Phenothiazine based nanocrystals with enhanced solid state emission

Article abstract of DOI:10.1016/j.molliq.2018.05.125

The paper focuses on the preparation of phenothiazine based nanoparticles, both in liquid and solid states, by reprecipitation in water and solvent induced phase separation respectively. Polymethylmethacrylate was used as an inert matrix and polyfluorene

Full text of DOI:10.1016/j.molliq.2018.05.125

Accepted Manuscript
Phenothiazine based nanocrystals with enhanced solid state
emission
Andrei Bejan, Luminita Marin
PII:
DOI:
Reference:
S0167-7322(18)32165-2
doi:10.1016/j.molliq.2018.05.125
MOLLIQ 9183
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Journal of Molecular Liquids
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24 April 2018
25 May 2018
28 May 2018
Please cite this article as: Andrei Bejan, Luminita Marin , Phenothiazine based
nanocrystals with enhanced solid state emission. Molliq (2017), doi:10.1016/
j.molliq.2018.05.125
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ACCEPTED MANUSCRIPT
Phenothiazine based nanocrystals with enhanced solid state emission
Andrei Bejan, Luminita Marin*
“Petru Poni” Institute of Macromolecular Chemistry of Romanian Academy, Iasi, Romania
*lmarin@icmpp.ro
Abstract
The paper focuses on the preparation of phenothiazine based nanoparticles, both in
liquid and solid states, by reprecipitation in water and solvent induced phase separation
respectively. Polymethylmethacrylate was used as an inert matrix and polyfluorene as a
luminescent matrix. The morphological features of the nanoparticles were investigated by
dynamic light scattering, and SEM, AFM and POM microscopy, which revealed their
supramolecular ordering as nanocrystals. Their photophysical behaviour was monitored by
UV-vis and photoluminescence spectroscopy. The nanocrystals emitted green light, with
higher efficiency compared to the bulk crystals, reaching an absolute quantum yield of 35 %
in water, 45 % in polymethylmethacrylate, and 39 % in polyfluorene.
Keywords: phenothiazine, nanocrystals, efficient luminescence, polymethylmethacrylate,
polyfluorene
Introduction
Nanoparticles are a class of nanomaterials intensely studied in the last decades due to their
outstanding properties, which make them promising for optoelectronics (light emitting
devices, transistors, solar cells) and bioengineering (imaging, ultrasensitive detection at
molecular and cellular level, drug delivery) [1-4]. Among nanoparticles, remarkable
endeavour has been directed to the development of inorganic crystals (e.g. quantum dots) and
conjugated polymeric nanoparticles. On the other hand, the nanocrystals based on organic low
molecular weight compounds were less studied, despite the fact that they are cheaper,
biologically more compatible and environmentally safer, bringing the advantage of reversible
assembling/disassembling by crystallization/dissolving processes, and thus prompting an easy
recycling [5,6]. The main drawback which hampered the developing of the organic
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nanocrystals is the difficulty to control their morphology and to prevent their precipitation. To
address both challenges, aromatic units which favour an easy crystallization should be used
[5-9].
Phenothiazine is a fused heterocycle that proved an easy crystallization [10-12]. Its
strong electron-donating capability made it an important building block in the chemical
engineering of the highly conjugated donor-acceptor materials for optoelectronics. A literature
survey reveals that the phenothiazine was extensively involved to design dyes for solar cells,
while less attention was devoted to its application for organic light emitting diodes. This
happens despite the fact that its butterfly geometry favours the light emission in solid state,
since it prevents strong intermolecular forces, limiting by this the non-radiative decay [10-12].
The few studies reported in literature to date, targeted mainly the obtaining of donor-acceptor
polymers, with the phenothiazine units incorporated in the backbone, as an alternant moiety to
an acceptor counterpart (oxadiazole, fluorene or cyano substituted aromatic rings) [13-15] or
as side chains on a polyfluorene backbone [16]. Also, the phenothiazine was used as a
building block for dendrimers [17] or molecules with RIR (restricted intermolecular rotation)
design [18, 19]. In all cases, the quantum efficiency gave encouraging values. The most
promising properties, a quantum yield of almost 60 % in thin solid films, were attained by its
grafting on the polyfluorene chains via a flexible spacer which decoupled the donor-acceptor
conjugation [16]. A critical view of these data indicates that phenothiazine is a challenging
building block in designing materials for optoelectronics, which deserves further efforts for
improving its performances. The progress in the optoelectronic field revealed that the
quantum yield in solid state could be greatly improved by manufacturing materials as blends,
nanocomposites, nanocrystals or cocrystals [2, 15, 19-23]. While extensive attention has been
directed to the use of these strategies to build luminescent materials based on traditional
luminophores, little interest was paid to the phenothiazine [24-27]. Considering the
phenothiazine’s ability to facilitate an easy crystallization when it is involved as a building
block in different compounds, the obtaining of phenothiazine based nanocrystals appears as a
challenge from both academic and applicative points of view.
In this contribution we report the preparation of nanocrystals based on a phenothiazine
derivative bearing bromine and carbonyl units. They were obtained as water suspensions by
the reprecipitation method, and as nanohybrid films by dispersing in polymethylmethacrylate
and polyfluorene matrix. Their morphological features were investigated using DLS, SEM,
AFM and POM technics and their photophysical behaviour was monitored by UV-vis and
photoluminescence spectroscopy.
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Experimental
Materials
Poly(9,9-di-n-octylfluorenyl-2,7-diyl) (99 % purity, Mn=20 000) and poly(methyl
methacrylate) (99 % purity, Mn=120 000) have been purchased from Sigma-Aldrich and used
as received. 7-Bromo-10-(-4-hexyloxy-phenyl)-10H-phenothiazine-3-carbaldehyde (FhABr)
has been prepared in our laboratory by bromination of the corresponding formyl derivative
with N-bromosuccinimide, following a published procedure [11]. The product has been
obtained as yellow-greenish needle single crystals, by recrystallization from ethyl acetate.
1H- NMR (400.13 MHz, DMSO-d6, ppm) δ = 9.73 (s, 1H), 7.53 (s, 1H), 7.45 (d, 1H), 7.37
(d, 2H), 7.31 (s, 1H), 7.23 (d, 2H), 7.11 (d, 1H), 6.20 (d, 1H), 6.02 (d, 1H), 4.08 (t, 2H), 1.82-
1.75 (m, 2H), 1.52-1.33 (superposed bands, 6H), 0.91 (t, 3H); 13C-NMR (400.16 MHz,
DMSO-d6, ppm) δ = 190.22 (C27), 158.83 (C18), 148.21 (C11), 141.83 (C12), 131.36 (C16,
C20), 131.16 (C15), 130.95 (C3), 130.02 (C4), 129.92 (C8), 128.30 (C6), 127.34 (C2), 120.58
(C14), 118.22 (C13), 117.80 (C7), 116.93 (C17, C19), 115.28 (C9), 114.89 (C1), 67.83 (C21),
30.95 (C24), 28.59 (C22), 25.17 (C23), 22.04 (C25), 13.88 (C26); FT-IR (KBr, cm-1): 3069,
3053 (νCHaromatic), 2954, 2935, 2866 (νCH3, νCH2), 1682 (νC=O), 1607, 1511 (νC=Caromatic), 1240
(νC-O-C), 1027 (νC-Br), 838, 803, 745 (δCHaromatic); Elemental analysis calc. for C25H24BrNO2S
(482): C 62.24, H 5.01, N 2.9, S 6.65. Found: C 62.43, H 5.21, N 3.03, S 6.71.
Nanoparticle preparation
Phenothiazine nanocrystals have been prepared by the reprecipitation method, which was
chosen in order to avoid the influence of undesirable impurities, which drastically would
affect their optical properties [2]. To this end, the FhABr solution in THF, 2x10-3 M, has been
dropped into a vial containing 6 mL of water nonsolvent, under vigorous magnetic stirring,
for 10 minutes. To establish the optimum concentration towards nanoparticles of best optical
properties, five different volumes of FhABr solution were used, leading to five different
systems (Table 1). The nanoparticles features were measured on fresh solutions and after one
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weak, in order to establish the ageing influence upon their dimensions. FhABr solution in
THF was prepared in similar conditions and used as control sample (Table 1).
Table 1. Preparation of nanoparticles suspended in water and their codes
Code
0
1
2
3
4
5
THF solution of FhABr 2x10-3 M (mL) 0.05 0.01 0.02 0.03 0.04 0.05
Water (mL)
THF (mL)
0
6
6
0
6
0
6
0
6
0
6
0
0
-
99.8 99.6 99.5 99.3 99.1
0.33 0.66 1 1.33 1.66
Water fraction
FhABr in water (x10-5 M)
Further, in order to obtain solid thin films proper for optoelectronic applications, the
phenothiazine nanoparticles were dispersed into a polymeric matrix, by solution induced
phase separation method [28, 29], into polymethylmethacrylate (PMMA) and polyfluorene
(PF). A volume of 2 % solution of FhABr in toluene was slowly dropped into 6 mL of a 2 %
solution of polymer in toluene under vigorous magnetic stirring (Table 2) [29]. Different
volumes of FhABr solution (Table 2) were used, in order to reach a final concentration of the
FhABr in polymer from 0.33 to 1.33 %, to see the influence of the concentration on the
morphology and properties. The clear solutions were vigorously magnetically stirred for 10
minutes, casted onto glass lamellas, and then incubated into a closed case in order to ensure
the slowly removing of the solvent in order to facilitate the FhABr crystallization as the
polymer solution harden.
Table 2. Preparation of the polymer dispersed nanoparticles and their codes
FhABr 2 % (mL)
PMMA 2 % (mL)
Code
0
6
0.02 0.03 0.04 0.06 0.08 0.1
6
6
6
6
6
6
PMMA 1a
6
2a
6
3a
6
4a
6
5a
6
6a
6
PF 2% (mL)
6
Code
PF
1b
2b
3b
4b
5b
6b
FhABr in PMMA/PF (%)
0.33 0.5 0.66 1
1.33 1.66
Equipment
The size and distribution of the nanoparticles were analysed by dynamic light
o
scattering on a Delsa Nano C, Beckman Coulter equipment, at room temperature (25 C). The
scattering light was measured at the fixed angle of 160 oC.
The phase segregation of the nanoparticles dispersed into polymeric matrix was firstly
attributed with a field emission scanning electron microscope (Scanning Electron Microscope
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SEM EDAX – Quanta 200) at lower accelerated electron energy of 20 Kev, to avoid the
sample decomposition. For a more accurate attribution of the nanoparticles dimension, atomic
force microscopy images of the film samples were acquired with a Solver PRO-M, NT-MDT,
Russia instrument, in semi contact mode. Nova v.1443 software was used to record and to
analyse the AFM topographic and phase contrast images. To further ascribe the crystalline
nature of the nanoparticles dispersed into the polymeric matrix, the samples were observed
with an Olympus BH-2 polarized light microscope.
Ultraviolet–visible (UV–vis) absorption and photoluminescence (PL) spectra were
recorded on a Carl Zeiss Jena SPECORD M42 spectrophotometer and Perkin Elmer LS 55
spectrophotometer, respectively, in solutions (nanoparticle suspensions) and thin films.
The fluorescence quantum yield (QY) and chromaticity diagrams of the nanoparticle
suspensions and polymer dispersed nanoparticles were measured on a FluoroMax-4
spectrofluorometer equipped with a Quanta-phi integrating sphere accessory Horiba Jobin
Yvon, by exciting at the absorption maxima, at room temperature. The slit widths and detector
parameters were optimized to maximize but not saturate the excitation Rayleigh peak, in order
to obtain a good optical luminescence signal-to-noise ratio.
Results and discussions
Rational design
Nanocrystals based on a phenothiazine derivative (FhABr) have been prepared by (i)
reprecipitation in water and (ii) solvent induced phase separation methods, using as a polymer
matrix polymethylmethacrylate (PMMA) and polyfluorene (PF). Three series of nanocrystals
samples were obtained by varying (i) the water fraction from 99.1 up to 99.8 (coded 1-6), and
the amount of FhABr from 0.33 to 1.66 % in (ii) PMMA (coded 1a-6a) and (iii) PF (coded
1b-6b) (Table 1, 2). Their design was thought starting from a series of premises which should
allow the obtaining of multifunctional materials useful for biological and opto-electronic
applications, as follows. (i) The phenothiazine has strong electron donating ability which
renders it an important building block for opto-electronic materials [13-19]. (ii) Its derivatives
proved a large range of biological activities and good biocompatibility, being presently
studied for their antibacterial, antifungal, anticancer, antiviral, anti-inflamatory, antimalarial,
anticonvulsant, analgesic, immunosuppressive, antifilarial, trypanocidal, and multidrug
resistance reversal properties [30, 31]. (iii) The FhABr has been chosen because it emitted
pure green light with a high quantum yield in solution and had the ability to easily grow as
crystals from a large variety of solvents [11]. Thus, it was estimated that FhABr has a high
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potential to form nanocrystals into the water nonsolvent or into a viscous solution of polymer
respectively. This should result in an increase of the surface-to-volume ratio and consequently
in an improvment of the quantum yield in solid state. Moreover, combining the FhABr with
the polyfluorene luminescent matrix, should bring the advantage of an easy manufacturing of
the thin films and should raise potential synergistic effects, providing new materials with a
good balance of photophysical properties. (iv) Water has been chosen as a nonsolvent because
it is the friendliest biodispersant, enabling the obtaining of biocompatible materials. (v)
PMMA was used as matrix for nanocrystals because it is colourless and has good insulating
properties, high light transmittance, chemical resistance, low optical absorption and low cost,
being an excellent inert matrix without interference from electron-transfer dynamics or
emission [32]. (vi) The blue light emitting polyfluorene has been chosen as matrix for the
nanocrystals, due to its high photoluminescence in solid state which ranked it as the most
prospective blue light emitting candidate for building organic light emitting diodes [33].
Previous studies reported that the combination of phenothiazine and fluorene chromophores
by decoupling their conjugation led to materials with efficient luminescence in solid films
[16]. Thus, by dispersing the phenothiazine chromophore into the polyfluorene matrix,
materials with improved luminescence are expected to be obtained, as already demonstrated
for other dyes [34]. All these premises indicated the obtaining of the phenothiazine based
nanoparticles as a promising route towards bio- and opto-electronic applications.
Nanocrystals. Morphology and supramolecular arrangement
FhABr has a great potential to crystalize from a wide range of solvents due to its
rigid-flexible structure which favours the segregation of phenothiazine arrays with weak π-π
intermolecular distances [11] (Figure 1a). To monitor its ability of nanocrystallization by
reprecipitation in water and by its dispersing into a polymeric matrix, dynamic light scattering
method (DLS), scanning electron microscopy (SEM), atomic force microscopy (AFM),
polarized light microscopy (POM) and fluorescence microscopy (MF) measurements were
performed.
Reprecipitation of FhABr in water led to macroscopic clear solutions. However, DLS
measurements revealed the presence of nanoparticles with mean hydrodynamic diameter from
113 to 155 nm and narrow polydispersity index from 0.09 to 0.2 (Figure 1c). The sample 3
showed the narrowest size distribution (0.098) (Table 1s), indicating the water fraction of 99.5
as being the optimum for providing homogeneous samples. By contrast, the pristine solution
of FhABr in THF didn’t reveal any nanoparticle formation. The ageing of the samples lead to
an increase of the nanoparticle dimensions up to 985 nm and a polydispersity index up to 0.4
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(Figure 1d, Table 1s). The samples with average amounts of FhABr (2, 3) suffered less
alteration of the dimensional parameters, suggesting once more the water fraction of 99.5 as
being the best, being able to provide nanoparticles with increased stability, at least for one
week.
SEM images acquired on the suspension samples casted on glass revealed a flower-
like crystalline morphology of the nanoparticles highlighting their tendency of crystallization
in aqueous medium (Figure 1b).
a)
b)
c)
d)
Figure 1. a) X-ray molecular structure of FhABr; b) SEM image of the sample 3 casted on
glass; c) Distribution of the mean hydrodynamic diameter of the samples 1 – 5; d) The
influence of ageing on the mean hydrodynamic diameter of the sample 3
The dispersion of the FhABr into the PMMA and PF matrix respectively, led to self-
standing films. To investigate the segregation of the FhABr in form of nanoparticles into the
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polymeric matrix, and besides to gain three-dimensional information related to their shape,
location, size and distribution, the 1a-6a and 1b-6b samples were analysed by scanning
electron microscopy (SEM) and atomic force microscopy (AFM).
SEM imaging revealed a smooth surface at micrometric level for all the films.
Nanometric round shapes could be distinguished, suggesting that the FhABr grew as
spherical shapes into the polymeric matrix (Figure 2a,b,c). Compared to the flower-like
crystals grown from water (Figure 1b), the spherical shape indicates enough interfacial
tension between the FhABr and the polymeric matrix to force the growing of round
nanoparticles. Regarding the variation of the mean diameter of the nanoparticles, in the case
of PMMA matrix it was observed a similar trend as for water suspensions; lower diameter
values of 160 nm and a standard deviation (SD) of 0.02 were registered for the sample 4a
with a median concentration of FhABr (Figure 1s). A less evident correlation has been
registered in the case of using PF matrix, when the mean diameter varied from 200 nm up to
350 nm in a random manner, with a lower SD for the samples 1b and 2b (Figure 1s). This was
attributed to the higher viscosity of the PF solution compared to the one of PMMA. In
comparison with the nanoparticles formed in water, it could be observed a much narrower
polydispersity attributed to the influence of the viscous polymer solution which hinders the
mobility of the nanocrystals and thus their aggregation. Related to the dimension of the
nanoparticles measured by SEM, it should be considered that the FhABr nanocrystals were
covered by a polymer shell, and their real dimension were lower [30]. To have an insight at
nanometric level, the films surface has been scanned by AFM, topographic and phase contrast
methods. The surface of the PMMA based films was very smooth, with roughness between
2.6 and 48 nm, while those based on PF were rougher, with roughness between 29 and 129
nm (Table 2s). On the surface, well defined spherical shapes were monitored, with similar
diameters as measured by SEM. The AFM phase contrast images confirmed the lower real
dimension of the nanoparticles, with diameter values less than 105 nm (Table 3s).
c) 1b
a) 2a
b) 4a
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d) 2a
e) 2a, phase contrast
f) 2a
Figure 2. Representative images of the film samples, acquired by SEM (a,b,c) and AFM
(d,e,f)
To confirm the supramolecular ordering of the nanoparticles, the samples were
visualized by polarized light microscopy. The crystalline FhABr, as recrystallized from
solution, showed under polarized light sharp wedge geometrical shapes with strong
birefringence (Figure 3a) [11]. When dispersed as nanoparticles into the polymer matrix, it
appeared as birefringent round droplets with different coloured concentric rings (Figure 3b), a
texture usually developed by the layered supramolecualar arrangement of a smectic A
mesophase in bulk or when dispersed into a polymer matrix [35-37]. Its occurring is in
agreement with the tendency of layered self-organization of the FhABr molecules, driven by
the rigid-flexible segregation (Figure 1a). The birefringent droplets could be seen in both
polymer matrices, more difficult to be distinguished into PF because of its own birefringence
(Figure 3b-d) [38]. The birefringent droplets confirmed the crystalline structure of the FhABr
nanoparticles as indicated by SEM and AFM microscopy. Moreover, a closer view of the
droplets reveals an extension of the birefringent concentric rings into the polymeric matrix
surrounding the FhABr nanocrystals, signature of a coupling of the ordering at interface [36,
39]. Thus, it is expected that the FhABr nanocrystals to be strongly anchored into the PMMA
and PF polymeric matrix by the interfacial forces.
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a) FhABr, (200x)
b) 3a, (200x)
c) 3b, (200x)
d) 2a, (400x)
Figure 3. Polarized light microscopy of a) the FhABr single crystals and b-d) FhABr
dispersed in PMMA and PF matrix (the magnification is given in brackets)
Photophysical behavior
The UV-vis spectrum of the FhABr solution in THF (1.33x10-5M) exhibited four
absorption maxima. The two maxima at lower wavelength, at 273 and 293 nm, were attributed
to the spin allowed π-π* benzenoid transitions of the local aromatic units. The two maxima at
higher wavelength, at 342 and 395 nm, were attributed to the localized conjugated system
between the phenothiazine donor (D) and the formyl acceptor (A), and to the D-A
intramolecular charge transfer complex (ICT) respectively (Figure 4a) [40]. By comparison,
the absorption spectra of the FhABr nanocrystals in water revealed a bathochromic shift of
the absorption maxima (to 277, 299, 348 and 404 nm), suggesting electronic transitions to
lower state, characteristic to a head-to-tail arrangement of a J-type coupling into nanocrystals
(Figure 4b, Figure 2s) [40]. The absorption intensity characteristic to the π-π* benzenoid
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transitions slightly decreased, while the absorption intensity characteristic to the conjugated
system and to the intramolecular charge transfer (ICT) slightly increased. This is in agreement
with the restriction of the intramolecular rotations in solid state which favoured the better
conjugation and ICT from phenothiazine donor to formyl acceptor [41]. The absorption
maxima followed a progressive bathochromic shifting as the suspensions aged and further for
the crystalline films (Figure 4b), in agreement with the enhanced intermolecular forces as the
crystal size increased [41, 42].
0
1
1'
crystalline film
4
3
2
300
350
400
450 500
300
350
400
450
500
Wavelength (nm)
Wavelength (nm)
b)
a)
Figure 4. a) Absorption spectra of the nanocrystals samples in water (2, 3, 4) and b)
Comparative absorption spectra of the FhABr in different states: solution in THF (0),
nanocrystals in water (1), nanocrystals in water aged for one week (1’) and crystalline film
To establish the influence of the nanocrystallization on the luminescence properties,
the emission spectra of the FhABr nanocrystals dispersed in water, and PMMA and PF
polymer matrix were recorded, by exciting with the absorption maxima of 348 and 404 nm.
The spectra of the FhABr solution, and pristine PMMA and PF films were recorded also as
reference. At first, no influence of the excitation wavelength on the emission profile was
noted (Figure 5, Figure 3s). The emission spectrum of the FhABr nanocrystals in water and
PMMA had a similar shape to that of the FhABr solution, with the main difference of a blue
shifting of 10 nm and 35 nm respectively, from 535 nm to 525 and 500 nm (Figure 5). This
blue shifting of the emission was attributed to the interfacial phenomena, which affected the
FhABr ordering into the crystalline state [23, 24, 29, 30, 43], and also to the different size of
the nanocrystals [44, 45]. A similar behaviour was observed in the case of the hydrogels
obtained by the formation of phenothiazine ordered clusters into the aqueous matrix [46]. On
the contrary, the emission of the FhABr bulk crystals was red shifted (545 nm) compared to
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that of the FhABr solution, clearly indicating the influence of the surrounding medium on the
supramolecular ordering (Figure 5). This influence was more evident for the FhABr
nanocrystals in PF, which showed an emission spectrum similar to that of the pristine PF
matrix, with three emission maxima (466, 493, 538 nm) slightly red shifted around 7 nm.
20000
FhABr sol
FhABr cr
3
3a
3b
10000
0
450
500
550
600
650
Wavelength (nm)
Figure 5. Comparative emission spectra of FhABr solution and FhABr nanocrystals
in water (3), PMMA (3a) and PF (3b), excited at 348 nm
The intensity of the emission was drastically affected by the nature of the
supramolecular ordering of the FhABr: it was the highest in the case of the isolated molecules
in solution, the lowest for the bulk crystalline state and medium for the nanocrystals, both in
water or polymer matrix. The variation of the concentration of the FhABr into the water
suspensions and polymer films didn’t significantly affect the emission profile (Figure 6a, b,
c), but significantly influenced the emission intensity. Interesting enough, the emission
intensity was correlated with the size of the nanocrystals and not with the content of the
chromophore; the highest emission intensity has been recorded for the nanocrystals with the
lowest size.
While no significant influence of the excitation wavelength was observed on the
emission profile, its drastic influence has been registered on the emission intensity, quantified
by measuring of the quantum yield (Table 3, Table 4). The FhABr nanocrystals showed, in a
similar manner with the THF solution and bulk crystalline state, that the excitation with the
absorption maximum of the intramolecular charge-transfer (ICT) band resulted in a more
effective fluorescence than the excitation with the absorption maximum of the conjugated
system. It is expected that the interaction of the FhABr chromophore with the polar
environment (THF, PMMA) to stabilize its planar intramolecular charge transfer state by
diminishing the torsion motions around the single bonds [47, 48]. Considering the larger
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intermolecular distances between the phenothiazine cores, the improvement of the quantum
yield when excited with the lower ICT energy should be attributed to the weakening of the
non-radiative de-excitation channels. This should be favoured by the diminishing of the
excimer formation by π-π stacking and of the energy dissipation to the surrounding
environment.
14000
12000
1
18000
16000
14000
12000
10000
8000
1b
2b
3b
4b
5b
6b
PF
2
3
4
5
1a
2a
3a
4a
5a
6a
12000
10000
8000
6000
4000
2000
10000
8000
6000
4000
2000
6000
4000
2000
450
500
550
600
650
450
500
550
600
650
450
500
550
600
650
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)
b)
c)
a)
CIE 1931
CIE 1931
CIE 1931
0,9
0,9
0,9
0,8
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0,0
0,8
0,7
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0,1
0,0
0,8
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0,4
0,3
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0,0
1
5
FhABr
2
4
FhABr
FhABr
3
5a6a
1a 3a
4a
6b1b
5b
2b
3b
2a
PF
4b
0,0
0,1
0,2
0,3
0,4
x
0,5
0,6
0,7
0,8
0,0
0,1
0,2
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0,4
x
0,5
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0,8
0,0
0,1
0,2
0,3
0,4
x
0,5
0,6
0,7
0,8
e)
d)
f)
i)
h)
g)
Figure 6. Emission spectra, chromaticity diagram and images of the samples illuminated with
an UV lamp of the nanocrystals in a), d), g) water, b), e), h) PMMA and c), f), i) PF, when
excited at 395 nm
Comparing the emission of the FhABr nanocrystals in water to that of the FhABr
bulk crystal, it could be observed an amazing increase of the luminescence efficiency
attributed to the nano-size, which increased the surface-to-volume ratio [2]. Among the 1-5
samples, it was observed a variation of the luminescence intensity inversely proportional with
the variation of the nanocrystals size and their polydispersity; better quantum yield was
registered for smaller nanoparticles with narrower polydispersity (Figure 7). A similar trend
was also reported for nanocrystals of polyfluorene, and was attributed to the increase of the
surface-to-volume ratio, to the better quality of the small crystals, and their ability to
accelerate the emission [2].
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- nanocrystals diameter
- QY, ex. 404 nm
- QY, ex. 348 nm
1
2
3
4
5
sample codes
Figure 7. Graphical representation of the variation of the size of the nanocrystals in water
(black) versus their quantum yield when excited with 348 (red) and 404 nm (blue)
Table 3. Quantum yield (QY) of the FhABr in solution, nanocrystals in water and bulk crystal
mol/L
QY (λex = 348 nm) QY (λex = 404 nm)
Code
1.33 * 10-5
0.66*10-5
10-5
24.8
11.3
54.6
18.7
35.4
34.7
23.51
19.43
4.2
0
1
2
3
4
5
15.4
1.33 * 10-5
2 *10 -5
20.06
11.64
11.73
2.12
2.66 * 10-5
bulk crystal
FhABr
A similar behaviour was observed for the film samples of the FhABr nanoparticles
dispersed into the polymeric matrix of PMMA and PF. The quantum yield provided lower
values when excited with the wavelength of the absorption maximum of the conjugated
system, and higher values when excited with the wavelength corresponding to the
intramolecular charge transfer (Table 4). A maximum value of the quantum yield of 45% was
reached for the films in PMMA matrix and one of 39% for the films in polyfluorene. The
values significantly surpass those of the pure components, FhABr and polymer matrix, when
excited with light of similar wavelength.
When excited with light of wavelength corresponding to the conjugated system, the
nanoparticles dispersed in PMMA presented lower values of the quantum yield compared to
those in water. This was attributed to the luminescence quenching because of the transfer of
the excited electrons of FhABr to the ester groups of PMMA, which play the role of acceptor
sites [49]. The intermolecular charge transfer seems to be avoided when the 1a-6a samples
were excited with the energy corresponding to the intramolecular charge transfer of FhABr,
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when the quantum yield has higher values in PMMA films compared to those of the
nanocrystals in water. The explanation can be given by the disagreement between the
HOMO/LUMO orbitals of the FhABr and PMMA [49].
Table 4. Quantum yield of the FhABr nanocrystals in PMMA (1a-6a) and PF (1b-6b)
Code
PMMA/PF
1a/1b
QY (λex = 348 nm) QY (λex = 404 nm)
0 / 7.01
8.17 / 9.37
7.2 / 13.29
7.12 / 18.06
11.21 / 17.25
9.41 / 16.72
9.44 / 14.64
2.12
0 / 19.07
35.98 / 26.93
31.4 / 32.44
27.55 / 39.04
45.38 / 37.28
33.14 / 38.68
31.44 / 36.25
4.2
2a/2b
3a/3b
4a/4b
5a/5b
6a/6b
FhABr bulk crystal
As can be seen in figure 6, all the samples emitted light in the gamut of human vision.
The blue shifting of the emission was reflected in the shifting of the colour of the emitted
light: from green-yellowish (FhABr solution) to green (FhABr nanocrystals in water), green-
bluish (FhABr nanocrystals in PMMA) and blue (FhABr nanocrystals in PF).
Conclusions
Nanocrystals based on a phenothiazine chromophore were successfully prepared, in
both water and a polymeric matrix, namely polymethylmethacrylate and polyfluorene.
Nanocrystals with diameter around 113 nm were obtained by reprecipitation in water, and
around 105 nm by solvent induced phase separation in PMMA and PF matrix. They presented
high efficient luminescence when excited with visible light, reaching an absolute quantum
yield of 35 % in water, 45 % in PMMA, and 39 % in polyfluorene.
The high luminescence efficiency of the nanoparticles in the water biodispersant,
when excited with light of low energy makes them attractive for bio-applications, while the
high efficiency of the nanoparticles in solid films recommends them as reliable candidates for
optoelectronics.
Acknowledgements
The paper is part of a project financed through a Romanian National Authority for
Scientific Research MEN – UEFISCDI grant, project number
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Graphical abstract
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Highlights
•
•
Phenothiazine nanocrystals were prepared in water, PMMA and polyfluorene
Their crystalline nature and nanometric size were proved by DLS, SEM, AFM and
POM
•
They showed high quantum efficiency, reaching values of 45 % in PMMA
22