Article pubs.acs.org/JPCA
Theoretical Study of the Photochemical Initiation in NitroxideMediated Photopolymerization Miquel Huix-Rotllant* and Nicolas Ferré* Aix-Marseille UniversitéCNRS, Institut de Chimie Radicalaire, Marseille 13397, France S Supporting Information *
ABSTRACT: Nitroxide-mediated photopolymerization (NMP2) is a promising novel route to initiate radical polymerization. In NMP2, alkoxyamines bounded to a monomer are attached to a chromophore. Upon light absorption, the excitation energy is transferred from the chromophore to the alkoxyamine moiety, inducing the cleavage of the oxygen−carbon bond and thus initiating the polymerization. The NMP2 mechanism depends strongly on several factors like the type of chromophore, the monomer, the connectivity pattern, etc. This complexity makes it difficult to design new NMP2 initiators with increased polymerization efficiency and selectivity. In the present article, we characterize by means of quantum mechanical calculations the main steps of the NMP2 initiation for alkoxyamines attached to aromatic ketones. We show how the excitation energy can be transferred from the chromophore to the alkoxyamine moiety, and present two easily computed parameters which can account for the selectivity of the O−C bond photocleaveage. Finally, using results obtained for a series of isomers, we give some rules that may help the design of more efficient NMP2 initiators.
1. INTRODUCTION The nitroxide-mediated polymerization (NMP) process is commonly used in controlled radical polymerization.1−7 In NMP, an alkoxyamine containing a monomer is used to initiate the polymerization by thermal decomposition of an oxygen− carbon single bond, leading to two radical species: a stable nitroxide and a transient monomer required for polymerization initiation.8 Despite its success, one of the main drawbacks of this technique lies in the relatively large dissociation energy to break a single bond (frequently larger than 100 kJ mol−1), which limits the applicability of NMP to some well-selected monomers.9 Recently, a possible extension of NMP has been proposed, in which light instead of heat is used to dissociate the O−C bond.10 This technique has been dubbed nitroxide-mediated photopolymerization (NMP2). NMP2 iniferters (herefter denoted photoiniferters) are similar to NMP iniferters (alkoxyamine and monomer) but include also a chromophore, which is able to transfer absorbed light energy to the alkoxyamine, eventually inducing the cleavage of the O−C bond.11 The experimental proof of principle of an NMP2 process was published in 2010, using aromatic ketones as chromophores.12 In the original work of Guillaneuf et al., the benzophenone chromophore was directly linked to the aminoxyl moiety in order to maximize the excitation transfer needed for the homolysis. Since then, several modifications of the chromophore, as well as the way it is linked to the alkoxyamine, have been tried and better polymerizations have been obtained.11,13−15 However, advances in this field are slow, due to the inherent complexity of the NMP2 mechanism. A successful NMP2 initiation requires a selective photocleavage of the O−C bond. Unlike the thermal polymerization, in which selectivity can be rationalized by the simple knowledge © XXXX American Chemical Society
of the bond dissociation energies (BDE), the products of an NMP2 reaction depend on several factors, such as the initial photon energy, the strengths of the labile bonds in the excited states, the statistical competition between the different photocleavege pathways (without mentioning other radiative or nonradiative processes), and the excess of vibrational energy after bond dissociation. Therefore, a rational design of photoiniferters requires the knowledge of the photochemical mechanism during NMP2 initiation. An ideal NMP2 photoinitiation mechanism happens in a twostep pathway, as represented in Figure 1. In the first step, the chromophore is locally excited to a singlet state. An excitation energy transfer occurs through internal conversion and the
Figure 1. General schema of the photochemically induced NMP2 initiation mechanism. The red part of the structure indicates the localization of the excitation energy in the structure. Received: February 19, 2014 Revised: May 28, 2014
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calculations use density-functional theory (DFT) and the B3LYP exchange-correlation functional. Excited states have been calculated with linear-response time-dependent DFT (TDDFT) unless otherwise stated. TDDFT in conjunction with a hybrid exchange-correlation functional has been shown to give qualitatively correct results, but energies are only in semiquantitative agreement.22 Therefore, the present study is not expected to give quantitative agreement with experiments. For the charge-transfer transitions of the type (nNπ*), which are sometimes problematic with TDDFT, we have crosschecked with second-order extended multireference quasidegenerate perturbation theory (XMCQDPT2)23 calculations for selected key structures in order to confirm the correct description of TDDFT for such charge transfer states performed with the Firefly package.24 It is noteworthy that transition states in the excited states are only approximate, since the analytic TDDFT Hessian is not yet implemented in GAUSSIAN09. Accordingly, transition states have been approximately located by performing a relaxed scan of the selected coordinate and taking the maximum value of the potential energy surface along this path. Nevertheless, the computed barriers are meaningful in the framework of nonequilibrium transformations like photochemical reactions. Because we are interested only in comparisons between different paths (or isomers), BDEs have been calculated using the unrestricted Kohn−Sham (UKS) energies. For the 3BDE, the geometries of the radical products have been optimized at the UKS level, while for 1 and its isomers, the TDDFT optimized geometries and energies were used. The use of different levels of theory for reactant and products was imposed by the inability of UKS to locate one important intermediate, see the Supporting Information. No corrections due to the zero-point energy have been included. Given these limitations, the present BDE calculations should only be considered as qualitatively correct.
excitation becomes delocalized over the alkoxyamine moiety. Depending on the type of chromophore, a singlet-to-triplet intersystem crossing may be expected to occur before the excitation energy transfer. In the second step, the (ideally selective) dissociation of the O−C bond occurs. An energy barrier may exist for this reaction, in which case the remaining photon energy must be large enough to overcome it. In this scheme, we introduce the photochemical bond dissociation energy 3BDE, a static parameter which is defined with respect to the excited state minimum of the delocalized triplet state, in contrast with the thermal bond dissociation energy 1BDE, which is defined with respect to the ground-state minimum. In the present work, we characterize the main photochemical properties (i.e., reactants, products and intermediates, transition states, and bond dissociation energies) based on the general scheme shown in Figure 1. We have selected a simplified model system (hereafter denoted 1) inspired by the original photoiniferter of Guillaneuf et al. In this model (see Figure 2), we have selected acetophenone (Ap) as the chromophore
Figure 2. Structure of 1 as a model of NMP2 iniferter, with tert-butyl as the monomer. Labile bonds of interest are σ-labeled.
and tert-butyl (tBu) as the monomer. The other substituent in the alkoxyamine is a methyl group. The present model is not intended to induce an efficient O−C dissociation, but it is general enough for establishing the general photochemical mechanism of the NMP2 photoreaction when aromatic ketones are used. While Ap is known to undergo Norrish type I photocleavage reactions,16 we focus only on the bond dissociations involving the alkoxyamine moiety, as required for the photopolymerization initiation. The first part of the photoreaction involves the Ap moiety only, which can undergo an almost 100% efficient singlet to triplet conversion when it is initially excited to S1. The underlying mechanism has extensively been studied in the literature, and it will not be repeated here (for a complete study of the photochemistry leading to triplet Ap, we refer to ref 17). Accordingly, we will mainly focus on the photochemical pathway starting with the Ap lowest triplet state, which electronic structure can be characterized as 3(nOπ*). This article is organized as follow: Section 2 contains the technical details of the computations and also describes the way in which approximate transition states have been found. The main photochemical steps of model 1 are studied in sections 3.1−3.3. In section 3.1, we will briefly demonstrate that the Ap photophysics is not affected by the presence of the alkoxyamine. In section 3.2, the excitation energy transfer to the alkoxyamine is described. In section 3.3, the photocleavage reactions are discussed. In the subsequent section 3.4, we compare the efficiency of several isomers based on 1. We end up with some conclusions and we sketch out some practical rules for designing new photoiniferters in section 4.
3. RESULTS AND DISCUSSION 3.1. Absorption Spectrum. The first step of the photoreaction of 1 is localized in the Ap moiety. Ap is wellknown for an ultrafast population of the triplet manifold.17 Ap photochemistry is strongly affected by the substituents, which can even change the nature of the lowest excited triplet state.25 Therefore, we first demonstrate that the low-lying states of Ap are preserved in the presence of the alkoxyamine moiety. For this purpose, we compare singlet and triplet excited state energies both at the ground state equilibrium structure and at the lowest triplet minimum-energy structure of isolated and substituted Ap. These results can be found in Figure 3a. When Ap is substituted with the alkoxyamine, the electronic structures of the low-lying excited states are essentially preserved both at the ground state and at the 3(nOπ*) minimum-energy structures. The main difference lies in the appearance of the 3(nNπ*) state between the 1(nOπ*) and 1 (ππ*) states. Taking into account that the 1(nOπ*) is the absorbing state in common NMP2 experiments, we can consider that the appearance of the (nNπ*) states does not alter the first stages of the photoreaction. Accordingly, we can safely assume that the first part of the photoreaction occurs as the isolated Ap. Accordingly, and similarly to other work in the literature (see ref 16, for instance), we assume that the excitation energy transfer to the alkoxyamine moiety happens from the lowest triplet state 3(nOπ*).
2. COMPUTATIONAL DETAILS All calculations have been performed using the GAUSSIAN09 package,18 employing a 6-31+G** basis set.19−21 Ground-state B
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(nNπ*) configuration is dominant and characterized by an almost-planar alkoxyamine structure. Accordingly, the 3(nOπ*) and 3(nNπ*) minima can be connected adiabatically as illustrated in the general schema given in Figure 1. In Figure 4, we compare the structural changes occurring in the transition from the 3(nOπ*) to the 3(nNπ*) minima. The 3
Figure 4. Side and top views of the two minimum energy structures on the lowest triplet surface corresponding to the 3(nOπ*) (left) and 3 (nNπ*)twi (right) states. Notice the rotation of the tBu group and the planarization of the N-center.
(nNπ*) minimum structure is characterized by not only a planarization at the nitrogen center but also a twist of the tBu group due to the modification of the steric interactions between the nitrogen substituents. Probably, the twist of the monomer group is possible only due to the small size of the methyl substituent of the N-center. Typical NMP2 iniferters feature much larger substituents, and therefore, the twisting of the monomer is probably hindered. The carbonyl (CO) and the alkoxyamine bond lengths give further evidence of the different electronic structures of the two triplet state minima (see Table 1.) The CO bond length 3
Figure 3. (a) Jablonksy diagrams of 1 and Ap at the 1(ππ) and 3 (nOπ*) minimum energy structures; (b) attachment (red) and detachment (blue) density analysis of the lowest excited states of model 1.
Table 1. Comparison of the Main Structural Parameters in the Ground State (GS), the 3(nOπ*) and 3(nNπ*) State Minima, and the Connecting Transition State (TS*)a 1
(ππ)
The electronic nature of the reported transitions can be appreciated more deeply by means of an attachment and detachment density analysis26 for the corresponding excitations (see Figure 3b). From such analysis, we can readily observe that 3 (nOπ*) and 3(ππ*) states feature electronic excitations localized in the chromophore, while the 3(nNπ*) state is characterized by both an intrachromophore (ππ*) transition and an electron transfer from the nitrogen lone pair toward the Ap π* orbital. 3.2. Energy Transfer. The excitation energy at the 3(nOπ*) minimum is initially localized on the Ap moiety. For the O−C bond dissociation to occur, this energy has to spread out over the alkoxyamine moiety. Starting from the 3(nOπ*) state minimum, the 3(nNπ*) state is the most plausible candidate for such a delocalization to occur. A (nN → π*) transition creates a partial positive charge on the nitrogen center and the planarization of the alkoxyamine. Indeed, we have found that the lowest triplet state features a local minimum in which the
CO N−O O−C N−C CNOC
1.225 1.446 1.458 1.474 120.3
(nOπ*)
3
1.314 1.448 1.456 1.478 120.1
(nNπ*)
3
1.260 1.338 1.539 1.514 40.8
a
The CO label indicates the carbonyl bond, while the other labels are defined in Figure 2. Distances are in Å and dihedral angles in deg.
is larger in the 3(nOπ*) minimum, while it remains close to the ground-state value at the 3(nNπ*) minimum. On the other hand, the alkoxyamine structure is barely modified in 3(nOπ*) while, in 3(nNπ*), the N−O bond is strongly compressed and the O−C and N−C bonds are elongated. These results give a structural evidence of the charge transfer characterizing the 3 (nOπ*) to 3(nNπ*) transition. Moreover, with regard to the O−C bond length increase, the 3(nNπ*) minimum can be C
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bound to the exact minimum energy path, are still meaningful in the framework of nonequilibrium transformations like photochemical reactions. Along the twist-then-planar pathway, the first barrier (twi only) is 0.90 eV while the successive planarization is barrierless. Along the planar-then-twist pathway, the first barrier (pla only) is 0.29 eV while the successive twisting is barrierless. Taking the direct path (simultaneous twi and pla directions), the barrier is 0.51 eV. From these observations, we can conclude that the energy transfer from the chromophore to the alkoxyamine moiety is possible, owing to the low energy barrier to be overcome, which is lower than or equal to 0.29 eV. This energy is still available since 1 roughly loses 1/3 of the absorbed photon energy between the Franck−Condon region and the 3 (nNπ*) minimum-energy structure. Additionally, we can infer from the energy barriers that the minimum-energy path has a larger component of the pla direction. This is important since the twi direction is possibly hindered in realistic in NMP2 iniferters. 3.3. Photodissociation Pathways. For the design of new photoiniferters, it is necessary to determine the possible dissociative pathways that can occur after the energy transfer. An extensive study of the different photocleavage reactions and of their competition would require a complete quantum dynamical simulation, which is out of the scope of the present study. Instead, we will establish which are the most probable photocleavage reactions that can occur in the alkoxyamine moiety and we will determine some simple set of energetic and structural parameters that can guide a rational design of photoiniferters. After the energy transfer, the alkoxyamine moiety experiences important structural changes (see Table 1 in the previous section). Owing to the elongation of the O−C bond length, we proposed that the 3(nNπ*) minimum is a predissociative intermediate, preceding the complete dissociation of the O−C bond. Nevertheless, the accumulated energy in the alkoxyamine moiety can also be used to dissociate some other bonds. Accordingly, we consider also the dissociation of the N−O (known to be thermally labile) and the N−C bonds (see σ bond labels in Figure 2). As a matter of fact, the N−C bond is expected to be easily broken, since it is found in a β-position with respect to Ap, aromatic ketones being well-known for undergoing β-photocleavege reactions.27 In Table 2, we report the 3BDE values, as well as the approximate energy barriers leading to dissociation of the three considered bonds. On one hand, the good agreement between XMCQDPT2 and DFT-based 3BDEs, and especially their relative differences, demonstrates that the discussion based on the latter results is qualitatively meaningful. On the other hand,
considered as a predissociative intermediate before the subsequent photocleavage reaction. We now consider the adiabatic pathway connecting the 3 (nOπ*) and the 3(nNπ*) minima. According to the structural changes reported in Figure 4, two reaction coordinates play a major role. The first one characterizes the planarization (pla) at the N center, while the other coordinate is the twist (twi) of the tBu group. Roughly, the pla coordinate affects the bond lengths and the valence angles of the alkoxyamine, while the twi coordinate changes the CNOC dihedral angle (see Table 1). We recall that the twi coordinate might simply be an artifact of the photoiniferter model, due to the small size of the methyl substituent on the alkoxyamine. However, we keep the discussion of the twi dimension for completeness. The true reaction coordinate connecting the 3(nOπ*) and 3 (nNπ*) minima involves a combination of the pla and twi degrees of freedom. Such a pathway is depicted with a green arrow in Figure 5. Due to the unavailability of TDDFT
Figure 5. Schematic ideal pathways connecting the 3(nOπ*) and 3 (nNπ*) state minima (marked as green spots). Red spots indicates local lowest energy structures along pure twisting or planar coordinates after crossing the corresponding barrier. The green dashed line indicates the ideal minimum-energy connection between the two structures. Energies in eV.
Hessians, we were not able to compute the exact minimumenergy path directly. Other density-functional methods such as unrestricted Kohn−Sham, which could potentially be used to calculate the Hessian for the lowest triplet state, are failing due to the intrinsic multireference character of the 3(nOπ*) to 3 (nNπ*) process (see the Supporting Information). Instead, we consider three idealized pathways: (i) first planarization then twisting, (ii) first twisting then planarization, and (iii) the linear interpolation between the two minima. Each of these pathways (see gray arrows in Figure 5) has been obtained in a two-step procedure. First, a set of geometries is obtained by linear interpolation between the initial and final structures. Second, a partial relaxation of each generated geometry is performed, keeping frozen the alkoxyamine backbone (N−O−C atom positions and C atoms bonded to N keep their interpolated coordinates). This constraint is necessary for the separation of the twi and pla directions. Energy barriers characterizing each of these paths are obtained as energy differences between the highest energy partially relaxed structure and the 3(nOπ*) minimum reference. These barriers, though giving an upper
Table 2. Photochemical Bond Dissociation Energies and Distance Difference between the 3(nNπ*) and Ground-State Minima for Model 1 (XMCQDPT2/6-31+G* Values in Parentheses)a barrier σOC σNC σNO
0.32 0.10 0.42
BDE
Δd
−0.87 (−1.10) −0.92 (−1.16) −0.79 (−1.03)
0.08 0.04 −0.11
3
The active spaces contain π/π* and nonbonding orbitals as well as the σ/σ* pair describing the bond of interest. Energies are in eV and distances in Å. a
D
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both the barriers and the 3BDEs give the same kind of information: when the photoproducts are more stable, both the 3 BDEs and the barriers are lower. Since the 3BDEs are computationally cheaper to obtain, we will simply discuss the latter values hereafter. The comparison of the 3BDE values gives a qualitative hint regarding the easiness to break these bonds, but ultimately, all the reported barriers can be crossed if enough energy (given by the absorbed photon) is still available. In order to complement this information, we added in Table 2 the bond length difference between the 3(nNπ*) minimum and the ground state for the dissociative bonds. This structural parameter carries additional information to understand the selectivity: positive values indicate a weakening of the bond, while negative values indicate its strengthening. Accordingly, we can easily observe that the N−O bond is expected to be more difficult to break than the O−C and the N−C bonds, owing to its smaller 3BDE and its 0.11 Å bond length shortening. Accordingly, we can safely conclude that the amount of the corresponding photoproduct would be of minor importance. As for the competition between N−C and O−C bond breaking, it appears that the former is energetically favored, even though the O−C bond is slightly more elongated than the N−C one. In such a case, we expect the quantum yield of the N−C photodissociation to be of similar or even higher importance than the O−C one. In order to improve the selectivity, an efficient photoinitiator should protect the βposition with respect to the atomatic ketone chromophore, hence favoring the generation of desired photoproducts and eventually the expected radical monomers. 3.4. Modulation of the Photopolymerization Initiation with Isomers. The results reported up to now establish a simple route to evaluate and compare the efficiency of several alkoxyamines bonded to chromophores belonging to the aromatic ketone family. In this last part of the study, we address the NMP2 efficiency with respect to several isomers of model 1. The three considered isomers are depicted in Figure 6. In structures 2 and 3, the alkoxyamine moiety is bonded to
transfer stops the reaction, and therefore, the 3′-position should not be considered for an efficient NMP2 photoreaction. In fact, this conclusion could be predicted by noting that most aromatic ketones are strong photoreducing agents, especially when the lowest triplet state is of (nOπ*) type. For compounds 3 and 4, we first study the delocalization of the excitation energy and then the dissociation pathways of the same three bonds: O−C, N−O, and N−C. In Figure 7, the
Figure 7. Side and top views of the two minimum energy structures on the lowest triplet surface corresponding to the 3(nOπ*) (left) and 3 (nNπ*)twi (right) states for models 3 and 4. As in the case of 1, notice the rotation of the tBu group and the planarization of the N center.
structures of the 3(nOπ*) and 3(nNπ*) minima are depicted for each isomer. In both models, we essentially observe the same principal structural changes already reported in 1, i.e., the planarization of the alkoxyamine and the twisting of the tBu group. Other geometrical modifications of 3 (Figure 7a) are similar to those found for 1. In model 4 instead (Figure 7b), we observe a noticeable rotation of the Ap moiety in going from 3 (nOπ*) to 3(nNπ*): the corresponding minimum-energy structures differ by almost 90° twist of Ap. This large rotation is possible because the alkoxyamine and the chromophore are connected through an ethyl linker. Therefore, more rotamers are accessible through rotation around the connecting C−C bonds. The main consequence of such easy rotations is the quasi-perfect planarity at the N center in the 3(nNπ*) state. Other important parameters, characterizing each of the equilibrium structures (geometries, relative energies), and the corresponding energy barriers calculated for the direct pathway are summarized in Table 3. They shall be compared to the same data for model 1 reported already in Figure 5 and Table 1. As expected from their structural similarities, 3 (Table 3a) resembles 1, although its 3(nOπ*) to 3(nNπ*) energy barrier is slightly larger. Probably, the energy delocalization would occur less efficiently for this model. Again, in the planar triplet state, both N−C and O−C are elongated with respect to their ground states values and it is expected that both bond dissociation would compete, ultimately affecting the photo-
Figure 6. Schematic structure of the three isomers of model 1.
the Ap phenyl ring in either the 2′- or 3′-positions, while in structure 4, the alkoxyamine is linked to the Ap ketone group instead of the phenyl one. In the case of structure 2, a simple geometry optimization on the lowest triplet energy surface leads to a photoproduct in which a proton from the methyl group of the alkoxyamine is transferred to the Ap carbonyl oxygen atom. The proton E
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have unraveled the main steps of NMP2 initiation when aromatic ketones are used as chromophores. The photoreaction features three principal steps: (i) light absorption and a subsequent singlet-to-triplet conversion localized in the Ap moiety; (ii) a triplet energy transfer in which the excitation energy is delocalized over the alkoxyamine moiety; and (iii) the cleavage of the alkoxyamine O−C bond, in competition with two other bond photodissociations. The delocalization of the excitation toward the alkoxyamine moiety is the most important step: it is necessary to form the predissociative intermediate of the photocleavage reactions. Such a transformation is possible through the creation of a partial positive charge on nitrogen which in turn induces a planarization of the alkoxyamine moiety. Concomitantly, the N−O bond is shortened, while the O−C and the C−N bonds are elongated. In order to understand qualitatively the competition between the photodissociation pathways, we have selected two simply calculated parameters: on the one hand, the 3BDE, always negative, which is related to the energy barrier to be overcome in the triplet excited state; on the other hand, the bond distance difference between the predissociative structure and the ground-state minimum. Using these parameters, we could easily rationalize the selectivity of the photocleavage reactions. Using these two parameters, we have shown that the βphotocleavage (N−C bond) competes with the O−C bond dissociation. This might explain the present low efficiency of NMP2 photoiniferters. As a first step toward the design of improved photoiniferters, we have studied how the alkoxyamine and the chromophore are connected and how this will impact the selectivity of the photodissociations. While two isomers show equal or even lower preference toward O−C photodissociations, the last isomer in which the alkoxyamine is connected to the ketone side of the chromophore significantly lowers the importance of the N−C dissociation, by quenching the usual β-dissociation of aromatic ketones. Molecules based on this model are interesting candidates to improve the efficiency of present NMP2 photoiniferters. To conclude, we summarize the present findings by a set of simple rules for the chemical design of efficient NMP2 photoiniferters: (i) favor the energy transfer to induce the planarization of the alkoxyamine moiety; (ii) link the chromophore to the alkoxyamine in a position that can easily accommodate the large structural rearrangement induced by the planarization; and (iii) protect the β-position of photoiniferters containing aromatic ketones. Such a reaction is in direct competition with the monomer bond breaking and can severely affect the efficiency of the NMP2.
Table 3. Energetic (in eV) and Geometric (in Å and deg) Parameters for the Minimum Energy Structures and the Triplet TS* of Models 3 and 4a (nOπ*)
1
3
(ππ)
energy CO N−O O−C N−C CNOC
1.224 1.457 1.447 1.475 120.2
energy CO N−O O−C N−C CNOC
1.226 1.429 1.481 1.458 60.9
(a) Model 3 0 1.314 1.456 1.447 1.475 120.3 (b) Model 4 0 1.307 1.437 1.464 1.464 116.9
3
TS*
(nNπ*)
0.556 1.272 1.388 1.329 1.466 81.5
−0.024 1.267 1.340 1.513 1.503 41.3
0.445 1.286 1.407 1.482 1.468 92.01
−0.340 1.281 1.335 1.534 1.476 31.2
Energy barriers connecting the 3(nOπ*) and 3(nNπ*) structures correspond to the direct pathway as depicted in Figure 5. a
polymerization yield. The similarity between 1 and 3 is confirmed by their almost equal 3BDE values and bond length variations (Tables 2 and 4a). Table 4. Photochemical Bond Dissociation Energies and Distance Differences between the 3(nNπ*) and Ground-State Minimaa 3
σOC σNC σNO σOC σNC σNO a
BDE
(a) Model 3 −0.89 −0.85 −0.81 (b) Model 4 −0.92 −0.29 −0.91
Δd 0.07 0.03 −0.12 0.05 0.02 −0.09
Energies are in eV and distances in Å.
Model 4 (Tables 2b and 3b) seems to feature noticeable and interesting differences with respect to the other models. First, the planar structure is 0.34 eV more stable than the 3(nOπ*) structure and the corresponding energy barrier is significantly 0.06 eV lower than the ones characterizing the models 1 and 3. Therefore, the energy transfer will probably be occurring faster in 4. This larger stabilization can be rationalized by the large structural reorganization allowed by the less constraining environment of the alkoxyamine moiety. Second, the N−C bond length remains close to its ground-state value while the O−C one is still significantly elongated (0.05 Å). Additionally, the N−C 3BDE is significantly smaller (0.6 eV) than the two other ones. This result can be rationalized easily: the βdissociation typical of aromatic ketones is not possible in model 4. Therefore, photoiniferters inspired by model 4 are expected to feature a significant increase of the NMP2 efficiency.
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ASSOCIATED CONTENT
S Supporting Information *
Discussion about TDDFT vs lowest triplet state. Cartesian energy structures for models available free of charge via the
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4. CONCLUSION NMP2 is a promising (photochemical) alternative to the usual (thermal) nitroxide-mediated polymarization. In the present quantum mechanical study using a simple molecular model, we
DFT for intermediates in the coordinates for the minimum1, 3, and 4. This material is Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. F
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Notes
(18) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09 Revision D.01; Gaussian Inc.: Wallingford, CT, 2009. (19) Hehre, W.; Ditchfield, R.; Pople, J. Self-Consistent Molecular Orbital Methods. XII. Further Extensions of Gaussian-Type Basis Sets for Use in Molecular Orbital Studies of Organic Molecules. J. Chem. Phys. 1972, 56, 2257. (20) Dill, J.; Pople, J. Self-consistent molecular orbital methods. XV. Extended Gaussian-type basis sets for lithium, beryllium, and boron. J. Chem. Phys. 1975, 62, 2921. (21) Clark, T.; Chandrasekhar, J.; Spitznagel, G.; Schleyer, P. Efficient Diffuse Function-Augmented Basis Sets For Anion Calculations. III. The 3-21+G Basis Set For First-Row Elements, LiF. J. Comput. Chem. 1983, 4, 294. (22) Huix-Rotllant, M.; Ferré, N. Triplet State Photochemistry and the Three-State Crossing of Acetophenone Within Time-Dependent Density-Functional Theory. J. Chem. Phys. 2014, 140, 134305. (23) Granovsky, A. A. Extended Multi-Configuration QuasiDegenerate Perturbation Theory: The New Approach to Multi-State Multi-Reference Perturbation Theory. J. Chem. Phys. 2011, 134, 214113. (24) Granovsky, A. A. Firefly version 7.1.G. http://classic.chem.msu. su/gran/firefly/index.html. (25) Yabumoto, S.; Shigeto, S.; Lee, Y.; Hamaguchi, H. Ordering, Interaction, and Reactivity of the Low-Lying nπ* and ππ* Excited Triplet States of Acetophenone Derivatives. Angew. Chem., Int. Ed. 2010, 49, 9201−9205. (26) Head-Gordon, M.; Grana, A. M.; Maurice, D.; White, C. A. Analysis of Electronic Transitions as the Difference of Electron Attachment and Detachment Densities. J. Phys. Chem. 1995, 99, 14261−14270. (27) Cai, X.; Sakamoto, M.; Hara, M.; Tojo, S.; Ouchi, A.; Sugimoto, A.; Kawai, K.; Endo, M.; Fujitsuka, M.; Majima, T. Stepwise Photocleavage of C−O Bonds of Bis(substituted-methyl)naphthalenes with Stepwise Excitation by Two-Color Two-Laser and Three-Color Three-Laser Irradiations. J. Phys. Chem. A 2005, 109, 3797−3802.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the Agence Nationale de la Recherche for funding through the IMPACT Project ANR-11-BS08-0016. This work was granted access to the HPC resources of Aix-Marseille Université financed by the project Equip@Meso (ANR-10EQPX-29-01) of the program “Investissements d’Avenir” supervised by the Agence Nationale pour la Recherche.
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