Laser action from two dipyrrinone dyesunder flash-lamp pumping

Theodore G. Pavlopoulos, David A. Lightner, and Justin O. Brower

Two quasi-aromatic compounds, namely, two dipyrrinone dyes �Xanthoglow and Kryptoglow�, exhibitedstrong fluorescence. Therefore they were tested for laser action under flash-lamp pumping. These twodyes outperformed Coumarin 540A and exhibited a high degree of photostability when dissolved in1,4-dioxane. However, in all other solvents used, considerable photodecomposition was observed.© 2003 Optical Society of America

OCIS codes: 140.2050, 140.3380.

1. Introduction

Although dye lasers have been with us for more than35 years, they constitute one of the few unfinishedfields of laser physics.

Dye lasers are widely used in spectroscopic re-search because they provide tunable, coherent radi-ation. Dye lasers employ highly fluorescent organiccompounds dissolved in suitable solvents. Impor-tantly, these liquid media can be cooled by circula-tion. In addition, these liquid media have no scalinglimitations and therefore should allow the construc-tion of large dye lasers. However, large dye lasersthat generate high-energy and high average poweroutputs do not yet exist because of the lack of a scal-able pump source. Therefore dye lasers have notfound wide technical applications. At present,mainly pulsed and cw lasers and small flash lampsare used for pumping. However, all three pumpsources have scaling limitations.1 Only these pumpsources are able to overcome excessive triplet-statelosses �TSLs� present in commercially available laserdyes.1 TSLs result from transient triplet-state dyemolecules generated during excitation. These mol-ecules absorb laser light. Absorption of light by

T. G. Pavlopoulos �pavlopou@spawar.navy.mil� is with the U.S.Space and Naval Warfare Systems Center—San Diego, Navigationand Applied Sciences Department Code 2361, B-111, San Diego,California 92152-5001. D. A. Lightener and J. O. Brower are withthe Department of Chemistry, University of Nevada, Reno, Ne-vada 89557-0020.

Received 20 December 2002.0003-6935�03�183555-03$15.00�0© 2003 Optical Society of America

triplet-state molecules is called triplet–triplet ab-sorption.

These TSLs depend on the triplet-state absorptioncoefficients �T��F� of the triplet-state dye moleculesand their concentration NT: When �F is the wave-length of the fluorescence �laser action� spectral re-gion,

TSL � NT�T��F�. (1)

The quantum fluorescence yields �F, and the triplet-state lifetime �T of the triplet-state molecule in solu-tion determine NT.1 In general, for highlyfluorescent compounds, NT is small. For efficientlaser action to occur, �T��F� must also be small, pref-erably very small.1

Approximately 120 laser dyes are commerciallyavailable, of which approximately 50% of these dyeshave been found by trial and error. The rest consistsmostly of derivatives of laser dyes, again found bytrial and error. They are dyes that have been newlysynthesized on the basis of structural and spectro-scopic considerations. Unfortunately, in most com-mercially available laser dyes, �T��F� values are notsufficiently small to allow the pumping of dye laserswith scalable pump sources such as large flash lampsemitting long pulses, laser-diode arrays, and incoher-ent light sources.1,2

2. Quasi-Aromatic Compounds

It has been proposed that efficient laser dyes may befound among the so-called quasi-aromatic com-pounds. These are five-and six-membered heterocy-clic systems in which vibrational spin-orbitinteractions are present.1,3 Often, these interac-tions reduce triplet–triplet absorption intensities, re-

20 June 2003 � Vol. 42, No. 18 � APPLIED OPTICS 3555

sulting in small �T��F� values. If, in addition, strongfluorescence is present, efficient laser action may beobserved.1

Two families of such quasi-aromatic laser dye sys-tems are known. The syn-bimanes,1,3 with theirchemical structure depicted in Fig. 1, constitute onesuch family.

Many of the syn-bimanes exhibit striking fluores-cence and exhibit laser action under flash-lamppumping in the violet as well as in the blue-greenspectral region. The anti-bimanes, also depicted inFig. 1, do not exhibit any fluorescence and, conse-quently, no laser action.1

The Pyrromethene-BF2 complexes constitute thesecond family. They exhibit efficient laser actionfrom the green to the red portions of the spectrum.1The chemical structure of this family of laser dyes isdepicted in Fig. 2.

3. Dipyrrinone Dyes

In different solvents the two dipyrronine dyes exhib-ited strong fluorescene, with quantum yields gener-ally in the range of �F � 0.95.4 Because they alsofulfilled the definitions of quasi-aromatics, they weretested for laser action in our small flash-lamp-pumped dye lasers.5

N,N�-carbonyl-dipyrrinone �3H,5H-dipyrrolo�1,2-c:2�,1�-f pyrimidin-3,5-dione�4 is a highly fluorescentchromophore. The chemical structure of this com-pound is shown in Fig. 3 and the substituents presentin Xanthoglow and Kryptoglow are shown in Table 1.

4. Experimental Results

The visible–near-UV absorption and fluorescence spec-tra of Xanthoglow dissolved in 1,4-dioxane are shownin Fig. 4. Kryptoglow exhibited the same fluores-cence spectrum; its absorption intensity maximumwas located at 425 nm, and ε 15,100 �L�mol cm�.

To obtain data on laser action efficiency underflash-lamp pumping, we used 2 � 10�4 molar solu-

tions in different solvents for both dyes. These out-puts were compared with the outputs obtained from a2 � 10�4 molar solution of Coumarin 540A in meth-anol. The dipyrronine dyes and Coumarin 540Ahave similar absorption spectra. Coumarin 540Aexhibits laser action in the same spectral region asthe two dipyrronine dyes.

In Fig. 5 we present the outputs in energies E asa function of pump energies E of these three com-pounds. By far, the best results were obtained when1,4-dioxane was used as a solvent for the two dipyr-ronine dyes. Xanthoglow lased with approximately60% higher efficiency compared with the Coumarindye with broadband output at 536 nm. When meth-anol and dimethylformamide were used as solvents;outputs were reduced to 75% and 60%, respectively.

Xanthoglow did not exhibit any laser action in ben-zene, iso-octane, and cyclohexane. Kryptoglowlased at 537 nm with approximately 44% higher ef-ficiency compared with Coumarin 540A. When eth-anol and dimethyl sulfoxide were used as solvents,efficiency fell to 95% and 75%, respectively.

We also tested the three dyes for photostability.The pumping pulse energy was 10 J. In Fig. 6 wepresent data on the photostability of Xanthoglow,Kryptoglow, and Coumarin 540A. Coumarin 540A

Table 1. Substituents Present in Xanthoglow and Kryptoglowa

Dyes R1 R2 R3 R4 R5

Xanthoglow -CH3 -C2H5 -CH3 P -CH3

Kryptoglow -CH3 -C2H5 -CH3 -C2H5 -CH3

aR1, R2, R3, R4, and R5 substituents, and P -CH2CH2CO2H.

Fig. 3. Chemical structure of N,N�-carbonyl-dipyrrinone.

Fig. 4. Near-UV–visible �S-S� and fluorescence spectra �FL� ofXanthoglow.

Fig. 1. Chemical structures of syn-bimane and anti-bimane.

Fig. 2. Chemical structure of the Pyrromethene-BF2 complexes.

3556 APPLIED OPTICS � Vol. 42, No. 18 � 20 June 2003

is one of the more photostable Coumarin dyes. How-ever, according to Fig. 6, Kryptoglow is the most pho-tostable laser dye we ever tested, outperformingCoumarin 540A. Xanthoglow is also a very photo-stable laser dye.

In Fig. 6, the two dipyrrinone dyes exhibit a dip inphotostability after approximately five pulses of ex-citation. Most likely, oxygen from air might act as aquencher of fluorescence intensity, reducing laser ac-tion efficiency. With continuing excitation, how-ever, the dissolved oxygen may be involved in

photochemical reactions with the solvent or dye. Itsremoval from the dye solution may increase the quan-tum fluorescence yield of the dye, resulting in im-proved efficiency.

Both Xanthoglow and Kryptoglow dissolved in 1,4-dioxane were chemically exceptionally unstable.Within only a few hours, these solutions visibly de-composed, with the yellow color of the two dipyrri-none dyes disappearing rapidly. These data aredifficult to explain.

5. Discussion

Obviously, Xanthoglow and Kryptoglow are notsuited for any practical use as laser dyes. However,closely related derivatives should outperform thesetwo dyes with regard to efficiency and photostability.If the Pyrromethene-BF2 complexes provide anyguidelines, substitution of R1 to R4 and the hydrogenin the eight-positions with-CH3 groups provided im-proved photostability.6

The data presented in the foregoing section aresignificant from the theoretical point of view. It hasbeen suggested that efficient laser dyes could befound among quasi-aromatic compounds that exhibitstrong fluorescene.1,3 After the syn-bimanes andthe Pyrromethene-BF2 complexes, the dipyrrinoneare the third family of such compounds found in a rowthat exhibit efficient laser action under flash-lamppumping. Although the two dipyrrinone dyes westudied have problems with chemical and photo-chemical stability, derivatives or closely related ana-logues may possess all the desired properties of laserdyes, such as exhibiting high efficiency and good pho-tostability. The crown jewels of laser dye properties,however, belong to dyes that, besides possessingthese two desired properties, are also water soluble.With improved laser dyes, many technical applica-tions should become feasible.1

References1. T. G. Pavlopoulos, “Laser dyes: structure and spectroscopic

properties,” in Colorants for Non-Textile Applications, H. S.Freeman and A. T. Peters, eds. �Elsevier Science, Amsterdam,2000�, pp. 275–338.

2. T. G. Pavlopoulos, “A figure of merit for laser dyes,” Opt. Com-mun. 38, 397–401 �1981�.

3. T. G. Pavlopoulos, “Spectroscopy and molecular structure ofefficient laser dyes: vibronic spin–orbit interactions in het-erocyclics,” Appl. Opt 36, 4969–4980 �1997�.

4. J. O. Brower and D. A. Lightner, “Synthesis and spectroscopicproperties of a new class of strongly fluorescent dipyrrinones,”J. Org. Chem. 67, 2713–2717 �2002�.

5. E. Schimitschek, J. A. Trias, P. R. Hammond, R. A. Henry, andR. L. Atkins, “New laser dyes with blue-green emission,” Opt.Commun. 16, 313–316 �1975�.

6. J. H. Boyer, A. M. Haag, G. Sathyamoorthi, M.-L. Soong, andT. G. Pavlopoulos, “Pyrromethene-BF2 complexes as laserdyes: 2,” Heteroatom Chem. 4, 39–49 �1993�.

Fig. 5. Laser action efficiencies of Xanthoglow, Kryptoglow, andCoumarin 540A.

Fig. 6. Photostability of Xanthoglow, Kryptoglow, and Coumarin540A.

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