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Complexation of 6-hydroxyquinoline with trimethylamine in polar and non-polar solvents
Chemical Physics Letters 366 (2002) 628–635 www.elsevier.com/locate/cplett
Complexation of 6-hydroxyquinoline with trimethylamine in polar and non-polar solvents M.S. Mehata *, H.C. Joshi 1, H.B. Tripathi Photophysics Laboratory, Department of Physics, DSB Campus, Kumaon University, Nainital 263002, India Received 13 June 2002; in final form 27 September 2002
Abstract Spectral characteristics of 6-hydroxyquinoline (6-HQ) in presence of trimethylamine (TMA) were investigated in polar and non-polar solvents. The steady-state absorption, emission and excitation spectra along with the transient parameters reveal a strong ground state hydrogen-bonded complex formation between the 6-HQ and TMA molecules in both the media. A large Stokes shifted emission due to the formation of contact ion-pairs is observed in these media. However, in acetonitrile the longer decay time ( 12 ns) with relatively broadened emission spectra can be attributed to the presence of solvent separated ion-pairs in addition to contact ion-pairs. The ground state equilibrium constant for complex formation has been determined. The observed quenching behaviour of the fluorescence emission from the normal molecule with TMA appears to be static in nature. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction It is well known that hydrogen-bonding interactions can affect the photophysical properties of the organic molecules [1–10] and can provide a pathway for fluorescence quenching [1]. When two conjugate p electron systems are directly combined by hydrogen bonding, the fluorescence of the proton donor or the acceptor is strongly quenched [9–13] which has been explained by charge transfer (CT) interaction between the donor and acceptor p
*
Corresponding author. Fax: +91-5942-35576. E-mail address: [email protected] (M.S. Mehata). 1 Present address: Institute for Plasma Research, Bhat, Gandhinagar 382428, India.
electron system [14–16] via the hydrogen bond as well as hydrogen atom transfer [17,18]. Intermolecular charge transfer interaction can result in the formation of ground state (hydrogen bonded) donor–acceptor (GDA) complex or one hetero excimer (EDA) in the excited state. Fluorescence quenching is often accompanied by the appearance of a new fluorescence emission, usually attributed to an exciplex [19]. Ambiguity persists in the interpretation of the observed data as excitation of both GDA and EDA complexes can emit at common wavelength. Moreover, it is possible that the species formed in the excited state via these two routes may exhibit different behaviour. However, in many kinetic treatments the two pathways of formation of an excited CT state have been found to be identical [8,20].
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 1 5 7 9 - 8
M.S. Mehata et al. / Chemical Physics Letters 366 (2002) 628–635
Recently, Chou et al. [21,22] have reported the single and double hydrogen-bonded complexes of 3-hydroxyisoquinoline (3-HIQ) and 7-hydroxyquinoline (7-HQ) with trimethylamine (TMA) triethylamine (TEA) and acetic acid, respectively, and also their self-association. They have tentatively suggested that the single hydrogen-bonded complex of 3-HIQ and TMA being highly endergonic, i.e., enol/TMA to keto/TMA reaction is not possible. A comprehensive study of the fluorescence and prototropic equlibria of 6-HQ in different media has been reported in literature [6,7,23–25]. Due to the extremely high simultaneous photoacidity and photobasicity in the S1 (excited) state, 6-HQ undergoes deprotonation and protonation in 10 M HClO4 and 12 M NaOH, respectively [7]. Comparative studies of 6-HQ and 8-HQ with the Aerosol-OT surfactant, reveal that 6-HQ is unable to form dimers whereas 8-HQ undergoes dimerization [26]. In earlier reports [27,28] we have proposed a mechanism of proton transfer for 6-HQ in methanol in presence of water molecules and also in Nafionâ polymer matrix. Here, we report the fluorescence characteristic of 6-HQ-trimethylamine (TMA) system at room temperature in polar and non-polar solvents.
2. Experimental details 6-HQ (Aldrich) was recrystallized from ether. All the solvents used were of spectroscopic grade. The purity of 6-HQ was checked from thin layer chromatography and the fluorescence excitation spectrum in different solvents. TMA (solid) was taken from Aldrich. The concentration of the sample used in our experiments was 105 M. The details of experimental measurements are same as given in earlier papers [3,4,27,28].
3. Results and discussion Figs. 1 and 2A,B show the absorption, emission and excitation spectra, respectively, of 6-hydroxyquinoline (6-HQ) in polar solvent acetoni-
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Fig. 1. Absorption spectra of 6-HQ in acetonitrile for various concentration of TMA (M); for 0.0 (a), 0.003 (b), 0.018 (c), 0.048 (d), 0.198 (e), 0.498 (f), and 0.798 (g).
trile (ACN), and with addition of varying concentration of trimethylamine (TMA). Fig. 1a shows that the lowest absorption band has maximum at 332 nm. With the addition of TMA a significant change in absorption spectrum is observed; the absorption band is red shifted and broadened with an increase in the absorbance which finally gives rise to a single band ðkmax 337:5 nmÞ at 0.798 M concentration of TMA. In toluene, a non-polar solvent, however, the shift is negligible but a small change in emax and the appearance of a shoulder in a region 340–350 nm is observed (not shown). Isosbestic points in both the solvents are obtained which indicates the formation of ground state hydrogen-bonded complex. The values of the ground state equilibrium constant (Kg ) were calculated to be 40:0 and 8.0 M1 in acetonitrile and in toluene, respectively, following Belletele et al. [29]. The changes in the fluorescence emission spectra of 6-HQ caused by the addition of TMA concentration in ACN are shown in Fig. 2A. It can be seen that the intensity of normal emission band (363 nm) of 6-HQ decreases continuously with increasing concentration of TMA in ACN, a new broad emission band with Stokes shift 7390 cm1 appears at 440 nm with full-width at
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Fig. 2. (A) Fluorescence emission spectra of 6-HQ in acetonitrile for various concentrations of TMA (M); for 0.0 (a), 0.003 (b), 0.018 (c), 0.048 (d), 0.198 (e), 0.498 (f) and 0.798 (g); excited by 310 nm wavelength. (B) Normalized excitation spectra of 6-HQ for 0.018 M TMA monitored at emission wavelengths 350 nm (a) and 450 nm (b) in acetonitrile.
half maximum (FWHM) 3830 cm1 . Whereas in toluene, where the solute–solvent interactions are comparatively low, the normal emission is observed at 359 nm and a new broad emission band at 417 nm with a relatively less Stokes shift (6090 cm1 ) and FWHM (3400 cm1 ) appears on addition of TMA. These emission bands which arise due to complex formation in both the solvents become predominant with decrease in the intensity of normal band as the concentration of TMA is increased. Especially in toluene, the intensity of complex emission (for max conc. of TMA) is approximately eight times higher than the intensity of normal emission (TMA free solution). Isoemissive points in the emission spectra are noticed in ACN (400 nm) and in toluene (370 nm) which indicate the existence of more than one emitting species. Further, the emission bands corresponding to complex show higher intensity on red edge excitation compared to excitation at the blue edge.
Excitation spectra are recorded for different TMA concentrations in both the solvents. For TMA free 6-HQ the excitation spectrum is almost identical to the observed absorption spectrum within experimental limits. However, addition of 0.018 M of TMA in 6-HQ solution in ACN the excitation spectra for 350 and 450 nm emissions are not identical and show a shift of about 792 cm1 (Fig. 2B). A similar situation is observed in toluene where the shift is found to be 785 cm1 . It can be mentioned that the shift observed in the excitation spectra in polar and non-polar solvents are nearly identical. These spectra confirm the formation of ground state hydrogen-bonded complex (GHBC) between 6-HQ and TMA in both the solvents. 3.1. Decay parameters The fluorescence decay for the normal emission (350 nm) of 6-HQ in neat ACN fits with a
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Fig. 3. Fitted decay curves of 6-HQ-TMA in acetonitrile (kexc ¼ 310 nm): (a) Monoexponential fit for free (0.0 TMA) at 350 nm; Biexponential fit for 0.018 M TMA at (b) 410 nm and (c) 490 nm with the value of v2 and residuals (a), (b) and (c), respectively; (b0 ) shows residuals and v2 value for monoexponential fit at 410 nm.
monoexponential function (Fig. 3a) with decay time of 1.6 ns. To explore the fluorescence quenching mechanism, the decay data are recorded at normal emission band (350 nm), for 310 nm excitation with varying TMA concentrations in ACN. It is observed that the decay time of the normal molecule does not follow the same trend as shown by intensity variation (decrease in the intensity of normal emission) and is constant for the entire concentration of TMA studied in this work. The results indicate that quenching is static rather than dynamic in nature [30–32] and hence gives evidence of the complex formation between the 6-HQ and TMA molecules. The transient parameters for 0.018 M of TMA concentration monitored across the emission profile are given in Table 1. It is observed that between 330 and 370 nm
emission regions, the decay shows monoexponential fit. Above 370 nm a clear departure from monoexponential is noticed and the decay fits with biexponential function (Fig. 3b). The amplitude corresponding to shorter component decreases and for longer one increases while monitoring towards the longer wavelength side of the emission spectrum. However, above 430 nm emission where the smaller component (1.6 ns) corresponding to normal emission is absent the decay again fits to a biexponential function and another longer decay component having decay time 12 ns appears (Fig. 3c). The amplitude of this longer decay component shows increase at higher emission wavelength. No rise time could be detected for longer wavelength emission with the present experimental setup.
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Table 1 Decay times and amplitudes for the 6-HQ-TMA system in acetonitrile (0.018 M, TMA) at room temperature kem (nm)
s1 and Stand. dev. (ns)
s2 and Stand. dev. (ns)
a1
a2
v2
For kexc ¼ 310 nm 330 350 370 390 410 430 450 470 490
1.60 1.65 1.68 1.80 2.07 2.09 11.20 11.60 12.20
– – – 7.03 6.04 5.93 5.64 5.43 6.19
(0.05) (0.07) (0.05) (0.11) (0.15) (0.35)
100.00 100.00 100.00 82.11 29.29 09.64 12.33 45.49 73.12
– – – 17.89 70.11 90.36 87.67 54.51 26.88
1.17 1.22 1.16 1.09 1.00 0.94 1.19 0.79 0.92
For kexc ¼ 340 nm 450
10.06 (0.6)
4.84 (0.09)
20.44
79.56
1.07
(0.01) (0.01) (0.01) (0.03) (0.08) (0.26) (1.20) (0.31) (0.21)
On excitation at the red edge of the absorption band (340 nm excitation) and monitoring at 450 nm emission band, the decay time values decrease and amplitude corresponding to shorter decay component increases (Table 1). This may result due to direct excitation of the species responsible for shorter decay ( 5 ns) time [10]. For higher TMA concentration (0.798 M), when the decay is collected at 500 nm emission wavelength and excited by the 310 nm, the decay fits with monoexponential function with a decay time of 12:21 0:04 ns (Fig. 4). The decay curve for the various concentrations of TMA at 440 nm emission are analysed, keeping the longer decay time fixed at 12 ns and the decay times are listed in Table 2. It can be seen that the smaller decay time ( 5 ns) component also remains unchanged for the various concentration of TMA. At this emission the major contribution to intensity is due to smaller decay time component. In toluene, on the other hand, the decay fits with a biexponential function even in the absence of TMA ðs1 ¼ 0:21 0:03 and s2 ¼ 0:86 0:03 ns) with higher amplitude corresponding to shorter decay component (56:44). It was difficult to ascertain it to some aggregate formation in toluene because of low solubility of 6-HQ in toluene. The decay parameters for 0.018 M of TMA concentration in toluene at various emission wavelengths are given in Table 3. The decays at 330 nm (blue edge) and 390 nm (towards red) of the normal emission band show good
Fig. 4. Monoexponential fitted decay curve of 6-HQ in acetonitrile (0.798 M of TMA), excited by 310 nm and emission monitored at 500 nm. Instrumental response function is also shown.
fit with biexponential function whereas at higher emission wavelengths (450 and 470 nm) the decay shows monoexponential fit with a decay time of 5 ns. In addition, the decay parameters (excited by
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Table 2 Decay times and amplitudes of 6-HQ-TMA system in acetonitrile for various concentration of TMA at room temperature (kexc ¼ 310 nm and kem ¼ 440 nm) Conc. of TMA (M)
s1 fixed (ns)
s2 and Stand. dev. (ns)
a1
a2
v2
0.048 0.198 0.498 0.798
12.00 12.00 12.00 12.00
4.82 4.99 4.98 4.99
09.43 07.69 06.92 07.96
90.57 92.31 93.08 92.04
1.06 1.13 1.17 1.15
(0.04) (0.04) (0.04) (0.03)
Table 3 Decay times and amplitudes for the 6-HQ-TMA system in toluene (0.018 M, TMA) at room temperature (kexc ¼ 310 nm) kem (nm)
s1 and Stand. dev. (ns)
s2 and Stand. dev. (ns)
a1
a2
v2
330 350 370 390 410 430 450 470
0.34 0.44 0.60 0.62 0.80 0.74 – –
1.37 2.11 4.87 5.05 5.08 5.06 5.05 5.06
85.26 86.81 63.62 22.04 7.86 2.74 – –
14.74 13.79 36.38 77.96 92.14 97.26 100.00 100.00
1.04 1.29 1.19 0.98 1.05 1.13 1.25 1.19
(0.01) (0.01) (0.01) (0.04) (0.01) (0.20)
(0.10) (0.10) (0.07) (0.03) (0.03) (0.02) (0.01) (0.01)
310 nm) for various concentration of TMA at both the emission bands (359 and 417 nm) are also recorded. The smaller decay time corresponding to normal emission for the various concentration of TMA is almost constant (s 0:3 ns). At higher concentration of TMA (0.048 M), even at 350 nm emission wavelength (normal band), the decay shows poor fitting with biexponential function and fits well with triple exponential function with decay times s1 ¼ 0:23 0:03 ns; s2 ¼ 1:01 0:09 and s3 ¼ 5:65 0:5 ns (the amplitude corresponding to
s3 component is small). On increasing the concentration of TMA, the contribution of s3 amplitude increases regularly as expected. However, for emission wavelength above 400 nm (kexc ¼ 310 nm) the decay shows biexponential fit. Decay parameters at 410 nm emission for the different concentration of TMA are given in Table 4. It can be seen that the longer decay component (5 ns) having higher amplitude is constant for the different TMA concentrations studied. When excited by red edge of the absorption band (340 nm) the decay
Table 4 Decay times and amplitudes of 6-HQ-TMA system in toluene for various concentration of TMA at room temperature (kexc ¼ 310 nm and kem ¼ 410 nm) Conc. of TMA (M)
s1 and Stand. dev. (ns)
s2 and Stand. dev. (ns)
a1
0.003 0.018 0.048 0.198
0.35 0.80 0.99 1.10
5.05 5.08 5.08 5.05
13.05 7.86 5.69 5.07
(0.05) (0.01) (0.20) (0.20)
For (kexc ¼ 340 nm and kem ¼ 450 nm) 0.198 –
(0.02) (0.03) (0.03) (0.03)
5.11 (0.01)
–
a2
v2
86.95 92.14 94.31 94.93
1.26 1.05 0.99 1.07
100.00
1.28
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shows monoexponential fit with decay time 5.11 ns. This decay time can be attributed to the complex in the form of contact ion-pair (discussed later). In alcoholic solvents, on the other hand, no change in the fluorescence characteristics is observed even in presence of sufficient amount of TMA concentration, and no emission corresponding to complex is observed. This fact was explained on the basis of the stronger hydrogenbonding nature of the alcohols as discussed in an earlier report [27]. It can also be mentioned here that in case of 2-naphthol and 6-methoxyquinoline, the quenching behaviour was found to be dynamic in nature in the presence of guest quenchers, e.g., TEA and chloride ion [11,30,31] unlike in the present case. In the above measurement, TMA is chosen as a guest molecule and possesses a proton-accepting nature. The 1:1 6-HQ/TMA complex formed are shown below:
the presence of the longer decay component (12 ns) and the broad emission spectrum reveals that the complex is also transformed to solvent separated ion-pairs. In fact the decay time of the anion is very close to this longer decay component [7] which is observed here for solvent separated ions. The absence of the rise time component in the decay indicates that the formation of contact ion-pair and solvent separated ion-pair from the H-bonded complex is very fast.
From the above results, it is evident that there are two kinds of absorbing species of 6-HQ viz. normal form and ground state 1:1 hydrogenbonded complex both in ACN and toluene. It is likely that in the excited state this H-bonded complex may be present as contact ion-pair in non-polar as well as polar solvents because of very high photoacidity of the hydroxylic proton [7]. In polar solvent (ACN), on the other hand,
solvent both contact ion-pair and solvent separated ion-pair are formed, leading to red shifted emission. Presence of contact ion-pair as well as solvent separated ion-pair leads to the broadening of the emission band in ACN. In both the solvents the contact ion-pair has a decay time value 5 ns. The longest decay time (12 ns), which is observed at longer emission wavelengths in ACN, can be attributed to solvent separated ions.
4. Conclusions It is observed that 6-HQ with TMA formed 1:1 complex both in polar acetonitrile and non-polar toluene in the ground state. This complex undergoes strong charge transfer and subsequent proton transfer in the excited state and leads to contact ion-pair formation in non-polar toluene. In polar
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Acknowledgements UGC and DST New Delhi are acknowledged for providing necessary experimental facilities. References [1] H. Leonhartd, A. Weller, Ber. Bunsenges. Phys. Chem. 67 (1963) 791. [2] I.G. Ochoa, P.B. Bisht, F. Sanchez, E. M-Ataz, L. Santos, H.B. Tripathi, A. Douhal, J. Phys. Chem. A 102 (1998) 8871. [3] M.S. Mehata, H.C. Joshi, H.B. Tripathi, J. Lumin. 93 (2001) 275. [4] M.S. Mehata, H.C. Joshi, H.B. Tripathi, Spectrochim. Acta Part A 58 (2002) 1589. [5] M.S. Mehata, H.B. Tripathi, Ind. J. Phys. 75 (2001) 189. [6] T.G. Kim, Y. Kim, D.J. Jang, J. Phys. Chem. A 105 (2001) 4328. [7] E. Bardez, A. Chatelain, B. Larrey, B. Valeur, J. Phys. Chem. 98 (1994) 2357. [8] A. Matsuzaki, S. Nagakura, K. Yoshihara, Bull. Chem. Soc. Jpn. 47 (1974) 1152. [9] A.E.W. Knight, B.K. Selinger, Chem. Phys. Lett. 10 (1971) 43, and references therein. [10] P.B. Bisht, G.C. Joshi, H.B. Tripathi, D.D. Pant, Chem. Phys. Lett. 142 (1987) 291. [11] P.B. Bisht, H.B. Tripathi, J. Lumin. 55 (1993) 153. [12] P.B. Bisht, H.B. Tripathi, D.D. Pant, Chem. Phys. 147 (1990) 173. [13] L. Biczok, T. Berces, F. Marta, J. Photochem. Photobiol. 48 (1989) 265.
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[14] N. Mataga, Y. Torihoshi, Y. Kaifu, Z. Phys. Chem. 34 (1962) 379. [15] M. Martin, W.R. Ware, J. Phys. Chem. 82 (1978) 2770. [16] M. Martin, H. Miyasaka, A. Karen, N. Mataga, J. Phys. Chem. 89 (1985) 182. [17] K. Kikachi, H. Watarai, M. Koizumi, Bull. Chem. Soc. Jpn. 46 (1973) 749. [18] D. Rehm, A. Weller, Isr. J. Chem. 8 (1970) 259. [19] P.B. Bisht, H.B. Tripathi, D.D. Pant, J. Photochem. Photobiol. A Chem. 58 (1991) 295. [20] N. Mataga, Y. Kaifu, Mol. Phys. 7 (1963) 137. [21] P.T. Chou, C.Y. Wei, C.R.C. Wang, F.T. Hung, C.P. Chang, J. Phys. Chem. A 103 (1999) 1939. [22] C.Y. Wei, W.S. Yu, P.T. Chou, F.T. Hung, C.P. Chang, T.C. Lin, J. Phys. Chem. B 102 (1998) 1053. [23] S.F. Mason, J. Philp, B.E. Smith, J. Chem. Soc. A (1968) 3051. [24] H. Yu, H.J. Kwon, D.J. Jang, Bull. Korean Chem. Soc. 18 (1997) 156. [25] A. Bach, J. Hewel, S. Leutwyler, J. Phys. Chem. 102 (1998) 10476. [26] E. Bardez, I. Devol, A. Chatelain, J. Colloid Interf. Sci. 205 (1998) 178. [27] M.S. Mehata, H.C. Joshi, H.B. Tripathi, Chem. Phys. Lett. 359 (2002) 314. [28] M.S. Mehata, H.C. Joshi, H.B. Tripathi, Spectrochem. Acta Part A (2002), in press. [29] M. Belletete, G. Lessard, J. Richer, G. Durocher, J. Lumin. 34 (1986) 279. [30] J.R. Lakowicz, Principle of Fluorescence Spectroscopy, Plenum Press, New York, London, 1983. [31] M.S. Mehata, H.B. Tripathi, J. Lumin. 99 (2002) 47. [32] C.D. Geddes, Meas. Sci. Technol. 12 (2001) R53.
Complexation of 6-hydroxyquinoline with trimethylamine in polar and non-polar solvents M.S. Mehata *, H.C. Joshi 1, H.B. Tripathi Photophysics Laboratory, Department of Physics, DSB Campus, Kumaon University, Nainital 263002, India Received 13 June 2002; in final form 27 September 2002
Abstract Spectral characteristics of 6-hydroxyquinoline (6-HQ) in presence of trimethylamine (TMA) were investigated in polar and non-polar solvents. The steady-state absorption, emission and excitation spectra along with the transient parameters reveal a strong ground state hydrogen-bonded complex formation between the 6-HQ and TMA molecules in both the media. A large Stokes shifted emission due to the formation of contact ion-pairs is observed in these media. However, in acetonitrile the longer decay time ( 12 ns) with relatively broadened emission spectra can be attributed to the presence of solvent separated ion-pairs in addition to contact ion-pairs. The ground state equilibrium constant for complex formation has been determined. The observed quenching behaviour of the fluorescence emission from the normal molecule with TMA appears to be static in nature. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction It is well known that hydrogen-bonding interactions can affect the photophysical properties of the organic molecules [1–10] and can provide a pathway for fluorescence quenching [1]. When two conjugate p electron systems are directly combined by hydrogen bonding, the fluorescence of the proton donor or the acceptor is strongly quenched [9–13] which has been explained by charge transfer (CT) interaction between the donor and acceptor p
*
Corresponding author. Fax: +91-5942-35576. E-mail address: [email protected] (M.S. Mehata). 1 Present address: Institute for Plasma Research, Bhat, Gandhinagar 382428, India.
electron system [14–16] via the hydrogen bond as well as hydrogen atom transfer [17,18]. Intermolecular charge transfer interaction can result in the formation of ground state (hydrogen bonded) donor–acceptor (GDA) complex or one hetero excimer (EDA) in the excited state. Fluorescence quenching is often accompanied by the appearance of a new fluorescence emission, usually attributed to an exciplex [19]. Ambiguity persists in the interpretation of the observed data as excitation of both GDA and EDA complexes can emit at common wavelength. Moreover, it is possible that the species formed in the excited state via these two routes may exhibit different behaviour. However, in many kinetic treatments the two pathways of formation of an excited CT state have been found to be identical [8,20].
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 1 5 7 9 - 8
M.S. Mehata et al. / Chemical Physics Letters 366 (2002) 628–635
Recently, Chou et al. [21,22] have reported the single and double hydrogen-bonded complexes of 3-hydroxyisoquinoline (3-HIQ) and 7-hydroxyquinoline (7-HQ) with trimethylamine (TMA) triethylamine (TEA) and acetic acid, respectively, and also their self-association. They have tentatively suggested that the single hydrogen-bonded complex of 3-HIQ and TMA being highly endergonic, i.e., enol/TMA to keto/TMA reaction is not possible. A comprehensive study of the fluorescence and prototropic equlibria of 6-HQ in different media has been reported in literature [6,7,23–25]. Due to the extremely high simultaneous photoacidity and photobasicity in the S1 (excited) state, 6-HQ undergoes deprotonation and protonation in 10 M HClO4 and 12 M NaOH, respectively [7]. Comparative studies of 6-HQ and 8-HQ with the Aerosol-OT surfactant, reveal that 6-HQ is unable to form dimers whereas 8-HQ undergoes dimerization [26]. In earlier reports [27,28] we have proposed a mechanism of proton transfer for 6-HQ in methanol in presence of water molecules and also in Nafionâ polymer matrix. Here, we report the fluorescence characteristic of 6-HQ-trimethylamine (TMA) system at room temperature in polar and non-polar solvents.
2. Experimental details 6-HQ (Aldrich) was recrystallized from ether. All the solvents used were of spectroscopic grade. The purity of 6-HQ was checked from thin layer chromatography and the fluorescence excitation spectrum in different solvents. TMA (solid) was taken from Aldrich. The concentration of the sample used in our experiments was 105 M. The details of experimental measurements are same as given in earlier papers [3,4,27,28].
3. Results and discussion Figs. 1 and 2A,B show the absorption, emission and excitation spectra, respectively, of 6-hydroxyquinoline (6-HQ) in polar solvent acetoni-
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Fig. 1. Absorption spectra of 6-HQ in acetonitrile for various concentration of TMA (M); for 0.0 (a), 0.003 (b), 0.018 (c), 0.048 (d), 0.198 (e), 0.498 (f), and 0.798 (g).
trile (ACN), and with addition of varying concentration of trimethylamine (TMA). Fig. 1a shows that the lowest absorption band has maximum at 332 nm. With the addition of TMA a significant change in absorption spectrum is observed; the absorption band is red shifted and broadened with an increase in the absorbance which finally gives rise to a single band ðkmax 337:5 nmÞ at 0.798 M concentration of TMA. In toluene, a non-polar solvent, however, the shift is negligible but a small change in emax and the appearance of a shoulder in a region 340–350 nm is observed (not shown). Isosbestic points in both the solvents are obtained which indicates the formation of ground state hydrogen-bonded complex. The values of the ground state equilibrium constant (Kg ) were calculated to be 40:0 and 8.0 M1 in acetonitrile and in toluene, respectively, following Belletele et al. [29]. The changes in the fluorescence emission spectra of 6-HQ caused by the addition of TMA concentration in ACN are shown in Fig. 2A. It can be seen that the intensity of normal emission band (363 nm) of 6-HQ decreases continuously with increasing concentration of TMA in ACN, a new broad emission band with Stokes shift 7390 cm1 appears at 440 nm with full-width at
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M.S. Mehata et al. / Chemical Physics Letters 366 (2002) 628–635
Fig. 2. (A) Fluorescence emission spectra of 6-HQ in acetonitrile for various concentrations of TMA (M); for 0.0 (a), 0.003 (b), 0.018 (c), 0.048 (d), 0.198 (e), 0.498 (f) and 0.798 (g); excited by 310 nm wavelength. (B) Normalized excitation spectra of 6-HQ for 0.018 M TMA monitored at emission wavelengths 350 nm (a) and 450 nm (b) in acetonitrile.
half maximum (FWHM) 3830 cm1 . Whereas in toluene, where the solute–solvent interactions are comparatively low, the normal emission is observed at 359 nm and a new broad emission band at 417 nm with a relatively less Stokes shift (6090 cm1 ) and FWHM (3400 cm1 ) appears on addition of TMA. These emission bands which arise due to complex formation in both the solvents become predominant with decrease in the intensity of normal band as the concentration of TMA is increased. Especially in toluene, the intensity of complex emission (for max conc. of TMA) is approximately eight times higher than the intensity of normal emission (TMA free solution). Isoemissive points in the emission spectra are noticed in ACN (400 nm) and in toluene (370 nm) which indicate the existence of more than one emitting species. Further, the emission bands corresponding to complex show higher intensity on red edge excitation compared to excitation at the blue edge.
Excitation spectra are recorded for different TMA concentrations in both the solvents. For TMA free 6-HQ the excitation spectrum is almost identical to the observed absorption spectrum within experimental limits. However, addition of 0.018 M of TMA in 6-HQ solution in ACN the excitation spectra for 350 and 450 nm emissions are not identical and show a shift of about 792 cm1 (Fig. 2B). A similar situation is observed in toluene where the shift is found to be 785 cm1 . It can be mentioned that the shift observed in the excitation spectra in polar and non-polar solvents are nearly identical. These spectra confirm the formation of ground state hydrogen-bonded complex (GHBC) between 6-HQ and TMA in both the solvents. 3.1. Decay parameters The fluorescence decay for the normal emission (350 nm) of 6-HQ in neat ACN fits with a
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Fig. 3. Fitted decay curves of 6-HQ-TMA in acetonitrile (kexc ¼ 310 nm): (a) Monoexponential fit for free (0.0 TMA) at 350 nm; Biexponential fit for 0.018 M TMA at (b) 410 nm and (c) 490 nm with the value of v2 and residuals (a), (b) and (c), respectively; (b0 ) shows residuals and v2 value for monoexponential fit at 410 nm.
monoexponential function (Fig. 3a) with decay time of 1.6 ns. To explore the fluorescence quenching mechanism, the decay data are recorded at normal emission band (350 nm), for 310 nm excitation with varying TMA concentrations in ACN. It is observed that the decay time of the normal molecule does not follow the same trend as shown by intensity variation (decrease in the intensity of normal emission) and is constant for the entire concentration of TMA studied in this work. The results indicate that quenching is static rather than dynamic in nature [30–32] and hence gives evidence of the complex formation between the 6-HQ and TMA molecules. The transient parameters for 0.018 M of TMA concentration monitored across the emission profile are given in Table 1. It is observed that between 330 and 370 nm
emission regions, the decay shows monoexponential fit. Above 370 nm a clear departure from monoexponential is noticed and the decay fits with biexponential function (Fig. 3b). The amplitude corresponding to shorter component decreases and for longer one increases while monitoring towards the longer wavelength side of the emission spectrum. However, above 430 nm emission where the smaller component (1.6 ns) corresponding to normal emission is absent the decay again fits to a biexponential function and another longer decay component having decay time 12 ns appears (Fig. 3c). The amplitude of this longer decay component shows increase at higher emission wavelength. No rise time could be detected for longer wavelength emission with the present experimental setup.
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Table 1 Decay times and amplitudes for the 6-HQ-TMA system in acetonitrile (0.018 M, TMA) at room temperature kem (nm)
s1 and Stand. dev. (ns)
s2 and Stand. dev. (ns)
a1
a2
v2
For kexc ¼ 310 nm 330 350 370 390 410 430 450 470 490
1.60 1.65 1.68 1.80 2.07 2.09 11.20 11.60 12.20
– – – 7.03 6.04 5.93 5.64 5.43 6.19
(0.05) (0.07) (0.05) (0.11) (0.15) (0.35)
100.00 100.00 100.00 82.11 29.29 09.64 12.33 45.49 73.12
– – – 17.89 70.11 90.36 87.67 54.51 26.88
1.17 1.22 1.16 1.09 1.00 0.94 1.19 0.79 0.92
For kexc ¼ 340 nm 450
10.06 (0.6)
4.84 (0.09)
20.44
79.56
1.07
(0.01) (0.01) (0.01) (0.03) (0.08) (0.26) (1.20) (0.31) (0.21)
On excitation at the red edge of the absorption band (340 nm excitation) and monitoring at 450 nm emission band, the decay time values decrease and amplitude corresponding to shorter decay component increases (Table 1). This may result due to direct excitation of the species responsible for shorter decay ( 5 ns) time [10]. For higher TMA concentration (0.798 M), when the decay is collected at 500 nm emission wavelength and excited by the 310 nm, the decay fits with monoexponential function with a decay time of 12:21 0:04 ns (Fig. 4). The decay curve for the various concentrations of TMA at 440 nm emission are analysed, keeping the longer decay time fixed at 12 ns and the decay times are listed in Table 2. It can be seen that the smaller decay time ( 5 ns) component also remains unchanged for the various concentration of TMA. At this emission the major contribution to intensity is due to smaller decay time component. In toluene, on the other hand, the decay fits with a biexponential function even in the absence of TMA ðs1 ¼ 0:21 0:03 and s2 ¼ 0:86 0:03 ns) with higher amplitude corresponding to shorter decay component (56:44). It was difficult to ascertain it to some aggregate formation in toluene because of low solubility of 6-HQ in toluene. The decay parameters for 0.018 M of TMA concentration in toluene at various emission wavelengths are given in Table 3. The decays at 330 nm (blue edge) and 390 nm (towards red) of the normal emission band show good
Fig. 4. Monoexponential fitted decay curve of 6-HQ in acetonitrile (0.798 M of TMA), excited by 310 nm and emission monitored at 500 nm. Instrumental response function is also shown.
fit with biexponential function whereas at higher emission wavelengths (450 and 470 nm) the decay shows monoexponential fit with a decay time of 5 ns. In addition, the decay parameters (excited by
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Table 2 Decay times and amplitudes of 6-HQ-TMA system in acetonitrile for various concentration of TMA at room temperature (kexc ¼ 310 nm and kem ¼ 440 nm) Conc. of TMA (M)
s1 fixed (ns)
s2 and Stand. dev. (ns)
a1
a2
v2
0.048 0.198 0.498 0.798
12.00 12.00 12.00 12.00
4.82 4.99 4.98 4.99
09.43 07.69 06.92 07.96
90.57 92.31 93.08 92.04
1.06 1.13 1.17 1.15
(0.04) (0.04) (0.04) (0.03)
Table 3 Decay times and amplitudes for the 6-HQ-TMA system in toluene (0.018 M, TMA) at room temperature (kexc ¼ 310 nm) kem (nm)
s1 and Stand. dev. (ns)
s2 and Stand. dev. (ns)
a1
a2
v2
330 350 370 390 410 430 450 470
0.34 0.44 0.60 0.62 0.80 0.74 – –
1.37 2.11 4.87 5.05 5.08 5.06 5.05 5.06
85.26 86.81 63.62 22.04 7.86 2.74 – –
14.74 13.79 36.38 77.96 92.14 97.26 100.00 100.00
1.04 1.29 1.19 0.98 1.05 1.13 1.25 1.19
(0.01) (0.01) (0.01) (0.04) (0.01) (0.20)
(0.10) (0.10) (0.07) (0.03) (0.03) (0.02) (0.01) (0.01)
310 nm) for various concentration of TMA at both the emission bands (359 and 417 nm) are also recorded. The smaller decay time corresponding to normal emission for the various concentration of TMA is almost constant (s 0:3 ns). At higher concentration of TMA (0.048 M), even at 350 nm emission wavelength (normal band), the decay shows poor fitting with biexponential function and fits well with triple exponential function with decay times s1 ¼ 0:23 0:03 ns; s2 ¼ 1:01 0:09 and s3 ¼ 5:65 0:5 ns (the amplitude corresponding to
s3 component is small). On increasing the concentration of TMA, the contribution of s3 amplitude increases regularly as expected. However, for emission wavelength above 400 nm (kexc ¼ 310 nm) the decay shows biexponential fit. Decay parameters at 410 nm emission for the different concentration of TMA are given in Table 4. It can be seen that the longer decay component (5 ns) having higher amplitude is constant for the different TMA concentrations studied. When excited by red edge of the absorption band (340 nm) the decay
Table 4 Decay times and amplitudes of 6-HQ-TMA system in toluene for various concentration of TMA at room temperature (kexc ¼ 310 nm and kem ¼ 410 nm) Conc. of TMA (M)
s1 and Stand. dev. (ns)
s2 and Stand. dev. (ns)
a1
0.003 0.018 0.048 0.198
0.35 0.80 0.99 1.10
5.05 5.08 5.08 5.05
13.05 7.86 5.69 5.07
(0.05) (0.01) (0.20) (0.20)
For (kexc ¼ 340 nm and kem ¼ 450 nm) 0.198 –
(0.02) (0.03) (0.03) (0.03)
5.11 (0.01)
–
a2
v2
86.95 92.14 94.31 94.93
1.26 1.05 0.99 1.07
100.00
1.28
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shows monoexponential fit with decay time 5.11 ns. This decay time can be attributed to the complex in the form of contact ion-pair (discussed later). In alcoholic solvents, on the other hand, no change in the fluorescence characteristics is observed even in presence of sufficient amount of TMA concentration, and no emission corresponding to complex is observed. This fact was explained on the basis of the stronger hydrogenbonding nature of the alcohols as discussed in an earlier report [27]. It can also be mentioned here that in case of 2-naphthol and 6-methoxyquinoline, the quenching behaviour was found to be dynamic in nature in the presence of guest quenchers, e.g., TEA and chloride ion [11,30,31] unlike in the present case. In the above measurement, TMA is chosen as a guest molecule and possesses a proton-accepting nature. The 1:1 6-HQ/TMA complex formed are shown below:
the presence of the longer decay component (12 ns) and the broad emission spectrum reveals that the complex is also transformed to solvent separated ion-pairs. In fact the decay time of the anion is very close to this longer decay component [7] which is observed here for solvent separated ions. The absence of the rise time component in the decay indicates that the formation of contact ion-pair and solvent separated ion-pair from the H-bonded complex is very fast.
From the above results, it is evident that there are two kinds of absorbing species of 6-HQ viz. normal form and ground state 1:1 hydrogenbonded complex both in ACN and toluene. It is likely that in the excited state this H-bonded complex may be present as contact ion-pair in non-polar as well as polar solvents because of very high photoacidity of the hydroxylic proton [7]. In polar solvent (ACN), on the other hand,
solvent both contact ion-pair and solvent separated ion-pair are formed, leading to red shifted emission. Presence of contact ion-pair as well as solvent separated ion-pair leads to the broadening of the emission band in ACN. In both the solvents the contact ion-pair has a decay time value 5 ns. The longest decay time (12 ns), which is observed at longer emission wavelengths in ACN, can be attributed to solvent separated ions.
4. Conclusions It is observed that 6-HQ with TMA formed 1:1 complex both in polar acetonitrile and non-polar toluene in the ground state. This complex undergoes strong charge transfer and subsequent proton transfer in the excited state and leads to contact ion-pair formation in non-polar toluene. In polar
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