Photochemical properties of excited triplet state of 6H-purine-6-thione investigated by laser flash photolysis

Article abstract of DOI:10.1021/jp973274q

Photochemical reactions of 6H-purine-6-thione (PuT) via the excited triplet state [³(PuT)*] have been studied by means of laser flash photolysis in organic solvents. Transient absorption bands at 475 and 690 nm were assigned to ³(PuT)*. Intersystem quantum yield and the lowest triplet energy of ³(PuT)* were evaluated to be 0.99 and 63 kcal/mol, respectively. The self-quenching rate constant is quite large (2.3×10⁹ M^-1 s^-1 in THF). In photoinduced electron transfer, ³(PuT)* acts as electron acceptor for tetramethylbenzidine, while ³(PuT)* acts as electron donor for p-dinitrobenzene. Rate constants for H-atom abstraction (k_hT) of ³(PuT)* from benzenethiols, tocopherol, and 1,4-cyclohexadiene are on the order of 10⁸ M^-1 s^-1. From the Hammett plots of k_hT for substituted benzenethiols, a negative ρ value indicates that ³(PuT)* has electrophilic character. In the addition reaction of ³(PuT)* toward various alkenes, the electrophilic character of ³(PuT)* was also confirmed. By steady-light photolysis of PuT, purine was produced via ³(PuT)* after H-atom abstraction. On combination of these results, the character of the lowest ³(PuT)* was presumed.

Full text of DOI:10.1021/jp973274q

1338
J. Phys. Chem. A 1998, 102, 1338-1344
Photochemical Properties of Excited Triplet State of 6H-Purine-6-thione Investigated by
Laser Flash Photolysis
Maksudul M. Alam, Mamoru Fujitsuka, Akira Watanabe, and Osamu Ito*
Institute for Chemical Reaction Science, Tohoku UniVersity, Katahira, Aoba-ku, Sendai, 980-77, Japan
ReceiVed: October 8, 1997; In Final Form: December 10, 1997
Photochemical reactions of 6H-purine-6-thione (PuT) via the excited triplet state [3(PuT)*] have been studied
by means of laser flash photolysis in organic solvents. Transient absorption bands at 475 and 690 nm were
3
3
assigned to (PuT)*. Intersystem quantum yield and the lowest triplet energy of (PuT)* were evaluated to
be 0.99 and 63 kcal/mol, respectively. The self-quenching rate constant is quite large (2.3 × 109 M-1 s-1 in
3
THF). In photoinduced electron transfer, (PuT)* acts as electron acceptor for tetramethylbenzidine, while
3(PuT)* acts as electron donor for p-dinitrobenzene. Rate constants for H-atom abstraction (khT) of 3(PuT)*
from benzenethiols, tocopherol, and 1,4-cyclohexadiene are on the order of 108 M-1 s-1. From the Hammett
plots of khT for substituted benzenethiols, a negative F value indicates that 3(PuT)* has electrophilic character.
3
3
In the addition reaction of (PuT)* toward various alkenes, the electrophilic character of (PuT)* was also
confirmed. By steady-light photolysis of PuT, purine was produced via (PuT)* after H-atom abstraction.
On combination of these results, the character of the lowest (PuT)* was presumed.
3
3
Introduction
SCHEME 1
The photochemistry of aliphatic thiones and aromatic thiones
has been well studied;1-6 the importance of the triplet states of
thiones has been revealed in the photochemical reactions.7,8 In
the case of N-atom-containing thiones, however, the photo-
chemical properties have not been studied compared with thiones
without N atoms; i.e., even in various recent reviews about the
photoexcited thiones, the descriptions of the thiones with N
atoms are excluded because of lack of the systematic studies.9,10
The photochemistry of thiones with N atoms is important in
chemical viewpoints11,12 and biological viewpoints.13-15 Chemi-
cally, various interesting photochemical reactions are known
especially for pyridine thiones.11,12 Biochemically, photolysis
of these thiones was used for specific fission of DNA sequence.
For example, 6H-purine-6-thione (PuT in Scheme 1) can
photosensitize the destruction of free-radical-scavenging anti-
oxidants, thereby depleting the system of its biological de-
fenses.15 Hemmens and Moore reported that photoionization
in aqueous solution is strongly related to biological activities.15,16
Thus, it is important to investigate the photochemical reactivities
of PuT in more detail in organic solvents.
zation. Commercially available dienes were purified by distil-
lation. Solvents were of spectroscopic grade.
Steady-state photolysis was performed with light of wave-
length longer than 310 nm from Xe-Hg lamp (150 W). The
photodesulfurization reaction of PuT yielding purine was
confirmed by the characteristic UV band and the GC-MS peak
of purine. Formation of H2S was confirmed by its reduction
ability from CuSO4 to CuS. The quantum yield of the
photochemical reaction was evaluated by the actinometric
method using potassium ferric oxalate.17 Absorption spectra
of the radical ions of PuT were measured by γ irradiation in
frozen glassy Me-THF solution at 77 K. The phosphorescence
spectrum was measured at 77 K in glassy Me-THF.
Transient Absorption Measurements. The nanosecond
time-scale laser photolysis apparatus was a standard design with
a Nd:YAG laser (6 ns fwhm).18,19 Transient spectra were
recorded with a multichannel photodiode (MCPD) system in
the visible region by photolysis of PuT with third-harmonic-
generated (THG) light (355 nm). The time profiles were
followed by a photomultiplier tube (PMT) in the visible region.
In the near-IR region, a Ge APD (germanium avalanche
photodiode) module was employed, using a pulsed Xe lamp
(60 µs fwhm). Picosecond laser photolysis was performed with
a mode-locked Nd:YAG laser (35 ps duration). The transient
absorption spectra were observed by monitoring light from Xe
breakdown with a streak camera detector.20,21 Laser photolysis
was performed for deaerated solutions obtained by Ar-bubbling
in a rectangular quartz cell with a 10-mm optical path at 23 °C.
In the present paper, we report photochemical behavior such
as energy transfer, electron transfer, H-atom abstraction, and
addition reaction toward alkenes of the triplet state of PuT
[3(PuT)*] measured mainly by nano-second laser flash pho-
tolysis. By prolonged steady-light illumination, the photo-
chemical reaction of PuT was also observed in organic solvents
with H-atom donor ability.
Experimental Section
Materials and Steady-State Measurements. PuT was
obtained from Aldrich Chemical Co. in a purity of >99%.
3,3′,5,5′-Tetramethylbenzidine (TMB), p-dinitrobenzene (DNB),
and other aromatic hydrocarbons were purified by recrystalli-
S1089-5639(97)03274-X CCC: $15.00 © 1998 American Chemical Society
Published on Web 01/30/1998

6H-Purine-6-thione
J. Phys. Chem. A, Vol. 102, No. 8, 1998 1339
Figure 1. Transient absorption spectra observed after laser photolysis
of PuT (0.5 mM) with 355-nm light (detector, MCPD) in Ar-saturated
THF. Inset shows time profiles of absorption bands: (a) in Ar-saturated
and (b) aerated solution (detector, PMT).
Figure 2. Concentration dependence of first-order plots for decay of
3(PuT)* at 475 nm in Ar-saturated THF (detector, PMT); [3(PuT)*] (a)
0.1, (b) 0.5 and (c) 1.0 mM. Inset shows pseudo-first-order plot.
TABLE 1: Absorption Maximum (λmax), Viscosity (η),
Intrinsic Triplet Lifetime (τT°), and Self-Quenching Rate
SCHEME 2
3
Constant (ksq) of (PuT)* in Various Solvents
solvents
λ
max/nm a
τT°/µsb
ksq/M-1 s-1 b
η/cP
THF
2-PrOH
ethanol
acetonitrile
476c
471
470
469
3.53
2.16
5.33
4.48
2.3 × 109
8.6 × 108
1.7 × 109
4.9 × 109
0.55
2.86
1.08
0.35
Results and Discussion
a Each λmax value contains estimation error of (1 nm. b Infinite
lifetime (τT°) was estimated by the extrapolation to infinite dilution in
the plots of kobs(1/τT) vs [PuT]; ksq was evaluated from the slope. c The
Ground-State Absorption of PuT. The UV-visible ab-
sorption spectrum of PuT shows the absorption peak at 331 nm
with high molar extinction coefficient (ꢀ ) 18 000 M-1 cm-1),
suggesting that the 331-nm band has the π-π* character. At
higher concentration, a very weak band with fine structure was
observed at ca. 430 nm with ꢀ ) 20-30 M-1 cm-1, which can
be ascribed to the forbidden n-π* transition. These spectral
features are similar to those of aliphatic and aromatic thiones
without N atoms except that the n-π* transition of PuT is
extremely low in intensity and close to the π-π* transition.4,6,10
No fluorescence was observed at room temperature for PuT.
This suggests that the properties of the excited singlet states of
PuT are quite different from those of thiones without N atoms,
for which intense fluorescence from the second-excited singlet
state was reported.4,6,22
ꢀT value in THF was estimated to be 6100 M-1 cm-1 for (PuT)* at
3
475 nm.
laser excitation at 355 nm. From the time profile of the rise of
3(PuT)*, the kisc value can be evaluated to be >1.3 × 1010 s-1
.
Self-Quenching of 3(PuT)*. It is reported that self-quench-
ing reaction 1 is one of the characteristics of the excited triplet
states of thiones.4
ksq
3(PuT)* + PuT
98 PuT + PuT
(1)
The triplet-decay lifetimes (τT) of 3(PuT)* were measured as a
function of ground-state PuT concentration in the limit of the
low laser powers (2.0-5.0 mJ/pulse) as shown in Figure 2.
For the decay of 3(PuT)*, the first-order rate constant (kT )
1/τT) increases with [PuT]. The self-quenching rate constant
(ksq) and the triplet lifetime (τT°), which are defined as intrinsic
in each solvent, can be evaluated from eq 2:3
The spectral shape of the dilute PuT solution (0.1 mM) is
the same as that of the concentrated solution (up to 0.5 mM).
The absorption intensities show a linear increase with the
concentration, indicating that aggregation of PuT may not be
appreciable in the concentration range up to ca. 0.5 mM.
Laser Flash Photolysis. The transient absorption spectra
observed by the laser photolysis of PuT in Ar-saturated THF
with 355-nm light exhibit an absorption band at 475 nm in
addition to a weak band around 690 nm as shown in Figure 1.
The absorption intensities of both bands, which decrease within
a few microseconds in THF, were quenched very quickly in
the presence of oxygen (inset in Figure 1) and other triplet
quenchers, suggesting that both 475 and 690 nm absorptions
1/τT ) 1/τT° + ksq[PuT]
(2)
Similarly, high self-quenching ability [ksq ) (0.8-5) × 109
M-1 s-1] was observed for (PuT)* in various solvents (Table
3
1). The ksq values seem to depend on the viscosity of the
solvent, suggesting that they are close to the diffusion-controlled
limit. These ksq values are similar to those reported for
xanthione triplet state in perfluoroalkane solvent.5
3
were due to (PuT)* (Scheme 2).
The 690-nm band was not effectively scavenged by solvated
The τT° values are smaller than those of aromatic carbonyl
compounds. Small τT° values are one of the characteristics of
the thiones, which may be caused by rapid T1-S1 equilibrium
of PuT in addition to the spin-orbit coupling of the S atom.
electron quenchers such as m-terphenyl,23 indicating that the
3
690-nm peak was the absorption band of (PuT)* but not the
absorption band of the solvated electron. By excitation of PuT
in aqueous solution with the 266-nm light, the broad absorption
band was observed at ca. 520 nm, which can be attributed to
the solvated electron formed by photoionization.15,16
The rate constant of the intersystem-crossing process of PuT
(kisc) was measured by the picosecond laser-photolysis method
because of its absence of fluorescence.22 The transient absorp-
tion band of 3(PuT)* was observed immediately after the 35-ps
3
The τT° value for (PuT)* in 2-propanol (2-PrOH) is slightly
smaller than those in other solvents. Since τT° was defined in
each solvent, the observed short τT° value may be related to
3
the H-abstraction reaction of (PuT)* from 2-PrOH.
In the presence of well-known triplet quenchers such as O2,
the decay rates of the transient absorption bands at 475 and
690 nm increase as shown in Figure 1. From these decay rates,

1340 J. Phys. Chem. A, Vol. 102, No. 8, 1998
Alam et al.
3
TABLE 2: Rate Constants for Reactions of (PuT)* with
Triplet Quenchers and Reactants at 23 °C in THF
quenchers
ET /kcal M-1
kq/M-1 s-1 a
reaction type
1
â-carotene
O2
ferrocene
1,3-CHD g
isoprene
19.4b
22.5c
42.9d
52.4c
60.1c
62.0c
64.3c
65.8c
78.0e
9.7 × 1010
4.6 × 109
7.1 × 109
6.3 × 109
4.1 × 109
3.5 × 109
2.4 × 107
5.8 × 105
6.6 × 107 f
T-energy transfer
T-energy transfer
T-energy transfer
T-energy transfer
T-energy transfer
T-energy transfer
T-energy transfer
T-energy transfer
H-abstraction
phenanthrene
m-terphenyl
diphenyl
1,4-CHD g
a Estimation error is (5%. b Reference 25. c Reference 27. d Refer-
ence 26. e Reference 28. f Rate constant is due to H-atom abstraction
(khT). g CHD: cyclohexadiene.
Figure 3. Laser-power dependence absorption-time profiles of
3(PuT)* at 475 nm obtained by 355-nm laser photolysis of PuT (0.2
mM) in Ar-saturated THF (detector, PMT). Inset shows plot of ∆k1st
vs ∆A0.
the second-order triplet-quenching rate constant (kO in reaction
2
3) was evaluated to be 6.4 × 109 M-1 s-1 for O2 in THF. In
our study, the formation of singlet O2 was confirmed by
consumption of 1,3-diphenylisobenzofuran.24 Hemmens et al.
reported that both 1O2 and superoxide (O2•-) are formed by the
laser flash photolysis of PuT in aqueous solution. Formation
constant for the growth of the â-carotene triplet in solutions
containing benzophenone or PuT (optically matched at the
excitation wavelength 355 nm). The k0 is the rate constant for
the decay of the triplet energy donors in the absence of
â-carotene. The direct excitation of only the â-carotene solution
did not result in any significant triplet formation because of
negligible triplet yield confirmed by 355-nm laser flash pho-
tolysis.3 Thus, the ΦT value in THF was evaluated to be 0.99
•-
1
of O2 is possible via electron transfer from PuT to O2 in
aqueous solution.15
3
for (PuT)*. The kisc value larger than ca. 1.3 × 1010 s-1
measured by the picosecond laser photolysis method in this
study supports such a high ΦT.
The triplet extinction coefficient (ꢀT) of (PuT)* was also
For other triplet quenchers (reaction 4),25-28 the kq values
thus obtained are summarized in Table 2. Although the kq value
3
3
estimated from the reported ꢀ value of (benzophenone)* as a
for phenanthrene (3.5 × 109 M-1 s-1; ET ) 62.0 kcal/mol)27
1
standard (7220 M-1 cm-1) by substituting the observed ΦT and
absorbance.3,30 The ꢀT value in THF was determined to be 6100
is still close to the diffusion-controlled limit, the kq value for
m-terphenyl (2.4 × 107 M-1 s-1; ET ) 64.3 kcal/mol)27 is about
1
3
M-1 cm-1 for (PuT)* at 475 nm. Such a relatively large ꢀ
3
1/100 of the diffusion limit, indicating that ET of (PuT)* is
1
value is also characteristic of the triplet states of thiones.3,5
Triplet-Triplet Annihilation. When the laser power was
increased under the same concentration of PuT, the initial decay
rate increased with changing decay kinetics from first order to
a mixed order (first and second order) because of the triplet-
triplet annihilation as shown in Figure 3 (reaction 6).
close to 62.5 kcal/mol as calculated from the Sandros equation:29
3
The ET of (PuT)* was also calculated from the phospho-
1
rescence band at 453 nm measurement at 77 K in glassy Me-
THF to be 63.8 ( 0.2 kcal/mol, which is in agreement with the
kTT
3(PuT)* + 3(PuT)* 8 1(PuT)* + PuT
(6)
ET value evaluated by quenching of 3(PuT)* within ca. 1 kcal/
1
mol. In ethanol, a slight shift of the phosphorescence band (457
nm) was observed, suggesting that the polar character of the T1
state is not high.
For mixed-order decay kinetics, eq 7 can be used to calculate
both first- and second-order rate constants:31
3
The rate constant of (PuT)* for 1,4-cyclohexadiene (1,4-
CHD; ET ) ca. 78 kcal/mol)28 is larger than those of
-d[ln(∆A0)]
dt
2k
1
) ∆k1st ) k0
+
2nd∆A0
(7)
m-terphenyl and diphenyl with lower ET values. This indicates
1st
1
ꢀT
that H-atom abstraction from 1,4-CHD by 3(PuT)* takes place.5
The rate constants for H-atom abstraction (khT) for 1,4-CHD
evaluated in this study are similar to that reported for the
xanthione triplet.5
where ∆A0 is the initial absorption intensity of 3(PuT)* at each
laser power, k2nd is the T-T annihilation rate constant (kTT),
and k0 is the triplet-decay rate constant at low laser power.
Triplet Quantum Yield. The quantum yield (ΦT) of 3(PuT)*
formation by intersystem crossing was evaluated by energy
transfer from 3(PuT)* to â-carotene in THF by comparison with
ΦT ) 1.0 of benzophenone (abbreviated as bp) as a standard
using eq 5:3,30
1st
The slope of the linear plot of ∆k1st vs ∆A0 yielded 2kTT/ꢀT
(inset in Figure 3), from which the kTT value was calculated to
3
be 3.3 × 109 M-1 s-1 for (PuT)* in THF by substituting ꢀT
3
for (PuT)* estimated above. This kTT value is considerably
lower than the diffusion-controlled limit (kd ) 1.2 × 1010 M-1
s-1 in THF), which is reasonable by taking the spin-statistical
factor into consideration. At a higher laser power of 30 mJ
pulse-1, ca. 33% of the T-T annihilation reaction participated
in the total decay of 3(PuT)*, while the participation of the T-T
annihilation reaction is ca. 10% at 15 mJ pulse-1 in THF. At
a higher concentration of PuT, T-T annihilation takes place
concomitant with self-quenching.
PuT
obs
bp
obs
k
k
- kb0p
∆APuT
ΦPTuT ) ΦbTp
(5)
∆Abp koPbusT - k0PuT
k
bp
obs
where ∆A’s are the respective maximal absorption intensities
observed by the laser excitation of PuT (0.1 mM) and ben-
zophenone (1.0 mM) and kobs is the pseudo-first-order rate

6H-Purine-6-thione
J. Phys. Chem. A, Vol. 102, No. 8, 1998 1341
Figure 4. Transient absorption spectra in the visible and near-IR region
obtained by 355-nm laser photolysis of PuT (0.1 mM) in the presence
of TMB (1.0 mM) in Ar-saturated acetonitrile. Inset shows time profiles
of absorption bands at 470, 570 (detector, PMT), and 900 nm (detector,
Ge APD).
Figure 5. Absorption-time profiles (A) at 900 nm for decay of TMB•+
and (B) at 910 nm for the decay of DNB•- obtained by 355-nm laser
photolysis (detector, Ge APD) with PuT (0.1 mM) in Ar-saturated
acetonitrile. Inset shows second-order plots.
TABLE 3: Rate Constants for Forward Electron-Transfer
of DNB•- in acetonitrile. Thus, it is evident that the reaction
Reactions (kfet) and Back-Electron-Transfer Reactions (kbet
)
3
in Acetonitrilea
rate constants for the quenching of (PuT)* by DNB and for
the formation of DNB•- in acetonitrile are attributed to the rate
constants (kfet) for electron transfer from 3(PuT)* to DNB,
forming DNB•- and PuT•+ (reaction 9):21,33
kfet/M-1 s-1 kfet/M-1 s-1
reactants
(rise)b
(decay)b
kbet/ꢀ/cm s-1 c kbet/M-1 s-1 c
TMB
DNB
4.3 × 109
d
3.1 × 105
6.3 × 109
1.1 × 1010
9.4 × 109
3.1 × 106
1.8 × 1010
a Estimation error is (5%. b Decay of (PuT)* at 470 nm. ꢀTMB
3
c
•+
) 20 200 M-1 cm-1 at 900 nm33 and ꢀDNB ) 5900 M-1 cm-1 at 910
•-
nm21 in acetonitrile. d Owing to the overlap with the rise of TMB•+
,
the kfet value was not obtained from the decay.
The absorption intensities of the radical ions (TMB•+, PuT•-,
and DNB•-) decay slowly after the formation of the radical ions
in acetonitrile. These decays obey second-order kinetics as
shown in Figure 5 and can be attributed to the back-electron
transfer reactions (kbet in reactions 10 and 11):21,33
Photoinduced Electron Transfer. The transient absorption
spectra observed by the laser photolysis of PuT (0.1 mM) in
the presence of TMB with 355-nm light in acetonitrile are shown
in Figure 4. The transient absorption bands at 470, 800, and
900 nm are attributed to TMB•+,32,33 due to the electron-transfer
kbet
PuT•- + TMB•+ 8 PuT + TMB
(10)
(11)
3
reaction from TMB to (PuT)* (reaction 8). A weak band at
570 nm can be assigned to PuT•- because a similar absorption
band was observed by γ irradiation of PuT in Me-THF at 77
K. The rise-time profiles were observed at 470, 900, and 570
nm (inset in Figure 4). From the rise curve of TMB•+ at 900
nm, the first-order rate constant was evaluated by curve-fitting
with a single exponential. The second-order rate constant for
electron transfer (kfet) can be evaluated by the TMB concentra-
tion dependence of the first-order rate constant as listed in Table
3:
kbet
PuT•+ + DNB•- 8 PuT + DNB
From the slopes of the second-order plots (inset in Figure 5),
the ratio of the rate constant to molar extinction coefficient
(kbet2nd/ꢀradical ion) can be obtained as listed in Table 3. By
21,33
2nd
substitution of the reported values of ꢀradical ion
,
the kbet
values were evaluated for TMB•+ and DNB•- in acetonitrile
(Table 3).
H-Atom Abstraction Reaction of 3(PuT)*. In the presence
of p-X-C6H4SH, formation of p-X-C6H4S• 35-37 was observed
in THF as shown in Figure 6 (Scheme 3). 3(PuT)* decreases
with the rise of the absorption band of p-X-C6H4S• (or
R-tocopheroxy radical) due to the H-atom abstraction reaction
3
by (PuT)* (inset in Figure 6). The formation of the radicals
3
At 470 nm, the rise of TMB•+ and decay of (PuT)* are
was suppressed in the presence of triplet quenchers such as O2,
1,3-cyclohexadiene, and phenanthrene, which suggests that the
overlapping. The finding that the rise-time profile was observed
at 470 nm comes from the larger ꢀ of TMB•+ compared with
H-atom abstraction reaction occurs via (PuT)*. The H-atom
3
that of (PuT)* at 470 nm. Indeed, ꢀ of (PuT)* is 6100 M-1
cm-1, and ꢀ of TMB•+ is 19 000 M-1 cm-1 at 470 nm.33
In the laser flash photolysis of PuT in the presence of DNB
with a light wavelength of 355 nm, the absorption band of
3(PuT)* at 470 nm disappeared within a few hundred nanosec-
onds, and new absorption bands appeared at 870 and 910 nm
with a rising absorption-time profile in the near-IR region in
acetonitrile. These absorption bands are attributed to the radical
anion of DNB (DNB•-).21,34 The first-order rate constant thus
obtained from the decay profiles of 3(PuT)* by DNB is in good
agreement with those obtained from the formation rise profiles
abstraction rate constants (khT) by (PuT)* are summarized in
3
3
3
Table 4 with the absorption maxima of the radicals formed from
the different H-atom donors.
Although the decay rate of 3(PuT)* in the presence of phenol
was very slow, tocopherol is highly reactive to 3(PuT)*.38 For
benzimidazole thiol and benzoxazole thiol, the rise-time profiles
of the corresponding thio radicals were observed at 590 nm
3
accompanied by the decay of (PuT)*, suggesting the H-atom
abstraction reaction (Scheme 4; PuTH• denotes an H-atom
adduct to PuT). On the other hand, the thio radical was not
observed for benzothiazole thiol. Nevertheless, rapid decay of

1342 J. Phys. Chem. A, Vol. 102, No. 8, 1998
Alam et al.
Figure 7. Hammett plot of log khT vs σ+ constants of p-X-C6H4SH
Figure 6. Transient absorption spectra observed by laser photolysis
(355-nm light) of PuT (0.1 mM) with p-HO-C6H4SH (1.0 mM) in
Ar-saturated THF (detector, PMT). Inset shows decay profiles of
3(PuT)* at 475 and 690 nm and rise profile of p-HO-C6H4S• at 520
nm.
3
for reaction with (PuT)* in Ar-saturated THF.
TABLE 5: Addition-Reaction Rate Constants (kad), log kad,
and Alfrey-Price’s e Values of Alkenes for Reaction with
3(PuT)* in THF
SCHEME 3
alkenes
kad/M-1 s-1 a
e valueb
CH2dC(CH3)Ph
CH2dC(CH3)CO2CH3
CH2dC(CH3)CN
CH2dCHCN
CH2dCHOBu
CH2dCHOEt
(R-MSt)
(MMA)
(MAN)
(AN)
(BVE)
(EVE)
(VAc)
2.0 × 109
7.3 × 108
7.6 × 108
4.8 × 108
2.4 × 108
2.7 × 108
1.3 × 108
-0.81
+0.40
+0.61
+1.20
-1.77
-1.17
-0.22
TABLE 4: Rate Constants (khT) for Hydrogen-Abstraction
CH2dCHOCOCH3
3
Reaction of (PuT)* with Different H-Atom Donors in THF
a Estimation error is (5%. b e values of alkenes are cited from ref
36.
and Hammett Parameter σ+ of p-X-C6H4SH
H-atom donor
p-NH2-C6H4SH p-NH2-C6H4S•
p-HO-C6H4SH
p-HO-C6H4S•
radicala
λ
max/nm khT/M-1 s-1 b σ+ c
580d
1.2 × 109 -1.3
9.3 × 108 -0.92
6.6 × 108 -0.78
2.5 × 108 -0.26
520d
p-CH3O-C6H4SH p-CH3O-C6H4S• 520d
p-tert-C4H9-
C6H4SH
C6H5SH
p-tert-C4H9-
490d
C6H4S•
C6H5S•
480d
510d
510d
420e
590 f
590 f
i
1.9 × 108
1.0 × 108
1.3 × 108
1.8 × 108
2.5 × 108
5.4 × 107
1.4 × 109
0.0
0.11
0.15
p-Cl-C6H4SH
p-Br-C6H4SH
R-tocopherol
2-MBI g
p-Cl-C6H4S•
p-Br-C6H4S•
R-tocopheroxy•
2-MBI• h
2-MBO g
2-MBO• h
i
2-MBS g
3
a Radical formed by the hydrogen abstraction of 3(PuT)* from
different H-atom donors. b Estimation error is (5%. c σ+ values are
cited from ref 36. d References 35 and 36. e Reference 38. f Reference
37. g 2-MBI, 2-mercaptobenzimidazole; 2-MBO, 2-mercaptobenzox-
azole; 2-MBS, 2-mercaptobenzothiazole. h 2-MBI•, benzimidazole-2-
Figure 8. Plots of log kad for (PuT)* vs Alfrey-Price’s e values of
different alkenes in Ar-saturated THF.
3
The electrophilic nature of (PuT)* suggesting that sulfur is
3
the active site for the reaction of (PuT)* is similar to that of
thio radical; 2-MBO•, benzoxazole-2-thio radical. Benzothiazol-2-thio
i
the aromatic and aliphatic thio radicals.36,39 Thus, this observa-
tion suggested that the C-centered radical (Scheme 3) was
formed first by H-atom abstraction of (PuT)*.
radical at 590 nm was not observed.
3
SCHEME 4
Addition Reaction to Alkenes. On addition of alkenes, the
3
decay rate of (PuT)* increases with increasing concentration
of alkenes because of the reaction with 3(PuT)*. The cycload-
dition reaction is most probable, like for other aromatic
thiones.8,39 The values of the addition reaction rate constants
(kad) for several alkenes are summarized in Table 5 with Alfrey-
Price’s e values,36 which are a measure of the polar nature of
CdC. The kad values are plotted vs e values as shown in Figure
8. Conjugated are more reactive than nonconjugated alkenes
3(PuT)* was observed, suggesting the electron-transfer reaction
between (PuT)* and benzothiazole thiol (Scheme 4).
3
toward (PuT)*. Conjugated alkenes substituted by electron-
3
donating groups such as R-methylstyrene are more reactive than
those substituted by electron-withdrawing groups such as
CH2dCHCN.
To investigate the substituent effect for the reactions of
3(PuT)* with p-X-C6H4SH as the H-atom donors, a Hammett
plot of log khT vs σ+ constants of p-X-C6H4SH is shown in
Figure 7. A negative slope (F+ ) ca. -1.0) of the Hammett
The plots can be divided into two groups according to the
conjugative abilities of vinyl monomers. Conjugative ability
seems to be a main factor for determining reactivity of alkenes
to 3(PuT)*. In each group, the slope is a measure of the polar
character of the attacking 3(PuT)*. Negative slopes suggest the
3
relation (eq 12)36 implies a strong electrophilicity of (PuT)*
with respect to H-S in p-X-C6H4SH.35,36
∆(log khT) ) F+σ+
(12)
3
X
electrophilicity of (PuT)* with respect to CdC.36,39 This is

6H-Purine-6-thione
J. Phys. Chem. A, Vol. 102, No. 8, 1998 1343
nitrogen atoms. The lowest triplet-state energy (ET ) 63 kcal)
1
is relatively high, which is the origin of efficient electron-transfer
3
3
ability of (PuT)*. Therefore, (PuT)* acts as both electron
donor and acceptor. The electrophilic character of 3(PuT)* was
revealed by the substituent effects in H-atom abstraction and
addition reactions. The high electrophilicity of 3(PuT)* implies
that the S atom is the reactive center. The T1 of PuT may have
3
3
a mixed character of (n,π*) and (π,π*).
Acknowledgment. The present work was partly supported
by a Grant-in-Aid on Priority-Area-Research on “Carbon
Alloys” (No. 09243201) from the Ministry of Education,
Science, Sports and Culture. The authors express thanks to
Takeda Science Foundation and to Professor H. Hiratsuka of
Gunnma University.
Figure 9. Steady-state absorption spectra observed by photolysis of
PuT (0.1 mM) in Ar-saturated THF with light of wavelength longer
than 310 nm at room temperature. The 265-nm band is due to purine.
References and Notes
the same trend for the electrophilic thio radicals.35,36,39 The
electrophilic nature of (PuT)* indicates that a partial charge
transfer is generated in the transition state during the addition
of 3(PuT)* toward alkenes such as the triplet state of xanthione.39
Thus, the initial bond formation with alkenes would be expected
to occur between the C atom in CdC and the S atom in
3(PuT)*.8,39
3
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caused by steady-light (>310 nm) illumination were studied.
By the steady photolysis of PuT (Figure 9), the thione band at
336 nm decreases slowly with irradiation time (t1/2 ) 45 min
in Ar-saturated THF) accompanied by an appearance of a new
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On addition of triplet quenchers such as 1,3-CHD and isoprene,
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PuT in the presence of H-atom donors.
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3
this basis, it is presumed that T1 has (n,π*) character by an
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mol (ca. 1000 cm-1) also supports the 3(n,π*) character.1 The
high reactivity and electrophilicity of 3(PuT)* correspond to the
3(n,π*) character of the T1 state from the analogy of the CdO
triplet state. On the other hand, the nonpolar nature of the T1
state evaluated from the shift of the phosphorescence data
suggests 3(π,π*) character. The MO calculation of 3(PuT)* by
the MNDO method42 employing the restricted open-shell
Hartree-Fock method with configuration interactions suggests
that both the upper and lower SOMO’s have π character. Thus,
it is difficult to determine definitively the character of T1.
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3
Probably, the T1 of PuT has a mixed character of (n,π*) and
3(π,π*) as observed for other thiones.10
(28) Evaluated from T1 of 2-butene.27
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Summary
The importance of 3(PuT)* in photophysical and photochemi-
cal processes was proved by laser flash photolysis methods. The
high ΦT value, a short intrinsic triplet lifetime, and a high self-
quenching rate are similar to the properties of the thiones without
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Science Data 34; Elsevier: Amsterdam, 1988.

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