Photochemistry and Photobiology Vol. 49, No. 4, pp. 395-399, 1989 Printed in Great Britain. All rights reserved 0031 -8655189 $03 .OO +0.00 Copyright 0 1989 Pergamon Press plc OPTICAL ABSORPTION AND EMISSION STUDIES ON LASER DYE INCORPORATED IN NAFION MEMBRANE HARI MOHAN, P. N. MOORTHY and R. M. IYER* Chemistry Division, Bhabha Atomic Research Centre, Bombay 400 085, India (Received 20 July 1988; accepted 7 November 1988) Abstract-Incorporation of the laser dye 7-amino-4-methyl coumarin (C-120) in a perfluoro sulfonate cation exchange membrane (Nafion) from aqueous solution has been studied by following its light absorption and emission (fluorescence) characteristics. It is shown that in the H' form of the membrane the dye exists in its rnonoprotonated cationic form, whereas in different cation exchanged forms of the membrane it is incorporated as the unprotonated neutral molecules. By repeated equilibration with the aqueous solution, high concentrations of the dye could be loaded into the membrane. As compared to an aqueous solution, the polymer matrix was found to confer very good photostability to the dye INTRODUCTION Photostability of the dyes used as the lasing medium in dye lasers is an essential requirement in connec- tion with the use of these devices in research and industry. As photodegradation of the dye results from the reaction of dye triplets, enhanced stability can be achieved by the use of triplet scavengers and by employing viscous and solid matrices where diffusion controlled triplet reactions can be expected to be slower than in fluid media. Limited studies on the use of laser dyes in viscous media and polymer matrices have been reported. Water soluble poly- mers such as polyvinyl alcohol (Leutwyler et al., 1976) have been recommended for jet stream dye lasers for high stability and high power applications. Laser emission from rhodamine dyes (Peterson and Snavely, 1968) in polymethylmethacrylate rods was observed at lower lamp intensities. Higher optical stability of dyes in polymer matrices was observed (Peterson and Snavely, 1968; Ivanova et al., 1981; Danilov et al., 1983; Gromov et al., 1984). However the solubility of dye in such rigid matrices is rather limited and restricts their utility. Recently, the per- fluoro sulfonate cation exchange membrane (Nafion) has been shown to be a very good medium for incorporation of high concentration of aromatic molecules such as pyrene (Lee and Mesel, 1985). The membrane consists of a tetrafluoroethylene backbone with perfluorinated ether side chain ter- minating with sulphonic acid groups: [(CFrCF2),,,-CF-CF2],, I (O-CF,-CF-CFj)k I 0 I CFZ-CF2-SO1H *To whom correspondence should be addressed. where m=5 to 13.5, n=lOOO and k=l, 2, 3. Such a membrane has molecular weights in the range of 1000-1500. The membrane provides very good chemical, mechanical and thermal stability (Komo- roski and Mauritz, 1978). Photochemical processes on a number of probe molecules have been studied in this matrix (Childs and Gibala, 1982; Olah et al., 1977, 1980). We report here the results of our stud- ies on the absorption characteristics of this mem- brane with respect to a typical laser dye C-120 (7- amino-4-methyl coumarin), and photophysical prop- erties in this polymer matrix. MATERIALS AND METHODS Materials. The matrix employed was 0.18 rnm thick sheet of Nafion 117, a commercial product of E. I. duPont Corporation, Wilmington, DE (density = 1.99 g cm-3). Laser grade C-120 has been synthesized in our laboratory (Rao et at., 1988) and was used as such. All other chemi- cals used were of the highest grade of purity. Instruments. Aminco-Bowman spectrophotofluoro- meter model 4-8202B was used for steady state fluoresc- ence emission measurements and optical absorption spec- tra were recorded on Hitachi spectrophotometer model 330. For fluorescence life time measurements, the single photon counting technique was employed making use of fluorescence decay time spectrometer model 199 (Edin- burgh Instruments Ltd). Photostability of C-120 in Nafion membrane and in aqueous solutions was studied by observ- ing the decrease in its concentration on photolysis with Rayonet Photochemical Reactor. The photon flux from this source as determined by using potassium ferrioxalate actinometer (Hatchard and Parker, 1956) was 1.63 x 10'' photons me-' min-I. Procedure. Nafion membrane in H' form was washed with deionized water, dried at 30°C and used for all the experimental work. other forms of the membrane were prepared from the H' form by equilibration with 1.0 mol dm-? solutions of hydroxide or chloride of the desired metal ion. Membrane of size 1 x 5 cm was equilibrated with 15 me of the dye solution which was kept continuously stirred. After equilibration for a known interval of time the membrane was washed with water, dried and used for further studies. High absorbance values in the membrane were measured with the use of 'auto zero' mode of the spectrophotometer. For life time measurements, the fluo- 395
Transcript
Photochemistry and Photobiology Vol. 49, No. 4, pp. 395-399, 1989 Printed in Great Britain. All rights reserved
OPTICAL ABSORPTION AND EMISSION STUDIES ON LASER DYE INCORPORATED IN NAFION MEMBRANE
HARI MOHAN, P. N. MOORTHY and R. M. IYER* Chemistry Division, Bhabha Atomic Research Centre, Bombay 400 085, India
(Received 20 July 1988; accepted 7 November 1988)
Abstract-Incorporation of the laser dye 7-amino-4-methyl coumarin (C-120) in a perfluoro sulfonate cation exchange membrane (Nafion) from aqueous solution has been studied by following its light absorption and emission (fluorescence) characteristics. It is shown that in the H' form of the membrane the dye exists in its rnonoprotonated cationic form, whereas in different cation exchanged forms of the membrane it is incorporated as the unprotonated neutral molecules. By repeated equilibration with the aqueous solution, high concentrations of the dye could be loaded into the membrane. As compared to an aqueous solution, the polymer matrix was found to confer very good photostability to the dye
INTRODUCTION
Photostability of the dyes used as the lasing medium in dye lasers is an essential requirement in connec- tion with the use of these devices in research and industry. As photodegradation of the dye results from the reaction of dye triplets, enhanced stability can be achieved by the use of triplet scavengers and by employing viscous and solid matrices where diffusion controlled triplet reactions can be expected to be slower than in fluid media. Limited studies on the use of laser dyes in viscous media and polymer matrices have been reported. Water soluble poly- mers such as polyvinyl alcohol (Leutwyler et al . , 1976) have been recommended for jet stream dye lasers for high stability and high power applications. Laser emission from rhodamine dyes (Peterson and Snavely, 1968) in polymethylmethacrylate rods was observed at lower lamp intensities. Higher optical stability of dyes in polymer matrices was observed (Peterson and Snavely, 1968; Ivanova et al . , 1981; Danilov et a l . , 1983; Gromov et al., 1984). However the solubility of dye in such rigid matrices is rather limited and restricts their utility. Recently, the per- fluoro sulfonate cation exchange membrane (Nafion) has been shown to be a very good medium for incorporation of high concentration of aromatic molecules such as pyrene (Lee and Mesel, 1985). The membrane consists of a tetrafluoroethylene backbone with perfluorinated ether side chain ter- minating with sulphonic acid groups:
[(CFrCF2),,,-CF-CF2],, I
(O-CF,-CF-CFj)k I 0
I CFZ-CF2-SO1H
*To whom correspondence should be addressed.
where m=5 to 13.5, n = l O O O and k = l , 2, 3. Such a membrane has molecular weights in the range of 1000-1500. The membrane provides very good chemical, mechanical and thermal stability (Komo- roski and Mauritz, 1978). Photochemical processes on a number of probe molecules have been studied in this matrix (Childs and Gibala, 1982; Olah et a l . , 1977, 1980). We report here the results of our stud- ies on the absorption characteristics of this mem- brane with respect to a typical laser dye C-120 (7- amino-4-methyl coumarin), and photophysical prop- erties in this polymer matrix.
MATERIALS AND METHODS
Materials. The matrix employed was 0.18 rnm thick sheet of Nafion 117, a commercial product of E. I. duPont Corporation, Wilmington, DE (density = 1.99 g cm-3). Laser grade C-120 has been synthesized in our laboratory (Rao et at., 1988) and was used as such. All other chemi- cals used were of the highest grade of purity.
Instruments. Aminco-Bowman spectrophotofluoro- meter model 4-8202B was used for steady state fluoresc- ence emission measurements and optical absorption spec- tra were recorded on Hitachi spectrophotometer model 330. For fluorescence life time measurements, the single photon counting technique was employed making use of fluorescence decay time spectrometer model 199 (Edin- burgh Instruments Ltd). Photostability of C-120 in Nafion membrane and in aqueous solutions was studied by observ- ing the decrease in its concentration on photolysis with Rayonet Photochemical Reactor. The photon flux from this source as determined by using potassium ferrioxalate actinometer (Hatchard and Parker, 1956) was 1.63 x 10'' photons me-' min-I.
Procedure. Nafion membrane in H' form was washed with deionized water, dried at 30°C and used for all the experimental work. other forms of the membrane were prepared from the H' form by equilibration with 1.0 mol dm-? solutions of hydroxide or chloride of the desired metal ion. Membrane of size 1 x 5 cm was equilibrated with 15 me of the dye solution which was kept continuously stirred. After equilibration for a known interval of time the membrane was washed with water, dried and used for further studies. High absorbance values in the membrane were measured with the use of 'auto zero' mode of the spectrophotometer. For life time measurements, the fluo-
395
396 HARI MOHAN et al.
rescence emission from the membrane was counted for about lo4 counts at the peak position (Ac,,,). The errors in the life time were < ?10%.
RESULTS AND DISCUSSION
Figure la-lc show the optical absorption spec- trum of C-120 in aqueous solution (8.8 x lop5 mol dm-3, pH = 6.0) in which Nafion membrane (H+ form) has been kept for different intervals of time. After 24 h the solution shows complete absence of the characteristic band of C-120 at 345 nm. Figure 2a shows the optical absorption spectrum of the membrane after equilibrating with the aqueous sol- ution (8.8 x rnol dm-') for 24 h. It shows absorption bands at 300 and 260 nm in contrast to one absorption band at 345 nm observed in aqueous solutions (Fig. la) . The background absorption of the blank membrane is shown in Fig. 2b. The absorption spectrum of the membrane after equili- brating with the aqueous solution of C-120 must be due to the incorporation of C-120 in the membrane. The extinction coefficient of C-120 in water has been determined to be 1.58 X lo4 dm3 mol-' cm-'. On this basis, the concentration of C-120 in the membrane corresponds to 3.7 x lo-' rnol dm-' of the membrane. The concentration of C-120 incor- porated into the membrane was also calculated from decrease in the concentration of C-120 in the aque-
W
z a m a I.0- 0 v) m 4
0.0- , 200 300 400
WAVELENGTH (nm)
Figure 1 . optical absorption spectra of aqueous solution (pH = 6.0) of C-120 on equilibration with Nafion mem- brane after (a) 0 h; (b) 5 h; (c) 24 h; (d) aqueous solution (pH = 1.0) of C-120 after 0 h and (e) variation in absorb-
ance (345 nm) of C-120 with pH of aqueous solution.
I 5 . 0 r ?
WAVELENGTH (nm)
Figure 2. Optical absorption spectra of Nafion membrane on equilibration with aqueous solution (pH = 6.0) after (a) 24 h; (b) 0 h and (c) increase in the absorption (300 nrn) of C-120 in the membrane with equilibration time.
ous solution. The calculated value is 6.5 x lo-' mol dm-3, which is close to the value determined directly by spectrophotometer. The concentration of C-120 in the aqueous solution decreases linearly up to 5 h of equilibration with the membrane (Fig. 3) . After 5 h of equilibration, the membrane (H') is again kept immersed in a fresh solution of C-120, for another 5 h. Further decrease in the concen- tration of C-120 in the solution and increase in the membrane is observed (Fig. 2c). The concentration of C-120 in the membrane reaches a plateau value at 5.0 x lo-* mol dm-3.
The H+ ion capacity in the dry membrane is 0.935 meq g-' (Pushpa et al., 1988) which corresponds to 50.6 x 10IH H' ions in the film of dimensions: 1 x 5 x 0.018 cm. The saturation concentration of C- 120 in the membrane is 5.0 x lo-* rnol dm-3, which corresponds to 1.3 x 10lx C-120 molecules in the film of above size. This is -40 times lower than the capacity of the membrane indicating that C-120 is not able to replace all the H+ in the membrane.
In aqueous solution the dye is known to exist in two forms (Drexhage 1973):neutral form (A) and cationic form (B) with a pK value of 2.2 (Fig. le). Aqueous solution of neutral form of C-120 (pH = 6.0) shows absorption bands at 345 nm (Fig. l a ) whereas cationic form shows absorption bands at 265 and 305 nm (Fig. Id).
The incorporation of C-120 in the membrane may either result from exchange with H+ of sulphonic acid group or as a local precipitate in the water clusters in the fluorocarbon backbone network of the membrane. In the former case, C-120 would
Optical and emission studies 397
exist in the cationic form whereas in the latter case, it would be in the neutral form. Since the absorption spectrum of membrane containing C-120 agrees closely with the absorption spectrum of cationic form of C-120 (except for slight blue shift of the bands), we infer that C-120 is largely present in the cationic form in the membrane. If the neutral form of C-120 was present in the membrane, then it would have shown absorption band at 345 nm (with slight shift).
There are two ways for the incorporation of cat- ionic form of C-120 in the membrane from neutral aqueous solution. (a) The cationic form in equilib- rium (1) may directly exchange with Hi of sulphonic acid groups or (b) H+ form of the membrane has a very low p H inside the membrane. The neutral form of C-120 may be converted to cationic form inside the membrane. An attempt was made to distinguish between these two processes by comparing the rates of incorporation of C-120 in the membrane from neutral and acidic aqueous solutions. This, however was inconclusive as the rates were observed to be comparable in both the cases.
Incorporation of dye in cationic form
The following experiments further confirm the presence of C-120 in the cationic form in the mem- brane: (1) The equilibration of the membrane with the aqueous solution of C-120 at pH = 1.0, where the dye exists in cationic structure, also resulted in the incorporation of C-120 in the membrane whose optical absorption spectrum was similar to the one shown in Fig. 2a confirming that C-120 is present in the membrane in cationic form only. Using the value of 0.82 x lo4 dm' mol-' cm-' for the extinc- tion coefficient of C-120 (305 nm) in aqueous sol- ution at pH = 1.0, the concentration of the dye incorporated into the membrane was estimated to be equal to 5.4 x mol dm-'. This is close to the value of C-120 incorporated into the membrane from neutral aqueous solution. (2) It was also observed that compounds such as rhodamine B,
Table 1. Physical properties of dyes
thionine and tryptophan which exist in cationic form in aqueous solutions even at neutral p H could also be incorporated in the membrane and their absorp- tion spectra were similar to those of aqueous sol- utions. (3) The physical properties of the dyes in the membrane and in aqueous solutions are shown in Table 1 and are observed t o be similar to those of their cationic forms.
Incorporation of dye in neutral form
If the dye molecules are not able to exchange with the H+ of sulphonic acid groups of the mem- brane, then they would be present as a local precipi- tate in water clusters in the polymer backbone net- work of the membrane. Membrane in which H+ of sulphonic acid groups was replaced by different metal ions was used for absorption of C-120 from the aqueous solutions. Figure 3 shows the decrease in the concentration of C-120 (1.64 x mol dm-3) in aqueous solution ( p H = 6.0) in which the membrane with different metal ions was kept immersed for different time intervals. At this pH, different metal ions are expected to remain in the membrane. Almost complete absorption of C-120 by Nafion (H+) takes place in 30 h (Fig. 3). O n the other hand, absorption of C-120 by the different cation exchanged forms of Nafion was very small for the same period of time (Fig. 3). The saturation concentration of C-120 in Nafion (Na+) is estimated to be 1.2 x mol dm-3. Optical absorption spectra of the membrane in which C-120 was incor- porated show absorption band in the region of 330-340 nm (Table 2), which is in agreement with the absorption spectrum of neutral form of C-120 in aqueous solution. The fluorescence emission spectra and decay life times of C-120 in these membrane also agreed with those of the neutral form of C-120 in the aqueous solution at pH = 6.0 (Table 2). This shows that dye molecules are not able to replace different metal ions and hence must be present as local precipitates in the water clusters in the polymer backbone network. The atomic absorption spectro-
4.01r------ incorporation of C-120 in the cationic form in Nafion (Na+) from acidic aqueous solution would not be due to exchange of the cationic form of C- 120 with Na+. Therefore we infer that the mem- brane (Na') is first converted to the H+ form and then C-120 is incorporated in the cationic form from the acidic aqueous solution.
Stability of C-120 in Nafion
0.0 Photodegradation of C-120 in organic solvents 0 10 20 30 40 has been investigated (Kunjappu and Rao, 1987)
and the dve is found to be stable in uolar solvents. TIME ( H I
Figure 3. Decrease in the concentration of C-120 in the
equilibration with Nafion membrane containing cations.
oxygen ehhanced the rate of decom~osition of the
(1.1 x mol dmP3) in aerated and deaerated
aqueous (2.0 mol dm- '? pH = 6'0) On dye. Figure 4a and b show the stability of C-120
scopic studies of the aqueous extract obtained on equilibration of Nafion (Na+) with neutral aqueous solution of C-120 also showed the absence of Na+ showing that the dye molecules are not able to replace Na+ in the membrane.
When the different cation exchanged forms of the membrane were equilibrated with C-120 in acidic aqueous solutions (pH = l.O), the optical absorp- tion spectra of the membrane showed that the dye is present in the cationic form. The atomic absorption spectroscopic studies of aqueous extract obtained on equilibration of Nafion (Na+) with acidic aqueous solution of C-120 showed the presence of Na+ in the aqueous solution. The cations of C-120 may either replace M' or the membrane is converted to H+ form and then the dye is incorporated in the membrane in the cationic form. To distinguish between these possibilities we studied the incorpor- ation of thionine in the membrane (Na+) from neu- tral aqueous solution, where it exists in the cationic form (TH'). After 24 h of equilibration of the membrane (Na+) with neutral aqueous solution of thionine (3.3 X mol dm-"), the aqueous extract showed the absence of Na+ indicating that (TH+) is unable to replace Na' in the membrane. Therefore
aqueous solutions on photolysis by 350 nm light. Appreciable decomposition is observed in aerated solutions. On the contrary Nafion membranes (H') containing different concentrations of C-120 were found t o be stable on photolysis (300 nm) during the same period of time (Fig. 4c-f). C-120 present in Nafion membrane (Na') was also found to be stable on photolysis at 350 nm. These results show that C-120 in Nafion membrane is not amenable to photodegradation at least up to a photon flux of 2.9 x l0ly photons me-'. Also when the membrane containing C-120 was kept immersed in water (pH = 6.0) for 4 days, there was no leaching out of the dye.
CONCLUSIONS
The present studies show that incorporation of high concentration of laser dyes in Nafion mem- brane is possible. The dye molecules are present in the membrane (H+) in the cationic form. In cation exchanged membranes (M'), dye molecules are unable t o replace M+ and are present in the mem- brane in the polymer network. These dyes are pho- tostable in this matrix, which may be due either to low availability of oxygen, or due to the highly polar nature of the membrane.
Table 2. Physical properties of C-120 in Nafion membrane and aqueous solution
Fluorescence
S. No. System
1 2 3
C-l20/Nafion (H') C-120iNafion (Na + ) C-l20/Nafion (K') C-I20/Nafion (Mg2+) C-l20/Nafion (Co2+) C-l20/Nafion ( A P ) C-I20/Nafion (Cu*') C-l20/Water (pH = 6.0) C-12WWater (pH = 1.0)
Figure 4. Effect of photolysis on the concentration of C-120 in aqueous solutions (pH = 6.0, concentration = 1.1 x mol IT-^) (a) aerated and (b) deaerated solution; in Nafion membrane containing (c) 5.6 x 10V; (d) 2.8 x mol d m 3 of the dye. (e) 2.0 x and (f) 1.2 x
Acknowledgemenfs-Sincere thanks are due to Dr. D. Nandan for providing Nafion membrane and for helpful discussions. Thanks are also due to Dr. T. Mukherjee for help in fluorescence life time measurements.
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