ARTICLE IN PRESS

0022-2313/$ - se

doi:10.1016/j.jlu

�CorrespondE-mail addr

Journal of Luminescence 128 (2008) 457–461

www.elsevier.com/locate/jlumin

Energy-transferred photoluminescence from thiophene/phenyleneoligomer thin films

T. Shimadaa, S. Hottab, H. Yanagic,�

aGraduate School of Science and Technology, Kobe University, Rokkodai, Nada-ku, Kobe 657-8501, JapanbGraduate School of Science and Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan

cGraduate School of Materials Science, Nara Institute of Science and Technology, 8916-5 Takayama-cho, Ikoma, Nara 630-0192, Japan

Received 9 April 2007; received in revised form 18 September 2007; accepted 20 September 2007

Available online 29 September 2007

Abstract

Photoluminescence (PL) based on Forster energy transfer between p-sexiphenyl (p-6P) and 5,50-bis(4-phenylyl)-2,20-bithiophene

(BP2T) was investigated for their coevaporated and laminated thin films. In the former films, fluorescence quenching of the p-6P was

accompanied by appearance of BP2T fluorescence, which indicated existence of the energy transfer between the donors and the

acceptors. The latter films were fabricated by successive depositions of p-6P, MgF2 and BP2T in which the thickness of the MgF2 spacer

was varied. The energy-transferred acceptor fluorescence was suppressed by the spacer thicker than the Forster distance (�10 nm).

r 2007 Elsevier B.V. All rights reserved.

PACS: 78.66.Qn; 78.55.�m

Keywords: Forster energy transfer; Photoluminescence; Thiophene/phenylene oligomer; p-Sexiphenyl; 5, 50-Bis(4-phenylyl)-2, 20-Bithiophene;

Coevaporated film; Laminated film

1. Introduction

In the last decades, organic light-emitting diodes(OLEDs) have attracted attention because of theirpotential as full-color displays and panel light sources[1,2]. For practical applications, further improvements intuning of emission color and quantum efficiency aswell as durability are still required. To meet theserequirements, energy-transferred luminescence fromguest dopants dispersed in a host film is extensively studied[3,4]. Intermolecular energy transfer proceeds in threeways by radiative, Dexter [5] and Forster [6,7] mechanisms.The radiative transfer is simply that in which the guestmolecule absorbs the host emission. The Dexter andForster transfers are nonradiative based on wave functionoverlapping and resonant dipole–dipole interaction,respectively. The Forster type is more beneficial than theDexter one, especially in the solid state, due to its longer

e front matter r 2007 Elsevier B.V. All rights reserved.

min.2007.09.015

ing author. Tel./fax: +81 743 72 6011.

ess: yanagi@ms.naist.jp (H. Yanagi).

effective distance. Therefore, the Forster energy transferdynamics has been studied in a variety of donor/acceptorsystems such as emissive polymer blends [8], light-harvest-ing dendrimers [9] and small OLED molecules [10]. TheForster distance, typically �10 nm, increases in proportionto the energy overlapping between the host fluorescenceand the guest absorption bands. For resonant dipole–dipole interaction, mutual orientation between the donorand acceptor molecules is also an important factor toelongate the Forster radius.In this study, we chose thiophene/phenylene oligomers

[11–13] as the donor–acceptor pair. They are attractivematerials for organic optoelectronic devices such asOLEDs [14–16], organic field effect transistors (OFETs)[17–19] and organic lasers [20–23], because of their highluminous efficiency and electrical conducting properties.A variety of modification in the monomer units andp-conjugating length enables us to tune their highestoccupied and lowest unoccupied molecular orbitals(HOMO and LUMO) to get transitions between them inthe visible wavelength region.

ARTICLE IN PRESST. Shimada et al. / Journal of Luminescence 128 (2008) 457–461458

Here, we fabricated two types of thin film samples withthiophene/phenylene oligomers: one is coevaporated do-nor–acceptor blends and another is laminated donor/acceptor layers. Using the former films, the Forsterenergy-transferred photoluminescence (PL) was investi-gated by changing the acceptor concentration, and theacceptor distribution was observed by confocal laserscanning microscopy (CLSM). In the latter films, depen-dences of the energy transfer on the donor–acceptordistance were evaluated by inserting a spacer with variedthickness between the donor and acceptor layers.

2. Experimental

Thiophene/phenylene oligomers used are p-sexiphenyl(p-6P) and 5,50-bis(4-phenylyl)-2,20-bithiophene (BP2T) asthe donor and the acceptor molecules, respectively (Fig. 1).The p-6P sample was purchased from Tokyo Kasei KogyoCo., Ltd., and was used without purification. The synthesisand purification of BP2T was carried out according tothe described method [12]. Their HOMO and LUMOlevels were calculated by the density functional theory(DFT) using an Accelrys Material Studio/Dmol3 program.The p-6P thin film doped with BP2T was fabricated bycoevaporation using a vacuum deposition apparatus(JEOL, JEE-400) onto a pre-cleaned quartz substrate.The doping concentrations of the BP2T acceptor werevaried in the range of 0–0.24mol% by controlling theirevaporation rates under monitoring with a quartz crystalmicrobalance (QCM). The laminated film was fabricatedby successive depositions of the p-6P donor, MgF2 spacerand BP2T acceptor onto a quartz substrate. The thicknessof the p-6P and BP2T layers was fixed to be 10 nm whilethat of the MgF2 spacer was changed in the range of2.6–13.2 nm by using QCM. Donor-only (p-6P/MgF2) andacceptor-only (MgF2/BP2T) control samples were alsofabricated as reference. All depositions were performedunder a vacuum of 1.5� 10�4 Pa while the substrate washeld at room temperature.

Absorption and PL spectra were taken by a UV/visspectrophotometer (JASCO, V-530ST) and a spectro-fluorometer (JASCO, FP-750), respectively. For the PLmeasurements the excitation wavelength was chosen at256 nm in order to maximize the excitation of the p-6Pdonor. The excitation beam irradiated the sample from the

Fig. 1. Molecular structures of p-6P (a) and BP2T (b).

quartz substrate side with an incident angle of 451 and theemitted light reflected through the substrate was collected.PL images of the coevaporated p-6P/BP2T films were takenby CLSM using a scanning probe microscope (JEOL,JSPM-4300). The PL signal was imaged through a low-pass filter (l4420 nm) under blue laser diode excitation(l ¼ 403 nm). All measurements and observations wereperformed in the atmosphere at room temperature.

3. Results and discussion

We chose the p-6P donor and BP2T acceptor accordingto their different HOMO/LUMO energies. Although bothmolecules consist of a six-ring p-conjugated chain, theHOMO/LUMO energies estimated by the DFT calculationare �5.31/�2.37 eV for p-6P and �4.83/�2.76 eV forBP2T; therefore, the HOMO–LUMO gap is 2.94 eV forp-6P and 2.07 eV for BP2T. This energy difference isattributed to less p-conjugation in the alternately twisted-ring structure of p-6P as compared to the planarbithiophene structure in BP2T. Fig. 2 shows absorptionand PL spectra of p-6P and BP2T in thin films evaporatedonto quartz substrate. As consistent with the energycalculation, the absorption and fluorescence bands ofp-6P appear in the shorter wavelength region than thoseof BP2T. Their absorption peaks are apart each other whilethe fluorescence band of p-6P overlaps widely with theabsorption tail of BP2T. This spectral configuration isexpected suitable for efficient Forster energy transfer fromp-6P to BP2T.First, we examined energy-transferred PL characteristics

of the coevaporated donor/acceptor films. Fig. 3 shows PLspectra of the p-6P films doped with BP2T. The weightratio of BP2T to p-6P was varied to be 0/4000, 1/4000,2/4000 and 3/4000, which corresponds to the BP2Tconcentration from 0 to 0.073mol%. With increasingBP2T concentrations, the emission bands of p-6P at 425and 450 nm were decreased while those of BP2T at 510 and550 nm were increased, suggesting that the energy transfersfrom p-6P to BP2T. It is noted that the fluorescence bandof BP2T in the coevaporated film is blue-shifted about

Fig. 2. Absorption (dashed line) and PL (solid line) spectra of p-6P

(donor) and BP2T (acceptor) in thin films.

ARTICLE IN PRESS

Fig. 3. PL spectra of coevaporated p-6P/BP2T thin films. The concentra-

tions of BP2T are 0mol% (solid line), 0.024mol% (dashed line),

0.049mol% (dotted line) and 0.073mol% (dashed-dotted line).

Fig. 4. CLSM images under excitation at 403 nm of undoped p-6P film

(a) and coevaporated p-6P/BP2T films with BP2T concentration of 0.024

(b) and 0.24mol% (c). The scanning area is 70� 70mm2.

T. Shimada et al. / Journal of Luminescence 128 (2008) 457–461 459

30 nm as compared to that of the pure BP2T film (seeFig. 2). This discrepancy is attributed to the solid-statesolvation effect [24], which is caused by the destabilizedexciton of the polar BP2T molecule doped in the non-polarp-6P host.In order to observe the donor/acceptor distribution in

the coevaporated films, PL imaging was carried out byCLSM. Fig. 4 shows CLSM images taken for the undopedp-6P and coevaporated p-6P/BP2T films. The pure p-6Pfilm exhibits homogeneous distribution of fluorescenceintensity from the p-6P donor (Fig. 4a) since the excitationat 403 nm still gives rise to weak emission of p-6P. Onthe other hand, the coevaporated films exhibited randomlydistributed emission peaks (Fig. 4b). Those PL peaksare attributed to the directly excited fluorescence from

Fig. 5. PL spectra of laminated p-6P/MgF2/BP2T films (solid line) with

MgF2 thickness of 2.6 nm (a), 7.9 nm (b) and 13.2 nm (c). As reference, PL

spectra of p-6P/MgF2 (dashed line) and MgF2/BP2T (dotted line) are also

shown.

ARTICLE IN PRESS

Table 1

Efficiency of energy transfer in p-6P/MgF2/BP2T films as a function of MgF2 thickness

MgF2 thickness (nm) Correction factor f ¼ IDA (at

420 nm)/ID (at 420 nm)

Fraction of direct donor

emission (IDA�fID�IA)/ID

Fraction of total emission

(IDA�fID�IA)/IDA

2.6 0.119 1.196 0.612

4.0 0.082 1.077 0.664

5.3 0.137 0.714 0.492

7.9 0.276 0.424 0.362

10.5 0.301 – –

13.2 0.265 – –

T. Shimada et al. / Journal of Luminescence 128 (2008) 457–461460

the BP2T acceptors. As the BP2T concentration increases,the distance between the BP2T acceptors should decreaseif its homogeneous doping is maintained. However,the PL image in Fig. 4c (0.24mol%) shows ratherinhomogeneous distribution of the acceptor emissionthan that in Fig. 4b (0.024mol%). This observation issuggestive of molecular aggregation in the coevaporatedfilm with high concentration of BP2T, and it makesdifficult to evaluate a dependence of the donor–acceptordistance on the energy-transferred PL in the coevaporatedfilms.

Next, we investigated energy-transferred PL character-istics in laminated structures composed of the p-6P donorand BP2T acceptor layers. In order to examine an effectivedistance for the energy transfer, a spacer layer of MgF2

with different thicknesses in the range of 2.6–13.2 nmwas inserted between the p-6P and BP2T layers whiletheir thickness was fixed to be 10 nm. Fig. 5 shows PLspectra of those laminated p-6P/MgF2/BP2T films takenunder excitation at l ¼ 256 nm. As reference, donor-only(p-6P/MgF2) and acceptor-only (MgF2/BP2T) structures,where the MgF2 thickness was regulated to be as same asthat of the respective p-6P/MgF2/BP2T sample, werefabricated and their PL spectra were also measured.When the MgF2 thickness is 2.6 nm, thinner than atypical Forster distance, the PL spectrum shape of thep-6P/MgF2/BP2T film is identical to that of the MgF2/BP2T reference sample. However, its PL intensity ismuch enhanced as compared to the acceptor-only struc-ture. It demonstrates that the absorbed energy in thep-6P layer is efficiently transferred to the BP2T layer acrossthe MgF2 spacer. As the MgF2 thickness is increased to7.9 nm, the emission from p-6P begins to overlap with thatof BP2T at 420–500 nm, suggesting that the suppressedenergy transfer causes the donor fluorescence. Furtherincrease of the MgF2 thickness to 13.2 nm significantlyaltered the spectral shape of the p-6P/MgF2/BP2T film.The acceptor emission is no longer observed and the donorfluorescence is also attenuated as compared to the donor-only reference sample. This attenuation of the donoremission is probably due to radiative transfer, i.e. the p-6Pfluorescence is absorbed or scattered by the MgF2/BP2Tlayer. According to those spectral changes with increasingspacer thickness, a clear variation of the emission color

from green of BP2T to blue of p-6P was observed byfluorescence microscopy.For quantitative characterization of the energy-trans-

ferred emission in the laminated films, the spectral datashown in Fig. 5 were analyzed based on the reported methodby Andrew and Barnes [25]. The result was summarized inTable 1 where IDA, ID and IA are the spectral intensityintegrated over 400–650nm taken from the donor/acceptor,donor-only and acceptor-only samples, respectively. Thecorrection factor f stands for the decreasing ratio of thedonor emission intensity at 420 nm in the p-6P/MgF2/BP2Tfilm as compared to that in the p-6P/MgF2 reference sample.Then, the energy-transferred emission IDA�fID�IA gives theefficiency against the donor emission (IDA�fID�IA)/ID,and the contribution of energy-transferred emission(IDA�fID�IA)/IDA. The both parameters decrease withincreasing MgF2 thickness, in particular they are reducedby the spacer thicker than 5 nm. Those parameters were notobtained for the films with MgF2 thickness larger than10nm, because the attenuated IDA resulted in a negativevalue of IDA�fID�IA. This spectral quantification as afunction of the spacer thickness reveals that the energytransfer between the donor and acceptor layers is based onthe Forster-type dipole–dipole interaction.

4. Conclusion

We investigated fluorescence resonance energy transferbetween the p-conjugating oligomeric p-6P donor andBP2T acceptor, using two types of structures: coevaporatedand laminated thin films. In the former film, the donor-excited acceptor luminescence enhanced with increasingBP2T concentration suggested the Forster-type energytransfer. In the latter structure, the energy transfer betweenthe donor/acceptor layers was controlled by inserting theMgF2 spacer. Efficient energy-transferred emission wasobtained from the laminated film with spacer thicknessshorter than the Forster distance (�10 nm) although theefficiency significantly fell as the spacer thickness wasbeyond 5 nm. In order to improve the energy transferefficiency, further attention should be paid to the orienta-tion factor that is controlled by the molecular alignment inthe donor and acceptor layers.

ARTICLE IN PRESST. Shimada et al. / Journal of Luminescence 128 (2008) 457–461 461

References

[1] C.W. Tang, S.A. Van Slyke, Appl. Phys. Lett. 51 (1987) 913.

[2] J.R. Sheats, H. Antoniadis, M. Hueschen, W. Leonard, J. Miller,

R. Moon, D. Roitman, A. Stocking, Science 273 (1996) 884.

[3] J.-I. Lee, G. Klaerner, M.H. Davey, R.D. Miller, Synth. Met. 102

(1999) 1087.

[4] I. Tanaka, Y. Tabata, S. Tokito, Chem. Phys. Lett. 400 (2004) 86.

[5] D. Dexter, J. Chem. Phys. 21 (1953) 836.

[6] T. Forster, Excitation Transfer in Comparative Effects of Radiation,

Wiley, New York, 1959, p. 300.

[7] K.B. Eisenthal, S. Siegel, J. Chem. Phys. 41 (1964) 652.

[8] J.-W. Yu, J.K. Kim, D.Y. Kim, C. Kim, N.W. Song, D. Kim, Curr.

Appl. Phys. 6 (2006) 59.

[9] I. Akai, A. Okada, K. Kanemoto, T. Karasawa, H. Hashimoto,

M. Kimura, J. Lumin. 119–120 (2006) 283.

[10] K. Nakai, K. Mizoguchi, M. Nakayama, in: Proceedings of the 16th

Symposium of Association for Condensed Matter Photophysics,

Japan, 2005, p. 175.

[11] K.N. Baker, A.V. Fratini, T. Resch, H.C. Knachel, W.W. Adams,

E.P. Socci, B.L. Farmer, Polymer 34 (1993) 1571.

[12] S. Hotta, H. Kimura, S.A. Lee, T. Tamaki, J. Heterocycl. Chem. 37

(2000) 281.

[13] S. Hotta, T. Katagiri, J. Heterocycl. Chem. 40 (2003) 845.

[14] M. Era, T. Tsutsui, S. Saito, Appl. Phys. Lett. 67 (1995) 2436.

[15] H. Yanagi, S. Okamoto, Appl. Phys. Lett. 71 (1997) 2563.

[16] T. Noda, H. Ogawa, N. Noma, Y. Shirota, Appl. Phys. Lett. 70

(1997) 699.

[17] F. Garnier, R. Hajlaoui, M. El Kassmi, Appl. Phys. Lett. 73 (1998)

1721.

[18] H.E. Katz, J.G. Laquindaum, A.J. Lovinger, Chem. Mater. 10 (1998)

633.

[19] H. Yanagi, Y. Araki, T. Ohara, S. Hotta, M. Ichikawa, Y. Taniguchi,

Adv. Funct. Mater. 13 (2003) 767.

[20] M. Ichikawa, R. Hibino, M. Inoue, T. Haritani, S. Hotta,

K. Araki, T. Koyama, Y. Taniguchi, Adv. Mater. 17 (2005)

2073.

[21] D. Fichou, S. Delysse, J.-M. Nunzi, Adv. Mater. 9 (1997) 1178.

[22] G. Horowitz, P. Valat, F. Garnir, F. Kouki, V. Wintgens, Opt.

Mater. 9 (1998) 46.

[23] H. Yanagi, A. Yoshiki, S. Hotta, S. Kobayashi, Appl. Phys. Lett. 83

(2003) 1941.

[24] V. Bulovic, R. Deshpande, M.E. Thompson, S.R. Forrest, Chem.

Phys. Lett. 308 (1999) 317.

[25] P. Andrew, W.L. Barnes, Science 306 (2004) 1002.