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Preferential solvation of fluorenone and 4-hydroxyfluorenone in binary solvent mixtures
Chemical Physics 294 (2003) 105–116 www.elsevier.com/locate/chemphys
Preferential solvation of fluorenone and 4-hydroxyfluorenone in binary solvent mixtures Marek J ozefowicz, Janina R. Heldt
*
Institute of Experimental Physics, University of Gda~ nsk, ul. Wita Stwosza 57, PL 80-952 Gda~ nsk, Poland Received 12 May 2003; accepted 16 July 2003
Abstract Preferential solvation of fluorenone and 4-hydroxyfluorenone in binary solvent mixtures has been studied using steady-state spectroscopic measurements. This study concerns the solvent-induced shift of the absorption and fluorescence spectra of both molecules in two solvent mixtures, i.e., cyclohexane–tetrahydrofuran and cyclohexane–ethanol. The first system contains polar solute molecules, fluorenone and 4-hydroxyfluorenone, in a mixture of polar aprotic (tetrahydrofuran) and non-polar (cyclohexane) solvents. In the second solvents mixture, hydrogen bonding with solute molecules (ethanol) may occur. The results of spectroscopic measurements are analysed using theoretical models of Bakshiev, Mazurenko and Suppan which describe preferential solvation phenomena. In the case of cyclohexane–tetrahydrofuran mixtures, the deviation from linearity in the absorption and fluorescence solvatochromic shifts vs. the solution polarity is due to non-specific dipolar solvent–solute interactions. For cyclohexane–ethanol binary mixtures, both non-specific and specific (hydrogen bond and proton-relay tautomerization) interactions contribute to the observed solvatochromism. 2003 Published by Elsevier B.V. Keywords: Fluorenone; 4-Hydroxyfluorenone; Binary solvent; Preferential solvation
1. Introduction For nearly half of the past century, the importance of chemical reactions in solution has motivated research aimed at understanding solvation of polar solute molecules in solvents of varying permittivity. Understanding of solvation of electronically excited molecules was greatly advanced
*
Corresponding author. Fax: +48-583-413-175. E-mail address: fi[email protected] (J.R. Heldt).
0301-0104/$ - see front matter 2003 Published by Elsevier B.V. doi:10.1016/S0301-0104(03)00379-3
over 40 years ago by the fundamental works of Lippert [1] and Mataga et al. [2]. In the 1960s and 1970s, the study of transient solvatochromism was pioneered by Bakshiev and Mazurenko [3,4]. Several experimental studies have shown that, for solutions where specific solute–solvent interactions are absent, these theories provide an excellent starting point for understanding the energetics of solute–solvent system [5,6]. It has also been shown that polar solute molecules dissolved in a binary solvent mixture interact differently with each of the solvent components. In mixtures of solvents of
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different polarity, a process of preferential solvation (PS) occurs. This process involves dielectric enrichment in the solvation shell of the dipolar solute molecule [7–11], resulting from a change in the relative concentrations of more and less polar solvent molecules in the immediate neighbourhood (solvation shell) of the polar solute molecule, in comparison with the bulk solvent concentrations. This is a consequence of electrostatic interactions between the solute and solvent molecules in the binary solvent mixture. Several spectral parameters are used for monitoring preferential solvation, for example, shifts in the absorption and emission wavelength maxima, non-linear changes of the quantum yield, and decay time of the fluorescence as functions of the mole fraction/concentration of polar component in the binary mixture [7–11]. It has been shown that the photophysical properties, i.e., radiative (kR ) and non-radiative (kNR ) rate constants, quantum yield, and decay time, of fluorenone and its derivatives are very sensitive to temperature [12,13] and solution microenvironment [14,15]. These properties attracted considerable attention to this group of molecules as sensitive luminescence probes for solute–solvent molecular interaction and relaxation processes [16–18]. In previous reports [19,20], we presented the results of studies concerning intramolecular proton-transfer reactions, solvent-cage relaxation processes (red-edge effect) and fluorescence quenching phenomena for fluorenone (9Fl) and 4hydroxyfluorenone (4HOFl) in binary solvents of cyclohexane–ethanol and acetonitrile–ethanol. This paper presents the results of studies on solvent-induced shifts of the absorption and emission spectra of 9Fl and 4HOFl in binary mixed solvents, e.g., cyclohexane–tetrahydrofuran (CH–THF) and cyclohexane–ethanol (CH– EtOH). The first system contains a polar solute molecule (9Fl or 4HOFl) in a mixture of polar aprotic and non-polar solvents. The second solvent mixture contains EtOH which is capable of forming hydrogen bonds with the luminescent molecules. This allows examination of the preferential solvation phenomenon in a system where, in addition to general solvent relaxation, hydrogen bonding between the solute and solvent molecules also occurs, allowing comparison with the general
solvation process in which only dielectric enrichment occurs. The solvation parameters of the selected molecular systems are determined from the experimental data obtained.
2. Experimental details Fluorenone and 4-hydroxyfluorenone were purchased from Aldrich Chemical Co. and purified by recrystallization from toluene. Their purities were checked chromatographically. All of the solvents used were of the highest grade commercially available: ethanol (99.9%) was reagent grade; all others solvents were Aldrich spectral or HPLC grade. CH was distilled before use from a sodium–potassium amalgam, to ensure that was anhydrous. Additionally all solvents were checked in both steady-state and time-resolved fluorescence apparatuses for lack of fluorescent impurities in the wavelength ranges of interest. Absorption, excitation, and fluorescence spectra were recorded using, respectively, a Shimadzu UV-2401 PC spectrophotometer and a Shimadzu RF-5301 spectrofluorometer with 5.0 nm bandwidths in both excitation and emission. The fluorescence emitted was observed perpendicularly to the direction of the exciting beam. Samples for fluorescence measurements were prepared using a 0.2 cm high sensitivity micro quartz cell. The concentration of the solutions studied was ca. 5 104 M. The luminescence spectra reported have been corrected for the spectral response of the photomultiplier (Hamamatsu R-928) and monochromator pass, but not for reabsorption which was negligible in these samples.
3. Results and discussion 3.1. Experimental results The electronic absorption and emission spectra of molecules in solution, respectively, give reliable information about solvation effects in the ground and excited states [21,22]. Solvatochromic shifts provide an experimental measure of the energy changes between the S0 and S1 state caused by
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interaction solvent molecules with the solute. When a solute molecule is dissolved in a binary solvent mixture, the local composition of the mixture is, in general, different from that in the bulk solution. This is confirmed by the observed non-linearities of solvatochromic shifts of the absorption and fluorescence spectra as a function of the mole fraction of the polar solvent. This nonlinearity has been explained as due to three main causes: non-ideal behaviour of the solvent mixture, specific solute–solvent association, and dielectric enrichment of the solvent around the polar solutes. The absorption and steady-state fluorescence spectra of 9Fl and 4HOFl in mixtures of CH–THF at different THF concentration are shown in Fig. 1. As can be seen, the absorption and emission of 9Fl and 4HOFl spectra show an approximate mirror symmetry, providing convincing evidence that
there is only a single excited electronic state contributing to the absorption and fluorescence spectra. In both of these fluorescent molecules, the absorption and emission bands become more and more red-shifted on increasing the concentration of the polar component in the solvent mixture. Figs. 2A and B show the normalized solvatochromic shifts of 9FL and 4HOFl in this binary mixture as a function of the mole fraction of THF. The shifts are independent of the concentration of the solute in the concentration range studied, indicating the absence of solute–solute interactions. Non-linear solvatochromic shifts of the absorption and fluorescence bands are observed for both fluorophores. Here we would like to note that, in plots of the same normalized solvatochromic shift data vs. the microenvironmental dielectric constant of the solvent mixtures, f ðenp Þ ¼ xn f ðen Þ þ xp f ðep Þ (see Section
Fig. 1. Long-wavelength absorption band and fluorescence spectra of 9Fl (A) and 4HOFl (B) in neat solvents of CH and THF and their mixtures at different mole fraction, xp , of THF.
Fig. 2. Normalized shift of the absorption and fluorescence bands maxima of 9Fl (A) and 4HOFl (B) determined for different mole fraction, xp , of THF.
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3.2), a departure from linearity is also evident. Such behaviour is often observed for polar molecules dissolved in binary mixtures of polar and non-polar solvents, and is taken as evidence for dielectric enrichment [7,9–11,23–26]. As can be seen in Fig. 1, the absorption spectrum of 4HOFl in pure cyclohexane, with a maximum at 24,650 cm1 , shows pronounced vibrational structure. At low THF concentrations (xp < 0:02) this vibronic structure is still visible, but it vanishes at higher concentrations (xp > 0:02), where the whole spectrum additionally undergoes a pronounced red-shift. The fluorescence spectra of 9Fl and 4HOFl do not show any vibrational structure (see Fig. 1). The observed red-shifts of these fluorescence spectra caused by increasing solvent polarity are larger than those of the absorption spectra. The fluorescence spectral maxima of 9Fl and 4HOFl are centred at 21,770, 20,840 cm1 and 21,040, 19,760 cm1 in CH and THF solutions, respectively. Fig. 3 shows the normalized absorption and fluorescence spectra of 9Fl and 4HOFl in CH– EtOH at different mole fractions of ethanol. The absorption and steady-state fluorescence spectra show the same general dependence on increasing the alcohol concentration as observed for both molecules in CH–THF mixture on increasing the tetrahydrofuran concentration (see Fig. 1). Fig. 4 shows their normalized shifts plotted against the EtOH concentration. Both fluorophores clearly show that a pronounced shift of the absorption and fluorescence spectra occurs already at very low alcohol concentration. For both solute molecules, about 50% of the total fluorescence spectrum shift observed between pure CH and pure EtOH has occurred by an EtOH mole fraction of 0.04 (see Fig. 4). This unusually large shift is understandable in terms of hydrogen bonding between the solute and the alcohol. It is well known that the hydrogen bonding breaks down at higher temperatures (>350 K), therefore the normalized shifts of the absorption and fluorescence spectra vs. EtOH mole fraction for both solvent mixtures were measured at different temperatures. Fig. 5 shows the fluorescence spectra of 9Fl in binary mixtures of CH– THF and CH–EtOH (polar component mole fraction 0.04) determined over the temperature range from 293 to 373 K. It is seen that:
Fig. 3. Long-wavelength absorption band and fluorescence spectra of 9Fl (A) and 4HOFl (B) in neat solvents of CH and EtOH and their mixtures at different mole fraction, xp , of EtOH.
• Neither the shape of the fluorescence spectrum of 9Fl in CH–THF nor its wavelength of maximum intensity is shifted on changing the temperature from 293 to 373 K (Fig. 5A). In addition, the intensity of the fluorescence decreases linearly with increasing temperature as a result of temperature quenching [12,13]. • The wavelength of maximum intensity of fluorescence of 9Fl in CH–EtOH is blue-shifted from 494 nm at 293 K to 476 nm at 373 K (Fig. 5B). The kmax (373 K) ¼ 476 nm agrees well with that determined in neat CH. The fluorescence spectral behaviours of 4HOFl in CH– THF and CH–EtOH (mole fractions of polar component 0.04 in both cases) are very similar, and not shown here.
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Fig. 4. Normalized shift of the absorption and fluorescence bands maxima of 9Fl (A) and 4HOFl (B) determined for different mole fraction, xp , of EtOH.
3.2. Review of the theories of preferential solvation Theoretical descriptions of preferential solvation have been given by Bakshiev et al. [5], Mazurenko et al. [6] and Suppan et al. [7]. All of the theories of solvatochromic shift in absorption and fluorescence spectra are based on the Onsager [27] description of non-specific electrostatic solute–solvent interactions. According to Bakhshiev and co-workers [5], the average degree of fill of the solute shell with polar solvent molecules (p), i.e., its mole fraction in the solvent shell, for a binary system in which nonpolar (n) and polar (p) solvent molecules possess similar radii (rn ffi rp ¼ r) and the solvents have similar refractive indices (nn ffi np ¼ n) is given by D E he i e eff n xBp ¼ ð1Þ ep en and the effective dielectric constant heeff i by
Fig. 5. Fluorescence spectra of 9Fl in THF (A) and EtOH (B) determined at different temperatures: 1, 293 K; 2, 303 K; 3, 313 K; 4, 323 K; 5, 333 K; 6, 350 K; 7, 363 K; and 8, 373 K.
D E heeff i ¼ en xBn þ ep xBp ;
ð1aÞ
where en and ep are the static dielectric constants of the non-polar and polar solvents, respectively. The microenvironmental dielectric constant heeff i appearing in Eq. (1), can be determined from the absorption and fluorescence spectral shifts, d~ mA and d~ mF , respectively, of fluorophores in mixed and neat solvents. According to Liptay [28], and as has been shown by Kawski and Czajko [8], for spherical solute molecules centred in a closed spherical microenvironment (first shell), the solvent spectral shifts are 1
1
The subscripts A and F in the spectral shift d~ mA;F in Eq. (2), and in other constants refer to the absorption and fluorescence, respectively.
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d~ mA;F ¼ ðD~ mnþp mnA;F Þ A;F D~ 2n2 þ 1 heeff i 1 n2 1 ¼ m1;2 2 ; n þ 2 heeff i þ 2 n2 þ 2
ð2Þ
determined from spectroscopic measurements of the maxima of the absorption (A) and fluorescence mpA;F ), (F) spectra in neat non-polar (~ mnA;F ), polar (~ nþp and binary mixtures (~ mA;F ), respectively, of the solvents, using a simplified formula [4]
ð3Þ
hA;F ¼
ð4Þ
(6)
where ~ le ~ lg Þ lg ð~ ; m1 ¼ 2pe0 hca3
where e0 ¼ 8:8542 1012 F m1 , h is the Planck constant, c the velocity of light in vacuo, lg and le the dipole moments in the ground and excited states, respectively, a the Onsager cavity radius of the solute molecule. The solvent spectral shifts d~ mA;F are determined as the differences between the maxima in the mixed solution and those in the neat solvents. Knowing the experimental values of d~ mA , m1 , and refractive index n we are able to calculate heeff i for the solvent shell with the solute molecule in its ground state (S0 ). A similar procedure is applied in order to determine the corresponding value for the solute molecule in its first excited state (S1 ). Mazurenko and co-workers [6], on the basis of statistical considerations for the cage model of a binary solvent, have shown that the probability with which polar and non-polar molecules fill the Nm solvent cells in the first solvation shell (cage) around a single solute with Np polar and Nn nonpolar molecules (Nm ¼ Np þ Nn ) at thermal equilibrium is given by the binomial distribution Nm ! Np hA;F ð1 hA;F ÞNm Np ; Np !ðNm Np Þ!
ð5Þ
ln
hA;F xp ¼ ln þ cA;F ; 1 hA;F 1 xp
Wmðg;eÞ 1 Nm kT
:
ð8Þ
where cA;F ¼
Wmðg;eÞ 1 DWmðg;eÞ ¼ Nm kT kT
ð9Þ
and the subscripts g and e indicate ground and excited states, respectively. Nm , the solvation number, can be calculated [4] as Nm ¼
hcð~ mnF m~pF þ m~nA m~pA Þ : 2kT ðcF cA Þ
ð10Þ
The average free orientational interaction energy for a single molecule in the solvation shell is obtained from Eqs. (8) and (10) as DWmðg;eÞ ¼ Wmðg;eÞ =Nm :
ð11Þ
According to Suppan and co-workers [7], the deviation of the shifts in absorption and emission maxima in binary mixtures from linearity with the mole fraction of polar solvent xp results from dielectric enrichment. In the single-shell approximation, the mole fraction Y ¼ yp =yn in the solvent shell and that in the bulk solution X ¼ xp =xn are related by a preferential solvation index, Z Y ¼ X eZ ;
where hA;F xp ¼ exp 1 hA;F 1 xp
ð7Þ
On taking the natural logarithmic form of Eq.
~ le ~ lg Þ le ð~ m2 ¼ ; 2pe0 hca3
/eq ðNp Þ ¼
m~nA;F m~nþp A;F : m~nA;F m~pA;F
ð12Þ
where ð6Þ
Assuming that replacement of any non-polar molecule by a polar one in any of the cells leads to the same gain in energy. Wm is the minimum solvent reorientation energy for filling Nm cells with polar solvent molecules, xp the mole fraction of the polar component. The ratio hNp i=Nm ¼ hA;F can be
Z¼
1 Cl2 MDf ðenp Þ 6 4pe0 RT drs–s
ð13Þ
in which ~ l is the solute dipole moment, M the mean molecular weight of polar and non-polar solvent, Df ðenp Þ the difference between the Onsager polarity functions of the neat polar and nonpolar solvents, Df ðenp Þ ¼ f ðep Þ f ðen Þ, where
M. Jozefowicz, J.R. Heldt / Chemical Physics 294 (2003) 105–116
f ðep Þ ¼ 2ðep 1Þ=ð2ep þ 1Þ and f ðen Þ ¼ 2ðen 1Þ= ð2en þ 1Þ; d the mean density of the two components, and rs–s the mean separation between the solute and solvent molecules, i.e., rs–s ¼ a þ r (a and r are the Onsager cavity and solvent molecule radii, respectively), C is a numerical constant formally equal to (3=8p) when both the solvent and solute molecules are considered to be spherical [27], which value will be used here. The other symbols possess the standard meaning. It should be noted that, if preferential solvation effects are absent in the mixed solvent, then Z ¼ 0. For many solvent mixtures, the bulk dielectric polarity function f ðenp Þ defined as f ðenp Þ ¼ xn f ðen Þ þ xp f ðep Þ using the static dielectric constants en and ep , does not follow the f ðeeff Þ curve as a function of polar solvent molar fraction. In such cases, the microenvironmental dielectric constant heeff i determined using the spectral shift method (see Eq. (2)) is appropriate to be used in the calculation of Z (Eq. (13)). 3.3. Determination of the solvation parameters The theories reviewed above allow evaluation of the main parameters describing the solvation phenomenon. As can be seen in Figs. 1–4, the rapid change in absorption and fluorescence maxima of 9Fl and 4HOFl in the binary CH–THF and CH– EtOH mixtures clearly show the effect of preferential solvation. In all cases, the experimental points deviate from linearity with mole fraction of the polar solvent, and their departure indicates that the solute molecules are preferentially solvated by the more polar component. This effect is even more pronounced when the more polar component is of the protic type (EtOH) (see Figs. 2 and 4). As can be seen, the curve of normalized spectral shifts of the long-wavelength absorption band does not always follow that of the fluorescence. The effective dielectric constants heeff i of the solvent shell of 9Fl, in agreement with [5,6], can be calculated using Eq. (2) only for CH–THF system, in which specific interactions between solute and solvent molecules do not occur, and the additional conditions, e.g., the refractive index and molecular and radii of CH and THF, nCH ¼ nTHF ffi 1:43 A rCH ffi rTHF ¼ 2:1 A, respectively, fulfil the as-
111
sumptions made by Bakshiev et al. [5] and Mazurenko et al. [6]. Using the spectral shift data experimentally determined from Figs. 1 and 3, the effective dielectric constant heeff i of the solvent shell around the 9Fl molecule in the ground and excited states as a function of the THF molar fraction has been calculated using Eq. (2) (see Table 1). Using the heeff i and enp data assembled in Table 1, the Onsager polarity functions have been calculated for different xp values. Fig. 6 presents graphically obtained data for 9Fl and 4HOFl in CH–THF mixtures. As can be seen in Figs. 6A and B, neither of the polarity functions f ðeeff Þ and f ðenp Þ follows a linear dependence on xp . This finding indicates preferential solvation for 9Fl and 4HOFl dissolved in CH–THF mixtures. Further, using the experimental spectral shift data, hA;F , values have been determined and used to plot lnðhA;F =1 hA;F Þ vs. lnðxp =1 xp Þ for both fluorophores in both binary solvent mixtures (see Fig. 7). As seen in Figs. 7A and B, a linear dependence is obtained for both 9Fl and 4HOFl in CH–THF over the whole range of xp , but not in CH–EtOH. cA and cF values for the fluorophores in CH–THF were therefore obtainable and used to calculate DWmðg;eÞ , and Nm using Eqs. (9) and (10). The results are assembled in Table 2. From the DWmðgÞ and DWmðeÞ values obtained for both of the fluorophores in CH–THF, it is evident that the packing of solvent molecules around the polar solutes in their ground and excited states is very different. The ratio of the corresponding 9Fl 9Fl free reorientational energies, DWmðeÞ =DWmðgÞ ffi 4HOFl 4HOFl DWmðeÞ =DWmðgÞ , is about 2. On the other hand, the free energy of the reorientational interaction for each of 9Fl and 4HOFl in their ground and ex4HOFl 9Fl cited states are different: DWmðgÞ =DWmðgÞ ffi 2:18 4HOFl 9Fl and DWmðeÞ =DWmðeÞ ffi 1:75. The Nm values, calculated using Eq. (9), are nearly the same for 9Fl and 4HOFl. It is worth noting that, for CH–EtOH, lnðhA;F =1 hA;F Þ, vs. lnðxp =1 xp Þ is linear only up to xEtOH ¼ 0:08, this simple dependence breaking down at higher EtOH concentrations, indicating that both fluorophores are undergoing hydrogenbonding interactions with EtOH at these higher concentrations [19,20]. According to the MazurenkoÕs model, the reorientational energy of a single molecule in the
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Table 1 Determined data of spectral shifts d~ mA and d~ mF , microenvironmental dielectric constants heeff i, and filling up degree, hxBp i, of the solvent shell as a function of the THF and EtOH mole fraction xp
9Fl
4HOFl
Ground state dmA (cm1 )
heeff i
Excited state hxBp i
dmF (cm1 )
heeff i
Ground state hxBp i
dmA (cm1 )
heeff i
Excited state hxBp i
dmF (cm1 )
heeff i
hxBp i
CH–THF 0 0 0.015 30 0.030 40 0.073 70 0.135 140 0.238 250 0.612 410 0.882 500 0.938 510 1 530
2.03 2.18 2.33 2.63 3.54 3.93 5.74 6.62 6.74 7.00
0 3.10 6.21 12.45 31.38 39.88 77.80 94.41 96.64 100
0 30 60 120 300 380 730 880 900 930
2.02 2.30 2.39 2.67 3.32 4.35 5.85 6.71 6.81 7.00
0 5.56 7.44 13.05 26.11 46.80 77.07 94.23 96.16 100
0 240 290 530 650 830 840 850 860 890
2.01 2.21 2.90 3.36 3.90 4.32 5.76 6.69 7.00 7.20
0 4.09 17.32 26.20 36.57 44.73 72.54 90.28 96.14 100
0 470 570 640 810 890 1020 1170 1240 1280
2.01 2.41 3.21 3.50 4.20 4.54 5.95 6.89 7.06 7.20
0 7.58 23.13 28.55 41.98 48.41 75.16 93.08 96.57 100
CH–EtOH 0 0 0.021 30 0.042 100 0.098 170 0.179 240 0.303 330 0.685 430 0.916 500 0.956 510 1 520
2.02 5.3 7.6 11.1 14.9 16.3 21.0 22.9 23.1 24
0 14.92 25.39 41.31 63.15 74.06 86.35 95.00 95.90 100
0 1500 1910 2320 2530 2810 2840 2970 2990 3070
2.02 8.7 11.1 12.3 17.1 19.5 21.6 23.3 23.8 24
0 30.39 41.31 46.77 68.60 79.53 89.08 96.81 99.00 100
0 580 760 930 950 1000 1290 1300 1320 1340
2.02 6.3 8.8 13.1 16.4 19.1 21.0 23.0 23.5 24
0 19.47 30.85 50.41 65.42 77.71 86.35 95.45 97.72 100
0 1180 1680 2030 2230 2280 2390 2730 2800 2820
2.02 8.2 10.8 12.1 16.1 18.6 22.6 23.1 23.6 24
0 28.11 39.94 45.86 64.06 75.42 93.63 95.90 98.18 100
solvation shell is given by ERo ¼ Nm hDW i. Using the hDW i and Nm values obtained from the CH– THF data and collected in Table 2, we find ERo ¼ 1100 and 2000 cm1 for 9Fl and 4HOFl, respectively. 2 Since the normalized spectral shift curves (Figs. 2 and 4) show large departures from linearity for both systems of binary solvents, it was tempting to calculate the deviation parameter, describing the
2
This value can be compared with that calculated on the basis of the Franck–Condon transition energy partitions hc~ mA ¼ DGsolv þ ERo , and hc~ mF ¼ DGsolv ERo , where DGsolv denotes the free energy difference between the solvated ground and relaxed solvated excited states [29,30]. From these, it follows that: 2ERo ¼ chð~ mA m~F Þ. The ERo values obtained are 1300 and 1900 cm1 in CH–THF and 2250 and 3050 cm1 in CH–EtOH for 9Fl and 4HOFl, respectively. They are in good agreement with the data presented in Table 2.
departure from linearity, introduced by Suppan [26] as R ½FD ðxp Þ F ðxp Þ dx q¼ ; ð14Þ ð1=2ÞDF where FD ðxp Þ is the measured wavenumber of the spectral peak at THF mole fraction xp , F ðxp Þ the band position that is expected according to linear behaviour, and DF is the difference between these values in the neat solvents, i.e., CH (xp ¼ 0) and THF or EtOH (xp ¼ 1) (see Fig. 2A). As follows from Eq. (14) q is a dimensionless quantity between 0 and 1, corresponding to linear additivity of f ðxn Þ and f ðxp Þ at q ¼ 0, and infinite deviation from linearity at q ¼ 1. Since for both fluorophores in both solvent mixtures evident deviations from linearity occur, the respective q values were calculated, and are presented in Table 3. In these cases, the deviation from linearity (as seen in Figs.
M. Jozefowicz, J.R. Heldt / Chemical Physics 294 (2003) 105–116
Fig. 6. Values of the polarity function of the binary mixture CH–THF determined for different mole fraction of THF using the formula: f ðenp Þ ¼ xp f ðep Þ þ xn f ðen Þ, and f ðeeff Þ ¼ 2ðheeff i 1Þ=ð2heeff i þ 1Þ where heeff i data are calculated using Eq. (2) for absorption (A) and fluorescence (B) solvent spectral shifts.
113
Fig. 7. The dependence of lnðhA;F =1 hA;F Þ vs. lnðxp =1 xp Þ of both fluorophores in CH–THF (A) and CH–EtOH (B) solutions.
calculated using Eq. (12) and the corresponding experimental data assembled in Table 1. The indices of preferential solvation thus determined from the spectral shifts in absorption, ZA , and fluorescence, ZF , are shown in Table 4. Z can alternatively be calculated by means of Eq. (13), using MCH–THF ¼78:14 g mol1 , MCH–EtOH ¼65:11 g mol1 , dCH–THF ¼ 0:83 g cm3 , dCH–EtOH ¼ 0:72 g cm3 ,
2 and 4) is due to preferential solvation or to specific interaction between solute and protic solvent molecules. The normalized spectral shift curves for 9Fl and 4HOFl as a function of xp are very similar for the two solvent mixtures. In view of this, the indices of preferential solvation Z were
Table 2 Free orientational interaction energy, ERo , of the solution molecules in the solvation shell calculated according to Mazurenko et al. [6] Solvent
DWmðgÞ (cm1 )
CH–THF
9Fl 112.0 4HOFl 244.4
CH–EtOH
a
9Fl 228.1 4HOFl 325.8
hDW i ¼ ðDWmðgÞ þ DWmðeÞ Þ=2.
DWmðeÞ (cm1 )
cA
cF
Nm
ERo ¼ Nm hDW ia (cm1 )
244.4
0.55
1.2
6
1095
427.6
1.2
2.1
6
2010
712.8
1.12
3.5
4
1900
692.4
1.6
3.4
6
3050
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M. Jozefowicz, J.R. Heldt / Chemical Physics 294 (2003) 105–116
Table 3 The Suppan deviation factor (see Eq. (14)) CH–THF qA
qFCH–THF
qCH–EtOH A
qCH–EtOH F
9Fl 0.3
0.29
0.77
0.52
4HOFl 0.43
0.78
0.67
0.69
dissolved in CH–THF and CH–EtOH solvent mixtures can be rationalized in terms of nonspecific (i.e., dipolar) and specific (especially Hbonding with EtOH) reactions of the fluorophore with the polar solvent. In previous work [20], we have shown that the absorption and the fluorescence spectra of 9Fl and 4HOFl in mixed binary solvents undergo very complex changes on adding ethanol to cyclohexane, i.e., they are shifted to longer wavelengths, with broadened half-widths and changed emission spectral profiles. This behaviour indicates that, in the binary CH–EtOH solution, a specific interaction, i.e., hydrogen-bonding between 9Fl or 4HOFl and the alcohol molecules, is occurring in addition to the well-known dipole–dipole interaction. In order to confirm the existence of hydrogen bonding, the temperature dependence of the fluorescence spectra of 9Fl and 4HOFl in the binary solvent was investigated. As seen in Fig. 5B, the fluorescence maximum is blue-shifted on increasing the temperature of the CH–EtOH mixture, a result of thermally induced hydrogen-bond breaking. The fluorescence spectra of 9Fl and 4HOFl in CH–THF (see Fig. 5A) show only a
l9Fl ¼ 3:1 D, l4HOFl ¼ 2:4 D, together with and for 9Fl and r ¼ 5:4 A for 4HOFl in CH– r ¼ 4:9 A for 9Fl and r ¼ 5:8 A THF mixtures and r ¼ 5:2 A for 4HOFl in CH–EtOH mixtures. Z was found to be 0.93 and 2.1 for fluorenone and 4-hydroxyfluorenone in CH–THF mixtures, respectively, and 1.46 and 2.64 in CH–EtOH. The values thus obtained are in a reasonable agreement with the average, hZi, for ground and excited states presented in Table 4.
4. Summary and discussion Non-linear plots of the solvatochromic shifts vs. mole fraction of polar solvent for 9Fl and 4HOFl
Table 4 Preferential solvation indexes, ZA and ZF , calculated using Eqs. (12) and (13) xp
9Fl ZA
CH–THF 0.015 0.030 0.073 0.135 0.238 0.612 0.882 0.938
1.37a 0.97a 0.65a 0.83a 1.05a 0.77a 0.80a 0.52
CH–EtOH 0.021 0.042 0.098 0.179 0.303 0.685 0.916 0.956
1.77a 1.60a 1.42a 1.30a 1.33a 0.75 0.68 0.61
a b
4HOFl hZA i
ZF
0.92 0.22 0.93b
0.78a 0.80a 0.63a 1.11a 0.79a 0.84a 0.86a 0.68
1.48 0.18 1.46b
3.64a 3.47a 3.20a 2.91a 3.05a 1.56 0.79 0.34
Data used to calculated the average value. Value calculated using Eq. (13).
hZF i
ZA
0.83 0.13 0.93b
3.18a 2.74a 2.93a 2.85a 2.79a 2.36 1.04 0.64
3.25 0.27 1.46b
3.52a 3.33a 2.97a 2.36a 1.84a 2.31 0.93 0.96
hZA i
ZF
hZF i
2.89 0.15 2.1b
3.64a 3.25a 2.54a 2.40a 1.99a 0.90 0.89 0.71
2.76 0.59 2.1b
2.80 0.62 2.64b
3.36a 3.37a 3.02a 2.70a 2.13a 0.80 0.84 0.73
2.92 0.46 2.64b
M. Jozefowicz, J.R. Heldt / Chemical Physics 294 (2003) 105–116
decreased intensity of fluorescence without change in spectral distribution. This is understandable in that specific polar solvent–solute associations appear in CH–EtOH, but not in CH–THF, mixtures. The non-linear behaviour of the absorption and fluorescence spectral shifts of 9Fl and 4HOFl in CH–EtOH solvent mixtures are also partly caused by these specific polar solute–polar protic solvent association phenomenon occurring in addition to dielectric enrichment. The solvent reorganization energy of 9Fl and 4HOFl in CH–THF mixture, determined as ERo ¼ N hDW i, correlates well with the alternatively calculated ERo ¼ h~ mA m~F i=2 value. The ERo values of 9Fl and 4HOFl determined by these two independent methods differ by about 5% and 18%, respectively. From the ERo values of the two fluorophores, it follows that the solvent reorganizational energy for 9Fl is smaller than that for 4HOFl (see Table 2). On the other hand, the average number of solvent molecules, Nm , interacting with each solute molecule, is essentially the same: 5.5 and 5.9, for 9Fl and 4HOFl, respectively. On examining the q values obtained for each of the fluorophores in CH–THF and CH–EtOH mixtures (see Table 3), it is evident that the deviations from linearity of the spectral shifts of 9Fl and 4HOFl in CH–THF are due to dielectric enrichment. In the case of CH–EtOH mixtures, the very sharp decrease of spectral peak absorption energy at low EtOH concentrations observed for both fluorophores at xp > 0:01 (see Fig. 4) arises in part from the formation of hydrogen bonds with EtOH [20]. It should be noted that, in the CH– EtOH mixture, the qA and qF values of 9Fl differ significantly, whereas, those for 4HOFl are almost the same (see Table 3 and Fig. 4). These findings indicate that the hydrogen-bond complexes of fluorenone and 4-hydroxyfluorenone, either in their ground states or in their excited states or both, are of a different nature. The comparable values qA and qF in the case of 4HOFl confirm the existence of a proton-transfer relay chain, formed between the oxygen atom and the functional hydroxyl group of 4HOFl. In their excited (S1 ) states, both surrounded by their solvation shells, this complex is more stable than the equivalent
115
9Fl HOR complex. The different stabilities of the hydrogen-bond complexes indicated by the earlier determined equilibrium constants for hydrogen-bond formation in the ground (S0 ) and excited (S1 ) states of these solute molecules [20], are confirmed by these qA and qF differences. The Z values for both fluorophores in CH–THF, calculated using Z ¼ lnðbÞ (where b ¼ X =Y ), and Eq. (13), are constant within the error limit of their determination (see Table 4), and about three times larger for 4HOFl than for 9Fl. The values of Z calculated using Eq. (13) show good agreement with the Z ¼ lnðbÞ experimental one, an agreement which, since Z values depend on r6 , is very sensitive to the choice of r. The Z values determined for the CH–EtOH solvent system differ from those calculated for the CH–THF system. For 9Fl, the Z values obtained using absorption and emission spectral shift data is different, i.e., ZA 6¼ ZF , by a factor of about two, whereas for 4HOFl they are approximately the same. This indicates that the 4HOFl hydrogenbond relay complex itself acts as a solvent, forming its own solvation shell. In conclusion, it is evident that the theoretical models of the preferential solvation developed by Bakshiev, by Mazurenko and by Suppan allow the determination of interesting and useful physico-chemical parameters describing solvation of polar fluorophores in binary solvent mixtures containing polar and non-polar components. The strong non-linear dependence of the solvatochromic shifts on the mole fraction of EtOH in CH–EtOH mixtures for both fluorophores studied is attributed to formation of hydrogen-bond between solute and solvent in addition to preferential solvation. The Z values determined for both fluorophores in CH–THF and CH–EtOH indicate that this preferential solvation does take place and depends strongly on the functional groups (electron density distribution) of the interacting molecules. The theoretical of the preferential solvation phenomenon for binary protic solvent mixtures provides a qualitative rather than quantitative rationalization of the experimental results. In future work, the dynamics of the preferential solvation process will be examined.
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Acknowledgements The authors express their gratitude to Professor Jozef Heldt for scientific discussion and Dr Robert Dale for the assistance in preparation of the manuscript. This work was partially supported by the research grant of the University of Gda~ nsk DS 5200-4-0024-03. References [1] E. Lippert, Z. Naturforsch. 10a (1955) 541. [2] N. Mataga, Y. Kaifu, M. Koizumi, Bull. Chem. Soc. Jpn. 28 (1955) 690. [3] N.G. Bakshiev, Opt. Spectrosc. 16 (1964) 446 [Opt.Spektrosk. 16 (1966) 821]. [4] Y.T. Mazurenko, N.G. Bakshiev, Opt. Spectrosc. 26 (1970) 490 [Opt.Spektrosk. 28 (1970) 905]. [5] N.G. Bakshiev, W.P. Wolkow, A.W. Altajskaja, Opt. Spektrosk. 28 (1970) 51. [6] J.T. Mazurenko, Opt. Spektrosk. 33 (1972) 1060. [7] P. Suppan, J. Photochem. Photobiol. A 50 (1990) 293. [8] A. Kawski, J. Czajko, Z. Naturforsch. 29a (1974) 84. [9] M.B. Ledger, P. Suppan, Spectrochim. Acta. 25A (1969) 953. [10] K.S. Nitsche, P. Suppan, Chimia 36 (1982) 346. [11] P. Suppan, J. Chem. Soc. Faraday. Trans. 1 88 (1992) 963. [12] L. Bicz ok, T. Berces, J. Phys. Chem. 92 (1998) 3842. [13] L. Bicz ok, T. Berces, F. Marta, J. Phys. Chem. 97 (1993) 8895.
[14] P.A. Thompson, J.D. Simon, J. Chem. Phys. 97 (1992) 4792. [15] P.A. Thompson, J.D. Simon, J. Am. Chem. Soc. 115 (1993) 5657. [16] L. Bicz ok, T. Berces, H. Inoue, J. Phys. Chem. A. 103 (1999) 3837. [17] T. Yatsuhashi, Y. Nakajima, T. Shimada, H. Inoue, J. Phys. Chem A. 102 (1998) 3018. [18] R.S. Murphy, C.P. Moorlag, W.H. Green, C. Bohne, J. Photochem. Photobiol A 110 (1997) 123. [19] M. J ozefowicz, J.R. Heldt, J. Heldt, Z. Naturforsch. 57a (2002) 787. [20] M. J ozefowicz, J.R. Heldt, J. Karolczak, J. Heldt, Z. Naturforsch. 58a (2003) 144. [21] N. Mataga, R. Kubota, Molecular Interactions and Electronic Spectra, Marcel Dekker, New York, 1990. [22] E. Lippert, in: Laser Spectroscopic Studies of Reorientation and Other Relaxation Processes in Solution, in: J.B. Birks (Ed.), Organic Molecular Photophysics, vol. 2, Wiley, London, 1995, p. 17 (Chapter 1). [23] J. Midwinter, P. Suppan, Spectrochim. Acta. 25A (1969) 953. [24] E. Butty, P. Suppan, Polym. Photochem. 5 (1984) 171. [25] M. Belletete, M. Lachapelle, G. Durocher, J. Phys. Chem. 94 (1990) 5337. [26] P. Suppan, J. Chem. Soc. Faraday. Trans. 1 83 (1987) 495. [27] L. Onsager, J. Am. Chem. Soc. 58 (1936) 1486. [28] W. Liptay, Z. Naturforsch. 20a (1964) 1441. [29] R.A. Marcus, J. Chem. Phys. 38 (1963) 1858; J. Chem. Phys. 43 (1965) 1261. [30] B.S. Brunschwig, S. Ehrenson, N. Sutin, J. Phys. Chem. 91 (1987) 4714.
Preferential solvation of fluorenone and 4-hydroxyfluorenone in binary solvent mixtures Marek J ozefowicz, Janina R. Heldt
*
Institute of Experimental Physics, University of Gda~ nsk, ul. Wita Stwosza 57, PL 80-952 Gda~ nsk, Poland Received 12 May 2003; accepted 16 July 2003
Abstract Preferential solvation of fluorenone and 4-hydroxyfluorenone in binary solvent mixtures has been studied using steady-state spectroscopic measurements. This study concerns the solvent-induced shift of the absorption and fluorescence spectra of both molecules in two solvent mixtures, i.e., cyclohexane–tetrahydrofuran and cyclohexane–ethanol. The first system contains polar solute molecules, fluorenone and 4-hydroxyfluorenone, in a mixture of polar aprotic (tetrahydrofuran) and non-polar (cyclohexane) solvents. In the second solvents mixture, hydrogen bonding with solute molecules (ethanol) may occur. The results of spectroscopic measurements are analysed using theoretical models of Bakshiev, Mazurenko and Suppan which describe preferential solvation phenomena. In the case of cyclohexane–tetrahydrofuran mixtures, the deviation from linearity in the absorption and fluorescence solvatochromic shifts vs. the solution polarity is due to non-specific dipolar solvent–solute interactions. For cyclohexane–ethanol binary mixtures, both non-specific and specific (hydrogen bond and proton-relay tautomerization) interactions contribute to the observed solvatochromism. 2003 Published by Elsevier B.V. Keywords: Fluorenone; 4-Hydroxyfluorenone; Binary solvent; Preferential solvation
1. Introduction For nearly half of the past century, the importance of chemical reactions in solution has motivated research aimed at understanding solvation of polar solute molecules in solvents of varying permittivity. Understanding of solvation of electronically excited molecules was greatly advanced
*
Corresponding author. Fax: +48-583-413-175. E-mail address: fi[email protected] (J.R. Heldt).
0301-0104/$ - see front matter 2003 Published by Elsevier B.V. doi:10.1016/S0301-0104(03)00379-3
over 40 years ago by the fundamental works of Lippert [1] and Mataga et al. [2]. In the 1960s and 1970s, the study of transient solvatochromism was pioneered by Bakshiev and Mazurenko [3,4]. Several experimental studies have shown that, for solutions where specific solute–solvent interactions are absent, these theories provide an excellent starting point for understanding the energetics of solute–solvent system [5,6]. It has also been shown that polar solute molecules dissolved in a binary solvent mixture interact differently with each of the solvent components. In mixtures of solvents of
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M. Jozefowicz, J.R. Heldt / Chemical Physics 294 (2003) 105–116
different polarity, a process of preferential solvation (PS) occurs. This process involves dielectric enrichment in the solvation shell of the dipolar solute molecule [7–11], resulting from a change in the relative concentrations of more and less polar solvent molecules in the immediate neighbourhood (solvation shell) of the polar solute molecule, in comparison with the bulk solvent concentrations. This is a consequence of electrostatic interactions between the solute and solvent molecules in the binary solvent mixture. Several spectral parameters are used for monitoring preferential solvation, for example, shifts in the absorption and emission wavelength maxima, non-linear changes of the quantum yield, and decay time of the fluorescence as functions of the mole fraction/concentration of polar component in the binary mixture [7–11]. It has been shown that the photophysical properties, i.e., radiative (kR ) and non-radiative (kNR ) rate constants, quantum yield, and decay time, of fluorenone and its derivatives are very sensitive to temperature [12,13] and solution microenvironment [14,15]. These properties attracted considerable attention to this group of molecules as sensitive luminescence probes for solute–solvent molecular interaction and relaxation processes [16–18]. In previous reports [19,20], we presented the results of studies concerning intramolecular proton-transfer reactions, solvent-cage relaxation processes (red-edge effect) and fluorescence quenching phenomena for fluorenone (9Fl) and 4hydroxyfluorenone (4HOFl) in binary solvents of cyclohexane–ethanol and acetonitrile–ethanol. This paper presents the results of studies on solvent-induced shifts of the absorption and emission spectra of 9Fl and 4HOFl in binary mixed solvents, e.g., cyclohexane–tetrahydrofuran (CH–THF) and cyclohexane–ethanol (CH– EtOH). The first system contains a polar solute molecule (9Fl or 4HOFl) in a mixture of polar aprotic and non-polar solvents. The second solvent mixture contains EtOH which is capable of forming hydrogen bonds with the luminescent molecules. This allows examination of the preferential solvation phenomenon in a system where, in addition to general solvent relaxation, hydrogen bonding between the solute and solvent molecules also occurs, allowing comparison with the general
solvation process in which only dielectric enrichment occurs. The solvation parameters of the selected molecular systems are determined from the experimental data obtained.
2. Experimental details Fluorenone and 4-hydroxyfluorenone were purchased from Aldrich Chemical Co. and purified by recrystallization from toluene. Their purities were checked chromatographically. All of the solvents used were of the highest grade commercially available: ethanol (99.9%) was reagent grade; all others solvents were Aldrich spectral or HPLC grade. CH was distilled before use from a sodium–potassium amalgam, to ensure that was anhydrous. Additionally all solvents were checked in both steady-state and time-resolved fluorescence apparatuses for lack of fluorescent impurities in the wavelength ranges of interest. Absorption, excitation, and fluorescence spectra were recorded using, respectively, a Shimadzu UV-2401 PC spectrophotometer and a Shimadzu RF-5301 spectrofluorometer with 5.0 nm bandwidths in both excitation and emission. The fluorescence emitted was observed perpendicularly to the direction of the exciting beam. Samples for fluorescence measurements were prepared using a 0.2 cm high sensitivity micro quartz cell. The concentration of the solutions studied was ca. 5 104 M. The luminescence spectra reported have been corrected for the spectral response of the photomultiplier (Hamamatsu R-928) and monochromator pass, but not for reabsorption which was negligible in these samples.
3. Results and discussion 3.1. Experimental results The electronic absorption and emission spectra of molecules in solution, respectively, give reliable information about solvation effects in the ground and excited states [21,22]. Solvatochromic shifts provide an experimental measure of the energy changes between the S0 and S1 state caused by
M. Jozefowicz, J.R. Heldt / Chemical Physics 294 (2003) 105–116
107
interaction solvent molecules with the solute. When a solute molecule is dissolved in a binary solvent mixture, the local composition of the mixture is, in general, different from that in the bulk solution. This is confirmed by the observed non-linearities of solvatochromic shifts of the absorption and fluorescence spectra as a function of the mole fraction of the polar solvent. This nonlinearity has been explained as due to three main causes: non-ideal behaviour of the solvent mixture, specific solute–solvent association, and dielectric enrichment of the solvent around the polar solutes. The absorption and steady-state fluorescence spectra of 9Fl and 4HOFl in mixtures of CH–THF at different THF concentration are shown in Fig. 1. As can be seen, the absorption and emission of 9Fl and 4HOFl spectra show an approximate mirror symmetry, providing convincing evidence that
there is only a single excited electronic state contributing to the absorption and fluorescence spectra. In both of these fluorescent molecules, the absorption and emission bands become more and more red-shifted on increasing the concentration of the polar component in the solvent mixture. Figs. 2A and B show the normalized solvatochromic shifts of 9FL and 4HOFl in this binary mixture as a function of the mole fraction of THF. The shifts are independent of the concentration of the solute in the concentration range studied, indicating the absence of solute–solute interactions. Non-linear solvatochromic shifts of the absorption and fluorescence bands are observed for both fluorophores. Here we would like to note that, in plots of the same normalized solvatochromic shift data vs. the microenvironmental dielectric constant of the solvent mixtures, f ðenp Þ ¼ xn f ðen Þ þ xp f ðep Þ (see Section
Fig. 1. Long-wavelength absorption band and fluorescence spectra of 9Fl (A) and 4HOFl (B) in neat solvents of CH and THF and their mixtures at different mole fraction, xp , of THF.
Fig. 2. Normalized shift of the absorption and fluorescence bands maxima of 9Fl (A) and 4HOFl (B) determined for different mole fraction, xp , of THF.
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M. Jozefowicz, J.R. Heldt / Chemical Physics 294 (2003) 105–116
3.2), a departure from linearity is also evident. Such behaviour is often observed for polar molecules dissolved in binary mixtures of polar and non-polar solvents, and is taken as evidence for dielectric enrichment [7,9–11,23–26]. As can be seen in Fig. 1, the absorption spectrum of 4HOFl in pure cyclohexane, with a maximum at 24,650 cm1 , shows pronounced vibrational structure. At low THF concentrations (xp < 0:02) this vibronic structure is still visible, but it vanishes at higher concentrations (xp > 0:02), where the whole spectrum additionally undergoes a pronounced red-shift. The fluorescence spectra of 9Fl and 4HOFl do not show any vibrational structure (see Fig. 1). The observed red-shifts of these fluorescence spectra caused by increasing solvent polarity are larger than those of the absorption spectra. The fluorescence spectral maxima of 9Fl and 4HOFl are centred at 21,770, 20,840 cm1 and 21,040, 19,760 cm1 in CH and THF solutions, respectively. Fig. 3 shows the normalized absorption and fluorescence spectra of 9Fl and 4HOFl in CH– EtOH at different mole fractions of ethanol. The absorption and steady-state fluorescence spectra show the same general dependence on increasing the alcohol concentration as observed for both molecules in CH–THF mixture on increasing the tetrahydrofuran concentration (see Fig. 1). Fig. 4 shows their normalized shifts plotted against the EtOH concentration. Both fluorophores clearly show that a pronounced shift of the absorption and fluorescence spectra occurs already at very low alcohol concentration. For both solute molecules, about 50% of the total fluorescence spectrum shift observed between pure CH and pure EtOH has occurred by an EtOH mole fraction of 0.04 (see Fig. 4). This unusually large shift is understandable in terms of hydrogen bonding between the solute and the alcohol. It is well known that the hydrogen bonding breaks down at higher temperatures (>350 K), therefore the normalized shifts of the absorption and fluorescence spectra vs. EtOH mole fraction for both solvent mixtures were measured at different temperatures. Fig. 5 shows the fluorescence spectra of 9Fl in binary mixtures of CH– THF and CH–EtOH (polar component mole fraction 0.04) determined over the temperature range from 293 to 373 K. It is seen that:
Fig. 3. Long-wavelength absorption band and fluorescence spectra of 9Fl (A) and 4HOFl (B) in neat solvents of CH and EtOH and their mixtures at different mole fraction, xp , of EtOH.
• Neither the shape of the fluorescence spectrum of 9Fl in CH–THF nor its wavelength of maximum intensity is shifted on changing the temperature from 293 to 373 K (Fig. 5A). In addition, the intensity of the fluorescence decreases linearly with increasing temperature as a result of temperature quenching [12,13]. • The wavelength of maximum intensity of fluorescence of 9Fl in CH–EtOH is blue-shifted from 494 nm at 293 K to 476 nm at 373 K (Fig. 5B). The kmax (373 K) ¼ 476 nm agrees well with that determined in neat CH. The fluorescence spectral behaviours of 4HOFl in CH– THF and CH–EtOH (mole fractions of polar component 0.04 in both cases) are very similar, and not shown here.
M. Jozefowicz, J.R. Heldt / Chemical Physics 294 (2003) 105–116
109
Fig. 4. Normalized shift of the absorption and fluorescence bands maxima of 9Fl (A) and 4HOFl (B) determined for different mole fraction, xp , of EtOH.
3.2. Review of the theories of preferential solvation Theoretical descriptions of preferential solvation have been given by Bakshiev et al. [5], Mazurenko et al. [6] and Suppan et al. [7]. All of the theories of solvatochromic shift in absorption and fluorescence spectra are based on the Onsager [27] description of non-specific electrostatic solute–solvent interactions. According to Bakhshiev and co-workers [5], the average degree of fill of the solute shell with polar solvent molecules (p), i.e., its mole fraction in the solvent shell, for a binary system in which nonpolar (n) and polar (p) solvent molecules possess similar radii (rn ffi rp ¼ r) and the solvents have similar refractive indices (nn ffi np ¼ n) is given by D E he i e eff n xBp ¼ ð1Þ ep en and the effective dielectric constant heeff i by
Fig. 5. Fluorescence spectra of 9Fl in THF (A) and EtOH (B) determined at different temperatures: 1, 293 K; 2, 303 K; 3, 313 K; 4, 323 K; 5, 333 K; 6, 350 K; 7, 363 K; and 8, 373 K.
D E heeff i ¼ en xBn þ ep xBp ;
ð1aÞ
where en and ep are the static dielectric constants of the non-polar and polar solvents, respectively. The microenvironmental dielectric constant heeff i appearing in Eq. (1), can be determined from the absorption and fluorescence spectral shifts, d~ mA and d~ mF , respectively, of fluorophores in mixed and neat solvents. According to Liptay [28], and as has been shown by Kawski and Czajko [8], for spherical solute molecules centred in a closed spherical microenvironment (first shell), the solvent spectral shifts are 1
1
The subscripts A and F in the spectral shift d~ mA;F in Eq. (2), and in other constants refer to the absorption and fluorescence, respectively.
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M. Jozefowicz, J.R. Heldt / Chemical Physics 294 (2003) 105–116
d~ mA;F ¼ ðD~ mnþp mnA;F Þ A;F D~ 2n2 þ 1 heeff i 1 n2 1 ¼ m1;2 2 ; n þ 2 heeff i þ 2 n2 þ 2
ð2Þ
determined from spectroscopic measurements of the maxima of the absorption (A) and fluorescence mpA;F ), (F) spectra in neat non-polar (~ mnA;F ), polar (~ nþp and binary mixtures (~ mA;F ), respectively, of the solvents, using a simplified formula [4]
ð3Þ
hA;F ¼
ð4Þ
(6)
where ~ le ~ lg Þ lg ð~ ; m1 ¼ 2pe0 hca3
where e0 ¼ 8:8542 1012 F m1 , h is the Planck constant, c the velocity of light in vacuo, lg and le the dipole moments in the ground and excited states, respectively, a the Onsager cavity radius of the solute molecule. The solvent spectral shifts d~ mA;F are determined as the differences between the maxima in the mixed solution and those in the neat solvents. Knowing the experimental values of d~ mA , m1 , and refractive index n we are able to calculate heeff i for the solvent shell with the solute molecule in its ground state (S0 ). A similar procedure is applied in order to determine the corresponding value for the solute molecule in its first excited state (S1 ). Mazurenko and co-workers [6], on the basis of statistical considerations for the cage model of a binary solvent, have shown that the probability with which polar and non-polar molecules fill the Nm solvent cells in the first solvation shell (cage) around a single solute with Np polar and Nn nonpolar molecules (Nm ¼ Np þ Nn ) at thermal equilibrium is given by the binomial distribution Nm ! Np hA;F ð1 hA;F ÞNm Np ; Np !ðNm Np Þ!
ð5Þ
ln
hA;F xp ¼ ln þ cA;F ; 1 hA;F 1 xp
Wmðg;eÞ 1 Nm kT
:
ð8Þ
where cA;F ¼
Wmðg;eÞ 1 DWmðg;eÞ ¼ Nm kT kT
ð9Þ
and the subscripts g and e indicate ground and excited states, respectively. Nm , the solvation number, can be calculated [4] as Nm ¼
hcð~ mnF m~pF þ m~nA m~pA Þ : 2kT ðcF cA Þ
ð10Þ
The average free orientational interaction energy for a single molecule in the solvation shell is obtained from Eqs. (8) and (10) as DWmðg;eÞ ¼ Wmðg;eÞ =Nm :
ð11Þ
According to Suppan and co-workers [7], the deviation of the shifts in absorption and emission maxima in binary mixtures from linearity with the mole fraction of polar solvent xp results from dielectric enrichment. In the single-shell approximation, the mole fraction Y ¼ yp =yn in the solvent shell and that in the bulk solution X ¼ xp =xn are related by a preferential solvation index, Z Y ¼ X eZ ;
where hA;F xp ¼ exp 1 hA;F 1 xp
ð7Þ
On taking the natural logarithmic form of Eq.
~ le ~ lg Þ le ð~ m2 ¼ ; 2pe0 hca3
/eq ðNp Þ ¼
m~nA;F m~nþp A;F : m~nA;F m~pA;F
ð12Þ
where ð6Þ
Assuming that replacement of any non-polar molecule by a polar one in any of the cells leads to the same gain in energy. Wm is the minimum solvent reorientation energy for filling Nm cells with polar solvent molecules, xp the mole fraction of the polar component. The ratio hNp i=Nm ¼ hA;F can be
Z¼
1 Cl2 MDf ðenp Þ 6 4pe0 RT drs–s
ð13Þ
in which ~ l is the solute dipole moment, M the mean molecular weight of polar and non-polar solvent, Df ðenp Þ the difference between the Onsager polarity functions of the neat polar and nonpolar solvents, Df ðenp Þ ¼ f ðep Þ f ðen Þ, where
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f ðep Þ ¼ 2ðep 1Þ=ð2ep þ 1Þ and f ðen Þ ¼ 2ðen 1Þ= ð2en þ 1Þ; d the mean density of the two components, and rs–s the mean separation between the solute and solvent molecules, i.e., rs–s ¼ a þ r (a and r are the Onsager cavity and solvent molecule radii, respectively), C is a numerical constant formally equal to (3=8p) when both the solvent and solute molecules are considered to be spherical [27], which value will be used here. The other symbols possess the standard meaning. It should be noted that, if preferential solvation effects are absent in the mixed solvent, then Z ¼ 0. For many solvent mixtures, the bulk dielectric polarity function f ðenp Þ defined as f ðenp Þ ¼ xn f ðen Þ þ xp f ðep Þ using the static dielectric constants en and ep , does not follow the f ðeeff Þ curve as a function of polar solvent molar fraction. In such cases, the microenvironmental dielectric constant heeff i determined using the spectral shift method (see Eq. (2)) is appropriate to be used in the calculation of Z (Eq. (13)). 3.3. Determination of the solvation parameters The theories reviewed above allow evaluation of the main parameters describing the solvation phenomenon. As can be seen in Figs. 1–4, the rapid change in absorption and fluorescence maxima of 9Fl and 4HOFl in the binary CH–THF and CH– EtOH mixtures clearly show the effect of preferential solvation. In all cases, the experimental points deviate from linearity with mole fraction of the polar solvent, and their departure indicates that the solute molecules are preferentially solvated by the more polar component. This effect is even more pronounced when the more polar component is of the protic type (EtOH) (see Figs. 2 and 4). As can be seen, the curve of normalized spectral shifts of the long-wavelength absorption band does not always follow that of the fluorescence. The effective dielectric constants heeff i of the solvent shell of 9Fl, in agreement with [5,6], can be calculated using Eq. (2) only for CH–THF system, in which specific interactions between solute and solvent molecules do not occur, and the additional conditions, e.g., the refractive index and molecular and radii of CH and THF, nCH ¼ nTHF ffi 1:43 A rCH ffi rTHF ¼ 2:1 A, respectively, fulfil the as-
111
sumptions made by Bakshiev et al. [5] and Mazurenko et al. [6]. Using the spectral shift data experimentally determined from Figs. 1 and 3, the effective dielectric constant heeff i of the solvent shell around the 9Fl molecule in the ground and excited states as a function of the THF molar fraction has been calculated using Eq. (2) (see Table 1). Using the heeff i and enp data assembled in Table 1, the Onsager polarity functions have been calculated for different xp values. Fig. 6 presents graphically obtained data for 9Fl and 4HOFl in CH–THF mixtures. As can be seen in Figs. 6A and B, neither of the polarity functions f ðeeff Þ and f ðenp Þ follows a linear dependence on xp . This finding indicates preferential solvation for 9Fl and 4HOFl dissolved in CH–THF mixtures. Further, using the experimental spectral shift data, hA;F , values have been determined and used to plot lnðhA;F =1 hA;F Þ vs. lnðxp =1 xp Þ for both fluorophores in both binary solvent mixtures (see Fig. 7). As seen in Figs. 7A and B, a linear dependence is obtained for both 9Fl and 4HOFl in CH–THF over the whole range of xp , but not in CH–EtOH. cA and cF values for the fluorophores in CH–THF were therefore obtainable and used to calculate DWmðg;eÞ , and Nm using Eqs. (9) and (10). The results are assembled in Table 2. From the DWmðgÞ and DWmðeÞ values obtained for both of the fluorophores in CH–THF, it is evident that the packing of solvent molecules around the polar solutes in their ground and excited states is very different. The ratio of the corresponding 9Fl 9Fl free reorientational energies, DWmðeÞ =DWmðgÞ ffi 4HOFl 4HOFl DWmðeÞ =DWmðgÞ , is about 2. On the other hand, the free energy of the reorientational interaction for each of 9Fl and 4HOFl in their ground and ex4HOFl 9Fl cited states are different: DWmðgÞ =DWmðgÞ ffi 2:18 4HOFl 9Fl and DWmðeÞ =DWmðeÞ ffi 1:75. The Nm values, calculated using Eq. (9), are nearly the same for 9Fl and 4HOFl. It is worth noting that, for CH–EtOH, lnðhA;F =1 hA;F Þ, vs. lnðxp =1 xp Þ is linear only up to xEtOH ¼ 0:08, this simple dependence breaking down at higher EtOH concentrations, indicating that both fluorophores are undergoing hydrogenbonding interactions with EtOH at these higher concentrations [19,20]. According to the MazurenkoÕs model, the reorientational energy of a single molecule in the
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Table 1 Determined data of spectral shifts d~ mA and d~ mF , microenvironmental dielectric constants heeff i, and filling up degree, hxBp i, of the solvent shell as a function of the THF and EtOH mole fraction xp
9Fl
4HOFl
Ground state dmA (cm1 )
heeff i
Excited state hxBp i
dmF (cm1 )
heeff i
Ground state hxBp i
dmA (cm1 )
heeff i
Excited state hxBp i
dmF (cm1 )
heeff i
hxBp i
CH–THF 0 0 0.015 30 0.030 40 0.073 70 0.135 140 0.238 250 0.612 410 0.882 500 0.938 510 1 530
2.03 2.18 2.33 2.63 3.54 3.93 5.74 6.62 6.74 7.00
0 3.10 6.21 12.45 31.38 39.88 77.80 94.41 96.64 100
0 30 60 120 300 380 730 880 900 930
2.02 2.30 2.39 2.67 3.32 4.35 5.85 6.71 6.81 7.00
0 5.56 7.44 13.05 26.11 46.80 77.07 94.23 96.16 100
0 240 290 530 650 830 840 850 860 890
2.01 2.21 2.90 3.36 3.90 4.32 5.76 6.69 7.00 7.20
0 4.09 17.32 26.20 36.57 44.73 72.54 90.28 96.14 100
0 470 570 640 810 890 1020 1170 1240 1280
2.01 2.41 3.21 3.50 4.20 4.54 5.95 6.89 7.06 7.20
0 7.58 23.13 28.55 41.98 48.41 75.16 93.08 96.57 100
CH–EtOH 0 0 0.021 30 0.042 100 0.098 170 0.179 240 0.303 330 0.685 430 0.916 500 0.956 510 1 520
2.02 5.3 7.6 11.1 14.9 16.3 21.0 22.9 23.1 24
0 14.92 25.39 41.31 63.15 74.06 86.35 95.00 95.90 100
0 1500 1910 2320 2530 2810 2840 2970 2990 3070
2.02 8.7 11.1 12.3 17.1 19.5 21.6 23.3 23.8 24
0 30.39 41.31 46.77 68.60 79.53 89.08 96.81 99.00 100
0 580 760 930 950 1000 1290 1300 1320 1340
2.02 6.3 8.8 13.1 16.4 19.1 21.0 23.0 23.5 24
0 19.47 30.85 50.41 65.42 77.71 86.35 95.45 97.72 100
0 1180 1680 2030 2230 2280 2390 2730 2800 2820
2.02 8.2 10.8 12.1 16.1 18.6 22.6 23.1 23.6 24
0 28.11 39.94 45.86 64.06 75.42 93.63 95.90 98.18 100
solvation shell is given by ERo ¼ Nm hDW i. Using the hDW i and Nm values obtained from the CH– THF data and collected in Table 2, we find ERo ¼ 1100 and 2000 cm1 for 9Fl and 4HOFl, respectively. 2 Since the normalized spectral shift curves (Figs. 2 and 4) show large departures from linearity for both systems of binary solvents, it was tempting to calculate the deviation parameter, describing the
2
This value can be compared with that calculated on the basis of the Franck–Condon transition energy partitions hc~ mA ¼ DGsolv þ ERo , and hc~ mF ¼ DGsolv ERo , where DGsolv denotes the free energy difference between the solvated ground and relaxed solvated excited states [29,30]. From these, it follows that: 2ERo ¼ chð~ mA m~F Þ. The ERo values obtained are 1300 and 1900 cm1 in CH–THF and 2250 and 3050 cm1 in CH–EtOH for 9Fl and 4HOFl, respectively. They are in good agreement with the data presented in Table 2.
departure from linearity, introduced by Suppan [26] as R ½FD ðxp Þ F ðxp Þ dx q¼ ; ð14Þ ð1=2ÞDF where FD ðxp Þ is the measured wavenumber of the spectral peak at THF mole fraction xp , F ðxp Þ the band position that is expected according to linear behaviour, and DF is the difference between these values in the neat solvents, i.e., CH (xp ¼ 0) and THF or EtOH (xp ¼ 1) (see Fig. 2A). As follows from Eq. (14) q is a dimensionless quantity between 0 and 1, corresponding to linear additivity of f ðxn Þ and f ðxp Þ at q ¼ 0, and infinite deviation from linearity at q ¼ 1. Since for both fluorophores in both solvent mixtures evident deviations from linearity occur, the respective q values were calculated, and are presented in Table 3. In these cases, the deviation from linearity (as seen in Figs.
M. Jozefowicz, J.R. Heldt / Chemical Physics 294 (2003) 105–116
Fig. 6. Values of the polarity function of the binary mixture CH–THF determined for different mole fraction of THF using the formula: f ðenp Þ ¼ xp f ðep Þ þ xn f ðen Þ, and f ðeeff Þ ¼ 2ðheeff i 1Þ=ð2heeff i þ 1Þ where heeff i data are calculated using Eq. (2) for absorption (A) and fluorescence (B) solvent spectral shifts.
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Fig. 7. The dependence of lnðhA;F =1 hA;F Þ vs. lnðxp =1 xp Þ of both fluorophores in CH–THF (A) and CH–EtOH (B) solutions.
calculated using Eq. (12) and the corresponding experimental data assembled in Table 1. The indices of preferential solvation thus determined from the spectral shifts in absorption, ZA , and fluorescence, ZF , are shown in Table 4. Z can alternatively be calculated by means of Eq. (13), using MCH–THF ¼78:14 g mol1 , MCH–EtOH ¼65:11 g mol1 , dCH–THF ¼ 0:83 g cm3 , dCH–EtOH ¼ 0:72 g cm3 ,
2 and 4) is due to preferential solvation or to specific interaction between solute and protic solvent molecules. The normalized spectral shift curves for 9Fl and 4HOFl as a function of xp are very similar for the two solvent mixtures. In view of this, the indices of preferential solvation Z were
Table 2 Free orientational interaction energy, ERo , of the solution molecules in the solvation shell calculated according to Mazurenko et al. [6] Solvent
DWmðgÞ (cm1 )
CH–THF
9Fl 112.0 4HOFl 244.4
CH–EtOH
a
9Fl 228.1 4HOFl 325.8
hDW i ¼ ðDWmðgÞ þ DWmðeÞ Þ=2.
DWmðeÞ (cm1 )
cA
cF
Nm
ERo ¼ Nm hDW ia (cm1 )
244.4
0.55
1.2
6
1095
427.6
1.2
2.1
6
2010
712.8
1.12
3.5
4
1900
692.4
1.6
3.4
6
3050
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M. Jozefowicz, J.R. Heldt / Chemical Physics 294 (2003) 105–116
Table 3 The Suppan deviation factor (see Eq. (14)) CH–THF qA
qFCH–THF
qCH–EtOH A
qCH–EtOH F
9Fl 0.3
0.29
0.77
0.52
4HOFl 0.43
0.78
0.67
0.69
dissolved in CH–THF and CH–EtOH solvent mixtures can be rationalized in terms of nonspecific (i.e., dipolar) and specific (especially Hbonding with EtOH) reactions of the fluorophore with the polar solvent. In previous work [20], we have shown that the absorption and the fluorescence spectra of 9Fl and 4HOFl in mixed binary solvents undergo very complex changes on adding ethanol to cyclohexane, i.e., they are shifted to longer wavelengths, with broadened half-widths and changed emission spectral profiles. This behaviour indicates that, in the binary CH–EtOH solution, a specific interaction, i.e., hydrogen-bonding between 9Fl or 4HOFl and the alcohol molecules, is occurring in addition to the well-known dipole–dipole interaction. In order to confirm the existence of hydrogen bonding, the temperature dependence of the fluorescence spectra of 9Fl and 4HOFl in the binary solvent was investigated. As seen in Fig. 5B, the fluorescence maximum is blue-shifted on increasing the temperature of the CH–EtOH mixture, a result of thermally induced hydrogen-bond breaking. The fluorescence spectra of 9Fl and 4HOFl in CH–THF (see Fig. 5A) show only a
l9Fl ¼ 3:1 D, l4HOFl ¼ 2:4 D, together with and for 9Fl and r ¼ 5:4 A for 4HOFl in CH– r ¼ 4:9 A for 9Fl and r ¼ 5:8 A THF mixtures and r ¼ 5:2 A for 4HOFl in CH–EtOH mixtures. Z was found to be 0.93 and 2.1 for fluorenone and 4-hydroxyfluorenone in CH–THF mixtures, respectively, and 1.46 and 2.64 in CH–EtOH. The values thus obtained are in a reasonable agreement with the average, hZi, for ground and excited states presented in Table 4.
4. Summary and discussion Non-linear plots of the solvatochromic shifts vs. mole fraction of polar solvent for 9Fl and 4HOFl
Table 4 Preferential solvation indexes, ZA and ZF , calculated using Eqs. (12) and (13) xp
9Fl ZA
CH–THF 0.015 0.030 0.073 0.135 0.238 0.612 0.882 0.938
1.37a 0.97a 0.65a 0.83a 1.05a 0.77a 0.80a 0.52
CH–EtOH 0.021 0.042 0.098 0.179 0.303 0.685 0.916 0.956
1.77a 1.60a 1.42a 1.30a 1.33a 0.75 0.68 0.61
a b
4HOFl hZA i
ZF
0.92 0.22 0.93b
0.78a 0.80a 0.63a 1.11a 0.79a 0.84a 0.86a 0.68
1.48 0.18 1.46b
3.64a 3.47a 3.20a 2.91a 3.05a 1.56 0.79 0.34
Data used to calculated the average value. Value calculated using Eq. (13).
hZF i
ZA
0.83 0.13 0.93b
3.18a 2.74a 2.93a 2.85a 2.79a 2.36 1.04 0.64
3.25 0.27 1.46b
3.52a 3.33a 2.97a 2.36a 1.84a 2.31 0.93 0.96
hZA i
ZF
hZF i
2.89 0.15 2.1b
3.64a 3.25a 2.54a 2.40a 1.99a 0.90 0.89 0.71
2.76 0.59 2.1b
2.80 0.62 2.64b
3.36a 3.37a 3.02a 2.70a 2.13a 0.80 0.84 0.73
2.92 0.46 2.64b
M. Jozefowicz, J.R. Heldt / Chemical Physics 294 (2003) 105–116
decreased intensity of fluorescence without change in spectral distribution. This is understandable in that specific polar solvent–solute associations appear in CH–EtOH, but not in CH–THF, mixtures. The non-linear behaviour of the absorption and fluorescence spectral shifts of 9Fl and 4HOFl in CH–EtOH solvent mixtures are also partly caused by these specific polar solute–polar protic solvent association phenomenon occurring in addition to dielectric enrichment. The solvent reorganization energy of 9Fl and 4HOFl in CH–THF mixture, determined as ERo ¼ N hDW i, correlates well with the alternatively calculated ERo ¼ h~ mA m~F i=2 value. The ERo values of 9Fl and 4HOFl determined by these two independent methods differ by about 5% and 18%, respectively. From the ERo values of the two fluorophores, it follows that the solvent reorganizational energy for 9Fl is smaller than that for 4HOFl (see Table 2). On the other hand, the average number of solvent molecules, Nm , interacting with each solute molecule, is essentially the same: 5.5 and 5.9, for 9Fl and 4HOFl, respectively. On examining the q values obtained for each of the fluorophores in CH–THF and CH–EtOH mixtures (see Table 3), it is evident that the deviations from linearity of the spectral shifts of 9Fl and 4HOFl in CH–THF are due to dielectric enrichment. In the case of CH–EtOH mixtures, the very sharp decrease of spectral peak absorption energy at low EtOH concentrations observed for both fluorophores at xp > 0:01 (see Fig. 4) arises in part from the formation of hydrogen bonds with EtOH [20]. It should be noted that, in the CH– EtOH mixture, the qA and qF values of 9Fl differ significantly, whereas, those for 4HOFl are almost the same (see Table 3 and Fig. 4). These findings indicate that the hydrogen-bond complexes of fluorenone and 4-hydroxyfluorenone, either in their ground states or in their excited states or both, are of a different nature. The comparable values qA and qF in the case of 4HOFl confirm the existence of a proton-transfer relay chain, formed between the oxygen atom and the functional hydroxyl group of 4HOFl. In their excited (S1 ) states, both surrounded by their solvation shells, this complex is more stable than the equivalent
115
9Fl HOR complex. The different stabilities of the hydrogen-bond complexes indicated by the earlier determined equilibrium constants for hydrogen-bond formation in the ground (S0 ) and excited (S1 ) states of these solute molecules [20], are confirmed by these qA and qF differences. The Z values for both fluorophores in CH–THF, calculated using Z ¼ lnðbÞ (where b ¼ X =Y ), and Eq. (13), are constant within the error limit of their determination (see Table 4), and about three times larger for 4HOFl than for 9Fl. The values of Z calculated using Eq. (13) show good agreement with the Z ¼ lnðbÞ experimental one, an agreement which, since Z values depend on r6 , is very sensitive to the choice of r. The Z values determined for the CH–EtOH solvent system differ from those calculated for the CH–THF system. For 9Fl, the Z values obtained using absorption and emission spectral shift data is different, i.e., ZA 6¼ ZF , by a factor of about two, whereas for 4HOFl they are approximately the same. This indicates that the 4HOFl hydrogenbond relay complex itself acts as a solvent, forming its own solvation shell. In conclusion, it is evident that the theoretical models of the preferential solvation developed by Bakshiev, by Mazurenko and by Suppan allow the determination of interesting and useful physico-chemical parameters describing solvation of polar fluorophores in binary solvent mixtures containing polar and non-polar components. The strong non-linear dependence of the solvatochromic shifts on the mole fraction of EtOH in CH–EtOH mixtures for both fluorophores studied is attributed to formation of hydrogen-bond between solute and solvent in addition to preferential solvation. The Z values determined for both fluorophores in CH–THF and CH–EtOH indicate that this preferential solvation does take place and depends strongly on the functional groups (electron density distribution) of the interacting molecules. The theoretical of the preferential solvation phenomenon for binary protic solvent mixtures provides a qualitative rather than quantitative rationalization of the experimental results. In future work, the dynamics of the preferential solvation process will be examined.
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