# Versatile Bottom-Up Approach to Nanostructured Functional Materials for Optoelectronic Applications

Authors: Giorgio Macchi; Francesco Meinardi; Patrizia Valsesia; Riccardo Tubino
Journal: International Journal of Photoenergy
 (2008)

---

## Abstract

A versatile strategy to load ordered mesoporous silica-based materials with functional molecular building blocks in order to obtain host-guest systems exhibiting specific functions is described. Optical microscope examination of the obtained material confirms that the addition of a chromophoric moiety to the reaction mixture is not detrimental in achieving the desired macroscopic morphology of the silica particles. A micro-photoluminescence investigation of the obtained material gave strong evidence that both surfactant micelles and the porous oxide matrix preserve the spectral features of the included molecular species in a nonconventional chemical environment.


---

## Body

## 1. Introduction

Since the discovery of conducting polymers,  much
effort has been put into the development of polymer-based optoelectronic
devices (OEDs) [1–3]. Organic polymers are ideal low-cost and high-versatility
materials for the rapidly evolving markets of plastic-based information and
energy technologies [4, 5]. Nevertheless, there are still problems associated
with the application of organic materials in OEDs (e.g., aggregation-induced
emission quenching, low quantum yield [6]). The idea described in this paper
aims at overcoming some of these problems by developing photoresponsive
organic/inorganic hybrid systems obtained by hierarchical organization of functional
organic molecules in arrays of nanostructured
host-guest compounds. Hybrid materials provide advantages which cannot
be achieved with the single components. For example, the design of hybrid solar
cells offers both, efficient light absorption of organic materials and
efficient conversion of light to electric energy in inorganic materials [5, 7].In recent years, among the
great variety of porous hosts, both inorganic and organic [8–10], the synthesis
of ordered mesoporous inorganic-based materials [11, 12], especially MCM-41, by
employing supramolecular assembly of surfactant to template the reaction of
inorganic species has been the subject of considerable research, due to their
large surface area (1000 m2/g) and storage capacity for applications
in catalysis, gas, and vapours adsorption, drug delivery, molecular
confinement, flow cytometry, and skincare products [13–20]. Such
mesostructurally ordered porous materials have been produced in the form of
micrometric powders [21], thin films [22], and bulk monoliths [23]. The key
idea underlying the present paper is the addition of functional molecular
building blocks to the reaction mixture, in order to obtain a host-guest
material exhibiting specific functions. This bottom-up approach will lead to
many advantages: (i) the oxide matrix effectively shields the guest species
form environmental moisture and chemical impurities (solvents, oxygen); (ii)
the proper choice of the molecular components and host geometrical constraints
produces a precise control of the supramolecular organization of the inserted
molecules, thus tailoring the properties of the resulting system by preventing
aggregation processes even at high concentration;  (iii) the obtained nanostructured material
should maintain the desired macroscopic morphology.The results that we will describe in
this paper would open up the way to the preparation of active materials in
various applications, such as optoelectronic devices and luminescent probes.


## 2. Experimental

### 2.1. Synthesis of Dye-Loaded Silica Particles

Silica-based organic-inorganic hybrid systems are
obtained by a templating process: MCM-like materials were synthesized following
the synthetic route described in [24]. The surfactant used was
cetyltrimethylammonium chloride (CTACl, 98%, Fluka Co., Buchs,
Switzerland).   Tetraethyl orthosilicate (TEOS, >99%, from Fluka Co., Buchs, Switzerland) was the silica precursor.
Oxide growth takes place in acidic conditions, and hydrochloric acid (HCl) was
consequently used. In order to prepare micrometer-sized gyroidal particles
[21] (see Figure 1(a)), the following molar ratio was used:
100:4:0.11:0.13 H2O/HCl/CTACl/ TEOS.(a) Sketch of a silica particle with the
expectedgyroidal morphology, showing the arrangement of surfactant
micelles. (b) Molecular structure of DPA.
(a)(b)Accordingly to the literature [25], the length of CTACl alkane chains
determines both micelle diameter and the pore size of the obtained material,
which is of about 3.5 nm. Under quiescent condition, cylindrical micelles
arrange themselves into gyroidal shapes containing concentric channels coiling
around the main axis (see Figure 1(a)).In a typical synthesis, 355 mg of CTACl were mixed in
15.52 mL of deionized water in a polypropylene bottle. The solution was kept
under mild stirring at room temperature (RT) and 3.29 mL of HCl (37%W, Fluka
Co.,Buchs, Switzerland
) was added dropwise as well as 0.29 mL of TEOS. 2.06 mg of
diphenylanthracene (DPA, ≥98.0%, Fluka Co., Buchs,
Switzerland, see Figure 1(b)) was finally mixed to the mixture. After a few minutes
of mild stirring, the polypropylene flask was closed and kept at RT for 3 hours.
The obtained suspension was centrifugated 5000 rpm for 5 minutes, the
supernatant removed, and the resulting white powder was extensively washed with
10 mL of deionized water and dried in air. The white powder was then placed
between a microscope slide and the cover glass for optical data acquisition. A
test solution of DPA in deionized water (DPA-W) with different concentration of
CTACl was prepared in order to check DPA solubility dependence on the
hydrophobicity of the solvent.


### 2.2. Optical Data Acquisition

Optical absorption and
photoluminescence (PL) spectra on aqueous solutions were recorder using a
Varian Cary 50 Scan UV-visible spectrophotometer (bandwidth 1 nm) and a Varian
Cary Eclipse spectrofluorimeter (bandwidth 1 nm), respectively.Reflected-light microimages were acquired using a doubled Nd:YAG
laser (532 nm) as light source, coupled via single-mode optical fiber to an 80i
Nikon confocal microscope, and a 100X oil-immersion objective (1.3 NA) to
collect the light from sample. The signal is then amplified with a
photomultiplier and a software-based scanning system reconstructs the overall
image. Micro-PL spectra of silica particles were obtained with the same setup,
using a filtered (3.26–3.76 eV bandpass)
high-pressure Hg lamp as excitation source and by coupling the microscope to a
N2-cooled CCD camera through a 190 nm polychromator via a multimodal
optical fiber.


## 2.1. Synthesis of Dye-Loaded Silica Particles

Silica-based organic-inorganic hybrid systems are
obtained by a templating process: MCM-like materials were synthesized following
the synthetic route described in [24]. The surfactant used was
cetyltrimethylammonium chloride (CTACl, 98%, Fluka Co., Buchs,
Switzerland).   Tetraethyl orthosilicate (TEOS, >99%, from Fluka Co., Buchs, Switzerland) was the silica precursor.
Oxide growth takes place in acidic conditions, and hydrochloric acid (HCl) was
consequently used. In order to prepare micrometer-sized gyroidal particles
[21] (see Figure 1(a)), the following molar ratio was used:
100:4:0.11:0.13 H2O/HCl/CTACl/ TEOS.(a) Sketch of a silica particle with the
expectedgyroidal morphology, showing the arrangement of surfactant
micelles. (b) Molecular structure of DPA.
(a)(b)Accordingly to the literature [25], the length of CTACl alkane chains
determines both micelle diameter and the pore size of the obtained material,
which is of about 3.5 nm. Under quiescent condition, cylindrical micelles
arrange themselves into gyroidal shapes containing concentric channels coiling
around the main axis (see Figure 1(a)).In a typical synthesis, 355 mg of CTACl were mixed in
15.52 mL of deionized water in a polypropylene bottle. The solution was kept
under mild stirring at room temperature (RT) and 3.29 mL of HCl (37%W, Fluka
Co.,Buchs, Switzerland
) was added dropwise as well as 0.29 mL of TEOS. 2.06 mg of
diphenylanthracene (DPA, ≥98.0%, Fluka Co., Buchs,
Switzerland, see Figure 1(b)) was finally mixed to the mixture. After a few minutes
of mild stirring, the polypropylene flask was closed and kept at RT for 3 hours.
The obtained suspension was centrifugated 5000 rpm for 5 minutes, the
supernatant removed, and the resulting white powder was extensively washed with
10 mL of deionized water and dried in air. The white powder was then placed
between a microscope slide and the cover glass for optical data acquisition. A
test solution of DPA in deionized water (DPA-W) with different concentration of
CTACl was prepared in order to check DPA solubility dependence on the
hydrophobicity of the solvent.


## 2.2. Optical Data Acquisition

Optical absorption and
photoluminescence (PL) spectra on aqueous solutions were recorder using a
Varian Cary 50 Scan UV-visible spectrophotometer (bandwidth 1 nm) and a Varian
Cary Eclipse spectrofluorimeter (bandwidth 1 nm), respectively.Reflected-light microimages were acquired using a doubled Nd:YAG
laser (532 nm) as light source, coupled via single-mode optical fiber to an 80i
Nikon confocal microscope, and a 100X oil-immersion objective (1.3 NA) to
collect the light from sample. The signal is then amplified with a
photomultiplier and a software-based scanning system reconstructs the overall
image. Micro-PL spectra of silica particles were obtained with the same setup,
using a filtered (3.26–3.76 eV bandpass)
high-pressure Hg lamp as excitation source and by coupling the microscope to a
N2-cooled CCD camera through a 190 nm polychromator via a multimodal
optical fiber.


## 3. Results and Discussion

In order to test our
approach, we test DPA as a suitable molecule for the inclusion in the silica
matrix by means of spectroscopic investigation of its optical features in
different environments.In Figure2, optical absorption spectrum of a DPA-water mixture (solid line) is reported,
showing no significant bands or peaks in the range 3.0–4.0 eV.Figure 2
Comparison between
optical absorption measurements of an aqueous solution of DPA alone (solid
line) and after addition of a sample amount of CTACl (dashed line), along
with  PL spectrum of the resulting system
(dotted line) excited at 3.15 eV.With DPA being water-insoluble, aggregates are probably formed and
precipitation consequently takes place.  Upon addition of surfactant (CTACl), a
well-resolved vibronic progression becomes evident (see Figure2, dashed line),
with the 0–0 purely electronic transition centered at 3.15 eV. Both the shape
and position of the spectrum are known in literature to be fingerprint of
isolated molecule behaviour [26]. PL spectrum of the DPA-water-CTACl ternary
mixture is plotted in Figure 2 (dotted line), with the maximum at 2.85 eV, accordingly
to the reported emission of isolated DPA in dilute solution [26]. These findings suggest that the
interior of micelles formed in the water-surfactant system is able to host DPA
molecules, preventing aggregation process. Moreover, following the key idea
described in Section 1, silica growth should take place around such all-organic
host-guest systems.Figure3 reports microscope images of the sample
prepared (MCM-DPA) accordingly to procedure described in Section 2.1. In Figure
3(a), a layer of silica particles dried in air shows quite good homogeneity
both in shapes and in dimensions. Figure 3(b) reports the same portion of the
material under UV light, showing that emitted light is coming entirely from the
silica beads. Figure 3(c) depicts a magnification of single silica particles: gyroidal shape confirms (see Figure 1(a) as reference) that the synthesis took place
as expected and that the insertion of the chromophore does not affect the
macroscopic morphology of the sample.Microscope images of
MCM-DPA sample, (a) and (b) reflected and emitted light, respectively, from
silica particles on glass substrate and (c) software-assisted zoom, showing
silica powder morphology (scale bars 10μm).
(a)(b)(c)Micro-PL
investigation of the particles in Figure3(c) let us to inspect the emissive
properties of the nanostructured material. The obtained PL spectrum is reported
in Figure 4 (solid line) compared to emission spectrum of DPA-water-CTACl
ternary system described above (dashed line). MCM-DPA emission shows a
structured, quite resolved band spanning from 2.0 to 3.2 eV, with a maximum
centered at 2.88 eV. The vibronic progression underlying this PL band is quite
easily recognizable, with the purely electronic transition at about 3.0 eV and
0–1, 0–2, and 0–3 replica at 2.7, 2.5, and 2.3 eV, respectively.Figure 4
PL spectra of MCM-DPA sample (solid line)
and DPA-water-CTACl ternary system (dashed line), excited with Hg lamp in the
range 3.26–3.76 eV and with
Xe lamp at 3.15 eV, respectively.The main difference between the two spectra reported in Figure4
concerns the intensity of the 0–0 purely electronic transitions, being the
shoulder in MCM-DPA spectrum about 70% of the corresponding peak in DPA-water-CTACl
system emission. This is probably due to self-absorption phenomena, which is an
evidence for the high concentration of the chromophore achievable within the
silica microparticles.


## 4. Conclusions

In summary, an effective procedure to obtain hierarchically-organized
nanostructured systems has been tested and described. The material prepared
maintains the desired spectral features as well as the expected macroscopic
morphology. A ~30% of self-absorption observed in the silica microparticles emission
spectrum demonstrates that a high
concentration of the loaded chromophore can be achieved, producing very bright
luminescent material by preventing aggregation processes (which are known to
quench PL) that usually take place in more conventional systems.These results therefore appear to be very promising for the inclusion of
functional molecular and supramolecular units in inorganic host matrices as
active materials in various applications, such as optoelectronic devices,
luminescent probes, and hybrid solar cells.



---

*Source: 784691-2008-06-15.xml*