- Email: [email protected]
Photoreactions of rhodamine dyes in basic solvents
13 February 1998
Chemical Physics Letters 283 Ž1998. 350–356
Photoreactions of rhodamine dyes in basic solvents St. Becker, I. Gregor, E. Thiel Department of Physical Chemistry II, UniÕersity of Siegen, Adolf-Reichwein-Straße 2, D-57068 Siegen, Germany Received 21 October 1997; in final form 22 November 1997
Abstract Using a highly sensitive spectrometer we measured the absorption of transient states of different dyes in liquid solution. Analysing the temporal and spectral dependency of the absorption we determined lifetimes as well as absorption spectra of transient states. The kinetic parameters are specifically influenced by interaction with certain bases. On investigating the acid–base equilibrium of the triplet state of different rhodamine dyes we found the deprotonated triplet state to be an efficient source of a long-lived transient species. We suggest a reaction scheme which explains the dependence of the observed triplet lifetime on the concentration of amine. The possible consequences for the photostability of the dye are discussed. q 1998 Elsevier Science B.V.
1. Introduction There is a wide spectrum of applications for fluorescent dyes. Beside the classical fields such as dye lasers or mode locking w1–4x, new applications in biology w5x, engineering and science are profiting from the properties of these dyes. Especially the fact that fluorescence can be detected easily and is extremely sensitive makes them first choice in ultra analytical problems down to the single molecule level w6x. Nowadays, the wavelengths of absorption and fluorescence, quantum yield of fluorescence or the influence of acids and bases on the ground state can be quite well predicted from the molecular structure w7–12x. In contrast, there are only poor Žif any. models to predict the physical properties of excited states and their reactivity. However, these properties
are important not only for photostability but also for applicational suitability of the dyes. In Ref. w13x a technique is described which allows a highly precise detection of transient absorption in the microsecond region. With this technique the physical properties of some fluorescent dyes have been investigated w14x. In the present Letter this method is modified. Using an achromatic optic and an incoherent source of probe light we are able to measure the transient absorption spectrum in the range of UV to NIR. The new method is found to be useful for studying primary processes in the photochemistry of rhodamine dyes. Photoproducts can be observed due to their specific absorption. Rhodamine dyes which obtain secondary amino groups can be easily deprotonated, being in their triplet state. The deprotonated triplet state undergoes a fast reaction, leading to a new comparatively long-lived state. The
0009-2614r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 0 9 - 2 6 1 4 Ž 9 7 . 0 1 3 9 5 - X
St. Becker et al.r Chemical Physics Letters 283 (1998) 350–356
deprotonated triplet state of a rhodamine dye can be an efficient source of a remarkably long-lived transient species. 2. Experimental 2.1. Apparatus 2.1.1. Working principle Experimental details are described elsewhere w13x. Briefly, the sample consists of the dye solved in ethylene glycol which is pressed through a nozzle producing a fast flowing jet stream. This jet is commonly known from continuous dye lasers. The excitation beam ŽAr-ion laser, Inova 70, Coherent Radiation Inc.. is focused by a lens Ž f s 50 mm. onto the sample. This produces a characteristic distribution of transient states. A typical situation is shown in Fig. 1a. The transmission of the sample is measured using a focussed probe beam. Due to the adjustable distance between the foci of probe and excitation beam, one can detect the complete distribution of transient states in the sample. Due to the motion of the sample, the spatial evolution of the transmission can be transformed into the time domain. Consequently, it is possible to identify individual states by analysing the temporal decrease of the transient absorption. In order to measure the transient amount of total transmission we compare the intensity I1 of the
351
transmitted probe beam, with transient states populated Žexcitation beam on. and, the intensity I, with no transient states populated Žexcitation beam off.. The recorded value is D IrI s Ž I1 y I .rI. Positive values of D IrI indicate an increased transmission of the excited sample compared to the sample with just a ground-state population. 2.1.2. Highly sensitiÕe transient spectrometer With the aim of measuring the full spectra of the transient state we used a Xe-arc lamp as probe light source of a laser beam Žsee Fig. 2a.. Due to the poor spectral intensity of the lamp the experimental apparatus had to be modified. The use of an incoherent source of probe light demands special attention on the quality of mapping. Fig. 2a shows our experimental apparatus. The major problem is to achieve a probe light focus of definite Žsmall. size, regular shape and fixed position. For this reason we project the arc of a Xe-arc lamp Žtype 959C 1980, Canrad Hannovia. on a pinhole Ž d s 20 mm. by an UVachromat Ž f s 60 mm.. The illuminated pinhole, which is stable in shape and location, is used as the light source. It is projected onto the sample at a fixed point by a concave mirror which is specially corrected for this angle of mapping in order to maintain size and shape of the image. The transmitted probe light is collected by a second concave mirror and projected onto the entrance slit Ž d s 1 mm. of a monochromator ŽH20 IR,
Fig. 1. Ža. Typical distribution of transient states in the sample as function of the flow direction x Žcalculated on parameters of Table 1.. Žb. Time-resolved transient absorption of rhodamine 6G in ethylene glycol. The experimental parameters are given in Table 1.
352
St. Becker et al.r Chemical Physics Letters 283 (1998) 350–356
Fig. 2. Ža. Experimental apparatus used for recording time-resolved spectra of transient absorption. el: ion laser Žcw excitation light.; pc: pockels cell Ž v E .; m: plane mirror; al: achromatic lens Ž f s 50 mm.; bs: beam splitter; s: sample; pl: Xe arc lamp Žprobe light.; uva: achromatic UV lens; a 1: aperture Ž d s 20 mm.; a 2 : aperture Ž d s 2 cm.; ch: chopper Ž v P .; cm 1 , cm 2 : concave mirrors; mc: monochromator; pd: photodiode. Žb. Investigated dyes.
ISA Jobin-Yvon.. A photodiode ŽSP 1337 8BQ, Hamamatsu. at the exit slit Ž d s 1 mm. detects the intensity of the transmitted probe beam at the selected wavelength. The use of mirror ensures that position and size of the probe light focus are completely independent of the wavelenght. Thus it is feasible to measure spectra by sweeping the wavelength setting at the monochromator. The new experimental apparatus has important advantages compared to the technique described in Ref. w13x. The absorption of transient states can be detected in the UV–VIS and NIR region without further correction of adjustment and allows comfortable detection of new states with unknown absorption bands.
In order to improve the signal to noise ratio, the intensities of excitation and probe beam are modulated with frequency v E and v P , respectively. The photocurrent is analysed with a lock-in amplifier tuned on v L s v E q v P . 2.2. Chemicals Fig. 2b shows the structures of the dyes. Rhodamine 6G in the form of perchlorate was supplied by Radiant Dyes. DR11, DR21, DR25 and rhodamine 3B also in the form of perchlorates were synthesised by the group of Professor Drexhage. For purification of these dyes we used a RP18 column ŽMerck LichroPrep, medium size.. The eluent solvent was a mixture of 0.1 M aqueous triethylammo-
St. Becker et al.r Chemical Physics Letters 283 (1998) 350–356
353
Table 1 Experimental conditions Žtypical values. Parameter
Magnitude
jet velocity lifetime of first excited singlet state lifetime of first triplet state intersystem crossing rate constant dye concentration excitation power mode width of excitation beam excitation wavelength absorption coefficient at excitation wavelength frequency of probe beam chopper frequency of excitation beam modulation
12 m sy1 4 ns 5 ms 0.5 msy1 1 mmol ly1 100 mW 20 mm 514 nm 5000 moly1 cmy1 2 kHz 1.5 kHz
niumacetate buffer ŽpH s 7. with a gradient rising from pure acetonitrile to pure buffer solution. All dyes were extensively checked for purity using TLC and HPLC. Acetonitrile ŽMerck, p.a.., ethylene glycol ŽMerck, p.a.., 2,2,6,6-tetramethylpiperidine ŽTMP, Aldrich 99%. and sodium hydroxide ŽMerck, p.a.. were used without any further purification. The buffer for chromatography was prepared from triethylamine ŽMerck, p.a.., glacial acetic acid ŽMerck, p.a.. and deionized water. 3. Results and discussion Fig. 1b shows a typical time-resolved transient transmission spectrum of rhodamine 6G dissolved in ethylene glycol. For analysing it is useful to distinguish between different spectral regions. 3.1. Ground-state depletion measurements If the wavelength of the probe beam falls in the main absorption band of the dye the ground-state depletion causes an increase of the transmitted beam intensity. With respect to the high extinction coefficient of the ground state the absorption of the transients states at this wavelength usually can be neglected. This way information about the total yield of all populated transient states can be obtained. We investigated the ground-state repopulation of all dyes Ž c s 1 mM in ethylene glycol. shown in Fig. 2b. Under these conditions all dyes show a strictly monoexponential repopulation due to a first-order
Fig. 3. Ground-state depletion of DR11 with and without TMP recorded at 531 nm Žnitrogen-flushed solution..
decay of the dye’s triplet state. The triplet lifetime in nitrogen-flushed solution is ; 15 ms. Fig. 3 shows the result of a nitrogen-flushed solution of DR11. After the addition of 1 mM 2,2,6,6-tetramethylpiperidine ŽTMP. to the solution of DR11 the recorded signal shows a different kinetic behaviour ŽFig. 3. which cannot be explained by a first-order decay of triplet states. We found similar results with the dyes rhodamine 6G and DR21 and with the addition of 1 mM piperidine, triethylamine or sodium hydroxide instead of TMP Žnot shown.. We also examined the effect of bases on rhodamine 3B and DR25. Addition of TMP or sodium hydroxide in concentrations up to 50 mM to the nitrogen-flushed solution of rhodamine 3B or DR25 does not have any influence on the ground-state repopulation. These results indicate that the influence of the amines and sodium hydroxide is caused by a deprotonation reaction of those dyes which possess secondary amino groups. 3.2. Triplet–triplet absorption measurements One should bear in mind that the ground-state depletion signal arises from all populated transient states. Therefore to specifically study the influence of a base on the triplet state of a dye it is much more advantageous to measure the time dependence of the triplet–triplet ŽT1 –Tn . absorption. It is known that for rhodamine 6G T1 –Tn absorption can be observed at 633 nm using a He–Ne laser w13x. Since the ground state does not absorb in this spectral region there is no interference by ground-state depletion.
354
St. Becker et al.r Chemical Physics Letters 283 (1998) 350–356
obviously caused only by T1 –Tn absorption and not influenced by the second species. 3.3. Kinetic analysis Fig. 4b shows the dependence of the observed decay rate constant 1rt B Žobtained by analysis of the signal at 633 nm. on the amine concentration. A gradual addition of the amine leads to a decrease of triplet lifetime at first, but at a given concentration of the additive a stationary value of t B is observed. Obviously, the system is not conform with the Stern–Volmer equation of dynamic quenching w15x, which predicts a linear dependence between the reciprocal triplet lifetime and the quencher concentration. To explain the kinetic behaviour ŽFig. 4b. we assume the following set of reactions:
ŽA.
Fig. 4. Ža. Triplet–triplet absorption of DR11 with and without TMP recorded at 633 nm Žnitrogen-flushed solution.. Žb. Dependence of observed inverse triplet lifetime of DR11 Ž k B s1rt B . on the concentration of TMP Ždots.. The solid curve is calculated by Eq. Ž8. with the parameters shown in Table 2.
DR11 shows almost the same ground-state absorption spectrum as rhodamine 6G and a T1 –Tn absorption at 633 nm, as well. The result of time-resolved T1 –Tn absorption measurements with DR11 at 633 nm is given in Fig. 4a. The signals clearly show the reduction of triplet lifetime in the presence of the amine. Notice that the T1 –Tn absorption decays via first-order kinetics also after the addition of the amine. Similar results are obtained with rhodamine 6G and DR21. Again, rhodamine 3B and DR25 do not show any influence by bases at this wavelength. These experiments indicate that the addition of a base has two effects: Ž1. The shortening of the triplet lifetime. Ž2. The formation of a second, unknown transient species, in addition to the triplet state. The nonmonoexponential ground-state recovery can be rationalised. At the wavelength of 633 nm the signal is
In protic solvents the triplet state T is in equilibrium with the deprotonated triplet state D. The acid–base equilibrium is established much faster than the T or D decay via the k D or k T route. Therefore both triplet states show the same effective lifetime t B . The reaction scheme implies that without base the lifetime will be t B s 1rk T whereas in the case of complete deprotonation the observed lifetime will be t B s 1rk D . In order to describe the dependence of t B on the base concentration we use the mass action law with the equilibrium constant K of the deprotonation reaction as Ž c i : concentration of compound i . Ks
c D c BH cT cB
s
c D2 cT cB
.
Ž 1.
We define a total concentration c as sum of triplet state and deprotonated triplet state concentration cD q cT s c Ž 2.
St. Becker et al.r Chemical Physics Letters 283 (1998) 350–356
and the degree of protolysis cT c BH as s . cD cB K
Table 2 Kinetics parameters of the reaction
Ž 3.
The rate equation of the sum concentration c can be formulated as dc y s cD k D q cT k T Ž 4. dt which may be written using Eq. Ž3. dc D y sc Ž k qa kT . . Ž 5. dt 1qa D Eq. Ž5. cannot be solved analytically as a is a function of time Žnotice that the concentration of the protonated base c BH increases during reaction.. Although, assuming a small observation interval for which a is constant, the integration of Eq. Ž5. leads to 1 kD qa kT s . Ž 6. tB 1qa Under our experimental conditions the relations c B 4 c BH and c B f c B0 hold Ž c B0 : initial concentration of base. and Eqs. Ž1. and Ž2. lead to 1 asy " 2
(
1
c q
4
.
c B0 K
Ž 7.
Eq. Ž3. demands a ) 0. Therefore only the positive sign of the root is of interest. With Eqs. Ž6. and Ž7. one obtains an equation which describes the dependence between the observed triplet lifetime and the concentration of base kD ykT
1 s
tB
1
1
ž ( ( 2
4
q
4
c q
1
q 2
1
y
c c B0
355
c B0 K
/
.
Ž 8.
K
Fig. 4b demonstrates the experimental results of DR11 and the fit using Eq. Ž8.. The calculated values are given in Table 2. In contrast to our results, the literature usually reports identical decay rates of acid and base form of a dye’s triplet state Ž k D f k T . w16x. This is ascribed to the fact that the rate-determining process of triplet decay is the diffusion-controlled quenching by molecular oxygen via triplet–triplet annihilation w17x.
Parameter
DR11
Rhodamine 6G
kT kD cr K kX
0.06 msy1 0.24 msy1 0.26 mM 0.18 msy1
0.05 msy1 0.60 msy1 0.91 mM 0.55 msy1
From this point of view our findings of an influence of base on the decay rates is surprising. The observed dependence of triplet lifetime on base concentration suggests an additional deactivation channel for the deprotonated triplet state D which is not given for the acid form T. We assume this channel to be the Žfirst order. reaction that produces the long-lived transient X. yH q
kX
T | D ™ X.
Ž B.
In this model k X is the only difference between k D and k T . We can calculate k X s k D y k T , which leads to the values for k X that are given in Table 2. 3.4. Absorption spectrum of the long-liÕed transient state In order to identify the unknown transient state we recorded the complete spectra of DR11 and rhodamine 6G in the region between 370 and 950 nm. The addition of 10 mM TMP to the solution of 1 mM DR11 leads to a transient absorption band with a maximum near 550 nm. The band stands out conspicuously in an oxygen-enriched solution ŽFig. 5.. In a nitrogen-flushed solution there is a severe overlap of the new transient absorption and the ground-state depletion caused by long-lived dye triplets. Using the 545 nm emission of a green He–Ne laser we determined the lifetime of the transient to be 50 ms in oxygen-enriched, as well as in nitrogenflushed, solutions. In the case of rhodamine 6G Ž1 mM. a new absorption band with a maximum near 475 nm is observed in the presence of 10 mM TMP both in nitrogen- and oxygen-flushed solutions. Korobov and Chibisov w18x and Dempster et al. w19x assigned the band with an absorption maximum at 475 nm to the half-oxidised form of rhodamine 6G
356
St. Becker et al.r Chemical Physics Letters 283 (1998) 350–356
build-up of a distinct concentration of, probably radical ions, that might be chemically reactive. Thus the presence of a base probably opens a pathway of irreversible photodecomposition of the dye. A further notable finding is that our spectroscopic method turned out to be a powerful tool in analysing reaction pathways in photochemistry. Especially electron transfer reactions, with long-lived ion radicals as products, may be easily studied. Fig. 5. Absorption spectrum of new transient state in ethylene glycol Žsee text..
Žion radical.. This indicates that the measured transient is probably a product of an electron transfer reaction of the deprotonated triplet state acting as donor. Our experiments give no answer concerning the nature of the corresponding acceptor.
Acknowledgements The authors express their thanks to Dr. J. ArdenJacob, G. Deltau and J. Marx for dye preparation and technical assistance. Financial support by the Deutsche Forschungsgemeinschaft is gratefully acknowledged. References
4. Conclusions The analysis of the experimental results shows a consistent picture. Rhodamine dyes which contain secondary amino groups can be easily deprotonated being in their triplet state. The deprotonated triplet state undergoes a fast reaction leading to a new comparatively long-lived state Žt X s 50 ms.. The absorption spectrum of this new state is quite close to the ground state. It has an absorption maximum at 550 nm ŽDR11. or 475 nm Žrhodamine 6G.. Thus we suppose the state to be the radical ion of the dye produced by electron transfer of the deprotonated triplet state. This fast reaction, which is coupled to the deprotonation equilibrium, significantly decreases the lifetime of the triplet state. The ground-state depletion shows biexponential recovery due to superposition of accelerated triplet decay and slow decay of the new state. Until now proton phototransfer involving rhodamine dyes in the triplet state has hardly been studied. The deprotonated triplet state is an efficient source of a remarkably long-lived transient species. The results are of particular interest in view of the photostability of widely used rhodamine dyes. Under constant irradiation this long lifetime causes the
w1x F.P. Schafer ¨ ŽEd.., Dye Lasers, Springer, Berlin, 1977. w2x M. Stucke ŽEd.., Dye Lasers: 25 Years, Springer, Berlin, 1992. w3x O.G. Peterson, in: C.L. Tang ŽEd.., Methods of Experimental Physics, Academic Press, New York, 1979. w4x F.J. Duarte, L.W. Hillmann ŽEd.., Dye Laser Principles, Academic Press, New York, 1990. w5x D.L. Taylor, A.S. Waggoner, R.F. Murphy, F. Lanni, R.R. Birge ŽEds.., Applications of Fluorescence in the Biomedical Sciences, Alan R. Liss, New York, 1986. w6x R.A. Keller, W.P. Ambrose, P.M. Goodwin, J.H. Jett, J.C. Martin, M. Wu, Appl. Spectrosc. 50 Ž1996. 12A. w7x H. Kuhn, Helv. Chim. Acta 31 Ž1948. 1441. w8x H. Kuhn, Helv. Chim. Acta 34 Ž1951. 2373. w9x R. Sens, Ph.D. Thesis, University of Siegen, Siegen, 1984. w10x J. Arden-Jacob, Ph.D. Thesis, University of Siegen, Siegen, 1992. w11x W.J. Siebrand, J. Chem. Phys. 47 Ž1967. 2411. w12x K.H. Drexhage, in: F.P. Schafer ¨ ŽEd.., Dye Lasers, Springer, Berlin, 1977. w13x E. Thiel, K.H. Drexhage, Chem. Phys. Lett. 199 Ž1992. 329. w14x E. Thiel, Eigenschaften angeregter Rhodamin-Farbstoffe und deren Wirkung im Farbstofflaser, Shaker, 1996. w15x O. Stern, M. Volmer, Z. Phys. 20 Ž1919. 155. w16x V. Bruckner, K.-H. Feller, U.-W. Grummt, Applications of ¨ Time-Resolved Optical Spectroscopy, Elsevier, Amsterdam, 1990. w17x T. Forster, Fluoreszenz organischer Verbindungen, Vanden¨ hoeck & Ruprecht, Gottingen, 1961 Žrepublication of 1st ¨ ed... w18x V.E. Korobov, A.K. Chibisov, J. Photochem. 9 Ž1978. 411. w19x D.N. Dempster, T. Morrow, M.F. Quinn, J. Photochem. 2 Ž1973. 343.
Chemical Physics Letters 283 Ž1998. 350–356
Photoreactions of rhodamine dyes in basic solvents St. Becker, I. Gregor, E. Thiel Department of Physical Chemistry II, UniÕersity of Siegen, Adolf-Reichwein-Straße 2, D-57068 Siegen, Germany Received 21 October 1997; in final form 22 November 1997
Abstract Using a highly sensitive spectrometer we measured the absorption of transient states of different dyes in liquid solution. Analysing the temporal and spectral dependency of the absorption we determined lifetimes as well as absorption spectra of transient states. The kinetic parameters are specifically influenced by interaction with certain bases. On investigating the acid–base equilibrium of the triplet state of different rhodamine dyes we found the deprotonated triplet state to be an efficient source of a long-lived transient species. We suggest a reaction scheme which explains the dependence of the observed triplet lifetime on the concentration of amine. The possible consequences for the photostability of the dye are discussed. q 1998 Elsevier Science B.V.
1. Introduction There is a wide spectrum of applications for fluorescent dyes. Beside the classical fields such as dye lasers or mode locking w1–4x, new applications in biology w5x, engineering and science are profiting from the properties of these dyes. Especially the fact that fluorescence can be detected easily and is extremely sensitive makes them first choice in ultra analytical problems down to the single molecule level w6x. Nowadays, the wavelengths of absorption and fluorescence, quantum yield of fluorescence or the influence of acids and bases on the ground state can be quite well predicted from the molecular structure w7–12x. In contrast, there are only poor Žif any. models to predict the physical properties of excited states and their reactivity. However, these properties
are important not only for photostability but also for applicational suitability of the dyes. In Ref. w13x a technique is described which allows a highly precise detection of transient absorption in the microsecond region. With this technique the physical properties of some fluorescent dyes have been investigated w14x. In the present Letter this method is modified. Using an achromatic optic and an incoherent source of probe light we are able to measure the transient absorption spectrum in the range of UV to NIR. The new method is found to be useful for studying primary processes in the photochemistry of rhodamine dyes. Photoproducts can be observed due to their specific absorption. Rhodamine dyes which obtain secondary amino groups can be easily deprotonated, being in their triplet state. The deprotonated triplet state undergoes a fast reaction, leading to a new comparatively long-lived state. The
0009-2614r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 0 9 - 2 6 1 4 Ž 9 7 . 0 1 3 9 5 - X
St. Becker et al.r Chemical Physics Letters 283 (1998) 350–356
deprotonated triplet state of a rhodamine dye can be an efficient source of a remarkably long-lived transient species. 2. Experimental 2.1. Apparatus 2.1.1. Working principle Experimental details are described elsewhere w13x. Briefly, the sample consists of the dye solved in ethylene glycol which is pressed through a nozzle producing a fast flowing jet stream. This jet is commonly known from continuous dye lasers. The excitation beam ŽAr-ion laser, Inova 70, Coherent Radiation Inc.. is focused by a lens Ž f s 50 mm. onto the sample. This produces a characteristic distribution of transient states. A typical situation is shown in Fig. 1a. The transmission of the sample is measured using a focussed probe beam. Due to the adjustable distance between the foci of probe and excitation beam, one can detect the complete distribution of transient states in the sample. Due to the motion of the sample, the spatial evolution of the transmission can be transformed into the time domain. Consequently, it is possible to identify individual states by analysing the temporal decrease of the transient absorption. In order to measure the transient amount of total transmission we compare the intensity I1 of the
351
transmitted probe beam, with transient states populated Žexcitation beam on. and, the intensity I, with no transient states populated Žexcitation beam off.. The recorded value is D IrI s Ž I1 y I .rI. Positive values of D IrI indicate an increased transmission of the excited sample compared to the sample with just a ground-state population. 2.1.2. Highly sensitiÕe transient spectrometer With the aim of measuring the full spectra of the transient state we used a Xe-arc lamp as probe light source of a laser beam Žsee Fig. 2a.. Due to the poor spectral intensity of the lamp the experimental apparatus had to be modified. The use of an incoherent source of probe light demands special attention on the quality of mapping. Fig. 2a shows our experimental apparatus. The major problem is to achieve a probe light focus of definite Žsmall. size, regular shape and fixed position. For this reason we project the arc of a Xe-arc lamp Žtype 959C 1980, Canrad Hannovia. on a pinhole Ž d s 20 mm. by an UVachromat Ž f s 60 mm.. The illuminated pinhole, which is stable in shape and location, is used as the light source. It is projected onto the sample at a fixed point by a concave mirror which is specially corrected for this angle of mapping in order to maintain size and shape of the image. The transmitted probe light is collected by a second concave mirror and projected onto the entrance slit Ž d s 1 mm. of a monochromator ŽH20 IR,
Fig. 1. Ža. Typical distribution of transient states in the sample as function of the flow direction x Žcalculated on parameters of Table 1.. Žb. Time-resolved transient absorption of rhodamine 6G in ethylene glycol. The experimental parameters are given in Table 1.
352
St. Becker et al.r Chemical Physics Letters 283 (1998) 350–356
Fig. 2. Ža. Experimental apparatus used for recording time-resolved spectra of transient absorption. el: ion laser Žcw excitation light.; pc: pockels cell Ž v E .; m: plane mirror; al: achromatic lens Ž f s 50 mm.; bs: beam splitter; s: sample; pl: Xe arc lamp Žprobe light.; uva: achromatic UV lens; a 1: aperture Ž d s 20 mm.; a 2 : aperture Ž d s 2 cm.; ch: chopper Ž v P .; cm 1 , cm 2 : concave mirrors; mc: monochromator; pd: photodiode. Žb. Investigated dyes.
ISA Jobin-Yvon.. A photodiode ŽSP 1337 8BQ, Hamamatsu. at the exit slit Ž d s 1 mm. detects the intensity of the transmitted probe beam at the selected wavelength. The use of mirror ensures that position and size of the probe light focus are completely independent of the wavelenght. Thus it is feasible to measure spectra by sweeping the wavelength setting at the monochromator. The new experimental apparatus has important advantages compared to the technique described in Ref. w13x. The absorption of transient states can be detected in the UV–VIS and NIR region without further correction of adjustment and allows comfortable detection of new states with unknown absorption bands.
In order to improve the signal to noise ratio, the intensities of excitation and probe beam are modulated with frequency v E and v P , respectively. The photocurrent is analysed with a lock-in amplifier tuned on v L s v E q v P . 2.2. Chemicals Fig. 2b shows the structures of the dyes. Rhodamine 6G in the form of perchlorate was supplied by Radiant Dyes. DR11, DR21, DR25 and rhodamine 3B also in the form of perchlorates were synthesised by the group of Professor Drexhage. For purification of these dyes we used a RP18 column ŽMerck LichroPrep, medium size.. The eluent solvent was a mixture of 0.1 M aqueous triethylammo-
St. Becker et al.r Chemical Physics Letters 283 (1998) 350–356
353
Table 1 Experimental conditions Žtypical values. Parameter
Magnitude
jet velocity lifetime of first excited singlet state lifetime of first triplet state intersystem crossing rate constant dye concentration excitation power mode width of excitation beam excitation wavelength absorption coefficient at excitation wavelength frequency of probe beam chopper frequency of excitation beam modulation
12 m sy1 4 ns 5 ms 0.5 msy1 1 mmol ly1 100 mW 20 mm 514 nm 5000 moly1 cmy1 2 kHz 1.5 kHz
niumacetate buffer ŽpH s 7. with a gradient rising from pure acetonitrile to pure buffer solution. All dyes were extensively checked for purity using TLC and HPLC. Acetonitrile ŽMerck, p.a.., ethylene glycol ŽMerck, p.a.., 2,2,6,6-tetramethylpiperidine ŽTMP, Aldrich 99%. and sodium hydroxide ŽMerck, p.a.. were used without any further purification. The buffer for chromatography was prepared from triethylamine ŽMerck, p.a.., glacial acetic acid ŽMerck, p.a.. and deionized water. 3. Results and discussion Fig. 1b shows a typical time-resolved transient transmission spectrum of rhodamine 6G dissolved in ethylene glycol. For analysing it is useful to distinguish between different spectral regions. 3.1. Ground-state depletion measurements If the wavelength of the probe beam falls in the main absorption band of the dye the ground-state depletion causes an increase of the transmitted beam intensity. With respect to the high extinction coefficient of the ground state the absorption of the transients states at this wavelength usually can be neglected. This way information about the total yield of all populated transient states can be obtained. We investigated the ground-state repopulation of all dyes Ž c s 1 mM in ethylene glycol. shown in Fig. 2b. Under these conditions all dyes show a strictly monoexponential repopulation due to a first-order
Fig. 3. Ground-state depletion of DR11 with and without TMP recorded at 531 nm Žnitrogen-flushed solution..
decay of the dye’s triplet state. The triplet lifetime in nitrogen-flushed solution is ; 15 ms. Fig. 3 shows the result of a nitrogen-flushed solution of DR11. After the addition of 1 mM 2,2,6,6-tetramethylpiperidine ŽTMP. to the solution of DR11 the recorded signal shows a different kinetic behaviour ŽFig. 3. which cannot be explained by a first-order decay of triplet states. We found similar results with the dyes rhodamine 6G and DR21 and with the addition of 1 mM piperidine, triethylamine or sodium hydroxide instead of TMP Žnot shown.. We also examined the effect of bases on rhodamine 3B and DR25. Addition of TMP or sodium hydroxide in concentrations up to 50 mM to the nitrogen-flushed solution of rhodamine 3B or DR25 does not have any influence on the ground-state repopulation. These results indicate that the influence of the amines and sodium hydroxide is caused by a deprotonation reaction of those dyes which possess secondary amino groups. 3.2. Triplet–triplet absorption measurements One should bear in mind that the ground-state depletion signal arises from all populated transient states. Therefore to specifically study the influence of a base on the triplet state of a dye it is much more advantageous to measure the time dependence of the triplet–triplet ŽT1 –Tn . absorption. It is known that for rhodamine 6G T1 –Tn absorption can be observed at 633 nm using a He–Ne laser w13x. Since the ground state does not absorb in this spectral region there is no interference by ground-state depletion.
354
St. Becker et al.r Chemical Physics Letters 283 (1998) 350–356
obviously caused only by T1 –Tn absorption and not influenced by the second species. 3.3. Kinetic analysis Fig. 4b shows the dependence of the observed decay rate constant 1rt B Žobtained by analysis of the signal at 633 nm. on the amine concentration. A gradual addition of the amine leads to a decrease of triplet lifetime at first, but at a given concentration of the additive a stationary value of t B is observed. Obviously, the system is not conform with the Stern–Volmer equation of dynamic quenching w15x, which predicts a linear dependence between the reciprocal triplet lifetime and the quencher concentration. To explain the kinetic behaviour ŽFig. 4b. we assume the following set of reactions:
ŽA.
Fig. 4. Ža. Triplet–triplet absorption of DR11 with and without TMP recorded at 633 nm Žnitrogen-flushed solution.. Žb. Dependence of observed inverse triplet lifetime of DR11 Ž k B s1rt B . on the concentration of TMP Ždots.. The solid curve is calculated by Eq. Ž8. with the parameters shown in Table 2.
DR11 shows almost the same ground-state absorption spectrum as rhodamine 6G and a T1 –Tn absorption at 633 nm, as well. The result of time-resolved T1 –Tn absorption measurements with DR11 at 633 nm is given in Fig. 4a. The signals clearly show the reduction of triplet lifetime in the presence of the amine. Notice that the T1 –Tn absorption decays via first-order kinetics also after the addition of the amine. Similar results are obtained with rhodamine 6G and DR21. Again, rhodamine 3B and DR25 do not show any influence by bases at this wavelength. These experiments indicate that the addition of a base has two effects: Ž1. The shortening of the triplet lifetime. Ž2. The formation of a second, unknown transient species, in addition to the triplet state. The nonmonoexponential ground-state recovery can be rationalised. At the wavelength of 633 nm the signal is
In protic solvents the triplet state T is in equilibrium with the deprotonated triplet state D. The acid–base equilibrium is established much faster than the T or D decay via the k D or k T route. Therefore both triplet states show the same effective lifetime t B . The reaction scheme implies that without base the lifetime will be t B s 1rk T whereas in the case of complete deprotonation the observed lifetime will be t B s 1rk D . In order to describe the dependence of t B on the base concentration we use the mass action law with the equilibrium constant K of the deprotonation reaction as Ž c i : concentration of compound i . Ks
c D c BH cT cB
s
c D2 cT cB
.
Ž 1.
We define a total concentration c as sum of triplet state and deprotonated triplet state concentration cD q cT s c Ž 2.
St. Becker et al.r Chemical Physics Letters 283 (1998) 350–356
and the degree of protolysis cT c BH as s . cD cB K
Table 2 Kinetics parameters of the reaction
Ž 3.
The rate equation of the sum concentration c can be formulated as dc y s cD k D q cT k T Ž 4. dt which may be written using Eq. Ž3. dc D y sc Ž k qa kT . . Ž 5. dt 1qa D Eq. Ž5. cannot be solved analytically as a is a function of time Žnotice that the concentration of the protonated base c BH increases during reaction.. Although, assuming a small observation interval for which a is constant, the integration of Eq. Ž5. leads to 1 kD qa kT s . Ž 6. tB 1qa Under our experimental conditions the relations c B 4 c BH and c B f c B0 hold Ž c B0 : initial concentration of base. and Eqs. Ž1. and Ž2. lead to 1 asy " 2
(
1
c q
4
.
c B0 K
Ž 7.
Eq. Ž3. demands a ) 0. Therefore only the positive sign of the root is of interest. With Eqs. Ž6. and Ž7. one obtains an equation which describes the dependence between the observed triplet lifetime and the concentration of base kD ykT
1 s
tB
1
1
ž ( ( 2
4
q
4
c q
1
q 2
1
y
c c B0
355
c B0 K
/
.
Ž 8.
K
Fig. 4b demonstrates the experimental results of DR11 and the fit using Eq. Ž8.. The calculated values are given in Table 2. In contrast to our results, the literature usually reports identical decay rates of acid and base form of a dye’s triplet state Ž k D f k T . w16x. This is ascribed to the fact that the rate-determining process of triplet decay is the diffusion-controlled quenching by molecular oxygen via triplet–triplet annihilation w17x.
Parameter
DR11
Rhodamine 6G
kT kD cr K kX
0.06 msy1 0.24 msy1 0.26 mM 0.18 msy1
0.05 msy1 0.60 msy1 0.91 mM 0.55 msy1
From this point of view our findings of an influence of base on the decay rates is surprising. The observed dependence of triplet lifetime on base concentration suggests an additional deactivation channel for the deprotonated triplet state D which is not given for the acid form T. We assume this channel to be the Žfirst order. reaction that produces the long-lived transient X. yH q
kX
T | D ™ X.
Ž B.
In this model k X is the only difference between k D and k T . We can calculate k X s k D y k T , which leads to the values for k X that are given in Table 2. 3.4. Absorption spectrum of the long-liÕed transient state In order to identify the unknown transient state we recorded the complete spectra of DR11 and rhodamine 6G in the region between 370 and 950 nm. The addition of 10 mM TMP to the solution of 1 mM DR11 leads to a transient absorption band with a maximum near 550 nm. The band stands out conspicuously in an oxygen-enriched solution ŽFig. 5.. In a nitrogen-flushed solution there is a severe overlap of the new transient absorption and the ground-state depletion caused by long-lived dye triplets. Using the 545 nm emission of a green He–Ne laser we determined the lifetime of the transient to be 50 ms in oxygen-enriched, as well as in nitrogenflushed, solutions. In the case of rhodamine 6G Ž1 mM. a new absorption band with a maximum near 475 nm is observed in the presence of 10 mM TMP both in nitrogen- and oxygen-flushed solutions. Korobov and Chibisov w18x and Dempster et al. w19x assigned the band with an absorption maximum at 475 nm to the half-oxidised form of rhodamine 6G
356
St. Becker et al.r Chemical Physics Letters 283 (1998) 350–356
build-up of a distinct concentration of, probably radical ions, that might be chemically reactive. Thus the presence of a base probably opens a pathway of irreversible photodecomposition of the dye. A further notable finding is that our spectroscopic method turned out to be a powerful tool in analysing reaction pathways in photochemistry. Especially electron transfer reactions, with long-lived ion radicals as products, may be easily studied. Fig. 5. Absorption spectrum of new transient state in ethylene glycol Žsee text..
Žion radical.. This indicates that the measured transient is probably a product of an electron transfer reaction of the deprotonated triplet state acting as donor. Our experiments give no answer concerning the nature of the corresponding acceptor.
Acknowledgements The authors express their thanks to Dr. J. ArdenJacob, G. Deltau and J. Marx for dye preparation and technical assistance. Financial support by the Deutsche Forschungsgemeinschaft is gratefully acknowledged. References
4. Conclusions The analysis of the experimental results shows a consistent picture. Rhodamine dyes which contain secondary amino groups can be easily deprotonated being in their triplet state. The deprotonated triplet state undergoes a fast reaction leading to a new comparatively long-lived state Žt X s 50 ms.. The absorption spectrum of this new state is quite close to the ground state. It has an absorption maximum at 550 nm ŽDR11. or 475 nm Žrhodamine 6G.. Thus we suppose the state to be the radical ion of the dye produced by electron transfer of the deprotonated triplet state. This fast reaction, which is coupled to the deprotonation equilibrium, significantly decreases the lifetime of the triplet state. The ground-state depletion shows biexponential recovery due to superposition of accelerated triplet decay and slow decay of the new state. Until now proton phototransfer involving rhodamine dyes in the triplet state has hardly been studied. The deprotonated triplet state is an efficient source of a remarkably long-lived transient species. The results are of particular interest in view of the photostability of widely used rhodamine dyes. Under constant irradiation this long lifetime causes the
w1x F.P. Schafer ¨ ŽEd.., Dye Lasers, Springer, Berlin, 1977. w2x M. Stucke ŽEd.., Dye Lasers: 25 Years, Springer, Berlin, 1992. w3x O.G. Peterson, in: C.L. Tang ŽEd.., Methods of Experimental Physics, Academic Press, New York, 1979. w4x F.J. Duarte, L.W. Hillmann ŽEd.., Dye Laser Principles, Academic Press, New York, 1990. w5x D.L. Taylor, A.S. Waggoner, R.F. Murphy, F. Lanni, R.R. Birge ŽEds.., Applications of Fluorescence in the Biomedical Sciences, Alan R. Liss, New York, 1986. w6x R.A. Keller, W.P. Ambrose, P.M. Goodwin, J.H. Jett, J.C. Martin, M. Wu, Appl. Spectrosc. 50 Ž1996. 12A. w7x H. Kuhn, Helv. Chim. Acta 31 Ž1948. 1441. w8x H. Kuhn, Helv. Chim. Acta 34 Ž1951. 2373. w9x R. Sens, Ph.D. Thesis, University of Siegen, Siegen, 1984. w10x J. Arden-Jacob, Ph.D. Thesis, University of Siegen, Siegen, 1992. w11x W.J. Siebrand, J. Chem. Phys. 47 Ž1967. 2411. w12x K.H. Drexhage, in: F.P. Schafer ¨ ŽEd.., Dye Lasers, Springer, Berlin, 1977. w13x E. Thiel, K.H. Drexhage, Chem. Phys. Lett. 199 Ž1992. 329. w14x E. Thiel, Eigenschaften angeregter Rhodamin-Farbstoffe und deren Wirkung im Farbstofflaser, Shaker, 1996. w15x O. Stern, M. Volmer, Z. Phys. 20 Ž1919. 155. w16x V. Bruckner, K.-H. Feller, U.-W. Grummt, Applications of ¨ Time-Resolved Optical Spectroscopy, Elsevier, Amsterdam, 1990. w17x T. Forster, Fluoreszenz organischer Verbindungen, Vanden¨ hoeck & Ruprecht, Gottingen, 1961 Žrepublication of 1st ¨ ed... w18x V.E. Korobov, A.K. Chibisov, J. Photochem. 9 Ž1978. 411. w19x D.N. Dempster, T. Morrow, M.F. Quinn, J. Photochem. 2 Ž1973. 343.