ARTICLE pubs.acs.org/JPCA
Shedding Light into the Detailed Excited-State Relaxation Pathways and Reaction Mechanisms of Thionaphthol Isomers Yasser M. Riyad,*,†,‡ Sergej Naumov,§ Ralf Hermann,† and Bernd Abel† †
Wilhelm-Ostwald-Institute for Physical and Theoretical Chemistry, Faculty of Chemistry and Mineralogy, University of Leipzig, Permoserstrasse 15, 04318 Leipzig, Germany ‡ Chemistry Department, Faculty of Science, Al-Azhar University, Nasr City, 11884, Cairo, Egypt § Leibniz Institute of Surface Modification, Permoserstrasse 15, 04303 Leipzig, Germany ABSTRACT: Nanosecond laser flash photolysis employing transient detection of emission and absorption in combination with pulse radiolysis and quantum theory has been employed to shed light into the kinetics, quantum yields, and mechanisms of the deactivation of the first excited singlet state of 1- and 2-thionaphthols (NpSH(S1)). In contrast to thiophenols (ArSH(S1)), the results revealed that the decay of the first excited singlet state of 1- and 2-thionaphthols (NpSH(S1)) is governed by radiationless internal conversion (ΦIC = 0.29-0.46; 0.016-0.190) and intersystem crossing (ΦISC = 0.14-0.15; 0.4-0.6), respectively, with pronounced S-H photodissociation (ΦD = 0.40-0.55; 0.35-0.40). Fluorescence as a deactivation channel plays a minor role (ΦF = 0.001-0.010; 0.010-0.034). Quantum chemical calculations helped in understanding the formation of naphthylthiyl radicals and rationalizing the differences in the efficiency of intersystem crossing of the 1- and 2-thionaphthol systems.
’ INTRODUCTION Due to the low ionization potential and weak S-H bond, aliphatic and aromatic thiols as well as phenols are used as antioxidants for organic matter ranging from polymers to living systems.1,2 The antioxidant action of thiols is understood in terms of electron and hydrogen atom donor ability.3,4 In biological systems, the thiols play a prominent role. Thiols are often thought to act as protectors against ionizing radiation via their radical scavenging activity. In addition, it was found that thiyl radicals surprisingly also cause biologically important chemical changes such as the efficient cis-trans isomerization of mono- and poly-unsaturated fatty acid residues in model membranes via a catalytic action.5-8 Photoexcitation of aromatic thiols results in the formation of phenylthiyl radicals,9-11 and only limited knowledge exists about the intermediates of aromatic thiols generated by photoexcitation such as their excited singlet and triplet states. In a previous paper,12 we provided a deeper insight into the photophysical and photochemical properties of thiophenol and its substituted derivatives in polar and nonpolar solvents at room temperature. For all thiophenols, however, we revealed that neither intersystem crossing nor radiative channels are important in the deactivation mechanism of the first excited singlet state. Instead both the S-H bond photodissociation (with few exceptions) and r 2010 American Chemical Society
radiationless internal conversion channels play a dominant role. Indeed, because of the similar symmetry properties of the excited singlet and triplet states of thiophenols, their triplet states could not be formed by photoexcitation but rather by pulse radiolysis via sensitization with appropriate compounds of higher triplet energy.13 As an exception and due to the influence of the carboxylic group substituent on the photophysical parameters of 2- and 4-thiosalicylic acids, we found that the triplets of those compounds are formed also by photoexcitation.14 Furthermore, a comparison shows that there are significant differences in the deactivation mechanisms of the first excited singlet states of thiophenol and phenol analogues.15 To characterize the fate of the first excited singlet state of a larger, compared to the parent thiophenol, aromatic thiol moiety, laser flash photolysis and pulse radiolysis studies with 1- and 2-thionaphthol have been performed. The photophysical properties of thionaphthols have not yet been characterized, and the knowledge about light- and radiation-induced transient reaction behavior of those compounds is still very limited too. In a recent paper of ours16 attention was paid only to the identification of Received: November 9, 2010 Revised: December 13, 2010 Published: December 30, 2010 718
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triplet-state properties of 1- and 2-thionaphthols. The main point of that study was the first realization and observation of a directly photogenerated triplet state of those compounds and the characterization of their triplets by quenching and energy-transfer experiments. With this background, we intend in this paper to give a comprehensive study and quantitative description of the remaining deactivation channels of the first excited singlet state of thionaphthols induced by monophotonic excitation. These channels include fluorescence, S-H fragmentation and radiationless internal conversion channels; see eqs 2, 3, and 5. In the current investigations quantum chemical calculations helped in rationalizing and understanding the large number of experimental results. Our experimental conditions were chosen in such a way that self-quenching, and a possible photoionization, which occurs at higher laser power, could be neglected. The interest was focused on the remaining processes solely. hν1 ¼ 266nm
f NpSHðS1 Þ NpSHðS0 Þ s kF
NpSHðS1 Þ f NpSHðS0 Þ þ hν2 kIC
f NpSHðS0 Þ þ E kISC
f NpSHðT1 Þ kD
•
f NP S þ H
fluorescence
ð1Þ ð2Þ
internal conversion
ð3Þ
intersystem crossing
ð4Þ
•
fragmentation
ð5Þ
’ EXPERIMENTAL SECTION Materials. Water, acetonitrile (ACN), ethanol (EtOH), and 1-chlorobutane (1-BuCl) of highest spectroscopic grade were chosen as solvents (99.9%, VWR). Benzophenone (99.9%) and phenol (99.5%) were obtained from Aldrich and Riedel-de Haen, respectively. Water treated in a Millipore Milli-Q Plus system was used for the experiments in aqueous solutions. 1-Thionaphthol and 2-thionaphthol (NpSH) are of the highest spectroscopic grade (99%, VWR) and used without further treatment. Apparatus and Methods. The ground-state optical absorption and fluorescence spectra were recorded with an UV-vis spectrophotometer (UV-2101 PC, Shimadzu) and a FluoroMax-2 (Instruments S. A., Jobin Yvon-Spex), respectively. Nanosecond Laser Photolysis. Excitation of the thionaphthols was performed with the fourth harmonics (266 nm) of a Quanta-Ray GCR-11 Nd:YAG laser (Spectra Physics), 0.5-4.0 mJ/pulse. Pulse duration was 3 ns (fwhm). The optical detection system consisted of a pulsed xenon lamp (XBO 150, Osram), a monochromator (SpectraPro-275, Acton Research), a R955 photomultiplier tube (Hamamatsu Photonics) or a fast Si-photodiode with 1 GHz amplification, and a 500 MHz digitizing oscilloscope (DSA 602 A, Tektronix). The laser power was monitored for every pulse using a bypath with a fast Si photodiode. The laser flash photolysis setup is reported elsewhere in more detail.17 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 between 50 and 100 Gy. Detection of the transient species was performed using an optical absorption setup consisting of a pulsed xenon lamp (XPO 450), a SpectraPro-500 monochromator (Acton Research Corp.), a R4220 photomultiplier (Hamamatsu Photonics),
Figure 1. Emission spectra of (A) 1-NpSH (0.15 mmol dm-3) and (B) 2-NpSH (0.06 mmol dm-3) in 1-chlorobutane (9), ethanol (2), and acetonitrile (b) upon excitation at 266 nm, together with normalized fluorescence excitation spectrum (O) in acetonitrile (λemission = 360 nm).
and a 1 GHz digitizing oscilloscope (TDS 640, Tektronix). Further details of this equipment are given elsewhere.18 All experiments were performed at room temperature (298 K). Freshly prepared solutions were used, flowing continuously through a 5 or 10 mm quartz sample cell in laser photolysis or in pulse radiolysis, respectively. Prior to the laser photolysis and the pulse radiolysis experiments, the solutions were deoxygenated by bubbling with purest grade N2 or saturated with the desired gas for 15 min and were used within 1 h. Quantum Chemical Approach. The calculations were carried out using the Gaussian 03 package19 applying the density functional theory (DFT) approach with B3LYP hybrid functionals.20-22 Stationary points were characterized by frequency calculations. For geometry optimizations, the standard 6-31G(d) basis set was used. The electronic transition spectra were calculated with the unrestricted time-dependent (UTD DFT)23 B3LYP/6-31þG(d,p) method.
’ RESULTS AND DISCUSSION Steady-State and Time-Resolved Fluorescence Measurements . Steady-State Fluorescence Studies. The fluores-
cence emission spectra were studied upon excitation of 1- and 2-thionaphthols in polar (acetonitrile and ethanol) and nonpolar surroundings (1-chlorobutane) at 266 nm, see Figure 1, and the 719
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(1-BuCl, EtOH) to 355 nm in ACN. The quite identical spectral fluorescence properties, together with fluorescence kinetics and quantum yields given below, indicate that the first excited states of both 1- and 2-NpSH most likely have the same geometry at room temperature. The fluorescence excitation spectra for the emission at 360 nm of both 1- and 2-NpSH in ACN solution are also shown in Figure 1A,B, respectively. The normalized fluorescence excitation and emission spectra intersect at ∼332 nm, due to the lowest excited singlet state energy value (E0-0) of ∼86.13 kcal mol-1. A similar value was obtained using other solvents. Fluorescence Kinetics. Generally, the fluorescence decay kinetics is determined by all of the relaxation processes 2-5 competing for the excited singlet state of a molecule. Lightinduced emission measurements give information about excited singlet state deactivations. The fluorescence kinetic measurements have been performed by using a cutoff filter (λ = 308 nm) to exclude the contributions of the 290 nm emission caused by the quartz cell and the scattering light due to the excitation pulse. Therefore, the fluorescence lifetimes, τF, have been determined by fitting the fluorescence decay profile for the S1-S0 transition of 1- and 2-NpSH in the solvents mentioned above. The kinetic analysis of the traces of 1- and 2-NpSH given in Figure 2 has revealed a monoexponential decay, and the fluorescence lifetimes, τF, thus obtained are demonstrated in Table 1. The fluorescence lifetimes of 1- and 2-NpSH measured in polar acetonitrile or ethanol are comparable, but shorter than those in the nonpolar 1-chlorobutane. Accordingly, the fluorescence kinetics under the polar conditions are shorter than in nonpolar solutions. This might be caused by the low stability of the first excited singlet state S1, and it points to a more efficient nonradiative relaxation. Fluorescence Quantum Yields (ΦF). Fluorescence quantum yields of thionaphthols were determined by steady-state fluorescence measurements using phenol in cyclohexane, acetonitrile, and ethanol as standards (Φ = 0.083, 0.19, and 0.16, respectively15). Under optically matching conditions at λexc = 266 nm, i.e., OD266nm(ArOH)=OD266nm(NpSH), ΦF was determined for diluted solutions (e0.5 mmol dm-3) with the method of comprising the integrals of reference and sample substance over the whole emission range.24,25 The calculated fluorescence quantum yield values determined in 1-BuCl, EtOH, and ACN are given in Table 1. It is obvious that the ΦF values of 2-NpSH are higher than those of 1-NpSH and this is consistent with the steady-state results, but both isomers fluoresce with a low quantum yield. Comparing the fluorescence quantum yield of each isomer in different solvents, one can find that the ΦF values particularly of 1-thionaphthol increase significantly and systematically from nonpolar to polar surroundings. A notable exception to this trend is the lower ΦF value of 2-thionaphthol in ethanol than in chlorobutane. Using the fluorescence quantum yields, the radiative rate constant values (kF = ΦF/τF) of thionaphthols in the employed solvents were determined to be ∼105-106 s-1 (see Table 2). Therefore, there are large differences between the experimental fluorescence (τF ∼ 4-11 ns) and the overall radiative (τ0 = 1/kF, ∼0.12-11 μs) lifetimes. Consequently, this emphasizes that radiationless (ISC or IC) and/or photodissociation processes are predominant over fluorescence pathway in the deactivation of the first excited singlet state of thionaphthols. Solvents of different nature have been employed, and the surrounding influence on the S1 decay kinetics and other
Table 1. Spectral and Kinetic Fluorescence Properties and Quantum Yields of All Deactivation Channels of the First Excited Singlet State of 1- and 2-Thionaphthols (NpSH) and Thiophenol (ArSH) in 1-Chlorobutane (1-BuCl), Ethanol (EtOH), and Acetonitrile (ACN) at Room Temperaturea b compound solvent λmax em (nm) τF (ns)
ArSH d
1-NpSH
2-NpSH
ΦF
ΦISC c
ΦD
ΦIC
1-BuCl
295
0.58
0.003
0.000
0.30 0.697
EtOH ACN
310 310
1.20 2.70
0.004 0.004
0.000 0.000
0.30 0.696 0.30 0.696
1-BuCl
345
11.80
0.001
0.15
0.50 0.349
EtOH
355
4.60
0.004
0.14
0.40 0.456
ACN
360
4.60
0.010
0.15
0.55 0.290
1-BuCl
350
4.50
0.014
0.60
0.35 0.036
EtOH
350
4.10
0.010
0.40
0.40 0.190
ACN
355
4.20
0.034
0.60
0.35 0.016
The yields are within (10% accuracy. b The error of τF is about (10%. c Data from ref 16. d Data from ref 12. a
Figure 2. Time-resolved fluorescence of 1-thionaphthol (0.15 mmol dm-3) in 1-chlorobutane (- 3 3 ), ethanol (---), and acetonitrile (--) solutions after excitation at 266 nm. The inset describes the fitting (O) of the fluorescence kinetics in 1-chlorobutane and acentonitrile by applying a monoexponential decay.
corresponding spectral data are summarized in Table 1. Upon the excitation of the thionaphthols at 275 or 325 nm (the data are not shown) the emission displayed identical spectra and emission maxima similar to those observed at 266 nm, due to S1f S0 transition; see Figure 1. In ACN, for instance single bands with maxima at 360 and 355, respectively, are observed for the 266 nm excitation, clearly attributable to emission from the lowest lying singlet excited state. By comparing the emission spectra given in Figure 1A,B, it can generally be observed that 2-NpSH has a higher fluorescence intensity than 1-NpSH, irrespective of the solvent. Moreover, the emission maxima generally exhibit a small bathochromic shift with increasing polarity of the solvent used. This is particularly pronounced for 1-NpSH with λF(max) = 345 nm in 1-BuCl which shifts to 355 and 360 nm in ACN and EtOH, respectively (see Figure 1A), whereas for the corresponding 2-thionaphthol the shifts are small, i.e., from 350 nm 720
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Table 2. Radiative (kF) and Radiationless (kISC, kIC, and kD) Rate Constants for 1- and 2-Thionaphthols in 1-BuCl, EtOH, and ACN compound 1-NpSH
2-NpSH
a
solvent
kF a (106 s-1)
kISC b (108 s-1)
kD c (108 s-1)
kIC d (108 s-1)
1-BuCl
0.085
0.127
0.424
0.296
EtOH ACN
0.870 2.174
0.304 0.326
0.870 1.196
0.991 0.630
1-BuCl
3.11
1.33
0.778
0.080
EtOH
2.44
0.976
0.976
0.463
ACN
8.10
1.43
0.833
0.040
kF = ΦF/τ. b kISC = ΦISC/τF. c kD = ΦD/τF. d kIC = ΦIC/τF.
processes. In the next sections, the fragmentation and radiationless internal conversion processes will be characterized. Fragmentation Channel . Naphthylthiyl Radical Absorption Characterized by Laser Photolysis. In a previous paper,16 laser photolysis measurements with 266 nm excitation of thionaphthols in oxygen-free acetonitrile solution recorded at early times (∼400-500 ns) showed that the triplet and radical formations are generated synchronously. The different kinetic behavior of the transient upon saturating the solution with oxygen (as an excited-state quencher) together with sensitization experiments, enabled us to identify thionaphthol triplet states and to separate their absorption bands and those of naphthylthiyl radicals. Because of the shorter lifetime of 1- and 2-NpSH(T1) (e.g., τT ∼ 4-7 μs in ACN) in comparison with those of NpS• radicals (τR > 15 μs), laser photolysis of thionaphthols in oxygen free acetonitrile solution taken after a long time (8-10 μs) allowed us to observe absorption spectra of 1- and 2-naphthylthiyl radical. Parts A and B of Figure 3 respectively show mainly the absorption spectra of 1-NpS•, peaking at 410 (double band structure), 370, and 670 nm, and that of 2-NpS•, peaking at 310, 370, 490, and 720 nm. The absorption maxima of 1- and 2-NpS• radicals agree with a previous pulse radiolysis measurement.27,28 Indeed, the time profiles given as insets of Figure 3 reveal that 1- or 2-NpS• radicals monitored at their absorption maxima show similar decay kinetics and are insensitive toward oxygen. According to our experimental results we found that the naphthylthiyl radical (NpS•) is only formed by the homolytic S-H bond scission of the first excited singlet state of thionaphthols (NpSH(S1); cf. eq 5). In the same manner as in ACN and under similar conditions, laser photolysis of thionaphthols in EtOH and 1-BuCl shows a similar trend (data are not given here), and the absorption maxima of the NpS• radicals in the employed solvents are summarized in Table 3. Indeed, it seems that the absorption maxima of the 1- and 2-NpS• do not show any solvent dependence; see Table 3. Molar Absorption Coefficient Determination of the Naphthylthiyl Radicals by Pulse Radiolysis. To obtain clear spectra of 1- and 2-naphthylthiyl radicals (NpS•) as well as to calculate their molar absorption coefficients, in analogy to the known thiophenolate oxidation,12 azide radicals have been used for oxidizing thionaphtholate via a pulse radiolysis experiment in aqueous alkaline (pH 12) solution under N2O atmosphere.
Figure 3. Transient absorption spectrum obtained after laser photolysis of (A) 1-NpSH (0.15 mmol dm-3) and (B) 2-NpSH (0.06 mmol dm-3) purged with N2 in acetonitrile taken 8 (b) or 10 μs (b) after the pulse, respectively (laser power = 3 mJ). Experimental time profiles of N2 (-) and O2 ( 3 3 3 ) saturated solutions taken at the absorption maxima of the observed transients are given in the insets.
spectroscopic data given in Table 1 was observed. The fluorescence behavior is expected to be sensitive to the chemical and physical properties of the solvent, but their theoretical descriptions are frequently too complex for generalization. At least, one has to divide into general and specific solvent effects.26 The influence of such interaction phenomena on the thionaphtholS1 kinetics is under investigation and will be presented in a further publication. A thorough analysis of the deactivation mechanism of the first excited singlet state entails a detailed study of nonradiative
NpS - þ N3 • f NpS• þ N3 -
k6 g7 109 dm3 mol - 1 s - 1 ð6Þ
Figure 4 shows the naphthylthiyl radical spectra of 1- and 2-thionaphthols obtained in the pulse radiolysis of alkaline N2O 721
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Table 3. Spectral Parameters of Naphthylthiyl Radicals (NpS•) of 1- and 2-Thionaphthols λmax(NpS•) (nm) compound 1-NpSH 2-NpSH
a
1-BuCl 370, 410, 670 310, 370, 500, 710
EtOH 380, 410, 650 310, 360, 490, 720
ACN 370, 410, 670 310, 370, 490, 720
H2O
ε(NpS•) a in H2O (dm3 mol-1cm-1)
420
6800 ( 200
680
2500 ( 200
390
8200 ( 300
510
4200 ( 200
720
1200 ( 200
The spectral data of the naphthylthiyl radicals were measured in N2O-purged aqueous alkaline solutions at pH 12 by pulse radiolysis.
disulfide radical anions, as observed for aliphatic thiols.31,32 These εR values also enable us to calculate the radical quantum yields. Quantum Yields of the Naphthylthiyl Radical (ΦD). The radical quantum yields of the photodissociation of the S-H bond of the NpSH(S 1) were determined by timeresolved nanosecond-laser photolysis using the benzophenone triplet, BP(T1), as an external standard. Under optically matched conditions at 266 nm, i.e., OD266nm(BP) = OD266nm(NpSH), the ΦD values were derived for N2-purged diluted solutions (e0.5 mmol dm-3) by comparing the concentration of reference (BP(T1)) and sample radical (NpS•) transients. The process was performed at low laser energy ∼ 0.6 mJ/pulse to avoid twophoton ionization. The data are given in Table 1 for the three solvents 1-chlorobutane, ethanol, and acetonitrile. At first sight, the results showed that the ΦD values of 1-NpSH(S1) are larger than those of the 2-NpSH(S1). Indeed, the ΦD values of both thionaphthols are comparable in polar solvents but slightly higher than those in nonpolar surroundings. These results point to the significant role of photodissociation (∼35-55%) together with either radiationless intersystem crossing or internal conversion processes in the deactivation mechanism of the first excited singlet state of thionaphthols. Radiationless Deactivation. The remaining process is the radiationless deactivation from the first excited singlet (S1) to the singlet ground state (S0) which occurs via internal conversion (IC). Here the excitation energy is consumed by changes in the molecular geometry or is dissipated to the surroundings. The overall IC process can be understood in terms of the rate controlling resonance interaction (RI) between the S1 and the S0 vibration levels followed by rapid vibration relaxation (VR). In contrast to the other deactivation channels studied, there is no direct measure of the internal conversion efficiency (3) which can, however, be evaluated simply as difference from the other NpSH(S1) deactivation processes
Figure 4. Transient optical absorption spectra of 1- and 2-naphthylthiyl radicals recorded on pulse radiolysis of N2O purged aqueous solutions of (A) 1- and (B) 2-thionaphthols (0.5 mmol dm-3) in the presence of 5 mmol dm-3 NaN3 at pH = 12.
ΦIC ¼ 1 - ðΦF þ ΦISC þ ΦD Þ
ð7Þ
These data are given in Table 1 together with the experimentally determined quantum yields of the other deactivation channels for 1-chlorobutane, ethanol, and acetonitrile solutions. The results showed that the ΦIC values of 1-NpSH in all solvents range between 0.29 and 0.456 and are significantly larger than those of 2-NpSH which range between 0.016 and 0.24. This reflects a negligible contribution of the internal conversion process in the deactivation mechanism of 2-NpSH(S1). Although the approach used cannot distinguish between the RI and VR contribution, for physical reasons the vibration relaxation is nearly unaffected by the influences considered, whereas relaxation by resonance interaction depends on the overlap of the
purged solutions of 5 mmol dm-3 sodium azide and 0.5 mmol dm-3 of the corresponding thionaphthols. We determined the molar absorption coefficient values (εR) given in Table 3 from the comparison of the azide molar absorption coefficient ε(N3•)275nm = 2300 dm 3 mol-1 cm-1 29 and via electron dosimetry.30 Unfortunately, the demonstration of the absorption of naphthylthiyl radicals in the UV region was limited because of the high thionaphthol concentrations and the corresponding selfabsorption. The rate constants k6 at pH 12 were calculated to be in the range of (7-10) 109 dm3 mol-1 s-1 for thionaphthols. In the kinetic analysis, we had no indication for the formation of 722
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Figure 5. Electron-density distribution of the molecular orbitals for thiophenol and 1- and 2-thionaphthols in the most stable structure calculated with the DFT B3LYP/6-311þG(d,p) method.
Table 4. B3LYP/6-31þG(d,p) Calculated First Excited Singlet and Triplet State Energies, Calculated Ground-State Bond Dissociation Energies (BDE(S0), Calculated Dissociation Enthalpies for S-H Bond Cleavage in the First Excited Singlet State (ΔH(S1)), and Calculated Changes of Dipole Moment (Δμ) in First Excited Singlet State compound
E(S1) (kcal mol-1)
E(T1) (kcal mol-1)
μ, μ* (D)
Δμ (D)
BDES-H(S0) (kcal mol-1)
ΔHS-H(S1) (kcal mol-1)
82.10
1.221, 1.921
0.70
80
-29.3
1-NpSH
90.63
59.30
1.077, 1.262
0.185
75
-15.5
2-NpSH
90.86
61.10
1.345, 0.779
0.566
77
-14.3
ArSH
107
corresponding vibration levels (Franck-Condon factors). For both thionaphthols studied, within the experimental errors, the internal conversion seems to be unaffected by the nature (dielectric constant) of the surrounding molecular environment. Therefore vibrational relaxation determines the IC channel of nonradiative relaxation to a high extent. From the fluorescence lifetimes and quantum yields, rate constants for radiationless (intersystem crossing (kISC), internal conversion (kIC), and dissociation (kD)) decays of the first excited state of thionaphthols were estimated and the data are presented in Table 2 together with the corresponding radiative decays (kF). The determined rate constants indicate that the radiationless pathways are dominant. However, with 1- and 2-thionaphthol-S1 the internal conversion and intersystem crossing are the main excited state deactivation pathways, respectively, with S-H photodissociation. As known for all of the vibrational relaxations considered, the relaxations from SnfS1 take place extremely fast (1013 s-1),33 whereas the S1f S0 relaxation appears with slower rate constants 105-108 s-1; see Table 3. Quantum Chemical Based Interpretation. To understand the influence of a structural variation between the parent thiophenol and its larger aromatic thionaphthol moiety on their first excited singlet states deactivation, the excited state energy properties (singlet and triplet) of 1- and 2-NpSH and their corresponding structures as well as those of the parent thiophenol
were calculated using the density functional theory (DFT) method at the B3LYP/6-31þG(d,p) level. At first sight, comparing between the molecular orbitals of different aromatic thiols given in Figure 5 one can observe that the extended aromatic moiety of thionaphthols increases the π-electron delocalization and consequently lowers significantly the excited-state energies which are given in Table 3. Consequently, increasing π-electron delocalization ought to stabilize the S1 electronic state of thionaphthols (nanosecond time scale) and cause a remarkable spectral red shift (about ∼50-60 nm) of their fluorescence in comparison with those of the parent thiophenol molecule (ArSH(S1) in subpicosecond time scale). The calculations indicated that the S1 and T1-T3 electronic states of thionaphthols are mostly formed by the excitation of electrons from the two highest occupied π- and n-MO's to the lowest unoccupied π-MO's; see Figure 5. Thus, excitation of 1- or 2-NpSH by a 266 nm laser pulse brings the molecule to the electronic S3 state 4.63 or 4.60 eV, respectively. Then the excited molecule in the S3 state subsequently relaxes to the lowest energy level of S1 due to the fast vibrational relaxation (in picosecond time scale). Therefore, the S1 state of thionaphthols is of a n,π* nature, and their corresponding energies are 3.94 eV (315 nm, f = 0.1174) and 3.95 eV (314 nm, f = 0.0405) for 1- and 2-NpSH, respectively; see Table 4. These data are well correlated with our steady-state experimental results. 723
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The Journal of Physical Chemistry A As far as the triplets are concerned and as calculated for thionaphthols with the time-dependent density functional theory (TDDFT) method, the second (T2) and third (T3) triplet states lie below and near the S1 state and both T2 and T3 are very close at 3.77 and 3.91 eV for 1-NpSH and 3.64 and 3.86 eV for 2-NpSH. Furthermore, the calculations revealed that S1 and T1 states possess a similar molecular orbital configuration, whereas those of S1 and Tn (T2 and T3) states are different. Due to the small energy gap between S1 and Tn=2,3 states and to their corresponding different orbital configurations, and according to Sayed's rules,34-36 therefore the S1 excitation energy of thionaphthols ought to be deactivated through the ISC channel. Furthermore, the ISC process in thionaphthols takes place rather by spin-orbital coupling to one of the higher Tn=2,3 states followed by rapid internal conversion Tn-T1, and not by direct spin-orbital coupling of S1 to the higher vibrational level of T1. Comparing between the geometrical parameters calculated for 1- and 2-NpSH and their corresponding vibration spectra, calculations showed that LUMO and LUMO þ 1 molecular orbitals of thionaphthols are involved in the formation of T2 and T3 electronic states, respectively. The structures of the LUMO molecular orbitals of 1- and 2-thionaphthols are identical, whereas those of LUMO þ 1 are different. Because the S-H vibration of 2-NpSH (1.789 Å distance) is closer to the π-electron density in the LUMO þ 1 molecular orbital than that of 1-NpSH (2.776 Å distance), it is obvious that the S-H vibration plays an important role in the radiationless deactivation. In spite of the very small energy gap between T3 and S1 states as well as their different orbital symmetry, the difference in LUMO þ 1 structures between 1- and 2-NpSH reflects probably a distinct efficiency in the extent of overlap between electronic and vibrational levels of 1- and 2-NpSH; see Figure 5. Therefore, the larger the extent of the overlap, the more efficient ISC takes place. This explains that radiationless intersystem crossing of 2-NpSH is dominating over the other decay processes and most of the 2-NpSH(S1) excitation energy ought to be deactivated through the ISC channel. The calculated enthalpies of the homolytic S-H bond dissociation of the first excited singlet state of thionaphthols, ΔHS-H(S1), are presented in Table 4 together with those of the corresponding ground-state bond dissociation energies (BDES-H(S0)). The results showed that the ΔHS-H(S1) values of 1- and 2-thionaphthols are -15.5 and -14.3 kcal mol-1, respectively, and they are substantially smaller than that of thiophenol (-29.3 kcal mol-1), due to the inherent weakness of the S-H bond of NpS-H(S1). As a result, a considerable amount of the S1 excitation energy ought to deactivate via the photodissociation channel. In contrast to the bond dissociation energy of NpSH(S1), the BDES-H(S0) values of 1- and 2-thionaphthols are very high and comparable to that of thiophenol. This reflects the difficulty of S-H bond dissociation of NpS-H(S0). Although, it is noteworthy that BDES-H(S0) values of thionaphthols are higher than those of the first excited triplet state energies of 59.26 and 61.10, respectively. This means that NpSH(T1) is not involved in the NpS• radical formation which is only generated via the homolytic S-H bond fission of the first excited singlet state of thionaphthols NpSH(S1), and this is correlated with our experimental findings. The calculated dipole moments (μ) for the singlet ground and excited states of thionaphthols are given in Table 4. While the dipole moment of 1-NpSH is slightly enlarged by excitation into
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the excited singlet state, that of 2-NpSH is decreased. Nevertheless, the dipole moment changes upon excitation of both thionaphthols are very small.
’ CONCLUSION For the first time, this paper provides a deep insight into the deactivation mechanism of the first excited singlet state of thionaphthols. After excitation of thionaphthol molecules from S0 to the S3 state at 4.6 eV by a 266 nm laser pulse, they subsequently relax extremely rapidly (1013 s-1) to the lowest energy level of S1 according to Kasha's rule.37 The NpSH(S1) level is the central point where the relaxation processes display branching ratios to various channels. Because of our systematics studies (cf. Table 1), we are able to state that deactivation channels (eqs 2-5) follow in a first approximation the same dynamic rules in identical surroundings and show similar kinetic parameters when expressed as rates, e.g., in 1-chlorobutane: kF = 105 to 3 106 s-1, kISC = 107 to 1 108 s-1, kD = 8 106 to 3 107 s-1, and kIC = (4-8) 107 s-1. The spectral properties of the first excited singlet state of 1- and 2-thionaphthols as well as its lifetimes and the quantum yields of the involved photophysical processes together with the photodissociation pathway have been determined. The results showed a marked difference in the deactivation mechanism of the S1 state between a larger aromatic thiol moiety and the parent thiophenol (see Table 1). The first excited singlet state of 1- and 2-thionaphthols mainly decays via radiationless internal conversion or intersystem crossing process, respectively, together with the S-H photodissociation. In contrast to thionaphthols, the ArSH(S1) is mainly decayed by internal conversion (∼70%) and photofragmentation (∼30%) channels, while intersystem crossing (0%) as well as radiative (∼0.30%) channels are of minor importance in the deactivation mechanism. Hence a structural variation causes significant influences in the photophysical parameters as well as in the S-H photodissociation. Quantum chemical calculations have provided a reasonable estimation of the excited-state and radical energies. Indeed, the calculations showed that naphthylthiyl radicals are only generated from the NpSH(S1) singlet state. Furthermore, the S-H vibration plays an important role in the radiationless deactivation. Indeed, the difference in the efficiency of intersystem crossing between 1- and 2-thionaphthols is rationalized by the difference in the extent of overlap.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Fax: þ49-(0)341-235-2317.
’ ACKNOWLEDGMENT Financial support from the Deutsche Forschungsgemeinschaft (DFG, Bonn) and the Egyptian Academy of Scientific Research and Technology (ASRT, Cairo) is gratefully acknowledged. We give special thanks to Professor Ortwin Brede for his valuable discussions. Our thanks are also extended to Dipl. Ing. J€urgen J€aschke for his technical assistance. ’ REFERENCES (1) Scott, G. Atmospheric Oxidation and Antioxidants; Elsevier: Amsterdam, 1993; Vols. I and II. 724
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