6
28
IVANOV et al.
Table 1. Quantum yields of dimerization Φ and anthracene anthracene) by alcohols [9]. The authors found that the
1
formation Φ2 during irradiation of 9-trimethylstannylan-
thracene with the 365-nm mercury line in various solvents at
various pK values of the alcohols
fluorescence lifetime and quantum yield of 9-trimethyl-
silylanthracene are half as large in methanol as in ace-
tonitrile and methylcyclohexane and concluded that the
photoprotonation of this compound with alcohols pro-
ceeds according to the second mechanism.
2
2
No.
Solvent
Heptane
Φ × 10
Φ × 10
pK [11]
1
2
1
2
3
4
5
6
7
0.21
0.37
3.35
0.6
–
–
We measured quantum yields and lifetimes of 1 flu-
orescence in various solvents (Table 2). To determine
the fluorescence quantum yields of 1, we used
anthracene as a reference with known fluorescence
quantum yield (0.3 [10]). In contrast to the results
obtained in [9] for 9-trimethylsilylanthracene, the fluo-
rescence of 1 in alcohols exhibits neither an anomalous
quantum yield nor an anomalous lifetime as compared
with the other solvents. The quantum yields of
anthracene formation during irradiation of 1 in alcohols
Benzene
Acetonitrile
Methanol
–
4.8
2.6
0.72
28
15.22
15.84
16.94
12.37
Ethanol
0.4
Isopropanol
Trifluoroethanol
0.24
–
Note: The quantum yields are given for a 9-trimethylstannylan-
–
4
thracene concentration of 1 × 10 M.
(Table 1) correlate with the pK values for the alcohols.
Based on the obtained experimental data, we cannot
favor any of the mechanisms.
Table 2. Lifetimes τ and quantum yields ϕ of 9-trimethyl-
0
fl
The quantum yield of dimerization of 1 and that of
its cleavage with the formation of anthracene did not
change after the solutions were blown with argon;
therefore, we concluded that the reagents were in a sin-
glet excited state. Thus, we can write down the follow-
ing kinetic scheme of the dimerization of 1:
stannylanthracene fluorescence in various solvents
9
Solvent
τ × 10 , s
ϕfl
0
1
2
3
4
5
6
7
8
Methanol
Heptane
3.0
3.2
3.5
3.5
3.8
4.6
5.6
6.4
0.13
0.15
0.17
0.18
0.19
0.22
0.3
Ethanol
A + hν
A* A (k = k + k ),
A* + A
A* + A
A* (w0),
sec-Butanol
Isopropanol
Acetonitrile
Benzene
0
f
isc
2A (kd),
A2 (kR),
Toluene
0.34
where w is the rate of light absorption by the sample;
0
Note: The quantum yields and lifetimes were measured with an
accuracy of 10%.
kf and kisc are, respectively, the fluorescence and inter-
system-crossing rate constants; k is the rate constant of
d
the deactivation of the excited molecule; and k is the
R
photodimerization rate constant.
–
1
–1
8
500 å cm [7]. When trifluoroethanol is used as
Thus, the quantum yield of photoreaction is
the solvent, the formation of anthracene proceeds rap-
idly even without irradiation, at a rate constant of
(
k + k )τ [A]
d R 0
–4
–1
2
.7 × 10 s . For this solvent, the quantum yield of
Φ
R
= -------------------------------------------- γ,
+ (k + k )τ [A]
(1)
1
d
R
0
anthracene formation under irradiation is higher than for
the other alcohols by an order of magnitude (Table 1).
where γ is the ultimate yield of photodimerization, a
quantity defined as
The formation of anthracene during photoexcitation
of 1 in alcohols can proceed by one of the two mecha-
nisms. The basicity of aromatic molecules in an excited
state is higher by several orders of magnitude [8]; there-
γ = k /(k + k ).
(2)
R
d
R
Since photodimerization was conducted at low con-
–
4
fore, position 9 of 1 in an excited state can be proto- centrations of 1 (<3 × 10 M), the value (k + k )τ [A]
d
R
0
nated by an alcohol, as its ground state is protonated by is negligibly small compared to unity, and, therefore,
strong acids. When a droplet of concentrated acetic acid the quantum yield of the reaction is proportional to the
was added to acetonitrile (5 ml), anthracene was concentration of 1. The quantum yield of the photo-
formed without irradiation. In this case, the C–Sn bond dimerization of 1 was plotted against its concentration
is broken via the direct attack of the electrophile. (Fig. 2). This dependence is nearly linear, with a slope
Another possible mechanism involves the formation of (k + k )τ γ. Assuming that the sum of the rate con-
d
R
0
of a complex alcohol molecule with an excited mole- stants of deactivation and dimerization is equal to the dif-
10
–1 –1
cule 1, which then decays via the concerted cleavage of fusion rate constant (1 × 10 å s for benzene [12]),
–1
the Sn–C and RO–H bonds. These two mechanisms the slope of the plot in Fig. 2 (equal to 38.5 å ) yields
were discussed to interpret the photochemical cleav- an ultimate yield of photodimerization of ~0.7. This
9
–1 –1
9
–1 –1
age of 9-trimethylsilylanthracene (a silicon analog of means that k = 7 × 10 å s and k = 3 × 10 å s .
R
d
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY Vol. 80 No. 4 2006
Ivanov
