right

Type Article

Textversion publisher

Kyoto University

Lecturer (pt)ProfHIRATSUKA, Hiroshi (D Sc)

AssocProfASAMI,Koji

(D Sc)

Gunma University

InstrKITA, Yasuo

(D Sc)

InstrYOSHIDA, Hiroyuki

(D Sc)

StudentsHIRAMATSU, Takaaki (Dl)YAMAGAMI, Takashi (M2)YAMAGUCIll, Takayuki (Ml)

TSUTSUMI, Jun'ya (Dl)MUROI, Kouji (Ml)

HARADA, Youichiro (M2)TSUJIKAWA, Susumu (Ml)

VisitorProfFELDMAN, Yuri Hebrew University ofJerusalem, Israel, 10 November 2003

Japan (Okayama, Japan), 20 - 23 September.

Polymorphism and Electronic Structures of Unoccupied

States in Lithium Phthalocyanine Thin Films Studied us-

ing Inverse Photoemission Spectroscopy, Sato N, Tsutsurni

K, Yoshida H, Fujimoto H (Kumamoto U.), Sumimoto M(Dept. Mol. Eng.), Sakaki S (Dept. Mol. Eng.), 2nd Japan-

Sweden Workshop on Advanced Spectroscopy of Organic

Materials for Electronic Applications (Hayama, Japan), 21 -

25 October.

GrantsAsarni K, Manipulation ofSingle Biological Cells by AC

Fields and Their Dielectric Spectroscopy, Grant-in-Aid for

Scientific Research (C)(2), 1 April 2002 - 31 March 2004.

Sato N, Development of Novel Electronic Systems

Based on Hybridization of Characteristic Molecular Prop-

erties and Specific Aggregate Structures, Grant-in-Aid

for Scientific Research (2) on Priority Areas of Molecular

Conductors, 17 October 2003 - 31 March 2008.

f---------------------

The research at this subdivision is devoted to correlation studies on structures and properties of both natural and arti-

ficial molecular aggregates from two main standpoints: photoelectric and dielectric properties. The electronic structure

of organic thin films is studied using photoemission and inverse photoemission spectrosocpies in connection with the

former, and the results are applied to create novel molecular systems with characteristic electronic functions. The latter is

concerned with heterogeneous structures in microcapsules, biopolymers, biological membranes and biological cells, and

the nonlinearity in their dielectric properties is also studied in relation to molecular motions.

f-----------------

PresentationsElectron Correlation in Molecular Films: Impact on

Electronic Structure of Unoccupied States, Sato N, Inter-

national WE-Heraeus Seminar (No. 306) on Energetics

of Interfaces between Organic Molecules and Conductors

(Bad Honnef, Germany), 20 - 22 March.

Macroscopic Current-voltage Relations of Alamethicin

Ion Channels, Asami K, 25th Annual Meeting of Mem-

brane Society ofJapan (Tokyo, Japan), 8 - 9 May.

Correlation Between Polymorphism and Electronic

Properties of Lithium Phthalocyanine Studied on the Ba-

sis of Electronic Structures Observed for the Unoccupied

States, Sato N, Tsutsurni K, Yoshida H, Surnimoto M (Dept.Mol. Eng.), Fujimoto H (Kumamoto U.), Sakaki S (Dept.

Mol. Eng.), The European Conference on Organic Elec-

tronics & Related Phenomena 2003 (Wye, u.K.), 21 - 26September.

Dielectric Spectroscopy of Colloid and Biological cells,

Asami K, 2003 Autumn Meeting of Physical Society of

10 TOPICS AND INTRODUCTORY COLUMNS OF LABORATORIES------

Direct observation of electronic structures ofunoccupied states in lithium phthalocyaninethin films in relation to their polymorphism

forms of LiPc as mentioned above appear to be essentiallyelucidated to a large extent.

Intensity -

N-terminal insertion of alamethicin helices involtage-activated ion-channel formation

0

-1b

2u

~2U

> -2 b9(]) 1u

uf -3 e, 9~ -4

a1U

-5 /a-form x-form

-6

Fig. 1 Comparison of IPE spectra between the a-form and the x-form of LiPc films with respective molecular stacking manners shownschematically. The ordinate is the state energy with reference to thevacuum level and as for the abscissae spectral intensities increase towardboth sides. The spectral deconvolution using Gaussian functions isshown for each datum with the assignments; the arrows point the energypositions of the notable spectral features. Inserted pictures exhibit

molecular stacking manners schematically.

Alamethicin, a 20-residue peptide, is supposed toform ion channels of a helix-bundle type by insertingthe N-terrninus into lipid bilayer membranes under suf-ficient voltages. In this study, the N-terrninal insertion hasbeen examined with an alamethicin dimer (di-alm) thatis N-terminally linked by a disulfide bond. When di-almwas added to the cis-side membrane, it forms long lastingchannels with the lifetime r ofabout 100 ms at Cis-positivevoltages. The lifetime was reduced to a few millisecondsby addition of dithiothreitol (DTT) to the cis-side mem-brane, indicating that most of the channels were formed bythe monomers (alm-SH) resulted from the cleavage of thedisulfide bond in di-alm by DTT. The succeeding additionof tetrathionate (IT) to the trans-side produced channels ofr = 10-20 ms besides the channels of alm-SR. The resultssuggested that TT reacted with the N-terrninal thiol groupof alm-SR locating at the trans-side of the membrane toalter the lifetime. The N-terrninal insertion of alamethicinhelices by voltage activation, therefore, was confirmed.

Lithium phthalocyanine (LiPc) is a neutral radicalwhich is stable even in the air; it has an unpaired electronderiving a singly occupied molecular orbital (SOMa)according to its Pc macrocyclic ring, and then its physi-cal properties, in particular, its intrinsic semiconductivityand magnetic behaviors, have attracted interests of manypeople in the research field of organic materials science.Besides, LiPc shows polymorphism as most Pc's do andthe three different polymorphs, i.e. the X-, the a- and the ~forms, of crystals have been characterized so far by severalresearch groups. Brinkmann et af. have reported that thex-form tetragonal crystal shows a notably low electricresistivity along the molecular stacking axis and that themonoclinic a-form one exhibits a Curie-Weiss susceptibil-ity with a mean-field temperature characteristic of ferro-magnetic couplings.

In this study, LiPc thin films in the x- and the a-formswere successfully prepared on gold substrates in the firstplace. Then their inverse photoemission (IPE) spectrawere measured to examine any differences in the elec-tronic structures of unoccupied states between two formsof the films, as the electronic structures of their valencestates have been observed to be almost the same from apreliminary measurements of their ultraviolet photoemis-sion spectra by Fujimoto et af. The obtained IPE spectrawere interpreted with the aid of the results calculated for aLiPc molecule and its dimers in the forms corresponding tothe x- and the a-forms using the density functional theory(DFT) method in collaboration with Sakaki et af.

The obtained IPE spectra for two kinds of the filmswere different from each other as shown in Fig. I: the low-est energy feature originated from SOMa is located lowerin energy for the a-form film, and the second lowest en-ergy feature corresponding to the next lowest unoccupiedmolecular orbital (NLUMO) appears to be much broaderfor the x-form one. With referring to the obtained resultsfrom the calculations above, we have come to a conclusionon the spectral interpretation as follows: the broad featurefor 6eg orbital (NLUMO) for the x-form is understood bythe removal of its degeneracy due to a large intermolecularinteraction along the molecular stacking axis in the film,and the lower energy of 2aJu orbital (SOMa) for the a-form is explained by higher contribution of electron corre-lation, both Coulomb and exchange repulsions, workingin the x-form film in particular. On the basis of these con-clusions, the different characteristics in the electrical andmagnetic properties revealed between the x- and the a-

ICRANNUAL REPORT, 2003 __11 _