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Photostability of lasers based on pyrromethene 567 in liquid and solid-state host media
15 March 2002
Optics Communications 203 (2002) 327–334 www.elsevier.com/locate/optcom
Photostability of lasers based on pyrromethene 567 in liquid and solid-state host media Mohammad Ahmad a, Terence A. King a,*, Do-Kyeong Ko b, Byung Heon Cha b, Jongmin Lee c a
Laser Photonics Research Group, Department of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK b Laboratory for Quantum Optics, Korea Atomic Energy Research Institute, P.O. Box 105, Yusong, Taejon 305-600, South Korea c Advanced Photonics Research Institute, Kwangju Institute of Science and Technology, 1 Oryong-dong, Buk-gu, Kwangju 500-712, South Korea Received 5 December 2001; accepted 10 January 2002
Abstract The photostability of the laser dye pyrromethene P567 in various liquid and solid samples has been measured by determining the longevity of laser operation when pumped by the second harmonic of a Q-switched Nd:YAG laser. A large improvement in photostability for pyrromethene was found in deoxygenated non-polar solvents, with a normalised photostability value of 1000 GJ/mol compared to deoxygenated polar solvents (100 GJ/mol). A high photostability has been observed in a heptane–1,4-dioxane azeotrope mixture and improvements observed in systems incorporating triplet quenching agents. Doped solid polymer samples showed an increased photostability up to 200 GJ/ mol for the pyrromethene laser dye when samples were modified with triplet quenchers. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 42.55.Mv; 42.70.Hj; 42.70.Jk; 82.35.Lr Keywords: Dye lasers; Laser materials; Polymers; Solvent; Pyrromethene; Triplet state; Singlet oxygen; Photostability
1. Introduction Solid-state dye lasers (SSDLs) are versatile tunable laser sources giving a compact and maintenance-free system and a low cost gain medium. Their applications range over spectroscopy, nonlinear optics, medicine and industry where tunable
*
Corresponding author. Fax: +44-161-275-5509. E-mail address: [email protected] (T.A. King).
high power pulsed laser beams are required. Useful properties for the dye in an SSDL are: 1. the dye should have a broad absorption band, 2. high conversion efficiency into laser emission, 3. high solubility in many solvents including water, 4. high photostability. Water solubility is advantageous since water is non-hazardous and has high thermo-optic properties. Rhodamine dyes have a high quantum yield but pyrromethene dyes have additional
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M. Ahmad et al. / Optics Communications 203 (2002) 327–334
benefit of reduced triplet absorption coefficient, being one-fifth that of rhodamine [1–8]. The pyrromethene family of dyes provides the best SSDL performance in polymers [9–13] and glass [14–20], however, photodegradation still occurs and its rate is dependent on the host composition. The photostability of laser dyes both in solution and solid is relatively low [5–8] for long operation of the dye lasers. This is due to the formation of large numbers of triplet states with longer lifetimes than the singlet levels contributing to the laser action. The longer triplet lifetimes affect the laser performance in both reduced photostability and efficiency. Efforts have been made to decrease the triplet states concentration both in liquid and solid hosts by addition of various additives triplet quenchers. These additives are triphenylamine (TPA) [21], 2,2,6,6 tetramethylpiperdin (TMP) [22], Tinuvin 770 [23], 1,3,5,7-cyclooctatetraene (COT) [24] and 1,4-diazobicyclo [2,2,2] octane (DABCO) [25–28]. SSDLs are usually based on the doping with laser dye molecules in either glass or polymer hosts. By modifying the components of the host, a variety of host media with different optical and physical properties can be made [29]. These include various modifications of acrylic polymers which include modified PMMA (mPMMA) [9,10,12,13], sol–gel glasses [16,19], organically modified silicon alkoxide glass (ormosil glass) [20] and porous sol– gel glass impregnated with poly(methyl methacrylate) (PMMA) [18]. Polyceram [15] and epoxy [17] materials have also been used as a host of laser dye because of their high optical transparencies and easy synthesis. Laser performance of pyrromethene dye in solid matrices of PMMA with various monomers has been studied [12]. In this study 2-hydroxyethyl methacrylate, 2-hydroxyethyl acrylate, 1-vinyl-2-pyrrolidone, 2-phenoxyethyl acrylate and 2,2,2-trifluromethyl methacrylate have been used to show favourable lasing properties. The photostability of pyrromethene in nonpolar solvents (benzene) is more than that of the polar solvents (methanol) [5]. For a polar laser dye like pyrromethene, the photostability of the dye in a solvent with a high dipole moment (ethanol) is
lower than in a solvent with low dipole moment (dioxane). A large increase in photostability of pyrromethene dye in non-polar solvents (inert) compared to polar solvent (active) is due to the formation of a reduced number of free radicals. Free radical production is one of several mechanisms responsible for activating the triplet states in organic dyes. The aprocity is the availability of protons, which determines whether anions or cations can be dissolved more strongly through hydrogen bonding. Aprotic solvents such as dioxane and THF (tetrahydrofuran) are inert. On the other hand the protic solvents like water and alcohol influence the reaction kinetics by either increasing the strength of nucleophiles or by decreasing the strength of the electrophiles. It is seen that nonpolar and aprotic solvents show the highest lasing efficiencies [30]. The combination of two solvents to form a liquid mixture such that the mixture has a higher vapour pressure or a lower boiling point than that of the individual solvents is termed as azeotrope. This kind of mixing scheme can be used to increase the solubility of laser dyes. As an example rhodamine is not fully soluble in MMA but may be if first dissolved in ethanol and then mixed in MMA. Recent experiments revealed a large increase in photostability of pyrromethene laser dye when using binary azeotrope solvents instead of a single solvent [31,32]. The reason for increased photostability of the laser dye in azeotropic mixture is still not clear. This needs more attention on the measurements of triplet state loss in these solvents. Non-polar or electrically inert solvents are unlikely to form free radicals and have been used to increase the photostability of P567 [5] while antioxidants reduce the predominant photodegradation mechanism of pyrromethene 567 [21–28]. By selecting a suitable solvent and adding antioxidant the stability can be increased many-fold both in liquid and solid samples. A photostability of 20 GJ/mol for polar (ethanol) and 40 GJ/mol for nonpolar (cyclohexane) has been achieved and 1000 GJ/mol for a deoxygenated non-polar heptane– dioxane solvent. In solid samples 200 GJ/mol was achieved in mPMMA with antioxidants, compared to 50 GJ/mol in pure PMMA.
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2. Experimental The photostability of pyrromethene laser dye 567 has been investigated using both solution and solid polymer samples and by addition of various quenchers to block the degradation process. Methyl methacrylate monomer (Aldrich Chemical) was distilled to remove the initiator (hydroquinone-monomethyl-ether). The laser dye pyrromethene was dissolved into the monomer and the mixture was placed in a water-filled ultrasonic bath until the dye was completely dissolved. The singlet oxygen quencher or the triplet quencher were added according to the required concentration along with 3 mg/ml of 2,2-azobis (2-methylpropionitrile) polymerisation initiator. Finally the mixture was replaced in an ultrasonic bath to completely dissolve the solid constituents. Air tight glass tubes containing the dye mixtures were placed in a water bath at a temperature 40 °C for a week until a viscous liquid was formed. The tubes were then transferred to an oven where the temperature was increased step-wise at 5 °C per day up to 90 °C. Then the temperature was reduced over two days to room temperature. The glass tubes were broken to remove the polymerised samples, which were then cut into disks and optically polished. Laser action was studied to determine the photostability of the sample under repetitive pulse excitation. The laser cavity was a compact plane– plane configuration, as used in [11]. The pump source was a Q-switched Nd:YAG laser operating at the second harmonic of 532 nm. This delivered up to 60 mJ/pulse in 10 ns at 1–10 Hz repetition rate or in a single pulse. The input mirror was dichroic with 90% transmission at 532 nm and 95%
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reflectivity between 560 and 600 nm. The output mirror was a 70% broadband reflector that was not necessarily of optimum reflectivity. A short cavity length of 15 mm was used to reduce the cavity losses due to a highly divergent output. A 20-mm focal length lens focused the pump beam onto the sample, which was placed before the focus such that the diameter of the pump beam was 2 mm at the sample’s input face. The pump beam was aligned off-axis at a slight tilt angle of 16° to the resonator axis so that any transmitted pump light was not collinear with the output beam and was not incident on the volume absorbing power meter. The efficiency, defined as the ratio of laser pulse energy output to absorbed pump pulse energy, and the photostability, equivalent to the total energy absorbed by one mole of the laser dye in the sample when the output pulse energy has decreased to one-half of its initial value were measured. Photostability experiments on the solid samples were performed using a pump fluence of 0.16 J/cm2 and a repetition rate of 10 Hz. The laser performance of solution samples was evaluated using 1 ml of dye solution with a dye concentration of 5 10 5 M in a 10 mm optical path length silica cuvette. The pump laser pulse energy was 30 mJ at a 10 Hz repetition rate.
3. Photostability measurements 3.1. Variation of aprocity The photostability of P567 in solutions has been determined for selected protic and aprotic solvents as shown in Table 1. The protic solvent with a high
Table 1 Dielectric constants and dipole moments of protic (p) and aprotic (ap) solvents and their effect on laser performance Solvents
Dielectric constant
Water (p) Methanol (p) Ethanol (p) THF (ap) 1,4-Dioxane (ap)
78.5 32.6 24.3 7.3 2.21
Dipole moment (debye) 1.84 1.70 1.69 1.63 0
Conversion efficiency (%)
Photostability (GJ/mol) of P567
60 60 55 62
20 20 15 70
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dipole moment (ethanol) does not show as high a photostability as an aprotic solvent with low dipole moment of a solvent like 1,4-dioxane [32]. In the extensive study of Arbeloa et al. [30] it was found that the fluorescence quantum yield and laser efficiency was the highest in polar protic solvents. 3.2. Azeotropic mixing of two solvents The photostability of a few binary azeotrope solutions doped with a pyrromethene 567 laser dye of 1 ml in 5 10 5 M concentration have been studied in 10 mm cuvette. A pump energy of 30 mJ/pulse was used at 10 Hz. The efficiency and photostability are compared in Table 2 with a single solvent medium. A large increase in photostability of pyrromethene laser dye in binary azeotrope mixtures has been achieved when oxygen is reduced in the solution. The removal of oxygen was carried out by bubbling nitrogen through the solution and this is expected to have removed a large fraction of the oxygen. Fig. 1 shows the longevity of laser operation of three pyrromethene 567 doped azeotrope solutions. The conversion efficiency of a heptane–1,4-
dioxane azeotrope solution with number of exciting pulses is shown in Fig. 1 and is compared to the performance in ethanol and deoxygenated ethanol. In some azeotrope mixtures no appreciable improvement of photostability has been noticed. This may be due to an increase in the polarity of the mixture by the addition of a polar solvent, e.g. ethanol in toluene. Non-polar solvents exhibit higher photostability for pyrromethene and the increase is five times higher than the air-saturated solution sample and 10 times higher than the deoxygenated ethanol sample. 3.3. Action of quenching additives The photostability of pyrromethene in solution has been studied with triplet quenching additives, 2,2,6,6-tetramethyl piperidine (TMP), triphenylamine (TPA), Tinuvin 770, 1,4-daizobicyclo[2,2,2]octane (DABCO), and 1,3,5,7cyclooctatetraene (COT). Previous studies showed an increased effect on the photostability of pyrromethene laser dye when DABCO is added in either liquid (ethanol) or solid PMMA [11] and glass [16] samples. The results of various additives for pyrromethene in ethanolic solutions are shown in
Table 2 Laser performance of pyrromethene laser dye in various solutions Solution 1
Solution 2
Conversion efficiency (%)
Number of pulses to 50% reduction
Photostability (GJ/mol)
Benzene Ethanol Ethyl acetate Cyclohexane Heptane 1,4-Dioxane Toluene Ethanol (68%) Ethanol (68%) Benzene (68%) Ethanol (30.5%) Ethanol (31%) Ethanol (48%) Heptane (19%) Ethanol Heptane (19%)
0 0 0 0 0 0 0 Toluene (32%) Benzene (32%) Cyclohexane (69.5%) Cyclohexane (69.5%) Ethyl acetate (69%) Heptane (52%) 1,4-Dioxane (81%) 0 (Reduced oxygen) 1,4-Dioxane (81%) (reduced oxygen)
55 55 52 50 56 62 56 51 52 45 54 46 60 54 55 50
70,000 33,500 60,000 64,000 84,000 117,000 83,500 86,500 83,000 85,200 75,000 83,500 126,000 340,000 170,000 1720,000
42 20 36 41 50 70 50 52 50 51 45 50 75 200 [31] 100 1000
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two times increase when COT or TPA are used, have been measured. These air-saturated results are still far less than the results of deoxygenated ethanol without any quencher (which gave a five times increase in photostability). In air-saturated liquid samples the oxygen-induced intersystem crossing increases the triplet state quantum yield and effects the fluorescence of the laser dye. These triplet states trap the dye molecules in an inactive mode for a longer time (106 times) than the lifetime of the singlet states of the dye molecule, which actively takes part in the laser action. 3.4. Photostability of solid media Fig. 1. Conversion efficiency with number of laser excitation pulses for heptane–1,4-dioxane azeotropic solution compared to ethanol and deoxygenated ethanol solutions.
Table 3. The photostability of a 1 ml sample in a 10 mm cuvette and a concentration of 5 10 5 M pyrromethene 567 in ethanol with additives was determined with pump energy of 30 mJ/pulse at 10 Hz repetition rate. The most stabilising agent for pyrromethene in solution was found to be Tinuvin 770 and the least stabilising are TPA and COT. A three times increase in photostability when Tinuvin is used and
Table 3 Photostability of 5 10 Additive No additive TMP TMP TMP TPA TPA TPA Tinuvin 770 Tinuvin 770 Tinuvin 770 DABCO DABCO DABCO COT COT COT
5
Various additives as triplet quenchers have been found to increase the photostability of pyrromethene laser dye in solution. Fabrication of PMMA samples with these additives enables the photostability of pyrromethene 567 in the solid phase to be compared with the solution systems. Doped polymer samples of lengths ranging from 2 to 4 mm were pumped with a fluence of 0:16 J cm 2 by the second harmonic of the Nd:YAG laser at 532 nm. The concentration of pyrromethene in each sample was 3:4 10 4 M. The laser performance of these solid polymer samples is shown in Table 4.
M P567 in ethanol with various additives Additive conc. (1 10 5 M) 1.25 2.5 5 1.25 2.5 5 1.25 2.5 5 0.5 1 5 0.6 1.25 2.5
Conversion efficiency (%)
Photostability (GJ/mol)
50 46 45 45 46 45 43 46 42 42 48 47 36 49 49 48
20 38 50 45 24 40 30 30 60 35 38 45 38 24 40 12
Factor increase in photostability 1 1.8 2.5 2.25 1.2 2 1.5 1.5 3 1.75 1.9 2.25 1.9 1.2 2 0.6
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Table 4 Photostability of pyrromethene 567 in PMMA host with various additives Additive
Concentration
Conversion efficiency (%)
Photostability (GJ/mol)
Factor increase in photostability
No additive TPA TPA TPA TMP TMP TMP Tinuvin 770 Tinuvin 770 Tinuvin 770 DABCO DABCO DABCO COT COT COT COT
0 5 10 20 1 2 3 1 5 10 5 10 25 0.25 0.5 1 2.5
30 30 31 20 27 28 25 23 26 25 30 35 25 23 25 23 20
50 95 140 100 85 140 110 60 145 90 90 190 32 60 80 50 40
1.00 1.90 2.80 2.00 1.70 2.80 2.20 1.20 2.90 1.80 1.80 3.80 0.64 0.46 1.60 1.00 0.80
The samples were pumped at a fluence of 0.16 J/cm2 . The concentration of P567 was 3:34 10
Pyrromethene 567 in PMMA is most stable with the DABCO additive, giving a photostability of 190 GJ/mol compared to 50 GJ/mol for a pure PMMA sample with no additive, while COT is the least stablising agent with an increase to only 80 GJ/mol. Also the DABCO sample gave the highest slope efficiency (35%) while COT sample gave the lowest slope efficiency (25%) at the maximum photostability point.
4. Discussion and conclusions Significantly enhanced photostability of pyrromethene 567 in solution and solid-state media has been achieved by the use of triplet quenchers. A three times increase in photostability is seen, 60 GJ/mol in ethanol using Tinuvin 770 compared to 20 GJ/mol for ethanol alone, and nearly four times increased in solid polymer PMMA (190 GJ/mol) with the use of DABCO compared to 50 GJ/mol in pure PMMA, have been achieved. In a deoxygenated azeotropic solution of heptane and 1,4dioxane, a high photostability reaching over 1000 GJ/mol has been observed. This is five times higher than the air-saturated mixture.
4
M in each sample.
It has previously been established that the fluorescence quantum yield (/), lifetime (s) and lasing efficiency (%) of pyrromethene have no linear correlation with any of the solvent parameters with as dipole moment, solubility and polarity of the solvent [32]. However, some trends have been observed that higher value of all these parameters for very polar and protic solvents (ethanol, methanol) and lower values for apolar and aprotic solvents (benzene and toluene). Interestingly a high photostability of P567 has been observed for non-polar and aprotic solvent (1,4-dioxane, benzene). High photostability has also been observed in various solvents when mixed to form an azeotrope, 200 GJ/mol for heptane–1,4-dioxane azetrope mixture compared to 70 GJ/mol for 1,4-dioxane itself. A three times increase in photostability in azeotropic mixture has been seen. This increase may be due to a reduced number of free radicals or less triplet states generated than individual solvents. Oxygen is also an agent which severely reduces the photostability of pyrromethene dyes. Ten times increase in photostability has been observed in deoxygenated solution. The highest reported photostability of 1000 GJ/mol in a
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deoxygenated solution of a mixture of heptane– 1,4-dioxane has been achieved. It may be noted that at the pump fluence used in this work, 0.16 J/cm2 , excellent performance has been demonstrated without any modifications to the PMMA host material. The photostability of P567 in pure PMMA was found to be 50 GJ/mol at 10 Hz repetition rate. The preparation methods used have resulted in a polymer structure that is resistant to laser damage. Modification of the host matrix to form mPMMA appears to reduce the resistance to optical damage [33]. An observation from this work is the substantial increase of photostability of pyrromethene 567 when quenching additives are incorporated: in solid hosts (four times) compared with solutions (three times). In solution the single-most important photodegradation mechanism is expected to be is self-sensitised photo-oxidation. Self-sensitised photo-oxidation is a diffusion-controlled photochemical reaction, its reaction rate is greatly reduced in the solid-state. If any competing photodegradation reactions are not diffusion controlled, such as bond cleavage or electron transfer with the host, then the degradation mechanisms competing with self-sensitised photo-oxidation would quickly become dominant in the solid-state and the photostability would be only marginally improved. Pyrromethene 567 in the solid-state, however, is improved in photostability compared to the solution, even when no additives are used. This indicates that all significant degradation mechanisms for pyrromethene 567 are mainly diffusion controlled and are significantly reduced by going from solution to the solid-state. Improvement in photostability by a few hundred percent has been achieved by the use of singlet oxygen and triplet quenchers [21–28]. DABCO has been used previously to improve the longevity of laser operation in epoxy based SSDLs [17]. DABCO is known to be a singlet oxygen quencher [25], a reducing agent [26] and possibly a free radical scavenger [11,16] in various systems. For the other additives used in this study, triphenylamine (TPA), 2,2,6,6 tetramethylpiperdin (TMP), Tinuvin 770 and 1,3,5,7-cyclooctatetraene (COT), all either block the degradation process by quenching singlet oxygen or by intercepting free
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radicals that are likely to compete with singlet oxygen in the photodegradation process. The addition of DABCO to the PMMA host increased the operational longevity from 50 GJ/ mol (210,000 pulses) to 19 GJ/mol (800,000 pulses). This, combined with a high conversion efficiency of 35% and no appreciable change in efficiency observed in the concentration range used with these additives, makes this composition to have the best reported performance. The performance with DABCO (380%), Tinuvin 770 (290%), TMP (280%) and TPA (280%) additives in PMMA improved the photostability whilst COT did only slightly (60%). DABCO has been shown to quench singlet oxygen in PMMA [11], and is the favoured explanation for the stabilisation mechanism of DABCO. Singlet oxygen quenching depends on the diffusion constants of oxygen and DABCO. The concentration of all the additives (TMP, TPA, Tinuvin 770 and DABCO) required to improve the photostability of P567 is higher than the dye itself in both ethanol and PMMA, which is also consistent with the effect of reduced diffusion. However, the concentrations of each of the additives either in solution or solid is greater than the concentration of the laser dye, the laser efficiency reduces due to quenching of the excited singlet state of the dye.
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Optics Communications 203 (2002) 327–334 www.elsevier.com/locate/optcom
Photostability of lasers based on pyrromethene 567 in liquid and solid-state host media Mohammad Ahmad a, Terence A. King a,*, Do-Kyeong Ko b, Byung Heon Cha b, Jongmin Lee c a
Laser Photonics Research Group, Department of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK b Laboratory for Quantum Optics, Korea Atomic Energy Research Institute, P.O. Box 105, Yusong, Taejon 305-600, South Korea c Advanced Photonics Research Institute, Kwangju Institute of Science and Technology, 1 Oryong-dong, Buk-gu, Kwangju 500-712, South Korea Received 5 December 2001; accepted 10 January 2002
Abstract The photostability of the laser dye pyrromethene P567 in various liquid and solid samples has been measured by determining the longevity of laser operation when pumped by the second harmonic of a Q-switched Nd:YAG laser. A large improvement in photostability for pyrromethene was found in deoxygenated non-polar solvents, with a normalised photostability value of 1000 GJ/mol compared to deoxygenated polar solvents (100 GJ/mol). A high photostability has been observed in a heptane–1,4-dioxane azeotrope mixture and improvements observed in systems incorporating triplet quenching agents. Doped solid polymer samples showed an increased photostability up to 200 GJ/ mol for the pyrromethene laser dye when samples were modified with triplet quenchers. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 42.55.Mv; 42.70.Hj; 42.70.Jk; 82.35.Lr Keywords: Dye lasers; Laser materials; Polymers; Solvent; Pyrromethene; Triplet state; Singlet oxygen; Photostability
1. Introduction Solid-state dye lasers (SSDLs) are versatile tunable laser sources giving a compact and maintenance-free system and a low cost gain medium. Their applications range over spectroscopy, nonlinear optics, medicine and industry where tunable
*
Corresponding author. Fax: +44-161-275-5509. E-mail address: [email protected] (T.A. King).
high power pulsed laser beams are required. Useful properties for the dye in an SSDL are: 1. the dye should have a broad absorption band, 2. high conversion efficiency into laser emission, 3. high solubility in many solvents including water, 4. high photostability. Water solubility is advantageous since water is non-hazardous and has high thermo-optic properties. Rhodamine dyes have a high quantum yield but pyrromethene dyes have additional
0030-4018/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 0 - 4 0 1 8 ( 0 2 ) 0 1 1 7 9 - 3
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benefit of reduced triplet absorption coefficient, being one-fifth that of rhodamine [1–8]. The pyrromethene family of dyes provides the best SSDL performance in polymers [9–13] and glass [14–20], however, photodegradation still occurs and its rate is dependent on the host composition. The photostability of laser dyes both in solution and solid is relatively low [5–8] for long operation of the dye lasers. This is due to the formation of large numbers of triplet states with longer lifetimes than the singlet levels contributing to the laser action. The longer triplet lifetimes affect the laser performance in both reduced photostability and efficiency. Efforts have been made to decrease the triplet states concentration both in liquid and solid hosts by addition of various additives triplet quenchers. These additives are triphenylamine (TPA) [21], 2,2,6,6 tetramethylpiperdin (TMP) [22], Tinuvin 770 [23], 1,3,5,7-cyclooctatetraene (COT) [24] and 1,4-diazobicyclo [2,2,2] octane (DABCO) [25–28]. SSDLs are usually based on the doping with laser dye molecules in either glass or polymer hosts. By modifying the components of the host, a variety of host media with different optical and physical properties can be made [29]. These include various modifications of acrylic polymers which include modified PMMA (mPMMA) [9,10,12,13], sol–gel glasses [16,19], organically modified silicon alkoxide glass (ormosil glass) [20] and porous sol– gel glass impregnated with poly(methyl methacrylate) (PMMA) [18]. Polyceram [15] and epoxy [17] materials have also been used as a host of laser dye because of their high optical transparencies and easy synthesis. Laser performance of pyrromethene dye in solid matrices of PMMA with various monomers has been studied [12]. In this study 2-hydroxyethyl methacrylate, 2-hydroxyethyl acrylate, 1-vinyl-2-pyrrolidone, 2-phenoxyethyl acrylate and 2,2,2-trifluromethyl methacrylate have been used to show favourable lasing properties. The photostability of pyrromethene in nonpolar solvents (benzene) is more than that of the polar solvents (methanol) [5]. For a polar laser dye like pyrromethene, the photostability of the dye in a solvent with a high dipole moment (ethanol) is
lower than in a solvent with low dipole moment (dioxane). A large increase in photostability of pyrromethene dye in non-polar solvents (inert) compared to polar solvent (active) is due to the formation of a reduced number of free radicals. Free radical production is one of several mechanisms responsible for activating the triplet states in organic dyes. The aprocity is the availability of protons, which determines whether anions or cations can be dissolved more strongly through hydrogen bonding. Aprotic solvents such as dioxane and THF (tetrahydrofuran) are inert. On the other hand the protic solvents like water and alcohol influence the reaction kinetics by either increasing the strength of nucleophiles or by decreasing the strength of the electrophiles. It is seen that nonpolar and aprotic solvents show the highest lasing efficiencies [30]. The combination of two solvents to form a liquid mixture such that the mixture has a higher vapour pressure or a lower boiling point than that of the individual solvents is termed as azeotrope. This kind of mixing scheme can be used to increase the solubility of laser dyes. As an example rhodamine is not fully soluble in MMA but may be if first dissolved in ethanol and then mixed in MMA. Recent experiments revealed a large increase in photostability of pyrromethene laser dye when using binary azeotrope solvents instead of a single solvent [31,32]. The reason for increased photostability of the laser dye in azeotropic mixture is still not clear. This needs more attention on the measurements of triplet state loss in these solvents. Non-polar or electrically inert solvents are unlikely to form free radicals and have been used to increase the photostability of P567 [5] while antioxidants reduce the predominant photodegradation mechanism of pyrromethene 567 [21–28]. By selecting a suitable solvent and adding antioxidant the stability can be increased many-fold both in liquid and solid samples. A photostability of 20 GJ/mol for polar (ethanol) and 40 GJ/mol for nonpolar (cyclohexane) has been achieved and 1000 GJ/mol for a deoxygenated non-polar heptane– dioxane solvent. In solid samples 200 GJ/mol was achieved in mPMMA with antioxidants, compared to 50 GJ/mol in pure PMMA.
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2. Experimental The photostability of pyrromethene laser dye 567 has been investigated using both solution and solid polymer samples and by addition of various quenchers to block the degradation process. Methyl methacrylate monomer (Aldrich Chemical) was distilled to remove the initiator (hydroquinone-monomethyl-ether). The laser dye pyrromethene was dissolved into the monomer and the mixture was placed in a water-filled ultrasonic bath until the dye was completely dissolved. The singlet oxygen quencher or the triplet quencher were added according to the required concentration along with 3 mg/ml of 2,2-azobis (2-methylpropionitrile) polymerisation initiator. Finally the mixture was replaced in an ultrasonic bath to completely dissolve the solid constituents. Air tight glass tubes containing the dye mixtures were placed in a water bath at a temperature 40 °C for a week until a viscous liquid was formed. The tubes were then transferred to an oven where the temperature was increased step-wise at 5 °C per day up to 90 °C. Then the temperature was reduced over two days to room temperature. The glass tubes were broken to remove the polymerised samples, which were then cut into disks and optically polished. Laser action was studied to determine the photostability of the sample under repetitive pulse excitation. The laser cavity was a compact plane– plane configuration, as used in [11]. The pump source was a Q-switched Nd:YAG laser operating at the second harmonic of 532 nm. This delivered up to 60 mJ/pulse in 10 ns at 1–10 Hz repetition rate or in a single pulse. The input mirror was dichroic with 90% transmission at 532 nm and 95%
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reflectivity between 560 and 600 nm. The output mirror was a 70% broadband reflector that was not necessarily of optimum reflectivity. A short cavity length of 15 mm was used to reduce the cavity losses due to a highly divergent output. A 20-mm focal length lens focused the pump beam onto the sample, which was placed before the focus such that the diameter of the pump beam was 2 mm at the sample’s input face. The pump beam was aligned off-axis at a slight tilt angle of 16° to the resonator axis so that any transmitted pump light was not collinear with the output beam and was not incident on the volume absorbing power meter. The efficiency, defined as the ratio of laser pulse energy output to absorbed pump pulse energy, and the photostability, equivalent to the total energy absorbed by one mole of the laser dye in the sample when the output pulse energy has decreased to one-half of its initial value were measured. Photostability experiments on the solid samples were performed using a pump fluence of 0.16 J/cm2 and a repetition rate of 10 Hz. The laser performance of solution samples was evaluated using 1 ml of dye solution with a dye concentration of 5 10 5 M in a 10 mm optical path length silica cuvette. The pump laser pulse energy was 30 mJ at a 10 Hz repetition rate.
3. Photostability measurements 3.1. Variation of aprocity The photostability of P567 in solutions has been determined for selected protic and aprotic solvents as shown in Table 1. The protic solvent with a high
Table 1 Dielectric constants and dipole moments of protic (p) and aprotic (ap) solvents and their effect on laser performance Solvents
Dielectric constant
Water (p) Methanol (p) Ethanol (p) THF (ap) 1,4-Dioxane (ap)
78.5 32.6 24.3 7.3 2.21
Dipole moment (debye) 1.84 1.70 1.69 1.63 0
Conversion efficiency (%)
Photostability (GJ/mol) of P567
60 60 55 62
20 20 15 70
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dipole moment (ethanol) does not show as high a photostability as an aprotic solvent with low dipole moment of a solvent like 1,4-dioxane [32]. In the extensive study of Arbeloa et al. [30] it was found that the fluorescence quantum yield and laser efficiency was the highest in polar protic solvents. 3.2. Azeotropic mixing of two solvents The photostability of a few binary azeotrope solutions doped with a pyrromethene 567 laser dye of 1 ml in 5 10 5 M concentration have been studied in 10 mm cuvette. A pump energy of 30 mJ/pulse was used at 10 Hz. The efficiency and photostability are compared in Table 2 with a single solvent medium. A large increase in photostability of pyrromethene laser dye in binary azeotrope mixtures has been achieved when oxygen is reduced in the solution. The removal of oxygen was carried out by bubbling nitrogen through the solution and this is expected to have removed a large fraction of the oxygen. Fig. 1 shows the longevity of laser operation of three pyrromethene 567 doped azeotrope solutions. The conversion efficiency of a heptane–1,4-
dioxane azeotrope solution with number of exciting pulses is shown in Fig. 1 and is compared to the performance in ethanol and deoxygenated ethanol. In some azeotrope mixtures no appreciable improvement of photostability has been noticed. This may be due to an increase in the polarity of the mixture by the addition of a polar solvent, e.g. ethanol in toluene. Non-polar solvents exhibit higher photostability for pyrromethene and the increase is five times higher than the air-saturated solution sample and 10 times higher than the deoxygenated ethanol sample. 3.3. Action of quenching additives The photostability of pyrromethene in solution has been studied with triplet quenching additives, 2,2,6,6-tetramethyl piperidine (TMP), triphenylamine (TPA), Tinuvin 770, 1,4-daizobicyclo[2,2,2]octane (DABCO), and 1,3,5,7cyclooctatetraene (COT). Previous studies showed an increased effect on the photostability of pyrromethene laser dye when DABCO is added in either liquid (ethanol) or solid PMMA [11] and glass [16] samples. The results of various additives for pyrromethene in ethanolic solutions are shown in
Table 2 Laser performance of pyrromethene laser dye in various solutions Solution 1
Solution 2
Conversion efficiency (%)
Number of pulses to 50% reduction
Photostability (GJ/mol)
Benzene Ethanol Ethyl acetate Cyclohexane Heptane 1,4-Dioxane Toluene Ethanol (68%) Ethanol (68%) Benzene (68%) Ethanol (30.5%) Ethanol (31%) Ethanol (48%) Heptane (19%) Ethanol Heptane (19%)
0 0 0 0 0 0 0 Toluene (32%) Benzene (32%) Cyclohexane (69.5%) Cyclohexane (69.5%) Ethyl acetate (69%) Heptane (52%) 1,4-Dioxane (81%) 0 (Reduced oxygen) 1,4-Dioxane (81%) (reduced oxygen)
55 55 52 50 56 62 56 51 52 45 54 46 60 54 55 50
70,000 33,500 60,000 64,000 84,000 117,000 83,500 86,500 83,000 85,200 75,000 83,500 126,000 340,000 170,000 1720,000
42 20 36 41 50 70 50 52 50 51 45 50 75 200 [31] 100 1000
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two times increase when COT or TPA are used, have been measured. These air-saturated results are still far less than the results of deoxygenated ethanol without any quencher (which gave a five times increase in photostability). In air-saturated liquid samples the oxygen-induced intersystem crossing increases the triplet state quantum yield and effects the fluorescence of the laser dye. These triplet states trap the dye molecules in an inactive mode for a longer time (106 times) than the lifetime of the singlet states of the dye molecule, which actively takes part in the laser action. 3.4. Photostability of solid media Fig. 1. Conversion efficiency with number of laser excitation pulses for heptane–1,4-dioxane azeotropic solution compared to ethanol and deoxygenated ethanol solutions.
Table 3. The photostability of a 1 ml sample in a 10 mm cuvette and a concentration of 5 10 5 M pyrromethene 567 in ethanol with additives was determined with pump energy of 30 mJ/pulse at 10 Hz repetition rate. The most stabilising agent for pyrromethene in solution was found to be Tinuvin 770 and the least stabilising are TPA and COT. A three times increase in photostability when Tinuvin is used and
Table 3 Photostability of 5 10 Additive No additive TMP TMP TMP TPA TPA TPA Tinuvin 770 Tinuvin 770 Tinuvin 770 DABCO DABCO DABCO COT COT COT
5
Various additives as triplet quenchers have been found to increase the photostability of pyrromethene laser dye in solution. Fabrication of PMMA samples with these additives enables the photostability of pyrromethene 567 in the solid phase to be compared with the solution systems. Doped polymer samples of lengths ranging from 2 to 4 mm were pumped with a fluence of 0:16 J cm 2 by the second harmonic of the Nd:YAG laser at 532 nm. The concentration of pyrromethene in each sample was 3:4 10 4 M. The laser performance of these solid polymer samples is shown in Table 4.
M P567 in ethanol with various additives Additive conc. (1 10 5 M) 1.25 2.5 5 1.25 2.5 5 1.25 2.5 5 0.5 1 5 0.6 1.25 2.5
Conversion efficiency (%)
Photostability (GJ/mol)
50 46 45 45 46 45 43 46 42 42 48 47 36 49 49 48
20 38 50 45 24 40 30 30 60 35 38 45 38 24 40 12
Factor increase in photostability 1 1.8 2.5 2.25 1.2 2 1.5 1.5 3 1.75 1.9 2.25 1.9 1.2 2 0.6
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Table 4 Photostability of pyrromethene 567 in PMMA host with various additives Additive
Concentration
Conversion efficiency (%)
Photostability (GJ/mol)
Factor increase in photostability
No additive TPA TPA TPA TMP TMP TMP Tinuvin 770 Tinuvin 770 Tinuvin 770 DABCO DABCO DABCO COT COT COT COT
0 5 10 20 1 2 3 1 5 10 5 10 25 0.25 0.5 1 2.5
30 30 31 20 27 28 25 23 26 25 30 35 25 23 25 23 20
50 95 140 100 85 140 110 60 145 90 90 190 32 60 80 50 40
1.00 1.90 2.80 2.00 1.70 2.80 2.20 1.20 2.90 1.80 1.80 3.80 0.64 0.46 1.60 1.00 0.80
The samples were pumped at a fluence of 0.16 J/cm2 . The concentration of P567 was 3:34 10
Pyrromethene 567 in PMMA is most stable with the DABCO additive, giving a photostability of 190 GJ/mol compared to 50 GJ/mol for a pure PMMA sample with no additive, while COT is the least stablising agent with an increase to only 80 GJ/mol. Also the DABCO sample gave the highest slope efficiency (35%) while COT sample gave the lowest slope efficiency (25%) at the maximum photostability point.
4. Discussion and conclusions Significantly enhanced photostability of pyrromethene 567 in solution and solid-state media has been achieved by the use of triplet quenchers. A three times increase in photostability is seen, 60 GJ/mol in ethanol using Tinuvin 770 compared to 20 GJ/mol for ethanol alone, and nearly four times increased in solid polymer PMMA (190 GJ/mol) with the use of DABCO compared to 50 GJ/mol in pure PMMA, have been achieved. In a deoxygenated azeotropic solution of heptane and 1,4dioxane, a high photostability reaching over 1000 GJ/mol has been observed. This is five times higher than the air-saturated mixture.
4
M in each sample.
It has previously been established that the fluorescence quantum yield (/), lifetime (s) and lasing efficiency (%) of pyrromethene have no linear correlation with any of the solvent parameters with as dipole moment, solubility and polarity of the solvent [32]. However, some trends have been observed that higher value of all these parameters for very polar and protic solvents (ethanol, methanol) and lower values for apolar and aprotic solvents (benzene and toluene). Interestingly a high photostability of P567 has been observed for non-polar and aprotic solvent (1,4-dioxane, benzene). High photostability has also been observed in various solvents when mixed to form an azeotrope, 200 GJ/mol for heptane–1,4-dioxane azetrope mixture compared to 70 GJ/mol for 1,4-dioxane itself. A three times increase in photostability in azeotropic mixture has been seen. This increase may be due to a reduced number of free radicals or less triplet states generated than individual solvents. Oxygen is also an agent which severely reduces the photostability of pyrromethene dyes. Ten times increase in photostability has been observed in deoxygenated solution. The highest reported photostability of 1000 GJ/mol in a
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deoxygenated solution of a mixture of heptane– 1,4-dioxane has been achieved. It may be noted that at the pump fluence used in this work, 0.16 J/cm2 , excellent performance has been demonstrated without any modifications to the PMMA host material. The photostability of P567 in pure PMMA was found to be 50 GJ/mol at 10 Hz repetition rate. The preparation methods used have resulted in a polymer structure that is resistant to laser damage. Modification of the host matrix to form mPMMA appears to reduce the resistance to optical damage [33]. An observation from this work is the substantial increase of photostability of pyrromethene 567 when quenching additives are incorporated: in solid hosts (four times) compared with solutions (three times). In solution the single-most important photodegradation mechanism is expected to be is self-sensitised photo-oxidation. Self-sensitised photo-oxidation is a diffusion-controlled photochemical reaction, its reaction rate is greatly reduced in the solid-state. If any competing photodegradation reactions are not diffusion controlled, such as bond cleavage or electron transfer with the host, then the degradation mechanisms competing with self-sensitised photo-oxidation would quickly become dominant in the solid-state and the photostability would be only marginally improved. Pyrromethene 567 in the solid-state, however, is improved in photostability compared to the solution, even when no additives are used. This indicates that all significant degradation mechanisms for pyrromethene 567 are mainly diffusion controlled and are significantly reduced by going from solution to the solid-state. Improvement in photostability by a few hundred percent has been achieved by the use of singlet oxygen and triplet quenchers [21–28]. DABCO has been used previously to improve the longevity of laser operation in epoxy based SSDLs [17]. DABCO is known to be a singlet oxygen quencher [25], a reducing agent [26] and possibly a free radical scavenger [11,16] in various systems. For the other additives used in this study, triphenylamine (TPA), 2,2,6,6 tetramethylpiperdin (TMP), Tinuvin 770 and 1,3,5,7-cyclooctatetraene (COT), all either block the degradation process by quenching singlet oxygen or by intercepting free
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radicals that are likely to compete with singlet oxygen in the photodegradation process. The addition of DABCO to the PMMA host increased the operational longevity from 50 GJ/ mol (210,000 pulses) to 19 GJ/mol (800,000 pulses). This, combined with a high conversion efficiency of 35% and no appreciable change in efficiency observed in the concentration range used with these additives, makes this composition to have the best reported performance. The performance with DABCO (380%), Tinuvin 770 (290%), TMP (280%) and TPA (280%) additives in PMMA improved the photostability whilst COT did only slightly (60%). DABCO has been shown to quench singlet oxygen in PMMA [11], and is the favoured explanation for the stabilisation mechanism of DABCO. Singlet oxygen quenching depends on the diffusion constants of oxygen and DABCO. The concentration of all the additives (TMP, TPA, Tinuvin 770 and DABCO) required to improve the photostability of P567 is higher than the dye itself in both ethanol and PMMA, which is also consistent with the effect of reduced diffusion. However, the concentrations of each of the additives either in solution or solid is greater than the concentration of the laser dye, the laser efficiency reduces due to quenching of the excited singlet state of the dye.
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