Highly efficient blue solid emitters and tautomerization-induced ON/OFF fluorescence switching based on structurally simple 3(5)-phenol-1: H -pyrazoles

Article abstract of DOI:10.1039/c6cc07300j

3(5)-Phenyl-1H-pyrazoles are employed in this study to develop highly efficient organic crystalline solids with deep-blue ESIPT fluorescence as well as provide novel experimental insight into the mechanism of ESIPT fluorescence and generate an intriguing

Full text of DOI:10.1039/c6cc07300j

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Highly efficient blue solid emitters and tautomerization‐induced
ON/OFF fluorescence switching based on structurally simple 3(5)‐
Phenol‐1H‐pyrazoles
Received 00th January 20xx,
Accepted 00th January 20xx
Baolei Tang, Houyu Zhang, Kaiqi Ye, Hongyu Zhang* and Yue Wang
DOI: 10.1039/x0xx00000x
www.rsc.org/
3(5)‐Phenyl‐1H‐pyrazoles are employed in this study to develop ESIPT‐active organic fluorophores effectively decreases self‐
highly efficient organic crystalline solids with deep‐blue ESIPT absorption and hence plays a positive role in emission
fluorescence as well as provide novel experimental insight into intensity.[10] Indeed, organic materials with bright ESIPT
mechanism of ESIPT fluorescence and generate an intriguing fluorescence have been designed and some of them even
fluorescence “ON/OFF” switching.
perform well in optoelectronics.[11] However, most of the
ESIPT‐active materials are restricted to green and longer
wavelength fluorophores due to the large Stokes shift ( 100

‐Conjugated luminescent organic materials have attracted
great attentions of scientists in material science due to their
wide applications including organic light‐emitting diodes
(OLEDs)[1], organic solid‐state lasers (OSLs)[2], sensors[3], bio‐
imaging[4], etc. Recently, great efforts have been paid to
synthesize organic luminescent materials by either developing
nm) and that exhibiting deep blue solid‐state emission is
extremely rare. Another notable feature of ESIPT fluorophores
is that the proton transfer in the excited state can be blocked
by perturbing the intramolecular hydrogen bond in solutions.
Thus, ESIPT emitters are good candidates for sensors. However,
the switching of ESIPT fluorescence in the solid state has not
been developed which restrict the application of this class of
fluorophores.
new design strategies or constructing novel

‐conjugated
systems.[5 8] However, the deep blue emissive organic solids
with desired properties are still scarce. The typical


‐
conjugation framework for blue emitters such as anthracene,
fluorence or pyrene takes a rigid and flat framework which
readily forms face‐to‐face packing structures through
intermolecular  interactions in solid states. Thus, the
To achieve ESIPT‐active blue solid emitters, extended
conjugation or donor‐acceptor structure should be avoided
whenever possible. Taking this into account, we attempt to
‐
fluorescence of organic solids containing such ‐electron units
a
OH
OH
N
NH
HN
N
usually suffers from aggregation‐caused quenching and/or
bathochromic shift effects. Although some blue emissive
organic solids with high fluorescence efficiency have been
Ar
Ar
R
R
1: R = H
tautomer a
enol form
tautomer b
enol form
2: R = F
constructed by introducing bulk substituents into the ‐system,
3: R = OMe
4: R = Me
the corresponding synthesis and purification is time‐
consuming and laborious. In this sense, the exploration of
novel design strategy towards efficient blue emissive materials
based on structurally simple and easily available organic
molecules is highly important and urgently demanded.

O
HN NH


ESIPT active
ESIPT inactive
h
h
Organic
compounds
that
undergo
excited‐state
keto* form
intramolecular proton transfer (ESIPT) have been widely
investigated recently since the photochemical process
produces a tautomer with a quite different electronic structure
from the original excited state.[9] The large Stokes shift of
b
+5.04 Kcal/mol
tautomer b
Energy:
HF = 532.49737 a.u.
tautomer a
Energy:
HF = 532.50537 a.u.
Fig. 1 Tautomeric structures and ESIPT process of 14 (a) and conformation and energy
of two tautomers calculated based on 1 (b).
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a
2
construct deep blue ESIPT fluorescent organic solids based on
the structurally simple 3(5)‐phenol‐1H‐pyrazole skeleton (Fig.
1a) in which the phenol ring act as proton donor and pyrazole
unit serve as proton acceptor. As expected, some of these
compounds produced highly efficient blue emissive crystalline
solids with quantum yield as high as 0.50. Notably, the proton
1
3
4
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1.0
0.8
0.6
0.4
0.2
0.0
1.0
DOI: 10.1039/C6CC07300J
0.8
0.6
0.4
0.2
0.0
1
2
I
I
3
4
acceptor unit has two tautomeric forms (tautomer
a and b)
which essentially determine the ESIPT generation. In this sense,
the present system provides a model to demonstrate the
possibility of the ESIPT fluorescence switching in the solid state
through tautomerization. Single crystals composed of either
250
300
350
400
450
500
Wavelength / nm
b
1.0
0.8
0.6
0.4
0.2
0.0
1
1.0
0.8
0.6
0.4
0.2
0.0
1
2
3
tautomer
a (13) or b (4) with ESIPT fluorescence “ON” or
2
“OFF”, respectively, have been obtained. Optical behaviors,
crystal structures, and ESIPT mechanism of these molecules
were carefully studied. And the high‐contrast solid‐state
fluorescence interconversion due to tautomerization was
finally investigated.
I
I
3
250
300
350
400
450
500
550
600
Compound 1[12,13] is a known molecule and
24 are newly
Wavelength / nm
600
c
synthesized according to the literature method[12,13] to
demonstrate the substituent effect. The fluorescence of
500
400
300
200
100
0
 20
compounds
14 in solution are very weak (f  0.01), as
shown in the inset of Fig. 2a. They exhibit dual‐band emission
spectra in dichloromethane (DCM), displaying a typical
fluorescence feature of ESIPT‐active molecules. The high‐
energy bands originated from the excited‐state enol forms
locate in the UV region, ranging from 323 to 334 nm. And the
low‐energy bands peaked at around 380 nm comes from the
excited‐state keto form species generated through an ESIPT
process. Although the substituents don’t affect the
fluorescence wavelength greatly, they show certain influence
on the proportion of the enol and keto emission bands in DCM
I
300
350
400
450
500
550
600
650
Wavelength / nm
Fig. 2 a) absorption and emission spectra of 14 in DCM solution, b) absorption and
emission spectra of crystals 13 and c) “ON” and “OFF” emission spectra of solid 4.
(inset: photographic images of solid samples under 254 nm UV irradiation)
together with aggregation‐induced emission (AIE) has been
proposed by Tang as a novel and powerful strategy for
constructing organic luminescent functional materials.[14]
To disclose the CEE behaviors of crystals
unusual emission property of , we have determined the
crystal structures of all compounds. Crystals hold the same
space group and crystal system (P212121, orthorhombic), in
consistent with their similar crystalline‐state emissions. As for
crystal 1, the intramolecular hydrogen bond OHN (distance:
1.826 Å) formed between the hydroxyl group and the nitrogen
atom of the pyrazole ring generates a planar framework which
not only facilitates ESIPT but also rigidifies the molecular
structure (Fig. 3a). The dihedral angle between phenol and
pyrazole units is 1.52
the packing structure, intermolecular C
allow the molecules to pack into a herringbone structure (Fig.
3b) which is stabilized by intermolecular C O hydrogen
solution. For instance, fluoro‐substituted compound
a balanced dual‐band emission while the fluorescence
spectrum of is dominated by the enol form emission. Similar
2 displays
1
13 as well as the
to solution samples, the spin‐coated thin films (Fig. S1) also
show very weak fluorescence (f  0.01).
4
13
As shown in Fig. 2b, the crystalline solids of
intense single‐band blue emissions. Crystal exhibits strong
fluorescence peaked at about 463 nm with f of 0.52. And
crystals and emit slightly blue‐shifted fluorescence centred
13 show
1
2
3
at 454 (f = 0.49) and 453 nm (f = 0.49), respectively. The
Stokes shifts of these crystals are about 130 nm, reflecting that
the intense blue emissions of crystals originate from the ESIPT
fluorescence. Enol form bands are not found in the emission

, reflecting a rather planar
‐skeleton. In
 interactions
spectra of crystals
different fluorescence compared with
similar optical behaviors in solutions and thin films. Crystals
show much weaker emission (f  0.01) compared with
1

3
. Surprisingly, crystals
4 exhibit totally

H
13
, despite of their
4
3

H
1

bonds. No  interactions are observed in the crystal
structure, despite of the planar molecular structure. As shown
(Table S1). As shown in Fig. 2c, the emission spectrum of
crystal shows a major band peaked at around 350 nm and a
weak broad band centred at about 460 nm. In this sense, ESIPT
might easily take place in crystals but difficult in crystal
The luminescent behaviors indicate that the excited‐state enol
form species of have intrinsic weak emission nature and
the excited‐state keto emissions of are significantly
enhanced by crystallization. Thus, compounds display a
4
in Fig. S2 and Fig. 3, crystals
structures with crystal . The molecules in crystals
the form of tautomer , namely, 2‐(1H‐pyrazol‐3‐yl)phenol. In
line with the unusual emission property, crystal
different molecular structure compared with
2
and
3
show the similar molecular
1
13
take
1
3
4.
a
4
1
shows totally
. The N
14

3
H
13
moiety of pyrazole ring in crystal
phenol group. Thus, the molecules in crystalline state take
4 is located close to the
13
crystallization‐enhanced emission (CEE) feature which
2 | Chem. Commun., 2016, 52, 1‐4
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In DCM solution, tautomer
than tautomer
and E) is in the order of
compound has the greatest possibility to undergo
isomerization during crystallization since
energy barrier between tautomer and
understanding of the different behaviors of
had better compare both and form in their crystal state,
unfortunately, we only obtained the crystal states of
their tautomer form and crystal state of in its tautomer
a
is energetic more favourable
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b
. The energy differenceDbOeI:t1w0.e10e3n9/tCa6uCtCo0m73e0r0aJ
b
(

4 < 1 < 2 < 3 (Table S2). Thus,
4
4
has the smallest
a
b. For the better
4
in solid state, it
a
b
1
3
in
a
4
b
form, so we could not compare the conformation and energy
of its tautomers directly. The calculated energies of
compounds in their crystal states are higher than those in
solution (Table S2). Such higher energy states in crystal need
the strong intermolecular interaction to stabilize their
conformation. It is the easiest for compound
tautomer transformation from to and once compound
its tautomer is formed, it will be stabilized by the
surrounding intermolecular interactions as demonstrated by
the crystal structure analysis. So only belongs to different
space group and crystal system, significantly different from
. Though short of direct comparison of and in solid
form seems reasonable from our
4
to occur such
a
b
4
in
b
Fig. 3 Molecular (a, c), packing structures (b, d) and intermolecular hydrogen
bonding interactions (e, f) of crystals and
1
4.
4
1
3
a
b
the structure of 2‐(1H‐pyrazol‐5‐yl)phenol (tautomer
hamper the formation of intramolecular O N hydrogen
bond. As a result, the ESIPT luminescence is prevented in
crystal . Instead of the O N interaction, N O hydrogen
b) which
state, the existence of
calculation results.
4 in b

H
Molecule
during crystallization. Thus, N
the solids of and lead to fluorescence change. Indeed, the
reversible heating‐cooling controlled “ON/OFF” fluorescence
switching of solid has been realized (Fig. 4a). The faint
emissive crystals (tautomer ) converted into the melt state
when heated over 130 C. During natural cooling, the sample
exhibited bright ESIPT fluorescence immediately after
solidification, indicating a certain amount of tautomer
4
underwent isomerization while
13 did not
4

H
H

H switching might take place in
bond (distance: 2.103 Å) forms which also endows the entire
molecule with a completely planar structure (Fig. 3c).
4
Molecules
4 form a two‐dimensional network in the packing
4
structure by intermolecular hydrogen bonding and 
interactions, as shown in Fig. 3d.
Crystal structure analysis has unambiguously confirmed that
4
b

the tautomeric form of molecules
14 decides the
a
fluorescence nature of the solids. The geometry optimizations
of two tautomers were performed with the density functional
theory using the B3LYP functional and 6–31G (d, p) basis set,
as implemented in Gaussian 09 package.[15] Both of tautomers
form flat structures because of the existence of an
intramolecular hydrogen bond (Fig. 1b), consistent with the
molecules existed in the deformed solid. The freshly generated
ESIPT fluorescence gradually diminished during cooling and
disappeared after standing in air for several hours. When the
crystalline solids were heated to melt, molecules underwent
isomerization from tautomer
130 C, the reverse isomerization took place right away. This
b to a. After solidified at about

crystal structure analysis. Tautomer
a is more stable than b by
tautomerization in solid form takes a relatively long time and
some ESIPT‐active molecules could retain to several hours.
Therefore, the just solidified sample exhibited bright emission
which became weaker and weaker due to the on‐going reverse
isomerization of ESIPT‐active molecules. It is worthy to note
that the ESIPT fluorescence could recover when the cooled
sample was heated to melt and followed by the cooling again.
Thus, the reversible fluorescence “ON/OFF” switching due to
the interconversion between ESIPT and local emissions which
are CEE‐active and faint emissive, respectively, has been
obtained.
0.008 a.u (5.04 Kcal/mol), indicating that these compounds
exist as the structure of 2‐(1H‐pyrazol‐3‐yl)phenol in solution.
Indeed, there are only one set of proton signals, corresponding
to tautomer
Although the reason why compound
tautomer in solid form is not clear at the current state, the
intermolecular hydrogen bonding interactions (Fig. 3e and f)
demonstrate that tautomer can stably survive in crystal
The O(OH) in acts as intermolecular hydrogen bond
acceptor and the H(OH) approaches to nearest of two nitrogen
atoms. In sharp contrast, H(OH) in crystal is away from the
a, in the NMR spectra of compounds
14.
4
prefer the structure of
b
b
4.
13
4
Although the conventional crystallization of
4 produces
adjacent N atom due to the formation of intermolecular
hydrogen bond with a neighbor molecule. As a result, H(N) is
located on the N atom close to phenol ring and form a NHO
intramolecular hydrogen bond. In addition, the rich
inter/intramolecular hydrogen bonds facilitate the formation
of a tetramer which greatly stabilizes tautomer b.
weak emissive solids, the rapid evaporation gives rise to
different results. As shown in Fig. 4b, when the DCM solution
was rapidly volatilized by hot air, the freshly obtained solids
exhibited bright ESIPT fluorescence which became weaker and
weaker and finally disappeared in air. The molecules have
enough time to undergo isomerization (tautomer ab) during
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the routine slow crystallization, leading to faint emissive example of blue solid‐state ESIPT fluorescence but also has
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DOI: 10.1039/C6CC07300J
crystals. However, in the rapid evaporation and solidification scientific significance in disclosing the ESIPT fluorescence
process, some ESIPT‐active molecules retained and gradually mechanism. In addition, the reversible luminescent chromism
underwent tautomerization, leading to the observed of compound
4 suggests the potential application of this
fluorescence change. Thus, the same fluorescence “ON/OFF” material in sensors or memory devices.
switching due to the solid phase tautomerization as mentioned
above was realized in another way.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (51622304).
Notes and references
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3
4
Fig. 4 The solid‐state ESIPT fluorescence “ON/OFF” switching of compound
4
realized through two different processes (a and b) and Switching mechanism (c).
To experimentally verify the ESIPT fluorescence nature of an
organic compound, replacing the hydroxyl group by methoxyl
substituent followed by the comparison of emission properties
is one of the routine ways. In that case, the additional
synthesis is required and the spectral difference that supports
ESIPT comes from different molecules. The solid‐state
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fluorescent chromism of compound
4 offers a novel evidence
7
for the ESIPT fluorescence. This compound produces ESIPT‐
active and inactive tautomers in solid phase by controlling
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solids depend on the ratio of ESIPT‐active tautomer. Thus, the
mechanism of ESIPT process has been verified in a new way by
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8
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Conclusions
In summary, 3(5)‐phenol‐1H‐pyrazole derivatives 13 were
employed in this study to construct organic crystalline solids
with intense deep blue ESIPT fluorescence. Despite of the
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similar structure as
faint ultraviolet emission. The tautomer form of molecules in
crystals facilitates ESIPT reaction whereas that in crystal 4
hampers the generation of ESIPT, which endow the similar
organic molecules with quite different solid‐state emission
properties. The high‐contrast “ON/OFF” fluorescence
13, compound 4 produced crystals with
1
3
switching of solids
4 based on the reversible isomerization
between two tautomeric forms has been realized through two
convenient procedures. The present study not only gives a rare
4 | Chem. Commun., 2016, 52, 1‐4
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Graphical Abstract
3(5)-Phenyl-1H-pyrazoles are employed in this study to develop highly efficient organic crystalline solids
with deep-blue ESIPT fluorescence as well as intriguing fluorescence “ON/OFF” switching.
This journal is © The Royal Society of Chemistry 2016
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