New Aggregation-Induced Emitters: Tetraphenyldistyrylbenzenes

Article abstract of DOI:10.1002/chem.201502877

The synthesis of five novel distyrylbenzene (DSB) derivatives, featuring a central tetraphenylbenzene core, is reported. The targets show aggregation-induced emission (AIE), which, however, is substituent-dependent. For the pure hydrocarbon and derivative

Full text of DOI:10.1002/chem.201502877

DOI: 10.1002/chem.201502877
Communication
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Photochemistry |Hot Paper|
New Aggregation-Induced Emitters: Tetraphenyldistyrylbenzenes
Jan Freudenberg, Frank Rominger, and Uwe H. F. Bunz*[a]
unlike their diphenylated analogues,[8] reveal AIE properties.
We discuss the influence of the substituents on the AIE proper-
ties of these DSBs as well as their photostability. Compounds
of this type undergo photocyclization, which is not limited to
cyclization in solution.
Abstract: The synthesis of five novel distyrylbenzene
(DSB) derivatives, featuring a central tetraphenylbenzene
core, is reported. The targets show aggregation-induced
emission (AIE), which, however, is substituent-dependent.
For the pure hydrocarbon and derivatives that do not
carry (+M) or (ÀM) substituents, classic AIE behavior is
observed, that is, the DSBs are non-fluorescent in solution,
but are highly fluorescent in cold matrices, upon aggre-
gate formation in poor solvents and in the solid, crystal-
line state. If aldehyde or dibutylamino groups are attached
in the para-position of the DSB unit, non-classic AIE-
phores result. These are fluorescent both in dilute solution
as well as in the solid state. Prolonged irradiation of the
targets leads to benzotetraphene derivatives by a double
cyclization.
The synthetic approach is shown in Scheme 1. Starting from
the dimeric 1, Diels–Alder reaction with tolane affords the hex-
asubstituted benzene derivative 2. Radical bromination fur-
nishes the bis(bromomethyl)benzene 3,[9] which is heated in
Herein, we describe the synthesis and properties of five new
aggregation-induced fluorophores and their photochemical
conversion.
Aggregation-induced emission (AIE) is now an important
concept in organic and materials chemistry, first proposed by
Tang et al.[1] Interruption of rotational freedom of multiple
phenyl groups (restriction of intramolecular rotation) or restric-
tion of intramolecular vibration suppresses radiationless excit-
ed state deactivation and leads to a turn on of the fluores-
cence in either poor solvents or in the solid state. AIE fluoro-
phores are generally non- or only weakly fluorescent in solu-
tion, but very fluorescent in the solid state. A series of different
fluorophore types shows this behavior, the most well-known
of which is hexaphenylsilole,[2] but also tetraphenylethylene is
a good example.[3] AIE is used in different sensors,[4] including
biosensory applications[5] and tribological schemes.
Scheme 1. Synthesis of 5a–e. 4-(Diethoxymethyl)benzaldehyde was em-
ployed for the synthesis of 5d.
triethylphosphite to give the bisphosphonate 4. This central
core 4 is then reacted with a series of aryl aldehydes in the
presence of KOtBu. The targeted distyrylbenzenes 5a–e are
isolated in yields between 44–92% as crystalline materials after
chromatography and crystallization.
Although some distyrylbenzene (DSB) derivatives are known
to exhibit AIE characteristics,[6] to the best of our knowledge,
however, di(styryl)tetraphenylbenzenes have never been inves-
tigated before. Expanding our portfolio of DSB fluorophores,[7]
we describe the synthesis and property evaluation of a series
of five tetraphenylated distyrylbenzene derivatives, which,
Compounds 5a–c display a very low fluorescence quantum
yield in THF, but are brightly emissive in the solid state
(Figure 1, Table 1). Upon attachment of electronically active
substituents featuring free electron pairs, that is, aldehyde or
dibutylamino groups, the fluorescence in solution increases
significantly. Hence, the DSB 5e behaves like a “conventional”
fluorophore, in which the emissive quantum yield is higher
in solution than in the solid state. The quantum yield of nor-
phenyl 5e is 0.60 in acetonitrile[10] and 0.88 in toluene,[11] sug-
gesting that the phenyl groups in 5e still have a negative
[a] J. Freudenberg, Dr. F. Rominger, Prof. U. H. F. Bunz
Organisch-Chemisches Institut
Ruprecht-Karls-Universität
Im Neuenheimer Feld 270, 69120 Heidelberg (Germany)
Fax: (+49)6221-54-8401
E-mail: uwe.bunz@oci.uni-heidelberg.de
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201502877.
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Figure 1. UV/Vis and emission spectra of 5a–5e in solution (THF) and in the
solid state (thin-film).
Figure 2. Top: Temperature-dependent emission spectra of 5b in 2-MeTHF
(Abs298K =0.1, I298K =0.01). Bottom: Emission spectra of solutions or suspen-
sions of molecules or aggregates of 5b in THF/water mixtures (3 mm) with
different volumetric fractions of water (fwater).
Table 1. Photophysical properties of compounds 5a–e.[a]
lmax, abs
lmax, em
Stokes shift
[cmÀ1
e
QY
[%]
[nm]
[nm]
]
[mÀ1 cmÀ1
]
5a
THF
film
5b
THF
film
5c
THF
film
5d
THF
film
5e
333
336
425
429
6501
6425
36590
–
<1
60
lower temperatures. Also, one observes a significant blue shift
of the emission of 5b upon cooling down. We explain this ob-
servation by the assumption of a “freezing out” of the phenyl
rotation at low temperature, minimizing conjugation between
the phenyl rings and the central benzene unit. We also observe
AIE upon the addition of water to a solution of 5b in THF
(Figure 2, bottom). With up to 80% water content the emission
stays weak, but at 90% water content it turns on significantly;
this behavior is quite typical for AIE fluorophores. We note the
sharpening of the fluorescence after the addition of water. The
splitting is caused by vibronic progression bands that show up
in the solid state as a consequence of the conformational
freezing of the molecules in their planar form, when going
into the nanocrystalline state.
334
368
436
430
7005
4238
35412
–
<1
84
337
358
438
437
6843
5050
43881
–
<1
30
360
365
451
477
6843
5050
48463
–
45
70
THF
film
389
404
506
478
5944
3832
52924
–
24
19
[a] Films prepared by spin-coating from chlorobenzene (10 mgmLÀ1) on
glass substrates.
Figure 3 shows photographs of the crystalline materials 5a–
e. While 5a–c are colorless crystals, 5d and particularly 5e are
yellowish-green and deep yellow, respectively. The emission
color of 5d and 5e in the solid state are similarly red shifted
(Figures 1 and 3). We obtained single-crystal X-ray structures
(Figure 4 and Supporting Information).[12] For 5a–c,e the DSB
units are close to planar, while the four phenyl rings are—not
surprisingly—almost perpendicular with respect to the central
benzene core, effectively shielding the different DSB axes from
p–p interactions with their next neighbors (see Supporting In-
formation, Table S1). The bond lengths and bond angles are all
effect on its solution quantum yield, just, that it is less distinct
than for 5a.
Attempts to determine emissive lifetimes of 5a–e in solution
were not met by success, as these phenylated DSBs perform
photochemical reactions under the intense irradiation through
a laser (vide infra).
To demonstrate typical AIE behavior we obtained tempera-
ture-dependent emission spectra in 2-methyltetrahydrofurane
(Figure 2, top and Supporting Information). While at room tem-
perature the emissive intensity is negligible, it increases at
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Figure 4. A) ORTEP of 5a. B) Molecular Packing of 5a along the b axis. C)
Molecular packing of 5b. D) Carbonyl–carbonyl interactions of one of the
crystallographic independent molecules of 5d. CH–p, CF–p and CÀO short
contacts are highlighted.
we assume that—due to the DSBs’ quinoid character of the S1
state—rotation around the inner single bond of the DSB axes
for 5d,e appears to be even more hindered than that for 5a–c,
leading to a “higher degree of planarity” of the DSB axis and
thus a higher quantum yield in solution.
An important issue was the photostability of these materials.
Here we found that upon irradiation the DSBs transform.
Scheme 2 shows the gestation of the tetrahydrobenzo[k]tetra-
phene 6 by double cyclization of the two distyryl units and
subsequent [1,5]-hydrogen shift.[13] Upon looking at the time-
dependent reaction monitoring by NMR spectroscopy
Figure 3. Photographs of single crystals of compounds 5a–e either in day-
light (left) or under UV irradiation (lmax =365 nm, right).
in excellent agreement with the expected values. Only the dia-
ldehyde 5d shows an all-twisted conformation, that is, here in
the solid state the styryl units are also twisted by approximate-
ly 908 with respect to the central benzene unit. This molecular
conformation is probably enforced by the aldehyde–aldehyde
interactions that are visible in the packing picture (Figure 4D).
What is the reason for +M/ÀM-substituted 5d,e to fluoresce
in solution in contrast to their electronically non-elongated
counterparts? As AIE phenomena are attributed to the efficient
depopulation of the excited state through rotation of rotors
(here: phenyl rings, styryl units) versus a stator (central core),
Scheme 2. Photocyclization of 5a,b.
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Figure 5. Left: Time-dependent monitoring [mm:ss] of the photocyclization reaction ([5a]=23.2 mgmLÀ1) by NMR spectroscopy in [D6]benzene under argon
atmosphere (photoreactor, l=350 nm). Evolution of trans-double bond signal for 7 is highlighted. Right: Time-dependent relative concentrations of 5a, 7
(inset) and 6a during the photoreaction.
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In conclusion we prepared a series of novel tetraphenylated
distyrylbenzenes 5a–e, some of which show attractive aggre-
gation-induced emission. The attachment of electronically
active functional groups, such as aldehyde or dialkylamino
units, changes the character of the AIE fluorophore, in the
sense that in these species emission from the dilute solution is
also possible, while the electronically unperturbed, that is, only
hydrocarbon AIEs, only emit in the agglomerated state. While
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This work was supported by a grant from the “Landesgraduier-
tenfçrderung (LGFG)”.
Keywords: aggregation-induced emission · distyrylbenzene ·
fluorescence · photochemistry · photosynthesis
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Received: July 22, 2015
Published online on September 29, 2015
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