Continuous-wave dye lasers in the DCM region Peter Hammond and Diane Cooke Laser dye DCM in an ethylene glycol solution is a favored medium for converting the argon-ion 488- and 514-nm pump lines to tunable radiation in the 600-730-nm region. However, the dye precipitates from solution, is a powerful mutagen, and the glycol solvent is hygroscopic. Replacement dyes in 3-phenyl-1- propanol or 2-phenoxyethanol, particularly the latter, are proposed. Key words: Jet-stream dye lasers; DCM dyes, solubility, structure, performance, mutagenicity; glycols, hygroscopicity. Introduction Laser dye DCM {4-dicyanomethylene-2-methyl-6-p- dimethylaminostyryl-4H-pyran, Chemical Abstract's name is now [2-[2-[4-(dimethylamino)phenyl]ethenyl]- 6-methyl-4H-pyran-4-ylidene]-propanedinitrileI has a quite large, nonionic, merocyanine structure. This determines its low solubility and indeed finding an appropriate solvent has been a continuing problem since inception. ' 2 Attempts to prepare solutions for cw jet-stream dye lasers based on the customary ethylene glycol have used propylene carbonate/ ethylene glycol (40:60, vol.:vol.); benzyl alcohol/ dimethyl sulfoxide/glycerol (25:37.5:37.5)3; benzyl alcohol/ethylene glycol/lauryldimethylamine N-oxide/ water (10:23:20:47) with and without dipropylacet- amide 4 ; benzyl alcohol/ethylene glycol (33:67),4 (2: 5),5 (40:60)2,3; benzyl alcohol/ethylene glycol/glycerol (4:5:2),6 and benzyl alcohol/glycerol (80:20).6 A stan- dard solution, benzyl alcohol/ethylene glycol (40:60, vol.:vol.), although giving initial acceptable perfor- mance, quickly degrades over several days whether the argon-ion pump laser is running or not. The dye precipitates on the filter, and the solution has to be changed, typically within two weeks. DCM is a powerful mutagen 7 ; hence, it is likely to be a carcinogen. Ethylene glycol is a solvent of choice for jet streams 8 because of its viscosity/temperature range; low dn/ dt; quite high thermal capacity and density; low volatility, flammability, and toxicity. Some other glycols that are shown in Table 1 were better sol- The authors are with Lawrence Livermore National Laboratory, L-463, P.O. Box 808, Livermore, California 94550. Received 14 February 1992. 0003-6935/92/337095-05$05.00/0. c 1992 Optical Society of America. vents, thus benzyl alcohol/2-hydroxyethyl-ether (40: 60, vol.:vol.) 9 gave stable operation over several weeks with both DCM and the dyes described in this paper. Although ethylene glycol is described as hygro- scopic, this fact does not appear to have been recog- nized in the laser literature or by users of cw dye lasers. Simple tests were conducted on ethylene glycol and other solvents. Samples of 100 mL were stored in 250-mL open beakers, swirled from time to time (every few days), and their weights measured. The results are shown in Figs. 1 and 2. All the diol solvents showed 5-15-g weight gains within ten days, volume expansion, and schlieren effects that are ascribed to water uptake. The 1-3-g variations seen occurred simultaneously and must have been caused by moisture exchange with a varying atmosphere. A free, exposed jet promotes even more rapid exchange. Doubtless, within three to four days of operation, the ethylene glycol solvent of these lasers contains as much as 15% water, and at this stage varies according to humidity. The measurements have not been taken further, but viscosity and refractive index must change. This will not seriously affect the laser; thus, viscosities below 10 cP are practicable, but for critical operation continuous tweaking would seem necessary. The other glycols are usable solvents in terms of viscosity and low dn/dt but are still hygroscopic. In this paper we examine the solvents 3-phenyl-1- propanol and 2-phenoxyethanol. 0- CH 2 CH 2 CH 2 OH (ID- 0 CH 2 CH 2 0H 3-Phenyl-1-propanol 2-Phenoxyethanol 20 November 1992 / Vol. 31, No. 33 / APPLIED OPTICS 7095
Transcript
Continuous-wave dye lasers in the DCM region
Peter Hammond and Diane Cooke
Laser dye DCM in an ethylene glycol solution is a favored medium for converting the argon-ion 488- and514-nm pump lines to tunable radiation in the 600-730-nm region. However, the dye precipitates fromsolution, is a powerful mutagen, and the glycol solvent is hygroscopic. Replacement dyes in 3-phenyl-1-propanol or 2-phenoxyethanol, particularly the latter, are proposed.
Laser dye DCM {4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran, Chemical Abstract'sname is now [2-[2-[4-(dimethylamino)phenyl]ethenyl]-6-methyl-4H-pyran-4-ylidene]-propanedinitrileI has aquite large, nonionic, merocyanine structure. Thisdetermines its low solubility and indeed finding anappropriate solvent has been a continuing problemsince inception. ' 2 Attempts to prepare solutions forcw jet-stream dye lasers based on the customaryethylene glycol have used propylene carbonate/ethylene glycol (40:60, vol.:vol.); benzyl alcohol/dimethyl sulfoxide/glycerol (25:37.5:37.5)3; benzylalcohol/ethylene glycol/lauryldimethylamine N-oxide/water (10:23:20:47) with and without dipropylacet-amide4; benzyl alcohol/ethylene glycol (33:67),4 (2:5),5 (40:60)2,3; benzyl alcohol/ethylene glycol/glycerol(4:5:2),6 and benzyl alcohol/glycerol (80:20).6 A stan-dard solution, benzyl alcohol/ethylene glycol (40:60,vol.:vol.), although giving initial acceptable perfor-mance, quickly degrades over several days whetherthe argon-ion pump laser is running or not. The dyeprecipitates on the filter, and the solution has to bechanged, typically within two weeks.
DCM is a powerful mutagen7; hence, it is likely tobe a carcinogen.
Ethylene glycol is a solvent of choice for jet streams8
because of its viscosity/temperature range; low dn/dt; quite high thermal capacity and density; lowvolatility, flammability, and toxicity. Some otherglycols that are shown in Table 1 were better sol-
The authors are with Lawrence Livermore National Laboratory,L-463, P.O. Box 808, Livermore, California 94550.
Received 14 February 1992.0003-6935/92/337095-05$05.00/0.c 1992 Optical Society of America.
vents, thus benzyl alcohol/2-hydroxyethyl-ether (40:60, vol.:vol.)9 gave stable operation over several weekswith both DCM and the dyes described in this paper.
Although ethylene glycol is described as hygro-scopic, this fact does not appear to have been recog-nized in the laser literature or by users of cw dyelasers. Simple tests were conducted on ethyleneglycol and other solvents. Samples of 100 mL werestored in 250-mL open beakers, swirled from time totime (every few days), and their weights measured.The results are shown in Figs. 1 and 2. All the diolsolvents showed 5-15-g weight gains within ten days,volume expansion, and schlieren effects that areascribed to water uptake. The 1-3-g variations seenoccurred simultaneously and must have been causedby moisture exchange with a varying atmosphere.A free, exposed jet promotes even more rapid exchange.Doubtless, within three to four days of operation, theethylene glycol solvent of these lasers contains asmuch as 15% water, and at this stage varies accordingto humidity. The measurements have not been takenfurther, but viscosity and refractive index mustchange. This will not seriously affect the laser; thus,viscosities below 10 cP are practicable, but for criticaloperation continuous tweaking would seem necessary.The other glycols are usable solvents in terms ofviscosity and low dn/dt but are still hygroscopic.
In this paper we examine the solvents 3-phenyl-1-propanol and 2-phenoxyethanol.
aAll quoted at 250C unless otherwise stated.b200C.cEstimated.d5°Co.
DCM solutions show a brighter fluorescence and thesolution in phenoxyethanol gave improved cw out-puts compared with both the benzyl alcohol-ethyleneglycol system10 and the benzyl alcohol-dimethylsulfoxide-glycerol system."1 For solvents in which itis readily soluble (dimethyl sulfoxide, N-methylpyrro-lidone) DCM shows efficient fluorescence and lasingaction. This contrasts with the low solubility,marked quenching, and inefficiency in aliphatic, hy-droxylic solvents (ethanol, methanol, ethylene glycol).Cis-trans isomerization of the dye is undoubtedlyimportant,' 2 but at the high concentrations in the cwoscillator (3.3 x 10-3 M) there seems to be an addi-tional effect, namely, aggregation. According to thisview, some aggregation and quenching of fluores-cence occurs for DCM in media rich in ethylene glycolcompared with phenoxyethanol.
Another approach is to engineer dye propertiesmolecularly. In this manner improved alcohol solu-
15
02C
la 10
.0.5
U.0s 10 20Time (days)
30 40
Fig. 1. Water uptake by 100-mL samples of ethylene glycol,propane-1,3-diol, and phenoxyethanol.
20
15
E
5 10
5
10 20 30Time (days)
40
Fig. 2. Water uptake by 100-mL samples of propane-1,2-diol and2-hydroxyethyl ether.
bility for DCM has been achieved.' 3 Ring constraintof the terminal nitrogen atoms of a chromophore is acustomary method of maximizing fluorescence quan-tum yield and optimizing laser performance. Herewe also examine the ring-constrained dyes 4 D2 andD3.
Experimental
3-Phenyl-l-propanol was used as obtained from Ald-rich Chemical Company. We favored the best grade of2-phenoxyethanol (98%) from Penta Manufacturing,West Caldwell, N.J. The material sold as Dowanol EPh (Dow Chemical Company) contained a 10% dieth-ylene glycol phenyl ether impurity and appearedtranslucent. It is an effective solvent for plastics andglues although Dow states that it is compatible withViton, Teflon, polypropylene, and polyethylene.
Refractive index/temperature gradients (phenylpro-panol and phenoxyethanol) were measured on aBausch & Lomb Abb6 refractometer under sodiumlamp illumination.
Experiments were conducted on a Coherent Model699-21 stabilized ring dye laser. The pump laserwas a Spectra Physics Model 171 argon-ion laser thatoperated all lines. All the tuning curves were mea-sured with the pump at 6 W and the laser insingle-frequency operation for power and wavelengthstability, namely, all the tuning elements (birefrin-gent filter, intracavity 6talon assembly, and scanningBrewster plate) were installed and the laser peakedfor minimum linewidth. Open cavity data were gath-ered with no tuning elements in the dye laser cavity,whereas for broadband data only the birefringentfilter was used. DCM optics were used throughout,optimized for a tuning range of 610-725 nm.
Dye concentrations were optimized at 3.3 x 10-3 M,which provided 93% absorption of the laser beam.They were dissolved by sonication or gentle warmingand for phenylpropanol and phenoxyethanol gavestable solutions.
Tuning curves for DCM, D2, and D3 in phenoxyetha-nol are shown in Fig. 3. In all cases the performancewas more stable than for customary DCM in ethyleneglycol mixtures; also less day-to-day adjustment wasneeded. Peak output for DCM itself is shifted from645 nm in benzyl alcohol/ethylene glycol (40:60,vol. :vol.) to 633 nm in phenoxyethanol. D2 inphenoxyethanol (655-nm peak output) now coversmost of the former DCM tuning range (see Fig. 4).Figures 5 and 6 compare power/performance curvesfor the DCM/benzyl alcohol/ethylene glycol systemand for the D2/phenoxyethanol solution at their peakwavelengths. Single-frequency operation maximizedat 9-10-W pump power in both cases. The D2system has lased more than 104 h (6-W pump; i.e.,624 Wh/L) and has been used intermittently for eightmonths without solution darkening or significantfalloff of performance. The D3 system in phen-oxyethanol peaked at 663 nm with a 640-710-nmtuning range.
DCM also lased in 3-phenyl-1-propanol. The peakemission wavelength was even further shifted to theblue (620 nm), and the tuning range was 600-670nm. Peak power was slightly inferior to that inphenoxyethanol (550 versus 620 mW), and the sys-tem was less stable, falling to 200 mW after 200Wh/L. No other measurements were made withthis solvent.
Dye Mutagenicity and Solvent Toxicity
DCM is a strong mutagen toward a number ofbacterial strains.7 Its potency is comparable to ben-
0.6
0.4
0.2
0
, 0.6o
xw 0.4
.
o 0.2
!;O2
0.6
0.4 1
0.2
0400 500 600
Wavelength (nm)700
Tuning Curve02 vs. OCM
0.4 -0
N
.
QU,
0.3 -
02 -
0 -0
0010 6o 60 670 6o
WaveIerVh (nm)0 DCM In E.0.- N.w + D2 In Ph.O.E.- New
Fig.4. Tuning curves for freshly prepared DCM inbenzyl alcohol/ethylene glycol and D2 in phenoxyethanol.
zo(a)pyrene, a powerful carcinogen found in tobaccosmoke, coal combustion, and coal tar. The measure-ments in Table 2 were made on dimethyl sulfoxidesolutions as described previously'5 on the most respon-sive strain, with and without S9 metabolic activation.A sample was judged mutagenic if it produced greaterthan twice the spontaneous background colonies atmore than one dose; certainly activity at less than athousandth of the levels shown was detectable.Quite minor changes in structure from DCM to D2 or
(CH 3)2 N H
0.8
= CH O CH3
CCN CN
DCM
0.6
CH3
N
~~CH =
0.4
0.2
0
0.8
0.6 9S
0.4 3
0 )
O.
0
0.8
0.6
0.4
0.2
0
Fig. 3. Absorption and tuning curves for (a) DCM, (b) D2, and (c)D3 in phenoxyethanol.
CH 0 H 3
CN CN
CH = CH 0CH3
CN N CN
D2
D3
D3 destroyed the potency. Also (not shown) DCMwas considerably more powerful than its synthesisintermediates-4-dicyanomethylene-2,6-dimethyl-4H-pyran (inactive) and p-dimethylaminobenzalde-hyde (a weak mutagen).
Phenoxyethanol has been used for some time as afixative for perfumes, a bactericide, and as an insect
repellant; hencetoxic (LD 50 orlirritant but sevtions, if they prcvented enclosureits E. Coli DNAhigh concentrati
Discussion
D2 and D3 are nments for DCM.readily in phenlasing solutions.terminal aminocence, and lasinchanges the oscilimproves laser p
+^.--4 4.x1 son
DCM In Ethylene GlycolPower In V. Power Out
Table 2. Mutagenic Potency of DCM Dyes
Chemical Mutagenicitya Revertants/ Lgb
DCM ++ (+S9) 400D2 - -D3 -Benzo(a)pyrene + + 480
aSalmonella strain TA 1538.bRevertants/lpg were taken from the
response curves (+S9).linear portion of dose
>9 attached to the dimethine unit could be the most_____.__,_.__._,__,_,__,_,__, important channel for nonradiative decay.3 a 7 ; n 1 ] S Apart from their poor solubility for DCM dyes, all
Puepy Power In M the glycols of Table 1 are extremely hygroscopic, andSingle Freq.- New jet streams that operate in air change their composi-
ormance curves for DCM in benzyl alcohol/ tion by absorbing moisture. Phenoxyethanol has asuitable viscosity but has a steeper refractive index/temperature gradient and lower thermal capacitythan pure ethylene glycol, although it should not beits toxicology is known. It is mildly significantly different from the original (40:60) ben-
-rat 1.26 g/kg); it is a moderate skin zyl alcohol/ethylene glycol mixture. The glycols are'ere to the eye. Jet-stream opera- small molecules that owe their viscosity, low dn/dt,)duce solvent mist, should be run in nonvolatility to the cohesive properties of internales. There is one report that it inhib- hydrogen bonding. Hydrogen bonding, in turn, pro-and RNA biosynthesis at the very motes hygroscopic behavior. Phenoxyethanol, on
on of 2000 ppm.16 the other hand, owes its viscosity to the size andshape of the molecules. Hydrogen bonding is muchless, dn/dt is larger, and water uptake is minimal.
onmutagenic and are usable replace- The large Stokes loss for these dyes may add to theThey are less soluble but dissolve thermal distortion of the phenoxyethanol jet (ther-
oxyethanol to give stable, efficient mal lensing). Even so, at 6-W pump power we weregr Although ring constraint of the able to achieve single-mode outputs in a CR 699-1group shifts the absorption, fluores- ring dye laser by using a 2.0-mm cavity aperture (notg wavelengths to the red, it hardly the smallest available) in front of the fold mirror.iatformstregtheshape of Thibas or At higher pump fluxes (above 10 W) there were'erformance (see Fig. 3). This con- marked amplitude and frequency fluctuations, and no
LiabaLb Wilil bile UCLIdVIUI U1 nxnuuwmIIIUt, ryruniii aaiuCoumarin dyes for which improved quantum yieldsand laser outputs are found. The DCM dyes are notcompletely rigid structures and rotation of a group
D2 In 2-PhenoxyethanolPower In vs. Power Out
1.71.8
1.43 .312
0.00.8-
06-
1 3ol70d Z I
Off0.4-0.3-0.2
01
Punyp Power In MWIa Open Cavity-New + Single Freq.- 500who Boadband- 0Owh w Open Cavity- 600wh
Fig. 6. Power/performance curves for D2 in phenoxyethanol.
amount of tweaking improved performance or pre-vented multimode operation. For single-frequencyperformance less day-to-day adjustment was needed,probably a result of its nonhygroscopic nature. Also,solutions that faded in benzyl alcohol (LD 700, Pyri-dine 1 and 2, perhaps a consequence of air oxidationof the CH 2 solvent group) were stable in phenoxyeth-anol. It is a useful replacement in some cases forethylene glycol in jet-stream dye lasers'7 and is atleast a substitute for benzyl alcohol.
This research was performed under the auspices ofthe U.S. Department of Energy by the LawrenceLivermore National Laboratory under contractW-7405-ENG-48.
References and Notes1. F. C. Webster and W. C. McColgin, "Arylidene dye lasers,"
U.S. patent 3,852,683, (3 December 1974); P. R. Hammond,"Laser dye DCM, its spectral properties, synthesis and compar-ison with other dyes in the red," Opt. Commun. 29, 331-333(1979).
2. E. G. Marason, "Laser dye DCM: CW, synchronously pumped,cavity dumped and single-frequency performance, " Opt. Com-mun. 37, 56-58 (1981).
3. Spectra-Physics, Inc., 1250 West Middlefield Road, MountainView, Calif. 94039.
4. T. F. Johnstone, R. H. Brady, and W. Proffit, "Powerfulsingle-frequency ring dye laser spanning visible spectrum,"Appl. Opt. 21, 2307-2316 (1982).
5. Coherent, Inc., 3210 Porter Drive, Palo Alto, Calif. 94304.6. J. Heber and A. Szabo, "Argon pumped dye laser operation in
the 690-700 nm region," IEEE J. Quantum Electron. QE-20,9-11 (1984).
7. B. J. Y. Wuebbles and J. S. Felton, "Evaluation of laser dyemutagenicity using the Ames/salmonella microsome test,"Environ. Mutag. 7, 511-522 (1985).
8. P. K. Runge and R. Rosenberg, "Unconfined flowing-dye filmsfor cw dye lasers," IEEE J. Quantum Electron. QE-8, 910-911(1972); S. Leutwyler, E. Schumacher, and L. W6ste, "Extend-ing the solvent palette for cw jet-stream dye lasers," Opt.Commun. 19, 197-200 (1976).
9. We thank M. Rotter and D. Jander, Lawrence LivermoreNational Laboratory, Livermore, Calif., for help with thesemeasurements.
10. Measurements made by Jerry Benterou, Lawrence LivermoreNational Laboratory, Livermore, Calif., 1988.
11. Measurement made by Rick Young, Lawrence LivermoreNational Laboratory, Livermore, Calif., 1988.
12. J. M. Drake, M. L. Lesiecki, and D. Camaioni, "Photophysicsand cis-trans isomerization of DCM," Chem. Phys. Lett. 113,530-534 (1985); M. Meyer, J. C. Mialocq, and B. Perly,"Photoinduced intramolecular change-transfer and trans-cisisomerization of the DCM styrene dye. Picosecond and nano-second laser spectroscopy, high-performance liquid chromatog-raphy, and nuclear-magnetic resonance studies," J. Phys.Chem. 94,98-104 (1990).
13. J. Bourson, D. Doizi, D. Lambert, T. Sacaze, and B. Valeur, "Aderivative of laser dye DCM highly soluble in alcohols," Opt.Commun. 72, 367-370 (1989); J. Said and J. P. Boquillon,"Lasing characteristics of a new DCM derivative under flash-lamp pumping," Opt. Commun. 82, 51-55 (1991).
14. P. R. Hammond, "Dye system for dye laser application," U.S.patent 5,018,160 (21 May 1991).
15. We thank Barbara Wuebbles and James Felton, LawrenceLivermore National Laboratory, Livermore, Calif., for thesemeasurements.
16. P. Gilbert, E. G. Beveridge, and P. B. Crone, "Effect of2-phenoxyethanol upon RNA, DNA and protein biosynthesisin E. Coli NCTC 5933," Microbios 28, 7-17 (1980).
17. L. E. Knaack, "Laser dye liquids, laser dye instruments andmethods," U.S. patent 4,896,329 (23 January 1990).
