Crystal-packing modes determine the solid-state ESIPT fluorescence in highly dipolar 2′-hydroxychalcones

Article abstract of DOI:10.1039/d1tc03096e

This work describes the systematic study of the structure-luminescence relationship of 15 hydroxy-chalcones directly in the crystal state. Chalcones are easily assembled at the gram scale allowing for efficient variation of their substitution motifs. Our molecule variants combine two modes of fluorescence generation, ESIPT and ICT, both known for their potential to achieve significant quantum yields even with emission in the red to near infrared, a region preferred for technologies as diverse as optoelectronics and chemical sensing. Quantum yields as high as 48% (at 665 nm) and emission wavelengths in the deep red region (710 nm, 5%) were achieved with variants equipped with a strained amino substituent in the donor portion (azetidinyl). Systematic XRD analysis of large monocrystals allowed for the identification of the subtle interplay of several inter- and intra-molecular parameters in achieving such performances, be it intramolecular planarity, non-classical H-bonds, and stacking modes.

Full text of DOI:10.1039/d1tc03096e

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Crystal-packing modes determine the solid-state
ESIPT fluorescence in highly dipolar 20-
Cite this: DOI: 10.1039/d1tc03096e
hydroxychalcones†
a
Alix Tordo,a Erwann Jeanneau,b Mathieu Bordy,a Yann Bretonniere
and
`
a
Jens Hasserodt
*
This work describes the systematic study of the structure–luminescence relationship of 15 hydroxy-
chalcones directly in the crystal state. Chalcones are easily assembled at the gram scale allowing for
efficient variation of their substitution motifs. Our molecule variants combine two modes of
fluorescence generation, ESIPT and ICT, both known for their potential to achieve significant quantum
yields even with emission in the red to near infrared, a region preferred for technologies as diverse as
optoelectronics and chemical sensing. Quantum yields as high as 48% (at 665 nm) and emission
wavelengths in the deep red region (710 nm, 5%) were achieved with variants equipped with a strained
amino substituent in the donor portion (azetidinyl). Systematic XRD analysis of large monocrystals
allowed for the identification of the subtle interplay of several inter- and intra-molecular parameters in
achieving such performances, be it intramolecular planarity, non-classical H-bonds, and stacking modes.
Received 3rd July 2021,
Accepted 31st July 2021
DOI: 10.1039/d1tc03096e
rsc.li/materials-c
such as hydrogen bonds, electrostatics, orbitals, oscillator strength,
etc., whose subtle interplay determines whether solid-state fluores-
Introduction
Solid-state luminescent materials (quantum dots, polymers, cence is observed. The understanding of this interplay was turned
etc.) have been intensively explored over the last decade because into
a theory named solid-state luminescent enhancement
of their attractive properties (high quantum yield and photo- (SLE),13–16 which emphasizes the multiparametric aspect of any
stability) for applications such as chemical sensing, bio- quest to tune solid-state fluorescence.
imaging and optoelectronics.1–6 On the other hand, molecular
Consequently, red-shifting the emission wavelength of a
approaches to solid-state fluorescent materials (organic fluor- fully organic solid-state emitter is a challenge that significantly
ophores and coordination compounds) have arguably been less surpasses that of red-shifting the emission wavelength of
frequently investigated due to the well-known quenching effect solution-phase fluorophores. The usual approach of increasing
in the condensed phase, and notably in the crystalline state. p– the size of the p-electron network or enhancing the push–pull
p interactions or excimer formation in the crystal lattice is character of the substitution pattern often results in fluores-
largely responsible for the extinction of the luminescence cence quenching through detrimental p-stacking interactions
properties exhibited by these molecules in solution.7–11 In or excimer formation once the crystalline state is reached.4,17–19
selected cases, this quenching phenomenon is reversed, and Also, the more red-shifted the fluorophore’s emission is, the lower
has been coined aggregation-induced emission (AIE).7,12 AIE its S1–S0 energy gap is, and the lower its quantum yield is through
has been largely studied in the amorphous phase upon aggre- non-radiative decay.20 Among the red-shifting strategies for solid-
gation from the solution state by increasing the water content. state emitters, the four-level energy system, excited-state intra-
Restriction of intramolecular motions was found to be princi- molecular proton transfer (ESIPT) and internal charge transfer
pally responsible for AIE. Recently, detailed attention to the (ICT or push/pull), attracted much attention because of their large
crystalline phase has identified numerous further parameters Stokes shift and efficient long-wavelength emission.21–23 Many
compounds already reported showed favorable properties with a
strong photon emission efficiency.24–26 Combining these two stra-
tegies to develop high-performance solid-state fluorophores has
thus become a busy research area. In the present study, we elected
to work with ESIPT-capable 20-hydroxychalcones to rationalize their
photophysical properties in the solid state. Their easy synthetic
assembly allows for the establishment of a variety of substitution
a
´
Univ. Lyon, ENS Lyon, CNRS, Universite Lyon 1, Laboratoire de Chimie, UMR
´
5182, 46 Allee d’Italie, 69364 Lyon, France. E-mail: jens.hasserodt@ens-lyon.fr
b
´
Univ. Lyon 1, Centre de Diffractometrie Henri Longchambon, 5 rue de la Doua,
69100 Villeurbanne, France
† Electronic supplementary information (ESI) available. CCDC 2089012–2089026.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/d1tc03096e
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motifs, and the systematic X-ray diffraction analysis of their crystal-
line forms might help us to identify intra- and inter-molecular
effects thereby promoting high quantum yields and emissions in
the red and near-infrared regions.
Experimental methods
20-Hydroxychalcones are a class of molecules exhibiting ESIPT-
based fluorescence as shown in Scheme 1. Ground state chal-
cone I can reach an excited state (II) upon high-energy photon
absorption. Besides the possible non-radiative events, II can
then decay through two different radiative pathways: (i) direct
de-excitation via emission of a photon (hn0) with a small Stokes
shift (typically r50 nm), regenerating ground state I, or (ii)
intramolecular proton transfer (ESIPT) concomitant with an
electronic rearrangement generating III (enol form), a lower-
energy excited state. Fluorescence from III (hn00) is thus
observed with a much larger Stokes shift (usually Z100 nm)
which leads to the formation of the ground state keto form (IV)
that rapidly tautomerizes to regenerate I. With this mechanism
in mind, we explored the effects of various substitution pat-
terns on the ESIPT quantum yield and emission wavelength.
Although unsubstituted 20-hydroxychalcones are reported to
show almost no fluorescence in the solid state, placing an
electron-donating substituent (EDG) on the phenyl unit leads
to intense red fluorescence in this condensed phase.27 Two
radiative pathways are therefore possible: the ESIPT mecha-
nism described above, and an internal charge transfer (ICT)
promoted by a push–pull effect between the EDG and the keto
group (Scheme 2A).28 A few substitution patterns had already
been explored for their potential merits for nano-lasers or
bioimaging probes.26,27,29–33 However, the structure–property
relationship remains unclear, even if recent studies tried to
rationalize the spectroscopic properties via computational or
crystallographic analysis.27,30 Indeed, only a few chalcone deri-
vatives with a push–pull substitution have been reported to
date. We therefore embarked on the design and synthesis of
several new derivatives in order to gain predictive insights into
the spectral tuning of this system. Among the fifteen variants
prepared in the present work (Scheme 2B), the five derivatives
in the 1st series are based on 20-hydroxyacetophenone (1a–e). In
the 2nd group, we investigated the extension of the p system by
using a naphthol unit (2a–e), a widely accepted method to
obtain polyaromatic near-infrared fluorophores.22,34,35 The 3rd
group is characterized by the presence of a methoxy substituent
Scheme 2 Synthesis (A) and structural variation (B) of compounds 1a–e,
2a–e, and 3a–e.
(3a–e) in the para position relative to the carbonyl group in
order to reinforce the ESIPT tautomerization in the excited
state, and thus favoring radiative decay and red-shifted emis-
sion. For these three groups, we studied the effect of electron-
donating substituents on the p-position of the phenyl ring,
either in the form of a methoxy group (1–2–3e) or a dimethy-
lamino (1–2–3a) group (with Hammett constants sp of ꢀ0,23
and ꢀ0,83, respectively).36,37 We further examined the influ-
ence on crystal packing, and thus fluorescence properties, by
varying the substitution motif on the nitrogen to diethylamino
(1–2–3b), azetidinyl (1–2–3c) or pyrrolidinyl (1–2–3d). These
latter modifications have been chosen to study the impact of
the level of molecular planarity on the spectroscopic properties,
as their inductive effect is not significantly modulated from one
molecule to another. Based on such structural permutations,
this study is meant to extract the precious trends that will be
key to the design of future near-infrared solid-state emitters.
Compounds 1a–e, 2a–e, and 3a–e (Scheme 2B) were synthe-
sized at the gram scale by simple pyrrolidine-catalyzed aldol
condensation in ethanol from the corresponding acetophe-
nones and benzaldehydes, a protocol adapted from a reported
method.38 Good to high yields are obtained for our fifteen
compounds (57–89%), boding well for further SPR screening in
the future. Centimeter-sized crystals of all fifteen chalcones
were obtained by liquid/liquid diffusion in CHCl3/MeOH
(Fig. 1), which allowed for the determination of their fluores-
cence properties in the crystalline state (Fig. S1–S15, ESI†).
Results
For all crystals, large emission bands were obtained (Fig. 2),
which are not only characteristics of solid-state fluorescence,
but also of fluorophores operating by an ESIPT mechanism. To
our delight, compounds 3c, 3a and 2d showed intense fluores-
cence in the far-red region (F = 48%, lem = 665 nm; F = 30%,
lem = 635 nm; F = 13%, lem = 690 nm, respectively, Table 1),
and this occurs despite the fairly small-sized p system, and the
Scheme 1 Excited-state intramolecular proton transfer mechanism for
20-hydroxychalcones.
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almost non-emissive (1e, 2e, and 3e). The methoxy group (sp =
ꢀ0,23) might simply be too weak to promote an efficient push–
pull mechanism in the excited state.38 In contrast, our candi-
dates with cyclic amine substituents (1, 2, and 3c–d) showed
high quantum yields and appreciably red-shifted emissions
compared to the ones carrying a dimethyl- or diethylamino
group (1, 2, and 3a–b). Particularly, the azetidine-substituted
derivatives (1c, 2c, and 3c) stood out for their efficient emission
in the far-red region. As the cyclic and open-chain amino
substituents exert similar electronic effects (see Hammett con-
stants), these results already hinted at the influence of other
intra- or inter-molecular parameters on the fluorescence prop-
erties in the crystal lattice. Moreover, extending the p-system by
annellation of the acceptor phenol with a 2nd aromatic ring (2a–
e) did not deliver the desired red-shift of the ESIPT emission
band, and a decrease in the quantum yield was observed.
While methoxy substitution of the donor site led to negligible
fluorescence, placing a methoxy group at the para position of the
acceptor portion of the fluorophore led to medium to high quantum
yields (F = 9% to 30% for 1a/3a, F o 1% to 15% for 1b/3b, and F =
9% to 48% for 1c/3c, Table 1). Unfortunately, this functionalization
always blue-shifted the emission wavelength, irrespective of the
nature of the amino substituent present. Indeed, the effect of a
reduced electron-withdrawing character of the acceptor unit on blue-
shifting has already been observed in other A–D systems.37,39 This
was explained with its drastic effect on the internal charge transfer in
the excited state, and thus the maximum emission wavelength.
To better understand these experimental results, single-crystal
X-ray diffraction (XRD) analysis was carried out for each variant and
rationalized in the next section. XRD analysis allowed us to deter-
mine for each variant whether it aggregates in J- or H modes and
whether it packs in 3D herringbone (HB) or p-stacking modes
(Table 1). The obtained data also included intramolecular para-
meters in the solid state, namely the distance between the phenolic
proton and the carbonyl oxygen (dESIPT), and the torsion angle
between planes formed by the A phenol and D phenyl units
(Fig. S16, ESI†). As expected, all compounds showed strong innate
intramolecular H-bonds between the carbonyl oxygen and the
hydroxyl group (1.724–1.802 nm), which is typical of the ESIPT
mechanism. Despite this strong H-bonding, which is known to
increase the rigidity and the degree of overall molecular planarity (a
phenomenon highly favorable for radiative decay), intramolecular
torsion angles differed significantly from one chalcone to the other,
implying that additional intra- and/or inter-molecular interactions
might be responsible for the observed difference in the lumines-
cence properties.
Fig. 1 Pictures of crystals 1a–e, 2a–e, and 3a–e under sunlight (left) and
UV irradiation (right).
Fig. 2 Crystal-state fluorescence emission spectra of compounds 1a–d,
2a–e, and 3a–d measured with an integration sphere, with an excitation
wavelength of 450 nm (see ESI† for methods). 1e and 3e were not
fluorescent enough to measure their emission spectrum (F = 0%).
Table 1 Crystal-state fluorescence properties of compounds 1a–e, 2a–e
and 3a–e, with their intra- and inter-molecular parameters extracted from
DRX analysis. HB = Herringbone, — = compound not fluorescent enough
to measure their emission spectrum
Crystal F (%) lmax (nm) Arrangement H/J dESIPT (nm) Torsion (1)
1a
1b
1c
1d
1e
9
675
660
670
690
—
HB
HB
HB
HB
J
J
J
J
1.783
1.802
1.783
1.787
1.794
10.86
24.47
12.68
9.95
o1
9
14
o1
p-Stack
H
6.89
2a
2b
2c
2d
2e
2
4
5
13
o1
670
665
710
690
630
p-Stack
HB
HB
HB
p-Stack
J
J
H
H
—
1.738
1.7578
1.724
1.729
1.777
8.28
3.34
9.34
10.80
3.83
Discussion
3a
3b
3c
3d
3e
30
15
48
16
o1
635
605
665
680
—
HB
HB
HB
HB
J
J
J
J
1.760
1.765
1.777
1.774
1.802
6.13
4.60
3.25
6.97
26.68
It is well known that the mode of crystal packing is paramount
to the observed spectroscopic properties in the condensed
phase.14,27,31 For example, the crystal lattice for 1a revealed
that excimer formation can be avoided even with linear and flat
p-Stack
H
absence of bulky substituents which are often deliberately molecules if they are packed as infinite molecular chains with a
introduced to avoid dense molecular packing and thus fluores- typical slip-packing structure (or J aggregate, Fig. 3). Conver-
cence quenching. Curiously, all anisole-derived molecules are sely, the anisole-based chalcone 1e also displayed a relatively
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Fig. 4 Degree of planarity and its influence on the quantum yield: amino
substituted chalcones.
Fig. 3 Comparison of J- with H aggregates as found in crystals of 1a and
1e, respectively.
In the series of 3 variants (Scheme 2B), a p-methoxy group
was placed on the phenolic acceptor unit. This did not greatly
change the molecular conformation and packing modes for
amino-substituted chalcones (3a–d), as they were still packed as
J aggregates in an HB 3D arrangement (Table 1). However, the
XRD analysis revealed that this new presence of an H bond
acceptor (O substituent) somehow altered the microenviron-
ment through non-classical H-bonds (Fig. 4). Torsion angles
were all small, thus resulting generally in higher quantum
yields compared to the unsubstituted series 1, except for 3e.
The passage from an N to an O substituent on the phenyl donor
unit once again led to a harmful face-to-face packing (or H-
aggregate), but this time with a far greater torsion angle (26.681
for 3e vs. 6.891 for 1e) (F o 1%, Fig. S20, ESI†). Unfortunately,
the emission wavelength was strongly affected by this functio-
nalization, as a blue-shift was observed for all compounds
compared to variants with no substitution on the phenolic
acceptor unit (1a–e). As the ESIPT tautomerization should be
reinforced by the presence of such a donor substituent
(resulting in a smaller S1–S0 energy gap, and thus red-shifted
emission), the observed blue-shift suggested the push/pull
fluorescence is dominating the ESIPT mechanism in this series.
planar conformation (torsion angle of 6.891), but pairs of
molecules in the crystal lattice adopted a face-to-face packing
mode (or H aggregate), with strong p–p and dipole–dipole
interactions; this drastically reduced the quantum yield (F o
1%). Apart from 1e, all variants in series 1 (phenol motifs
without any substituent) adopted the desirable J aggregate with
an HB 3D arrangement. Intriguingly, all these four variants
differed from 1e by displaying on the donor phenyl an N rather
than an O substituent. But even within the N series, significant
differences in the quantum yield can be detected (from 0 to
14%). For example, 1b showed practically no fluorescence in
the solid state. It is also the variant whose torsion angle is
substantially greater than that of its congeners (24.471/1% vs.
9.951/14% for 1d, see Table 1). This correlation has already
been observed in a series of stilbene variants.13 High degrees of
planarity not only promoted the push–pull effect but also the
ESIPT mechanism.
In the second series of this study, the phenolic acceptor
unit’s p-system was enlarged by annellation with an extra benzo
ring (Scheme 2B). The resulting naphthol derivatives 2a–e did
not show the desired shift of their solid-state fluorescence in
the red region (arguably apart from 1d, lmax = 690 nm). The
Conclusions
observed quantum yields are also not as easily rationalized To conclude, fifteen ESIPT-active and D–A structured 20-
from the XRD data. All five compounds showed mostly planar hydroxychalcones were obtained in the crystalline state in this
conformations (torsion angles between 3.341 and 10.801). Yet, study. This allowed us to determine their suitability as solid-
only crystalline 2d showed intense fluorescence (F = 13%). state emitters in the red to near-infrared regions. Candidates
Strong p–p interactions between naphthol units might be the with highly promising properties were found, such as 3a and
origin for the insignificant fluorescence of crystalline 2a, 3c, with red fluorescence and high quantum yields in the
despite its favorable J-aggregate slip-packing structure crystal state (F = 30% and F = 48%, respectively). X-ray diffrac-
(Fig. S17, ESI†). p–p interactions have already been suggested tion analysis allowed us to rationalize the influence of sub-
to be the origin for the weak solid-state fluorescence in poly- stituents on intra- and inter-molecular interactions on the
aromatic systems such as porphyrines.40–42 The observed pack- observed spectral properties. The push/pull character of such
ing modes for other variants in this series were more difficult to chalcones needed to be promoted with para-amino groups in
correlate with the observed spectral properties. Candidates 2b the donor portion as they tend to adopt favorable J aggregates
and 2e were almost non-emissive (F = 4% and F o 1%, with a Herringbone 3D arrangement, both known to promote
respectively), but their crystal lattice exhibited highly favorable radiative decay. On the other hand, replacing such amino
HB 3D arrangements with J aggregates (Fig. S18, ESI†). On the substituents with alkoxy donors suppresses the emission. Gen-
other hand, 2c and 2d showed a priori unfavorable H aggrega- erally, the degree of planarity correlated well with the emission
tion (Fig. S19, ESI†); however, while 2c was only weakly fluor- intensity, as demonstrated in the variant series 1 and 3.
escent (F = 5%), 2d was a good contender (F = 13% at 690 nm). Unfortunately, the strategy to extend the p system via benzo
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annellation of the phenol unit (20-hydroxynaphtol series 2) did
not deliver the promised benefits in red-shifting and likewise
furnished disappointing emission intensities. As another devia-
tion from the rule, the p stacking between polyaromatic cycles
in crystalline 3a did not cause notable quenching of its fluores-
cence. As XRD analysis could not explain all observed trends,
molecular modeling might contribute to rationalize the photo-
physical properties of such compounds. In conclusion, mole-
cular conformation and crystal packing of 20-hydroxychalcone
derivatives could be strongly tuned by simple chemical mod-
ifications. We hope that this study will help in future efforts to
rationally design solid-state molecular fluorophores.
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