Energy down converting organic fluorophore functionalized mesoporous silica hybrids for monolith-coated light emitting diodes

Article abstract of DOI:10.3762/bjoc.13.76

The covalent attachment of organic fluorophores in mesoporous silica matrices for usage as energy down converting phosphors without employing inorganic transition or rare earth metals is reported in this article. Triethoxysilylpropyl-substituted derivatives of the blue emitting perylene, green emitting benzofurazane, and red emitting Nile red were synthesized and applied in the synthesis of mesoporous hybrid materials by postsynthetic grafting to commercially available MCM-41. These individually dye-functionalized hybrid materials are mixed in variable ratios to furnish a powder capable of emitting white light with CIE chromaticity coordinates of x = 0.33, y = 0.33 and an external quantum yield of 4.6% upon irradiation at 410 nm. Furthermore, as a proof of concept two different device setups of commercially available UV light emitting diodes, are coated with silica monoliths containing the three triethoxysilylpropyl-substituted fluorophore derivatives. These coatings are able to convert the emitted UV light into light with correlated color temperatures of very cold white (41100 K, 10700 K) as well as a greenish white emission with correlated color temperatures of about 5500 K.

Full text of DOI:10.3762/bjoc.13.76

Energy down converting organic fluorophore functionalized
mesoporous silica hybrids for monolith-coated light
emitting diodes
Markus Börgardts and Thomas J. J. Müller*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2017, 13, 768–778.
Institut für Organische Chemie und Makromolekulare Chemie,
Heinrich-Heine-Universität Düsseldorf, Universitätsstr. 1, D-40225
Düsseldorf, Germany
doi:10.3762/bjoc.13.76
Received: 21 December 2016
Accepted: 05 April 2017
Published: 25 April 2017
Email:
Thomas J. J. Müller* - ThomasJJ.Mueller@uni-duesseldorf.de
This article is part of the Thematic Series "Organic porous materials".
Guest Editor: S. Bräse
* Corresponding author
Keywords:
benzofurazane; LED; luminophores; mesoporous silica hybrids;
monoliths; Nile red; perylene; postsynthetic grafting; white light
emission
© 2017 Börgardts and Müller; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
The covalent attachment of organic fluorophores in mesoporous silica matrices for usage as energy down converting phosphors
without employing inorganic transition or rare earth metals is reported in this article. Triethoxysilylpropyl-substituted derivatives of
the blue emitting perylene, green emitting benzofurazane, and red emitting Nile red were synthesized and applied in the synthesis of
mesoporous hybrid materials by postsynthetic grafting to commercially available MCM-41. These individually dye-functionalized
hybrid materials are mixed in variable ratios to furnish a powder capable of emitting white light with CIE chromaticity coordinates
of x = 0.33, y = 0.33 and an external quantum yield of 4.6% upon irradiation at 410 nm. Furthermore, as a proof of concept two dif-
ferent device setups of commercially available UV light emitting diodes, are coated with silica monoliths containing the three
triethoxysilylpropyl-substituted fluorophore derivatives. These coatings are able to convert the emitted UV light into light with
correlated color temperatures of very cold white (41100 K, 10700 K) as well as a greenish white emission with correlated color
temperatures of about 5500 K.
Introduction
The development of new efficient illumination materials has concept bears the advantage of producing white light with a
received increasing attention in the past years [1-3]. For the high color rendering index (CRI) as well as the generation of
generation of white light the combination of a UV emitting light white light with a warm correlated color temperature (CCT)
source and energy down converting phosphor emitting in the [4-6]. Usually several UV excitable red, green and blue
full visible spectrum is an intriguing approach [4,5]. This (RGB) emitting phosphors generally relying on inorganic
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Beilstein J. Org. Chem. 2017, 13, 768–778.
luminophores, such as rare earth metals as Ce3+ or Eu2+, are 4-chloro-7-nitrobenzo[c][1,2,5]oxadiazole (3) with propargyl-
employed for white light generation [3,4,7]. However, the de- amine. The alkyne-functionalized derivatives 4, 5 and 6 were
velopment of purely organic down converting phosphors would then reacted with azide 7 via CuAAC to furnish the triethoxy-
offer a sustainable and potentially cost-effective access alterna- silyl-substituted luminophore precursor molecules 8 (red emis-
tive, eventually starting from renewable organic materials [8,9]. sion), 9 (blue emission), and 10 (green emission) (Scheme 1). In
contrast to usually high yields of the CuAAC the precursor syn-
Other than inorganic luminophores these organic analogues thesis only gives yields between 44 to 45% after purification by
cannot simply be intercalated in a robust inorganic host struc- column chromatography on silica gel [22,23]. The diminished
ture and, therefore, ligating materials are required. Since the yields are a consequence of the reaction of the terminal tri-
host structure should not undergo photodegradation, stable ethoxysilyl groups with the free silanol groups of the silica gel
structures like silica-based materials, in particular mesoporous taking place already at room temperature.
silica, offer favorable properties [4,10]. Eventually, organic–in-
organic hybrid materials can be easily accessed by covalent For generating white light emission with good color rendering
ligation of the functional guest chromophore to a silica host index and tunable color temperature the characteristics of red
matrix [11,12]. As a consequence the inherent robustness and emissive Nile red, blue emissive perylene, and green emissive
stability of the rigid inorganic silica material can be combined benzofurazane have to be matched with comparable emission
with the functionality and tunability of the organic chromo- intensity. All three chromophores exhibit absorption in the
phore. In addition, these mesoporous materials possess defined UV–vis region with comparable molar absorption coefficients
pore structures in which the organic dye can be homogeneously (Figure 1a). Neglecting energy transfer between the
distributed [13]. Thereby aggregation induced self-quenching luminophores superposition of the emission spectra of the three
and singlet oxygen dependent dye degradation can be effi- dyes covers the whole range of the visible region (400–800 nm)
ciently prevented [14,15]. By the rigid silica host framework (Figure 1b).
bimolecular photochemical reactions are also suppressed [16].
Additionally, mesoporous materials possess the crucial advan- The three dyes were additionally analyzed with respect to their
tage that they can be synthesized as transparent monoliths in CIE chromaticity coordinates, i.e., their fluorescence color in
different shapes [17-19]. Here, we communicate our first efforts order to determine the color space which should be accessible
to design red, green, and blue light-emitting mesoporous materi- by additive color mixing [25-28]. As shown in Figure 2 CIE
als from organic chromophores, their blending to white light- chromaticity coordinates of the blue, green and red colors were
emitting silica hybrids based upon additive color mixing, and as determined as follows: perylene precursor 10: x = 0.14,
a proof of principle, the implementation in a white light emit- y = 0.11; benzofurazane precursor 9: x = 0.33, y = 0.60; Nile
ting monolith for down converting commercially available UV red precursor 8: x = 0.68, y = 0.32.
light emitting diodes.
Thus, by mixing of the three precursors 8, 10, and 9 the color
Results and Discussion
space within the triangle defined by the pure colors should be
A general approach to mesoporous organo–silica hybrid materi- accessible. As this triangle comprises the spectrum of a black
als takes advantage of the condensation of triethoxysilyl-func- body radiator an emission of white light with tunable color tem-
tionalized functional organic molecules with inorganic silica perature is possible. The synthesis of silica hybrid materials
hosts [11,20]. Therefore, a terminal triethoxysilyl group has to functionalized with a single dye was performed via postsyn-
be coupled to luminescent dyes with blue, green, and red emis- thetic grafting of commercially available MCM-41 according to
sion characteristics for generating hybrid materials with white literature procedures [12,13,17]. Therefore, the respective dye
light emission based upon additive color mixing. A rapid and precursors 9, 10, and 8 at variable concentrations were reacted
versatile functionalization can be achieved upon ligating a with free silanol groups displayed on the surface of MCM-41
triethylsiloxy-functionalized azide and a terminal alkynyl-func- furnishing hybrid materials with different dye loadings in a
tionalized luminophore by CuAAC (Cu-catalyzed azide–alkyne μmol·g−1 range. The amount of dye incorporated into the hybrid
cycloaddition) [21-23].
material was estimated by UV–vis spectroscopic analysis ac-
cording to our previous report [24].
Commencing from a 2-hydroxy-substituted Nile red 1 or
3-hydroxymethylperylene (2) the alkyne-substituted fluoro- The synthesized materials with the corresponding dye loading
phore derivatives 4 and 5 were accessible by Williamson ether and solid state quantum yields are summarized in Table 1.
synthesis [24]. The alkyne-functionalized green benzofurazane Especially for the Nile red-functionalized hybrid materials the
derivate 6 was obtained by nucleophilic aromatic substitution of highest quantum yield of 23% was obtained at a low dye
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Beilstein J. Org. Chem. 2017, 13, 768–778.
Scheme 1: Synthesis of the triethoxysilyl-functionalized dye precursors 8, 9, and 10.
loading of 1.5 μmol·g−1 (8@MCM-2). A further increase of molecular energy transfer causing losses in quantum yield can
Nile red loading causes a drop of quantum yields, presumably be expected at higher dye loadings.
due to self-quenching effects.
Likewise, for the perylene and benzofurazane-functionalized
Based on the assumption that energy transfer typically occurs at hybrid materials 9@MCM-3 and 10@MCM-6 similar dye
distances of less than 10 nm between two dye molecules, self- loadings in the lower μmol·g−1 range were ascertained for
quenching should be observed if more than one molecule is furnishing optima of their solid-state fluorescence quantum
found in an area of 10 × 10 nm [29]. If dye loadings of the yields Φf (Figure 4).
hybrids are correlated with the surface area of the silica materi-
al, typically 700 m2·g−1, self-quenching effects should occur at The optical properties of the dye-functionalized hybrid materi-
loadings higher than 10 μmol·g−1 as illustrated in Figure 3 [24]. als differ slightly from the properties of their precursor mole-
Although aggregation-induced self-quenching can be prevented cules in solution. For all hybrid materials a broad band can be
by covalently ligating dye molecules to the silica matrix, inter- observed in the spectra, which can be attributed to an inhomo-
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Beilstein J. Org. Chem. 2017, 13, 768–778.
Table 1: Calculated dye loadings and solid state fluorescence quan-
tum yields Φf of grafted hybrid materials.
dye@MCM
calculated dye
Φf [%]
loading [μmol·g−1]
8@MCM
8@MCM-1
8@MCM-2
8@MCM-3
8@MCM-4
8@MCM-5
8@MCM-6
8@MCM-7
8@MCM-8
8@MCM-9
9@MCM
0.6 of 8
1.5 of 8
2.9 of 8
5.9 of 8
8.8 of 8
12 of 8
12 of 8
18 of 8
23 of 8
14
23
20
20
17
12
11
8.6
6.7
9@MCM-1
9@MCM-2
9@MCM-3
9@MCM-4
9@MCM-5
9@MCM-6
10@MCM
10@MCM-1
10@MCM-2
10@MCM-3
10@MCM-4
10@MCM-5
10@MCM-6
10@MCM-7
0.8 of 9
2.0 of 9
4.0 of 9
6.0 of 9
8.0 of 9
20 of 9
5.8
7.6
12
7.3
6.6
7.6
1.2 of 10
2.3 of 10
4.6 of 10
6.9 of 10
9.3 of 10
12 of 10
16 of 10
10
17
14
14
16
19
14
Figure 1: Absorption (a) and emission (b) spectra of perylene 9,
benzofurazane 10, and Nile red precursors 8 (recorded in ethanol at
T = 298 K; ([9] = 4.1 × 10−6 M, [10] = 4.0 × 10−6 M, [8] = 4.1 × 10−6 M;
λexc(9) = 409 nm, λexc(10) = 450 nm, λexc(8) = 540 nm).
Figure 2: CIE 1931 color space chromaticity diagram (2° observer) with the CIE chromaticity coordinates of the emitting precursors 9 (blue),
10 (green), and 8 (red) in ethanol as well as the color space accessible by mixing these three precursors shown as triangle.
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Beilstein J. Org. Chem. 2017, 13, 768–778.
solvatochromism of the Nile red hybrid materials in compari-
son to the precursor molecule 8 was previously reported and ex-
tensively discussed [24].
Also for perylene-functionalized hybrids a peculiar influence on
the optical properties by incorporation into a silica matrix was
found. Most striking is the change of the vibrational fine struc-
ture. The solution spectra of the perylene precursor 9 in ethanol
show an increasing intensity of the vibrations with increasing
wavelength in the absorption spectrum as well as a typical mir-
ror image of the emission spectrum (vide supra Figure 1a and
b). But as shown in Figure 7 (vide infra) for the perylene-func-
tionalized hybrid materials, the second vibrational band
becomes the most intense in the emission spectrum. This behav-
ior clearly reflects the change in the fluorescence spectra of the
free precursor at higher concentrations. This is typical for dyes
with small Stokes shifts capable of inner filter and energy
transfer effects.
Figure 3: Number of molecules per 100 nm² and quantum yields of
8@MCM in relation to the loading of hybrid materials.
With single dye-functionalized silica hybrid materials in hand
displaying highest quantum yields, blends with tunable color
temperature should be realized. In order to get an estimate of
the mixing ratio of blue, green and red emitting hybrids to yield
a white light emitting material upon excitation at 410 nm, a
blend of the hybrid materials displaying the second highest
quantum yields (8@MCM-4, 9@MCM-2, 10@MCM-2) was
prepared.
Starting from a blend of 8@MCM-4, 9@MCM-2, 10@MCM-
2 with a molar ratio of 1:0.1:0.1 the CIE coordinates were deter-
mined as x = 0.17 and y = 0.19. Thus, as this first blend was
blue emissive and lacked portions of the green and red compo-
nents, the amount of the green and red hybrid was increased as
depicted in Table 2 with their corresponding CIE coordinates
shown in Figure 5. Thereby, a blend [8@MCM-4 + 9@MCM-
2 + 10@MCM-2]-5 with CIE coordinates in the proximity of
the white point could be obtained. The molar ratio of that blend
was used as a starting point for the mixing of the hybrid materi-
als with the highest quantum yields. Thus after determination of
the CIE coordinates of a blend of 8@MCM-2, 9@MCM-3 and
10@MCM-6 with a similar molar ratio, the missing color com-
Figure 4: Solid-state fluorescence quantum yields Φf of grafted hybrid
materials in relation to the calculated dye loading (λexc(9@MCM) =
409 nm, λexc(10@MCM)) = 450 nm, λexc (8@MCM)) = 540 nm).
geneous line broadening in the solid state. As benzofurazane
and Nile red dyes exhibit solvatochomism, their spectral proper-
ties are clearly affected by incorporation into the polar silica
environment. A detailed study of the spectral properties and
Table 2: Molar ratios of the hybrid blends [9@MCM-2 + 10@MCM-2 + 8@MCM-4] with their corresponding CIE coordinates x and y (λex = 410 nm).
hybrid blends
9@MCM-2
10@MCM-2
8@MCM-4
x
y
[9@MCM-2 + 10@MCM-2 + 8@MCM-4]-1
[9@MCM-2 + 10@MCM-2 + 8@MCM-4]-2
[9@MCM-2 + 10@MCM-2 + 8@MCM-4]-3
[9@MCM-2 + 10@MCM-2 + 8@MCM-4]-4
[9@MCM-2 + 10@MCM-2 + 8@MCM-4]-5
1
1
1
1
1
0.1
0.5
0.9
2.7
2.7
0.1
0.6
0.7
1.4
2.6
0.17
0.23
0.24
0.27
0.27
0.19
0.28
0.30
0.38
0.34
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Beilstein J. Org. Chem. 2017, 13, 768–778.
Figure 5: CIE 1931 color space chromaticity diagram (2° observer) with the color space accessible by mixing these three precursors 9 (blue),
10 (green), and 8 (red) in ethanol shown as triangle as well as the CIE coordinates of the hybrid blends
[9@MCM-2 + 10@MCM-2 + 8@MCM-4]-1 to 5.
ponents could be admixed to yield a white light emitting 8@MCM-4, 9@MCM-2, and 10@MCM-2 upon UV excita-
powder [8@MCM-2 + 9@MCM-3 +10@MCM-6]-1 with CIE tion (λexc = 365 nm) in suspension as well as in the solid state
chromaticity coordinates of x = 0.33 and y = 0.33, an external are shown in Figure 6.
solid state fluorescence quantum yield Φf of 4.6% and a corre-
lated color temperature of 5500 K (λexc = 410 nm, molar ratio: Investigation of the excitation and emission spectra of the single
1.0:4.9:1.3). Pictures of a blend of the hybrid materials dye-functionalized hybrid materials 8@MCM-3, 9@MCM-3,
Figure 6: a) Suspensions of the dye-functionalized silica hybrid materials 8@MCM-3, 9@MCM-3, and 10@MCM-6 as well as their mixture in dichloro-
methane under UV light (λexc = 365 nm). b) Solids of dye-functionalized silica hybrid materials 8@MCM-4, 9@MCM-2, and 10@MCM-2 as well as
their mixture under daylight. c) Solids of dye-functionalized silica hybrid materials 8@MCM-4, 9@MCM-2, and 10@MCM-2 as well as their mixture
under UV light (λexc = 365 nm).
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Beilstein J. Org. Chem. 2017, 13, 768–778.
and 10@MCM-6 as well as their white light-emitting mixture tionalized material 10@MCM-6. This dissimilarity arises from
[8@MCM-2 + 9@MCM-3 + 10@MCM-6]-1 revealed in first the fact that at the emission wavelength of 535 nm not only the
approximation that blending did not alter the spectral appear- benzofurazane-functionalized hybrid 10@MCM-6 shows an
ance in comparison to the constituents, i.e., the recorded spec- emission, but also the perylene-functionalized hybrid
tra are in agreement with an additive spectral behavior (super- 9@MCM-3 (Figure 7b). Thus the excitation spectra at an emis-
imposition of the individual spectra of 8@MCM-3, 9@MCM- sion wavelength of 535 nm gives a superimposition of the exci-
3, and 10@MCM-6) void of intermolecular energy transfer tation spectra of the blue and green component in the blend, re-
effects (Figure 7). However, minor differences in the excitation sulting in a virtual blue shift of the excitation of the green com-
and emission spectra can be found upon comparison of the ponent relative to the pure hybrid 10@MCM-6.
single dye-functionalized hybrids and their blend, especially for
the benzofurazane part.
Similarly, the emission spectra are affected as both the blue and
green constituents show emissions in the range of 475–600 nm
resulting in a superimposition of both emissions leading to a
virtual hypsochromic shift of the emission maximum of the
green component of the white light emitting powder
[8@MCM-2 + 9@MCM-3 + 10@MCM-6]-1 relative to the
emission spectra of the benzofurazane-functionalized hybrid
10@MCM-6.
Additional information on the excitation mechanism of all three
chromophores was gained upon excitation with UV light and
recording of emission spectra at different excitation wave-
lengths (Figure 8).
Figure 8: Emission spectra of blend [8@MCM-2 + 9@MCM-3 +
10@MCM-6]-1 at different excitation wavelengths (2nd order of excita-
tion beam not shown).
Figure 7: a) Excitation and b) emission spectra of the single dye-func-
tionalized hybrid materials 8@MCM-2, 9@MCM-3, and 10@MCM-6 as
well as their blend [8@MCM-2 + 9@MCM-3 + 10@MCM-6]-1.
Upon excitation at 320 nm green as well as intense red emis-
sion was found. With increasing excitation wavelength to
350 nm the emissions in the red and green component decrease
In the excitation spectra the excitation of the green constituent while the blue component starts to emit. Upon excitation at
of the blend [8@MCM-2 + 9@MCM-3 + 10@MCM-6]-1 higher wavelengths, the emissions of the blue and green compo-
detected at an emission wavelength of 535 nm is bathochromi- nents increase while the emission intensity of the red compo-
cally shifted compared to the excitation of the single dye-func- nent decreases.
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Beilstein J. Org. Chem. 2017, 13, 768–778.
These observations are in accordance with the excitation spec- quantum yield Φf of 4.6% as the red component is only poorly
tra shown in Figure 7a, where only the benzofurazane and Nile excited at 410 nm.
red hybrids are excited at 320 nm. With increasing excitation
wavelength, the excitation spectra of benzofurazane and Nile Encouraged by generating white light emitting dye-functionali-
red hybrids show a decrease in intensity thus explaining the de- zed silica hybrids upon blending pure dye hybrids we consid-
crease in fluorescence intensity for the green and red compo- ered incorporating all three dyes simultaneously for the forma-
nents at an excitation wavelength of 350 nm. Upon further tion of samples of monolithic silica according to literature pro-
increase of the excitation wavelength the perylene and benzofu- cedures [17]. For the synthesis of monolithic materials the pre-
razane hybrids show increasing fluorescence intensity as their cursor molecules 8, 9, and 10 were mixed in ethanol in a ratio
excitation spectra concomitantly reveal increasing excitation in- of 1:6:17 furnishing a perfect white emission with the CIE chro-
tensity starting from 310 nm for the perylene hybrid and from maticity coordinates of x = 0.33 and y = 0.33. This mixture was
355 nm for the benzofurazane material.
then mixed with TEOS (tetraethoxysilane), surfactant P123
(poly(ethylene glycol)-poly(propylene glycol)-poly(ethylene
As the excitation intensity of the Nile red hybrid decreases up to glycol/(EtO)20(iPrO)70(EtO)20), HCl and water. After 36 h of
400 nm, the decrease in emission intensity shown in Figure 8 is aging at room temperature, the samples were covered with
presumably due to this decrease in excitation intensity. These n-octane and aged at 60 °C for a period of 1–4 d, depending on
findings thus indicate a direct excitation of all three chro- the shape of the reaction vessel, until the samples were dry.
mophores in the blend of [8@MCM-2 + 9@MCM-3 +
10@MCM-6]-1. Although reabsorption effects are possible, With this formation of white light emitting monoliths, as a
they seem to affect the spectra only to a minor extent, as other- proof of concept, we prepared four examples of white light
wise an increase of the emission intensity of the red component emitting LEDs with organic phosphors. Therefore, UV-emit-
in Figure 8 should be obtained as the emission intensity of the ting LEDs were early immersed into the reaction mixture upon
green component increases. This direct excitation of all three monolith formation. After aging, according to the described
dyes in the blend of [8@MCM-2 + 9@MCM-3 + 10@MCM- method, the UV emitting diodes were coated with the white
6]-1 also rationalizes the low external solid-state fluorescence light emitting hybrid materials as shown in Figure 9. Two dif-
Figure 9: Coating of the a) conventional diode setup and b) surface-mounted device (SMD) (left: prior to the addition of the reaction mixture, right:
after the addition), c) coated conventional diode setup, d) coated surface-mounted device.
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Beilstein J. Org. Chem. 2017, 13, 768–778.
ferent LED designs were investigated, a conventional diode matrices. This was achieved by postsynthetic grafting of com-
setup as well as a much smaller surface mounted device (SMD). mercially available MCM-41 yielding single dye-functionali-
zed powders, which could be mixed to give white light-emit-
These coated LEDs were spectroscopically analyzed displaying ting powders upon excitation with UV light. Furthermore, com-
correlated color temperatures of 10700 K and 41100 K for the mercially available UV emitting diodes were coated with silica
conventional LED setup, overlapping with the spectrum of the monoliths composed of a mixture of the perylene, benzofu-
black body radiator. For the SMD setup warmer color tempera- razane and Nile red precursor molecules serving as UV light
tures were obtained, but their CIE chromaticity coordinates are converting phosphors to yield white light emission without em-
slightly shifted from the emission of the black body radiator ploying of inorganic luminophores. Further optimization of the
(Table 3, Figure 10, Figure 11).
dye loadings of these monoliths as well as fine tuning of the
correlated color temperatures and determination of their lumi-
nescence efficiencies is currently under way. Additionally,
Conclusion
In summary, we were able to synthesize triethoxysilylpropyl- studies on the photostability of the hybrid materials are in the
substituted perylene, benzofurazane, and Nile red precursor focus of further research. Further investigations are directed to
molecules and to incorporate them into sol–gel derived silica the determination of quantum yields of the monolith-functional-
Table 3: CIE-coordinates x and y and correlated color temperatures of the four monolith coated LEDs.
Sample
x
y
correlated color temperature [K]
W-LED-1
W-LED-2
W-SMD-1
W-SMD-2
0.25
0.27
0.32
0.33
0.23
0.29
0.40
0.41
41104
10686
5785
5530
Figure 10: Pictures of the coated LEDs in compact device set-up (SMD) and conventional diode design (LED).
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Beilstein J. Org. Chem. 2017, 13, 768–778.
Figure 11: CIE chromaticity coordinates of the coated LEDs in compact device set-up (SMD) and conventional diode design (LED) in the
CIE 1931 color space (2° standard observer).
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The authors cordially thank the Strategic Research Fund of the
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