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Lasing Properties of Novel near Infrared Laser Dyes

Mark R.W. Venner*a, Antony D. Case a, David J. Fulker a, John Griffiths b, John Mama b a Sensors, Processing and Integration Centre, QinetiQ, Ively Road, Farnborough, GU14 0LX, UK

b Department of Color Chemistry, Leeds University, Leeds, LS2 9JT, UK

ABSTRACT A number of novel near infrared oxazine laser dyes have been designed, synthesized and purified. The photophysical and lasing properties of these near infrared laser dyes are reported in this paper. The dyes have been found to exhibit moderately high fluorescence quantum efficiencies. Laser testing has been undertaken on the novel oxazine dyes and the results have been compared with those obtained with commercially available near infrared laser dyes. Keywords: Dye laser, near infrared dye, flashlamp pump, oxazine

1. INTRODUCTION Dye lasers have found application in a wide variety of fields,1,2 because of their advantageous properties. These properties include continuous tunability from the UV to the near-IR region using a selection of different dyes, operation from picosecond pulses to continuous wave, output power scaleability, and inherent simplicity. The majority of the work involving dye lasers has been conducted in liquid solution using a variety of fluorescent laser dyes developed over a number decades. In addition almost since the first demonstration of the liquid dye laser, researchers have been trying to incorporate laser dyes in to solid matrices in order to increase further the utility of dye lasers. The bulk of this work has concentrated upon incorporation of different dye classes (coumarins, xanthenes, pyrromethenes, perylenes) into solid media for optical pumping using lasers. Solid state samples pumped with a variety of these lasers delivering nanosecond and microsecond pulses have been reported. The host materials for the samples have included acrylic polymers, epoxy polymers, sol-gels, organically modified sol-gels (ORMOSILs), and sol-gel/polymer composites (POLYCOMs). Improvements to both the laser efficiency and sample lifetime have been achieved through the continuous improvement in host materials coupled more recently with the introduction of the pyrromethene class of laser dyes. The evolution of the field of solid state dye lasers and the host materials used has recently been reviewed.3

2. NEAR INFRARED DYE LASERS The great majority of reports of dye lasers both in the solid state and in solution concern visible dyes, in particular dyes from the coumarin,4 xanthene,5 and pyrromethene families.6,7 In comparison there have been far fewer reports of near infrared (700-1000 nm) dye lasers and many of these reports date from the 1970’s and 1980’s. In recent years there has been some renewed interest in the synthesis and laser testing of near infrared laser dyes,8,9 but also an increasing interest in fluorescent near infrared dyes particularly for biolabelling applications.10-12 The near infrared dyes that have been reported13-17 to lase include oxazines, extended rhodamines, pyrromethenes, styryl dyes and cyanines. In comparison with visible dyes, the laser efficiency obtainable with near infrared dyes is lower, particularly for flashlamp pumping.16,17 This is as a result of the lower fluorescence quantum yield of near infrared dyes, which can be rationalised theoretically in terms of the probability of internal conversion within the dye being inversely proportional to the S1-S0 energy gap - the Energy Gap Law.18

* [email protected]

Solid State Lasers XIV: Technology and Devices, edited by Hanna J. Hoffman,Ramesh K. Shori, Proceedings of SPIE Vol. 5707 (SPIE, Bellingham, WA, 2005)0277-786X/05/$15 · doi: 10.1117/12.600694

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In a previous paper19 we reported the results of a study to characterize and laser test a number of commercially available near infrared laser dyes in order to benchmark their performance. Under flashlamp excitation the most efficient dyes at near infrared wavelengths were found to be the oxazine dye LD690 and the extended rhodamines Rhodamine 700 (LD700) and Rhodamine 800 (LD 800) (Scheme 1). In this paper, we report the results achieved to date in a study aimed at demonstrating near infrared fluorescent dyes with enhanced laser efficiency under flashlamp excitation. The work has featured the design, synthesis and characterization of a number of near infrared fluorescent dyes. The dyes have been characterized to determine their solubility and spectroscopic properties, and the most appropriate dye/solvent combinations have been tested in solution using a QinetiQ-designed flashlamp pumped dye laser.

3. EXPERIMENTAL 3.1 Materials The commercial near infrared dyes used in the study (Scheme 1) – LD 690 (Oxazine 4), Rhodamine 700 (LD 700), Rhodamine 800 (LD 800) - were all purchased from Exciton and were used without further purification. The solvents were purchased from Sigma-Aldrich, and were of spectroscopic grade and used without further purification. The methanol/water for the laser testing was a deuterated 50/50 (v:v) mixture. A number of near infrared oxazine dyes were synthesized in the Department of Color Chemistry at the University of Leeds using standard synthetic procedures for this dye type. These synthetic experimental details will be reported elsewhere. 3.2 Absorption and fluorescence spectroscopy Absorption spectra were recorded using an Agilent HP8453 diode array spectrophotometer using 2 mm cuvettes. Fluorescence emission and relative quantum yield values were obtained in a 1 cm fluorescence cuvette using a Spex fluorescence spectrometer. The optical density of the solutions was kept below 0.1 in a 1 cm path length cell in order to minimise self-absorption effects. 3.3 Flashlamp pumped lasing tests For these tests a QinetiQ-designed and built flashlamp pumped dye laser was used, the experimental setup is shown in Scheme 2. The two flashlamps were driven by a 2 µF capacitor giving a flashlamp pulse of approximately 7 µs duration. The laser had a dye cell with a 14 mm diameter and 270 mm length. The laser cavity comprised a plane-parallel resonator of length 570 mm having a 60% reflective output coupler and a 100% reflective rear mirror. Alignment of the cavity was aided by the use of an auxiliary diode laser beam (786 nm). The combination of a CCD camera attached to Spiricon beam analyser (LBA-100A) was used to monitor the laser alignment. The output energies were measured behind a wedged beamsplitter using a calibrated calorimeter (Laser Instrumentation Model 6000 display meter and 17AN thermopile head). The energy output was measured over a range of input energies from which the slope efficiency was calculated. The lasing wavelength of each dye was measured using a 200 mm flat field spectrograph (Jobin Yvon, model UFS200) calibrated using the lines from HeNe lasers operating at 632.8 nm and 594.1 nm. Each dye solvent combination was made with sufficient dye concentration so as to give an optical density of the solution of 0.5 in a 2 mm path length cell when measured by absorbance spectroscopy.

4. RESULTS AND DISCUSSION

4.1 Novel near infrared oxazine laser dyes In our previous paper19 we reported that under flashlamp excitation the most efficient commercially available laser dyes at near infrared wavelengths were found to be the oxazine dye LD690 and the extended rhodamine dyes Rhodamine 700 (LD700) and Rhodamine 800 (LD 800) (Scheme 1). It was found that the laser efficiency of the dyes was lower than that of R6G, and it was found to decrease with the lasing wavelength of the dye (Figure 1).

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Potential fluorescent dye systems that have been shown to lase, are stable, and which have the potential to be shifted into the near infrared, are very few in number. In the search for more efficient dyes at the longer wavelengths similar to Rhodamine 800 it was noted that the extended rhodamines Rhodamine 700 and Rhodamine 800 had been derived from the dye Rhodamine 6G (R6G) by synthetic modification to produce a bathochromic shift. Whilst in the resulting dyes the wavelength had been successfully extended, the chemical stability of both dyes in protic solvents was found to be poor.19 This is as a direct result of the structural modifications embodied to produce the bathochromic shift. In the light of this evidence, oxazine dyes and in particular LD 690 seemed a natural starting point in order to produce efficient longer wavelength laser dyes for flashlamp excitation. Therefore, the goal of the work was to extend the lasing wavelength of LD 690 by synthetic modification, whilst maintaining the laser efficiency of the dye. In wavelength terms oxazine dyes have the advantage compared with rhodamines of an inherent bathochromic shift of approximately 90 nm as a result of the nitrogen for oxygen atom replacement in the chromophore (Scheme 3). In pursuit of the goal to extend the chromophore of LD 690 a number of oxazine structures were identified as having been reported in the literature. Hammond and coworkers have reported20 the synthesis and laser-pumped laser performance of oxazine dyes with the general structures shown in Scheme 4. Dyes with similar structures have also been reported by Drexhage and coworkers21 for use as fluorescent markers in biolabelling applications. Therefore based upon the literature reports, and underpinning synthetic experience of functional dyes, a number of oxazine dye structures were targeted for synthesis as potentially efficient flashlamp pumped laser dyes during this study. As a result of the synthetic study carried out at Leeds University a total of five oxazine dyes (Scheme 5) have been synthesized and purified to a level suitable for laser testing.

4.2 Spectroscopic properties of novel near infrared oxazine laser dyes Following their synthesis and purification, solubility and stability trials of the oxazine dyes in solution were initially carried out. All of the oxazine dyes were readily soluble in α,α,α-trifluorotoluene (TFT), dichloromethane (DCM), propylene carbonate (PC), methanol and methanol/water mixtures. Furthermore, the dyes were all found to be extremely stable in solution over extended periods. Absorption and fluorescence spectra were recorded for a variety of dye/solvent combinations, the results (Table 1), show the desired bathochromic shifts compared to the benchmark oxazine dye LD 690. The longest wavelength dyes were found to have spectroscopic properties similar to the extended rhodamine Rh800. All of the oxazine dyes had extinction coefficients of between 1.17x105 and 1.31x105. The comparative fluorescence quantum yield (QF) for each dye in the most appropriate solvents was measured using the Parker-Reas method.22 The following fluorescence standards were employed in the study: LD 690 in methanol QF = 0.63,23 and Rh 800 in methanol QF = 0.16.24 The new oxazine dyes all have fluorescence quantum yields somewhat lower than the benchmark oxazine dye LD 690, but higher than Rh700 and Rh800. Between the oxazine dyes the fluorescence yields were found to be similar. As was found in our earlier study19 QF values for near infrared dyes are reduced from those obtained with the benchmark visible laser dye Rhodamine 6G (R6G) QF = 0.94 in methanol.25

4.3 Flashlamp pumped lasing properties of novel near infrared oxazine dyes Following their characterization the five oxazine dyes were tested in different solvents in a QinetiQ flashlamp-pumped dye laser, the results are shown in Table 2. The visible dye R6G, lasing at 589 nm and the commercial near infrared laser dyes LD 690, Rh700 and Rh800 were used as the comparative baseline for the laser testing. All of the oxazine dyes were found to lase with good lasing efficiency at wavelengths considerably longer than LD 690 and similar to Rh800. In comparison to Rh800 all of the new dyes are more efficient (Figure 1). The best of the new dyes OX13 and OX12 exhibit more than twice the laser efficiency of Rh800.

5. CONCLUSIONS In summary, building upon the results of previous studies at QinetiQ, a number of near infrared oxazine laser dyes have been successfully designed, synthesized, characterized and laser tested in solution, under flashlamp excitation. The

Proc. of SPIE Vol. 5707 229


laser efficiency of the new dyes has been compared with the commercial dyes previously tested.19 All of the oxazine dyes have been found to lase at considerably longer wavelengths than the benchmark oxazine dye LD690, thus achieving the first objective of the work. The laser efficiency is somewhat reduced compared to LD690. However, the dyes were found to be more efficient than the dye Rh800 lasing at similar wavelengths, with the most efficient dye OX13 being twice as efficient as Rh800. In addition the solution stability of the new oxazine dyes has been found to be superior to the extended rhodamine dyes Rh700 and Rh800. It is anticipated26 that the photostability under flashlamp excitation of the new oxazine dyes will also be superior to the dyes Rh700 and Rh800. Work is now underway to confirm that the photostability in solution of the new oxazine dyes under flashlamp excitation is superior to the extended rhodamine dyes Rh700 and Rh800.

ACKNOWLEDGEMENTS This work was funded as part of the Corporate Research Programme of the United Kingdom Ministry of Defence. We would like to acknowledge Professor David Titterton (Defence Science Technology Laboratory) for useful discussions during the course of the work.

REFERENCES

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224, 2002. 3. M. D. Rahn, T. A. King, “Comparison of solid state dye laser performance in various host media”, Proc. SPIE-Int.

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1,3,5,7-tetramethyl-pyrromethene-BF2 complexes: part 1”, Appl. Opt., 29, 3885-3886, 1990. 7. M. Shah, K. Thangaraj, M-L. Soong, L. T. Woloford, J. H. Boyer, T. G. Pavlopoulos, “Pyrromethene-BF2

complexes as laser dyes”, Heteroatom. Chem., 1, 385-387, 1990. 8. G. Jones, II, Z. Huang, S. Kumar, “Fluorescence and Lasing properties of benzo-fused pyrromethene dyes in poly

methylmethacrylate solid host media”, Proc. SPIE-Int. Soc. Opt. Eng., 4630, 65-69, 2002. 9. G. Jones, II, Z. Huang, D. Pacheco, “Fluorescence and Lasing properties of Meso-substituted Benzo-fused

pyrromethene dyes in solid host media”, Proc. SPIE-Int. Soc. Opt. Eng., 4968, 24-34, 2003. 10. M. Sauer, K.-T. Han, R. Muller, S. Nord, A. Schulz, S. Seeger, J. Wolfrum, J. Arden-Jacob, G. Deltau, N. J. Marx,

C. Zander, K. H. Drexhage, “New Fluorescent Dyes in the red Region for Biodiagnostics”, J. Fluoresc., 5, 247–261, 1995.

11. B. Oswald, M. Gruber, M. Böhmer, F. Lehmann, M. Probst, O.S. Wolfbeis, “Novel Diode laser-compatible fluorophores and their application to single molecule detection, protein labelling and fluorescence resonance energy transfer immunoassay”, Photechemistry and Photobiology, 74, 237-245, 2001.

12. J. Liu, Z. Diwu, W-Y. Leung, Y. Lu, B. Patch, R.P. Haughland, “Rational design and synthesis of a novel class of highly fluorescent rhodamine dyes that have strong absorption at long wavelengths”, Tetrahedron Letters, 44, 4355-4359, 2003.

13. K.Kato, “Near–infrared dye laser pumped by a Carbazine-122 dye laser”, IEEE J. Quantum Electron., QE-12, 442-442, 1976.

14. B.M Pierce, R.R. Birge, “Lasing properties of several near IR dyes for a nitrogen laser pumped dye laser with an optical amplifier”, IEEE J. Quantum Electron., QE-18, 1164-1170, 1982.

15. C-H. Lin, B. Marshall, “Flashlamp pumped Styryl 9 dye laser”, Applied Optics, 23, 2228, 1984. 16. M. Maeda, Y. Miyazoe, “Flashlamp excited organic liquid laser in the range from 342nm to 889 nm”, Japanese

Journal of Applied Physics, 11, 692-698, 1972.



17. J.B. Marling, J.G. Hawle, E.M. Liston, W.B. Grant, “Lasing characteristics of seventeen visible wavelength dyes using a coaxial flashlamp pumped dye laser”, Applied Optics, 13, 2317-2320, 1974.

18. N.J Turro, Modern Molecular Photochemistry, Ch. 6, University Science Books, California, 1991. 19. M.R.W. Venner, A.D. Case, D.J. Fulker “Lasing properties of near-infrared laser dyes in the liquid and solid state”,

Proc. SPIE-Int. Soc. Opt. Eng., 5332, 189-199, 2004. 20. P.R. Hammond, G.F Field, “Oxazine laser dyes”, US Patent 5,149,807, 1992. 21. R. Herrmann,H-P. Josel, K-H Drexhage, N-J Marx, “New oxazine dyes and their use as fluorescent markers”, US

Patent Application 0224421, 2003. 22. J.D Ingle Jr., S.R. Crouch, Spectrochemical Analysis, Prentice Hall, Upper Saddle River, NJ, 1988. 23. R. Sens, K. H. Drexhage, “Fluorescence quantum yield of oxazine and carbazine laser dyes”, J. Lumin., 24-25, 709-

712, 1981. 24. P. Sperber, W. Spangler, B.Meier, A.Penzkofer, “Experimental and theoretical investigation of tunable picosecond

pulse generation in longitudinally pumped dye laser generators and amplifiers”, Optical and Quantum Electronics, 20, 395-431, 1988.

25. J. Georges, N. Arnaud, L.Parise, “Limitations arising from optical saturation in fluorescence and thermal lens spectrometries using pulsed laser excitation: application to the determination of the fluorescence quantum yield of rhodamine 6G”, Applied Spectroscopy, 50, 1505-1511, 1996.

26. K.H Drexhage, Dye Lasers – Topics in Applied Physics Volume 1, Ch.5, Ed. F.P Schafer, Springer Verlag, Berlin, 1990.



Scheme 1. Structures of the commercial near infrared dyes used as the comparative baseline in the study.

Scheme 2. The experimental setup for the flashlamp pumped dye laser testing.

Screen

Back mirror

Output coupler Wedged beam

splitter

Calorimeter Dye laser

1 m FL lens

Integrating sphere

Neutral density filters

CCD camera Neutral

density filters

Spectrometer Quartz fibre

1 m

N O

N

N+C2H5H5C2

HH

CH3H3C

ClO4-

N O N+

R1

ClO4-

LD 690 R1 = CF3 - Rh 700, R1 = CN - Rh 800

Spiricon



R6G − λmax 525 nm LD690 − λmax 611 nm

Scheme 3. The structures of the rhodamine dye R6G and the oxazine dye LD690 illustrating the intrinsic bathochromic shift in the oxazine family of dyes.

Scheme 4. The structures of oxazine dyes reported by Hammond and coworkers.20

N O N+

C2H5H5C2

HH

CH3CH3

COOMe

N O

N

N+

C2H5H5C2

HH

CH3CH3

N

ON+ N

N

ON+

N

R1R2



Scheme 5. The structures of the oxazine dyes synthesized and laser tested during the study.

N

ON+ N

C2H5C2H5

CH3 CH3

CH3

CH3

CH3

CH3

N

ON+ N

CH3

CH3

CH3

N

ON+ N

C2H5

CH3

CH3

CH3

N

ON+ N

C2H5

CH3

CH3

CH3

CH3

CH3

CH3

N

ON+ N

OX11 OX12

OX13 OX14

OX16



Dye Solvent Absorption Maximum

(nm)

Fluorescence Maximum

(nm)

Fluorescence Quantum Yield

QF Methanol 610 629 0.63

Methanol/Water 617 634 0.51 LD 690 PC 612 626 0.70

Methanol 644 665 0.23 Rh 700 Methanol/Water 646 667 0.21

Methanol 679 701 0.16 Rh 800 PC 688 703 0.18

Methanol 670 699 0.19 OX11 DCM 678 694 0.37

Methanol 673 701 - OX12 DCM 680 693 0.42

Methanol 672 - - OX13 DCM 676 690 0.39

Methanol 664 - - OX14 DCM 664 676 0.47

Methanol 679 705 - OX16 DCM 686 702 0.40

Table 1. Absorption and fluorescence properties of near infrared oxazine dyes used in this work.

Dye Solvent Output Energy

Relative to R6G

Laser Wavelength

(nm)

R6G Methanol/Water 1.00 589 LD 690 Methanol/Water 0.87 700 Rh 700 Methanol/Water 0.75 730 Rh 800 Methanol/Water 0.29 789 OX11 Methanol/Water 0.49 781 OX12 Methanol/Water 0.57 783

TFT 0.30 762 Methanol 0.41 770 OX13

Methanol/Water 0.64 777 TFT 0.73 755

Methanol 0.66 764 OX14 Methanol/Water 0.50 768

OX16 Methanol/Water 0.45 791

Table 2. Flashlamp pumped properties of near infrared dyes in solution compared to commercially available laser dyes.



0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

580 600 620 640 660 680 700 720 740 760 780 800

Wavelength (nm)

Lase

r Ene

rgy

Rel

ativ

e to

R6G

LD690

OX13

OX12

LD800

LD700

OX14OX16

R6G

OX11

Figure 1. Graph illustrating the relative laser efficiency under flashlamp excitation, of commercial laser dyes (closed diamond) and new near infrared oxazine dyes (open circles). Copyright © QinetiQ Ltd 2004.



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