Excited state interactions of thiopyrylium salts

Article abstract of DOI:10.1039/a707307k

The photophysics and photochemistry of thiopyrylium salts have been investigated by UV-VIS absorption spectroscopy and nanosecond laser absorption spectroscopy. The effect of substituents of the heterocycle on the photophysical properties was analysed. Dissociation of peroxidic compounds photosensitized by these salts was studied. The interaction with monomers and many other additives has also been investigated. A mechanism is proposed for the initiation of acrylic photopolymerization reactions.

Full text of DOI:10.1039/a707307k

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Excited state interactions of thiopyrylium salts
F. Morlet-Savary,a S. Parret,a J. P. Fouassier,a K. Inomatab and T. Matsumotob
a L aboratoire de Photochimie Generale, Unite de Recherche Associee au C.N.R.S. no. 431, Ecole
Nationale Superieure de Chimie, 3 rue A. W erner, 68093 Mulhouse Cedex, France




b NOF Corporation, T sukuba Research L aboratory, T okodai 5-10, T sukubaÈIbaraki 300-26,
Japan
The photophysics and photochemistry of thiopyrylium salts have been investigated by UVÈVIS absorption spectroscopy and
nanosecond laser absorption spectroscopy. The e†ect of substituents of the heterocycle on the photophysical properties was
analysed. Dissociation of peroxidic compounds photosensitized by these salts was studied. The interaction with monomers and
many other additives has also been investigated. A mechanism is proposed for the initiation of acrylic photopolymerization
reactions.
2.2 Absorption
1
Introduction
The UVÈVIS light absorption spectra of the various com-
pounds were obtained by using a Beckman DU 7400 spectro-
photometer connected to a Hewlett Packard plotter. The
analysing sources were a deuterium lamp for spectra in the
UV range and a tungsten lamp for those recorded in the
visible range. The molar absorption coefficients (e) were calcu-
lated by using the BeerÈLambert law A \ elc, where A is the
measured absorbance, l the optical pathlength of the quartz
cell (1 cm) and c is the concentration of the solution. Typical
absorption spectra are shown in Fig. 1.
Most of the commonly used polymerization photoinitiators
exhibit a spectral sensitivity restricted to the UV range of light
sources.1 Only compounds such as benzyls, thioxanthones
and ketocoumarins exhibit an absorption spectrum extending
beyond 400 nm. Many light sources display emission bands or
lines lying in the visible range and currently used laser sources
emitting beyond 400 nm are e.g. Ar` at 488 and 514 nm and
HeÈNe at 544 and 633 nm. It was, therefore, necessary to
develop photoactive compositions whose spectral sensitivity
extends to higher wavelengths. Using pigmented photosensi-
tive resins such as paints also requires the development of ini-
tiating systems absorbing in the spectral window of the
formulation that, for most of the pigmented compositions, is
located beyond 400 nm. In the medical Ðeld (e.g. dental resins
in odontology), it is safer to use photoactive formulations
capable of polymerizing on visible light illumination, thus,
avoiding deep burns of the buccal tissues. These processes also
require the use of sensitizers absorbing between 400 and 700
nm. Pyrylium and thiopyrylium salts meet this requirement.
Their use as photosensitizers of polymerization reactions has
already found many applications e.g. resins for artiÐcial Ðnger
nails,2 acrylic dental resin,3 polymers involved in laser
imaging,4 printing plate compositions.5 It is our aim in this
paper to study the photophysical properties of thiopyrylium
salts with regard to their use as photosensitizers of the decom-
position of peroxidic compounds for the initiation of radical
photopolymerization reactions. Several aspects will be
explored and discussed e.g. S and T reactivity transient
2.3 Emission
The Ñuorescence emission spectra of the various products
were recorded by using a Perkin-Elmer LS5B spectro-
Ñuorometer connected to a Perkin-Elmer R100A plotter.
Table 1 Compounds studied
thiopyrylium
salt
R \ R
R4
2
6
1
1
TP1
TP4
TP7
C H
C H
6
C H
absorption spectra, and interactions with peroxides, peresters,
monomers, amines, iodonium salt and carbon tetrabromide.
6
5
5
5
5
p-ClC H
C H
6
6 4
p-BrC H
6
4
6
TP8
C H
p-N(CH ) C H
6
5
3 2 6
4
TP10
TP11
TP12
TP13
p-OMeC H
C H
6
4
4
4
4
6
5
2
Experimental
p-OMeC H
p-OMeC H
6
6
4
p-OMeC H
p-OC H C H
6
p-OMeC H
6
4
9 6
4
H
2.1 Chemicals
The solvents required for the spectroscopic measurements:
acetonitrile (for UV spectroscopy, supplied by the Fluka
Corp.), denatured ethanol (purchased from Aldrich Corp.)
were used without further puriÐcation.
R \ R \ H; A~ is BF ~.
3
5
4
The chemical formulae of the various compounds studied
are shown in Table 1. The thiopyrylium salts were synthesized
as reported in ref. 6 and references therein. No further puriÐ-
cation was carried out.
BTTB is a product developed by the NOF corporation.
J. Chem. Soc., Faraday T rans., 1998, 94(6), 745È752
745

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The probe light was sent through the sample solution orthog-
onal to the pump pulse onto the slit of a monochromator
(HUET M25) which was used to select the observation wave-
length. A photomultiplier acquired in real time the variation
of the transmitted probe light, the electronic transient signal
was fed into a transient digitizer (TEKTRONIX DSA 601A)
and then computerized. The synchronization between the
laser pulse excitation and the analysing light was Ðxed by
using a home-made electronic device.
3
Results and Discussion
Work reported in e.g. ref. 8È11 amply covers the chemistry of
pyrylium salts: their di†erent methods of synthesis, their
chemical reactivities, their physical properties and their practi-
cal applications. The thiopyrylium salts, their isoelectronic
counterparts, have been much less extensively studied. Their
methods of synthesis and their reactivities have been described
in ref. 12. On the basis of calculated values of the p electron
distribution,13 it is apparent that, both for pyrylium and
thiopyrylium salts, the electron deÐciency is more substantial
for carbons in the a position than for C . The pyrylium and
4
the thiopyrylium cation, that contain the O and the S hetero-
atom, respectively, are more stable than the oxonium and the
sulfonium cation, owing to the possible aromatic conjugation.
Owing to their cationic structure, pyrylium and thiopyrylium
salts are excellent electron acceptors and can easily be reduced
when a one-electron reaction takes place, leading to the corre-
sponding pyranyl and thiopyranyl neutral radicals in degassed
aprotic solvent media.14 These radicals are stable if the sub-
stituents of the heterocycle are electron attracting. In the pres-
ence of electron-donating substituents, the radicals dimerize
Fig. 1 Absorption spectra of thiopyrylium salts in acetonitrile solu-
tion: a, TP1 (ÈÈÈ), b, TP4 (ÈÈÈ), TP7 (ÉÉÉÉÉÉÉ) and TP8 (È È È È);
c, TP10 (ÈÈÈ), TP11 (É É É É É É É), TP12 (È È È È) and TP13 (ÈÉÈÉÈÉ)
spontaneously at the C carbons.
4
2.4 Time-resolved absorption spectroscopy
3.1 UV–VIS light absorption of the thiopyrylium salts
The triplet state manifold was analysed by using a typical
apparatus for time resolved absorption spectroscopy with
nanosecond time resolution. With this set-up, one can
monitor the electronic absorption spectrum of transient
species after laser pulse excitation. Both the absorption spec-
trum and kinetics of formation and decay of the transient
were measured. The experimental set-up combines a nanose-
cond Nd : YAG laser as excitation source (pump) to a xenon
pulsed lamp as analysing light source (probe).7 Thus, the laser
delivered a 6 ns pulse up to 80 mJ at 355 nm. The pulses were
focused on the sample by a system of cylindrical lenses. A
second and a fourth harmonic, 532 and 266 nm emissions,
respectively, may also be used to excite the sample in solution.
The absorption of transient species was monitored by using
an analysing device capable of producing a bright light pulse
with a homogeneous spectral bandwidth in agreement with
the detection system. According to this principle, a xenon
pulsed lamp (XBO 450W OSRAM) pulsing unit (MULLER
SVX 1450 and MSP 05) was used as probe analysing light.
The absorption intensities of pyrylium and thiopyrylium salts
may vary to a considerable extent according to the purity of
the solvent that was used15 because, as mentioned previously,
the heterocyclic cations under consideration are excellent elec-
tron acceptors and, following a one-electron reduction reac-
tion, they convert to pyranyl or thiopyranyl radicals that
absorb at higher wavelengths.16 The presence of electron
donors in the solvent, even in trace amounts, induces, there-
fore, an appreciable change in absorption intensity. The
absorption spectra in acetonitrile of the various thiopyrylium
salts that were investigated are shown in Fig. 1. Table 2 lists
the measured absorption maxima and the molar absorption
coefficients for all the salts. The absorption by the salt is due
to the ion associated with its anion.15 Many workers have
given attention to the bathochromic and hypsochromic shifts
of the x and y bands resulting from the introduction of alkyl,
phenyl or para-substituted phenyls on the heterocycle.12,17h19
A phenyl group has a much more substantial e†ect on the
wavelengths of the absorption maxima than an alkyl substit-
Table 2 Absorption maximum (j ) and molar absorption coefficients (e) of the thiopyrylium salts in acetonitrile
max
thiopyrlium
salt
j
/nm (e/dm3 mol~1 cm~1)
max
TP1
TP4
TP7
[225 (14 000)]; 248 (17 000); 273 (15 200); 372 (26 300); [405]
254 (19 100); 276 (18 800); 372 (17 700); [406 (15 700)]
256 (19 100); 280 (20 500); 370 (27 400); [414 (23 400)]
[239]; 260 (21 900); 381 (16 100); 579 (58 300)
TP8
TP10
TP11
TP12
TP13
298 (19 400); 354 (17 500); 388 (15 200); 478 (25 900)
230 (18 000); 263 (14 500); 294 (18 700); 439 (41 200)
230 (18 000); 262 (14 500); 294 (18 400); 444 (42 400)
232 (16 100); 294 (17 900); 378 (4700); 475 (29 200)
Wavelengths in square brackets correspond to shoulders.
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Table 3 Singlet state quenching of TP10: upper values of k
SV in ace-
uent: the electronic e†ect of an aryl group stabilizes the posi-
tive charges of the a and the c position to a larger extent than
the inductive e†ect of an alkyl substituent. Generally speaking,
it is apparent that a substitution in the a position causes band
x (polarized along the C wC axis) to shift, while a substitut-
tonitrile for di†erent quenchers
quencher
k
/dm3 mol~1
SV
MMA
MA
BTTB
BPO
0.02
0.02
1
2
6
ion in the c position induces a shift of band y (polarized along
the XwC axis) (Table 1). The introduction of a substituent in
4
1.5
the b position only exercises very minor, identical e†ects on
band x and band y. These shifts are hypsochromic for
electron-attracting and bathochromic for electron-donating
substituents. Their intensity depends on the degree of donat-
ing or accepting character of the substituents. Thus, one may
observe a polarization reversal of the absorption band with
the highest wavelength by substituting the cationic hetero-
cycle.
presence of a quencher Q such as BTTB or monomer:
kST
ÈÈÈ
S1
Õ
T1
kr
@
@
@
ÈÈÈ
Õ
Ñuorescence
internal conversion
quenching
The absorption spectra of thiopyrylium salts are quite
similar. The two absorption bands with the highest wave-
lengths, however, are slightly separated for the thiopyrylium.
Thiopyrylium salts are also quite insensitive to the electron-
donating character of the substituents of the heterocycle. For
instance, band y of TP1 undergoes a 207 nm bathochromic
shift when the phenyl group in the c position to the hetero-
atom is substituted in the para-position by the group
N(CH ) . Studies have been carried out on some typical pyry-
kIC
ÈÈÈ
Õ
kQ
ÈÈÈ
Õ
/
with:
0 \ k q0 with q0 \ 1/(k ] k ] k ) and / \ k
q
ST
ST f
f
ST
r
IC
ST
ST f
1
1
\
] k [Q]
q
q0
q
f
f
3
2
thus:
lium, and on the S and T properties of thiopyrylium salts.20
1
1
/0
q0
y \ /ST \ f \ 1 ] k [Q]
q
SV
3.2 Singlet state reactivity
ST
f
for [BTTB] \ 5 ] 10~2 M; y O 1.05; for [monomer] \ 2 M;
y O 1.04.
Fluorescence properties (quantum yields, lifetimes) have been
reported in ref. 21. A typical Ñuorescence spectrum is shown in
Fig. 2. Quenching of the Ñuorescence of TP10, for example,
has been investigated in the presence of (BPO) benzoyl perox-
ide, BTTB, methyl methacrylate (MMA) and methyl acrylate
However, owing to the short lifetimes of the S state (ca. 1
1
ns), high quenching rate constants can be attained (ca. 109
dm3 mol~1 s~1) which allow a possible weak generation of
transient species through a BTTBÈTP interaction.
(
MA). No signiÐcant quenching is observed in the concentra-
tion range used: MMA or MA (0È2 M); BTTB (0È5 ] 10~2
3
.3 Triplet state reactivity
M); BPO (0È3 ] 10~2 M). An upper value of the SternÈVolmer
coefficients k can be deduced (Table 3) from the usual plot:
SV
3.3.1 Under laser excitation at 355 nm. The triplet states of
the di†erent salts in acetonitrile solution were investigated.
Upon laser excitation at 355 nm, all thiopyrylium salts gave
rise to transients that were analysed via their absorption
spectra. The signals obtained are Ðtted according to a pseudo-
Ðrst-order decay. The absorption spectra of salt TP4 in argon-
saturated acetonitrile solution, that were recorded
immediately (*t \ 0 ls) and 10 ls after laser excitation, are
shown in Fig. 3. Another example of salt TP10 is presented in
Fig. 4: the transient spectra of TP10 in argon-saturated ace-
tonitrile solution after laser excitation at 355 nm. Small di†er-
ences appear between the spectra (slight shifts of the maxima).
Unfortunately, a strong depletion occurs at 480 nm owing to
the large molar absorption coefficient of the S ] S absorp-
tion at this wavelength. Because of this phenomenon, it is dif-
Ðcult to ascribe any maximum for the tripletÈtriplet
absorption or to extract any information, such as substituent
e†ects, from transient absorption spectra. Di†erent decay
curves for TP10 monitored at 380, 470 and 590 nm are pre-
sented in Fig. 5. The signal monitored at 380 nm is Ðtted
according to a pseudo-Ðrst-order decay with a lifetime of ca.
I0
f \ 1 ] k [Q] \ 1 ] k q0[Q]
I
SV
q f
f
where I0 and I are the Ñuorescence intensities measured in
the absence and in the presence of a quencher Q, respectively,
f
f
k the rate constant of singlet state deactivation and q0 the
q
f
singlet state lifetime in the absence of quencher. If a triplet
state is formed after light excitation of TP10, the quantum
yield of intersystem crossing / should not be a†ected by the
ST
0
1
11 ls; the situation is totally di†erent if one looks at the signal
monitored at 590 nm: this signal consists mainly of two com-
ponents, a fast one and a long-lived one. The fast one is of the
same nature as that observed at 380 nm and the long-lived
one presents a longer lifetime and red-shifted absorption
toward 700 nm (Fig. 5). These results were already reported
previously7 when the fast component was ascribed to a
tripletÈtriplet absorption (triplet state lifetime ca. 10 ks) and
the long-lived component to a species arising from the triplet
state, since this species disappears in aerated acetonitrile solu-
tion.
Fig. 2 Typical Ñuorescence spectrum of TP1 in methanol.
In all cases, the lifetime of the transient, which is of the
order of 10 ls, falls abruptly when oxygen is added. In salt
j
\ 450 nm.
excitation
J. Chem. Soc., Faraday T rans., 1998, V ol. 94
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saturated medium (*t \ 0 ls), one may conclude that the
latter results from the absorption of the triplet state of the salt.
The spectrum observed at 10 ls after a laser pulse is not
exactly similar to that obtained immediately after (*t \ 0 ls).
A slight hypsochromic shift of ca. 10 to 20 nm was observed
for the absorption maximum (with thiopyrylium salt TP4, for
instance, the maximum for *t \ 0 ls was located at 490 nm,
and at 480 nm for *t \ 10 ls), together with an increased
absorbance on the red side of the visible spectrum. The signals
observed in argon-saturated solution (Fig. 3) do not clearly
demonstrate the presence of a second species. The lifetime of
the latter must certainly be as long as that of the triplet state,
which would explain why it cannot be observed on the oscillo-
gram. Since no residual absorption was observed in aerated
media, this second transient must arise from the triplet state of
the salt. One might consider that this species can be attributed
to the radical but, from the radical absorption spectrum
obtained after one electron reduction of the 2,4,6-tri-
phenylthiopyrylium by THF,22 the main absorption of the
radical is situated in the blue region of the spectrum and, in
our case, the TP10 transient decay at 380 nm presents only
one pseudo-Ðrst-order component ascribed to tripletÈtriplet
absorption relaxation. From this result, we can say that this
long-lived species cannot be attributed with certainty to the
thiopyranyl radical absorption. On the other hand, one-
electron reduction may occur between the salt and acetonitrile
solvent, but under continuous irradiation with an HPK
medium-pressure mercury lamp, no changes in the UVÈVIS
absorption spectrum of the thiopyrylium salt TP10 were
noticed. Another possibility to consider is that the thiopyry-
lium salt reacts with residual oxygen in acetonitrile solution.
It has been reported in23 that triplet-state of 2,4,6-triphenyl
thiopyrylium reacts with ground-state oxygen in alcoholic
solutions. However, the transient absorption experiments
carried out in aerated acetonitrile solution led us to observe
only the tripletÈtriplet absorption.
Fig. 3 Absorption spectrum of TP4 in argon-saturated acetonitrile
solution immediately (…) and 10 ls (L) after laser pulse. Signal
obtained at wavelength of 480 nm.
TP12, for instance, the lifetime at 580 nm decreases from 14.0
ls in argon-saturated medium to 390 ns in the presence of
oxygen. The spectrum obtained at *t \ 0 ls (Fig. 3) has been
attributed to the tripletÈtriplet absorption of the salt or to a
species arising from this state. However, since the absorption
spectra of the transient are the same in aerated and argon-
Finally, the reactivity of the triplet state can be summarized
by the following diagram:
hl
TP ÈÈÈ
ISC
1TP* ÈÈÈ
Fig. 4 Transient spectra of TP10 in argon-saturated acetonitrile
solution taken immediately (…), 10 ls (L) and 30 ls (K) after the
laser pulse (excitation at 355 nm).
Õ
Õ
3TP*
ISC: intersystem crossing
3
TP* ÈÈÈ
Õ
relaxation
K
O2
ÈÈÈÈ
kq
Õ
inhibition by oxygen
For salt TP12, the constant of quenching by oxygen is ca.
.5 ] 109 dm3 mol~1 s~1 if an oxygen concentration of
.6 ] 10~3 mol dm~3 in acetonitrile is assumed.24
1
1
3.3.2 Under visible laser light excitation. As mentioned
before, only Ðve thiopyrylium salts: TP8 and TP10ÈTP13
show absorption at 532 nm, the most important being for the
TP8 compound.
Transient spectra of TP10, presented in Fig. 6, were
obtained after laser excitation in argon-saturated acetonitrile
solution. They show similar characteristics to those recorded
under similar experimental conditions after laser excitation at
3
55 nm. In these experiments, after excitation at 532 nm, the
absorbance of the sample solution was kept below or around
.1, in order to avoid strong depletion of the transient absorp-
tion caused by the S ] S transition.
0
0
1
For TP10 thiopyrylium salt, the tripletÈtriplet absorption
shows pseudo-Ðrst order decay [Fig. 7, (a) and (c)]; the decays
monitored at 470 and 590 nm, (Fig. 7), respectively, show the
same characteristics as those reported in Fig. 5. For the
decays monitored at 470 nm (Fig. 7) of the other salts (TP11È
TP13), a fast component and a slow component, ascribed in
the same manner as described above, are found. The measured
lifetimes for the triplet state range between 10 and 20 ls.
Fig. 5 Transient decays of TP10 in acetonitrile solution monitored
at a, 380; b, 470 and c, 590 nm after laser excitation at 355 nm.
748
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the dissociation of two compounds with an OwO bond:
BTTB and BPO.
The triplet state of tetraperester BTTB has already been
investigated.7 Since BTTB has an n ] p* transition around
350 nm, direct excitation by laser light is possible and we have
observed, in previous experiments, a transient absorption fol-
lowing laser excitation at 355 nm.7 A lifetime of ca. 1 ns has
been determined for this excited state. The results reported at
that time permitted us to suggest the following mechanism for
the dissociation of the perester induced by UV light:
Fig. 6 Transient spectra of TP10 after laser excitation at 532 nm in
argon-saturated acetonitrile solution, immediately (…), 10 ls (L) and
3
0 ls (K) after the laser pulse at 532 nm.
3
.4 Using thiopyrylium salts as photosensitizers for the
visible-light-induced dissociation of peroxidic compounds:
triplet state reactivity
Photosensitizers for the homolytic cleavage of aromatic per-
oxides, such as benzophenone or anthracene can only be effi-
cient under UV light illumination. Therefore, it was
worthwhile to search for sensitizers that can be used in the
visible region. In a Ðrst stage, it was shown25,26 that poly-
merization can be initiated by an organic peroxide sensitized
by thiopyrylium salt TP12 under illumination from an Ar`
laser at 488 nm. On the basis of these results, it has been
demonstrated27 that TP12 can sensitize the dissociation of
BTTB.
Quenching of the triplet state of the salt TP12 by BTTB
and BPO was carried out (Fig. 8). Values of the quenching
rate constants are shown in Tables 4 and 5 and these also
indicate a strong interaction.
Indeed, when BTTB or the thiopyrylium salt TP12 is used
alone, MMA cannot be polymerized by illumination with a
wavelength [450 nm. On the other hand, when these two
compounds are used together, such illumination allows the
polymerization of acrylate and methacrylate monomers, thus
revealing the sensitizing role of the thiopyrylium salt in the
dissociation of BTTB wherein it produces radicals capable of
initiating the polymerization. In the present investigation,
pyrylium and thiopyrylium salts were used to photosensitize
3
.5 Quenching of the salts by monomers
3.5.1 Interaction with a vinylic monomer. A value of
1.0 ] 106 dm3 mol~1 s~1 was obtained for the quenching rate
constant of the triplet state of TP11 and TP10 salts by MMA
in argon-saturated acetonitrile solution.28 This value indicates
a comparatively weak interaction of MMA with the triplet
state of the salts, since it is ca. 80 times lower than the rate
constant of quenching of TP11 by BTTB (kT \ 7.7 ] 107 dm3
q
mol~1 s~1) and 35 times lower than with BPO (kT \ 3.5
q
]
107 dm3 mol~1 s~1). Therefore, the quenching of the triplet
Fig. 7 Transient traces of TP10 after laser excitation at 532 nm
under low light excitation (avoiding TÈT annihilation) monitored at a,
Fig. 8 Triplet state quenching of TP12 by BTTB and BPO moni-
tored at 580 nm
3
80; b, 470 and c, 590 nm
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Table 4 Quenching rate constant of the triplet state by BPO for the
thiopyrylium salts in argon-saturated acetonitrile solution
salt
kT/107 dm3 mol~1 s~1
q
TP1
TP4
TP7
TP10
TP11
TP12
TP13
2.8
4.7
5.1
4.2
3.5
3.0
3.9
state of the salts by BTTB is faster than that by MMA.
The efficiency of the interaction of a thiopyrylium salt with
BTTB in a medium containing a monomer in acetonitrile
solution can be estimated by measuring the interaction
yields.7 For an MMA concentration of 2 mol dm~3 and a
BTTB concentration of 5 ] 10~2 mol dm~3 in acetonitrile
solution, the interaction yield of thiopyrylium salt TP11 with
BTTB was 0.65 and 0.42 in degassed and aerated medium,
respectively.
Fig. 9 Quenching of TP12 triplet state by EB600 (4.3 ] 10~4 M).
j \ 380 nm.
The quenching rate constants for salt TP10 measured in
degassed and aerated acetonitrile medium were of the same
order of magnitude: 0.6 and 0.35, respectively.
The values obtained for the interaction yield demonstrate
the efficiency of the sensitization process between a thiopyry-
lium salt and BTTB. This sensitization process is slightly less
efficient with benzoyl peroxide because, in the case of
thiopyrylium salt TP11, for example, the interaction yields are
only 0.46 in degassed and 0.25 in aerated medium (with a
BPO concentration of 5 ] 10~2 mol dm~3).
Nd:YAG laser) may take place in the triplet state of TP12 via
an electron-transfer process.
Triplet state quenching by PVP and PETA was also con-
sidered under visible light excitation (532 nm). For each salt,
TP10ÈTP13, the measured lifetimes extracted from Ðrst-order
decay are plotted according to a SternÈVolmer plot. The mea-
sured quenching rate constants of thiopyrylium triplet state
are reported in Table 6. During quenching experiments, for-
mation of the corresponding thiopyranyl radicals is observed
when monitoring the transient decays at 380 nm. Thus, the
interaction mechanism between PVP and PETA also proceeds
via an electron transfer.
3.5.2 Quenching by an epoxy acrylate monomer, polyvinyl
pyrrolidone (PVP), pentaerythritol triacrylate (PETA). In the
past, several authors demonstrated the possibility of using
visible light for the curing of acrylate monomerÈPVP,6 epoxy
acrylates28 and phenoxy resin esters of cinnamylidene acetic
acid29 using thiopyrylium salts as sensitizers. The interaction
between the TP triplet state and an epoxy acrylate monomer
called EB600 (Ebecryl 600) was studied.
3.6 Quenching by various additives
Rate constants for the triplet interaction of TP10 with mono-
mers (MMA; EB 600: an epoxyacrylate), amines [DABCO,
N-phenylglycine (NPG), triphenylamine (TPA)], peroxides
The quenching of the TP10 and TP12 triplet state is found
to be very fast according to the high rate constants measured
[
benzoylperoxide (BPO)], iodobenzene (IDB), CBr , onium
in argon-saturated acetonitrile solution. k \ 4.9 ] 109 dm3
4
q
salt [diphenyliodonium salt (DPI)] and peresters (BTTB) have
been determined (Table 7). The thiopyranyl radical is
observed e.g. in the presence of quenchers Q such as: EB 600,
amines, BTTB, IDB according to:
mol~1 s~1 for TP10 and 3.6 ] 109 dm3 mol~1 s~1 for TP12.
Both rate constants were measured at 540 nm. A new tran-
sient is observed at 380 nm following the tripletÈtriplet
absorption decay that might be ascribed to the thiopyranil
radical TP12~~ (Fig. 9).30 Thus, the interaction between TP12
and EB600 under visible light excitation (532 nm of the
3
TPy* ] Q ] TPy~~ ] Q~`
In the presence of DPI and CBr , only deactivation of the
4
triplet state manifold following a SternÈVolmer law is
Table 5 Quenching rate constant of the triplet state by BTTB for the
thiopyrylium salts in argon-saturated acetonitrile solution
Table 6 Rate constants for the quenching of the thiopyrylium triplet
state by monomers
salt
kT/107 dm3 mol~1 s~1
q
k /dm3 mol~1 s~1
q
TP1
TP4
TP7
TP10
TP11
TP12
TP13
7.0
8.0
7.1
6.6
7.7
6.2
6.4
PETA
PVP
EB 600
TP10
TP11
TP12
TP13
3.6 ] 107
3.5 ] 107
4.1 ] 107
3.7 ] 107
2.3 ] 109
2.7 ] 109
2.3 ] 109
2.2 ] 109
4.9 ] 109
È
3.6 ] 109
È
EB600
750
J. Chem. Soc., Faraday T rans., 1998, V ol. 94

View Article Online
Table 7 Quenching of TP10 by various additives
quencher
MMA
1
EB 600
4800
DABCO
14000
TPA
NPG
30000
IDB
1400
CBr4
3400
DPI
17000
BTTB
66
k /106 dm3 mol~1 s~1
16000
q
observed. No thiopyranyl radical absorption was detected
when quenching was monitored at 380 nm and in the quen-
cher concentration range used: [CBr ] (2 ] 10~5)È(2 ] 10~4)
4
mol dm~3, [DPI] (2 ] 10~6)È(2 ] 10~5) mol dm~3.
However, since these quenching compounds contain iodine
and bromine atoms, triplet state deactivation through an
external heavy atom e†ect should be considered. Possible elec-
tron transfer should not be discounted, based upon thermody-
namic data, considering TP12 and DPI. The reduction
potential of DPI has been measured by Bachofner et al.31 as
Scheme 1
action TP10ÈBTTB has already been evaluated to be ca. 0.3
in bulk MA monomer.7
E1@2 \ [0.193 eV vs. SCE in 1 : 1 ethanolÈwater solution with
red
The yield (calculated according to the same scheme) is con-
siderably lower in the presence of other monomers, e.g. epoxy-
acrylates or PVP (owing to the high rate constants for
monomer quenching). The primary process for the initiation
step of the photopolymerization shown in Scheme 1 might be
unable to explain the activity of the combination TP saltÈ
BTTB as a photoinitiating system. It might be postulated that
BTTB can undergo further reactions involving the radical
anion of the thiopyrylium salt (thermodynamic calculations
suggest that this process should not be feasible) or the cation
radical on the monomer:
0
.4 ] 10~3 mol dm~3 iodonium salt and 0.1 mol dm~3 tetra-
ethylammonium phosphate supporting electrolyte. According
to the RehmÈWeller relationship32
*
G \ Eox (TP12) [ Ered(DPI) [ E
(A)
1
@2 1@2 S or T
E (A) \ 2.32 eV, E (A) \ 2.12 eV for the singlet and triplet
state energy for TP12 and, neglecting the coulombic term for
S
T
a polar medium such as acetonitrile, we found that *G \
T
0
.073 eV and *G \ [0.127 eV. The Gibbs energy change for
S
the electron-transfer reaction:
3
TPy* ] DPI ] TPy~`
monomer
3
TP ÈÈÈ
Õ
TP~~ ] M~`
]
DPI~~[(C H ) I~ ] C H I ] C H ~]
6
5 2 6 5 6 5
M
is slightly positive for the triplet state or negative for the
singlet state, rendering electron transfer feasible between both
compounds. Moreover, as mentioned above, no thiopyranyl
radical was observed during the quenching experiment moni-
tored at 380 nm.
TP~~ ] BTTB ÈÈÈ
Õ
TP ] BTTB~~
M ] BTTB~`
radicals
M~` ] BTTB ÈÈÈ
Õ
BTTB~~ or BTTB~` [ [ [
Õ
The generation of the radical anion of BTTB has already
been proposed for the photosensitized reaction between dyes
such as coumarin derivatives and thioxanthene derived dyes)
In the presence of CBr , one might have (since no thiopy-
4
ranyl radical is observed):
(
3
TPy* ] CBr ] TPy~` ] ~CBr ] Br~
and BTTB.31 Generation of BTTB~` as a formal intermediate
species has been proposed28 to explain the quenching of TP
salts by BTTB.
Under blue light exposures (e.g. Ar` laser line at 488 m), the
photoinitiation step of the polymerization reaction is related
to the generation of initiating radicals through (i) a direct
decomposition of BTTB and (ii) a sensitized decomposition of
this compound in the presence of the TP salt.
4
3
Further experiments are underway to study the interaction
mechanism between thiopyrylium salts and these halogeno-
compounds in more detail.
3
.7 Sensitization mechanism
The photosensitization of BTTB by a salt in the triplet state
does not appear to take place via a conventional mechanism
as discussed elsewhere.33 On the other hand, BTTB is effi-
ciently photosensitized by dyes of the thioxanthene or the
coumarin type33 through an electron transfer from the singlet
excited state of the dye to BTTB. In a monomer medium, the
radicals arising from the cleavage should react with the
monomer by a double-bond-breaking process. The photo-
sensitization by thiopyrylium of the cleavage of perester can
be suggested as leading to:
4 Conclusion
The present study has shown that thiopyrylium salts are good
sensitizers for the dissociation of peroxides such as BTTB on
illumination with visible light. Structural modiÐcation did not
lead to any signiÐcant e†ect on the excited states. The photo-
physics and photochemistry of a typical representative of the
thiopyrylium series have been discussed and have helped to
explain the interactions of these components with various
additives.
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