Aggregation-enhanced emission in fluorophores containing pyridine and triphenylamine terminals: Restricted molecular rotation and hydrogen-bond interaction

Article abstract of DOI:10.1039/c3tc31592d

Restriction on molecular rotation of fluorophores reduces non-radiative decay channels and promotes strong fluorescence due to aggregation-enhanced emission (AEE) behavior. To evaluate the important role of restricted molecular rotation on AEE behavior, tetraphenylthiophene (TP) derivatives with two pyridine (Py) or two triphenylamine (TPA) terminals were synthesized and characterized to be AEE-active fluorophores. Because of the efficient hindered molecular rotation of the larger TPA terminals, TP-2TPA emitted with higher emission efficiency than TP-2Py with smaller Py terminals. In addition, Py and TPA terminals can serve as hydrogen-bond (H-bond) accepting groups to bind with H-bond donating hydroxyl groups in poly(vinyl phenol) (PVPh) and poly(vinyl alcohol) (PVA) to further reinforce rotational restriction on the TP-2Py and TP-2PTA fluorophores. TP-2Py and TP-2PTA were therefore blended with PVPh and PVA and the emissive properties of the resultant blends were characterized and compared with the unblended TP-2Py and TP-2PTA to emphasize the role of H-bond on restricted molecular rotation.

Full text of DOI:10.1039/c3tc31592d

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DOI: 10.1039/C3TC31592D
Aggregation-Enhanced Emission in Fluorophores Containing
Pyridine and Triphenylamine Terminals: Restricted Molecular
Rotation and Hydrogen-bond Interaction
Shiang-Lin Deng, Tai-Lin Chen, Wei-Lun Chien and Jin-Long Hong*
Department of Materials and Optoelectronic Science
National Sun Yat-Sen University
Kaohsiung 804214, Taiwan
Restriction on molecular rotation of fluorohphores reduces non-radiative decay
channels and promotes strong fluorescence due to aggregation-enhanced emission
(AEE) behavior. To evaluate the important role of restricted molecular rotation on
AEE behavior, tetrephenylthiophene (TP) derivatives with two pyridine (Py) or two
triphenylamine (TPA) terminals were synthesized and characterized to be AEE-active
fluorophores. Because of the efficient hindered molecular rotation of the larger TPA
terminals, TP-2TPA emitted with higher emission efficiency than TP-2Py with smaller
Py terminals. In addition, Py and TPA terminals can serve as hydrogen-bond (H-bond)
accepting groups to bind with H-bond donating hydroxyl groups in poly(vinyl phenol)
(PVPh) and poly(vinyl alcohol) (PVA) to further reinforce rotational restriction on the
TP-2Py and TP-2PTA fluorophores. TP-2Py and TP-2PTA were therefore blended
with PVPh and PVA and the emissive properties of the resultant blends were
characterized and compared with the unblended TP-2Py and TP-2PTA to emphasize
the role of H-bond on restricted molecular rotation.
1

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Introduction
Conventional planar, disk-like organic fluorophores exhibit high fluorescence in dilute
solutions but suffer from detrimental aggregation-caused quenching (ACQ) in
concentrated solutions and solid states. The fluorescence quenching of conventional
fluorophores in the solid state greatly limits their real-world application. In contrast,
1-methyl-1,2,3,4,5-pentaphenylsilole, the first fluorophore with aggregation-induced
emission (AIE) property, has the unusual behavior that it emitted strongly in the
solution aggregated or in the solid state despite its non-emissive dilute solution state.1
The AIE effect of silole derivatives was attributed to the restricted intramolecular
rotation (RIR) of the phenyl peripheries2–5 against the central silole core in the
aggregate state. It was rationalized that effective RIR in the aggregated states of
AIE-active fluorophores reduces the possible nonradiative decay channels and leads
to enhanced radiative recombination of the excited state and the strong fluorescence.
In view that RIR is the main mechanism responsible for AIE activity, substituent with
large molecular size was covalently-linked to AIE-active centers (e.g. silole6 and
tetraphenylethene7) to induce hindered rotation and the desired emission enhancement.
Since the first discovery of the silole system, lots of organic and polymeric materials8–
15
with AIE or aggregation-enhanced emission (AEE) properties have been prepared
and characterized.
Since physical interaction forces can be used to reinforce restriction on
molecular rotation, several reports had been focused on evaluating the relationship
between hydrogen-bond (H-bond) and molecular rotation of AIE (or AEE) active
materials. A fluorenone-derivative of 2,7-bis(4-(tert-butylthio)phenyl)-fluorenone
(DSFO)16 was reported to have enhanced excimer emissions due to intermolecular
H-bonds. The dimer structure of DSFO was locked by intermolecular H-bonds and
the corresponding excimer emission underwent fewer non-radiative decay pathways.
2

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The salicylideneaniline (SA)17 compound formed gel in certain organic solvents and
the resultant gel solution was reported to have a fluorescent quantum yield (ΦF) 600
times higher than that of the homogeneous solution. The AEE property is ascribed to
the formation of J-aggregates and the inhibition of molecular rotation by H-bond
interactions in the viscous gel. Other systems relating compounds with hydrazine18,
benzoxazole19, acrylamido20 and naphthalide21 functions were also found to exhibit
enhanced emission in the gelled states promoted by H-bond interactions.
Poly(fluorene-alt-naphthol)22 previously prepared in our laboratory was also found to
exhibit AEE properties due to the restricted molecular rotation promoted by inter and
intrachain H-bonds among the inherent hydroxyl side groups. The restricted
molecular rotation of PFN can be further enhanced by blending it with different
amounts of poly(N-vinyl pyrrolidone)23 (PVR). Through the facile intermolecular
H-bond interactions between PFN and PVR, a PFN/PVR blend with small amounts
(2.3 wt%) of PFN can emit with a high ΦF value of 93%. Blend using H-bond
interactions were also conducted in small molecule system. An AEE-active
fluorophore with pyridine terminals was mixed with either small-mass bisphenol A or
polymeric poly(vinyl phenol)24 to generate pseudo-linear or pseudo-crosslinked
blends with higher emission intensity compared to the starting flurophore. Use of
H-bond interactions to lock the molecular rotations of organic and polymeric
fluorophores was therefore illustrated by the above examples and with effective
restriction on molecular rotation, the AEE-active fluorophores possess a beneficial
strong emission in the solid state, which facilitates their solid-state applications (e.g.
as a light-emitting layer in an organic light-emitting diode (OLED)). Under the
premise that the emission performance can be maintained or enhanced, blending the
valuable AEE-active fluorophores with other inexpensive, commercialized materials
provides alternative routes to cut down the costs in practical applications.
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In this study, the idea of using large substituent and H-bond interaction to
reinforce restricted molecular rotation was further tested on new AEE-active
fluorophores. To execute the idea, a former AEE-active tetraphenylthiophene (TP)25
fluorophore was used as the main structural center to link with either pyridine (Py) or
two triphenylamino (TPA) terminals via double bond (Scheme 1). The AEE-active
fluorophores of TP-2Py and TP-2PTA differ in the size of terminal groups used and
with the large TPA terminals, TP-2PTA emitted strongly with a high solid ΦF value of
80% as compared to 50% obtained for TP-2Py with smaller Py terminals. This result
is correlated with the size effect on restricted molecular rotation. In addition, we
intended to use Py and TPA terminals, as the potential H-bond acceptors, in the
respective TP-2Py and TP-2TPA to H-bond to hydroxyl groups in poly(vinyl phenol)
(PVPh) and poly(vinyl alcohol) (PVA) (Scheme 2) to reinforce restriction on
molecular rotation. However, unexpected fluorescence reduction was found when
TP-2Py was blended with PVPh, which is distinguished from the enhanced emissions
observed in blends of TP-2Py/PVA, TP-2PTA/PVPh and TP-2PTA/PVA. The varied
fluorescence responses in terms of potential side reaction and degree of molecular
rotation are therefore the focuses of this research.
Results and Discussion
Synthesis
As illustrated in Scheme 1, the fluorescent molecules of TP-2Py and TP-2TPA
were synthesized from the Heck-coupling reaction, with which reaction between
4-vinylpyridine (V-Py) and 2,5-bis(4-bromophenyl)-3,4-diphenylthiophene (TP-2Br)
yielded TP-2Py whereas reaction between TP-2Br and N,N-diphenyl-4-vinylaniline
(V-TPA) obtained TP-2TPA. Beforehand, the key intermediate of TP-2Br needed to be
prepared from bromination of TP26,27 in the presence of bromine/FeCl3. For
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preparation of TP-2PTA, the required compound of V-TPA was synthesized from a
two-step reaction scheme, with the first step formylation of triphenylamine (TPA) to
obtain F-TPA and the following Wittig reaction of F-TPA in the presence of
PPh3CH3Br /t-BuOK. Chemical structures of all intermediates and products were
confirmed from the corresponding 1H NMR (Fig. S1-S5) and elemental analysis. The
double bonds in TP-2Py and TP-2TPA are all trans in geometry as judged by the
resolved coupling constants (Jtrans > 15 Hz) of the alkene protons in the corresponding
1H NMR spectra. Both products of TP-2Py and TP-2PTA are amorphous materials
o
with glass transition temperatures (Tgs) at 102 and 140 C (Fig. S6-S7), respectively.
The higher molar mass of TP-2TPA contributes to its higher Tg as compared to
TP-2Py molecule with lower molar mass.
Aggregation-enhanced emission (AEE) behavior of TP-2Py and TP-2TPA
Effect of concentration on TP-2Py and TP-2TPA were described in the
unnormalized solution PL emission spectra in Fig. 1A and 1B, respectively. Both
solution systems exhibited the continuous gain of the emission intensity as
concentration of the fluorophore in THF was increased from 10-6 to 10-3 M. Whereas
solution thickening often weakens (or quenches) fluorophore’s emission, the solutions
of TP-2Py and TP-2TPA become more emissive with higher concentration and this
concentration-enhanced emission is typical of AEE-active fluorophores. For TP-2Py
(Fig.1A), solution of very low fluorophore content (10-6 M) is weakly-emissive but
increasing concentration from 10-5 to 10-3 M, two emission peaks at 452 and 481 nm
started to emerge and gained their intensity with increasing concentration. These two
emissions should be attributed to varied vibronic electronic states instead of monomer
and aggregate emissions based on the small spacing (1389 cm-1 < 1400 cm-1)28
between these two emission peaks and the proportional increase of both bands with
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increasing concentration. In case of preferable aggregation in more concentrated
solution, we would expect an inevitable increase on the long-wavelength aggregate
emission band. The two-peak pattern however disappeared and was replaced by a
broad peak at 500 nm in the spectrum of solid TP-2Py. Compared to solution sample,
red-shift of the emission peak in the solid TP-2Py indicates a more extended
conformation existing in the solid TP-2Py. For TP-2PTA, only one broad emission
peak at 500 nm was resolved (Fig. 1B). The continuous emission gain with increasing
concentration also suggests that TP-2PTA is an AEE-active fluorophore. Compared to
TP-2Py under the same concentration, TP-2PTA emitted with larger intensity.
The AEE effect was also characterized by the PL emission spectra of TP-2Py and
TP-2TPA in THF/hexane solvent/poor solvent pair. Primary evaluation indicated that
hexane is a poor solvent for both fluorophores, the inclusion of hexane in the
homogeneous fluorophore solutions in THF induced molecular aggregations and the
corresponding spectral responses can be used to study the effect of aggregation on
fluorescence behavior. The solution emission intensities for TP-2Py and TP-2TPA
(Fig. 2) are all increased with increasing hexane content, which fits the characteristics
of AEE behavior that aggregation due to inclusion of poor solvent leads to emission
enhancement. For TP-2Py, the proportional increase of both emission bands once
again confirms the vibronic nature of these two bands. Development of aggregated
structure of TP-2Py upon hexane inclusion can be evaluated from dynamic light
scattering (DLS) (Fig. S8). Aggregated particles of TP-2Py in the 30 vol%-hexane
solution possess an average hydrodynamic diameter (Dh) of 610 nm whereas the 60
and 80 vol%-hexane solutions contains nanoparticles with smaller Dh values of 390
and 110 nm, respectively. Addition of hexane clearly reduced the particle size;
therefore, the congested environment in the shrunk nanoparticles is effective in
imposing rotational restriction on the fluorophores; thus, leading to enhanced
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emission of the aggregated nanoparticles.
Restricted molecular rotations of TP-2Py and TP-2TPA
It is known that rotational energy relaxation can non-radiatively deactivate the
excited specie and consequently weaken (or quench) the fluorescence. In the dilute
solution state, the excited species are easily deactivated through molecular collisions
with the surrounding solvent molecules, thus making the fluorescent molecules
non-emissive. In the solid aggregates, molecular motions are considerably frozen and
consequently, the possibility of non-radiative decay channels are largely reduced and
the radiative process acts to result in emission enhancement. The effectiveness of the
restriction forces determines the resolved fluorescence intensity of the AIE or AEE-
active fluorophores. Under the same solution conditions (cf. Fig. 1 and 2), TP-2TPA
all have higher emission efficiency than TP-2Py and the solid TP-2TPA also has a
higher quantum yield (ΦF = 80%, Table 1) than that (= 50%) of the solid TP-2Py. This
result suggests that molecular rotation of TP-2PTA should be more effectively
restricted in comparison to TP-2Py.
Since molecular rotations can be thermally activated, the emission intensity at
different temperatures is therefore useful for evaluating relative degree of rotational
restriction on the fluorophores. It is known that cooling increases the viscosity and
friction of the solution mixtures and thereby, hampers the non-radiative decay process
detrimental for fluorescence. We therefore measured the emission spectra of TP-2Py
and TP-2TPA in THF at reduced temperatures from 293 to 153 K. The emission
spectra at reduced temperatures (Fig. 3) showed that both samples emitted with
increasing intensity as the system temperature was decreased; however, a less
pronounced temperature effect was resolved for TP-2TPA as compared to the
significant emission enhancement observed for TP-2Py. Through the whole cooling
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cycle, the emission intensity of TP-2Py became twice of the starting emission band at
room temperature, which is in distinction to the minor (< 10%) emission enhancement
observed for TP-2TPA. The less pronounced temperature effect on TP-2PTA should
be attributed to its higher rotation energy barrier (more effective restriction on
molecular rotation) as compared to TP-2Py. Molecular rotation of TP-2PTA is said to
be effectively prohibited and decreasing temperature did not help much on hampering
the molecular motion. With lower rotation barrier, TP-2Py molecules in dilute
solution rotates freely at room temperature and upon cooling, the involved molecular
rotations can be efficiently prohibited in the frozen media, which is particularly true
when the temperature was lowered to below melting point (= 258 K) of THF, to result
in large emission enhancements.
To demonstrate that restricted molecular rotation is the key factor leading to the
varied emission responses of TP-2Py and TP-2TPA toward cooling, computations on
rotation barriers were conducted by Molecular Studio (MS) software. The resultant
single-molecule conformers of TP-2Py and TP-2TPA with minimum energy were
given at top of Fig. 4, which illustrates the fundamental difference between the
C2-substituents of both molecules. The two (A and B) rings in the C2-substutuent of
TP-2Py are tilted to each other to adopt a non-coplanar arrangement, which is in wide
distinction to the coplanar C and D rings in the C2-substituent of TP-2PTA. The
C2-substituent should be responsible for the observed fluorescence and its
non-coplanar geometry may contribute to the observed two-band emission pattern. On
the contrast, more extensive electron delocalization over the more coplanar
C2-substituent of TP-2TPA may homogenize the energy levels of the excited
electronic state, leading to the broad one-band emission pattern at longer wavelengths
than the two bands in TP-2Py.
Rotation barrier is considered to be the average energy required to rotate the
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single bond (bond 1) connecting the C2-substituent and the central thiophene ring
from the equilibrium position of the minimum-energy conformer to other conformers
with rotation angles deviated from the equilibrium one. The calculated energy barrier
was plotted against the rotation angle at the bottom of Fig. 4. For TP-2Py, the
maximum energy barrier (< 300 Kcal/mol) occurred at a rotation angle of 160o; in
contrast, the calculated energy barrier of TP-2PTA arose quickly as the rotation angle
exceeded ∼ 140 ° and reached a maximum value of 600 Kcal/mol at an angle of 180o.
Supposedly, this result is directly related to the large steric repulsion forces between
the bulky C-2 substituent and the central thiophen ring. Rotation of the bulky C-2
substituent of TP-2TPA is surely a more difficult task than rotating the smaller C-2
substituent of TP-2Py, which is considered to be responsible for the more efficient
emission of TP-2PTA when compared to TP-2Py.
H-bond interactions of TP-2Py and TP-2TPA
With the respect that the terminal Py and TPA groups are H-bond acceptors, the
AEE-active TP-2Py and TP-2TPA were therefore reacted with H-bond donating
hydroxyl groups in poly(vinyl phenol) (PVPh) and poly(vinyl alcohol) (PVA) for the
sake of reinforcing rotational restriction. To maximize restriction forces, polymeric
poly(vinyl phenol) (PVPh) and poly(vinyl alcohol) (PVA) with hydroxyl function
equivalent to Py and TPA terminals in the respective TP-2Py and TP-2TPA
fluorophores were used. With the mutual H-bond interaction, molecular rotation of the
C2-substituents can be effectively restricted to result in better emission efficiency than
pure TP-2Py and TP-2TPA without polymer additives.
Primarily, the solid TP-2Py/PVPh and TP-2TPA/PVPh blends containing equal
amounts of H-bond donating and accepting groups were prepared from the
homogeneous fluorophor/PVPh solutions in THF. Dilute solutions were virtually
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homogeneous but during the slow evaporation of THF solvent certain amounts of
blended products, presumably with physical crosslinked network structure,
precipitated from the solutions.
The mutual H-bond interactions between polymers and TP-2Py (or TP-2PTA)
should change the chain dimensions of the polymers in THF. Once H-bonded to
polymeric PVA and PVPh, TP-2Py and TP-2PTA molecules should serve as anchoring
junctions in between hydroxyl groups of functions of PVA and PVPh and therefore,
they will function to shorten the intra- and inter-chain distances of the polymers to
result in aggregates smaller than the pure polymers. The hypothetical size variations
can be conveniently evaluated from DLS analysis (Fig. S9) on the dilute (10-5 M)
solutions of pure PVA, PVPh, TP-2Py/PVA and TP-2PTA/PVPh mixtures in THF.
Chain segments of pure PVA and PVPh formed large aggregates with the detected Dhs
> 1 ꢀm and as expected, preferable H-bond interactions between fluorophores and
polymers considerably shrunk the aggregated particles (with the detected Dhs smaller
than 500 nm). Variations on the sizes of the aggregates can be used to demonstrate the
presence of H-bond interactions between Py terminals of TP-2Py (or TPA terminals of
TP-2PTA) and hydroxyl functions of polymers.
After complete solvent removal, the resultant solid TP-2Py/PVPh and
TP-2PTA/PVPh blends all exhibited emission at the same wavelengths as the pure
TP-2Py and TP-2PTA (Fig. 5A). However, the solid TP-2Py/PVPh blend showed an
unexpected reduction on the emission intensity (Fig. 5A) when compared to the pure
TP-2Py itself, this result is quite different from the TP-2PTA/PVPh blend (Fig. 5B)
that PVPh in this case actually functioned as rotational restrictor to result in better
emission efficiency than the unblended TP-2PTA itself.
Long chain polymers should impose effective restriction on molecular mobility
of the small-mass TP-2Py and TP-2PTA; however, the multiple hydroxyl pendant
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groups along the polymer chain of PVPh and PVA will generate lots of intermolecular
physical junction points when H-bonded to TP-2Py and TP-2PTA molecules in the
solution state and the resultant crosslinked networks in the solution should prevent the
hydroxyl functions in polymer chains from complete H-bonding. Blend systems were
prepared from mixing of both components in THF solvent. To avoid the premature
precipitation of the crosslinked products, all solution mixtures were agitated
vigorously during the solution mixing step; however, visible precipitates still occurred
during the mixing process. After removal of THF solvent, the resultant solid blends
were sent for spectral measurement immediately.
Incorporation of PVPh in the blends caused different emission responses
between TP-2Py and TP-2PTA (Fig. 5). Obviously, emission intensity was drastically
reduced by PVPh in the blend; on the contrary, further emission enhancement was
observed in TP-2PTA system. What is the origin for the varied responses toward
PVPh addition? It should relate to the potential acid-base reaction, which is less
probable in TP-2TPA/PVPh system, between the weakly-alkaline Py terminals of
TP-2Py and the acidic phenolic pendant groups of PVPh. The protonated TPA
(TPAH+X-) is a highly acidic species with a pKa = -2 while the protonated Py (PyH+X-)
is weakly acidic with a pKa = 5.3.29 With the respect of the relative acid-base strength,
it is envisaged that certain fractions of the Py terminals in TP-2Py will be protonated
by the phenolic protons in PVPh, resulting in weakly (or non-) emissive protonated
species responsible for the reduced emission observed in the TP-2Py/PVPh blend.
Here emission efficiency of TP-2Py was considerably reduced by complexation to
PVPh as indicated by ΦF values of the solid samples (50 vs. 25%, Table 1). In contrast,
little emission enhancement was observed in TP-2PTA system (ΦF = 80 and 85% for
TP-2PTA and TP-2PTA/PVPh, respectively).
After recognizing that acidic phenolic function is harmful for the emission of
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TP-2Py, neutral PVA was used to replace PVPh as the H-bond donating component.
Unlike the acidic phenol groups in PVPh, the neutral hydroxyl groups of PVA should
not protonate TP-2Py and produce no pyridinium species harmful for fluorescence.
Indeed, the solid TP-2Py/PVA blend emit with stronger intensity than pure TP-2Py
(Fig. 6A). Through steric constraint imposed by the polymer framework of PVA,
molecular rotations of the pyridine and therefore the whole C2-substituents of TP-2Py
can be effectively restricted, resulting in further emission enhancement of the
AEE-active TP-2Py. The same result also holds for TP-2TPA as the corresponding
emission spectrum (Fig. 6B) of the TP-2TPA/PVA blend is higher in emission
intensity compared to pure TP-2TPA itself. The resultant ΦF values of all solid blends
(Table 1) are in line with the emission spectra shown in Fig. 5 and 6.
Experimential
Synthesis
Triphenylamine
(TPA),
phosphorous(v)
oxychloride
(POCl3),
triphenyl-methyl-phosphonium bromide (Ph3PCH3Br), potassium tert-butoxide
(t-BuOK), bromine, Iron(III) chloride (FeCl3), palladium(II) acetate (Pd(OAc)2),
tri-o-tolyamine, sodium bicarbonate and 4-vinylpyridine (V-Py) were all purchased
from Aldrich Chemical Co. and used directly without further purification. THF and
DMF were refluxed over sodium and benzophenone under nitrogen for more than 2
days before distillation for use. Triethylamine (TEA) was distilled from CaH2 under
nitrogen before use. TP compound was prepared according to the reported
procedures.26,27
Preparation of 4-(diphenylamino)benzaldehyde(F-TPA). Triphenylamine
(5g, 20.3mmol) was dissolved in 10 mL of DMF. POCl3 (9.4 mL, 102 mmol) was
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added drop to drop to the mixture at 0oC. After mixing, the temperature was raised to
room temperature and during which, color of the solution turned red. The reaction
mixture was then heated to 45oC and stirred for an additional 2 hr. The product
mixture was poured in an ice-bath and neutralized with sodium bicarbonate.
Recrystallization of resulting precipitates from ethanol. Pale yellow solid (5.4g, 96%)
1
was collected by filtration.. H NMR (500MHz, d6-DMSO): δ 9.79 (s, 1H, Ha), 7.7–
7.75 (d, 2H, Hb), 7.4−7.5 (t, 4H, Hc), 7.15–7.25 (m, 6H, Hd), 6.85−6.9 (d, 2H, He) (Fig.
S1); m/z (EI MS): calcd for C19H15NO, 273.12, found, 273.11; Anal calcd for
C19H15NO: C, 83.49%; H, 5.53%; N, 5.12%, found: C, 83.47%; H, 5.54%; N, 5.13%.
Preparation of N,N-diphenyl-4-vinylaniline (V-TPA). To a solution of (9.8 g
27 mmol) of methyltriphosphonium bromide in 20 ml of THF (3.3 g, 29 mmol),
potassium tert-butylate were added under cooling, during which the solution turned
into yellow. The reaction solution was stirred for further 30 minutes and then (5 g, 18
mmol) of the 4-(diphenylamino)benzaldehyde (F-TPA) were added. The reaction was
stirred for 24 hr at room temperature. The yellow solution was then extracted with
methylene chloride. The product (3.7 g, 70%) was purified by column
chromatography with toluene as eluent. 1H NMR (500 MHz, d6-DMSO):
δ 6.89−7.4 ( d, 14H, Hd-h), 6.61−6.7 (dd, 1H, Hb), 5.65−5.73 (d, 1H, Hc), 5.12− 5.19 (d,
1H, Ha) (Fig. S2); m/z (EI MS): calcd for C20H17N, 271.14, found 271.10; Anal calcd
for C20H17N: C, 88.59%; H,6.31%; N, 5.1%, found: C, 88.52%; H, 6.34%; N, 5.14%.
Preparation of 2,5-bis(4-bromophenyl)-3,4-diphenylthiophene (TP-2Br). To
a solution of 2,3,4,5-tetraphenylthiophene (5 g, 12.8 mmol) in 40 mL of CHCl3 (40
mL) at 0 oC, ferric chloride (32 mg, 0.2 mmol) and bromine (4.2 g, 26.2 mmol) were
added. This procedure was operated in the dark to avoid the undesired bromination on
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the other portion of molecule. The solution was then warmed to room temperature
before continuous reaction for 4 hr. The resultant slurry was poured into water and
washed with sodium thiosulfate to the extent that the red color disappeared. The
aqueous layer was extracted with CHCl3 twice, and the combined organic layers were
1
dried over magnesium sulfate. (5.1g, 83%). H NMR (500 MHz, d6−DMSO) :
δ6.97−6.99 (d, 4H, Ha), 7.12−7.18 (m, 10H, Hb), 7.47−7.48 (d, 4H, Hc) (Fig. S3); m/z
(EI MS): calcd for C28H18Br2S, 545.95, found, 545,93; Anal calcd for C28H18Br2S: C,
61.57%; H, 3.32%. Found: C, 61.50%; H, 3.32%.
Preparation of
4,4'-((1E,1'E)-((3,4-diphenylthiophene-2,5-diyl)bis(4,1-phenylene))bis(ethene-2,1-
diyl))dipyridine (TP-2Py). A reaction mixture of 4-vinylpyridine (1.1 mL, 10
mmol), TP-2Br (2.5 g, 4.58 mmol), triethylamine (30 mL), Pd(OAc)2 (0.33g, 1.47
mmol) and tri-o-tolylphosphine (0.22 g, 0.72 mmol) in dry DMF (30 mL) were placed
in a predried round-bottomed flask (100 mL). The reaction mixture was then degassed
by freeze–pump–thaw 3 times before heated at 110 oC for 24 hr. After cooling to room
temperature, the reaction mixtures were poured into water to obtain yellow powder
product. The crude product was then purified by flash column chromatography (v/v
hexane/ethyl acetate = 4/1). (1.5g, 55%). 1H NMR (500 MHz, d6−DMSO): δ 8.53 (d,
4H, Ha), 7.1−7.5 (m, 22H, Hc-h), 7.03 (d, 4H, Hb, Jtrans = 16.1 Hz). 13C NMR (500
MHz, d6−DMSO): δ 120.88, 126.55, 127.19, 127.27, 128.12, 128.91, 130.45, 132.16,
133.55, 135.46, 135.91, 137.58, 140.16, 144.17, 150.06 (Fig. S4); m/z (EI MS): calcd
for C42H30N2S, 594.21, found, 594.20; Anal calcd for C42H30N2S: C, 84.9%; H, 5.09%;
N, 4.71%, found: C, 84.83%; H, 5.14%; N, 4.72%.
Preparation of
4,4'-((1E,1'E)-((3,4-diphenylthiophene-2,5-diyl)bis(4,1-phenylene))bis(ethene-2,1-
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diyl))bis(N,N-diphenylaniline) (TP-2TPA). A reaction mixture of V-TPA (2.12 g, 10
mmol), TP-2Br (2.5 g, 4.58 mmol), triethylamine (30 mL), Pd(OAc)2 (0.33 g, 1.47
mmol) and tri-o-tolylphosphine (0.22 g, 0.72 mmol) in dry DMF (30 mL) were placed
in a predried round-bottomed flask (100 mL). The reaction mixture was then degassed
by freeze–pump–thaw 3 times before heated at 110 oC for 24 hr. After cooling to room
temperature, the reaction mixtures were poured into water to obtain yellow powder
products. The crude products were then purified by flash column chromatography (v/v
hexane/ethyl acetate = 4/1). (2.5g, 58.8 %). 1H NMR (500 MHz, d6−DMSO):
δ 7.02−7.47 (m, 46H, Hb-i). 6.93−6.94 (d, 4H, Ha, Jtrans=16.3 Hz). 13C NMR (500
MHz, d6−DMSO): δ 122.91, 123.35, 124.21, 125.95, 126.39, 127.09, 127.70, 128.07,
128.51, 128.80, 129.62, 130.47, 131.18, 132.24, 136.07, 136.61, 137.56, 139.78 (Fig.
S5); m/z (EI MS): calcd for C68H50N2S, 926.37, found, 926.35; Anal Calcd for
C68H50N2S: C, 88.17%; H, 5.44%; N, 3.02%, found: C,88.2%; H, 5.4%; N, 3.1%.
Instrumentations
The emission spectra were obtained from a LabGuide X350 FL spectrophotometer
using a 450W Xe lamp as the continuous light source. Temperature-programming PL
emission spectra were conducted using a Peltier cell from Ocean Optics connecting
to HCS 302 hot stage. Quantum yield (ΦF) of the solid sample was measured in an
integrating sphere made by Ocean Optics. For liquid solutions, value of ΦPL was
obtained by using quinine sulfate as reference standard. The UV-vis absorption
spectra were recorded with an Ocean Optics DT 1000 CE 376 spectrophotometer. To
measure the solution UV-Vis absorption and emission spectra, quartz cell with
dimensions of 0.2 x 1.0 x 4.5 cm3 was used. Stock solutions of the fluorophores were
primarily prepared and aliquot of the stock solution was transferred to 10 mL
volumetric flask, into which appropriate volumes of hexane and THF was added
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dropwise under vigorous stirring to furnish solution with desired concentration and
hexane content. Particle sizes of fluorophores and fluophore/polymer mixtures in
different solvent media were measured by DLS instrument on a Malvern ZetaSizer
Nano ZS90 spectrometer at room temperature. A He–Ne laser operating at 633 nm
was used as light source. Fluorophore/polymer blends were prepared from mixing
the respective components in THF solvent. Mixing was conducted by vigorous
stirring till precipitation occurred. Solid samples were prepared by dropping the
solution over quartz plate to obtain solid specimen for measurement. 1H NMR spectra
were recorded with a VarianVXR-500 MHz FT-NMR instrument. Tetramethylsilane was
used as internal standard. Elemental analyses were performed on an Elementary Vario
EL-III C, H and N analyzer. A mass spectrum was obtained by using a Bruker Daltonics
Autoflex III MALDI-TOF mass spectrometer.
Conclusion
TP-2Py and TP-2PTA with TP center connected by the respective Py and TPA
terminals were successfully synthesized from the Heck-coupling reactions between
TP2Br and the corresponding diene monomers. The resultant TP-2Py and TP-2PTA
were found to exhibit AEE fluorescence behavior that the solution fluorescence
intensity increases with increasing aggregation upon adding hexane poor solvent in
the solution mixtures. With the large TPA terminals, TP-2PTA has higher solution and
solid emission intensities than TP-2Py under the same experimental conditions.
Therefore, the large TPA terminals in TP-2PTA imposed effective rotational restriction,
rendering in high emission efficiency (e.g. solid TP-2PTA has a high ΦF of 80%). In
contrast, less effective rotational restriction of TP-2Py resulted in a lower emission
efficiency (e.g. solid TP-2Py has a ΦF of 50%) than TP-2PTA.
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Theoretically, H-bonding to polymers will further enhance fluorescence of
TP-2Py and TP-2PTA because that molecular rotation of the fluorophores can be
hampered by the viscous polymeric segments in close vicinities. However, the
alkaline TP-2Py tends to react with the acidic phenol groups in PVPh, resulting in
protonated species with reduced emission intensity compared to TP-2Py itself. The
acid-base reaction can be avoided by using neutral PVA instead; therefore, further
emission enhancement was observed in TP-2Py/PVA blend. For TP-2PTA,
complexation to PVPh and PVA all resulted in further emission enhancements
because the neutral TPA terminals can be immune from acid-base reaction. The idea
using H-bond interaction to enhance emission of AEE-active fluorophores was
therefore demonstrated in this study.
Acknowledgement
We appreciate the financial support from National Science Council, Taiwan,
under the contract no. NSC 102-2221-E-110-084-MY3
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Scheme 1 Syntheses of AEE-active fluorophores of TP-2Py and TP-2TPA.
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OH
N
N
N
OH
N
HO
HO
TP-2Py
or
HO
OH
PVP blends
PVA blends
HO
HO
OH
N
N
N
N
OH
TP-2PTA
PVP
OH
HO
OH
or
HO
OH
PVA
OH
OH
OH
Scheme 2 Blends of TP-2Py and TP-2PTA with poly(vinyl phenol) (PVPh) and
poly(vinyl alcohol) (PVA) through H-bond interaction.
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Table 1 Quantum yield (ΦF)a of solid TP-2Py, TP-2TPA and their complexes with
PVPh and PVA.
sample TP-2Py TP2TPA
TP-2Py/
PVPh blend PVA blend PVPh blend PVA blend
25 54 85 84
TP-2Py/
TP-2TPA/
TP2TPA/
50
80
ΦF, %
a quantum yield was obtained from integrating sphere.
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20000
15000
10000
5000
0
TP-2Py
in THF
A
10-6 M
10-5 M
10-4 M
10-3 M
Solid
400
450
500
550
600
650
Wavelength (nm)
TP-2TPA
in THF
B
30000
20000
10000
0
10-6 M
10-5 M
10-4 M
10-3 M
Solid
450
540
630
Wavelength (nm)
Fig. 1 Solution PL emission spectra of (A) TP-2Py and (B) TP-2PTA in THF of
different concentrations. (λex = 365 nm for TP-2Py and 390 nm for TP-2PTA)
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6000
4000
2000
0
TP-2Py (10-5 M)
THF : Hexane
1 : 0
A
7 : 3
4 : 6
2 : 8
1 : 9
400
450
500
550
600
Wavelength (nm)
Fig. 2 Solution PL emission spectra of (A) TP-2Py and (B) TP-2PTA in THF/hexane
mixtures of different volume ratios. (λex = 365 nm for TP-2Py and 390 nm for
TP-2PTA)
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Fig. 3 Solution PL emission spectra of (A) TPA-2Py and (B) TP-2PTA in THF (10-2
M) at different reduced temperatures from 298 to 153 K. ((λex = 365 nm for TP-2Py
and 390 nm for TP-2PTA)
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TP2Py
A
B
TP-2PTA
C
D
600
550
500
450
400
350
300
250
200
150
100
50
TP-2Py
TP-2TPA
0
0
20 40 60 80 100 120 140 160 180
Rotation Angle(degree)
Fig. 4 The simulated conformers (top) of TP-2Py and TP-2PTA with minimum energy
and the calculated rotation energy barriers versus rotation angle (bottom).
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20000
15000
10000
5000
0
TP-2Py
TP-2Py / PVPh
A
400
500
600
700
Wavelength (nm)
50000
40000
30000
20000
10000
0
TP-2TPA
TP-2TPA / PVPh
B
500
600
700
Wavelength (nm)
Fig. 5 The solid PL emission spectra of (A) TP-2Py and TP-2Py/PVPh blend and (B)
TP-2PTA and TP-2PTA/PVPh blend. (λex = 365 nm for TP-2Py system and 390 nm
for TP-2PTA system)
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30000
20000
10000
0
TP-2Py
TP-2Py / PVA
A
400
500
600
700
Wavelength (nm)
50000
40000
30000
20000
10000
0
TP-2TPA
TP-2TPA / PVA
B
500
600
700
Wavelength (nm)
Fig. 6 The solid PL emission spectra of (A) TP-2Py and TP-2Py/PVA blend and (B)
TP-2PTA and TP-2PTA/PVA blend. (λex = 365 nm for TP-2Py system and 390 nm for
TP-2PTA system)
Content of TOC
28