Hydrogen Bonding-Induced H-Aggregation for Fluorescence Turn-On of the GFP Chromophore: Supramolecular Structural Rigidity

Article abstract of DOI:10.1002/chem.202000358

To turn on the fluorescence of the native green fluorescence protein (GFP) chromophore, 4-hydroxybenzylidene-dimethylimidazolinone (HBDI), in an artificial supramolecular system has been a challenging task, because it requires high local environmental rigidity. This work shows that the formation of H-aggregates of an HBDI-containing organogelator results in two orders of magnitude fluorescence enhancement (Φ_f=2.9 vs. 0.02 %), in which the inter-HBDI OH???OH H-bonds play a crucial role. The aggregation-induced fluorescence enhancement of HBDI has important implications on the origin of the high fluorescence quantum efficiency of HBDI in the GFP β-barrel and on the supramolecular strategy for a full fluorescence recovery of HBDI. These results reveal a new approach to designing rigid chromophore aggregates for high-performance optoelectronic properties.

Full text of DOI:10.1002/chem.202000358

A Journal of
Accepted Article
Title: Hydrogen Bonding-Induced H-Aggregation for Fluorescence
Turn-On of the GFP Chromophore: Supramolecular Structural
Rigidity
Authors: Meng-Shiue Tsai, Sung-Yu Tsai, Yi-Fan Huang, Chien-Lung
Wang, Shih-Sheng Sun, and Jye-Shane Yang
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To be cited as: Chem. Eur. J. 10.1002/chem.202000358
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Hydrogen Bonding-Induced H-Aggregation for Fluorescence
Turn-On of the GFP Chromophore: Supramolecular Structural
Rigidity
Meng-Shiue Tsai,[a][b] Sung-Yu Tsai,[c] Yi-Fan Huang,[c] Chien-Lung Wang,[c] Shih-Sheng Sun,*[b] and
Jye-Shane Yang*[a]
Abstract: To turn on the fluorescence of the native green
fluorescence protein (GFP) chromophore HBDI in an artificial
supramolecular system has been a challenging task, because it
requires high local environmental rigidity. This work shows that the
formation of H-aggregates of an HBDI-containing organogelator
results in two orders of magnitude fluorescence enhancement (f =
2.9 vs 0.02%), in which the inter-HBDI OH∙∙∙OH H-bonds play a crucial
role. The aggregation-induced fluorescence enhancement of HBDI
has important implications on the origin of the high fluorescence
quantum efficiency of HBDI in the GFP -barrel and on the
supramolecular strategy for a full fluorescence recovery of HBDI.
These results reveal a new approach to designing rigid chromophore
aggregates for high-performance optoelectronic properties.
studies6,7 have shown that the fluorescence activity is much lower
for HBDI relative to its structural analogues, revealing an inherent
challenge for the recovery of its fluorescence. Indeed, previous
attempts7,8 to turn on the fluorescence of HBDI in host-guest
systems have found limited success (≤ 10-fold enhancement),
indicating insufficient microenvironmental rigidity (Figure 1b).
In an effort to test whether the rigidity of self-assembled
nanofibers could turn on the fluorescence of HBDI, we have
designed compound 1 that consists of the HBDI chromophore and
the pyridyldiamide (PDA, Figure 1a) group. Several PDA-arene
hybrids form gels in organic solvents with fibrillar morphology, in
which the  interactions of the arene moiety and the
intermolecular H-bonding between the PDA groups are involved.9
The results show that 1 forms two types of organogels with distinct
fluorescence activities (Figure 1b). The fluorescent gels possess
a fibrillar network of H-type aggregation and the fluorescence of
HBDI is enhanced by two orders of magnitude (f = 2.9 vs 0.02%).
The occurrence of a spontaneous supramolecular polymorphic
transition and fluorescence on-off switching in a medium H-bond-
accepting solvent reveals the H-bonding effects on the observed
fluorescence activity. With the O-methylated reference compound
2, the critical role of the inter-HBDI OH∙∙∙OH H-bonding in the
fluorescence turn-on of 1 is manifested.
Supramolecular chromophore aggregation is an important
subject in both fundamental and applied chemistry, because
different molecular packing modes (e.g., H- vs J-aggregates)
deliver distinct photophysical properties.1 Among the various
aspects of chromophore aggregates reported to date,2-4 the
fundamental issue about local structural rigidity is much less
explored. Herein, we report an approach to investigate the local
structural rigidity of chromophore aggregates with a built-in
fluorescent probe of the native GFP chromophore.
The native GFP chromophore, 4-hydroxybenzylidene-
dimethylimidazolinone (HBDI, Figure 1a), is an ultrasensitive
probe of environmental rigidity. While HBDI displays high
fluorescence quantum efficiency (f ~ 80%) when embedded
inside the -barrel of GFP matrix, it becomes nearly
nonfluorescent (f ~ 0.01%) in fluid solutions as a result of the
near barrierless torsional motions in the excited state.5 Many
[a]
[b]
[c]
Dr. M.-S. Tsai and Prof. Dr. J.-S. Yang
Department of Chemistry
National Taiwan University
No 1, Sec 4, Roosevelt Rd, Taipei, Taiwan 10617
E-mail: jsyang@ntu.edu.tw
Dr. M.-S. Tsai and Prof. Dr. S.-S. Sun
Institute of Chemistry
Academia Sinica
No. 128, Sec. 2, Academia Rd., Nankang, Taipei, Taiwan 11529
E-mail: sssun@chem.sinica.edu.tw
Mr. S.-Y. Tsai, Ms. Y.-F. Huang and Prof. Dr. C.-L. Wang
Department of Applied Chemistry
National Chiao-Tung University
Figure 1. (a) Structures of HBDI, 1, 2, and the precursors 1-N3, 2-N3, and EPDA
and (b) schematic representations of supramolecular effects on the
fluorescence of HBDI.
No. 1001, University Rd, Hsinchu, Taiwan 30010
Supporting information for this article is given via a link at the end of
the document.
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Compounds 1 and 2 were prepared by the click reactions
between the ethynyl PDA (EPDA) and the HBDI derivatives 1-N3
and 2-N3, respectively (Figure 1a). Details of the synthesis
(Scheme S1) are supplied in the Supporting Information.
distinct fluorescence activity of these two types of gels (denoted
as 1Gsolvent) are called fl-ON and fl-OFF gels, respectively
(Figures 2a and S5). Interestingly, the fluorescence activity of the
organogels formed in 1,2-dichloroethane (DCE) depends on the
cooling rate: a fast-cooling rate (45 deg min-1) resulted in a fl-ON
gel (1GDCE-F) and a slow cooling rate (10 deg min-1) afforded a fl-
OFF gel (1GDCE-S). Remarkably, a spontaneous transformation of
In dilute solutions (10 M), the fluorescence of 1 in good
solvents such as THF is as weak as that of HBDI (f = 0.02%),
but the fluorescence is largely enhanced in poor solvents such as
cyclohexane (f = 1.43%). The absorption profile is broadened
and blue-shifted (25 nm) in cyclohexane vs THF (Figure S1),
revealing the formation of H-aggregates. Indeed, fibrillar
aggregates of 1 were detected by SEM (Figure S2), and DLS
measurements show a multi-domain distribution (6, 100, and
2000 nm) of the particle size (Figure S3). The red-shifted
fluorescence in cyclohexane (546 nm) relative to THF (438 nm) is
also consistent with aggregate emission. Notably, aggregates are
also formed in mixed H2O/THF, but they show negligible
fluorescence enhancement (Figure S4), indicating a different
structure from that in cyclohexane.
The fluorescence of 1 is further enhanced (f = 1.7−2.9%)
at higher concentration in benzene (Ben), toluene (Tol), and p-
xylene (Xyl) when organogels are formed by a heating-cooling
cycle. While 1 also forms organogels in butanone (But), dioxane
(Diox), and 1,1,2,2,-tetrachloroethane (TCE), the fluorescence
quantum yields are lower than 0.1% (f = 0.01−0.06%). The
1GDCE- → 1GDCE-S occurred over a time span of 160 min (Figure
F
2b), indicating that the former is a metastable assembly and its
formation is under kinetic control. Besides optical properties, the
1GDCE-
→ 1GDCE- transformation involves a change in
S
F
morphology from entangled fibrils to amorphous islands (Figure
S6), attributable to the collapse and merging of the fibrils. The
morphological features of the other fl-ON and fl-OFF gels are
similar to those of 1GDCE- and 1GDCE-S, respectively (Figure S7),
F
showing a correlation between morphology and fluorescence
activity. Note that the effect of cooling rate on f was observed
exclusively for 1 in DCE among the solvents investigated. Also
note that the critical gelation concentrations (CGC) is in the range
2.8−13.5 mM (Table S1), and a dependence of f on the
concentration of 1 occurs at conditions both below and above the
CGC (Figure 2c). The concentration also affects the absorption
profiles, but the outcome is different for the fl-ON and fl-OFF gels,
as represented by 1GTol and 1GTCE (Figure 2d). Upon increasing
the concentration of 1 in toluene, the absorption maximum shifts
from 359 to 344 nm, indicating the formation of H-aggregates, as
is the case in cyclohexane (vide supra). In general, H-aggregates
are nonemissive,1 because the optical transition is forbidden (i.e.,
a low emission rate constant). However, examples of fluorescent
H-aggregates are known.10,11 Indeed, the presence of the 410-nm
absorption shoulder for 1GTol indicates a nonvanishing transition
probability, attributable to vibronic coupling or a small rotational
twist.10 In contrast, there is little spectral shift but broadening for 1
in TCE upon increasing the concentration, indicating an ill-defined
packing mode for the aggregates.
The comparison of IR spectra for the xerogels of 1 (denoted
as 1XGsolvent) prepared from the corresponding gels and for the
reference compounds HBDI and PDA-C12 in THF and in solid
forms reveals distinct H-bonding characters for the fl-ON and fl-
OFF gels (Figure 3). In the lack of H-bonding with the carbonyl
groups (i.e., HBDI and PDA-C12 in THF), the HBDI C=O is 1712
cm-1 and the PDA-C12 C=O is 1684 cm-1. The shift of C=O to lower
frequency for HBDI in solids (1677 cm-1) is consistent with the
presence of O−H∙∙∙O=C H-bonds in single crystals,12 and thus the
corresponding H-bonded O−H is 3125 cm-1. In contrast, the
unchanged C=O for PDA-C12 on going from THF to solid denotes
the absence of intermolecular H-bonding between the amide C=O
and N−H (3375 cm-1) groups. Accordingly, the observations of (i)
well-resolved PDA N−H at ~3370 cm-1 for the fl-OFF but not for
the fl-ON gels, (ii) lower PDA C=O (~1652 vs ~1675 cm-1) in the fl-
ON vs fl-OFF gels, and (iii) higher HBDI C=O (~1709 vs ~1693 cm-
1) for the fl-ON vs fl-OFF gels indicate that the PDA amides are
H-bonded but the HBDI C=O is non-H-bonded in the fl-ON gels,
and, on going from the fl-ON to the fl-OFF gels, the H-bonding is
weakened for the PDA amides but enhanced for the HBDI
carbonyl. This conclusion is depicted in the inset of Figure 3.
The importance of H-bonding interactions for gelation and
thus the fluorescence activity is further confirmed by the
Figure 2. Preparation and optical properties of the fl-ON and fl-OFF gels of 1:
(a) effects of solvent and cooling rate on the fluorescence activity of the gels,
(b) time-dependent fluorescence spectra (ex = 380 nm) for the transition of
1GDCE-F → 1GDCE-S, (c) f against concentration plots of the fl-ON gels, and (d)
concentration-dependent absorption spectra of 1 in toluene and TCE.
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Figure 4. SAXS and WAXS experiments of (a) 1XGTol and (b) 1XGTCE and (c)
the thus-deduced supramolecular structures.
shorter d-spacing (Figure 4c). Moreover, for the fl-ON 1XGTol the
d-spacings at ~20, ~15, and ~10 Å reasonably fit with a tetragonal
system (1: 1/√2 : 1/2). In contrast, the absence of the higher order
diffraction peaks but only a broad amorphous halo around 1.5 Å -1
for 1XGTCE indicate that the fl-OFF gels contain molecules that
pack into a mesophase that includes more structural disorder than
the fl-ON gels. The SAXS and WAXS patterns of the fl-ON
1XGDCE-F lie in-between the above two cases, indicating a mixture
of the two supramolecular polymorphs (Figure S11).
Figure 3. FTIR spectra of HBDI and PDA-C12 in THF (10 mM) and in solids
and 1 in xerogels, where the HBDI and PDA C=O vibrational peaks (cm-1) are
highlighted with red and blue background, respectively. The marks * and #
denote the peaks participated in H-bonding with adjacent molecules and THF
solvent, respectively. Insects are the structure of PDA-C12 and the proposed
H-bonding modes for the fl-ON vs fl-OFF xerogels
observation of degelation and fluorescence quenching in
response to external stimuli that interrupts the H-bonds for 1GTol
.
Implications of our results are at least twofold. First, the H-
aggregate structure has a high local rigidity for the fluorescence
turn-on of the HBDI chromophore. This might result from the
presence of intermolecular H-bonds at both ends of the
chromophore: the PDA amide N−H∙∙∙O=C H-bonding interactions
at one end and the inter-HBDI O−H∙∙∙O−H H-bonds at the other
end. When these H-bonds are weakened or interrupted by H-
bond-accepting solvent molecules such as TCE, butanone, and
dioxane, the conformational confinement of the HBDI is released,
leading to fluorescence quenching (i.e., the fl-OFF gels).
Regarding that the exact mechanism of conformational
confinement for HBDI in the GFP -barrel remains an open
question,5 our results indicate that the H-bonds associated with
the HBDI OH group in the -barrel should play an important role.
Another implication of our results is about a supramolecular
strategy for a full fluorescence recovery of HBDI. The host-guest
approach that mimics the conformational confinement of a single
HBDI chromophore in the -barrel of GFP has been shown to be
ineffective.7,8 The current approach of chromophore aggregation
that achieves a fluorescence turn-on up to 2.9% of quantum
efficiency is, to the best of our knowledge, the highest to date for
HBDI in a supramolecular system other than the -barrel. This is
particuarly encouraging, given that the crystalline HBDI is
expected to have a high local environmental rigidity but turns out
to be nonfluorescent.13 The fluorescence quenching in the HBDI
The stimuli includes amine base such as pyridine or
diisopropylamine (Figure S8) and heating. Note that heating from
298 to 368 K also reverses the characteristics of the absorption
spectra from aggregates back to the monomer form (Figure S9).
The critical role of the HBDI hydroxyl group in the
aggregate/gel structure and thus the fluorescence turn-on
property of 1 is evidenced by the comparison with 2 (Figure S5).
While 2 also forms aggregates in cyclohexane, the particle size is
smaller and more uniform (30-40 nm), and the degree of
absorption blue-shift (9 nm) and fluorescence turn-on (f ~
0.08%) is much smaller (Figures S2-3). In addition, the solvents
for gelation of 2 are more limited (i.e., in Ben, Tol, and Xyl only)
and it requires a higher CGC in the same solvent (Table S1).
Moreover, all the 2-based gels are essentially nonfluorescent (f
≤ 0.06%), and the island-like morphology resembles the case of
the fl-OFF gels of 1 (Figure S10).
The small- and wide-angle X-ray scattering (SAXS and
WAXS) experiments provide further structural information. As
shown in Figures 4a and 4b, the d-spacing of the first diffraction
peak of 1XGTol is ~51.5 Å, whereas that of 1XGTCE is ~38.1 Å. In
conjunction with the above IR and electronic spectral information,
a columnar H-aggregate molecular packing assisted by head-to-
head inter-HBDI OH∙∙∙OH H-bonding is proposed for the fl-ON
gels to explain the larger d-spacing, while a loose head-to-tail
HBDI -stacking is proposed for the fl-OFF gels to explain the
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In summary, we have shown that supramolecular
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outside the GFP  barrel has been achieved with the formation of
a rigid H-aggregate motif. The concept of structural rigidity
engineering toward high-performance chromophore aggregates
might prove of value in the design of new materials for
optoelectronic applications.
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Acknowledgements
We thank the Ministry of Science and Technology (MOST 107-
2113-M-002-022-MY3 and MOST 106-2113-M-001-014-MY3) of
Taiwan, Academia Sinica (AS), and National Taiwan University
(108L880101 and 108L880118) for financial support. We would
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Keywords: Supramolecular chemistry • Aggregation •
Fluorescence • Gels • Hydrogen bonds
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Table of Contents
COMMUNICATION
A Meng-Shiue Tsai, Sung-Yu Tsai, Yi-
Fan Huang, Chien-Lung Wang, Shih-
Sheng Sun,* and Jye-Shane Yang*
Page No. – Page No.
Hydrogen Bonding-Induced H-
Aggregation for Fluorescence Turn-
On of the GFP Chromophore:
The formation of H-aggregates for an HBDI-containing organogelator enhances the
fluorescence quantum efficiency by two orders of magnitude, a new record for HBDI
in an artificial supramolecular system.
Supramolecular Structural Rigidity
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