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Determination of dimers concentration of chromophores by the method of fluorescence synchronous scanning
Journal of Luminescence 81 (1999) 79—84
Determination of dimers concentration of chromophores by the method of fluorescence synchronous scanning O.A. Khakhel’, V.E. Krykunova* Poltava Cooperative Institute, Koval+ 3, Poltava 314601, Ukraine Received 27 March 1997; received in revised form 20 July 1998; accepted 20 July 1998
Abstract In order to determine the concentration ratio of monomers to dimers of a fluorescent chromophore in a polymer matrix we have applied fluorescence synchronous scanning. We have studied compositions poly(methyl methacrylate) with (9-anthryl)glycidyl ester and have estimated the share of aggregated chromophore and the efficiency of radiationless energy transfer from monomers to dimers. 1999 Elsevier Science B.V. All rights reserved. Keywords: Fluorescence synchronous scanning; Dimer of (9-anthryl)glycidyl ester; Radiationless transfer of energy
1. Introduction In almost any system containing organic chromophores, the phenomenon of aggregation occurs. Physical dimers of molecules often function as aggregates. Here, the issue of the concentration ratio of monomers to dimers in the objects under study arises. Also relevant to this issue is the efficiency of radiationless energy transfer between these species. It is apparently impossible to attain an answer to these questions using traditional spectral methods. In this paper, we describe techniques applying synchronous scanning spectroscopy to resolve these two problems. The recording of the fluorescence spectrum by synchronous scanning using excitation and emission monochromators has been proposed by Lloyd [1—4] for analyzing multicomponent fluorescent mixtures. The spectrum of an individual species obtained in such a manner (FSS spectrum) appears * Corresponding author. Fax: #380 5322 74542.
as a comparatively narrow band lying in the overlap region of its absorption and fluorescence spectra if the scanning regime meets the condition j "j . Generally, this band satisfactorily ap proximates the Gaussian function [5]. An FSS spectrum on a system of several species appears as a superposition of bands of individual species. For a wide range of chromophores the dimeric state is lower in energy than the monomeric state. Therefore, the monomeric fluorescence is quenched by the dimers. So far as we know, the process of radiationless transfer of energy between species has not been considered before this in the context of FSS spectra.
2. Experiment We have conducted our investigations with samples of poly(methyl methacrylate) dyed with
0022-2313/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 8 ) 0 0 0 4 9 - 0
80
O.A. Khakhel+, V.E. Krykunova / Journal of Luminescence 81 (1999) 79—84
(9-anthryl)glycidyl ester:
in which the content of chromophoric addition made up 2.0 mol% (samples I and II) and 0.15 mol% (sample III). We conducted the synthesis via polymerization in the mass at two-stage temperature conditions: at 50°C for 24 h with subsequent after-polymerization at 115°C. Azoisobutyronitrile (0.06 mol%) was used as an initiator. We obtained polymers I and III from mixtures prepared in monomeric form before synthesis. However, to ensure a higher degree of chromophoric aggregation in sample II, we introduced a chromophoric addition, but only after reaching the 50% conversions of methyl methacrylate. We purified the methyl methacrylate via the ordinary procedure, i.e. with distillation and a membrane filter. We degassed the prepared mixtures of polymers I, III (monomer, dye, initiator) and polymer II (monomer, polymer, dye, initiator) using several freeze—thaw cycles and then placing them into glass moulds. After polymerization, we removed the samples (1 mm thick plates) from the moulds for measurement. Absorption and fluorescence spectra were registered with spectrometers (both LOMO) SF-46 and SDL-2, respectively, at room temperature. Upon frontal excitation of the samples, without dissarranging the monochromators (j "j ), we ob tained FSS spectra.
3. Method The usual cantitative criterion of energy transfer is the Fo¨rster radius [6,7]. Other parameters describing the mobility of excitons could also be named. However, it is complicated to operate these parameters for the purpose of interpreting FSS spectra. In this paper, we shall estimate energy
transfer efficiency by a magnitude which, in a sense, is a fictitious coefficient of extinction. Calling it a, we can record the ratio of emission intensities of dimers (I ) and monomers (I ) at a given " + wavelength in a fluorescence spectrum thus: I
(e #a)C " " R" "" " , I
e (C!2C )!aC + ++ " "
(1)
where e and e are true coefficients of the extinc" + tion of dimers and monomers; and are their " + relative quantum yields of emission. We have taken into account that the concentration of monomeric centers, C , is connected to the overall chromo+ phoric concentration C and the concentration of dimers C as C "C!2C . The values a, e , e , " + " " +
and depend on the excitation or emission " + wavelength. By somehow upsetting the monomer—dimer balance in a system, it becomes possible to trace fluorescence spectra dynamics, hence determining the meaning of some values in Eq. (1) and likewise establishing inter-dependencies among them. So, the resolution of a system of already two equation types (1) yields a functional tie between the values a/e and C : + " K"(2#a/e )C . + "
(2)
Parameter K depending on C is defined through " the concentration ratio of dimers in various samples and the values R, measured in their FSS spectra at one and the same wavelength: K(C )"C "
1!C R /C R " " , 1!R /R
C K(C )" "K(C ). " " C " Here, indices 1 and 2 relate to the given samples 1 and 2 in which the values of chromophoric concentration C are equal. It is easy to determine the concentration ratio of the dimers by comparing their absorption spectra in the long-wave section beyond the edge of the absorption band of non-aggregated molecules. Obviously, one can disregard a possible concentrational influence of dimers on the efficiency of
O.A. Khakhel+, V.E. Krykunova / Journal of Luminescence 81 (1999) 79—84
81
to the energy transfer from the monomer. On some wavelength of j , we can estimate the fluorescence intensity in spectrum 2 according to the intensity IY at a wavelength of jY (see Fig. 1). In view of this " fact, we can also measure the ratio: I
(e #a)C ""1#a/e . RY" "" " " " IY
e C " "" "
Fig. 1. See explanation in the text.
energy transfer if one considers those systems as having values of C that are sufficiently close to " each other. As is evident from this observation, determining dimeric concentration and determining energy transfer efficiency are equivalent tasks (see Eq. (2)). Using the usual spectral methods, one can arrive only at the determination of parameter K which projects, as it were, the value: the indeterminacy of magnitudes C and a/e . And perhaps, the one " + thing allowing separate estimation of the values of a/e and C employing the FSS method. + " We shall explain our further speculation in Fig. 1 regarding the FSS spectra of a hypothetical chromophore (curve 1) and another specie (curve 2), which might be its dimer. It is obvious that the FSS spectrum system of these two species will not be a simple superpositioning of spectrum 1 upon spectrum 2. Due to energy transfer from monomer to dimer, the intensity of the first will decrease (curve 3). At the same time, the intensity of the dimeric component in the area overlapping the FSS spectrum of the monomeric chromophore will increase (curve 4). The resulting FSS spectrum of this system, recorded in experiment, is a superposition of spectra 3 and 4. We shall consider the spectra shown in Fig. 1 as corresponding to the scanning regime without disarranging the monochromators (j "j ). Then, spectrum 2 should be symmetrical in relation to the maximum (on a linear energy scale) [5]. This circumstance allows taking into consideration an intensity growth of the dimeric FSS spectrum due
(3)
More precisely, we can determine R at wavelength j since IY (j )"IY (jY). " " For an arbitrary wavelength on which R is measured, parameter a/e can be evaluated only with " a precision up to the magnitude value of e . In " general, the dimer extinction coefficient is unknown. However, one can point to wavelength j on which e "2e . This has been determined " + according to the isobestic point in the absorption spectra of samples with varied monomer—dimer ratios. In this way, while exciting the fluorescence at this wavelength, then measuring the FSS spectra magnitudes R and R one can calculate a/e and + C from Eqs. (3) and (2). " Conditions caused by the energy donor’s band and the energy acceptor’s band overlapping in FSS spectra in a particular position of j limit the application of the method just described. More concretely let us consider such a class of organic chromophores as the aromatic hydrocarbons. The spectral properties of their dimers are known. The absorption spectra of dimers have shifted relative to the monomeric one to the red side and have experienced some hypochromism (85—90% [8]). That is the wavelength on which e "2e is posi" + tioned on the long-wave edge of monomeric absorption spectra, i.e. in the spectral range between the monomer and dimer bands in FSS spectra. However, the magnitude of a dimeric spectrum shift can be too large and the dimer and monomer bands may not overlap in FSS spectra. In particular, such a situation is observed in the solid solutions of anthracene in poly(methyl methacrylate) [9]. This problem can be solved by changing the synchronous scanning regime. Introducing some displacement of monochromators of *j (scanning regime is j "j #*j), one can achieve overlapping of dimer and monomer bands in FSS spectra. The data for calculating the C and a/e " +
82
O.A. Khakhel+, V.E. Krykunova / Journal of Luminescence 81 (1999) 79—84
Fig. 2. Absorption spectra of samples I(1),II(2),III(3) and fluorescence spectrum (j "360 nm) of sample III(4).
magnitudes can be arrived at analytically by examining the form of the recorded FSS spectra. The main provisions of FSS analysis are stated in the works [5,10,11]. 4. Result For experiment we chose samples of poly(methyl methacrylate) which had been dyed with (9-anthryl)glycidyl ester. The spectral properties of this chromophore and its dimers in a polymeric matrix allow us to work in the scanning regime j "j , which according to our own point of view makes the findings of this work more convincing. One can get an idea about the spectral properties of monomeric form (9-anthryl)glycidyl ester from Fig. 2 in which the absorption and fluorescence spectra of sample III are shown. We do not show the fluorescence excitation spectra of this polymer. They do not depend on the registration wavelength which proves the absence of a chromophoric aggregation. We support this statement with the data of synchronous scanning spectroscopy. The FSS spectrum of sample III (curve 3 in Fig. 3b) is represented by one band with a maximum at 396 nm indicating the position of 0—0-transition of the monomer. Chromophoric aggregation in polymers I and II appears in their absorption spectra (curves 1 and 2 in Fig. 2) by means of the red shift of the longwave edge. Comparing the spectra one can discover that the ratio of the optical densities of samples
I and II makes up on the average 0.89 in the area of j'415 nm. That is, the dimeric concentration in sample II exceeds the value C in sample I by " +12%. The isobestic point of these spectra is j "410 nm. Let us now refer to the FSS spectra of samples I and II. In Fig. 3a they are shown as normalized in the short-wave part. Unfortunately, the conditions by which we recorded these spectra (scanning regime is j "j ) do not permit complete elimina tion of the influence of diffused light from the excitation source, whose spectrum appears as a background of characteristic maxima of approximately 450—500 nm. However, its contribution to these spectra is sufficiently minimized and can easily be excluded. Corrected spectra of polymers I, II and III are shown below in Fig. 3b. First of all, we must determine the region of FSS spectra in which the emission band of dimers appears. We can see in Fig. 3b that this area is limited by wavelengths of 395—435 nm. In this connection we draw your attention to the maximum. The shift of the dimeric component in the spectrum of polymer II (j "410.5 nm) relative to the maximum
in the spectrum of the comparison sample (j "409 nm) occurs. One can forecast that the
limit of the shift of the dimeric band maximum is j"414 nm. This wavelength is positioned on a linear energy scale according to the center of the interval 395—435 nm. In order to determine the values of R, it is necessary to break down the initial FSS spectra into
O.A. Khakhel+, V.E. Krykunova / Journal of Luminescence 81 (1999) 79—84
Fig. 3. (a) Spectra of fluorescence synchronous scanning of samples I(1) and II(2). The background is cut off by the dot-and dash line. In the inset, apparatus function of the spectrophotometer is shown. The function is normalized to one at 410 nm. (b) Spectra of fluorescence synchronous scanning of samples I (1), II(2) and III(3); 4 is the difference of spectra 2 and 1. (c) Fluorescence spectrum of sample II(1) and its resolution into dimeric (2) and monomeric (3) components. The contribution of exciting band is singled out by a dashed line.
their constituent components. One can perform the break down by substracting the normalized FSS spectrum of sample III (curve 3 in Fig. 3b) from FSS spectra of samples I and II. It will be correct if we put parameter a proportionally to e (i.e. on the + condition that a(j)/e (j)"const). The spectra + shown in Fig. 3c are evidence of this condition. Here we show the result of the break down of the fluorescence spectrum of sample II obtained at j "403 nm into monomeric and dimeric compo
83
nents. As we see, the ratio of the emission intensities of dimers and monomers at the excitation wavelength in spectra 2 and 3 in Fig. 3c agrees with the value obtainable from spectra 2 and 3 in Fig. 3c. The ratios of R measured at j"410 nm in spectra 1 and 2 (Fig. 3b) are equal to 3.80 and 4.87. Then, taking into account that C"2.0 mol% and C /C "0.89 where indices 1 and 2 relate " " to the data for polymers I and II, respectively, we obtain from Eq. (1): K "0.996 mol% and K "1.119 mol%. The monomeric band of (9-antryl)glycidyl ester in FSS spectra stretches from 378 to 415 nm which is evident from the spectrum of polymer III in Fig. 3b. Consequently, the wavelength of 418 nm symmetrical to j"410 nm relative to the center of the emission range of dimers in FSS spectra lies outside the absorption region of the monomer. An intensity value of dimeric fluorescence at this point IY can be used for evaluating magnitude R (at " j"410 nm) from Eq. (3). Taking into consideration the apparatus function of the spectral device (see the inset in Fig. 3a), we have determined: R "1.79. The apparatus function reflects the spectral relationships of the photodetecting device sensitivity and of the exciting lamp light-emission intensity. The final result is: a/e "1.58; C " + " 0.278 mol% and C "0.312 mol%, i.e. in samples " I and II +30% of the chromophore contained in them is in the aggregated state. It is also not at all complicated to estimate the value of / at j"410 nm. For example, from " + Eq. (1): / "3.8. Although this magnitude is " + not the ratio of total fluorescence quantum yields of species, as it is, one can conclude that the monomeric fluorescence experiences considerable concentration quenching.
5. Conclusion This paper proposes using fluorescence synchronous scanning to determine both the relative quantity of monomers and dimers of organic chromophores and the efficiency of radiationless transfer of energy from monomers to dimers.
84
O.A. Khakhel+, V.E. Krykunova / Journal of Luminescence 81 (1999) 79—84
This method is useful when the energy donor and acceptor bands overlap in the FSS spectra of the system. Efficiency of energy transfer is determined by the ratio a/e , where e is a monomer extinction + + coefficient and a is a conditional growth of dimer extinction due to energy transfer. The techniques for determining magnitudes a/e and C is + " based on the known property of FSS spectra [5], namely, on the symmetry of the fluorescence band of each type of chromophore contained in the mixture. We used samples of (9-anthryl)glycidyl ester solid solution (2.0 mol%) in poly(methyl methacrylate) for demonstrating the use of FSS for the purposes stated above. We found that the share of aggregated chromophore of the same system makes up +30% and the a/e value is close to 1.58. The last +
magnitude does not show a dependence on the excitation wavelength.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
J.B.F. Lloyd, Nature (London), Phys. Sci. 231 (1971) 64. J.B.F. Lloyd, J. Forensic Sci. Soc. 11 (1971) 83. J.B.F. Lloyd, J. Forensic Sci. Soc. 11 (1971) 153. J.B.F. Lloyd, J. Forensic Sci. Soc. 11 (1971) 235. J.B.F. Lloyd, I.W. Evett, Anal. Chem. 49 (1977) 1710. R.C. Powell, J. Chem. Phys. 58 (1973) 920. T.T. Ndou, R. von Wandruszka, J. Lumin. 46 (1990) 33. E.A. Chandross, J. Ferguson, J. Chem. Phys. 45 (1966) 3554. T.V. Sakhno, N.N. Barashkov, R.N. Nurmukhametov, O.A. Khakhel’, Polymer 37 (1996) 1109. [10] T. Vo-Dinh, Anal. Chem. 50 (1978) 396. [11] E.L. Inman Jr., J.D. Winefordner, Anal. Chem. 54 (1982) 2018.
Determination of dimers concentration of chromophores by the method of fluorescence synchronous scanning O.A. Khakhel’, V.E. Krykunova* Poltava Cooperative Institute, Koval+ 3, Poltava 314601, Ukraine Received 27 March 1997; received in revised form 20 July 1998; accepted 20 July 1998
Abstract In order to determine the concentration ratio of monomers to dimers of a fluorescent chromophore in a polymer matrix we have applied fluorescence synchronous scanning. We have studied compositions poly(methyl methacrylate) with (9-anthryl)glycidyl ester and have estimated the share of aggregated chromophore and the efficiency of radiationless energy transfer from monomers to dimers. 1999 Elsevier Science B.V. All rights reserved. Keywords: Fluorescence synchronous scanning; Dimer of (9-anthryl)glycidyl ester; Radiationless transfer of energy
1. Introduction In almost any system containing organic chromophores, the phenomenon of aggregation occurs. Physical dimers of molecules often function as aggregates. Here, the issue of the concentration ratio of monomers to dimers in the objects under study arises. Also relevant to this issue is the efficiency of radiationless energy transfer between these species. It is apparently impossible to attain an answer to these questions using traditional spectral methods. In this paper, we describe techniques applying synchronous scanning spectroscopy to resolve these two problems. The recording of the fluorescence spectrum by synchronous scanning using excitation and emission monochromators has been proposed by Lloyd [1—4] for analyzing multicomponent fluorescent mixtures. The spectrum of an individual species obtained in such a manner (FSS spectrum) appears * Corresponding author. Fax: #380 5322 74542.
as a comparatively narrow band lying in the overlap region of its absorption and fluorescence spectra if the scanning regime meets the condition j "j . Generally, this band satisfactorily ap proximates the Gaussian function [5]. An FSS spectrum on a system of several species appears as a superposition of bands of individual species. For a wide range of chromophores the dimeric state is lower in energy than the monomeric state. Therefore, the monomeric fluorescence is quenched by the dimers. So far as we know, the process of radiationless transfer of energy between species has not been considered before this in the context of FSS spectra.
2. Experiment We have conducted our investigations with samples of poly(methyl methacrylate) dyed with
0022-2313/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 8 ) 0 0 0 4 9 - 0
80
O.A. Khakhel+, V.E. Krykunova / Journal of Luminescence 81 (1999) 79—84
(9-anthryl)glycidyl ester:
in which the content of chromophoric addition made up 2.0 mol% (samples I and II) and 0.15 mol% (sample III). We conducted the synthesis via polymerization in the mass at two-stage temperature conditions: at 50°C for 24 h with subsequent after-polymerization at 115°C. Azoisobutyronitrile (0.06 mol%) was used as an initiator. We obtained polymers I and III from mixtures prepared in monomeric form before synthesis. However, to ensure a higher degree of chromophoric aggregation in sample II, we introduced a chromophoric addition, but only after reaching the 50% conversions of methyl methacrylate. We purified the methyl methacrylate via the ordinary procedure, i.e. with distillation and a membrane filter. We degassed the prepared mixtures of polymers I, III (monomer, dye, initiator) and polymer II (monomer, polymer, dye, initiator) using several freeze—thaw cycles and then placing them into glass moulds. After polymerization, we removed the samples (1 mm thick plates) from the moulds for measurement. Absorption and fluorescence spectra were registered with spectrometers (both LOMO) SF-46 and SDL-2, respectively, at room temperature. Upon frontal excitation of the samples, without dissarranging the monochromators (j "j ), we ob tained FSS spectra.
3. Method The usual cantitative criterion of energy transfer is the Fo¨rster radius [6,7]. Other parameters describing the mobility of excitons could also be named. However, it is complicated to operate these parameters for the purpose of interpreting FSS spectra. In this paper, we shall estimate energy
transfer efficiency by a magnitude which, in a sense, is a fictitious coefficient of extinction. Calling it a, we can record the ratio of emission intensities of dimers (I ) and monomers (I ) at a given " + wavelength in a fluorescence spectrum thus: I
(e #a)C " " R" "" " , I
e (C!2C )!aC + ++ " "
(1)
where e and e are true coefficients of the extinc" + tion of dimers and monomers; and are their " + relative quantum yields of emission. We have taken into account that the concentration of monomeric centers, C , is connected to the overall chromo+ phoric concentration C and the concentration of dimers C as C "C!2C . The values a, e , e , " + " " +
and depend on the excitation or emission " + wavelength. By somehow upsetting the monomer—dimer balance in a system, it becomes possible to trace fluorescence spectra dynamics, hence determining the meaning of some values in Eq. (1) and likewise establishing inter-dependencies among them. So, the resolution of a system of already two equation types (1) yields a functional tie between the values a/e and C : + " K"(2#a/e )C . + "
(2)
Parameter K depending on C is defined through " the concentration ratio of dimers in various samples and the values R, measured in their FSS spectra at one and the same wavelength: K(C )"C "
1!C R /C R " " , 1!R /R
C K(C )" "K(C ). " " C " Here, indices 1 and 2 relate to the given samples 1 and 2 in which the values of chromophoric concentration C are equal. It is easy to determine the concentration ratio of the dimers by comparing their absorption spectra in the long-wave section beyond the edge of the absorption band of non-aggregated molecules. Obviously, one can disregard a possible concentrational influence of dimers on the efficiency of
O.A. Khakhel+, V.E. Krykunova / Journal of Luminescence 81 (1999) 79—84
81
to the energy transfer from the monomer. On some wavelength of j , we can estimate the fluorescence intensity in spectrum 2 according to the intensity IY at a wavelength of jY (see Fig. 1). In view of this " fact, we can also measure the ratio: I
(e #a)C ""1#a/e . RY" "" " " " IY
e C " "" "
Fig. 1. See explanation in the text.
energy transfer if one considers those systems as having values of C that are sufficiently close to " each other. As is evident from this observation, determining dimeric concentration and determining energy transfer efficiency are equivalent tasks (see Eq. (2)). Using the usual spectral methods, one can arrive only at the determination of parameter K which projects, as it were, the value: the indeterminacy of magnitudes C and a/e . And perhaps, the one " + thing allowing separate estimation of the values of a/e and C employing the FSS method. + " We shall explain our further speculation in Fig. 1 regarding the FSS spectra of a hypothetical chromophore (curve 1) and another specie (curve 2), which might be its dimer. It is obvious that the FSS spectrum system of these two species will not be a simple superpositioning of spectrum 1 upon spectrum 2. Due to energy transfer from monomer to dimer, the intensity of the first will decrease (curve 3). At the same time, the intensity of the dimeric component in the area overlapping the FSS spectrum of the monomeric chromophore will increase (curve 4). The resulting FSS spectrum of this system, recorded in experiment, is a superposition of spectra 3 and 4. We shall consider the spectra shown in Fig. 1 as corresponding to the scanning regime without disarranging the monochromators (j "j ). Then, spectrum 2 should be symmetrical in relation to the maximum (on a linear energy scale) [5]. This circumstance allows taking into consideration an intensity growth of the dimeric FSS spectrum due
(3)
More precisely, we can determine R at wavelength j since IY (j )"IY (jY). " " For an arbitrary wavelength on which R is measured, parameter a/e can be evaluated only with " a precision up to the magnitude value of e . In " general, the dimer extinction coefficient is unknown. However, one can point to wavelength j on which e "2e . This has been determined " + according to the isobestic point in the absorption spectra of samples with varied monomer—dimer ratios. In this way, while exciting the fluorescence at this wavelength, then measuring the FSS spectra magnitudes R and R one can calculate a/e and + C from Eqs. (3) and (2). " Conditions caused by the energy donor’s band and the energy acceptor’s band overlapping in FSS spectra in a particular position of j limit the application of the method just described. More concretely let us consider such a class of organic chromophores as the aromatic hydrocarbons. The spectral properties of their dimers are known. The absorption spectra of dimers have shifted relative to the monomeric one to the red side and have experienced some hypochromism (85—90% [8]). That is the wavelength on which e "2e is posi" + tioned on the long-wave edge of monomeric absorption spectra, i.e. in the spectral range between the monomer and dimer bands in FSS spectra. However, the magnitude of a dimeric spectrum shift can be too large and the dimer and monomer bands may not overlap in FSS spectra. In particular, such a situation is observed in the solid solutions of anthracene in poly(methyl methacrylate) [9]. This problem can be solved by changing the synchronous scanning regime. Introducing some displacement of monochromators of *j (scanning regime is j "j #*j), one can achieve overlapping of dimer and monomer bands in FSS spectra. The data for calculating the C and a/e " +
82
O.A. Khakhel+, V.E. Krykunova / Journal of Luminescence 81 (1999) 79—84
Fig. 2. Absorption spectra of samples I(1),II(2),III(3) and fluorescence spectrum (j "360 nm) of sample III(4).
magnitudes can be arrived at analytically by examining the form of the recorded FSS spectra. The main provisions of FSS analysis are stated in the works [5,10,11]. 4. Result For experiment we chose samples of poly(methyl methacrylate) which had been dyed with (9-anthryl)glycidyl ester. The spectral properties of this chromophore and its dimers in a polymeric matrix allow us to work in the scanning regime j "j , which according to our own point of view makes the findings of this work more convincing. One can get an idea about the spectral properties of monomeric form (9-anthryl)glycidyl ester from Fig. 2 in which the absorption and fluorescence spectra of sample III are shown. We do not show the fluorescence excitation spectra of this polymer. They do not depend on the registration wavelength which proves the absence of a chromophoric aggregation. We support this statement with the data of synchronous scanning spectroscopy. The FSS spectrum of sample III (curve 3 in Fig. 3b) is represented by one band with a maximum at 396 nm indicating the position of 0—0-transition of the monomer. Chromophoric aggregation in polymers I and II appears in their absorption spectra (curves 1 and 2 in Fig. 2) by means of the red shift of the longwave edge. Comparing the spectra one can discover that the ratio of the optical densities of samples
I and II makes up on the average 0.89 in the area of j'415 nm. That is, the dimeric concentration in sample II exceeds the value C in sample I by " +12%. The isobestic point of these spectra is j "410 nm. Let us now refer to the FSS spectra of samples I and II. In Fig. 3a they are shown as normalized in the short-wave part. Unfortunately, the conditions by which we recorded these spectra (scanning regime is j "j ) do not permit complete elimina tion of the influence of diffused light from the excitation source, whose spectrum appears as a background of characteristic maxima of approximately 450—500 nm. However, its contribution to these spectra is sufficiently minimized and can easily be excluded. Corrected spectra of polymers I, II and III are shown below in Fig. 3b. First of all, we must determine the region of FSS spectra in which the emission band of dimers appears. We can see in Fig. 3b that this area is limited by wavelengths of 395—435 nm. In this connection we draw your attention to the maximum. The shift of the dimeric component in the spectrum of polymer II (j "410.5 nm) relative to the maximum
in the spectrum of the comparison sample (j "409 nm) occurs. One can forecast that the
limit of the shift of the dimeric band maximum is j"414 nm. This wavelength is positioned on a linear energy scale according to the center of the interval 395—435 nm. In order to determine the values of R, it is necessary to break down the initial FSS spectra into
O.A. Khakhel+, V.E. Krykunova / Journal of Luminescence 81 (1999) 79—84
Fig. 3. (a) Spectra of fluorescence synchronous scanning of samples I(1) and II(2). The background is cut off by the dot-and dash line. In the inset, apparatus function of the spectrophotometer is shown. The function is normalized to one at 410 nm. (b) Spectra of fluorescence synchronous scanning of samples I (1), II(2) and III(3); 4 is the difference of spectra 2 and 1. (c) Fluorescence spectrum of sample II(1) and its resolution into dimeric (2) and monomeric (3) components. The contribution of exciting band is singled out by a dashed line.
their constituent components. One can perform the break down by substracting the normalized FSS spectrum of sample III (curve 3 in Fig. 3b) from FSS spectra of samples I and II. It will be correct if we put parameter a proportionally to e (i.e. on the + condition that a(j)/e (j)"const). The spectra + shown in Fig. 3c are evidence of this condition. Here we show the result of the break down of the fluorescence spectrum of sample II obtained at j "403 nm into monomeric and dimeric compo
83
nents. As we see, the ratio of the emission intensities of dimers and monomers at the excitation wavelength in spectra 2 and 3 in Fig. 3c agrees with the value obtainable from spectra 2 and 3 in Fig. 3c. The ratios of R measured at j"410 nm in spectra 1 and 2 (Fig. 3b) are equal to 3.80 and 4.87. Then, taking into account that C"2.0 mol% and C /C "0.89 where indices 1 and 2 relate " " to the data for polymers I and II, respectively, we obtain from Eq. (1): K "0.996 mol% and K "1.119 mol%. The monomeric band of (9-antryl)glycidyl ester in FSS spectra stretches from 378 to 415 nm which is evident from the spectrum of polymer III in Fig. 3b. Consequently, the wavelength of 418 nm symmetrical to j"410 nm relative to the center of the emission range of dimers in FSS spectra lies outside the absorption region of the monomer. An intensity value of dimeric fluorescence at this point IY can be used for evaluating magnitude R (at " j"410 nm) from Eq. (3). Taking into consideration the apparatus function of the spectral device (see the inset in Fig. 3a), we have determined: R "1.79. The apparatus function reflects the spectral relationships of the photodetecting device sensitivity and of the exciting lamp light-emission intensity. The final result is: a/e "1.58; C " + " 0.278 mol% and C "0.312 mol%, i.e. in samples " I and II +30% of the chromophore contained in them is in the aggregated state. It is also not at all complicated to estimate the value of / at j"410 nm. For example, from " + Eq. (1): / "3.8. Although this magnitude is " + not the ratio of total fluorescence quantum yields of species, as it is, one can conclude that the monomeric fluorescence experiences considerable concentration quenching.
5. Conclusion This paper proposes using fluorescence synchronous scanning to determine both the relative quantity of monomers and dimers of organic chromophores and the efficiency of radiationless transfer of energy from monomers to dimers.
84
O.A. Khakhel+, V.E. Krykunova / Journal of Luminescence 81 (1999) 79—84
This method is useful when the energy donor and acceptor bands overlap in the FSS spectra of the system. Efficiency of energy transfer is determined by the ratio a/e , where e is a monomer extinction + + coefficient and a is a conditional growth of dimer extinction due to energy transfer. The techniques for determining magnitudes a/e and C is + " based on the known property of FSS spectra [5], namely, on the symmetry of the fluorescence band of each type of chromophore contained in the mixture. We used samples of (9-anthryl)glycidyl ester solid solution (2.0 mol%) in poly(methyl methacrylate) for demonstrating the use of FSS for the purposes stated above. We found that the share of aggregated chromophore of the same system makes up +30% and the a/e value is close to 1.58. The last +
magnitude does not show a dependence on the excitation wavelength.
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