- Email: [email protected]
Thioanisole triplet: Laser flash photolysis and pulse radiolysis studies
Journal of Photochemistry and Photobiology A: Chemistry 335 (2017) 294–299
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
Invited paper
Thioanisole triplet: Laser flash photolysis and pulse radiolysis studies Yasser M. Riyada,b,c,* a
Leibniz Institute of Surface Modification, Permoserstr. 15, 04303 Leipzig, Germany Department of Chemistry, Faculty of Science, Al-Azhar University, Nasr City, 11884, Cairo, Egypt c Wilhelm-Ostwald-Institute for Physical and Theoretical Chemistry, Faculty of Chemistry and Mineralogy, University of Leipzig, Permoserstr. 15, 04318 Leipzig, Germany b
A R T I C L E I N F O
Article history: Received 19 September 2016 Received in revised form 25 November 2016 Accepted 10 December 2016 Available online 14 December 2016 Keywords: Thioanisole excited triplet state Triplet quantum yield Triplet lifetime Energy transfer Laser photolysis Pulse radiolysis
A B S T R A C T
Direct photogeneration of excited triplet states of thiophenol and its derivatives is substantially influenced by substitutions and structural changes. In this study the impact of the methyl substituent of thioanisole (a derivative of thiophenol, denoted as PhS-Me) relative to the basic molecular structure (PhS-H) on the dynamics of thioanisole triplet and its features has been explored by direct excitation (lexc. = 266 nm) and by sensitization. The T1-Tn absorption spectrum is characterized by three bands peaking at 320 nm, 360 nm and 490 nm. The triplet reactivity with triplet quenchers was determined and the triplet energy level was identified by sensitization with beta-carotene, naphthalene, and benzophenone using laser photolysis as well as solvent benzene using pulse radiolysis. The results indicate that the replacement of the hydrogen atom by the methyl substituent leads to a significant change in the thioanisole triplet properties (t(PhSMe(T1)) = 1.20 ms, fT(PhSMe) = 0.35, 353 kJ mol1 > E(PhSMe(T1)) > 287 kJ mol1) compared with those of thiophenol (t(PhSH(T1)) = 40 ns, fT(PhSH) = 0.00, E(PhSH(T1)) = 248 kJ mol1). © 2016 Elsevier B.V. All rights reserved.
1. Introduction Owing to the relatively low ionization potential and weak carbon-sulfur bond in general, sulfur compounds are used as antioxidants for organic matter ranging from polymers to living systems [1,2]. The antioxidant action of sulfur compounds is understood in terms of electron and hydrogen atom donor ability [3,4]. Indeed, they are active in many biochemical processes, including those connected with biological aging [5], pathologies such as Creutzfeldt-Jacob and Alzheimer’s diseases [6,7], radical repair mechanisms [8],cis-trans-isomerisation of mono- and polyunsaturated fatty acid residues [9], and with oxidative stress [10]. Generally, intersystem crossing process (ISC) is expected to be enhanced by heavy atom substitution (such as the sulfur atom) which affects spin-orbit coupling interaction. It has been previously reported that direct photogeneration of excited triplet states of thiophenols is substantially influenced by substitutions and structural changes [11–15]. Triplets of thiophenol (PhSH) and its derivatives (e.g. methyl, methoxy, and chloro substituents) [11]
* Correspondence to: Leibniz Institute of Surface Modification, Permoserstr. 15, 04303 Leipzig, Germany. E-mail address: [email protected] (Y.M. Riyad). http://dx.doi.org/10.1016/j.jphotochem.2016.12.007 1010-6030/© 2016 Elsevier B.V. All rights reserved.
were not observed either by direct photoexcitation with 266 nm or by photosensitization experiment, but rather by pulse radiolysis via triplet energy transfer with solvent benzene [12]. On the other hand, triplets of thiosalicylic acids and thionaphthols were directly generated by photoexcitation [13–15]. These studies emphasized that sulfur substitution in aromatic thiophenols results in promotion of the radiationless processes over the radiative pathway. Now an obvious question arises as to whether these dynamical behaviours can be extended to structural systems such as thioanisole (a derivative of thiophenol, in which the thiyl hydrogen is replaced by the methyl group, denoted as PhSMe) where the leaving species is a methyl group instead of a hydrogen atom. The knowledge regarding light-induced transient reaction behaviour of thioanisole remains limited [16–20]. In fact the excited singlet state of PhSMe has been characterized to some extent. To the best of my knowledge, thioanisole triplet or other possible transients, such as radicals and radical ions, generated by direct photoexcitation of PhSMe in solution have not yet been studied. The aim of this paper is to study the effect of the structural changes of PhSMe, compared to the parent thiophenol molecule (PhSH), on the dynamic of the excited triplet state of thioanisole (PhSMe(T1)) and its properties such as absorption spectrum, lifetime, quantum yield, energy level, and reactivity with triplet
Y.M. Riyad / Journal of Photochemistry and Photobiology A: Chemistry 335 (2017) 294–299
quenchers. Indeed, it is of particular interest to identify the other possible transients, such as radicals and radical ions, generated by direct excitation of PhSMe. This approach has been performed by laser flash photolysis and complemented by electron pulse radiolysis. The results show that the substitution of the methyl group for the S H hydrogen in thiophenol results in an essential change on the triplet parameters of the thioanisole molecule.
295
digitizing oscilloscope (TDS 640, Tektronix). Further details of this equipment are given elsewhere [22]. All experiments were performed at room temperature. Freshly prepared solutions were used flowing continuously through a 5 mm or 10 mm quartz sample cell in laser photolysis or in pulse radiolysis, respectively. Prior to the experiments, the solutions were bubbled with purest grade N2 or O2 for 15 min, and were used within one hour.
2. Experimental 3. Results and discussion 2.1. Chemicals 3.1. Nanosecond-laser flash photolysis Thioanisole (PhSMe, purity 99%) was purchased from SigmaAldrich and used as received. Acetonitrile (MeCN) was purchased from VWR and was of highest spectroscopic grade (purity 99.9%, VWR). Benzene (purity 99.8%) was obtained from VWR too. Other chemicals including ferrocene (purity 98%), benzophenone (BP, purity 99.9%), naphthalene (Np, purity 99%) and ß-carotene (ß-C, purity 95%) were obtained from Aldrich. 2.2. Apparatus and methods
In nitrogen saturated MeCN solution, the transient absorption spectra obtained after excitation of PhSMe (0.04 mmol dm3) at 266 nm is shown in Fig. 1. The spectrum taken 150 ns after the pulse displayed two broad absorption bands in the 300–320 nm and 450–500 nm regions as well as a shoulder in the range between 350 nm and 375 nm. After 8 ms the shoulder completely disappeared, the absorption intensity of other bands was decreased, and the absorption spectrum which became narrow exhibited only two bands; a sharp and intense one centered at
2.2.1. Nanosecond-laser flash photolysis Spectral and kinetic data of the triplet state of PhSMe were measured with the 266 nm 4th harmonic of a Quanta-Ray GCR-11 Nd:YAG laser (Spectra Physics). Pulse widths of 3 ns and energies between 0.5 and 3.0 mJ/pulse at 266 nm were selected. The optical detection is based on a pulsed xenon lamp (XBO 150, Osram), a monochromator (Spectra Pro 275, Acton Research), R955 photomultiplier tube (Hamamatsu Photonics) or a fast Si-photodiode and a 1 GHz digital oscilloscope (TDS 684A, Tektronix). The laser power of every laser pulse was registered using a bypath with a fast Silicon photodiode. A more detailed description is reported elsewhere [21]. 2.2.2. Electron pulse radiolysis The liquid samples were irradiated with high energy electron pulses (1 MeV, 12 ns duration) generated by a pulse transformer type accelerator ELIT (Institute of Nuclear Physics, Novosibirsk, Russia). The dose delivered per pulse was measured with an electron dosimeter and was usually between 50 and 100 Gy. Detection of the transient species was performed using an optical absorption set-up consisting of a pulsed xenon lamp (XPO 450), a SpectraPro-500 monochromator (Acton Research Corporation), a R4220 photomultiplier (Hamamatsu Photonics) and a 1 GHz
Fig. 1. Nanosecond transient absorption spectra of PhSMe (0.04 mmol dm3) in MeCN: purged with N2 taken 150 ns (filled circles), 8 ms (filled boxes), and purged with O2 taken 150 ns (filled triangles) after the pulse. The spectrum shown by open circles or open squares; is obtained from the difference between spectra of 150 ns and 8 ms or 150 ns in the N2- or O2-containing sample, respectively; represents the T1-Tn absorption spectrum of PhSMe (lexc. = 266 nm, laser energy = 3 mJ).
Fig. 2. Experimental time profiles of PhSMe (0.04 mmol dm3) in N2 (black lines) and O2 (red lines) saturated MeCN solutions at different wavelengths (lexc. = 266 nm, laser energy = 3 mJ). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
296
Y.M. Riyad / Journal of Photochemistry and Photobiology A: Chemistry 335 (2017) 294–299
Fig. 3. (a) Decay dynamics of T-T absorption at 490 nm obtained after nanosecond laser photolysis of MeCN solution of 0.1 mmol dm3 PhSMe in the presence of different ferrocene concentrations (lexc. = 266 nm, laser energy = 2 mJ). (b) Plot of observed rate constants (kobs) vs. ferrocene concentrations.
300 nm and a moderate and broad band centered at 460 nm. In the presence of oxygen, the spectrum taken after 150 ns was similar to that observed after a long time (8 ms) in the nitrogen containing sample as shown in Fig. 1. The kinetic analysis of the traces of PhSMe in N2-saturated MeCN solution revealed the decay of short and long-lived transients as shown in the insets of Fig. 2. The short-lived component whose decay was adequately fitted to a first-order kinetic and was quenched by known triplet quenchers such as oxygen and ferrocene, is assigned to the first excited triplet state PhSMe(T1) as shown in Figs. 2 and 3 (see also below, sensitization experiment). Moreover, the triplet lifetime (tT) in N2-saturated MeCN solution was measured to be 1.20 0.1 ms (see Table 1). Indeed, the quenching rate constant (kq) determined from the linear dependence of the rates of the triplet decay (kobs, measured at 490 nm) on the concentrations of ferrocene is 9 0.5 109 dm3 mol1 s1 (see Fig. 3), while that of oxygen is 7 0.5 109 dm3
mol1 s1 (obtained using the triplet lifetime of PhSMe in the presence of oxygen 17 ns and oxygen concentration 9.1 mmol dm3 in MeCN [23]). The long-lived transient (tR 15 ms as shown in the insets of Fig. 2) which exhibited two absorption maxima at 300 nm and 460 nm and was insensitive toward oxygen is attributed to the phenylthiyl radical (PhS) (see also Fig. 2). Therefore, the residual absorption in the oxygen containing sample taken 150 ns after the laser pulse represents the absorption of the PhS radical. These features of the sulfur-centered radical, PhS, agree well with the literature [11–15,24]. Indeed, the kinetic analysis of the traces of PhSMe monitored between 300 nm and 400 nm showed no indication of the formation of the carbon-centered (PhSCH2) radicals which are characterized with an absorption maximum around 330 nm [25,26]. These results indicate that the absorption spectrum is dominated by a superposition of PhSMe(T1) and the long-lived PhS radical, and both transients are generated synchronously. Subtracting the radical background absorption of 8 ms in the N2- or 150 ns in the O2-containing sample from that of the overall absorption of 150 ns in the N2-containing sample results in a separation of the triplet spectrum of PhSMe(T1) which exhibits two maxima centered around 320 nm and 490 nm and a shoulder centered around 360 nm as shown in Fig. 1 (see also Table 1). Now, the question remains whether the thioanisole triplet contributes to the formation of the phenylthiyl radicals (PhS). Obviously, the absorption band taken after 8 ms monitored at 300 nm in the nitrogen-containing sample belongs only to the PhS radicals (see Fig. 1). In the presence of oxygen, the PhSMe(T1) was efficiently quenched after 150 ns, while the yield of the PhS radical observed in the UV (300 nm) and visible (460 nm) regions remained unchanged (see Fig. 1). Therefore, there seems to be no evidence for the phenylthiyl radical formation via a reaction of PhSMe(T1). In the light of these experimental findings, it can be concluded that the PhS radical formed here occurs by the sulfurcarbon bond fission of the excited singlet state of PhSMe(S1) [16–20]. 3.2. Triplet energy transfer Further characterization of the thioanisole triplet was carried out by the energy transfer process in order to identify the properties of this species, such as its energy level, quantum yield, and energy transfer rate constant. In this regard, sensitization experiments were performed with ß-carotene (ß-C, ET1 = 88 kJ mol1, eT(515nm) = 187000 dm3 mol1 cm1), naphthalene (Np, ET1 = 253 kJ mol1, eT(415nm) = 24500 dm3 mol1 cm1), and benzophenone (BP, ET1 = 287 kJ mol1, eT(520nm) = 6500 dm3 mol1 cm1)
Table 1 Photophysical parameters of thioanisole (PhSMe), thiophenol (PhSH), phenol (PhOH), and anisole (PhOMe) in solution at room temperature. S1a
tF
T1a
KF
[ns] PhSHb PhSMe PhOHc,d PhOMed a b c d e f
2.70 – 5.50 7.50
0.004 – 0.16 0.24
Da
ICa
tT [ms]
KT
KD
KIC
[nm] 380 320, 360, 490 250, 400 252, 350–430
0.04 1.20 3.30 3.30
0.00 0.35 0.50 0.64
0.30 – 0.07 –
0.696 – 0.270 –
lT1 max
S1 = first excited singlet state, T1 = first excited triplet state, D = dissociation, IC = internal conversion. Data from Refs. [11] and [12]. Data from Ref. [21]. Data from Ref. [23]. kr = radiative rate constant (kr = KF/tF). knr = nonradiative rate constant (knr = 1/tF–kr).
kre [ 107 s1]
knrf [ 108 s1]
0.15 – 2.91 3.20
3.70 – 1.53 1.01
Y.M. Riyad / Journal of Photochemistry and Photobiology A: Chemistry 335 (2017) 294–299
297
Fig. 4. Transient absorption spectra of N2-purged MeCN solution of PhSMe (0.10 mmol dm3) in the presence of ß-C (0.08 mmol dm3) taken 1.65 ms after the pulse (lexc. = 266 nm, laser energy = 0.6 mJ). The Inset (a) shows time profiles of N2 saturated MeCN solutions without (blue line) and with ß-C (black line). The inset (b) shows the plot of observed rate constants (kobs) vs. ß-C concentrations. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. Transient absorption spectra of N2-purged MeCN solution of PhSMe (0.10 mmol dm3) in the presence of Np (0.04 mmol dm3) taken 350 ns after the pulse (lexc. = 266 nm, laser energy = 0.6 mJ). The Inset (a) shows time profiles of N2 saturated MeCN solutions without (blue line) and with Np (black line). The inset (b) shows the plot of observed rate constants (kobs) vs. Np concentrations. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
according to Eqs. (1a), (1b) [23,27]. When the triplet energy difference between donor and acceptor is 20 kJ mol1, a diffusioncontrolled rate is assumed for the energy transfer formulated in Eqs. (1a) and (1b) [28–30]. The T-T transfer reaction (Eqs. (1a) and (1b)) is realized by adding a small concentration of acceptors and can generally be observed experimentally for the donor triplet decay as well as for the acceptor triplet formation.
In contrast, exploring the energy transfer from the thioanisole triplet to BP or vice versa by lexc = 266 nm (Eq. (1a)) is inappropriate for a quantitative treatment due to the possible direct photogeneration of the triplet of benzophenone and superposition with other absorptions. To overcome these difficulties, a sensitization experiment employing BP(T1) as triplet energy donor was performed at lexc = 355 nm (3rd harmonic of Nd:YAG laser, pulse duration = 5 ns) according to Eq. (1b), where no direct thioanisole triplet formation occurred at this excitation wavelength. Therefore, laser photolysis of BP by 355 nm in N2-satuarted MeCN solution led to the formation of the BP(T1) which exhibited strong absorptions at 320 nm and 520 nm [23,27] and was quenched in the presence of thioanisole (kq 1 109 dm3 mol1 s1) and no transient products of the quenching reaction were observed (data not shown). Obviously, the quenching of BP(T1) in MeCN by thioanisole was resulted in neither formation of the PhSMe(T1) nor generation of any free radical ion products, namely the thioanisole radical cation (PhSMe+, lmax = 530 nm, 310 nm) [31] and BP radical anion (BP, lmax > 580 nm) [24]. The former finding indicates that PhSMe(T1) is energetically higher than that of BP(T1), i.e. E(PhSMe(T1)) > 287 kJ mol1. A possible explanation for the later observation is that the quenching of benzophenone triplet by thioanisole in a non-protic MeCN environment might occur via a partial charge-transfer process rather than full electron transfer reaction as reported in the literature for similar systems (e.g. BP (T1)/anisole) [32,33]. Further investigations on the environment effect (protic vs. non-protic) for BP(T1)/PhSMe system on the photoinduced electron transfer efficiency will be necessary to gain insights in the quenching mechanism. In order to determine the quantum yield for thioanisole triplet, a sensitization experiment was carried out by using the energy transfer from PhSMe(T1) to ß-C according to Eq. (2). Since this triplet spectrum is superimposed with the ground-state absorption of ß-C, the difference in extinction coefficients of ß-C triplet and ground state ß-C (bleaching at longer times after the decay of both sensitizer triplets Eqs. (2) and (3)) was adjusted in a further sensitization experiment with BP, a substance which exhibits a complete intersystem crossing [23,27], according to Eq. (3). Monitoring the experiment at l = 520 nm, the immediately formed BP(T1) decays under the formation of the ß-C(T1) enabling a direct comparison of the absorptions under the employed conditions. The concentrations of ß-C(S0) for both solutions (Eqs. (2) and (3)) should be high enough to fully quench (or the same fraction of thioanisole and benzophenone triplets) and should be equal in
PhSMe(T1) + A(S0) ! PhSMe(S0) + A(T1) (lexc = 266 nm)
where
A = ß-C
BP(T1) + PhSMe(S0) ! BP(S0) + PhSMe(T1) (lexc = 355 nm)
or
Np, (1a)
(1b)
In the case of ß-C, a substance which possesses low triplet energy and exhibits a pronounced T1-Tn absorption spectrum in the visible range, a 510 nm long pass filter was used to avoid the degradation of ß-C by the analyzing light. Laser photolysis of N2saturated MeCN solution of PhSMe (0.1 mmol dm3) in the presence of low concentrations of ß-C (0.02-0.10 mmol dm3) resulted in a pronounced band with a maximum at 520 nm which is characteristic for the ß-C(T1) that formed according to the T-T transfer reaction (Eq. (1a)), see Fig. 4. The time profile given as inset (a) of Fig. 4 clearly shows the kinetic of ß-C(T1) formation. The kinetic decay of thioanisole triplet monitored above 510 nm in the presence of ß-C could not be analyzed due to the high absorption of ß-C(T1) which closed the absorption window. Therefore, the energy transfer rate constant (k1a) was determined by analyzing the kinetic of the ß-C triplet formation at 520 nm. Using different concentrations of ß-C, the k1a value was determined to be 1.3 0.15 1010 dm3 mol1 s1 as derived from the linear dependence of the observed growth rates (kobs) of ß-C triplet formation on the concentrations of ß-C (see inset b of Fig. 4). An analogous process is expected to take place between the thioanisole triplet and naphthalene (Np). Therefore, laser photolysis of N2-saturated MeCN solution of PhSMe (0.1 mmol dm3) in the presence of low concentrations of Np (0.02–0.08 mmol dm3) led to the characteristic transient absorption spectrum of Np(T1) with a maximum at 420 nm as shown in Fig. 5. The Np(T1) formation can also be seen from the time profile given as inset (a) of Fig. 5. Using different naphthalene concentrations, the energy transfer rate constant was determined to be 1.4 0.15 1010 dm3 mol1 s1 as determined from the linear dependence of the observed growth rates (kobs) of Np triplet formation (kobs) on the concentrations of Np (see inset (b) of Fig. 5).
298
Y.M. Riyad / Journal of Photochemistry and Photobiology A: Chemistry 335 (2017) 294–299
order to absorb the same amount of 266 nm photons. The experiments were carried out under optically matching conditions (Eq. (4)) at 0.6 mJ excitation energy. The low ground-state absorption of ß-C at 266 nm has a minor effect on the experiment because its direct excitation does not lead to the generation of the excited triplet molecules. Therefore, the triplet quantum yield of thioanisole (KT) was determined by the ratio of the DOD520-values of ß-carotene triplet formed via triplet energy transfer (Eqs. (3) and (4)) under the conditions mentioned above, according to Eq. (5). The KT value of PhSMe(T1) was estimated to be 0.35 0.05 in MeCN solution. PhSMe(T1) + ß-C(S0) ! PhSMe(S0) + ß-C(T1)
(2)
BP(T1) + ß-C(S0) ! BP(S0) + ß-C(T1)
(3)
OD266 nm (BP) = OD266 nm (PhSMe)
(4)
fT ðPhSMeðT1ÞÞ ¼
DOD520nm ðPhSMeðT1 Þ ! ß CðT 1 ÞÞ DOD520nm ðBPðT 1 Þ ! ß C ðT 1 ÞÞ
ð5Þ
According to the results obtained (see Table 1 vide supra), it was found that the excited state dynamic of thioanisole triplet in acetonitrile solution is very different from that of thiophenol in that the thioanisole triplet is observed either directly by 266 nm photons or by photosensitization and its lifetime (tT(PhSMe) = 1.20 ms) is 30 times longer. In contrast, the decay of the excited triplet state of both phenol and anisole molecules is similar (tT = 3.3 ms [21,23], see Table 1), but longer than that of thiophenols. This shows that the substitution of methyl for the SH hydrogen in thiophenol results in substantial changes in the triplet properties of thioanisole (see Table 1). In contrast, the substitution of methyl for the O H hydrogen in phenol has little effect on the decay of the excited-triplet state of the anisole molecule. At the same time and in the light of available data, aromatic thiophenols [11,13,15] exhibit shorter fluorescence lifetimes and lower quantum yields than those of phenols [21,23,34] (see Table 1). Furthermore, the generation of the PhS radicals occurs via the S Me bond fission of the PhSMe(S1) (as found in thiophenols [11] and phenol [34]), while the formation of the PhSCH2 radicals via the MeH bond cleavage seems energetically unlikely to happen. In contrast, both phenoxyl (PhO) and carboncentered (PhOCH2) radicals are observed after excitation of anisole. [34] Also, it should be noted that the quantum yield of sulfur radical [11,13,15] of thiophenol, for instance, was reported to be four times higher than that of phenol [21] (see Table 1) due to the inherently weak S H bond of thiophenol singlet excited state compared with those of the O H bond of phenol. Hence, sulfur substitution results generally in the promotion of the radiationless processes. Indeed, radiationless pathways are expected to predominate over the radiative channel in the excited state dynamics of thioanisole (knr >>> kr in the case of thiophenol, see Table 1). For comparison, methyl substitution in anisole enhances the radiative process, compared with that of phenol (kr > knr in the case of phenols see Table 1). Further studies will be necessary to understand how the methyl substitution in thioanisole affects the radiative and non-radiative (internal conversion and photodissociation) pathways in order to provide a complete picture. 3.3. Pulse-radiolytic sensitization with benzene In order to verify the photogenerated thioanisole triplet and its energy level, pulse radiolysis studies of thioanisole were
performed in a N2-saturated benzene solution. Radiolysis of pure benzene produces its first excited singlet C6H6(S1) (ES = 459 kJ mol1) and triplet C6H6(T1) (ET1 = 353 kJ mol1) states via geminate ion-recombination in the primary process (Eq. (6)) as a result of fast ions mobility [35]. Benzene triplet immediately reacts with another benzene molecule with a lifetime of 4.5 ns forming a biradical (lmax = 320 nm, t = 150 ns) (Eq. (7)), whereas benzene singlet rapidly forms singlet excimer (lmax = 520 nm, t = 30 ns) (Eq. (8)). Therefore, the benzene singlet excimer and biradical are observed as short-living species [36,37]. In spite of the short lifetime of benzene triplet, it can produce solute triplets of low energy by using relatively high solute concentrations (Eq. (9)). C6H6 ! (C6H6+ + e) ! C6H6(S1), C6H6(T1)
(6)
C6H6(T1) + C6H6 ! (C6H6)2 k7 = 1.8 107 dm3 mol1 s1
(7)
C6H6(S1) + C6H6 ! (C6H6)2*
(8)
C6H6(T1) + PhSMe ! PhSMe(T1) + C6H6
(9) 3
At high thioanisole concentrations (10 mmol dm ), the energy transfer (Eq. (9)) successfully competes with benzene biradical formation (Eq. (7)), see Fig. S1. In the same manner as described vide supra in laser photolysis, the triplet spectrum of the PhSMe(T1) which was separated from the radical background absorption exhibits three major absorptions (at 320 nm, 360 nm and 490 nm, see Fig. S1) and displays identical kinetics (see Fig. S2), indicating that they represent the same chemical species. Therefore, it is evident that the spectral and kinetic parameters of the thioanisole triplet are independent of their generation (direct (Figs. 1 and 2) vs. indirect (Figs. S1 and S2)). These results indicate an energy transfer from benzene triplet to PhSMe and the formation of PhSMe(T1). Indeed, the long-lived phenylthiyl radicals are formed by a reaction between the thioanisole triplet and another thioanisole molecule but this reaction is not taken into account in the flash photolysis experiments because the thioanisole concentration is too low in these experiments. Moreover, the long-lived-transient absorption is caused not only by phenylthiyl radicals, but also by the remaining biradicals and their decay products (phenylcyclohexadienyl radicals). In the light of these results, thioanisole triplet is thus lower than that of benzene, i.e. 353 kJ mol1 > E(PhSMe(T1)) > 287 kJ mol1. 4. Conclusions Generation of the thioanisole triplet is performed directly by photolysis (lexc. = 266 nm) or indirectly by pulse radiolysis via sensitization with benzene triplet. It was revealed here that the excited state dynamic of thioanisole triplet in acetonitrile solution is very different from that of thiophenol (see Table 1). For comparison, the decay of the excited triplet state of both phenol and anisole is similar. This emphasizes that the substitution of methyl group for the S H hydrogen in thiophenol results in substantial changes in the triplet properties of thioanisole, whereas that in phenol leads to a little effect on the decay of the excited-triplet state of the anisole molecule. Furthermore, the generation of the sulfur-centered (PhS) radicals occurs via the S Me bond fission of the PhSMe(S1), while the formation of the carbon-centered (PhSCH2) radicals via the Me H bond cleavage seems energetically unlikely to happen. Further studies will be necessary to understand how the methyl substitution in thioanisole influences on the other processes (e.g. fluorescence,
Y.M. Riyad / Journal of Photochemistry and Photobiology A: Chemistry 335 (2017) 294–299
internal conversion, and photodissociation processes) in order to provide a complete picture. Conflict of interest The authors declare no competing financial interest. Acknowledgements The author thanks Prof. Dr. Bernd Abel (Leibniz Institute of Surface Modification, Leipzig), Prof. Dr. Ortwin Brede (Leipzig University), and Dr. Ralf Hermann (Leipzig University), for interesting discussions and support. The author gratefully acknowledges financial support from the Deutsche Forschungsgemeinschaft (DFG, Bonn) and the Egyptian Academy of Scientific Research and Technology (ASRT, Cairo). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. jphotochem.2016.12.007. Transient absorption spectra and time profiles are obtained after pulse radiolysis of thioanisole in benzene. References [1] G. Scott, Atmospheric Oxidation and Antioxidants, vols. I and II, Elsevier, Amsterdam, 1993. [2] K.-D. Asmus, Sulfur-centered free radicals, Methods Enzymol. 186 (1990) 168– 180. [3] C. von Sonntag, H.-P. Schuchmann, The chemistry of functional groups, The Chemistry of Ethers Suppl. E, Hydroxyl Groups and Their Sulphur Analogues, Part 2, Crown Ethers, Wiley, NY, 1980, pp. 923–934. [4] I.W.J. Still, Organic Sulfur Chemistry: Theoretical and Experimental Advances, in: F. Bernardi, I.G. Csizmadia, A. Mangini (Eds.), Elsevier, Amsterdam, 1985, pp. 595–659. [5] B.S. Berlett, E.R. Stadtman, Protein oxidation in aging disease, and oxidative stress, J. Biol. Chem. 272 (1997) 20313–20316. [6] C. Schöneich, Reactive oxygen species and biological aging: a mechanistic approach, Exp. Gerontol. 34 (1999) 19–34. [7] W.R. Markesbery, Oxidative stress hypothesis in Alzheimer’s disease, Free Radic. Biol. Med. 23 (1997) 134–147. [8] C. Chatgilialoglu, K.-D. Asmus, Sulfur-Centered Reactive Intermediates in Chemistry and Biology, Plenum Press, New York, 1989. [9] H. Sprinz, J. Schwinn, S. Naumov, O. Brede, Mechanism of thiyl radical catalyzed isomerization of unsaturated fatty acid residues in homogeneous solution and in liposomes, Biochim. Biophys. Acta 1483 (2000) 91–100. [10] P. Wardman, Thiyl radicals in biology: their role as a molecular switch central to cellular oxidative stress, in: Z.B. Alfassi (Ed.), S-Centered Radicals, Wiley, Chichester, UK, 1999, pp. 289–309. [11] Y.M. Riyad, S. Naumov, R. Hermann, O. Brede, Deactivation of the first excited singlet state of thiophenols, Phys. Chem. Chem. Phys. 8 (2006) 1697–1706. [12] Y.M. Riyad, R. Hermann, O. Brede, Hidden triplet states of aromatic thiols characterized by radiation-chemical sensitization, Chem. Phys. Lett. 376 (2003) 776–780. [13] Y.M. Riyad, R. Hermann, O. Brede, Quantitative description of the deactivation channels of the first excited singlet state of 2- and 4-thiosalicylic acids, J. Phys. Chem. A 109 (2005) 6457–6464. [14] Y.M. Riyad, S. Naumov, R. Hermann, O. Brede, Structure dependence of the photogeneration of thionaphthol triplets, Chem. Phys. Lett. 477 (2009) 290– 297.
299
[15] Y.M. Riyad, S. Naumov, R. Hermann, B. Abel, Shedding light into the detailed excited-state relaxation pathways and reaction mechanisms of thionaphthol isomers, J. Phys. Chem. A 115 (2011) 718–725. [16] M. Hoshino-Nagasaka, T. Suzuki, T. Ichimura, Rotationally resolved highresolution spectrum of the S1-S0 transition of jet-cooled thioanisole, Phys. Chem. Chem. Phys. 12 (2010) 13243–13247. [17] S.L. Li, X. Xu, D.G. Truhlar, Computational simulation and interpretation of the low-lying excited electronic states and electronic spectrum of thioanisole, Phys. Chem. Chem. Phys. 17 (2015) 20093–20099. [18] A.M. Wenge, T.N.V. Karsili, J.D. Rodriguez, M.I. Cotterell, B. Marchetti, R.N. Dixon, M.N.R. Ashfold, Tuning photochemistry: substituent effect on ps* state mediated bond fission in thioanisoles, Phys. Chem. Chem. Phys. 17 (2015) 16246–16256. [19] S. Han, J.S. Lim, J.H. Yoon, J. Lee, S.Y. Kim, S.K. Kim, Conical intersection seam and bound resonance embedded in continuum observed in the photodissociation of thioanisole-d3, J. Chem Phys. 140 (2014) 0543071– 0543078. [20] G.M. Roberts, D.J. Hadden, L.T. Bergendahl, A.M. Wenge, S.J. Harris, T.N.V. Karsili, M.N.R. Ashfold, M.J. Paterson, V.G. Stavros, Exploring quantum phenomena and vibrational control in s* mediated photochemistry, Chem. Sci. 4 (2013) 993–1001. [21] R. Hermann, G.R. Mahalaxmi, T. Jochum, S. Naumov, O. Brede, Balance of the deactivation channels of the first excited singlet state of phenols: effect of alkyl substitution, sterical hindrance, and solvent polarity, J. Phys. Chem. A 106 (2002) 2379–2389. [22] M.R. Ganapathi, R. Hermann, S. Naumov, O. Brede, Free electron transfer from several phenols to radical cations of non-polar solvents, Phys. Chem. Chem. Phys. 2 (2000) 4947–4955. [23] Handbook of Photochemistry, in: S.L. Murov, I. Carmichael, G.L. Hug (Eds.), Marcel Dekker, New York, 1993. [24] M. Ioele, S. Steenken, E. Baciocchi, Generation and reactivity of aromatic thioether radical cations in aqueous solution as studied by pulse radiolsis, J. Phys. Chem. A 101 (1997) 2979–2987. [25] P. Filipiak, G.L. Hug, B. Marciniak, Photochemistry of carboxylic acids containing the phenyl and thioether groups: steady-state and laser flash photolysis, J. Photochem. Photobiol. A Chem. 177 (2006) 295–306. [26] P. Filipiak, G.L. Hug, K. Bobrowski, B. Marciniak, J. Photochem. Photobiol. A Chem. 172 (2005) 322–330. [27] I. Carmichael, G.L. Hug, Triplet-triplet absorption spectra of organic molecules in condensed phases, J. Phys. Chem. Ref. Data 15 (1986) 1–250. [28] O. Brede, R. Mehnert, W. Naumann, J. Teply, Laser photolysis and pulse radiolysis investigations on the triplet energy transfer in aromatic donoracceptor system, Ber. Bunsenges. Phys. Chem. 89 (1985) 1036–1041. [29] O. Brede, R. Mehnert, W. Naumann, Kinetics of excitation and charge transfer reactions in non-polar media, Chem. Phys. 115 (1987) 279–296. [30] H.L.J. Backstrom, K. Sandros, Transfer of triplet energy in fluid solutions. I. Sensitized phosphorescence and its application to the determination of triplet state lifetimes, Acta Chem. Scand. 14 (1960) 48–62. [31] T.D. Giacco, F. Elisei, O. Lanzalunga, Photoinduced hydrogen- and electrontransfer processes between chloranil and aryl alkyl sulfides in organic solvents. Steady-state and time-resolved studies, Phys. Chem. Chem. Phys. 2 (2000) 1701–1708. [32] A. Lewandowska-Andralojc, G.L. Hug, G. Hörner, T. Pedzinski, B. Marciniak, Unusual photobehavior of benzophenone triplets in hexfluoroisopropanol. Inversion of the triplet character of benzophenone, J. Photochem. Photobiol. A Chem. 244 (2012) 1–8. [33] A. Lewandowska-Andralojc, G.L. Hug, G. Hörner, T. Pedzinski, P. Filipiak, B. Marciniak, Efficient photochemical oxidation of anisole in protic solvents: electron transfer driven by specific solvent-solute interactions, ChemPhysChem 11 (2010) 2108–2117. [34] D.V. Bent, E. Hayon, Excited state chemistry of aromatic amino acids and related peptides. I. Tyrosine, J. Am. Chem. Soc. 97 (1975) 2599–2606. [35] R.V. Bensasson, J.T. Richards, J.K. Thomas, The pulsed laser photolysis of benzene in cyclohexane, Chem. Phys. Lett. 9 (1971) 13–16. [36] J.K. Thomas, I. Mani, Pulse radiolysis of benzene cyclohexane mixtures, J. Chem. Phys. 51 (1969) 1834–1838. [37] O. Brede, W. Helmstreit, R. Mehnert, Nanosekunden-pulsradiolyse und laserphotolyse von benzophenon in benzol und cyclohexan, Z. Phys. Chem. (Leipzig) 256 (1975) 505–512.
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
Invited paper
Thioanisole triplet: Laser flash photolysis and pulse radiolysis studies Yasser M. Riyada,b,c,* a
Leibniz Institute of Surface Modification, Permoserstr. 15, 04303 Leipzig, Germany Department of Chemistry, Faculty of Science, Al-Azhar University, Nasr City, 11884, Cairo, Egypt c Wilhelm-Ostwald-Institute for Physical and Theoretical Chemistry, Faculty of Chemistry and Mineralogy, University of Leipzig, Permoserstr. 15, 04318 Leipzig, Germany b
A R T I C L E I N F O
Article history: Received 19 September 2016 Received in revised form 25 November 2016 Accepted 10 December 2016 Available online 14 December 2016 Keywords: Thioanisole excited triplet state Triplet quantum yield Triplet lifetime Energy transfer Laser photolysis Pulse radiolysis
A B S T R A C T
Direct photogeneration of excited triplet states of thiophenol and its derivatives is substantially influenced by substitutions and structural changes. In this study the impact of the methyl substituent of thioanisole (a derivative of thiophenol, denoted as PhS-Me) relative to the basic molecular structure (PhS-H) on the dynamics of thioanisole triplet and its features has been explored by direct excitation (lexc. = 266 nm) and by sensitization. The T1-Tn absorption spectrum is characterized by three bands peaking at 320 nm, 360 nm and 490 nm. The triplet reactivity with triplet quenchers was determined and the triplet energy level was identified by sensitization with beta-carotene, naphthalene, and benzophenone using laser photolysis as well as solvent benzene using pulse radiolysis. The results indicate that the replacement of the hydrogen atom by the methyl substituent leads to a significant change in the thioanisole triplet properties (t(PhSMe(T1)) = 1.20 ms, fT(PhSMe) = 0.35, 353 kJ mol1 > E(PhSMe(T1)) > 287 kJ mol1) compared with those of thiophenol (t(PhSH(T1)) = 40 ns, fT(PhSH) = 0.00, E(PhSH(T1)) = 248 kJ mol1). © 2016 Elsevier B.V. All rights reserved.
1. Introduction Owing to the relatively low ionization potential and weak carbon-sulfur bond in general, sulfur compounds are used as antioxidants for organic matter ranging from polymers to living systems [1,2]. The antioxidant action of sulfur compounds is understood in terms of electron and hydrogen atom donor ability [3,4]. Indeed, they are active in many biochemical processes, including those connected with biological aging [5], pathologies such as Creutzfeldt-Jacob and Alzheimer’s diseases [6,7], radical repair mechanisms [8],cis-trans-isomerisation of mono- and polyunsaturated fatty acid residues [9], and with oxidative stress [10]. Generally, intersystem crossing process (ISC) is expected to be enhanced by heavy atom substitution (such as the sulfur atom) which affects spin-orbit coupling interaction. It has been previously reported that direct photogeneration of excited triplet states of thiophenols is substantially influenced by substitutions and structural changes [11–15]. Triplets of thiophenol (PhSH) and its derivatives (e.g. methyl, methoxy, and chloro substituents) [11]
* Correspondence to: Leibniz Institute of Surface Modification, Permoserstr. 15, 04303 Leipzig, Germany. E-mail address: [email protected] (Y.M. Riyad). http://dx.doi.org/10.1016/j.jphotochem.2016.12.007 1010-6030/© 2016 Elsevier B.V. All rights reserved.
were not observed either by direct photoexcitation with 266 nm or by photosensitization experiment, but rather by pulse radiolysis via triplet energy transfer with solvent benzene [12]. On the other hand, triplets of thiosalicylic acids and thionaphthols were directly generated by photoexcitation [13–15]. These studies emphasized that sulfur substitution in aromatic thiophenols results in promotion of the radiationless processes over the radiative pathway. Now an obvious question arises as to whether these dynamical behaviours can be extended to structural systems such as thioanisole (a derivative of thiophenol, in which the thiyl hydrogen is replaced by the methyl group, denoted as PhSMe) where the leaving species is a methyl group instead of a hydrogen atom. The knowledge regarding light-induced transient reaction behaviour of thioanisole remains limited [16–20]. In fact the excited singlet state of PhSMe has been characterized to some extent. To the best of my knowledge, thioanisole triplet or other possible transients, such as radicals and radical ions, generated by direct photoexcitation of PhSMe in solution have not yet been studied. The aim of this paper is to study the effect of the structural changes of PhSMe, compared to the parent thiophenol molecule (PhSH), on the dynamic of the excited triplet state of thioanisole (PhSMe(T1)) and its properties such as absorption spectrum, lifetime, quantum yield, energy level, and reactivity with triplet
Y.M. Riyad / Journal of Photochemistry and Photobiology A: Chemistry 335 (2017) 294–299
quenchers. Indeed, it is of particular interest to identify the other possible transients, such as radicals and radical ions, generated by direct excitation of PhSMe. This approach has been performed by laser flash photolysis and complemented by electron pulse radiolysis. The results show that the substitution of the methyl group for the S H hydrogen in thiophenol results in an essential change on the triplet parameters of the thioanisole molecule.
295
digitizing oscilloscope (TDS 640, Tektronix). Further details of this equipment are given elsewhere [22]. All experiments were performed at room temperature. Freshly prepared solutions were used flowing continuously through a 5 mm or 10 mm quartz sample cell in laser photolysis or in pulse radiolysis, respectively. Prior to the experiments, the solutions were bubbled with purest grade N2 or O2 for 15 min, and were used within one hour.
2. Experimental 3. Results and discussion 2.1. Chemicals 3.1. Nanosecond-laser flash photolysis Thioanisole (PhSMe, purity 99%) was purchased from SigmaAldrich and used as received. Acetonitrile (MeCN) was purchased from VWR and was of highest spectroscopic grade (purity 99.9%, VWR). Benzene (purity 99.8%) was obtained from VWR too. Other chemicals including ferrocene (purity 98%), benzophenone (BP, purity 99.9%), naphthalene (Np, purity 99%) and ß-carotene (ß-C, purity 95%) were obtained from Aldrich. 2.2. Apparatus and methods
In nitrogen saturated MeCN solution, the transient absorption spectra obtained after excitation of PhSMe (0.04 mmol dm3) at 266 nm is shown in Fig. 1. The spectrum taken 150 ns after the pulse displayed two broad absorption bands in the 300–320 nm and 450–500 nm regions as well as a shoulder in the range between 350 nm and 375 nm. After 8 ms the shoulder completely disappeared, the absorption intensity of other bands was decreased, and the absorption spectrum which became narrow exhibited only two bands; a sharp and intense one centered at
2.2.1. Nanosecond-laser flash photolysis Spectral and kinetic data of the triplet state of PhSMe were measured with the 266 nm 4th harmonic of a Quanta-Ray GCR-11 Nd:YAG laser (Spectra Physics). Pulse widths of 3 ns and energies between 0.5 and 3.0 mJ/pulse at 266 nm were selected. The optical detection is based on a pulsed xenon lamp (XBO 150, Osram), a monochromator (Spectra Pro 275, Acton Research), R955 photomultiplier tube (Hamamatsu Photonics) or a fast Si-photodiode and a 1 GHz digital oscilloscope (TDS 684A, Tektronix). The laser power of every laser pulse was registered using a bypath with a fast Silicon photodiode. A more detailed description is reported elsewhere [21]. 2.2.2. Electron pulse radiolysis The liquid samples were irradiated with high energy electron pulses (1 MeV, 12 ns duration) generated by a pulse transformer type accelerator ELIT (Institute of Nuclear Physics, Novosibirsk, Russia). The dose delivered per pulse was measured with an electron dosimeter and was usually between 50 and 100 Gy. Detection of the transient species was performed using an optical absorption set-up consisting of a pulsed xenon lamp (XPO 450), a SpectraPro-500 monochromator (Acton Research Corporation), a R4220 photomultiplier (Hamamatsu Photonics) and a 1 GHz
Fig. 1. Nanosecond transient absorption spectra of PhSMe (0.04 mmol dm3) in MeCN: purged with N2 taken 150 ns (filled circles), 8 ms (filled boxes), and purged with O2 taken 150 ns (filled triangles) after the pulse. The spectrum shown by open circles or open squares; is obtained from the difference between spectra of 150 ns and 8 ms or 150 ns in the N2- or O2-containing sample, respectively; represents the T1-Tn absorption spectrum of PhSMe (lexc. = 266 nm, laser energy = 3 mJ).
Fig. 2. Experimental time profiles of PhSMe (0.04 mmol dm3) in N2 (black lines) and O2 (red lines) saturated MeCN solutions at different wavelengths (lexc. = 266 nm, laser energy = 3 mJ). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
296
Y.M. Riyad / Journal of Photochemistry and Photobiology A: Chemistry 335 (2017) 294–299
Fig. 3. (a) Decay dynamics of T-T absorption at 490 nm obtained after nanosecond laser photolysis of MeCN solution of 0.1 mmol dm3 PhSMe in the presence of different ferrocene concentrations (lexc. = 266 nm, laser energy = 2 mJ). (b) Plot of observed rate constants (kobs) vs. ferrocene concentrations.
300 nm and a moderate and broad band centered at 460 nm. In the presence of oxygen, the spectrum taken after 150 ns was similar to that observed after a long time (8 ms) in the nitrogen containing sample as shown in Fig. 1. The kinetic analysis of the traces of PhSMe in N2-saturated MeCN solution revealed the decay of short and long-lived transients as shown in the insets of Fig. 2. The short-lived component whose decay was adequately fitted to a first-order kinetic and was quenched by known triplet quenchers such as oxygen and ferrocene, is assigned to the first excited triplet state PhSMe(T1) as shown in Figs. 2 and 3 (see also below, sensitization experiment). Moreover, the triplet lifetime (tT) in N2-saturated MeCN solution was measured to be 1.20 0.1 ms (see Table 1). Indeed, the quenching rate constant (kq) determined from the linear dependence of the rates of the triplet decay (kobs, measured at 490 nm) on the concentrations of ferrocene is 9 0.5 109 dm3 mol1 s1 (see Fig. 3), while that of oxygen is 7 0.5 109 dm3
mol1 s1 (obtained using the triplet lifetime of PhSMe in the presence of oxygen 17 ns and oxygen concentration 9.1 mmol dm3 in MeCN [23]). The long-lived transient (tR 15 ms as shown in the insets of Fig. 2) which exhibited two absorption maxima at 300 nm and 460 nm and was insensitive toward oxygen is attributed to the phenylthiyl radical (PhS) (see also Fig. 2). Therefore, the residual absorption in the oxygen containing sample taken 150 ns after the laser pulse represents the absorption of the PhS radical. These features of the sulfur-centered radical, PhS, agree well with the literature [11–15,24]. Indeed, the kinetic analysis of the traces of PhSMe monitored between 300 nm and 400 nm showed no indication of the formation of the carbon-centered (PhSCH2) radicals which are characterized with an absorption maximum around 330 nm [25,26]. These results indicate that the absorption spectrum is dominated by a superposition of PhSMe(T1) and the long-lived PhS radical, and both transients are generated synchronously. Subtracting the radical background absorption of 8 ms in the N2- or 150 ns in the O2-containing sample from that of the overall absorption of 150 ns in the N2-containing sample results in a separation of the triplet spectrum of PhSMe(T1) which exhibits two maxima centered around 320 nm and 490 nm and a shoulder centered around 360 nm as shown in Fig. 1 (see also Table 1). Now, the question remains whether the thioanisole triplet contributes to the formation of the phenylthiyl radicals (PhS). Obviously, the absorption band taken after 8 ms monitored at 300 nm in the nitrogen-containing sample belongs only to the PhS radicals (see Fig. 1). In the presence of oxygen, the PhSMe(T1) was efficiently quenched after 150 ns, while the yield of the PhS radical observed in the UV (300 nm) and visible (460 nm) regions remained unchanged (see Fig. 1). Therefore, there seems to be no evidence for the phenylthiyl radical formation via a reaction of PhSMe(T1). In the light of these experimental findings, it can be concluded that the PhS radical formed here occurs by the sulfurcarbon bond fission of the excited singlet state of PhSMe(S1) [16–20]. 3.2. Triplet energy transfer Further characterization of the thioanisole triplet was carried out by the energy transfer process in order to identify the properties of this species, such as its energy level, quantum yield, and energy transfer rate constant. In this regard, sensitization experiments were performed with ß-carotene (ß-C, ET1 = 88 kJ mol1, eT(515nm) = 187000 dm3 mol1 cm1), naphthalene (Np, ET1 = 253 kJ mol1, eT(415nm) = 24500 dm3 mol1 cm1), and benzophenone (BP, ET1 = 287 kJ mol1, eT(520nm) = 6500 dm3 mol1 cm1)
Table 1 Photophysical parameters of thioanisole (PhSMe), thiophenol (PhSH), phenol (PhOH), and anisole (PhOMe) in solution at room temperature. S1a
tF
T1a
KF
[ns] PhSHb PhSMe PhOHc,d PhOMed a b c d e f
2.70 – 5.50 7.50
0.004 – 0.16 0.24
Da
ICa
tT [ms]
KT
KD
KIC
[nm] 380 320, 360, 490 250, 400 252, 350–430
0.04 1.20 3.30 3.30
0.00 0.35 0.50 0.64
0.30 – 0.07 –
0.696 – 0.270 –
lT1 max
S1 = first excited singlet state, T1 = first excited triplet state, D = dissociation, IC = internal conversion. Data from Refs. [11] and [12]. Data from Ref. [21]. Data from Ref. [23]. kr = radiative rate constant (kr = KF/tF). knr = nonradiative rate constant (knr = 1/tF–kr).
kre [ 107 s1]
knrf [ 108 s1]
0.15 – 2.91 3.20
3.70 – 1.53 1.01
Y.M. Riyad / Journal of Photochemistry and Photobiology A: Chemistry 335 (2017) 294–299
297
Fig. 4. Transient absorption spectra of N2-purged MeCN solution of PhSMe (0.10 mmol dm3) in the presence of ß-C (0.08 mmol dm3) taken 1.65 ms after the pulse (lexc. = 266 nm, laser energy = 0.6 mJ). The Inset (a) shows time profiles of N2 saturated MeCN solutions without (blue line) and with ß-C (black line). The inset (b) shows the plot of observed rate constants (kobs) vs. ß-C concentrations. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. Transient absorption spectra of N2-purged MeCN solution of PhSMe (0.10 mmol dm3) in the presence of Np (0.04 mmol dm3) taken 350 ns after the pulse (lexc. = 266 nm, laser energy = 0.6 mJ). The Inset (a) shows time profiles of N2 saturated MeCN solutions without (blue line) and with Np (black line). The inset (b) shows the plot of observed rate constants (kobs) vs. Np concentrations. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
according to Eqs. (1a), (1b) [23,27]. When the triplet energy difference between donor and acceptor is 20 kJ mol1, a diffusioncontrolled rate is assumed for the energy transfer formulated in Eqs. (1a) and (1b) [28–30]. The T-T transfer reaction (Eqs. (1a) and (1b)) is realized by adding a small concentration of acceptors and can generally be observed experimentally for the donor triplet decay as well as for the acceptor triplet formation.
In contrast, exploring the energy transfer from the thioanisole triplet to BP or vice versa by lexc = 266 nm (Eq. (1a)) is inappropriate for a quantitative treatment due to the possible direct photogeneration of the triplet of benzophenone and superposition with other absorptions. To overcome these difficulties, a sensitization experiment employing BP(T1) as triplet energy donor was performed at lexc = 355 nm (3rd harmonic of Nd:YAG laser, pulse duration = 5 ns) according to Eq. (1b), where no direct thioanisole triplet formation occurred at this excitation wavelength. Therefore, laser photolysis of BP by 355 nm in N2-satuarted MeCN solution led to the formation of the BP(T1) which exhibited strong absorptions at 320 nm and 520 nm [23,27] and was quenched in the presence of thioanisole (kq 1 109 dm3 mol1 s1) and no transient products of the quenching reaction were observed (data not shown). Obviously, the quenching of BP(T1) in MeCN by thioanisole was resulted in neither formation of the PhSMe(T1) nor generation of any free radical ion products, namely the thioanisole radical cation (PhSMe+, lmax = 530 nm, 310 nm) [31] and BP radical anion (BP, lmax > 580 nm) [24]. The former finding indicates that PhSMe(T1) is energetically higher than that of BP(T1), i.e. E(PhSMe(T1)) > 287 kJ mol1. A possible explanation for the later observation is that the quenching of benzophenone triplet by thioanisole in a non-protic MeCN environment might occur via a partial charge-transfer process rather than full electron transfer reaction as reported in the literature for similar systems (e.g. BP (T1)/anisole) [32,33]. Further investigations on the environment effect (protic vs. non-protic) for BP(T1)/PhSMe system on the photoinduced electron transfer efficiency will be necessary to gain insights in the quenching mechanism. In order to determine the quantum yield for thioanisole triplet, a sensitization experiment was carried out by using the energy transfer from PhSMe(T1) to ß-C according to Eq. (2). Since this triplet spectrum is superimposed with the ground-state absorption of ß-C, the difference in extinction coefficients of ß-C triplet and ground state ß-C (bleaching at longer times after the decay of both sensitizer triplets Eqs. (2) and (3)) was adjusted in a further sensitization experiment with BP, a substance which exhibits a complete intersystem crossing [23,27], according to Eq. (3). Monitoring the experiment at l = 520 nm, the immediately formed BP(T1) decays under the formation of the ß-C(T1) enabling a direct comparison of the absorptions under the employed conditions. The concentrations of ß-C(S0) for both solutions (Eqs. (2) and (3)) should be high enough to fully quench (or the same fraction of thioanisole and benzophenone triplets) and should be equal in
PhSMe(T1) + A(S0) ! PhSMe(S0) + A(T1) (lexc = 266 nm)
where
A = ß-C
BP(T1) + PhSMe(S0) ! BP(S0) + PhSMe(T1) (lexc = 355 nm)
or
Np, (1a)
(1b)
In the case of ß-C, a substance which possesses low triplet energy and exhibits a pronounced T1-Tn absorption spectrum in the visible range, a 510 nm long pass filter was used to avoid the degradation of ß-C by the analyzing light. Laser photolysis of N2saturated MeCN solution of PhSMe (0.1 mmol dm3) in the presence of low concentrations of ß-C (0.02-0.10 mmol dm3) resulted in a pronounced band with a maximum at 520 nm which is characteristic for the ß-C(T1) that formed according to the T-T transfer reaction (Eq. (1a)), see Fig. 4. The time profile given as inset (a) of Fig. 4 clearly shows the kinetic of ß-C(T1) formation. The kinetic decay of thioanisole triplet monitored above 510 nm in the presence of ß-C could not be analyzed due to the high absorption of ß-C(T1) which closed the absorption window. Therefore, the energy transfer rate constant (k1a) was determined by analyzing the kinetic of the ß-C triplet formation at 520 nm. Using different concentrations of ß-C, the k1a value was determined to be 1.3 0.15 1010 dm3 mol1 s1 as derived from the linear dependence of the observed growth rates (kobs) of ß-C triplet formation on the concentrations of ß-C (see inset b of Fig. 4). An analogous process is expected to take place between the thioanisole triplet and naphthalene (Np). Therefore, laser photolysis of N2-saturated MeCN solution of PhSMe (0.1 mmol dm3) in the presence of low concentrations of Np (0.02–0.08 mmol dm3) led to the characteristic transient absorption spectrum of Np(T1) with a maximum at 420 nm as shown in Fig. 5. The Np(T1) formation can also be seen from the time profile given as inset (a) of Fig. 5. Using different naphthalene concentrations, the energy transfer rate constant was determined to be 1.4 0.15 1010 dm3 mol1 s1 as determined from the linear dependence of the observed growth rates (kobs) of Np triplet formation (kobs) on the concentrations of Np (see inset (b) of Fig. 5).
298
Y.M. Riyad / Journal of Photochemistry and Photobiology A: Chemistry 335 (2017) 294–299
order to absorb the same amount of 266 nm photons. The experiments were carried out under optically matching conditions (Eq. (4)) at 0.6 mJ excitation energy. The low ground-state absorption of ß-C at 266 nm has a minor effect on the experiment because its direct excitation does not lead to the generation of the excited triplet molecules. Therefore, the triplet quantum yield of thioanisole (KT) was determined by the ratio of the DOD520-values of ß-carotene triplet formed via triplet energy transfer (Eqs. (3) and (4)) under the conditions mentioned above, according to Eq. (5). The KT value of PhSMe(T1) was estimated to be 0.35 0.05 in MeCN solution. PhSMe(T1) + ß-C(S0) ! PhSMe(S0) + ß-C(T1)
(2)
BP(T1) + ß-C(S0) ! BP(S0) + ß-C(T1)
(3)
OD266 nm (BP) = OD266 nm (PhSMe)
(4)
fT ðPhSMeðT1ÞÞ ¼
DOD520nm ðPhSMeðT1 Þ ! ß CðT 1 ÞÞ DOD520nm ðBPðT 1 Þ ! ß C ðT 1 ÞÞ
ð5Þ
According to the results obtained (see Table 1 vide supra), it was found that the excited state dynamic of thioanisole triplet in acetonitrile solution is very different from that of thiophenol in that the thioanisole triplet is observed either directly by 266 nm photons or by photosensitization and its lifetime (tT(PhSMe) = 1.20 ms) is 30 times longer. In contrast, the decay of the excited triplet state of both phenol and anisole molecules is similar (tT = 3.3 ms [21,23], see Table 1), but longer than that of thiophenols. This shows that the substitution of methyl for the SH hydrogen in thiophenol results in substantial changes in the triplet properties of thioanisole (see Table 1). In contrast, the substitution of methyl for the O H hydrogen in phenol has little effect on the decay of the excited-triplet state of the anisole molecule. At the same time and in the light of available data, aromatic thiophenols [11,13,15] exhibit shorter fluorescence lifetimes and lower quantum yields than those of phenols [21,23,34] (see Table 1). Furthermore, the generation of the PhS radicals occurs via the S Me bond fission of the PhSMe(S1) (as found in thiophenols [11] and phenol [34]), while the formation of the PhSCH2 radicals via the MeH bond cleavage seems energetically unlikely to happen. In contrast, both phenoxyl (PhO) and carboncentered (PhOCH2) radicals are observed after excitation of anisole. [34] Also, it should be noted that the quantum yield of sulfur radical [11,13,15] of thiophenol, for instance, was reported to be four times higher than that of phenol [21] (see Table 1) due to the inherently weak S H bond of thiophenol singlet excited state compared with those of the O H bond of phenol. Hence, sulfur substitution results generally in the promotion of the radiationless processes. Indeed, radiationless pathways are expected to predominate over the radiative channel in the excited state dynamics of thioanisole (knr >>> kr in the case of thiophenol, see Table 1). For comparison, methyl substitution in anisole enhances the radiative process, compared with that of phenol (kr > knr in the case of phenols see Table 1). Further studies will be necessary to understand how the methyl substitution in thioanisole affects the radiative and non-radiative (internal conversion and photodissociation) pathways in order to provide a complete picture. 3.3. Pulse-radiolytic sensitization with benzene In order to verify the photogenerated thioanisole triplet and its energy level, pulse radiolysis studies of thioanisole were
performed in a N2-saturated benzene solution. Radiolysis of pure benzene produces its first excited singlet C6H6(S1) (ES = 459 kJ mol1) and triplet C6H6(T1) (ET1 = 353 kJ mol1) states via geminate ion-recombination in the primary process (Eq. (6)) as a result of fast ions mobility [35]. Benzene triplet immediately reacts with another benzene molecule with a lifetime of 4.5 ns forming a biradical (lmax = 320 nm, t = 150 ns) (Eq. (7)), whereas benzene singlet rapidly forms singlet excimer (lmax = 520 nm, t = 30 ns) (Eq. (8)). Therefore, the benzene singlet excimer and biradical are observed as short-living species [36,37]. In spite of the short lifetime of benzene triplet, it can produce solute triplets of low energy by using relatively high solute concentrations (Eq. (9)). C6H6 ! (C6H6+ + e) ! C6H6(S1), C6H6(T1)
(6)
C6H6(T1) + C6H6 ! (C6H6)2 k7 = 1.8 107 dm3 mol1 s1
(7)
C6H6(S1) + C6H6 ! (C6H6)2*
(8)
C6H6(T1) + PhSMe ! PhSMe(T1) + C6H6
(9) 3
At high thioanisole concentrations (10 mmol dm ), the energy transfer (Eq. (9)) successfully competes with benzene biradical formation (Eq. (7)), see Fig. S1. In the same manner as described vide supra in laser photolysis, the triplet spectrum of the PhSMe(T1) which was separated from the radical background absorption exhibits three major absorptions (at 320 nm, 360 nm and 490 nm, see Fig. S1) and displays identical kinetics (see Fig. S2), indicating that they represent the same chemical species. Therefore, it is evident that the spectral and kinetic parameters of the thioanisole triplet are independent of their generation (direct (Figs. 1 and 2) vs. indirect (Figs. S1 and S2)). These results indicate an energy transfer from benzene triplet to PhSMe and the formation of PhSMe(T1). Indeed, the long-lived phenylthiyl radicals are formed by a reaction between the thioanisole triplet and another thioanisole molecule but this reaction is not taken into account in the flash photolysis experiments because the thioanisole concentration is too low in these experiments. Moreover, the long-lived-transient absorption is caused not only by phenylthiyl radicals, but also by the remaining biradicals and their decay products (phenylcyclohexadienyl radicals). In the light of these results, thioanisole triplet is thus lower than that of benzene, i.e. 353 kJ mol1 > E(PhSMe(T1)) > 287 kJ mol1. 4. Conclusions Generation of the thioanisole triplet is performed directly by photolysis (lexc. = 266 nm) or indirectly by pulse radiolysis via sensitization with benzene triplet. It was revealed here that the excited state dynamic of thioanisole triplet in acetonitrile solution is very different from that of thiophenol (see Table 1). For comparison, the decay of the excited triplet state of both phenol and anisole is similar. This emphasizes that the substitution of methyl group for the S H hydrogen in thiophenol results in substantial changes in the triplet properties of thioanisole, whereas that in phenol leads to a little effect on the decay of the excited-triplet state of the anisole molecule. Furthermore, the generation of the sulfur-centered (PhS) radicals occurs via the S Me bond fission of the PhSMe(S1), while the formation of the carbon-centered (PhSCH2) radicals via the Me H bond cleavage seems energetically unlikely to happen. Further studies will be necessary to understand how the methyl substitution in thioanisole influences on the other processes (e.g. fluorescence,
Y.M. Riyad / Journal of Photochemistry and Photobiology A: Chemistry 335 (2017) 294–299
internal conversion, and photodissociation processes) in order to provide a complete picture. Conflict of interest The authors declare no competing financial interest. Acknowledgements The author thanks Prof. Dr. Bernd Abel (Leibniz Institute of Surface Modification, Leipzig), Prof. Dr. Ortwin Brede (Leipzig University), and Dr. Ralf Hermann (Leipzig University), for interesting discussions and support. The author gratefully acknowledges financial support from the Deutsche Forschungsgemeinschaft (DFG, Bonn) and the Egyptian Academy of Scientific Research and Technology (ASRT, Cairo). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. jphotochem.2016.12.007. Transient absorption spectra and time profiles are obtained after pulse radiolysis of thioanisole in benzene. References [1] G. Scott, Atmospheric Oxidation and Antioxidants, vols. I and II, Elsevier, Amsterdam, 1993. [2] K.-D. Asmus, Sulfur-centered free radicals, Methods Enzymol. 186 (1990) 168– 180. [3] C. von Sonntag, H.-P. Schuchmann, The chemistry of functional groups, The Chemistry of Ethers Suppl. E, Hydroxyl Groups and Their Sulphur Analogues, Part 2, Crown Ethers, Wiley, NY, 1980, pp. 923–934. [4] I.W.J. Still, Organic Sulfur Chemistry: Theoretical and Experimental Advances, in: F. Bernardi, I.G. Csizmadia, A. Mangini (Eds.), Elsevier, Amsterdam, 1985, pp. 595–659. [5] B.S. Berlett, E.R. Stadtman, Protein oxidation in aging disease, and oxidative stress, J. Biol. Chem. 272 (1997) 20313–20316. [6] C. Schöneich, Reactive oxygen species and biological aging: a mechanistic approach, Exp. Gerontol. 34 (1999) 19–34. [7] W.R. Markesbery, Oxidative stress hypothesis in Alzheimer’s disease, Free Radic. Biol. Med. 23 (1997) 134–147. [8] C. Chatgilialoglu, K.-D. Asmus, Sulfur-Centered Reactive Intermediates in Chemistry and Biology, Plenum Press, New York, 1989. [9] H. Sprinz, J. Schwinn, S. Naumov, O. Brede, Mechanism of thiyl radical catalyzed isomerization of unsaturated fatty acid residues in homogeneous solution and in liposomes, Biochim. Biophys. Acta 1483 (2000) 91–100. [10] P. Wardman, Thiyl radicals in biology: their role as a molecular switch central to cellular oxidative stress, in: Z.B. Alfassi (Ed.), S-Centered Radicals, Wiley, Chichester, UK, 1999, pp. 289–309. [11] Y.M. Riyad, S. Naumov, R. Hermann, O. Brede, Deactivation of the first excited singlet state of thiophenols, Phys. Chem. Chem. Phys. 8 (2006) 1697–1706. [12] Y.M. Riyad, R. Hermann, O. Brede, Hidden triplet states of aromatic thiols characterized by radiation-chemical sensitization, Chem. Phys. Lett. 376 (2003) 776–780. [13] Y.M. Riyad, R. Hermann, O. Brede, Quantitative description of the deactivation channels of the first excited singlet state of 2- and 4-thiosalicylic acids, J. Phys. Chem. A 109 (2005) 6457–6464. [14] Y.M. Riyad, S. Naumov, R. Hermann, O. Brede, Structure dependence of the photogeneration of thionaphthol triplets, Chem. Phys. Lett. 477 (2009) 290– 297.
299
[15] Y.M. Riyad, S. Naumov, R. Hermann, B. Abel, Shedding light into the detailed excited-state relaxation pathways and reaction mechanisms of thionaphthol isomers, J. Phys. Chem. A 115 (2011) 718–725. [16] M. Hoshino-Nagasaka, T. Suzuki, T. Ichimura, Rotationally resolved highresolution spectrum of the S1-S0 transition of jet-cooled thioanisole, Phys. Chem. Chem. Phys. 12 (2010) 13243–13247. [17] S.L. Li, X. Xu, D.G. Truhlar, Computational simulation and interpretation of the low-lying excited electronic states and electronic spectrum of thioanisole, Phys. Chem. Chem. Phys. 17 (2015) 20093–20099. [18] A.M. Wenge, T.N.V. Karsili, J.D. Rodriguez, M.I. Cotterell, B. Marchetti, R.N. Dixon, M.N.R. Ashfold, Tuning photochemistry: substituent effect on ps* state mediated bond fission in thioanisoles, Phys. Chem. Chem. Phys. 17 (2015) 16246–16256. [19] S. Han, J.S. Lim, J.H. Yoon, J. Lee, S.Y. Kim, S.K. Kim, Conical intersection seam and bound resonance embedded in continuum observed in the photodissociation of thioanisole-d3, J. Chem Phys. 140 (2014) 0543071– 0543078. [20] G.M. Roberts, D.J. Hadden, L.T. Bergendahl, A.M. Wenge, S.J. Harris, T.N.V. Karsili, M.N.R. Ashfold, M.J. Paterson, V.G. Stavros, Exploring quantum phenomena and vibrational control in s* mediated photochemistry, Chem. Sci. 4 (2013) 993–1001. [21] R. Hermann, G.R. Mahalaxmi, T. Jochum, S. Naumov, O. Brede, Balance of the deactivation channels of the first excited singlet state of phenols: effect of alkyl substitution, sterical hindrance, and solvent polarity, J. Phys. Chem. A 106 (2002) 2379–2389. [22] M.R. Ganapathi, R. Hermann, S. Naumov, O. Brede, Free electron transfer from several phenols to radical cations of non-polar solvents, Phys. Chem. Chem. Phys. 2 (2000) 4947–4955. [23] Handbook of Photochemistry, in: S.L. Murov, I. Carmichael, G.L. Hug (Eds.), Marcel Dekker, New York, 1993. [24] M. Ioele, S. Steenken, E. Baciocchi, Generation and reactivity of aromatic thioether radical cations in aqueous solution as studied by pulse radiolsis, J. Phys. Chem. A 101 (1997) 2979–2987. [25] P. Filipiak, G.L. Hug, B. Marciniak, Photochemistry of carboxylic acids containing the phenyl and thioether groups: steady-state and laser flash photolysis, J. Photochem. Photobiol. A Chem. 177 (2006) 295–306. [26] P. Filipiak, G.L. Hug, K. Bobrowski, B. Marciniak, J. Photochem. Photobiol. A Chem. 172 (2005) 322–330. [27] I. Carmichael, G.L. Hug, Triplet-triplet absorption spectra of organic molecules in condensed phases, J. Phys. Chem. Ref. Data 15 (1986) 1–250. [28] O. Brede, R. Mehnert, W. Naumann, J. Teply, Laser photolysis and pulse radiolysis investigations on the triplet energy transfer in aromatic donoracceptor system, Ber. Bunsenges. Phys. Chem. 89 (1985) 1036–1041. [29] O. Brede, R. Mehnert, W. Naumann, Kinetics of excitation and charge transfer reactions in non-polar media, Chem. Phys. 115 (1987) 279–296. [30] H.L.J. Backstrom, K. Sandros, Transfer of triplet energy in fluid solutions. I. Sensitized phosphorescence and its application to the determination of triplet state lifetimes, Acta Chem. Scand. 14 (1960) 48–62. [31] T.D. Giacco, F. Elisei, O. Lanzalunga, Photoinduced hydrogen- and electrontransfer processes between chloranil and aryl alkyl sulfides in organic solvents. Steady-state and time-resolved studies, Phys. Chem. Chem. Phys. 2 (2000) 1701–1708. [32] A. Lewandowska-Andralojc, G.L. Hug, G. Hörner, T. Pedzinski, B. Marciniak, Unusual photobehavior of benzophenone triplets in hexfluoroisopropanol. Inversion of the triplet character of benzophenone, J. Photochem. Photobiol. A Chem. 244 (2012) 1–8. [33] A. Lewandowska-Andralojc, G.L. Hug, G. Hörner, T. Pedzinski, P. Filipiak, B. Marciniak, Efficient photochemical oxidation of anisole in protic solvents: electron transfer driven by specific solvent-solute interactions, ChemPhysChem 11 (2010) 2108–2117. [34] D.V. Bent, E. Hayon, Excited state chemistry of aromatic amino acids and related peptides. I. Tyrosine, J. Am. Chem. Soc. 97 (1975) 2599–2606. [35] R.V. Bensasson, J.T. Richards, J.K. Thomas, The pulsed laser photolysis of benzene in cyclohexane, Chem. Phys. Lett. 9 (1971) 13–16. [36] J.K. Thomas, I. Mani, Pulse radiolysis of benzene cyclohexane mixtures, J. Chem. Phys. 51 (1969) 1834–1838. [37] O. Brede, W. Helmstreit, R. Mehnert, Nanosekunden-pulsradiolyse und laserphotolyse von benzophenon in benzol und cyclohexan, Z. Phys. Chem. (Leipzig) 256 (1975) 505–512.