Unusual photoluminescence characteristics of tetraphenylpyrene (TPPy) in various aggregated morphologies

Article abstract of DOI:10.1016/j.cplett.2005.12.102

We found that 1,3,6,8-tetraphenylpyrene (TPPy) demonstrates unusual photoluminescence (PL) characteristics in the solid-state morphologies. We investigated the PL characteristics of TPPy in various morphologies including powder, deposited film, and soluti

Full text of DOI:10.1016/j.cplett.2005.12.102

Chemical Physics Letters 421 (2006) 295–299
www.elsevier.com/locate/cplett
Unusual photoluminescence characteristics of tetraphenylpyrene
(TPPy) in various aggregated morphologies
a
b
c
d
d
Takahito Oyamada , Seiji Akiyama , Masayuki Yahiro , Mari Saigou , Motoo Shiro ,
a
a,e,
*
Hiroyuki Sasabe , Chihaya Adachi
a
Department of Photonics Materials Science, Chitose Institute of Science and Technology (CIST) 758-65 Bibi, Chitose, Hokkaido 066-8655, Japan
b
Optoelectronic Materials Laboratory, Research and Technology Development Division, Mitsubishi Chemical Group, Science and Technology
Research Center, Inc. 1000 Kamoshida, Aoba, Yokohama 227-8502, Japan
c
International Innovation Center (IIC), Kyoto University, Yoshida-Honmachi, Sakyo, Kyoto 606-8501, Japan
d
Rigaku Co., Application Laboratory, 3-9-12 Matsubara, Akishima, Tokyo 196-8666, Japan
Center for Future Chemistry, Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan
e
Received 21 November 2005
Available online 17 February 2006
Abstract
We found that 1,3,6,8-tetraphenylpyrene (TPPy) demonstrates unusual photoluminescence (PL) characteristics in the solid-state mor-
phologies. We investigated the PL characteristics of TPPy in various morphologies including powder, deposited film, and solutions. The
TPPy powder (A), which was prepared through column chromatography, recrystallization, and train sublimation, showed blue fluores-
cence with a peak of maximum wavelength of kmax = 451 nm. The TPPy powder (B), which was obtained by thermal annealing of TPPy
powder (A) in a quartz tube in nitrogen, showed green fluorescence with kmax = 510 nm. Furthermore, the TPPy powder (B) was revers-
ibly converted into TPPy powder (A) by recrystallization. We conclude that TPPy dimers form locally in the TPPy monomer aggregates
during thermal annealing and redissociate into the monomer states during recrystallization.
ꢀ 2006 Elsevier B.V. All rights reserved.
Recent research has actively investigated various
organic electronic devices, such as organic light-emitting
diodes (OLEDs) [1–3], organic thin-film transistors (TFTs)
[4–9], organic switching and memory devices [10–12], and
organic solar cells (OSCs) [13–15]. Various molecular
structures have been used widely in these devices. Con-
densed aromatic hydrocarbons, such as pentacene and rub-
rene, have been revealed to be particularly useful for use in
the active layer in organic field-effect transistors (OFETs).
In particular, very high hole mobilities (lhole) exceeding
1 cm2/V s [16,17] using a pentacene deposited film as an
active layer have been achieved. Furthermore, an OFET
using a rubrene single crystal showed extremely high hole
mobility (lhole) of 10 cm2/V s [18], indicating that the
highly packed molecular aggregates having tight intermo-
lecular p-stacking demonstrate notably high carrier mobil-
ities in organic light-emitting FETs (OLEFETs). In
addition to the high carrier mobility, some of the polyacene
derivatives have been found to show high PL quantum effi-
ciency (UPL) even in the solid state films, making it possible
to have a novel organic light-emitting device, i.e., an OLE-
FET, which requires both light-emitting and field-effect
transistor characteristics [9]. In this device, aromatic com-
pounds including tetracene [7], oligothiophene [8], and
1,3,6,8-tetraphenylpyrene (TPPy) [9] were revealed to have
excellent OLEFET characteristics. Using TPPy in OLE-
FETs, we attained bright electroluminescence (EL) with
an external EL efficiency of gext = of 0.5%.
During the OLEFET study using TPPy, we encountered
unusual photoluminescence (PL) characteristics of TPPy.
The TPPy deposited film showed different PL spectra
depending on the purification procedures, such as thermal
annealing and recrystallization. In this study, we investi-
*
Corresponding author. Fax: +81 123 27 6048.
E-mail address: adachi@cstf.kyushu-u.ac.jp (C. Adachi).
0009-2614/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2005.12.102

296
T. Oyamada et al. / Chemical Physics Letters 421 (2006) 295–299
gated detailed PL characteristics of TPPy in various mor-
phologies such as powder, deposited film, and solution,
and examined the unique PL characteristics with special
focus on dimer formation.
Fig. 1(I) shows the PL characteristics of the TPPy (A)
and TPPy (B) powders. The TPPy (A) powder showed fluo-
rescence with a maximum wavelength (kmax) of 451 nm,
while the TPPy (B) powder showed fluorescence with
We synthesized TPPy by the Suzuki coupling of 1,3,6,8-
tetrabromopyrene and 4-phenylbronic acid with Pd(PPh3)4
in a toluene solution by refluxing for 9 h under nitrogen
gas, yielding 68% in the crude product. The synthesized
TPPy powder was purified by column chromatography (sil-
ica gel, toluene) to remove the palladium catalyst, recrys-
tallized from toluene, and finally purified twice using the
train sublimation method [19] (<3 · 10À3 Pa). Here, we
abbreviate this purified TPPy powder as TPPy (A). We
thermally annealed the TPPy (A) powder at T = 300 ꢁC
for 2 h in nitrogen and obtained TPPy powder (B). The
purities of these powders were confirmed to be higher than
99 +% by high performance liquid chromatography
(HPLC); the exact purity of the powders was (A): 99.5%
and (B): 99.4%.
kmax = 510 nm. Further, the TPPy (B) powder can be
reversibly converted to the TPPy powder (A) by recrystal-
lization with toluene. Thus, blue to green by thermal
annealing and green to blue by recrystallization processes
are mutually reversible.
Fig. 1(II) shows the PL characteristics of 100 nm-thick
TPPy deposited films using these powders as the deposition
sources. The TPPy (A) deposited film showed fluorescence
(I)
1
TPPy(A)
Heating
TPPy(B)
TPPy(A)
Re-crystallization
In the formation of the TPPy deposited films, the
organic sources were heated in
a
high-vacuum
TPPy(B)
(<3 · 10À3 Pa) thermal evaporation system. The deposition
speed was monitored with a quartz sensor and maintained
at 0.3 nm sÀ1. To fabricate single layer organic light-emit-
ting diodes (OLEDs), we used a clean glass substrate pre-
coated with a 110 nm thick indium–tin-oxide (ITO) layer
with a sheet resistance of approximately 20 X/h. Prior to
use, the substrate was degreased with solvents and cleaned
in a UV–ozone chamber (Nippon Laser & Electronics
Lab., NL-UV253) before it was loaded into an evaporation
system. A 100 nm-thick TPPy layer was deposited and a
shadow mask with 1 mm-diameter openings was used to
define the cathode consisting of 100 nm-thick MgAg and
10 nm-thick Ag layers as a cover layer.
0
200
300
400
500
600
700
Wavelength (nm)
(II)
Absorption and PL spectra of the organic layers were
measured using a fluorospectrometer (FP-6300, Jasco)
and a UV–Vis spectrometer (UV-3100, Shimadzu). In the
PL measurement of the TPPy powders, the powders were
carefully sandwiched between two quartz plates. The abso-
lute PL quantum efficiency was measured using an integrat-
ing sphere [20]. The highest occupied molecular orbital
(HOMO) levels of the metal and the organic layers were
measured using an ultraviolet photoelectron spectroscope,
AC-1 (Riken Keiki Co.). Current density–voltage–lumi-
nance (J–V–L) characteristics were measured using a semi-
conductor parameter analyzer (HP4145, Agilent), with the
external EL quantum efficiency directly obtained by plac-
ing and centering the OLEDs on the surface of a large-
diameter calibrated Si photodiode.
X-ray analysis of the deposited film and the single crys-
tal were measured, respectively, using X-ray diffraction sys-
tems [21] (out of plane measurement: RINT-TTRIII and
refraction measurement: RINT-UlatimaIII, Rigaku) and
an X-ray crystal structure analysis system [18] (RAXIS
RAPID, Rigaku). The TPPy single crystals were grown
using the train sublimation method [22] at T = 300 ꢁC with
a flow of argon gas (60 ml/min.).
1
1
TPPy(B)
TPPy(A)
0
200
0
300
400
500
600
700
Wavelength (nm)
Fig. 1. Photoluminance (PL) and absorption spectra of TPPy powders
and deposited films. (I) TPPy powders: (A) recrystallized form and (B)
after thermal annealing of TPPy (A) powders at T = 300 ꢁC. (II) TPPy
deposited films evaporated from the TPPy (A) and TPPy (B) powders. The
deposited films have similar PL spectra to those of the powders. Inset
shows TPPy molecular structure.

T. Oyamada et al. / Chemical Physics Letters 421 (2006) 295–299
297
with kmax = 455 nm, transient PL lifetime of sPL ꢀ 2 ns and
absolute PL efficiency of UPL = 68 2%; the TPPy (B)
deposited film showed fluorescence with kmax = 503 nm,
sPL ꢀ 6 ns and UPL = 45 2%. Thus, the PL spectra of
the TPPy deposited films are very similar to those of the
powders, indicating that the locally aggregated structures
in the TPPy powder’s deposition sources are preserved in
their deposited films. Thus, the sublimation occurs as a
TPPy cluster and does not necessarily occurs as an individ-
ual isolated molecule. Here, the absorption spectra of the
deposited films showed no appreciable differences despite
the significant differences of their PL characteristics.
To investigate the correlation between PL characteristics
and morphologies of the TPPy films and powders, we con-
ducted X-ray analysis for both the deposited films and the
single crystals. In the X-ray diffraction patterns, no sharp
diffraction peaks were observed in either of the TPPy films,
indicating that the TPPy deposited films have amorphous
morphologies. We also calculated the densities of the TPPy
deposited films from the X-ray reflection curves, suggesting
that the densities of both TPPy deposited films are the same
at 1.25 0.03 g/cm3; no significant difference between A
and B was observed. We next conducted X-ray analysis
for the single crystals of TPPy (A) and (B). Fig. 2 shows
the PL characteristics and photographs of TPPy (A) and
(B) single crystals prepared by the gas flow-train sublima-
tion method [22]. The TPPy (A) single crystals had flake-
like shapes with blue fluorescence with kmax = 450 nm,
while the TPPy (B) single crystal had needle-like shapes
with green fluorescence with kmax = 540 nm. Although
the PL characteristics are completely different, the structure
analysis clearly confirmed that the single crystals have
exactly the same crystal form, an orthorhombic structure
˚
with the lattice parameters of a = 11.324(2) A,
˚
˚
b = 12.400(2) A, and c = 19.105(3) A (Fig. 2b).
Next, we investigated the PL and absorption spectra of
TPPy (A) and (B) in a dichloromethane (DCM) solution.
We dissolved the TPPy powders (A) and (B) in separate
(a)
(ii) TPPy (B)
(i) TPPy (A)
1
0
300 350 400 450 500 550 600 650 700 750
Wavelength (nm)
(b)
Fig. 2. (a) Photoluminescence (PL) spectra of TPPy single crystals (A and B). Inset shows photographs of the crystals. (i) TPPy single crystals having
flake-like shapes have blue PL and (ii) those having needle-like shapes have green PL. (b) Both TPPy (A) and (B) structures have the same crystal form, an
˚
˚
˚
orthorhombic structure with lattice parameters of a = 11.324(2) A, b = 12.400(2) A and c = 19.105(3) A.

298
T. Oyamada et al. / Chemical Physics Letters 421 (2006) 295–299
Based on the PL characteristics we found in the various
1
morphologies of the single crystal, deposited film, and solu-
tion, we can discuss the unique PL characteristics of TPPy.
In the TPPy (B) solid-state aggregates, it is probable that a
very small amount of dimer formation occurs in the TPPy
monomer host matrix and the excitons created in the TPPy
monomer diffuse into the dimer sites through exciton
migration, resulting in exciton decay at the dimer states
(green emission). The longer sPL in the green emission com-
pared with that in the blue emission also support the idea
of the dimer formation. In the TPPy (B) solutions, we
clearly observed dimer formation at the high concentra-
tion, while no dimer formation occurred at the low concen-
tration. In the case of TPPy (A), however, no dimer
formation was detected in either the solid states or the solu-
tion, leading to monomer emission (blue emission). There-
fore, a very small number of the TPPy molecules must have
formed dimer aggregates during the thermal annealing of
the TPPy (A) powder. We infer that the large p–p stacking
interaction between the pyrene cores induced such dimer-
ization during thermal annealing, as is well known for
un-substituted pyrene [23]. Activation on the rotation of
the attached phenyl rings by thermal annealing probably
enhanced this aggregation. However, the number of such
dimer formations was so small that we could not detect
them by X-ray analysis. We have to conclude that once
the dimer was formed, it would not be dissociated easily
in the DCM solution and it dissociated through recrystalli-
zation. In a dilute solution, the energy transfer between the
isolated TPPy molecules is inefficient, resulting in TPPy
monomer emission. At the high concentration, however,
the energy transfer became more active and TPPy dimer
emission occurred through efficient energy transfer. No
possibility exists that the TPPy molecules will decompose
during thermal annealing; we carefully checked the purities
by HPLC and column chromatography and detected no
appreciable decomposition.
1
TPPy(A)
10-3
10-4
10-5
M
M
M
0
400
450
500
550
600
Wavelength (nm)
0
400
450
500
550
600
Wavelength (nm)
1
1
TPPy(B)
10-3
10-4
10-5
M
M
M
0
400
450
500
550
600
Wavelength (nm)
0
400
450
500
550
600
Wavelength (nm)
Fig. 3. Absorption spectra of TPPy (A and B) depending on the TPPy
concentrations in dichloromethane (DCM) solutions of 10À3 M (ꢁ),
10À4 M (n) and 10À5 M (,). Inset: photoluminescence (PL) spectra of
TPPy (A and B) depending on the concentration.
Finally, to confirm a related aspect of the unusual PL
characteristics, we examined the electrical characteristics
of the TPPy (A) and TPPy (B) deposited films. Fig. 4a
shows the current density–voltage (J–V) and external EL
quantum efficiency–current density (gext–J) characteristics
of single layer OLEDs with TPPy (A) and TPPy (B) as
an active layer in the configuration of ITO/TPPy
(100 nm)/MgAg (100 nm)/Ag (10 nm). Even the EL spectra
showed differences between TPPy (A) and (B). The OLED
with TPPy (A) showed blue EL with an EL spectra at
solutions of DCM. Fig. 3 shows the PL and absorption
spectra of TPPy (A) and TPPy (B) in a DCM solution
depending on the TPPy concentrations. We changed the
TPPy concentrations from 10À5 to 10À3 M and with this
increase, the PL spectra of TPPy (A) showed a slight red
shift of 13 nm from k = 420 nm to k = 433 nm. The PL
spectra of TPPy (B) under the low concentration of
10À5 M showed blue fluorescence with kmax = 421 nm,
which coincides well with that of TPPy (A). With a higher
concentration of >10À3 M, however, the PL showed addi-
tional peaks (shoulders) at
max = 525 nm in addition to the similar red shift. Further-
more, although the absorption spectra of TPPy (A) showed
no significant difference when the TPPy concentration was
changed, TPPy (B) showed new absorption peaks at
k
max = 450 nm, while the OLED with TPPy (B) showed
green EL with an EL spectra at kmax = 503 nm. Also, the
J–V characteristics with TPPy (B) showed a shift of the
driving voltage to high voltage compared with that of
TPPy (A), and gext with TPPy (B) showed 20 times higher
efficiency than that of TPPy (A), suggesting that charge
carriers were well trapped by the dimer sites. Although
the HOMO level and the lowest unoccupied molecular
orbital (LUMO) level showed the same values of 5.7 eV
and 2.7 eV in both TPPy (A) and (B) deposited films, the
kmax = 500 nm and
k
k
max = 450 nm and kmax = 478 nm with the higher concen-
tration of 10À3 M, indicating the formation of dimer aggre-
gates only in the TPPy (B) solution.

T. Oyamada et al. / Chemical Physics Letters 421 (2006) 295–299
299
104
103
102
101
100
10-1
10-2
10-3
550 nm originates from the formation of TPPy dimer
states. We examined other TPPy derivatives having various
substituents and observed that the shift of the PL spectrum
depends heavily on the substituent. Comparing the substi-
(a)
TPPy(A)
TPPy(B)
tuent effects would provide
discussion.
a more comprehensive
10-1
10-2
10-3
This work was supported by the Integrated industry aca-
demia partnership (IIAP), Kyoto University International
Innovation Center. We especially thank Prof. Katsumi
Tokumaru for his insightful advice.
TPPy(B)
1x10-4
1x10-5
10-6
TPPy(A)
References
100
101
102
103
104
Current density (mA/cm2)
10-7
[1] C.W. Tang, S.A. Van Slyke, Appl. Phys. Lett. 51 (1987) 913.
[2] C. Adachi, M.A. Baldo, S.R. Forrest, M.E. Thompson, Appl. Phys.
Lett. 77 (2000) 904.
100
101
Volt (V)
102
[3] T. Oyamada, S. Murase, T. Tominaga, C. Maeda, H. Sasabe, C.
Adachi, Appl. Phys. Lett. 86 (2005) 033503.
[4] Z. Bao, A. Dodabalapur, A.J. Lovinger, Appl. Phys. Lett. 69 (1996)
4108.
(b)
1
[5] A.R. Brown, C.P. Jarrett, D.M. de Leeeuw, M. Matters, Synth. Met.
88 (1997) 37.
TPPy(A)
TPPy(B)
[6] D.X. Wang, Y. Tanaka, M. Iizuka, S. Kuniyoshi, K. Kudo, K.
Tanaka, Jpn. J. Appl. Phys. 38 (1999) 256.
[7] A. Hepp, H. Heil, W. Weise, M. Ahles, R. Schmechel, H. von
Seggern, Phys. Rev. Lett. 91 (2003) 157406.
[8] T. Oyamada, S. Okuyama, N. Shimoji, K. Matsushige, H. Sasabe, C.
Adachi, Appl. Phys. Lett. 86 (2005) 093505.
[9] T. Oyamada, H. Uchiuzou, Y. Oku, N. Shimoji, K. Matsushige, H.
Sasabe, C. Adachi, J. Appl. Phys. 98 (2005) 074506-1.
[10] L. Ma, J. Liu, S. Pyo, Y. Yang, Appl. Phys. Lett. 80 (2002) 362.
[11] X.-C. Gao, D.-C. Zou, K. Fujita, T. Tsutsui, Appl. Phys. Lett. 81
(2002) 4508.
0
350
400
450
500
550
600
650
700
[12] T. Oyamada, H. Tanaka, K. Matsushige, H. Sasabe, C. Adachi, Appl.
Phys. Lett. 83 (2003) 1252.
Wavelength (nm)
[13] C.W. Tang, Appl. Phys. Lett. 48 (1986) 183.
[14] Z.H. Kafafi, Proc. SPIE. (2002) 4465.
[15] B.P. Rand, P. Peumans, S.R. Forrest, J. Appl. Phys. 96 (2004) 7519.
[16] Y.-Y. Lin, D.J. Dundlach, S. Nelson, T.N. Jackson, IEEE Electr.
Dev. Lett. 18 (1997) 606.
Fig. 4. (a) Current density (J) depending on voltage (V) and external
quantum efficiency (gext) and current density (J) characteristics in OLEDs
using the TPPy powders (A and B) as the deposition sources. (b) EL
spectra in the OLEDs with the TPPy powders (A and B).
[17] Y. Inoue, S. Tokito, K. Ito, T. Suzuki, J. Appl. Phys. 95 (2004) 5795.
[18] V.C. Sundar, J. Zaumseil, V. Podzorov, E. Menard, R.L. Willett, T.
Someya, M.E. Gershenson, J.A. Rogers, Science 303 (2004) 1644.
[19] H.J. Wagner, R.O. Loutfy, C.K. Hshiao, J. Mat. Sci. 17 (1982) 2781.
[20] Y. Kawamura, H. Sasabe, C. Adachi, Jpn. J. Appl. Phys. 43 (2004)
7729.
[21] M. Ofuji, K. Inaba, K. Omote, H. Hoshi, Y. Takanishi, K. Ishikawa,
H. Ta·kezoe, Jpn. J. Appl. Phys., Part I 41 (2002) 5467.
[22] Ch. Kloc, P.G. Simpkins, T. Siegrist, R.A. Laudise, J. Cryst. Growth
182 (1981) 416.
dimer sites should form localized states within the HOMO
and LUMO levels and contribute to increasing direct car-
rier injection from the cathode, improving exciton genera-
tion efficiency. These electrical measurements are consistent
with the PL characteristics, supporting our conclusions.
In summary, we demonstrated unique PL characteristics
in TPPy powders and deposited films. The thermal anneal-
ing and recrystallization reversibly converted the PL spec-
tra. We concluded that the fluorescence around 500–
[23] H. Sumi, Chem. Phys. 130 (1989) 433.