Interaction of Photoexcited Photoinitiators with Nitroxyl Radicals
by Dr. Igor V. Khudyakov
Catawba County, North Carolina
SCHEME 1. Set-up of TR ESR
SCHEME 2. Chemical structures of mono- and polynitroxyls
SCHEME 3. Photodissociation of phosphine oxides (MAPO and others). isc stands for intersystem crossing of initially formed S1* – to the reactive T-state.
SCHEME 4. Interaction of a free radical with the triplet state
FIGURE 1. TR ESR spectra of TEMPO (three components) and of several polynitroxyls (Scheme 2) obtained under laser photoexcitation of BP in the presence of corresponding (poly)nitroxyls. Spectra are taken at 300-600 ns after a laser flash.8
SCHEME 5. Processes involved in the photodissociation of MAPO in a magnetic field
FIGURE 2. TR ESR spectra of free radicals that are primary products of the photolysis of MAPO. Spectra are taken at 50-150 ns after a laser flash.
FIGURE 3. TR ESR spectrum obtained under photolysis of solutions of polarization donors (a) Irgacure 651 and (b) MAPO in the presence of TEMPO. Spectra are taken at 300-800 ns after a laser flash.
SCHEME 6. Interaction of a free radical with binitroxyl
SCHEME 7. Different mechanisms of interaction of a reactive free radical with mono- and binitroxyls
Formulations that undergo photopolymerization have, in most cases, photoinitiators (PIs) such as benzophenone and others. These formulations may have nitroxyl (aminoxyl) radicals, which are added for prevention of degradation of the cured coatings. Nitroxyls also are formed during oxidation of hindered amine light stabilizers (HALS). In some cases, it is possible to increase pot life of unstable UV-curable formulations by the addition of namely nitroxyls. Thus, there is a possibility of interaction of photoexcited PI with nitroxyl. This process was studied by ns laser flash photolysis and by time-resolved (TR) electron spin resonance (ESR).
Flash photolysis of photoinitiators (PIs) of free radical polymerization in the cavity of the ESR spectrometer often is accompanied by chemically-induced dynamic electron polarization (CIDEP), i.e., by formation of radicals with non-Boltzmann population of electron Zeeman levels.1,2 CIDEP manifests itself in enhanced absorption or emission of all or of certain components in ESR spectra of photogenerated free radicals. Radicals that manifest CIDEP usually are called polarized. The main mechanisms leading to CIDEP in photoinduced reactions are well established and have been investigated theoretically and experimentally.1,2
Analysis of CIDEP pattern of time-resolved (TR) ESR spectra allows for a solid conclusion on spin multiplicity of molecular precursors of polarized free radicals (singlet or triplet excited molecules) and the tracking of fast reactions of polarized radicals leading to secondary radicals. Thus, TR ESR is a convenient method in mechanistic photochemistry and free radical chemistry.2 The pattern of TR ESR spectra provides valuable information on the interaction of one or another excited state with a stable radical. Interestingly, polarization can be created – in the absence of any chemical reaction but – during “physical” interaction of species with non-zero spin orbital momenta.
Devices and chemicals
We used TR ESR of X-band, cf. Scheme 1: The screen of boxcar integrator in Scheme 1 demonstrates a transient emissive ESR signal of nitroxyl radical – three components or three lines. Minor modification of the device allows returning it to a standard ESR spectrometer. One can get the well-known ESR spectrum of the same nitroxyl – in its usual form – as the first derivative. The studied flash irradiated solution is always refreshed by flowing though the cavity, as shown in Scheme 1.
We used common nitroxyls – 2,2,6,6- tetramethylpiperidine-1-oxyl – (TEMPO) and others, as well as polynitroxyls of the following structures (Scheme 2).
Several identical fragments combined into one molecule can be more efficient stabilizers or antioxidants than the equivalent amount of individual molecules. It is known that a certain amount of added antioxidant Irganox 1010 [tetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate)] is more efficient than monophenol 2,6-di-tert-butyl-4-methylphenol (BHT) taken in four times higher concentration.
In this work, we report results obtained with common PIs – or MAPO – or Lucirin TPO (diphenyl-2,4,6-trimethylbenzoyl phosphine oxide), benzil dimethyl monoketal or Irgacure 651 and benzophenone (BP). Laser flash photolysis experiments and TR ESR experiments were performed at room temperature in nonviscous solvents.
Results and discussion
1. Quenching. Electronically excited states of organic molecules are effectively quenched by stable free radicals. Often quenching is not accompanied by a net photochemical reaction (eqs 1, 2) and represents the interaction of an electronically excited state and a paramagnetic species, a radical:
S1* + R. __> So + R.# (1)
T + R. __> So + R.# (2)
Sign # stands here and below for polarization of a radical in the magnetic field, which means non-Boltzmann population of Zeeman levels of a radical in magnetic field.1,2 (Seminal contributions of research groups from several countries into TR ESR and CIDEP studies are summarized in ref. 1 and are cited in our publications2-8).
PI usually initiates polymerization being in the excited triplet state T, which produces reactive radicals, and reaction 2 is important. Fortunately, most of Irgacures and Darocur dissociate very fast into reactive free radicals (Scheme 3).
Lifetime τ of MAPO triplet is very short, namely ~100 ps, τ of BAPO triplet is ~300 ps. That means that nitroxyl radicals existing in a relatively low concentration in the coating, as well as dissolved in the coating air dioxygen, should not affect photodissociation of Irgacures and Darocur.
BP, a Type II PI, participates in a hydrogen abstraction (or in electron transfer with a subsequent proton transfer).4 At the same time triplet of BP (T) can interact with nitroxyl radical R. by reactions presented in Scheme 4.
Triplet and a radical form two different pairs: One is in a doublet, and another is in a quartet electronic state. As a result of this interaction, nitroxyl R. becomes polarized, cf. for details refs 1-8. It is possible to conclude occurrence of reaction 2 not only by the fact of accelerated disappearance of T but by the observation of polarized R.# as well. TR ESR spectra of nitroxyl, which interacted with T molecules of PI, always appear as emission spectra, cf. Figure 1.
>N-O. fragments of polynitroxyls that are spatially close interact with each other and demonstrate not three but more components in the ESR spectra. In particular, binitroxyl – in the case of a relatively strong magnetic interaction of >N-O. fragments – demonstrates five components and most of such binitroxyl is in the triplet state 3(RXR), cf. Scheme 2. (Here and Scheme 6 R stands for a nitroxyl fragment and X is a chemical bridge connecting R-fragments). It is a case of so-called strong spin exchange. Binitroxyl, which has >N-O. fragments spatially separated, is a case of weak exchange, 2RX2R (Scheme 2). ESR spectrum of the latter binitroxyl has three components; >N-O. do not interact with each other.
Contrary to the triplet molecules case, nitroxyls, which interact with singlet excited molecules (eq 1), demonstrate absorptive TR ESR spectra. In our experience, singlet dioxygen 1O2 produced by photolysis of endoperoxide, is quenched by nitroxyl, and TR ESR spectrum of nitroxyl appears in absorption.3
We omit discussion of time profile of nitroxyl TR ESR spectra and a complex case of strong dependence of intensities of components upon hyperfine coupling (HFC) constants of nitroxyl.
2. Chemical reaction. Free radicals of Type I PI manifest polarization in their TR ESR spectra (CIDEP). Scheme 5 is a simplified presentation of origination of such polarization by a triplet mechanism. Scheme 5 presents a case of MAPO as an example. Well-documented spectra of two spin polarized radicals r# of MAPO are presented in Figure 2.
Polarized radicals of PI are designated as r# in the present work. One can see from Figure 2 that r# of MAPO are in absorption. Observed under photolysis of Irgacure 651, r# are in emission.
Photolysis of MAPO or Irgacure 651 in the presence of TEMPO (Scheme 2) leads to the TR ESR spectra of TEMPO in Figure 3, which demonstrates that absorptive or emissive polarization of initial r# is completely transferred to TEMPO. There are no residual spectra of r#.
More interesting is the case of interaction of r# with binitroxyl in a strong exchange 3(RXR) (cf. above). The interaction of r# with binitroxyl is two competitive processes. They are “physical” processes of electron spin polarization transfer (ESPT) and ESPT in the fast chemical reaction (Scheme 6).
In summary, Scheme 7 is a pictorial demonstration of TR ESR spectra observed under interaction of absorptively or emissively polarized r# with mono- and binitroxyls in a strong and weak exchange.
It is possible to study kinetics of elementary reactions during photopolymerization (UV-cure) of neat vinyl monomer up till conversion of 5-10 percent.9 The system may be approximately considered as Newtonian liquid, and one can get more or less reliable rate constants of elementary reactions, identify transient radicals and quantitatively estimate efficiency of the used PI. High rates of photodissociation of Type I common PI do not depend upon addition of nitroxyls. Quenching of triplet BP by (poly)nitroxyl leads to emissive TR ESR spectrum of the latter.
Free radicals of PI r# are highly reactive towards mono- and polynitroxyls and r# demonstrates overall emissive or absorptive polarization. That polarization is transferred to (poly)nitroxyl radical by ESPT: non-reactive polarization transfer and in the course of concurrent chemical addition. Polarization causes radicals to be “labeled” but does not affect their chemical reactivity in chemical reactions. The magnetic energy is negligible compared with the thermal energy kBT. Polarization transfer is an excellent method for following the pathways of photoinduced free-radical chemical reactions or “physical” interaction of radicals with other radicals or excited states.
TEMPO and other more complex nitroxyls (Scheme 2) are used by coatings chemists as additives to the formulations undergoing free radical photopolymerization. It was mentioned in the introduction that nitroxyls are powerful inhibitors of spontaneous polymerization. Nitroxyls can be added in a typical concentration of 10-4 M to act as a stabilizer. Contrary to phenolic stabilizers, nitroxyls do not need oxygenated solution in order to be effective. Drums with UV-curable formulations containing nitroxyls can be filled in completely for storage and shipment, whereas such barrels with formulations stabilized by phenolic compounds are usually shy – due to the goal of keeping enough of molecular oxygen.
The role of nitroxyls during photopolymerization is complex. In a relatively high concentration, nitroxyls interact with free radicals formed from PI, and that way decrease rate of photopolymerization or lead to an induction period. Thus, concentration of the added nitroxyl should be optimal: Nitroxyl should provide the required pot life and have a minor effect on the rate of photopolymerization. Individual reactions of nitroxyls with free radicals formed in the system can be studied by TR ESR with ~0.1 μs time resolution. Such study will help in formulation and tweaking of coating, which usually has several vinyl (acrylate) ingredients. Examples of tracing photoinduced reactions with TR ESR are presented in this paper.
ns Laser flash photolysis with optical detection of triplet states and radicals is a popular technique for studying photoinduced reactions. Unfortunately, most of free radicals of PIs, radical adducts and macroradicals adsorb light at very short wavelengths and have low extinction coefficients. Due to these reasons they can’t be monitored by flash photolysis. (Type II PI BP, cf. above, is one of few exceptions among common PIs: Its triplet state and the corresponding ketyl radical can be monitored easily by flash photolysis with detection in visible area of the light spectrum.) That is why laser flash photolysis with ESR detection of radicals (TR ESR) is a valuable tool for the coatings chemist interested in the individual radical reactions in his formulation.
The late Professor N.J. Turro, Dr. S. Jockusch and all other co-authors of the cited publications.
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- Khudyakov, I.V.; Turro, N.J. In Photochemistry and UV-Curing: New Trends; Fouassier, J.P., Ed, Research Signpost, Trivandrum, 2006.