A R T I C L E S
Park et al.
transfer (ILET),12 and excited-state intramolecular proton
transfer (ESIPT) systems.13 However, these “frustrated
energy transfer” strategies for constructing a white-emitting
molecular dyad have yet to be demonstrated, although few cases
of white-emitting molecule based on “partial energy transfer”
have been reported.14 As a promising step toward our goal of
designing white-light-emitting molecules devoid of the energy
transfer problem, we exploited a pair of hydroxy-substituted
tetraphenyl imidazole derivatives as complementary and inde-
pendent emissive components based on frustration of energy
transfer between these materials. This arises because of ESIPT,
as previously demonstrated with a molecular mixture,13 leading
to an energy state mismatch arising from the lack of a ground-
state population in the acceptor.15
of fluorescence kinetic profiles with excitation at the front face by
347 nm laser pulses. Emission wavelengths were selected by
combining band-pass and cutoff filters. Fluorescence decay times
were extracted by biexponential fitting procedures through decon-
volution employing instrumental response functions. Differential
scanning calorimetry (DSC) was carried out under a nitrogen
atmosphere at a heating rate of 20 °C/min on a Perkin-Elmer DSC7.
Cyclic voltammetric experiments were performed using a model
273A machine (Princeton Applied Research) with a one-compart-
ment electrolysis cell consisting of a platinum working electrode,
a platinum wire counter electrode, and a quasi Ag+/Ag electrode
as reference. Measurements were performed in 0.5 mM CH2Cl2
solution, with tetrabutylammonium tetrafluoroborate as supporting
electrolyte, at a scan rate of 50 mV/s. Each oxidation potential was
calibrated using ferrocene as a reference.
Calculations. All (time-dependent) density functional theory
(TD-DFT) calculations were carried out in the gas phase using the
TURBOMOLE 5.10 quantum-chemical package, employing the
B3LYP function and the 6-311G** basis set.32 TD-DFT was shown
to reproduce the E* f K* ESIPT process until the biradical
character began to dominate, which, in the gas phase, led to fast
internal conversion via a conical intersection governed by torsional
relaxation within the hydroxyphenyl imidazole unit.33 Thus, TD-
DFT sufficiently described the radiative processes of HPI and HPNI.
To this end, vertical emission was calculated using a coplanar
geometry of the K* form of the hydroxyphenyl imidazole backbone.
EL Device Fabrication. EL devices were fabricated with a
configuration of (A) ITO/NPD (40 nm)/EML (W1, 30 nm)/BPhen
(50 nm)/LiF (1 nm)/Al (100 nm) and (B) ITO/2-TNATA (60 nm)/
NPD (20 nm)/EML (W1, 40 nm)/BPhen (20 nm)/LiF (1 nm)/Al
(100 nm). All organic layers were deposited onto UV-O3-treated
indium-tin-oxide (ITO)-coated glass. All organic layers were
deposited by thermal evaporation under a base pressure of <10-7
Torr. Deposition of a hole-transport layer of 600 Å (device B only;
see Figure S10 in Supporting Information for details), a thick layer
of 4,4′,4′′-tris(2-naphthylphenylamino)triphenylamine (2-TNATA),
and a 400 or 200 Å thick layer of N,N′-diphenyl-N,N′-bis(1-
naphthalenyl)-(1,1′-biphenyl)-4,4′-diamine (NPD) was followed by
deposition of the emitting layer (W1). Over this layer, hole-blocking
and electron-transport layers (500 or 200 Å of 4,7-diphenyl-1,10-
phenanthroline [BPhen]) were sequentially deposited. Finally, 10
Å thick LiF and 1000 Å thick aluminum were successively
deposited to form a cathode. The active area was 2 mm × 2 mm.
Current-voltage-luminescence characteristics were obtained using
a Keithley 237 source measurement unit and a Photoresearch PR-
650 spectrometer.
Here, we report the first example of a concentration-
independent ultimate white-light-emitting molecular dyad based
on two ESIPT keto-emitting units. These molecules are co-
valently linked blue- and orange-light-emitting moieties between
which energy transfer is frustrated, and the combination thus
shows broad white photoluminescence covering the entire visible
range after single-wavelength excitation, as well as stable white
electroluminescence, upon simple OLED device fabrication.
Experimental Section
Measurements. Chemical structures were fully identified by 1H
NMR (JEOL, JNM-LA300), 13C NMR (Bruker, Avance DPX-300),
MALDI-TOF mass spectrometry (Applied Biosystems Inc.; Voyager-
DE STR Biospectrometry Workstation), GC-mass spectrometry
(JEOL, JMS-AX505WA), and elemental analysis (CE Instruments,
EA1110). Absorption spectra were obtained using either of two
UV-vis spectrophotometers (Scinco S-2040 and Shimadzu UV-
1650PC). Photoluminescence emission and excitation spectra were
obtained using a fluorescence spectrophotometer (Varian, Cary
Eclipse); excitation spectra were corrected for characteristics of the
light source. Emission spectra were corrected for wavelength-
dependent sensitivity of the detector. Photoluminescence quantum
efficiencies (ΦPL values) for solutions were obtained using 9,10-
diphenylanthracene as reference.16 The ΦPL values of thin films
on quartz plates were measured using a 6 in. integrating sphere
(Labsphere, 3P-GPS-060-SF) equipped with a 325 nm CW He-Cd
laser (Omnichrome, Series 56) and a PMT detector (Hamamatsu,
PD471) attached to a monochromator (Acton Research, Spectrapro-
300i). The detailed procedure followed to obtain solid-state ΦPL
data has been described elsewhere.17 Pulse-excited emission spectra
were measured using an actively/passively mode-locked 25 ps Nd:
YAG laser (Quantel, YG701) and an intensified CCD (Princeton
Instruments, ICCD576G) attached to a spectrometer (Acton Re-
search, Spectrapro-500) as the excitation source and detector,
respectively. Samples were excited at the front face with pulses of
347 nm, generated through a Raman shifter filled with methane at
15 atm, and pumped by the fourth-harmonic pulses (266 nm) of
the laser. A 10 ps streak camera (Hamamatsu, C2830) with a CCD
detector (Princeton Instruments, RTE128H) was used for detection
Results and Discussion
The standard method for white-light generation typically
involves mixing of the three primary colors red, green, and
blue2,18-20 or combining two complementary colors.8,21 How-
ever, the simple mixing of luminescent materials can be
problematic because color balancing is difficult to control as a
result of near-inevitable energy transfer, leading to dominant
emission from lower band-gap components, even when they
are present in very small amounts.22 This energy transfer
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(13) Kim, S.; Seo, J.; Jung, H. K.; Kim, J. J.; Park, S. Y. AdV. Mater.
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(19) (a) Aharon, E.; Kalina, M.; Frey, G. L. J. Am. Chem. Soc. 2006, 128,
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(14) (a) Klymchenko, A. S.; Pivovarenko, V. G.; Ozturk, T.; Demchenko,
A. P. New J. Chem. 2003, 27, 1336. (b) Klymchenko, A. S.;
Yushchenko, D. A.; Mely, Y. J. Photochem. Photobiol. A 2007, 192,
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(20) Kido, J.; Hongawa, K.; Okuyama, K.; Nagai, K. Appl. Phys. Lett. 1994,
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(15) (a) Goodman, J.; Brus, L. E. J. Am. Chem. Soc. 1978, 100, 7472. (b)
Kasha, M. J. Chem. Soc., Faraday Trans. 2 1986, 82, 2379.
(16) deMello, J. C.; Wittmann, H. F.; Friend, R. H. AdV. Mater. 1997, 9,
230.
(21) (a) Liu, J.; Guo, X.; Bu, L. J.; Xie, Z. Y.; Cheng, Y. X.; Geng, Y. H.;
Wang, L. X.; Jing, X. B.; Wang, F. S. AdV. Funct. Mater. 2007, 17,
1917. (b) Qin, D. S.; Tao, Y. Appl. Phys. Lett. 2005, 86, 113507.
(22) (a) Van Der Meer, B. W.; Coker, G.; Chen, S. Y. S. Resonance Energy
Transfer: Theory and Data; Wiley: New York, 1994. (b) Berggren,
M.; Dodabalapur, A.; Slusher, R. E.; Bao, Z. Nature 1997, 389, 466.
(17) Berlman, I. B. Handbook of Fluorescence Spectra of Aromatic
Molecules; Academic Press: New York, 1971.
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14044 J. AM. CHEM. SOC. VOL. 131, NO. 39, 2009