Acylgermanes: Excited state processes and reactivity

Chemical Physics Letters 469 (2009) 298–303

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Acylgermanes: Excited state processes and reactivity J. Lalevée *, X. Allonas, J.P. Fouassier Department of Photochemistry, UMR 7525 CNRS, University of Haute Alsace, ENSCMu, 3 rue Alfred Werner, 68093 Mulhouse Cedex, France

a r t i c l e

i n f o

Article history: Received 20 November 2008 In final form 30 December 2008 Available online 4 January 2009

a b s t r a c t The excited state processes of an acetyltriphenylgermane as well as the generation and the reactivity of the germyl radical are studied by laser flash photolysis, ESR spin trapping experiments and MO calculations. The transient spectra, the dissociation quantum yield, the singlet excited state S1 lifetime, the S1 quenching by an iodonium salt, the triplet state energy level ET, the Ge–C bond dissociation energy BDE, the interaction of the germyl radical with oxygen, double bonds and an iodonium salt, and the ESR spectra are determined. A triplet cleavage process is clearly evidenced; a singlet cleavage should likely contribute. The overall reactivity is discussed and compared to that of parent compounds. The acetyltriphenylgermane behaves as a high performance photoinitiator in the free radical polymerization and free radical promoted cationic polymerizations. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction The search for new photoinitiators (PI) and co-initiator (coI) for radical polymerization exhibiting enhanced reactivity remains of great interest [1–4]. In this context, for example, new efficient PI and PI/coI systems based on sulfur, silyl or germyl radical chemistry (S, Si, Ge) were recently proposed by us [5–10] and the excited state processes have been investigated in various S–S [5], Si–Si [7a], S–Si [7b], Si–C [7b] cleavable PI systems or PI/coI combinations where coI stand for a thiol S–H [5], a silane Si–H [8,9], a germane Ge–H [10]. The S and Si chemistry is well documented (see e.g. [11] and references therein). A better knowledge of the Ge radical chemistry and the search for another source of germyl radicals are still of interest. Indeed, scarce data are available on C–Ge bond containing structures: the steady state photolysis was carried out a long time ago [12]; the fluorescence properties and the T1 state reactivity of benzoyl trialkyl phenyl germane (b1), benzoyl triphenyl germane (b2) and benzoyl dialkyl phenyl germane (b3) shown in Scheme 1 were studied [13]. The formation and the spectral characterization of Ge was provided but the reactivity of this radical was not checked [14]. Very recently, cleavable C–Ge bond containing acylgermanes (more specifically b1) were shown to play [15–17] a promising role as PIs; the steady state photolysis of compound b1 was reported [15,16]. All these facts prompted us to deeply investigate the excited states processes and the germyl radical formation/ reactivity of a different and carefully selected, commercially available, acylgermane representative a (Scheme 1) through laser flash photolysis (LFP), Electron Spin Resonance spin trapping experiments (ESR-ST) and molecular orbital calculations. * Corresponding author. Fax: +33 3 89 33 68 95. E-mail address: [email protected] (J. Lalevée). 0009-2614/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2008.12.096

Compound a strongly differs from the molecules b1, b2 and b3 studied in [13] and b1 proposed as PI [15,16]: the benzoyl was changed for an acetyl group and the three alkyl for three phenyl substituents. Both higher excited state energy levels and higher stabilization of the germyl radical are expected. As a consequence, the dissociation process should be enhanced and a high photoinitiation ability of a in free radical polymerization (FRP) and free radical promoted cationic polymerizations (FRPCP) should be observed. The reactivity and the efficiency of a will be checked here and the reactivity difference of a vs. b1, b2, b3 will be discussed.

2. Experimental section and computational procedure 2.1. Samples Compounds a (acetyltriphenylgermane), 2,20 -dimethoxyphenyl acetophenone (DMPA) (used here as a reference photoinitiator) and diaryliodonium hexafluorophosphate (U2I+) were obtained from Aldrich. In the case of liquid monomers (vinyl ethyl ether VE, vinyl acetate VA, methyl acrylate MA and acrylonitrile AN – purchased from Aldrich), the stabilizer (hydroquinone methylether-HQME) was removed by column purification (Aldrich AL-154). For the FRPCP experiments, a di-(cycloaliphatic epoxide) (CE) monomer (Cyracure 6110 from Dow) was used. For the FRP experiments, a bulk oligomer/monomer formulation based on 75/25 w/w epoxyacrylate/tripropyleneglycoldiacrylate (Ebecryl 605 from Cytec) was selected. 2.2. Computational procedure Molecular orbital calculations were carried out with the GAUSSIAN 03 suite of programs [18]. The triplet energy level (ET) and the bond

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O

Ph Ge Ph Ph

a

O

O Ge

Ph

Ph

Ph Ge Ph Ph

O Ph

b2

b1

Ge

Ph

b3

Scheme 1.

dissociation energy (BDE) were calculated at the UB3LYP/6-31+G* level. The different optimized geometries were frequency checked.

800

2.3. Laser flash photolysis

600

2.4. Fluorescence properties

ε (M-1cm-1)

Nanosecond laser flash photolysis (LFP) experiments were carried out using a Q-switched nanosecond Nd/YAG laser (kexc = 266 nm, 9 ns FWHM pulses; energy reduced down to 10 mJ) from Continuum (Powerlite 9010) and an analyzing system consisting of a pulsed xenon lamp, a monochromator, a fast photomultiplier and a transient digitizer [19].

400

200

0 300

The steady state and time resolved fluorescence properties were examined with a Fluoromax-4 (Jobin-Yvon-Horiba) apparatus using a nano LED emitting at 340 nm as the excitation source and a time-correlated single-photon counting (TCSPC) accessory for the lifetime determinations.

b

a

400

500

λ (nm)

LUMO

2.5. ESR spin trapping experiments The ESR spin trapping experiments were carried out using a X-Band spectrometer (MS 200 Magnettech). This technique is efficient for the identification of radicals [20]. The radicals generated under the light irradiation (Xe–Hg lamp (Hamamatsu, L8252, 150 W; k > 310 nm) were trapped by phenyl-N-tbutylnitrone (PBN). The ESR spectra simulations were carried out with WINSIM software [21].

HOMO

2.6. Polymerization experiments In film FRP experiments, the laminated or aerated films (50 lm thick) deposited on a BaF2 pellet were irradiated with a polychromatic irradiation (incident light intensity: I0  10 mW cm2) delivered by a Xe–Hg lamp (Hamamatsu, L8252, 150 W) and filtered by a glass window (k > 300 nm) or a monochromatic irradiation (Xe– Hg lamp, 366 nm, I  0.2 mW cm2). The evolution of the double bond content was continuously followed by real time FTIR spectroscopy at about 1640 cm1 (Nexus 870, Nicolet) as described in [5,22]. The rates of polymerization Rp were calculated from the linear part of the conversion vs. time curves and can be corrected to take into account the amount of absorbed light Iabs as done in detail in [7a,7b]. In FRPCP experiments, the evolution of the epoxy group content is continuously followed by real time FTIR at about 800 cm1 [8]. 3. Results and discussion 3.1. Photochemical properties of a The UV absorption spectrum of a, depicted in Fig. 1A, presents a maximum at about 360 nm. The extinction coefficient at 366 nm is also quite high (e366 = 710 M1 cm1) compared to that of a benzoin ether derivative DMPA (e366 = 217 M1 cm1). This spectrum is characteristic for the acylgermane structure [12,13] and exhibits a np* character. The observed vibrational structure (m  1600

Fig. 1. (A) Absorption (a) and fluorescence (b) spectra of a in acetonitrile. (B) Orbitals involved in the electronic transition (HOMO–LUMO; the Ge atom is indicated by an arrow).

cm1), typical for a carbonyl group, is also in agreement with this np* character. The red shift of the transition (by about 80 nm) due to the carbonyl–Ge substitution compared to that of classical alkylketones is ascribed to the participation of the d orbitals of Ge as evidenced in Fig. 1B. This delocalization also explains the measured extinction coefficients higher than for the pure np* transition of usual carbonyl groups [23]. Very interestingly, a is characterized by a much better absorption at 366 nm than b1, b2 and b3 [13b]. For example, for a and b1, the extinction coefficients are 710 and 37 M1 cm1, respectively [16]. For b1, b2 and b3, however, the maximum absorption spreads over the visible region (415, 420, 420 with e = 147, 223, 321 M1 cm1, respectively) [13b]. The aromatic substituents on Ge lead to higher e. This is ascribed to a partial pp* character, as evidenced in Fig. 1B for a, due the participation of the p system to the HOMO and LUMO orbitals. As for b1, b2 and b3 [13b], the fluorescence spectrum of a corresponds to the mirror image of the absorption (Fig. 2). A vibrational structure is also noted with m  1600 cm1. A fluorescence lifetime of 33 ns +/ 4 ns is determined by time resolved fluores-

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J. Lalevée et al. / Chemical Physics Letters 469 (2009) 298–303 0.20

0.03

0.02

ΔOD

Intensity

0.15

0.10

0.05

0.08

0.01

0.00 50

100

ΔOD

0

0.00 0

200

400

150

200

Time (μs) 0.04

600

Time (ns) 4

I0 /I

0.00 350

400

450

500

550

λ (nm)

2

1600000

0.12

1 0.00

1200000

0.01

0.02

[Ph2I+] M 0.08

ΔOD

Intensity (a.u.)

300

3

800000

0.04 400000

0 350

0.00 400

450

500

550

λ (nm)

0

100

300

400

0.018

ΔOD

cence (Fig. 2): this value is longer than those reported for b1, b2 and b3. The excitation in the S0–S2 band leads to a similar emission spectrum to that obtained for the S0–S1 band: this demonstrates that, in contrast to b1, b2 and b3, no fluorescence arises from S2 [13a]. The S1 excited singlet state is very efficiently quenched by an electrophilic structure such as U2I+ with a rate constant of 4.5 109 M1 s1 as determined by the usual Stern Volmer plot (Fig. 2). The fluorescence quantum yield is low (Ufluo < 103).

200

Time (ns)

Fig. 2. (A) Fluorescence decay of a at 420 nm in acetonitrile. (B) Fluorescence quenching of a by U2I+ in acetonitrile. Insert: Stern–Volmer plot.

0.009

3.2. Laser flash photolysis experiments The laser excitation of a at 266 nm leads to the transient absorption spectrum (maximum absorption close to 330 nm) shown in Fig. 3. As this spectrum is similar to that found for the triphenylgermyl radical (Ph3Ge) [10,13a,14], the transient is ascribed to the germyl radical. The rise time for Ph3Ge (29 ns +/ 5 ns) is quite close to the S1 lifetime (Fig. 3): this behavior usually suggests a singlet cleavage. The decay of the radical corresponds to a second order kinetic with a rate constant kr/e where kr is the recombination rate constant (kr/e = 7.4  105 s1; k = 330 nm). Considering that kr is equal to the diffusion rate constant corrected by the spin multiplicity factor as usually done [24], a e molar extinction coefficient of 4050 M1 cm1 is calculated at 330 nm. Using the triplet state of benzophenone (BP) as an actinometer allows a calculation (Eq. (1)) of the overall dissociation quantum yield (Udiss) where eBP and ePh3 Ge are the molar extinction coefficients of the BP triplet state at 525 nm and of Ph3Ge at 330 nm, DODBP and DODPh3 Ge

0.000

0

5

10

15

Time (μs) Fig. 3. (A) Time resolved transient spectrum recorded immediately after the laser excitation of a at 266 nm in acetonitrile. Insert: decay of Ph3Ge at 330 nm. (B) Rise of Ph3Ge at 330 nm. (C) Decay of 3CQ at different [CQ] for the a/CQ system at 800 nm in acetonitrile (excitation wavelength 355 nm; the [CQ] range is 0–0.19 M).

the produced transient absorbances of the corresponding species obtained for the same amount of light absorbed [23]. Udiss was determined as 0.95 +/ 0.1 for a. On the other side, the photolysis quantum yields for b1 was determined as 0.4 [16].

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Udiss ¼ ðeBP =ePh3 Ge Þ  ðDODPh3 Ge =DODBP Þ

ð1Þ

3

No triplet state a was directly observed. However, its existence is revealed through its quenching by camphorquinone (CQ) which is a low triplet energy acceptor (ET = 51.6 kcal/mol [23]). Indeed (Fig. 3), the laser excitation of a at 355 nm leads to an increase of the optical density of 3CQ with [CQ]. From the usual Stern–Volmer treatment [25], the deduced triplet state lifetime (22 ns +/ 5 ns) is a little shorter than the fluorescence lifetime: in that case, the rise of the Ph3Ge radical will be governed by the S1 lifetime if originating from S1 or by the slowest step if formed from T1 (i.e. the ISC process; the rise time still corresponds to the S1 lifetime). As a consequence, the observed rise time of the germyl radical (see above) cannot be used here to demonstrate a S1 cleavage. The excitation of thioxanthone TX (ET = 63 kcal/mol) in the presence of a generates the germyl radical through T–T energy transfer from TX to a: this confirms the existence of 3a and the dissociation pathway from 3 a. The comparison of 3CQ formed in a/CQ vs. benzophenone(BP)/ CQ for the same light absorbed and assuming an intersystem crossing quantum yield (Uisc) of 1.0 for BP allows a rough estimation of Uisc > 0.5 for a. The dissociation yield Uc from T1 was evaluated from the TX to a energy transfer by comparing the TX triplet absorption at 650 nm and the triphenylgermyl radical absorption at 330 nm. Using an extinction coefficient of 30 000 M1 cm1 at 650 nm for TX [26], a Uc value of about 0.7 +/ 0.1 is calculated for 3a. The dissociation quantum yield UTdiss ¼ Uc Uisc in T1 is thus >0.35: the efficient dissociation pathway from T1 is clearly outlined. A cleavage from S1 can contribute ðUSdiss ¼ Udiss  UTdiss < 0:6Þ. A similar triplet cleavage was found for b1, b2, b3 from the phase patterns of the ESR spectra observed by TR-ESR [13a]. No information about the triplet state lifetimes and a possible S1 cleavage (b1, b2, b3) was available. 3.3. The ESR spin trapping experiments Upon light irradiation of a, two radicals are generated and trapped by PBN (Fig. 4). The hyperfine splittings (HFS) for both

A

B

3310

3320

3330

3340

3350

3360

B (G) Fig. 4. ESR spin trapping spectrum (A) using PBN (0.05 M) obtained under the UVlight irradiation of a (0.01 M) at k > 310 nm; simulated spectrum (B) with two adducts: (aN = 14.7; aH = 5.4 G) and (aN = 14.3; aH = 2.95 G).

the nitrogen (aN) and the hydrogen (aH) of the adducts are 14.7, 5.4 G and 14.3, 2.95 G, respectively. In agreement with the HFS reported in [27,28], these species can clearly be ascribed to a triphenylgermyl and an acyl radical (CH3C(O)). This confirms an efficient C(O)–Ge bond cleavage. Methyl radicals are not observed: the decarbonylation of this acyl radical is slow in agreement with previous data for this structure [29]. 3.4. MO calculations Using molecular orbital (MO) calculations, the ETs and BDE(Ge–C)s were calculated for a and b1: ET = 62.9, 55.2 kcal/mol and BDE(Ge–C) = 58.2, 65.0 kcal/mol, respectively. From the absorption and fluorescence spectra (Fig. 1), a singlet excited state energy level ES1  72 kcal/mol is evaluated for a: the BDE is clearly lower than ES1 and ET, supporting a possible exothermic dissociation process from both these excited states in agreement with the efficient bond cleavage reported here. For b1, an efficient cleavage process from T1 was observed [13b], albeit an endothermic character is noted here. On the other hand, due to the more red shifted absorption of b1, ES1 is lower than for a. With a higher BDE(Ge–C), the singlet dissociation process is less exothermic than for a. Both results probably also explain the lower decomposition observed for b1 [16]. It appears, as expected, that the benzoyl/acetyl and alkyl/phenyl changes on the Ge atom both lead to higher energy levels for the excited states and a higher stabilization of the germyl radical, thereby enhancing the cleavage process. 3.5. Radical reactivity The addition rate constants kadd of Ph3Ge to different double bonds were determined (Table 1). For the different investigated alkenes, the striking feature is that the kadd values are higher than for classical carbon centered radicals which were already considered as reactive structures [30]. The substrates being ranging from alkenes bearing a withdrawing substituent to electron rich alkenes, the germyl radical therefore appears as highly efficient and exhibits a rather low selectivity toward the addition process. This low selectivity was ascribed previously by us to antagonist polar and enthalpy effects [6a]. Concerning the acyl radical (not observable in our LFP set-up), no experimental data are available in the literature. From the general r character of the acyl radicals, it can be expected that this structure can efficiently add to a double bond i.e. for the benzoyl radical, the addition rate constant to MA is 105 M1 s1 (a weak effect of the phenyl/alkyl substitution can be expected) [31]. The oxygen quenching of Ph3Ge is diffusion controlled (4.5  109 M1 s1). Due to its low ionization potential (IP = 5.6 eV) [10], Ph3Ge is also easily oxidized by U2I+ (5.5  107 M1 s1). 3.6. Free radial FRP and free radial promoted cationic PRPCP photopolymerization ability Film photopolymerization experiments outline the high cleavage quantum yield for the C–Ge bond and the high reactivity of the germyl radicals. Under a polychromatic light exposure (Fig. 5

Table 1 Rate constants of the reaction of Ph3Ge (generated from a) with different additives in acetonitrile at RT. Radical

AN kadd (107 M1 s1)

MA kadd (107 M1 s1)

VA kadd (107 M1 s1)

VE kadd (107 M1 s1)

CEa kadd (107 M1 s1)

O2 kadd (109 M1 s1)

Ph2I+ PF 6 kadd (107 M1 s1)

Ph3Ge

13

22

0.49

0.32

<0.01

4.5

5.5

a

CE: di-(cycloaliphatic epoxide) monomer (Cyracure 6110 from Dow).

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J. Lalevée et al. / Chemical Physics Letters 469 (2009) 298–303

1 2

Conversion (%)

60

Table 2 Radical polymerization ability of the studied compound (0.5% w/w) in the absence/ presence of ITX. Xe–Hg lamp. Polychromatic irradiation: (I0  10 mW/cm2). Monochromatic irradiation: k = 366 nm, I0  0.2 mW/cm2 ([M0] is the initial monomer concentration). Photoinitiator (0.5% w/w)

40

20 a DMPA

0 20

40

60

Monochromatic irradiation no ITX

Rp/[M0] * 100 laminated

Rp/[M0] * 100c laminated

Rp/[M0] * 100 laminated

32.1 –

2.0 (2.85a; 1.8b) 0.7

Rp/ [M0] * 100 air 21.5 (1.5a; 1.6b) 17.5 (1.6a) 13.9 10.8

50

1

40

2

30 20 10

0 0

20

40

60

80

The Rp values are relative to DMPA. The Rp values are corrected from the amount of light absorbed (see Eq. (1)); by taking into account the respective light absorbed by the PI (integration of the lamp emission spectrum with the absorption of PI), Iabs were calculated as in a ratio 2.5 for DMPA and a for a monochromatic irradiation, respectively. c Photosensitized experiment with isopropylthioxanthone (ITX 1% w/w). b

Time (s)

Conversion (%)

Polychromatic irradiation with ITX

a

0

100

Time (s)

1 60

Conversion (%)

Polychromatic irradiation no ITX

ciency of TPO (corrected from the light absorption) compared to DMPA is 0.7 [22], it can be deduced that a is more reactive than b1. Using 2-isopropylthioxanthone (ITX) as a photosensitizer (ET  63 kcal/mol) [23], an increase of Rp is noted (Table 2). This evidences the occurrence of a triplet state cleavage after an efficient ITX to a energy transfer. In FRPCP, b1 was only tested in solution photopolymerization [17]. As seen in Fig. 5 from the film polymerization profiles of an epoxy monomer, the a/U2I+ cationic photoinitiating system is clearly much more efficient than DMPA/U2I+ (an already mentioned reference system for FRPCP [1,2,8,10,17]) with higher polymerization rates and final conversions (the initiation ability of U2I+ alone in our irradiation conditions at k > 300 nm is negligible). 4. Conclusion

2 40

20

In the present paper, the excited state processes in the acetyltriphenylgermane a were presented. The reactivity of a and three related parent compounds was discussed. Higher excited state energy levels, a higher stabilization of the germyl radicals and a higher reactivity are demonstrated for a. A remarkable ability of a to initiate both a free radical polymerization and a free radical promoted cationic polymerization processes is found. Acknowledgments

0 0

50

100

150

200

250

300

350

400

Time (s) Fig. 5. (A) Radical photopolymerization ability of a (1) and DMPA (2) (0.5% w/w in Ebecryl 605, laminated). Polychromatic irradiation. (B) Radical photopolymerization ability of a (1) and DMPA (2). 0.5% w/w in Ebecryl 605. Laminated. Monochromatic irradiation. (C) Cationic photopolymerization ability of a (1) and DMPA (2). Polychromatic irradiation. Photoinitiator/U2I+: 0.5%/1% w/w in Cyracure 6110 under air.

– Table 2), a exhibits a better efficiency in FRP than that of DMPA (Rp ratio = 1.5 and 1.6 in laminated and aerated conditions, respectively). Under a monochromatic irradiation and a low light intensity (0.2 mW/cm2), a also exhibits a much better efficiency (Rp ratio: 2.85) together with a shorter inhibition period and a tack free surface for the final polymer (even when the polymerization is carried out under air). Considering the same amount of absorbed light (Table 2), the intrinsic photochemical reactivity in laminated conditions remains better for a than for DMPA (almost two times). For comparison with b1, no experimental data are available. However, b1 exhibits a practical efficiency close to that of 2,4,6trimethylbenzoyl diphenylphosphine oxide (TPO) [16]. As the effi-

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