Phenylbenzimidazole proton-transfer laser dyes: Spectral and operational properties

Volume 64, number 5

OPTICS COMMUNICATIONS

l December 1987

PHENYLBENZIMIDAZOLE PROTON-TRANSFER LASER DYES: S P E C T R A L AND OPERATIONAL P R O P E R T I E S A. C O S T E L A a), F. A M A T b), j. C A T A L A N c), A. D O U H A L a), J.M. F I G U E R A a), J.M. MUI~IOZ a) and A.U. ACUI~A a) a~ lnstituto de Quimica Fisica "Rocasolano", CSIC, Serrano 119, 28006 Madrid. Spain b~ lnstituto de Quimica Orgdnica General, CSIC, Juan de la Cierva, 1, 28006 Madrid, Spain ~ Departamento de Quimica, Universidad Aut6noma de Madrid, Cantoblanco, Madrid, Spain

Received 4 August 1987

The efficient emission of tunable stimulated light in the 420-550 nm range from solutions of a number of derivatives of 2-(2 hydroxyphenyl) benzimidazole is reported. The lasing mechanism is based on an intramoleeular proton-transfer reaction taking place in the initially populated excited state. These dyes, when pumped with a XeC1 or N2 lasers, produce tunable radiation covering a range of more than 100 nm with efficiencies up to 15.5%.

I. Introduction Dye lasers offer a series o f interesting spectroscopic properties, such as large spectral coverage a n d very, small b a n d w i d t h , which m a k e t h e m essential tools in a n u m b e r o f applications. Two p a r a m e t e r s of the lasing dyes are usually the limiting factors in the operation o f a particular c o m p o u n d : p h o t o c h e m ical stability a n d tunable range. In addition, unwanted absorption o f the laser radiation by long lived transient species o f the dye itself (triplets, radicals, etc) m a y b e c o m e a severe deterrent for cw operation or flash-lamp pumping. Recently, a n u m b e r o f laser dyes have been described [ 1 - 3 ] where the generation o f stimulated r a d i a t i o n is due to a relatively novel photochemical m e c h a n i s m which might lead to the overcoming o f some o f these limitation. In brief, an intramolecular proton-transfer ( I P T ) reaction o f the electronically excited molecule results in an excited p r o d u c t ( t a u t o m e r ) having zero concentration in the ground state and emitting fluorescence with a high yield a n d a large Stokes shift ( 6 0 0 0 - 1 0 0 0 0 cm - ~) [ 4 - 6 ]. This m e c h a n i s m produces a large p o p u l a t i o n inversion and efficient laser emission. In addition, those molecules showing IPT in the excited state are often very stable p h o t o c h e m ically with no i n d i c a t i o n o f triplet f o r m a t i o n [ 7 ], al-

though in some o f them microsecond transients have been observed [8] and interpreted as the phototaut o m e r ground state. Here we discuss the I P T laser operation o f a new family o f b e n z i m i d a z o l e derivatives. In ref. [ 3 ] we reported IPT laser operation o f a dioxane solution o f 2-(2-hydroxyphenyl) benzimidozole (OH-PBIM) with an efficiency higher than 10%. N o w laser operation has been extended to other m e m b e r s o f the family: 2-(2,5-dihydroxyphenyl) benzimidazole (DIPBIM), 2- (2-hydroxy-5-methyl) benzimidazole (MeB I M ) , 2-(2-hydroxy-5-methoxy) b e n z i m i d a z o l e ( O M e - B I M ) , and 2- ( 2-hydroxy- 5-fluoro) benzimidazole ( F - B I M ) , as well as O H - P B I M in a n u m b e r o f solvents, covering an emission range o f more than 100 nm and with much higher efficiencies.

2. Experimental O H - P B I M was p r e p a r e d from p h e n y l e n e d i a m i n e and salicylic acid as described previously [3]. The synthesis o f the remaining derivatives o f O H - P B I M was carried out following essentially the same procedure and using the corresponding 5-derivative o f salicylic acid. The purity o f the dyes was checked by HPLC, TLC and combustion analysis.

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OPTICS C O M M U N I C A T I O N S

A conventional transvers-pumping configuration was first used for excitation of the IPT laser solution. The dye cell was a magnetically stirred 10 mm length quartz cell. The IPT laser cavity was formed by a ~90% reflectivity aluminium mirror and an uncoated quartz parallel plate as an output coupler, with a cavity length of 8 cm. For tuning and narrowing the linewidth a Shoshan type [9] oscillator was locally built, consisting of a transversally-pumped, magnetically stirred 10 mm dye cell, two ~90% reflectivity aluminium mirrors and holographic grating (2400 lines/mm, PTR) set up at grazing incidence. The pump radiation was obtained from a home-made automatic preionization laser, similar to that described by Kearsley et al. [10], that can be used as XeC1 or as N2 laser. When used as a XeC1 laser it produced energies up to 40 mJ in ~ 10 ns fwhm pulses at 308 nm. When working as N2 laser the energy was 3 mJ in ~ 7 ns fwhm pulses at 337 nm. The pump light was focused into the dye cell by a combination of spherical ( f = 5 0 cm) and cylindrical ( f = 15 cm) quartz lenses. The repetition rate of the system was 1 Hz and the linewidth of the IPT laser output in the tuned cavity less than 0.2 cm The IPT and pump laser pulses were characterized with the following instruments: GenTec ED-100A and GenTec ED-200 energy meters, ITL TF1850 fast rise time photodiode, Tektronix 7912AD transient digitizer, 7934 and 468 Storage oscilloscopes, Bausch and Lomb 33-86-79 high-intensity monochromator, EMI 9783-B photomultiplier, and 6.35 mm thick solid etalon. Absorption coefficients were estimated ( ~ 20%) measuring the absorbance of a few solutions of the dyes with a CARY 219 spectrophotometer.

3. Results and discussion

Laser emission was obtained in a number of compounds of the benzimidazole family following the mechanisms shown in fig. 1 where a picosecond proton shift takes place between acidic and basic centers placed in close vicinity on the same molecule. The emission originates in the proton-transferred tautomer, which is a different species from that initially excited. Therefore, the (observed) Stokes shift is very large ( ~ 9000 cm - ~) and the absorption of the laser 458

1 December 1987

H-O

~

H,

N) 0 i

f

I

R

H

H

~PUMp(308,337nm)

H-O

I

H

R

I~LASER

~

R

Fig. 1. R = H : OH-PBIM; R = O H : DI-PBIM; R=CH3: Me-BIM; R = O C H 3 : OMe-BIM; R = F : F-BIM.

light by the triplets of the initially excited compound negligeable. This probably contributes to the photochemical stability of the dye. In addition, the large fwhm of the tautomer fluorescence (60-80 nm) allows wide tunable ranges. Some representative absorption and fluorescence spectra are given in fig. 2. Table 1 summarizes the spectral data for the fluorophores studied. All the tuning curves are reproduced in fig. 3. As can be seen from table 1 and fig. 3, both the emission of the XeCI excimer laser at 308 nm and the emission of the N2 laser at 337 nm fall within the absorption band of the studied fluorophores. All of them can be pumped with either the XeC1 or the N2 laser but the energy conversion efficiency of compounds 1-5 is higher when pumped with the 308 nm radiation, whereas the efficiency of compounds 6-13 is higher when pumped at 337 nm. The energy conversion efficiencies quoted in table 1 are maximum values obtained using the single untuned oscillator cavity. These values would be lower when using the grazing-grating cavity, necessary to obtain narrow output linewidth as well as tuning. For comparison, a Coumarin 120/ethanol solution placed in the same untuned resonator gives lasing with a 12.5% efficiency. The peaks of the tuning curves can be shifted by the use of mixtures of the neighbouring fluorophores. In this way the tuning gap at 512 nm can be easily covered by the use of a mixture of Me-BIM/DIPBIM in dioxane (fig. 4a) or OH-PBIM/DI-PBIM in acetonitrile (fig. 4b). With the peak of the tuning

Volume 64, number 5

OPTICS COMMUNICATIONS

December 1987

(a) (b) 41/---,\

41-

/

] I

i [

F F

l

i

250

....... 300

1

/ I . . . .

350

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.... 500

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.

550

250

300

350

400 Mnm)

A(nm)

450

500

550

600

Fig. 2. Absorption (solid line) and corrected fluorescence spectra (dashed line) of: (a) 3 X 10 -4 M OH-PBIM in acetonitrile; (b) 3 × 10 -5 M Me-BIM in dioxane. Table 1 Spectroscopic and laser data for the investigated organic fluorophores. ~= extinction coefficient; c=concentration of the fluorophor solution; Eft= energy conversion efficiency Fluorophor

Solvent a)

Fluorophor absorption parameter -~max

Emax

6308

Laser data 6377

( nm )

C

Tuning

"~max

( mole/l )

range (nm)

(nm)

Effma. (%)

2.0X 10 -3 1.9X 10 3 2.1X l0 3 2.6X 10 -3 1.9X 10 -3

450-480 454-480 460-490 459-489 463-495 465-510 475-522 509-537 507-540 509-542 514-552 515-549 515-553

463 465 473 474 477 487 495 520 524 524 530 531 535

6.0 3.0 14.5 9.0 11.5 10.0 15.5 5.5 10.0 8.0 9.0 13.5 7.0

(1/mole cm) 1 2 3 4 5 6 7 8 9 10 11 12 13

OH-PBIM OH-PBIM OH-PBIM OH-PBIM OH-PBIM F-BIM Me-BIM OMe-BIM DI-PBIM DI-PBIM OMe-BIM DI-PBIM DI-PBIM

MeOH EtOH d an DMF an d ch/d an ch/d d d DMF

316 317 319 316 318 326 326 302/342 300 301 301/341 302 303

28000 27300 27200 22100 27500 24100 22800 20200 19600 34000 20200 34000 22000

21300 19000 17800 11300 16400 12800 10900 7200 5900 8000 7200 8000 9000

9200 11500 17200 14600 14300 22700 21200 19400 15100 19000 19400 19000 15000

7 . 0 X 10 - 4 1 . 2 X 10 - 3

1.0X 10 3 1 . 0 × 10 - 3

1.0× 1.4× 1.4X 1.5X

10 3 10 -3 10 .3 10 .3

a) DMF= Dimethylformamide; EtOH=ethanol; d=dioxane, MeOH=methanol; an=acetonitrile; ch = cyclohexane

c u r v e at 512 n m t h e e n e r g y c o n v e r s i o n efficiency was 11.5% for t h e M e - B I M / D I - P B I M m i x t u r e a n d 7% for the OH-PBIM/DI-PBIM mixture. W e n o t e in p a s s i n g t h a t n o t all t h e s o l v e n t s w h i c h are o f t e n u s e d i n d y e lasers a r e s u i t a b l e f o r I P T l a s e r o p e r a t i o n . I n t h i s case, n o n - h y d r o g e n b o n d i n g solv e n t s a r e b e s t s u i t e d b e c a u s e t h e y f a v o u r t h e form a t i o n o f t h e g r o u n d - s t a t e species h a v i n g a n i n t e r n a l H bond between the hydroxy and imino groups

( s c h e m e 1 ). W h e n t h i s i n t e r n a l l y b o n d e d s t r u c t u r e is e x c i t e d a p r o t o n - t r a n s f e r r e a c t i o n t a k e s place, d u e to the change (increase) in the basicity of the imino g r o u p [ 11 ]. F i n a l l y , it s h o u l d b e p o i n t e d o u t t h a t t h e s o l u t i o n s of the above compounds have shown a good photochemical stability at the pumping energies availa b l e to use: a f t e r s e v e r a l h u n d r e d s s h o t s t h e r e is n o noticeable photodegradation of the solutions. 459

Volume 64, number 5

OPTICS COMMUNICATIONS

i

1 December 1987

525 52OI 12 515 fi c 51(

O0

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1

(a) 495

i

210

~

~O

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470

490

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550

;',(nm)

Fig. 3. Laser tuning spectra of the investigated organic fluorophores. Areas are efficiency-normalized. 1-5: XeCI pumped; 6-13: N2 pumped.

~495 485

(b)

475 210

4. C o n c l u s i o n

We h a v e r e p o r t e d the efficient e m i s s i o n o f t u n a b l e s t i m u l a t e d light f r o m solutions o f a n u m b e r o f m e m bers o f the b e n z i m i d a z o l e f a m i l y p u m p e d w i t h XeC1 and N2 laser r a d i a t i o n . T h e e m i s s i o n w a v e l e n g t h s c o v e r a range o f m o r e t h a n 100 n m a n d the energy c o n v e r s i o n efficiencies can be as high as 15.5%. T h e w o r k i n g m e c h a n i s m o f these lasers is based on an excited-state i n t r a m o l e c u l a r p r o t o n transfer reaction.

Acknowledgements

We wish to t h a n k Dr. M.P. Lillo for h e r assistance. J . M . M . a n d A.D. t h a n k the C S I C for a Scholarship. T h i s w o r k was s u p p o r t e d by P r o j e c t s 84/145 ( C A I C Y T ) a n d 653/070 C S I C ) .

460

410

610

I

8'0 ~ 100 °/o OF DI-PBIM

Fig. 4. Shifting of the peak of the tuning curves in mixtures of neighbouring fluorophores. (a) Mixture of Me-BIM/DI-PBIM in dioxane. (b) Mixture of OH-PBIM/DI-PBIM in acetonitrile. References

[ 1] P. Chou, D. McMarrow, T.J. Aartsma and M. Kasha, J. Phys. Chem. 88 (1984) 4596. [2] A.U. Acufia, A. Costela and J.M. Mufioz, J. Phys. Chem. 90 (1986) 2807. [3] A.U. Acufia, F. Amat, J. Catalan, A. Costela, J.M. Figuera and J.M. Mufioz, Chem. Phys. Lett. 132 (1986) 567. [ 4 ] A. Weller, Progr. Reaction Kinetics 1 (1961 ) 189. [5] W. K16pffer, Adv. Photochem. 10 (1977) 311. [6] A.U. Acufia, J. Catalan and F. Toribio, J. Phys. Chem. 85 (1981) 241. [7] LEA. Ottersted, J. Chem. Phys. 58 (1973) 5716. [8] H. Shizuka, M. Machii, Y. Higaki, M. Tanaka and I. Tanaka, J. Phys. Chem. 89 (1985) 320. [9] I. Shoshan, N.N. Danon and U.P. Oppenheim, J. Appl. Phys. 48 (1977) 4495. [ 10] A.J. Kearsley, A.J. Andrews and C.E. Webb, Optics Comm. 31 (1979) 181. [ 11 ] H.K. Sinha and S,K. Dogra, Chem. Phys. 102 (1986) 336.