A biomolecular light-emitting
Hiroyuki Tajima, Masaki Matsuda, and Shingo Ikeda
An LED fabricated with biomocular materials may signal the start of
Light-emitting diodes (LEDs) have become key components of
numeric and alphanumeric display technology. The most com-
mon are p-n-junction diodes fabricated from inorganic semicon-
ductors. In the past decades, nanotechnology advances gave rise
Figure 1. Shown is the molecular structure of heme, a porphyrin
to a new-type of LED, the organic light-emitting diode (OLED),
macrocycle coordinated to iron and found in many heme proteins.
a prototype of which was ﬁrst fabricated by Vincett et al.1 OLED
displays with small screens are now commercially available.
In the basic OLED design, a thin ﬁlm of highly phosphorescent emission of our biomolecules. The observed electroluminescence
or ﬂuorescent material is sandwiched between various types spectra were broad but consistent with the absorption spectra.
of metal electrode. These light-emitting materials are also espe- The quantum efﬁciencies of the fabricated devices were 4 × 10−7
cially designed for efﬁcient charge injection from the electrodes. at most (in the case of a hemin BIODE 6 ), thus signiﬁcantly lower
In this context, questions come to mind with no textbook to pro- than those of commercially available devices. However, one of
vide answers. What kind of organic materials can be used to fab- the reasons for their small quantum efﬁciencies was their lack of
ricate OLEDs? Can we make them using common bio-functional transport layers.
materials? As for observing electroluminescence in compounds that did
Our work has allowed to provide answers in that we have not exhibit photoluminescence, our interpretation was to at-
recently succeeded in fabricating the ﬁrst biomolecular light- tribute the phenomenon to different excitation states in the
emitting diode (BIODE).2 We decided to use metalloporphyrins electroluminescence and photoluminescence processes. In the
and related biomolecules—such as cytochrome c,2–4 myoglobin,3 case of electroluminescence, carriers are injected directly into
hemin,5 and chlorophyll a.6 —for this purpose. Cytochrome c and d-orbitals whose energy levels are close to those of electrodes.
myoglobin are known as heme proteins because their active site On the other hand, photoexcitation produces electron-hole pairs
consists of an iron-porphyrin, called a heme (shown in Figure 1). in the π and π ∗ orbitals. Therefore, d electrons (or d holes) are
In living systems, cytochromes are electron carriers, while myo- hardly formed in photoluminescence, since the probability of
globin transports and stores oxygen. Chlorophyll a is the well- d − π and d − π ∗ transitions are quite small.4
known photosynthetic pigment and hemin is the ferric oxidation Figure 3 shows the drastic changes observed in the electrolu-
product of heme. Except for chlorophyll a, these molecules ex- minescence spectrum of hemin under applied voltages.6 On the
hibit no photoluminescence. We were, accordingly, greatly sur- basis of magnetic susceptibility and Raman measurements, we
prised to observe current ﬂow and electroluminescence in all concluded that these changes resulted from the high-spin to low-
BIODE devices fabricated using them. spin transition of the iron. Since the spin state of the heme iron is
Figure 2 shows a schematic illustration of a BIODE. The device associated with various bio-functionalities, this phenomenon is
is a simple sandwich-type junction. We did not prepare carrier- of signiﬁcant interest. Although the transition is irreversible, re-
transport layers, as done in most OLEDs, to balance the den- versibility may be induced by chemical modiﬁcation of hemin.
sity of electrons and holes. This is because we wanted to exclude
light emission from the transport layers to observe the intrinsic Continued on next page
10.1117/2.1200609.0316 Page 2/2
Hiroyuki Tajima, Masaki Matsuda, and Shingo Ikeda
Institute for Solid State Physics
University of Tokyo
Kashiwa, Chiba, Japan
Professor Tajima is a physical chemist specializing in optical and
electrical measurements. Currently, his research effort is focused
on molecular devices.
Dr Matsuda has been studying molecular materials based on
metal complexes. He has been designing, synthesizing and
exploring related new materials.
Figure 2. Shown is a schematic illustration of a biomolecular light- Dr Ikeda acquired his PhD in organic photochemistry. He
emitting diode. The image at the lower right bottom is the emission of is currently involved in photolithography and impedance-
a cytochrome c device measured using a CCD detector. spectroscopy measurements.
1. P. S. Vincett, W. A. Barlow, R. A. Hann, and G. G. Roberts, Electrical conduction
and low voltage blue electroluminescence in vacuum-deposited organic ﬁlms, Thin Solid
Films 94, pp. 171–183, 1982. doi:0040-6090/82/0000-0000/$02.75
2. H. Tajima, S. Ikeda, M. Matsuda, N. Hanasaki, Ji-Won Oh, and H. Akiyama, A
light-emitting diode fabricated from horse-heart cytochrome c, Solid State Commun. 126,
pp. 579–581, 2003. doi:10.1016/S0038-1098(03)00305-3
3. H. Tajima, S. IkedaK.Shimatani, M. Matsuda, Y. Anodo, J. Oh, and H. Akiyama,
Light-emitting diodes fabricated from cytochrome c and myoglobin, Synth. Met. 153,
pp. 29–32, 2005. doi:10.1016/j.synthmet.2005.07.241
4. S. Ikeda, H. Tajima, M. Matsuda, Y. Ando, and H. Akiyama, External Quantum Ef-
ﬁciency and Electroluminescence Spectra of BIODE (Biomolecular Light-emitting Diode)
Fabricated from Horse-heart Cytochrome c, Bull. Chem. Soc. Jpn. 78 (9), pp. 1608–1611,
5. K. Shimatani, H. Tajima, T. Komino, S. Ikeda, M. Matsuda, Y. Ando, and
H. Akiyama, The Electroluminescence Spectrum of Chlorophyll a, Chem. Lett. 34 (7),
pp. 948–949, 2005. doi:10.1246/cl.2005.948
6. H. Tajima, K. Shimatani, T. Komino, M. Matsuda, S. Ikeda, Y. Ando,
and H. Akiyama, A Voltage Induced Transition of Hemin in BIODE (Biomolec-
ular Light-Emitting Diode), Bull. Chem. Soc. Jpn. 79, pp. 549–554, 2006.
Figure 3. Shown is a voltage-induced transition of hemin. The red and
blue curves illustrate the electroluminescence spectra in the low- and
high-voltage states, respectively.
Such a transition could be applied to the design of new biosen-
Biomolecular compounds have a wide range of functionali-
ties. If we can learn to use them in electric devices, we may
decrease environmental pollution and realize more eco-friendly
technologies for the future. Our BIODE studies are a ﬁrst step
towards such a dream.
c 2006 SPIE—The International Society for Optical Engineering