High Fluorescence Efficiencies and Large Stokes Shifts of Folded Fluorophores Consisting of a Pair of Alkenyl-Tethered, π-Stacked Oligo-p-phenylenes

Article abstract of DOI:10.1021/acs.orglett.5b03152

A series of pure hydrocarbon fluorophores containing a pair of π-stacked oligo-p-phenylenes have been synthesized and analyzed by NMR and X-ray crystallography. They show good fluorescence in solutions and enhanced fluorescence in the aggregated state. Large Stokes shifts (up to 214 nm) have been achieved in these folded fluorophores in virtue of intramolecular energy transfer, and balanced structural rigidity and flexibility. These folded fluorophores provide perfect models for understanding the energy and charge transfer process in π-stacked systems.

Full text of DOI:10.1021/acs.orglett.5b03152

Letter
pubs.acs.org/OrgLett
High Fluorescence Efficiencies and Large Stokes Shifts of Folded
Fluorophores Consisting of a Pair of Alkenyl-Tethered, π‑Stacked
Oligo‑p‑phenylenes
Bairong He,† Han Nie,† Long Chen,† Xiaoding Lou,§ Rongrong Hu,† Anjun Qin,† Zujin Zhao,*,†
and Ben Zhong Tang*,†,‡,∥
†State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China
‡Department of Chemistry, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China
§School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China
∥Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, Hong Kong,
China
S
* Supporting Information
ABSTRACT: A series of pure hydrocarbon fluorophores containing a pair of π-
stacked oligo-p-phenylenes have been synthesized and analyzed by NMR and X-
ray crystallography. They show good fluorescence in solutions and enhanced
fluorescence in the aggregated state. Large Stokes shifts (up to 214 nm) have been
achieved in these folded fluorophores in virtue of intramolecular energy transfer,
and balanced structural rigidity and flexibility. These folded fluorophores provide
perfect models for understanding the energy and charge transfer process in π-
stacked systems.
a series of folded pure hydrocarbon fluorophores comprising a
pair of alkenyl-bridged, π-stacked oligo-p-phenylenes. Large
Stokes shifts and high fluorescence efficiencies are attained in
these fluorophores. By conducting a comparative study on
individual oligo-p-phenylenes and the folded archetypes, (Z)-o-
BPTPE and (Z)-o-BBPTPE (Scheme 1), the intramolecular
igh fluorescence efficiency and large Stokes shifts are
Himportant parameters for fluorescent dyes, and large
Stokes shifts are highly preferred to diminish self-absorption and
light scattering in optical materials, such as fluorescent probes,1
organic light-emitting diodes,2 organic lasers,3 etc. However, in
most cases, high fluorescence efficiency and large Stokes shifts
are not compatible, especially for conventional dyes, such as
BODIPY,4 coumarin,5 rhodamine,6 and fluorescein.7 They show
strong fluorescence in dilute solutions but become weakly
fluorescent in concentrated solutions or in the solid state, which
is partially caused by the severe self-absorption among closely
located dye molecules due to their intrinsic small Stokes shifts.
The trials of enlarging Stokes shifts are always present,
commonly by virtue of intramolecular charge/proton transfer or
excimer/exciplex formation.8 However, these approaches cannot
fully meet practical demands in many cases, exhibiting variations
that are difficult to control. For example, excited state
intramolecular proton transfer (ESIPT) is often accompanied
by a large Stokes shift but is highly sensitive to surrounding
conditions, such as pH or solvent.9 Strong intramolecular charge
transfer (ICT) in high-polar media can prolong the Stokes shifts
but can also drastically depress the fluorescence.10 Therefore, it
remains a challenging task to prepare fluorophores with both
high fluorescence efficiencies and large Stokes shifts, particularly
pure hydrocarbon systems containing no heteroatoms to
function in ESIPT and ICT. To address this issue, we elaborate
Scheme 1. Chemical Structures and Syntheses of the Folded
Fluorophores
Received: November 3, 2015
© XXXX American Chemical Society
A
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Figure 1. Crystal structures of folded fluorophores with indicated distances (Å) between π-stacked phenyl rings. Hydrogen atoms are omitted for clarity.
energy transfer and the balance of structural rigidity and
flexibility are demonstrated to be responsible for both merits,
that is, large Stokes shifts and high fluorescence efficiency.
The folded fluorophores were readily synthesized in moderate
yields by McMurry coupling of oligo-p-phenylenes carrying a
benzoyl group in the presence of TiCl4 and zinc dust (Scheme 1).
Whereas most McMurry couplings were non-stereospecific, the
present reaction proceeded stereoselectively, affording products
with a cis-conformation after forming an alkenyl group from two
carbonyl groups. The possible mechanism was described in our
previous work.11 These folded fluorophores show distinctive
doublet peaks at ∼ 6.0 ppm, assigned to the protons on the π-
stacked phenyl rings, which are notably upfield-shifted compared
to those of common aromatic protons, due to the shielding effect
from another π-stacked phenyl ring.11b,12 Examples of NMR
spectra of f-3Ph, f-3Ph(Me), f-4Ph, and f-4Ph(Me) are displayed
in Figure S1 in Supporting Information.
Single crystals of f-3Ph, f-3Ph(Me), f-4Ph, f-4Ph(Me), and
(Z)-o-BBPTPE11b grown from THF/methanol or dichloro-
methane/hexane mixtures were analyzed by X-ray diffraction
crystallography. Detailed parameters for the geometries of the
crystal structures are given in Figures S2−S6. As displayed in
Figure 1, the crystal structures validate a folded cis-conformation,
where two oligo-p-phenylene chains, linked with an alkenyl
bridge, are located closely in a roughly parallel manner. The
shortest distances between slightly slipped, face-to-face phenyl
rings are 3.16−3.46 Å, which are shorter than the typical distance
for a π−π stacking interaction (3.5 Å), indicative of efficient
intramolecular through-space conjugation13 in these folded
fluorophores. However, a longer oligo-p-phenylene chain does
not mean a stronger π−π stacking interaction, as the longer oligo-
p-phenylene chain becomes more twisted at both ends, making it
difficult to form a regular π−π stacking between phenyl rings.
This finding is consistent with the packing manner of linear oligo-
p-phenylenes in the crystalline state.14
These folded fluorophores show absorption peaks at 275−308
nm in THF, which gradually move to the long-wavelength region
with the increase of chain length (Figure 2). For instance, the
absorption peak of f-3Ph is located at 275 nm, while that of f-5Ph
shifts to 302 nm, equal to a red shift of 27 nm. The substitution of
methyl groups on oligo-p-phenylene chains further pushes the
absorption spectra to longer wavelengths. The absorption peaks
of these folded fluorophores are consistent with the absorption of
linear oligo-p-phenylenes, such as p-terphenyl (3Ph, 281 nm)
and p-quaterphenyl (4Ph, 299 nm) (Figure S8, Table S1),
demonstrating that the peaks are associated with the absorption
of oligo-p-phenylene chains rather than the alkenyl-bridged
entire molecular backbone.
Contrary to the chain-length sensitive absorption property, the
photoluminescence (PL) peaks of the folded fluorophores in
solutions are much more stable. Their PL maxima vary slightly in
the range of 489−498 nm, which are very close to that of the
archetypal folded fluorophore (Z)-o-BPTPE (493 nm)11b but
extremely longer than those of linear oligo-p-phenylenes, such as
B
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Figure 2. Normalized absorption and photoluminescence spectra of the
folded fluorophores in THF solution (10−5 M).
Figure 3. Photoluminescence spectra of f-4Ph(Me) in THF−water
mixtures with different water fractions (fw), excited at 330 nm. Inset:
Photos of f-4Ph(Me) in THF−water mixtures (fw = 0 and 90%), taken
under the illumination of a UV lamp (365 nm).
3Ph (340 nm) and 4Ph (368 nm). This implies that the
emissions originate from the central folded part (alkenyl-linked
two biphenyl fragments) rather than from the entire
fluorophores or the oligo-p-phenylene moieties. Hence, an
interesting photophysical process occurs where the oligo-p-
phenylene chains actually function as antennas, which harvest
and transfer photonic energy to the central alkenyl-linked part for
light emission. These fluorophores show only slight PL variations
in nonpolar and high-polar solvents (Figure S9), showing the
absence of an ICT effect.
These folded fluorophores show good PL emissions in THF,
with high absolute fluorescence quantum yields (ΦF) of 30−38%
(Table 1). Their fluorescence lifetimes in THF are 6.5−8.6 ns
(Table S2, Figure S10), which fall within the fluorescence region,
revealing that the PL emissions originate from the radiative decay
of the singlet excited state. The efficient PL emissions in THF of
these folded fluorophores are really interesting because most
tetraarylethenes are almost nonfluorescent in solutions with
much shorter fluorescence lifetimes because the severe intra-
molecular rotation of aromatic rings nonradiatively deactivates
the excited state.15 The folded conformation should have greatly
improved the molecular rigidity, and the rotational or torsional
motions of the oligo-p-phenylene chains are suppressed
significantly. Therefore, the nonradiative decay of the excited
state is blocked, enabling molecules to fluoresce strongly in
solution.
insoluble in water, aggregates of f-4Ph(Me) are formed in the
aqueous environment. The PL enhancement occurs along with
the aggregate formation, demonstrating the aggregation-
enhanced emission (AEE) attribute. f-4Ph(Me) in the solid
state is highly emissive, and its ΦF increases to 58% in solid film,
being nearly 2-fold higher than that in THF (30%), which further
verifies its AEE feature. This is attributed to the two unstacked
phenyl rings attached on the alkenyl group, which are free to
rotate in solution and nonradiatively deplete part of the excited
state energy. In aggregates, the rotation of the unstacked phenyl
rings is restricted by the spatial constraint, and the nonradiative
excited state decay is further depressed, rendering fluorophores
even more emissive.
The aggregate formation gives rise to blue-shifted PL spectra,
and varied PL peaks are observed for the folded fluorophores in
different aggregate states (Table 1, Figure S12). For example, the
PL peak of f-4Ph(Me) is blue-shifted from THF solution (498
nm) to amorphous film (478 nm) and then to crystals (463 nm).
This phenomenon should result from a more rigid molecular
structure in the aggregated state, particularly in the crystalline
state, in which the reorganization energy is lowered, leading to a
decreased Stokes shift and, thus, blue-shifted emission.16
Notably, these folded fluorophores exhibit very large Stokes
shifts in THF. As indicated in Figure 2, the differences between
the absorption and emission peaks are in the range of 190−214
nm, which are rarely observed for pure hydrocarbon
fluorophores. For instance, the Stokes shifts of linear oligo-p-
These folded fluorophores give enhanced emissions in the
aggregated state (Figure S11). For example, the PL emission of f-
4Ph(Me) intensifies with the increase of water fraction in THF/
water mixtures (Figure 3). Since the hydrophobic f-4Ph(Me) is
Table 1. Photophysical Properties of Folded Fluorophores
c
λabs (nm)
λem (nm)
Δ(λem−λabs) (nm)
ΦF (%)
a
a
b
a
a
b
d
compound
soln
soln
film
crystal
soln
soln
film
⟨τ⟩ (ns)
(Z)-o-BBPTPE
f-3Ph
280
275
282
291
295
302
308
492
489
492
494
498
496
498
472
471
470
478
478
471
470
454
464
451
463
463
212
214
210
203
203
194
190
32
30
32
38
30
32
30
61
35
37
57
58
52
67
7.0
6.5
8.0
7.9
8.6
8.2
8.5
f-3Ph(Me)
f-4Ph
f-4Ph(Me)
f-5Ph
f-5Ph(Me)
a
b
c
In THF solution (10−5 M). Film drop-casted on a quartz plate. Absolute fluorescence quantum yield measured by an integrating sphere.
d
Fluorescence lifetime determined in THF solution at room temperature in air.
C
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phenylenes in the literature are <90 nm,13 and those of 3Ph and
4Ph measured by us are merely 59 and 69 nm, respectively. Since
the structural rigidity is improved in the aggregated state, the
reorganization energy is decreased as discussed above, which
diminishes the Stokes shifts (Table 1). Therefore, it becomes
clear that, in THF, after the energy transfer to the central alkenyl-
linked fragment, the rotational motion of the unstacked phenyl
rings on alkenyl group consumes part of the energy, that is,
enhanced reorganization energy, which red shifts the PL
emissions, and leads to large Stokes shifts. Therefore, the freely
rotatable unstacked phenyl rings should be crucial to the large
Stokes shifts of these folded fluorophores. This is also evidenced
by the much smaller Stokes shift (113 nm) of a folded terphenyl
dimer without freely rotatable phenyl rings.17 Hence, a balance
between the rigidity from the π-stacked oligo-p-phenylenes and
the flexibility from the unstacked phenyl rings accounts for the
high fluorescence efficiency and large Stokes shift of these folded
fluorophores.
In conclusion, a series of intriguing folded fluorophores
composed of a pair of π-stacked oligo-p-phenylenes tethered with
an alkenyl group have been readily synthesized and characterized.
They showed good fluorescence in solutions and further
enhanced fluorescence in aggregates (AEE). Meanwhile,
remarkably large Stokes shifts of up to 214 nm were obtained.
These interesting photophysical properties were realized by the
energy transfer from absorptive oligo-p-phenylene chains to the
central alkenyl-linked two biphenyl fragments and the balance
between structural rigidity and flexibility of the fluorophores. The
present folded fluorophores are excellent models for the study of
hydrocarbon fluorophores with both high fluorescence efficiency
and large Stokes shifts.
(201101C0105067115), the ITC-CNERC14S01 and the
Fundamental Research Funds for the Central Universities
(2015PT020 and 2015ZY013).
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.5b03152.
Synthesis information, single-crystal structures of f-3Ph, f-
3Ph(Me), f-4Ph, and f-4Ph(Me), PL spectra of folded
fluorophores in different states and different solvents,
absorption and PL spectra of 3Ph and 4Ph, fluorescence
decay curves, X-ray crystallography information, and parts
of photophysical data (PDF)
X-ray data of (Z)-o-BBPTPE (CIF)
X-ray data of f-3Ph(Me) (CIF)
X-ray data of f-4Ph(Me) (CIF)
X-ray data of f-3Ph (CIF)
X-ray data of f-4Ph (CIF)
(14) Banerjee, M.; Shukla, R.; Rathore, R. J. Am. Chem. Soc. 2009, 131,
1780.
AUTHOR INFORMATION
Corresponding Authors
■
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23726. (b) Hong, Y.; Lam, J. W. Y.; Tang, B. Z. Chem. Soc. Rev. 2011, 40,
5361.
(16) Wu, Q.; Zhang, T.; Peng, Q.; Wang, D.; Shuai, Z. Phys. Chem.
Chem. Phys. 2014, 16, 5545.
*E-mail: mszjzhao@scut.edu.cn.
*E-mail: tangbenz@ust.hk.
Notes
(17) Nehls, B. S.; Galbrecht, F.; Bilge, A.; Brauer, D. J.; Lehmann, C.
W.; Scherf, U.; Farrell, T. Org. Biomol. Chem. 2005, 3, 3213.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge the financial support from the National Natural
Science Foundation of China (51273053), the 973 Program
(2013CB834702), the Guangdong Natural Science Funds for
Distinguished Young Scholar (2014A030306035), the Guang-
d o n g I n n o v a t i v e R e s e a r c h T e a m P r o g r a m
D
DOI: 10.1021/acs.orglett.5b03152
Org. Lett. XXXX, XXX, XXX−XXX