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Ground- and excited-state solvation of a squaraine dye by water in dioxane
27 March 1998
Chemical Physics Letters 285 Ž1998. 385–390
Ground- and excited-state solvation of a squaraine dye by water in dioxane Cesar ´ A.T. Laia, Sılvia ´ M.B. Costa
)
Centro de Quımica Estrutural, Complexo 1, Instituto Superior Tecnico, 1096 Lisboa Codex, Portugal ´ ´ Received 9 December 1997
Abstract Dioxanerwater mixtures were used in order to assess the water effect on the photophysics of two squaraine dyes. It was found that a long-range hydrogen-bonding interaction exists in the ground state, without preferential solvation by water. However, in the excited state a more specific interaction is responsible for a pronounced quenching effect. Indeed, in the unsubstituted squaraine bisw4-Ždimethylamine.phenylx squaraine an intermolecular hydrogen bonding leads to an increase of internal conversion due to accepting vibrational modes created by this interaction, while in the symmetrically dihydroxy-substituted squaraine, an intramolecular hydrogen bond prevents the occurrence of the intermolecular interaction with water. q 1998 Elsevier Science B.V.
1. Introduction Hydrogen bonding between a given solute and the solvent constitutes one of the most important interactions which influence the photochemistry of organic dyes w1x. These interactions may lead to a proton transfer reaction or solute solvation, changing decisively the spectroscopic characteristics w2–5x. Among the molecules affected by hydrogen interactions are the squaraine derivatives w6–8x. These are important dyes used in several applications such as xerographic processes and sensitization of semiconductor materials w8,9x. The main spectroscopic feature of these dyes is the intense charge transfer band observed both in the ground and excited state transitions, associated with a planar D–A–D structure Ždonor–
)
Corresponding author
acceptor–donor., where two aniline moieties act like electron donors to the central squaric moiety, C 4 O 2 , the electron acceptor group w10x. The first attempt to understand the photophysics of these dyes was made by Law in 1987 w6x. It was found that the photophysical behaviour of these molecules is strongly solvent dependent. A dependence on both the temperature and solvent was observed on the absorption and emission properties of several substituted squaraines, leading to the proposal of a solute–solvent complex in the ground and excited state w7x. Hole-burning and other studies in polymer films w11,12x stressed the importance of the protic character of the medium as well as the presence of hydroxyl groups in the squaraine, pointing to specific interactions between the squaric group and the protic solvent w7,11,12x. The squaraines used in this study were bisw4-Ždimethylamine.phenylx squaraine ŽHSq. and bisw4-Ždimethylamine.-2-hydroxyphenylx squaraine ŽHOSq. Žsee Fig. 1.. The aim of this work was directed to the
0009-2614r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 0 0 9 2 - X
386
C.A.T. Laia, S.M.B. Costar Chemical Physics Letters 285 (1998) 385–390
water used was bidistilled. Dye concentrations were always below 1 = 10y6 M due to the low solubility and in order to keep the optical density below 0.1. Samples were made from stock solutions in chloroform by careful solvent evaporation. All measurements were made one day after the sample preparation in order to ensure a complete dye solubilization. 2.2. Apparatus
Fig. 1. Chemical structure of bisw4-Ždimethylamine.phenylx squaraine ŽHSq. and bisw4-Ždimethylamine.-2-hydroxyphenylx squaraine ŽHOSq..
understanding of the interaction of these dyes with water. A considerable increase of the fluorescence quantum yield in the hydroxyl derivative was found previously w6–8x. That was caused by the increase of the dye rigidity created by a double intramolecular hydrogen bond, preventing deactivation of the excited state. That does not happen in the case of HSq; thus the presence of water is expected to change greatly the fluorescence efficiency. Since squaraines are insoluble in water, dioxanerwater mixtures were used in order to study the interaction of HSq and HOSq with water w5,13– 19x. We wish to report that there is neither preferential solvation due to the complex formation with water nor dielectric enrichment w15,16x. However, there is a strong effect of water in the non-radiative processes of these dyes. The possible consequence of the non-ideality of these mixtures w13,15,16,19x in the photophysics of both HSq and HOSq is also discussed. The final goal is to assess the feasibility of these dyes as water probes, using the standard but highly sensitive fluorescence spectroscopy.
2. Experimental section 2.1. Materials The squaraines were obtained from Xerox and used as supplied. Dioxane was purchased from Merck with spectroscopic grade and used as received. The
Absorption spectra were recorded with a JASCO V-560 UVrVIS Spectrophotometer. Steady-state emission measurements were recorded with a Perkin–Elmer LS 50B Spectrofluorimeter with the sample holder thermostatized at 228C. All data were stored in a computer. The instrumental response at each wavelength was corrected by means of a curve provided with the instrument. For each sample, emission spectra were recorded immediately after the measurement of the absorption spectra. Oxazin 1 was used as fluorescent standard Ž f f s 0.11. w20x. Transient measurements were carried out using the time-resolved single photon counting technique with a mode-locked Coherent Innova 400-10 argonion laser, synchronously pumped with a cavity dumped Coherent 701-2 dye Žrhodamine 6G. laser, giving 3–4 ps pulses Žwith f 40 nJrpulse, 3.4 MHz. with lex s 592 nm. Emission polarizer was set at the magic angle. Further description may be found in Ref. w21x. The decays were obtained at lem s 650 nm. Decay data analysis was performed using the software provided by Photon Technology International ŽCanada..
3. Results and discussion The emission spectra of HSq and HOSq were taken in pure dioxane and for different water concentrations in dioxanerwater mixtures ŽFig. 2.. The absorption and emission spectra are structureless in agreement with previous results published in the literature w8,12,22x. The maximum absorption and emission wavelengths have a red shift with increasing water concentration reflecting the increase of polarity. Using the dielectric constants and refractive indexes found in the literature w5,23x, both absorption
C.A.T. Laia, S.M.B. Costar Chemical Physics Letters 285 (1998) 385–390
Fig. 2. Representative fluorescence decay of HSq in dioxanerwater mixture ŽwH 2 Ox s 2 M. with monoexponential fit. Insert: emission spectra of HSq in dioxanerwater ŽwH 2 Ox s 0, 0.56, 1.1, 2.8, 5.6 and 11 M from top to bottom..
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solvation is expected on account of the high insolubility of these dyes in water. Therefore, there is neither dielectric enrichment in the surroundings of both squaraine dyes nor squarainerwater complex formation in the ground state. The Stokes shift correlation with the Lippert– Mataga function also gives a good linear relationship for HSq, with approximately the same slope obtained in pure solvents w26x ŽFig. 4.. A complex relation with f Ž ´ ,n. is found for HOSq: at small water concentrations the slope is similar to the one found in pure solvents w26x, whereas a plateau is reached at high concentrations. The different behaviour observed for the two dyes is an indication of the importance of the double intramolecular hydrogen bond formed in HOSq. In Table 1 the fluorescence quantum yields of HSq and HOSq are listed. Large differences are observed for each solute: in the HSq case, the fluorescence quantum yield decreases with the water concentration while in HOSq it remains almost constant. The fluorescence steady-state data indicate that
and emission were correlated with the Lippert– Mataga dielectric function w24,25x ŽFig. 3.: Õa y Õ f s
2 Ž me y mg .
f Ž ´ ,n . s
hca3
f Ž ´ ,n . , n2 y 1
´y1 y 2´q1
2
2 n2 q 1
Ž 1. Ž 2.
A good linear relationship for the absorption maximum was found for both dyes, while the HOSq emission maximum deviates from linearity ŽFig. 3b.. When the correlation was made with the water mole fraction, the linearity was lost ŽFig. 3a.. This is due to the fact that dioxanerwater mixtures are not ideal and the following equation: f Ž ´ ,n . s x Dx f Dx Ž ´ ,n . q x W f W Ž ´ ,n .
Ž 3.
where x is the mole fraction ŽDx stands for dioxane and W for water. does not hold w13,15,16,19x. The linear relation of the absorption with f Ž ´ ,n. for both dyes shows that in the ground state there is no solute–solvent interaction. The lack of preferential
Fig. 3. Maximum absorption Ž^,I. and emission Ž',B. wavenumbers for HSq Ž^,'. and HOSq ŽI,B. against water mole fraction Ža. and f Ž ´ ,n. Žb..
C.A.T. Laia, S.M.B. Costar Chemical Physics Letters 285 (1998) 385–390
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accordance with fluorescence quantum yield measurements, the HSq fluorescence lifetime also decreases with the increase of water concentration. A Stern–Volmer dependence of the HSq fluorescence quantum yields and lifetimes should be expected w5x. However, should there be formation of specific complexes between HSq and water in the ground state, the static quenching fraction contribution should appear in the fluorescence quantum yields:
Ž forf . s 1 q K b wH 2 Ox , Ž tort . Fig. 4. Stokes shift of HSq Ž D . and HOSq ŽI. in dioxanerwater mixtures.
the intramolecular hydrogen bond in HOSq increases the rigidity of the molecule as seen earlier w6–8x and also prevents the squaric moiety from having interactions with the protic molecules in the excited state. HSq feels a strong fluorescence quenching in the presence of water molecules similar to the one found for other organic molecules like, for example substituted anthraquinones and anthroates w2,5,27x. The formation of the hydrogen bond creates an accepting vibrational mode w2,28x and therefore an increase of the internal conversion is observed. The fluorescent decays of HSq and HOSq were always mono-exponential Žsee Table 1 and Fig. 2. in pure dioxane and for all the water concentrations studied, indicating one spectroscopic species only. In
Ž 4.
where K b is the equilibrium constant for the association of the solute with water in the ground state. In Fig. 5 it is shown that this fraction is negligible within experimental error, indicating no association between HSq and water in the ground state, as found in the solvatochromism study. At this point, it must be said that hydrogen interactions between ground state HSq and water do exist, otherwise HSq absorption would not be sensitive to the protic character of the solvent as observed
Table 1 Experimental data of HSq and HOSq dyes in dioxanerwater mixtures: fluorescence quantum yield and fluorescence lifetimes wH 2 OxrM
f HSq
t HSq rns
f HOSq
t HOSq rns
0 0.28 0.56 0.83 1.11 2.00 2.78 4.00 5.56 8.00 11.11 15.00
0.57 0.48 0.42 0.37 0.32 0.22 0.21 0.16 0.13 0.11 0.09 0.08
2.101 1.829 1.588 1.373 1.167 0.935 0.779 0.643 0.536 0.435 0.360 0.318
0.84 0.82 0.87 0.86 0.83 0.79 y 0.79 0.83 0.78 0.77 0.78
2.755 2.796 2.646 2.686 2.743 2.487 y 2.434 2.377 2.410 2.489 2.309
Fig. 5. Stern–Volmer plots for HSq Ž', fo rf ; D, to rt . and HOSq ŽB, fo rf ; I, to rt . in dioxanerwater mixtures Ža. and static quenching fraction for HSq Ž D . and HOSq ŽI. Žb..
C.A.T. Laia, S.M.B. Costar Chemical Physics Letters 285 (1998) 385–390
with pure solvents w7x but they are long range in nature w29x. Should short range interactions be already present in the ground state, non-linearity would appear in Fig. 3 w15x and static effects would contribute in the fluorescence quenching as happened in the 9-anthroate fluorescence quenching study case w5x. The HSq ratio tort has a negative deviation from the Stern–Volmer equation. The origin of this deviation may be due to the existence of ineffective water molecules, which cannot quench the dye fluorescence. This result must be interrelated with the fact that these mixtures are not ideal w13,15,16x. An attempt to account for this phenomenon was made by assuming water dimer formation: 2H 2 O ° Ž H 2 O . 2
K dim s
Ž H 2 O. 2 . wH 2 Ox 2
Ž 5.
The possibility of water aggregation in liquid mixtures was speculated w5,13,30x and shown earlier w31x. It was found that in a hydrophobic solvent the water molecules self aggregate easily, leading to a great variety of dioxanerwater complexes in the present mixture w31x. Thus, this equilibrium is only a rough approximation of what is happening at the molecular level. The water dimer has a binding energy around 5 kcalrmol w32x. In the gas phase an estimate of K dim s 1 My1 was obtained for water dimerization w33x. When this extra process is considered in the Stern–Volmer kinetic scheme and assuming that the dimer does not quench the squaraine, one obtains the following equation:
to t
s 1 q k qto
(1 q 8 K
dim
wH 2 Ox o y 1
4 K dim
,
Ž 6.
where wH 2 Oxo is the analytical water concentration. Fitting the experimental results with Eq. 6 Žsee Fig. 5. it is possible to extract a dimerization constant Ž K dim . equal to 0.0794 My1 and a quenching rate constant Ž k q . equal to 3.87 = 10 8 My1 sy1. The dimerization constant obtained is reasonable when compared with the one predicted for the gas phase w33x, since competition from dioxane may occur w31,34x. The quenching rate constant is slower than diffusion, meaning that the process is reaction con-
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trolled Žas found in another case w5x.. The quenching inefficiency of water dimers may be due to the high reversibility in the formation of the excited state HSqrwater dimer complex, lack of accepting vibrational modes or to the complexity of these mixtures at the molecular level w31x. With the present results it is impossible to establish an exact picture of the HSq interaction with aggregated water, so we can only speculate that these aspects may contribute to the quenching inefficiency of aggregated water. The use of HSq as a fluorescent water probe looks promising, due to the fact that its sensitivity to media dielectric changes is linear in the ground state Žthere is no preferential solvation. and that the effect of water concentration in the emission is selective Ždue to quenching by hydrogen bonding.. However, a wide use of this dye in solution is not possible due to its low solubility in most liquids. The use of other functionalized squaraines w6,7x which may be better dissolved in organic media would make these dyes an important class of fluorescent water probes.
Acknowledgements This w ork w as supported by Project PRAXISr2r2.1rQUIr22r94. The authors thank Dr. R.O. Loutfy ŽXerox, Canada. for the generous gift of squaraine samples. Professor J.M.G. Martinho is acknowledged for the use of the SPC equipment and Dr. A. Fedorov for the valuable assistance in the time-resolved measurements. Cesar A.T. Laia ac´ knowledges a PhD Grant BD No. 961 from PRAXIS XXI.
References w1x M. Kasha, J. Chem. Soc., Faraday Trans. II 82 Ž1986. 2379. w2x S.R. Flom, P.F. Barbara, J. Phys. Chem. 89 Ž1985. 4489. w3x Y.R. Kim, J.T. Yardley, R.M. Hochstrasser, Chem. Phys. 136 Ž1989. 311. w4x P.J. Reid, P.F. Barbara, J. Phys. Chem. 99 Ž1995. 3554. w5x A.L. Mac¸anita, F.P. Costa, S.M.B. Costa, E.C. Melo, H. Santos, J. Phys. Chem. 93 Ž1989. 336. w6x K.-Y. Law, J. Phys. Chem. 91 Ž1987. 5184. w7x K.-Y. Law, J. Photochem. Photobiol. A: Chem. 84 Ž1994. 123. w8x P.V. Kamat, S. Das, K.G. Thomas, M.V. George, J. Phys. Chem. 96 Ž1992. 195.
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C.A.T. Laia, S.M.B. Costar Chemical Physics Letters 285 (1998) 385–390
w9x K.-Y. Law, Chem. Rev. 93 Ž1993. 449. w10x R.W. Bigelow, H.-J. Freund, Chem. Phys. 107 Ž1986. 159. w11x E. Vauthey, J. Voss, C. De Caro, A. Renn, U.P. Wild, J. Luminesc. 56 Ž1993. 61. w12x G.W. Scott, K.J. Tran, J. Phys. Chem. 98 Ž1994. 11563. w13x H. Langhals, Angew. Chem. Int. Ed. Engl. 21 Ž1982. 724. w14x C.J. Drummond, F. Grieser, T.W. Healy, Faraday Discuss. Chem. Soc. 81 Ž1986. 95. w15x P. Suppan, J. Chem. Soc., Faraday Trans. I 83 Ž1987. 495. w16x M. Belletete, ˆ G. Lessard, G. Durocher, J. Luminesc. 42 Ž1989. 337. w17x M. Belletete, ˆ M. Lachapelle, G. Durocher, J. Phys. Chem. 94 Ž1990. 5337. w18x T. Sa´ e Melo, M. Bazin, J.-C. Ronfard-Haret, R. Santus, Photochem. Photobiol. 58 Ž1993. 19. w19x L.P. Novaki, O.A. El Seoud, Ber. Bunsenges. Phys. Chem. 101 Ž1997. 105. w20x U. Kringel, Ph.D. Thesis, Siegen Ž1988.. w21x F.A. Salazar, A. Fedorov, M.N. Berberan-Santos, Chem. Phys. Lett. 271 Ž1997. 361. w22x L.F.V. Ferreira, S.M.B. Costa, J. Luminesc. 48r49 Ž1991. 395.
w23x F.E. Critchfield, J.A. Gibson, Jr., J.L. Hall, J. Am. Chem. Soc., 75 Ž1953. 1991. w24x E.Z. Lippert, Naturforsch. A 10 Ž1955. 541. w25x N. Mataga, Y. Kaifu, M. Koizumi, Bull. Chem. Soc. Jpn 29 Ž1956. 465. w26x C.A.T. Laia, S.M.B. Costa, unpublished results. w27x S. Lin, W.S. Struve, Photochem. Photobiol. 54 Ž1991. 361. w28x P. Avouris, W.M. Gelbart, M.A. El-Sayed, Chem. Rev. 77 Ž1977. 793. w29x D.V. Matyushov, R. Schmid, B.M. Ladanyi, J. Phys. Chem. B. 101 Ž1997. 1035. w30x A. Wakisaka, Y. Shimizu, N. Nishi, K. Tokumaru, H. Sakuragi, J. Chem. Soc., Faraday Trans. 88 Ž1992. 1129. w31x E. Zoidis, J. Yarwood, T. Tassaing, Y. Danten, M. Besnard, J. Mol. Liq. 64 Ž1995. 197. w32x M.W. Feyereisen, D. Feller, D.A. Dixon, J. Phys. Chem. 100 Ž1996. 2993. w33x Z. Slanina, J.-F. Crifo, Int. J. Thermophys. 13 Ž1992. 465. w34x H. Kleeberg, W.A.P. Luck, Z. Phys. Chem. Leipzig 270 Ž1989. 613.
Chemical Physics Letters 285 Ž1998. 385–390
Ground- and excited-state solvation of a squaraine dye by water in dioxane Cesar ´ A.T. Laia, Sılvia ´ M.B. Costa
)
Centro de Quımica Estrutural, Complexo 1, Instituto Superior Tecnico, 1096 Lisboa Codex, Portugal ´ ´ Received 9 December 1997
Abstract Dioxanerwater mixtures were used in order to assess the water effect on the photophysics of two squaraine dyes. It was found that a long-range hydrogen-bonding interaction exists in the ground state, without preferential solvation by water. However, in the excited state a more specific interaction is responsible for a pronounced quenching effect. Indeed, in the unsubstituted squaraine bisw4-Ždimethylamine.phenylx squaraine an intermolecular hydrogen bonding leads to an increase of internal conversion due to accepting vibrational modes created by this interaction, while in the symmetrically dihydroxy-substituted squaraine, an intramolecular hydrogen bond prevents the occurrence of the intermolecular interaction with water. q 1998 Elsevier Science B.V.
1. Introduction Hydrogen bonding between a given solute and the solvent constitutes one of the most important interactions which influence the photochemistry of organic dyes w1x. These interactions may lead to a proton transfer reaction or solute solvation, changing decisively the spectroscopic characteristics w2–5x. Among the molecules affected by hydrogen interactions are the squaraine derivatives w6–8x. These are important dyes used in several applications such as xerographic processes and sensitization of semiconductor materials w8,9x. The main spectroscopic feature of these dyes is the intense charge transfer band observed both in the ground and excited state transitions, associated with a planar D–A–D structure Ždonor–
)
Corresponding author
acceptor–donor., where two aniline moieties act like electron donors to the central squaric moiety, C 4 O 2 , the electron acceptor group w10x. The first attempt to understand the photophysics of these dyes was made by Law in 1987 w6x. It was found that the photophysical behaviour of these molecules is strongly solvent dependent. A dependence on both the temperature and solvent was observed on the absorption and emission properties of several substituted squaraines, leading to the proposal of a solute–solvent complex in the ground and excited state w7x. Hole-burning and other studies in polymer films w11,12x stressed the importance of the protic character of the medium as well as the presence of hydroxyl groups in the squaraine, pointing to specific interactions between the squaric group and the protic solvent w7,11,12x. The squaraines used in this study were bisw4-Ždimethylamine.phenylx squaraine ŽHSq. and bisw4-Ždimethylamine.-2-hydroxyphenylx squaraine ŽHOSq. Žsee Fig. 1.. The aim of this work was directed to the
0009-2614r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 0 0 9 2 - X
386
C.A.T. Laia, S.M.B. Costar Chemical Physics Letters 285 (1998) 385–390
water used was bidistilled. Dye concentrations were always below 1 = 10y6 M due to the low solubility and in order to keep the optical density below 0.1. Samples were made from stock solutions in chloroform by careful solvent evaporation. All measurements were made one day after the sample preparation in order to ensure a complete dye solubilization. 2.2. Apparatus
Fig. 1. Chemical structure of bisw4-Ždimethylamine.phenylx squaraine ŽHSq. and bisw4-Ždimethylamine.-2-hydroxyphenylx squaraine ŽHOSq..
understanding of the interaction of these dyes with water. A considerable increase of the fluorescence quantum yield in the hydroxyl derivative was found previously w6–8x. That was caused by the increase of the dye rigidity created by a double intramolecular hydrogen bond, preventing deactivation of the excited state. That does not happen in the case of HSq; thus the presence of water is expected to change greatly the fluorescence efficiency. Since squaraines are insoluble in water, dioxanerwater mixtures were used in order to study the interaction of HSq and HOSq with water w5,13– 19x. We wish to report that there is neither preferential solvation due to the complex formation with water nor dielectric enrichment w15,16x. However, there is a strong effect of water in the non-radiative processes of these dyes. The possible consequence of the non-ideality of these mixtures w13,15,16,19x in the photophysics of both HSq and HOSq is also discussed. The final goal is to assess the feasibility of these dyes as water probes, using the standard but highly sensitive fluorescence spectroscopy.
2. Experimental section 2.1. Materials The squaraines were obtained from Xerox and used as supplied. Dioxane was purchased from Merck with spectroscopic grade and used as received. The
Absorption spectra were recorded with a JASCO V-560 UVrVIS Spectrophotometer. Steady-state emission measurements were recorded with a Perkin–Elmer LS 50B Spectrofluorimeter with the sample holder thermostatized at 228C. All data were stored in a computer. The instrumental response at each wavelength was corrected by means of a curve provided with the instrument. For each sample, emission spectra were recorded immediately after the measurement of the absorption spectra. Oxazin 1 was used as fluorescent standard Ž f f s 0.11. w20x. Transient measurements were carried out using the time-resolved single photon counting technique with a mode-locked Coherent Innova 400-10 argonion laser, synchronously pumped with a cavity dumped Coherent 701-2 dye Žrhodamine 6G. laser, giving 3–4 ps pulses Žwith f 40 nJrpulse, 3.4 MHz. with lex s 592 nm. Emission polarizer was set at the magic angle. Further description may be found in Ref. w21x. The decays were obtained at lem s 650 nm. Decay data analysis was performed using the software provided by Photon Technology International ŽCanada..
3. Results and discussion The emission spectra of HSq and HOSq were taken in pure dioxane and for different water concentrations in dioxanerwater mixtures ŽFig. 2.. The absorption and emission spectra are structureless in agreement with previous results published in the literature w8,12,22x. The maximum absorption and emission wavelengths have a red shift with increasing water concentration reflecting the increase of polarity. Using the dielectric constants and refractive indexes found in the literature w5,23x, both absorption
C.A.T. Laia, S.M.B. Costar Chemical Physics Letters 285 (1998) 385–390
Fig. 2. Representative fluorescence decay of HSq in dioxanerwater mixture ŽwH 2 Ox s 2 M. with monoexponential fit. Insert: emission spectra of HSq in dioxanerwater ŽwH 2 Ox s 0, 0.56, 1.1, 2.8, 5.6 and 11 M from top to bottom..
387
solvation is expected on account of the high insolubility of these dyes in water. Therefore, there is neither dielectric enrichment in the surroundings of both squaraine dyes nor squarainerwater complex formation in the ground state. The Stokes shift correlation with the Lippert– Mataga function also gives a good linear relationship for HSq, with approximately the same slope obtained in pure solvents w26x ŽFig. 4.. A complex relation with f Ž ´ ,n. is found for HOSq: at small water concentrations the slope is similar to the one found in pure solvents w26x, whereas a plateau is reached at high concentrations. The different behaviour observed for the two dyes is an indication of the importance of the double intramolecular hydrogen bond formed in HOSq. In Table 1 the fluorescence quantum yields of HSq and HOSq are listed. Large differences are observed for each solute: in the HSq case, the fluorescence quantum yield decreases with the water concentration while in HOSq it remains almost constant. The fluorescence steady-state data indicate that
and emission were correlated with the Lippert– Mataga dielectric function w24,25x ŽFig. 3.: Õa y Õ f s
2 Ž me y mg .
f Ž ´ ,n . s
hca3
f Ž ´ ,n . , n2 y 1
´y1 y 2´q1
2
2 n2 q 1
Ž 1. Ž 2.
A good linear relationship for the absorption maximum was found for both dyes, while the HOSq emission maximum deviates from linearity ŽFig. 3b.. When the correlation was made with the water mole fraction, the linearity was lost ŽFig. 3a.. This is due to the fact that dioxanerwater mixtures are not ideal and the following equation: f Ž ´ ,n . s x Dx f Dx Ž ´ ,n . q x W f W Ž ´ ,n .
Ž 3.
where x is the mole fraction ŽDx stands for dioxane and W for water. does not hold w13,15,16,19x. The linear relation of the absorption with f Ž ´ ,n. for both dyes shows that in the ground state there is no solute–solvent interaction. The lack of preferential
Fig. 3. Maximum absorption Ž^,I. and emission Ž',B. wavenumbers for HSq Ž^,'. and HOSq ŽI,B. against water mole fraction Ža. and f Ž ´ ,n. Žb..
C.A.T. Laia, S.M.B. Costar Chemical Physics Letters 285 (1998) 385–390
388
accordance with fluorescence quantum yield measurements, the HSq fluorescence lifetime also decreases with the increase of water concentration. A Stern–Volmer dependence of the HSq fluorescence quantum yields and lifetimes should be expected w5x. However, should there be formation of specific complexes between HSq and water in the ground state, the static quenching fraction contribution should appear in the fluorescence quantum yields:
Ž forf . s 1 q K b wH 2 Ox , Ž tort . Fig. 4. Stokes shift of HSq Ž D . and HOSq ŽI. in dioxanerwater mixtures.
the intramolecular hydrogen bond in HOSq increases the rigidity of the molecule as seen earlier w6–8x and also prevents the squaric moiety from having interactions with the protic molecules in the excited state. HSq feels a strong fluorescence quenching in the presence of water molecules similar to the one found for other organic molecules like, for example substituted anthraquinones and anthroates w2,5,27x. The formation of the hydrogen bond creates an accepting vibrational mode w2,28x and therefore an increase of the internal conversion is observed. The fluorescent decays of HSq and HOSq were always mono-exponential Žsee Table 1 and Fig. 2. in pure dioxane and for all the water concentrations studied, indicating one spectroscopic species only. In
Ž 4.
where K b is the equilibrium constant for the association of the solute with water in the ground state. In Fig. 5 it is shown that this fraction is negligible within experimental error, indicating no association between HSq and water in the ground state, as found in the solvatochromism study. At this point, it must be said that hydrogen interactions between ground state HSq and water do exist, otherwise HSq absorption would not be sensitive to the protic character of the solvent as observed
Table 1 Experimental data of HSq and HOSq dyes in dioxanerwater mixtures: fluorescence quantum yield and fluorescence lifetimes wH 2 OxrM
f HSq
t HSq rns
f HOSq
t HOSq rns
0 0.28 0.56 0.83 1.11 2.00 2.78 4.00 5.56 8.00 11.11 15.00
0.57 0.48 0.42 0.37 0.32 0.22 0.21 0.16 0.13 0.11 0.09 0.08
2.101 1.829 1.588 1.373 1.167 0.935 0.779 0.643 0.536 0.435 0.360 0.318
0.84 0.82 0.87 0.86 0.83 0.79 y 0.79 0.83 0.78 0.77 0.78
2.755 2.796 2.646 2.686 2.743 2.487 y 2.434 2.377 2.410 2.489 2.309
Fig. 5. Stern–Volmer plots for HSq Ž', fo rf ; D, to rt . and HOSq ŽB, fo rf ; I, to rt . in dioxanerwater mixtures Ža. and static quenching fraction for HSq Ž D . and HOSq ŽI. Žb..
C.A.T. Laia, S.M.B. Costar Chemical Physics Letters 285 (1998) 385–390
with pure solvents w7x but they are long range in nature w29x. Should short range interactions be already present in the ground state, non-linearity would appear in Fig. 3 w15x and static effects would contribute in the fluorescence quenching as happened in the 9-anthroate fluorescence quenching study case w5x. The HSq ratio tort has a negative deviation from the Stern–Volmer equation. The origin of this deviation may be due to the existence of ineffective water molecules, which cannot quench the dye fluorescence. This result must be interrelated with the fact that these mixtures are not ideal w13,15,16x. An attempt to account for this phenomenon was made by assuming water dimer formation: 2H 2 O ° Ž H 2 O . 2
K dim s
Ž H 2 O. 2 . wH 2 Ox 2
Ž 5.
The possibility of water aggregation in liquid mixtures was speculated w5,13,30x and shown earlier w31x. It was found that in a hydrophobic solvent the water molecules self aggregate easily, leading to a great variety of dioxanerwater complexes in the present mixture w31x. Thus, this equilibrium is only a rough approximation of what is happening at the molecular level. The water dimer has a binding energy around 5 kcalrmol w32x. In the gas phase an estimate of K dim s 1 My1 was obtained for water dimerization w33x. When this extra process is considered in the Stern–Volmer kinetic scheme and assuming that the dimer does not quench the squaraine, one obtains the following equation:
to t
s 1 q k qto
(1 q 8 K
dim
wH 2 Ox o y 1
4 K dim
,
Ž 6.
where wH 2 Oxo is the analytical water concentration. Fitting the experimental results with Eq. 6 Žsee Fig. 5. it is possible to extract a dimerization constant Ž K dim . equal to 0.0794 My1 and a quenching rate constant Ž k q . equal to 3.87 = 10 8 My1 sy1. The dimerization constant obtained is reasonable when compared with the one predicted for the gas phase w33x, since competition from dioxane may occur w31,34x. The quenching rate constant is slower than diffusion, meaning that the process is reaction con-
389
trolled Žas found in another case w5x.. The quenching inefficiency of water dimers may be due to the high reversibility in the formation of the excited state HSqrwater dimer complex, lack of accepting vibrational modes or to the complexity of these mixtures at the molecular level w31x. With the present results it is impossible to establish an exact picture of the HSq interaction with aggregated water, so we can only speculate that these aspects may contribute to the quenching inefficiency of aggregated water. The use of HSq as a fluorescent water probe looks promising, due to the fact that its sensitivity to media dielectric changes is linear in the ground state Žthere is no preferential solvation. and that the effect of water concentration in the emission is selective Ždue to quenching by hydrogen bonding.. However, a wide use of this dye in solution is not possible due to its low solubility in most liquids. The use of other functionalized squaraines w6,7x which may be better dissolved in organic media would make these dyes an important class of fluorescent water probes.
Acknowledgements This w ork w as supported by Project PRAXISr2r2.1rQUIr22r94. The authors thank Dr. R.O. Loutfy ŽXerox, Canada. for the generous gift of squaraine samples. Professor J.M.G. Martinho is acknowledged for the use of the SPC equipment and Dr. A. Fedorov for the valuable assistance in the time-resolved measurements. Cesar A.T. Laia ac´ knowledges a PhD Grant BD No. 961 from PRAXIS XXI.
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