Photoinitiator Triplet States and Photoinitiating Radical-Monomer

Chapter 12

Downloaded by UNIV OF SYDNEY on May 3, 2015 | http://pubs.acs.org Publication Date: March 3, 2003 | doi: 10.1021/bk-2003-0847.ch012

Photoinitiator Triplet States and Photoinitiating Radical-Monomer Interactions: Investigation through Time-Resolved Photothermal Techniques Xavier Allonas, Jacques Lalevée, and Jean-Pierre Fouassier Département de Photochimie Générale, CNRS UMR 7525, Ecole Nationale Supérieure de Chimie de Mulhouse, 3 rue Alfred Werner, 68093 Mulhouse, France

This paper provides a general presentation of photothermal methods for the study of kinetic or thermodynamic properties of photopolymerization processes. It was shown that photoacoustic (PAS) and thermal lensing (TLS) spectroscopies allow the determination of the triplet quantum yields and energy levels of photoinitiators. Beyond the possibility to determine easily and accurately bond dissociation energies of coinitiators, providing important information on their reactivity, the application of PAS was extended for the first time to the study of the initiation step. A specific data treatment was developed to determine the rate constant and the enthalpy of the addition reaction of a radical onto a monomer unit.

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© 2003 American Chemical Society

In Photoinitiated Polymerization; Belfield, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

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Introduction In photoinduced polymerization reactions, the photochemical reactivity of the photoinitiator plays a deciding role towards the practical efficiency of the process, which is a very important point for industrial applications in the U V curing area (1-2). More precisely, the characteristics of the excited states as well as the processes involved in these excited states and in the intermediate species generated through secondary reactions will have a strong influence on the ability of a molecule to be a good candidate as a photoinitiator. For many years, time resolved absorption spectroscopy (TRAS) has been routinely used for the investigation of the excited state processes of photoinitiators (1-2), although a severe pitfall is that the transients must absorb the light to be optically detected. The aim of this paper is to underline some advantages of time-resolved thermal lens (TLS) and photoacoustic (PAS) spectroscopies, that can circumvent some limitations of TRAS. Measurement of the heat produced in the medium from the non-radiative processes (that results in a change of the refractive index and the generation of acoustic waves) can provide important kinetic and thermodynamic information on the system studied. TLS and PAS give access to triplet quantum yields, triplet energy levels, lifetimes, rate constants of interaction, enthalpies of radical formation and bond dissociation energy of amines (3-5). Several recent examples are briefly recalled in this paper showing the efficiency of these techniques for the investigation of triplet states and radicals reactivity. The study of the photoinitiation step of a radical polymerization reaction is shown to be possible by PAS and a specific data treatment is developed for this purpose. Through several examples, it is evidenced that both enthalpy and rate constant of initiation can be measured.

Experimental An optical parametric oscillator device pumped by a nanosecond Nd:YAG laser operating at 10 Hz allows the production of any wavelength between 225 nm and 1600 nm with an energy about 10 mJ (4). A solution of 2hydroxybenzophenone (BPOH) in acetonitrile was used as calorimetric standard (5) and also as chemical attenuator to decrease the incident pulse energy in the range 0.5 to 100 μ]. The optical density of the samples was about 0.1 for TLS and 0.2 for PAS at the excitation wavelength. Oxygen was removed by bubbling argon for 15 minutes. Experimental setups for TLS and PAS have been fully described elsewhere (3-4).


142

Triplet states studied by photothermal techniques


Excitation of absorbing species in solution results in the creation of excited states that are doomed to deactivate or to react. Photophysical deactivations occurring through non-radiative processes and exothermic photochemical reactions release heat in solution. The subsequent temperature jump causes a variation of pressure ΔΡ, leading to the generation of acoustic waves, and a variation of the solvent refractive index An that can be probed by a continuous wave laser. Therefore, the amplitude and the time evolution of the signal is intimately related to the kinetics of the heat deposited in the media and to the experimental setup. Roughly, the heat deposited can be considered as fast when arising within few nanoseconds or slow if it corresponds to microsecond processes (Figure 1).

— :

radiative processes

> : 'fast' non-radiative processes 1

> : 'slow non-radiative processes 1 : absorption, 2 : internal conversion, 3 : fluorescence, 4 : intersystem crossing.

Figure 1. Separation between fast and slow heat deposit processes. With the typical experimental setups described here, internal conversion and intersystem crossing to the triplet state are considered to be fast processes. In contrast, the deactivation of the triplet state to the ground state is considered as a slow process. In the following, fluorescence and phosphorescence will be neglected for the systems studied.

PAS methodology Figure 2 shows typical signals obtained by PAS in the case of BPOH and the afunctional photoinitiator Esacure 1001 (Lamberti Spa., eq 1) in acetonitrile and under argon. As can be seen, the amplitude of the acoustic wave corresponding to Esacure 1001 (S ) was found lower than that of BPOH (S POH)The latter is known to release the whole excitation energy in solution in less than 1 ns, and therefore was used as a calorimetric reference (5). On the contrary, Esacure 1001 gave rise to a triplet state that deactivated slowly in solution, and therefore the fast heat deposition resulted in a lower acoustic wave. PI


B

143

d)

c—γ—so

2


CH

2

4 time (μβ)

3

6

Figure 2. Typical PAS signal obtained in the case of BPOH and Esacure 1001 in argon saturated acetonitrile solution. The amplitude of the acoustic waves are related to the photophysical parameters of the triplet state through the following expression: F

φ · ET = Av I Γ

S PI SBPOH

Λ

(2)

Ù

where E and φτ· stands respectively for the energy level and the quantum yield of triplet state (corresponding generally to the quantum yield of intersystem crossing). In order to avoid biphotonic absorption, photothermal experiments are performed at very low pump intensity. Therefore, the most accuratetyi*E value is deduced by extrapolation at zero pimp intensity. If E is known, φτ- can be easily deduced. A value of φ = 0.78±0.02 was found for Esacure 1001, in good agreement with the literature (6). T

T

T

Γ

TLS methodology The analysis of the thermal lens signal obtained under direct excitation of the sample can also allows the determination of the φτ- values of photoinitiators. Figure 3 shows a typical signal obtained in the case of camphorquinone in acetonitrile.


144


0

5 10 time (μβ)

15

Figure 3. TLS signal obtainedfor camphorquinone in argon saturated acetonitrile solution and bestfirst-orderfit

The time-dependence of the signal rise S(t) can be fitted with a first-order kinetics: S(t) = S + S (1 - exp (-t/τ)), where S is the fast heat release and S stands for the slow heat deposition, i.e. the triplet state deactivation. From the results of the fits one can derive the lifetime of the excited state and the ratio S IS (with S = S + S ). In the case of triplet formation, and if no fluorescence nor subsequent photoreaction occurs, eq 2 can be written as: F

S

T

T

s

F

F

s

s

χ

s

s

*v

Φ Γ = / ~

(3)

As for PAS measurements, TLS experiments were carried out at low pump intensity, and the most accurate φ value was derived from eq 3 using the extrapolated S^S ratio at zero pump energy. Under excitation at 466 nm, the φ/- of camphorquinone was found to be unity, in accordance with the literature (7). Γ

T

Reactivity of Radicals One of the most promising capabilities of photothermal techniques is to provide thermodynamic and kinetic information on secondary species such as radicals or dark consecutive reactions. This could be very useful in the study of the reactivity of the initiating radicals, by determining the bond dissociation energy (BDE) of the coinitiators or by measuring directly the rate constant of interaction with monomer units.


145


Bond dissociation energy of coinitiators The C-H bond cleavage of a coinitiator leading to the production of radicals represents one of the first steps of photoinitiation processes when aromatic ketones are used as photoinitiators (1-2). Therefore, the measurement of the bond dissociation energy (BDE) is useful in order to compare the relative reactivity of radicals. Amines are extensively used as coinitiators in U V curing so a better knowledge of their reactivity is important. Their aC-H bond dissociation energies have been the subject of intensive experimental and theoretical studies for more than 20 years. Despite the different experimental methods used, the published values of amine aC-H BDEs have a large degree of variability (8,9). Both TLS and PAS allows the determination of the BDEs (4): indeed, the value obtained for triethylamine by TLS matches quite well that determined by PAS (91.5 kcal/mol found by TLS instead of 91.2 kcal/mol obtained by PAS) as well as the valuefromthe literature (9).

Rate constant of interaction between radicals and monomers The mechanism of radical addition to monomers is a subject of great interest. Indeed, in basic chemistry, it represents a fundamental bond forming process, while in the polymerfield,the addition of radical to double-bond in the initiation step is probably one of the most important reactions. Consequently, factors controlling these reactions have been the subject of experimental and theoretical works. Due to their low extinction coefficients at wavelengths higher than 300 nm, aminoalkyl radicals are difficult to detect by classical TRAS. Different experimental methods have been used to solve this problem : ESR (10), CIDNP (77), and optical detection of the radical adduct (72). In this part, a new approach for the determination of the rate constants of photoinitiation kbased on photoacoustic spectroscopy, is presented. Efficient production of radical can be obtained by using the two following methods : 1/ Photoreduction of benzophenone (BP) by amine (AH) according to the following reaction: 3

BP + A H -> BPH* + A*

(4)

It is well known that this reaction produces a large amount of corresponding aminoalkyl radicals (Α'). 21 Direct cleavage of photoinitiator which can undergo a Norrish type I cleavage. Compounds I to III (eq 5) are expected to produce respectively benzoyl/ketyl, benzoyl/aminoalkyl or benzoyl/dimethoxybenzyl pairs of radicals (R).


146

I

II

HI

Both processes produce initiating radicals that can react with monomers (M):


(R- or Α·) + M

4

R-M' or A - M

AH

e

r

(6)

where AH is the enthalpy accompanying the reaction and k the rate constant of initiation. r

t

Interaction A'/monomer Adding various amounts of monomer into a solution of benzophenone/amine results in a decrease of the radical lifetimes and the release in the media of the corresponding heat of reaction, as depicted on Figure 4. The fast heat released was probed by PAS for different amounts of monomers.

Figure 4. Schematic representation of the energy diagram for the differen processes. Considering that A-Nf is a long-lived species (72) and that BPH* does not react efficiently with M (73), the variation of the fast PAS signal with the monomer concentration should allow the measurement of AH and the extraction of k . Indeed, the heat H(t) released at a time t is given by the following relationship : r


t

147 H(t) =

hv-$ -E -$ -AH rad

rad

rad

r

1 - expl — τ

(7)

9

§md Erad represents the energy stored in the radicals (BPH" and A") preceding the addition of monomer, and τ is the lifetime of the initiating radical A". In fact, only the heat H(t) released during the time resolution x of the experimental setup is detected and corresponds to the fast signal S , normalized for one photon absorbed and measured at very low pump intensity. However, the volume change associated with the reaction (àV ) can give rise to a fast signal (5), and therefore must be taken into account together with the enthalpy of reaction àH . This leads to an apparent enthalpy of reaction AH . In the following discussion we will consider the term άΗ, =

Photoinitiator Triplet States and Photoinitiating Radical-Monomer

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