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Time-resolved laser-pumped dye laser spectroscopy: energy and electron transfer in ketones
Volume 135, number I ,2
TIME-RESOLVED
27 March 1987
CHEMICAL PHYSICS LETTERS
LASER-PUMPED
DYE LASER SPECTROSCOPY:
ENERGY AND ELECTRON TRANSFER IN KETONES
J.P. FOUASSIER,
D.J. LOUGNOT,
A. PAVERNE
and F. WIEDER
Laboratoire de Photochimie G&n&ale, Unit&eAssocitfe au CNRS No. 431, Ecole Nationale Supkrieure de Chimie, 3, Rue Alfred Werner, 68093 Mulhouse Cedex, France
Received 28 October 1986; in final form 20 January 1987
The synergic effects which are generally invoked to account for the specific features of a system of two ketones used as polymerization photoinitiators are reconsidered. The increase in reactivity observed when mixing these two initiators is reinterpreted in terms of a simultaneous energy and electron transfer in the pair. The relative efficiencies of these processes depend on the energy gap between the triplet states involved, which is known to be influenced by the polarity of the medium.
1. Introduction
Time-resolved laser spectroscopy is a convenient method to study directly the temporal behaviour of excited chromophores. The irradiation source is usually a solid (YAG ruby) or a gas (nitrogen excimer) laser and as a consequence, the available wavelengths of excitation are.rather limited. In order to extend the feasibility of this technique to other excitation wavelengths, one can resort to the emission laser-pumped dye laser (LPDL) . Its spectral characteristics are obviously dependent on the laser dye used: thus, tunable wavelengths are available over the whole visible part of the spectrum. Most of the time, due to the rather broad electronic absorption spectra of the emitting species, a LPDL device is not required. However, one may be faced with specific problems demanding a very selective excitation in a mixture of different molecules with closely related absorption spectra. The present paper deals with the description of such a technique and its use in the investigation of the excitation exchange (energy and electron transfer) between two molecules used as polymerization photoinitiators. ETX, a substituted thioxanthone ( 3-carboxyethyl7-ethylthioxanthone) and TPMK, a substituted acetophenone (2-methyl-l-( 4-methylthiophenyl)-2morpholinopropan1-one ) are both efficient pho30
toinitiators for the polymerization of acrylic monomers. It was recently shown that mixing ETX (in the absence of any H or e- donor) with TPMK extends the photosensitivity towards the visible part of the spectrum [ 11.
H.8
TPnK
Such a procedure involving a mixture of several photoinitiators often enhances the efftciency of the initiation process and up to now, synergetic effects were generally invoked to explain this behaviour [ 21. The present investigation attempts to clarify our understanding of the exchange of excitation energy which takes place between TPMK and ETX and the role of synergism will be made explicit.
2. Experimental 2.1. Apparatus Our laser photolysis equipment has been built following basic ideas used by several authors for the development of very sophisticated devices. Two Quantel sources are commonly used as the excitation source: a ruby laser working in the mode-locked 0 009-2614/87/$ (North-Holland
03.50 0 Elsevier Science Publishers Physics Publishing Division)
B.V.
CHEMICAL PHYSICS LETTERS
Volume 135, number 1,2 secondary laser output
Ampll
PUMP
LASER
Fig. 1.Nd/YAG-pumped dye laser device.
regime and a neodymium-YAG Q-switched by a Pockels cell. The first laser [ 31 produces a train of 5 to 7 ps pulses (300 ps fwhm) of up to 150 mJ separated by 10 ns at 694.3 nm. By passing through a KDP frequency-doubling crystal this emission can be partly converted into a UV beam of light (347.1 nm) which exhibits the same temporal characteristics as the parent red beam. The second source produces a 3 ns pulse of up to 300 mJ at 1.06 pm. In the same way, the use of KDP crystals allows this emission to be converted to secondary emissions at 532 nm (120 mJ) and at 355 nm (40 mJ). The pulses are focused on the samples by a system of spherical lenses. In some cases, when the wavelengths of the excitation pulses which are directly available from the Nd/YAG or ruby lasers are not adapted to the requirements of a specific experiment, a laserpumped dye laser is used. This system - a schematic of which is shown in fig. 1 - has been built (SOPRA) according to the following ideas: the pulse corresponding to the second (532 nm) or third (355 nm) harmonic of the Nd/YAG laser is split into two parts; the first part (about 10%) is focused by a system of cylindric lenses onto a Suprasil cell in which a fluorescent dye circulates. This cell is placed into a short cavity resonator and the laser emission which escapes from this oscillator is collected and directed by a system of mirrors to a second laser cell which is used as an amplification stage. The different elements of this system have been arranged so as to produce an optical delay between excitation and emission: the laser beam travelling from the oscillator which is used to stimulate the emission in the second cell must arrive
27 March 1987
just as the population inversion reaches its maximum. The output yield of this system is dependent upon the laser dye excited: when the third harmonic is used as the pumping source with coumarin 420 in methanol, a 10% conversion is achieved - about 4 mJ are available at 430 nm - whereas about 25 mJ can be obtained at 600 nm when a rhodamine dye is pumped by the second harmonic. The monitoring system consists of a 450 W xenon arc, a Huet M 25 monochromator and a R 212 Hamamatsu photomultiplier at the detection end; only six dynodes are used. The intensity of the monitoring source is increased during the measurement by discharging a capacitor bank through its power supply for a short and adjustable period of time; this light pulse starts 100 t.tsbefore the laser pulse reaches the sample. The samples are contained in 10x 10 mm* Suprasil tubing. The monitoring and excitation beams are arranged orthogonally and the excited volume is limited by a set of rectangular diaphragms. The electric signal delivered by the photomultiplier is displayed onto the screen of a wide band memory scope (Tektronix 7834); it is then read by a video camera, numerized and stored. A Texas Instrument microcomputer controls the experiment, gathers the data, processes the information, produces suitable tiles on storage devices and provides graphics and other useful information. Several synchronizers activate the different units (shutters, pulser, lasers) with suitable delays. The risetime of the detection system is x 2 ns when terminated into 50 n and the actual “window” in which measurements can be carried out is from 10 ns to 200 ps. 2.2. Materials The compounds studied ETX and TPMK were obtained from Ciba Geigy Base1 [ 1] and coumarin 420 was purchased from Exciton. All the chemicals (Janssen Chimica) were used as received. The UV spectra of ETX and TPMK are reported in fig. 2; they show intense absorption around 400 and 3 10 nm respectively.
31
OD
E =I8600
27 March 1987
CHEMICAL PHYSICS LETTERS
Volume 135, number 1,2
M-‘cm-’
ETX
A
hv
‘ETX
__t
3ETX tit
I
’ IEii
i$
H+ +
I ._ 3[ETX
.+ N;]
K’:‘=K’
+
Nt pi-
Fig. 2. Absorption spectra of ETX and TPMK in methanol.
3. Results
In the case of TPMK, a fast cleavage occurs in a short-lived triplet state, generating a methyl thiobenzoyl morpholino isopropyl radical pair (which undergoes disproportionation or separation as shown by CIDNP measurements [ 1I): TPtlK
hv
-----+
‘TPtlK
3 TPIIK
+
3.1. Primary processes in ETX and TPMK
1 CH3-S
The two photoinitiators ETX and TPMK exhibit the usual behaviour of thioxanthone derivatives [ 4-71 and aryl alkyl [ 81 or aryl aryl [ 9,101 ketones. The triplet state of ETX is long lived in deaerated solution (r > 10 us); the absorption maximum of the triplet-triplet transition is blue-shifted when going from non-polar (A,,, = 670 nm in toluene) to polar solution (A,,, = 625 nm in methanol); this could be readily understood in terms of a change of electronic configuration of the lowest triplet state (as reported in the case of xanthone [ 111) or by a decrease of the dipole moment (in the triplet manifold) upon photoexcitation (as in the cases of xanthone, thioxanthone or methylacridone [ 121). The fluorescence intensity of ETX increases and is red-shifted with increasing solvent polarity; a similar behaviour is found with thioxanthone, a fact which supports the view of either an increase of the dipole moment (in the singlet manifold) upon excitation or an inversion of the relative position of the nrc* and xx* states. Fluorescence and triplet absorption are quenched when methyldiethanol amine (MDER) is added to deoxygenated or aerated solutions of ETX. A new long-lived absorption, assigned to the thioxanthonederived ketyl radical K’, is detected between 400 and 750 nm; the red portion of the transient absorption spectrum contains an important contribution from the radical anion KT :
32
-0
(-J
H/ escape
CH3m
c*
‘hi
0
e CH3-S
u
0
1 k”3-J CM2 II A pl + c-n C”,y” 0
3.2. Interaction between ETX and TPMK
The interaction between the excited states of ETX and TPMK as well as between morpholine and thio anisole - used as models of the TPMK moieties were investigated by LPDL spectroscopy. Figs. 3 and 4 clearly show that the thioether group and the morpholino moiety of TPMK are responsible for the interaction with the S, and T1 states of ETX, respectively. The triplet quenching rate constant is affected by the solvent or by changing ETX to ITX (2-isopropylthioxanthone) as observed in table lwhere the rate constants of electron transfer with morpholine (k,) and of excitation transfer (k,) are reported. It is worth noting that K’ radicals are only formed on excitation of ETX/TPMK in toluene and not in the other systems. Fluorescence quenching will not be taken into consideration since it yields a singlet charge transfer complex which mainly undergoes deactivation through an electron back-transfer process.
Volume 135, number 1,2
CHEMICAL PHYSICS LETTERS
4. Discussion
I
3
I
27 March 1987
1
Morphollne _o--
1
, lO’[Q]
2
1
3
Fig. 3. Fluorescence quenching of ETX in aerated methanol. A,.,=375 nm; As..=470 nm. [Q] is in mole/B.
k x lo+ s-’ 6-
6-
3 3
6
9
12
, 103[al 16
Fig. 4. Triplet state quenching of ETX in methanol at 1= 625 nm. [Q ] is in mole/!?.
Table 1 Rate constant (M-’ s-‘) for electron transfer in ETX and ITX in the presence of morpholine in aerated toluene and methanol solutions. Rate constant of excitation transfer in the presence of TPMK
ETX ITX
10-s%, morpholine
1O-8k,, TPMK
toluene
methanol
toluene
methanol
6.3 12
3.1 1.1
0.12 0.6
1.1 0.55
The rate constants for electron transfer (k,) between thioxanthones (ETX and ITX) and morpholine decrease when toluene is replaced by methanol; investigation of a series of solvents shows that k, decreases as the solvent polarity increases (as defined by the ET Dimroth parameter). The solvent effect on the rate constants k, is much more pronounced in the case of ITX than with ETX. On the other hand, a strong increase of the reaction rate constant (k,) between ETX and TPMK (and not between ITX and TPMK) is observed on increasing the solvent polarity although the contribution of the electron transfer process (e.g. k, between ETX and morpholine) to the overall effect is considerably less affected. These results strongly suggest that the change in the interaction rate of the triplet state of ETX with TPMK as a function of the solvent cannot be accounted for solely on the basis of ability to undergo electron transfer: if this were the case, the kq would be almost solvent independent for the system ETX/TPMK and strongly affected for the system ITX/TPMK. The results reported in table 1 are at variance with this model. A possible way out of this discrepancy is to introduce the idea of an energy transfer process competing with the electron transfer, the relative efficiencies of these processes being dependent on the stabilization of the lowest excited state by solvation and on the relative positions of the triplet energy levels of TPMK, ETX and ITX (61 f. 1; 58.4; 61.4 kcal mol- ’ respectively in EPA glasses at 77 K [ 1] ) . It is thus inferred that, in toluene, the excitation transfer occurs mainly through electron transfer for ETX and energy transfer for ITX whereas, in methanol, energy transfer would predominate for both ITX and ETX. The energy transfer rate constant is a function of the energy gap between the donor and the acceptor: thus, this process is expected to be favoured for ITX relative to ETX in toluene. If the change in reactivity is assumed to reflect the Boltzmann distribution between the nx* and rccx*levels of the donor [ 111, it is evident that a more or less important mixing of these two states will influence the interaction rate constant. The nrt * triplet state of ETX (or ITX) is expected 33
CHEMICAL PHYSICS LETTERS
Volume 135, number 1,2 ETX E Methanol
TOlU~fle
f
27 March 1987
processes involving its lowest triplet state should be solvent dependent. The reality of the excitation exchange between ETX and TPMK is demonstrated by the fact that a polymerization can be initiated under laser irrradiation at 440 nm (a wavelength which is only absorbed by ETX) in the presence of ETX/TPMK and not in the presence of ETX alone. The relative quantum yield of polymerization obtained is similar to that of the system ETX/morpholine (at 1= 363 nm under cw Ar+ laser) but considerably less than that of TPMK at the same irradiation wavelength. This observation is in good agreement with the assumption of an electron transfer process predominating in non-polar media. A thorough investigation of this question will be the subject of a forthcoming paper.
Scheme 1.
Acknowledgement
to be the lowest lying state in non-polar solvents and to become more and more stabilized (scheme 1) to the point of mixing with the roll:* state as the solvent becomes more polar (by analogy with the case of xanthone [ 111 which behaves similarly, we suggest that the energy gap between the nrc* state in toluene and the AX* state in methanol accounts for most of the spectral shift of the triplet-triplet absorption recorded in these solvents). ‘Likewise, the apparent change of k, as a function of the solvent polarity is accounted for by a change of reactivity of the lowest state in an electron transfer process since mt* states are known to be more easily photoreducible than xx* states: this statement is consistent with the k, values measured in toluene and in methanol (table 1). The energy of the lowest excited state of TPMK is also expected to be solvent sensitive. Acetophenone possesses an nlc* state in non-polar solvents and a IF A* state in polar solvent [ 131 whereas 4-CH30acetophenone has a xx* lowest triplet state in any solvent presumably because of a substantial lowering of the XR* level due to the presence of the methoxy substituent. Thus, in TPMK, the thioether group can be expected to induce strong modifications in the energy levels of the benzoyl chromophore; consequently solvent effects are to be expected in this product and hence the rate of the energy transfer
34
Drs. G. Berner and W. Rutsch (Ciba Geigy Basel) are greatly acknowledged for supplying the samples used in this work.
References [ 1] W. Rutsch, G. Bemer, R. Kirchmayer, R. Hustler, G. Rist and N. Buhler, Techn. Paper Radcure Base1 (1985). [2] S.P. Pappas, ed., UV curing: science and technology, Stamford Techn. Mark. (1978). [ 31 J. Faure, J.P. Fouassier, D.J. Lougnot and R. Saluin, Now. J. Chim. 1 (1977) 15. [4] G. Amirzadeh and W. Schnabel, Makromol. Chem. 182 (1981) 2821. [ 51S.F. Yates and G.B. Schuster, J. Org. Chem. 49 (1984) 3349. [ 61 J.P. Fouassier and D.J. Lougnot, to be published. [ 71 N.S. Allen, F. Catalina, P.N. Green and W.A. Green, European Polym. J. 21 (1985) 841. [8] J. Eichler, C.P. Herz, I. Naiko and W. Schnabel, J. Photothem. 12 (1980) 225. [9] R. Kiihlmann and W. Schnabel, Polymer 18 (1977) 1163. [lo] J.P. Fouassier and A. Merlin, J. Photochem. 12 (1980) 17. [ 111 J.C. Scaiano, J. Am. Chem. Sot. 102 (1980) 7747. [12] K.A. Abdullah and T.J. Kemp, J. Photochem. 32 (1986) 49. [ 131 N.J. Turro, Molecular photochemistry (Benjamin, New York, 1978).
TIME-RESOLVED
27 March 1987
CHEMICAL PHYSICS LETTERS
LASER-PUMPED
DYE LASER SPECTROSCOPY:
ENERGY AND ELECTRON TRANSFER IN KETONES
J.P. FOUASSIER,
D.J. LOUGNOT,
A. PAVERNE
and F. WIEDER
Laboratoire de Photochimie G&n&ale, Unit&eAssocitfe au CNRS No. 431, Ecole Nationale Supkrieure de Chimie, 3, Rue Alfred Werner, 68093 Mulhouse Cedex, France
Received 28 October 1986; in final form 20 January 1987
The synergic effects which are generally invoked to account for the specific features of a system of two ketones used as polymerization photoinitiators are reconsidered. The increase in reactivity observed when mixing these two initiators is reinterpreted in terms of a simultaneous energy and electron transfer in the pair. The relative efficiencies of these processes depend on the energy gap between the triplet states involved, which is known to be influenced by the polarity of the medium.
1. Introduction
Time-resolved laser spectroscopy is a convenient method to study directly the temporal behaviour of excited chromophores. The irradiation source is usually a solid (YAG ruby) or a gas (nitrogen excimer) laser and as a consequence, the available wavelengths of excitation are.rather limited. In order to extend the feasibility of this technique to other excitation wavelengths, one can resort to the emission laser-pumped dye laser (LPDL) . Its spectral characteristics are obviously dependent on the laser dye used: thus, tunable wavelengths are available over the whole visible part of the spectrum. Most of the time, due to the rather broad electronic absorption spectra of the emitting species, a LPDL device is not required. However, one may be faced with specific problems demanding a very selective excitation in a mixture of different molecules with closely related absorption spectra. The present paper deals with the description of such a technique and its use in the investigation of the excitation exchange (energy and electron transfer) between two molecules used as polymerization photoinitiators. ETX, a substituted thioxanthone ( 3-carboxyethyl7-ethylthioxanthone) and TPMK, a substituted acetophenone (2-methyl-l-( 4-methylthiophenyl)-2morpholinopropan1-one ) are both efficient pho30
toinitiators for the polymerization of acrylic monomers. It was recently shown that mixing ETX (in the absence of any H or e- donor) with TPMK extends the photosensitivity towards the visible part of the spectrum [ 11.
H.8
TPnK
Such a procedure involving a mixture of several photoinitiators often enhances the efftciency of the initiation process and up to now, synergetic effects were generally invoked to explain this behaviour [ 21. The present investigation attempts to clarify our understanding of the exchange of excitation energy which takes place between TPMK and ETX and the role of synergism will be made explicit.
2. Experimental 2.1. Apparatus Our laser photolysis equipment has been built following basic ideas used by several authors for the development of very sophisticated devices. Two Quantel sources are commonly used as the excitation source: a ruby laser working in the mode-locked 0 009-2614/87/$ (North-Holland
03.50 0 Elsevier Science Publishers Physics Publishing Division)
B.V.
CHEMICAL PHYSICS LETTERS
Volume 135, number 1,2 secondary laser output
Ampll
PUMP
LASER
Fig. 1.Nd/YAG-pumped dye laser device.
regime and a neodymium-YAG Q-switched by a Pockels cell. The first laser [ 31 produces a train of 5 to 7 ps pulses (300 ps fwhm) of up to 150 mJ separated by 10 ns at 694.3 nm. By passing through a KDP frequency-doubling crystal this emission can be partly converted into a UV beam of light (347.1 nm) which exhibits the same temporal characteristics as the parent red beam. The second source produces a 3 ns pulse of up to 300 mJ at 1.06 pm. In the same way, the use of KDP crystals allows this emission to be converted to secondary emissions at 532 nm (120 mJ) and at 355 nm (40 mJ). The pulses are focused on the samples by a system of spherical lenses. In some cases, when the wavelengths of the excitation pulses which are directly available from the Nd/YAG or ruby lasers are not adapted to the requirements of a specific experiment, a laserpumped dye laser is used. This system - a schematic of which is shown in fig. 1 - has been built (SOPRA) according to the following ideas: the pulse corresponding to the second (532 nm) or third (355 nm) harmonic of the Nd/YAG laser is split into two parts; the first part (about 10%) is focused by a system of cylindric lenses onto a Suprasil cell in which a fluorescent dye circulates. This cell is placed into a short cavity resonator and the laser emission which escapes from this oscillator is collected and directed by a system of mirrors to a second laser cell which is used as an amplification stage. The different elements of this system have been arranged so as to produce an optical delay between excitation and emission: the laser beam travelling from the oscillator which is used to stimulate the emission in the second cell must arrive
27 March 1987
just as the population inversion reaches its maximum. The output yield of this system is dependent upon the laser dye excited: when the third harmonic is used as the pumping source with coumarin 420 in methanol, a 10% conversion is achieved - about 4 mJ are available at 430 nm - whereas about 25 mJ can be obtained at 600 nm when a rhodamine dye is pumped by the second harmonic. The monitoring system consists of a 450 W xenon arc, a Huet M 25 monochromator and a R 212 Hamamatsu photomultiplier at the detection end; only six dynodes are used. The intensity of the monitoring source is increased during the measurement by discharging a capacitor bank through its power supply for a short and adjustable period of time; this light pulse starts 100 t.tsbefore the laser pulse reaches the sample. The samples are contained in 10x 10 mm* Suprasil tubing. The monitoring and excitation beams are arranged orthogonally and the excited volume is limited by a set of rectangular diaphragms. The electric signal delivered by the photomultiplier is displayed onto the screen of a wide band memory scope (Tektronix 7834); it is then read by a video camera, numerized and stored. A Texas Instrument microcomputer controls the experiment, gathers the data, processes the information, produces suitable tiles on storage devices and provides graphics and other useful information. Several synchronizers activate the different units (shutters, pulser, lasers) with suitable delays. The risetime of the detection system is x 2 ns when terminated into 50 n and the actual “window” in which measurements can be carried out is from 10 ns to 200 ps. 2.2. Materials The compounds studied ETX and TPMK were obtained from Ciba Geigy Base1 [ 1] and coumarin 420 was purchased from Exciton. All the chemicals (Janssen Chimica) were used as received. The UV spectra of ETX and TPMK are reported in fig. 2; they show intense absorption around 400 and 3 10 nm respectively.
31
OD
E =I8600
27 March 1987
CHEMICAL PHYSICS LETTERS
Volume 135, number 1,2
M-‘cm-’
ETX
A
hv
‘ETX
__t
3ETX tit
I
’ IEii
i$
H+ +
I ._ 3[ETX
.+ N;]
K’:‘=K’
+
Nt pi-
Fig. 2. Absorption spectra of ETX and TPMK in methanol.
3. Results
In the case of TPMK, a fast cleavage occurs in a short-lived triplet state, generating a methyl thiobenzoyl morpholino isopropyl radical pair (which undergoes disproportionation or separation as shown by CIDNP measurements [ 1I): TPtlK
hv
-----+
‘TPtlK
3 TPIIK
+
3.1. Primary processes in ETX and TPMK
1 CH3-S
The two photoinitiators ETX and TPMK exhibit the usual behaviour of thioxanthone derivatives [ 4-71 and aryl alkyl [ 81 or aryl aryl [ 9,101 ketones. The triplet state of ETX is long lived in deaerated solution (r > 10 us); the absorption maximum of the triplet-triplet transition is blue-shifted when going from non-polar (A,,, = 670 nm in toluene) to polar solution (A,,, = 625 nm in methanol); this could be readily understood in terms of a change of electronic configuration of the lowest triplet state (as reported in the case of xanthone [ 111) or by a decrease of the dipole moment (in the triplet manifold) upon photoexcitation (as in the cases of xanthone, thioxanthone or methylacridone [ 121). The fluorescence intensity of ETX increases and is red-shifted with increasing solvent polarity; a similar behaviour is found with thioxanthone, a fact which supports the view of either an increase of the dipole moment (in the singlet manifold) upon excitation or an inversion of the relative position of the nrc* and xx* states. Fluorescence and triplet absorption are quenched when methyldiethanol amine (MDER) is added to deoxygenated or aerated solutions of ETX. A new long-lived absorption, assigned to the thioxanthonederived ketyl radical K’, is detected between 400 and 750 nm; the red portion of the transient absorption spectrum contains an important contribution from the radical anion KT :
32
-0
(-J
H/ escape
CH3m
c*
‘hi
0
e CH3-S
u
0
1 k”3-J CM2 II A pl + c-n C”,y” 0
3.2. Interaction between ETX and TPMK
The interaction between the excited states of ETX and TPMK as well as between morpholine and thio anisole - used as models of the TPMK moieties were investigated by LPDL spectroscopy. Figs. 3 and 4 clearly show that the thioether group and the morpholino moiety of TPMK are responsible for the interaction with the S, and T1 states of ETX, respectively. The triplet quenching rate constant is affected by the solvent or by changing ETX to ITX (2-isopropylthioxanthone) as observed in table lwhere the rate constants of electron transfer with morpholine (k,) and of excitation transfer (k,) are reported. It is worth noting that K’ radicals are only formed on excitation of ETX/TPMK in toluene and not in the other systems. Fluorescence quenching will not be taken into consideration since it yields a singlet charge transfer complex which mainly undergoes deactivation through an electron back-transfer process.
Volume 135, number 1,2
CHEMICAL PHYSICS LETTERS
4. Discussion
I
3
I
27 March 1987
1
Morphollne _o--
1
, lO’[Q]
2
1
3
Fig. 3. Fluorescence quenching of ETX in aerated methanol. A,.,=375 nm; As..=470 nm. [Q] is in mole/B.
k x lo+ s-’ 6-
6-
3 3
6
9
12
, 103[al 16
Fig. 4. Triplet state quenching of ETX in methanol at 1= 625 nm. [Q ] is in mole/!?.
Table 1 Rate constant (M-’ s-‘) for electron transfer in ETX and ITX in the presence of morpholine in aerated toluene and methanol solutions. Rate constant of excitation transfer in the presence of TPMK
ETX ITX
10-s%, morpholine
1O-8k,, TPMK
toluene
methanol
toluene
methanol
6.3 12
3.1 1.1
0.12 0.6
1.1 0.55
The rate constants for electron transfer (k,) between thioxanthones (ETX and ITX) and morpholine decrease when toluene is replaced by methanol; investigation of a series of solvents shows that k, decreases as the solvent polarity increases (as defined by the ET Dimroth parameter). The solvent effect on the rate constants k, is much more pronounced in the case of ITX than with ETX. On the other hand, a strong increase of the reaction rate constant (k,) between ETX and TPMK (and not between ITX and TPMK) is observed on increasing the solvent polarity although the contribution of the electron transfer process (e.g. k, between ETX and morpholine) to the overall effect is considerably less affected. These results strongly suggest that the change in the interaction rate of the triplet state of ETX with TPMK as a function of the solvent cannot be accounted for solely on the basis of ability to undergo electron transfer: if this were the case, the kq would be almost solvent independent for the system ETX/TPMK and strongly affected for the system ITX/TPMK. The results reported in table 1 are at variance with this model. A possible way out of this discrepancy is to introduce the idea of an energy transfer process competing with the electron transfer, the relative efficiencies of these processes being dependent on the stabilization of the lowest excited state by solvation and on the relative positions of the triplet energy levels of TPMK, ETX and ITX (61 f. 1; 58.4; 61.4 kcal mol- ’ respectively in EPA glasses at 77 K [ 1] ) . It is thus inferred that, in toluene, the excitation transfer occurs mainly through electron transfer for ETX and energy transfer for ITX whereas, in methanol, energy transfer would predominate for both ITX and ETX. The energy transfer rate constant is a function of the energy gap between the donor and the acceptor: thus, this process is expected to be favoured for ITX relative to ETX in toluene. If the change in reactivity is assumed to reflect the Boltzmann distribution between the nx* and rccx*levels of the donor [ 111, it is evident that a more or less important mixing of these two states will influence the interaction rate constant. The nrt * triplet state of ETX (or ITX) is expected 33
CHEMICAL PHYSICS LETTERS
Volume 135, number 1,2 ETX E Methanol
TOlU~fle
f
27 March 1987
processes involving its lowest triplet state should be solvent dependent. The reality of the excitation exchange between ETX and TPMK is demonstrated by the fact that a polymerization can be initiated under laser irrradiation at 440 nm (a wavelength which is only absorbed by ETX) in the presence of ETX/TPMK and not in the presence of ETX alone. The relative quantum yield of polymerization obtained is similar to that of the system ETX/morpholine (at 1= 363 nm under cw Ar+ laser) but considerably less than that of TPMK at the same irradiation wavelength. This observation is in good agreement with the assumption of an electron transfer process predominating in non-polar media. A thorough investigation of this question will be the subject of a forthcoming paper.
Scheme 1.
Acknowledgement
to be the lowest lying state in non-polar solvents and to become more and more stabilized (scheme 1) to the point of mixing with the roll:* state as the solvent becomes more polar (by analogy with the case of xanthone [ 111 which behaves similarly, we suggest that the energy gap between the nrc* state in toluene and the AX* state in methanol accounts for most of the spectral shift of the triplet-triplet absorption recorded in these solvents). ‘Likewise, the apparent change of k, as a function of the solvent polarity is accounted for by a change of reactivity of the lowest state in an electron transfer process since mt* states are known to be more easily photoreducible than xx* states: this statement is consistent with the k, values measured in toluene and in methanol (table 1). The energy of the lowest excited state of TPMK is also expected to be solvent sensitive. Acetophenone possesses an nlc* state in non-polar solvents and a IF A* state in polar solvent [ 131 whereas 4-CH30acetophenone has a xx* lowest triplet state in any solvent presumably because of a substantial lowering of the XR* level due to the presence of the methoxy substituent. Thus, in TPMK, the thioether group can be expected to induce strong modifications in the energy levels of the benzoyl chromophore; consequently solvent effects are to be expected in this product and hence the rate of the energy transfer
34
Drs. G. Berner and W. Rutsch (Ciba Geigy Basel) are greatly acknowledged for supplying the samples used in this work.
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