J. Phys. Chem. B 2009, 113, 10611–10618
10611
Efficient Red-Edge Materials Photosensitized by Rhodamine 640 Inmaculada Garcia-Moreno,*,† Angel Costela,† Mercedes Pintado-Sierra,† Virginia Martin,† and Roberto Sastre‡ Instituto de Quı´mica-Fı´sica “Rocasolano”, CSIC, Serrano 119, 28006 Madrid, Spain, Instituto de Ciencia y Tecnologı´a de Polı´meros, CSIC, Juan de la CierVa 3, 28006 Madrid, Spain ReceiVed: May 12, 2009; ReVised Manuscript ReceiVed: June 22, 2009
We report on tunable, highly efficient and photostable solid-state dye laser emitting around 640 nm based on Rhodamine 640 incorporated into homopolymers, linear and cross-linked copolymers, and silicon-modified organic matrices. The effect on the lasing properties of both dye concentration and environmental conditions was analyzed. Under transversal pumping at 532 nm with 5.5 mJ/pulse, high-lasing efficiencies of up to 42% were recorded. The laser operation was highly stable with a drop in the laser output of ∼20% after 100 000 pump pulses at the same position of the sample at 10 Hz repetition rate. To the best of our knowledge, these results are the topmost achieved to date for organic, inorganic, and hybrid materials doped with rhodamine 640. When the samples were incorporated into a grazing-incidence grating oscillator, narrow-line-width operation with tunning ranges of up to 40 nm was obtained. 1. Introduction Tunable lasers are one of the most important tools in spectroscopy, underwater sensing, medicine, and polymer fiber networks.1,2 Compared with other commercially available multiwavelength laser sources, organic dye molecules offer obvious advantages such as low cost, ample spectral coverage from the ultraviolet to the near-infrared, and high conversion efficiency for applications that require tunable high-power pulsed beams.1,2 Unfortunately, their usual implementation in liquid state results in some limitations of convenience and flexibility. Solid-state dye lasers are an attractive alternative to the established sources in the visible region, since the high gain and broad tunability of liquid dye lasers are retained in solid host media with the advantage of clean and low cost active elements that are easily exchanged to access different spectral regions with different dyes.3 Much research has been carried out for a long time to incorporate dye molecules into solid matrices with appropriate chemical, mechanical, thermal, and optical properties.4-8 Over the past decade, considerable progress has been made in the development and preparation of efficient solid-state active media for tunable lasers based on organic dyes incorporated into different matrices (including homopolymers, linear and crosslinking copolymers, sol-gel matrices, and composite inorganic and organic materials).9-24 Solid materials doped with organic chromophores have also attracted increased interest in other applications such as nonlinear optical materials, biosensors, or luminescence solar collectors.25 The main drawback in dyedoped solid-state devices is impaired photostability because of the absence of active medium circulation to supply “fresh” molecules to the pumped region with the consequently fast degradation of the fluorophores. This is a complex process, which depends not only on laser dye and host composition and structure, but also on factors such as dye concentration, pump wavelength, pulse rate, sample thickness, and geometry.26-31 * To whom correspondence should be addressed.
[email protected]. † Instituto de Quı´mica-Fı´sica “Rocasolano”, CSIC. ‡ Instituto de Ciencia y Tecnologı´a de Polı´meros, CSIC.
E-mail:
Up to date, the most promising results in solid-state dye lasers have been obtained in the yellow region with pyrromethene-doped materials. Pyrromethene dyes exhibit good photostability, which ensures long operating lifetimes of the active laser media. Furthermore, the characteristics of their output power are on a par with those obtained in liquid solutions.20-24 In contrast, few results have been published on dye-doped solid-state lasers emitting in the red part of the visible spectrum in the range 610-650 nm. Rhodamine 640 (Rh640) and its sulfonated derivative sulforhodamine 640 (SulRh640) (also called rhodamine 101 and sulforhodamine 101, respectively) are red-emitting dyes that adapt very well to solid-state hosts because of their high fluorescence quantum yields in this spectral region.32-34 These xanthene dyes are used as fluorescent probes in proteins in biological applications35 and, more recently, in the development of polymer optical fiber (POF) based on dye-doped poly(methyl methacrylate) (pMMA) because of their property of emission very near to the second low-loss window of typical POF materials, which lies at 650 nm.36 In spite of the useful photophysical properties exhibited by these dyes,32-34 limited progress has been made in the development and preparation of solid-state active laser media based on them.6,37-40 To attempt producing a highly efficient and stable solid-state dye laser with emission in the red spectral region, we first analyzed, in a systematic way, the laser behavior of both dyes Rh640 and SulRh640 in liquid phase. Since the luminescent properties depends on the environmental, concentration of fluorophores, and excitation conditions,41,42 the results of this study guided the selection, among the quasi-limitless compositions and structures of solid materials, of the best dye/host matrix in order to optimize the laser action attending to both lasing efficiency and photostability. In this work, we report on wavelength-tunable laser action in solid state based on dyes Rh640 and SulRh640 incorporated into organic linear and cross-linked matrices. The appropriate rigidity of the matrix is of utmost importance, as caging and immobilization of the chromophores reduces
10.1021/jp904419j CCC: $40.75 2009 American Chemical Society Published on Web 07/10/2009
10612
J. Phys. Chem. B, Vol. 113, No. 31, 2009
Garcia-Moreno et al.
Figure 1. Molecular structures of Rh640 and SulRh640 dyes as well as the following monomers selected in this work: MMA, HEMA, TMSPMA, EGDMA, PETA, and PETRA.
the dynamic interactions with other species (i.e., oxygen, other dyes molecules, and radicals) avoiding early photobleaching of the dye. Taking into account that hybrid organic-inorganic materials have been used with good results to improve the thermal resistance of the host without losing the benefits provided by polymers,11,14,18,43 the dye was also dissolved in silicon-modified organic matrices. The incorporation of silicon atoms to the backbone of the monomers avoidstheproblemsrelatedtothesol-gelhybridorganic-inorganic matrices (complex and lengthy synthesis process, fragility of the final materials that renders difficult their mechanizing and polishing, and optical inhomogeneity caused by refractive index mismatch between organic and inorganic parts) and allows maintaining the organic character of the material, which means plasticity and easy synthesis, but with improved thermal properties.44 The results reported in this work allow also reaching a deeper insight in the origin and causes of the bichromatic laser emission of these dyes under certain circumstances, previously reported.45-49 2. Experimental Section Materials. Rhodamine 640 and sulforhodamine 640 (chloride salt, laser grade, Exciton) were used as received with a purity >99% (checked by spectroscopic and chromatographic methods). Solvents for laser studies were of spectroscopic grade (Merck, Aldrich, or Sigma) and were used without purification. Linear copolymers were obtained by copolymerization of 2-hydroxyethyl methacrylate (HEMA) with different volumetric proportions of methyl methacrylate (MMA) and of the sililated monomer 3-(trimethoxysilyl)propyl methacrylate (TMSPMA). Cross-linked matrices were obtained by copolymerization of HEMA with monomers with more than one polymerizable double bond per molecule. Di-, tri- and tetra-dentated comonomers, such as ethylene glycol dimethacrylate (EGDMA),
pentaerythritol triacrylate (PETA) and pentaerythritol tetraacrylate (PETRA), respectively, were selected. All monomers were purchased from Aldrich and used as received. Figure 1 shows the molecular structures of these monomers. Preparation of Solid Polymeric Samples. Rh640 was incorporated into the different solid matrices following the procedure previously described20 and rendering materials named as cop(HEMA/monomer). The solid monolith laser samples were cast in a cylindrical shape, forming rods of 10 mm diameter and 10 mm length. A cut was made parallel to the axis of cylinder to obtain a lateral flat surface of ∼6 × 10 mm. This surface as well as the ends of the laser rods were prepared for lasing experiments by using a grinding and polishing machine (Phoenix Beta 4000, Buehler) until optical-grade finished. The planar grinding stage was carried out with a Texmet 1000 sand paper (Buehler) using a diamond polishing compound of 6 µm as an abrasive in mineral oil as a lubricant. The final polishing stage was realized with a G-Tuch Microcloth (Buehler), using a cloth disk Mastertex (Buehler) with diamond of 1 µm in mineral oil as an abrasive type. Methods. Liquid solutions of dyes were contained in 1 cm optical-path quartz cells that were carefully sealed to avoid solvent evaporation during experiments. Both the liquid cells and the solid samples were transversely pumped at 532 nm with 5.5 mJ (3.5 mJ onto the sample), 6 ns fwhm pulses from a frequency-doubled Q-switched Nd:YAG laser (Monocrom OPL10) at a repetition rate of up to 10 Hz. Details of the experimental system can be found elsewhere.50 Narrow-line-width laser emission and tuning ranges of dye solutions were obtained by placing the samples in a homemade Shoshan-type oscillator,51 consisting of full-reflecting aluminum back and tuning mirrors and a 2400 lines · mm-1 holographic grating in grazing incidence with outcoupling via the grating zero order. Wavelength tuning was accomplished by rotation
Red-Edge Materials Photosensitized by Rhodamine 640
J. Phys. Chem. B, Vol. 113, No. 31, 2009 10613
Figure 2. Lasing efficiency and emission spectra of the dye Rh640 as a function of the dye concentration in ethanol.
Figure 3. Lasing efficiency and emission spectra of the dye SulfRh640 as a function of the dye concentration in ethanol.
of the tuning mirror. Tuning mirror and grating (both from Optometrics) were 5 cm wide and the angle of incidence on the grating was 88.5°. Laser line width was measured with a Fabry-Perot etalon (IC Optical Systems) with a free spectral range of 15.9 GHz. Absorption and fluorescence (after excitation at 532 nm) spectra were recorded on a fiber optic spectrophotometer (Ocean Optics USB2000) and on a spectrofluorimeter (Perkin-Elmer LS-50B), respectively, for diluted (5 × 10-4 M) and concentrated (1 × 10-3 M) dye solutions in ethanol contained in 0.01 cm optical path length quartz cuvettes. 3. Results and Discussion 3.1. Lasing Properties in the Liquid Phase. First, we carried out a systematic analysis of the laser behavior of the selected red-edge dyes in liquid phase as a guide to develop polymeric materials that could enhance their laser action in solidstate. The dependence of the laser action on the concentration of both dyes was analyzed in ethanol. The experiments were performed by varying the dye concentration whereas the other experimental parameters were kept constant. Dye solutions with optical densities (for 1 cm path length) in the range 1.5-30 were prepared and their lasing properties evaluated. The lasing efficiencies of both dyes, defined as the ratio between the energy of the dye laser output and the energy of the pump laser incident on the sample surface, as a function of the dye concentration are reported in Figures 2 and 3. As it was expected, the lasing efficiencies increased significantly with dye concentration up to a certain value. After an initial fast increase with dye concentration, the lasing efficiency levels off when the optical density of the solution reaches a value of ca.10. From this point on, further increases in the dye concentration have no effect on lasing efficiency. The dependence of the laser emission spectrum on the concentration of both dyes, Rh640 and SulRh640, is also shown
in Figures 2 and 3. The wavelength of the laser emission is red shifted as the concentration increases. This trend must be related to the effect of reabsorption/reemission phenomena on the laser emission, because the probability of exciting molecules by absorption of a photon previously emitted by another molecule in the medium depends on the overlapping between absorption and fluorescence spectra, which is affected by the dye concentration.52 At the highest dye concentration selected in this work (1 × 10-3 M), Rh640 exhibits dual laser emission with a main peak at 625 nm and a second one centered at 645 nm. Bichromatic laser emission from Rh640 has been previously reported by other authors under certain circumstances. When this laser dye was dissolved in methanol containing randomly distributed highly scattering titanium dioxide particles, bichromatic laser emission was observed, depending on dye concentration, pump energy, and scattering particle density.45-49 Under these conditions, a single laser peak (at about 617 nm) was emitted at low dye concentration (∼10-4 M), whereas at dye concentrations of about 10-2 M two peaks appeared simultaneously in the laser emission at 620 and 650 nm. With dual-wavelength emission established, the ratio between the intensities of the longwavelength and the short-wavelength emission peaks was found to increase with pump fluence.45-49 Although formation of aggregates has been suggested as an explanation for the bichromatic emission,49 in our case the shape of the absorption and fluorescence spectra at the highest dye concentration used is similar to those obtained in more diluted systems, which would indicate that there is not aggregation of the dye (see Figure 4). On the other hand, an influential factor in the observed bichromatic emission is pump fluence. It was found that the relationship between the intensities of the two peaks varies with pump fluence with the long-wavelength peak becoming more important as pump fluence increases. Under pump fluences at which the peak at 650 nm dominates the lasing
10614
J. Phys. Chem. B, Vol. 113, No. 31, 2009
Garcia-Moreno et al. TABLE 1: Laser Parametersa for Rh640 and SulRh640 Dyes in Different Solvents (Nd:YAG Laser (Second Harmonic) Pump Energy, 5.5 mJ/pulse; Dye Concentration, 6 × 10-4 M) Rh640 solvent F3-ethanol methanol ethanol acetone
Figure 4. Molar absorption and normalized fluorescence spectra of diluted (6 × 10-4 M, thin curves) and concentrated (1 × 10-3 M, bold curves) solutions of Rh640 in ethanol (0.01 cm optical pathway cuvette).
emission, removing from the oscillation cavity the 90% reflectivity aluminum mirror leads to the spectrum evolving to a single peak at 620 nm. This behavior can be understood in terms of reabsorption/reemission effects and inhomogeneous broadening of the absorption spectrum due to an effective vibrational temperature. Reabsorption/reemission effects could lead to an apparent new band at longer-wavelength. An examination of the absorption and fluorescence spectra of Rh640 shows that they exhibit enough overlap to allow energy transfer by reabsorption, which is a mechanism also confirmed by other authors.48 In the sample with lower concentration, the emission corresponding to the 0-0 band is dominant, and as result laser emission at the shorter wavelength of 620 nm is observed. When the dye concentration is raised to 1 × 10-3 M, reabsorption effects increase the losses at the short emission wavelengths, where there is the greatest overlapping with the absorption spectrum, and gain at the 0-1 band can contribute to the laser emission, resulting in lasing at 640 nm. Under high energy excitation, the vibrational temperature increases and the spectrum becomes inhomogeneously broadened, with the vibrational levels of the ground state being populated via the S1 manifold.53 The mechanism involves excited-state absorption. In short,53 as the pump intensity increases, S1-S2 transitions are excited by the same pump pulse. Relaxation from S2 to the vibrational manifold of S1 occurs very quickly in the subpicosecond time scale, and then vibrational population is transferred to the vibrational manifold of the ground state due to stimulated emission. As a consequence, during the nanosecond pulse a steady-state distribution of vibrational population in the ground state is created, which in turn produces inhomogeneous broadening due to coupling with vibronic levels of the S1 manifold.52 Thus, as the pump energy increases also does the contribution to the laser emission at lower energies, and the long-wavelength emission increases. Consequently, the short-wavelength emission corresponds to the usual homogeneous S0-S1 transition and dominates at low dye concentration. The long-wavelength emission appears when reabsorption/reemission and inhomogeneous broadening dominates, and gain at higher vibronic modes competes advantageously with that of the short-wavelength mode. The actual effect of the solvent on the lasing emission of Rh640 and SulRh640 was analyzed at a common concentration (6 × 10-4 M), selected for being the one that optimizes the lasing action of both dyes in ethanol. Table 1 summarizes the laser properties of these dyes in several solvents. The low solubility of both dyes in polar nonprotic and apolar solvents prevents the use of adequate solutions in cyclohexane and ethyl
b
SulRh640
eff (%)
λmax (nm)
eff (%)
λmax (nm)
38 50 51 49
613 620 621 624
56 62 60
616 618 620
a eff, energy conversion efficiency; λmax, peak wavelength of the laser emission. b F3-ethanol: 2,2,2-trifluoroethanol.
acetate. The protic nature of the solvents improves the lasing efficiency of both dyes. This behavior could be related to the fact that the dye can exist in different molecular forms, such as lactone (neutral form), zwitterion, and cation (ionic forms). The existence of the dye in a definite molecular form depends mainly on solvent polarity and proticity. It is well established that the zwitterions form, which is the molecular form with the highest fluorescence, is favored by polar protic solvents whereas the lactone form, with a very low luminescence yield, dominates in nonpolar and polar aprotic media.32 To analyze the lasing photostability of these dyes, some studies were carried out under experimental conditions identical to those selected to irradiate the fluorophores when embedded in solid polymeric matrices. In this way, comparison could be made in due time between the stability of the dyes in liquid and solid phases under laser irradiation. The solvent selected was ethanol because the highest lasing efficiencies of the selected dyes were obtained in solutions in ethanol. The irradiated volume in the solid samples under the selected experimental conditions was estimated to be 8 µL. Thus, capillary tubes into which the liquid solutions were incorporated offer the best geometry to reproduce an irradiated volume similar to that in the solid samples in order to maintain the same laser pump conditions in both cases. Although the optical quality of the capillary prevents laser emission from the dyes, information on photostability can be obtained by monitoring the decrease in laser-induced fluorescence intensity, excited transversally to the capillary, as a function of the number of pump pulses. The fluorescence emission was monitored perpendicular to the exciting beam, and its collection and analysis was carried out with the same setup selected to characterize the laser emission from dyes incorporated into solid samples. The concentration of the dyes was 6 × 10-4 M, and both samples had the same optical density at 532 nm (OD ∼ 18). The results obtained from both dyes, Rh640 and SulRh640 are plotted in Figure 5. The results obtained indicate that under laser irradiation Rh640 is a dye more photostable than the sulfonated derivative. In previous works, it has been established that the photostability of rhodamine fluorophores is essentially determined by both specific interactions of their amine groups with the environment and generation of free radicals from the substituents in the phenyl groups.54 The presence of substituents SO2- in orto- and SO2OH in para-positions of the xanthene chromophore could favor, under laser excitation, the formation of free radicals and/ or induce a redistribution of the aromatic system increasing specific interactions and, consequently, could increase the photobleaching rate of the SulRh640 dye with respect to that exhibited by Rh640. In addition, the analysis of the laser-induced fluorescence spectra before and after irradiation reveals that the photodeg-
Red-Edge Materials Photosensitized by Rhodamine 640
J. Phys. Chem. B, Vol. 113, No. 31, 2009 10615 TABLE 2: Laser Parametersa for Rh640 Dye in Linear, Cross-Linked and Silicon-Containing Organic Matrices (Nd:YAG (second harmonic) Pump Energy, 5.5 mJ/pulse; Repetition Rate, 10 Hz; Dye Concentration, 6 × 10-4 M)
Figure 5. Normalized laser-induced fluorescence emission as a function of the number of pump pulses at 10 Hz repetition rate for the dyes Rh640 (a) and SulRh640 (b) in ethanolic solutions.
radation is an irreversible process leading to the generation of products in low concentration or without significant absorption at the laser irradiation wavelength, because no new features appear in the spectrum registered after irradiation. 3.2. Lasing Properties in Solid State. As the studies in liquid phase showed that both dyes, Rh640 and SulRh640, exhibited similar lasing efficiencies but dye Rh640 has higher laser photostability under the selected pump conditions, the influence of both composition and structure of the solid matrix on the laser action was only studied in a systematic way for Rh640. To this aim, a number of organic and hybrid polymeric formulations with different plasticity and/or degree of crosslinking were synthesized as host materials for the Rh640 dye. The actual composition of these materials was decided in the light of the information obtained in our previous studies of the lasing action of dyes dissolved in different organic and hybrid solid materials.3,19-24 The dye concentration was selected to be 6 × 10-4 M because this was the concentration producing the highest lasing efficiencies in liquid solution with no dualemission band. When the samples were placed in a simple plane-plane nontunable resonator, single peaked laser emission, centered at ∼645 nm was obtained from the materials under study. Beam divergence was ∼5 mrad and pulse duration was ∼5 ns fwhm. The tuning capability of the dye-doped solid matrices, one of the most important features of dye lasers, was determined placing the samples in a grazing-incidence grating cavity in Shoshan configuration. Tunable laser emission with line-width of the order of 0.15 cm-1 was obtained with a tuning range of 40 nm from 630 to 670 nm. 3.2.1. Laser Operation in Linear Copolymers. As discussed above, improvements in the lasing properties of Rh640 were found when the solvent was protic but not very polar medium. In the solid samples, the polarity of the polymeric medium can be modulated by use of appropriate copolymers. Thus, copolymers of MMA with monomers functionalized with polar groups, such as HEMA were prepared in volumetric proportions ranging from 10/0 (pMMA homopolymer) to 0/10 (pHEMA homopolymer). The results obtained are summarized in Table 2. Lasing efficiencies in the range 15-39% were obtained in these linear copolymers where the presence of MMA induces a drastic reduction of this laser parameter. All the efficiencies registered are lower than those registered in liquid phase. In this regard, it has to be taken into account that the polishing of the surfaces of the solid samples relevant to laser operation was not laser grade. The laser emission spectra of these linear copolymers consist of a single-band centered at the long-wavelength (∼650 nm) in
material
eff (%)
λmax (nm)
I100 000 (%)b
pHEMA cop(HEMA/MMA) 7/3 cop(HEMA/MMA) 5/5 cop(HEMA/MMA) 3/7 pMMA cop(HEMA/EGDMA) 9/1 cop(HEMA/EGDMA) 8/2 cop(HEMA/EGDMA) 7/3 cop(HEMA/PETA) 9/1 cop(HEMA/PETA) 8/2 cop(HEMA/PETA) 7/3 cop(HEMA/PETRA) 9/1 cop(HEMA/PETRA) 8/2 cop(HEMA/PETRA) 7/3 cop(HEMA/TMSPMA) 9/1 cop(HEMA/TMSPMA) 8/2 cop(HEMA/TMSPMA) 7/3 cop(HEMA/TMSPMA) 5/5 cop(HEMA/TMSPMA) 4/6
39 35 28 26 15 30 34 40 42 39 30 35 36 35 32 31 30 27 23
650 648 646 641 628 649 647 641 641 644 649 641 643 645 648 648 646 644 643
57 35 24 25 24 30 34 38 78 (20)c (15)c 38 (32)c (24)c 42 35 29 25 27
a As defined in Table 1. b Intensity of the dye laser output after 100 000 pump pulses in the same position of the sample referred to the initial intensity I0, In(%) ) (In/I0) × 100. c Intensity of the dye laser output after 60 000 pump pulses.
spite of the moderate dye concentration in the samples. This band does not disappear when the pump fluences are reduced down to the laser threshold or when the 90% reflectivity aluminum mirror is removed from the oscillation cavity. A decrease in the polarity of the copolymer due to an increase in the proportion of MMA leads to a significant hypsochromic shift of the laser peak until the short-wavelength band centered at 620 nm is reached when the matrix was composed of just homopolymer pMMA. As discussed above, this behavior could be related to the low solubility of Rh640 in ethyl acetate. As this solvent mimics the MMA monomer, it is to be expected also a low solubility of the dye in MMA, which leads to a real concentration of the dye in the matrix much lower than the selected one. The lasing stability was studied by following the evolution of the laser emission with the number of pump pulses in the same position of the sample at 10 Hz repetition rate. Values of the laser output normalized to the initial lasing intensity after 100 000 pump pulses in the same position of the sample are collected in Table 2. To facilitate comparison, some of these data have been represented graphically in Figure 6. When the spectra of the laser emission are registered as a function of the number of pump pulses, a slight hypsochromic shift (∼6 nm) in the wavelength of the peak of the laser emission is observed. This shift is a consequence of the decrease of the concentration of the dye in the irradiated region as a result of the photodegradation of the dye molecules with the increased number of pump pulses.20 This behavior was consistently noticed in the laser emission of Rh640 in all the solid matrices investigated. In no case did two peaks appear in the laser emission spectral band. As seen in Table 2, the presence of MMA reduces significantly the lasing lifetime. The best photostability and the highest lasing efficiency are attained with Rh640 dissolved in the homopolymer pHEMA. This behavior is consistent with that observed in liquid phase, since HEMA monomer mimics the ethanol solvent. An increase of the concentration of HEMA
10616
J. Phys. Chem. B, Vol. 113, No. 31, 2009
Garcia-Moreno et al.
Figure 7. Laser emission energy of Rh640 doped into cop(HEMA/ PETA) 9/1 as a function of the pump energy. The solid line represents the linear fit of the data points.
Figure 6. Normalized laser output as a function of the number of pump pulses for Rh640 dye dissolved in (A) linear matrices (a) homopolymer pHEMA, (b) homopolymer pMMA, and (c) silicon-containing copolymer cop(HEMA/TMSPMA 8/2) and (B) cross-linked copolymers (a) cop(HEMA/EGDMA 7/3), (b) cop(HEMA/PETA 9/1), and (c) cop(HEMA/PETRA 8/2). Dye concentration: 6 × 10-4 M. Pump energy and repetition rate: 5.5 mJ/pulse and 10 Hz, respectively.
comonomer leads to a more plastic material with lower elastic limit and better damage resistance.55 However, as the internal plasticization increases, the protecting “polymer cage” that surrounds the dye groups weakens (the polymer becomes less rigid), and as a result the chromophore could be easily bleached.56 Consequently, increasing the rigidity of the matrix by copolymerization with cross-linked monomers could be of utmost importance in order to improve the laser action of Rh640 when incorporated into solid materials. 3.2.2. Laser Operation in Cross-Linked Copolymers. Methacrylic (EGDMA) and acrylic (PETA, PETRA) cross-linking monomers with different numbers of polymerizable groups per monomer molecule were chosen for preparing three-dimensional cross-linked copolymers with HEMA since, as we have just discussed, this monomer optimized the laser action of Rh640 when dissolved in linear copolymers. EGDMA is a double functionalized monomer (two double bonds), PETA is triple functionalized (three double bonds) and PETRA is quadruple functionalized (four double bonds). It has to be remarked that PETA incorporates in its structure a hydroxyl group, which should result in a more polar monomer and, consequently, more compatible with the HEMA comonomer. The lasing results obtained with Rh640 dye dissolved in the different cross-linked copolymers are summarized in Table 2. Comparison with the results obtained when the dye was incorporated into linear copolymers shows that whereas the position of the laser emission band is not significantly affected by cross-linking, the lasing efficiencies in all the cross-linked materials are in general similar to or lower than that obtained with the pHEMA homopolymer. The sample with the highest lasing efficiency (42%) was Rh640/cop(HEMA/PETA) 9/1. The slope efficiency for this material was also determined (Figure 7), and found to be 46%.
The photostability of the dye depends strongly on both the composition and the structure of the matrix. In fact, in the case of the methacrylic cross-linking monomer EGDMA the reduction of the medium polarity leads to lasing lifetime of Rh640 much lower to that registered in pHEMA. For the acrylic monomers, there is a composition, cop(HEMA/PETA 9/1), that improves the photostability exhibited by Rh640 in pHEMA, with the laser emission remaining at 78% of its initial value after 100 000 pump pulses in the same position of the sample. Further increases in the proportion of PETA result in a decrease of photostability. Likewise, a decrease of the matrix free volume induced by increasing the number of polymeric chains crosslinked by each monomer, such as happens in the materials based on PETRA, impairs significantly the lasing lifetime. The observed behavior points to the importance of the proper adjustment of the rigidity of the polymeric matrix in the laser performance of dyes. As the degree of cross-linking increases, the available free volume in the polymer decreases. It seems that to some extension this factor controls the photodegradation process of the dye dissolved in polymeric matrices at the concentration needed for laser operation in transversal pumping schemes. For a certain cross-linking degree, the free volume available within the polymeric matrix will be completely occupied by the dye. Increasing the degree of cross-linking beyond this point will result in the dye molecules being partially excluded from the shrinking free volume, and formation of dimers and/or reactions with other species, impairing the laser operation, will be forced. In the present case, the results obtained indicate that to optimize the photostability of Rh640 the crosslinking monomer must be polar, acrylic, and triple functionalized such as PETA, and must be added in volumetric proportions of up to 10%. The photobleaching curves are reproducible. For each dye/ host system, the photodegradation decay was registered in two different positions of a same monolith, and for some of the dye/ host systems more than one sample has been synthesized and analyzed. The photobleaching curves of Rh640-doped polymeric matrices deviate so much from an exponential-like decay, such as one produced by diffusional or heating effects, since no single cause is expected to fully explain the behavior of these rather complicated systems. 3.2.3. Laser Operation in Silicon-Containing Organic Matrices. Previous studies revealed that the laser action of organic dyes based on the BODIPY core in polymeric matrices was greatly enhanced by properly incorporating silicon atoms into the structure of the organic monomers.44 Trying to further improve the laser action of Rh640 in solid materials, the dye was incorporated into copolymers of HEMA
Red-Edge Materials Photosensitized by Rhodamine 640 with TMSPMA added in volumetric proportions ranging from 10 to 60%. Taking into account that there is one silicon atom per TMSPMA monomer repetition unit (see Figure 1), the concentration of silicon atoms in the material varies from 1% in the HEMA/TMSPMA 9/1 formulations to 7% in the 4/6 composition. The lasing results obtained are collected in Table 2. Comparison of the data summarized in this table shows that the laser performance of Rh640 dye in the silicon-based organic matrices does not improve that obtained in pure pHEMA homopolymer. In fact, as the presence of the sililated monomer into the matrix increases both the lasing efficiency and photostability of the dye decreases significantly. Although the photobleaching of dyes can occur by several different mechanisms and, from a general point of view, can be considered to be quite complex, photo-oxidation, in which chemically reactive singlet oxygen is formed by sensitization of ground-state triplet oxygen molecules by triplet state dyes,57 is maybe the degradation mechanism most often referred to and the one with strongest relevance. High silicon content plays an important role in the photodegradation processes of Rh640, probably as a consequence of the higher oxygen permeability in sililated materials and the longer lifetime of singlet molecular oxygen in these media.58 These photodegradation processes could become more relevant in xanthene dyes than in BODIPY chromophores due to both a higher polar character and a higher intersystem crossing quantum yield in the xanthene dyes. Thus, contrary to the general trend exhibited by the pyrromethene dyes, the laser action of Rh640 incorporated into silicon-containing organic matrices is not enhanced with respect to that registered in the pHEMA homopolymer, in spite of the improved thermal properties exhibited by the matrices incorporating the TMSPMA monomer. 3.3. Comparison with Previous Results. The laser action of Rh640 incorporated into the matrices synthesized in this work improves significantly the results reported in the two only previous studies on this dye incorporated into solid materials. In the first of these previous works, we reported a lasing efficiency of 4% with a very low photostability (the emission dropped to zero after only 800 pulses at 10 Hz repetition rate) for a sample of Rh640 doped in copolymer cop(MMA/HEMA 5/5) and pumped transversely at 337 nm, under otherwise identical experimental conditions than those selected in the present work.37 In the second previous work,38 a hot-press molding technique was used as an alternative to the common method of radicalinitiated polymerization for the fabrication of dye-doped pMMA impregnated with Rh640. The dye and granular pMMA were first dissolved in a solvent mixture of chloroform and methanol and then the mixture was heated to evaporate the solvents. Molding takes place at 175 °C and low pressure (
