Direct photophysical evidence for quenching of the triplet excited state of 2,4,6-triphenyl(thia)pyrylium salts by 2,3-diaryloxetanes

Article abstract of DOI:10.1021/jp027408y

The thiapyrylium salt 1b is an efficient electron-transfer photosensitizer in the preparative irradiation of trans,-trans-2,3-diphenyl-4-methyloxetane (2a). Previously, the reaction has been assumed to occur through the triplet excited state of 1b, which has a very high intersystem crossing yield (_φISC = 0.97). In the present report, direct evidence for triplet quenching in the oxidative cycloreversion of the 2,3-diaryloxetanes is provided. The rate constants k_q(T₁) and the free energy changes of the electron transfer ΔG_ET(T₁) have been determined for the reaction between 1b and oxetanes 2a-d and 2a′. The process has been found to be exergonic in all cases except for 2d; this agrees with the fact that no photoproduct was obtained in the preparative irradiation of 1b in the presence of 2d. Good correlation between k_q(T₁) and ΔG_ET(T₁) was observed, confirming the involvement of the excited triplet state in the reaction mechanism.

Full text of DOI:10.1021/jp027408y

2478
J. Phys. Chem. A 2003, 107, 2478-2482
Direct Photophysical Evidence for Quenching of the Triplet Excited State of
2,4,6-Triphenyl(thia)pyrylium Salts by 2,3-Diaryloxetanes
Miguel A. Miranda,* M. Angeles Izquierdo, and Rau´l Pe´rez-Ruiz
Departamento de Qu´ımica/ Instituto de Tecnolog´ıa Qu´ımica UPV- CSIC, UniVersidad Polite´cnica de Valencia,
Camino Vera s/n, Apdo. 22012, 46022, Valencia, Spain
ReceiVed: NoVember 7, 2002; In Final Form: February 10, 2003
The thiapyrylium salt 1b is an efficient electron-transfer photosensitizer in the preparative irradiation of trans,-
trans-2,3-diphenyl-4-methyloxetane (2a). Previously, the reaction has been assumed to occur through the
triplet excited state of 1b, which has a very high intersystem crossing yield (φISC ) 0.97). In the present
report, direct evidence for triplet quenching in the oxidative cycloreversion of the 2,3-diaryloxetanes is provided.
The rate constants kq(T1) and the free energy changes of the electron transfer ∆GET(T1) have been determined
for the reaction between 1b and oxetanes 2a-d and 2a′. The process has been found to be exergonic in all
cases except for 2d; this agrees with the fact that no photoproduct was obtained in the preparative irradiation
of 1b in the presence of 2d. Good correlation between kq(T1) and ∆GET(T1) was observed, confirming the
involvement of the excited triplet state in the reaction mechanism.
TABLE 1: Redox and Photophysical Properties of Pyrylium
Salts 1a-d
Introduction
Photoinduced electron transfer (PET) processes have attracted
growing interest in the last two decades.1 It is well-known that
their mechanism involves exchange of one electron between
the excited state of the sensitizer (singlet or triplet) and the
ground state of the substrate. In general, the synthetic utility of
PET is limited by deactivation of the resulting radical ion pairs
via back electron transfer (BET).1c,d
The effect of spin multiplicity on PET reactions has been
discussed as one of the most important factors from the
viewpoint of charge separation.2 The high efficiency of reactions
occurring from the triplet as opposed to the singlet has two main
causes: (a) the triplet states are long-lived, making it possible
for the reactants to diffuse and collide with each other and (b)
back electron transfer is slow within the triplet radical ion pairs,
allowing efficient escape of the partner ions. By contrast, singlet
excited states are very short-lived and the radical ion pairs
derived therefrom undergo back electron transfer very fast. This
makes PET for the excited singlets less effective, despite their
higher oxidizing ability.
E(S1)a E(T1)a φISC E(A/A
)
b
c,d
-•
sensitizer
X
Y
Ar
1a
1b
1c
1d
O
S
O
O
BF4 Ph
ClO4 Ph
BF4 p-Br-C6H4
BF4 p-MeO-C6H4 58
66
66
63
53d
52d
0.53 -0.29
0.94 -0.21
n.a.e 0.67 -0.19
51d
0.03 -0.52
a E(S1), singlet energy; E(T1), triplet energy (given in kcal/mol).
b φISC, intersystem crossing quantum yield, in acetonitrile, in the absence
of quencher; (φISC ) 1-φF). c In V, vs SCE. d Taken from the literature.6
e n.a., not available.
because of their absorption in the visible, and are extremely
good photooxidizing agents (Table 1). Depending on the
substitution pattern and the reaction conditions, pyrylium salts
can generate radical ion pairs of different multiplicity (singlet
or triplet);7-9 hence, they may be the sensitizers of choice to
study the influence of this factor on the reaction.
The 2,4,6-triphenyl derivative 1a is the most studied pyrylium
salt. Its triplet excited-state sensitizes the oxygenation7a and
isomerization7b of stilbenes. Other reactions, such as the
oxygenation of arylfurans,8a adamantylideneadamantanes,8b
and diarylethylenes,8c appear to be photosensitized both
from the singlet and the triplet of 1a. Finally, in some cases
the only possibility considered is reaction from the singlet
excited state9 (for instance, in the PET ring opening of
epoxides).9a
Recently, the PET cycloreversion (CR) of oxetanes has
attracted considerable interest as it is involved in the photo-
enzymatic repair of the (6-4) photoproducts of the DNA
dipyrimidine sites by photolyase.3 Besides, this process entails
cleavage of two bonds and may yield formal metathesis
products.4
Prior to our work, only two reports have appeared on the
CR of oxetane radical cations.5 In both cases, cyanoaromatic
compounds were used as electron-transfer photosensitizers, and
the cycloreversion was assumed to occur through the excited
singlet state. Although the mechanism was explained via
formation of radical cations, no intermediate of this type was
detected.
The only example of pyrylium sensitized CR of oxetanes has
been recently reported by our group.10 Different results were
obtained in the preparative irradiation of model 2,3-diphenyl-
oxetanes 2a and 2a′ depending on the employed photosensi-
tizers. Thiapyrylium salt 1b, with the highest intersystem
Pyrylium salts are well-established PET sensitizers;6 they can
be selectively excited in the presence of a variety of substrates,
* To whom correspondence should be addressed. E-mail: mmiranda@
qim.upv.es.
10.1021/jp027408y CCC: $25.00 © 2003 American Chemical Society
Published on Web 03/19/2003

Quenching of 2,4,6-Triphenyl(thia)pyrylium Salts
J. Phys. Chem. A, Vol. 107, No. 14, 2003 2479
crossing quantum yield (φISC),11 was found to be the most
efficient photosensitizer; by contrast, irradiation in the presence
of methoxy salt 1d, with the lowest φISC value,6a did not result
in the formation of any photoproduct.
From these preparative studies, electron-transfer CR was
assumed to proceed through the triplet sensitizer. This was
supported by the fact that intersystem crossing was found to be
efficient enough even at high oxetane concentration. However,
direct photophysical evidence for quenching of the triplet excited
state of pyrylium salts by oxetanes is still missing.
In view of the results obtained in the CR of model diphen-
yloxetane 2a,10a,b thiapyrylium salt 1b was chosen as the most
suitable PET sensitizer to explore the feasibility of oxidative
CR of oxetanes from the triplet excited state. Besides, the
thiapyranyl radical (the one-electron reduced species) does not
absorb in the 500-600 nm region,11c so it does not interfere
with the typical triplet-triplet absorption of 1b (460 and 600
nm).
In the present work, electron transfer between the thiapyr-
ylium salt 1b and diphenyloxetane 2a has been studied in detail,
and the photophysical parameters of the involved processes have
been determined. These studies have provided clear evidence
for triplet quenching. The quenching rate constant kq(T1) and
the free energy change of the electron-transfer process
∆GET(T1) have been calculated. The study has been extended
to oxetanes 2a′ and 2b-d with electron releasing or electron
withdrawing substituents attached to the aryl group(s), which
modify the oxidation potential of the model oxetane. A good
correlation between kq(T1) and ∆GET(T1) has been obtained,
which confirms the electron-transfer mechanism from the triplet
excited state.
Hz, 1H), 5.09 (dq, J 7.5 Hz, J 6.0 Hz, 1H), 5.72 (d, J 7.5 Hz,
1H), 7.31-7.44 (m, 5H), 7.50 (d, J 8.4, 2H),), 7.66 (d, J 8.4,
2H). 13C NMR (δ, ppm): 23.2 (CH3), 57.7 (CH), 81.6 (CH),
84.0 (CH), 112.1 (C), 119.4 (C), 126.2 (CH), 127.8 (CH), 128.2
(CH), 129.6 (CH), 133.0 (CH), 138.8 (C), 148.5 (C). MS (m/z
(%): 205 (24), 204 (25), 203 (11), 190 (11), 176 (5), 165 (3),
151 (2), 130 (5), 118 (100), 117 (66), 102 (8), 91 (12), 76 (4).
Exact mass (CI): required for C17H16NO: 250.1232 (MH+);
found: 250.1236.
trans,trans-2,3-Diphenyl-4-hydroxymethyloxetane (2a′). 1H
NMR (δ, ppm): 3.83 (dd, J 12.8 Hz, J 3.7 Hz, 1H), 3.92 (dd,
J 3.4 Hz, J 12.8 Hz, 1H), 4.13 (t, J 7.7 Hz, 1H), 4.95 (ddd, J
7.6 Hz, J 3.7 Hz, J 3.4 Hz, 1H), 5.75 (d, J 7.7 Hz, 1H), 7.31-
7.49 (m, 10H). 13C NMR (δ, ppm): 49.7 (CH2), 64.8 (CH),
84.3 (CH), 85.5 (CH), 125.5 (CH), 127.3 (CH), 128.5 (CH),
128.5 (CH), 138.6 (C), 141.4 (C). MS (m/z (%)): 240 (<1,
[M+•]), 222 (<1), 180 (92), 179 (100), 178 (74), 165 (51), 152
(15), 134 (96), 105 (34), 92 (92), 78 (29). Exact mass (CI):
required for C16H17O2 241.1229 (MH+); found: 241.1182.
Cycloreversion Reactions. Solutions of oxetanes 2a-d and
2a′ (4 × 10-2 M) with photosensitizer 1b (10-3 M) in CD3CN
(0.8 mL) were placed in NMR tubes and bubbled with argon.
Then, the solutions were irradiated during 20 min in a multilamp
photoreactor, using 8 W lamps (4×) with emission maximum
at λ ) 350 nm. The reaction was followed by 1H NMR, which
was recorded before and after the irradiation. Control experi-
ments showed that photocycloreversion does not take place in
the dark or in the absence of photosensitizer.
Cyclic Voltammetry. The redox potentials were measured
using the cyclic voltammetry technique. All measurements were
made in acetonitrile containing tetrabutylammonium perchlorate
(0.1 M) as supporting electrolyte, using a glassy carbon working
electrode and ferrocene in acetonitrile (5 mg in 30 mL) as
standard, EPa ) 423 mV and EPc ) 345 mV. The scan rate was
400 mV/s. Potentials are reported with respect to the saturated
calomel electrode (SCE) as reference.
Fluorescence Spectroscopy. The steady-state fluorescence
spectra were obtained with a FS 900 spectrofluorimeter equipped
with a 450 W xenon lamp. The samples were placed into quartz
cells of 1 cm path length. The pyrylium salt concentration was
fixed adjusting the absorbance of the solutions at arbitrary
concentration between 0.2 and 0.3.
Experimental Section
Chemicals. Sensitizer 1a was commercially available. Its
analogue 1b was synthesized according to the procedure
described by Wizinger and Ulrich,12 whereas 1c and 1d were
obtained following the method described by Steckhan and co-
workers.6b
Oxetane 2a and 2b were prepared via the Paterno-Bu¨chi
photocycloaddition of benzaldehyde and trans-â-methylstyrene,13a
or trans-anethole,13b respectively. A similar procedure was
followed in the synthesis of 2c and 2d. Briefly, solutions of the
aryl olefin and the aldehyde (ca. 0.5 M) derivatives were placed
in Pyrex tubes and irradiated for 72 h under argon atmosphere
with a high-pressure mercury lamp (400 W). After irradiation,
the solvent was evaporated and the reaction mixture was purified
by silica gel column chromatography using hexane:ethyl acetate
as eluent (gradient from 99:1 to 95:5 V/V).
Time-Resolved Absorption Spectroscopy. The laser flash
photolysis system was based on a pulsed Nd:YAG SL404G-10
Spectron Laser Systems, using 355 nm as excitation wavelength.
The single pulses were ca. 10 ns duration and the energy was
ca. 20 mJ/pulse. A Lo255 Oriel xenon lamp was employed as
detecting light source. The laser flash photolysis apparatus
consisted of the pulsed laser, the Xe lamp, a 77200 Oriel
monochromator, an Oriel photomultiplier (PMT) system made
up of 77348 side-on PMT tube, 70680 PMT housing, and a
70705 PMT power supply. The oscilloscope was a TDS-640A
Tektronix. The output signal from the oscilloscope was trans-
ferred to a personal computer for study.
1
All known compounds were characterized by H and 13C
NMR spectra, which were recorded on 300 and 75 MHz,
respectively. Data were consistent with those found in the
literature.
trans,trans-4-Methyl-2-(4-methylphenyl)-3-phenyloxe-
1
tane (2c). H NMR (δ, ppm): 1.57 (d, J 6.0 Hz, 3H), 2.34 (s,
3H), 3.58 (t, J 7.5 Hz, 1H), 5.02 (dq, J 7.5 Hz, J 6.0 Hz, 1H),
5.63 (d, J 7.5 Hz, 1H), 7.2-7.4 (m, 9 H). 13C NMR (δ, ppm):
21.2 (CH3), 23.2 (CH3), 57.7 (CH), 80.7 (CH), 85.0 (CH), 125.5
(CH), 127.2 (CH), 128.7 (CH), 129.0 (CH), 129.2 (CH), 136.0
(C), 139.0 (C), 140.0 (C). MS (m/z (%)): 194 (42), 180 (6),
179 (45), 178 (38), 165 (7), 152 (5), 119 (31), 118 (100), 117
(55), 105 (4), 91 (20), 77 (5). Exact mass (EI): required for
C17H18O: 238.1358 (M+); found: 238.1325.
Results and Discussion
Singlet Quenching vs Triplet Population. Oxetanes 2a-d
and 2a′ were synthesized according to the Paterno-Bu¨chi
cycloaddition, following the procedure described in the litera-
ture.13 Preparative irradiations were carried out in the presence
of catalytic amounts of thiapyrylium salt 1b. The reaction
pathway was different depending on the substitution pattern of
the oxetane: stilbenes (3a,c) and acetaldehyde were obtained
in the CR of 2a,c (pathway a, Scheme 1),10a,b whereas trans-
trans,trans-2-(4-Cyanophenyl)-4-methyl-3-phenyloxetane
(2d). 1H NMR (δ, ppm): 1.56 (d, J 6.0 Hz, 3H), 3.49 (t, J 7.5

2480 J. Phys. Chem. A, Vol. 107, No. 14, 2003
Miranda et al.
TABLE 2: Relative Contribution of the Different Pathways
Resulting in Deactivation of the Excited Singlet of 1b at Two
Concentrations of Oxetane
SCHEME 1
oxetane 10-9 × kq(S1)a [2] (M)b F (%)c Q(%)d ISC (%)e
2a
2b
2c
6.9
16.6
13.9
5.4
10-2
10-4
10-2
10-4
10-2
10-4
10-2
10-4
10-2
10-4
3
5
3
5
3
5
4
5
4
5
25
1
54
1
48
1
19
1
72
94
43
94
59
94
77
94
80
94
2d
2a′
4.1
16
1
a Rate constant for quenching of the fluroescence of 1b by 2a-d
and 2a′ (given in M-1 s-1). b Oxetane concentration. c F, Residual
fluorescence. d Fluorescence quenching. e ISC, Intersystem crossing.
anethole (4b) and benzaldehyde were found in the case of 2b
(pathway b, Scheme 1);14d no photoproduct was detected in the
PET irradiation of 2d. The hydroxymethyl derivative 2a′ also
followed pathway a, leading to stilbene 3a and the hydroxy-
acetaldehyde dimer.10c As stated above, the reaction was
assumed to occur from the triplet excited state of the thia-
pyrylium salt.
To make sure that the triplet of 1b was formed in sufficient
amount, even at high concentration of oxetane, it appeared
necessary to determine the relative intersystem crossing yield
under the employed reaction conditions. Estimations were made
taking into account the main deactivation mechanisms of the
singlet excited state: fluorescence emission, fluorescence
quenching, and intersystem crossing. Competition between them
should be governed by the respective rate constants and by the
quencher concentration.
The rate constant of fluorescence emission (kf) and inter-
system crossing (kISC) are intrinsic properties of the photo-
sensitizer. The values for 1b were calculated from eqs 1 and 2
Figure 1. Transient spectra obtained upon LFP (λ ) 355 nm) of 1b
(0.75 × 10-4 M), in acetonitrile, under argon: (a) in the absence of
quenchers (9) and (b) in the presence of 1.25 × 10-4 M of 2b (;).
Spectra recorded 1 µs after the laser pulse. Insert: growth and decay
of the bands at 600 nm (b) and at 470 nm (O).
and were kf ) 0.1 × 108 s-1 and kISC ) 2.1 × 108 s-1
,
respectively
τfkf ) φf
(1)
(2)
τfkISC ) φISC
presence of oxetanes 2a-c and 2a′, a faster decay of the triplet
was observed. Concomitantly, transients were formed corre-
sponding to olefin radical cations resulting from the cyclo-
reversion of oxetanes. As expected, no band related to the
thiapyranyl radical was found, because this species does not
absorb in the recorded spectral window.11c
For example, when 1b was submitted to LFP in the presence
of 2b, triplet quenching (measured at 470 nm) was accompanied
by the appearance of a new transient with absorption maximum
at 600 nm (Figure 1). The latter can be safely assigned to the
radical cation of trans-anethole (4b+•), on the basis of the
literature data.14 The insert of Figure 1 shows that, under the
employed experimental conditions (low concentration of oxe-
tane), growth of the 600 nm band occurs in the submicrosecond
time scale together with the decay at 470 nm.
The rate constants for triplet quenching of 1b in the presence
of 2a-c and 2a’were measured by monitoring the decay of the
T-T transition at different concentrations of oxetane. The
wavelength was selected case by case, to avoid interference with
species due to other transients. Thus, it was previously
established by our group that PET cycloreversion of 2a follows
pathway a of Scheme 1 and leads to trans-stilbene radical cation
(3a+•), which has an absorption maximum at 470 nm;10a,b hence,
The rate constants for quenching of the thiapyrylium salt 1b
fluorescence kq(F) by 2a-d and 2a′ were obtained from the
Stern-Volmer analysis (eqs 3 and 4)
φ0/φ ) 1 + KSV[oxetane]
KSV ) τfkq(F)
(3)
(4)
Their values are given in Table 2, together with the relative
contributions of the different singlet deactivation pathways at
two oxetane concentrations.
From these data, it is clear that the main deactivation route
is intersystem crossing, except when high concentrations of 2b
are present. That means that, even at high quencher concentra-
tions, the triplet excited state of the photosensitizer is sufficiently
populated and may photosensitize the electron-transfer reaction.
Evidence for Triplet Quenching. Laser flash photolysis
(λexc ) 355 nm) of thiapyrylium salt 1b resulted in a broad
transient absorption between 460 and 600 nm, with a lifetime
of ca. 4.2 µs in acetonitrile, which was assigned to the known
T-T transition of 1b.11c Depletion of the ground state of the
sensitizer was also observed between 390 and 440 nm. In the

Quenching of 2,4,6-Triphenyl(thia)pyrylium Salts
J. Phys. Chem. A, Vol. 107, No. 14, 2003 2481
TABLE 3: Thermodynamics and Kinetics of the
Photosensitized ET Reaction from the Triplet Excited State
of 1b
b
+•
oxetanes
10-9 × kq(T1)a
E(D
/D)
∆GET(T1)c
2a
2b
2c
2d
2a′
0.21
3.98
2.60
0.00
0.29
1.86d
1.48
1.75e
2.30
n.a.f
-4
-13
-7
+6
n.a.f
a Rate constant for quenching of triplet 1b by oxetanes 2a-d and
2a′, given in M-1 s-1 b Given in V vs SCE. c Given in kcal/mol.
.
d Obtained after the first scan. Previously, a value of 1.42 V has been
reported; however, this value was obtained after several scans, where
) 1.45 V vs SCE)15 resulting from CR of 2a
+•
some stilbene (E(D
/D)
could be already present. e Value calculated using p-xylene as a model,
because cyclic voltametry of 2c was unclear, probably due to
decomposition of the sample. f n.a.: not available.
Figure 2. Decay traces of the T-T absorption of 1b (1.25 × 10-4 M)
measured at 600 nm in the presence of increasing amounts of 2a: 0 M
(9), 0.68 × 10-3 M (0), 1.37 × 10-3 M (b), 2.40 × 10-3 M (O).
Insert: Stern-Volmer plot to obtain kq(T1). The insert also includes
results obtained at oxetane concentrations for which the decay traces
are not shown.
the quenching experiments were measured at 600 nm. In the
case of 2b, CR exhibits a different regioselectivity (pathway b
in Scheme 1) and proceeds through the radical cation of trans-
anethole (4b+•).14 As this species displays a band with maximum
at 600 nm (Figure 1), quenching experiments were followed at
470 nm.
When LFP of 1b was performed in the presence of 2c, CR
occurred following the same pathway as in the case of 2a. A
band with maximum at ca. 490 nm (4-methylstilbene radical
cation, 3c+•) was recorded, so quenching experiments were
measured at 600 nm.
In the LFP of mixtures of 1b and 2d, neither deactivation of
the triplet nor formation of new bands was detected, so no
quenching measurements could be done.
The triplet quenching rate constants, kq(T1) were determined
from the decay traces obtained for the T-T absorption of 1b
in the presence of increasing amounts of oxetane. Such traces
are shown in Figure 2 for 2a as a typical case. By plotting the
reciprocal lifetimes (1/τ) against the concentration of oxetane,
Stern-Volmer linear relationships were obtained (see insert of
Figure 2). The slopes of the straight lines correspond to kq(T1)
as deduced from Equation 5. Data are collected in Table 3 and
good correlation coefficients were found in all cases
Figure 3. Correlation between log kq(T1) and ∆GET(T1) for quenching
of triplet 1b by several donors: oxetanes 2a (0), 2b(O) 2c (b), 2d
(9), and trans-anethole 4b (2).
+•/D
The oxidation potentials ED were measured by cyclic
voltammetry in acetonitrile. Their values (Table 2) were found
to depend on the substitution pattern of the aryl groups. As
anticipated, electron withdrawing substituents increase the
oxidation potential, while the reverse is true for electron
+•/D
releasing groups. Thus, the ED
values decreased in the
order: 2d >2a > 2c > 2b.
-•
As the EA/A and the triplet energy of 1b have been previously
measured (Table 1), the ∆GET(T1) values were obtained by
fitting the above data into eq 6. The results are given in Table
3. According to them, the reaction would be possible in all cases,
except for oxetane 2d. The most exergonic reaction would be
expected in the case of the methoxy substituted derivative 2b.
A plot of the logarithm of the triplet quenching rate constants
log kq(T1) against ∆GET(T1) is shown in Figure 3. The resulting
correlation is typical for electron-transfer processes.11e,17 It
presents two clearly defined regions: for slightly exergonic
processes (from 0 kcal/mol to ca. -5 kcal/mol), the more
negative is ∆GET(T1), the higher is the rate of triplet quenching.
However, for ∆GET(T1) more negative than ca. -7 kcal/mol,
the curve reaches a plateau. In this region, the electron transfer
process progressively approaches diffusion control. The figure
also includes the experimental values obtained for trans-anethole
4b (the photoproduct of 2b) under the same conditions.
1/τ ) k0 + kq(T1)
(5)
It is noteworthy that the kq(T1) values were markedly
influenced by the substitution pattern of the aryl groups in the
oxetanes. They show that electron transfer occurs at almost
diffusion controlled rate for oxetanes 2b and 2c (with electron
releasing groups) while no triplet quenching was observed for
oxetanes with electron withdrawing substituents (cf. 2d).
Correlation between Thermodynamics and Kinetics of the
Electron-Transfer Reaction. The free energy changes associ-
ated with electron transfer from the triplet excited states of the
sensitizers were calculated using the Weller16 equation (eq 6)
Conclusions
Cycloreversion of 2,3-diaryloxetanes 2a-c and 2a′ follows
an electron-transfer mechanism through the triplet excited state
∆GET(kcal/mol) ) 23.06[E(D /D) - E(A/A-•)] - E* (6)
+•
A

2482 J. Phys. Chem. A, Vol. 107, No. 14, 2003
Miranda et al.
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of the thiapyrylium salt. The rate constants for quenching of
the triplet of 1b kq(T1) by the oxetanes have been determined;
they exhibit a marked dependence on the substitution at the
phenyl groups. Free energy changes ∆GET(T1) have also been
calculated, and the reaction has been found to be exergonic in
all cases except for PET between 1b and 2d. Accordingly, in
this case, no photoproduct was found in the preparative
irradiation. A good correlation between kq(T1) and ∆GET(T1) is
observed, as a confirmation of the electron transfer between
the triplet excited state of the thiapyrylium salt 1b and the 2,3-
diphenyloxetanes.
Acknowledgment. Financial support by the Spanish MCYT
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