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Effects of solvent polarity and hydrogen bonding on the fluorescence properties of trans-4-hydroxy-4′-nitrostilbenes
Chemical Physics Letters 489 (2010) 59–63
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Effects of solvent polarity and hydrogen bonding on the fluorescence properties of trans-4-hydroxy-40 -nitrostilbenes Mónika Megyesi a, László Biczók a,*, Helmut Görner b, Zsombor Miskolczy a a b
Chemical Research Center, Hungarian Academy of Sciences, P.O. Box 17, 1525 Budapest, Hungary Max-Planck-Institute for Bioinorganic Chemistry, P.O. Box 101365, D-45413 Mülheim an der Ruhr, Germany
a r t i c l e
i n f o
Article history: Received 12 January 2010 In final form 17 February 2010 Available online 21 February 2010
a b s t r a c t The excited singlet state properties of 40 -nitro- and 20 ,40 -dinitro-substituted trans-4-hydroxystilbenes were studied in various solvents. The fluorescence quantum yield (Uf) of the former compound in solvents unable to form strong hydrogen bonds goes through a maximum as a function of the solvent polarity parameter ENT , reaching the highest value of 0.31 in butyronitrile. In contrast, a monotonous diminution of Uf and moderate changes were found in hydrogen bonding media. The addition of nitrogen-heterocyclic compounds leads to both dynamic and static fluorescence quenching due to excited-state proton transfer along the hydrogen bond. trans-20 ,40 -Dinitro-4-hydroxystilbene emits only weak fluorescence in all solvents studied. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction The fluorescence of stilbene and its derivatives has been the subject of various investigations [1–15]. The quantum yield of trans ? cis isomerization is usually large, whereas the quantum yield of fluorescence (Uf) and intersystem crossing at the trans side are generally small [9–17]. This can be different for push–pull 4-R40 -R0 -stilbenes [1–9]. Solvent effects on the time-resolved fluorescence spectra of various stilbene derivatives have recently received special attention [18–20]. Despite the wealth of information available on the photochemistry of stilbenes, little attention has been devoted to the hydroxy-substituted derivatives. These substances have antioxidant activity and trans-3,5,40 -trihydroxystilbene (resveratrol) is a well-known anticancer agent [21–23]. Electron donating moieties on the hydroxystilbene skeleton enhance the radical scavenging efficiency and the compound bearing o-dihydroxyl groups proved to be the most reactive [23]. The rate of the various excited-state deactivation processes was found to depend on the location of the OH-moiety [24–27]. 3-Hydroxy-substitution of stilbene prolongs the fluorescence lifetime (sf) and enhances Uf [26], whereas attachment of a nitro group accelerates the transition into triplet-excited state resulting in a short sf and a small Uf. The efficient intersystem crossing facilitates photoisomerization via the triplet state [28–36]. This mechanism is operating for trans-4-nitrostilbene and its derivatives substituted with an electron donating substituent in 40 -position. In the present work, the combined effect of nitro- and hydroxysubstitution on the fluorescent behavior was studied in various
* Corresponding author. Fax: +36 1 325 7554. E-mail address: [email protected] (L. Biczók). 0009-2614/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2010.02.050
solvents at room temperature in order to compare the photophysical characteristics of trans-4-hydroxy-40 -nitrostilbene (trans-HN) and its 20 ,40 -dinitro analogue (trans-HDN), Scheme 1. So far, trans-HN has been used as non-linear-optical material [37,38] and the hyperpolarizability was determined [39]. Both trans-HN and trans-HDN show antibacterial and antiproliferative effects [40,41], but their photoinitiated processes are practically unknown. An exception is a paper from Splitter and Calvin, which described the formation of 2-(4-hydroxy-phenyl)-6-nitro-indol-3one-1-oxide, when trans-HDN in benzene was irradiated with sunlight [42]. In view of the biological activity of trans-HN and trans-HDN, it is important to reveal how the local polarity and hydrogen bonding affect their fluorescent properties. 2. Materials and methods trans-HN was synthesized by condensation of the corresponding phenyl acetic acid derivative with 4-hydroxybenzaldehyde [43], whereas trans-HDN was prepared from the reaction of 2,4dinitrotoluene with 4-hydroxybenzaldehyde [42,43]. The solvents (Aldrich, Merck) were of the purest spectroscopic quality (Uvasol or HPLC) and used as received. The absorption spectra were monitored on a spectrophotometer (Unicam UV 500 or HP 8453). A spectrofluorimeter (Cary Eclipse or Jobin–Yvon Fluoromax-P) was employed to record the fluorescence spectra. Uf was obtained using optically matched solutions and 9,10-diphenylanthracene in cyclohexane as reference, for which Uf = 0.70 was reported under air [44]. Fluorescence decay was measured with a timecorrelated single-photon counting technique: a Picoquant diode laser (pulse duration ca. 70 ps, wavelength 372 nm) excited the samples, and the fluorescence decays were detected with a
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M. Megyesi et al. / Chemical Physics Letters 489 (2010) 59–63
O2N OH
trans-HN NO2 O2N OH
the similar ENT parameter of acetonitrile and dimethyl sulfoxide (DMSO), a marked red-shift of the absorption and fluorescence bands is seen in the latter solvent. As the b parameter of DMSO is much larger, the displacement of the spectra in this solvent is due to hydrogen bonding of the phenolic OH group to the oxygen atoms of the solvent significantly modifying the energy of the first singlet-excited state. The behavior of trans-HN in dimethylformamide (DMF) also does not fit the trend observed in weak hydrogen bond acceptor solvents (Table 1), showing again the considerable effect of hydrogen bonding on the spectral characteristics. The Stokes shift increases substantially with growing polarity in nonhydrogen bonding solvents, but only small changes occur in hydrogen bonding media. 3.2. Photophysical behavior of trans-HN
trans-HDN Scheme 1.
Hamamatsu R3809U-51 microchannel plate photomultiplier. The time resolution, when appropriate connected to a Timeharp 100 electronics, is 36 ps/channel. Data were analyzed by a non-linear least-squares deconvolution method using the FLUOFIT software. All measurements were performed in air-saturated solutions at 297 ± 2 K. 3. Results and discussion 3.1. Solvent effect on the absorption and fluorescence spectra of transHN Fig. 1 shows the absorption and fluorescence spectra of transHN, whereas the location of the maximum of the first bands (kabs ~) are compiled in Table 1. and kf) as well as the Stokes shift (Dm Introduction of a nitro group in the 40 position of 4-hydroxystilbene brings about a significant bathochromic displacement of the absorption and fluorescence spectra. The maximum of the bands appears at even lower energy than that observed for 4-nitrostilbene. This and the considerable Stokes shift indicate the push–pull effect of the electron withdrawing nitro and the electron donating OH moieties, which emerges primarily in the excited state. The absorption spectrum in chlorobenzene barely differs from that observed in more polar solvents such as acetonitrile. As a measure of solvent polarity, the ENT parameter [45] was used, whereas b [46] reflects the hydrogen bond acceptor basicity (Table 1). Despite
Fig. 1. Absorption and fluorescence (normalized intensity) spectra of trans-HN in chlorobenzene (dotted line), acetonitrile (thin line) and dimethyl sulfoxide (thick line).
trans-4-Nitrostilbene is practically non-fluorescent [32] but its 40 -hydroxy derivative shows strongly solvent-dependent fluorescence characteristics. Fig. 2 displays the variation of Uf of transHN as a function of ENT solvent polarity parameter. The most remarkable feature of this plot is that the data in solvents capable of hydrogen bonding constitute an entirely distinct group, where Uf moderately diminishes with the increase of ENT . On the other hand, in media unable to form strong hydrogen bonds, Uf goes through a maximum, reaching the largest value in butyronitrile (Uf = 0.31). Fig. 2 is reminiscent to a similar behavior of trans-4alkylamino-40 -nitrostilbenes, where the presence of an excited charge transfer state has been postulated [34,35]. The maximum in the plot of Uf versus ENT of trans-4-amino-40 -nitrostilbene has been ascribed to contrasting solvent polarity dependences: efficient intersystem crossing at the trans side in solvents of low polarity and the involvement of deactivation via an ICT state, where a partial electron transfer from the amino entity to the nitro group takes place, in more polar solvents. The latter process is most efficient in a polar solvent. An analogous case is also proposed for trans-HN in non-hydrogen-bonding solvents; the increase of solvent polarity accelerates the internal conversion to the ground state as expected on the basis of the energy gap law. The very low quantum yield of UF 6 104 for trans-HN in water–methanol mixture (1:9) is in agreement with the trend shown in Table 1. Excited-state proton transfer to water may contribute to the rapid radiationless deactivation. The presence of phosphate buffer at pH 7 has no marked effect. The fluorescence intensity decays follow single-exponential kinetics. The lifetime is sf = 2.0 ns in butyronitrile and smaller in the other solvents. Moreover, sf and Uf are correlated (Tables 1 and 2). The low intensity and/or the short lifetime of the fluorescence thwarted a determination of sf in toluene and alcohols. The rate constant of fluorescence emission (kf = Uf/sf) is insensitive to the hydrogen bonding character of the solvent and grows slightly with increasing solvent polarity (Table 2). For trans-HN in butyronitrile kf = 1.6 108 s1 and the rate constant of radiationless decay is knr = 3.5 108 s1. The kf value of trans-HN is about an order of magnitude smaller than that reported for 4-hydroxystilbene in tetrahydrofuran [26]. This is in accord with the Strickler– Berg relationship [47], which predicts the deceleration of the radiative process when the fluorescence band shifts toward lower energy. As major deactivation processes of excited trans-HN, we propose intersystem crossing in solvents of low polarity. The marked red-shift of the fluorescence maximum with increasing solvent polarity reflects the diminution of the energy gap between the lowest singlet-excited and ground states, which leads to the acceleration of internal conversion. Interestingly, the interaction with DMF and DMSO, the solvents of considerable hydrogen bond acceptor character, significantly increases knr (Table 2) indicating that the vibrations associated with the hydrogen-bound OH-group
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M. Megyesi et al. / Chemical Physics Letters 489 (2010) 59–63 Table 1 Effects of solvent polarity and hydrogen bond acceptor basicity on kabs and the fluorescence properties of trans-HN. Data in parenthesis refer to trans-HDN.
a b
Solvent
a EN T
bb
kabs (nm)
kf (nm)
~ 103 Dm (cm1)
Uf 103
Toluene Cl-benzene 1,2-Cl2-benzene Ethyl acetate CH2Cl2 Butyronitrile Dimethylformamide Propionitrile Dimethyl sulfoxide Acetonitrile 2-Propanol 1-Butanol Ethanol Methanol
0.099 0.188 0.225 0.228 0.309 0.364 0.386 0.398 0.444 0.460 0.546 0.586 0.654 0.762
0.11 0.07
372 376 378 375 372 380 392 377 396 376 382 383 382 378
490 521 541 535 572 597 640 618 653 621 625 625 628 683
6.8 (5.2) 7.8 (5.9) 8.3 (5.8) 8.5 (6.8) 9.2 (7.6) 9.9 (7.6) 10.2 (8.3) 10.3 (8.5) 10.3 (8.5) 10.8 (8.6) 10.8 (7.0) 10.6 (6.8) 10.8 (7.1) 11.8 (7.1)
<0.5 47 135 91 240 310 31 260 17 110 7.0 7.9 2.0 1.4
0.45 0.00 0.69 0.37 0.76 0.31 0.95 0.88 0.77 0.62
(398) (400) (402) (399) (395) (400) (410) (399) (413) (399) (409) (409) (406) (399)
(494) (516) (518) (547) (550) (567) (613) (590) (630) (601) (554) (552) (556) (556)
Ref. [45]. Ref. [46].
trans-HN. The introduction of the second nitro substituent leads to about 24 nm red-shift of the first absorption maximum, whereas the second band peaking around 250 nm exhibits hyperchromicity. Similar to the behavior of trans-HN, the variation of weakly hydrogen bonding solvents exerts a small solvent effect on trans-HDN absorption. However, the absorption peak appears at lower energy in DMSO, DMF and alcohols, where hydrogen bonding with the solute plays a significant role. The Stokes shift of the fluorescence maximum is smaller for trans-HDN compared to that found for trans-HN. The fluorescence quantum yield of trans-HDN is low (Uf < 1 104) in all solvents studied, and the dependence of Uf versus ENT does not show a trend such as that observed for transHN. It has also been reported for other trans-2,4-dinitrostilbene derivatives that the solvent properties generally have no marked effect on Uf [32]. Fig. 2. Plot of the fluorescence quantum yield (kexc = 355 nm) of trans-HN as a function of the solvent polarity parameter ENT . Open symbols represent the data in solvents capable of hydrogen bonding.
Table 2 Solvent effect on the fluorescence decay properties of trans-HN.a Solvent Cl-benzene 1,2-Cl2-benzene Ethyl acetate CH2Cl2 Butyronitrile Dimethylformamide Propionitrile Dimethyl sulfoxide Acetonitrile
(ns)
kf 108 (s1)
knr 108 (s1)
0.38 1.3 0.90 1.8 2.0 0.20 1.6 0.13 0.79
1.2 1.0 1.0 1.3 1.6 1.6 1.6 1.3 1.4
25 6.7 10 4.2 3.5 48 4.6 76 11
sf
3.4. Hydrogen bonding with nitrogen-heterocyclic compounds In order to separate the effect of bulk polarity from that of hydrogen bonding, complexation with nitrogen-heterocyclic compounds was studied. The addition of pyridine, 2,4,6-trimethylpyridine (TMPy) or 1-methylimidazole (MIm) to trans-HN or transHDN solutions brings about bathochromic displacement of the first absorption band. The relatively small shifts suggest that that the phenolic OH groups are not deprotonated by pyridine and MIm. This is in accordance with the reported low acidity of trans-HN (pKa = 10.1) in 1:1 ethanol-mixtures [48]. As a representative
a For the other solvents the decay time is shorter than the time-resolution of our instrument.
of HN may act as effective accepting modes in the energy dissipation. Excited-state proton transfer to the solvent is not probable because DMSO and DMF are weak bases. 3.3. Spectral characteristics of trans-HDN For the sake of comparison, the absorption and fluorescence maxima of trans-HDN are given in parenthesis in Table 1. The absorption spectrum of trans-HDN resembles that found for
Fig. 3. Effect of pyridine on the absorption spectrum of HN in CH2Cl2. Inset displays the absorbance change at 410 nm and the line represents the fitted function.
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M. Megyesi et al. / Chemical Physics Letters 489 (2010) 59–63
Table 3 Effect of hydrogen bonding between trans-HN and nitrogen-heterocyclic compounds.
a b c
Additive
Solvent
pKa of additive
Pyridine 2,4,6-Trimethylpyridine 1-Methylimidazole Pyridine 1-Methylimidazole
CH2Cl2 CH2Cl2 CH2Cl2 CH3CN CH3CN
5.25a 7.43a 7.88c 5.25a 7.88c
K/M1 Absorption
Fluorescence
31 (41)b 69 (105) 110 (170) 0.7 (1.2) 7.7 (10)
33 46 120 0.5 6.4
kq/109 M1s1 11 7.9 14 2.7 4.1
Taken from Ref. [49]. In parentheses: the binding constants of trans-HDN. Taken from Ref. [50].
example, Fig. 3 shows the shift of the absorption spectra of transHN in dichloromethane with gradually increasing pyridine concentration. The isosbestic points at 311 and 376 nm suggest 1:1 complexation. The absorbance change (inset to Fig. 3) can be well described by the following function:
Ak ¼ A0k
1 þ e0k =ek K½additive 1 þ K½additive
ð1Þ
Here (e0k =ek ) is the ratio of the molar absorption coefficients for the complexed and unbound trans-HN at a particular wavelength, A0k and Ak denote the absorbance in the absence and the presence of pyridine, respectively. The non-linear least-squares fit of Eq. (1) to the experimental data gives K = 31 M1 for the equilibrium constant of hydrogen bonding in dichloromethane. Using TMPy and MIm as stronger hydrogen bond acceptor additives, K = 69 and 110 M1 are obtained, respectively. As expected for hydrogen bonding, K values significantly decrease in polar solvents (Table 3). The equilibrium constants for trans-HNpyridine and transHNMIm complex formation in acetonitrile are 0.7 and 7.7 M1, respectively. Substitution with a second nitro group enhances the strength of hydrogen bonding. K = 41 and 170 M1 are obtained for pyridine and MIm binding to trans-HDN in dichloromethane, respectively. The electron withdrawing effect of the two nitro moieties diminishes the charge density on the phenolic OH group, thereby augmenting the hydrogen bonding affinity. It is worth noting that an addition of tetrabutylammonium fluoride to trans-HN in acetonitrile leads to a violet solution, indicating that a deprotonation of the OH moiety takes place. The phenolate form, produced thereby, has more substantial charge transfer character than the hydrogen-bound complexes. Therefore, its absorption spectrum appears at much lower energy. The first absorption band is very broad extending from 370 to 760 nm with a maximum at 543 nm, whereas the second absorption peak is located at 331 nm. No emission is observed from the excited phenolate form. 3.5. Fluorescence quenching by hydrogen bond acceptors As trans-HDN is barely fluorescent, the singlet-excited state quenching was studied only for trans-HN. Gradual addition of a hydrogen bond acceptor expedites the transition into the ground state, but the fluorescence decay remains single-exponential. The reciprocal fluorescence lifetime always shows excellent linear correlation with the additive concentration. The slope of this type of plot gives the rate constant of singlet-excited trans-HN quenching by various N-heterocyclic compounds (kq in Table 3). In CH2Cl2, the quenching rates are close to the diffusion-controlled limit. The somewhat lower value obtained for TMPy may arise from steric hindrance of the excited hydrogen-bound complex formation by the methyl groups in the 2 and 6 position of the heterocyclic ring. The stronger solvent–solute interactions in the more polar acetonitrile slow down the dynamic quenching compared to that observed
Fig. 4. Stern–Volmer plot of the fluorescence intensity at 570 nm for quenching of trans-HN by pyridine in CH2Cl2. The line displays the best fit of Eq. (2). Inset: fluorescence spectra (normalized intensity) upon addition of 0, 0.4, 1.2, 2.8, 5.2, 9.0, 16, 30, 59, 107, 210 and 500 mM pyridine.
in CH2Cl2. Opposite trends would be expected if the quenching occurred via an electron transfer mechanism. The more basic MIm is a more efficient quencher than pyridine indicating that proton transfer within the excited hydrogen-bound complex leads to energy dissipation. The steady-state fluorescence measurements show that the addition of N-heterocyclic compounds brings about neither the appearance of a new band nor a shift of the fluorescence maximum. On the basis of these observations, we conclude that the hydrogen-bound complexes of trans-HN have negligible fluorescence yield. The Stern–Volmer plot of the fluorescence intensities in the absence (I0) and the presence (I) of an additive exhibits an upward curvature (Fig. 4). This suggests that the fluorophore can be quenched both in dynamic and static processes. If the static quenching is attributed entirely to ground state hydrogen-bonding, the modified form of the Stern–Volmer equation describes the variation of I0/I versus the quencher concentration [47],
I0 =I ¼ ð1 þ K½quencherÞð1 þ kq sf ½quencherÞ
ð2Þ
where kq is the rate constant of the dynamic quenching and K denotes the equilibrium constant of complex formation in the ground state. As a representative example, Fig. 4 shows the results on transHN fluorescence quenching by pyridine. Taking kq determined by the time-resolved fluorescence technique (vide supra), K values were calculated from the non-linear least-square fit of Eq. (2) to the experimental data. It is apparent from Table 3 that the ground-state binding constants obtained by the fluorescence method are in good agreement with the corresponding values derived from absorption measurements.
M. Megyesi et al. / Chemical Physics Letters 489 (2010) 59–63
4. Conclusions Substitution of 4-hydroxystilbene with a nitro group at 40 -position augments the extent of the intramolecular electron density redistribution upon photoexcitation and leads to a substantial decrease in the energy of the first singlet-excited state, while introduction of the second nitro moiety further enhances these effects. Hydrogen bonding of the phenolic OH group of trans-HN significantly diminishes the fluorescence quantum yield and shortens the fluorescence lifetime. The radiative rate constant barely changes upon binding to weakly basic compounds, such as DMF and DMSO, but the radiationless deactivation is accelerated. When stronger bases are bound to the OH moiety of trans-HN, the extent of the proton shift along the hydrogen-bond increases and the intramolecular charge transfer character of the singlet-excited state is enhanced. These effects decrease the energy of the lowest singlet-excited state. The vibrational coupling enhancement caused thereby leads to a substantial increase in the rate of internal conversion, as is expected on the basis of the energy gap law. Acknowledgements We thank Professor W. Lubitz for his support and Mr. L.J. Currell for technical assistance. The authors very much appreciate the support of this work by the Hungarian Scientific Research Fund (OTKA, Grant K75015) and the bilateral program between the Deutsche Forschungsgemeinschaft and the Hungarian Academy of Sciences. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
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Effects of solvent polarity and hydrogen bonding on the fluorescence properties of trans-4-hydroxy-40 -nitrostilbenes Mónika Megyesi a, László Biczók a,*, Helmut Görner b, Zsombor Miskolczy a a b
Chemical Research Center, Hungarian Academy of Sciences, P.O. Box 17, 1525 Budapest, Hungary Max-Planck-Institute for Bioinorganic Chemistry, P.O. Box 101365, D-45413 Mülheim an der Ruhr, Germany
a r t i c l e
i n f o
Article history: Received 12 January 2010 In final form 17 February 2010 Available online 21 February 2010
a b s t r a c t The excited singlet state properties of 40 -nitro- and 20 ,40 -dinitro-substituted trans-4-hydroxystilbenes were studied in various solvents. The fluorescence quantum yield (Uf) of the former compound in solvents unable to form strong hydrogen bonds goes through a maximum as a function of the solvent polarity parameter ENT , reaching the highest value of 0.31 in butyronitrile. In contrast, a monotonous diminution of Uf and moderate changes were found in hydrogen bonding media. The addition of nitrogen-heterocyclic compounds leads to both dynamic and static fluorescence quenching due to excited-state proton transfer along the hydrogen bond. trans-20 ,40 -Dinitro-4-hydroxystilbene emits only weak fluorescence in all solvents studied. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction The fluorescence of stilbene and its derivatives has been the subject of various investigations [1–15]. The quantum yield of trans ? cis isomerization is usually large, whereas the quantum yield of fluorescence (Uf) and intersystem crossing at the trans side are generally small [9–17]. This can be different for push–pull 4-R40 -R0 -stilbenes [1–9]. Solvent effects on the time-resolved fluorescence spectra of various stilbene derivatives have recently received special attention [18–20]. Despite the wealth of information available on the photochemistry of stilbenes, little attention has been devoted to the hydroxy-substituted derivatives. These substances have antioxidant activity and trans-3,5,40 -trihydroxystilbene (resveratrol) is a well-known anticancer agent [21–23]. Electron donating moieties on the hydroxystilbene skeleton enhance the radical scavenging efficiency and the compound bearing o-dihydroxyl groups proved to be the most reactive [23]. The rate of the various excited-state deactivation processes was found to depend on the location of the OH-moiety [24–27]. 3-Hydroxy-substitution of stilbene prolongs the fluorescence lifetime (sf) and enhances Uf [26], whereas attachment of a nitro group accelerates the transition into triplet-excited state resulting in a short sf and a small Uf. The efficient intersystem crossing facilitates photoisomerization via the triplet state [28–36]. This mechanism is operating for trans-4-nitrostilbene and its derivatives substituted with an electron donating substituent in 40 -position. In the present work, the combined effect of nitro- and hydroxysubstitution on the fluorescent behavior was studied in various
* Corresponding author. Fax: +36 1 325 7554. E-mail address: [email protected] (L. Biczók). 0009-2614/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2010.02.050
solvents at room temperature in order to compare the photophysical characteristics of trans-4-hydroxy-40 -nitrostilbene (trans-HN) and its 20 ,40 -dinitro analogue (trans-HDN), Scheme 1. So far, trans-HN has been used as non-linear-optical material [37,38] and the hyperpolarizability was determined [39]. Both trans-HN and trans-HDN show antibacterial and antiproliferative effects [40,41], but their photoinitiated processes are practically unknown. An exception is a paper from Splitter and Calvin, which described the formation of 2-(4-hydroxy-phenyl)-6-nitro-indol-3one-1-oxide, when trans-HDN in benzene was irradiated with sunlight [42]. In view of the biological activity of trans-HN and trans-HDN, it is important to reveal how the local polarity and hydrogen bonding affect their fluorescent properties. 2. Materials and methods trans-HN was synthesized by condensation of the corresponding phenyl acetic acid derivative with 4-hydroxybenzaldehyde [43], whereas trans-HDN was prepared from the reaction of 2,4dinitrotoluene with 4-hydroxybenzaldehyde [42,43]. The solvents (Aldrich, Merck) were of the purest spectroscopic quality (Uvasol or HPLC) and used as received. The absorption spectra were monitored on a spectrophotometer (Unicam UV 500 or HP 8453). A spectrofluorimeter (Cary Eclipse or Jobin–Yvon Fluoromax-P) was employed to record the fluorescence spectra. Uf was obtained using optically matched solutions and 9,10-diphenylanthracene in cyclohexane as reference, for which Uf = 0.70 was reported under air [44]. Fluorescence decay was measured with a timecorrelated single-photon counting technique: a Picoquant diode laser (pulse duration ca. 70 ps, wavelength 372 nm) excited the samples, and the fluorescence decays were detected with a
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M. Megyesi et al. / Chemical Physics Letters 489 (2010) 59–63
O2N OH
trans-HN NO2 O2N OH
the similar ENT parameter of acetonitrile and dimethyl sulfoxide (DMSO), a marked red-shift of the absorption and fluorescence bands is seen in the latter solvent. As the b parameter of DMSO is much larger, the displacement of the spectra in this solvent is due to hydrogen bonding of the phenolic OH group to the oxygen atoms of the solvent significantly modifying the energy of the first singlet-excited state. The behavior of trans-HN in dimethylformamide (DMF) also does not fit the trend observed in weak hydrogen bond acceptor solvents (Table 1), showing again the considerable effect of hydrogen bonding on the spectral characteristics. The Stokes shift increases substantially with growing polarity in nonhydrogen bonding solvents, but only small changes occur in hydrogen bonding media. 3.2. Photophysical behavior of trans-HN
trans-HDN Scheme 1.
Hamamatsu R3809U-51 microchannel plate photomultiplier. The time resolution, when appropriate connected to a Timeharp 100 electronics, is 36 ps/channel. Data were analyzed by a non-linear least-squares deconvolution method using the FLUOFIT software. All measurements were performed in air-saturated solutions at 297 ± 2 K. 3. Results and discussion 3.1. Solvent effect on the absorption and fluorescence spectra of transHN Fig. 1 shows the absorption and fluorescence spectra of transHN, whereas the location of the maximum of the first bands (kabs ~) are compiled in Table 1. and kf) as well as the Stokes shift (Dm Introduction of a nitro group in the 40 position of 4-hydroxystilbene brings about a significant bathochromic displacement of the absorption and fluorescence spectra. The maximum of the bands appears at even lower energy than that observed for 4-nitrostilbene. This and the considerable Stokes shift indicate the push–pull effect of the electron withdrawing nitro and the electron donating OH moieties, which emerges primarily in the excited state. The absorption spectrum in chlorobenzene barely differs from that observed in more polar solvents such as acetonitrile. As a measure of solvent polarity, the ENT parameter [45] was used, whereas b [46] reflects the hydrogen bond acceptor basicity (Table 1). Despite
Fig. 1. Absorption and fluorescence (normalized intensity) spectra of trans-HN in chlorobenzene (dotted line), acetonitrile (thin line) and dimethyl sulfoxide (thick line).
trans-4-Nitrostilbene is practically non-fluorescent [32] but its 40 -hydroxy derivative shows strongly solvent-dependent fluorescence characteristics. Fig. 2 displays the variation of Uf of transHN as a function of ENT solvent polarity parameter. The most remarkable feature of this plot is that the data in solvents capable of hydrogen bonding constitute an entirely distinct group, where Uf moderately diminishes with the increase of ENT . On the other hand, in media unable to form strong hydrogen bonds, Uf goes through a maximum, reaching the largest value in butyronitrile (Uf = 0.31). Fig. 2 is reminiscent to a similar behavior of trans-4alkylamino-40 -nitrostilbenes, where the presence of an excited charge transfer state has been postulated [34,35]. The maximum in the plot of Uf versus ENT of trans-4-amino-40 -nitrostilbene has been ascribed to contrasting solvent polarity dependences: efficient intersystem crossing at the trans side in solvents of low polarity and the involvement of deactivation via an ICT state, where a partial electron transfer from the amino entity to the nitro group takes place, in more polar solvents. The latter process is most efficient in a polar solvent. An analogous case is also proposed for trans-HN in non-hydrogen-bonding solvents; the increase of solvent polarity accelerates the internal conversion to the ground state as expected on the basis of the energy gap law. The very low quantum yield of UF 6 104 for trans-HN in water–methanol mixture (1:9) is in agreement with the trend shown in Table 1. Excited-state proton transfer to water may contribute to the rapid radiationless deactivation. The presence of phosphate buffer at pH 7 has no marked effect. The fluorescence intensity decays follow single-exponential kinetics. The lifetime is sf = 2.0 ns in butyronitrile and smaller in the other solvents. Moreover, sf and Uf are correlated (Tables 1 and 2). The low intensity and/or the short lifetime of the fluorescence thwarted a determination of sf in toluene and alcohols. The rate constant of fluorescence emission (kf = Uf/sf) is insensitive to the hydrogen bonding character of the solvent and grows slightly with increasing solvent polarity (Table 2). For trans-HN in butyronitrile kf = 1.6 108 s1 and the rate constant of radiationless decay is knr = 3.5 108 s1. The kf value of trans-HN is about an order of magnitude smaller than that reported for 4-hydroxystilbene in tetrahydrofuran [26]. This is in accord with the Strickler– Berg relationship [47], which predicts the deceleration of the radiative process when the fluorescence band shifts toward lower energy. As major deactivation processes of excited trans-HN, we propose intersystem crossing in solvents of low polarity. The marked red-shift of the fluorescence maximum with increasing solvent polarity reflects the diminution of the energy gap between the lowest singlet-excited and ground states, which leads to the acceleration of internal conversion. Interestingly, the interaction with DMF and DMSO, the solvents of considerable hydrogen bond acceptor character, significantly increases knr (Table 2) indicating that the vibrations associated with the hydrogen-bound OH-group
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M. Megyesi et al. / Chemical Physics Letters 489 (2010) 59–63 Table 1 Effects of solvent polarity and hydrogen bond acceptor basicity on kabs and the fluorescence properties of trans-HN. Data in parenthesis refer to trans-HDN.
a b
Solvent
a EN T
bb
kabs (nm)
kf (nm)
~ 103 Dm (cm1)
Uf 103
Toluene Cl-benzene 1,2-Cl2-benzene Ethyl acetate CH2Cl2 Butyronitrile Dimethylformamide Propionitrile Dimethyl sulfoxide Acetonitrile 2-Propanol 1-Butanol Ethanol Methanol
0.099 0.188 0.225 0.228 0.309 0.364 0.386 0.398 0.444 0.460 0.546 0.586 0.654 0.762
0.11 0.07
372 376 378 375 372 380 392 377 396 376 382 383 382 378
490 521 541 535 572 597 640 618 653 621 625 625 628 683
6.8 (5.2) 7.8 (5.9) 8.3 (5.8) 8.5 (6.8) 9.2 (7.6) 9.9 (7.6) 10.2 (8.3) 10.3 (8.5) 10.3 (8.5) 10.8 (8.6) 10.8 (7.0) 10.6 (6.8) 10.8 (7.1) 11.8 (7.1)
<0.5 47 135 91 240 310 31 260 17 110 7.0 7.9 2.0 1.4
0.45 0.00 0.69 0.37 0.76 0.31 0.95 0.88 0.77 0.62
(398) (400) (402) (399) (395) (400) (410) (399) (413) (399) (409) (409) (406) (399)
(494) (516) (518) (547) (550) (567) (613) (590) (630) (601) (554) (552) (556) (556)
Ref. [45]. Ref. [46].
trans-HN. The introduction of the second nitro substituent leads to about 24 nm red-shift of the first absorption maximum, whereas the second band peaking around 250 nm exhibits hyperchromicity. Similar to the behavior of trans-HN, the variation of weakly hydrogen bonding solvents exerts a small solvent effect on trans-HDN absorption. However, the absorption peak appears at lower energy in DMSO, DMF and alcohols, where hydrogen bonding with the solute plays a significant role. The Stokes shift of the fluorescence maximum is smaller for trans-HDN compared to that found for trans-HN. The fluorescence quantum yield of trans-HDN is low (Uf < 1 104) in all solvents studied, and the dependence of Uf versus ENT does not show a trend such as that observed for transHN. It has also been reported for other trans-2,4-dinitrostilbene derivatives that the solvent properties generally have no marked effect on Uf [32]. Fig. 2. Plot of the fluorescence quantum yield (kexc = 355 nm) of trans-HN as a function of the solvent polarity parameter ENT . Open symbols represent the data in solvents capable of hydrogen bonding.
Table 2 Solvent effect on the fluorescence decay properties of trans-HN.a Solvent Cl-benzene 1,2-Cl2-benzene Ethyl acetate CH2Cl2 Butyronitrile Dimethylformamide Propionitrile Dimethyl sulfoxide Acetonitrile
(ns)
kf 108 (s1)
knr 108 (s1)
0.38 1.3 0.90 1.8 2.0 0.20 1.6 0.13 0.79
1.2 1.0 1.0 1.3 1.6 1.6 1.6 1.3 1.4
25 6.7 10 4.2 3.5 48 4.6 76 11
sf
3.4. Hydrogen bonding with nitrogen-heterocyclic compounds In order to separate the effect of bulk polarity from that of hydrogen bonding, complexation with nitrogen-heterocyclic compounds was studied. The addition of pyridine, 2,4,6-trimethylpyridine (TMPy) or 1-methylimidazole (MIm) to trans-HN or transHDN solutions brings about bathochromic displacement of the first absorption band. The relatively small shifts suggest that that the phenolic OH groups are not deprotonated by pyridine and MIm. This is in accordance with the reported low acidity of trans-HN (pKa = 10.1) in 1:1 ethanol-mixtures [48]. As a representative
a For the other solvents the decay time is shorter than the time-resolution of our instrument.
of HN may act as effective accepting modes in the energy dissipation. Excited-state proton transfer to the solvent is not probable because DMSO and DMF are weak bases. 3.3. Spectral characteristics of trans-HDN For the sake of comparison, the absorption and fluorescence maxima of trans-HDN are given in parenthesis in Table 1. The absorption spectrum of trans-HDN resembles that found for
Fig. 3. Effect of pyridine on the absorption spectrum of HN in CH2Cl2. Inset displays the absorbance change at 410 nm and the line represents the fitted function.
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M. Megyesi et al. / Chemical Physics Letters 489 (2010) 59–63
Table 3 Effect of hydrogen bonding between trans-HN and nitrogen-heterocyclic compounds.
a b c
Additive
Solvent
pKa of additive
Pyridine 2,4,6-Trimethylpyridine 1-Methylimidazole Pyridine 1-Methylimidazole
CH2Cl2 CH2Cl2 CH2Cl2 CH3CN CH3CN
5.25a 7.43a 7.88c 5.25a 7.88c
K/M1 Absorption
Fluorescence
31 (41)b 69 (105) 110 (170) 0.7 (1.2) 7.7 (10)
33 46 120 0.5 6.4
kq/109 M1s1 11 7.9 14 2.7 4.1
Taken from Ref. [49]. In parentheses: the binding constants of trans-HDN. Taken from Ref. [50].
example, Fig. 3 shows the shift of the absorption spectra of transHN in dichloromethane with gradually increasing pyridine concentration. The isosbestic points at 311 and 376 nm suggest 1:1 complexation. The absorbance change (inset to Fig. 3) can be well described by the following function:
Ak ¼ A0k
1 þ e0k =ek K½additive 1 þ K½additive
ð1Þ
Here (e0k =ek ) is the ratio of the molar absorption coefficients for the complexed and unbound trans-HN at a particular wavelength, A0k and Ak denote the absorbance in the absence and the presence of pyridine, respectively. The non-linear least-squares fit of Eq. (1) to the experimental data gives K = 31 M1 for the equilibrium constant of hydrogen bonding in dichloromethane. Using TMPy and MIm as stronger hydrogen bond acceptor additives, K = 69 and 110 M1 are obtained, respectively. As expected for hydrogen bonding, K values significantly decrease in polar solvents (Table 3). The equilibrium constants for trans-HNpyridine and transHNMIm complex formation in acetonitrile are 0.7 and 7.7 M1, respectively. Substitution with a second nitro group enhances the strength of hydrogen bonding. K = 41 and 170 M1 are obtained for pyridine and MIm binding to trans-HDN in dichloromethane, respectively. The electron withdrawing effect of the two nitro moieties diminishes the charge density on the phenolic OH group, thereby augmenting the hydrogen bonding affinity. It is worth noting that an addition of tetrabutylammonium fluoride to trans-HN in acetonitrile leads to a violet solution, indicating that a deprotonation of the OH moiety takes place. The phenolate form, produced thereby, has more substantial charge transfer character than the hydrogen-bound complexes. Therefore, its absorption spectrum appears at much lower energy. The first absorption band is very broad extending from 370 to 760 nm with a maximum at 543 nm, whereas the second absorption peak is located at 331 nm. No emission is observed from the excited phenolate form. 3.5. Fluorescence quenching by hydrogen bond acceptors As trans-HDN is barely fluorescent, the singlet-excited state quenching was studied only for trans-HN. Gradual addition of a hydrogen bond acceptor expedites the transition into the ground state, but the fluorescence decay remains single-exponential. The reciprocal fluorescence lifetime always shows excellent linear correlation with the additive concentration. The slope of this type of plot gives the rate constant of singlet-excited trans-HN quenching by various N-heterocyclic compounds (kq in Table 3). In CH2Cl2, the quenching rates are close to the diffusion-controlled limit. The somewhat lower value obtained for TMPy may arise from steric hindrance of the excited hydrogen-bound complex formation by the methyl groups in the 2 and 6 position of the heterocyclic ring. The stronger solvent–solute interactions in the more polar acetonitrile slow down the dynamic quenching compared to that observed
Fig. 4. Stern–Volmer plot of the fluorescence intensity at 570 nm for quenching of trans-HN by pyridine in CH2Cl2. The line displays the best fit of Eq. (2). Inset: fluorescence spectra (normalized intensity) upon addition of 0, 0.4, 1.2, 2.8, 5.2, 9.0, 16, 30, 59, 107, 210 and 500 mM pyridine.
in CH2Cl2. Opposite trends would be expected if the quenching occurred via an electron transfer mechanism. The more basic MIm is a more efficient quencher than pyridine indicating that proton transfer within the excited hydrogen-bound complex leads to energy dissipation. The steady-state fluorescence measurements show that the addition of N-heterocyclic compounds brings about neither the appearance of a new band nor a shift of the fluorescence maximum. On the basis of these observations, we conclude that the hydrogen-bound complexes of trans-HN have negligible fluorescence yield. The Stern–Volmer plot of the fluorescence intensities in the absence (I0) and the presence (I) of an additive exhibits an upward curvature (Fig. 4). This suggests that the fluorophore can be quenched both in dynamic and static processes. If the static quenching is attributed entirely to ground state hydrogen-bonding, the modified form of the Stern–Volmer equation describes the variation of I0/I versus the quencher concentration [47],
I0 =I ¼ ð1 þ K½quencherÞð1 þ kq sf ½quencherÞ
ð2Þ
where kq is the rate constant of the dynamic quenching and K denotes the equilibrium constant of complex formation in the ground state. As a representative example, Fig. 4 shows the results on transHN fluorescence quenching by pyridine. Taking kq determined by the time-resolved fluorescence technique (vide supra), K values were calculated from the non-linear least-square fit of Eq. (2) to the experimental data. It is apparent from Table 3 that the ground-state binding constants obtained by the fluorescence method are in good agreement with the corresponding values derived from absorption measurements.
M. Megyesi et al. / Chemical Physics Letters 489 (2010) 59–63
4. Conclusions Substitution of 4-hydroxystilbene with a nitro group at 40 -position augments the extent of the intramolecular electron density redistribution upon photoexcitation and leads to a substantial decrease in the energy of the first singlet-excited state, while introduction of the second nitro moiety further enhances these effects. Hydrogen bonding of the phenolic OH group of trans-HN significantly diminishes the fluorescence quantum yield and shortens the fluorescence lifetime. The radiative rate constant barely changes upon binding to weakly basic compounds, such as DMF and DMSO, but the radiationless deactivation is accelerated. When stronger bases are bound to the OH moiety of trans-HN, the extent of the proton shift along the hydrogen-bond increases and the intramolecular charge transfer character of the singlet-excited state is enhanced. These effects decrease the energy of the lowest singlet-excited state. The vibrational coupling enhancement caused thereby leads to a substantial increase in the rate of internal conversion, as is expected on the basis of the energy gap law. Acknowledgements We thank Professor W. Lubitz for his support and Mr. L.J. Currell for technical assistance. The authors very much appreciate the support of this work by the Hungarian Scientific Research Fund (OTKA, Grant K75015) and the bilateral program between the Deutsche Forschungsgemeinschaft and the Hungarian Academy of Sciences. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
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