New Laser Hybrid Materials Based on POSS Copolymers - The

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2008, 112, 14710–14713 Published on Web 09/04/2008

New Laser Hybrid Materials Based on POSS Copolymers Olga Garcı´a,*,† Roberto Sastre,† Inmaculada Garcı´a-Moreno,‡ Virginia Martı´n,‡ and ´ ngel Costela‡ A Departamento de Fotoquı´mica, Instituto de Ciencia y Tecnologı´a de Polı´meros (CSIC), C/ Juan de la CierVa 3, and Departamento de Quı´mica La´ser, Instituto de Quı´mica- Fı´sica “Rocasolano” (CSIC), C/ Serrano 119, Madrid 28006, Spain ReceiVed: July 21, 2008; ReVised Manuscript ReceiVed: August 22, 2008

We report on the design and synthesis of new laser hybrid materials based on dye-doped poly(methyl methacrylate) cross-linked with octa(methyl methacrylate)-POSS (polyhedral oligomeric silsesquioxanes). The enhancement of the thermal and optical properties of these hybrid matrices lead to the best laser action reported to date for different dye-doped solid matrices, independently on the chromophore nature. These results are a strong indication that these new photosensitized materials could be a universal matrix to optimize solid-state laser actions of different dyes. The progress in the chemical synthesis of hybrid matrices has allowed the design and construction of new optical and optoelectronic materials with specific properties, spanning and impelling their applications in areas of high technical importance such as light emitting diodes,1 field effect transistors,2 photodetectors,3 or solar cells.4 The possibility to combine in a unique material the advantages of inorganic glasses (high thermal dissipation capability, low thermal expansion and thermal coefficient of refractive index dn/dT, and high damage threshold)5,6 with those offered by organic polymers (high capability to solve organic dyes, good homogeneity, adaptability to techniques of cheap production, and relative easiness to modify their material’s composition and chemical structure)6-8 allows us to define the hybrid matrices as the most promising and attractive materials to be used as solid hosts for organic dyes, in the attempt to develop solid-state dye lasers (SSDL).6-16 Highly emissive structures based on hybrid materials doped with laser dyes have been already synthesized via sol-gel.9-21 However, these materials exhibit some serious limitations such as a complex and lengthy synthesis process, fragility which results in difficult mechanization of the final material, and most important, frequent optical inhomogeneity caused by refractive index mismatch between organic and inorganic domains. A way to avoid these problems while maintaining the material’s hybrid character has been the synthesis of silicon-containing organic matrices.16 High lasing photostabilities but with only reasonable efficiencies were registered for these dye-doped matrices.16 In addition, the potential properties of these hybrid matrices are limited in this application by the low silicon content allowed to avoid obtaining very soft final materials unfit for subsequent mechanization and proper polishing. The design and synthesis of new materials with silica incorporated at molecular level could overcome all the above* Corresponding author. E-mail: [email protected]. † Departamento de Fotoquı´mica, Instituto de Ciencia y Tecnologı´a de Polı´meros (CSIC). ‡ Departamento de Quı´mica La ´ ser, Instituto de Quı´mica- Fı´sica “Rocasolano” (CSIC).

10.1021/jp8064097 CCC: $40.75

mentioned problems. Although in the commercially available silsesquioxanes (POSS) the silicon is coordinated with three oxygen atoms, and in the silica is a quadruple-coordination, the similarity between both allows considering POSS compounds like a silica of polyhedrical or prismatic structure (Figure 1). The self-assembling of nanosized POSS to the polymer matrix through the peripherical polymerizable groups on the POSS cage enhances significantly the thermal, mechanical, and physical properties of the final materials.22 Consequently, photosensitized materials based on POSS could improve the laser behavior to accomplish an operational SSDL as commercial alternative to conventional liquid solution dye lasers. In this Letter, we report, for the first time to the best of our knowledge, on the laser action of new lasing materials based on BODIPY dye Pyrromethene 567 (PM567) (Figure 2) doping copolymers of methyl methacrylate (MMA) with octa(methyl methacrylate)-POSS (8POSS), added in increasing weight proportions from 1% to 50%. Previous studies on the laser properties of this dye incorporated into the homopolymer PMMA, and into different organic-inorganic hybrid materials,7,8,11,14,16 allow us to asses directly the effect on the laser operation of the incorporation of the 8POSS in the matrix material. To define a general approach that could optimize the synthesis routes of hybrid matrices for optical applications with independence of structures and compositions, the laser behavior of two other different dyes, Pyrromethene 597 (PM597) and Rhodamine 6G (Rh6G) (Figure 2), incorporated into these new MMA-8POSS matrices was also systematically studied. The synthesis route followed to prepare the materials studied in this work was the block radical polymerization of the above monomers using AIBN in an appropriate concentration (1.0 wt %) with regard to the total amount of monomers in the final mixture. First, adequate amounts of the methacryl substitutedPOSS (8POSS) (from Hybrid Plastics) and the initiator were added to freshly distilled MMA (Aldrich) to prepared a number of different weight/weight copolymers proportions of MMA8POSS (from 1 to 50 wt %). Second, the adequate amount of the PM567 dye (laser grade from Exciton) was added to the  2008 American Chemical Society

Letters

Figure 1. Schematic representation of MMA-8POSS cross-linked hybrid materials.

Figure 2. Molecular structures of the laser dyes PM567, PM597 and Rh6G.

resulting hybrid mixture and was placed in an ultrasonic bath until complete dissolution of the dye. The resulting solution was filtered into appropriate cylindrical polypropylene molds using a 2 µm pore size filter (Whatman Laboratory, PTFE disposable filters). Polymerization was performed in a thermal bath at 40 °C over a period of 2 days and then at 45 °C for about 1 day. Afterward, the temperature was raised to 50 °C and increased slowly up to 80 °C over a period of 1 day, to decompose residual AIBN. Finally, the temperature was reduced in steps of 5 °C per day until room temperature was reached, and only then was the sample unmolded. This procedure was essential to reduce the buildup of stress in the polymer sample due the thermal shock. The weight content of silicon in the final materials ranges from 0.2% in the MMA-8POSS 99:1 formulation to 8% in the 50:50 copolymer. With the 8POSS content increases linearly the density of the final material, from 1.1810 to 1.2363 g/cm3,

J. Phys. Chem. C, Vol. 112, No. 38, 2008 14711 the refractive indices, from 1.4883 to 1.4977, and the thermal conductivity from 0.2050 to 0.2333 (W m1- K-1). The solid laser samples of the prepared materials were cylinders (10 mm length, 10 mm diameter) with a cut parallel to the axis of the cylinder to obtain a lateral flat surface of 4 × 10 mm2. This surface and the ends of the rods were prepared for lasing experiments by using a grinding and polishing machine (Phoenix Beta 4000, Buehler) until optical-grade finished. The samples were transversely pumped at 532 nm with 5.5 mJ, 6 ns fwhm pulses from a frequency-doubled Q-switched Nd:YAG laser (Monocrom OPL-10) at a repetition rate of 10 Hz. Some samples were also pumped at 30 Hz with 5.5 mJ, 10 ns fwhm pulses from a diode-pumped Nd:YAG laser (Monocrom EO Q-DPSSL 532-12) under otherwise identical experimental conditions. Details of the experimental system can be found elsewhere.16 Broad-line-width laser emission with pump threshold energy of ≈0.5 mJ, beam divergence of ≈5 mrad and pulse duration of ≈5 ns fwhm was obtained, in a simple plane-plane nontunable resonator, from all the materials under study. A summary of the results obtained is shown in Table 1, where data on lasing efficiencies, peak of the laser emission and intensity of the laser output (referred to the initial intensity) after 100,000 pump pulses in the same position of the sample are included. PM567 (1.5 × 10-3 M) dissolved in a copolymer of MMA with 8POSS at 13 wt % proportion exhibits a high nonoptimized lasing efficiency of 60% with a high photostability with no sign of degradation in the laser output after 100,000 pump pulses at 10 Hz repetition rate. Laser action of PM567 depends on the structure and composition of the functionalized-POSS more than on the actual silicon content of the final material. The dye-laser emission parameters seem nearly independent of the total content of 8POSS for matrices with weight proportions ranging from 5 to 30%; both higher and lower contents of silicon POSS result in noticeable decreases of the dye-laser efficiency. However, for a given weight proportion of POSS added to MMA, the substitution of 8POSS, where POSS cages become cross-linking points in the three-dimensional network (Figure 1), by the monofunctionalized heptaisobutyl-methacryl-POSS (1POSS), which defines a linear polymer with POSS cage as pendant groups of the polymeric chains, impairs the lasing efficiency and stability of PM567 by ≈25%. Thus, the rigidity of the matrix appears as being a structural parameter of utmost importance, caging and immobilizing the chromophores and, consequently, reducing the dynamic interactions with other species avoiding early photobleaching of the dye.7,8 For comparison, Table 1 also collects the best results obtained to date with dye PM567 incorporated into organic and hybrid materials with the same dye concentration, subjected to the same pump conditions, and under identical experimental arrangement than the POSS-based organic matrices. The presence of 8POSS in the final material clearly improves the laser efficiencies while the photostabilities remain or even enhance the highest lasing lifetimes previously recorded. To assess the long-term stability of the new photosensitized material, it was subjected to a longer run of pumping under more drastic conditions. Thus, the sample was pumped in a fixed position with the same energy (5.5 mJ/pulse) but at 30 Hz repetition rate until a downward slope tendency in the laser stability was detected. In previous studies, we presented evidence showing that the accumulation of heat into the material in polymeric-solid-state dye lasers increased significantly with the pump repetition rate, impairing lasing stability.16 This effect represents a serious drawback for some potential and new

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TABLE 1: Laser Parametersa for Dyes PM567, PM597 and Rh6G Incorporated into a Copolymer of MMA with 8POSS 13 wt % Proportionb dye PM567 (1.5 × 10

-3

M)

PM597 (0.6 × 10-3 M)

Rh6G (0.4 × 10-3 M)

matrix material

λmax (nm)

eff (%)

I100,000 (%)

COP(MMA-8POSS 87-13) PMMA COP(MMA-TMSPMA 40-60)d TER(MMA/HEMA(50/50)-TRIEOS-10%)e COP(MMA-8POSS 87-13) PMMA COP(MMA-TMSPMA 80-20)d TER(MMA/HEMA(50/50)-TRIEOS-10%)e COP(MMA-8POSS 87-13) PHEMA COP(HEMA-TMSPMA 40-60) COP(HEMA-TEOS 99-1)

568 562 560 559 583 580 576 573 576 580 571 568

60 39 21 25 63 40 40 23 42 25 18 16

110 30 110 60 111 56 104 135 97 0 50 91

a λmax: peak of the laser emission. Eff: energy conversion efficiency. I100,000 (%): intensity of the dye-laser output after 100,000 pump pulses in the same position of the sample referred to the initial intensity. b Data obtained with the same dyes doped organic and hybrid materialsc and pumped under otherwise identical experimental conditions are included for comparison. c TMSPMA: 3-(trimethoxysilyl)propyl methacrylate. HEMA: 2-(hydroxyethyl) methacrylate. TRIEOS: methyltriethoxysilane. TEOS: tetraethoxysilane. The numbers after these monomers refer to their vol/vol proportion in the final material. d Data from ref 16. e Data from ref 15.

Figure 3. Normalized laser output as a function of the number of pump pulses in the same position of the sample for dye PM567 in a copolymer (MMA-8POSS 87-13) pumped at 30 Hz (A) and pure PMMA pumped at 10 Hz (B). Both pumping at energy of 5.5 mJ/pulse.

important applications of solid-state dye lasers, such as photodynamic therapy, which would require the laser energy to be applied in high-repetition rate pulses. Under these more demanding conditions, dye-doped POSSmatrix exhibits again an excellent lasing lifetime because the laser output decreases slightly after 850,000 pump pulses (Figure 3). This is, to the best of our knowledge, the highest photostability achieved to date for solid-state dye lasers in organic, inorganic, or hybrid matrices doped with any laser dye without rotating or translating the medium to distribute the thermal load over a large volume. To assess if the enhancement of the laser operation with MMA-8POSS cross-linked hybrid materials is specific for dye PM567 or could be extend to other dyes, the lasing behavior of the nonpolar PM597, and the polar Rh6G, were also analyzed under identical experimental conditions. The behavior of these materials (Table 1) follows the same pattern previously observed with PM567: the addition of a 13 wt % proportion of 8POSS to MMA enhanced significantly the laser action of these two dyes with respect to the corresponding data recorded previously under identical experimental conditions. In conclusion, we report on the design and synthesis of new SSDL materials based on dye-doped copolymers of POSS with organic monomers. The silicon-content of POSS materials

enhance significantly the lasing properties of different organic dyes. These new hybrid matrices exhibit important advantages with respect to laser materials prepared via sol-gel because they remain organic, which means easy synthetic functionalization of the POSS periphery, improvement of the interphase with the organic components, and well-controlled network free volume by cross-linking with the polymeric chains. The ease of controlling in a precise way the structural, thermal, and optical properties of these optoelectronic materials has allowed the best laser action reported up to now for polar and nonpolar organic dyes. It should be remarked that this improved laser operation has been achieved in a nonoptimized cavity and under very demanding pumping conditions. To understand the physics of the laser emission in these photosensitized hybrid materials and for a more comprehensive assessment of their potential applications, a detailed investigation of their optical properties as a function of the composition and structure of the matrix in the solid state is ongoing. Extension of this study to other dyes with tunable laser emission in the blue- and red-edge spectral regions could define the polymeric POSS-organic cross-linked hybrid materials as a kind of universal matrix for lasing dyes. Thus, it would be possible obtaining optimized laser action in the solid state with a single matrix overcoming the dye/hosts specificity imposed up to date for the existing polymeric and hybrid matrices. The design and synthesis of POSS-based solid-state dye lasers opens new challenges for the science of materials and enhances novel properties, economic processes, and innovative applications, especially in optoelectronic fields. Acknowledgment. The materials described in this work and their utilization in solid-state dye lasers are covered by Spanish Patent No. P200800220 filed on January 2008. This work was financed by the Spanish CICYT (Project MAT2007-65778-C0201). V.M. thanks CSIC for her JAE-Doc postdoctoral contract. References and Notes (1) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. B.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Nature 1990, 347, 539. (2) Garnier, F.; Hajlaoui, R.; Yassar, A.; Shirakawa, P. Science 1994, 265, 1684. (3) Kanicki, J. Handbook of Conducting Polymers; Marcel Dekker: New York, 1986. (4) Hugah, W. H.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 469.

Letters (5) Nikogosian, D. N. Properties of Optical and Lasers Related Materials. A Handbook; Wiley: New York, 1997. (6) Rahn, M. D.; King, T. A. J. Mod. Opt. 1998, 45, 1259. (7) Costela, A.; Garcı´a-Moreno, I.; Sastre, R. Handbook of AdVances Electronic and Photonic Materials and DeVices; Academic Press: San Diego, 2001. (8) Garcı´a-Moreno, I.; Amat, F.; Liras, M.; Costela, A.; Infantes, L.; Sastre, R.; Lo´pez Arbeloa, F.; Ban˜uelos Prieto, J.; Lo´pez Arbeloa, I. AdV. Funct. Mater. 2007, 17, 3088. (9) Costela, A.; Garcı´a-Moreno, I.; Go´mez, C.; Garcı´a, O.; Sastre, R. Appl. Phys. B: Laser Opt. 2002, 75, 827. (10) Nung, T. H.; Canva, M.; Dao, T. T. A.; Chaput, F.; Brun, A.; Hung, N. D.; Boilot, J. P. Appl. Opt. 2003, 42, 2213. (11) Costela, A.; Garcı´a-Moreno, I.; Go´mez, C.; Garcı´a, O.; Sastre, R. Chem. Phys. Lett. 2003, 369, 656. (12) Yang, Y.; Wang, M.; Qian, G.; Wang, Z.; Fan, X. Opt. Mater. 2004, 24, 621. (13) Reisfeld, R.; Weiss, A.; Saraidarov, T.; Yariv, E.; Ishchenko, A. A. Polym. AdV. Technol. 2004, 15, 291.

J. Phys. Chem. C, Vol. 112, No. 38, 2008 14713 (14) Costela, A.; Garcı´a-Moreno, I.; Go´mez, C.; Garcı´a, O.; Sastre, R.; Roig, A.; Molins, E. J. Phys. Chem. B 2005, 109, 4475. (15) Garcı´a-Moreno, I.; Costela, A.; Cuesta, A.; Garcı´a, O.; del Agua, D.; Sastre, R. J. Phys. Chem B 2005, 109, 21618. (16) Costela, A.; Garcı´a-Moreno, I.; del Agua, D.; Garcı´a, O.; Sastre, R. J. Appl. Phys. 2007, 101, 073110. (17) Reisfeld, R. Handbook of Sol-Gel Technology; Springer: Berlin, 2004. (18) Marlow, F.; McGehee, M. D.; Zhao, D.; Chmelka, B. F.; Stucky, G. C. AdV. Mater. 1999, 11, 632. (19) Ahmad, M.; King, T. A.; Ko, D.; Cha, B. H.; Lee, J. J. Phys. D: Appl. Phys. 2002, 35, 1473. (20) Duarte, F. J.; James, R. O. Opt. Lett. 2003, 28, 2088. (21) Saraidarov, T.; Reisfeld, R.; Kazes, M.; Banin, U. Opt. Lett. 2006, 31, 356. (22) Markovic, E.; Clarke, S.; Matisons, J.; Simon, G. P. Macromolecules 2008, 41, 1685.

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