Emission Properties of Artificial Opals Infiltrated with a Heteroaromatic Quadrupolar Dye

Marco Cucini,a Davide Comoretto,a,* Matteo Galli,b Franco Marabelli,b Alessandro Abbotto,c,** Luca

Bellotto,c Chiara Marinzic

aDipartimento di Chimica e Chimica Industriale, Università di Genova, via Dodecaneso 31, I-16146 Genova;

bDipartimento di Fisica “A. Volta”, Università di Pavia, via Bassi 7, 27100 Pavia; cDipartimento di Scienza dei Materiali e INSTM, Università di Milano-Bicocca, Via Cozzi 53, I-

20125, Milano

ABSTRACT

Artificial opals are a simple and cheap playground to manipulate the propagation of light. The interest in these kind of photonic crystals is further increased by the possibility to be infiltrated with highly polarisable media like organic semiconductors, i.e. conjugated polymers, push-pull molecules and multipolar chromophores.

In this work, we report on the optical properties of polystyrene opals infiltrated with a heteroaromatic quadrupolar derivative endowed with strong nonlinear optical properties (two-photon absorption) in solution. The insertion of tris(ethylene glycol)monomethyl ether chains on the conjugated skeleton allows the molecule to be soluble in water, a non-solvent for polystyrene. This condition is fundamental in order to attempt opal infiltration.

Variable angle transmittance and photoluminescence spectroscopy are used to characterize the system. The bathochromic shift of the opal stop band upon immersion in the chromophore solution confirms that the infiltration process easily takes place preserving a dielectric contrast suitable for further investigations.

Photoluminescence spectra recorded at different emission angle with respect to the normal of the sample for both the chromophore solution and opals infiltrated with such solution show interesting characteristics. The presence of opal modifies the chromophore emission spectrum by filtering the light for wavelengths corresponding to those of the stop band and according to its dispersion.

Keywords: Artificial opals, photonic crystals, quadrupolar dyes

1. INTRODUCTION

Among photonic crystals (PC), i.e. materials possessing a periodic modulation of the dielectric constant on a length scale comparable to the wavelength of visible light, artificial opals are a simple, cheap and interesting playground to investigate optical effects on this class of materials. Despite opals do not show a complete photonic band gap (PBG) but only a pseudogap (stop band), their versatility allows to obtain interesting structures. As a matter of fact, they can be employed as templates to prepare inverse opals (which instead might have a complete PBG) and they can also be infiltrated with a variety of materials ranging from metals to organic semiconductors1. A large amount of work has been done in the last years on opals infiltration (see for instance the review in ref. 1). A proper choice of the infiltrating material allows tailoring the optical properties of opals for specific functions. For instance, opals infiltrated with highly efficient non linear emitters showed unusual effects on two-photon excited emission spectra when the chromophore * comorett@chimica.unige.it; phone +39-0103538736; fax +39-0103538733 ** alessandro.abbotto@mater.unimib.it; +39-026448 5227;+39-026448 5401

Organic Optoelectronics and Photonics III, edited by Paul L. Heremans, Michele Muccini, Eric A. Meulenkamp, Proc. of SPIE Vol. 6999, 69992D, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.783367

Proc. of SPIE Vol. 6999 69992D-12008 SPIE Digital Library -- Subscriber Archive Copy

emission was properly tuned with the photonic stop band of the opal2. Moreover, a dramatic enhancement of the third harmonic generation efficiency was observed when phase matching conditions are provided by the periodical structure of the opal photonic crystal3. Artificial opals infiltrated with π-conjugated polymers also exhibit random lasing action. With such system, simultaneous laser emission of different colors has been demonstrated4,5. Photonic crystal polaritons have also been observed in opals possessing the stop band tuned with the sharp and strong absorption of excitons both in molecular J-aggregates6 or pervoskites7.

In this work, we report the preliminary results on opal infiltration with solution of quadrupolar dyes derived from PEPEP molecule (scheme 1).

NMeN N

MeMe

CF3SO3 CF3SO3

1

Scheme 1. Chemical structure of PEPEP.

The reason of our interest in chromophore 1 is due to its large two photon absorbing (TPA) properties.8 PEPEP presents an A-π-D-π-A general quadrupolar framework, where A is a π-deficient heteroaromatic ring (pyridinium) and D a π-excessive pyrrolyl moiety. A spectroscopic characterization of this molecule shows that the maxima of linear absorption and emission spectra in DMSO solution are at 524 and 620 nm, respectively. The fluorescence quantum yield is 0.14. The TPA cross-section (σTPA) at 790 nm was measured to be 119 GM9 (Z-scan measurements, 150 fs pulses). More recently two-photon induced fluorescence studies have shown σTPA values in the 100-800 GM range, with a maximum at about 820 nm.10

In order to obtain polystyrene opals infiltrated with the heteroaromatic TPA quadrupolar dye 1, solutions of the chromophore in water or other non-solvents for polystyrene need to be used. We have therefore designed the new derivatives 2 (monoTEG-PEPEP) and 3 (bisTEG-PEPEP) where one and two tris(ethylene glycol) monomethyl ether (TEG) chains, respectively, were introduced in the pyridinium moieties in order to impart solubility in hydrophilic and other polar solvents (Scheme 2).

NMeN N

Me

CF3SO3

2

OO

O

TsO

NMeN N

3

OO

OOO

O

TsOTsO

Scheme 2. Chemical structure of monoTEG-PEPEP (2) e bisTEG-PEPEP (3)

2. EXPERIMENTAL

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Commercial polystyrene monodisperse microspheres (diameter a=260 and 300 nm, refractive index, nPS=1.59; standard deviation < 5%) water suspension 10% in volume (Duke Scientific) were used for opal preparation. In order to have a suitable sample thickness, we properly diluted these suspensions with distilled water, and then we grew opals films by using the meniscus technique.11

A glass substrate coated with an opal film (about 20 micron thick) is sandwiched to a window through a rubber spacer thus forming a cell which was filled with water dye solutions. In few seconds, the solution was infiltrated in the interstices between the microspheres.

Transmittance (T) and photoluminescence (PL) spectra were measured with optical set-ups based on fiber optical coupled Avantes 2048 or StellarNet EPP2000 compact spectrometers working in the range 300-1100 nm. The sample was mounted on a rotating stage allowing for T measurements at different incidence angle (θ). PL spectra were recorded in back-scattering geometry. For PL angle resolved measurements, the incident geometry was kept fixed, whereas the detection arm of our set-up was rotated of an angle θ’ with respect to normal to the surface. The excitation for PL spectra was provided both by a 405 nm laser diode or by high power and low radial divergence LEDs (405, 465 and 525 nm).

3. RESULTS AND DISCUSSION

3.1 Synthesis and linear optical properties of hydrophilic quadrupolar chromophores.

The two hydrophilic dyes 2 and 3 were synthesized according to Schemes 3 and 4, respectively. The monoTEG derivative 2 was obtained in 94% yield by alkylation of the mono-pyridinium precursor 412,13 with TEG-OTs14 in acetone in a microwave reactor for 3 h at a constant power of 90 W. The product is recovered as a dark precipitate after cooling to room temperature and washing with cold acetone and toluene. The same reaction can be carried out in CH3CN in good yields (85%) but in this case the isolation of the product is harder due to its large solubility in the reaction solvent. It is worth noting that the same reaction in acetone or CH3CN at reflux without the use of the microwave irradiation did not lead to any formation of the product. The bisTEG derivative 3 was obtained in good yields (74%) in a similar manner by alkylation of the bidentate precursor 5 with TEG-OTs in a microwave reactor. The pure product was obtained as a dark purple solid after precipitation with toluene from the reaction mixture, thorough washing with toluene and petroleum ether, and crystallization from acetone. The new dyes have been completely characterized by 1H and 13C NMR and high-resolution mass spectrometry.

NMeN N

Me

CF3SO3

4

CH3(OCH2CH2)3OTs

acetonemicrowave90 W, 3 h

2

Scheme 3

NMeN N

5

CH3(OCH2CH2)3OTs

acetonemicrowave100 W, 3 h

3

Scheme 4

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As expected, the absorption spectra of the hydrosoluble dyes 2 and 3 are similar to that of the parent chromophore 1 in DMSO solution. In fact, it is known that chromophore 1 in DMSO shows absorption/emission spectra maximum at 524/620 nm.10 Solutions of bisTEG-PEPEP in DMSO shows absorption peak at 526 nm while PL maximum is slightly changed since is observed at 609 nm, independently on excitation wavelength (λexc 405, 470, and 525 nm). When derivatives 3 is dissolved in water, the solvent used for opal infiltration, a more pronounced variation is observed. As a matter of fact the absorption peak is detected at 507 nm and PL maximum is shifted at 598 nm, again independently on λexc. The solvatochromism properties of PEPEP based dyes is currently under investigation.

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nce

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rban

ce

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λexc =470 nm λexc =525 nm λexc =405 nm

(b)

Fig. 1. Absorption and PL spectra of chromophore 3 in DMSO (a) and water (b).

3.2 Optical properties of bare and infiltrated opals.

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We infiltrated opals with both dyes 2 and 3, but, due to the superior solubility, we report here only data relative to dye 3.

Fig. 2 shows the typical T spectra recorded at different incidence angle for 300 nm bare opals (a), a bisTEG-PEPEP solution infiltrated opals (b), and bisTEG-PEPEP solutions (c). Bare opals show the typical transmission dip associated with the photonic pseudo gap (stop band) at 690 nm. The spectrum background is mainly due to incoherent Rayleigh scattering, which becomes more intense for higher photon energies.

Upon increasing the incidence angle, the stop band shifts toward high energies according to the opal photonic band structure up to 50° where a broad minimum is observed at 590 nm. For higher angles, the stop band seems to invert its dispersion and at 60° is detected at 610 nm. However, this behaviour must be analyzed in a different way. As a matter of fact, we notice that a change of slope appears in the spectra for θ=20° at about 480 nm. Upon increasing the incidence angle, this feature shows a bathochromic shifts (460 nm at 40°) and then merges with the stop band giving rise to a single minimum at 50°. If we look more carefully to the spectra recorded at θ=60°, two additional features can be also observed at 440 and 530 nm. The assignment of optical spectra of opals was recently provided by a theoretical investigation of their photonic band structure showing that a clear distinction can be done between diffraction in the direction of light propagation by (111) family planes (leading to the formation of the stop band) and diffraction in other directions by higher-order planes (corresponding to the excitation of photonic modes in the crystal) having higher energy for low incidence angles.15 According to this model, the proper interpretation of the spectra in Fig.2a is related to the presence of two spectral features, the stop band and an high energy mode, having opposite dispersion properties with a crossing at about 50°.

If we limit our interest to the dispersion properties up to 40°, i.e. up to angle preventing spectral overlap between the stop band and high energy photonic modes, we can fit these data with the Bragg-Snell law16

θλ 22

effsin-nD2m = (1)

where m is the diffraction order, D is the interplanar spacing in the [111] growth direction which is perpendicular to the opal surface in order to better characterize our materials. By fixing the value of D=a(2/3)0.5 with the nominal one assuming a face centered cubic compact structure, we get neff=1.40. This value is in good agreement with that theoretically calculated by the effective medium theory (1.41) being the difference (<1%) smaller than the dispersion of the sphere diameters (<5%), which can cause uncertainty in the determination of λ.

When opals are infiltrated by bisTEG-PEPEP water saturated solutions, some changes occur in T spectra recorded at different incidence angle (Fig. 2b). First of all, due to the high dye concentration, transmittance is negligible for photon wavelength shorter than 580 nm where the compound 3 absorption is located (Fig. 1). Then, it is interesting to notice the drastic reduction of the incoherent Rayleigh scattering which flattens the backgrounds of T spectra. More interesting for us is the bathochromic shift of the stop band, now located at 740 nm, as well as its full width half maximum reduction (FWHM). These effects are due to the reduced dielectric contrast of solution infiltrated opals with respect to bare ones.

Upon changing the incidence angle, again the dispersion of the stop band is observed but having modified properties since the fit with Eq. 1 provides a value of neff=1.52. As we recently showed for opals infiltrated with gold nanoparticles17, from the neff value we can obtain the refractive index of infiltrated interstices (ni) by using the Lorentz-Lorentz effective medium theory. In this case, we found ni=1.33 with an increase of 33% with respect to the case of bare opals (ni=1). Notice that the value of ni we obtained is coincident to the refractive index of water used to prepare the solutions used for infiltration.

A final comment is related to transmittance background intensity reduction for wavelength larger than the stop band observed both for bare and solution infiltrated opals upon increasing the incidence angle. This effect is related to increase of reflection losses on the cell window upon increasing the incidence angle (and not to the photonic crystal) since the same behaviour is observed for a cell containing just the dye solution as demonstrated in Fig. 2c where its angle dispersion properties are reported. Finally notice that no change of the molecular absorption onset (around 600 nm) is observed.

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Fig. 2. Transmittance spectra versus incidence angle for a bare opal (a), a bisTEG-PEPEP solution infiltrated opal (b), and a PEPEP solution (c). Sphere diameter 300 nm.

20000 18000 16000 14000

500 550 600 650 700 750 800

θ'=35° θ'=30° θ'=25° θ'=22° θ'=20° θ'=18° θ'=16° θ'=15° θ'=12° θ'=12° θ'=10° θ'=8° θ'=5° θ'=0°

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nsity

(a.u

.)

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Wavenumber (cm-1)

(b)

Fig. 3. Photoluminescence spectra versus detection angle θ’ for (a) a bisTEG-PEPEP solution and for (b) a bisTEG-PEPEP solution

infiltrated opal (a=260 nm). Spectra are properly offset for clarity; θ’=0° top, θ’=35° bottom.

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Fig. 3 shows the angle dependent (θ’) PL measurements both for a bisTEG-PEPEP solution and a bisTEG-PEPEP solution infiltrated opals (a=260 nm). Notice that now, the stop band is shifted to higher energies with respect to transmittance measurements. For θ’=0, the PL spectrum is identical to that reported in Fig. 1b. Upon increasing the detection angle, the major effect observed is an increase of the signal for angles up to θ’=12° followed by its reduction. The effect is particularly evident in the high energy part of the PL spectrum where a shoulder at about 550 nm gains intensity. This is due to the backscattering geometry used for the measurements which makes for θ’=12° the detection direction collinear with the reflection of the exiting laser beam, thus increasing the PL signal. Such an effect, joined to the change in the probed solution thickness (inducing a modification of self-absorption) due to the cell rotation, is also responsible for the slight change in the PL peak position observed in the spectra.

PL spectrum at θ’=0 for solution infiltrated opal shows a remarkable difference. Since the PL spectrum is tuned over the photonic stop band, a clear dip in PL is observed for the corresponding wavelengths. Upon changing θ’, the minimum moves toward higher energies and the shape o the PL spectrum is strongly modified indicating that the dispersion of the stop band on the PL spectrum acts as a selective filter. The effect of back scattering measuring geometry is still present even in this case and then the change in intensity does not allow to fully appreciate the role of opals on the emission spectrum. In order to better comment this last point, we report in Fig. 4 the PL peak position and intensity as a function of the detection angle for both the solution (a) and the opal infiltrated solution (b).

For the cell containing the solution a bell-like shape is observed as a function of the detection angle for both observables. When the solution infiltrates the opal, the shape of such curves is strongly modified and very different behaviours are observed. Moreover, the peak position is shifted due to the filtering action of the photonic crystal.

Let’s now compare the dispersion properties of T and PL spectra. Since they are recorded for opals having different sphere diameter (300 nm for T and 260 for PL spectra), they are spectrally shifted according to the scaling laws of photonic crystals18. However, by using the dimensionless photon energy parameter l/λ (where l=a√2 is the cell length for the FCC lattice of spheres) they can be directly compared (Fig. 5). The normalized spectral position and the dispersion properties of the two observable are identical thus confirming their common origin as due to the photonic crystal stop band.

This result is very important for different several reasons. First of all, in spite of the reduced dielectric contrast due to solution infiltration, the photonic stop band can be still clearly observed. This is a prerequisite to any further study on these systems. Moreover, the filtering effect of the stop band on the PL spectrum is clearly observed, thus demonstrating that the investigation of emission properties of proper dyes solution infiltrating opals is possible.

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k P

ositi

on (n

m) P

L Intensity (a.u.)

0 5 10 15 20 25 30 35600

605

610

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620

625

Peak

Pos

ition

(nm

)

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solution and for (b) a bisTEG-PEPEP solution infiltrated opal (a=260 nm).

It is interesting to notice that these results can be exploited to increase the time duration of interaction of photons with the active material. It is known that at the high (low) energy edges of the stop band, the electromagnetic field is confined in the low (high) dielectric constant material composing the photonic crystal18. In particular the field is not propagating but behaves as a stationary wave being confined in the interstices (microspheres). This effect has been recently shown to modify the T spectrum of opals doped with gold nanoparticles when the stop band is tuned over the aggregated nanoparticles broad absorption.19 In the present case, the low dielectric constant material are the interstices filled with the bisTEG-PEPEP solution, i.e. the active material. By exploiting the localization effect so far described, we can in principle amplify the non linear response of our dyes. This work is currently in progress.

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0

5

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θ, θ

' (de

g)

l /λ

PL T

Fig. 5. Dispersion properties of the stop band in transmission spectra and of dip in PL spectra for bisTEG-PEPEP solution infiltrated

opal as a function of photon energy in dimensionless units.

In conclusions, we showed that multipolar chromophores possessing a large two photon absorption cross section can be efficiently substituted with ethylene glycolic chains in order to promote their solubility in polar solvent like water. This property is particularly attractive since allows to use mono- and bis-substituted PEPEP for solution infiltration of polystyrene artificial opals. The infiltration process induces a red shift of the photonic stop band as well as a reduction of its FWHM. Even though the dielectric properties of the infiltrated photonic crystals result to be modified, the residual dielectric contrast is strong enough to modulate the emission properties of the chromophore inside the interstices. As a matter of fact, such excited chromophores emit light which is filtered by the opal for photon energies tuned on the stop band spectral range. These results open new perspectives toward the tailoring of the non linear optical properties of molecules by means of organic nanostructures.

ACKNOWLEDGMENTS

We gratefully acknowledge the finantial support of Italian Ministry of University and Research thorough program Progetti di Ricerca di Rilevante Interesse Nazionale 2006.

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