Ultrafast Electron Transfer across a Nanocapsular Wall: Coumarins

Article Cite This: J. Phys. Chem. B XXXX, XXX, XXX−XXX

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Ultrafast Electron Transfer across a Nanocapsular Wall: Coumarins as Donors, Viologen as Acceptor, and Octa Acid Capsule as the Mediator Chi-Hung Chuang,† Mintu Porel,‡ Rajib Choudhury,§ Clemens Burda,*,† and V. Ramamurthy*,‡ †

Center for Chemical Dynamics and Nanomaterials Research, Department of Chemistry, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106, United States ‡ Department of Chemistry, University of Miami, Coral Gables, Florida 33124, United States § Department of Physical Sciences, Arkansas Tech University, Russellville, Arkansas 72801, United States S Supporting Information *

ABSTRACT: Results of our study on ultrafast electron transfer (eT) dynamics from coumarins (coumarin-1, coumarin-480, and coumarin-153) incarcerated within octa acid (OA) capsules as electron donors to methyl viologen dissolved in water as acceptor are presented. Upon photoexcitation, coumarin inside the OA capsule transfers an electron to the acceptor electrostatically attached to the capsule leading to a long-lived radical−ion pair separated by the OA capsular wall. This charge-separated state returns to the neutral ground state via back electron transfer on the nanosecond time scale. This system allows for ultrafast electron transfer processes through a molecular wall from the apolar capsular interior to the highly polar (aqueous) environment on the femtosecond time scale. Employing femtosecond transient absorption spectroscopy, distinct rates of both forward (1−25 ps) and backward eT (700−1200 ps) processes were measured. Further understanding of the energetics is provided using Rehm−Weller analysis for the investigated photoinduced eT reactions. The results provide the rates of the eT across a molecular wall, akin to an isotropic solution, depending on the standard free energy of the reaction. The insights from this work could be utilized in the future design of efficient electron transfer processes across interfaces separating apolar and polar environments.

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communicate.32,33 While the internal cavity of the OA nanocapsule is hydrophobic, the exterior functionalized with carboxylate groups being hydrophilic makes it soluble in aqueous solution.34 Although the capsule separates the donor and acceptor into hydrophobic and hydrophilic environments, it keeps the donor−acceptor pairs spatially coupled within 1 nm.35 Keeping the central role of eT in several aspects of chemistry in mind, a few years ago, we initiated a preliminary femtosecond laser spectroscopic investigation of a supramolecular coumarin-153@(OA)2−acceptor complex, in which coumarin-153 resided in a nanocapsule made up of two molecules of octa acid and the acceptor 4,4′-dimethyl viologen dichloride (MV2+) was bound electrostatically to the OA2 capsule’s exterior surface.36 While in an isotropic solvent eT from C153 to methyl viologen had a time constant of ∼631 ps, limited by diffusion, the incarcerated C153 within an OA2 capsule showed an accelerated eT to methyl viologen with a time constant of 20 ps. The enhanced rate of eT is attributable to the supramolecular assembly in which the donor and acceptor are held at a close distance through hydrophobic and

INTRODUCTION Electron transfer (eT) reactions are important in chemistry, biology, and nanochemistry.1−10 Recently, supramolecular assemblies that mimic nature have become increasingly important in the development of novel optical, electronic, and energy conversion systems.10−14 Research that melded supramolecular assemblies and electron transfer has yielded considerable mechanistic insight into the relationship between energy, structure, and the dynamics of forward (eT) and back electron transfer (BeT).15−24 In recent years, reactions initiated by eT prompted by visible light have become a sought after strategy in organic synthesis.25−28 In addition, understanding light-induced electron transfer in supramolecular assemblies is important in generating and harvesting energy via transient radical ion pairs.29,30 This article presents results of our studies directed toward understanding the electron transfer (eT) process, in water, initiated by light in a supramolecular assembly made up of an organic host (octa acid, OA),31 entrapped guest donors (substituted coumarins), and acceptor 4,4′-dimethyl viologen dichloride (MV2+) held closer to the capsule via electrostatic interaction. Our recent recognition of the value of OA as a unique reaction container prompted us to probe whether an OA encapsulated guest molecule and a free molecule in solution can © XXXX American Chemical Society

Received: November 15, 2017 Revised: December 6, 2017 Published: December 6, 2017 A

DOI: 10.1021/acs.jpcb.7b11306 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B

Figure 1. Structure of the donors (guests), acceptor, and hosts. Coumarin-1, coumarin-480, coumarin-153, 4,4′-dimethyl viologen dichloride, and octa acid are abbreviated with C1, C480, C153, MV2+, and OA, respectively. Two OAs form an OA2 nanocapsule with a nonpolar interior.

from Sigma-Aldrich/Acros Organics. Deionized water was used for complexation studies. General Procedure for Guest Binding Studies Probed by NMR. A D2O stock solution (600 μL) of host OA (1 mM) and sodium tetraborate buffer (10 mM) taken in a NMR tube was titrated with the guest by sequential addition of guest (known amounts of 60 mM solution in DMSO-d6). The complexation was achieved by shaking the NMR tube for about 5 min. 1H NMR spectra were recorded at room temperature under aerated conditions on a Bruker 500 MHz NMR spectrometer. The 1:2 (guest:host) complex was achieved by adding 5 μL of guest solution (60 mM in DMSO-d6) to 600 μL of 1 mM OA host in 10 mM buffer. Completion of complexation was monitored by the disappearance of the free OA signals in 1H NMR spectra upon addition of guest. Fluorescence Studies. Fluorescence spectra were recorded on a FS920CDT Edinburgh steady-state fluorometer. A 30 mM stock solution of the guest was prepared in CHCl3. The host (OA) aqueous solution was prepared in a 10 mM sodium tetraborate buffer. The guest−OA complexes (guest@OA2) solutions were prepared by adding 1.5 mL of the host buffer solution to a thin film of the guest prepared by evaporating the solvent from a CHCl3 guest solution. The resulting aqueous mixture was sonicated for 30 min to obtain a transparent. Finally, a calculated amount of the quencher solution (MV2+ in water) was added and mixed thoroughly by shaking the test tube. The solutions containing different amounts of MV2+ were transferred to fluorescence cell, and the fluorescence spectra were recorded. For control experiments without OA, the solution of coumarins was prepared in 30% (v/v) acetonitrile− water solvent mixture. Femtosecond Transient Absorption Measurements. Femtosecond transient absorption (TA) measurements were performed using a Clark MXR 2001 fs laser system producing 780 nm, 150 fs pulses from a regenerative amplifier.5,7 The laser pulse train was split to generate a white light continuum probe

Coulombic interactions. Such a preorganization provided an opportunity to probe the inherent rate of eT in an isotropic aqueous solution bypassing the need for diffusion. Here, we expand on our preliminary observation of eT from C153@OA2 to methyl viologen and present a broader, more general picture of the photophysics of donor@OA2−acceptor pairs. We report results of our investigations on photoelectron transfer between three donors (coumarin-153, coumarin-1, and coumarin-480) and acceptor (4,4′-dimethyl viologen dichloride)37 and the comparative analysis of their eT dynamics. The forward eT and backward eT (BeT) dynamics are measured in this series of supramolecular assemblies using femtosecond laser spectroscopic transient absorption (TA) measurements. The chemical structures of the donors, acceptor, and host are provided in Figure 1. We also discuss the correlation between energetics and eT rates of the different donor@OA2−acceptor complexes using the Rehm−Weller equation.38 Results also bring out the importance of capsular wall in the electron transfer process. Our findings suggest that this unique system, consisting of a fast eT rate from a hydrophobic to a hydrophilic medium separated by a molecular wall of a nanocapsule, can be potentially useful for effective photocarrier generation. In addition, the presented nanocapsules can be applied for bioimaging since the internal hydrophobicity enhances the fluorescent yield of the encapsulated chromophore. Although a full understanding of the phenomenon is yet to be reached, results thus far obtained and presented here clearly establish that this process is general and is ready for further theoretical and experimental exploitation.

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EXPERIMENTAL SECTION Chemicals. The hosts octa acid and CB7 were synthesized following literature procedures.31,39 Laser grade coumarin-153 (C153), coumarin-480 (C480), and coumarin-1 (C1) and 4,4′dimethyl viologen dichloride (MV2+) were used as received B


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Figure 2. (a) Normalized UV−vis absorption (solid lines) and fluorescence (dashed lines) spectra of coumarin@OA2 complexes. The spectra for the C-1@OA2 (λex = 360 nm), C-480@OA2 (λex= 370 nm), and C-153@OA2 (λex= 400 nm) in the Na2B4O7 buffer are shown in red, black, and blue, respectively. Absorption and fluorescence spectra are shifted vertically for clarity. (b) Absorption (solid lines) and fluorescence (dashed lines) spectra of coumarin@OA2 complexes (2 × 10−5 M) in the presence of MV2+ (5 × 10−5 M). The fluorescence is quenched in each coumarin@OA2 case. The inset shows the absorption spectrum of MV2+. It has an absorption maximum at ∼256 nm and no absorption above 300 nm.

Frog algorithm; coordinate and velocities were saved every 10 ps. Trajectories and relevant data were analyzed with VMD and Pymol programs.47,48 Most representative structures were prepared by clustering (program: g_cluster) structurally similar frames (RMSD cutoff 0.3 nm). The frame with largest number of neighbors, a representative of that cluster, has been reported throughout this work. For coumarin@OA2−MV2+ complexes similar simulation strategies were employed. Initial structures for the simulations were prepared by taking equilibrated structures from the previous 100 ns coumarin@OA2 simulations. A molecule of MV2+ was added outside of the coumarin@OA2 capsule, followed by construction of a cubic box of dimensions 40 × 40 × 40 Å3. Box was filled with explicit waters, and the system was neutralized with 16 sodium and 2 chloride ions. To remove bad contacts, energy minimization was performed with steepest descent method with 3000 steps. Simulations were carried out for 100 ns while all bonds were maintained in constraint conditions with the LINCS algorithm. Periodic boundary conditions were applied for all the simulations. Newtonian equation of motion was integrated with the LEAP-FROG algorithm. Coordinate and velocities were saved every 10 ps. Most representative structures were obtained from g_cluster (GROMACS 5.1.1) by grouping structurally similar frames (RMSD cutoff 0.3) generated in the simulation trajectory.35

pulse in a sapphire crystal and a 390 nm pump pulse using second harmonic generation. The excitation power was ∼5 mJ/ cm2 per pulse. All femtosecond laser experiments were carried out in a 2 mm quartz cuvette at room temperature. The instrumental time resolution was determined to be ∼150 fs via a pump−probe cross-correlation analysis. Solutions of guest@ OA2 and guest@OA2 + MV2+ were prepared as described above for steady-state fluorescence measurements. Molecular Dynamics Simulations. All atom molecular dynamics simulations were carried out using the GROMACS 5.1.1 program with OPLS-AA force field.40−42 Three-dimensional coordinate structures of OA and guests were constructed on Spartan 03 and Chem3D programs, respectively.43 Coordinates of MV2+ were obtained from the Cambridge Structural Database. Initial geometries of both OA and guests were optimized with MM force field. Partial charges of guests were obtained by electronegativity equalization method using B3LYP/6-311G/NPA parameters.44 Topologies of the OA, coumarin guests, and MV2+ were prepared using the MKTOP program based on the work done by Ribeiro and co-workers.45 All simulations were carried out in a cubic box (40 × 40 × 40 Å3) filled with explicit water molecules. An extended simple point charge (SPC/E) model of water was utilized for all the simulations. Sixteen sodium ions were used to neutralize the systems. Prior to every simulation, energy minimization was performed with steepest descent method. Simulations were performed at constant temperature (300 K) and 1 atm pressure with constant number of particles. Temperature and pressure were held constant by coupling the systems with Berendsen thermostat and barostat, respectively. Electrostatics were calculated by particle-mesh Ewald method with Verlet cutoff (1 nm) scheme.46 A cutoff of 1 nm was also maintained for van der Waals interactions. All bonds constraints were applied with the LINCS algorithm. Simulations were run with periodic boundary conditions. Velocities for atoms were assigned according to a Maxwell distribution at 300 K. Newtonian equations of motion were integrated every 2 fs with the Leap-

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RESULTS AND DISCUSSION Three 7-amino-substituted coumarins C1, C480, and C153, referred to as coumarins, were used as electron donors (Figure 1). Steady-state UV−vis absorption and fluorescence spectroscopies were employed to explore the optical properties of coumarins and their complexes. Figure 2 shows the absorption (solid lines) and fluorescence (dashed lines) spectra of three coumarin@OA2 (one molecule of coumarin encapsulated within two molecules of OA) and coumarin@OA2−MV2+ complexes. The three different coumarins C1, C480, and C153 are represented with red, black, and blue curves, C


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Figure 3. Fluorescence titration spectra of coumarins with addition of OA: (a) C-1, λex = 360 nm; (b) C-480, λex = 370 nm; and (c) C-153, λex = 400 nm. With increased addition of OA, the emission maxima shift to shorter wavelength.

Figure 4. 1H NMR (500 MHz, 600 μL of D2O) spectra of (a) OA (1 mM, in sodium tetraborate buffered D2O), (b) C1@OA2 (0.5 mM: 1 mM in buffered D2O), (c) C480@OA2 (0.5 mM: 1 mM in buffered D2O), (d) C153@OA2 (0.5 mM: 1 mM in buffered D2O). Signals marked A−J in (a) represent uncomplexed OA protons; signals marked A, A′, B , B′, C, C′, F, F′, and G, G′ in (b) represent complexed OA protons, and signals marked * represent the guest alkyl proton signals.

respectively, with corresponding absorption/fluorescence maxima at 361/411, 376/425, and 420/482 nm (shown in Figure 2a). The same three coumarins included within OA have previously been used to demonstrate the occurrence of energy transfer from the encapsulated donor to free rhodamine-6G in solution.35 A large blue shift in the emission maxima of the three coumarins (Figure 3) in water upon addition of OA confirmed that in the presence of OA they are in a nonpolar environment.49 Ultrafast twisted intramolecular charge transfer behavior of coumarin in solution has thoroughly been studied. The observed large shift in the emission between polar and nonpolar solvents is the result of the well-known TICT and charge-shift behavior of these molecules in the excited state.49−51 We have reported previously based on the emission spectral maxima of coumarins, pyrene aldehyde and 2acetylantharecne and pyrene emission I1/I3 band intensities ratio that the interior of the OA capsule is benzene-like.34 The observed emission maximum is consistent with the conclusion that the three coumarins are within the OA capsule and experience a nonpolar environment and are not exposed to water molecules.

Characterization of the host−guest complexes was made by H NMR spectroscopy. Since the details are available in our previous publication,35 this is discussed briefly below. Further confirmation of the inclusion of coumarins within the OA capsule came from 1H NMR spectra of the 1:2 guest−host complexes (Figure 4). Encapsulation of the guest within OA is inferred from the significant upfield shift of the guest proton signals (note the starred peaks in the spectra are the upfield shifted signals of the 7-methylene and 4-methyl protons of the guests).52,53 Upon encapsulation of the guest, a splitting of several of the host OA proton signals (for example, A is split into AA′) occurred resulting from the nonsymmetrical nature of the top and bottom halves of the capsule upon inclusion of unsymmetrical guest.29 1H NMR titration spectra of the three coumarins with OA and the diffusion constants for free OA and C-1@OA2, C-480@OA2, and C-153@OA2 complexes as 1.9 × 10−6, 1.3 × 10−6, 1.3 × 10−6, and 1.2 × 10−6 cm2/s, respectively,31 are consistent with the formation of 1:2 complex.34 The lower diffusion constant with respect to free OA is in agreement with the larger size of the OA capsule. Based on the above changes in emission maxima and 1H NMR chemical shifts and measured diffusion constants of the complex, we envisage the donor coumarin dye molecules to 1

D


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Figure 5. (a) Fluorescence titration spectra of C480@OA2 with MV2+; λex = 380 nm; [C480] = 2 × 10−5 M, [OA] = 1 × 10−4 M, and [MV2+] = 1.5 × 10−5 M to 5 × 10−5 M in 10 mM sodium tetraborate buffer. (b) Fluorescence titration spectra of C480@OA2 in the presence of MV2+ and MV2+@ CB7; λex = 380 nm; [C480] = 2 × 10−5 M, [OA] = 1 × 10−4 M, [MV2+] = 1.5 × 10−5 M to 5 × 10−5 M and [CB7] = 5 × 10−5 M in 10 mM sodium tetraborate buffer.

be tightly held within the OA capsule as a 1:2 complex and the capsule do not disassemble and expose the guest to water on the NMR time scale.54,55 Conclusions regarding host−guest inclusion (coumarin−OA) drawn from emission and 1H NMR spectra are consistent with various other photoactive molecules we have investigated in the past.32 We did not measure the binding constants of coumarins with OA but have estimated binding constants of organic guests with OA by isothermal calorimetric measurements.56 These studies were restricted to 1:1 complexes, and the Ka values were in the range of 104−107 M−1. We believe that binding constants of coumarins would also be in a similar range. Supporting this are our 1H NMR spectral results. At no stage did we see signals due to free coumarins when they were added to an aqueous solution of OA. Only when the amount of OA exceeded 0.5 equiv were free coumarin signals seen. This suggested that coumarin did not exist as free molecules when sufficient OA was present in solution. Upon addition of MV2+ to a solution of coumarin@OA2, the absorption and fluorescence spectra of the coumarin@OA2 + MV2+ have the same spectral shape as that of coumarin@OA2 (Figure 2b). However, the fluorescence intensity decreases with the increasing addition of MV2+ (Figure 5). Lack of direct influence on the spectral shape suggested that MV2+ is not forming a ground state complex with the coumarins. To probe the location of MV2+ in the presence of coumarin@OA2, we compared the 1H NMR spectra of MV2+ in D2O in the presence and absence of coumarin@OA2 capsule (Figure 6). The unaffected proton signals of MV2+ (compare (a) and (b)) suggested that the latter remained outside the capsule in D2O. However, the diffusion constant of MV2+ was identical to that of C-153@OA2 complex (1.2 × 10−6 cm2/s) indicating that MV2+ is strongly associated with the capsule. Based on the above data, we conclude that the donor coumarins are inside the capsule and the acceptor MV2+ is held at the exterior of the capsule through electrostatic interaction between the cationic MV2+ and the anionic COO− groups present at the exterior of the OA capsule. As illustrated in Figure 5a, the increasing concentration of MV2+ gradually quenched the fluorescence spectra of C-480@ OA2. Similar fluorescence quenching was observed in the case of C-153 and C-1. To confirm that MV2+ has to be close to the

Figure 6. 1H NMR (500 MHz, D2O) spectra of (a) MV2+; (b) C153@OA2 + MV2+; (c) C-153@OA2 + MV2+@CB7; [C-153] = 0.5 mM, [OA] = 1 mM, [MV2+] = 1 mM, and [CB7] = 1 mM in 10 mM sodium tetraborate buffer; *, ●, and ▲ represent bound C-153 protons, MV2+ protons, and residual proton signal in D 2O, respectively.

capsule for quenching to occur, we employed a second host cucurbit[7]uril (CB7) that is well-known to bind to MV2+.57 Inclusion of MV2+ within CB7 was confirmed by recording the 1 H NMR spectrum of the solution that contains C-153 and CB7 (compare (b) and (c) in Figure 6). The fact that the signal due to MV2+ is upfield shifted in the presence of CB7 confirmed that the host CB7 included MV2+ within its cavity. To probe the consequence of moving MV2+ away from OA, fluorescence spectra of the solution containing C480@OA2, MV2+, and CB7 were recorded. Upon addition of CB7 to a solution containing C480@OA2 and MV2+, the fluorescence of C480@OA2 was recovered (Figure 5b). Based on the above studies, we conclude that fluorescence quenching of an encapsulated dye is possible only when the quencher is associated with the capsule. Clearly, quenching is inhibited when both the donor and the acceptor are included within two separate organic hosts (donor in OA and acceptor in CB7). A point to note is that we do not know the distribution of the quencher MV2+ with respect to the capsule. The quencher distribution is not likely to be uniform, i.e., one quencher per one capsule. Most likely it follows a Boltzmann distribution leaving some capsules with no quencher and a few with more than one quencher. Such a distribution is likely to leave some E


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Figure 7. Transient absorption spectra and kinetics monitored at the positions of fluorescence maxima and MV+• absorption for three coumarin@ OA2−MV2+ complexes (λex = 390 nm). Transient absorption spectra recorded at different delay time for (a) C1@OA2−MV2+, (d) C480@OA2− MV2+, and (g) C153@OA2−MV2+; [coumarin] = 2 × 10−5 M, [OA] = 1 × 10−4 M, [MV2+] = 5 × 10−5 M. The pump−probe delay times of the recorded spectra are shown in the corresponding legends. The second column (b, e, and h) presents the fast eT kinetics within 100 ps while the third column (c, f, and i) shows the slower BeT processes in a 3000 ps time window. Blue data correspond to the formation of the MV+• absorption (λobs = 625 nm), and black data represent the fluorescence decay of the photoexcited coumarins. Red curves are the numerical fits using single exponential functions. The fit results are also shown directly in the figures. For C1, this data is not shown in the figure because the wavelength window of C1 fluorescence is out of the detection limit.

The time constants of the MV+• formation (rise time) are 1.1, 1.2, and 22.1 ps for C1, C480, and C153, respectively. The rise time constants are a measurement of the forward eT process from the photoexcited coumarins to MV2+. Another feature, which is observed in both C480@OA2 + MV2+ and C153@OA 2 + MV 2+ complexes, is the laser-induced fluorescence (LIF) from the photoexcited coumarins. Laserinduced fluorescence (LIF), allows us to observe the timeresolved fluorescence properties of the investigated samples, while probing the transient absorption properties. Compared with the usual singlet-state fluorescence lifetimes of the freely dissolved coumarins on the nanosecond time scale, the measured LIF in the coumarin@OA2 + MV2+ complexes shows an accelerated decay occurring within 23 ps for C153 and ∼1 ps for C480. C480 and C153 have their fluorescence maxima at 450 and 500 nm, respectively, while the fluorescence maximum of C1 is at 420 nm, which is out of the detection range of our instrument. Figures 7e and 7h show the fluorescence decay (black) with the rate constants of 1.8 and 23.5 ps for C480 and C153, respectively. These decays are consistent with the rise dynamics of the MV+• formation. MV+• ions generated by eT from excited C1, C480, and C153 have lifetimes of 1158, 544 and 727 ps, respectively. The lifetimes were obtained by using single-exponential functions to fit the kinetic decays. The kinetics (blue) and numerical fits (red curves) shown in the right column of Figure 7 reveal the lifetimes of the radical−pair complexes. The lifetime of the radical−pair complexes is limited by BeT from MV+• to

capsules with donors with no quenchers. Our inability to fully quench the donor emission in Figure 5 could be a reflection of the presence of some capsules with no quencher. Our next experiments aimed to find out the origin of the quenching of coumarin@OA2 emission by MV2+. The basis of the quenching became clearer from the absorption spectra of the transient intermediates of coumarin@OA2 in the presence of MV2+ recorded by femtosecond pump−probe spectroscopy. All three TA spectral data (Figure 7a, d, and g) sets show a common feature, namely, a positive broad transient absorption centered at 625 nm (550−700 nm). Based on literature report, the spectrum is assigned to MV+·.58 The MV+· must be generated as a result of electron transfer from photoexcited coumarin@OA2 to MV2+. This provided unequivocal support for electron transfer across the capsular wall. The ultrafast nature of the photoelectron transfer was probed with femtosecond time-resolved transient absorption spectroscopy. A 390 nm pump pulse was used to photoexcite the coumarin inside the OA2 capsule, and the transient pump− probe spectra were recorded as a function of the relative delay time. Figure 7 presents in addition to TA spectra (left column) the eT kinetics in 15 and 100 ps time windows, as well as the back electron transfer (BeT) kinetics in a 3000 ps time window (right column) monitored at the spectral positions of fluorescence maxima and radical ion absorption. The numerical fitting results are shown as solid red exponential curves. The spectral data of C1, C480, and C153 are provided in three rows of Figure 7. F


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The Journal of Physical Chemistry B Table 2. Parameters for the Coumarin@OA2−MV2+ Complexes Used in the Rehm−Weller Analysisa

coumarin+• ions across the OA2 capsule. In Table 1, the lifetimes of the various photoinduced processes for all studied donor@OA2−acceptor combinations are tabulated. Table 1. Lifetimes of Various Photoinduced Processes As Obtained by Femtosecond Transient Absorption Spectroscopy τdeca/ps

τdecb/ps

coumarin 1 coumarin 480 coumarin 153

1.1 ± 0.1 1.2 ± 0.1 22.1 ± 2.0

1158 ± 66 544 ± 18 727 ± 37

− 1.8 ± 0.2 23.5 ± 0.8

Measured at 625 nm, absorption of MV+•. bMeasured at fluorescence maxima for C480 and C153. a

(1)

To map out the energetics of the coumarin@OA2−acceptor systems, we performed the Rehm−Weller analysis to derive ΔGeT, which dictates the photoinduced eT reactions by employing the Rehm−Weller equation38,61 ΔGeT = EOX − E RED − E00 − E IPS

E00d

EIPSe

ΔGeT (eV)

keTf (s−1)

C1 C480 C153

1.09 0.72 0.89

−0.69 −0.69 −0.69

3.19 3.06 2.75

0.03 0.03 0.03

−1.44 −1.68 −1.2

9.1 × 1011 8.3 × 1011 4.5 × 1010

Figure 8 represents all electronic states and pathways involved in the photoelectron process between coumarin@ OA2 and MV2+. At the beginning, a coumarin@OA2−MV2+ complex in the ground-state equilibrium (Figure 8a) is excited and results in a locally excited coumarin complex (Figure 8b). Then, the photoexcited coumarin could either relax to the ground state by fluorescence or transfer an electron from the OA2 cavity across the molecular wall to the MV2+ ions yielding the radical−ion pair complex (Figure 8c). The later one is an eT process through an apolar-to-high-dielectric interface. The resulting radical−pair complex, which is relatively stable with a lifetime of ∼1000 ps, finally returns to the ground state (Figure 8a) by BeT. The cyclic process of the coumarin@OA2−MV2+ complexes provide a stable and functional assembly of organic electron donor−acceptor pairs in aqueous solution. To gain an understanding of the mechanism of eT from OA entrapped coumarin to free MV2+, we felt that it is important to have an insight into the structure of coumarin@OA2−V2+ complex. To accomplish this goal, we performed MD simulations of C-1, C-480, and C-153 within two molecules of OA in explicit water for 100 ns with OPLS-AA force-field parameters on the GROMACS software package.41,42,63 In each simulation, guest was randomly placed in the space between two OA cavitands. A cubic box of dimensions (40 × 40 × 40 Å3) was constructed surrounding the OA−guest assembly. The box was filled with water, and sodium ions were added to neutralize the system. Several simulations were performed by arranging them in different orientations with random velocity distribution. Root mean square deviation (RMSDs) analysis indicated formation of equilibrium host−guest assembly. The structures obtained for the three complexes are shown in Figure 9a, b, and c (for corresponding initial structures and trajectories see the Supporting Information Figures S1−S3 and Movies movie_C1@OA2, movie_C480@OA2, and movie_C153@ OA2). It is clear that coumarins are present within OA capsule and they are fully protected from water. The simulated structures account for the observed shift in the 1H NMR and the emission wavelengths. To obtain the consolidated structure of the coumarin@OA2−MV2+ complex, to the above structures MV2+ was introduced into water and simulations were performed (for details see the experimental section). The obtained structures are illustrated in Figure 9d, e, and f (for corresponding initial structures and trajectories see the Supporting Information Figures S4−S6 and Movies movie_C1@OA2−MV2+, movie_C480@OA2−MV2+, and movie_C153@OA2−MV2+). Expectedly, the acceptor MV2+ stays close to the walls of the capsule. Interestingly, it is held by

Marcus theory provides an understanding for eT reactions as the interplay of the free energy difference between the reactant and product states and the extent of reorganization which the environment must undergo to accommodate the charge redistribution.1,59,60 In the classical limit, the rate of eT at temperature T is determined by the activation barrier ΔG#eT for the reaction: keT ∝ exp( −ΔG # eT /kBT)

EREDc

All the potentials are expressed in volt (V vs SCE). bThe oxidation potentials were reported in ref 26. All measurements were taken in CH3CN. cThe reduction potential of MV2+ = −0.69 V in aqueous solution was reported in ref 27. dE00 is obtained from the intersection point of absorption and fluorescence spectra in Figure 2a. ee = 1.602 × 10−19 C, εs ∼ 78.5, r0 ∼ 6 × 10−10 m, which is estimated based on the size of the OA2 capsule reported in ref 22. fThe rates refer to keT as MV2+ is the acceptor. keT is the reciprocal of the rise time constants of the MV+• signals (the first column in Table 1).

methyl viologen (MV ) τrisea/ps

EOXb

a

2+

sample

donor

(2)

where EOX and ERED are the oxidation and reduction potentials of the donors (coumarins) and acceptor (MV2+), respectively. E00 is the energy difference between S0 and S1 states of the donors that is being excited, obtained at the wavelength of the intersection of the absorption and fluorescence spectra. The last term, EIPS, is the ion pair stabilization energy in the medium. EIPS equals e2/εsr0, where e, εs, and r0 are the electron charge, the dielectric constant of the medium, and the interaction distance between the donor and acceptor, respectively. The oxidation potentials of the coumarins are reported in the literature,37 as well as the reduction potentials of the MV2+ ions.62 From Figure 2a, we are able to estimate the E00 values for the three coumarins. The EIPS is calculated here using εs = 78.5 (water) and r0 ∼ 6 Å. This results in very small ion pair stabilization energies. But even if the relative dielectric constant of the chemical environment would average to ∼1, it would still affect all three systems in exactly the same manner and simply add a constant stabilization energy for all three systems. We do not know the exact value of εs in our system as the donor is within the capsule, nonpolar benzene-like, and the acceptor is in water attached closer to the walls of the capsule. Our estimate, at best, has only semiqualitative value. All the parameters required for the Rehm−Weller analysis are summarized in Table 2. As the MV2+ ion is the acceptor, we obtained the ΔGeT values of −1.44, −1.68, and −1.2 eV for C1, C480, and C153 complexes, respectively. To examine the relationship between ΔGeT and keT, we obtained the keT values by converting the fitted time constants into their reciprocal values. The forward eT rates are tabulated in Table 2 for comparison. G


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Figure 8. Schematic showing three states of coumarin@OA2−MV2+ complexes: (a) ground state, (b) locally excited state, and (c) radical−pair complex. Red arrows indicate transitions between states. Other symbols include lowest unoccupied molecular orbital (LUMO), highest occupied molecular orbital (HOMO), electron transfer (eT), photoexcitation (hν), back electron transfer (BeT), and fluorescence (Flu).

that upon excitation a charge shift occurs from encapsulated C480 to the capsule.64 It is not obvious what role such a shift plays in the eT observed in this study. Until detailed computational results become available, we cannot be certain of the role of the above suggested charge shift. At this stage, all we could state is that the capsule is not inert when coumarins present within them are excited. It is quite likely the OA participates in the eT process. One should note that the capsule is electron rich and has an oxidation potential close to 1.5 eV.65 In the current examples, it is not expected to donate electrons to methyl viologen since the excited coumarins are more easily oxidized. We are yet to fully understand the detailed mechanism of the eT from encapsulated coumarins to free MV2+. The role of capsular wall in this ultrafast eT is yet to be fully understood.

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CONCLUSIONS

Using three different donor@OA2−acceptor pairs, a first comprehensive investigation of the ultrafast charge separation from a nonpolar to a polar medium has been carried out. In this femtosecond time-resolved study of the coumarin@OA2− acceptor complexes, we focused on the eT dynamics across the molecular wall of an OA2 capsule by choosing different coumarins as the donors and MV2+ as the acceptor and measuring the eT process across the apolar-to-polar interface OA2. The forward and backward eT across the capsule were directly observed and examined using ultrafast laser spectroscopy with femtosecond time resolution. The photoinduced eT reactions were analyzed using the Rehm−Weller equation. This work provides a design for photoinduced charge separation, by transporting the excited electron from a highly apolar environment (ε ∼ 2.3) to a high dielectric environment of water (ε ∼ 80).

Figure 9. Most representative structures of (a) C1@OA2, (b) C480@ OA2, (c) C153@OA2, (d) C1@OA2−MV2+, (e) C480@OA2−MV2+, and (f) C153@OA2−MV2+.

electrostatic interaction between the two COO− present at the bottom and middle regions of the capsule and the two N+ of the MV2+. Although there are COO− groups at the middle regions of the capsule as per the MD simulation data, it seems to prefer the butanoic acid anions at the bottom and top of the capsule. Such a structure ensures that the donor and acceptor are close and they are separated only by the thin electron rich walls of the capsule. We have shown previously with a variety of guests that the partial opening of the capsule takes about 5 μs.55 Given that the eT occurs on sub-nanosecond time scale (Table 1), we believe that during that time scale the capsule is tightly closed. Thus, the eT must occur through the walls of the capsule. Recently, TDDFT computations carried out by Dunietz’s group revealed H


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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/acs.jpcb.7b11306. Figures of molecular dynamics simulations of various coumarin@octa acid complexes (PDF) Movies of various coumarin@octa acid complexes (ZIP)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Clemens Burda: 0000-0002-7342-2840 V. Ramamurthy: 0000-0002-3168-2185 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS V.R. thanks the National Science Foundation for financial support (CHE-1411458). C.B. gratefully acknowledges funds from the Chemical Professorship at CWRU.

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REFERENCES

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Ultrafast Electron Transfer across a Nanocapsular Wall: Coumarins

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