- Email: [email protected]
On the aggregation of rhodamine B in ethanol
8 July 1988
CHEMICAL PHYSICS LETTERS
Volume 148, number 2,3
ON THE AGGREGATION OF RHODAMINE B IN ETHANOL F. LOPEZ ARBELOA, P. RUIZ OJEDA and I. LbPEZ ARBELOA Depattamento de Quimica Fisica. Universidaddel Pais Vasco-EHU,Apdo 644, 48080 Bilbao, Spain Received 15February 1988; in final form 3 May 1988
The dimer and the trimer aggregation constants of the cationic form of rhodamine B are determined in very concentrated ethanolic solutions of the dye. The absorption spectrum, the geometrical structure and the association bonding of the aggregates are discussed and compared with those previously obtained in aqueous solution.
1. Introduction
The optical properties of rhodamines in solution have attracted the attention of numerous workers since the investigation of such properties is essential for the development of tunable lasers using organic dyes as media. In particular, the spectroscopic and photophysical properties of rhodamine B, RB, have been extensively studied as a function of solvent [ l41, concentration [5-121, andpH [3,4,9-161. The equilibrium between the zwitterion, RB’, and the catiopic, RBH+, molecular forms of the dye (fig. 1) can be shifted depending on the concentration of H+ ions in the solution. In spite of the number of papers, the optical characteristics of both molecular forms of RB continue to be the subject of some controversy. The effect produced on the absorption spectrum by increasing the concentration of the dye in ethanolic solutions has
been interpreted in different ways. First of all, changes in the absorption spectra have been attributed by some authors to a shift in the equilibrium of RB from the zwitterionic to the cationic form with increasing dye concentration [ 13,14,16 1. Other authors have considered the dimerization process which has a large equilibrium constant between lo4 and 2.4 X lo4 (concentration standard = 1 mol dmd3) [ 7,8]. Finally, the occurrence of both processes, with a small degree of aggregation which becomes appreciable only in very concentrated solutions of the dye (higher than 5~10-~ mol dmm3), has been also taken into account [9,10]. This paper aims to obtain further information on the occurrence of aggregation for RB in ethanol. To this purpose, absorption spectra and vapour-pressure osmometty measurements have been taken up to very high concentrations of RB. The results indicate that not only the dimer but also the trimer is
COOH
Cationic (protonated
form R BH+)
Zwitterion (RB’I
form
Fig. 1.Motecular structure of the zwitterionic, RB’, and cationic RBH+, forms of rhodamine B.
0 009-2614/8&X/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division )
253
Volume 148,number 2,3
CHEMICALPHYSICSLETTERS
formed in the concentration range used in this work. The aggregation constant, the geometrical structure and the association bonding of the aggregates are discussed and compared with those obtained in aqueous solution [ 11,121.
2. Experimental Rhodamine B was supplied from Merck (proanalysis). It was recrystallized twice from ethanol and dried under vacuum. Other chemical compounds were Merck suprapur grade quality. The zwitterionic and cationic forms of the dye were generated by addition of suitable amounts of NaOH (ethanolic sol. ) and HCl (ethanolic sol.), respectively, to the solutions [ 3,4,9-161. Without such addition the molecular form equilibrium of RB is displaced to the cationic side in concentrated solutions of the dye (> 10m3mol dms3) [g-16]. Absorption spectra were recorded at a dye concentration range from 8X lo-’ up to 5X 10m2 mol dme3 in a Beckman 5260 spectrophotometer with a double monochromator using quartz cells with different optical path lengths. A Beckman RIIC cell (model BC-14) with a variable optical path length was used to record the absorption spectra of RB in very concentrated solutions. Vapour-pressure osmometry measurements were carried out in a Knauer 1974 osmometer using benzyl as the standard [ 18,191, The concentration ranged from 2.5~10-~ to 0.14 mol kg-’ and the temperature from 27 to 50°C at 6°C intervals.
3. Results and discussion When the concentration of RB in ethanol is increased to 10m3mol dm13 a displacement to smaller energies of about 350 cm-’ in the absorption and the fluorescence spectra is observed. This was attributed by Ferguson and Mau [ 131 to a change in the molecular form of RB from the zwitterionic to the cationic form since an increase in the dye concentration leads to an increase in the acidity of the solution. Later this was extensively confirmed [ 9- 16 1. However, we have observed that the shape of the absorption spectra of both molecular forms in ethanol, 254
8 July 1988
generated by controlling the pH, remains constant up to a concentration of around 5 x 10B3mol dm-3. Some workers have reported the absence of aggregates of RB in ethanol, but they used concentrations smaller than 5 x 10e3 mol drne3 [ 13,161. However, the changes observed in the shape of the absorption spectra (tig. 2) for very concentrated solutions in ethanol greater than 5X 10m3mol dm-3) should be attributed to aggregation. Leshin et al. [ 91 have confirmed not only the molecular change of RB, but also the aggregation of the cationic form at concentrations higher than 5 x 10m3mol dmm3, although nonquantitative evaluation of the association was used. In very concentrated solutions, they reported a sharp decrease of the fluorescence yield; this has also been observed by Bojarski et al. [ lo], together with an increase of the apparent molecular weight of the dye. If it is considered that only one kind of molecular
12
lO_
8_
6_
4_
2_
”
I
1.7
I ts
I 19
I 2D
Fig. 2. Absorption spectra of the cationic form of rhodamine B in ethanol: 1.4x 10e6mol dm-’ (-) and 4 X lo-* mol dm-’ (---)at2O”C.
Volume 148, number 2,3
CHEMICAL PHYSICS LETTERS
association takes place, its equilibrium constant and absorption spectrum can be calculated using the iterative method previously reported [20]. The aggregation constant obtained in this way for both molecular forms of RR increases progressively with dye concentration. The calculated absorption spectrum of the aggregate has three bands, their relative intensities change with the dye concentration as fig. 3 shows for the cationic form, These results could indicate that different aggregates are formed at high concentrations of RB in ethanol, although this treatment does not permit the determination of the spectroscopic properties of all aggregates separately. In order to elucidate which aggregates are formed and to determine the corresponding equilibrium constants, vapour-pressure osmometry was used. Unfortunately, this technique cannot be applied to the zwitterion of RB in ethanol. The displacement of the equilibrium of RB with dye concentration towards the acid form [9-161 implies the use of a buffer for the osmometry measurements of the zwitterionic form, which will produce a change in the experimental conditions. Consequently only the
solution 2x lo-’ mol dm-3 (-) -).
and 4x lo-* mol dm-’ (- -
8 July 1988
aggregation of the cationic form of RB in ethanol was studied by osmometric measurements since this form is naturally generated in very concentrated solutions of the dye [ 9- 161. The association of the zwitterionic form is presumably not very different, as is indicated by the similarity observed in the aggregate absorption spectrum for both molecular forms of RB, and as is the case in aqueous solution [ 11,121. Moreover, the fluorescence quantum yield of the zwitterionic form of RB in concentrated ethanolic solution [ 2 1] follows the Stern-Volmer equation generated by considering the aggregation constants of the cationic form reported below. The experimental molality of the sample, determined from vapour-pressure osmometry [ 191, is compared with the theoretical value obtained by considering several aggregation models [ 18,19 1, i.e. the closed mechanism with the formation of a unique aggregate, K, (n is the aggregation order), and various open mechanisms considering the formation of aggregates of diverse orders as well as different relations between the association constants &, K3, .. .. K,,. The standard deviation of the complete set of concentrations used is minimized. A detailed description of the method has recently been published and applied to the aggregation of rhodamine 6G in ethanol [ 18 1. Similarly to rhodamine 6G [ 18 1, the cationic form of RB in the most concentrated ethanolic solution used in this work seems to be aggregated as dimers and trimers (standard deviation with experimental results, u* 1.2). Models considering the formation of higher-order aggregates give larger standard deviations (CT>15 ) . These results have been confirmed by emission measurements since the decrease of the fluorescence quantum yield of both molecular forms of RB in concentrated ethanolic solutions follows the Stem-Volmer equation for the quenching produced by dimers and trimers [ 2 11. The average value of the dimerization, &, and trimerization, K,, constants obtained are given in table 1 at the two extreme temperatures used in this work (27 and SO’C) and at 20°C obtained using the van? Hoff equation. The results obtained in this work contrast with the larger dimerization constant of RB in ethanol reported by Selwyn and Steinfeld [ 71 Kdx 1O4 (concentration standard: 1 mol dme3). This difference 255
CHEMICAL PHYSICS LEl-l-ERS
Volume 148, number 2,3
Table I Dimerization constant, Kd,and trimerization constants, K, (standard concentration = 1 mol dm-‘) for the cationic form of rhodamine B in ethanolic and aqueous [ 121 solutions at different temperatures Ethanol 2O’C
27’C
5O’C
1.70 24.7
1.78 23.1
2.05 18.9
Estimated error
Water 20°C
kO.1 fO.5
1400
is probably due to the neglect by these authors of the molecular form change of RB with dye concentration, as could be concluded by comparing the spectra reported by these authors to those obtained in studies of the molecular form equilibrium of RB [ 9- 17 1. Similar reasons are probably responsible for the large dimerization constant ( Kd= 2.8 x 104) for this dye reported by Wong and Schelly [ 8 1. As table 1 shows, the aggregation of the cationic form of RB in ethanol is some orders of magnitude smaller than that in aqueous solutions [ 5-7,11,12 1. This can be explained by the affinity of ethanol molecules for RB due to specific interactions with the ethylamino groups of the dye [ 3,4], hindering the association. Water molecules are avoided by the hy drophobic ethyl groups [3,4] favouring the aggregation of RB. It has not been possible to separately determine the absorption spectrum of both aggregates due to the fact that their concentrations are of the same order of magnitude in the concentration range studied. However, it is possible to assign the bands in fig. 3 if one assumes that the increase of the dye concentration implies a higher proportion of the trimer with respect to the dimer. So, the most energetic band in fig. 3 at x 19220 cm-’ should be attributed to the trimer since it increases in intensity with the dye concentration. For the same reason, the dimer should contribute to the central band, at % 18650 cm-‘, to a higher degree than the trimer. In contrast, the most intense band that appears at lower energies, at x 17630 cm-‘, does not depend appreciably on the dye concentration and therefore both aggregates seem to have an absorption band in this spectral region whose intensity and energy are similar. The same assignment has been previously proposed for the di256
8 July 1988
mer and trimer of rhodamine 6G in ethanol [ 181, where an approximate calculation of the absorption spectrum of both aggregates could be carried out. Hence, it is considered that the dimer of the cationic form of RB in ethanol has two bands at PZ17 630 and 18650 cm-‘, whereas the trimer has at least two bands at x 17630 and 19920 cm-‘. Exciton theory [ 211 predicts a third absorption band for the trimer but this could be masked or its intensity could be very low, as excitonic and experimental results have confirmed for other xanthene dyes such as rhodamine 6G [ 18 ] and fluorescein [ 201. Though the spectrum of both aggregates are not exactly stabilized, it is possible to draw some conclusions as to their geometrical structure from the band assignment of fig. 3. Exciton theory [ 221 predicts a band splitting in the dimer spectrum, with the higher energy band the most intense in a sandwich aggregate and the lower for a linear configuration. Fig. 3 suggests that the monomeric units in the cationic form of RB are disposed linearly in ethanolic solutions. A linear geometry has also been suggested for the dimer of RB in ethanol [ $1 and in other organic solvents [ 61. This contrasts with the sandwich structure adopted by the monomer units in the dimer of RB in aqueous solutions [ 5,11,12 1. This important difference could be due to the hydrophobic character of the ethylamino groups of RB [ 3,4] favouring the parallel-plane disposition of the chromophores in the aqueous dimer, thus avoiding the water molecules. Instead, the interaction of ethanol molecules with the ethylamino groups of RB [24,231 avoids the stacking of monomers. Moreover, a linear disposition of monomers maintains more of the solvation structure of ethanol. For the same reason a further linear attachment of another monomer is expected in order to generate the trimer. This can give rise to two structures, a linear or a cyclic trimer [ 18 1: Although the structure of the trimer cannot be determined from the absorption spectra, a cyclic linear geometry for the trimer does not allow the incorporation of a fourth monomer to form the tetramer, as is suggested by the results from the osmometry treatment carried out in the present work. In order to study the bonding between the monomers in the aggregates, thermodynamic parameters for both association processes have been determined
CHEMICAL PHYSICS LETTERS
Volume 148, number 2,3
Table 2 Thermodynamic parameters for the dimerization and trimerization of the cationic form of rhodamine B in ethanol at 20°C
dimerization trimerization estimated error
GO (kJ mol-I)
HO (klmol-‘)
-1.3 -1.8 !: 0.2
5.0 -1.0 f0.5
SO (Jmol-’ K-l) 21 3 f2
8 July 1988
Acknowledgement One of us (FLA) wishes to thank the Basque Country Government for a Reincorporation grant. The Universidad de1 Pais Vasco (EHU) is thanked for financial support.
References by vapour-pressure osmometry. The ethalpies of formation have been calculated using the van ‘t Hoff equation for the aggregation equilibrium constants at different temperatures. The changes produced in the standard free energy and entropy have been evaluated from AGO=RTln K and ASo= (AH’-AGO)/ T,respectively. The corresponding average values at 20°C for the dimerization and trimerization processes using different concentrations and temperatures are given in table 2. The dimerization enthalpy change obtained in this work is different to values previously reported [ 7,8 ] (- 17 and - 10kJmol- ’) , probably due to the neglect by these authors of both the molecular form change with dye concentration and the trimer formation. Our results show that the dimerization of the cationic form of RB in ethanol generates a slightly positive enthalpic change, whereas a small negative variation is observed for the trimerization. Dimer formation should imply the breaking of two hydrogen bonds between the solvent and the ethylamino groups of the dye, which is energetically compensated by the bonding between the monomer units in the dimer. The attachment of a third monomer to form the trimer should give a higher stabilization in this aggregate, especially if the cyclic structure is formed [ 18 1. Consequently a decrease in AH’ is expected and could explain the greater tendency to form the trimer than the dimer (table 1). The increase in the entropy change, despite the fact that these processes are associative, is attiibuted to the breakdown of the solvation structure. These results contrast with the large enthalpy decrease observed in the dimerization of molecular forms of RB in aqueous solution, AH02 20 kJ mol-’ [5-7,11,12]. In this case the self-aggregation of the dye easily takes place because there is no need to break any specific dye-solvent interaction [3,4].
[ 1] M.J. Snare, F.E. Treloar, K.P. Ghiggino and P.J. Thistlethwaite, J. Photochem. 18 ( 1982) 335.
[ 21 R.S. Moog, M.D. Ediger, S.G. Boxer and M.D. Fayer, J. Phys. Chem. 86 (1982) 4694; V. Sundstrom, T. Gillbro and M. Bergstrom, Chem. Phys. 13 (1982) 439. [ 31 I. L.&z Arbeloa and K.K. Rohatgi-Mukherjee, Chem. Phys. Letters 128 (1986) 414; 129 (1986) 601. [4] I. Mpez Arbeloa and K.K. Rohatgi-Mukhejke, to be published. [ 51K.K. Rohatgi and G.S. Singhal, J. Phys. Chem. 10 ( 1966) 1965; 16 (1972) 162; K.K. Rohatgi, J. Mol. Spectry. 21 ( 1968) 545; M.E. Gal, G.R. Kelly and T. Kurucsev, J. Chem. Sot. Faraday Trans. II 69 (1973) 395; T. Kajiwara, R.W. Chambers and D.R. Keams, Chem. Phys. Letters 22 (1973) 31; G. Obcrmuller and C. Bojarski, Acta Phys. Pol. A 52 ( 1977) 431. [ 61 J. Muto, J. Phys. Chem. 80 ( 1916) 1342. [7] J.E. Selwyn and J.I. Steinfeld, J. Phys. Chem. 16 (1912) 762. [S] M.M. Wong and 2-A. Schelly, J. Phys. Chem. 78 (1914) 1891. [ 9 ] L.V. Levshin, T.D. Slavnova, V.I. Jushakov, N.B. Zorov and V.Z. Pastchenko, Russian J. Phys. Chem. 48 ( 1974) 46. [lo] C. Bojarski, G. Zurkowska and J. Tyrzyk, Z. Naturforsch. 37a (1982) 14. [ 111I. Mpez Arbeloa and P. Ruiz Ojeda, Chem. Phys. Letters 79 (1981) 347. [ 1211.Ldpez Arbeloa and P. Ruiz Ojeda, Chem. Phys. Letters 87 (1982) 556. [ 131J. Ferguson and A.W. Mau, Chem. Phys. Letters 17 ( 1972) 543;Australian J. Chem. 26 (1973) 1617. [ 141 K.H. Drexhage, Laser Focus 9 (1973) 35; J. Res. 80A ( 1976) 42 1; in: Topics in applied physics, Vol. 1. Dye laser, ed. F.P. Schiifer (Springer, Berlin, 1977); B.R. Russell, Chem. Phys. Letters 78 ( 198I ) 456; A.L. Smirl, J.B. Clark, E.W. van Stryland and B.R. Russell, J. Chem. Phys. 77 ( 1982) 63 1. [IS] P.J. Sadowski and G.R. Fleming, Chem. Phys. Letters 57 (1982) 526. [ 161 M. Faraggi, P. Peretz, I. Rosenthal and D. Weinraub, Chem. Phys. Letters 103 (1984) 310.
257
Volume 148, number 2,3
CHEMICAL PHYSICS LETTERS
[ 171J.B. Clark, A.L. Smirl, E.W. van Stryland and B.R. Russell, Chem. Phys. Letters 78 ( 1981) 456. [ 18] P. Ruiz Ojeda, I. Katime, J.R. Ochoa and I. Mpez Arbeloa, J. Chem. Sot. Faraday Trans. II 84 ( 1988) 1. [ 191I.KatimeandF. Aguilar,Tbemwchim. Acta (1981) 139. [ 201 I. Mpez Arbeloa, J. Chem. !kx. Faraday Trans. II 77 ( 1981) 725. [ 2 1 ] F. Mpez Arbeloa, P. Ruiz Ojeda and I. Mpez Arbeloa, to be published.
258
8 July 1988
[22] E.G. McRae and M. Kasha, in: Physical processes in radiation biology (Academic Press, New York, 1964); M. Kasha, H.R. Rawls and A. El Bayoumi, Pure Appl. Chem. 11 (1965) 371. [ 231 A. von Jena and H.E. Lessing, Chem. Phys. 40 ( 1979) 245; V. Sundstrom, T. Gillbro and M. Bergstrom, Chem. Phys. 73 (1982) 439.
CHEMICAL PHYSICS LETTERS
Volume 148, number 2,3
ON THE AGGREGATION OF RHODAMINE B IN ETHANOL F. LOPEZ ARBELOA, P. RUIZ OJEDA and I. LbPEZ ARBELOA Depattamento de Quimica Fisica. Universidaddel Pais Vasco-EHU,Apdo 644, 48080 Bilbao, Spain Received 15February 1988; in final form 3 May 1988
The dimer and the trimer aggregation constants of the cationic form of rhodamine B are determined in very concentrated ethanolic solutions of the dye. The absorption spectrum, the geometrical structure and the association bonding of the aggregates are discussed and compared with those previously obtained in aqueous solution.
1. Introduction
The optical properties of rhodamines in solution have attracted the attention of numerous workers since the investigation of such properties is essential for the development of tunable lasers using organic dyes as media. In particular, the spectroscopic and photophysical properties of rhodamine B, RB, have been extensively studied as a function of solvent [ l41, concentration [5-121, andpH [3,4,9-161. The equilibrium between the zwitterion, RB’, and the catiopic, RBH+, molecular forms of the dye (fig. 1) can be shifted depending on the concentration of H+ ions in the solution. In spite of the number of papers, the optical characteristics of both molecular forms of RB continue to be the subject of some controversy. The effect produced on the absorption spectrum by increasing the concentration of the dye in ethanolic solutions has
been interpreted in different ways. First of all, changes in the absorption spectra have been attributed by some authors to a shift in the equilibrium of RB from the zwitterionic to the cationic form with increasing dye concentration [ 13,14,16 1. Other authors have considered the dimerization process which has a large equilibrium constant between lo4 and 2.4 X lo4 (concentration standard = 1 mol dmd3) [ 7,8]. Finally, the occurrence of both processes, with a small degree of aggregation which becomes appreciable only in very concentrated solutions of the dye (higher than 5~10-~ mol dmm3), has been also taken into account [9,10]. This paper aims to obtain further information on the occurrence of aggregation for RB in ethanol. To this purpose, absorption spectra and vapour-pressure osmometty measurements have been taken up to very high concentrations of RB. The results indicate that not only the dimer but also the trimer is
COOH
Cationic (protonated
form R BH+)
Zwitterion (RB’I
form
Fig. 1.Motecular structure of the zwitterionic, RB’, and cationic RBH+, forms of rhodamine B.
0 009-2614/8&X/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division )
253
Volume 148,number 2,3
CHEMICALPHYSICSLETTERS
formed in the concentration range used in this work. The aggregation constant, the geometrical structure and the association bonding of the aggregates are discussed and compared with those obtained in aqueous solution [ 11,121.
2. Experimental Rhodamine B was supplied from Merck (proanalysis). It was recrystallized twice from ethanol and dried under vacuum. Other chemical compounds were Merck suprapur grade quality. The zwitterionic and cationic forms of the dye were generated by addition of suitable amounts of NaOH (ethanolic sol. ) and HCl (ethanolic sol.), respectively, to the solutions [ 3,4,9-161. Without such addition the molecular form equilibrium of RB is displaced to the cationic side in concentrated solutions of the dye (> 10m3mol dms3) [g-16]. Absorption spectra were recorded at a dye concentration range from 8X lo-’ up to 5X 10m2 mol dme3 in a Beckman 5260 spectrophotometer with a double monochromator using quartz cells with different optical path lengths. A Beckman RIIC cell (model BC-14) with a variable optical path length was used to record the absorption spectra of RB in very concentrated solutions. Vapour-pressure osmometry measurements were carried out in a Knauer 1974 osmometer using benzyl as the standard [ 18,191, The concentration ranged from 2.5~10-~ to 0.14 mol kg-’ and the temperature from 27 to 50°C at 6°C intervals.
3. Results and discussion When the concentration of RB in ethanol is increased to 10m3mol dm13 a displacement to smaller energies of about 350 cm-’ in the absorption and the fluorescence spectra is observed. This was attributed by Ferguson and Mau [ 131 to a change in the molecular form of RB from the zwitterionic to the cationic form since an increase in the dye concentration leads to an increase in the acidity of the solution. Later this was extensively confirmed [ 9- 16 1. However, we have observed that the shape of the absorption spectra of both molecular forms in ethanol, 254
8 July 1988
generated by controlling the pH, remains constant up to a concentration of around 5 x 10B3mol dm-3. Some workers have reported the absence of aggregates of RB in ethanol, but they used concentrations smaller than 5 x 10e3 mol drne3 [ 13,161. However, the changes observed in the shape of the absorption spectra (tig. 2) for very concentrated solutions in ethanol greater than 5X 10m3mol dm-3) should be attributed to aggregation. Leshin et al. [ 91 have confirmed not only the molecular change of RB, but also the aggregation of the cationic form at concentrations higher than 5 x 10m3mol dmm3, although nonquantitative evaluation of the association was used. In very concentrated solutions, they reported a sharp decrease of the fluorescence yield; this has also been observed by Bojarski et al. [ lo], together with an increase of the apparent molecular weight of the dye. If it is considered that only one kind of molecular
12
lO_
8_
6_
4_
2_
”
I
1.7
I ts
I 19
I 2D
Fig. 2. Absorption spectra of the cationic form of rhodamine B in ethanol: 1.4x 10e6mol dm-’ (-) and 4 X lo-* mol dm-’ (---)at2O”C.
Volume 148, number 2,3
CHEMICAL PHYSICS LETTERS
association takes place, its equilibrium constant and absorption spectrum can be calculated using the iterative method previously reported [20]. The aggregation constant obtained in this way for both molecular forms of RR increases progressively with dye concentration. The calculated absorption spectrum of the aggregate has three bands, their relative intensities change with the dye concentration as fig. 3 shows for the cationic form, These results could indicate that different aggregates are formed at high concentrations of RB in ethanol, although this treatment does not permit the determination of the spectroscopic properties of all aggregates separately. In order to elucidate which aggregates are formed and to determine the corresponding equilibrium constants, vapour-pressure osmometry was used. Unfortunately, this technique cannot be applied to the zwitterion of RB in ethanol. The displacement of the equilibrium of RB with dye concentration towards the acid form [9-161 implies the use of a buffer for the osmometry measurements of the zwitterionic form, which will produce a change in the experimental conditions. Consequently only the
solution 2x lo-’ mol dm-3 (-) -).
and 4x lo-* mol dm-’ (- -
8 July 1988
aggregation of the cationic form of RB in ethanol was studied by osmometric measurements since this form is naturally generated in very concentrated solutions of the dye [ 9- 161. The association of the zwitterionic form is presumably not very different, as is indicated by the similarity observed in the aggregate absorption spectrum for both molecular forms of RB, and as is the case in aqueous solution [ 11,121. Moreover, the fluorescence quantum yield of the zwitterionic form of RB in concentrated ethanolic solution [ 2 1] follows the Stern-Volmer equation generated by considering the aggregation constants of the cationic form reported below. The experimental molality of the sample, determined from vapour-pressure osmometry [ 191, is compared with the theoretical value obtained by considering several aggregation models [ 18,19 1, i.e. the closed mechanism with the formation of a unique aggregate, K, (n is the aggregation order), and various open mechanisms considering the formation of aggregates of diverse orders as well as different relations between the association constants &, K3, .. .. K,,. The standard deviation of the complete set of concentrations used is minimized. A detailed description of the method has recently been published and applied to the aggregation of rhodamine 6G in ethanol [ 18 1. Similarly to rhodamine 6G [ 18 1, the cationic form of RB in the most concentrated ethanolic solution used in this work seems to be aggregated as dimers and trimers (standard deviation with experimental results, u* 1.2). Models considering the formation of higher-order aggregates give larger standard deviations (CT>15 ) . These results have been confirmed by emission measurements since the decrease of the fluorescence quantum yield of both molecular forms of RB in concentrated ethanolic solutions follows the Stem-Volmer equation for the quenching produced by dimers and trimers [ 2 11. The average value of the dimerization, &, and trimerization, K,, constants obtained are given in table 1 at the two extreme temperatures used in this work (27 and SO’C) and at 20°C obtained using the van? Hoff equation. The results obtained in this work contrast with the larger dimerization constant of RB in ethanol reported by Selwyn and Steinfeld [ 71 Kdx 1O4 (concentration standard: 1 mol dme3). This difference 255
CHEMICAL PHYSICS LEl-l-ERS
Volume 148, number 2,3
Table I Dimerization constant, Kd,and trimerization constants, K, (standard concentration = 1 mol dm-‘) for the cationic form of rhodamine B in ethanolic and aqueous [ 121 solutions at different temperatures Ethanol 2O’C
27’C
5O’C
1.70 24.7
1.78 23.1
2.05 18.9
Estimated error
Water 20°C
kO.1 fO.5
1400
is probably due to the neglect by these authors of the molecular form change of RB with dye concentration, as could be concluded by comparing the spectra reported by these authors to those obtained in studies of the molecular form equilibrium of RB [ 9- 17 1. Similar reasons are probably responsible for the large dimerization constant ( Kd= 2.8 x 104) for this dye reported by Wong and Schelly [ 8 1. As table 1 shows, the aggregation of the cationic form of RB in ethanol is some orders of magnitude smaller than that in aqueous solutions [ 5-7,11,12 1. This can be explained by the affinity of ethanol molecules for RB due to specific interactions with the ethylamino groups of the dye [ 3,4], hindering the association. Water molecules are avoided by the hy drophobic ethyl groups [3,4] favouring the aggregation of RB. It has not been possible to separately determine the absorption spectrum of both aggregates due to the fact that their concentrations are of the same order of magnitude in the concentration range studied. However, it is possible to assign the bands in fig. 3 if one assumes that the increase of the dye concentration implies a higher proportion of the trimer with respect to the dimer. So, the most energetic band in fig. 3 at x 19220 cm-’ should be attributed to the trimer since it increases in intensity with the dye concentration. For the same reason, the dimer should contribute to the central band, at % 18650 cm-‘, to a higher degree than the trimer. In contrast, the most intense band that appears at lower energies, at x 17630 cm-‘, does not depend appreciably on the dye concentration and therefore both aggregates seem to have an absorption band in this spectral region whose intensity and energy are similar. The same assignment has been previously proposed for the di256
8 July 1988
mer and trimer of rhodamine 6G in ethanol [ 181, where an approximate calculation of the absorption spectrum of both aggregates could be carried out. Hence, it is considered that the dimer of the cationic form of RB in ethanol has two bands at PZ17 630 and 18650 cm-‘, whereas the trimer has at least two bands at x 17630 and 19920 cm-‘. Exciton theory [ 211 predicts a third absorption band for the trimer but this could be masked or its intensity could be very low, as excitonic and experimental results have confirmed for other xanthene dyes such as rhodamine 6G [ 18 ] and fluorescein [ 201. Though the spectrum of both aggregates are not exactly stabilized, it is possible to draw some conclusions as to their geometrical structure from the band assignment of fig. 3. Exciton theory [ 221 predicts a band splitting in the dimer spectrum, with the higher energy band the most intense in a sandwich aggregate and the lower for a linear configuration. Fig. 3 suggests that the monomeric units in the cationic form of RB are disposed linearly in ethanolic solutions. A linear geometry has also been suggested for the dimer of RB in ethanol [ $1 and in other organic solvents [ 61. This contrasts with the sandwich structure adopted by the monomer units in the dimer of RB in aqueous solutions [ 5,11,12 1. This important difference could be due to the hydrophobic character of the ethylamino groups of RB [ 3,4] favouring the parallel-plane disposition of the chromophores in the aqueous dimer, thus avoiding the water molecules. Instead, the interaction of ethanol molecules with the ethylamino groups of RB [24,231 avoids the stacking of monomers. Moreover, a linear disposition of monomers maintains more of the solvation structure of ethanol. For the same reason a further linear attachment of another monomer is expected in order to generate the trimer. This can give rise to two structures, a linear or a cyclic trimer [ 18 1: Although the structure of the trimer cannot be determined from the absorption spectra, a cyclic linear geometry for the trimer does not allow the incorporation of a fourth monomer to form the tetramer, as is suggested by the results from the osmometry treatment carried out in the present work. In order to study the bonding between the monomers in the aggregates, thermodynamic parameters for both association processes have been determined
CHEMICAL PHYSICS LETTERS
Volume 148, number 2,3
Table 2 Thermodynamic parameters for the dimerization and trimerization of the cationic form of rhodamine B in ethanol at 20°C
dimerization trimerization estimated error
GO (kJ mol-I)
HO (klmol-‘)
-1.3 -1.8 !: 0.2
5.0 -1.0 f0.5
SO (Jmol-’ K-l) 21 3 f2
8 July 1988
Acknowledgement One of us (FLA) wishes to thank the Basque Country Government for a Reincorporation grant. The Universidad de1 Pais Vasco (EHU) is thanked for financial support.
References by vapour-pressure osmometry. The ethalpies of formation have been calculated using the van ‘t Hoff equation for the aggregation equilibrium constants at different temperatures. The changes produced in the standard free energy and entropy have been evaluated from AGO=RTln K and ASo= (AH’-AGO)/ T,respectively. The corresponding average values at 20°C for the dimerization and trimerization processes using different concentrations and temperatures are given in table 2. The dimerization enthalpy change obtained in this work is different to values previously reported [ 7,8 ] (- 17 and - 10kJmol- ’) , probably due to the neglect by these authors of both the molecular form change with dye concentration and the trimer formation. Our results show that the dimerization of the cationic form of RB in ethanol generates a slightly positive enthalpic change, whereas a small negative variation is observed for the trimerization. Dimer formation should imply the breaking of two hydrogen bonds between the solvent and the ethylamino groups of the dye, which is energetically compensated by the bonding between the monomer units in the dimer. The attachment of a third monomer to form the trimer should give a higher stabilization in this aggregate, especially if the cyclic structure is formed [ 18 1. Consequently a decrease in AH’ is expected and could explain the greater tendency to form the trimer than the dimer (table 1). The increase in the entropy change, despite the fact that these processes are associative, is attiibuted to the breakdown of the solvation structure. These results contrast with the large enthalpy decrease observed in the dimerization of molecular forms of RB in aqueous solution, AH02 20 kJ mol-’ [5-7,11,12]. In this case the self-aggregation of the dye easily takes place because there is no need to break any specific dye-solvent interaction [3,4].
[ 1] M.J. Snare, F.E. Treloar, K.P. Ghiggino and P.J. Thistlethwaite, J. Photochem. 18 ( 1982) 335.
[ 21 R.S. Moog, M.D. Ediger, S.G. Boxer and M.D. Fayer, J. Phys. Chem. 86 (1982) 4694; V. Sundstrom, T. Gillbro and M. Bergstrom, Chem. Phys. 13 (1982) 439. [ 31 I. L.&z Arbeloa and K.K. Rohatgi-Mukherjee, Chem. Phys. Letters 128 (1986) 414; 129 (1986) 601. [4] I. Mpez Arbeloa and K.K. Rohatgi-Mukhejke, to be published. [ 51K.K. Rohatgi and G.S. Singhal, J. Phys. Chem. 10 ( 1966) 1965; 16 (1972) 162; K.K. Rohatgi, J. Mol. Spectry. 21 ( 1968) 545; M.E. Gal, G.R. Kelly and T. Kurucsev, J. Chem. Sot. Faraday Trans. II 69 (1973) 395; T. Kajiwara, R.W. Chambers and D.R. Keams, Chem. Phys. Letters 22 (1973) 31; G. Obcrmuller and C. Bojarski, Acta Phys. Pol. A 52 ( 1977) 431. [ 61 J. Muto, J. Phys. Chem. 80 ( 1916) 1342. [7] J.E. Selwyn and J.I. Steinfeld, J. Phys. Chem. 16 (1912) 762. [S] M.M. Wong and 2-A. Schelly, J. Phys. Chem. 78 (1914) 1891. [ 9 ] L.V. Levshin, T.D. Slavnova, V.I. Jushakov, N.B. Zorov and V.Z. Pastchenko, Russian J. Phys. Chem. 48 ( 1974) 46. [lo] C. Bojarski, G. Zurkowska and J. Tyrzyk, Z. Naturforsch. 37a (1982) 14. [ 111I. Mpez Arbeloa and P. Ruiz Ojeda, Chem. Phys. Letters 79 (1981) 347. [ 1211.Ldpez Arbeloa and P. Ruiz Ojeda, Chem. Phys. Letters 87 (1982) 556. [ 131J. Ferguson and A.W. Mau, Chem. Phys. Letters 17 ( 1972) 543;Australian J. Chem. 26 (1973) 1617. [ 141 K.H. Drexhage, Laser Focus 9 (1973) 35; J. Res. 80A ( 1976) 42 1; in: Topics in applied physics, Vol. 1. Dye laser, ed. F.P. Schiifer (Springer, Berlin, 1977); B.R. Russell, Chem. Phys. Letters 78 ( 198I ) 456; A.L. Smirl, J.B. Clark, E.W. van Stryland and B.R. Russell, J. Chem. Phys. 77 ( 1982) 63 1. [IS] P.J. Sadowski and G.R. Fleming, Chem. Phys. Letters 57 (1982) 526. [ 161 M. Faraggi, P. Peretz, I. Rosenthal and D. Weinraub, Chem. Phys. Letters 103 (1984) 310.
257
Volume 148, number 2,3
CHEMICAL PHYSICS LETTERS
[ 171J.B. Clark, A.L. Smirl, E.W. van Stryland and B.R. Russell, Chem. Phys. Letters 78 ( 1981) 456. [ 18] P. Ruiz Ojeda, I. Katime, J.R. Ochoa and I. Mpez Arbeloa, J. Chem. Sot. Faraday Trans. II 84 ( 1988) 1. [ 191I.KatimeandF. Aguilar,Tbemwchim. Acta (1981) 139. [ 201 I. Mpez Arbeloa, J. Chem. !kx. Faraday Trans. II 77 ( 1981) 725. [ 2 1 ] F. Mpez Arbeloa, P. Ruiz Ojeda and I. Mpez Arbeloa, to be published.
258
8 July 1988
[22] E.G. McRae and M. Kasha, in: Physical processes in radiation biology (Academic Press, New York, 1964); M. Kasha, H.R. Rawls and A. El Bayoumi, Pure Appl. Chem. 11 (1965) 371. [ 231 A. von Jena and H.E. Lessing, Chem. Phys. 40 ( 1979) 245; V. Sundstrom, T. Gillbro and M. Bergstrom, Chem. Phys. 73 (1982) 439.