Extension of Intramolecular Charge-Transfer State Lifetime by

Institute for Energy Studies, Western Washington University, Bellingham, WA, USA .... This Article reports a mechanism for the CT state lifetime exten...
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Extension of Intramolecular Charge-Transfer State Lifetime by Encapsulation in Porous Frameworks Natalya A. Garcia† and Tim Kowalczyk* Department of Chemistry, Advanced Materials Science and Engineering Center, and Institute for Energy Studies, Western Washington University, Bellingham, Washington 98225, United States S Supporting Information *

ABSTRACT: Control over excited-state lifetimes and relaxation pathways in chromophoric materials is a major challenge in the optimization of photovoltaic energy conversion and photochemical energy storage mechanisms. In light of experimental evidence that the charge-transfer excited-state lifetime of 9-mesityl-10-methylacridinium ion (MesAcr+) can be dramatically extended via encapsulation in the mesoporous aluminosilicate AlMCM-41, we employ multiscale simulations to uncover a mechanism for the suppression of charge recombination of MesAcr+ in AlMCM-41. The simulations reveal that solvation in acetonitrile and encapsulation in AlMCM-41 have opposing effects on the reorganization energy of MesAcr+: solvation substantially raises the reorganization energy while the encapsulation lowers it. Because charge recombination in MesAcr+ takes place deep within the Marcus inverted region, the smaller reorganization energy observed in solvent-free AlMCM41 raises the activation barrier for excited-state charge recombination. Implications for the design of encapsulated chromophores to extend excited-state lifetimes for energy conversion applications are discussed.

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INTRODUCTION Electronic excitation is a transient but essential first stage in a variety of solar energy conversion and storage processes, from photosynthesis and photovoltaics to the photoelectrochemical production of solar fuels. In organic chromophores, the excitation energy acquired from an absorbed solar photon is typically retained for a time scale on the order of fs to ns before the energy is used to drive downstream chemical processes or is dissipated through radiative relaxation or internal conversion.1,2 The ability to extend the lifetime over which this initial excitation energy is available could increase the efficiency of a variety of photophysical processes that rely on this energy to generate useful electrical work, as in organic photovoltaics, or to produce high-value chemical fuels3 and other feedstocks4 from CO2. The challenge of channeling the energy associated with this photoexcitation into useful work by sustaining an electrical current or initiating an energetically uphill photochemical reaction is also a central issue in artificial photosynthesis.5−7 Among efforts to engineer extended excited-state lifetimes into organic materials, approaches based on small-molecule donor−acceptor dyads offer a high degree of structural control.8,9 These dyads undergo rapid charge separation upon photoexcitation, yielding a charge-transfer (CT) excited state. The redistribution of charge accompanying CT state formation reduces the probability of radiative losses through fluorescence, and with judicious matching of donor and acceptor moieties, the CT state lifetime can be extended by several orders of magnitude in frozen matrices. Achieving the same excited-state lifetime extension at room temperature has proven more challenging.10,11 In this Article, atomistic simulations of charge © XXXX American Chemical Society

recombination (CR) kinetics from the CT state of a dyad in three different molecular environments − vacuum, dilute solution, and encapsulated in a porous aluminosilicate material − are compared to uncover a mechanism for the experimentally observed extension of CT state lifetime in porous media. The 9-mesityl-10-methylacridinium ion (MesAcr+, 1, Figure 1) is among the most intensively investigated small donor−

Figure 1. MesAcr+ cation 1.

acceptor chromophores. In its ground electronic state, the positive charge of the ion resides primarily on the 10methylacridinium (Acr) moiety, but photoexcitation can in principle generate a biradicaloid CT state, with the positive charge shifted to the Mes moiety, via local S1 excitation on Acr. Since the introduction of 1 by Fukuzumi’s group,12 the formation and stability of the CT state in 1 has been a controversial finding.13−16 Debate over the interpretation of cyclic voltammetry and spectroscopic measurements of 1 in Received: July 10, 2017 Revised: August 19, 2017 Published: September 6, 2017 A

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The Journal of Physical Chemistry C benzonitrile centered around whether the long-lived excited state corresponded to the CT state or a long-lived locally excited triplet state. Despite the controversy, 1 has proven in practice to be a versatile platform for organic photoredox catalysis,17−20 and interest in this donor−acceptor dyad has recently surged on demonstrations of this catalytic versatility. An investigation of other 9-substituted 10-methylacridinium ions by Fukuzumi’s group suggests that the exceptionally large driving force for charge recombination in 1 uniquely suits the dyad for long-lived CT state formation.21 Recent experimental evidence for a long-lived CT state of 1 sustained at elevated temperatures through encapsulation in solvent-free AlMCM-41 pores22,23 points to the possibility of temporarily “storing” solar energy in a hybrid material for which the active storage medium is a collection of electronically excited, charge-separated organic donor−acceptor dyads encapsulated in a mesoporous aluminosilicate. Spectroscopic studies of other organic radical ions encapsulated in mesoporous aluminosilicates have suggested that electron donating and accepting sites on the pore wall can modulate the energetics of intramolecular charge transfer.24 The charge-stabilizing effects of encapsulation have been observed in other electron transfer processes in porous media, including the Fe(II)−Fe(III) electron self-exchange reaction in an ionic liquid environment confined by nanoporous carbon25 and exchange between oxo-bridged Ti(IV) and Mn(II) centers anchored on a silica nanopore.26 The favorable influence of nanoconfinement on CT state lifetimes extends to intermolecular CT between the encapsulated species and the porous medium. For example, Hureau et al. showed that confinement within zeolites can lower the rate of charge recombination between oxidized trans-stilbene and reduced sites on the zeolite wall.27 Rigid spacer groups have been employed to control excited-state relaxation pathways in Ru-based photosensitizers anchored to mesoporous TiO2 thin films.28 This Article reports a mechanism for the CT state lifetime extension of 1 in AlMCM-41, supported by density functional theory (DFT)-based molecular simulations and interpreted through the Marcus theory of electron transfer. Long CT state lifetimes correspond to a slow rate k CR for charge recombination from the CT excited state back to the ground electronic state. Classical Marcus theory establishes the following well-known expression for kCR in terms of the driving force for CR, −ΔGCR, the reorganization energy λ, and the electronic coupling VCR,29,30 k CR =

2π ℏ

⎡ (λ + ΔG )2 ⎤ CR ⎥ exp⎢ − k T 4 λ 4πλkBT ⎦ ⎣ B

− ΔGCR =

λ=

1 (⟨ΔE⟩ID + ⟨ΔE⟩CT ) 2

1 (⟨ΔE⟩ID − ⟨ΔE⟩CT ) 2

(2)

(3)

Here the subscripts indicate the electronic state for which the sampled configurations are equilibrium configurations. The methods employed to compute energy gaps between ID and CT states are based on DFT with the PBE0 functional33 with environment-dependent corrections as discussed below. Gas Phase. Diabatic representations of the ground state and charge-transfer excited state of 1 were obtained through constrained density functional theory (CDFT)34 as implemented in Q-Chem 4.2.35 The idealized ground (ID) and charge-transfer (CT) diabatic states correspond to localization of the +1 charge on the Acr and Mes moieties, respectively. The PBE0 functional33 and 6-31G* basis set were employed throughout, unless otherwise specified. Solution Phase. A periodic box containing a single molecule of 1 in explicit acetonitrile solvent was prepared from the organic solvent structure database at virtualchemistry. org.36,37 After classical energy minimization with the CGenFF force field,38,39 several QM/MM molecular dynamics trajectories with electronic embedding of 1 in MeCN were obtained along both electronic states (ID and CT). These simulations sampled the NVT ensemble at T = 298 K using a Nosé− Hoover thermostat and an integration time step of 2 fs. The QM energy and forces in the QM/MM dynamics were obtained through same CDFT approach used for 1 in vacuum except for the adoption of the smaller 3-21G basis for computational efficiency. Energies of the ID and CT states were computed for snapshots sampled every 20 steps from each trajectory at the constrained PBE0/6-31G* level for consistency with simulations in the gas-phase and encapsulated environments. The resulting 200 configurations per electronic state represent modest sampling at best, but the energy gap distributions were sufficiently converged to estimate Marcus parameters in solution. The solvent itself was modeled with the CGenFF force field. QM/MM simulations were performed through the CHARMM/Q-Chem interface.40 Encapsulation in AlMCM-41. Given the large size of the MesAcr@AlMCM-41 model system, we employed a multiscale modeling approach in which the MesAcr+ dyad (1) was modeled with DFT (PBE0/6-31G*) in Q-Chem while the AlMCM-41 pore and its interactions with the dyad were modeled with DFTB (including empirical dispersion) using the DFTB+ package.41 This approach, similar in spirit to ONIOM methods,42 applies the difference between DFT and DFTB energies of 1 as a correction factor to the DFTB energy of the full 1@AlMCM-41 system,

VCR 2

(1)

By measuring the environment-dependence of Marcus parameters as predicted by the simulations, we can explain how encapsulation of 1 in AlMCM-41 extends its chargetransfer state lifetime. Computational Methods. Our simulation approach follows closely a strategy based on the linear free energy relation introduced by Warshel.31 The equilibrium distribution of the energy gap ΔE between the diabatic electronic ground state (denoted ID for “idealized ground state”) and the diabatic CT excited state (simply denoted CT) in each environment is approximated from the simulations and used to calculate −ΔGCR and λ within the linear response approximation,32

E K (1@AlMCM‐41) K K K ≈ E DFTB (1@AlMCM‐41) + [E DFT (1) − E DFTB (1)]

(4)

where K ∈ {ID,CT} is the electronic state of interest. Configurational sampling of a total 7128 ID state configurations and 6163 CT state configurations was achieved through a combined Monte Carlo and molecular dynamics (MC/MD) approach. MC/MD simulation details, including the validation of Slater−Koster parameters, are available in the Supporting Information. B

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Figure 2. Simulation of Marcus ET parameters for 1@AlMCM-41 through the linear free energy relation. (a) Representative snapshot of 1@ AlMCM-41. (b,c) Potential energy curves of the ID (blue) and CT (red) states as a function of distance between the centroid of 1 and the nearest atom in the pore wall. The curves represent averages over configurations sampled in (b) the CT state and (c) the ID state, respectively. (d,e) Distribution of the energy gap ΔE observed along (d) CT state and (e) ID state MC/MD trajectories, respectively.

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RESULTS AND DISCUSSION A slab of AlMCM-41 was prepared from a DFT-optimized structure of MCM-4143 through Al doping and Na+ cation exchange of the model by analogy to the procedures described in ref 23. For computational tractability, the energy of the pore wall and its interactions with 1 were modeled using the multiscale approach employing DFT for 1 and densityfunctional tight-binding (DFTB)44 for the AlMCM-41 pore wall. A sampled snapshot from the DFT-in-DFTB simulations of 1@AlMCM-41 is shown in Figure 2a. Energy scatter plots of the ID and CT states in Figure 2b,c show that configurations sampled from hybrid Monte Carlo/ molecular dynamics simulations in both states reflect a weak, configuration-dependent noncovalent attraction between 1 and the pore wall. The distribution of energy gaps ΔE in Figure 2d,e reveal a skew toward smaller ΔE for CT configurations and toward larger ΔE for ID configurations, but the average energy gaps only differ by 0.2 eV (Table 1). The calculated

state. Because of the greater density of solvent molecules required in the simulation, we adopted a QM/MM approach; further details are provided in the Computational Methods and Supporting Information. The solution-phase simulations revealed qualitative differences between the energy gap distributions for 1 in solution versus encapsulated in AlMCM-41. Applying eqs 2 and 3 with average energy gaps taken from the distributions in Figure 3d,e, we obtained a driving force −ΔGCR = 2.74 eV which hardly differs from the driving force in AlMCM-41. In contrast, the reorganization energy was substantially elevated in MeCN solution at λ = 1.45 eV due to solvent stabilization of the localized charge in each electronic state. Marcus parameters for 1 in vacuum, in MeCN solution, and in solvent-free AlMCM-41 are summarized in Table 1. Overall, the driving force for CR appears only weakly sensitive to the environment, while the reorganization energy shows a strong dependence. In stark contrast to the considerable outer-sphere reorganization energy in solution, we observe a reduction in the total reorganization energy of 1@ AlMCM-41 relative to the gas phase. This observation suggests that for donor−acceptor dyads encapsulated in porous materials, λ cannot necessarily be decomposed into positive inner-sphere and outer-sphere contributions, λ = λinner + λouter, as is frequently assumed in homogeneous solution. Figure 4 illustrates the differences between CR in vacuum, in MeCN solution, and encapsulated in AlMCM-41 from the perspective of Marcus theory. By dramatically lowering the reorganization energy relative to the solution phase, and even below that in vacuum, encapsulation in AlMCM-41 raises the activation

Table 1. Comparison of Marcus ET Parameters Derived from Simulations of 1 in Vacuum, in MeCN Solution, and Encapsulated in AlMCM-41a

a

environment

method

⟨ΔE⟩ID

⟨ΔE⟩CT

−ΔGCR

Λ

vacuum MeCN AlMCM-41

PBE0 PBE0/MM PBE0-in-DFTB

3.10 4.19 3.01

2.28 1.29 2.42

2.70 2.74 2.71

0.40 1.45 0.30

All energies are reported in eV.

driving force −ΔGCR = 2.71 eV is remarkably close to the calculated value in the gas phase (2.70 eV) and greater than Fukuzumi’s voltammetric estimate of 2.37 eV in MeCN.12 To better understand differences between the homogeneous MeCN environment and the environment of the AlMCM-41 pore interior, we performed MD simulations of 1 in MeCN solution at room temperature and applied the same linear free energy relationship used above to sample ΔE in each electronic

(λ + ΔG )2

‡ CR = barrier ΔGCR in the Marcus inverted region and 4λ substantially lowers the CR rate. To probe more precisely how AlMCM-41 encapsulation reduces the reorganization energy of 1, we analyzed the dependence of the energy gap on structural features of the interface between 1 and the pore wall. Hypothesizing that

C

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Figure 3. Simulation of Marcus ET parameters for 1 in MeCN solution through the linear free energy relation. (a) Representative snapshot of 1 solvated in MeCN. (b,c) Evolution of the ID (blue) and CT (red) state energies along several ps of an example QM/MM MD trajectory propagated on the (b) CT, (c) ID potential energy surfaces. (d,e) Distribution of the energy gap ΔE observed along (d) CT state and (e) ID state QM/MM MD trajectories, respectively.

reduce the reorganization energy. Since this CR takes place in the Marcus inverted region, a further reduction of λ without a change in ΔGCR should increase the CR activation barrier, trapping the excitation energy in the charge-transfer state for longer. To provide further evidence for the role of charged sites on the pore in modulating the reorganization energy, at each MC/ MD sampled configuration we measured the distance from the strongest Lewis acidic and basic sites in the pore wall to the centroid of the Acr moiety (for the ID state) and of the Mes moiety (for the CT state). We then computed the electrostatic interaction between the DFTB Mulliken charge at these Lewis acid/base sites on the pore wall and a model +1 charge at each centroid. Further details of the model are provided in the Supporting Information. Table S1 shows that Wall-Acr-Mes configurations experience a stronger destabilization of the ID state by a Lewis acidic site than Wall-Mes-Acr configurations. Conversely, the CT state is more strongly destabilized in the Wall-Mes-Acr configurations than in Wall-Acr-Mes configurations. The net result is a decrease in the difference between the energy gaps in ID and CT configurations, and hence (via eq 3) a decrease in the reorganization energy.

charge transfer parameters would be sensitive to the orientation of 1 relative to the pore wall, we classified MC/MD snapshots according to which moiety of the dyad (Mes or Acr) was closer to the pore wall. We then computed the driving force and reorganization energy separately for Wall-Mes-Acr and WallAcr-Mes configurations. Table 2 reveals a smaller difference in the energy gaps, and therefore (via eq 3) a smaller reorganization energy for snapshots in the Wall-Acr-Mes orientation. The orientation dependence of the sampled energy gaps suggests a mechanism involving Lewis acidic and basic sites in the pore wall to explain the smaller reorganization energy of 1@AlMCM-41 relative to other phases. Our proposed mechanism is closely related to the argument put forth by Zilberg45 to help resolve a debate in the literature13−16 regarding the role of triplet versus CT excited states in the photophysics of 1. The presence of both Lewis acidic and basic sites in AlMCM-41 is well documented,24 and the role of counteranions in stabilizing the CT state of 1 has been corroborated experimentally in laser-pump, X-ray probe crystallographic characterization of MesAcr(ClO−4).46 When 1 is oriented with the Acr unit in close proximity to the pore wall, as in Figure 5a, electrostatic attraction between the electron hole on the dye and a Lewis basic site on the wall reduces the difference between the energy gaps for equilibrium ID and CT configurations, facilitating nuclear reorganization. Conversely, when 1 is oriented with the Mes unit closer to the pore wall, as in Figure 5b, reorganization can be facilitated by a lowering of electrostatic repulsion between the hole and a Lewis acidic site. In both cases, the availability of partially charged sites on the interior surface of AlMCM-41 lowers the energetic cost of reorganizing the dye’s geometry. Furthermore, our simulations indicate that the lowering is slightly more pronounced for the case where the Acr ion is oriented closer to the wall. These results suggest that orientational control over the dyad, for example, via strategic functionalization with electronically noninteracting bulky substituents or selective narrowing of the pore wall, could be harnessed to further

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CONCLUSIONS We have rationalized the observed extension of charge-transfer excited state lifetime in the photocatalyst MesAcr+ via encapsulation in the mesoporous aluminosilicate AlMCM-41. Electronic structure-based simulations reveal a significant environment-dependence of the reorganization energy λ of MesAcr+, including a reduced λ when the MesAcr+ is encapsulated in AlMCM-41. The smaller λ can be explained in terms of orientation-dependent electrostatic interactions between MesAcr+ and the AlMCM-41 wall. This understanding, and the ability to quantify Marcus parameters in complex environments through molecular simulation, will aid in the rational design of photofunctional organic materials that D

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Figure 5. Orientation dependence of the proposed mechanism for reorganization energy lowering of 1 by encapsulation in AlMCM-41. The δ+ and δ− charges on AlMCM-41 represent generic Lewis acidic and basic sites, respectively. The curved arrow indicates electron flow.

absorb and manipulate solar energy for catalysis, charge carrier generation, and production of solar fuels.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b06770. Additional details of MC/MD simulations, including Slater−Koster parameter validation; details of the electrostatic dye-pore interaction model; Cartesian coordinates of optimized configurations of 1 in vacuum and of the model AlMCM-41 pore wall (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Tim Kowalczyk: 0000-0003-1806-059X Present Address †

N.A.G.: Department of Chemistry, Curtin Institute for Computation, Curtin University, PO Box U1987, Perth, Western Australia 6845, Australia.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Acknowledgment is made to the Donors of the American Chemical Society Petroleum Research Fund (57240-UNI6) for partial support of this research. T.K. gratefully acknowledges start-up support from Western Washington University. The QM/MM simulations reported in this work were enabled in part through a WWU Research and Creative Opportunities for Undergraduates award to N.A.G.

Figure 4. Marcus curves for CR in photoexcited 1 (a) in the gas phase (b) in MeCN solution at room temperature (c) encapsulated in solvent-free AlMCM-41 at room temperature. The differing environments have negligible influence on the driving force for back electron transfer, but the reorganization energy is raised in MeCN solution and depressed within AlMCM-41.

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Table 2. Orientation Dependence of Average Energy Gaps ⟨ΔE⟩, in eV, between ID and CT States of 1@AlMCM-41 ⟨ΔE⟩ID ⟨ΔE⟩CT

Wall-Acr-Mes

Wall-Mes-Acr

2.97 2.45

3.04 2.39

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DOI: 10.1021/acs.jpcc.7b06770 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.7b06770 J. Phys. Chem. C XXXX, XXX, XXX−XXX