Microporous and Mesoporous Materials 51 (2002) 91–138 www.elsevier.com/locate/micromeso
Review
Chromophores in porous silicas and minerals: preparation and optical properties G€ unter Schulz-Ekloff a,*, Dieter W€ ohrle b, Bast van Duffel c, Robert A. Schoonheydt c a b
Institut f€ ur Angewandte und Physikalische Chemie, Universit€at Bremen, Postfach 330 440, D-28334 Bremen, Germany Institut f€ ur Organische und Makromolekulare Chemie, Universit€at Bremen, Postfach 330 440, D-28334 Bremen, Germany c Departement Interfasechemie, Katholieke Universiteit Leuven, Kardinaal Mercierlaan 92, B-3001 Leuven, Belgium Received 15 September 2000; received in revised form 25 September 2001; accepted 1 October 2001 Dedicated to Prof. J. Weitkamp on the occasion of his 60th birthday
Abstract Various synthesis strategies for the preparation of chromophores in porous silicas and minerals like zeolites, zeolite analogues and clays are described focusing on soft procedures like the sol–gel technique, microwave-assisted crystallization inclusion, and deposition in solution in combination with the Langmuir–Blodgett technique. The mild preparation methods protect sensitive dye molecules from chemical degradations. The influences (i) of the chemical properties of the mineral hosts, e.g., polarity and acidity, and (ii) of their spatial constraints on the guest molecules, i.e., their organized arrangement and photophysical properties, are demonstrated exemplarily. 2002 Elsevier Science B.V. All rights reserved. Keywords: Organically modified porous silicas; Optically functional chromophores; Soft inclusion chemistry; Supramolecular selforganization and self-assembly; Photophysics and photochemistry of organized host–guest composites
Contents 1.
Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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2.
Dyes in porous sol–gel silicas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Preparation of sol–gel-derived silicas and hybrid matrices. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Dye-doped porous silicas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Host–guest interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5. Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
93 93 93 95 95 96
*
Corresponding author. Tel.: +49-421-218-2373; fax: +49421-218-4918. E-mail address:
[email protected] (G. Schulz-Ekloff). 1387-1811/02/$ - see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 7 - 1 8 1 1 ( 0 1 ) 0 0 4 5 5 - 3
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2.5.1. Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.2. Lasers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.3. Optical switching. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.4. Miscellaneous . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
96 97 98 99 99
3.
Molecular-sieve-encapsulated dye molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Synthetic methods for the inclusion of dyes in molecular sieves. . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1. Dye loading by sorption. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1.1. Sorption from solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1.2. Sorption from the gas phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1.3. Sorption by solid-state reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2. Dye loading by ion exchange . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.3. Covalent grafting of dye molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.4. Dye synthesis in nanopores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.5. Encapsulation during the hydrothermal synthesis of molecular sieves . . . . . . . . . . . . . . . . . . . . . 3.3. Fundamental requirements for technical applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1. Photophysical and photochemical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.2. Linear optical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.2.1. Micrometer-sized lasers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.2.2. Optical switches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.2.3. Optical sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.2.4. Molecular sieve pigments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.3. Non-linear optical effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.3.1. Frequency doubling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.3.2. Spectral hole burning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
100 100 100 100 101 102 104 104 106 107 109 116 117 120 120 122 124 126 126 126 127
4.
Orientational order of dyes in clay minerals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Clay minerals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Self-assembly and fuzzy assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4. Langmuir–Blodgett technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5. Functional films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
128 128 129 130 131 132
5.
Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
1. Preface Whereas optical functionalities of doped glasses play an increasing and well-observed role in various photonic applications [1], chromophores in porous silicas and minerals are still at the beginning of their perception. A variety of favorable properties of mineral–dye composites can be expected based on peculiar host–guest interactions. Firstly, Stokes shifts, i.e., significantly different wavelengths for the absorption and emission of
the (0; 0) transitions, which occur if the potential functions of the p-electron system are different in the ground and excited states, are considered to be enlarged by strong host–guest interactions, causing variations of bond distances and decreases of the perfection of conjugation in a dye molecule [2]. Further, increased probabilities for (0; 1) transitions occur frequently for the distorted dyes in minerals. Secondly, strong changes of refractive indices are observed for photoinduced switching between different conformers of dye mole-
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cules, surpassing the corresponding values of many proven materials by a large factor. This effect is attributed to a mechanism of photoinduced change of the degree of orientation of host–guest arrays simultaneously to the photoinduced change of the conformation [3]. Further effects occur, like (i) uni-directional transfer of electromagnetic energy based on the oriented arrangement of chromophore dipoles in porous mineral crystals [4], and (ii) a large macroscopic second-order nonlinear susceptibility due to non-centrosymmetric orientation of polar molecules in the channels of crystalline minerals [5]. Favorable waveguide characteristics can be used for (i) super-radiance emissions based on back-reflection and quasiresonator configuration, or even (ii) microlaser emissions due to whispering gallery type waveguide in microcrystals (vide infra). The porous structure enables the access of analyte molecules to the indicator dye centers, which are anchored in the pores of the mineral host and, thus, a successful application for various sensing tasks. Combined with glass fiber optics, applications of optical sensors are envisaged in medicine, environmental monitoring or industrial process control. The complex mechanisms of photobleaching, comprizing thermal and chemical degradations, photosplitting with subsequent radical attacks or reactions with singlet oxygen from photoexcitations, are some of the main reasons for the reserved occupation of this field of composite materials. Observations of a decrease of the rates of photodegradation by two orders of magnitude have been reported sporadically for chromophores in porous silica or minerals. The prevention of harmful singlet oxygen formation by keeping O2 away from the mineral–dye composites obviously favors the photostability of the chromophore. However, the influence of spatial constraints on photolytic processes is largely unexplored. This review aims at an inventory of composites, consisting of chromophores in porous silicas, crystalline molecular sieves and layered minerals, focusing on (i) synthesis strategies and proven arsenals for their characterization and (ii) optical properties. The potential uses of these materials in the field of highly integrated and nanostructured analytical systems are indicated.
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2. Dyes in porous sol–gel silicas 2.1. Introduction In the past decade an increasing number of studies on the optical properties of dye-doped porous silicas was published. The advantages of an inorganic matrix for the embedding of functional chromophores are attributed to the more rigid environment as well as much higher migration stability and strongly increased photostability compared to organic polymer matrices. These properties render such composites attractive for second harmonic generation (SHG) [6], solid-state tunable lasers [7] or delayed fluorescence [8]. The pore structure of silicas derived from sol–gel synthesis enables a ready access of gas molecules or dissolved ions to embedded fluorescing chromophores. This is the basis for the rapidly increasing research activities on dye-doped silicas for optical detection of gases or dissolved analytes with a view to their application in industrial, environmental, and biomedical monitoring [9]. The recent progress in the tailored preparation of inorganic–organic (hybrid) polymers via hydrolysis and condensation of precursor alkoxysilanes [10] stimulates the use of the hybrid polymers for dye-containing composites. The hybrid polymers combine the advantages of the inorganic component, i.e., high mechanical rigidity and thermal stability as well as light fastness, with those of the organic one, i.e., high optical transparency and shrinking stability as well as low brittleness. These hybrid polymers are nowadays nearly exclusively applied as matrices for dyedoped silica composites, the preparation, characterization, and potential use of which will be described in the following. 2.2. Preparation of sol–gel-derived silicas and hybrid matrices The preparation of pure silica matrices can favorably start from alkoxysilane (Si(OR)4 , R: usually short-chain alkyl groups) precursors, where the complex mechanisms of hydrolysis and condensation can be catalyzed by acids, bases, metal salts, or metal complexes [10,11]. The use of
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alkoxysilanes (i) provides monomeric Si species in a uniform distribution from the start of the sol–gel process, whereas inorganic silicon sources require dissolution processes which enter additionally the complex reaction network and (ii) makes roomtemperature processing possible, enabling the addition of sensitive molecules like chromophores to the synthesis mixture. The kinetics are influenced by various factors, like (i) the activities of Hþ or OH in acidic or basic solutions, (ii) the type of organic solvent, (iii) the H2 O/TEOS ratio, or (iv) the nature of the alkoxy group in the precursor. The ratio of the rate constants of hydrolysis and condensation determines the size and nature of the polycondensate structures in the sol, e.g., the length of linear chains, the degree of branching, or the density. Consequently, the physical properties of the gels depend on these polycondensate structures and their degree of crosslinking. The pore structures, i.e., the average pore sizes and the corresponding histograms, depend on the conditions of aging, drying, and thermal stabilization. The maintenance of structures with high porosity requires careful removal of solvents to avoid pore collapse via the high surface tensions in the capillaries. Materials with high density are obtained by sintering processes at high temperatures. The broad applicability of the sol–gel technology is based on the possibilities to tailor the shapes of the desired devices, i.e., whether usages of monoliths, films, fibers, or powders are required. Comprehensive pertinent information has been summarized in textbooks [10,12]. Hybrid polymers are prepared starting from organically modified silanes (ormosils), i.e., organoalkoxysilanes, e.g. R0x –Si(OR)4x (R0x : hydrocarbons with desired properties), where only the alkoxy groups undergo hydrolysis and condensation [13,14]. The organic component in the hybrid polymer (i) facilitates the removal of solvent by preventing shrinkage, pore collapse, or formation of cracks, which would reduce the optical transparency of composite materials, and (ii) can form an additional network by standard methods of organic polymerization, using catalysts, radicalforming initiators, or light (Fig. 1) [15–17]. Further modifications of the inorganic network can be
Fig. 1. Scheme of the organic crosslinkage for the examples methacryloxypropyltrimethoxysilane 1 and 3-glycidyloxypropyl-trimethoxysilane 2.
achieved by substitution of the silicon by other elements like aluminum, titanium, or zirconium [18]. The kinetics of hydrolysis of organically modified trimethoxysilanes have been followed by Fourier-transform Raman spectroscopy, monitoring the bands at 612 cm1 (mas Si(OCH3 )3 ) and 643 cm1 (msym Si(OCH3 )3 ) assigned to the combination of antisymmetric and symmetric trigonalpyramidal vibration of the methoxy groups at the silicon atom [19]. The influence of pH change, attributed to a neutralization reaction between acidic silanol groups and hydroxyl ions, on the kinetics of hydrolysis was elucidated [19]. The rigidity of the polymer matrix has been studied by means of 13 C crosspolarization magic angle spinning nuclear magnetic resonance (CPMAS-NMR) spectroscopy on the basis of line broadening of CH2 groups in a-position to the silicon atom [20], attributed to a restricted proton
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mobility [19,21]. A similar information is gleaned from line broadening in 1 H-MAS-NMR spectra [19]. The degree of network linkage for the inorganic component is usually monitored by 29 Si-CPMAS-NMR [19,21]. 2.3. Dye-doped porous silicas The development of the room-temperature sol– gel process for the preparation of porous silicas opened the possibility to add organic chromophores, exhibiting photophysical and photochemical functionality, to the synthesis mixtures. This procedure results in a uniform dispersion of the dyes in the porous host. Incorporation of the chromophores by wetness impregnation methods favors undesirable migration and aggregation. Further, the sol–gel process enables the fabrication of monolithic bulk pieces as well as films and fibers, representing suitable components for various potential applications in photonic devices. In the majority of papers describing the preparation of dye-doped sol–gel silicas, the dissolved dye is added to the reaction mixture, yielding entrapped chromophores in the final solid composite [13,22,23]. Recently, covalent bonding of the dye molecules to the silica matrix has been increasingly considered with respect to improved fastness towards migration and leaching. Two different strategies are applied for the covalent bonding of dye molecules to silica matrices. In the first strategy, the silanol groups of a porous silica, prepared by sol–gel processing, are functionalized by a suitable silane, e.g., 3-aminopropyltriethoxysilane (APTES) (Fig. 2, step 1). The latter reacts with corresponding functionalized derivatives of the dye molecule (Fig. 2, step 2) [24]. One has, however, to consider, that the surface Si–O–Si bonds are sensitive to hydrolysis, i.e. attack by water vapor could result in leaching of the chromophore. Therefore, the other strategy, called co-condensation, starts with triethoxysilane derivatives of the chromophore (Fig. 3) and exposes the hydrolysisresistant Si–R bonds at the surface of the dyemodified porous silica [24,25], yielding more stable composites. The quality of the products, e.g., desired porosity, optical transparency, brittleness, leaching stability, light fastness, etc., depends
Fig. 2. Scheme of the functionalization of silanol groups at silica surfaces (step 1) and covalent bonding to functionalized chromophores (step 2).
Fig. 3. Scheme of anchoring of functionalized chromophores R(R–Si(OC2 H5 )3 ) via co-condensation with, e.g., tetraethoxysilane (Si(OC2 H5 )4 ) in a desired ratio m=n.
sensitively on the processes of polycondensation, aging, and drying, which in turn depend on the starting composition of the sol–gel batch. This means that no general synthesis recipes exist, which lead to products of high quality, and that each composite requires its own optimization. 2.4. Host–guest interactions Dye-doped porous silicas inherently contain their internal indicator for the physical and chemical character of the host structure, since the optical properties of the entrapped dyes depend on the chemical interactions with the silanol groups and the physical interactions with the cage walls. For example, changes in the photochromic behavior of trapped spiropyrans reveal that the normal photochromism, i.e., formation of cismerocyanine from spiropyran upon irradiation of the latter at 365 nm, appears as long as the solvent
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methanol is present in the cages during sol–gel aging. Upon removal of methanol, the cis-merocyanine is stabilized by interaction with the silanol groups of the silica cages, and the reversed photochromism takes place, i.e., spiropyran appears upon irradiation and thermally relaxes to the stable constitutional isomer [22]. The gel formation favors the dispersion of monomers at the expense of dimeric aggregates as concluded from the study of the temporal development of the fluorescence spectra of pyrene monomers and excimers [26]. A study of the decay times of room-temperature phosphorescence and delayed fluorescence of 4-biphenylcarboxylic acid in aging sol–gel silica revealed the influence of the increasing rigidity on (i) intersystem crossing and (ii) thermal deactivation of excited triplet states T1 [23]. Usually, the luminescence decay times are increased by one order of magnitude for dyedoped sol–gel glasses, increasing with growing densification of the host material [27]. Static and dynamic fluorescence spectroscopy of Rhodamine 6G (structure 36) in a sol–gel matrix are used to study the effect of aging on the local microviscosity [28]. The different influences of (i) interaction between dye and pore wall and (ii) physical constraints imposed on the dye molecule by the rigidity of the silica xerogel cage are elucidated for 3,30 -diethyloxadicarbocyanine iodide (DODCI) [29]. A study of 1-naphthol as a fluorescent probe for the sol to gel to xerogel reaction of mixed alkoxysilane and alkoxyalumino-alkoxysilane revealed that the encapsulation of the dye in the gel network exhibits band shifts similar to those in a polar solvent at low temperature [30]. The photostability of dyes in porous silicas has not been studied very broadly, up to now. One reason is the large number of parameters influencing the complex process of photobleaching, like (1) photolysis, presumably influenced by the spatial confinements of the corresponding transition states, (2) photochemical reactions with photoproduced free radicals, influenced by changes in the microviscosities, and (3) reaction with singlet oxygen produced by energy transfer from the triplet states of the excited dye. Various methods to improve the photostability have been proposed, e.g., covalent attachment of the dye to the sol–gel
host, alteration of the porosity and the acidity, or addition of an antioxidant [31,32]. Photostability alterations of Rhodamine 6G in porous silicas over three orders of magnitude have been reported, yet the reasons for these are not clear [33]. Absorption and emission spectroscopic parameters were found to be largely independent of the pore size, as long as no dimers are formed [34]. A clear increase of photostability by two orders of magnitude upon immobilization in organically modified porous silicas has been found, e.g., for pyromethane or Rhodamine 6G. This effect is, however, counterbalanced by thermal degradation upon increase of the laser intensity [35]. 2.5. Applications 2.5.1. Sensors Optical oxygen sensing based on luminescence intensity quenching via the Dexter energy transfer has been studied intensively because of the great importance to determine oxygen concentrations for many industrial, medical, and environmental applications [36]. Optical sensing offers advantages over other sensor types, e.g., amperometric devices, since it has a fast response, does not consume oxygen, is not easily poisoned, and has a larger potential for miniaturization. At present, fluorescence intensity-based sensors are developed predominantly (Fig. 4) [37–40]. Although they are susceptible to detector drift due to photobleaching of the dye, this disadvantage is overcome using the luminescence lifetime of an indicator as measuring method [36,41–43]. The viability of phase fluorimetry, applying continuous sinusoidally modulated excitation combined with phase-sensitive detection, is demonstrated for a LED-based oxygen sensor using evanescent wave excitation of a sol–gel-immobilized RuII –tris(4,7-diphenyl-1,10phenanthroline) [36]. A planar waveguide configuration from dye-doped sol–gel films is shown to be favorable for the separation of fluorescence and emission light without the use of optical filters. This process can be controlled and improved by adjusting the refractive index of the fluorescent sol–gel coating (Fig. 5) [44]. Optical sensing can also be based on UV/VIS absorption spectroscopy using changes of reflec-
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Fig. 4. Structure of coated fiber sensor (a) and sensor characterization system (b). Collimating lens A, launching lens B, collecting lens C, short-wave pass filter D, long-wave pass (sensing) filter E, lock-in amplifier (LIA) and coated fiber sensor F.
Fig. 5. Scheme of compact and efficient capture of fluorescence emitted from a coating on a planar waveguide.
tance. The applicability of this method is demonstrated for optical NO2 sensing based on sol–gel-entrapped azobenzene dyes, where variations of n–p transition intensities with changes of NO2 concentrations are attributed to transient charge-transfer type bonds between indicator and analyte [45]. Fiber optic pH sensors find rapidly increasing application in medical research, e.g., for the study of the influence of local pH on stroke [46] or gastric diseases [47]. Fiber optic pH sensors have the advantage of very small size, flexibility, possibility for invasive analysis, and remote sensing. A number of fiber optic pH sensors have been employed, based on bulk absorption, optical reflectance, fluorescence, or energy transfer [48]. Evanescent wave spectroscopy offers a variety of
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advantages, especially using dye-doped porous sol–gel coatings enabling a rapid response [46,48, 9a]. If the optically less dense medium, penetrated by the evanescent field passing the boundary between fiber and coating, contains a fluorescing dye, the generated fluorescence can couple back to the core and the amount of power being transmitted by the core will be attenuated. Further, the possibility of distributed sensing, e.g., for short-range pH variations, exists especially if dye mixtures are used covering a broad range of pH values [48]. Optical ammonia sensors employ colorimetric or fluorimetric indicators, i.e., ammonium-selective ionophores, indicator ion pairs, or receptor molecules such as porphyrins [9c]. Also fluorescence lifetime-based ammonia sensors are studied [49]. Indicators can change their optical properties via protonation and deprotonation [50–55]. However, other mechanisms of acid–base indicators exist as well, e.g., ammonia sensors on porous silicas or ormosils have been developed for (i) near-infrared dyes as photometric indicators [56], (ii) fluorophore–absorber pairs [57], and (iii) aminofluoresceine [58]. Solvatochromic dyes like Malachite Green have been immobilized in thin films of porous glass from sol–gel processes and used as polar sensitive indicator for ethanol in water down to a detection limit of 0.6 vol.% ethanol [59]. Sol–gel-derived porous silica matrices are also applied for the immobilization of metal-complexing colorimetric reagents like Eriochrome Cyanine R for Cu(II) [60], Zincon for Cu(II) [61], or 2-(5amino-3,4-dicyano-2H-pyrrol-2-ylidene)-1,1,2-tricyanoethanide for Hg(II) [9f]. Good prospects are expected for optical biosensors using sol–gel techniques, since enzyme systems in porous glass monoliths like pyridine nucleotide co-enzymes, sorbitol dehydrogenase/ NADH, or lactate dehydrogenase/NADH exhibit strong absorption and fluorescence as well [62]. 2.5.2. Lasers Interest in solid-state dye lasers originates from advantages like compactness, i.e., high potential for miniaturization or absence of toxic solution i.e. safe handling and deposition of spent laser media,
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since the shortcoming of solvent-based dye lasers are bulky dye-flow systems and reservoirs, needing significant maintenance and high costs for running and removal of toxic wastes. The main barrier for a preferential use of solid-state dye lasers is the limited photostability of dyes. Recent developments in solid-state dye lasers concentrate on dye-doped porous glasses, derived from sol–gel processes [63–68], since low thermoconductivity and low laser damage threshold limit the applicability of polymer dye lasers. Sol–gelderived porous silicas improve the monomeric dispersion and fixation of dye guests, reduce rotational relaxation of excited states of lasers dyes, which is the main source of non-radiative energy loss, and exhibit superior physical properties like low thermal expansion, low strain birefringence, or low non-linear refractive index [7]. The better transmittance of silicas for UV radiation as compared to polymeric hosts enables the study of solid-state UV dye lasers [69,70]. For example, silica slabs in the form of parallelepipeds with a cross-sectional area of 4:5 4:5 mm2 were prepared from tetraethoxysilane, doped with UV dye (e.g. p-terphenyl or 2-phenyl-5-biphenyl-1,3,4oxadiazole) by impregnation, cut to samples (0.5 mm thickness) by a diamond saw, placed between reflector and output coupler and pumped by a short pulse (1 ns) N2 laser. A pump threshold energy of 20 lJ and a slop efficiency of 35% can be achieved without measures to reduce overheating [69]. Feedback effects in dye-doped sol–gel films are observed, where these films are waveguides for the spontaneous emission or super-radiance obtained upon pumping with a frequency-doubled laser (Fig. 6) [71,72]. Here, Fresnel back-reflection acts as a quasi-resonator configuration. Output energies were enhanced by factors of up to five and pumping threshold energies reduced down to 15 lJ pulse1 [72]. The effects are influenced by the refractivity and thickness of the film, the polarization of the emitted light and the incident angle of the excitation light [73]. Further improvements are expected for resonators based on (i) surface grating structures like distributed Bragg resonators or (ii) distributed feedback mirrors (Fig. 7) [72].
Fig. 6. Schematic representation of the effects of lasing RB– SiO2 /TiO2 films (RB: Rhodamine B dye).
Fig. 7. Experimental layout of the narrow line width laser experiment based on surface grating structures. Sample: Rhodamine 6G dye in sol–gel silica.
Narrow line width operation of solid-state dye lasers based on sol–gel silica is reported using a double-grating resonator cavity, one grating for wavelength tuning and a holographic grating serving as beam expander [74]. Narrow line width lasing was achieved for Rhodamine 6G (560 nm), Coumarin 460 (474 nm), and Exalite 377 (378 nm). 2.5.3. Optical switching Extended studies on the properties of photoactive dyes, immobilized in porous silicas and ormosils by co-condensation, reveal (i) weak electron–phonon coupling in hybrid matrices and (ii) strong dipolar interactions in inorganic matrices allowing to create a local anisotropy or an adjustable index change [25]. Consequently, normal photochromism is found for spiropyran or spirooxazine in ormosil hosts, and reverse photochromism of the stabilized merocyanine for the pure silica (Eq. (13)) [25,75]. Optical memory systems and waveguides based on dye-doped silica matri-
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ces, which can be easily adapted to optical fiber devices, have been proposed [76]. Spiropyran-doped gel–glass waveguides are studied for application as optical intensity-induced shutter, resulting from the photochromic properties. Such a device behavior is based on light-induced changes in the imaginary component of the complex index of refraction of the material, i.e., changes in the absorption due to the light-induced population changes [77]. Spirooxazine, embedded in a sol–gel material having strongly hydrophobic Si–H bonds, is found to exhibit the fastest thermal bleaching after strong photocoloration [78]. Sulfonated spiropyran exhibits normal photochromism in a porous silica matrix in the presence of dodecylbenzenesulfonic acid with a half-life time of 30 days [79]. 2.5.4. Miscellaneous A stable and large SHG in sol–gel-processed poled silica waveguides doped with organic azo dye has been achieved [80]. The corona-poled silica film doped with Disperse Red 1 exhibits an effective SHG coefficient of deff ¼ 75 pm V1 [80]. Organic-doped sol–gel glasses for electrooptics and display applications have been proposed [81]. Microdroplets of nematogenic organic compounds, i.e. liquid crystals, were tapped in sol–gel glass films. The glass-dispersed liquid crystals (LCs) are birefringent depending on the orientation of the LCs and the optical angle of incidence. If the film is coated with transparent electrodes and an electric field is applied, a reorientation of the LC director in the droplet occurs, varying the refractive index [23]. Various fluorescing organic molecules like phenanthrene, naphthalene, quinine (potassium channel blocker, CAS registry no. 130-95-0) and pyrene, exhibit increased triplet state population upon embedding in sol–gel glasses, resulting in observed room-temperature fluorescence or delayed fluorescence, attributed to different influences like dissociation processes, basic conditions at the interface, adsorption onto cage walls, or increased rigidity by steric constraints [23]. The latter effect was confirmed for Erythrosin B and Eosin Y in sol–gel silica from measurements of decay lifetimes [8].
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Azo dye-doped films, prepared from co-condensation of triethoxysilane derivatives of the dye and tetraethoxysilane, were subjected to photoinscription via surface relief grating resulting in diffraction efficiencies of >30% [82]. Dye-doped porous silicas have been used for persistent spectral hole burning, which has attracted interest for high-density frequency domain optical storage [8,82–86]. High-temperature spectral hole burning was achieved using dye-doped sol–gel-derived titania [87] and alumina [88]. Photostable solar concentrators based on fluorescent dye-doped sol–gel films have been developed to collect, concentrate, store, and convert solar radiation, which is diffuse and intrinsically intermittent [13]. The efficiency of photostable perylimide embedded in ormosil from 3-glycidoxypropylethoxysilane was found to be close to 20% [89]. Materials with properties of optical saturation are candidates for use in optical pulse compression, and chromophores exhibiting reverse saturation absorption (RSA) have received much attention for use in optical limiting [90–93]. RSA as well as upconverted luminescence are assumed to result from stronger absorption of the higher excited states [94]. The phenomena have been attributed to a stepwise two-photon absorption transition from both singlet and triplet states to their higher excited states via the first excited states. Sol–gel-derived ormosils and aluminosilicates doped with metalloporphyrins were found to be favorable candidates for RSA [94,95]. 2.6. Conclusions Chromophores in inorganic–organic polymers have proved to exhibit various optical properties for potential applications, e.g., as sensors, lasers, frequency doublers and optical switches, limiters, amplifiers, or storage devices as well as solar concentrators. The combination of inorganic condensation and organic polymerization properties in the hybrid ormosil structure enables nearly unlimited possibilities for varying the environment for chromophore guest molecules, i.e., the pore size and the chemical nature of the pore walls, thus strongly tuning their optical behavior. Further, the
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diversity of multi-lateral reactivities, which the various ormosil functional groups can undergo, enables stable anchoring of such dye-containing composite materials to nearly all kinds of films or fibers from glasses, ceramics, or metals. These aspects make dye-loaded hybrid polymers attractive for future uses, either as advanced materials with optical functions or as hybrid pigments, e.g., with fluorescing properties, having improved migration stability or light fastness.
3. Molecular-sieve-encapsulated dye molecules 3.1. Introduction Inorganic molecular sieves comprizing (i) zeolites and zeolite analogues in the range of micropores (
