Article Cite This: J. Am. Chem. Soc. 2018, 140, 17800−17804
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Internal Oriented Electric Fields as a Strategy for Selectively Modifying Photochemical Reactivity Nicholas S. Hill and Michelle L. Coote* ARC Centre of Excellence for Electromaterials Science, Research School of Chemistry, Australian National University, Canberra, Australian Capital Territory 2601, Australia
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S Supporting Information *
ABSTRACT: Time-dependent density functional theory calculations have been performed on acetophenone derivatives to explore the possibility of using charged functional groups as internal electric fields, the orientation of which can be altered to change photochemical behavior at will. Results demonstrate that nonconjugated charged groups can significantly alter, by up to −1.44 eV, the stabilities of excited states. Specifically, a nonconjugated negatively charged group in the para position will destabilize the nπ* and stabilize the ππ* transitions, while a positively charged group will do the opposite. These electrostatic effects can be tuned by moving these substituents into the meta and ortho positions. Through use of acids and bases, these charged groups can be switched on or off with pH, allowing for selective alteration of the energy levels and photochemical reactivity. Solvent effects are shown to attenuate the electric field effect with increasing dielectric permittivity; however electrostatic effects are shown to remain significant even in quite polar solvents. Using charged functional groups to deliver the position-dependent electrostatic (de)stabilization effects is therefore a potential route to improving the efficiency of desirable photochemical processes.
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protonation states, although charged ligands,9 Lewis acids,10 and ionic aggregates11 are also recognized as having potential as electrostatic catalysts. Despite this recent attention, studies into the applicability of CFGs have focused primarily on ground state, thermal reactions. In this work their application to excited state chemistry is explored. Photochemistry is a promising avenue for application of electrostatic effects due to the presence of electron density in high-energy antibonding orbitals, orbitals that are inherently more diffuse and polarizable than ground state equivalents. As well as this, under mild conditions photoactive molecules can give selective access to reactions that are extremely unfavorable in the ground state, and therefore the ability to tune and enhance photochemical reactivity is highly desirable. Current approaches to changing the photochemical behavior of molecules focus on increasing the amount of conjugation in the molecule to red-shift absorption,12 although Lewis acid complexation has also been found to induce similar effects.13 However, increasing conjugation to an extent where absorption is shifted significantly can result in excitation character in regions of a molecule too far from the desired reaction center to be useful, while Lewis acid complexation was found to reduce the initiation efficiency. Introduction of CFGs could provide a
INTRODUCTION Until relatively recently, electrostatic effects were largely thought to be applicable only to redox chemistry, where reactions involve electron transfer and as such respond predictably to an external voltage bias. However, work by Shaik, Coote, and others have highlighted the ability of carefully oriented electric fields to catalyze and alter nonelectrochemical chemical reactions.1−3 Specifically, the relative stability of the high-energy charge-separated resonance contributors of the transition state can be altered significantly upon introduction of electrostatic effects, which in turn can alter the reaction barrier. The magnitude of an electrostatic effect is dependent on the polarities involved, the strength of the applied field, and, importantly, the direction of the field relative to the reaction axis dipole. This direction dependence introduces complexities when attempting to apply oriented external electric fields (OEEFs) on a large scale to solution-based chemistry. To date these challenges have been overcome on the nanoscale using surface chemistry techniques in conjunction with scanning tunneling microscopy,4,5 but movement toward larger scale systems involving electrodes or charged insulators has been limited to unimolecular reactions and/or surface-tethered systems.5−7 A more promising approach to applying electrostatic effects to solution chemistry is by introduction of charged functional groups (CFGs) to a specific region of a molecule.8 Typically these CFGs are Brønsted acids and bases in their relevant © 2018 American Chemical Society
Received: November 8, 2018 Published: November 23, 2018 17800
DOI: 10.1021/jacs.8b12009 J. Am. Chem. Soc. 2018, 140, 17800−17804
Article
Journal of the American Chemical Society more balanced approach to tuning a molecule’s photochemistry based on the following considerations: • ππ* and nπ* excited states exhibit different dipole moments and can therefore be targeted independently of each other; • the stabilities of different states can be increased or decreased by introduction of either a positively or negatively charged group; • the magnitude of the resultant electrostatic effect can be tuned by changing the position of the CFG on the molecule. This study does not aim to fully explore the excited state properties, which often requires high-level, multireference ab initio calculations.14 Instead, time-dependent density functional theory (TD-DFT) is employed to demonstrate that vertical excitation energies can be changed at will with electrostatic effects. For simplicity, acetophenone has been taken as a model system, as it is the simplest in the class of aromatic ketone photoinitiators, and TD-DFT is found to reproduce the correct ordering of its excited states.15 Herein we show that the inclusion of (nonconjugated) charged functional groups can be used to selectively stabilize or destabilize its various transitions according to the sign of the charge and its location.
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Figure 1. Energy level diagram highlighting excited state radiationless processes in acetophenone that give rise to radical dissociation.
Figure 2. CFG-functionalized acetophenone.
COMPUTATIONAL METHODOLOGY
All calculations were performed using time-dependent density functional theory (DFT) with the Gaussian 16 electronic structure package.16 Geometries were optimized with the M06-2X17 exchange− correlation functional with the 6-31+G(d,p) basis set; TD-DFT calculations of excited states were also performed at M06-2X/631+G(d,p). Solvent effects (toluene, dichloromethane, acetonitrile, H2O) were considered with the SMD continuum solvent model.18 All energies given are from the conformationally searched, global minimum energy structures in both gas and solvent phases, unless stated otherwise. Carboxylate pH switches for the nπ* and ππ* transitions were calculated with a range of basis sets for benchmarking purposes, finding that the switches predicted by 6-31+G(d,p) were consistent with both larger and smaller basis sets.
in the ortho, meta, and para positions, and it should be noted that upon conformational optimization, the two ortho positions are equivalent to each other, as are the two meta positions. Singlet States. The formation of charges on the carboxylate and amino groups to acetophenone results in stabilization/destabilization effects that vary as a function of ring position (Figure 3) and protonation state. For the singlet states, positive charge is found to stabilize the 1nπ* and destabilize the 1ππ* state, except at the ortho position, where the 1ππ* state is stabilized. To understand these effects, it is helpful to examine the electronic structure of the excited states. As is well known,20 the 1nπ* state is associated with a dipole with a significant charge-transfer contribution (Scheme 1), and hence appropriately placed charged groups should be able to electrostatically stabilize this dipole. From Scheme 1 it is clear that substituents in the para position should be best aligned with the dipole and hence most effective, and this alignment is poorer in the meta and, to a greater extent, the ortho position. It is also clear from Scheme 1 that a negatively charged carboxylate will destabilize the dipole and the positively charged protonated amine will stabilize it. This is consistent with the trends observed in Figure 3. In contrast, the negatively charged carboxylate group is found to have the opposite effect on transition energies; deprotonation drastically stabilizes the ππ* state at all positions on the acetophenone ring, while the nπ* transition is destabilized. The result of this large stabilization upon deprotonation, based on the transition orbitals for the S1 states for both acetophenone and its carboxylate-functionalized equivalents (Figure S1, Supporting Information), is that the ordering of the nπ* and ππ* states actually inverts between the meta and para positions. More generally, the opposing effects of charge on the nπ* and ππ* transitions provide a strategy for, say, red-shifting one with respect to the other.
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RESULTS AND DISCUSSION The photochemistry of acetophenone is known to change depending on whether it is excited to nπ* S1, upon which it will fluoresce, or to ππ* S2, which will give rise to radical generation via the mechanism shown in Figure 1.19 The nπ* S1 transitions involve electron donation from the lone pair on oxygen to the π* orbital of the phenyl to form a chargeseparated biradical,13 while the ππ* S2 involves excitation from the π-bonding orbital to its antibonding orbital. The nπ* state can undergo dissociation but only after intersystem crossing to the triplet (nπ* T1). The relative energies of the various states influence the population of the dissociative nπ* T1 state and can thus have a major impact on the efficiency of acetophenone as a photoinitiator. Charged functional groups were added to the acetophenone phenyl ring in combination with a methylene spacer group to prevent direct π-conjugation between the charged group and the orbitals on the acetophenone fragment. This separation does not completely prevent through-bond effects such as sigma donation or withdrawal, but will to a first approximation help to isolate the electrostatic effects from other orbital effects. Deprotonated carboxylic acid groups were used to form negative charges, and protonated tertiary amine groups were used to form positive charges (Figure 2). Groups were placed 17801
DOI: 10.1021/jacs.8b12009 J. Am. Chem. Soc. 2018, 140, 17800−17804
Article
Journal of the American Chemical Society
Figure 3. Change in 1nπ*, 1ππ*, 3nπ*, and 3ππ* vertical excitation energies (eV), upon charge formation (i.e., deprotonation of COOH, red, or protonation of NMe2, blue) relative to the corresponding neutral systems.
Scheme 1. Charge-Separated Representation of the 1nπ* Excited State
In the ortho position, the positively charged group lowers the 1ππ* transition, in opposition to the other trends. This is because upon protonation the ortho-amino group forms a hydrogen bond between the N−H and the carbonyl moiety (Figure S1, Supporting Information). This bond is not possible in the neutral species (lacking the proton) and thus results in a net stabilization despite the destabilizing electrostatic effects. Removal of the hydrogen bond by selecting a higher energy conformation without the hydrogen bond present returns the pH switch to the predicted trend of (de)stabilization. However, the lowest conformer results, i.e., the conformer with the H-bond, are presented here, as they highlight how different intramolecular interactions can prevent a designed electric field effect from taking place. Triplet States. Acetophenone is an example of a molecule that exhibits an intersystem crossing (ISC) to its triplet excited states, regardless of whether it is excited to the S1 or S2 states. ISC is favored in situations where the Sn and Tn states have different orbital symmetries; for example 1nπ* → 3ππ* ISC is more efficient than 1ππ* → 3ππ* ISC.24 Tuning the triplet energies of acetophenone is undoubtedly unnecessary, due to the near-degenerate 1nπ*/3ππ* states. However, examining the triplet excited states of acetophenone functionalized with (un)charged groups suggests that these transition energies can also be altered with OEFs (Figure 3). For the nπ* transitions the electrostatic effects in the singlet and triplet states are essentially identical. For the ππ* transitions, however, there are differences with the electrostatic effects on the triplet states generally smaller than the corresponding singlet states. This is likely due to TD-DFT inadequacies,25,26 rather than an electric field effect. As we move further into the excited state manifold with the T2(ππ*) state (inaccessible with open-shell calculations), these errors are likely exacerbated. A more thorough exploration of the excited states with high-level multireference calculations would provide more insight into the true behavior of the triplet states; however this is beyond the scope of the current work. It is sufficient to conclude here that electrostatic effects on the triplet nπ* and ππ* are qualitatively in line with those on the corresponding singlet states. Isolation of Electrostatic Effects. While the CH2 linker prevents π-conjugation between the pH-switchable group and the acetophenone, hyperconjugation between the phenyl ring and CH2−R σ bond (where R is COOH or NMe2) is possible. Indeed, in the lowest energy conformations of the various neutral and charged species this CH2−R σ bond is clearly orthogonal to the phenyl ring, allowing for orbital overlap with the π system (Figure S1, Supporting Information). Formation of a charge on R can stabilize or destabilize the C−R sigma*
To understand the ππ* effects in more detail, we turn to valence bond (VB) theory. Scheme 2 shows the primary Scheme 2. Resonance Contributors to the S2 ππ* State
resonance contributors to the 1ππ* state. While there are no apparent dipoles as drawn, VB studies of simpler compounds such as ethylene21 show that additional “asymmetric” contributors can contribute as much as a quarter of the VB wave function (Scheme 3). In a symmetrical compound such Scheme 3. VB Contributors to the First Singlet Excited State of Ethylene, Adapted from Ref 21
as ethylene this results in no net dipole, as both asymmetric contributors are degenerate; it does, however, explain the highly polarizable nature of the ππ* excited state. However, in acetophenone, the equivalent asymmetric VB structures for IV would involve a choice between a net negative charge on the electronegative oxygen or a net positive charge. The former would be more stable, and this contributor would thus carry more weight, giving the excited acetophenone a net dipole in which the para-carbon is delta positive and the oxygen is delta negative. As a result, one would expect a negatively charged group to stabilize the ππ* state and the positively charged group to destabilize it, in accordance with observations in Figure 3. At the same time, the ππ* state remains highly polarizable, and this polarizability enhances the stabilizing effects of the negative charge and diminishes the destabilizing effects of the positive charge, analogous to the asymmetry seen previously in ground state electrostatic catalysis.22,23 17802
DOI: 10.1021/jacs.8b12009 J. Am. Chem. Soc. 2018, 140, 17800−17804
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Journal of the American Chemical Society orbital and, hence, indirectly enhance or diminish hyperconjugation. To separate through-bond orbital effects and through-space electrostatic effects, two independent approaches can be used: application of an increasingly large external electric field to the protonated form of the lowest energy deprotonated carboxylate structure and removal of the CH2 spacer group to instead terminate the carboxylate and acetophenone groups with hydrogen atoms. The external electric field was applied to the neutral form of the lowest energy m-carboxy-substituted structure. Moving from the protonated to the deprotonated, charged form of the carboxylate structure results in a large change in the overall dipole moment, and the electric field was applied along the axis in which this change was largest (Figure S4, Supporting Information). Figure 4 shows the destabilization of the nπ* transition and stabilization of the ππ* transition is almost totally recovered at a field strength of 1.6 × 10−2 au.
Figure 5. Removal of methylene spacer groups demonstrates the through-space effect deprotonation has on the 1nπ* and 1ππ* transition energies.
8.93) has previously been found to be satisfactory.27 The effect of solvation on the change in energy of the nπ* and ππ* states, relative to uncharged para-substituted acetophenone, is shown in Figure 6 for a range of solvents. There is a significant
Figure 4. Change in 1nπ* and 1ππ* vertical excitation energies on uncharged, m-carboxy-functionalized acetophenone with increasing static electric field strength.
The external electric field calculations in Figure 5 still include any through-bond sigma effects, which can be removed by deleting the methyl spacer group, i.e., studying an equivalent complex of HC(O)O− (or H2NMe2+) and acetophenone (Figure 5) and comparing the excitation energies of the resulting protonated and deprotonated forms. To be as consistent as possible when the CH2 linker was removed, the positions of all conserved atoms were held constant, the missing bonds were capped with hydrogens, and these were added using standard default bond lengths and angles. Figure 5 confirms that electrostatic effects persist in the absence of a sigma-bonded connection between the charged group and the acetophenone and also suggests the sigma effects are attenuating the electrostatic field in the case of the 1 ππ* transition for the carboxylate-substituted compound. This is consistent with the idea that, when connected by a CH2 linker, the charge is also diminishing hyperconjugative stabilization of the excited state. Solvent Effects. Calculations so far have focused primarily on electrostatic effects exhibited in the gas phase; however, in real systems solvation is a key concern for future application of OEFs, as high-polarity solvents, for example, acetonitrile and water, significantly attenuate through-space OEFS.23 It is therefore necessary to find a balance between solubility and polarity for the charged species, and dichloromethane (ϵ =
Figure 6. Change in 1nπ* and 1ππ* vertical excitation energies upon charge formation (i.e., deprotonation of COOH, red, or protonation of NMe2, blue) for para-substituted acetophenone as a function of increasing solvent polarity.
reduction in both stabilization and destabilization effects for both states, as solvent polarity is increased. Promisingly, the electrostatic effects for negative carboxylate remain significant in both dichloromethane and even acetonitrile (ϵ = 35.7), exhibiting changes in ππ* energy of −0.53 and −0.47 eV, respectively. This suggests that electrostatic effects on photochemistry could remain large and usable in a range of solvation environments.
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CONCLUSIONS To conclude, electrostatic effects have been found to originate from the introduction of CFGs to acetophenone. These effects have a significant effect on the stability of ππ* transitions and a smaller opposite effect on nπ* transitions. Specifically, a nonconjugated negatively charged group in the para position will destabilize the nπ* and stabilize the ππ*, while a positively charged group will do the opposite. These electrostatic effects can be tuned by moving these substituents into the meta and ortho positions and/or increasing their distance from the acetophenone through the use of additional linkers. Given the 17803
DOI: 10.1021/jacs.8b12009 J. Am. Chem. Soc. 2018, 140, 17800−17804
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Journal of the American Chemical Society
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position dependence for both the magnitude and direction of the effective (de)stabilizing field, it should be possible to enhance the efficiency of low-yielding photochemical processes. As well as this, unlike effects on ground states,23,27 electrostatic effects are found to remain significant even in quite polar solvents, such as acetonitrile. This is likely due to the increased sensitivity to electric fields exhibited by the more diffuse antibonding orbitals. This gives great promise for the general application of this approach to solvated photochemical systems.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b12009. Geometries and total energies for all structures in this work (PDF)
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AUTHOR INFORMATION
Corresponding Author
*
[email protected] ORCID
Michelle L. Coote: 0000-0003-0828-7053 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge financial support from the Australian Research Council (ARC) Centre of Excellence for Electromaterials Science, an ARC Laureate Fellowship (to M.L.C.), generous supercomputing time from the National Computational Infrastructure, and helpful discussions with Kam Fung.
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DOI: 10.1021/jacs.8b12009 J. Am. Chem. Soc. 2018, 140, 17800−17804
