Designing highly fluorescent, arylated poly(phenylene vinylene)s of intrinsic microporosity

Article abstract of DOI:10.1039/c9tc06028f

Three new polymers containing tetraphenylethylene and diphenyl-dinaphthylethylene cores and their corresponding monomeric model compounds were synthesized and fully characterized aiming to investigate their photoluminescence efficiency, microporosity and Brunauer-Emmett-Teller-derived surface areas (S_BET). Comprehensive photophysical characterization was undertaken in the solid state (powder and thin films), in tetrahydrofuran (THF) solution and in mixtures of "good" and "poor" solvent to induce aggregation (THF:water mixtures). Aggregation induced emission (AIE) was found for the tert-butyl-TPE monomer and polymer and diphenyl-dinaphthylethylene monomer with the increase of the water amount in THF:water mixtures and in the solid state. The tert-butyl substituted TPE derivatives display the highest fluorescence quantum yield (?_F) values: 0.14 to 0.30 (in powder) and 0.46 to 0.64 in thin films. In contrast, with the diphenyl-dinaphthylethylene (meta and para-phenylene) polymers aggregation caused quenching (ACQ) occurs in THF:water mixtures (?_F ≤ 0.011) and in the solid state (?_F ≤ 0.012). The microporosity of the soluble conjugated polymers as potential conjugated polymers of intrinsic microporosity (cPIMs) was further investigated. The S_BET of the polymers were related to their optical properties. The polymers show an attractive combination of high S_BET surface area (417 m² g^-1) and the occurrence of distinct AIE effects for the tert-butyl-TPE polymer while the diphenyl-dinaphthylethylene polymers do not exhibit microporosity (S_BET ≤ 17 m² g^-1) and show ACQ behavior.

Full text of DOI:10.1039/c9tc06028f

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Designing highly fluorescent, arylated
poly(phenylene vinylene)s of intrinsic
Cite this:DOI: 10.1039/c9tc06028f
microporosity†
b
a
Ana Clara B. Rodrigues, ‡a Isabell S. Geisler,‡b Patrick Klein,
Joao Pina,
˜
Fabian J. H. Neuhaus,b Elke Dreher,c Christian W. Lehmann,c Ullrich Scherf
*
b
a
and J. Sergio Seixas de Melo
*
´
Three new polymers containing tetraphenylethylene and diphenyl-dinaphthylethylene cores and their
corresponding monomeric model compounds were synthesized and fully characterized aiming to
investigate their photoluminescence efficiency, microporosity and Brunauer–Emmett–Teller-derived
surface areas (SBET). Comprehensive photophysical characterization was undertaken in the solid state
(powder and thin films), in tetrahydrofuran (THF) solution and in mixtures of ‘‘good’’ and ‘‘poor’’ solvent
to induce aggregation (THF:water mixtures). Aggregation induced emission (AIE) was found for the tert-
butyl-TPE monomer and polymer and diphenyl-dinaphthylethylene monomer with the increase of the
water amount in THF:water mixtures and in the solid state. The tert-butyl substituted TPE derivatives
display the highest fluorescence quantum yield (fF) values: 0.14 to 0.30 (in powder) and 0.46 to 0.64 in
thin films. In contrast, with the diphenyl-dinaphthylethylene (meta and para-phenylene) polymers
aggregation caused quenching (ACQ) occurs in THF:water mixtures (fF r 0.011) and in the solid state
(fF r 0.012). The microporosity of the soluble conjugated polymers as potential conjugated polymers
of intrinsic microporosity (cPIMs) was further investigated. The SBET of the polymers were related to their
Received 12th November 2019,
Accepted 4th December 2019
optical properties. The polymers show an attractive combination of high SBET surface area (417 m2 gꢀ1
)
DOI: 10.1039/c9tc06028f
and the occurrence of distinct AIE effects for the tert-butyl-TPE polymer while the diphenyl-
dinaphthylethylene polymers do not exhibit microporosity (SBET r 17 m2
g
ꢀ1) and show ACQ behavior.
rsc.li/materials-c
restriction of intramolecular motions (RIM), including rotations
and vibrations, that would deactivate non-radiatively the excited
Introduction
Aggregation induced emission (AIE, or even aggregation state of the luminogens.2 This phenomenon occurs in some
induced enhanced emission, AIEE) corresponds to the description luminogens in contrast to aggregation caused quenching (ACQ)
of the luminescence phenomenon in which the luminescence commonly described for traditional fluorophores in which the
efficiency of a luminogen is increased upon its aggregation.1 The p–p-stacking of the molecules at high concentration or in the solid
most accepted mechanism for the occurrence of AIE is through a state leads to the formation of H-aggregates, which decreases the
emission efficiency.3 Typically, low molecular weight AIE chromo-
phores possess a propeller-shaped structure with rotatable
peripheral phenyl rings (rotors),4 such as 1,1,2,3,4,5-hexaphenylsilole
(HPS),5 2,3,3-triphenylacrylonitrile (TPAN)6 and 1,1,2,2,-tetra-
phenylethylene (TPE).7 TPE derivatives have demonstrated a
wide range of applications in optoelectronic devices,8–10 bio-
imaging,11 biosensors,12 etc. due to their AIE properties.
TPE-based conjugated polymers – poly(arylene-diphenyl-
a CQC, Department of Chemistry, University of Coimbra, P3004-535 Coimbra,
Portugal. E-mail: sseixas@ci.uc.pt
b Bergische Universitat Wuppertal, Macromolecular Chemistry Group (buwmakro)
and Institute for Polymer Technology, Gauss-Str. 20, D-42119 Wuppertal,
Germany. E-mail: scherf@uni-wuppertal.de
c
¨
Max-Planck-Institut fur Kohlenforschung, Kaiser-Wilhelm-Platz 1,
¨
D-45470 Mulheim an der Ruhr, Germany
† Electronic supplementary information (ESI) available: Detailed synthesis and
structural characterization of model compounds and polymers; crystal data and vinylenes) – have been known since the 1960s.13 Soluble, high
structure refinement of monomers; nitrogen gas adsorption and desorption
molecular weight poly-TPEs (Mn 10–40 kDa) can be made in
reductive polyolefinations of aromatic, bisgeminal tetrachlorides
isotherms of polymers and their BET surface area fits and room temperature
fluorescence decays for Poly-t-Bu. CCDC 1961126 and 1961127. For ESI and
as diketone derivatives with Cr2ac4 or Co2(CO)8 as reducing
crystallographic data in CIF or other electronic format see DOI: 10.1039/
agents14–16 or in carbonyl polyolefinations after McMurry with
c9tc06028f
‡ These authors contributed equally to the work.
TiCl3/LiAlH4.17 Some of them show distinct AIE properties
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leading to high solid state photoluminescence quantum yields Belsorp-max system at 77 K. The surface areas were calculated
of 470% for some of the poly-TPEs.4,18 Since 2011,19,20 the using the BET model in the pressure range p/p0 from 0.05–0.25.
easily functionalized four phenyl rings of the TPE unit were All samples were degassed offline at 4100 1C for 16 hours
combined with a variety of reactive groups in construction of under a vacuum. Single crystal X-ray structures were obtained
TPE-based polymers with inherent porosity.21 The development on a Bruker-AXS Kappa Mach3 APEX-II-diffractometer with a
of microporous organic polymers (MOPs) with very high surface FR591 rotating anode, equipped with graded multilayer optics
area is an active field of current research,22 with high potential emitting copper radiation (1.54178 Å), and measured at 100 K.
in a variety of applications4,23–26 such as energy storage, light For structure solution and refinement, the SHELX-package29,30
harvesting, catalysis and sensing of hazardous chemicals and was used as integrated in Olex2.31
explosives. Several classes of MOPs, such as amorphous hyper-
cross-linked polymers (HCPs), polymers of intrinsic microporosity
Solutions and film preparation
(PIMs), and conjugated microporous polymers (CMPs), have been An appropriate amount of powder of each compound was
reported.27 CMPs are also potential fluorescent materials due to diluted in THF to prepare a stock solution with an optical
their extended conjugated frameworks and tunable optoelectronic density of 1.0 at the excitation wavelength used for the experi-
properties.28
ments. Then, 100 mL of the stock solution was then diluted with
In this work, two types of poly(1,4-phenylene-diarylvinylene)s the proper amount of THF or THF:water mixture to obtain the
with 4-tert-butylphenyl or naphthyl as aryl substituents at the desired water fraction ( fW = 0–95%, v/v) in 2 mL of final volume.
vinylene units as well as the corresponding poly(1,3-phenylene-
Thin films from the compounds were obtained with a desktop
dinaphthylvinylene) are investigated for their photophysical precision spin-coating system, model P6700 series from Speedline
properties in tetrahydrofuran (THF) and THF:water mixtures Technologies, as described elsewhere.32 Briefly, thin films from the
(a mixture of a ‘‘good solvent’’, THF, and water, a ‘‘poor samples were obtained by deposition of ca. 50 mL from a solution
solvent’’ to induce the formation of aggregates) as well as in of the compounds onto a circular sapphire substrate (10 mm
the solid state (powder and films) for studying the effect of diameter) followed by spin-coating (2500 rpm) in a nitrogen-
aggregation on the photoluminescence efficiency. The results saturated atmosphere (2 psi). The solutions for spin-coating were
are further rationalized with investigations on monomeric prepared by adding 2 mg of the samples to 200 mL of chloroform
model monomer compounds and with the parent TPE molecule solution, with stirring, at environment temperature, overnight.
under the same conditions to also elucidate the effect of the 15 mg of Zeonexs was added to the chloroform solution of the
substituent and polymerization on the photophysical properties model compounds, TPE, Mono-t-Bu and Mono-Np, as a polymeric
of these compounds. Furthermore, the porosity of these new matrix to obtain thin films of these samples.
TPE-based polymers is also investigated.
Steady state and time-resolved fluorescence measurements
The absorption spectra were recorded using Shimadzu UV-2450
or Agilent Cary 5000 UV-vis-NIR spectrometers. Absorption
spectra of the transparent thin films were obtained in absorption
Experimental section
Materials
mode using a clean sapphire substrate as the reference sample. The
Most chemicals were purchased from Sigma-Aldrich, Fischer
absorption spectra of the amorphous powder samples were
Scientific or TCI and used without further purification, unless
recorded by collecting diffuse reflectance using a Cary 5000 DRA
described otherwise. For the photophysical studies, the solutions
(an integrating sphere accessory with detection in the 200–2500 nm
were prepared with solvents of spectroscopic grade or equivalent:
range). Background correction was performed by collecting the
tetrahydrofuran (THF, Uvasol Merck), analytical grade chloroform
baseline with 100% and 0% reflectance (using a polytetrafluoro-
(Fischer Chemical) or deionized water (18.2 MO cm at 25 1C,
Milli-Q, Millipore).
ethylene, PTFE, reference sample and a blocked beam, respec-
tively) prior to the determination of the spectra of the solid
samples. Conversion to absorption was performed assuming the
Kubelka–Munk function, F(R).33
Synthesis and structural characterization of the model
compounds and polymers
Fluorescence spectroscopic studies were performed using a
All reactions were carried out under an argon atmosphere using Horiba-Jobin-Yvon Fluorolog 3-22 spectrofluorimeter. The
flame dried glassware. The detailed synthetic procedure and fluorescence quantum yields (fF) of all compounds, in solution
structural characterization are provided in the ESI.† NMR or in the solid state, were measured using the absolute method
spectra were recorded on a Bruker AVANCE 400 or AVANCE with a Hamamatsu Quantaurus QY absolute photoluminescence
III 600. APCI (Atmospheric Pressure Chemical Ionization) and quantum yield spectrometer, model C11347 (integrating sphere).
ESI (electrospray ionisation) mass spectra were obtained on a A clean sapphire substrate was used as a reference for the fF
Bruker Daltronik micrOTOF system. Gel permeation chromato- measurements of solid-state thin films.
graphy (GPC) measurements were carried out on a PSS/Agilent
Fluorescence decays were measured using a home-built
SECurity GPC system equipped with polystyrene gel columns Time-Correlated Single Photon Counting (TCSPC) apparatus
using chloroform or THF as the eluent. Nitrogen adsorption– described previously.32 An IBH nanoLED (339 nm, 1.0 kHz)
desorption isotherms were recorded on a BEL Japan Inc was used as the excitation source. The fluorescence decays and
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the instrumental response function (IRF) were collected using developed by us in the 1990s.15 The required tetrachlorides were
1024 channels on a time scale of up to 48.8 ps per channel; generated starting from the commercially available tere- or
alternate measurements (500 counts) of the pulse profile at the isophthaloyl chlorides in two steps (Scheme 2a). In the first
excitation wavelength and the sample emission were performed step diketones were synthesized following a modified Friedel–
until 3000 counts at the maximum were reached. Deconvolution Crafts-acylation procedure using tere-/isophthaloyl chloride and
of the fluorescence decay curves was performed using the tert-butylbenzene or naphthalene.40,41 Next, the bisgeminal
modulation function method in the SAND program, as previously tetrachlorides were obtained by reaction of the diketones with
14,38
described.34
phosphorous pentachloride as described by Horhold et al.
The monomeric model compounds were prepared in a two-
step synthesis. First benzoyl chloride and naphthalene or tert-
butylbenzene were converted to the corresponding diarylketones
in a Friedel–Crafts-acylation protocol42 (Scheme 2b). In the case of
naphthalene, the resulting mixture of 1-benzoylnaphthalene and
2-benzoylnaphtalene was separated using flash column chromato-
graphy. In the second step the desired tetraarylethylenes were
obtained by reductive coupling of the ketones using titanium
tetrachloride and zinc.43 The 1 : 1 mixtures of E/Z-isomers in
¨
Results and discussion
Synthesis and physical characterization of the polymers
and monomeric model compounds
The structures and acronyms of the investigated compounds
are depicted in Scheme 1.
Three new polymers comprising tetraphenylethylene and
diphenyl-dinaphthylethylene backbones together with their
corresponding monomeric analogues were synthesized according
to the general strategic methodology described in Scheme 2.
These polymers were inspired by the well-known aggregation
induced emission, AIE, luminogen, 1,1,2,2-tetraphenylethylene
(TPE).1,7,24,35–37
1
the products (as determined from the H-NMR spectrum) were
separated by recrystallization.
%
The average molecular weight, Mn, weight average molecular
weight, Mw, and polydispersity (PD) values of the polymers are
listed in Table 1. Based on the Mn values the degrees of
%
%
polymerization (DP) have been calculated. The two naphthyl-
substituted polymers, Poly-Np1 and Poly-Np2, can be assigned
to the same model compound, Mono-Np; Poly-Np1 incorpo-
rates para-phenylene main chain linker units, while Poly-Np2
contains meta-phenylene main chain units (Scheme 1). Poly-Np1
and Poly-Np2 display reduced molecular weights, most probably
caused by the increased steric demand coupled with decreased
polydispersity (PD) values. Table 1 also lists the BET-based surface
areas (SBET) of the solid polymer powders extracted from nitrogen
sorption/desorption isotherms (Fig. SI1 and SI2, ESI†). Obviously,
only Poly-t-Bu exhibits intrinsic microporosity with a high SBET
surface area of up to 417 m2 gꢀ1 as a so-called conjugated polymer
of intrinsic microporosity (cPIM).23
All polymers were synthesized via reductive polyolefinations
of aromatic, bisgeminal tetrachlorides following a procedure
14,38,39
¨
initially described by Horhold and co-workers in the 1970s.
In contrast to the original procedure dicobalt octacarbonyl was
¨
used as a condensing agent, a variation of Horhold’s method
The investigated phenyl-terminated monomeric model
compounds, Mono-t-Bu and Mono-Np, are both of E-configuration
as determined by single crystal X-ray crystallography (see Fig. 1). In
the crystalline state, Mono-t-Bu adopts a propeller like conformation
with dihedral angles between the plane subtended by the central
double bond and the phenyl rings of 52.31 and 55.91. The ring
planes of the two tert-butylphenyl groups are rotated slightly less, by
38.81 and 41.11. The molecular conformation of Mono-Np is similar
to that of Mono-t-Bu. The molecule is located on a crystallographic
twofold axis and consequently only one independent dihedral angle
between the two phenyl rings and the olefinic bond exists.
Scheme 1 Representative structures and acronyms of the investigated
polymers, and TPE and tert-butylbenzyl and naphthyl monomeric model
compounds.
Table 1 Physical characteristics of the polymers investigateda
Compound Mn (g molꢀ1
)
Mw (g molꢀ1
)
PD
DP SBET (m2 gꢀ1
)
%
%
Poly-t-Bu
Poly-Np1
Poly-Np2
29 300
11 600
5300
164 000
17 800
9400
5.62 80 417
1.54 33
1.77 15
14
17
a
%
%
Mean and weight average molecular weights (Mn and Mw), polydispersity
Scheme 2 General synthesis of polymers (a) and phenyl-terminated
monomers (b).
(PD) and the degree of polymerization (DP) calculated based on the
average molecular weight (Mn) and BET-derived surface area (SBET).
%
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side groups tend to aggregate (p-stack) with neighbouring side
groups in the solid state, the 4-tert-butylphenyl groups with their
bulky tert-butyl substituents create free volume and microporosity
(please see also the discussion of the X-ray crystal structures of the
monomers).
Electronic spectral data
The photophysical properties of the polymers and model
compounds, TPE and phenyl-terminated monomers, were
investigated in the solid state (amorphous powders and thin
films), in tetrahydrofuran (THF) solution and in mixtures of
good and poor solvent (THF:water mixtures) to evaluate the
effect of aggregation on the fluorescence emission efficiency.
Fig. 3 presents the absorption and fluorescence emission
spectra of the polymers and model compounds in the solid-
state (powder and thin films) and in THF solutions. The
spectroscopic data in Fig. 3 are designed and structured in
order to compare the polymer with the respective monomeric
model compound plus the model AIGen, TPE.
The same wavelength scale is presented for all the com-
pounds, which further allows a direct comparison between the
two types of polymers.
Fig. 1 Molecular structures of the phenyl-terminated monomeric model
compounds, Mono-t-Bu and Mono-Np, determined by single crystal
X-ray crystallography.
The absorption and emission spectra of the investigated
compounds in the amorphous powders are red-shifted and
broader than in THF solution, indicating an extent of conjugation
through intermolecular packing in the solid state (see Fig. 3 and
Table 2). Additional observation of the spectral behavior shows
that there is a good match between the solution absorption band
profiles and those obtained in thin films for the polymers and
monomers (although with a different maximum, Table 2).
Observation of Fig. 3 and Table 2 shows that the emission
spectra of the polymers are red-shifted relative to the monomeric
model compounds, emitting in the visible green region of the
light spectrum: in solution, the emission maxima correspond to
530 nm, for Poly-t-Bu, and 545 nm and 518 nm for Poly-Np1 and
This amounts to 56.21. The same symmetry restriction holds for the
naphthyl rings, which are rotated by 53.31 against the plane of the
double bond. While the molecular conformations of the two
propeller shaped molecules are similar, a marked difference is
observed regarding the intermolecular interactions. Mono-t-Bu does
not form p–p interactions, either between the tert-butyl rings or the
phenyl rings. In contrast, Mono-Np exhibits p-stacking between the
naphthyl rings. The shortest C–C contact between the rings is 3.32 Å
and slightly shorter than in graphite (Fig. 2). The distance between
the centroids of the overlapping halves of the naphthyl rings is 4.8 Å,
indicating ring slippage and reduced overlap of the p-systems.
However, the p-stacking is not limited to pairs of molecules but
extends infinitely through the crystal along the c-axis. Also, the
observed differences in the surface area values of the polymers
should result from the different packing behaviour of the 4-tert-
butylphenyl vs. naphthyl side groups. Indeed, while the naphthyl
Fig. 3 Normalized absorption and emission spectra for the compounds in
powder and thin films (i.e., in a solid environment) and in THF solution. For
better comparison the polymers together with the corresponding mono-
mer are presented the same graph. Solid line: normalized absorption
spectra; dashed line: normalized emission spectra. For comparison, the
TPE absorption and emission spectra are shown (grey lines).
Fig. 2 View of the crystal packing of Mono-Np, showing short Cp–Cp
interactions (in cyan) parallel to the c-axis.
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Table 2 Room temperature spectroscopic data (absorption and fluores-
cence emission maxima together with Stokes shift, DSS) for the polymers
and model compounds in THF, amorphous powders and thin films
Table 3 Room temperature fluorescence quantum yields (fF) for the
investigated model compounds and polymers in different media
THF
90W : 10 THF
Powder
Film
Compound
Medium
labs (nm)
lem (nm)
DSS (cmꢀ1
)
TPE
0.003
0.024
0.046
0.003
0.028
0.008
0.25
0.20
0.62
0.043
0.011
0.003
0.23a
0.14
0.30
0.14
0.012
0.004
0.26b
0.46b
0.64
Powdera
Filmb
THFa
Powder
Filmb
THF
Powder
Film
THF
Powder
Filmb
THF
Powder
Film
THF
Powder
Film
381
454
482
—
441
473
358, 490
546
537
530
468
479
407, 490 (sh)
590
553
545
511
611
518
4220
11 304
—
Mono-t-Bu
Poly-t-Bu
Mono-Np
Poly-Np1
Poly-Np2
TPE
243/312
238/308
382
245/320
242/314
421
252/355
250/366
362
226/337
220/334
391
222/351
222/343
346
268/348
221/335
0.12b
0.010
0.002c
Mono-t-Bu
Poly-t-Bu
Mono-Np
Poly-Np1
Poly-Np2
3052
10 108
3914
5438
9547
8454
6257
8797
5370
8626
10 407
10 806
9332
12 369
10 546
a
b
Data from ref. 44. Prepared using Zeonexs as a polymeric matrix.
f
c
F determined by comparison with the emission area of the Poly-Np1
film, with the same absorption at the excitation wavelength.
thin films where a small red shift is observed when compared to
Mono-Np (see Table 2). This shows that the chromophoric unit is in
these polymers limited to the monomeric counterpart.
In the case of Mono-Np the two bands at maxima 407 nm
and 498 nm (Fig. 3, right hand bottom panel) indicate the
coexistence of monomer and aggregate species. This is further
complemented with a concentration dependence study in
Fig. SI3 (ESI†).
THF
a
b
Data from ref. 44. Prepared using Zeonexs as a polymeric matrix.
Poly-Np2, respectively. In films, Poly-Np2 presents the highest
It is also worth noting that in the solid state (powder and
emission wavelength found for the studied compound, with a thin films) the investigated phenyl-terminated monomers
maximum at 611 nm (Table 2). Generally, the polymers also have retain the electronic spectral absorption features and maxima
higher Stokes shifts (DSS) when compared to their respective found for TPE (Fig. 3). On going to solution (THF), although
monomers, in the same medium. Since charge transfer bands red-shifted (B6 nm for Mono-t-Bu and B26 nm for Mono-Np,
cannot occur in these polymers this behavior points to the see Table 2), similar broad absorption bands are observed for
adoption of different conformations in the ground- and excited- the monomeric model compounds when compared with TPE.
state of the polymers (probably adopting a more planar conforma- The higher bathochromic shift found for Mono-Np can be
tion in the excited-state, which can lead to an extended degree attributed to the increase in the conjugation segment promoted
of conjugation). The more extended nature of the p-conjugation in by the naphthyl moieties when compared with the phenyl units
Poly-Np1 compared to Poly-Np2 in THF solution may be explained in TPE.37
by the fact that the para-phenylene units in Poly-Np1 formally allow
Clearly contrasting with the behavior found for TPE in good
extended p-conjugation along the main chain (not much due to its solvents, where fluorescence emission is almost negligible (with
strongly distorted conformation), while in Poly-Np2, the meta- fF values of 0.002 or 0.0024 and of 0.003 in Table 3),37,45,46 the
phenylene main chain units act now as effective conjugation phenyl-terminated monomeric model compounds, Mono-t-Bu
barriers. This is further reflected in the differences observed in and Mono-Np, display fluorescence both in solution and in the
the solution absorption and emission (solvent:THF) spectra solid state (Fig. 3 and Table 3). Indeed, TPE itself fluoresces when
(Fig. 3, bottom right hand panel). In contrast with these results, aggregated in solution37,47 or in the solid state (Fig. 3), i.e., when
a different behavior is found in thin films (Fig. 3, middle right restriction of intramolecular rotation (RIR) occurs, thus hindering
hand panel) where the fluorescence spectrum of Poly-Np2 is the excited state radiationless processes.
red-shifted when compared to Poly-Np1. While the THF solution
In THF solution, when TPE is compared with the tert-butyl-
and thin film emission maxima of Poly-Np1 are quite similar TPE (Mono-t-Bu) derivative, the absorption spectra almost
(545 nm in THF solution vs. 553 nm in the thin film), Poly-Np2 totally match and the emission is absent in TPE. However,
shows a distinctly red-shifted PL maximum when compared to the in the case of Mono-t-Bu, the emission spectrum presents
solution value (611 nm in the thin film vs. 518 nm for the THF two bands: a vibronically resolved band with a maximum at
solution). This finding may be explained by increased side chain B358 nm (corresponding to the monomer) and a second broad
aggregation (and AQC) in Poly-Np2 with the aggregate emission as band with a maximum at B490 nm corresponding to the
the dominating feature. This explanation is supported by the emission of the aggregate. This last band increases and red-
reduced thin film fF for Poly-Np2 (PLQY: 0.02 for Poly-Np1 vs. shifts with the addition of water (Fig. 4). Moreover the fF value
0.001 for Poly-Np2, which will be further discussed in the next of Mono-t-Bu in THF is one order of magnitude higher than
section on Table 3).
that of TPE (Table 3).
Upon going from the monomeric model compound, Mono-t-Bu,
Although the emission spectra of the model compounds
to the polymer Poly-t-Bu a significant red shift of the absorption Mono-t-Bu and Mono-Np in films quite overlap with that of
spectra is observed (39 nm in powder, 35 nm in film and 52 nm in TPE, in powder the emission of Mono-t-Bu is blue-shifted when
THF). The same is not found for the Poly-Np polymers in THF and compared to TPE, with emission maxima of 441 nm and
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Fig. 4 Room temperature fluorescence emission (photoluminescence, PL) spectra for the model compounds (TPE, Mono-t-Bu and Mono-Np) and
polymers (Poly-t-Bu, Poly-Np1 and Poly-Np2) in THF:water mixtures and their respective correlation of the fluorescence quantum yield and emission
maxima with increasing water fraction, fW = 0–95%. The lines in the right hand panels are just meant to be a guide to the eye.
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454 nm for Mono-t-Bu and TPE, respectively (Fig. 3 and reported46 in comparison with our fF of 0.25 (for fW = 90%).
Table 2). In the case of Mono-Np and again compared with The values obtained in this study are slightly different from
TPE, there is now a red-shift of the emission maximum of those reported previously in films. This is likely because the
14 nm (Table 2). For the latter monomer the observed red-shift photophysical properties are intrinsically dependent on the
together with the increase in the fluorescence quantum yield on morphological properties of the films. Indeed, and exemplify-
going from solution to the solid state gives support to the ing this, the fF of TPE in powder was described as 24.1%,52
formation of J-aggregates.48
found to be in good agreement with the value previously
obtained by us (23%)44 with the same equipment that was used
to perform all the fF measurements.
Aggregation induced emission (AIE) studies
To further explore the occurrence of AIE in the investigated
Usually, when AIEgens, such as TPE and its derivatives, are
compounds, the emission behavior was studied in THF:water chemically incorporated into polymeric structures, polymers with
mixtures (Fig. 4). TPE blue emission is only visible in high water AIE characteristics could be obtained.32,36,53 Thus, it was anticipated
content, either in acetonitrile:water7 or in THF:water mixtures.49 that Poly-t-Bu and Poly-Np1 and Poly-Np2, whose monomeric model
The broad emission band associated with this emissive aggregate, compounds are AIE active, would also maintain AIE properties.
centered at ca. 470 nm, becomes detectable at a water fraction Although Poly-t-Bu presents an increase of fluorescence quantum
( fW) Z 85%.
yield upon the increase of the water fraction, the fluorescence of the
Enhancing the steric effect is an efficient means to activate naphthyl polymers is quenched with the addition of water in
the restriction of intramolecular motion (RIM) process,47 which THF:water mixtures. Indeed, as can be seen from Fig. 2 Poly-Np1
is considered one of the possible mechanisms to explain the and Poly-Np2 present ACQ. Moreover, as shown in Table 3, the fF of
AIE phenomenon.50 The phenyl rings in TPE have been sub- Poly-Np2 is smaller than Poly-Np1 in solution and the solid state
stituted with multiple methyl groups at the o-positions to and when compared at the same fW in THF:water mixtures. The
examine how the intramolecular steric effects affect its photo- ACQ behaviour of Poly-Np1,2 may be caused by the stacking
physical properties and AIE properties.46 It was shown that the tendency of the naphthyl groups, leading to the formation of
sterically crowded TPE derivative with four methyl groups loses H-type aggregates. This is supported by the observed intermolecular
its AIE activity, directly validating the RIM hypothesis.51 How- p-interactions of the naphthyl substituents in the single crystal
ever, it is expected that the t-Bu groups substituted in the structure of Mono-Np.
p-position of the benzyl rings of TPE should enhance the steric
In the solid state, the highest photoluminescence efficiency
effect and not affect the rotation of the aromatic rotors; there- was obtained for Poly-t-Bu, with fF = 64% in film – an increase
fore, the Mono-t-Bu monomer should present AIE properties. of ca. 14ꢁ when compared to THF and higher than TPE and
To verify this hypothesis, the emission spectra and fluorescence Mono-t-Bu – and when aggregated in THF:water mixtures it
quantum yields (fF) for Mono-t-Bu were obtained in THF:water shows a value of fF = 62% for fW = 90%.
mixtures. Naphthalene-substituted ethenes, such as Mono-Np,
The combination of TPE and carbazole groups also generated
porous organic polymers with higher SBET area, varying from 472
are AIE-active luminogens.37
The spectral behavior of the investigated model compounds to 2200 m2 gꢀ1 24,25,54 with fluorescence quantum yields of up to
,
and polymers in THF:water mixtures, aiming to observe the 40% in films55 (see also Table SI5, ESI,† for a literature review of
presence of and to quantify the AIE effect, is shown in Fig. 4 SBET and fluorescence quantum yield values for porous TPE based
and Table 3. From Fig. 4, it can be seen that Mono-t-Bu presents polymers). In our set of TPE derived polymers, a new compound
a structured emission spectrum in THF:water mixtures up to with moderate surface area (412 m2 gꢀ1), but higher fluorescence
fW = 60% ( fW: water:THF fraction, v/v), increasing in intensity quantum yields (64% in spin coated films), is described.
with the addition of water. From fW Z 70% on, the emission
Time resolved fluorescence decays were also obtained in
maximum for Mono-t-Bu red-shifts and the fF value increases order to gain further insight into the aggregate formation, i.e.,
with the addition of water, up to fF = 0.2 for fW = 90%, i.e., AIE in Poly-t-Bu. The fluorescence decays of Poly-t-Bu in THF
B1 order of magnitude higher than in pure THF (Table 3).
and THF:water mixtures were found to be well fitted with a
Mono-Np shows a similar behavior (to that found for Mono- bi-exponential decay law (Fig. SI4, ESI†), thus indicating
t-Bu), i.e., a vibronically resolved spectrum up to fW = 60% and the presence of two emitting species in the aggregate region
a red-shifted emission band, followed by an increase in the fF (lem = 525 nm). This possibly indicates that the emissive
values for fW Z 70%. This observation is likely due to the aggregate results from two main conformers with different
formation of J-aggregates in the two phenyl-terminated mono- contributions. Indeed, the fractional contribution (Ci), in
mers. Indeed, although the presence of the naphthalene rings Table 4, of each species is given by the following equation56
could favor p–p interactions, leading to the formation of H-
aggregates and consequently to aggregation caused quenching
(ACQ), the opposite behavior was found.
aiti
Cið%Þ ¼
ꢁ 100
(1)
n
P
aiti
i¼1
The fF values for TPE and Mono-Np in films have been
previously reported as 0.49 and 0.30, respectively.37 Our values where n stands for the number of exponential terms and ai the
in Table 3 are 0.24 (TPE) and 0.46 (Mono-t-Bu), while for TPE in contribution of each exponential term at t = 0 and ti is the
a THF:water mixture fF = 0.14 for fW = 95% was previously associated decay time.
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Table 4 Room temperature fluorescence quantum yields (fF) and lifetimesa (t) for Poly-t-Bu in selected THF:water mixtures. Also present are the
associated pre-exponential values (ai) and fractional contribution of each decay time (%Ci) and the chi-square values (w2) for the judgment of the quality
of the fits. Radiative (kF) and radiationless rate (kNR) constants associated with the second decay component (t2) are also presented
w2
fF
%C1
%C2
kF (nsꢀ1
b
)
kNR (nsꢀ1
)
c
Solvent
a1
t1 (ns)
a2
t2 (ns)
100% THF : 0% water
80% THF : 20% water
50% THF : 50% water
30% THF : 70% water
10% THF : 90% water
0.947
0.768
0.692
0.627
0.601
0.23
0.5
0.78
0.98
1.23
0.053
0.232
0.308
0.373
0.399
1.45
1.87
2.36
2.74
3.3
1.01
1.29
1.27
1.35
1.05
0.046
0.161
0.332
0.485
0.612
73.9
47.0
42.6
37.5
36.0
26.1
53.0
57.4
62.5
64.0
0.032
0.086
0.141
0.177
0.185
0.658
0.449
0.283
0.188
0.118
fF
c
tF
ð1 ꢀ fFÞ
a
b
Experimental conditions: nanoLed lex = 339 nm; lem = 525 nm; 48.1 ps per channel, 3k counts. kF
¼
.
kNR
¼
.
tF
tert-butyl substituents and naphthalene-substituted ethenes,
respectively, showed AIE active properties in the solid state and
in THF:water mixtures. This behavior is attributed to RIR in the
solid state and when the compounds aggregate with the addition
of the ‘‘poor solvent’’ (water), which hinders the excited state
radiationless channels. The polymer Poly-t-Bu was found to be
more fluorescent than its monomer Mono-t-Bu and to retain its
AIE active properties. The fF value of Poly-t-Bu is found to be one
order of magnitude higher in the solid state than in solution
(fF = 64% in film vs. 4.6% in THF). This is accompanied by
distinct microporosity in the solid state with a high SBET surface
area of 417 m2 gꢀ1, probably driven by the steric demand of the
tert-butyl groups. Poly-t-Bu, therefore, represents a so-called con-
jugated polymer of intrinsic microporosity (cPIM) with an attrac-
tive combination of intrinsic microporosity and occurrence of
distinct AIE effects, thus predestining further experiments into
stimuli-responsive properties in contact with suited analytes. In
contrast, the diphenyl-dinaphthylethylene derivative polymers,
Poly-Np1 and Poly-Np2, are less emissive than Mono-Np and
presented ACQ properties. These properties are assigned to the
occurrence of strong p–p interactions in these polymers, leading
to the formation of non-fluorescent H-aggregates. The occurrence
of p–p interactions may also lead to tight packing of Poly-Np1
and Poly-Np2, since these polymers do not show intrinsic
microporosity (SBET o 17 m2 gꢀ1).
Fig. 5 Correlation of fluorescence quantum yields and fluorescence
decay times for Poly-t-Bu in THF:water mixtures with increasing water
fraction (fW).
The decay time values together with the fractional contribu-
tion of each species (Ci) at the aggregate emission wavelength
(525 nm) change with the increase of the water fraction in the
mixture (Fig. 5). For Poly-t-Bu in THF a fast decay component,
t1, in the 0.23–1.2 ns range and a longer decay time, t2, ranging
from 1.5 ns to 3.3 ns were found on going from 0% to 90%
water content (Table 4). As shown in Fig. 5 the fluorescence
quantum yields and the fluorescence lifetimes follow a similar
trend, i.e., a concomitant increase of these parameters with the
increase of the water fraction. In addition, the determination of
the radiative (kF) and radiationless (kNR) rate constants clearly
shows a decrease of kNR and an increase of kF with fW. This is
valid for the two decay components, but in Table 4 the data are
only shown for the longer (t2) component. The radiationless
decay is therefore dominant up to fW = 70%, and from there on
the radiative deactivation begins to dominate.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by Project ‘‘Hylight’’ (no. 031625) 02/
SAICT/2017 which is funded by the Portuguese Science Foun-
dation and Compete Centro 2020. We acknowledge funding
from Fundo Europeu de Desenvolvimento Regional (FEDER)
Conclusions
In summary, in this work we have synthesized propeller-shaped through Programa Operacional Factores de Competitividade
structures derived from tetraphenylethylene (TPE). The optical (COMPETE) and project ROTEIRO/0152/2013. The Coimbra
˜
ˆ
and PL properties of these compounds were studied in the solid Chemistry Centre is supported by the Fundacao para a Ciencia e
state (powder and films) and in solution (THF and THF:water a Tecnologia (FCT), Portuguese Agency for Scientific Research,
mixtures). Their properties were compared to the model through the Project PEst-OE/QUI/UI0313/2014. The research leading
compound TPE in similar experimental conditions. Mono-t-Bu to these results has received funding from Laserlab-Europe (grant
and Mono-Np, monomeric model compounds with para-position agreement no. 284464, EC’s Seventh Framework Programme).
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Paper
26 Y. Xu, S. Jin, H. Xu, A. Nagai and D. Jiang, Chem. Soc. Rev.,
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