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Twisted intramolecular charge transfer of methyl p-dimethylaminobenzoate in aqueous β-cyclodextrin solution
SpectrochimicaAeta, Vol. 51A, No. 2, pp. 275-282, 1995
Pergamon 0584-8539(94)E0006-V
Copyright (~) 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0584-8539/95 $9.50 + 0.00
Twisted intramolecular charge transfer of methyl p-dimethylaminobenzoate in aqueous ~-cyciodextrin solution Y U N - B A O JIANG Department of Chemistry and Institute of Analytical Chemistry, Xiamen University, Xiamen 361005, People's Republic of China
(Received 1 November 1993; in final form 20 December 1993; accepted 22 December 1993) Abstract--This paper reports the investigation of the twisted intramolecular charge transfer (TICT) of methyl p-dimethylaminobenzoate (MDMAB) in aqueous fl-cyclodextrin (fl-CD) solution by the TICT-typical dual fluorescence. In pure water, MDMAB emits only LE fluorescence, and the TICT fluorescence band is developed with the addition of fl-CD. Both the LE and TICT fluorescence bands are continuously enhanced upon the increase of fl-CD concentration. The intensity ratio of the TICT band to the LE band shows a hillshaped dependence on fl-CD concentration, and a blue shift in both TICT and LE bands is observed with the increase of fl-CD concentration, of which the blue shift in TICT band is more appreciable. Formation of a 1 : 1 MDMAB-fl-CD inclusion complex, with an association constant of 580 _+801 mol- t, is evaluated. The effect of fl-CD on TICT of MDMAB is discussed, with consideration of the fact that aqueous fl-CD solution is pseudo° aquaorganic binary mixture and that TICT in aqueous solution acts differently than in organic solvents. A comparison is made between the TICT of MDMAB and of DMABN in aqueous fl-CD solution.
1. INTRODUCTION
TWISTED intramolecular charge transfer (TICT) [1-3] in aqueous cyclodextrin (CD) solutions for p-dimethylaminobenzonitrile (DMABN) has been examined in two capacities: (a) the TICT photophysics as a tool to understand the microenvironment of the CD cavity [4]; and (b) the influence of the microenvironment of the CD cavity on TICT photophysics [5-7]. TICT of alkyl p-dimethylaminobenzoate is easier than that of D M A B N in organic solvents [2, 8] and supercritical fluids [9], which is evidenced by the fact that the former emits a relatively stronger TICT fluorescence even in nonpolar solvents. Recently, we have shown that, for p-dimethylaminobenzaldehyde (DMABA) and p-dimethylaminobenzoic acid (DMABOA), the medium polarity correlation of the intensity ratio of the TICT band to the LE band, la/lb, is opposite in aqueous micellar and cyclodextrin solutions [10-16] to that in pure organic solvents [17, 18], i.e. enhanced Ia/Ib is observed on decreasing the medium polarity [10-16]. A similar situation was reported for alkyl p-dimethylaminobenzoate in aquaorganic binary mixtures [19]. Therefore, it was argued that the TICT in aqueous medium may be somewhat different from that in organic solvent, and that the difference is due to the high polarity of water compared with that of common organic solvents [10-16]. One must then wonder about the TICT of alkyi p-dimethylaminobenzoate in aqueous CD solution compared with that of DMABN. To answer this question, the TICT of alkyl p-dimethylaminobenzoate needs firstly to be examined in aqueous CD solution. In the present paper, we describe the TICT behavior of methyl p-dimethylaminobenzoate (MDMAB) in aqueous fl-CD solution as monitored by the TICT-typical dual fluorescence and conduct a comparison between the TICT behavior of MDMAB and DMABN.
2. EXPERIMENTAL M D M A B was s y n t h e s i z e d according to REVILL a n d BROWN [20] a n d purified by d o u b l e recrystallization f r o m a b s o l u t e ethyl alcohol, fl-CD was used as received from Fluka. T w i c e - d e i o n i z e d w a t e r was used for all m e a s u r e m e n t s . F l u o r e s c e n c e spectra of M D M A B were r e c o r d e d on a Hitachi 650-10S s p e c t r o f l u o r o m e t e r . T h e excitation w a v e l e n g t h was 280 n m a n d the slits for excitation a n d emission m o n o c h r o m a t o r s were 6 a n d 4 n m , respectively. Total fluorescence intensity was o b t a i n e d by weighing the p a p e r c o v e r e d by 275
276
Y.-B. JIANG
8 e-
g
320
360 400 440 480 Wavelength (nm)
520
Fig. l. Fluorescence spectra of M D M A B in a q u e o u s fl-CD solutions. M D M A B concentration is 2.5 x 10 -5 mol 1-I. fl-CD concentrations are 0 (a), 2.0 x 10 4 (b), 4.0 x 10 -4 (c), 8.0 x 10 4 (d), 1 . 5 x 10 3 (e), 2.5 × 10-3 (f), 3 . 5 × 10 3 (g), 4.5 × 10 3 (h), 6 . 0 × 10 -3 (i), and 8 . 0 × l0 3 m o l l (j), respectively.
the emission spectrum. The G-factor correction [21] was adopted in the determination of the fluorescence polarization.
3.
RESULTS AND DISCUSSION
Shown in Fig. 1 are the fluorescence spectra of the aqueous solution containing 2 . 5 x 10-Smoll -~ M D M A B and different concentrations of fl-CD. In pure water, M D M A B emits only LE fluorescence at short wavelength. The TICT fluorescence band at longer wavelengths is developed upon the addition of fl-CD. With increase in fl-CD concentration, the LE and TICT fluorescence bands are enhanced and shifted to blue in different rates, indicative of the formation of an MDMAB-fl-CD inclusion complex [22], i.e. the penetration of M D M A B into the nonpolar fl-CD cavity. The blue shifting in the T1CT band is more appreciable than that in the LE band, which is a reflection of the high polarity of the TICT state [1, 2]. The rate of enhancement of TICT fluorescence by the presence of fl-CD is initially higher than that of the LE band and becomes lower when the fl-CD concentration is 1.5 x 10 ~mol l- L, although the total fluorescence intensity, If, is progressively increased, as can be easily seen in Fig. 2. Figure 2 plots the intensity ratio of the TICT band to the LE band, Ia/Ib, and /f, of MDMAB vs fl-CD concentration. Similar fl-CD concentration dependence of Ia/Ib has been reported for D M A B N [6] and D M A B A [10]. For the D M A B A - f l - C D system [10], the opposite fl-CD concentration dependence of lflIb was attributed to the different stoichiometries in the formed inclusion complex between D M A B A and fl-CD. The positive dependence is related to the formation of the 1 : 1 inclusion complex, while the negative dependence is due to the formation of a 1 : 2 complex. Unfortunately, the results displayed in Fig. 3 suggest that this is not the case in the present system. In Fig. 3, the fluorescence intensity data of the present system are shown processed against fl-CD concentration by two different methods [23, 24], including double reciprocal plotting [24]. Both methods yield one straight line, indicating that only the 1:1 MDMAB-fl-CD inclusion complex is formed. The obtained association
TICT of M D M A B
277
1.6 1.4 1.2
~
1.0
10
0.8
8
0.6
6
0.4
/
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0.2 I
0.2
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0.4
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0.8
I
1
1.0
I
1.2
I
1.4
o 2.0
I
1.6
l.S
10 2 [~-CD] (mol 1"1) Fig. 2. la/l b (a) and If (b) of M D M A B in aqueous solution as a function of fl-CD concentration. As a comparison, la/Ib data of D M A B N in aqueous fl-CD solutions (c), taken from Ref. [6], are also given.
constant of the inclusion complex, 580+801 mol -l, is compatible with that of the DMABN-fl-CD inclusion complex (440 + 1001 mol- 1) [4]. NAG et al. [6] have given an explanation for TICT behavior in the DMABN-fl-CD system in terms of different forms of the 1 : 1 DMABN-fl-CD inclusion complex. The enhanced TICT fluorescence was said to be due to DMABN molecules that are partially inside the fl-CD cavity, and that thus experience a higher polarity; while the enhanced LE fluorescence is due to DMABN molecules that are totally inside the fl-CD cavity. Such an explanation, although it seems reasonable on the basis of the medium-polarity controlled TICT process [17, 18], cannot explain the fact that the enhancement of TICT fluorescence is accompanied by a blue shift in the TICT band in the DMABN-fl-CD system [6]. This explanation is, at least, not applicable in the present system because of the same contradiction. REVILE and BROWN [20] have attributed the long wavelength emission of MDMAB in a polar organic solvent acetonitrile to the excited dimer. This is unlikely in the present aqueous MDMAB-fl-CD solution, because, if this is the case, the formation of 1:1
I/[IS-CD] (I mol l ) 5OOO o
400
800
1200
1600
2000
2400
I
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4
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If Fig. 3. Plot of (ldlo)/[fl-CD[ versus If (a) and double reciprocal plot (b) of M D M A B - f l - C D system, where 10 refers to the relative fluorescence intensity in the absence of fl-CD.
278
Y.-B. JIANG 0.15 - 0 0 0 0 0
000
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Wavelength ( n m ) Fig. 4. Fluorescence polarization emission spectra of MDMAB in the presence of 0 ( a ) , 4 . 0 x 10 4 ( b ) , 1.0 x 10 -3 (c), 5 . 0 x 10 -3 ( d ) , a n d 9 . 0 x 10 -3 m o l I ' (e) f l - C D , respectively.
M D M A B - f l - C D inclusion complexes will hinder the ground-state dimerization of M D M A B and in turn weaken its emission. This is opposite to the experimental results shown in Figs 1 and 2. Also, the blue shift in the long-wavelength band in the presence of fl-CD cannot be in the line of the excited dimer since no charge transfer takes place in the excited dimer. Figure 4 shows the fluorescence polarization emission spectra of the M D M A B - f l - C D system, which also indicates that the long wavelength emission is not due to the excited dimer. The fluorescence polarization emission spectrum of M D M A B in pure water has only one plateau, while two plateaux are observed in those of M D M A B in aqueous fl-CD solutions. Because fluorescence polarization is independent of emission wavelength [21], the polarization spectra in Fig. 4 should suggest that only one emissive excited state is present for M D M A B in pure water, while two emissive excited states, corresponding to the dual fluorescence bands in Fig. 1, are present for M D M A B in fl-CD solutions, of which the excited state emitted at longer wavelengths possesses a lower polarization. If the long-wavelength emission of M D M A B is due to the excited dimer, its polarization should be larger than that of the LE state because of the larger volume [21]; unfortunately, this is not the fact. On the other hand, attributing the long-wavelength emission to the TICT state is reasonable. Because the angle shifting between absorption and emission dipoles in the TICT state is larger than that in the LE state, the TICT state possesses lower polarization [10, 21]. We therefore present an alternative explanation for the fl-CD concentration dependent la/Ib of M D M A B in aqueous solution from the viewpoint of the solvent polarity effects on the TICT emission [25]. It can be derived from Refs [17, 18, 25] that, with the increase of solvent polarity, the rate of TICT state formation and in turn the yield of TICT emission increase, while the yield of LE emission automatically decreases because TICT is the main nonradiative pathway in the LE state. As a consequence, the ratio la/lb increases. At the same time, the nonradiative rates of the TICT state also increase, which causes the decrease of the yield of TICT emission and thus the decrease of the la/lh. Obviously, the two factors compete with each other. The fact that the IJA, of the TICT fluorophore in organic solvents increases with the solvent polarity indicates that the former factor is predominant, whereas in aqueous solution, e.g. the present system, the case should be controlled by the latter one due to the opposite correlation of l,,/lb with
TICT of MDMAB
279
the medium polarity, which means that the TICT in aqueous solution is different from that in organic solvents [10-16]. Comparison of the TICT behaviors of MDMAB and of DMABN in aqueous fl-CD solution by the ratio lallb would strengthen the above conclusion. It is clear in Fig. 2 that, when fl-CD concentration is lower than ca. 1.6 x 10 -2 mol 1-~, the ratio l~/lb of MDMAB is lower than that of DMABN at each fl-CD concentration. This occurs when the two fluorophores experience the same polarity in the CD cavity because the association constant between MDMAB and fl-CD is nearly the same as that between DMABN and fl-CD [4], and la/Ib of MDMAB becomes higher than that of DMABN in solution with fl-CD concentrations higher than 1.6 x 10 -2 mol 1-I. These results indicate that, when the fluorophores experience a relatively higher polarity in aqueous fl-CD solution, the ratio la/Ib of DMABN is higher than that of MDMAB, which is opposite to the case in organic solvents [2, 8]; and when the fluorophore is solubilized in an environment with a relatively lower polarity in aqueous fl-CD solution, the ratio la/Ib of DMABN is lower than that of MDMAB, which is similar to the situation in pure organic solvents. Calculation from the association constant (5801 mol -I) indicates that more than 90% of MDMAB is included in the nonpolar fl-CD cavity when fl-CD concentration is higher than 1 . 6 x l 0 - 2 m o l l -l, i.e. the MDMAB molecules are almost the same as being solubilized in nonpolar organic solvent. Thus MDMAB in aqueous fl-CD solution with flCD concentration lower than 1.6 x 10--' mol 1-t is, apparently, similar to be in aquaorganic binary mixtures. Increase of fl-CD concentration in the aqueous solution is the same as as the increase of the fraction of organic component in the aquaorganic binary mixture and the decrease of the polarity of the mixture. We have ascribed the different behavior of TICT in aqueous solution to the much higher polarity of water compared with that of common organic solvents [10-16]. In aqueous solution, the highly polar TICT state is so strongly solvated and stabilized that the energy barrier for the TICT process dramatically decreases and is not a factor affecting the formation of TICT state in a range of solvent polarity; thus the TICT emission is solely or mainly controlled by the radiationless decay of the TICT state. Therefore, the variations of la/Ib of MDMAB with fl-CD concentration shown in Fig. 2 can be explained in terms of the given solvent polarity effects on the TICT processes by considering that aqueous fl-CD solution as a pseudo aquaorganic binary mixture. When fl-CD concentration is relatively lower, the polarity of the 'aquaorganic mixture' is higher, and the TICT process is controlled by the latter factor in the solvent polarity effects; thus l~/lb enhances with the decrease of the polarity or with the increase of fl-CD concentration. Further increase of fl-CD concentration appreciably decreases the polarity of the 'aquaorganic mixture', the TICT process is controlled by the former factor in the solvent polarity effects, and la/Ib decreases upon decrease of the polarity or upon increase in fl-CD concentration. From the same point of view, it should also be expected that the solvent polarity or the fl-CD concentration of aqueous solution, required to reverse the correlation of 1Jlb with polarity or fl-CD concentration, if starting from the high polarity terminal (i.e. starting with pure water as a solvent), is lower in polarity or higher in fl-CD concentration for MDMAB than for DMABN. This is because of the higher energy barrier for the TICT process in DMABN [2], and is indeed the case, as shown in Fig. 2. It is worth pointing out that a mysterious excitation wavelength dependence of the TICT fluorescence of MDMAB in aqueous fl-CD solution is observed. If using the curvature of the curve of 1Jlt, vs excitation wavelength as a measurement of dependence, we note that the dependence is also fl-CD concentration-dependent (Fig. 5) in the same way as that of la/Ib of MDMAB vs fl-CD concentration, implying that the excitation wavelength dependence of TICT fluorescence in aqueous solution is also correlated to the medium polarity. The excitation wavelength dependence of the TICT fluorescence of methyl or ethyl p-dimethylaminobenzoate in acetonitrile-protic solvent mixtures [26] and in supercritical CHF3 (but not in less polar supercritical CO2 and C2I-I6 [27]) has been reported, although the dependence in supercritical CHF 3 has recently been found to be due to the impurity in the solvent [28]. Similar excitation wavelength dependence was also observed for the TICT fluorescence of DMABOA in aqueous fl-CD [16] and buffer
280
Y.-B. JIANG
0
0.50
0 o
o
(d)
o
0.40 0.60-
0 0 0
0.50 -
0 0 0
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0
(b)
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Ca)
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270 280 290 Wavelength (nm)
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Fig. 5. la/Ib of M D M A B in aqueous fl-CD solutions as a function of excitation wavelength, fl-CD concentrations are 4 . 0 x 1 0 -4 (a), 1 . 0 x 1 0 -3 (b), 5 . 0 x 1 0 -3 (c), and 9 . 0 x l 0 - 3 m o l l -l (d), respectively.
E 0
o
~d
320
360
400 440 480 Wavelength (nm)
520
Fig. 6. Excitation wavelength dependent fluorescence spectra of M D M A B in 1.0 x 10 -3 mol I -~ fl-CD solution. Excitation wavelengths are 275 (a), 280 (b), 285 (c), 290 (d), and 295 nm (e), respectively.
TICT of MDMAB
281
solutions [29]. Although such a dependence in fluid medium has not been rationalized up to now, it is apparently similar to that of the polar fluorophore in polar glass medium [30] where the presence of the different excited state solvation sites was considered to be the cause of the dependence, and is similar to that of the TICT fluorescence of D M A B A [31] and D M A B N [32] in rigid media where the existence of different ground-state rotamers or of broad ground-state distribution was believed to be the cause of the dependence. In the present system, only one peak at the same position of c a . 325 nm has been observed in both the excitation spectra of M D M A B obtained by monitoring TICT fluorescence and LE fluorescence at the corresponding maximum wavelength in aqueous solutions with different/3-CD concentrations. In addition, both the excitation spectra have the same shape, with the width at half of the peak height of c a . 4200 cm- t. These parameters are different from those of D M A B A and D M A B N in rigid media [31, 32] where two peaks were present in the excitation spectra and correspond to the LE state and the TICT state, respectively, and where the positions of the excitation spectra varied with the monitored fluorescence wavelengths. Together with the consideration of the present fluid medium vs the rigid medium of the reported systems [31, 32], we conclude that the ground-state inhomogeneity should not be the cause of the excitation wavelength dependence of TICT fluorescence observed in aqueous M D M A B - / 3 - C D system. Due to the high polarity of the TICT state and of water, a strong solvation of the TICT state must be expected. Thus the explanation that the excitation wavelength dependence of TICT fluorescence is the result of the heterogeneity in excited state solvation seems more likely [16, 29, 30], although it has not been finally determined. The fact that the TICT fluorescence, but not LE fluorescence, slightly red-shifts as the excitation wavelength is lengthened (as shown in Fig. 6), may serve as added support for the explanation of the heterogeneity in excited state solvation.
Acknowledgements--This project was supported by the Natural Science Foundation of Fujian province, China, under the grant of B92005, and by the National Natural Science Foundation of China under the grant of 29303023. I gratefully thank Professor Peng-Yuan Yang for his help with translation of the manuscript.
REFERENCES [1] Z. R. Grabowski, K. Rotkiewicz, A. Siemiarczuk, D. J. Cowley and W. Baumann, Nouv. J. Chim. 3,443 (1979). [2] W. Rettig, Angew. Chem. Int. Ed. Engl. 25,971 (1986). [3] Z. R. Grabowski and J. Dobkowski, Pure Appl. Chem. 55,245 (1983). [4] G. S. Cox, P. J. Hauptman and N. J. Turro, Photochem. Photobiol. 39,597 (1984). ]5] A. Nag and K. Bhattacharyya, Chem. Phys. Lett. 151,474 (1988). [6] A. Nag, R. Dutta, N. Chattopadhyay and K. Bhattacharyya, Chem. Phys. Lett. 157, 83 (1989). [7] A. Nag and K. Bhattacharyya, J. Chem. Soc. Faraday Trans. 86, 53 (1990). [8] W. Rettig and G. Wermuth, J. Photochem. 28, 351 (1985). [9] Y.-P. Sun, G. Bennett, K. P. Johnston and M. A. Fox, J. Phys. Chem. 96, 10001 (1992). [10] Y. B. Jiang and X. Z. Huang, Science in China (Ser. B), Chinese Edn 23, 7 (1993); English Edn 36, 1025 (1993). [11] Y. B. Jiang, J. G. Xu and X. Z. Huang, Acta Chim. Sinica 51,748 (1993). [12] Y. B. Jiang and J. S. Hu, Chem. J. Chinese Univ. 14, 1068 (1993). [13] Y. B. Jiang and X. J. Wang, Acta Physico-Chimica Sinica 10, 856 (1994). [14] Y. B. Jiang, Chin. Sci. Bull. (in English) 39, 1179 (1994). 115] Y. B. Jiang, J. G. Xu and X. Z. Huang, Chem. J. Chinese Univ. 15,552 (1994). [16] Y. B. Jiang, J. Photochem. Photobiol. A: Chem., submitted. [17] J. M. Hicks, M. T. Vandersall, Z. Babarogic and K. B. Eisenthal, Chem. Phys. Lett. 116, 16 (1985). [18] J. M. Hicks, M. T. Vandersall, E. V. Sitzmann and K. B. Eisenthal, Chem. Phys. Lett. 135,413 (1987). [19] Z. Zhen and C. H. Tung, Chem. Phys. Lett. 180,211 (1991). [20] J. A. T. Revill and R. G. Brown, Chem. Phys. Lett. 188,433 (1992). [21] J. R. Lakowicz, Principles o f Fluorescence Spectroscopy, Chap. 5. Plenum Press, New York (1986). [22] J. Szejtli, Cyclodextrins and their Inclusion Complexes. Akademiai Kiado, Budapest (1982). [23] Y. B. Jiang, X. Z. Huang and G. Z. Chen, Chin. Sci. Bull. 35, 120 (1990). [24] G. C. Catena and F. V. Bright, Analyt. Chem. 61,905 (1989).
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A. Nag, T. Kundu and K. Bhattacharyya, Chem. Phys. Chem. 160, 257 (1989). D. J. Cowley and P. J. Healy, Proc. R. It. Acad. Sec. B 77B, 397 (1977); Chem. Abstr. 89, 33714b (1978). B. J. Hrnjez, P. T. Yazdi, M. A. Fox and K. P. Johnston, J. A m . Chem. Soc. 111, 1915 (1989). Y.-P. Sun, M. A. Fox and K. P. Johnston, J. Am. Chem. Soc. 114, 1187 (1992). Y. B. Jiang, J. Photochem. Photobiol. A: Chem. 78, 205 (1994). T. C. Werner, Modern Fluorescence Spectroscopy (Edited by E. L. Wehry), Vol. 2, p. 311. Plenum Press, New York (1976). [31] J. Dobkowski, E. Kirkov-Kaminska and A. Siemiarczuk, J. Lumin. 27, 339 (1982). [32] K. A. AI-Hassan and W. Rettig, Chem. Phys. Left. 126, 273 (1986).
Pergamon 0584-8539(94)E0006-V
Copyright (~) 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0584-8539/95 $9.50 + 0.00
Twisted intramolecular charge transfer of methyl p-dimethylaminobenzoate in aqueous ~-cyciodextrin solution Y U N - B A O JIANG Department of Chemistry and Institute of Analytical Chemistry, Xiamen University, Xiamen 361005, People's Republic of China
(Received 1 November 1993; in final form 20 December 1993; accepted 22 December 1993) Abstract--This paper reports the investigation of the twisted intramolecular charge transfer (TICT) of methyl p-dimethylaminobenzoate (MDMAB) in aqueous fl-cyclodextrin (fl-CD) solution by the TICT-typical dual fluorescence. In pure water, MDMAB emits only LE fluorescence, and the TICT fluorescence band is developed with the addition of fl-CD. Both the LE and TICT fluorescence bands are continuously enhanced upon the increase of fl-CD concentration. The intensity ratio of the TICT band to the LE band shows a hillshaped dependence on fl-CD concentration, and a blue shift in both TICT and LE bands is observed with the increase of fl-CD concentration, of which the blue shift in TICT band is more appreciable. Formation of a 1 : 1 MDMAB-fl-CD inclusion complex, with an association constant of 580 _+801 mol- t, is evaluated. The effect of fl-CD on TICT of MDMAB is discussed, with consideration of the fact that aqueous fl-CD solution is pseudo° aquaorganic binary mixture and that TICT in aqueous solution acts differently than in organic solvents. A comparison is made between the TICT of MDMAB and of DMABN in aqueous fl-CD solution.
1. INTRODUCTION
TWISTED intramolecular charge transfer (TICT) [1-3] in aqueous cyclodextrin (CD) solutions for p-dimethylaminobenzonitrile (DMABN) has been examined in two capacities: (a) the TICT photophysics as a tool to understand the microenvironment of the CD cavity [4]; and (b) the influence of the microenvironment of the CD cavity on TICT photophysics [5-7]. TICT of alkyl p-dimethylaminobenzoate is easier than that of D M A B N in organic solvents [2, 8] and supercritical fluids [9], which is evidenced by the fact that the former emits a relatively stronger TICT fluorescence even in nonpolar solvents. Recently, we have shown that, for p-dimethylaminobenzaldehyde (DMABA) and p-dimethylaminobenzoic acid (DMABOA), the medium polarity correlation of the intensity ratio of the TICT band to the LE band, la/lb, is opposite in aqueous micellar and cyclodextrin solutions [10-16] to that in pure organic solvents [17, 18], i.e. enhanced Ia/Ib is observed on decreasing the medium polarity [10-16]. A similar situation was reported for alkyl p-dimethylaminobenzoate in aquaorganic binary mixtures [19]. Therefore, it was argued that the TICT in aqueous medium may be somewhat different from that in organic solvent, and that the difference is due to the high polarity of water compared with that of common organic solvents [10-16]. One must then wonder about the TICT of alkyi p-dimethylaminobenzoate in aqueous CD solution compared with that of DMABN. To answer this question, the TICT of alkyl p-dimethylaminobenzoate needs firstly to be examined in aqueous CD solution. In the present paper, we describe the TICT behavior of methyl p-dimethylaminobenzoate (MDMAB) in aqueous fl-CD solution as monitored by the TICT-typical dual fluorescence and conduct a comparison between the TICT behavior of MDMAB and DMABN.
2. EXPERIMENTAL M D M A B was s y n t h e s i z e d according to REVILL a n d BROWN [20] a n d purified by d o u b l e recrystallization f r o m a b s o l u t e ethyl alcohol, fl-CD was used as received from Fluka. T w i c e - d e i o n i z e d w a t e r was used for all m e a s u r e m e n t s . F l u o r e s c e n c e spectra of M D M A B were r e c o r d e d on a Hitachi 650-10S s p e c t r o f l u o r o m e t e r . T h e excitation w a v e l e n g t h was 280 n m a n d the slits for excitation a n d emission m o n o c h r o m a t o r s were 6 a n d 4 n m , respectively. Total fluorescence intensity was o b t a i n e d by weighing the p a p e r c o v e r e d by 275
276
Y.-B. JIANG
8 e-
g
320
360 400 440 480 Wavelength (nm)
520
Fig. l. Fluorescence spectra of M D M A B in a q u e o u s fl-CD solutions. M D M A B concentration is 2.5 x 10 -5 mol 1-I. fl-CD concentrations are 0 (a), 2.0 x 10 4 (b), 4.0 x 10 -4 (c), 8.0 x 10 4 (d), 1 . 5 x 10 3 (e), 2.5 × 10-3 (f), 3 . 5 × 10 3 (g), 4.5 × 10 3 (h), 6 . 0 × 10 -3 (i), and 8 . 0 × l0 3 m o l l (j), respectively.
the emission spectrum. The G-factor correction [21] was adopted in the determination of the fluorescence polarization.
3.
RESULTS AND DISCUSSION
Shown in Fig. 1 are the fluorescence spectra of the aqueous solution containing 2 . 5 x 10-Smoll -~ M D M A B and different concentrations of fl-CD. In pure water, M D M A B emits only LE fluorescence at short wavelength. The TICT fluorescence band at longer wavelengths is developed upon the addition of fl-CD. With increase in fl-CD concentration, the LE and TICT fluorescence bands are enhanced and shifted to blue in different rates, indicative of the formation of an MDMAB-fl-CD inclusion complex [22], i.e. the penetration of M D M A B into the nonpolar fl-CD cavity. The blue shifting in the T1CT band is more appreciable than that in the LE band, which is a reflection of the high polarity of the TICT state [1, 2]. The rate of enhancement of TICT fluorescence by the presence of fl-CD is initially higher than that of the LE band and becomes lower when the fl-CD concentration is 1.5 x 10 ~mol l- L, although the total fluorescence intensity, If, is progressively increased, as can be easily seen in Fig. 2. Figure 2 plots the intensity ratio of the TICT band to the LE band, Ia/Ib, and /f, of MDMAB vs fl-CD concentration. Similar fl-CD concentration dependence of Ia/Ib has been reported for D M A B N [6] and D M A B A [10]. For the D M A B A - f l - C D system [10], the opposite fl-CD concentration dependence of lflIb was attributed to the different stoichiometries in the formed inclusion complex between D M A B A and fl-CD. The positive dependence is related to the formation of the 1 : 1 inclusion complex, while the negative dependence is due to the formation of a 1 : 2 complex. Unfortunately, the results displayed in Fig. 3 suggest that this is not the case in the present system. In Fig. 3, the fluorescence intensity data of the present system are shown processed against fl-CD concentration by two different methods [23, 24], including double reciprocal plotting [24]. Both methods yield one straight line, indicating that only the 1:1 MDMAB-fl-CD inclusion complex is formed. The obtained association
TICT of M D M A B
277
1.6 1.4 1.2
~
1.0
10
0.8
8
0.6
6
0.4
/
~4 2
0.2 I
0.2
I
I
0.4
I
0.6
0.8
I
1
1.0
I
1.2
I
1.4
o 2.0
I
1.6
l.S
10 2 [~-CD] (mol 1"1) Fig. 2. la/l b (a) and If (b) of M D M A B in aqueous solution as a function of fl-CD concentration. As a comparison, la/Ib data of D M A B N in aqueous fl-CD solutions (c), taken from Ref. [6], are also given.
constant of the inclusion complex, 580+801 mol -l, is compatible with that of the DMABN-fl-CD inclusion complex (440 + 1001 mol- 1) [4]. NAG et al. [6] have given an explanation for TICT behavior in the DMABN-fl-CD system in terms of different forms of the 1 : 1 DMABN-fl-CD inclusion complex. The enhanced TICT fluorescence was said to be due to DMABN molecules that are partially inside the fl-CD cavity, and that thus experience a higher polarity; while the enhanced LE fluorescence is due to DMABN molecules that are totally inside the fl-CD cavity. Such an explanation, although it seems reasonable on the basis of the medium-polarity controlled TICT process [17, 18], cannot explain the fact that the enhancement of TICT fluorescence is accompanied by a blue shift in the TICT band in the DMABN-fl-CD system [6]. This explanation is, at least, not applicable in the present system because of the same contradiction. REVILE and BROWN [20] have attributed the long wavelength emission of MDMAB in a polar organic solvent acetonitrile to the excited dimer. This is unlikely in the present aqueous MDMAB-fl-CD solution, because, if this is the case, the formation of 1:1
I/[IS-CD] (I mol l ) 5OOO o
400
800
1200
1600
2000
2400
I
I
I
I
I
I
1.0
0.8 o
3OO0
-
0.6
I000
0
0.2
I
2
I
4
I
I
6
8
I
I0
I
12
0
If Fig. 3. Plot of (ldlo)/[fl-CD[ versus If (a) and double reciprocal plot (b) of M D M A B - f l - C D system, where 10 refers to the relative fluorescence intensity in the absence of fl-CD.
278
Y.-B. JIANG 0.15 - 0 0 0 0 0
000
0
(e)
0 O0
0
0 0
0
0
0
0
0
0
0
0
0.05 0
0.15
0 000
o O0
O0
(d)
0
0.05 o
0.15 - 0 0 0 0
0 0 000
sO la,
0 0
(c)
0
0
0.05 0.15
"000000000 O0 o
(b)
0
0.05 0.15
_0_00 O 0 °
~
.
.
.
.
0 .
.
-o-
.-o_
o.05
320
1
360
_
_
-o_
_
O
(a) I
400
I
440
I
480
520
Wavelength ( n m ) Fig. 4. Fluorescence polarization emission spectra of MDMAB in the presence of 0 ( a ) , 4 . 0 x 10 4 ( b ) , 1.0 x 10 -3 (c), 5 . 0 x 10 -3 ( d ) , a n d 9 . 0 x 10 -3 m o l I ' (e) f l - C D , respectively.
M D M A B - f l - C D inclusion complexes will hinder the ground-state dimerization of M D M A B and in turn weaken its emission. This is opposite to the experimental results shown in Figs 1 and 2. Also, the blue shift in the long-wavelength band in the presence of fl-CD cannot be in the line of the excited dimer since no charge transfer takes place in the excited dimer. Figure 4 shows the fluorescence polarization emission spectra of the M D M A B - f l - C D system, which also indicates that the long wavelength emission is not due to the excited dimer. The fluorescence polarization emission spectrum of M D M A B in pure water has only one plateau, while two plateaux are observed in those of M D M A B in aqueous fl-CD solutions. Because fluorescence polarization is independent of emission wavelength [21], the polarization spectra in Fig. 4 should suggest that only one emissive excited state is present for M D M A B in pure water, while two emissive excited states, corresponding to the dual fluorescence bands in Fig. 1, are present for M D M A B in fl-CD solutions, of which the excited state emitted at longer wavelengths possesses a lower polarization. If the long-wavelength emission of M D M A B is due to the excited dimer, its polarization should be larger than that of the LE state because of the larger volume [21]; unfortunately, this is not the fact. On the other hand, attributing the long-wavelength emission to the TICT state is reasonable. Because the angle shifting between absorption and emission dipoles in the TICT state is larger than that in the LE state, the TICT state possesses lower polarization [10, 21]. We therefore present an alternative explanation for the fl-CD concentration dependent la/Ib of M D M A B in aqueous solution from the viewpoint of the solvent polarity effects on the TICT emission [25]. It can be derived from Refs [17, 18, 25] that, with the increase of solvent polarity, the rate of TICT state formation and in turn the yield of TICT emission increase, while the yield of LE emission automatically decreases because TICT is the main nonradiative pathway in the LE state. As a consequence, the ratio la/lb increases. At the same time, the nonradiative rates of the TICT state also increase, which causes the decrease of the yield of TICT emission and thus the decrease of the la/lh. Obviously, the two factors compete with each other. The fact that the IJA, of the TICT fluorophore in organic solvents increases with the solvent polarity indicates that the former factor is predominant, whereas in aqueous solution, e.g. the present system, the case should be controlled by the latter one due to the opposite correlation of l,,/lb with
TICT of MDMAB
279
the medium polarity, which means that the TICT in aqueous solution is different from that in organic solvents [10-16]. Comparison of the TICT behaviors of MDMAB and of DMABN in aqueous fl-CD solution by the ratio lallb would strengthen the above conclusion. It is clear in Fig. 2 that, when fl-CD concentration is lower than ca. 1.6 x 10 -2 mol 1-~, the ratio l~/lb of MDMAB is lower than that of DMABN at each fl-CD concentration. This occurs when the two fluorophores experience the same polarity in the CD cavity because the association constant between MDMAB and fl-CD is nearly the same as that between DMABN and fl-CD [4], and la/Ib of MDMAB becomes higher than that of DMABN in solution with fl-CD concentrations higher than 1.6 x 10 -2 mol 1-I. These results indicate that, when the fluorophores experience a relatively higher polarity in aqueous fl-CD solution, the ratio la/Ib of DMABN is higher than that of MDMAB, which is opposite to the case in organic solvents [2, 8]; and when the fluorophore is solubilized in an environment with a relatively lower polarity in aqueous fl-CD solution, the ratio la/Ib of DMABN is lower than that of MDMAB, which is similar to the situation in pure organic solvents. Calculation from the association constant (5801 mol -I) indicates that more than 90% of MDMAB is included in the nonpolar fl-CD cavity when fl-CD concentration is higher than 1 . 6 x l 0 - 2 m o l l -l, i.e. the MDMAB molecules are almost the same as being solubilized in nonpolar organic solvent. Thus MDMAB in aqueous fl-CD solution with flCD concentration lower than 1.6 x 10--' mol 1-t is, apparently, similar to be in aquaorganic binary mixtures. Increase of fl-CD concentration in the aqueous solution is the same as as the increase of the fraction of organic component in the aquaorganic binary mixture and the decrease of the polarity of the mixture. We have ascribed the different behavior of TICT in aqueous solution to the much higher polarity of water compared with that of common organic solvents [10-16]. In aqueous solution, the highly polar TICT state is so strongly solvated and stabilized that the energy barrier for the TICT process dramatically decreases and is not a factor affecting the formation of TICT state in a range of solvent polarity; thus the TICT emission is solely or mainly controlled by the radiationless decay of the TICT state. Therefore, the variations of la/Ib of MDMAB with fl-CD concentration shown in Fig. 2 can be explained in terms of the given solvent polarity effects on the TICT processes by considering that aqueous fl-CD solution as a pseudo aquaorganic binary mixture. When fl-CD concentration is relatively lower, the polarity of the 'aquaorganic mixture' is higher, and the TICT process is controlled by the latter factor in the solvent polarity effects; thus l~/lb enhances with the decrease of the polarity or with the increase of fl-CD concentration. Further increase of fl-CD concentration appreciably decreases the polarity of the 'aquaorganic mixture', the TICT process is controlled by the former factor in the solvent polarity effects, and la/Ib decreases upon decrease of the polarity or upon increase in fl-CD concentration. From the same point of view, it should also be expected that the solvent polarity or the fl-CD concentration of aqueous solution, required to reverse the correlation of 1Jlb with polarity or fl-CD concentration, if starting from the high polarity terminal (i.e. starting with pure water as a solvent), is lower in polarity or higher in fl-CD concentration for MDMAB than for DMABN. This is because of the higher energy barrier for the TICT process in DMABN [2], and is indeed the case, as shown in Fig. 2. It is worth pointing out that a mysterious excitation wavelength dependence of the TICT fluorescence of MDMAB in aqueous fl-CD solution is observed. If using the curvature of the curve of 1Jlt, vs excitation wavelength as a measurement of dependence, we note that the dependence is also fl-CD concentration-dependent (Fig. 5) in the same way as that of la/Ib of MDMAB vs fl-CD concentration, implying that the excitation wavelength dependence of TICT fluorescence in aqueous solution is also correlated to the medium polarity. The excitation wavelength dependence of the TICT fluorescence of methyl or ethyl p-dimethylaminobenzoate in acetonitrile-protic solvent mixtures [26] and in supercritical CHF3 (but not in less polar supercritical CO2 and C2I-I6 [27]) has been reported, although the dependence in supercritical CHF 3 has recently been found to be due to the impurity in the solvent [28]. Similar excitation wavelength dependence was also observed for the TICT fluorescence of DMABOA in aqueous fl-CD [16] and buffer
280
Y.-B. JIANG
0
0.50
0 o
o
(d)
o
0.40 0.60-
0 0 0
0.50 -
0 0 0
o
(c)
0.40 0
0.60-
0 0 0
0.50 -
0
(b)
o 0.40 0.55 0 0 0
0.45 0
Ca)
0
I
I
I
270 280 290 Wavelength (nm)
o
3OO
Fig. 5. la/Ib of M D M A B in aqueous fl-CD solutions as a function of excitation wavelength, fl-CD concentrations are 4 . 0 x 1 0 -4 (a), 1 . 0 x 1 0 -3 (b), 5 . 0 x 1 0 -3 (c), and 9 . 0 x l 0 - 3 m o l l -l (d), respectively.
E 0
o
~d
320
360
400 440 480 Wavelength (nm)
520
Fig. 6. Excitation wavelength dependent fluorescence spectra of M D M A B in 1.0 x 10 -3 mol I -~ fl-CD solution. Excitation wavelengths are 275 (a), 280 (b), 285 (c), 290 (d), and 295 nm (e), respectively.
TICT of MDMAB
281
solutions [29]. Although such a dependence in fluid medium has not been rationalized up to now, it is apparently similar to that of the polar fluorophore in polar glass medium [30] where the presence of the different excited state solvation sites was considered to be the cause of the dependence, and is similar to that of the TICT fluorescence of D M A B A [31] and D M A B N [32] in rigid media where the existence of different ground-state rotamers or of broad ground-state distribution was believed to be the cause of the dependence. In the present system, only one peak at the same position of c a . 325 nm has been observed in both the excitation spectra of M D M A B obtained by monitoring TICT fluorescence and LE fluorescence at the corresponding maximum wavelength in aqueous solutions with different/3-CD concentrations. In addition, both the excitation spectra have the same shape, with the width at half of the peak height of c a . 4200 cm- t. These parameters are different from those of D M A B A and D M A B N in rigid media [31, 32] where two peaks were present in the excitation spectra and correspond to the LE state and the TICT state, respectively, and where the positions of the excitation spectra varied with the monitored fluorescence wavelengths. Together with the consideration of the present fluid medium vs the rigid medium of the reported systems [31, 32], we conclude that the ground-state inhomogeneity should not be the cause of the excitation wavelength dependence of TICT fluorescence observed in aqueous M D M A B - / 3 - C D system. Due to the high polarity of the TICT state and of water, a strong solvation of the TICT state must be expected. Thus the explanation that the excitation wavelength dependence of TICT fluorescence is the result of the heterogeneity in excited state solvation seems more likely [16, 29, 30], although it has not been finally determined. The fact that the TICT fluorescence, but not LE fluorescence, slightly red-shifts as the excitation wavelength is lengthened (as shown in Fig. 6), may serve as added support for the explanation of the heterogeneity in excited state solvation.
Acknowledgements--This project was supported by the Natural Science Foundation of Fujian province, China, under the grant of B92005, and by the National Natural Science Foundation of China under the grant of 29303023. I gratefully thank Professor Peng-Yuan Yang for his help with translation of the manuscript.
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