Time-resolved laser spectroscopy of the sensitized photolysis of iodonium salts

Journal of Photochemistry

and Photobiology,

A:

Chemistry,

49 (1989)

TIME-RESOLVED LASER SPECTROSCOPY SENSITIZED PHOTOLYSIS OF IODONIUM J. P. FOUASSIER

317

- 324

317

OF THE SALTS

and D. BURR

Laboratoire de Photochimie Ge’ne’rale, Unite’ de Recherche Associe’e au CNRS 431, Ecole Nationale Supe’rieure de Chimie, 3 rue A. Werner, 68093 M&house Cedex (France) J. V. CRIVELLO Rensselaer (Received

Polytechnic

Institute,

Troy,

NY

12180-3590

(U.S.A.)

March 30, 1989)

summary Laser flash photolysis of thioxanthone derivatives in the presence of substituted iodonium salts gives rise to an electron transfer process. Rate constants are measured and the cation radical of the thioxanthone is determined. Other results obtained with anthracene, pyrene, benzophenone and ruthenium tris-2,2 ‘-bipyridine are discussed.

1. Introduction Recent papers reviewed the behaviour of onium salts as photoinitiators of cationic polymerization [ 1, 21, Such compounds, e.g. diaryliodonium salts, can initiate the reaction through the generation of a proton under light exposure. The chief mechanism proposed for acid production in the direct decomposition of diaryliodonium salts involves a homolytic bond cleavage yielding an aryliodonium radical cation and a hydrogen abstraction on the solvent or the monomer, leading to a protonated aryl iodide which acts as a strong Bronsted acid and may dissociate into acid species [2 - 41. Photosensitization of the diphenyliodonium salt decomposition occurs mostly through electron transfer: this has been observed in the case of photosensitizers, P, such as dyes, hydrocarbons and ketones ]1,2] : hv P +P*

+JP/yq+1*

Pt + $21’

(1) P + #)*I+*

A recent paper [5] explored the anthracene-sensitized photolysis of diphenyliodonium salts, using the techniques of steady state and laser flash photolysis. In the presence of ketones, the quenching of the phosphoreslOlO-6030/89/$3.50

@ Elsevier Sequoia/Printed

in The Netherlands

318

cence produced by acetophenone and benzophenone BP has been attributed to energy transfer [l], and was not clarified in the case of xanthone and thioxanthone. The role of the ketyl radical of benzophenone as an electron donor has been outlined [ 1,2] : 3BP + RH -

@-d-G

+ R’

8H

(2)

I

H+ + BP

The present paper is concerned with the photoinduced electron transfer between ruthenium tris-2,2’-bipyridine (Ru) and different photosensitizers in their triplet state: pyrene (Py), benzophenone (BP), chlorothioxanthone (CTX), 2-isopropylthioxanthone (ITX) and 4-substituted diaryliodonium salts, On+ 1 - 4 (the counterion being AsF,):

2. Experimental details The original iodonium salts 1 - 4 were prepared as described in ref. 2. The photosensitizers Py, BP, CTX, ITX and Ru are commercial products. A full description of the time-resolved laser spectroscopy apparatus has been published 161. Short light pulses (3 ns) at 355 nm were produced by an Nd-YAG laser. 3. Results and discussion 3.1. Excited state processes in the presence of CTX Laser excitation of CTX in methanol usually results in the generation of a long-lived triplet state (microsecond range) [7 - 91 (Fig. 1). The triplet

319

0 D (a.u.1

0.5

Fig. 1. Transient absorption spectra of CTX in methanol in the presence of 1: triplet state 3CTX and radical cation CTXZ; absorption spectrum of the ketyl radical of CTX in presence of methyldiethanolamine.

absorption spectrum exhibits a maximum in the range 600 - 640 nm, depending on the solvent used. Electron donors, such as amines, efficiently quench the triplet state and the usual ground state ketyl radical absorption is recorded. Addition of iodonium salts 1 - 4 to CTX in methanol shortens the triplet state lifetime to a very large extent (Fig. 2); the calculated bimolecular quenching rate constants k, are shown in Table 1. Substituting acetonitrile for methanol does not result in a marked variation in k,: this demonstrates that, in a given hydrogendonating medium (which improves the 1O-6 k

(s-l 1

104x[On+l M

I

0

I

10

Fig. 2. Stern-Volmer l-4;X=605nm.

30

20

plot

for the triplet

state

deactivation

of CTX

in the presence

of

320 TABLE

1

Rate constants (in 10’ M-’ s-l ) of deactivation of the excited states of different sensitizers by onium salts 1 - 4 in methanol (M) and acetonitrile (AN) 2

3

photo-

4

Solvent

I

31TX

M

250

33

80

28

3CTX

M AN

400 780

67 210

80 360

36 250

3pY

M AN

130 160

6.7 45

12.5 100

3BP

M

500

12

400

100

Ru

M

41

12

13

11

10 33

efficiency of the cationic polymerization), highly efficient electron transfer can be achieved. As revealed by CTX and ITX, commonly used in photosensitized polymerizations, substitution on the thioxanthone skeleton induces only slight changes in the k, values. Quenching of the triplet absorption also results in the appearance of a new absorption band in the 400 - 500 nm region (Fig. 1). This species is ascribed to the radical cation of the thioxanthone derivative CTXt, which would support the view of an electron transfer process from the ketone to the iodonium salt. In actual fact, this species does not appear during the quenching by amines; its absorption increases with [On+]. It cannot be attributed to a transient species or a photolysis product of the On+, since it is not detected in the presence of other sensitizers, such as BP and Ru. The ability of chlorothioxanthone to transfer an electron is also substantiated by the fact that methylviologen MV2+ deactivates the ketone triplet state and forms the cation radical MV?, clearly identified by its wellknown absorption at X = 600 nm [lo]. The rate constant of electron transfer measured in this case (8 X 10’ M-l s-l) compares well with the values reported for other ketones [ 10 - 131. 3.2. Interactions between photosensitizers and I - 4 Formation of the pyrene radical cation during the quenching of the pyrene triplet state by diaryliodonium cations is detected at X = 450 nm, the position of the transient absorption spectrum (Fig. 3) being in good agreement with the known Pyt absorption maximum [ 14,151. For comparison, in the presence of anthracene (An), the rate of triplet state quenching is very low (
321 0 D (a.uJ

Fig. 3. Transient absorption triplet state by 1 in methanol.

spectrum

observed

during the deactivation

of the pyrene

eration of the well-known long-lived (0.72 ps in methanol) emitting transient state [ 16 - 181 that is quenched by 1 - 4, as well as by methylviologen. The interaction between excited BP and 1 - 4 leads to a considerable shortening of its triplet state lifetime. The absence of generation of a new transient species (the benzophenone radical cation) around 700 nm {where PB+, formed through pulsed radiolysis [19], was shown to absorb) could be interpreted as a lack of electron transfer process and may support the view of an energy transfer process. This deduction, however, must be considered with caution: in water-soluble benzophenones/MV2+ where electron transfer does occur (the characteristic absorption of MVt is recorded), the cation radical of the benzophenone derivative is also not observed [20]. To the best of our knowledge, no BPt absorption spectrum has been recorded so far through laser spectroscopy. The rate constants of deactivation, k,, measured for the excited state of the photosensitizer, are listed in Table 1. In the presence of 3TXs, 3Py and Ru, they are significantly higher for 1. On the contrary, there is little difference between the rate constants for quenching of 3BP by iodonium salts 1 and 3. 3.3. Electron vs. energy transfer The free energy change AG and Weller equation [21] AG = E(D/D+)

- E(A-/A)

-

C -

can be calculated according to the Rehm

AE.,

322

where E(D/Dt) stands for the oxidation potential of the donor, E(AT/A) the reduction potential of the acceptor, AE,, the electronic energy corresponding to the excited state of the donor and C for the coulombic interaction between the radical ions. Considering that E 1, 2red for On+ measured in ref. 28 is of the same order (Table 2) as the corresponding value of the unsubstituted diphenyliodonium salt (although one would expect that the dinitro-substituted iodonium salt 1 would have a substantially greater El,zred than the other derivatives), it is apparent that the calculated AG values are negative and are very similar in magnitude for a given photosensitizer (Table 3). In addition, for 3TXs, 3Py and Ru, an energy transfer process can be ruled out, owing to the high triplet energy (estimated to be around 270 - 300 kJ mol-’ for &I+ [l, 41. As a consequence, the interaction between these photosensitizers and 1 - 4 should be thought of as involving an electron transfer process. The electron transfer also is favourable in ‘Py as well as TABLE

2

Oxidation potential, reduction potential and excited state energy for the different pounds studied; ground state absorption maxima h, of 1 - 4 in methanol PY

E 1nox %zred

W) 0’)

A.&,,0(kJ

mol-I)

I

2

3

4

-l-62

-0.2

-0.186

-0.14

-0.192

[271

1283

1281

1281

1283

215

246

264

240

[21

[21

121

[21

BP

GTXa

Ru

1.25

2.7b

1.7

1.23

1221

1231

~241

[251

-2.45

-l.72

1261

1273

201.5

289.2

275.9

200.6

1291

f291

1291

iI81

A, (nm)

aThese values referred to TX since they are unknown for CTX. bThe ionization potential (9.45 eV for BP) was converted into redox potential by means of E, ,zox = 0.89IP - 5.7 (as proposed in ref. 30 and used in ref. 1).

TABLE

us. SCE

3

Calculated AG values (in kJ mol-l) for the electron transfer process between different sensitizers; in this calculation, the term C has been disregarded

3CTX 3PY 3BP Ru aThese text).

com-

AG

should

1 - 4 and

Ia

2

3

4

-92 -62 -10 -63

-94 -63 -11 -64

-99 -67 -15.5 -68.5

-94 -63 -10 -63.5

be considered

with caution

owing to the value of E 1,2red for l (see

323

in ‘An (AG = -193 kJ mol-’ for &I+AsF6- [l]) and ‘Pe (perylene; AG = --170 kJ mol-’ [l]). The situation is less favourable for ketones in their -117 kJ mol-’ for acetophenone, thiotriplet state (AG = -10, -93, xanthone and xanthone respectively [ 11). For the time being, neither the high value of k, for CTX/l nor the significantly lower values for CTX/2 - 4 can be accounted for by using the published values of E,,zred determined by polarography 1281. However, preliminary data obtained through cyclic voltametry clearly suggest that E 1,2red for 1 is larger than for 2 - 4 : consequently, as expected, the higher the E1,2red for the onium salt, the more efficient it should be with respect to electron transfer quenching of the excited photosensitizer. In the case of BP, the absence of any cationic radical absorption, the high rate constant for the triplet deactivation and the very weak AG (= -9 kJ mol-‘) may support the view of an energy transfer process (as proposed in ref. 1). Whatever its nature,. this process is detrimental to the formation of the ketyl radical: e.g. an equal efficiency is expected for thse two competitive processes in the presence of a hydrogen donor (5 M) and 1 (10e3 M). Generation of the resulting substituted diaryliodonium radical and the cationic radical P+ on the photosensitizer governs the initiation efficiency of the polymerization: the possible back electron transfer process could be significant. Both this phenomenon and the reactivity of Pt or the BrBnsted acid release can explain why the extent of the polymerization does not correlate systematically with k,. This more specific point will be discussed later on.

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