Fluorescent liquid pyrene derivative-in-water microemulsions

Article abstract of DOI:10.1039/c6cc01517d

A fluorescent liquid pyrene derivative with a high fluorescence quantum yield (65%) in the bulk state is reported. With this as the sole oil phase, stable luminescent oil-in-water microemulsions have been prepared. Increasing the loading of liquid pyrene swells the droplets, as detected by small-angle neutron scattering. These larger droplets have a greater proportion of pyrene excimer emission contribution in their photoluminescence spectra, which leads to a red shift in the chromaticity of the emission.

Full text of DOI:10.1039/c6cc01517d

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Fluorescent liquid pyrene derivative-in-water microemulsions
M. J. Hollamby,a* A. E. Danks,b Z. Schnepp,b S. E. Rogers,c S. R. Hart,d and T. Nakanishie
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A fluorescent liquid pyrene derivative with a high fluorescence FOL-in-water (o/w) microemulsions. Microemulsions typically
quantum yield (65%) in the bulk state is reported. With this as the comprise nano-sized oil droplets, form spontaneously on
sole oil phase, stable luminescent oil-in-water microemulsions mixing, are thermodynamically stable15 and are consequently
have been prepared. Increasing the loading of liquid pyrene swells commonly investigated as a means to deliver hydrophobic
the droplets, as detected by small-angle neutron scattering. These drugs.16 As no other oil is required, a FOL/w microemulsion has
larger droplets have a greater proportion of pyrene excimer the potential to achieve a dye loading of >35% within the
emission contribution in their photoluminescence spectra, which droplet cores, based on the relative mass of the fluorescent π-
leads to a red shift in the chromaticity of the emission.
conjugated moiety versus the attached alkyl chains.
Luminescent FOL/w microemulsions therefore have potential
in sensing or imaging applications, provided that the
luminescence properties of the FOL can be retained upon
compartmentalisation into a nano-sized droplet.
Attaching branched alkyl chains to π-conjugated molecules can
yield a liquid state at room temperature.1,2 The added bulky,
flexible groups naturally reduce the solidification temperature,
but also disrupt the ability of the π-conjugated cores to pack
via π–π interactions. The resulting non-volatile functional
organic liquids (FOLs), including liquid pyrene derivatives,3–6
are being studied for their unique luminescence properties7,8
and potential in organic electronic3–6,9,10 applications.
Advantages include ease of application, ability to form a
continuous layer, ability to accommodate dopant molecules,
and high thermal and photo-stability.
Here, a new liquid pyrene derivative
which has a high photoluminescence quantum yield in the bulk
liquid state. Stable /C12E6/water microemulsions are prepared
1 (Fig 1a) is reported,
1
for the first time, stabilised by the non-ionic surfactant
hexaethylene glycol monododecyl ether, C12E6. C12E6 was
chosen as it is known to readily stabilise o/w microemulsions
without the need for a co-surfactant.17,18 The effect of the
concentrations of
photoluminescence of the microemulsions is investigated.
Derivative (Fig. 1a) was synthesised as described in the SI.
Under normal light, is a yellow oil, but under UV light (λ =
365 nm) an intense blue-green emission is observed (Fig. 1b,
1c). UV-visible absorption spectra of pyrene and in solution
(solvent = CH2Cl2, [ as a liquid film are
] = 10−5 M) and of
1 and C12E6 on the droplet size, and the
One unexplored advantage of luminescent FOLs might be
the ability to directly disperse the liquid in a second phase to
form an emulsion. Luminescent dyes are often dispersed into
micelles, vesicles or oil-in-water nano-emulsion droplets, to act
as sensors11 or contrast agents12,13 for biomedical imaging.
However, in those cases the luminescent properties can be
limited by the concentration of (solid) dye that can be
dissolved into an oil phase without self-quenching.12,14 The use
of FOLs as the sole oil phase may have a distinct advantage, as
they often retain significant luminescence in the liquid state.7,8
One way to disperse FOLs in water might be to prepare
1
1
1
1
1
shown in Fig. 1d. The absorption spectra of the two solutions
exhibit similar features, with several sharp maxima in the
detected range. The maxima of 1 are red-shifted in comparison
to pyrene, suggesting a narrowing of the HOMO-LUMO band
gap.19 Broader peaks, further red-shifted in comparison to the
solution state spectra, are observed in the absorption data for
the film of 1. This may indicate an increased proximity of the
pyrene moieties within the film.20
The normalised photoluminescence (PL) spectra of
1 and
pyrene in solution (in CH2Cl2, [1 1 as a liquid film
] = 10−5 M) and
are shown in Fig. 1e. The PL data for the solutions are
characteristic of pyrene monomer emission (λem ≈ 350 - 450
nm). A red-shift is again noted in the maxima of 1 vs. pyrene,
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Fig.
2 (a) Phase behaviour of the 1/C12E6/D2O microemulsions as a function of
temperature, X and [C12E6]. Regions representing a single phase microemulsion (1φ)
and two-phase system (2φ) are divided by a dashed (---) line where the position of the
boundary is known and (···) where it has been approximated. (b) SANS data for
1/C12E6/D2O microemulsions ([C12E6] = 50 mM). Data were measured at 25 °C in all
cases except for X = 0.2, which was measured at 45 °C. For clarity, the datasets are each
scaled by a constant factor, as detailed in the legend. Lines represent fits using a
cyindrical P(Q), with S(Q) as described in the text.
Fig. 1 (a) Structure of 1. (b) Photograph of 1, taken under normal light conditions. (c)
Photograph of 1, taken under UV light (λ = 365 nm), showing the bright excimer
emission, with a quantum yield of around 65%. (d) UV-visible absorption spectra of 1 in
the solution and bulk film state, with that for pyrene solution provided for comparison.
(e) Normalised photoluminescence emission spectra of pyrene (10-5 M) and 1 (10-5 M)
in solution in CH2Cl2 and of 1 in the bulk film state at 25 °C.
sections of
excimer state, but not so closely packed for self-quenching.
Secondly, air penetration into the more viscous liquid will be
1 may be sufficiently close to form a highly emissive
alongside a reduction in the fine structure of the spectrum, in
line with previous work on mono-substituted pyrenes.19 The PL
1
quantum yields (ΦF) of the solutions of pyrene and
were 0.06 and 0.13 (±0.01) respectively at 25 °C. The increase
in ΦF for
vs. pyrene is due to π-σ conjugation.19 The ΦF values
1 in CH2Cl2
slower than into CH2Cl2, which should again reduce quenching.
Microemulsions were prepared in water (D2O), comprising
1
are lower than ΦF = 0.28 (pyrene) and 0.41 (1-butylpyrene).19
However, in that work, deaerated solutions were used; the
oxygen in air is known to quench pyrene PL.21
varying concentrations of the surfactant C12E6 and
1. The
temperature-dependant phase behaviour is shown in Fig 2a. In
line with other work on o/w microemulsions,23,24 changing the
oil/surfactant ratio X = [1] / [C12E6] significantly alters the
The PL data for the film of
excimer emission (λem ≈ 400 - 650 nm).22 A CIE xy chromaticity
diagram for both the solution and film state of is shown in
1 is dominated by an intense
phase behaviour, while altering [C12E6] at constant X yields
only minor shifts in the phase boundaries. For 0 < X < 0.1,
single-phase microemulsions were observed within the range
10 – 46 °C. Outside the single phase region (1Φ on Fig 2a), the
samples became cloudy and separated into oil-rich and water-
rich layers (2Φ). Single-phase microemulsions were observed
up to X = 0.5, albeit over a much reduced T-range.
1
Fig. S1. At 25 °C in air, ΦF = 0.65 (±0.01) was obtained,
representing a significant enhancement in PL intensity vs. the
solution state. High PL ΦF have been found for other FOLs,
including those based on anthracene (ΦF = 0.55)7 and oligo(p-
phenylenevinylene) (ΦF = 0.48),8 although in those cases the
solution state ΦF was higher. Here, the enhancement is likely
to originate from two factors: In the liquid state, the pyrene
Small-angle neutron scattering, SANS probed the droplet
structure of the
1
/C12E6/water microemulsions (Fig 2b). SANS
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Table 1. Selected analysis results arising from SANS fitting for 1/C12E6/water
microemulsions, using a cylindrical P(Q) and S(Q) as described in the text.
X
0
T / °C
25
R / Å
21
σ
L / Å
64
L/2R
1.5
1.3
1.3
1.3
1.3
1.3
N1 / micelle
0.11
0.14
0.11
0.11
0.08
0.16
0
3
0.02
0.035
0.05
0.1
25
23
61
25
25
66
6
25
25
64
9
25
28
70
22
75
0.2
45
33
86
contains information on the size distribution and shape of the
droplets via the form factor P(Q), and on how droplets interact
via the structure factor S(Q). These contributions are scaled by
concentration and the difference Δρ between the scattering
length density of the droplets and the solvent medium. The
latter is maximised here by detecting 1H-rich droplets in D2O.
SANS data for 1/C12E6/D2O microemulsions for X = 0 – 0.2
are shown in Fig 2b. All data was collected in the single-phase
region (25 °C for X = 0 – 0.1, 45 °C for X = 0.2). The profiles are
typical of SANS from non-ionic surfactant micelles25–28 or
microemulsion droplets.18 When stabilised by C12E6,17,25,29 or
the closely-related C12E5,18,28,30,31 such assemblies are thought
to be elongated. However, the droplet aspect ratio tends to
decrease with the inclusion of greater amounts of oil.17
Consequently,
a
Schultz-Zimm32,33 distribution of solid
cylinders (see SI for details) was used as the P(Q).34 A full
report of the fit parameters and the significant values
calculated from them are given in Supporting Tables S1 and S2.
Fig. 3 (a) Photoluminescence, PL spectra for 1/C12E6/D2O microemulsions at different
values of X ([C12E6] = 50 mM). All spectra were collected in the single phase region (1φ
on Fig 1a). (inset, a) Plot of relative PL intensity at 477 nm and 377 nm as a function of
X. (b) Idealised depiction of the changes that take place within the microemulsion
droplets as X increases, based on the PL results, as explained in the text.
The latter includes (i) the number of C12E6 and
1 molecules per
micelle (Nsurf and N1), (ii) the interfacial area occupied by C12E6,
(ah and ah,IF), and (iii) the number of molecules in the system
approximated by fitting, divided by the number of molecules
known to be present, nfit/ncalc. In almost all cases, both the
goodness-of-fit parameter, χ2 and nfit/ncalc are around 1,
indicating that the fits agree with the data within error and
that the parameters adopt reasonable values.
As
L and aspect ratio, L/2R both drop, in line with previous
results.17 After this, increasing the number of molecules of
1 is added to the C12E6/D2O solution, the droplet length,
1
per micelle (N1) leads to increases in L and the radius, R at an
approximately constant aspect ratio, L/2R and distribution
width, σ. The swelling requires a larger Nsurf to maintain the
interface, as the approximate interfacial area occupied by
C12E6, ah - calculated as the droplet surface area divided by
Nsurf - remains relatively constant at around 85 Å2 in all cases
(Table S2). This value of ah is larger than previously reported
for C12E6 at the air-liquid interface (55 Å2)35, or in bi-continuous
systems (69 Å2).36 However, its calculation assumes a sharp oil-
water interface at the edge of the micelles. Moving the
position of the interface 3 Å towards the micelle centre, which
is reasonable as the extended length of the C12E6 headgroup is
10.7 Å,35 yields values (ah,IF in table S2) of around 70 Å2.
In addition to the P(Q), for X = 0 and 0.02, an attractive
Orstein-Zernicke S(Q) was required to account for additional
scattering at low Q.26,28 The parameter κ indicates the strength
of these interactions and noticeably reduces upon uptake of
1
(κ = 1.0 and 0.2 for X = 0 and 0.02 respectively - see Table S1).
Apparent attractive interactions are likely to be coupled to
increased length or
formation).28 A reduction in κ points to a reduced tendency of
the system to form clusters upon uptake of . After X = 0.035,
a flexible (e.g. end-on-end cluster
1
to X = 0.1 a repulsive effective hard sphere S(Q) gave better fits
to the data, which suggests a change from clustered or
wormlike assemblies to discrete microemulsion droplets.
Similar transitions with increased oil incorporation into o/w
microemulsion droplets have previously been noted.30
The photoluminescence, PL spectra of the
microemulsions are shown in Fig 3a. Despite incorporation
into microemulsion droplets, the bright luminescence of is
1/C12E6/D2O
For X = 0.2, increased scattering at low Q is again observed,
requiring a shift back to an attractive Orstein-Zernicke S(Q).
However, rather than indicating micelle elongation, these
apparent attractions may be indicative of critical concentration
fluctuations, due to the proximity of the measurement
temperature to the phase boundary (Fig 2).
1
retained. However, the relative intensity of the excimer (broad
peak, λem ≈ 400 - 650 nm) emission increases with increasing X.
Size and shape dependent emission is common in inorganic
quantum dots37 and has been demonstrated in organic
nanocrystals.38 In this case, the change in excimer/monomer
Selected results for [C12E6] = 50 mM are shown in Table 1.
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to stem from the relative proximity of molecules of
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1. This can
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TOC image:
TOC text (max 20 words): Using a liquid pyrene derivative as the oil, stable oil-in-water microemuslions are
prepared, with tunable fluorescence emission via droplet size.
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