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Chemical Physics Letters 394 (2004) 194–197

Molecular orbital engineering of single-molecular light emission

Daijiro Nozaki, Kazunari Yoshizawa *

Institute for Fundamental Research of Organic Chemistry, Materials Chemistry and Engineering, Kyushu University, Fukuoka 812-8581, Japan

Received 15 May 2004; in final form 10 June 2004

Available online 21 July 2004

Abstract

A concept of single-molecular light emission is theoretically proposed by using a p-conjugated molecule linked by meta-phenylene

couplers, gold as the anode, and calcium as the cathode. The molecule, which comprises perylene at the center as the light-emitting

moiety and two phenylacetylenes as the hole- and electron-transporting moieties, is designed to have localized p-MOs with an ap-

propriate energy gradient to efficiently transport carriers at the single-molecular level.

� 2004 Elsevier B.V. All rights reserved.

1. Introduction

Organic light-emitting diode (LED) is an attractive

electro-luminescent (EL) device because of its potential

applications such as flat-panel display [1,2]. It has sub-

stantial merits of low cost, low driving voltage, and wide

viewing angle compared to current liquid crystal display.

Since the milestone work of organic LED by Tang andVan Slyke [3], a great deal of experimental effort has

been devoted to the development of new organic materi-

als for optelectronic applications. Despite the consider-

able improvement and progress in organic LED,

important and fundamental challenges still remain; for

instance, needs for improving energy efficiency [4], oper-

ational [5,6] and thermal stability [7], lifetime [8,9], and

color purity [10]. The first idea of single-molecular de-vice was proposed 30 years ago by Aviram and Ratner

[11]. Possibility of preparing single-molecular light-emit-

ting device was recently discussed by Wada et al. [12].

The idea that electron-injection and hole-injection mol-

ecules are connected to both sides of a light-emitting

molecule to form a single-molecular LED is interesting.

Although the carrier transport through molecule-by-

0009-2614/$ - see front matter � 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.cplett.2004.06.131

* Corresponding author. Fax: +81-92-642-2735.

E-mail address: kazunari@ms.ifoc.kyushu-u.ac.jp (K. Yoshizawa).

molecule based on the hopping mechanism decreases

the quantum yield of light emission in current organic

LED, one can expect efficient carrier transport in such

a single-molecular LED. However, there is no reason-

able design for such a molecule that performs effective

carrier transport through it. The purpose of this study

is to propose a new carrier transport concept in a molec-

ular system in terms of molecular orbital (MO) theory.We show that a phenylacetylene system with localized

p-MOs should carry holes and electrons directionally

to emit light at the central perylene moiety at the sin-

gle-molecular level.

2. p-MO localization in a phenylacetylene dendrimer

It is known that meta-phenylene couplers localize the

MOs of dendrimers jointed by them. Shimoi and Fried-

man carried out MO calculations to look at the origin of

the localized state of meta-linked phenylene dendrimers

[13]. Although their models do not involve acetylene

blocks that are essential for dendrimers to stabilize their

planar structures, they reasonably obtained relevant lo-

calized levels and concluded that the electron–hole inter-action is not necessary for the localized electronic

excitations. In a previous Letter we reconsidered from

simple Huckel MO and density functional theory

D. Nozaki, K. Yoshizawa / Chemical Physics Letters 394 (2004) 194–197 195

(DFT) calculations the important relationship between

the light-harvesting function and the role of the meta-

phenylene couplers in a dendrimer [14]. Although the to-

tal density is homogeneous in the p-conjugated system,

the amplitude of an individual orbital is localized well

in a small fragment, due to the meta-phenylene couplersinvolved.

We considered a p-conjugated molecule that consists

of perylene at the core and two phenylacetylenes at-

tached to the perylene moiety. To reasonably determine

the structures and MO levels of this molecule, we per-

formed full geometry optimizations using the B3LYP

DFT method [15–18] with the 6-31G** basis set [19–

21] on the GAUSSIANAUSSIAN 98 program package [22]. Fig. 1shows computed MOs of the p-conjugated molecule

and their energies. The highest occupied MO (HOMO)

and lowest unoccupied MO (LUMO) of this system

are localized well at the perylene core. In contrast, the

HOMO�2, HOMO�1, LUMO+1, and LUMO+2

are localized in the first-generation branches; the HO-

MO�3, HOMO�4, LUMO+3, and LUMO+4 are lo-

calized in the second-generation branches although theLUMO+3 has partial orbital amplitude in the perylene

moiety. The localization of the p-MOs, which is inde-

Fig. 1. p-MO levels (in eV) of a phenylacetylene oligomer at the

B3LYP/6-31G** level of theory.

pendent of the level of theory used, is a unique electronic

feature of this meta-phenylene coupled molecule. Al-

though the localization of MOs is potentially applicable

to a single-molecular LED, we cannot put a directional

flow of electrons and holes into practice through it be-

cause the occupied and unoccupied levels are localizedsymmetrically.

3. Single-molecular light-emitting diode

Fig. 2 shows a proposed single-molecular LED,

which consists of a relevant p-conjugated molecule 1

that has a contact with a gold electrode as the anodeand a calcium electrode as the cathode, the work func-

tions of gold and calcium being 4.8 and 2.8 eV, respec-

tively. We have chosen gold and calcium as the anode

and cathode because the Fermi energies of these elec-

trodes fit well with certain MO levels when a voltage is

applied to this LED, as described below. Molecule 1

comprises perylene at the core and two phenylacetyl-

enes. The phenylacetylene attached to one side of theperylene moiety is partially fluorinated to adjust the

MO levels of the non-substituted one indicated in Fig.

1. Molecule 1 is partitioned into hole-transporting (or-

ange), light-emitting (red), and electron transporting

(blue) moieties. To efficiently transport carriers across

the different phases, we designed this molecule to have

localized p-MOs at the boundary regions of the anode

and cathode using the p-MO localization because ofthe meta-phenylene coupler as well as the electron-with-

drawing effect of the fluorine atom that lowers relevant

orbital levels in general. The optimized structure of 1

Fig. 2. Single-molecular light-emitting diode.

196 D. Nozaki, K. Yoshizawa / Chemical Physics Letters 394 (2004) 194–197

is planar. This result is reasonable because calculated

F� � �H distances of 4.2–4.3 A are longer than the van

der Waals contact (2.9 A) of the F and H atoms.

Fig. 3 shows calculated p-MOs of 1 and their energy

levels. Although 1 is a p-conjugated system, which is

generally considered to have a delocalized electronicstructure, it has localized p-MOs in different chromo-

phore segments. The holes and electrons injected can

travel through the localized p-MOs directionally to the

light-emitting moiety, due to the well-designed energy

gradient of the frontier orbitals. When 2 V is applied

to the LED cell, the Fermi energy of the cathode is

shifted up to �1.8 eV and electrons are injected from

the cathode to the LUMO+2, which is localized nearthe cathode tip. The injected electrons migrate to the

LUMO, which is localized well in the central perylene

moiety. The localization of the LUMO+2 near the cath-

ode plays an essential role for efficient electron-injection

because its energy is close to the Fermi energy of the

cathode. On the other hand, the Fermi energy of the an-

ode is shifted down to �5.8 eV and holes are injected

from the anode to the HOMO�2, which is localizednear the anode tip. The injected holes migrate to the

HOMO, which is localized in the central perylene moie-

ty. The localization of the HOMO�2 near the anode

also makes the hole-injection to the HOMO�2 more

efficiently than other energy levels. Thus, molecule 1

Fig. 3. p-MO levels (in eV) of molecule 1 calculated at the B3LYP/6-

31G** level of theory.

can transport the carriers directionally using these local-

ized p-MO levels, which are designed to have an appro-

priate energy gradient. The electrons and holes can be

radiatively recombined in the central perylene moiety.

The LED cell in Fig. 3 has functions of carrier blocking

barrier at the HOMO+3 and LUMO�3, which preventunproductive carrier migration to the opposite electrode

without radiative recombination [1,23,24].

We propose the carrier injection and transport mech-

anism on the assumption that the injected electrons and

holes non-radiatively migrate to the LUMO and the

HOMO, respectively. This is based on Moore�s pioneer-ing idea that a phenylacetylene-based dendrimer called

�nanostar�, which constitutes well-designed segments ofdifferent lengths that decrease in higher generations of

the peripheries, and serves as exciton funnel, due to

the presence of the well-designed energy gradient

[25,26]. Moore et al. [27] also reported that the meta

branching positions in this molecule disrupt the conju-

gation between adjacent aromatic rings and that direct

multistep exciton transfer occurs with a high transfer ef-

ficiency of 98% from the photoabsorbing backbone tothe perylene trap. This non-radiative exciton transfer

in similar phenylacetylene-based dendrimers and oligo-

mers has been theoretically and spectroscopically inves-

tigated [28–30]. We pointed out that there is MO

localization that plays an important role in the efficient

carrier transport in the nanostar dendrimer [14]. From

these experimental and theoretical studies, it is reason-

able to consider that in molecule 1 the injected electronsmigrate non-radiatively to the LUMO and the injected

holes migrate non-radiatively to the HOMO. The elec-

trons and holes eventually recombine in the perylene

moiety to emit light. In addition to the photoemission

process, unproductive processes such as the quenching

by metal electrons can take place in the contact regions.

We have to consider the interaction between the mole-

cule and the electrodes to control such unproductiveprocesses.

4. Conclusions

We propose a concept of single-molecular light emis-

sion by using a p-conjugated molecule linked by meta-

phenylene couplers, gold as the anode, and calcium asthe cathode. The molecule has unique localized p-MOs, which can play essential roles in the light-emitting

and carrier-transporting moieties at the single-molecular

level. Although we do not take the interactions between

the molecule and the electrodes in the present study, this

proposal can give a guiding principle for modeling func-

tional supramolecular systems. Fixing the molecule to

the electrodes through covalent bonds will improve theefficiency of the carrier injection if its electronic struc-

ture remains unaffected.

D. Nozaki, K. Yoshizawa / Chemical Physics Letters 394 (2004) 194–197 197

Acknowledgements

K.Y. acknowledges the Ministry of Culture, Sports,

Science and Technology of Japan (MEXT), Japan Soci-

ety for the Promotion of Science, �Nanotechnology Sup-

port Project� of MEXT, Japan Science and TechnologyCooperation, the Murata Science Foundation, and Kyu-

shu University P&P �GreenChemistry� for their supportof this work.

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