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The spectroscopic behaviour of rhodamine 6G in polar and non-polar solvents and in thin glass and PMMA films
Volume 147, number 2,3
CHEMICAL PHYSICS LETTERS
3 June 1988
THE SPECTROSCOPIC BEHAVIOUR OF RHODAMINE 66 IN POLAR AND NON-POLAR SOLVENTS AND IN THIN GLASS AND PMMA FILMS *
Renata REISFELD ‘, Rivka ZUSMAN, Yoram COHEN and Marek EYAL Department ofInorganic and Analytical Chemistry. The Hebrew University, 91904 Jerusalem, Israel
Received I5 February 1988; in fmal form 5 April 1988
Absorption, fluorescence, quantum yields and lifetimes of rhodamine 6G in water, methanol, dichloroethane and chloroform were measured. From a comparison of these quantities with those for thin glass and plastic films conclusions are drawn about dimer formation and the stability of rhodamine 6G in these materials. The stability in the films is increased as compared with the
1. Introduction
In contrast to the majority of organic dyes, Rh6G can be incorporated into both non-polar plastic materials and glasses, which provides a rare opportunity for studying the behaviour of a dye in polar and non-polar environments both liquid and solid. Rhodamine 6G (Rh6G) dissolved in liquid solvents is one of the most common laser dyes for continuous wave (cw) operation [ 11 and short-pulsed ( -c 1.5 ps generation) lasers [ 21. This dye has also been considered as a laser material in a PMMA (poly( methyl methacrylate) ) matrix [ 31 and in glasses [ 41. In designing lasers where the active medium is a solid the effect of the host matric on the dye characteristics is of the utmost importance. For example, it was claimed that Rh6G incorporated in a mixture of MMA (methyl methacrylate) and PMMA has a lasing efficiency of 36Oh[ 3 1; however the system is photo-unstable. The photostability of dyes may be increased considerably when the organic laser host is covered by a protective layer of UV absorbing glass [ 51 or when the organic host is replaced by an inorganic glass matrix [ 6,7]. The process of incorporating the organic dye into the glass is achieved by utilizing the sol-gel method [ 6-9 1. * This work was supported by the US-Israel Binational Fund. ’ Enrique Berman Professor of Solar Energy.
142
Rh6G incorporated in thin glass films deposited on glass plates may also be considered for luminescent solar concentrators (LSC) [ 5,1O-l 2 1. These devices collect direct and diffuse solar light on plane surfaces. The solar photons are subsequently converted into fluorescent light in a narrow spectral range, which is then concentrated at the collector’s edges. When solar cells are attached to the edges the concentrated light of specific wavelengths is converted into electricity. This simple device enables the photovoltaic cell area to be reduced. The operation of a LSC is based on the absorption of solar radiation in a collector containing a fluorescent species in which the emission bands have little or no overlap with the absorption bands. The fluorescence emission is trapped by total internal reflection and concentrated at the edges of the collector which is usually a thin plate [ lo,13 1. LSCs have the following advantages over conventional solar concentrators: they collect both direct and diffuse light; there is good heat dissipation of non-utilized energy by the large area of the collector plate in contact with air so that essentially “cold light” reaches the photovoltaic cells; tracking the sun is unnecessary; the luminescent species can be chosen to allow matching of the concentrated light to the maximum sensitivity of the photovoltaic cell [ 10-l 41. The performance of LSCs which absorb light in a large plate area and convert it to luminescence which
0 009-2614/88/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
CHEMICAL PHYSICS LETTERS
Volume 147, number 2,3
ultimately concentrates at the edges of the plate has been discussed in refs. [10,11,151. The optical efficiency of an LSC may be expressed as %pt =
( 1 -Rhbs%l?St?trfl~ar
>
(1)
where R is the Fresnel reflection COefflCient, qabs iS the efficiency of the absorbed light, qfl is the fluorescence efficiency, qs, is the Stokes efficiency, r,~~, is the efficiency of the trapped fluorescence due to internal reflections and qDaris the efficiency including parasitic losses. r&s, qfl and qst are parameters characteristic of a dye in a given medium. The trapping qt, and the parasitic qpar efficiencies are dependent on the material from which the collection is made. In general, the optical efficiencies of LSCs containing fluorescent species can be obtained from the calculated absorption and fluorescence efficiency [ 12 1. These values are helpful for determining which specific material should be studied. In the present paper we discuss the properties of LSCs prepared by coating transparent plates of PMMA and glass with thin films of PMMA and glass doped by the dye Rh6G. The advantage of thin films having optical contact with a transparent thicker plate is that the fluorescence is trapped in the bulk of the plate and thus selfabsorption is avoided. Since the thin films are prepared from appropriate solutions containing the dye, such behaviour as spectral dependence on the solvent, amount of dimerization, molar absorption and quantum efficiencies need to be studied. In this study Rh6G was chosen specifically since in contrast to the similar rhodamine B, Rh6G does not exist in a lactonic form and much higher quantum efficiencies may be obtained [ 16 1. Selwyn and Steinfeld [ 17 ] have found that Rh6G in aqueous solution tends to dimerize at a concentration of approximately 10e4 M/Q. The distinction between the non-fluorescent dimer and the highly luminescent monomer is a difficult task since part of the two spectra overlap. Penzkofer et al. [ 18-201 have studied the behaviour of Rh6G in methanol and found a rapid reduction of fluorescence lifetime at about lo-’ M/Q due to dynamic aggregates (different to the stable dimers formed in water). These authors [ 20 ] performed a separation of the monomer
3 June 1988
spectrum from those of other aggregates by a mathematical procedure. In what follows we show that the various species of Rh6G existing in solution can be distinguished by a straightforward difference spectroscopy method [ 2 1 ] which was recently applied to Rh B in water solution.
2. Experimental The following materials were used: rhodamine 6G chloride (Eastman Kodak Co. ), methanol, chloroform, 1,2-dichloroethane, and hydrochloric acid (all analytical grade, Frutarom) , tetramethoxysilane (TMOS ) (Aldrich), Triton X 100 and PMMA beads (laboratory grade) (both from BDH ) , PMMA plates GS 2 151 (BASF-24 1) (from Rohm GmbH ) and triply distilled water. A series of solutions of Rh6G at concentrations varying from 10M6to 10m4 M/Q were prepared in water, methanol, 1,2dichloroethane and chloroform. The absorption spectra were measured using Perkin-Elmer-Lambda double beam and Kontron Uvikon spectrophotometers using air as a standard. The measurements were performed at temperatures of 18 and 52°C after 1 h and then at intervals of 20, 40, 90, 130 and 160 h. The thin films of PMMA were prepared by dissolving the appropriate weights of PMMA beads and Rh6G in chloroform to give a concentration of 1.4x 1Om4M/Q of Rh6G in PMMA, and pouring 5 ml of this solution on to transparent PMMA plates ( 5 x 5 x 0.3 cm) while spinning in a centrifuge for 1 min at 30-40°C. A transparent uniform-emitting plate was thus obtained. The sol-gel glass films on glass plates were similarly prepared using a 4.5 x 10e4 M/Q solution of Rh6G in methanol to which was added 1.25 ml 0.03 N HCl, 10 drops of Tritonx 100 and 2.5 ml TMOS to give 25 ml of solution. The TMOS solution was allowed to hydrolyze for 30 min and the films prepared on low iron content glass plates of dimensions 5 X 5 X 0.3 cm. The glasses were kindly provided by Mr. M. Harari and Kibbutz Tzuba. The difference spectra were obtained by subtracting the spectra of 10T6 M/Q solutions of equal optical density in which no dimers or aggregates were 143
Volume 147, number 2,3
CHEMICAL PHYSICS LETTERS
3 June 1988
observed, e.g. the spectrum of 10s4 M/g was measured in a cell of 0.1 cm thickness while the spectrum of 10m6M/Q was measured in a 1 cm wide cell and multiplied by 10 in situ. The fluorescence spectra were measured on a commercial SLM 4800C spectrofluorimeter, which provides corrected emission spectra. The fluorescence lifetimes were obtained using the same apparatus with a phase-shift attachment. This attachment only allows measurement of lifetimes exhibiting exponential behaviour. The quantum efficiencies were measured by the comparative method [ 22 ] using as standard for solutions a solution of Rh6G in methanol which has a quantum efficiency of 98% [ 23 1, and for thin films of dye in PMMA a PMMA plane of BASF-24 1 with a known quantum efficiency of 98% f 2%.
3. Results and discussion Fig. 1 presents the absorption (A) and emission (B) of Rh6G in chloroform (curve 1), methanol (curve 2), thin glass film (curve 3) and thin plastic (PMMA) film (curve 4) on glass support. Absorption and emission maxima and lifetimes are summarized in table 1. Fig. 2 presents absorption and differential spectra of Rh6G in water (A), methanol (B), dichloroethane (C) and chloroform (D). Curves 1 are due to absorption of lob6 M/g solution of Rh6G measured in a 1 cm cell (multiplied by lo), curves 2 represent absorption of 10m4M/R solution measured in a 0.1 cm cell. The existence of dimers in water (a) can be clearly seen in curve 2; however, the exact position of the monomer peaks can be obtained from the minimum of the differential curves 3 (524 nm) and of the dimers from the maxima of the differential curve (499 and 560 nm). Quantitatively the absorption spectra of the dimers can be explained by LCAO theory, the bonding orbital being at about 17860 cm- ’ and the antibonding at about 20000 cm-‘. The higher intensity of the latter indicates, from symmetry considerations, that the transition probability from the ground state to the antibonding state is higher. Addition of pcyclodextrin decreases the dimer concentration as has been shown previously for rhodamine B [ 24 1. P_cyclodextrin (CD) is a cyclic polysugar com144
500
550
600
Wavelength
650
700
7 10
[ nm]
Fig. 1. Absorption (A) and emission (B) spectra of rhodamine 6G in: 1, methanol; 2, chloroform; 3, thin film of poly(methyl methacrylate) on poly (methyl methacrylate) support; 4, thin film of sol-gel glass on a glass support.
posed of glucose units linked by l-4 glycoside bonds. The hydrophobic cavity present in the structure is capable of binding the organic dye in a monomeric form since the cavity has a diameter of 6.2 A and height 7 A. We prepared a solution of 1O-2 M/I1 CD in water to which Rh6G was added to form a total concentration of lob4 M/B. The absorption spectra, measured in 0.1 cm cuvettes, are presented in fig. 3, where curve 2 presents the absorption of Rh6G without CD, curve 1 represents the absorption of Rh6G with CD and curve 3 is the difference spectrum. The decrease of concentration of dimers as reflected by the increase of the absorption of monomers is also accompanied by an increase in the luminescence quantum yield of Rh6G. Preliminary experiments on degradation of Rh6G dissolved in water indicate that CD hinders photodegradation
3 June 1988
CI-IEMICAL PHYSICS LETTERS
Volume 147, number 2,3
Table 1 Mole concentrations, absorption and emission maxima, quantum efficiencies and lifetimes of rhodamine 6G in water, CH,OH, CHCls and I ,2_dichloroethane Concentration
Abs. max-
Em. max-
Quantum
Lifetime
Mole ext.
(mole/P)
(nm)
(nm)
efficiency
(ns)
coeff.
Hz0
lo-*
CH,OH CHCl,
10-6 1O-6
3.95kO.l 3.9 3.6 (1 day) 3.4 (5 days)
18 300 110000 82000
1o-6
553 556 556 530 544
0.88 0.98 0.75 0.40
W-W12 thin films PMMA glass
526 527 530 505 522 534 534
563 566
0.75 0.82
Solvent
1.4x 1o-4 4.5x 1o-4
I .oo
110000
-
82000
120
30
-0.21 300
I 400
I 500
II 600
I 400
I 500
I 600
I 400
3 500
WAVELENGTH
I 400
600
y 500
, 600
400 500
600
.020 700
[ nm 1
Fig. 2. Absorption spectra of rhodamine 6G in: (A) water, (B ) methanol, (C ) dichloroethane and (D) chloroform. Curve designaton: 1, low6 M/P measured in a 1 cm cell (multiplied by 10); 2, 10e4 M/P measured in a 0.1 cm cell; 3, difference of 2 against 1. (a) Difference spectra enlarged. Assignments as in fig. 2.
Table 2 Position ofdimer and monomer peaks of rhodamine 6G in various solvents Solvent
Literature [ 17]
Experimental dimer peak (nm )
water methanol chloroform dichloroethane
monomer peak (nm)
1
2
499
552
499 499
538 538
526 526 519 519
dimer peak (nm)
monomer peak (nm)
1
2
502
542
526
145
Volume 147, number 2,3
400
CHEMICAL PHYSICS LEl-fERS
500
3 June 1988
hydrogen bonds are formed between the hydrogens of water and molecules of Rh6G. In this way the water molecule bridges between two molecules of the dye. In non-polar solvents the separation of Rh6G cation and chloride anion is inefficient, the molecule is uncharged and the electrostatic repulsion between the molecules is negligible. The energetics of such a situation favour the formation of dimers; however, the nature of the bonding orbitals is different from those in water. The different nature of dimers in water and in non-polar solvents is emphasized in fig. 2a. Non-polar solvents also display a slow decrease in absorption intensity accompanied by a blue-shift at high as well as at low concentrations of Rh6G, which is promoted by temperature and/or light. This behaviour is shown in fig. 4, where changes in the absorption and emission spectra of Rh6G in chloroform are recorded as a function of time, These spectral
600
Wavelength [nm] Fig. 3. Absorption spectra of 10e4 M/P of rhodamine 6G in water, measured in a 0.1 cm cell. Curve 1, with B-cyclodextrin added; curve 2, without addition of B-cyclodextrin; curve 3, difference of 2 against 1.
hinting that the dimers in water degrade faster than the monomers. Figs. 2B, 2C and 2D represent the absorption spectra of 10m6M/I1 (designated 1) and of 10e4 M/Q (designated 2) of Rh6G in methanol, dichloroethane and chloroform, respectively. The absorption spectra of the 10m6M/P solution were measured in a 1 cm cell (multiplied by 10) and for the 10V4M/I1 solution in a 0.1 cm cell. In methanol the spectral changes with concentration are small indicating the absence of dimers but the presence of closely spaced pairs arising from random distribution of the dye molecules [ 201, The results for water and methanol are in reasonable agreement with numerical methods (see table 2 ) . Aggregation of Rh6G in both chloroform and dichloroethane is different to that in water and these aggregates decompose slowly to a non-cationic form of Rh6G. The process is enhanced by heat and light. In these solvents the intensity of the low-energy peak of the dimers is higher than the high-energy peak, the reverse haviour
146
of the situation may be explained
in water.
This
different
be-
by the fact that in water
0 IOO-
6
I
550
600
Wavelength
650 [nm ]
Fig. 4. Absorption (A) and emission (B ) spectra of 10m6M/P of rhodamine 6G in chloroform as a function of time elapsed from preparation. 1, a few minutes after preparation; 2, after two days; 3, after four days; 4, after six days; 5, after eight days.
Volume147,number2,3
CHEMICAL PHYSICSLETTERS
changes in solution are rather slow and we tend to believe that the process is controlled by a surface reaction of the dye molecules with oxygen. The degradation can be significantly diminished by incorporating the dye into a polymeric matrix where diffusion of the molecules to the surface is hindered. This leads to long-term stability of the absorption and emission maxima and to a very slow change of emission intensity. The situation improves in the inorganic glassy matrix where in addition to the stability of the emission and absorption maxima no change in emission intensity is observed over a period of wekks ad the absence of dimers ensures long-term stability.
3 June 1988
Jorgensen for most enlightening discussions and to Professor Itamar Wilner for his help with the absorption measurements.
References [ I] F.P.Schaefer,Topicsin appliedphysics,Vol.1. Dye lasers
(Springer,Berlin,1973). [2] S.Singh,Appl.Opt.26 (1987) 66. [ 31V.V.Rodchenkova,S.A.Tsogocva,T.M.Muraveva,L.K. Denisovand B.M. Uzhinov, Opt. i Spectroskopiya 60 (1986) 35.
[ 41 R. Reisfeld, Criteria and prospects of new lasers based on fluorescent dyes in glasses, J. Phys. (Paris), to be published.
[ 51R. Reisfeld and R. Zusman, Solar Concentrator Plates, US Patent No. 4,661,649 (28 April 1987).
[ 61 D. Avnir, D. Levy and R. Reisfeld, J. Phys. Chem. 88 ( 1984)
41 Conclusions In the present work a straightforward method for estimating the amount of dimerization and the position of the dimer peaks is applied. The results lead to the following conclusions: (a) Different types of dimers of Rh6G are formed in water and in the non-polar solvents studied in the present work. (b) Addition of P-cyclodextrin to the starting solution of thin glass films ensures the maximum amount of monomers in the film. (c) Plastic thin films must be prepared from fresh solutions of Rh6G in chloroform. The degradation process is frozen in the plastic and, with the evaporation of chloroform, the dye molecules persist indefinitely in the state “inherited” from the solution.
Acknowledgement The authors are grateful to the National Council of Research and Development for a scholarship awarded to RZ. We are also grateful to Professor C.K.
5956.
[ 71 R. Reisfeld, M. Eyal and R. Gvishi, Chem. Phys. Letters 138 (1987) 377. [ 81 D. Avnir, V.R. Kaufman and R. Reisfeld, J. Non-Cryst. Solids 74 (1985) 395. [9] D. Brusilovsky and R. Reisfeld, Chem. Phys. Letters 141 (1987) 119. [ lo] R. Reisfeld and C.K. Jergensen, Struct. Bonding 49 ( 1982)
[ 111 i. Reisfeld, J. Less-Common Metals 93 ( 1983) 243. [ 12] R. Reisfeld, J. Less-Common Metals 112 ( 1985) 9. [ 131 R. Reisfeld, Inorg. Chim. Acta 140 ( 1987) 345. [ 14) R. Reisfeld, Inorg. Chim. Acta 95 (1984) 69. [IS] N. Neuroth and R. Haspel, Proc. SPIE 653 (1986) 88. [ 161 D.A. Hinckley, P.G. Seybold and D.P. Borris, Spectrochim. Acta 42A ( 1986) 747. [ 171J.E. Selwyn and J.I. Steinfeld, J. Phys. Chem. 76 (1972) 762.
[ 181A. Penzkofer and Y. Lu, Chem. Phys. 103 (1986) 399. [ 191 Y. Lu and A. Penzkofer, Chem. Phys. 107 (1986) 175. [ 201 A. Penzkofer and W. Leupacher, J. Luminescence 37 ( 1987) 61. [21] L. Bruzzone and M.E. Rosello, Appl. Spectry. 40 ( 1986) 1066. [ 221 R. Reisfeld, NBS 76A ( 1972) 613. [23] J.S. Batchelder, A.H. Zewail and T. Cole, Appl. Opt. 20 (1981) 3733. [24] Y. Degani, I. Wilner and Y. Haas, Chem. Phys. Letters 104 (1984) 496.
147
CHEMICAL PHYSICS LETTERS
3 June 1988
THE SPECTROSCOPIC BEHAVIOUR OF RHODAMINE 66 IN POLAR AND NON-POLAR SOLVENTS AND IN THIN GLASS AND PMMA FILMS *
Renata REISFELD ‘, Rivka ZUSMAN, Yoram COHEN and Marek EYAL Department ofInorganic and Analytical Chemistry. The Hebrew University, 91904 Jerusalem, Israel
Received I5 February 1988; in fmal form 5 April 1988
Absorption, fluorescence, quantum yields and lifetimes of rhodamine 6G in water, methanol, dichloroethane and chloroform were measured. From a comparison of these quantities with those for thin glass and plastic films conclusions are drawn about dimer formation and the stability of rhodamine 6G in these materials. The stability in the films is increased as compared with the
1. Introduction
In contrast to the majority of organic dyes, Rh6G can be incorporated into both non-polar plastic materials and glasses, which provides a rare opportunity for studying the behaviour of a dye in polar and non-polar environments both liquid and solid. Rhodamine 6G (Rh6G) dissolved in liquid solvents is one of the most common laser dyes for continuous wave (cw) operation [ 11 and short-pulsed ( -c 1.5 ps generation) lasers [ 21. This dye has also been considered as a laser material in a PMMA (poly( methyl methacrylate) ) matrix [ 31 and in glasses [ 41. In designing lasers where the active medium is a solid the effect of the host matric on the dye characteristics is of the utmost importance. For example, it was claimed that Rh6G incorporated in a mixture of MMA (methyl methacrylate) and PMMA has a lasing efficiency of 36Oh[ 3 1; however the system is photo-unstable. The photostability of dyes may be increased considerably when the organic laser host is covered by a protective layer of UV absorbing glass [ 51 or when the organic host is replaced by an inorganic glass matrix [ 6,7]. The process of incorporating the organic dye into the glass is achieved by utilizing the sol-gel method [ 6-9 1. * This work was supported by the US-Israel Binational Fund. ’ Enrique Berman Professor of Solar Energy.
142
Rh6G incorporated in thin glass films deposited on glass plates may also be considered for luminescent solar concentrators (LSC) [ 5,1O-l 2 1. These devices collect direct and diffuse solar light on plane surfaces. The solar photons are subsequently converted into fluorescent light in a narrow spectral range, which is then concentrated at the collector’s edges. When solar cells are attached to the edges the concentrated light of specific wavelengths is converted into electricity. This simple device enables the photovoltaic cell area to be reduced. The operation of a LSC is based on the absorption of solar radiation in a collector containing a fluorescent species in which the emission bands have little or no overlap with the absorption bands. The fluorescence emission is trapped by total internal reflection and concentrated at the edges of the collector which is usually a thin plate [ lo,13 1. LSCs have the following advantages over conventional solar concentrators: they collect both direct and diffuse light; there is good heat dissipation of non-utilized energy by the large area of the collector plate in contact with air so that essentially “cold light” reaches the photovoltaic cells; tracking the sun is unnecessary; the luminescent species can be chosen to allow matching of the concentrated light to the maximum sensitivity of the photovoltaic cell [ 10-l 41. The performance of LSCs which absorb light in a large plate area and convert it to luminescence which
0 009-2614/88/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
CHEMICAL PHYSICS LETTERS
Volume 147, number 2,3
ultimately concentrates at the edges of the plate has been discussed in refs. [10,11,151. The optical efficiency of an LSC may be expressed as %pt =
( 1 -Rhbs%l?St?trfl~ar
>
(1)
where R is the Fresnel reflection COefflCient, qabs iS the efficiency of the absorbed light, qfl is the fluorescence efficiency, qs, is the Stokes efficiency, r,~~, is the efficiency of the trapped fluorescence due to internal reflections and qDaris the efficiency including parasitic losses. r&s, qfl and qst are parameters characteristic of a dye in a given medium. The trapping qt, and the parasitic qpar efficiencies are dependent on the material from which the collection is made. In general, the optical efficiencies of LSCs containing fluorescent species can be obtained from the calculated absorption and fluorescence efficiency [ 12 1. These values are helpful for determining which specific material should be studied. In the present paper we discuss the properties of LSCs prepared by coating transparent plates of PMMA and glass with thin films of PMMA and glass doped by the dye Rh6G. The advantage of thin films having optical contact with a transparent thicker plate is that the fluorescence is trapped in the bulk of the plate and thus selfabsorption is avoided. Since the thin films are prepared from appropriate solutions containing the dye, such behaviour as spectral dependence on the solvent, amount of dimerization, molar absorption and quantum efficiencies need to be studied. In this study Rh6G was chosen specifically since in contrast to the similar rhodamine B, Rh6G does not exist in a lactonic form and much higher quantum efficiencies may be obtained [ 16 1. Selwyn and Steinfeld [ 17 ] have found that Rh6G in aqueous solution tends to dimerize at a concentration of approximately 10e4 M/Q. The distinction between the non-fluorescent dimer and the highly luminescent monomer is a difficult task since part of the two spectra overlap. Penzkofer et al. [ 18-201 have studied the behaviour of Rh6G in methanol and found a rapid reduction of fluorescence lifetime at about lo-’ M/Q due to dynamic aggregates (different to the stable dimers formed in water). These authors [ 20 ] performed a separation of the monomer
3 June 1988
spectrum from those of other aggregates by a mathematical procedure. In what follows we show that the various species of Rh6G existing in solution can be distinguished by a straightforward difference spectroscopy method [ 2 1 ] which was recently applied to Rh B in water solution.
2. Experimental The following materials were used: rhodamine 6G chloride (Eastman Kodak Co. ), methanol, chloroform, 1,2-dichloroethane, and hydrochloric acid (all analytical grade, Frutarom) , tetramethoxysilane (TMOS ) (Aldrich), Triton X 100 and PMMA beads (laboratory grade) (both from BDH ) , PMMA plates GS 2 151 (BASF-24 1) (from Rohm GmbH ) and triply distilled water. A series of solutions of Rh6G at concentrations varying from 10M6to 10m4 M/Q were prepared in water, methanol, 1,2dichloroethane and chloroform. The absorption spectra were measured using Perkin-Elmer-Lambda double beam and Kontron Uvikon spectrophotometers using air as a standard. The measurements were performed at temperatures of 18 and 52°C after 1 h and then at intervals of 20, 40, 90, 130 and 160 h. The thin films of PMMA were prepared by dissolving the appropriate weights of PMMA beads and Rh6G in chloroform to give a concentration of 1.4x 1Om4M/Q of Rh6G in PMMA, and pouring 5 ml of this solution on to transparent PMMA plates ( 5 x 5 x 0.3 cm) while spinning in a centrifuge for 1 min at 30-40°C. A transparent uniform-emitting plate was thus obtained. The sol-gel glass films on glass plates were similarly prepared using a 4.5 x 10e4 M/Q solution of Rh6G in methanol to which was added 1.25 ml 0.03 N HCl, 10 drops of Tritonx 100 and 2.5 ml TMOS to give 25 ml of solution. The TMOS solution was allowed to hydrolyze for 30 min and the films prepared on low iron content glass plates of dimensions 5 X 5 X 0.3 cm. The glasses were kindly provided by Mr. M. Harari and Kibbutz Tzuba. The difference spectra were obtained by subtracting the spectra of 10T6 M/Q solutions of equal optical density in which no dimers or aggregates were 143
Volume 147, number 2,3
CHEMICAL PHYSICS LETTERS
3 June 1988
observed, e.g. the spectrum of 10s4 M/g was measured in a cell of 0.1 cm thickness while the spectrum of 10m6M/Q was measured in a 1 cm wide cell and multiplied by 10 in situ. The fluorescence spectra were measured on a commercial SLM 4800C spectrofluorimeter, which provides corrected emission spectra. The fluorescence lifetimes were obtained using the same apparatus with a phase-shift attachment. This attachment only allows measurement of lifetimes exhibiting exponential behaviour. The quantum efficiencies were measured by the comparative method [ 22 ] using as standard for solutions a solution of Rh6G in methanol which has a quantum efficiency of 98% [ 23 1, and for thin films of dye in PMMA a PMMA plane of BASF-24 1 with a known quantum efficiency of 98% f 2%.
3. Results and discussion Fig. 1 presents the absorption (A) and emission (B) of Rh6G in chloroform (curve 1), methanol (curve 2), thin glass film (curve 3) and thin plastic (PMMA) film (curve 4) on glass support. Absorption and emission maxima and lifetimes are summarized in table 1. Fig. 2 presents absorption and differential spectra of Rh6G in water (A), methanol (B), dichloroethane (C) and chloroform (D). Curves 1 are due to absorption of lob6 M/g solution of Rh6G measured in a 1 cm cell (multiplied by lo), curves 2 represent absorption of 10m4M/R solution measured in a 0.1 cm cell. The existence of dimers in water (a) can be clearly seen in curve 2; however, the exact position of the monomer peaks can be obtained from the minimum of the differential curves 3 (524 nm) and of the dimers from the maxima of the differential curve (499 and 560 nm). Quantitatively the absorption spectra of the dimers can be explained by LCAO theory, the bonding orbital being at about 17860 cm- ’ and the antibonding at about 20000 cm-‘. The higher intensity of the latter indicates, from symmetry considerations, that the transition probability from the ground state to the antibonding state is higher. Addition of pcyclodextrin decreases the dimer concentration as has been shown previously for rhodamine B [ 24 1. P_cyclodextrin (CD) is a cyclic polysugar com144
500
550
600
Wavelength
650
700
7 10
[ nm]
Fig. 1. Absorption (A) and emission (B) spectra of rhodamine 6G in: 1, methanol; 2, chloroform; 3, thin film of poly(methyl methacrylate) on poly (methyl methacrylate) support; 4, thin film of sol-gel glass on a glass support.
posed of glucose units linked by l-4 glycoside bonds. The hydrophobic cavity present in the structure is capable of binding the organic dye in a monomeric form since the cavity has a diameter of 6.2 A and height 7 A. We prepared a solution of 1O-2 M/I1 CD in water to which Rh6G was added to form a total concentration of lob4 M/B. The absorption spectra, measured in 0.1 cm cuvettes, are presented in fig. 3, where curve 2 presents the absorption of Rh6G without CD, curve 1 represents the absorption of Rh6G with CD and curve 3 is the difference spectrum. The decrease of concentration of dimers as reflected by the increase of the absorption of monomers is also accompanied by an increase in the luminescence quantum yield of Rh6G. Preliminary experiments on degradation of Rh6G dissolved in water indicate that CD hinders photodegradation
3 June 1988
CI-IEMICAL PHYSICS LETTERS
Volume 147, number 2,3
Table 1 Mole concentrations, absorption and emission maxima, quantum efficiencies and lifetimes of rhodamine 6G in water, CH,OH, CHCls and I ,2_dichloroethane Concentration
Abs. max-
Em. max-
Quantum
Lifetime
Mole ext.
(mole/P)
(nm)
(nm)
efficiency
(ns)
coeff.
Hz0
lo-*
CH,OH CHCl,
10-6 1O-6
3.95kO.l 3.9 3.6 (1 day) 3.4 (5 days)
18 300 110000 82000
1o-6
553 556 556 530 544
0.88 0.98 0.75 0.40
W-W12 thin films PMMA glass
526 527 530 505 522 534 534
563 566
0.75 0.82
Solvent
1.4x 1o-4 4.5x 1o-4
I .oo
110000
-
82000
120
30
-0.21 300
I 400
I 500
II 600
I 400
I 500
I 600
I 400
3 500
WAVELENGTH
I 400
600
y 500
, 600
400 500
600
.020 700
[ nm 1
Fig. 2. Absorption spectra of rhodamine 6G in: (A) water, (B ) methanol, (C ) dichloroethane and (D) chloroform. Curve designaton: 1, low6 M/P measured in a 1 cm cell (multiplied by 10); 2, 10e4 M/P measured in a 0.1 cm cell; 3, difference of 2 against 1. (a) Difference spectra enlarged. Assignments as in fig. 2.
Table 2 Position ofdimer and monomer peaks of rhodamine 6G in various solvents Solvent
Literature [ 17]
Experimental dimer peak (nm )
water methanol chloroform dichloroethane
monomer peak (nm)
1
2
499
552
499 499
538 538
526 526 519 519
dimer peak (nm)
monomer peak (nm)
1
2
502
542
526
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CHEMICAL PHYSICS LEl-fERS
500
3 June 1988
hydrogen bonds are formed between the hydrogens of water and molecules of Rh6G. In this way the water molecule bridges between two molecules of the dye. In non-polar solvents the separation of Rh6G cation and chloride anion is inefficient, the molecule is uncharged and the electrostatic repulsion between the molecules is negligible. The energetics of such a situation favour the formation of dimers; however, the nature of the bonding orbitals is different from those in water. The different nature of dimers in water and in non-polar solvents is emphasized in fig. 2a. Non-polar solvents also display a slow decrease in absorption intensity accompanied by a blue-shift at high as well as at low concentrations of Rh6G, which is promoted by temperature and/or light. This behaviour is shown in fig. 4, where changes in the absorption and emission spectra of Rh6G in chloroform are recorded as a function of time, These spectral
600
Wavelength [nm] Fig. 3. Absorption spectra of 10e4 M/P of rhodamine 6G in water, measured in a 0.1 cm cell. Curve 1, with B-cyclodextrin added; curve 2, without addition of B-cyclodextrin; curve 3, difference of 2 against 1.
hinting that the dimers in water degrade faster than the monomers. Figs. 2B, 2C and 2D represent the absorption spectra of 10m6M/I1 (designated 1) and of 10e4 M/Q (designated 2) of Rh6G in methanol, dichloroethane and chloroform, respectively. The absorption spectra of the 10m6M/P solution were measured in a 1 cm cell (multiplied by 10) and for the 10V4M/I1 solution in a 0.1 cm cell. In methanol the spectral changes with concentration are small indicating the absence of dimers but the presence of closely spaced pairs arising from random distribution of the dye molecules [ 201, The results for water and methanol are in reasonable agreement with numerical methods (see table 2 ) . Aggregation of Rh6G in both chloroform and dichloroethane is different to that in water and these aggregates decompose slowly to a non-cationic form of Rh6G. The process is enhanced by heat and light. In these solvents the intensity of the low-energy peak of the dimers is higher than the high-energy peak, the reverse haviour
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of the situation may be explained
in water.
This
different
be-
by the fact that in water
0 IOO-
6
I
550
600
Wavelength
650 [nm ]
Fig. 4. Absorption (A) and emission (B ) spectra of 10m6M/P of rhodamine 6G in chloroform as a function of time elapsed from preparation. 1, a few minutes after preparation; 2, after two days; 3, after four days; 4, after six days; 5, after eight days.
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changes in solution are rather slow and we tend to believe that the process is controlled by a surface reaction of the dye molecules with oxygen. The degradation can be significantly diminished by incorporating the dye into a polymeric matrix where diffusion of the molecules to the surface is hindered. This leads to long-term stability of the absorption and emission maxima and to a very slow change of emission intensity. The situation improves in the inorganic glassy matrix where in addition to the stability of the emission and absorption maxima no change in emission intensity is observed over a period of wekks ad the absence of dimers ensures long-term stability.
3 June 1988
Jorgensen for most enlightening discussions and to Professor Itamar Wilner for his help with the absorption measurements.
References [ I] F.P.Schaefer,Topicsin appliedphysics,Vol.1. Dye lasers
(Springer,Berlin,1973). [2] S.Singh,Appl.Opt.26 (1987) 66. [ 31V.V.Rodchenkova,S.A.Tsogocva,T.M.Muraveva,L.K. Denisovand B.M. Uzhinov, Opt. i Spectroskopiya 60 (1986) 35.
[ 41 R. Reisfeld, Criteria and prospects of new lasers based on fluorescent dyes in glasses, J. Phys. (Paris), to be published.
[ 51R. Reisfeld and R. Zusman, Solar Concentrator Plates, US Patent No. 4,661,649 (28 April 1987).
[ 61 D. Avnir, D. Levy and R. Reisfeld, J. Phys. Chem. 88 ( 1984)
41 Conclusions In the present work a straightforward method for estimating the amount of dimerization and the position of the dimer peaks is applied. The results lead to the following conclusions: (a) Different types of dimers of Rh6G are formed in water and in the non-polar solvents studied in the present work. (b) Addition of P-cyclodextrin to the starting solution of thin glass films ensures the maximum amount of monomers in the film. (c) Plastic thin films must be prepared from fresh solutions of Rh6G in chloroform. The degradation process is frozen in the plastic and, with the evaporation of chloroform, the dye molecules persist indefinitely in the state “inherited” from the solution.
Acknowledgement The authors are grateful to the National Council of Research and Development for a scholarship awarded to RZ. We are also grateful to Professor C.K.
5956.
[ 71 R. Reisfeld, M. Eyal and R. Gvishi, Chem. Phys. Letters 138 (1987) 377. [ 81 D. Avnir, V.R. Kaufman and R. Reisfeld, J. Non-Cryst. Solids 74 (1985) 395. [9] D. Brusilovsky and R. Reisfeld, Chem. Phys. Letters 141 (1987) 119. [ lo] R. Reisfeld and C.K. Jergensen, Struct. Bonding 49 ( 1982)
[ 111 i. Reisfeld, J. Less-Common Metals 93 ( 1983) 243. [ 12] R. Reisfeld, J. Less-Common Metals 112 ( 1985) 9. [ 131 R. Reisfeld, Inorg. Chim. Acta 140 ( 1987) 345. [ 14) R. Reisfeld, Inorg. Chim. Acta 95 (1984) 69. [IS] N. Neuroth and R. Haspel, Proc. SPIE 653 (1986) 88. [ 161 D.A. Hinckley, P.G. Seybold and D.P. Borris, Spectrochim. Acta 42A ( 1986) 747. [ 171J.E. Selwyn and J.I. Steinfeld, J. Phys. Chem. 76 (1972) 762.
[ 181A. Penzkofer and Y. Lu, Chem. Phys. 103 (1986) 399. [ 191 Y. Lu and A. Penzkofer, Chem. Phys. 107 (1986) 175. [ 201 A. Penzkofer and W. Leupacher, J. Luminescence 37 ( 1987) 61. [21] L. Bruzzone and M.E. Rosello, Appl. Spectry. 40 ( 1986) 1066. [ 221 R. Reisfeld, NBS 76A ( 1972) 613. [23] J.S. Batchelder, A.H. Zewail and T. Cole, Appl. Opt. 20 (1981) 3733. [24] Y. Degani, I. Wilner and Y. Haas, Chem. Phys. Letters 104 (1984) 496.
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