Room-temperature phosphorescence in solution and in solid state from purely organic dyes

Article abstract of DOI:10.1016/j.dyepig.2012.04.003

Phosphorescence has seldom been observed in purely organic dyes in the solution at room temperature. We herein report efficient phosphorescence in the solution from (E)-3-benzylideneimidazo[1,2-a]pyridin-2(3H)-one and its derivatives. This opens up a new

Full text of DOI:10.1016/j.dyepig.2012.04.003

Dyes and Pigments 95 (2012) 161e167
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Room-temperature phosphorescence in solution and in solid state from purely
organic dyes
*
Guoping Yong , Wenlong She, Yiman Zhang
Department of Chemistry, University of Science and Technology of China, Hefei 230026, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Phosphorescence has seldom been observed in purely organic dyes in the solution at room temperature.
We herein report efficient phosphorescence in the solution from (E)-3-benzylideneimidazo[1,2-a]pyr-
idin-2(3H)-one and its derivatives. This opens up a new class of purely organic dyes to applications in the
phosphor design. In the solid state, the ordered columnar stacking arrangements, intermolecular close
contacts, and lateral intermolecular hydrogen bonding interactions between neighboring columns make
these luminogenic molecules highly phosphorescent at room temperature, and lead to phosphorescent
color changes (from yellow in the solution to aurantiacusered in the solid state). Here, using these
luminogenic molecules, a series of new purely organic phosphors are color-tuned to emit yellow, aur-
antiacus, orange, nacarat and red in the solid state, which are promising candidates for potential
applications in organic light-emitting diodes for display.
Received 20 December 2011
Received in revised form
6 March 2012
Accepted 5 April 2012
Available online 17 April 2012
Keywords:
(E)-3-benzylideneimidazo[1,2-a]pyridin-
2(3H)-one derivatives
Phosphorescent dyes
One-pot synthesis
Ó 2012 Elsevier Ltd. All rights reserved.
Phosphorescent colors
Solid state
Room-temperature phosphorescence
1. Introduction
materials, because the highly bonded nature of electrons in
metal-free organic molecules makes them little free and less
Organic conjugated compounds are nowadays still the focus of
intense investigations in the field of organic electronics, due to their
appealing applications such as electroluminescent devices, sensors,
lasers, and other semiconductor devices. The development of effi-
cient solid luminescent materials, however, is still a challenging
issue, because many dyes are highly emissive in dilute solutions but
become weakly luminescent or even nonemissive in the condensed
phase, attributed to the formation of detrimental excimers and
exciplexes in the condensed phase [1]. Such an aggregation-caused
quenching (ACQ) problem must be overcome, because luminogenic
molecules are commonly used as the condensed states. Although
various chemical, physical, and engineering approaches have been
used to solve the ACQ effect [2], the attempts have met with only
limited success. Fortunately, some purely organic molecules that
can display efficient aggregation-induced emissions in the solid
states have been reported [3].
impetus to emit from triplet states. Phosphorescence is therefore
generally regarded as a solely inorganic or organometallic property.
The phosphorescent emission of purely organic materials is so
weak and ineffective that they fail to enter discussions on modern
phosphorescent applications [5]. However, the development of
phosphorescent dyes is a goal of intensive research because of their
high application potentials in organic electronics, as well as
chemical and biological detection [6]. Although most purely
organic materials have been widely considered to be non-
phosphorescent in character, very recently, crystallization-
induced phosphorescence of the purely organic materials has
been reported [7], probably attributable to aromatic carbonyls, the
heavy halogen atom effect, or halogen bonding.
On the basis of 2,30-biimidazo[1,2-a]pyridin-20-one (Hbipoꢀꢁ
)
radical and itsderivatives, our group recently reported a series ofpurely
organic luminescent materials which also display unique magnetic
properties [8], and coordination polymers which exhibit interesting
magnetic properties and various luminescent colors, including direct
white-light phosphorescent emission [9]. Very recently, we discovered
two purely organic solid phosphorescent materials based on 2-(imi-
dazo[1,2-a]pyridin-2-yl)-2-oxoacetic acid radical, which exhibit exci-
tation wavelength independent of direct white-light and blue-light
To date, purely organic luminogenic molecules that are also
phosphorescent are very rare [4], in contrast to metal-containing
* Corresponding author. Tel.: þ86 551 360 1584; fax: þ86 551 360 1592.
E-mail address: gpyong@ustc.edu.cn (G. Yong).
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2012.04.003

162
G. Yong et al. / Dyes and Pigments 95 (2012) 161e167
phosphorescent emission, respectively [10]. The phosphorescence can
derive from carbonyl group which exhibit somewhat spineorbit
coupling at the carbonyl oxygen that allows for intrinsic triplet
generation through intersystem crossing [7b,11].
2.2. Synthesis
2.2.1. (E)-3-benzylideneimidazo[1,2-a]pyridin-2(3H)-one (1)
A mixture of hydrochloride salt of imidazo[1,2-a]pyridin-2(3H)-
one (1.72 g, 10 mmol) and benzaldehyde (1.06 g, 10 mmol) in acetic
acid (50 mL) was heated at 125 ꢂC for 2.5 h. After cooled to room
temperature, the resulting orange solid was collected by filtration
and washed with ethanol. The orange solid was immersed in
saturated NaHCO3 solution until no bubble was observed. The solid
then was filtered out and washed with water and ethanol; recrys-
tallization of orange solid from DMF solution afforded orange
crystals which are suitable for X-ray single-crystal diffraction
analysis. Yield: 1.59 g (7.16 mmol), 71.6%. 1H NMR (400 MHz,
DMSO-d6, 25 ꢂC, ppm): 8.67 (d, J ¼ 6.8 Hz, 1H), 8.48 (dd, J ¼ 6.6 Hz,
3.0 Hz, 2H), 7.94 (s, 1H), 7.74 (ddd, J ¼ 8.6 Hz, 7.0 Hz, 1.3 Hz, 1H), 7.51
(dd, J ¼ 5.1 Hz, 1.9 Hz, 3H), 7.06 (d, J ¼ 8.9 Hz, 1H), 6.86 (td,
In an effort to explore the novel purely organic phosphorescent
materials based on imidazo[1,2-a]pyridin-2-ones, we designed and
synthesized a new class of organic conjugated compounds ((E)-3-
benzylideneimidazo[1,2-a]pyridin-2(3H)-one derivatives, Fig. 1) to
examine their room-temperature phosphorescent characters in the
solution and solid state. To the best of our knowledge, this is the
first report on the evaluation of the phosphorescent properties in
the solution and solid state of (E)-3-benzylideneimidazo[1,2-a]
pyridin-2(3H)-one derivatives. We are aware of only two works
reporting the synthesis of few (E)-3-benzylidene- imidazo[1,2-a]
pyridin-2(3H)-ones [12] and no photophysical characterization for
these compounds was given.
13
J ¼ 6.9 Hz, 1.1 Hz, 1H). C NMR (101 MHz, DMSO-d6, 25 ꢂC, ppm):
172.82, 162.72, 141.56, 132.02, 131.87, 131.32, 129.80, 128.32, 127.86,
114.69, 111.34. HRMS (EI, m/z, [M]þ): Calc. for C14H10N2O: 222.24,
Found: 222.22.
2. Experimental
2.1. Materials and methods
2.2.2. Compounds 2e7
Under the same reaction conditions as 1, the reactions by the
replacement of benzaldehyde with different benzaldehyde deriv-
atives afforded a series of (E)-3-benzylideneimidazo[1,2-a]pyridin-
2(3H)-one derivatives.
All reagents were of analytical grades, and obtained from
commercial vendors and used without further purification. 1H and
13C NMR spectra were obtained on Bruker Avance 400 MHz NMR
spectrometers using DMSO-d6 as solvent and tetramethylsilane
(TMS) as the internal standard. High-resolution mass spectra
(HRMS) were obtained on Micromass GCT-MS instrument oper-
ating in electron impact (EI) mode and time of flight (TOF) mass
detector. UV/vis absorption spectra were measured using UV-3100
spectrophotometer in DMF and DMSO solutions at 298 K. The
2.2.2.1. (E)-3-(30-nitrobenzylidene)imidazo[1,2-a]pyridin-2(3H)-one
(2). Nacarat crystals, yield: 2.05 g (7.68 mmol), 76.8%. 1H NMR (400
MHz, DMSO-d6, 25 ꢂC, ppm): 9.48 (s, 1H), 8.76 (d, J ¼ 7.8 Hz, 1H),
8.63 (d, J ¼ 6.8 Hz, 1H), 8.34 (d, J ¼ 7.8 Hz, 1H), 8.09 (s, 1H),
7.84e7.75 (m, 2H), 7.09 (d, J ¼ 8.8 Hz, 1H), 6.89 (t, J ¼ 6.8 Hz, 1H). 13
C
solution (10 mM DMF) and solid photoluminescence (PL) spectra,
and the decay lifetimes were determined at room temperature on
a Fluorolog-3-TAU fluorescence spectrophotometer. The CIE-1931
NMR (101 MHz, DMSO-d6, 25 ꢂC, ppm): 172.85, 163.53, 149.39,
147.70, 142.44, 137.82, 133.32, 131.20, 129.99, 129.90, 125.96, 125.30,
125.14, 114.96, 111.72. HRMS (EI, m/z, [M]þ): Calc. for C14H9N3O3:
267.24, Found: 267.21.
chromaticity coordinates were calculated using
Coordinate.exe program. The same results can also be obtained
using gocie.exe program from http://www.geocities.com/
krjustin/gocie.html. The quantum yields of solution state were
measured at room temperature in a 10 M DMF solution using
a
Color-
a
2.2.2.2. (E)-3-(40-methoxybenzylidene)imidazo[1,2-a]pyridin-2(3H)-
one (3). Aurantiacus crystals, yield: 1.91 g (7.58 mmol), 75.8%. 1H
NMR (400 MHz, DMSO-d6, 25 ꢂC, ppm): 8.69e8.53 (m, 3H), 7.89 (s,
1H), 7.74e7.64 (m,1H), 7.11e6.99 (m, 3H), 6.83 (td, J ¼ 6.9 Hz,1.1 Hz,
1H), 3.86 (s, 3H). 13C NMR (101 MHz, DMSO-d6, 25 ꢂC, ppm): 173.04,
162.00, 161.81, 140.62, 134.64, 129.38, 128.00, 127.40, 124.83, 114.52,
113.96, 111.15, 55.45. HRMS (EI, m/z, [M]þ): Calc. for C15H12N2O2:
252.27, Found: 252.28.
m
quinine sulfate (V ¼ 0.54) as a reference. Corrections were made
due to the change in solvent refractive indices [13]. The quantum
yields of solid state were measured on a Fluorolog-3-TAU fluores-
cence spectrophotometer equipped with a BaSO4-coated inte-
grating sphere, upon excitation at 365 nm.
2.2.2.3. (E)-3-(40-(dimethylamino)benzylidene)imidazo[1,2-a]pyr-
idin-2(3H)-one (4). Bright orange red crystals, yield: 2.12 g (8.0
1
mmol), 80%. H NMR (400 MHz, DMSO-d6, 25 ꢂC, ppm): 8.62 (d,
J ¼ 6.8 Hz, 1H), 8.60e8.52 (m, 2H), 7.79 (s, 1H), 7.65e7.58 (m, 1H),
7.02 (d, J ¼ 8.9 Hz, 1H), 6.84e6.75 (m, 3H), 3.07 (s, 6H). 13C NMR
(101 MHz, DMSO-d6, 25 ꢂC, ppm): 173.10, 160.10, 152.49, 138.87,
135.00, 129.13, 128.68, 124.47, 119.79, 114.15, 111.15, 110.76, 40.31,
40.20. HRMS (EI, m/z, [M]þ): Calc. for C16H15N3O: 265.31, Found:
265.31.
2.2.2.4. (E)-3-(40-(1H-imidazol-1-yl)benzylidene)imidazo[1,2-a]pyr-
idin-2(3H)-one (5). Orange red crystals, yield: 1.26 g (4.34 mmol),
43.4%. 1H NMR (400 MHz, DMSO-d6, 25 ꢂC, ppm): 8.65 (t, J ¼ 8.0 Hz,
3H), 8.43 (s, 1H), 7.96 (s, 1H), 7.89 (t, J ¼ 1.3 Hz, 1H), 7.86e7.82 (m,
2H), 7.77e7.69 (m, 1H), 7.16 (s, 1H), 7.07 (d, J ¼ 8.9 Hz, 1H), 6.86 (td,
13
J ¼ 6.9 Hz, 1.1 Hz, 1H). C NMR (101 MHz, DMSO-d6, 25 ꢂC, ppm):
172.96, 162.71, 141.55, 138.52, 135.58, 133.82, 130.29, 130.22, 129.70,
129.46, 126.68, 119.42, 117.62, 114.75, 111.41. HRMS (EI, m/z, [M]þ):
Calc. for C17H12N4O: 288.30, Found: 288.32.
Fig. 1. (E)-3-benzylideneimidazo[1,2-a]pyridin-2(3H)-one derivatives.

G. Yong et al. / Dyes and Pigments 95 (2012) 161e167
163
2.2.2.5. (E)-3-(40-(1H-pyrazol-1-yl)benzylidene)imidazo[1,2-a]pyr-
idin-2(3H)-one (6). Aurantiacus crystals, yield: 1.65 g (5.73 mmol),
57.3%. 1H NMR (400 MHz, DMSO-d6, 25 ꢂC, ppm): 8.66 (dd,
J ¼ 11.0 Hz, 5.7 Hz, 4H), 8.05e7.94 (m, 3H), 7.83 (d, J ¼ 1.5 Hz, 1H),
7.73 (ddd, J ¼ 8.4 Hz, 7.0 Hz, 1.2 Hz, 1H), 7.07 (d, J ¼ 8.9 Hz, 1H), 6.86
(td, J ¼ 6.9 Hz, 1.0 Hz, 1H), 6.65e6.57 (m, 1H). 13C NMR (101 MHz,
DMSO-d6, 25 ꢂC, ppm): 172.95,162.54, 141.82,141.40, 141.23,133.73,
129.68, 129.66, 129.17, 128.08, 126.98, 117.68, 114.71, 111.38, 108.58.
HRMS (EI, m/z, [M]þ): Calc. for C17H12N4O: 288.30, Found: 288.29.
2.2.2.6. (E)-3-(40-(1H-1,2,4-triazol-1-yl)benzylidene)imidazo[1,2-a]
pyridin-2(3H)-one (7). Orange red crystals, yield: 1.86 g
(6.43 mmol), 64.3%. 1H NMR (400 MHz, DMSO-d6, 25 ꢂC, ppm): 9.43
(s,1H), 8.71e8.63 (m, 3H), 8.30 (s,1H), 8.05e7.98 (m, 3H), 7.78e7.71
Fig. 2. ORTEP diagram of molecular structure of 1 with ellipsoids drawn at 50%
probability.
(m, 1H), 7.07 (d, J ¼ 8.9 Hz, 1H), 6.87 (td, J ¼ 6.9 Hz, 1.1 Hz, 1H). 13
C
NMR (101 MHz, DMSO-d6, 25 ꢂC, ppm): 172.74, 162.78, 154.11,
142.75, 139.40, 133.62, 131.48, 129.57, 128.65, 126.06, 121.79, 118.80,
114.79, 111.46. HRMS (EI, m/z, [M]þ): Calc. for C16H11N5O: 289.29,
Found: 289.28.
Compound 1 forms one-dimensional (1D) columnar stacking
chain along the b-axis, in which the molecules appear to be
a slightly slipped stack. In 1D chain, (E)-3-benzylideneimidazo[1,2-
a]pyridin-2(3H)-one molecules adopt slipped head-to-tail stacking
modes, as shown in Fig. 3. Although the stacking distance is outside
van der Waals contact, there exist four kinds of intermolecular close
contacts (C1/C70 of 3.336 Å, C6/C100 of 3.369 Å, C7/C90 of
3.391 Å, and C8/C80 of 3.257 Å, shorter than van der Waals)
between neighboring molecules. The columnar chains are further
assembled into two-dimensional (2D) supramolecular network via
lateral intermolecular C14eH/N10 hydrogen bonds (3.155 Å)
between neighboring columns, as shown in Fig. S1 (in the Sup-
porting information).
2.3. Single-crystal X-ray crystallography
The X-ray diffraction measurement for 1 was performed on
a Gemini S Ultra CCD diffractometer (Oxford diffraction Ltd.) using
graphite monochromated Cu-K
a
radiation (
l
¼ 1.54184 Å). The
structure was solved by direct method and refined with full-matrix
least-squares technique on F2 by using the SHELXS-97 and SHELXL-
97 programs [14]. The empirical absorption correction (SCALE3
ABSPACK) was applied. All non-hydrogen atoms were refined with
anisotropic displacement parameters. The hydrogen atoms were
placed in geometrically calculated positions. Weighted R factor (Rw)
and all goodness of fit S are based on F2, conventional R factor (R) is
based on F.
3.2. UV/vis absorption spectra of compounds 1e7
The UV/vis absorption spectra of all seven compounds were
measured at room temperature in DMF solution (Fig. 4). Even if
measured in DMSO solution (Fig. S2, in the Supporting Informa-
tion), these compounds exhibit similar UV/vis absorption spectra in
DMF solution. Except for compound 4, other six compounds display
intense S0 / S2 bands at 326e368 nm that should be attributed to
3. Results and discussion
3.1. Syntheses and crystal structure description
p
/
p* type transitions of carbonyl groups [7b], and weak broad
bands centered at about 458 nm which are attributed to n /
p
*
As described in Scheme 1, the reaction between hydrochloride
salt of imidazo[1,2-a]pyridin-2(3H)-one and benzaldehyde deriva-
tives afforded a series of (E)-3-benzylideneimidazo[1,2-a]pyridin-
2(3H)-one derivatives by one-pot synthesis method, and the orange
single crystals, (E)-3-benzylideneimidazo[1,2-a] pyridin-2(3H)-one
(1) were obtained. Compound 1 crystallizes in triclinic space group
P-1. The molecular structure of 1 is shown in Fig. 2. Interestingly,
compound 1 exhibits the trans-configuration. According to the
mechanism of the Pschorr cyclization, the condensation of imidazo
[1,2-a]pyridin-2(3H)-one with benzaldehyde should have the cis-
configuration of the phenyl and pyridyl groups, however, the
product was the trans-compound. Abramovitch and Hey have
reported that (E)-3-(20-nitrobenzylidene)imidazo[1,2-a]pyridin-
2(3H)-one was the trans-configuration [12b]. They think that it
seems likely that there is dipolar repulsion between the pyridine
nitrogen atom and the electronegative nitro group, leading to the
observed stereochemistry.
type transitions, and even extend to the visible region. These
similar absorption spectra are owing to their similar molecular
structures. Compared to 1 that shows intense bands at 335 nm,
compound 2 with electron-withdrawing substituent on the phenyl
ring shows the blue-shifted absorption band (326 nm), conversely,
compounds 3, 5, 6 and 7 with electron-donating substituents on
the phenyl ring exhibit the red-shifted absorption bands in the
range of 353 nm and 368 nm. Interestingly, compound 4 with
strong electron-donating substituent (eN(CH3)2) on the phenyl
ring even leads to all absorption bands (440, 478 and 503 nm) in
the visible region. The blue-shifted absorption band of compound 2
may be ascribed to its lower HOMO energy level owing to the
electron-withdrawing effect of nitro group, while the red-shifted
absorption bands of compounds 3, 5, 6 and 7 can be attributed to
higher HOMO energy level owing to the electron-donating effect of
donating electron groups. It is obvious that compared to 3, 5 and 6,
compound 7 with weak electron-donating triazolyl group leads to
fewer red-shift, however, 4 with strong electron-donating N,N’-
dimethylamine group, that effectively elevates HOMO energy level
of phenyl ring, occurs obvious red-shift (Fig. 4).
3.3. Phosphorescence of compounds 1e7 in the solution
Fig. 5 shows the photoluminescence (PL) spectra of compounds
1e7 in the dilute (10
mM) DMF solution at room temperature, after
Scheme
1. Synthesis
of
(E)-3-benzylideneimidazo[1,2-a]pyridin-2(3H)-one
derivatives.
omitting the contributions from the DMF solvent. The signals of

164
G. Yong et al. / Dyes and Pigments 95 (2012) 161e167
Fig. 3. The columnar stacking chain in 1.
shorter than 500 nm may be attributed to contributions from the
DMF solvent (Fig. S3, in the Supporting Information). Indeed, in the
3.4. Phosphorescence of compounds 1e4 in the solid state
dilute (10
m
M) DMSO solution (Fig. S4, in the Supporting Informa-
Because many highly emissive dyes can become weak lumi-
nescent or even nonemissive in the solid state, to examine whether
1e7 were phosphorescent in the solid state, we investigated their
PL spectra decay behaviors. As shown in Fig. 7, the decay lifetime of
tion), these compounds exhibit similar PL spectra to in DMF solu-
tion. The PL spectra of compounds 1e7 in the DMF solution are
similar; they all exhibit broad emission peaks centered at about
546e578 nm, upon excitation at 365 nm. The similar emission
peaks are relative to their similar molecular structures, however,
the slight differences of maximal emission peaks among all seven
compounds should be ascribed to electronic and steric effect of
different substituents on the phenyl ring. Surprisingly, the decay
lifetime of compounds 1e7 in the dilute solution was determined to
1e7 in the solid state was determined to be
s
¼ 29.0, 26.6, 26.5,
24.3, 22.9, 23.7 and 23.7 s, respectively. The microsecond-order
m
decay lifetimes confirm their phosphorescent nature in the solid
state at room temperature. Different from solution state, 1e7
exhibit longer decay lifetimes in the solid state, probably attribut-
able to stacking effects.
be
s
¼ 16.8e18.5
m
s (Fig. 6), upon excitation at 365 nm, indicative of
In the solution, upon excitation at 365 nm, compounds 1e4
exhibit similar PL spectra (Fig. 5) and emit similar yellow light as
indicated by the CIE chromaticity diagrams (Fig. S5, in the Sup-
porting Information). In the solid state, the room-temperature PL
spectra of 1 and 3 display maximal emission peaks at about 570 nm
the phosphorescent nature which might be assigned to the
carbonyl effects in molecules, because carbonyl group can exhibit
somewhat spineorbit coupling at the carbonyl oxygen that allows
for intrinsic triplet generation through intersystem crossing [7b,11].
Fig. 4. UV/vis absorption spectra of 1e7 in 10
mM DMF solution at 298 K.
Fig. 5. PL spectra of compounds 1e7 (>500 nm) in the dilute DMF solution.

G. Yong et al. / Dyes and Pigments 95 (2012) 161e167
165
Fig. 8. Room-temperature solid PL spectra of 1e4 upon excitation at 365 nm.
3.5. Phosphorescence of compounds 5, 6 and 7 in the solid state
Fig. 6. The decay lifetime curves of compounds 1e7 in the dilute DMF solution at room
temperature. The lifetime ( ) is defined as the time in which the emission intensity
s
The room-temperature solid PL spectra of compounds 5, 6 and 7
are shown in Fig. 10, which display maximal emission peaks at
about 589, 579 and 596 nm, respectively, upon excitation at
365 nm. Regardless of their similar PL spectra (Fig. 5), and similar
yellow phosphorescence (excitation at 365 nm) as indicated by
their CIE chromaticity diagrams (Fig. S9, in the Supporting Infor-
mation) in the solution, in the solid state, 5 and 7 exhibit similar
aurantiacus phosphorescence (excitation at 365 nm) as indicated
by the CIE chromaticity diagrams (Fig. S10, in the Supporting
Information), which can also be attributed to the stacking-induced
phosphorescent color changes, but, 6 still emits yellow phospho-
rescence (Figs. S10 and S11, in the Supporting Information). Inter-
estingly, the solid phosphorescent colors (orange) for 5 and 7 taken
under ambient condition and 365-nm UV light illumination are
almost consistent, indicating that the phosphorescence can be
excited in the visible region, such as 550 nm excitation light (Fig. 9).
Seven phosphorescent dyes based on (E)-3-benzylideneimidazo
[1,2-a] pyridin-2(3H)-one derivatives display similar photophysical
properties (Figs. 4 and 5), and similar yellow phosphorescence
(excitation at 365 nm) in the solution at room temperature. The
slight differences in the photophysical properties among all seven
compounds should be ascribed to electronic and steric effect of
different substituents on the phenyl ring. To the best of our
knowledge, the purely organic dyes that are visible phosphorescent
in the solution at room temperature has not been reported.
decays to 1/e of the initial intensity (Io), where e is the natural log constant and is equal
s
)
to 2.718. (I ¼ Ioeꢀ(t/
ꢃ s ¼ t ꢃ I ¼ (1/e)Io) [15].
(Fig. 8), upon excitation at 365 nm, implying they emit yellow solid
phosphorescence, as indicated by their CIE chromaticity diagrams
(Fig. S6, in the Supporting Information). However, the room-
temperature solid PL spectra of 2 and 4 exhibit maximal emission
peaks at 598 and 615 nm, respectively (Fig. 8), upon excitation at
365 nm, indicating that 2 and 4 emit orange and red solid phos-
phorescence, respectively, as indicated by the CIE chromaticity
diagrams (Fig. S6, in the Supporting Information).
It should be noted that 2 and 4 reveal stacking-induced phos-
phorescent color changes in the solid state, regardless of their
similar phosphorescent colors in the solution. The red and bright
orange red phosphorescence for 2 and 4 can even be seen with the
naked eye when solid is illuminated with a 365-nm UV light at
room temperature (Figs. S7 and S8, in the Supporting Information).
Indeed, the yellow solid phosphorescence for 1 and 3 can also be
seen with the naked eye when solid is illuminated with a 365-nm
UV light at room temperature (Figs. S7 and S8, in the Supporting
Information). Interestingly, the bright orange red phosphorescence
for 4 even can be observed under laboratory lighting condition,
indicating that the bright orange red phosphorescence of 4 can be
excited in the visible region, such as 560 nm excitation light (Fig. 9).
It is noticeable that compounds 2, 4, 5 and 7 exhibit the stacking-
induced phosphorescent color changes (from yellow phosphores-
cence in the solution to aurantiacusered phosphorescence in the
Fig. 7. The decay lifetime curves of 1, 2 and 3e7 (inset) in the solid state at room
temperature.
Fig. 9. Solid excitation spectra of compounds 1e7 at room temperature.

166
G. Yong et al. / Dyes and Pigments 95 (2012) 161e167
contacts, and lateral intermolecular hydrogen bonding interactions
between neighboring columns make these luminogenic molecules
highly phosphorescent in the solid state at room temperature, and
lead to phosphorescent color changes (from yellow phosphores-
cence in the solution to aurantiacus to red phosphorescence in the
solid state). Interestingly, some dyes show phosphorescent colors
taken under ambient conditions closely resemble those measured at
365 nm UV light illumination, indicating that the phosphorescence
of these compounds can even be excited in the visible region. The
phosphorescence nature of these compounds is further supported
by the long decay lifetimes.
Acknowledgments
Fig. 10. Room-temperature solid PL spectra of 5, 6 and 7 upon excitation at 365 nm.
We appreciate the financial support from the National Nature
Science Foundation of China (21172210 and 20872135).
solid state, upon excitation at 365 nm) at room temperature.
Because 2, 4, 5 and 7 contain the hydrogen bonding acceptor group
(eNO2, eN(CH3)2, and N atom in imidazole and triazole ring,
respectively), these hydrogen bonding acceptor groups canprobably
increase lateral intermolecular hydrogen bonding interactions
between neighboring columns (similar to Fig. S1, in the Supporting
Information), which are helpful to charge transferbetween columns,
further leading to lower energy emissions, as a result, these
compounds emit aurantiacus to red phosphorescence in the solid
state. Interestingly, these four dyes show similar phosphorescent
colors taken under ambient condition and 365 nm UV light illumi-
nation, indicating that the phosphorescence can even be excited in
the visible region, such as about 555 nm excitation light (Fig. 9). The
similar solid-state phosphorescent colors for 2, 4, 5 and 7 in the
visible light and ultraviolet light (365 nm) are very interesting in
purelyorganic phosphorescent dyes. On the other hand, compounds
1, 3 and 6 do not display the stacking-induced phosphorescent color
changes, which still emit the yellow phosphorescence (excitation at
365 nm) in the solid state at room temperature, similar to that in the
solution. However, under ambient condition, they show aurantiacus
to orange phosphorescence. More strikingly, seven phosphorescent
dyes possess longer solid-state decay lifetimes than that in the
solution (Figs. 6 and 7), implying that stacking not only induces
phosphorescent enhancement effect, but leads to longer decay
lifetime as well. The stacking-induced phosphorescent enhance-
ment can be demonstrated by obvious difference in quantum yields
between solution state and solid state: in the DMF solution, the
quantum yield of 1e7 is 0.016, 0.016, 0.017, 0.0021, 0.0018, 0.017 and
0.018%, respectively; in the solid state, the quantum yield of 1e7 is
1.3, 1.2, 1.7, 1.0, 1.1, 1.5 and 1.2%, respectively.
Appendix A. Supplementary material
CCDC 853989 contains the supplementary crystallographic data
for 1. The data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif. Supplementary data associated with this article can
be found, in the online version, at doi:10.1016/j.dyepig.2012.04.003.
References
[1] (a) Birks JB. Photophysics of aromatic molecules. London, UK: Wiley; 1970;
(b) Malkin J. Photophysical and photochemical properties of aromatic
compounds. Boca Raton, FL: CRC; 1992.
[2] (a) Jakubiak R, Bao Z, Rothberg R. Dendritic side groups as three-dimensional
barriers to aggregation quenching of conjugated polymer fluorescence. Synth
Met 2000;114:61e4;
(b) Chen L, Xu S, McBranch D, Whitten D. Tuning the properties of conjugated
polyelectrolytes through surfactant complexation. J Am Chem Soc 2000;122:
9302e3;
(c) Hecht S, Frechet JMJ. Dendritic encapsulation of function: applying nature’s
site isolation principle from biomimetics to materials science. Angew Chem
Int Ed 2001;40:74e91;
(d) Setayesh S, Grimsdale AC, Weil T, Enkelmann V, Müllen K, Meghdadi F,
et al. Polyfluorenes with polyphenylene dendron side chains: toward non-
aggregating, light-emitting polymers. J Am Chem Soc 2001;123:946e53;
(e) Gaylord BS, Wang S, Heeger AJ, Bazan GC. Water-soluble conjugated
oligomers: effect of chain length and aggregation on photoluminescence-
quenching efficiencies. J Am Chem Soc 2001;123:6417e8.
[3] (a) Luo JD, Xie ZL, Lam JWY, Cheng L, Chen HY, Qiu CF, et al. Aggregation-
induced emission of 1-methyl-1,2,3,4,5-pentaphenylsilole. Chem Commun;
2001:1740e1;
(b) An BK, Kwon SK, Jung SD, Park SY. Enhanced emission and its switching in
fluorescent organic nanoparticles. J Am Chem Soc 2002;124:14410e5;
(c) Chen J, Law CCW, Lam LWY, Dong YQ, Lo SMF, Williams ID, et al. Synthesis,
light emission, nanoaggregation, and restricted intramolecular rotation of 1,1-
substituted 2,3,4,5-tetraphenylsiloles. Chem Mater 2003;15:1535e46;
(d) Li Z, Dong YQ, Lam JWY, Sun JX, Qin AJ, Haëussler M, et al. Functionalized
siloles: versatile synthesis, aggregation-induced emission, and sensory and
device applications. Adv Funct Mater 2009;19:905e17;
It should be noted that such a new class of purely organic
phosphorescent dyes are promising candidates for potential
applications including organic electronics such as organic light-
emitting diodes for display or ambient lighting [16]. Benefits for
purely organic materials (compared to organometallics) may exist
in terms of costs and tunable phosphorescent colors (achieve
yellow phosphorescence and low-energy red phosphorescence).
(e) Hong YN, Lam JWY, Tang BZ. Aggregation-induced emission: phenomenon,
mechanism and applications. Chem Commun; 2009:4332e53.
[4] Baldo MA, Thompson ME, Forrest SR. Phosphorescent materials for application
to organic light emitting devices. Pure Appl Chem 1999;71:2095e106.
[5] Kohler A, Wilson JS, Friend RH. Fluorescence and phosphorescence in organic
materials. Adv Mater 2002;14:701e7.
[6] (a) De Silva AP, Gunaratne HQN, Gunnlaugsson T, Huxley AJM, McCoy CP,
Rademacher JT, et al. Signaling recognition events with fluorescent sensors
and switches. Chem Rev 1997;97:1515e66;
4. Conclusion
(b) Baldo MA, O’Brien DF, You Y, Shoustikov A, Sibley S, Thompson ME, et al.
Highly efficient phosphorescent emission from organic electroluminescent
devices. Nature 1998;395:151e4;
(c) Rumsey WL, Vanderkooi JM, Wilson DF. Imaging of phosphorescence:
a novel method for measuring oxygen distribution in perfused tissue. Science
1988;241:1649e51;
(d) Shao Y, Yang Y. Efficient organic heterojunction photovoltaic cells based on
triplet materials. Adv Mater 2005;17:2841e4;
(e) Zhang G, Palmer GM, Dewhirts MW, Fraser CL. A dual-emissive-materials
design concept enables tumour hypoxia imaging. Nat Mater 2009;8:747e51.
In conclusion, in this work, for the first time, we discovered
a novelclassof purelyorganicphosphorescent dyes in the solution at
room temperature. This opens up the broad class of purely organic
compounds to new applications in phosphor design. A carbonyl
group has been found to be key structural feature to activate phos-
phorescent emission from pure organic dyes in the solution. The
ordered columnar stacking arrangements, intermolecular close

G. Yong et al. / Dyes and Pigments 95 (2012) 161e167
167
[7] (a) Yuan WZ, Shen XY, Zhao H, Lam JWY, Tang L, Lu P, et al. Crystallization-
induced phosphorescence of pure organic luminogens at room temperature.
J Phys Chem C 2010;114:6090e9;
of Cd(II)- and Fe(II)-anion radical frameworks: various networks or structures
influenced by metal ion sizes or in situ forming mechanisms of anion radical
ligand. Cryst Eng Comm 2012;14:1439e48.
(b) Bolton O, Lee K, Kim HJ, Lin KY, Kim J. Activating efficient phosphorescence
from purely organic materials by crystal design. Nat Chem 2011;3:205e10.
[8] (a) Yong GP, Li CF, Li YZ, Luo SW. New zwitterionic radical salts: dimers in
solution and unusual magnetic and luminescent properties in the solid state.
Chem Commun 2010;46:3194e6;
[10] Yong GP, Zhang YM, She WL, Li YZ. Stacking-induced white-light and blue-
light phosphorescence from purely organic radical materials. J Mater Chem
2011;21:18520e2.
[11] (a) Kearns DR, Case WA. Investigation of singlet/triplet transitions by the
phosphorescence excitation method. III. Aromatic ketones and aldehydes.
J Am Chem Soc 1966;88:5087e97;
(b) Itoh T. The evidence showing that the intersystem crossing yield of
benzaldehyde vapour is unity. Chem Phys Lett 1988;151:166e8.
[12] (a) Reindel F, Von Putzer-Reybegg A. Über die umsetzungs-produkte von
pyrimidazolon-2 mit aromatischen aldehyden und ringförmigen 1,2-
dicarbonylverbindungen. Ber Dtsch Chem Ges 1926;59:2926e32;
(b) Abramovitch RA, Hey DH. The condensation of 2,3-dihydro-2-oxoimidazo
[1,2-a]pyridine with o-nitrobenzaldehyde. J Chem Soc C; 1966:1095e6.
[13] Li CF, Yong GP, Li YZ. Phosphorescent iridium (III) 2-phenylpyridine
complexes: efficient color tuning by novel ancillary ligands. Inorg Chem
Commun 2010;13:179e82.
(b) Li YZ, Yong GP, Zhang YM, Li CF, She WL. 3-Carbaldehyde-substituted 2,30-
biimidazo[1,2-a]pyridin-20-one radicals: Interesting
magnetic properties. Synth Met 2011;161:713e7;
p-stacking structures and
(c) Zhang YM, Yong GP, Li CF, She WL, Li YZ. The zwitterionic radical and its
neutral radical derivative with interesting magnetic properties. Synth Met
2012;161:2708e13.
[9] (a) Yong GP, Qiao S, Wang ZY. A one-dimensional coordination polymer based
on novel radical anion ligand generated in situ: notable magnetic and lumi-
nescence properties. Cryst Growth Des 2008;8:1465e7;
(b) Yong GP, Li YZ, Li CF, Zhang YM, She WL. New metaleanion radical
framework materials: CoII compounds showing ferromagnetic to antiferro-
magnetic phase transition at about 344 K, and ZnII compounds exhibiting
terminal anion ligand induced direct white-light-emission. Dalton Trans
2011;40:4131e9;
(c) Yong GP, Li YZ, She WL, Zhang YM. Isostructural metaleanion radical
coordination polymers with tunable phosphorescent colors (deep blue, blue,
yellow, and white) induced by terminal anions and metal cations. Chem Eur J
2011;17:12495e501;
[14] Sheldrick GM. SHELXL-97 and SHELXS-97. Göttingen, Germany: University of
Göttingen; 1998.
[15] Stylianou KC, Heck R, Chong SY, Bacsa J, Jones JTA, Khimyak YZ, et al. A guest-
responsive fluorescent 3D microporous metalꢀorganic framework derived
from a long-lifetime pyrene core. J Am Chem Soc 2010;132:4119e30.
[16] Kim SH, Park S, Kwon JE, Park SY. Organic light-emitting diodes with a white-
emitting molecule: emission mechanism and device characteristics. Adv Funct
Mater 2011;21:644e51.
(d) Yong GP, Li YZ, Zhang YM, She WL. Magnetic and luminescent properties