Fine tuning the energetics of excited-state intramolecular proton transfer (ESIPT): White light generation in a single ESIPT system

Article abstract of DOI:10.1021/ja2062693

Using 7-hydroxy-1-indanone as a prototype (I), which exhibits excited-state intramolecular proton transfer (ESIPT), chemical modification has been performed at C(2)-C(3) positions by fusing benzene (molecule II) and naphthalene rings, (molecule III). I undergoes an ultrafast rate of ESIPT, resulting in a unique tautomer emission (λ_max ～530 nm), whereas excited-state equilibrium is established for both II and III, as supported by the dual emission and the associated relaxation dynamics. The forward ESIPT (normal to proton-transfer tautomer species) rates for II and III are deduced to be (30 ps)^-1 and (22 ps)^-1, respectively, while the backward ESIPT rates are (11 ps)^-1 and (48 ps)^-1. The ESIPT equilibrium constants are thus calculated to be 0.37 and 2.2 for II and III, respectively, giving a corresponding free energy change of 0.59 and -0.47 kcal/mol between normal and tautomer species. For III, normal and tautomer emissions in solid are maximized at 435 and 580 nm, respectively, achieving a white light generation with Commission Internationale de l'Eclairage (CIE) (0.30, 0.27). An organic light-emitting diode based on III is also successfully fabricated with maximum brightness of 665 cd m^-2 at 20 V (885 mA cm^-2) and the CIE coordinates of (0.26, 0.35). The results provide the proof of concept that the white light generation can be achieved in a single ESIPT system.

Full text of DOI:10.1021/ja2062693

ARTICLE
pubs.acs.org/JACS
Fine Tuning the Energetics of Excited-State Intramolecular Proton
Transfer (ESIPT): White Light Generation in A Single ESIPT System
Kuo-Chun Tang,† Ming-Jen Chang,‡ Tsung-Yi Lin,† Hsiao-An Pan,† Tzu-Chien Fang,‡ Kew-Yu Chen,*,‡
Wen-Yi Hung,*,§ Yu-Hsiang Hsu,§ and Pi-Tai Chou*,†
†Department of Chemistry, National Taiwan University, Taipei, 10617 Taiwan, R.O.C.
‡Department of Chemical Engineering, Feng Chia University, Taichung, 40724 Taiwan, R.O.C.
§Institute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung, 20224 Taiwan, R.O.C.
S Supporting Information
b
ABSTRACT: Using 7-hydroxy-1-indanone as a prototype (I), which exhibits excited-state
intramolecular proton transfer (ESIPT), chemical modification has been performed at
C(2)ꢀC(3) positions by fusing benzene (molecule II) and naphthalene rings, (molecule
III). I undergoes an ultrafast rate of ESIPT, resulting in a unique tautomer emission
(λmax ∼530 nm), whereas excited-state equilibrium is established for both II and III, as
supported by the dual emission and the associated relaxation dynamics. The forward ESIPT
(normal to proton-transfer tautomer species) rates for II and III are deduced to be (30 ps)ꢀ1
and (22 ps)ꢀ1, respectively, while the backward ESIPT rates are (11 ps)ꢀ1 and (48 ps)ꢀ1
.
The ESIPT equilibrium constants are thus calculated to be 0.37 and 2.2 for II and III,
respectively, giving a corresponding free energy change of 0.59 and ꢀ0.47 kcal/mol between
normal and tautomer species. For III, normal and tautomer emissions in solid are maximized
at 435 and 580 nm, respectively, achieving a white light generation with Commission
Internationale de l’Eclairage (CIE) (0.30, 0.27). An organic light-emitting diode based on III
is also successfully fabricated with maximum brightness of 665 cd mꢀ2 at 20 V (885 mA cmꢀ2) and the CIE coordinates of
(0.26, 0.35). The results provide the proof of concept that the white light generation can be achieved in a single ESIPT system.
combining solid polymers doped with ESIPT molecules.7d Very
1. INTRODUCTION
recently, via anchoring the designated ESIPT molecules onto
Excited-state intramolecular proton transfer (ESIPT) has long
ultrasmall CdSe quantum dots, we reported white light genera-
been an issue of fundamental importance.1 Most of the ESIPT
tion of the resulting nanocomposites with the fascinating Com-
reactions involve proton (on hydrogen atom) transfer from a pre-
mission Internationale de l’Eclairage (CIE) coordinate of (0.33,
existing hydrogen bond, giving rise to a proton-transfer tautomer
in the excited state. Due to the drastic structural alternation, the
tautomer possesses different photophysical properties from that of
0.33).7e Unfortunately, mixing multiple emissive materials com-
monly requires a delicate balance between chromophores in
terms of emission and excitation to attain a white light. Also,
spectral variation may arise due to different photostability and
the original (normal) species, offering great versatility in a variety
of applications, such as lasing materials,2 UV photostabilizer,3
environmental perturbation onto different chromophores upon
optical filter,4 radiation scintillator,5 molecular recognition,6 etc.
long-term use. Needless to say, the more emissive materials are
Most recent cutting edge application seems to be focusing on the
lighting materials.7 The ultrafast rate of ESIPT, together with a
large Stokes shift between absorption and tautomer emission
(peak-to-peak), provides a broad spectral window to fill in other
complementary emission color free from energy transfer and
used, the more cost ineffective the device is.
Thus, further advancement may lie in searching for a single
ESIPT system that is able to render both normal and tautomer
emissions intrinsically, spanning the entire visible region suited
for white light generation. Unfortunately, most ESIPT molecules
reabsorption. Also, due to the near UV absorption, the naked
possess a strong intramolecular hydrogen bond, so that ESIPT is
eye’s view of the film is transparent, beneficial to its versatility in
commercial applications.
either barrierless or small; its time scale is generally <1 ps,1
resulting in a lack of normal emission. If there is any exception,
Along the above advantages, attempts exploiting ESIPT mol-
one class of ESIPT system exhibiting dual emission should be
ecules to attain white light generation have recently been made.7
credited to recently developed excited-state proton coupled
Via mixing two ESIPT molecules, dual proton-transfer emission
charge-transfer molecules,1b for which either normal or tautomer
and hence color hue can be ratiometrically fine-tuned.7a Park
and co-workers reported the white-emitting molecules com-
prised of covalently linked blue and yellow color-emitting ESIPT
Received: July 6, 2011
Published: September 29, 2011
moieties.7b,c Sun et al. reported a white light generation
r
2011 American Chemical Society
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Scheme 1. Structures and ESIPT Mechanism of Systems
Scheme 2. Synthetic Routes of IIIa
I, II, and IIIa
a See experimental section for the detail of synthesis.
predicted to be more unstable than that of III, due to more
energy needed to reduce the aromaticity of benzene (in II) than
that of the naphthalene moiety (in III).10 Accordingly, the
relative energy between normal and tautomer in the excited state
can be systematically fine-tuned among I, II, and III. As a result,
for II and III excited-state equilibrium is established between
normal and tautomer forms. White light generation is achieved
serendipitously in a single ESIPT system III. Detail of synthesis,
photophysical properties, and OLED application is elaborated as
follows.
a Note that the effective conjugated double bonds responsible for the
proton-transfer tautomer emission are marked in red.
2. EXPERIMENTAL SECTION
species possesses strong charge-transfer character, giving a
drastic difference in dipole moment between normal and tauto-
mer forms. Accordingly, in solution, solvent reorganization may
channel into the reaction, inducing a barrier that leads to slow
ESIPT and hence dual emission. Unfortunately, the prohibition
of molecular skeleton relaxation in rigid media hampers further
fabrication toward solid devices.
In this study, we then switched the research direction toward
harnessing the thermodynamics of ESIPT. Our tactic is to sys-
tematically fine-tune the energetics of a series of designed ESIPT
molecules, so that thermal equilibrium can be reached between
normal and tautomer species in the excited state. If the pre-
equilibrium can be established prior to their respective popula-
tion deactivation, we then expect a dual (normal and tautomer)
emission covering the designated visible range.
2.1. Synthesis and Characterization. Except for acetonitrile-d3
(Cambridge Isotope Laboratories, Inc.), other solvents were purchased
from Merck and distilled from appropriate drying agents prior to use.
Commercially available reagents were used without further purification
unless otherwise stated. All reactions were monitored by thin-layer
chromatography (TLC) with MachereyꢀNagel precoated Glassic
sheets (0.20 mm with fluorescent indicator UV254). Compounds were
visualized with UV light at 254 and 365 nm. Flash column chromatog-
raphy was carried out using silica gel from Merck (230ꢀ400 mesh).
Compounds I and II were prepared according to a standard literature
procedure,11 for which the characterization is provided in the Supporting
Information. 1-Hydroxy-11H-benzo[b]fluoren-11-one (III) was syn-
thesized according to a four-step synthetic route depicted in Scheme 2,
in which each step is described below.
2.1.1. 3-Bromo-7-methoxy-2,3-dihydro-1H-inden-1-one (4). 7-Methoxy-
2,3-dihydro-1H-inden-1-one (1.0 g, 6.2 mmol) and N-bromosuccini-
mide (1.2 g, 6.8 mmol) were dissolved in 20 mL of CCl4, and 12 mg
(1 mol %) of 2,20-azobisisobutyronitrile (AIBN) was added. The mixture
was slowly stirred and heated to 80 ꢀC for 2 h. After cooling, the mixture
was poured into cold water, extracted with CH2Cl2, and dried with
anhydrous MgSO4. After the solvent was removed, the crude product
was purified by silica gel column chromatography with eluent ethyl
Herein, we report the strategic design and synthesis of a series
of potential ESIPT molecules I, II, and III, in which II and III are
chemical modifications of the ESIPT system I (7-hydroxy-1-
indanone)8 at C(2)ꢀC(3) positions (see Scheme 1). IꢀIII are
designed on a basis of the judicious balance between π-elonga-
tion and the stabilization of aromaticity. Upon forming the
proton-transfer tautomer in the excited state, using I (see Scheme 1)
as a prototype, the aromaticity of benzene is destabilized by the
elongated π-delocalization incorporating four conjugated double
bonds (see those double bonds marked in red). According to the
theory of effective conjugation length (ECL),9 the proton-
transfer tautomer with longer π-conjugation (cf. the normal
1
acetate/n-hexane (1/4) to afford 4 (1.1 g, 74%). H NMR (CDCl3,
ppm) 7.62 (t, J = 8.5 Hz, 1H), 7.20 (d, J = 8.0 Hz, 1H), 6.87 (d, J = 8.5
Hz, 1H), 5.52 (dd, J1 = 7.0 Hz, J2 = 2.0 Hz, 1H), 3.92 (s, 3H), 3.32 (dd,
J1 = 18.5 Hz, J2 = 7.0 Hz, 1H), 2.97 (dd, J1 = 18.5 Hz, J2 = 2.0 Hz, 1H).
MS (FAB) m/z 240 (M + H)+. HRMS calcd for C10H10BrO2, 240.9864;
found, 240.9866.
2.1.2. 7-Methoxy-1H-inden-1-one (3). 3-Bromo-7-methoxy-2,3-dihy-
dro-1H-inden-1-one (1.0 g, 4.1 mmol) in CCl4 was cooled in an ice bath,
and 1.93 mL (1.4 g, 14.0 mmol) of triethylamine was added a slowly. The
reaction mixture was allowed to warm to room temperature and stir
overnight. After the solvent was removed, the crude product was purified
by silica gel column chromatography with eluent ethyl acetate/n-hexane
(1/4) toafford3(0.63g, 95%). 1HNMR(CDCl3, ppm) 7.42(d,J= 6.0 Hz,
0
0
form) is expected to have smaller S1 ꢀS0 (prime denotes the
proton-transfer tautomer) energy gap (cf. normal S1ꢀS0,). The
interplay between π-conjugation and aromaticity is more sig-
nificant in II and III because of the partial reduction of aromaticity in
the fused benzene and naphthalene, respectively, in order to fulfill
extended five effective π-conjugated bonds (see Scheme 1).
The proton-transfer tautomer of II (relative to normal form) is
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1H), 7.34 (t, J = 8.0 Hz, 1H), 6.85 (d, J = 8.8 Hz, 1H), 6.70 (d, J =
8.0 Hz, 1H), 5.84 (d, J = 6.0 Hz, 1H), 3.95 (s, 3H). 13C NMR (100 MHz,
CDCl3, ppm) 196.57, 156.50, 147.38, 147.03, 144.48, 136.02, 127.70,
115.65, 114.67, 55.89. MS (FAB) m/z 161 (M + H)+. HRMS calcd for
C10H9O2, 161.0603; found, 161.0608.
2.1.3. 1-Methoxy-11H-benzo[b]fluoren-11-one (2). A mixture of
solution of α,α,α0α0-terarbromo-o-xylene (1.3 g, 3.1 mmol), 7-meth-
oxy-1H-inden-1-one (0.5 g, 3.1 mmol), sodium iodide (1.8 g, 12 mmol),
and dry DMF (30 mL) was stirred at 65 ꢀC for 24 h. The reaction
mixture was poured into cold water (70 mL) containing sodium bisulfite
(1.0 g). The yellow precipitate was purified by silica gel column
chromatography with eluent ethyl acetate/n-hexane (1/4) to afford 2
1
(0.73 g, 90%). H NMR (CDCl3, ppm) 8.14 (s, 1H), 7.79ꢀ7.88 (m,
3H), 7.43ꢀ7.54 (m, 3H), 7.31 (d, J = 7.6 Hz, 1H), 6.85 (d, J = 8.4 Hz,
1H), 3.99 (s, 3H). 13C NMR (100 MHz, CDCl3, ppm) 191.06, 158.54,
146.83, 137.56, 136.96, 136.50, 133.76, 133.10, 130.58, 128.68, 128.59,
126.78, 125.01, 122.74, 119.03, 113.26, 112.37, 55.89. MS (FAB) m/z
261 (M + H)+. HRMS calcd for C18H13O2, 261.0916; found, 261.0918.
2.1.4. 1-Hydroxy-11H-benzo[b]fluoren-11-one (III). 1-Methoxy-11H-
benzo[b]fluoren-11-one (300 mg, 1.1 mmol) was dissolved in 10 mL of
dichloromethane in a 50 mL round-bottom flask, and the flask was placed
in an ice bath at 0 ꢀC. A solution of boron tribromide (0.25 mL, 1.0 M
solution in dichloromethane) was added carefully to the stirred solution
under a nitrogen atmosphere. After 4 h, the reaction was cooled, and the
reaction mixture was then hydrolyzed bycarefully shaking it with 10 mL of
water and extracted twice with 10 mL of dichloromethane. The combined
organic phases were then dried over magnesium sulfate, filtered, and
evaporated in vacuo; the crude product was purified by silica gel column
chromatography with eluent ethyl acetate/n-hexane (1/10) to afford III
(269 mg, 95%). 1H NMR (CDCl3, ppm) 8.64 (s, 1H), 8.07 (s, 1H), 7.84
(d, J = 7.5 Hz, 1H), 7.78 (s, 1H), 7.77 (d, J = 9.0 Hz, 1H), 7.54 (t, J =
7.5 Hz, 1H), 7.38ꢀ7.46 (m, 2H), 7.15 (d, J = 7.0 Hz, 1H), 6.76 (d, J =
8.0 Hz, 1H). 13C NMR (100 MHz, CDCl3, ppm) 195.51, 157.66, 144.25,
137.99, 137.81, 136.74, 133.41, 132.68, 130.78, 129.13, 128.85, 127.08,
125.54, 120.16, 120.01, 117.40, 113.00. MS (FAB) m/z 247 (M + H)+.
HRMS calcd for C17H11O2, 247.0759; found, 247.0755.
Figure 1. (A) Molecular structure of III with thermal ellipsoids drawn
at the 50% probability level. (B) Packing view of III, viewed along the
b axis.
instrument response function, which was determined by measuring the
Raman scattering signal.
2.3. Computational Methodology. All the theoretical calcula-
tions were performed with the Gaussian 03 program.13 Geometry
optimization for the ground state of molecules IꢀIII in cyclohexane
solution (PCM model)14 was performed using density functional theory
(DFT) with B3LYP15 hybrid function. For the first singlet excited state,
we calculated the FranckꢀCondon states with time-dependent density
functional theory (TD-DFT) using the B3LYP hybrid function. The
6-311+G(d,p) basis sets16 were employed for all atoms.
2.2. Steady-State and Time-Resolved Fluorescence Spec-
troscopy. Steady-state absorption and emission spectra were recorded
on a Hitachi (U-3310) spectrophotometer and an Edinburgh (FS920)
fluorimeter, respectively. Both the wavelength-dependent excitation and
the emission response of the fluorimeter were calibrated.
Time-resolved spectroscopic measurements were carried out by
means previously reported elsewhere in detail.12 In brief words, sub-ns
to ns time-resolved studies were performed using a time-correlated sin-
gle photon counting (TCSPC) system (OB-900 L lifetime spectro-
meter, Edinburgh) with the excitation light from either second harmonic
generation (SHG, at 400 nm) or third harmonic generation (THG, at
266 nm) of pulse-selected femtosecond laser pulses at 800 nm (Tsunami
and Model 3980 pulse picker, Spectra-Physics). The fluorescence was
collected at a right angle with respect to the pump beam path and passed
through a polarizer, setting the polarization at the magic angle (54.7ꢀ)
with respect to the pump polarization, and located in front of the
detector to eliminate anisotropy. The temporal resolution, after partial
removal of the instrument time broadening, is ∼30 ps. Ultrafast
spectroscopic studies were performed by a femtosecond fluorescence
up-conversion system (FOG100, CDP) pumped by the same femtose-
cond oscillator. In the experiment, fluorescence from a rotating sample
cell, following the excitation by SHG of a femtosecond pulse, was
collected, focused, and frequency summed in a BBO crystal, along with
an interrogation gate pulse at designated delay time with respect to the
pump pulse. A λ/2 waveplate was used to set the polarization between
pump and gate pulses at magic angle (54.7ꢀ) to prevent fluorescence
anisotropy contributed by solute reorientation. Fluorescence up-conversion
data were fitted to the sum of exponential functions convoluted with the
2.4. OLED Device Fabrications. All chemicals were purified
through vacuum sublimation prior to use. The OLEDs were fabricated
through vacuum deposition of the materials at 10ꢀ6 Torr onto ITO-
coated glass substrates, having a sheet resistance of 15 Ω sqrꢀ1. The
3
ITO surface was cleaned ultrasonically—sequentially with acetone,
MeOH, and deionized water, followed by the treatment with UV-ozone.
A hole-injection layer (PEDOTꢀPSS) was spin coated onto the
substrates and dried at 130 ꢀC for 30 min to remove residual water.
Organic layers were then vacuum deposited at a deposition rate of ca.
1ꢀ2 Å sꢀ1. Subsequently, LiF was deposited at 0.1 Å sꢀ1 and then
capped with Al (ca. 5 Å sꢀ1) through shadow masking without breaking
the vacuum. The JꢀVꢀL characteristics of the devices were measured
simultaneously using a Keithley 6430 source meter and a Keithley 6487
picoammeter equipped with a calibration Si-photodiode in a glovebox.
EL spectra were measured using a photodiode array (OTO SD1000)
with a spectral range from 300 to 1100 nm and a resolution of 1.5 nm.
3. RESULTS AND DISCUSSION
Compounds I and II were prepared according to a standard
literature procedure.11 Synthetic route of III is depicted in
Scheme 2. In brief, the synthesis of III started from a bromination
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Scheme 3. Calculated Frontier Orbitals for I, II, and III
of Normal Form Involved in the First Singlet Excitation
by B3LYP/6-311+G(d,p)/PCM
Figure 2. Solid lines: steady-state UVꢀvis absorption spectra (gray) and
photoluminescence spectra (black) of (a) I, (b) II and (c) III in cyclo-
hexane. Dotted lines in (b) depict the decomposed PL spectra assigned for
normal (blue) and tautomer (red) species, respectively. Dashed line in (c)
depicts the PL spectrum of amorphous solid powder sample III, whose
corresponding calculated CIE 1931 chromaticity coordinates (x = 0.30, y =
0.27) is shown in (d). The photograph in (e) shows, in the dark room, a
piece of solid vinyl matrix doped with III and shed with laboratory UV lamp
(366 nm) through a square mask underneath.
of 7-methoxy-2,3-dihydro-1H-inden-1-one (5), followed by the
elimination of the bromo adduct (4), giving a reactive dienophile
3. The naphthalene moiety was then fused onto the C2ꢀC3
double bond (see Scheme 1) by placing 3 through a reaction with
tetrabromo-o-xylene,17 yielding 2. Compound 2 was then subject
to the deprotection with BBr3 to give III an overall product yield
of 60%.The structure of III was further confirmed by single-
crystal X-ray diffraction analysis. As shown in Figure 1A, the
molecular structure of III is nearly planar, which is also confirmed
by geometry optimization in our DFT calculation (see Scheme 3).
This, together with 2.879 Å of O(2)ꢀO(1) distance, supports
the existence of a six-membered ring intramolecular hydro-
for the emission. The difference in peak wavelength between
absorption and emission of as large as ∼13,000 cmꢀ1 unambigu-
ously supports the occurrence of ESIPT, forming a proton-
transfer tautomer adiabatically in the excited state. Further
attempts to resolve the dynamics of ESIPT were made by using
a time-correlated single-photon counting system with 266 nm
excitation. The result (see Figure S1b, Supporting Information)
reveals that the tautomer emission of I (monitored at 550 nm)
consists of a rise component that is beyond the limit of our sys-
tem response (<30 ps), followed by a population decay time
resolved to be 1.0 ns. Upon monitoring at, e.g., 370 nm, which is
supposed to be in the normal emission region and is barely
shown in the steady-state measurement, a fast, irresolvable decay
component of <30 ps was also acquired. Unfortunately, the
lowest lying absorption of <350 nm for I makes further femto-
second time-resolved study, such as fluorescence up-conversion,
infeasible, mainly due to the lack of excitation wavelength in the
current set up. Nevertheless, both steady-state and picosecond
time-resolved measurements lead us to conclude an ultrafast,
exergonic type of ESIPT for I, similar to most of the reported
ESIPT molecules.1
In stark contrast, as shown in Figure 2b, dual emission seems
to be resolved in the steady-state measurement of II, which upon
deconvolution, is composed of a normal emission, justified by
its mirror image with respect to the lowest lying absorption, and
a large Stokes shifted tautomer emission band maximized at
468 and 535 nm, respectively (also see deconvoluted curve in
Figure 2b). In III, both normal (λmax ∼450 nm) and tautomer
(λmax ∼ 620 nm) emissions are well distinguished. The early time
relaxation dynamics of the normal emission for both II and III
were measured by femtosecond fluorescence up-conversion
(λex: ∼400 nm). The resulting relaxation dynamics, shown in
Figure 3 and Table 1, consist of a fast decay with a time constant
of 8.2 ps for II and 15 ps for III, followed by a much longer popu-
lation decay component. Upon monitoring tautomer emission,
the relaxation pattern consists of a rise component, 1.9 and 8.3 ps
for II and III, respectively, followed by a long population decay
1
gen bond. However, in the H NMR spectrum (in CDCl3), as
opposed to the large downfield shift (δ > 10 ppm) of the ꢀOH
proton in most ESIPT molecules possessing strong intramole-
cular hydrogen bond,1 the ꢀOH proton signal of III appears at δ
8.64 ppm, revealing a relatively weaker hydrogen-bond forma-
tion. This is possible due to the fact that the carbonyl oxygen
(O(1), see Figure 1) sits at the five-membered ring cyclopenta-
2,4-dienone moiety, such that the —O(2)ꢀHꢀO(1) angle is
expected to be deviated from 120ꢀ, a perfect six-member ring
hydrogen-bonding formation. This viewpoint is supported by the
—O(2)ꢀHꢀO(1) angle of 142.6ꢀ, according to the X-ray
structure analysis. Careful examination of the crystal structure
also indicates that there exists substantial πꢀπ stacking between
the tetracyclic plane and its adjacent one, so the molecules are
packed into layers along the b axis shown in Figure 1B.
Steady-state absorption and emission spectra of compounds
IꢀIII in cyclohexane at room temperature are depicted in
Figure 2. Pertinent data are listed in Table 1. I exhibits the
lowest lying absorption band maximized at 315 nm, attributed to
a πꢀπ* transition, which is also supported by the calculated
frontier orbitals shown in Scheme 3. Scheme 3 also reveals that
the electron density around the intramolecular hydrogen binding
site is mainly populated at hydroxyl oxygen and carbonyl oxygen
at highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO), respectively. The results
clearly indicate that upon electronic excitation of all IꢀIII, the
hydroxyl proton is expected to be more acidic, whereas the
carbonyl oxygen is more basic with respect to their ground state,
driving the proton-transfer reaction.
Experimentally, upon excitation, a large Stokes-shiftedemission
maximized at ∼530 nm is resolved for I in cyclohexane. The sim-
ilar excitation and absorption spectral profiles (see Figure S1a,
Supporting Information) conclude the same ground-state origin
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Table 1. Photophysical Properties of IꢀIII in Cyclohexane at Room Temperature
steady-state measurement
time-resolved measurement
λabs [nm] λem [nm] (Q.Y.) λmonitor fitted time constants (prefactor) by femtosecond fluorescence up-conversion population decay time by TCSPC
I
315
354
530 (0.012)
468 (0.019)
535 (0.014)
550 nm
440 nm
480 nm
620 nm
480 nm
620 nm
650 nm
1.0 ns
1.2 ns
II
8.2 ps (0.27), 1.2 ns (0.73)a
1.9 ps (ꢀ0.34), 32 ps (0.16), 1.2 ns (0.50)a
1.2 ns
III
409
450 (0.006)
621 (0.020)
15 ps (0.69), 0.66 ns (0.31)a
0.67 ns
8.3 ps (ꢀ0.44), 0.66 ns (0.56)a
0.66 ns
a Population decay time constant was applied for the fitting.
The coupling eqs 1 and 2 can be solved with given initial
conditions, [N*](t) = [N*]0 and [T*](t) = [T*]0 = 0 at t = 0 and
the assumption of kpt and kꢀpt . kN* and kT*:
ꢁ
½N ꢂ0
½ðλ2 ꢀ XÞ eꢀλ t þ ðX ꢀ λ1Þ eꢀλ t
ꢂ
ð3Þ
ð4Þ
1
2
ꢁ
½N ꢂ ¼
3
3
3
λ2 ꢀ λ1
ꢁ
kpt ½N ꢂ0
3
1
2
ꢁ
½T ꢂ ¼
½eꢀλ t ꢀ eꢀλ t
ꢂ
3
λ2 ꢀ λ1
where,
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
ðX þ YÞ ꢀ ðX ꢀ YÞ þ 4 kpt
k
ꢀpt
3
3
λ1 ¼
ð5Þ
ð6Þ
2
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
ðX þ YÞ þ ðX ꢀ YÞ þ 4 kpt
k
ꢀpt
3
3
λ2 ¼
2
with X = kpt + kN* and Y = kꢀpt + kT*.
Under the assumption that there exists a fast, excited-state
equilibrium between the normal and tautomer species (i.e., kpt
and kꢀpt . kN* and kT*), X ≈ kpt and Y ≈ kꢀpt. eq 5 and 6 can be
written as follows:
Figure 3. Fluorescence up-conversion decay curves obtained in cyclo-
hexane of (a) II and (b) III. The excitation is at 400 nm and the data
points (O and 0) shown are with monitored emission wavelength as
depicted. Solid lines depict the best polyexponential fits. The fitting
results are summarized in Table 1.
ꢁ
ꢁ
ꢁ
ꢁ
kN
k
k
þ kT kpt
kN þ kT Keq
ꢀpt
3
3
3
λ1 ¼
¼
ð7Þ
þ kpt
1 þ Keq
ꢀpt
component. The results of subnanosecond time-correlated single-
photon counting experiments are shown in Figure 4, which resolve
the population decay of the normal emission to be 1.2 and 0.67 ns
for II and III, respectively. These time scales, within experimental
error, are identical with that of the corresponding tautomer emis-
sion, i.e., 1.2 ns in II and 0.66 ns in III. The results are thus fitted to a
reaction pattern, depicted in Scheme 4, involving ESIPT upon
excitation, followed by an establishment of fast equilibrium prior to
the population decay for both normal and tautomer excited states.
According to the reaction model shown in the Scheme 4, the
differential rate equations for change of concentrations of normal
and tautomer species can be expressed as follows:18
λ2 ¼ kpt þ k
ð8Þ
ꢀpt
The pre-exponential factors in eq 3 for the normal emission
can be derived further as
kpt
X ꢀ λ1
ꢁ
½N ꢂ0
≈
ð9Þ
3
3
λ2 ꢀ λ1 kpt þ k
ꢀpt
k
λ2 ꢀ X
ꢀpt
ꢁ
½N ꢂ0
≈
ð10Þ
λ2 ꢀ λ1 kpt þ k
ꢀpt
The equilibrium constant Keq (kpt/kꢀpt) for tautomer versus
normal species in the excited state is then obtained by dividing
eq 9 with eq 10. As a result, the equilibrium constants are
calculated to be 0.37 and 2.2, respectively, corresponding to the
free energy change of 0.59 and ꢀ0.47 kcal/mol between normal
and tautomer species for II and III, respectively. Furthermore,
ꢁ
d½N ꢂ
ꢁ
ꢁ
ꢁ
¼ ꢀðkN þ kptÞ ½N ꢂ þ k
½T ꢂ
ð1Þ
ð2Þ
ꢀpt
3
3
dt
ꢁ
d½T ꢂ
ꢁ
ꢁ
ꢁ
¼ ꢀðkT þ kꢀptÞ ½T ꢂ þ kpt ½N ꢂ
the kpt and k
can be derived as (30 ps)ꢀ1 and (11 ps)ꢀ1
,
3
3
dt
ꢀpt
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Table 2. Calculated Potential Energy (zero point energy) of
First Singlet Excited State (S1) and Ground State (S0) for
IꢀIII
normal form
tautomer form
I
S1 (hartree)
ꢀ498.08423
ꢀ498.09250
ꢀ498.21122
S0 (hartree)
ΔE (kcal/mol)a
ꢀ498.23214
ꢀ5.20 (ꢀ6.68)b
II
S1 (hartree)
ꢀ650.56476
ꢀ650.54962
ꢀ650.64651
S0 (hartree)
ΔE (kcal/mol)a
ꢀ650.66826
9.50 (3.48)b
Figure 4. TCSPC measurements of (a) II and (b) III in cyclohexane
with excitation at 400 nm. Open squares (0) and open circles (O, offset
for clarity) are data points obtained with monitored emission wave-
length as depicted, while solid lines are corresponded convoluted fitting
curves of single-exponential function to the data. See Table 1 for fitting
results.
III
S1 (hartree)
ꢀ804.20147
ꢀ804.19574
ꢀ804.28167
S0 (hartree)
ΔE (kcal/mol)a
ꢀ804.30266
3.60 (1.67)b
a Energy change calculated for ESIPT reaction. b Values in parentheses
are calculated based on spectroscopic measurements and calculated
energies of ground state (S0).
Scheme 4. Schematic ESIPT Reactiona
up to the tautomer ground state. As a result, the energy changes
(ΔE) of ESIPT are calculated to be ꢀ6.68, 3.48, and 1.67 kcal/
mol for IꢀIII, respectively (see Table 1), the trend of which is
consistent with that calculated from the aforementioned TD-
DFT. More importantly, in a qualitative manner, the results of
both methods correlate well with the experimental results. Both
experimental and computational results thus support the inter-
play between effective π-conjugation length and aromaticity. The
partial reduction of aromaticity in the fused benzene and naph-
thalene for II and III, respectively, at the C2ꢀC3 position has to
be taken into account in order to fulfill five effective π-conjugated
bonds for the tautomer species (see Scheme 1). Comparing II
and III, the proton-transfer tautomer of II (relative to normal
form, ΔE in Table 1) is more endergonic than that of III, due
to more energy required to destroy the aromaticity of benzene
(in II) than that of naphthalene (in III). Evidently, the energetics
of ESIPT have been fine-tuned among IꢀIII.
a [N*]: concentration of normal species of excited state; [T*]: concen-
tration of tautomer species of excited state; kpt: proton transfer rate con-
stant; kꢀpt: reverse proton-transfer rate constant; kN*: decay rate constant
of N* for all decay channel except kpt; kT*: decay rate constant of T* for all
decay channel except k
.
ꢀpt
respectively for II and (21 ps)ꢀ1 and (48 ps)ꢀ1 for III. The
higher endergonic ESIPT for II may partly explain its weaker
tautomer emission due to a lesser thermal population of the
tautomer species (see Figure 2). Similar rising kinetics following
the pattern of eq 4 can be resolved upon monitoring at the
tautomer emission (see Figure 3 and Table 1). However, due to
the possible overlap with normal emission, both proton transfer
kinetics and the deduction of the equilibrium constants are
mainly based on those resolved from normal emission using eq 3.
Supplementary support of the above experimental results is
provided by the computational approach. The ground-state
potential energies calculated at B3LYP/6-311+G(d,p) level
are shown in Scheme S1, Supporting Information. For all IꢀIII,
the normal form was calculated to be the dominant ground-state
species due to its lower energy than the tautomer form by 12 to
14 kcal/mol. Note that our focus in this study is mainly on the
relative thermodynamics between normal and tautomer excited
states, while the reaction potential energy surface (PES), i.e. the
kinetics, is not pursued. As for the reaction thermodynamics, the
energy level of the tautomer first excited state compared with the
normal form is lower by ꢀ5.20 kcal/mol for I, while it is higher by
9.50 and 3.60 kcal/mol for II and III, respectively. Table 2
summarizes the energetics calculated for the ground and first
singlet excited states of IꢀIII. In yet another approach, we may
simply add the experimentally resolved tautomer emission gap
The existence of excited-state equilibrium is also observed in
solid state. As shown in Figure 2c, III clearly reveals both normal
and tautomer emissions maximized at 435 and 580 nm, respec-
tively, which are blue shifted in comparison to those acquired in
solution (see Figure 2c). The results are consistent with X-ray
crystal analysis, concluding substantial πꢀπ stacking in between
the tetracyclic planes. The spectral blue shift can be ascribed to a
result of the H-aggregate type of interaction. Incidentally, in the
solid state, the dual emission achieves a nearly white light
generation with CIE (0.30, 0.27) and an overall PL Q.Y. of
∼0.1. We then moved one step further by using III to fabricate
the device for OLEDs in the configuration ITO/PEDOTꢀPSS-
(30 nm)/NPB (15 nm)/TCTA (5 nm)/5 wt % III doped CBP
(25 nm)/TPBI (50 nm)/LiF (0.5 nm)/Al (100 nm). In this
device, PEDOTꢀPSS functioned as the hole injection layer; 4,40-
bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB) was the
hole transport layer. To decrease the hole-injection barrier
between NPB and the emitting layer, we incorporated a layer
of 4,40,400-tris(N-carbazolyl)triphenylamine (TCTA) because
of its appropriately matching HOMO energy level. 4,40-bis-
(9-carbazolyl)biphenyl (CBP) is used as host for the III emitter.
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4. CONCLUSION
In conclusion, we report a rational tuning of the energetics
for normal versus tautomer species of a series of new ESIPT
molecules I, II, and III, based on a judicious balance between
destruction of aromaticity and compensation of π-conjugation.
As a result, the existence of excited-state equilibrium leads to
normal and tautomer dual emission spanning the entire visible
region for III. Its transformation to the bulk properties is also
facile, facilitating the device fabrication. OLEDs based on III have
been successfully fabricated, achieving a maximum brightness of
665 cd mꢀ2 at 20 V (885 mA cmꢀ2) with the CIE coordinates of
(0.26, 0.35). Further improvement can be made by introducing a
bulky substituent such as the tert-butyl group to the tetracyclic
moiety of III so that the π-stacking can be further minimized.
Comparing the recent proposal of utilizing excimer and/or
exciplex emission to attain white light emission,19 the dual emis-
sion in the ESIPT system is intrinsic, which is established at the
single molecular level in both solution and solid phase. Standing
on this basis, not only is fine tuning the energetics of excited-state
intramolecular proton transfer of fundamental importance but also
paves a new and feasible avenue en route to white light generation.
Figure 5. (a) Current densityꢀvoltageꢀluminance (JꢀVꢀL) charac-
teristics. (b) EL spectra for devices incorporating III as dopant at
different voltage. Inset: photo of the device in operation.
’ ASSOCIATED CONTENT
S
Supporting Information. Additional NMR, time-resol-
b
ved spectroscopic, crystallographic (CIF), and computational data
along with complete ref 13 are provided. This material is available
free of charge via the Internet at http://pubs.acs.org.
1,3,5-Tris(N-phenylbenzimidizol-2-yl)benzene (TPBI) func-
tioned as both hole-blocking material and ETL. Lithium fluoride
(LiF) was used as the electron-injection layer, and aluminum
(Al) was the cathode. In our preliminary test, the device reveals a
maximum brightness of 665 cd mꢀ2 at 20 V (885 mA cmꢀ2) as
Figure 5a shows. The maximum external quantum efficiency,
current, and power efficiency were 0.11%, 0.20 cd Aꢀ1, and 0.06
lm Wꢀ1, respectively. In Figure 5b, the EL spectrum of III pre-
sents both normal and tautomer emissions at 488 and 590 nm,
respectively, which is close to that acquired in solution (e.g.,
cyclohexane, see Figure 2c). The result can be rationalized by
only 5 wt % of III used as dopant in OLEDs, so that III is prone to
exhibit the π-stacking free behavior.
’ AUTHOR INFORMATION
Corresponding Author
kyuchen@fcu.edu.tw; wenhung@mail.ntou.edu.tw; chop@ntu.
edu.tw
’ ACKNOWLEDGMENT
We thank National Science Council, Taiwan for the financial
support.
Figure 5b also displays the EL spectra of III recorded at
various voltages. The spectra are constant when applied voltage
under 16 V. However, the relative intensity of the orange emis-
sion increases slightly when the voltage is over 16 V, leading to a
slight shift in the CIE coordinates from (0.26, 0.35) to (0.28,
0.40) at 16 and 20 V, respectively. This color shift possibly
originates from the energy transfer from the blue (normal excited
state) to the orange (tautomer excited state) fluorophore at
higher voltage. For this case, one would expect slow reverse pro-
ton transfer from tautomer to the normal form in the ground
state. This issue is pending further resolution.
At the current stage, due to its relatively low overall PL
quantum yield of 0.03 in the doped film of III, the EL
(or OLED) performance is only decent compared with the work
mentioned early utilizing multi-ESIPT moieties. For example,
Park and co-workers7b,c have reported the white light OLED
comprising a molecular framework that covalently links two
ESIPT chromophores. Nevertheless, spectral variation due to
different photostability of two ESIPT moieties may still need to
be concerned. On the other hand, our results clearly provide the
proof of concept that a white light generation is accessible
based on a single ESIPT system, which opens a new route of
white OLED.
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