Rational design of slightly twisted coumarin molecules with remarkable solution and solid dual efficient luminescence

Article abstract of DOI:10.1016/j.dyepig.2017.09.060

Endowing slightly twisted molecules highly emissive in both solution and solid state is of great importance for understanding the principle of maximizing the luminescent efficiency of luminophores. To this end, a series of slightly twisted coumarin luminophores CMs with different alkoxyl substituents at the 7-positions were synthesized. The effect of the substitutes on the diversity photophysical properties of the four compounds in solution, THF/H₂O mixtures and solid state were investigated. Comparing to the referenced compound CM (3-p-tolyl-2H-chromen-2-one) that without a substitute, the introduced electron-rich alkoxyl substitutes not only enhanced the intramolecular charge transfer (ICT) effect, but also significantly modified their molecular packing patterns in the crystals. The combined effect of increasing the radiative and suppressing the nonradiactive pathways boosted the luminescence efficiency of CM1-CM3 in solution and the solid state simultaneously. Eventually, compound CM2 with an ethoxyl substitute exhibited the strongest blue emission with fluorescence quantum yields as high as 73.2% and 96.7% in solution and the solid state, respectively. This work presents an efficient strategy towards dual strong fluorescent luminophores in both solution and the solid state.

Full text of DOI:10.1016/j.dyepig.2017.09.060

Accepted Manuscript
Rational design of slightly twisted coumarin molecules with remarkable solution and
solid dual efficient luminescence
Yue Sun, Tong Wu, Fang Zhang, Rong Zhang, Min Wu, Yuezhen Wu, Xiaozhong
Liang, Kunpeng Guo, Jie Li
PII:
S0143-7208(17)31625-X
10.1016/j.dyepig.2017.09.060
DYPI 6286
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 28 July 2017
Revised Date: 18 September 2017
Accepted Date: 25 September 2017
Please cite this article as: Sun Y, Wu T, Zhang F, Zhang R, Wu M, Wu Y, Liang X, Guo K, Li J,
Rational design of slightly twisted coumarin molecules with remarkable solution and solid dual efficient
luminescence, Dyes and Pigments (2017), doi: 10.1016/j.dyepig.2017.09.060.
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Graphical Abstract
Dual efficient slightly twisted coumarin derivatives in solution and solid were realized
by modulating intramoleculat charge transfer (ICT) and molecular pattern in crystal.

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Rational design of slightly twisted coumarin molecules with
remarkable solution and solid dual efficient luminescence
Yue Sun,b Tong Wu,a, b Fang Zhang,a Rong Zhang,a Min Wu,a Yuezhen Wu,a
Xiaozhong Liang,a Kunpeng Guo,a* Jie Lia*
a Ministry of Education Key Laboratory of Interface Science and Engineering in
Advanced Materials, Research Center of Advanced Materials Science and Technology,
Taiyuan University of Technology, Taiyuan, 030024, China.
b School of Chemistry and Chemical Engineering, Taiyuan University of Technology,
Taiyuan 030024, China.
Corresponding Author: guokunpeng@tyut.edu.cn; lijie01@tyut.edu.cn
Abstract
Endowing slightly twisted molecules highly emissive in both solution and solid state
is of great importance for understanding the principle of maximizing the luminescent
efficiency of luminophores. To this end, a series of slightly twisted coumarin
luminophores CMs with different alkoxyl substituents at the 7-positions were
synthesized. The effect of the substitutes on the diversity photophysical properties of
the four compounds in solution, THF/H2O mixtures and solid state were investigated.
Comparing to the referenced compound CM (3-p-tolyl-2H-chromen-2-one) that
without a substitute, the introduced electron-rich alkoxyl substitutes not only
enhanced the intramolecular charge transfer (ICT) effect, but also significantly
modified their molecular packing patterns in the crystals. The combined effect of
increasing the radiative and suppressing the nonradiactive pathways boosted the
luminescence efficiency of CM1-CM3 in solution and the solid state simultaneously.
Eventually, compound CM2 with an ethoxyl substitute exhibited the strongest blue

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emission with fluorescence quantum yields as high as 73.2% and 96.7% in solution
and the solid state, respectively. This work presents an efficient strategy towards dual
strong fluorescent luminophores in both solution and the solid state.
KEYWORDS: coumarin derivatives; slightly twisted conformation; intramolecular
charge transfer; molecular packing pattern; dual efficient luminescence
1. Introduction
Highly emissive organic materials have attracted great attention due to their
applicability in the fields of optoelectronic devices, chemosensors and bioprobes
[1-18]. Although the emission of a number of traditional organic luminophores is
efficient in dilute solution, it tends to decrease or even quench in the aggregated or
solid states [19-22]. This aggregation-caused quenching (ACQ), which mainly caused
by internal conversion, intersystem crossing, intermolecular electron transfer, as well
as excimer or exciplex formation and isomerization, significantly limits the organic
luminophores for their practical applications [23]. To prevent or alleviate these
nonradiative pathways, numerous endeavors through molecular engineering and
physical technology have been made, including the introduction of bulky substituents,
enhanced intramolecular charge transfer (ICT) transition, cross-dipole packing and
aggregated formation [24-34]. In 2001, Tang’s group reported a series of silole
derivatives with propeller-like conformations that was nonemissive in dilute solitions,
but highly luminescent when aggregated into solid state [35]. Their work has attracted
huge attention as an effective methodology to overcome ACQ, and this novel

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phenomenon was termed as aggregation-induced emission (AIE). Since then, various
AIE materials with efficient luminescence in the solid state have been prepared for
diverse applications [36-40].
To realize intense luminescence in the solid state, most AIE molecules adopt highly
twisted conformations to restrict intramolecular rotation (RIR) or adverse
intermolecular interactions [41-47]. However, such molecules often emit weak
emission in solution, which limits their wide range of applications. Studies of
developing molecules possess intense luminescence in both solution and the solid
state have, to the best of our knowledge, been focused on restricting free rotation of a
single bond in twisted AIE molecules through conjugation-induced rigidity or
increasing steric hindrance [48-50]. However, the exploration of dual efficient
luminescent materials based on slightly twisted molecules is still challenging. This is
probably arisen from the difficulties of luminescence optimization from molecular
level to molecular packing pattern control in solution and the solid state.
Coumarin (chromen-2-one) derivatives have been widely studied and become a
class of fluorescent dyes of intense interest due to their promising applications in
organic lasers, organic light-emitting diodes and fluorescent sensors [51-59]. It is well
known coumarin derivatives give intense emission in solution, but poor luminescence
in the aggregated and solid state because their planar skeletons are prone to form
strong π-π stacking [60]. To expand their real-world utilization, an AIE coumarin
derivative based on the RIR mechanism has been designed and synthesized [61].
However, the reported AIE coumarin derivative still emits weakly in its solution. Thus,

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the exploration of facile ways to obtain dual efficient fluorescent coumarin derivatives,
especially those with slightly twisted conformations, is of high importance for
structure-property understanding and practical applications. Herein, we developed
four coumarin derivatives (Scheme 1) and investigated their photophysical properties
systematically.
Comparing
with
the
referenced
compound
CM
(3-p-tolyl-2H-chromen-2-one), its alkoxyl substituted derivatives at the 7-positions
CM1-CM3 showed boosted luminescence in both solution and the solid state due to
the synergistic effect of the ICT characteristic and the molecular packing modification
by the alkoxyl tail. Among them, CM2 with an ethoxyl substitute exhibited the
strongest blue emission with absolute fluorescence quantum yields (ΦFs) of 73.2%
and 96.7% in THF solution and the solid state, respectively.
2. Experimental section
2.1. Materials and Characterization
All chemicals for synthesis were purchased from Energy Chemical and used without
purification. Reactions were monitored by TLC silica plate (60F-254). NMR spectra
measurements were carried out at Bruker 600 MHz for 1H NMR and 151 MHz for 13C
NMR, using chloroform-d as the solvent. Chemical shifts were reported in parts per
million (ppm) relative to internal TMS (0 ppm). Splitting patterns were described as
singlet (s), doublet (d), triplet (t), quartet (q), or multiplet (m). Mass spectra were
measured on Microflex MALDI-TOF mass spectrometer. UV-Vis spectra were
recorded in a HITIACH U-3900 spectrometer. Photoluminescent (PL) spectra were

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recorded in a HORIBA FluoroMax-4 spectrometer. The absolute fluorescence
quantum yields (ΦF) of solutions (10 µM) and powders were measured on HORIBA
FluoroMax-4 by using a calibrated integrating sphere. The quartz cuvettes used were
of 1 cm path length. Powder X-ray diffraction (XRD) of the samples was
characterized using a Philips high resolution X-ray diffraction system (model
PW1825). X-ray single-crystal diffractions of CM and CM2 were performed on a
Bruker SMART APEX II diffractometer with Mo Ka radiation (λ= 0.71000 Å). The
structures were solved with direct method (SHELX-97) and refined with full-matrix
least-squares technique. All non-hydrogen atoms were refined anistropically and
hydrogen atoms were geometrically placed. Relevant crystal collection data,
refinement data for the crystal structures and the cif files of CM and CM2 can be
found in the supporting information (ESI). The transient photoluminescence decay
profiles of the solids were recorded using an Edinburgh Instrument FLS980
spectrometer equipped with an EPL-375 picosecond pulsed diode laser.
2.2. Synthetic procedures
2.2.1. 3-p-Tolyl-2H-chromen-2-one (CM)
An acetic anhydride (15 mL) solution of compound 2-hydroxybenzaldehyde (2.0 g,
16.4 mmol), 2-p-tolylacetic acid (2.46 g, 16.4 mmol) and several drops of
triethylamine (5 mL) were charged sequentially into a three-necked flask. The mixture
was heated to reflux till no raw material was detected by the TLC plate. After cooling
the reaction mixture, the precipitation was filtered and recrystallized from ethanol to
1
produce CM as white crystals (2.83 g, yield 72%). H NMR (600 MHz, CDCl3) δ =

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7.79 (s, 1H), 7.61 (d, J = 8.1 Hz, 2H), 7.54 – 7.49 (m, 2H), 7.36 (d, J = 8.2 Hz, 1H),
7.30 – 7.26 (m, 2H), 7.25 (s, 1H), 2.40 (s, 3H). 13C NMR (151 MHz, CDCl3) δ =
163.59, 156.34, 142.10, 141.82, 134.73, 134.09, 132.09, 131.32, 131.21, 130.73,
127.35, 122.69, 119.32, 24.21. MALDI-TOF: m/z [M]+ cacld. C16H12O2, 236.2653;
found: 236.2651. Anal. Calcd for C16H12O2: C, 81.34; H, 5.12; O, 13.54. Found: C,
81.29; H, 5.15; O, 13.56.
2.2.2. 7-Hydroxy-3-p-tolyl-2H-chromen-2-one (CM-OH)
An acetic anhydride (20 mL) solution of compound 2,4-dihydroxybenzaldehyde
(4.0 g, 29 mmol), 2-p-tolylacetic acid (4.4 g, 29 mmol) and several drops of
triethylamine (5 mL) were charged sequentially into a three-necked flask. The mixture
was heated to reflux till no raw material was detected by the TLC plate. After cooling
the reaction mixture, the precipitation was filtered and recrystallized from ethanol to
produce CM-OH as a yellow solid (5.3g, yield 72%). 1H NMR (600 MHz, CDCl3) δ
= 7.78 (s, 1H), 7.60 (d, J = 8.1 Hz, 2H), 7.54 (d, J = 8.4 Hz, 1H), 7.27 (s, 1H), 7.26 (s,
13
1H), 7.15 (d, J = 2.1 Hz, 1H), 7.07 (dd, J = 8.4, 2.2 Hz, 1H), 2.40 (s, 3H). C NMR
(151 MHz, CDCl3) δ = 171.67, 163.22, 156.82, 155.57, 141.90, 141.47, 134.53,
132.11, 131.40, 131.27, 130.58, 121.34, 120.51, 112.84, 24.20, 24.04. MALDI-TOF:
m/z [M]+ cacld. C16H12O3, 252.2647; found: 252.2644. Anal. Calcd for C16H12O3: C,
76.18; H, 4.79; O, 19.03. Found: C, 76.21; H, 4.74; O, 19.05.

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2.2.3. 7-Methoxy-3-p-tolyl-2H-chromen-2-one (CM1)
A CH3CN (50 mL) solution of compound CM-OH (2.0 g, 7.9 mmol), methyl
o
iodide (1.3 g, 9.5 mmol ) and K2CO3 (1.5 g, 10.9 mmol) was stirred at 80 C for 12
hours till no raw material was detected by the TLC plate. After cooling to room
temperature, mixture was poured into water and the precipitated yellow solid was
filtered. The crude product was purified by silica gel column chromatography with
petroleum: ethylacetate (15: 1,v: v) as eluent to afford CM1 as a white powder (1.5 g,
yield 71%). 1H NMR (600 MHz, CDCl3) δ= 7.73 (s, 1H), 7.60 – 7.58 (m, 2H), 7.42 (t,
J = 5.2 Hz, 1H), 7.26 – 7.23 (m, 2H), 6.88 – 6.85 (m, 2H), 3.89 (s, 3H), 2.39 (s, 3H).
13C NMR (151 MHz, CDCl3) δ = 165.39, 163.80, 158.13, 142.20, 141.29, 135.05,
132.01, 131.64, 131.16, 127.71, 116.36, 115.56, 103.34, 58.66, 32.60, 24.13.
MALDI-TOF: m/z [M]+ cacld. C17H14O3, 266.2913; found: 266.2915. Anal. Calcd for
C17H14O3: C, 76.68; H, 5.30; O, 18.02. Found: C, 76.61; H, 5.26; O, 18.13.
2.2.4. 7-Ethoxy-3-p-tolyl-2H-chromen-2-one (CM2)
A procedure similar to the synthesis of CM1 was followed but using bromoethane
(1.0 g, 9.5 mmol) instead of methyl iodide. CM2 was obtained as a white powder (1.7
g, yield 79%). 1H NMR (600 MHz, CDCl3) δ 7.73 (s, 1H), 7.59 (d, J = 8.1 Hz, 2H),
7.41 (d, J = 8.4 Hz, 1H), 7.25 (d, J = 8.0 Hz, 2H), 6.85 (dt, J = 4.0, 2.2 Hz, 2H), 4.11
(q, J = 7.0 Hz, 2H), 2.39 (s, 3H), 1.47 (t, J = 7.0 Hz, 3H). 13C NMR (151 MHz,
CDCl3) δ = 164.76, 163.93, 158.11, 142.33, 141.28, 135.07, 132.02, 131.62, 131.16,
127.54, 116.21, 115.97，103.77, 67.07, 32.61, 24.16, 17.49. MALDI-TOF: m/z [M]+

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cacld. C18H16O3, 280.3718; found: 280.3715. Anal. Calcd for C18H16O3: C, 77.12; H,
5.75; O, 17.12. Found: C, 77.08; H, 5.77; O, 17.15.
2.2.5. 7-Butoxy-3-p-tolyl-2H-chromen-2-one (CM3)
A procedure similar to the synthesis of CM1 was followed but using
1-bromobutane (1.3 g, 9.5 mmol) instead of methyl iodide. CM3 was obtained as a
white powder (2.5 g, yield 85%). 1H NMR (600 MHz, CDCl3) δ = 7.73 (s, 1H), 7.60 –
7.58 (m, 2H), 7.41 (d, J = 8.4 Hz, 1H), 7.25 – 7.23 (m, 2H), 6.85 (dt, J = 3.3, 2.3 Hz,
2H), 4.03 (t, J = 6.5 Hz, 2H), 2.39 (s, 3H), 1.81 (dd, J = 14.8, 7.0 Hz, 2H), 1.51 (dd, J
= 10.3, 4.7 Hz, 2H), 1.00 (t, J = 7.4 Hz, 3H). 13C NMR (151 MHz, CDCl3) δ = 164.97,
163.94, 158.12, 142.34, 141.26, 135.09, 132.02, 131.60, 131.16, 127.49, 116.17,
116.00, 103.78, 71.26, 33.94, 32.62, 24.17, 22.10, 16.72. MALDI-TOF: m/z [M]+
cacld. C20H20O3, 308.3710; found: 308.3708. Anal. Calcd for C20H20O3: C, 77.90; H,
6.54; O, 15.57. Found: C, 77.86; H, 6.51; O, 15.63.
3. Results and discussion
3.1 Synthesisand characterization
The target coumarin homologues CM and CM1-CM3 were synthesized according
to literatures as depicted in Scheme 2 [62]. Compounds CM1-CM3 were synthesized
by the alkoxylation reaction of the key intermediate CM-OH with methyl iodide,
1
bromoethane and 1-bromobutane, respectively. They were all characterized by H
13
NMR, C NMR, and mass spectroscopy. All of the coumarin derivatives are soluble

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in common organic solvents such as toluene, tetrahydrofuran (THF), ethanol and
dimethyl sulfoxide (DMSO).
3.2 Theoretical calculations
To understand the conformations of these compounds, the optimized molecular
geometries were first evaluated by DFT calculation at the B3LYP/6-31G* level
(Fig.1). The dihedral angles between the chromen-2-one and the 4-methylphenyl
moiety for CM, CM1, CM2 and CM3 were 36.22°, 36.39°, 34.64° and 35.96°,
respectively, indicating all of the compounds adopted slightly twisted conformations.
Meanwhile, the four compounds presented similar frontier orbital distributions. As
shown in Fig. S1, large steric overlap between the highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular orbital (LUMO) was observed, which
were mainly distributed over the conjugated chromen-2-one and the 4-methylphenyl
segment. Although the alkoxyl substitutes in CM1-CM3 hardly made contributions to
the distribution of the molecular orbits, they did have a significant effect on the dipole
moment of the molecules. Comparing to the referenced CM (4.0758 D), it was found
increased dipole moments for CM1 (5.7458D), CM2 (5.9456D) and CM3 (5.7857D),
respectively, indicating an improved ICT effect of CM1, CM2 and CM3.
3.3 Photophysical properties in solution
To verify the increased ICT effect on the photophysical properties of the compounds,
UV-vis absorption and photoluminescence spectra were studied in solvents with
different polarities and the corresponding data were extracted in Table 1. A
bathochromic shift of the absorption bands from 328-330 nm for CM to 338-346 nm

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for CM1-CM3 was observed (Fig.2), which can be attributed to the introduced
electron-rich alkoxyl substitutes at the 7-position in coumarin skeleton [63].
Meanwhile, CM showed similar absorption maxima and profiles in solutions,
indicating a small change of dipole moment at the ground state in different solvents
[64]. However, the absorption maxima (λAbs) of CM1-CM3 red-shifted about 15 nm
on increasing the solvent polarity from hexane to DMSO. These features indicate that
ICT character has more evident influence on the absorption bands of CM1-CM3 than
that of CM.
The ICT effect introduced by the alkoxyl substitutes in coumarin skeleton was
further confirmed in the photoluminescence spectra (Fig. 3). All of the four
compounds gave blue emission in various solutions (Fig.4). The maximum emission
peak (λPL) of CM was blue shifted from 425 nm to 409 nm as increasing the solvent
polarity from nonpolar hexane to polar DMSO. This negative fluorosolvatochromism
indicated that the molecule became less polar in the excited state than in the ground
state [65]. On the contrary, the emission spectra of CM1-CM3 showed positive
fluorosolvatochromism with unstructured peaks shifted to the long wavelength as
increasing the solvent polarity owning to the ICT effect of the molecules (Fig. 3b-3d,
Table 1).
To further evaluate the influence of alkoxyl substitutes on the photoluminescent
properties of CM and CM1-CM3, their ΦFs were examined. The ΦFs of the
compounds increased with solvent polarity just in the primary region, and decreased
in the very strong polar solvents, producing a distinct maximum. This up-down

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phenomenon has been observed in some enone derivatives [66]. From hexane to THF,
the increased ΦF of “negative solvation effect” can be ascribed to the reduction of “the
proximity effect” of π-π* and n-π* transition of the molecule with the increased
solvent polarity [67]. With a further increase of the solvent polarity from THF to
DMSO, the decreased ΦF of “positive solvation effect” is attributed to the domination
of the ICT process. In addition, the Stokes shifts of CM in various solvents were
larger than those of CM1-CM3. This suggests a larger excitation-induced geometry
change exists in CM than those in CM1-CM3 in solution, and such “conformation
change” decreases the ΦF of CM [68]. All of the competing mechanisms resulted in
the observed maximum fluorescence quantum yields of 17.6%, 45.8%, 73.2% and
61.06% for CM, CM1, CM2 and CM3 in THF, respectively. Obviously, CM1-CM3
with enhanced ICT effect exhibited more efficient luminescence in solution than the
referenced compound CM (Fig.4). Among them, CM2 with an ethoxyl substitute
exhibited the strongest blue emission, whose ФF value was about 4.1 times larger than
that of CM in THF solution. Considering their similar slightly twisted skeletons, the
reasonable explanation for the ethoxyl effect is that CM2 possesses the largest dipole
moment among the four compounds, leading to more obvious ICT effect to give
highly efficient luminescence in solution.
3.4 Fluorescent properties in the aggregated states
To investigate the fluorescence behaviors of CM and CM1-CM3 in aggregated
states, the UV-Vis absorption and PL spectra in THF/water mixtures with different
water fractions (fw) were studied. When the fw was up to 90%, the absorption spectra

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exhibited significant changes with leveled-off tails clearly appearing in the
long-wavelength region (Fig. S2). This result can be attributed to the well-known Mie
effect, which indicates that CM and CM1-CM3 molecules aggregate in high water
fractions. In the PL spectra, all of the compounds exhibited an up-down tendency in
intensity upon increasing the water fraction. Taking CM2 as a typical example, its PL
intensity was increased upon increasing water content when the water fraction was
less than 80%, but sharply decreased with emission peak shifting 17 nm to long
wavelength when the fw reached 99%, indicating J-type nanoaggregates may be
formed during aggregation (Fig. 5). Similar behaviors were observed for compounds
CM, CM1 and CM3 (Fig.S3), respectively. This up-down phenomenon has been
observed in some compounds with AIE properties, suggesting the aggregates
morphology of the coumarin derivatives at high water content is different from that at
lower water content [69]. SEM images in Fig. 6 and Fig. S4 gave a visualized proof to
support our hypothesis. CM2 formed amorphous nanoscale aggregates in mixtures
with “low” water content (fw < 80%) (Fig. 6a), and further developed to nanoparticles
which gave intense emission when fw reached 80% (Fig. 6b). Further increasing the fw
to 99%, regular 1D rods were observed (Fig. 6c), which may cause emission quench
because of the dominated J-aggregates.
3.5 Fluorescent properties in the solid states
Although CM and CM1-CM3 aggregates exhibited similar photophysical
properties in THF/water mixtures, their crystals gave distinct fluorescence behaviors.
As shown in Fig. 7a and Table 2, CM crystals emitted blue fluorescence peaking at

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441 nm with a moderate ФF of 67.1% under UV irradiation. However, the crystals of
CM1, CM2 and CM3 gave red-shifted emission of 450 nm, with a dramatically
enhanced ФF value of 89.6%, 96.7% and 70.7%, respectively. Similar to the
fluorescence behavior in solutions, the crystal of CM2 showed the strongest blue
emission, whose ФF value was about 1.44 times larger than that of CM.
Time-resolved fluorescence measurements revealed a longest lifetime of CM2
crystals (4.39 ns) than any other crystals of the compounds (Fig. 7b and Table 2). The
highest ФF value, combined with a longest lifetime, led to a relatively large radiative
decay rate (kr 0.22 ns-1) and a smallest nonradiative decay rate (knr 0.0075 ns-1) of
CM2 crystals, which suggested the absolute domination of radiative decay.
To explore the underlying origins of the enhanced emission of alkoxyl substituted
coumarins, the molecular packing patterns in single crystals were analyzed through
X-ray diffraction experiments and the crystallographic data were summarized in Table
S1. As shown in Fig. 8, both CM (CCDC1561738) and CM2 (CCDC 1561739)
molecules adopted slightly twisted conformations, and the dihedral angles of between
the chromen-2-one and the 4-methylphenyl moiety at the 3- position were 28.7°, and
37.53°, respectively, which was close to their calculated results. Notably, their
molecular packing patterns were significantly different from each other. CM
crystallized in monoclinic P21/c symmetry (Fig. 9a and 9b). In the crystal, the
molecules were stacked in an anti-parallel arrangement to form a lamellar-packed
mode viewed down the b-axis. The interplanar distance (3.393 Å) between the
adjacent chromen-2-one moieties demonstrated efficient π-π stacking in crystal that

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would activate nonradiative pathways of excited states causing fluorescence
quenching. On the other hand, multiple O...H hydrogen bonds with distances about
2.56 Å were observed between the aromatic H and the carbonyl groups of adjacent
molecules. These hydrogen bonds can rigidify the molecular conformation of CM in
the solid thus suppress the nonradiative pathways and resulted emission intensity
increasing. As a result, CM crystals demonstrated a moderate ФF because of the
competitive intermolecular interactions of π-π stacking and hydrogen bonds. However,
in the case of CM2, dramatic changes took place after the introduction of an ethoxyl
substitute at the 7-position of CM in the stacking pattern and intermolecular
interactions (Fig. 9c and 9d): 1) CM2 crystallized in triclinic P-1 symmetry. The
molecules formed herringbone packing in a face-to-edge pattern without π-π overlap
between two adjacent molecules; 2) two main kinds of intermolecular weak
interactions were observed including multiple O…H hydrogen bonds with distances
in the range of 2.56-2.71 Å, and C-H…π interactions with distances about 2.8 Å
between the aromatic H and the chromen-2-one moiety as well as the alkoxyl H and
the aromatic ring of adjacent molecules. Therefore, the nonradiative pathways were
effectively weakened or eliminated due to the beneficial hydrogen bonds and C-H…π
interactions as well as the absence of the adverse π-πinteractions, raising the emission
intensity of CM2 crystal to give a ФF as high as 96.7%.
4. Conclusion
In summary, we developed a molecular design strategy for boosting the
luminescence efficiency both in solution and the solid state of coumarin derivatives

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with slightly twisted conformation. By attaching electron-rich alkoxyl tails at the
7-position of CM the fluorescent efficiencies of CM1-CM3 in solution were
enhanced up to 73.2% through the introduced ICT effect. Meanwhile, the alkoxyl tail
plays a key role in modulating the molecular packing patterns and controlling the
intermolecular interactions to enhance the solid efficiency of CM1-CM3. Both the
face-to-edge packing patterns that eliminated π-π intermolecular interactions and
beneficial weak intermolecular interactions suppressed nonradiative pathways in
crystal, endowing CM2 with remarkable solid efficiency of 96.7%. Such strategy of
simultaneously taking advantages of ICT effect and modulating molecular packing
patterns provides an effective method to construct dual-state highly luminescent
materials in both solution and the solid state, which would expand their practical
applications in optoelectronic and biological fields.
Acknowledgements
We thank the National Natural Science Foundation of China (61605138), the
Qualified Personnel Foundation of Taiyuan University of Technology (QPFT) (No:
tyutrc-201255a), Shanxi Province Science Foundation for Youths (201701D221038),
and the Shanxi Provincial Key Innovative Research Team in Science and Technology
(201513002-10).
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Table1
Photophysical properties of CM and CM1-CM3 in various solvents (10 µM).
Compound
Solvent
λAbs
(nm)
λPL
(nm)
Stokes shift
(cm-1)
ФF
(%)
CM
Hexane
THF
Ethanol
DMSO
Hexane
THF
Ethanol
DMSO
Hexane
THF
Ethanol
DMSO
Hexane
THF
328
328
328
330
338
342
342
345
338
342
344
345
338
343
344
346
425
410
408
409
414
424
429
431
416
425
431
433
416
423
429
431
6958
6098
5978
5853
6333
5655
5930
5784
5547
5710
5868
5891
5547
5514
5760
5700
6.3
17.6
4.7
4.2
CM1
CM2
CM3
28.5
45.8
41.2
39.4
38.9
73.2
71.5
68.1
56.4
61.1
59.7
58.5
Ethanol
DMSO
Table 2
Luminescent properties of CM and CM1-CM3 in solid sates.
Compound
CM
λPL (nm)
441
ФF (%)
67.1
τ (ns)
3.72
3.76
4.39
3.79
kr(ns-1)
0.18
knr(ns-1)
0.088
CM1
450
89.6
0.24
0.028
CM2
450
96.7
0.22
0.0075
0.077
CM3
450
70.7
0.19
1

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Captions
Scheme 1 Molecular structures of CM, CM1, CM2 and CM.
Fig. 1 Optimized molecular geometry of CM, CM1, CM2 and CM3 by DFT calculation at the
B3LYP/6-31G* level.
Scheme 2 The synthetic route for compounds CM, CM1, CM2 and CM3.
Fig. 2 Normalized absorption spectra of (a) CM, (b) CM1, (c) CM2 and (d) CM3 in various solvents (10
µM).
Fig. 3 Normalized PL spectra of (a) CM, (b) CM1, (c) CM2 and (d) CM3 in various solvents (10 µM).
Fig.4 Photographs of (a) CM, (b) CM1, (c) CM2 and (d) CM3 in various solvents under UV (λex=365
nm) illumination.
Fig.5 (a) PL spectra of CM2 (10 µM) in THF/water mixtures with different water fractions, (b) The effect
of water fractions on the maximum emission intensity of CM2.
Fig. 6 SEM images of CM2 aggregates in THF/water mixtures (10 µM) with different water fractions: (a)
in THF/H2O (60:40, v/v); (b) in THF/H2O (20:80, v/v); (c) in THF/H2O (1:99, v/v).
1

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Fig.7 (a) PL emission spectra of CM, CM1, CM2 and CM3 in the solid state. Inset: photographs of CM,
CM1, CM2 and CM3 powders under UV (λex=365 nm) illumination. (b) Time-resolved fluorescence
spectra of CM, CM1, CM2 and CM3 in the solid state (λex = 375 nm).
Fig.8 ORTEP diagram of (a) CM and (b) CM2.
Fig.9 (a) The packing pattern and (b) weak interactions in the crystals of CM. (c) The packing pattern and
(d) weak interactions in the crystals of CM2.
2