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Fluorescence and lasing properties of rhodamine dyes
352
Journal of Luminescence 48 & 49 (1991) 352 358 North-Holland
Fluorescence and lasing properties of rhodamine dyes Jutta Arden, Gerhard Deltau, Volker Huth, Ute Kringel, Dimitrios Peros and Karl H. Drexhage Fachbereich Chemie, Un,versitht Siegen, D-5900 Siegen, FRG
New rhodamine dyes with amino endgroups that are rigidized by 5- and 6-membered rings have been synthesized. The fluorescence quantum yield in ethanolic solution is, independent of temperature, close to unity. A derivative with amino groups incorporated in a single 6-membered ring lases more efficiently than rhodamine 6G. In an attempt to improve pumping by an excimer laser the rhodamine chromophore was connected via methylene bridges with 2,5 diphenyloxazole (PPO) which absorbs strongly at 308 nm. In these antenna dyes complete energy transfer from PPO to rhodamine is observed, yet lasing efficiency is influenced by some unexpected excited state interaction between the chromophores. We studied with other multichromophoric dyes also the influence of nitrobenzene and anthracene moieties on the fluorescence efficiency of the rhodamine chromophore. Linkages consisting of two methylene groups permit a stronger interaction between chromophores than those consisting of a single methylene group.
1. Introduction Rhodamine 6G is a widely used and very efficient laser dye. Its absorption spectrum is determined by a symmetrical IT-electron system extending over a diaminoxanthene frame (fig. 1). The carboxyphenyl group is introduced by the common synthesis from phthalic anhydride and an maminophenol. It has but a small influence on the absorption and fluorescence of the dye. If the carboxyl group is not esterified, absorption and fluorescence of protonated form and ethyl ester agree within 2 nm. On deprotonation a hypsochromic shift (about 10 nm in ethanolic solution) occurs [1]. Such dyes can be considered as derivatives of benzoic acid. Since this is a weak acid, the addition of only a small amount of a strong acid
0
~OC
2H5
C~0 H3C
~
.~,
H
0
...
—.
H5C2
CH3 N~ C2H5
Cl Fig. 1. Molecular structure of rhodamine 6G. 0022-2313/91/S03.50
©
1991
or base ensures either protonation or deprotonation of the dye. Dilution of a neutral solution also increases the ratio of unprotonated to protonated form and thereby causes a shift of absorption and fluorescence to shorter wavelengths. In some earlier studies this shift has been interpreted erroneously as being due to a monomer-dimer equilibrium. The esterified dyes are salts that are dissociated completely in ethanolic solution. Therefore, absorption and fluorescence are independent of the nature of the anion up to a concentration of about 10 ~mol/1. At higher concentrations certain anions interact with excited dye molecules and reduce the fluorescence efficiency [1]. Rhodamines are, owing to their structural rigidity, highly fluorescent dyes. Yet their fluorescence efficiency shows a peculiar dependence on the substitution pattern of the amino groups. If these are only partially substituted by alkyl groups, the fluorescence efficiency is close to unity and nearly independent of solvent polarity and temperature; e.g., the fluorescence quantum yield of rhodamine 6G dissolved in ethanol has the value 0.95 [1,2]. On the other hand, if the amino groups are fully alkylated, e.g., by ethyl groups, the fluorescence efficiency is strongly dependent on solvent and
Elsevier Science Publishers B.V. (North-Holland)
J. Arden et al.
/ Fluorescence and
lasing properties of rhodamine dyes
353
~ ~>
~0)4(c~
LNL~rLO)~5)
b
a
Fig. 2. Structures of (a) rhodamine B and (b) rhodamine 101.
temperature. This has been attributed to some kind of torsion or rotation of the amino groups occurring in the excited state. If the mobility is prevented by incorporation of the amino groups in 6membered rings, it was found that the fluorescence is independent of the molecular environment [1,3]. Whereas the fluorescence quantum yield of rhodamine B (acidified ethanol, 20°C) has the value of 0.50, it is 0.96 in the rigidized rhodamine 101 (fig. 2) [2]. For efficient dye laser operation, a high value of fluorescence quantum yield is generally useful, However, absorption by dye molecules in the lowest excited singlet and triplet state may cause a severe loss of pump and dye laser radiation, Therefore efficient laser dyes are only those cornpounds in which the excited state absorption coefficients are small in the pertinent wavelength regions. The loss due to T-T absorption also depends on the S-T intersystem crossing rate and the triplet decay rate. In rhodamines these quantities, taken together, have values very favorable for dye laser operation. Rhodamine 6G (fig. 1) appears to be the most efficient laser dye, at least in the orange-red region of the spectrum [4,5]. It is interesting, however, that rhodamine 101, in spite of its high fluorescence efficiency, is markedly less efficient in most pumping configurations [4-6]. These differences are not yet understood. Because the pertinent quantities have not yet been determined with sufficient accuracy, it is also not known so far whether the excited state properties of rhodamine 60 are such that resonator loss caused by them is negligible. Therefore we have set out to synthesize rhodamines with new types of amino group rigidization and report here their fluorescence and lasing properties.
Because the emission of dyes overlaps the low energy tail of the absorption band, dye laser radiation is partly reabsorbed by the dye in its ground state. This loss is especially high, if high dye concentration is required in order to absorb the pump light: a frequent situation in pumping by an excimer laser at 308 nm. It has been suggested to connect the laser dye with one or more chromophores that absorb strongly at the pump wavelength [7]. The energy absorbed by these antennas would then be transferred efficiently to the laser chromophore. In effect, one would increase the absorption coefficient of the laser dye at the wavelength of the pump radiation. We have applied this concept to the rhodarnine class of dyes and report on several multichromophoric rhodamines.
2. Experimental The structures of the new rhodamine dyes are shown in figs. 3 and 4. The synthesis of the PPOintermediate for the antenna dyes has been described earlier [8]. Further details of dye preparation will be published elsewhere. Unless stated otherwise the measurements were done using air-equilibrated absolute ethanol; however, no precautions were taken to keep out moisture. Solutions of dyes with unesterified carboxyl groups were acidified with a small amount of perchioric or trifluoroacetic acid, until no further change in the main absorption band occurred. Fluorescence quantum yields were determined at exactly 20°C;laser measurements were performed at “room temperature”.
J. Arden et a!. / Fluorescence and lasing properties of rhodamine dyes
354
E~IL,c,.0C2H5 N
~L~~L0 ~
H5C2-.....
el dO4 C2H5
C2H5
N~~r 0_’l~ ‘Nd’ C2H5 e dO4
b
a
Fig. 3. Structures of new dyes with rigidized amino endgroups: (a) rhodamine 630, (b) rhodamine 650.
~L~~~0C2H5
____________
Cl-~
(t!~H2)~
(~H2)~
ClO?
—
e i
-i--
—O--N02
fl~fl
‘2 Fig. 4. Molecular structures of trichromophoric dyes: (a) PPO-rhod. 1, (b) PPO-rhod. 2, (c) NP-rhod. 1, (d) NP-rhod. 2, (e) AN-rhod. 1, (f) AN-rhod. 2.
The fluorescence quantum yield of rhodamine 60, B and 101 was determined by the thermal blooming method, which is especially accurate at high yields [2,9]. The other values, reported here, were obtained by relative measurements in a spectrofluorimeter (Fluorolog 2, Spex) using the above dyes as basis. Excitation and detection took place at an angle of nearly 45°with respect to the normal of the front window of a 1 mm cell. In order to avoid the influence of fluorescence polarization, which may cause considerable errors, both exciting and emitted light were polarized in the plane of incidence by polarizing filters appropriately oriented [10]. Excitation took place at a wavelength closetothemaximumofthemainabsorptionband. Dye concentrations were 10 ~mol/l or smaller. Because the spectra of the dye solutions used for comparison are very similar, errors due to reabsorption of fluorescence are negligible, Dye laser operation was investigated in three different configurations. For excimer laser pump-
ing, the dye laser cavity (FL 105, Lambda Physik) consisted of a holographic grating with 2440 lines/mm for tuning and a quartz plate as output coupler. The dye solution, circulated through a 2 cm long quartz cell, was pumped transversely at 308 nm by pulses of 10 ns duration from a XeCl laser (EMG 500, Lambda Physik). The dye concentrations were optimized for maximum output. This was the case at an absorbance of 1.2/mm at 308 nm. Flashlamp pumping was performed using a system with two linear xenon flashlamps within an elliptical reflector, which produced pulses of 1 ~isrisetime (FL 3000, Lambda Physik). The dye solution was circulated through a quartz cell of 17cm length and 4mm inner diameter. The laser resonator was formed by flat broad band mirrors of 99% and 33% reflectivity. Optimum dye concentration was 3 x 10 ~mol/l in all cases. For continuous operation, a jet stream dye laser (CR 599, Coherent) was pumped by a Kr-ion laser (CR 3000K, Coherent). Pumping wavelength was
.1. Arden et al.
530 nm in case of rhodamine 60 case of rhodamine 630. The dye were optimized resulting in an 1.1/0.1mm at 530nm (rhod. 60)
/ Fluorescence and
and 568 nm in concentrations absorbance of and at 568nm
lasing properties of rhodamine dyes 10
manufacturer for rhod. 60, had a transmission of glycol. (rhod. 630), The output resp.; solvent coupler, in this recommended laser was ethylene by the 4% at 588 nm and of 2% at 639 nm.
355
Rh~d 60
~ 580
600
620
640
660
Wovelength, nm
Fig. 5. Tuning curves of excimer pumped dye laser with
3. Results
rhodamine 630 and rhodamines formerly known.
The long-wavelength transition in rhodamine dyes is clearly affected by rigidization of the amino endgroups, as is well known for rhodamine 101 [1,3]. Whereas the new rhodamine 630 shows only a small bathochromic shift compared to rhod. B, this is surprisingly large in rhod. 650, which absorbs with maximum at 594 nm (table 1). The shape of the main band of both dyes closely resembles that of rhod. 60 [1]. The fluorescence spectra also show a Stokes-shift very similar to that of other rhodamines (table 1). The fluorescence quantum efficiency of both dyes is high, independent of temperature. However, in the derivative with 5-membered ring rigidization (rhod. 650) it has a value of only 0.80, indicating an increased rate of nonradiative deactivation of the first excited singlet state.
Both new rhodamines are very efficient laser dyes. We will concentrate here on the lasing properties of rhodamine 630, which we have studied in several dye laser configurations. On pumping with 10 mJ pulses of an excimer laser (wavelength 308 nm), the energy-efficiency has its maximum at 618 nm and is slightly lower than that of rhod, 60 (maximum value at 585 nm). In terms of emitted photons both dyes are, within experimental error, equally efficient, The tuning range of rhod. 630 extends from 605 to 665 nm; between 610 and 650 nm rhod. 630 supersedes rhod. B and rhod. 101 in efficiency (fig. 5). On flashlamp pumping dye laser operation is quasi-continuous. In such a configuration rhod. 630 shows lower threshold and higher slope efficiency than rhod. 60 and rhod. 101 (fig. 6). Lasing wavelength, untuned, is 640 nm,
Table I Main absorption maximum Aa
5~,fluorescence maximum A5 and ethanol (acidified in case of rhod. B. and 101), temperature
20°C. Dye Aab~[nm] fluorescence quantum yield ~ of rhod. 6G 530 rhod. B 554 rhod. 101 574 rhod. 630 563 rhod. 650 594 PPO-rhod. 1 562 PPO-rhod. 2 572 NP-rhod. 1 557 NP-rhod. 2 567 AN-rhod. 1 565 AN-rhod. 2 570
A 7J~ rhodamine 5 [nm] dyes; solvent 553 0.95 576 0.50 595 0.96 585 0.95 616 0.80 584 0.94 600 0.75 580 0.90 588 0.29 586 0.22 586 0.07
1,Onrn Rhod. 630 6
—,
E ci,
~3 a,
100 80
600 nm Rhod 101
60
656 nm Rhod 60
40 20
Rhod B ~__~—
0
20
40
520 nm
60 80 Input Energy,
100
Fig. 6. Flashlamp pumped dye laser with rhodamine 630 and well-known derivatives for comparison. Output vs. electrical input. Untuned lasing wavelengths as indicated.
356
1 Arden et a!.
/ F!uorescence
and !asing properties of rhodamine dyes
400
10
300
~,
I CH
R~riS~O
2
‘:: 0
PPO Rhod 1
PPO-Rhod 2
(-CH2-CH2-)
______
200
400 600 Input, mW
800
1200~
Fig. 7. Continuous dye laser with rhodamine 630 and rhodamine 6G. Output vs. input power. Solvent: ethylene glycol. Untuned lasing wavelengths as indicated.
In a continuous dye laser, pumped by a Kr-ion laser, rhod, 630 is again more efficient than rhod. 6G (fig. 7). The low threshold of 150 mW may be explained, at least in part, by the lower transmission (higher reflectivity) of the output coupler at 639 nm, the lasing wavelength of rhod. 630. However, the markedly higher slope efficiency can only be attributed to superior dye properties of rhod. 630 compared to rhod. 6G. The fact that rhod. 630 shows higher efficiency in continuous dye lasers points to a reduced triplet loss in this dye. In addition to excited state absorption the lasing efficiency of a dye may be affected by ground state absorption of emitted radiation. At a pumping wavelength of 308 nm (excimer laser) the absorption coefficient of rhodamines is rather low [1,8]. Therefore, concentrations of about 10 mol/l are needed in order to absorb 90% of the pump light in a depth of 1 mm, Such a high concentration in turn causes a high absorbance in the longwavelength tail ofthe main absorption band, where the stimulated emission occurs. We have prepared dyes, in which two 2,5-diphenyloxazole (PPO) moieties are linked via methylene bridges to a rhodamine chromophore (fig. 4). The absorption coefficient PPO has The the newvalue 2.8 x 4 1 mol cmof at 308 nm. trichromoi0 phoric rhodamines have absorption spectra that agree to a first approximation with the superposition of the spectra of one rhodamine and two PPO molecules, and the absorption at 308 nm is quite strong now (fig. 8) [6]. If one applies Förster’s
Wavelength, rim Fig. 8. Absorption spectra of antenna dyes PPO-rhod. 1 and PPO rhod. 2 in ethanol.
theory to such a system of donors and acceptors, a very efficient energy transfer from the PPO antennas to the rhodamine is expected [8]. This is borne out by our experiments: the fluorescence quantum yield of PPO-rhod. 1 has a value 0.94, on excitation either at 308 or at 562 nm. At the same time the blue fluorescence of the PPO groups is quenched. Complete energy transfer from PPO to rhodamine is also found in PPO-rhod. 2, in which the chromophores are separated by two CH2 groups. However, the fluorescence quantum efficiency of this dye is markedly lower; it has only a value of 0.75 at 20°C.The absorption spectrum also deviates from that of PPO-rhod. 1 (fig. 8). We attribute both effects to a change of conformation: the PPOsubstituents interact more strongly with the rhodamine system because of the higher flexibility of a CH2 CH2 link (fig. 9). We have studied such intramolecular interactions also in related dyes, in which nitrobenzene or anthracene is connected with the rhodamine system (fig. 4). On excitation in the UV region no energy transfer takes place from nitrobenzene to rhodamine, probably due to the very fast nonradiative relaxation in this compound. Whereas the fluorescence quantum yield is very high in NP-rhod. 1, it is strongly reduced in the related dye to with a CH2-CH2 link in(table 1). Westate attribute this an electron transfer the excited from rhodamine to nitrobenzene. This should depend strongly on the distance between donor and acceptor and therefore is inefficient in NP-rhod. 1, if we assume the same conformations as in the PPOrhodamines (fig. 9). In the anthracene derivatives
.1. Arden et a!.
/ Fluorescence and /RhOd.
g
CH
2
__________
I
I
H2C
H2C
I
357
lasing properties of rhodamine dyes
______
c’ H2
a
H2
b
Fig. 9. Conformation of trichromophoric dyes in solution; (a) PPO rhod. 1, (b) PPO-rhod 2.
(fig. 4) energy transfer from anthracene to rhodamine is very efficient, if the excitation is in the region of the anthracene absorption. The fluorescence is already reduced in AN-rhod. 1, and again even more so in the derivative with the flexible CH2 CH2 link. The quenching of fluorescence due to anthracene is not yet well understood. We ascribe it tentatively to energy transfer from rhodamine to the triplet state of anthracene. We have investigated the lasing properties of the antenna dyes PPO-rhod. I and PPO-rhod. 2 in our excimer pumped dye laser (fig. 10). The dye PPO-rhod. 1 shows, compared to rhod. 630, increased efficiency in the short-wavelength region. This had been expected, because its lower concentration leads to a reduction of the reabsorption in this wavelength region. The increased efficiency of PPO-rhod. 2 in the long-wavelength region is due to the shifted fluorescence spectrum [6]. However, in the wavelength range 6 10-640 nm the
C 8~
0 Ui
4. Conclusion We have synthesized new rhodamine dyes with rigidized amino endgroups. They show a fluorescence efficiency close to unity, independent of temperature. The new dye rhodamine 630 surpasses rhod. 6G in laser efficiency, in particular in flashlamp-pumped and continuous dye lasers, In order to reduce resonator loss by ground state absorption, we have also synthesized trichromophoric dyes with UV-ahsorhing antennas. Energy transferfromantennatorhodamineisveryefficient as expected. However, the fluorescence quantum yield can be reduced severely by electron or energy transfer to the antenna, Furthermore a new type of excited state interaction between the chromophores reduces considerably the lasing efficiency.
Rhod 630
PPO-Rhod 1
Ui
efficiency of the antenna dyes is much lower than that of rhod. 630. This is evidence for some excited state interaction which causes an absorption and therefore additional resonator loss during the lasing process.
hod 2
04I
0L.
580
Acknowledgements ____
500
520
640
660
Wavelength. nm
Fig. 10. Tuning curves of excimer pumped dye laser with antenna dyes; rhodamine 630 for comparison (see fig. 5).
We thank Ms. Birgit Runde for part of the fluorescence data used in this work. We gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft, by the Minister für
358
1 Arden et a!.
/
Fluorescence and losing properties of rhodamine dyes
Wissenschaft und Forschung des Landes Nordrhein-Westfalen and by the Fonds der Chemischen Industrie
References [1] K.H. Drexhage, in: Topics in Applied Physics, Vol. 1, ed. F.P. Schafer (Springer, Berlin, 1973) ch. 4. [2] R. Sens, Ph.D. Thesis, Siegen (1984). [3] K.H. Drexhage, Laser Focus 9 (1973) 35.
[4] Laser dye catalogue, Exciton Chemical Company (Dayton, Ohio, 1983). [5] U. Brackmann, Lambdachrome Laser Dyes (Lambda
Physik, Göttingen, FRG, 1986). [6] G. Deltau, U. Kringel, D. Peros, B. Runde and K.H. Drexhage, in: Recent Developments in Molecular Spectroscopy, eds. B. Jordanov, N. Kirov and P. Simova (World Scientific, Singapore, 1989) p. 539. [7] F.P. Schafer, in: Tunable Lasers and Applications, eds. A. Mooradian, T. Jaeger and P. Stockseth (Springer, Berlin, 1976) p. 56. [8] V. Huth, Ph.D. Thesis, Siegen (1986). [9] J.H. Brannon and D. Magde, J. Phys. Chem. 82(1978) 705. [10] U. Kringel, Ph.D. Thesis, Siegen (1088).
Journal of Luminescence 48 & 49 (1991) 352 358 North-Holland
Fluorescence and lasing properties of rhodamine dyes Jutta Arden, Gerhard Deltau, Volker Huth, Ute Kringel, Dimitrios Peros and Karl H. Drexhage Fachbereich Chemie, Un,versitht Siegen, D-5900 Siegen, FRG
New rhodamine dyes with amino endgroups that are rigidized by 5- and 6-membered rings have been synthesized. The fluorescence quantum yield in ethanolic solution is, independent of temperature, close to unity. A derivative with amino groups incorporated in a single 6-membered ring lases more efficiently than rhodamine 6G. In an attempt to improve pumping by an excimer laser the rhodamine chromophore was connected via methylene bridges with 2,5 diphenyloxazole (PPO) which absorbs strongly at 308 nm. In these antenna dyes complete energy transfer from PPO to rhodamine is observed, yet lasing efficiency is influenced by some unexpected excited state interaction between the chromophores. We studied with other multichromophoric dyes also the influence of nitrobenzene and anthracene moieties on the fluorescence efficiency of the rhodamine chromophore. Linkages consisting of two methylene groups permit a stronger interaction between chromophores than those consisting of a single methylene group.
1. Introduction Rhodamine 6G is a widely used and very efficient laser dye. Its absorption spectrum is determined by a symmetrical IT-electron system extending over a diaminoxanthene frame (fig. 1). The carboxyphenyl group is introduced by the common synthesis from phthalic anhydride and an maminophenol. It has but a small influence on the absorption and fluorescence of the dye. If the carboxyl group is not esterified, absorption and fluorescence of protonated form and ethyl ester agree within 2 nm. On deprotonation a hypsochromic shift (about 10 nm in ethanolic solution) occurs [1]. Such dyes can be considered as derivatives of benzoic acid. Since this is a weak acid, the addition of only a small amount of a strong acid
0
~OC
2H5
C~0 H3C
~
.~,
H
0
...
—.
H5C2
CH3 N~ C2H5
Cl Fig. 1. Molecular structure of rhodamine 6G. 0022-2313/91/S03.50
©
1991
or base ensures either protonation or deprotonation of the dye. Dilution of a neutral solution also increases the ratio of unprotonated to protonated form and thereby causes a shift of absorption and fluorescence to shorter wavelengths. In some earlier studies this shift has been interpreted erroneously as being due to a monomer-dimer equilibrium. The esterified dyes are salts that are dissociated completely in ethanolic solution. Therefore, absorption and fluorescence are independent of the nature of the anion up to a concentration of about 10 ~mol/1. At higher concentrations certain anions interact with excited dye molecules and reduce the fluorescence efficiency [1]. Rhodamines are, owing to their structural rigidity, highly fluorescent dyes. Yet their fluorescence efficiency shows a peculiar dependence on the substitution pattern of the amino groups. If these are only partially substituted by alkyl groups, the fluorescence efficiency is close to unity and nearly independent of solvent polarity and temperature; e.g., the fluorescence quantum yield of rhodamine 6G dissolved in ethanol has the value 0.95 [1,2]. On the other hand, if the amino groups are fully alkylated, e.g., by ethyl groups, the fluorescence efficiency is strongly dependent on solvent and
Elsevier Science Publishers B.V. (North-Holland)
J. Arden et al.
/ Fluorescence and
lasing properties of rhodamine dyes
353
~ ~>
~0)4(c~
LNL~rLO)~5)
b
a
Fig. 2. Structures of (a) rhodamine B and (b) rhodamine 101.
temperature. This has been attributed to some kind of torsion or rotation of the amino groups occurring in the excited state. If the mobility is prevented by incorporation of the amino groups in 6membered rings, it was found that the fluorescence is independent of the molecular environment [1,3]. Whereas the fluorescence quantum yield of rhodamine B (acidified ethanol, 20°C) has the value of 0.50, it is 0.96 in the rigidized rhodamine 101 (fig. 2) [2]. For efficient dye laser operation, a high value of fluorescence quantum yield is generally useful, However, absorption by dye molecules in the lowest excited singlet and triplet state may cause a severe loss of pump and dye laser radiation, Therefore efficient laser dyes are only those cornpounds in which the excited state absorption coefficients are small in the pertinent wavelength regions. The loss due to T-T absorption also depends on the S-T intersystem crossing rate and the triplet decay rate. In rhodamines these quantities, taken together, have values very favorable for dye laser operation. Rhodamine 6G (fig. 1) appears to be the most efficient laser dye, at least in the orange-red region of the spectrum [4,5]. It is interesting, however, that rhodamine 101, in spite of its high fluorescence efficiency, is markedly less efficient in most pumping configurations [4-6]. These differences are not yet understood. Because the pertinent quantities have not yet been determined with sufficient accuracy, it is also not known so far whether the excited state properties of rhodamine 60 are such that resonator loss caused by them is negligible. Therefore we have set out to synthesize rhodamines with new types of amino group rigidization and report here their fluorescence and lasing properties.
Because the emission of dyes overlaps the low energy tail of the absorption band, dye laser radiation is partly reabsorbed by the dye in its ground state. This loss is especially high, if high dye concentration is required in order to absorb the pump light: a frequent situation in pumping by an excimer laser at 308 nm. It has been suggested to connect the laser dye with one or more chromophores that absorb strongly at the pump wavelength [7]. The energy absorbed by these antennas would then be transferred efficiently to the laser chromophore. In effect, one would increase the absorption coefficient of the laser dye at the wavelength of the pump radiation. We have applied this concept to the rhodarnine class of dyes and report on several multichromophoric rhodamines.
2. Experimental The structures of the new rhodamine dyes are shown in figs. 3 and 4. The synthesis of the PPOintermediate for the antenna dyes has been described earlier [8]. Further details of dye preparation will be published elsewhere. Unless stated otherwise the measurements were done using air-equilibrated absolute ethanol; however, no precautions were taken to keep out moisture. Solutions of dyes with unesterified carboxyl groups were acidified with a small amount of perchioric or trifluoroacetic acid, until no further change in the main absorption band occurred. Fluorescence quantum yields were determined at exactly 20°C;laser measurements were performed at “room temperature”.
J. Arden et a!. / Fluorescence and lasing properties of rhodamine dyes
354
E~IL,c,.0C2H5 N
~L~~L0 ~
H5C2-.....
el dO4 C2H5
C2H5
N~~r 0_’l~ ‘Nd’ C2H5 e dO4
b
a
Fig. 3. Structures of new dyes with rigidized amino endgroups: (a) rhodamine 630, (b) rhodamine 650.
~L~~~0C2H5
____________
Cl-~
(t!~H2)~
(~H2)~
ClO?
—
e i
-i--
—O--N02
fl~fl
‘2 Fig. 4. Molecular structures of trichromophoric dyes: (a) PPO-rhod. 1, (b) PPO-rhod. 2, (c) NP-rhod. 1, (d) NP-rhod. 2, (e) AN-rhod. 1, (f) AN-rhod. 2.
The fluorescence quantum yield of rhodamine 60, B and 101 was determined by the thermal blooming method, which is especially accurate at high yields [2,9]. The other values, reported here, were obtained by relative measurements in a spectrofluorimeter (Fluorolog 2, Spex) using the above dyes as basis. Excitation and detection took place at an angle of nearly 45°with respect to the normal of the front window of a 1 mm cell. In order to avoid the influence of fluorescence polarization, which may cause considerable errors, both exciting and emitted light were polarized in the plane of incidence by polarizing filters appropriately oriented [10]. Excitation took place at a wavelength closetothemaximumofthemainabsorptionband. Dye concentrations were 10 ~mol/l or smaller. Because the spectra of the dye solutions used for comparison are very similar, errors due to reabsorption of fluorescence are negligible, Dye laser operation was investigated in three different configurations. For excimer laser pump-
ing, the dye laser cavity (FL 105, Lambda Physik) consisted of a holographic grating with 2440 lines/mm for tuning and a quartz plate as output coupler. The dye solution, circulated through a 2 cm long quartz cell, was pumped transversely at 308 nm by pulses of 10 ns duration from a XeCl laser (EMG 500, Lambda Physik). The dye concentrations were optimized for maximum output. This was the case at an absorbance of 1.2/mm at 308 nm. Flashlamp pumping was performed using a system with two linear xenon flashlamps within an elliptical reflector, which produced pulses of 1 ~isrisetime (FL 3000, Lambda Physik). The dye solution was circulated through a quartz cell of 17cm length and 4mm inner diameter. The laser resonator was formed by flat broad band mirrors of 99% and 33% reflectivity. Optimum dye concentration was 3 x 10 ~mol/l in all cases. For continuous operation, a jet stream dye laser (CR 599, Coherent) was pumped by a Kr-ion laser (CR 3000K, Coherent). Pumping wavelength was
.1. Arden et al.
530 nm in case of rhodamine 60 case of rhodamine 630. The dye were optimized resulting in an 1.1/0.1mm at 530nm (rhod. 60)
/ Fluorescence and
and 568 nm in concentrations absorbance of and at 568nm
lasing properties of rhodamine dyes 10
manufacturer for rhod. 60, had a transmission of glycol. (rhod. 630), The output resp.; solvent coupler, in this recommended laser was ethylene by the 4% at 588 nm and of 2% at 639 nm.
355
Rh~d 60
~ 580
600
620
640
660
Wovelength, nm
Fig. 5. Tuning curves of excimer pumped dye laser with
3. Results
rhodamine 630 and rhodamines formerly known.
The long-wavelength transition in rhodamine dyes is clearly affected by rigidization of the amino endgroups, as is well known for rhodamine 101 [1,3]. Whereas the new rhodamine 630 shows only a small bathochromic shift compared to rhod. B, this is surprisingly large in rhod. 650, which absorbs with maximum at 594 nm (table 1). The shape of the main band of both dyes closely resembles that of rhod. 60 [1]. The fluorescence spectra also show a Stokes-shift very similar to that of other rhodamines (table 1). The fluorescence quantum efficiency of both dyes is high, independent of temperature. However, in the derivative with 5-membered ring rigidization (rhod. 650) it has a value of only 0.80, indicating an increased rate of nonradiative deactivation of the first excited singlet state.
Both new rhodamines are very efficient laser dyes. We will concentrate here on the lasing properties of rhodamine 630, which we have studied in several dye laser configurations. On pumping with 10 mJ pulses of an excimer laser (wavelength 308 nm), the energy-efficiency has its maximum at 618 nm and is slightly lower than that of rhod, 60 (maximum value at 585 nm). In terms of emitted photons both dyes are, within experimental error, equally efficient, The tuning range of rhod. 630 extends from 605 to 665 nm; between 610 and 650 nm rhod. 630 supersedes rhod. B and rhod. 101 in efficiency (fig. 5). On flashlamp pumping dye laser operation is quasi-continuous. In such a configuration rhod. 630 shows lower threshold and higher slope efficiency than rhod. 60 and rhod. 101 (fig. 6). Lasing wavelength, untuned, is 640 nm,
Table I Main absorption maximum Aa
5~,fluorescence maximum A5 and ethanol (acidified in case of rhod. B. and 101), temperature
20°C. Dye Aab~[nm] fluorescence quantum yield ~ of rhod. 6G 530 rhod. B 554 rhod. 101 574 rhod. 630 563 rhod. 650 594 PPO-rhod. 1 562 PPO-rhod. 2 572 NP-rhod. 1 557 NP-rhod. 2 567 AN-rhod. 1 565 AN-rhod. 2 570
A 7J~ rhodamine 5 [nm] dyes; solvent 553 0.95 576 0.50 595 0.96 585 0.95 616 0.80 584 0.94 600 0.75 580 0.90 588 0.29 586 0.22 586 0.07
1,Onrn Rhod. 630 6
—,
E ci,
~3 a,
100 80
600 nm Rhod 101
60
656 nm Rhod 60
40 20
Rhod B ~__~—
0
20
40
520 nm
60 80 Input Energy,
100
Fig. 6. Flashlamp pumped dye laser with rhodamine 630 and well-known derivatives for comparison. Output vs. electrical input. Untuned lasing wavelengths as indicated.
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/ F!uorescence
and !asing properties of rhodamine dyes
400
10
300
~,
I CH
R~riS~O
2
‘:: 0
PPO Rhod 1
PPO-Rhod 2
(-CH2-CH2-)
______
200
400 600 Input, mW
800
1200~
Fig. 7. Continuous dye laser with rhodamine 630 and rhodamine 6G. Output vs. input power. Solvent: ethylene glycol. Untuned lasing wavelengths as indicated.
In a continuous dye laser, pumped by a Kr-ion laser, rhod, 630 is again more efficient than rhod. 6G (fig. 7). The low threshold of 150 mW may be explained, at least in part, by the lower transmission (higher reflectivity) of the output coupler at 639 nm, the lasing wavelength of rhod. 630. However, the markedly higher slope efficiency can only be attributed to superior dye properties of rhod. 630 compared to rhod. 6G. The fact that rhod. 630 shows higher efficiency in continuous dye lasers points to a reduced triplet loss in this dye. In addition to excited state absorption the lasing efficiency of a dye may be affected by ground state absorption of emitted radiation. At a pumping wavelength of 308 nm (excimer laser) the absorption coefficient of rhodamines is rather low [1,8]. Therefore, concentrations of about 10 mol/l are needed in order to absorb 90% of the pump light in a depth of 1 mm, Such a high concentration in turn causes a high absorbance in the longwavelength tail ofthe main absorption band, where the stimulated emission occurs. We have prepared dyes, in which two 2,5-diphenyloxazole (PPO) moieties are linked via methylene bridges to a rhodamine chromophore (fig. 4). The absorption coefficient PPO has The the newvalue 2.8 x 4 1 mol cmof at 308 nm. trichromoi0 phoric rhodamines have absorption spectra that agree to a first approximation with the superposition of the spectra of one rhodamine and two PPO molecules, and the absorption at 308 nm is quite strong now (fig. 8) [6]. If one applies Förster’s
Wavelength, rim Fig. 8. Absorption spectra of antenna dyes PPO-rhod. 1 and PPO rhod. 2 in ethanol.
theory to such a system of donors and acceptors, a very efficient energy transfer from the PPO antennas to the rhodamine is expected [8]. This is borne out by our experiments: the fluorescence quantum yield of PPO-rhod. 1 has a value 0.94, on excitation either at 308 or at 562 nm. At the same time the blue fluorescence of the PPO groups is quenched. Complete energy transfer from PPO to rhodamine is also found in PPO-rhod. 2, in which the chromophores are separated by two CH2 groups. However, the fluorescence quantum efficiency of this dye is markedly lower; it has only a value of 0.75 at 20°C.The absorption spectrum also deviates from that of PPO-rhod. 1 (fig. 8). We attribute both effects to a change of conformation: the PPOsubstituents interact more strongly with the rhodamine system because of the higher flexibility of a CH2 CH2 link (fig. 9). We have studied such intramolecular interactions also in related dyes, in which nitrobenzene or anthracene is connected with the rhodamine system (fig. 4). On excitation in the UV region no energy transfer takes place from nitrobenzene to rhodamine, probably due to the very fast nonradiative relaxation in this compound. Whereas the fluorescence quantum yield is very high in NP-rhod. 1, it is strongly reduced in the related dye to with a CH2-CH2 link in(table 1). Westate attribute this an electron transfer the excited from rhodamine to nitrobenzene. This should depend strongly on the distance between donor and acceptor and therefore is inefficient in NP-rhod. 1, if we assume the same conformations as in the PPOrhodamines (fig. 9). In the anthracene derivatives
.1. Arden et a!.
/ Fluorescence and /RhOd.
g
CH
2
__________
I
I
H2C
H2C
I
357
lasing properties of rhodamine dyes
______
c’ H2
a
H2
b
Fig. 9. Conformation of trichromophoric dyes in solution; (a) PPO rhod. 1, (b) PPO-rhod 2.
(fig. 4) energy transfer from anthracene to rhodamine is very efficient, if the excitation is in the region of the anthracene absorption. The fluorescence is already reduced in AN-rhod. 1, and again even more so in the derivative with the flexible CH2 CH2 link. The quenching of fluorescence due to anthracene is not yet well understood. We ascribe it tentatively to energy transfer from rhodamine to the triplet state of anthracene. We have investigated the lasing properties of the antenna dyes PPO-rhod. I and PPO-rhod. 2 in our excimer pumped dye laser (fig. 10). The dye PPO-rhod. 1 shows, compared to rhod. 630, increased efficiency in the short-wavelength region. This had been expected, because its lower concentration leads to a reduction of the reabsorption in this wavelength region. The increased efficiency of PPO-rhod. 2 in the long-wavelength region is due to the shifted fluorescence spectrum [6]. However, in the wavelength range 6 10-640 nm the
C 8~
0 Ui
4. Conclusion We have synthesized new rhodamine dyes with rigidized amino endgroups. They show a fluorescence efficiency close to unity, independent of temperature. The new dye rhodamine 630 surpasses rhod. 6G in laser efficiency, in particular in flashlamp-pumped and continuous dye lasers, In order to reduce resonator loss by ground state absorption, we have also synthesized trichromophoric dyes with UV-ahsorhing antennas. Energy transferfromantennatorhodamineisveryefficient as expected. However, the fluorescence quantum yield can be reduced severely by electron or energy transfer to the antenna, Furthermore a new type of excited state interaction between the chromophores reduces considerably the lasing efficiency.
Rhod 630
PPO-Rhod 1
Ui
efficiency of the antenna dyes is much lower than that of rhod. 630. This is evidence for some excited state interaction which causes an absorption and therefore additional resonator loss during the lasing process.
hod 2
04I
0L.
580
Acknowledgements ____
500
520
640
660
Wavelength. nm
Fig. 10. Tuning curves of excimer pumped dye laser with antenna dyes; rhodamine 630 for comparison (see fig. 5).
We thank Ms. Birgit Runde for part of the fluorescence data used in this work. We gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft, by the Minister für
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/
Fluorescence and losing properties of rhodamine dyes
Wissenschaft und Forschung des Landes Nordrhein-Westfalen and by the Fonds der Chemischen Industrie
References [1] K.H. Drexhage, in: Topics in Applied Physics, Vol. 1, ed. F.P. Schafer (Springer, Berlin, 1973) ch. 4. [2] R. Sens, Ph.D. Thesis, Siegen (1984). [3] K.H. Drexhage, Laser Focus 9 (1973) 35.
[4] Laser dye catalogue, Exciton Chemical Company (Dayton, Ohio, 1983). [5] U. Brackmann, Lambdachrome Laser Dyes (Lambda
Physik, Göttingen, FRG, 1986). [6] G. Deltau, U. Kringel, D. Peros, B. Runde and K.H. Drexhage, in: Recent Developments in Molecular Spectroscopy, eds. B. Jordanov, N. Kirov and P. Simova (World Scientific, Singapore, 1989) p. 539. [7] F.P. Schafer, in: Tunable Lasers and Applications, eds. A. Mooradian, T. Jaeger and P. Stockseth (Springer, Berlin, 1976) p. 56. [8] V. Huth, Ph.D. Thesis, Siegen (1986). [9] J.H. Brannon and D. Magde, J. Phys. Chem. 82(1978) 705. [10] U. Kringel, Ph.D. Thesis, Siegen (1088).