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Photoionization of psoralen derivatives in micelles: Imperatorin and alloimperatorin
Spectrochimica Acta Part A 77 (2010) 811–815
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Photoionization of psoralen derivatives in micelles: Imperatorin and alloimperatorin Sameh R. El-Gogary ∗ Chemistry Department, Faculty of Science (Damietta), Mansoura University, New damietta, Egypt
a r t i c l e
i n f o
Article history: Received 11 April 2010 Received in revised form 27 July 2010 Accepted 3 August 2010 Keywords: Psoralen derivative Photoionization Micelle Radical cation Hydrated electron
a b s t r a c t The fluorescence properties of psoralen derivatives, 8-methoxypsoralen (8-MOP), imperatorin (IMP) and alloimperatorin (ALLOI), were investigated in various solvent and micellar solutions. The variation in intensity and maxima of the fluorescence in micellar solutions suggest that psoralens are located in the micelle–water interface region. Radical cations and hydrated electrons were generated by photoionization in micellar solution upon excitation at 266 nm. A nonlinear relationship between transient yield and photon fluency was obtained for each compound, indicating that a two-photon mechanism is predominant in the photoionization of the sensitizers. The photoionization efficiencies are significantly higher in anionic sodium dodecyl sulfate (SDS) than in cationic cetyltrimethylammonium bromide (CTAB) micelles, reflecting the influence of micelle charge on the efficiency of the separation of the photoproduced charge carriers. The photoionization efficiencies of IMP and ALLOI are similar. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Photochemical investigation in mimicking biological systems such as micelles is an extremely active area of research in supramolecular photochemistry [1–4]. Micelles have been used as models for understanding the effect of microheterogeneous environments on reaction dynamics and mechanisms [5,2,6–10]. The photochemical and photophysical studies in simple membranemimicking systems like micelles are particularly important in the case of phototoxic drugs. Although the photochemistry of the drugs in homogeneous media is the first step for understanding the molecular basis of the phototoxic effects, the photoreactivity in homogeneous media and phototoxicity is often not directly correlated to each other. This lack of correlation is due to the formation of the host–guest supramolecular aggregates between drug molecule and the biomolecules. These aggregates display different photochemical and photophysical properties when compared with free drug molecule. An important class of photoactive drugs is psoralens (furocoumarins) which are extensively used in the PUVA (psoralen plus UVA radiation) therapy for the treatment of dermatological disorders [11–13]. The phototherapeutic effects of psoralens are believed to result from intercalation of the drug between adjacent base pairs in the DNA duplex, followed by two successive photocycloaddition reactions that cross-link the DNA. This leads to
inhibition of replication and hence cell proliferation. The long term use of psoralens may have deleterious effects [14–16]. Recently, the photoionization of several psoralens and coumarins in aqueous solution [17–19], micelles [20] protein and liposomes [21] have been studied to elucidate the role of the environment on psoralen photochemistry. Previous results show that a variety of psoralens and coumarins undergo reasonably efficient photoionization upon excitation in aqueous solution [17–19]. The resulting radical cations react with easily oxidizable substrates such as amino acids and nucleotides; primarily via electron-transfer reactions. This raises the possibility that electron-transfer chemistry may play a role in the clinical use of these compounds. Moreover, the photoionization efficiencies are significantly higher in anionic sodium dodecyl sulfate (SDS) micelles than in aqueous solution as a result of favorable electrostatic effects that lead to rapid ejection of the electron into the aqueous phase. By contrast, much lower quantum yields are measured in neutral and cationic micelles (CTAB) [20]. The present study is dedicated to the investigation of the photoionization of imperatorin (IMP) and alloimperatorin (ALLOI), (Scheme 1), two natural psoralen derivatives have not been studied before, in aqueous cationic (CTAB) and anionic (SDS) micelles by means of laser flash photolysis, focusing in measurements of the efficiencies of the formation of hydrated electron and of radical cations. 2. Experimental
∗ Corresponding author. Tel.: +20 572403866; fax: +20 572403868. E-mail address: [email protected]. 1386-1425/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2010.08.009
8-Methoxypsoralen (8-MOP) (Aldrich, 98%), Imperatorin was supplied by Memphis Chemical Co., Egypt; Alloimperatorin was
812
S.R. El-Gogary / Spectrochimica Acta Part A 77 (2010) 811–815
Scheme 1. Chemical structures of psoralen derivatives.
prepared according to literature procedure [22], sodium dodecyl sulfate (SDS, Pharmacia Biotech, 99%) and cetyltrimethylammonium bromide (CTAB, Merck, 99%). All aqueous solutions were prepared with doubly distilled water. The concentrations used for the measurements in micelles were 5 × 10−5 M for the substrate molecules, 5 × 10−3 M for CTAB, and 2.5 × 10−2 M for SDS. The measurements were performed in O2 -saturated aqueous solutions or in air-equilibrated micellar solutions. Fluorescence measurements were performed with a Perkin-Elmer LSB-50 spectrofluorimeter. The nanosecond laser flash photolysis system has been described previously [23]. The fourth harmonics (266 nm) of a Qswitched Nd:YAG laser (Quanta-Ray DCR-1) with 8 ns duration was used to excite the solutions. Transient absorbances were measured in a right-angle set-up using a cell holder with incorporated rectangular apertures defining a reaction volume of dimensions 0.17 cm (height), 0.32 cm (width), and 0.13 cm (depth) within the cell. Pulse energies were measured using a ballistic calorimeter (Raycon-WEC 730). 3. Results and discussion
Fig. 1. Effect of SDS on the fluorescence intensity of MOP (), IMP (䊉) and ALLOI ().
upon addition of cationic micelle (CTAB) in the same order of SDS (IMP > ALLOI > MOP).
3.1. Fluorescence measurements Fluorescence spectra of substrates were measured in a number of different solvents and micellar solution to provide evidence for the location of the substrate in the micelle. The data presented in Table 1 show that the fluorescence maximum shifts to the longer wavelength with increasing solvent polarity for each of the substrate. By comparing the fluorescence maxima in micellar and homogeneous solutions, the microenvironment of the substrates in micellar solutions was found to be more polar than the organic solvents but less polar than water, suggesting that the substrates are located in the micelle–water interface region. The fluorescence spectra varied markedly on the addition of surfactants (SDS and CTAB). The change in fluorescence intensity as a function of surfactant concentration is presented in Figs. 1 and 2. In case of anionic micelle (SDS), the fluorescence intensity of substrates was increased in the order IMP > ALLOI > MOP. In contrast to SDS, the fluorescence intensity of substrates was decreased
3.2. Transient absorption spectra Transient absorption spectra obtained by direct excitation (exc = 266 nm) of an N2 -saturated solution of imperatorin (IMP)
Table 1 Fluorescence maxima of the substrates in various solvent at room temperature.
THF Methanol Water SDS CTAB
MOP
IMP
ALLOI
No 465 510 493 500
No 488 502 495 492
No 477 507 497 493
Fig. 2. Effect of CTAB on the fluorescence intensity of MOP (), IMP (䊉) and ALLOI ().
S.R. El-Gogary / Spectrochimica Acta Part A 77 (2010) 811–815
Fig. 3. Transient absorption spectra obtained via 266 nm excitation of airequilibrated solutions containing IMP (), ALLOI () and () 8-MOP in 25 mM SDS, 200 ns after the laser pulse. All spectra are normalized at A266 = 1, pulse energy = 1 mJ/pulse.
and alloimperatorin (ALLOI) in aqueous and ethanol solution are similar to those previously reported for 8-MOP [24–28], data not shown. The overall absorption in the near-UV had a maximum around 360 nm; a further intense broad absorption band extended to longer wavelengths, with a maximum around 700 nm. The transient spectra are expected to include contributions of the absorptions of triplet states, hydrated electrons, radical cations, and radical anions formed by reaction of hydrated electron with the substrate. The four contributed transients are differed in their life time, the very short-lived transient (hydrated electron), shortlived triplet state of the molecule and the very long-lived radical anion are quenched by oxygen. The residual absorption of oxygensaturated solution is attributed to the very long-lived radical cation. Thus, photolysis of the molecule in oxygen-saturated solution gives rise a transient of the radical cation. Fig. 3 shows the normalized radical cation absorption of airequilibrated aqueous anionic micellar (SDS) solution of IMP, ALLOI and 8-MOP, obtained by direct excitation at 266 nm. The radical cation spectra have a two-band structure with a maximum in the near-UV and another one in the red spectral region. Similar results have been obtained by excitation of both sensitizers in aqueous and cationic micellar (CTAB) solutions, data not shown. The spectra of the radical cations of IMP and ALLOI in micelles are virtually identical to those obtained in aqueous solution. This is consistent with the results of fluorescence which indicates that the IMP and ALLOI are localized near the micellar interface in a relatively polar environment. 3.3. Laser energy dependence of photoionization To get information about the influence of the micellar microenvironment, on the efficiency and nature of the photoionization process of psoralen derivatives, we performed a laser energy effect on the absorbance of radical cation and hydrated electron, monitored at 360 and 700 nm, respectively in the presence and, for comparison, in the absence of micelles. The dependence of the transient absorbance of IMP radical cation at 360 nm in air-equilibrated SDS and CTAB micellar solutions on pulse energy upon direct excitation (ex 266 nm), was shown in Fig. 4, has nonlinear relation, characteristic for the superposition of a monophotonic and a biphotonic ionization pathway with a predominance of the latter. Clearly, the radical cation yield
813
Fig. 4. Plot of signal intensity for the radical cation vs incident laser energy obtained via 266 nm excitation of an air-equilibrated SDS () and CTAB () micellar solutions of imperatorin. Inset shows the plot of the function A/P vs P (P is the laser energy) of radical cation formation of imperatorin in air-equilibrated micellar solutions upon excitation at 266 nm, 25 mM SDS () and 5 mM CTAB (). All data normalized to the same absorbance (1.0) at the wavelength of excitation.
is higher in the anionic SDS micellar solutions than in the cationic CTAB one. In these environments photoionization may occur through a mixture of mono- and biphotonic mechanisms. The monophotonic mechanism occurs from the fluorescent state (first excited state S1 ) when the molecule is excited in its first singlet [29]. The second mechanism requires the consecutive absorption of two photons via the intermediate first excited state S1 or triplet excited state T1 and is appreciable at high excitation intensities [30]. In this case the dependence of photoionization on pulse energy can be described, at low pulse energies, by the following equation [31]: A = aP + bP 2 P being the pulse energy, which is proportional to photon fluency, a is a coefficient depending on the quantum yield of the one-photon process, and b is related to the efficiency of the twophoton process [32]. The plot of the function A/P vs P for IMP in aqueous micelles systems upon excitation at 266 nm is shown in Fig. 4 inset. The very small value obtained for the coefficient a (intercept in Fig. 4 inset) indicates that the one-photon process is minor process in all systems. The significant enhancement of the biphotonic mechanism of the photoionization process in anionic micellar solution (SDS) relative to those in cationic micellar solution (CTAB) was achieved. Similar results have been obtained during direct excitation (ex 266 nm) of air-equilibrated micellar solution containing ALLOI. A plot of the function A/P vs P for ALLOI in micelles system upon excitation at 266 nm is presented in Fig. 5. Further evidence of the photoionization of psoralen derivatives (IMP and ALLOI) is the photoejection of electrons. The hydrated electron formation of IMP and ALLOI were measured in airequilibrated micellar solutions, using 266 nm. Fig. 6 shows the plots of intensities of hydrated electron at 700 nm of IMP and ALLOI (Fig. 6 inset) in air-equilibrated micellar solutions vs pulse energy upon direct excitation (ex 266 nm). The laser energy dependence of the yield of hydrated electron of air-equilibrated solutions (Fig. 6) is nonlinear for each of the compounds, consistent with biphotonic ionization as observed in radical cation measurements. Again the results of the hydrated electron measurements indicate that the photoionization of IMP and ALLOI is considerably higher in anionic micelles (SDS) as compared to cationic micelles (CTAB).
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S.R. El-Gogary / Spectrochimica Acta Part A 77 (2010) 811–815
The results obtained for the photoionization of IMP and ALLOI can be interpreted with regard to the effect of the micellar charge on the separation and recombination of the photoproduced charges. The formation of radical cation and the hydrated electron for each substrate (IMP and ALLOI) is significantly higher in SDS relative to CTAB. In case of SDS the initial photoionization of the micelle associated substrate will generate an electron that is rapidly repelled from the negatively charged micellar interface into the aqueous phase; conversely, the electrostatic effects will favor association of the radical cation with the micelle. This rapid separation of the initial electron/radical cation pair leads to an enhanced yield of detectable radical cations and hydrated electrons. In CTAB, the electron is not able to escape easily from the cationic micelle because of the electrostatic attraction; the recombination of the cation radical and the electron is consequently more efficient, and the yield of detectable species is low. Fig. 5. A plot of the function A/P vs P (P is the laser energy) of radical cation formation of alloimperatorin in air-equilibrated aqueous micellar solutions upon excitation at 266 nm, 25 mM SDS () and 5 mM CTAB (). All data normalized to the same absorbance (1.0) at the wavelength of excitation.
The previous results of psoralen and coumarin derivatives [18] had demonstrated that the radical cation yields in aqueous solution depended linearly on the laser energy for both 308 and 355 nm excitation indicating that the photoionization process is monophotonic with direct contrast to the first photoionization results of 8-MOP in aqueous solution [25]. This discrepancy is due to the use of high pulse energy in the recent photoionization measurements. Furthermore, the photoionization of psoralen and coumarin derivatives in micellar solutions has been reported, indicating that the photoionization yield in micelles increased linearly on the pulse energy for both 308 and 355 nm excitation [20]. Significant enhanced photoionization yields of psoralen and coumarin derivatives were measured in anionic SDS micelles relative to aqueous solution; by contrast, decreased yields were observed in cationic CTAB micelles. On the other hand, by consistent to the results of our work herein, the photoionization of variety aromatic hydrocarbons, amines, an aromatic ketone, modified furocoumarin and furochromones derivatives in aqueous and micellar solutions is biphotonic process [33–37].
Fig. 6. Plot of signal intensity of hydrated electron formation of IMP and ALLOI (inset) in air-equilibrated micellar solutions vs incident laser energy upon excitation at 266 nm, 25 mM SDS () and 5 mM CTAB (). All data normalized to the same absorbance (1.0) at the wavelength of excitation.
4. Conclusion Psoralen derivatives (IMP and ALLOI) are located at the micelle–water interface region in anionic sodium dodecyl sulfate (SDS) and cationic cetyltrimethylammonium bromide (CTAB) micellar solutions. The photoionization of two psoralens (IMP and ALLOI) has been studied in micellar solutions using 266 nm laser flash photolysis. The photoionization efficiencies are significantly higher in anionic (SDS) than in cationic (CTAB) micelles, reflecting the influence of micelle charge on the efficiency of the separation of the photoproduced charge carriers. Nonlinear energy dependence plots for radical cation or hydrated electron formation of IMP and ALLOI demonstrate that photoionization is mainly biphotonic. Acknowledgments Special thanks go to all members in professor Koehler’s group, Max F. Perutz Laboratories, University of Vienna, Vienna, Austria for their friendly reception and kind help. References [1] V. Ramamurthy, Photochemistry in Organized and Constrained Media, VCH Publisher, New York, 1991. [2] K. Kalyanasundaram, Photochemistry in Microheterogeneous Systems, Academic Press, Orlando, FL, 1987. [3] P. Bortolus, S. Monti, Adv. Photochem. 21 (1996) 1. [4] M.H. Kleinman, C. Bohne, Organic Photochemistry: Molecular and Supramolecular Photochemistry, Marcel Dekker, New York, 1997. [5] M. Gratzel, in: M. Gratzel (Ed.), Solar Energy Harvesting, Elsevier, New York, 1988, pp. 394–440. [6] M.A. Fox, Top. Curr. Chem. 159 (1991) 67–101. [7] N.J. Turro, Pure Appl. Chem. 67 (1995) 199–208. [8] K. Weidemaier, H.L. Tavernier, M.D. Fayer, J. Phys. Chem. B 101 (1997) 9352–9361. [9] I.V. Soboleva, J. van Stam, G.B. Dutt, M.G. Kuzmin, F.C. De Schryver, Langmuir 15 (1999) 6201–6207. [10] J.W. Hackett II, C. Turro, J. Phys. Chem. A 102 (1998) 5728–5733. [11] F.P. Gasparro, Psoralen DNA Photobiology, vols. 1 and 2, CRC Press, Boca Raton, FL, 1988. [12] M.A. Pathak, T.B. Fitzpatrick, J. Photochem. Photobiol. B: Biol. 14 (1992) 3–22. [13] R.V. Bensasson, E.J. Land, T.G. Truscott, Excited States and Free Radicals in Biology and Medicine, Oxford University Press, Oxford, 1993. [14] E. Gonzalez, Dermatol. Clin. 13 (1995) 851–866. [15] R.S. Stern, K.T. Nichols, L.H. Vakeva, N. Engl. J. Med. 336 (1997) 1041–1045. [16] A.R. Young, J. Photochem. Photobiol. B: Biol. 6 (1990) 237–247. [17] P.D. Wood, A. Mnyusiwalla, L. Chen, L.J. Johnston, Photochem. Photobiol. 72 (2000) 155–162. [18] P.D. Wood, L.J. Johnston, J. Phys. Chem. 102 (1998) 5585–5591. [19] P.D. Wood, L.J. Johnston, Photochem. Photobiol. 66 (1997) 642–648. [20] P.D. Wood, A. Mnyusiwalla, L. Chen, J. Marlinga, L.J. Johnston, Phys. Chem. B 105 (2001) 10927–10935. [21] L. Chen, O. Rinco, J. Popov, N. Vuong, L.J. Johnston, Photochem. Photobiol. 82 (2006) 31–37. [22] T. Noguti, M. Kawakami, J. Pharm. Soc. Jpn. 61 (1941) 77. [23] P. Baranyai, S. Gangl, G. Grabner, M. Knapp, G. Köhler, T. Vidóczy, Langmuir 15 (1999) 7577–7585.
S.R. El-Gogary / Spectrochimica Acta Part A 77 (2010) 811–815 [24] R.V. Bensasson, E.J. Land, C. Salet, Photochem. Photobiol. 27 (1978) 273–280. [25] R.W. Sloper, T.G. Truscott, E.J. Land, Photochem. Photobiol. 29 (1979) 1025–1029. [26] M.T. Sa e Melo, D. Averbeck, R.V. Bensasson, E.J. Land, C. Salet, Photochem. Photobiol. 30 (1979) 645–651. [27] M. Craw, R.V. Bensasson, J.C. Ronfard-Haret, M.T. Sa e Melo, T.G. Truscott, Photochem. Photobiol. 37 (1983) 611–615. [28] R.V. Bensasson, O. Chalvet, E.J. Land, J.C. Ronfard-Haret, Photochem. Photobiol. 39 (1984) 287–291. [29] J. Zechner, G. Köhler, G. Grabner, N. Getoff, Chem. Phys. Lett. 37 (1976) 297. [30] G. Grabner, G. Köhler, J. Zechner, N. Getoff, Photochem. Photobiol. 26 (1977) 449.
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[31] U. Lachish, A. Shaffermann, G. Stein, J. Chem. Phys. 64 (1976) 4205. [32] G. Grabner, N. Getoff, T. Gantchev, D. Angelov, M. Shopova, Photochem. Photobiol. 54 (1991) 673–681. [33] J.K. Thomas, P.L. Piciulo, J. Am. Chem. Soc. 101 (1978) 2502–2503. [34] S. Hashimoto, J.K. Thomas, J. Photochem. Photobiol. A: Chem. 55 (1991) 377–386. [35] R. Boch, M.K. Whittlesey, J.C. Scaiano, J. Phys. Chem. 98 (1994) 7854– 7857. [36] S. El-Gogary, G. Grabner, Photochem. Photobiol. Sci. 5 (2006) 311– 316. [37] S. El-Gogary, G. Grabner, G. Koehler, J. Photochem. Photobiol. B 92 (2008) 185–189.
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Photoionization of psoralen derivatives in micelles: Imperatorin and alloimperatorin Sameh R. El-Gogary ∗ Chemistry Department, Faculty of Science (Damietta), Mansoura University, New damietta, Egypt
a r t i c l e
i n f o
Article history: Received 11 April 2010 Received in revised form 27 July 2010 Accepted 3 August 2010 Keywords: Psoralen derivative Photoionization Micelle Radical cation Hydrated electron
a b s t r a c t The fluorescence properties of psoralen derivatives, 8-methoxypsoralen (8-MOP), imperatorin (IMP) and alloimperatorin (ALLOI), were investigated in various solvent and micellar solutions. The variation in intensity and maxima of the fluorescence in micellar solutions suggest that psoralens are located in the micelle–water interface region. Radical cations and hydrated electrons were generated by photoionization in micellar solution upon excitation at 266 nm. A nonlinear relationship between transient yield and photon fluency was obtained for each compound, indicating that a two-photon mechanism is predominant in the photoionization of the sensitizers. The photoionization efficiencies are significantly higher in anionic sodium dodecyl sulfate (SDS) than in cationic cetyltrimethylammonium bromide (CTAB) micelles, reflecting the influence of micelle charge on the efficiency of the separation of the photoproduced charge carriers. The photoionization efficiencies of IMP and ALLOI are similar. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Photochemical investigation in mimicking biological systems such as micelles is an extremely active area of research in supramolecular photochemistry [1–4]. Micelles have been used as models for understanding the effect of microheterogeneous environments on reaction dynamics and mechanisms [5,2,6–10]. The photochemical and photophysical studies in simple membranemimicking systems like micelles are particularly important in the case of phototoxic drugs. Although the photochemistry of the drugs in homogeneous media is the first step for understanding the molecular basis of the phototoxic effects, the photoreactivity in homogeneous media and phototoxicity is often not directly correlated to each other. This lack of correlation is due to the formation of the host–guest supramolecular aggregates between drug molecule and the biomolecules. These aggregates display different photochemical and photophysical properties when compared with free drug molecule. An important class of photoactive drugs is psoralens (furocoumarins) which are extensively used in the PUVA (psoralen plus UVA radiation) therapy for the treatment of dermatological disorders [11–13]. The phototherapeutic effects of psoralens are believed to result from intercalation of the drug between adjacent base pairs in the DNA duplex, followed by two successive photocycloaddition reactions that cross-link the DNA. This leads to
inhibition of replication and hence cell proliferation. The long term use of psoralens may have deleterious effects [14–16]. Recently, the photoionization of several psoralens and coumarins in aqueous solution [17–19], micelles [20] protein and liposomes [21] have been studied to elucidate the role of the environment on psoralen photochemistry. Previous results show that a variety of psoralens and coumarins undergo reasonably efficient photoionization upon excitation in aqueous solution [17–19]. The resulting radical cations react with easily oxidizable substrates such as amino acids and nucleotides; primarily via electron-transfer reactions. This raises the possibility that electron-transfer chemistry may play a role in the clinical use of these compounds. Moreover, the photoionization efficiencies are significantly higher in anionic sodium dodecyl sulfate (SDS) micelles than in aqueous solution as a result of favorable electrostatic effects that lead to rapid ejection of the electron into the aqueous phase. By contrast, much lower quantum yields are measured in neutral and cationic micelles (CTAB) [20]. The present study is dedicated to the investigation of the photoionization of imperatorin (IMP) and alloimperatorin (ALLOI), (Scheme 1), two natural psoralen derivatives have not been studied before, in aqueous cationic (CTAB) and anionic (SDS) micelles by means of laser flash photolysis, focusing in measurements of the efficiencies of the formation of hydrated electron and of radical cations. 2. Experimental
∗ Corresponding author. Tel.: +20 572403866; fax: +20 572403868. E-mail address: [email protected]. 1386-1425/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2010.08.009
8-Methoxypsoralen (8-MOP) (Aldrich, 98%), Imperatorin was supplied by Memphis Chemical Co., Egypt; Alloimperatorin was
812
S.R. El-Gogary / Spectrochimica Acta Part A 77 (2010) 811–815
Scheme 1. Chemical structures of psoralen derivatives.
prepared according to literature procedure [22], sodium dodecyl sulfate (SDS, Pharmacia Biotech, 99%) and cetyltrimethylammonium bromide (CTAB, Merck, 99%). All aqueous solutions were prepared with doubly distilled water. The concentrations used for the measurements in micelles were 5 × 10−5 M for the substrate molecules, 5 × 10−3 M for CTAB, and 2.5 × 10−2 M for SDS. The measurements were performed in O2 -saturated aqueous solutions or in air-equilibrated micellar solutions. Fluorescence measurements were performed with a Perkin-Elmer LSB-50 spectrofluorimeter. The nanosecond laser flash photolysis system has been described previously [23]. The fourth harmonics (266 nm) of a Qswitched Nd:YAG laser (Quanta-Ray DCR-1) with 8 ns duration was used to excite the solutions. Transient absorbances were measured in a right-angle set-up using a cell holder with incorporated rectangular apertures defining a reaction volume of dimensions 0.17 cm (height), 0.32 cm (width), and 0.13 cm (depth) within the cell. Pulse energies were measured using a ballistic calorimeter (Raycon-WEC 730). 3. Results and discussion
Fig. 1. Effect of SDS on the fluorescence intensity of MOP (), IMP (䊉) and ALLOI ().
upon addition of cationic micelle (CTAB) in the same order of SDS (IMP > ALLOI > MOP).
3.1. Fluorescence measurements Fluorescence spectra of substrates were measured in a number of different solvents and micellar solution to provide evidence for the location of the substrate in the micelle. The data presented in Table 1 show that the fluorescence maximum shifts to the longer wavelength with increasing solvent polarity for each of the substrate. By comparing the fluorescence maxima in micellar and homogeneous solutions, the microenvironment of the substrates in micellar solutions was found to be more polar than the organic solvents but less polar than water, suggesting that the substrates are located in the micelle–water interface region. The fluorescence spectra varied markedly on the addition of surfactants (SDS and CTAB). The change in fluorescence intensity as a function of surfactant concentration is presented in Figs. 1 and 2. In case of anionic micelle (SDS), the fluorescence intensity of substrates was increased in the order IMP > ALLOI > MOP. In contrast to SDS, the fluorescence intensity of substrates was decreased
3.2. Transient absorption spectra Transient absorption spectra obtained by direct excitation (exc = 266 nm) of an N2 -saturated solution of imperatorin (IMP)
Table 1 Fluorescence maxima of the substrates in various solvent at room temperature.
THF Methanol Water SDS CTAB
MOP
IMP
ALLOI
No 465 510 493 500
No 488 502 495 492
No 477 507 497 493
Fig. 2. Effect of CTAB on the fluorescence intensity of MOP (), IMP (䊉) and ALLOI ().
S.R. El-Gogary / Spectrochimica Acta Part A 77 (2010) 811–815
Fig. 3. Transient absorption spectra obtained via 266 nm excitation of airequilibrated solutions containing IMP (), ALLOI () and () 8-MOP in 25 mM SDS, 200 ns after the laser pulse. All spectra are normalized at A266 = 1, pulse energy = 1 mJ/pulse.
and alloimperatorin (ALLOI) in aqueous and ethanol solution are similar to those previously reported for 8-MOP [24–28], data not shown. The overall absorption in the near-UV had a maximum around 360 nm; a further intense broad absorption band extended to longer wavelengths, with a maximum around 700 nm. The transient spectra are expected to include contributions of the absorptions of triplet states, hydrated electrons, radical cations, and radical anions formed by reaction of hydrated electron with the substrate. The four contributed transients are differed in their life time, the very short-lived transient (hydrated electron), shortlived triplet state of the molecule and the very long-lived radical anion are quenched by oxygen. The residual absorption of oxygensaturated solution is attributed to the very long-lived radical cation. Thus, photolysis of the molecule in oxygen-saturated solution gives rise a transient of the radical cation. Fig. 3 shows the normalized radical cation absorption of airequilibrated aqueous anionic micellar (SDS) solution of IMP, ALLOI and 8-MOP, obtained by direct excitation at 266 nm. The radical cation spectra have a two-band structure with a maximum in the near-UV and another one in the red spectral region. Similar results have been obtained by excitation of both sensitizers in aqueous and cationic micellar (CTAB) solutions, data not shown. The spectra of the radical cations of IMP and ALLOI in micelles are virtually identical to those obtained in aqueous solution. This is consistent with the results of fluorescence which indicates that the IMP and ALLOI are localized near the micellar interface in a relatively polar environment. 3.3. Laser energy dependence of photoionization To get information about the influence of the micellar microenvironment, on the efficiency and nature of the photoionization process of psoralen derivatives, we performed a laser energy effect on the absorbance of radical cation and hydrated electron, monitored at 360 and 700 nm, respectively in the presence and, for comparison, in the absence of micelles. The dependence of the transient absorbance of IMP radical cation at 360 nm in air-equilibrated SDS and CTAB micellar solutions on pulse energy upon direct excitation (ex 266 nm), was shown in Fig. 4, has nonlinear relation, characteristic for the superposition of a monophotonic and a biphotonic ionization pathway with a predominance of the latter. Clearly, the radical cation yield
813
Fig. 4. Plot of signal intensity for the radical cation vs incident laser energy obtained via 266 nm excitation of an air-equilibrated SDS () and CTAB () micellar solutions of imperatorin. Inset shows the plot of the function A/P vs P (P is the laser energy) of radical cation formation of imperatorin in air-equilibrated micellar solutions upon excitation at 266 nm, 25 mM SDS () and 5 mM CTAB (). All data normalized to the same absorbance (1.0) at the wavelength of excitation.
is higher in the anionic SDS micellar solutions than in the cationic CTAB one. In these environments photoionization may occur through a mixture of mono- and biphotonic mechanisms. The monophotonic mechanism occurs from the fluorescent state (first excited state S1 ) when the molecule is excited in its first singlet [29]. The second mechanism requires the consecutive absorption of two photons via the intermediate first excited state S1 or triplet excited state T1 and is appreciable at high excitation intensities [30]. In this case the dependence of photoionization on pulse energy can be described, at low pulse energies, by the following equation [31]: A = aP + bP 2 P being the pulse energy, which is proportional to photon fluency, a is a coefficient depending on the quantum yield of the one-photon process, and b is related to the efficiency of the twophoton process [32]. The plot of the function A/P vs P for IMP in aqueous micelles systems upon excitation at 266 nm is shown in Fig. 4 inset. The very small value obtained for the coefficient a (intercept in Fig. 4 inset) indicates that the one-photon process is minor process in all systems. The significant enhancement of the biphotonic mechanism of the photoionization process in anionic micellar solution (SDS) relative to those in cationic micellar solution (CTAB) was achieved. Similar results have been obtained during direct excitation (ex 266 nm) of air-equilibrated micellar solution containing ALLOI. A plot of the function A/P vs P for ALLOI in micelles system upon excitation at 266 nm is presented in Fig. 5. Further evidence of the photoionization of psoralen derivatives (IMP and ALLOI) is the photoejection of electrons. The hydrated electron formation of IMP and ALLOI were measured in airequilibrated micellar solutions, using 266 nm. Fig. 6 shows the plots of intensities of hydrated electron at 700 nm of IMP and ALLOI (Fig. 6 inset) in air-equilibrated micellar solutions vs pulse energy upon direct excitation (ex 266 nm). The laser energy dependence of the yield of hydrated electron of air-equilibrated solutions (Fig. 6) is nonlinear for each of the compounds, consistent with biphotonic ionization as observed in radical cation measurements. Again the results of the hydrated electron measurements indicate that the photoionization of IMP and ALLOI is considerably higher in anionic micelles (SDS) as compared to cationic micelles (CTAB).
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The results obtained for the photoionization of IMP and ALLOI can be interpreted with regard to the effect of the micellar charge on the separation and recombination of the photoproduced charges. The formation of radical cation and the hydrated electron for each substrate (IMP and ALLOI) is significantly higher in SDS relative to CTAB. In case of SDS the initial photoionization of the micelle associated substrate will generate an electron that is rapidly repelled from the negatively charged micellar interface into the aqueous phase; conversely, the electrostatic effects will favor association of the radical cation with the micelle. This rapid separation of the initial electron/radical cation pair leads to an enhanced yield of detectable radical cations and hydrated electrons. In CTAB, the electron is not able to escape easily from the cationic micelle because of the electrostatic attraction; the recombination of the cation radical and the electron is consequently more efficient, and the yield of detectable species is low. Fig. 5. A plot of the function A/P vs P (P is the laser energy) of radical cation formation of alloimperatorin in air-equilibrated aqueous micellar solutions upon excitation at 266 nm, 25 mM SDS () and 5 mM CTAB (). All data normalized to the same absorbance (1.0) at the wavelength of excitation.
The previous results of psoralen and coumarin derivatives [18] had demonstrated that the radical cation yields in aqueous solution depended linearly on the laser energy for both 308 and 355 nm excitation indicating that the photoionization process is monophotonic with direct contrast to the first photoionization results of 8-MOP in aqueous solution [25]. This discrepancy is due to the use of high pulse energy in the recent photoionization measurements. Furthermore, the photoionization of psoralen and coumarin derivatives in micellar solutions has been reported, indicating that the photoionization yield in micelles increased linearly on the pulse energy for both 308 and 355 nm excitation [20]. Significant enhanced photoionization yields of psoralen and coumarin derivatives were measured in anionic SDS micelles relative to aqueous solution; by contrast, decreased yields were observed in cationic CTAB micelles. On the other hand, by consistent to the results of our work herein, the photoionization of variety aromatic hydrocarbons, amines, an aromatic ketone, modified furocoumarin and furochromones derivatives in aqueous and micellar solutions is biphotonic process [33–37].
Fig. 6. Plot of signal intensity of hydrated electron formation of IMP and ALLOI (inset) in air-equilibrated micellar solutions vs incident laser energy upon excitation at 266 nm, 25 mM SDS () and 5 mM CTAB (). All data normalized to the same absorbance (1.0) at the wavelength of excitation.
4. Conclusion Psoralen derivatives (IMP and ALLOI) are located at the micelle–water interface region in anionic sodium dodecyl sulfate (SDS) and cationic cetyltrimethylammonium bromide (CTAB) micellar solutions. The photoionization of two psoralens (IMP and ALLOI) has been studied in micellar solutions using 266 nm laser flash photolysis. The photoionization efficiencies are significantly higher in anionic (SDS) than in cationic (CTAB) micelles, reflecting the influence of micelle charge on the efficiency of the separation of the photoproduced charge carriers. Nonlinear energy dependence plots for radical cation or hydrated electron formation of IMP and ALLOI demonstrate that photoionization is mainly biphotonic. Acknowledgments Special thanks go to all members in professor Koehler’s group, Max F. Perutz Laboratories, University of Vienna, Vienna, Austria for their friendly reception and kind help. References [1] V. Ramamurthy, Photochemistry in Organized and Constrained Media, VCH Publisher, New York, 1991. [2] K. Kalyanasundaram, Photochemistry in Microheterogeneous Systems, Academic Press, Orlando, FL, 1987. [3] P. Bortolus, S. Monti, Adv. Photochem. 21 (1996) 1. [4] M.H. Kleinman, C. Bohne, Organic Photochemistry: Molecular and Supramolecular Photochemistry, Marcel Dekker, New York, 1997. [5] M. Gratzel, in: M. Gratzel (Ed.), Solar Energy Harvesting, Elsevier, New York, 1988, pp. 394–440. [6] M.A. Fox, Top. Curr. Chem. 159 (1991) 67–101. [7] N.J. Turro, Pure Appl. Chem. 67 (1995) 199–208. [8] K. Weidemaier, H.L. Tavernier, M.D. Fayer, J. Phys. Chem. B 101 (1997) 9352–9361. [9] I.V. Soboleva, J. van Stam, G.B. Dutt, M.G. Kuzmin, F.C. De Schryver, Langmuir 15 (1999) 6201–6207. [10] J.W. Hackett II, C. Turro, J. Phys. Chem. A 102 (1998) 5728–5733. [11] F.P. Gasparro, Psoralen DNA Photobiology, vols. 1 and 2, CRC Press, Boca Raton, FL, 1988. [12] M.A. Pathak, T.B. Fitzpatrick, J. Photochem. Photobiol. B: Biol. 14 (1992) 3–22. [13] R.V. Bensasson, E.J. Land, T.G. Truscott, Excited States and Free Radicals in Biology and Medicine, Oxford University Press, Oxford, 1993. [14] E. Gonzalez, Dermatol. Clin. 13 (1995) 851–866. [15] R.S. Stern, K.T. Nichols, L.H. Vakeva, N. Engl. J. Med. 336 (1997) 1041–1045. [16] A.R. Young, J. Photochem. Photobiol. B: Biol. 6 (1990) 237–247. [17] P.D. Wood, A. Mnyusiwalla, L. Chen, L.J. Johnston, Photochem. Photobiol. 72 (2000) 155–162. [18] P.D. Wood, L.J. Johnston, J. Phys. Chem. 102 (1998) 5585–5591. [19] P.D. Wood, L.J. Johnston, Photochem. Photobiol. 66 (1997) 642–648. [20] P.D. Wood, A. Mnyusiwalla, L. Chen, J. Marlinga, L.J. Johnston, Phys. Chem. B 105 (2001) 10927–10935. [21] L. Chen, O. Rinco, J. Popov, N. Vuong, L.J. Johnston, Photochem. Photobiol. 82 (2006) 31–37. [22] T. Noguti, M. Kawakami, J. Pharm. Soc. Jpn. 61 (1941) 77. [23] P. Baranyai, S. Gangl, G. Grabner, M. Knapp, G. Köhler, T. Vidóczy, Langmuir 15 (1999) 7577–7585.
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