Ultrafast Electron Transfer from Upper Excited State of Encapsulated

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Ultrafast Electron Transfer from Upper Excited State of Encapsulated Azulenes to Acceptors across an Organic Molecular Wall Mohan Raj Anthony Raj, Mintu Porel, Puspal Mukherjee, Xiuyuan Ma, Rajib Choudhury, Elena Galoppini, Pratik Sen, and Vaidhyanathan Ramamurthy J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07260 • Publication Date (Web): 24 Aug 2017 Downloaded from http://pubs.acs.org on August 25, 2017

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The Journal of Physical Chemistry

Ultrafast Electron Transfer from Upper Excited State of Encapsulated Azulenes to Acceptors across an Organic Molecular Wall A. Mohan Raj,1 Mintu Porel,1 Puspal Mukherjee,2 Xiuyuan Ma,3 Rajib Choudhury,4 Elena Galoppini,3,* Pratik Sen2,* and V. Ramamurthy1,* 1 2

Department of Chemistry, University of Miami, Coral Gables, FL 33146, USA

Department of Chemistry, Indian Institute of Technology, Kanpur, 208016, India 3

4

Chemistry Department, Rutgers University, Newark, NJ 07102, USA

Department of Physical Sciences, Arkansas Tech University, Russellville, AR 72801, USA

Abstract In the context of generating reactive organic radical cations within a confined capsule and exploring photoelectron transfer from encapsulated organic molecules to organic and inorganic acceptors through an organic molecular wall we have investigated electron transfer from upper excited state (S2) of azulene (Az) and guaiazulene (GAz) enclosed within octa acid (OA) capsule to water soluble 4,4’-dimethyl viologen2+ (DMV2+) and pyridinium+ (Py+) salts and colloidal TiO2. S2 fluorescence of OA encapsulated Az and GAz were quenched by electron acceptors such as DMV2+ and Py+ salts. Electron transfer being responsible for S2 fluorescence quenching was established by recording the transient absorption spectrum of DMV monoradical cation in the femtosecond time regime. Femtosecond time resolved fluorescence experiments suggested that the time constant for the forward and reverse electron transfer from encapsulated Az and GAz to DMV2+ to be 4 ps and 3.6, and 55.7 and 36.9 ps respectively. Observed S2 fluorescence quenching by colloidal TiO2 in aqueous buffer solution is attributed to electron transfer from encapsulated Az and GAz to TiO2. Lack of quenching by the wider band gap material ZrO2 supported the above conclusion. FT-IRATR experiments confirmed that OA capsules containing Az and GAz can be adsorbed on TiO2 films and excitation of these resulted in S2 fluorescence quenching. The observations presented here is important in the context of establishing the value of OA type cavitands where charge separation and donor shielding are critical.

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Introduction Electron transfer (eT) induced by light continues to play a fundamental role in several aspects of chemistry including natural- and artificial photosynthesis, environmental remediation and synthesis of organic molecules through visible light photocatalysis.1-3 This process has therefore resulted in a persistent quest for a fundamental understanding of eT across distances short and long. At the same time, the journey in organic chemistry has progressed from organic solvents and solid state to water as the medium resulting in the use of water-soluble hosts such as micelles and organic cavitands as reaction vessels that could solubilize hydrophobic organic molecules in water.4-7 In this context, motivated by the desire to achieve selectivity in photoreactions we have been interested in performing photoreactions of organic molecules confined in small spaces.8-9 One type of reaction we have become inquisitive is of organic radical ions enclosed within an organic capsule which requires generating them through eT from outside the capsule. This presentation concerns with such a study combining eT, supramolecular chemistry and photochemistry. Early examples of eT in confined spaces included open cavitands such as cyclodextrins, calixarenes and cucurbiturils that do not fully encapsulate the guest molecules.10-15 This results in fast back electron transfer that must be slowed to manipulate the radical ions generated by the forward eT. Carcerand and hemicarcerand synthesized by Cram and co-workers are the first group of fully closed, fascinating hosts that can physically separate the donor and acceptor molecules.16 Deshayes, Piotrowiak and co-workers probed various aspects of triplet excitation transfer through the walls of hemicarcerands, including the role of reorganization energy, by employing encapsulated biacetyl and a variety of donor molecules in organic solvents17,18 Balzani and co-workers employing methylene chloride soluble hemicarcerand as the host, triplet excited biacetyl as the guest acceptor, and free amines as the donors established the feasibility of eT across the host wall with a rate constant at least two orders of magnitude smaller than in its absence.19 Later, Deshayes, Piotrowiak and co-workers explored electron transfer from chromophore@hemicarcerand complexes to various acceptors. They used a water-soluble hemicarcerand synthesized by Yoon and Cram as the host to explore the feasibility of eT between caged excited azulene (Az) and colloidal, dilute solutions of TiO2 nanoparticles. 20 More recently, they employed caged ferrocene, quinones as acceptors and excited Zn-cytochrome C as donors.21-22 Since these reports, our


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ability to fully understand eT across fully enclosed guest and free host has not progressed due to the small cavity volume (~ 200 Å3) and the size of the portals severely limiting the ability to introduce a variety of guests within hemicarcerands. In fact the early experiments involving eT across hemicarcerand walls involved the biacetyl acceptor entrapped as the small molecular guest.19 The capsule we have been employing as a ‘molecular laboratory’ is formed by the assembly of two molecules of water-soluble cavitand known as octa acid (OA; Scheme 1), and it has allowed us to explore feasibility of eT across fully closed capsules.23-24 Unlike carcerands and hemicarcerands, OA does not have a pre-assembled capsular structure. Two molecules of OA spontaneously assemble in borate buffered aqueous solution in the presence of an organic guest molecule to form a capsule trapping the latter.25-26 The larger size of the internal cavity (~500 Å3), easy formation of capsules in water with a number of organic guest molecules and slow dissociation of the capsule make OA an attractive host for eT studies. Although the two halves of the cavitand forming the capsule are not covalently linked as in the case of carcerand and hemicarcerand, they form stable host-guest complexes with high binding constants and dissociation of the two OA molecules takes several seconds.27-28 Therefore during the excited state lifetime of the guests the capsule would be expected to remain completely closed. The OA approach has allowed us to revisit and further develop the study of eT between encapsulated chromophores at nanostructured semiconductor interfaces, especially of mesoporous nanoparticle films. The ability to control at the molecular level the bond between a photo-or redox-active molecule and a nanostructured wide band gap semiconductor such as TiO2, SnO or ZnO through designer linker units is of great interest to solar energy conversion, ranging from photovoltaics29 to artificial photosynthesis30 to solar fuels31. The encapsulation of chromophores (dyes) into a molecular host, which is in turn covalently bound to the metal oxide, is an attractive strategy11 to control interfacial eT to an unprecedented level by preventing dye aggregation, increasing their photostability and slowing eT recombination processes. The well-known process of eT in photochemistry32-38 has garnered the current interest in its utility in organic synthesis. During the last several years numerous synthetically relevant molecules have been synthesized in isotropic solution by employing photoelectron transfer process.39-43 Given our interest in ‘supramolecular photochemistry’8-9, 44-45 we


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believed merging photocatalysis and supramolecular chemistry would lead to highly selective photoreactions of radical ions. With this proviso we have explored the feasibility of, and the factors that control the eT across the OA barrier so that we can generate radical ions of molecules of synthetic interest in a confined space. We believed the closed capsule would favor a fast forward and slow back eT. This should allow us to exploit the long-lived guest radical cations in a confined space of ~500 Å3. Recently we had reported the feasibility of eT from OA encapsulated aminocoumarin to dimethylviologen present in aqueous solution.46 In the current study we have utilized water insoluble azulene (Az) and guaiazulene (GAz) as the donors and electron deficient 4,4’-dimethylviolgen (MV2+) and pyridinium salts and a semiconductor with a small bandgap TiO2 as the acceptors. The choice of Az and GAz was dictated by our desire to explore eT from upper excited states. Majority of the donors thus far investigated react from S1 or T1. Blue colored azulene, being an exception to Kasha’s rule, attracted our attention.47 A brief study of on excited azulene as an electron donor has been reported by one of the current authors.20 Based on our earlier studies with aromatic molecules we expected Az to form a 2:2 complex.8 To restrict the complexation to 1:2 (Az to OA) we explored a highly substituted and commercially available azulene derivative namely guaiazulene. To explore the fundamentals of eT, the well investigated 4,4’-dimethylviolgen (MV2+) and pyridinium salts seemed appropriate as the acceptors. We anticipated Az and GAz enclosed within OA capsule to transfer electron to the viologen and pyridinium salts through the walls of the capsule.46, 48 The study focused on addressing the following questions: (a) Where do the donor azulenes and acceptors reside in an aqueous solution containing the supramolecular host OA? (b) Is there an electron transfer upon excitation of Az to the upper excited state? (c) What is the time constant for the eT from excited Az to the acceptors and for the reverse transfer from radical ions to generate ground state molecules? (d) How such eT processes change in Az@OA2/TiO2 films with inorganic metal oxides as the acceptors? (e) Can the dye@OA2/TiO2 be used to inhibit electron recombination processes? This study does not address the exact nature of electronic coupling involved during eT across the molecular wall which we recognize requires independent in-depth theoretical investigation. The structures of molecules we have used as host, donors and acceptors to address the above questions are provided in Scheme 1.


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Hosts

octa acid (OA)

Cucurbit[7]uril (CB[7])

Donors

Acceptors

N

Azulene (Az)

Guaiazulene (GAz)

N

MV2+

N MePy+

N H

TiO2

Py+

Scheme 1. Structures of host, donor and acceptor molecules used in this study.

Experimental section Materials and methods: The hosts octa acid and cucurbiturils were synthesized following published procedures.23, 49 Laser grade azulene was purchased and recrystallized with ethanol. Guaiazulene , pyridinium trifluoromethanesulfonate (Py+) and 4,4dimethylviologen(MV2+) were purchased from Sigma-Aldrich and used as received. Nmethyl pyridinium iodide (MePy+) was synthesized following reported procedure.48 General procedure for guest binding studies probed by NMR: A D2O stock solution (600 µL) of host OA (1 mM) and sodium borate buffer (10 mM) taken in a NMR tube was titrated with the guest by sequential addition of 0.25 eq of guest (2.5µL of a 60 mM solution in DMSOd6). The complexation was achieved by shaking the NMR tube for about five minutes. 1H NMR spectra were recorded at room temperature under aerated conditions on a Bruker 500 MHz NMR. 1:2 (guest:host) and 2:2 complexes were achieved by adding 5 or 10 µL respectively of guest solution to 600 µL of 1 mM OA host in 10 mM buffer. Completion


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of complexation was monitored by the disappearance of the free OA signals upon addition of guest. General protocol for fluorescence study: Fluorescence emission spectra were recorded on a FS920CDT Edinburgh steady-state fluorimeter. The aqueous solution of host OA was prepared in 10 mM sodium tetraborate buffer (1 mM). Stock solutions of the guests were prepared in DMSO. 2:2 and 1:2 complex solutions were prepared by adding required amount of guest solution in a vial. These 2:2 and 1:2 complex were diluted to 10-5 M for the experiment. Calculated amounts of acceptor solutions (MV2+, MePy+, Py+ solutions, TiO2 and ZrO2 colloidal solutions) were added to 2:2 and 1:2 complex solutions and mixed thoroughly and fluorescence spectra recorded. In studies using the host cucurbit[7]uril (1 mM), required amount of it was added to the complex solutions with quenchers, mixed thoroughly and then fluorescence spectra were recorded. Sample preparation for adsorption on TiO2 and ZrO2: Capsular assemblies (Az2@OA2, GAz@OA2) (in this abbreviation the guest is indicated first, host last and the symbol @ means guest is complexed to host and the number indicates the number of molecules in the complex) were made in sodium tetraborate buffer (pH-8.9) and emission spectra were recorded. Aqueous HCl was added dropwise and the pH of the solution was checked. After adjusting to a certain pH, emission of the solution was recorded. It was observed that up to pH ~7, complex emission remained almost the same, which is suitable for binding studies with TiO2 and ZrO2. Mesoporous metal oxide (MO) film preparation and binding: Colloidal TiO2 and ZrO2 films were prepared by a sol−gel technique that produces mesoporous films of approximately 10 µm thickness and that consist of nanoparticles with an average diameter of ∼20 nm.50 The TiO2 films were prepared by casting the colloidal solutions by the doctorblade technique onto the substrate over an area of 1 × 2 cm2, followed by sintering at ∼450 °C for 30 min. For absorption and fluorescence studies the films were cast on cover glass slides (VWR). General protocol for FTIR-ATR study of the films: All FTIR-ATR spectra for OA (neat solid) and host@guest (at pH = 7) on TiO2/ZrO2 films were collected on a Thermo Electron Corporation Nicolet 6700 FTIR utilizing the SMART MIRacle-single bounce ATR


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accessory (ZnSe crystal, with 128 scans and spectral resolution of 8 cm-1). The films were dried in the oven to 110 ºC for 30 minutes for further use. General protocol for emission studies of the films: Fluorescence emission spectra of the host@guest (at pH 7) binding on TiO2/ ZrO2 films were recorded on a Horiba Fluorolog-3 instrument equipped with a Xenon short-arc lamp source for films at room temperature. The fluorescence spectra were recorded at 340 nm and 350 nm for Az2@OA2 and GAz@OA2 respectively. General protocol for transient absorption and up-conversion studies: Since the details of our femtosecond transient absorption spectroscopy setup (FemtoFrame-II, IB Photonics, Bulgaria) and fluorescence up-conversion setup (FOG-100, CDP Corp., Russia) have been discussed earlier3 we present here a brief overview. For transient absorption measurements the fundamental 800 nm light obtained from a Spitfire Pro (Spectra Physics, USA) amplifier was split into 2 parts. One part was focused on a BBO crystal to generate 400 nm light, which was used as the pump beam and the other part was focus on a sapphire plate after passing through a delay stage to generate the white light continuum (450 nm to 750 nm), which was used as the probe light. The instrument response function has a FWHM of 120 fs. For fluorescence up-conversion setup the fundamental 720 nm light was obtained from a MaiTai HP (Spectra Physics, USA), which was frequency doubled in a 0.2 mm BBO crystal to obtain 360 nm pump light. The pump beam was focused onto a rotating sample and the fluorescence was collected and up-converted with the residual fundamental light in another 0.5 mm BBO crystal. This was focused onto a monochromator and then to a photon counting device to obtain the fluorescence transient. The FWHM of the instrument response function is 200 fs. The ps-ns time resolved fluorescence transients were collected using a commercial TCSPC setup (Life Spec II, Edinburgh Instruments, UK). All samples were excited at 375 nm and the FWHM of the instrument response function is 120 ps. Molecular dynamics simulation: Fully atomistic molecular dynamics simulations were performed on a single graphical processing unit (GTX 900) using the GROMACS 5.1.1 program.51 Each simulation was carried out for 100 ns with OPLS-AA force-field parameters.52-53 Initially, OA cavitand was constructed and optimized with Merck Molecular Force Field (MMFF) on Spartan 03 program.54 It was then simulated for 40 ns in explicit water. Initial structures of guests were optimized with MM2 force field. Topologies of the


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host and guests were prepared using the MKTOP program. 55 Partial charges of guests were generated by electronegativity equalization method using B3LYP/6-311G/NPA parameters.56 Simulations were run by placing the respective molecule(s) in a cubic (40 × 40 × 40 3

Å ) box filled with SPC water. Sixteen sodium (Na+) ions were added to neutralize the simulating system. Energy minimization of the starting system was performed with steepest decent method for 1000 steps. Initial velocities for atoms were assigned according to the Maxwell distribution at 300 K. A periodic boundary condition (PBC) was applied and the equation of motion was integrated at 2 fs time step by using the LEAP-FROG algorithm under the NPT ensemble at 300 K temperature and 1 bar pressure. A non-bond pair list cutoff of 12 Å was used. Bond lengths were constrained using the LINCS algorithm. The long– range electrostatic interactions were calculated by the particle-mesh Ewald method with Verlet cut-off scheme.57 VMD and Pymol software packages were used for trajectory analysis, visualization and for preparation of structural diagrams.58-59 The most representative structures and root mean square deviations were calculated from the entire 100 ns simulation trajectory. In the cluster analysis, the trajectories were analyzed by grouping structurally similar frames (root mean square deviation cutoff = 0.30 nm), and the frame with the largest number of neighbors was denoted as the “most representative” structure, which represents that particular cluster.

Results Location of the donor and the acceptors: Below we present our results on photoinduced eT between donor Az and GAz entrapped within an organic capsule assembled by two molecules of the organic host OA and acceptors MV2+, N-methyl pyridinium iodide (MePy+), pyridinium sulfonate (Py+) and TiO2 present outside. Since the results for Az and GAz are similar, spectral details for Az are presented in this section and most of those for GAz are included in the Supporting Information (SI). Formation of host-guest complexes between Az and OA, and GAz and OA was established by 1H NMR studies. The 1H NMR spectra of OA, Az/GAz-OA in borate buffer-D2O are presented in Figure 1. Generally, the inclusion is signaled by the shifts in the host and guest 1H NMR peaks.25-26 In the case of GAz it is clear that the signals due to the two methyl and isopropyl groups are shifted upfield between δ -1 and 2 ppm with the signals due to the former appearing at δ -1 ppm. Such an


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upfield shift and changes in the signals in the downfield region between δ 6 and 8 ppm due to the host protons are clear indication of the guest being included within the lipophilic OA capsule (compare Figure 1 (i) and (iii)). In the case of Az, with most signals being in the region above δ 6 ppm there was no clear indication of the upfield shifted aromatic hydrogen signals. However, the signals due to OA have been altered and there is considerable difference in the NMR spectra between OA and Az-OA (compare Figure 1 (i) and (ii)). The host-guest ratio in the capsule was determined through 1H NMR titration and DOSY-NMR experiments. Select 1H NMR spectra from titration of Az into OA in borate buffer-D2O solution with slow addition of the former are presented in Figure S1 (see Supporting Information, SI). It is clear Az forms a 1:1 stoichiometric complex, which could be either 1:1 cavitandplex or 2:2 capsuleplex. The fact that a single set of signals are seen for the top and bottom halves of the capsule suggest that if it is a 2:2 complex the two azulene molecules would be associated in a head-tail symmetrical fashion or in some other unsymmetrical orientations where the two molecules would be able to rotate in the NMR timescale. Or else two sets of signals for the top and bottom OA would be expected. The later scenarios emerged from three independent atomistic MD simulations, suggesting encapsulation of two azulene molecules, which undergo rapid rotation along the horizontal axis of the OA capsule (see below for discussion). Comparison of 1H NMR spectra at various ratios of Az and OA in Figure S1 reveals changes in signals due to OA as the guest is added up to one equivalent. Further addition leads to no change. In contrast to Az, GAz forms 1:2 (guest:host) complex. The titration spectra are shown in Figure S2 in SI. In this case the changes observed in the spectrum stopped at addition of 0.5 equivalent of GAz to one equivalent of OA. Complexation was further confirmed by measuring the diffusion constant of the guest and host in borate bufferD2O at 1:1 and 1:2 ratios with Az and GAz respectively. Diffusion constant determined for free OA (in the absence of guest) to be 1.88x10-6 cm2/s was reduced in the presence of Az and GAz to 1.48x10-6 cm2/s and 1.50x10-6 cm2/s respectively (Figure S4 and S5 in SI). The identical diffusion constant of guests and OA confirming that the guest and host are complexed and diffuse together suggest the donors-Az and GAz are encapsulated within a OA capsule. The difference in the nature of the complex between Az and GAz is most likely due to their size; multiple methyl substitution prevented GAz from forming 2:2 capsuleplex.


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The nearly no change in the proton signals when acceptors MV2+, MePy+ or Py+ were added to OA in borate buffer-D2O was deduced to be due to their location in water adjacent to the OA capsule and not included within OA (Figure S3 in SI). Under the borate buffer conditions used in this study all the sixteen COOH groups of the capsule are expected to be ionized and remain as COO-. Given the negatively charged OA capsule from the sixteen carboxylate anion groups and the positively charged acceptor molecules one would expect them to be associated externally through Coulombic interaction. Consistent with this picture is the observation that diffusion constants of MV2+, MePy+ and Py+ were smaller in presence of OA than in its absence. In the case of MV2+ the diffusion constant was nearly the same as the capsule. In summary, the diffusion constants, NMR and UV-VIS spectral shifts indicate that the acceptor molecules are held outside the capsule while the donor molecules are held within.48 We had previously determined the polarity of the OA capsule’s interior to be closer to benzene’s and interior to be free of water molecules.60 Thus the donor molecules Az and GAz are present in a non-polar environment within a well-defined confined space.


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a

e d b

i)

c

f

gh

i

j

ii)

iii)

*

*

*

Figure 1. 1H NMR (500 MHz) spectra of (i) OA at 1 mM in 10 mM Na2B4O7 buffer/D2O. (ii) Az2@OA2 ([OA] = 1 mM), [Az] = 1 mM) in10 mM Na2B4O7 buffer/D2O. (iii) GAz@OA2 ([OA] = 1 mM), [GAz] = 0.5 mM) in10 mM Na2B4O7 buffer/D2O; “*” and “●”represent the bound guaiazulene protons and the residual D2O respectively. Quenching of upper excited state fluorescence: The absorption and emission spectra of Az2@OA2 and GAz@OA2 are shown in Figure 2 and Figures S6 in SI respectively. In these spectra one could see two absorptions, one due to S0 to S1 (λmax ~ 590 nm) and the other due to S0 to S2 ( λmax 338 nm). It is well known that azulenes do not emit from S1 but excitation of the S2 band results in fluorescence (λmax ~ 374 nm).47 The emission spectra both for Az and GAz were similar suggesting that association of two molecules in the case of Az had no effect on the S2 emission. MV2+ addition to a buffer solution containing Az2@OA2 quenched the fluorescence (Figure 3a). Similar quenching was observed with MePy+ and Py+ (Figure 3b and Figure S7). The absence of change in S2 lifetime of the emitting species in the ns time regime (measured with SPC with lamp IRF ~1 ns, Fig 3c


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inset) suggested the quenching to be static in character and the close proximity of the donor and acceptor molecules. To probe this observation further we used a second host molecule, cucurbit[7]uril (CB-7), well known to bind with high binding constants to MV2+in aqueous solution (MV2+: 2 x 105 L/mol)61 We anticipated a disruption of the Coulombic attraction between MV2+ and OA from the addition of CB-7 to a solution containing Az2@OA2 and MV2+ by binding of MV2+ to CB-7. This we envisioned would lead to increased distance between the excited Az and the quencher MV2+ that would ultimately result in reduced quenching. As expected, addition of CB-7 to the solution restored S2 fluorescence from Az (Figure 4a). Similar observations were made with MePy+ and Py+ (Figure 4b and Figure S7b in SI), as CB-7 binds to all three electron deficient molecules with high binding constants. GAz@OA2 showed behavior identical to Az2@OA2: (a) S2 fluorescence was quenched by MV2+, MePy+ and Py+ (Figure S8 in SI) and (b) The quenched fluorescence was completely recovered upon addition of one equivalent of CB-7 (Figure S9 in SI).

0.5

6

Absorbance

5

S0-S2 S2-S0

0.4

a

0.3

4

b 3

0.2 2

S0-S1

0.1

1

0

Intensity ( a.u )

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0 290

390

490

590

690

Wavelength (nm)

Figure 2. a) Absorption spectrum of Az2@OA2. [OA] = 1 x 10-4 M, [Az] = 1 x 10-4 M. b) Fluorescence spectrum of Az2@OA2: [OA] = 1 x 10-5 M, [Az] = 1 x 10-5 M in borate buffer.


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Fluorescence Intensity

a) c)

600 500

[MV2+]

Count s

700

400 300

Time (ns) 200 100 0 360

410

460

400

b)

350 300

[MePy+]

250 200 150 100 50 0

510

360

410

Wavelength (nm)

460

510

Wavelength (nm)

Figure 3. a) Fluorescence titration spectra of Az2@OA2 with MV2+. [OA] = 2 x 10-5 M, [Az] = 2 x 10-5M , [MV2+] = 0 to 2.5 x 10-5 M. b) Fluorescence titration spectra of Az2@OA2 with MePy+,[OA] = 4 x 10-5 M, [Az] = 4 x 10-5M ,[MePy+] = 0 to 8.5 x 10-5M in borate buffer. (c) Inset: Fluorescence decay in ns time scale of Az upon addition of MV2+.

a)

b)

Az2@OA2

Az2@OA2

Az2@OA2 + MV2+@CB7

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3 2

Az2@OA2 + MV2+ 1

Az2@OA2 + MePy+@CB7

5 4 3

Az2@OA2 + MePy+

2 1 0

400

450

500

550

600

400

Wavelength (nm)

450

50 0

55 0

60 0

Wavelength (nm)

Figure 4. a) Fluorescence recovery spectra of Az2@OA2. [OA] = 4 x 10-5 M, [Az] = 4 x 10-5 M , [MV2+] = 3 x 10-4 M, CB[7] = 1 mM. b) Fluorescence recovery spectra of Az2@OA2 [OA] = 4 x 10-5 M, [Az] = 4 x 10-5M , [MePy+] = 6 x 10-4 M, CB[7] = 1 mM in borate buffer.

Origin of fluorescence quenching: Given the higher singlet energy of the acceptors with respect to the donor (S2 energy of Az and S1 energy of MV2+ 3.47 and 3.9 eV respectively) singlet-singlet energy transfer is not expected to be responsible for the quenching. On the other hand, the oxidation and reduction potentials and the excitation energy of the Az and GAz suggest that eT from excited Az and GAz to MV2+ and Py+ is possibile (EOx (SCE): Az, 0.71 V and GAz, 0.65 V; ERed (SCE): MV2+, -0.45 V; Py+,-0.58


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V).62 We had demonstrated earlier that excited coumarins and trans-stilbene trapped within OA capsule donate electron to MV2+, MePy+ and Py+ present outside in aqueous media.46,48 We anticipate the same process would occur in this system as well. To ascertain this we carried out femtosecond transient absorption studies. A transient with a broad absorption between 500 and 750 nm with a broad maximum around 600 nm (Figure 5) resulted on excitation of the Az2@OA2 + MV2+ solution at 400 nm. A similar result was obtained with GAz@OA2 + MV2+ solution (Figure S10 in SI). This spectrum corresponding to that of MV+• supports63 the conclusion that S2 Az and GAz fluorescence quenching is due to electron transfer from excited Az and GAz to MV2+. Electron transfer to nanostructured TiO2 colloidal solutions and thin films: The observation that Az S2 can participate in eT led us to bind Az2@OA2 and GAz@OA2 capsuleplexes to TiO2 colloidal aqueous suspensions and films cast on glass to study TiO2 as the electron acceptor. Indeed it has been demonstrated that the S2 excited state of azulene derivatives lies above the conduction band of this metal oxide, as photoinduced electron transfer from the S2 state of azulene to the conduction band of TiO2 takes place efficiently, following selective excitation of the azulene chromophore20, 64 As carboxylic acids are known to bind covalently to metal oxide semiconductors, some of the COOH groups present on OA’s exterior can be expected to bind to the TiO2 nanoparticle surface, whether these are diluted colloidal solutions or mesoporous films.65-67


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Figure 5. Femtosecond transient absorption spectra for Az2@OA2 with 10 mM MV2+ at different times. Sample excited at 400 nm.

An important prerequisite is that the Az2@OA2 and GAz@OA2 capsuleplexes are stable in the experimental conditions used for binding. We recently established that once OA capsuleplexes are assembled at pH ~ 9, they are stable up to pH 6. Hence, quenching studies with TiO2 colloidal aqueous suspensions and films cast on glass were conducted at pH 7 at which pH the Az2@OA2 and GAz@OA2 are stable (Figure S11 in SI). It should be noted that we have shown that the OA capsule binds or physisorbs onto TiO2 surface at pH close to 7.0 and that binding at higher pH does not occur.68 As a control experiment, binding was conducted on nanostructured ZrO2, either colloidal nanoparticles or films of morphology similar to that of the TiO2 films used in the experiments. ZrO2, with a wider band gap (5.0 eV), acts as an insulator, allowing to study the excited state of bound Az2@OA2 and GAz@OA2. Fluorescence spectra of Az2@OA2 collected following the incremental addition of TiO2 colloidal aqueous suspension are shown in Figure 6a. As expected, the emission intensity showing an inverse relationship to the amount of TiO2 suspension in solution and the fluorescence was quenched while the emission was not quenched following the addition of aliquots of a colloidal ZrO2 aqueous solution (Figure 6b). A similar behavior was observed with GAz@OA2 (Figures 6c and d). Despite the fluorescence quenching, the lifetime (in the ns regime) was unchanged, suggesting the eT to be static in TiO2 as it was observed using MV2+ as the acceptor.


15


b) Az2@OA2

Az2@OA2



a) 6 5

TiO2

4 3

TiO2 in water

2 1 0 400

450

500

550

6

ZrO2

5 4 3

ZrO2 in water

2 1

600

400

450

Wavelength (nm)

c)

550

600

d)

GAz@OA2

4

TiO2

3

500

Wavelength (nm)


5


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2

TiO2 in water 1

GAz@OA2 4

ZrO2 3

ZrO2 in water

2 1

0 400

450

500

550

600

400

Wavelength (nm)

450

500

550

600

Wavelength (nm)

Figure 6. Fluorescence titration spectra of a) Az2@OA2 with TiO2 nanoparticles; b) Az2@OA2 with ZrO2 nanoparticles ( λexc = 340 nm); Fluorescence titration spectra of c) GAz@OA2 with TiO2 nanoparticles; d) GAz@OA2 with ZrO2 nanoparticles ( λexc = 350 nm) at pH = 7. Fluorescence titration spectra of Az2@OA2 with a) TiO2 nanoparticles b) ZrO2 nanoparticles ( λexc = 340 nm); Fluorescence titration spectra of GAz@OA2 with c)TiO2 nanoparticles d) ZrO2 nanoparticles ( λexc = 350 nm) at pH = 7.

To probe the feasibility of eT from Az2@OA2 and Gaz@OA2 to the surface of TiO2 nanostructured thin films, which are more technologically relevant substrates, the capsules were adsorbed on films of TiO2 cast on glass (ZrO2 films were used as the control), by immersing the films in a 1 mM aqueous solution of the host-guest complex at pH ~ 7 overnight.

FT-IR-ATR spectra of the Az2@OA2/MO2 and Gaz@OA2/MO2 with M = Zr

and Ti (Figures S12 and S13 in SI) showed both in the case of ZrO2 and TiO2 films, a band at 1707 cm-1 due to the ν(C=O) stretch of unbound COOH groups together with broad, intense bands in the 1500-1650 cm-1 region assigned to the ν(Ο…C…O) stretch of bound carboxylate groups, indicative of binding. As anticipated, not all COOH on the OA exterior are able to bind to the surface. A band at 1298 cm-1 was assigned to the v(C-O) stretch of the


16

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carboxylic acid, whereas the broad O-H stretch above 3000 cm-1 can be assigned to the COOH group as well as physisorbed water on the surface of the films. Comparison of these spectra with that of neat OA powder is consistent with binding of the OA capsuplexes to the TiO2 and ZrO2 films. The emission spectra of Az2@OA2 and GAz@OA2 bound to TiO2 and ZrO2 films shown in Figure 7 indicated that fluorescence emissions of azulenes were fully quenched on TiO2 film, whereas on ZrO2 a broad fluorescence spectrum persisted. It should be noted that broadening of absorption and emission spectra of chromophores bound on nanostructured metal oxide films is commonly observed and has been in part ascribed to binding heterogeneity onto the nanostructured mesoporous surfaces. Unlike in the case of MV2+ the Az radical cation spectrum was beyond the detection limit of our instrument. As in prior experiments, we observed low surface coverages, presumably a result of the large size of the capsuplexes. The observation presented here is important in the context of establishing the value of OA type cavitands where charge separation and donor shielding from heterogeneous environment of a semiconductor surface are critical.

a)

Az2@OA2 bound to ZrO2 Az2@OA2 bound to TiO2

28000

18000

8000

b)

29000

Intensity (CPS)

38000

Intensity (CPS)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


GAz@OA2 bound to ZrO2 GAz@OA2 bound to TiO2

23000 17000 11000 5000

-2000

-1000 350

400

450

500

550

600

360

Wavelength (nm)

410

460

510

560

Wavelength (nm)

Figure 7. Fluorescence spectra of a) Az2@OA2 bound to TiO2 and ZrO2 films (λexc = 340 nm); b) Fluorescence spectra of GAz@OA2 bound to TiO2 and ZrO2 films ( λexc = 350 nm).

Femtosecond studies: Femtosecond transient absorption (TA) and femtosecond time resolved fluorescence experiments were conducted with 1 mM Az (and 1 mM GAz) in OA solution (2 mM) with and without 10 mM MV2+. When Az2@OA2:10 mM MV2+ was excited using 400 nm light, a broad positive absorption band centered around 600 nm was observed


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due to the formation of MV+• (Figure 5 and Figure S10 in SI for GAz radical cation). The kinetic trace at 600 nm is shown in Figure 8a, where a rise time component of ~ 4 ps due to MV+• formation is observed almost immediately after excitation. This rise component was assigned to the forward electron transfer rate from excited Az to MV2+. We interpret the fast rise time to be due to the nullified diffusion in the eT process by MV2+ molecules being held close to the OA capsule by Coulombic attraction. The control experiment, carried out with Az2@OA2, showed no excited state dynamics feature at 600 nm confirming that the observed decay is due to MV+•. To ensure that we can measure the rate constant accurately and there is no wavelength dependence as such, global fitting was performed in the wavelength region 550 nm – 650 nm using Glotaran software.69 The data were best fitted with a sum of 3 exponential function convoluted with a Gaussian function representing the IRF and reported the time constants in Table 1. We attribute the first rise time component of ~4 ps to the rate of forward electron transfer and the second time component (τ2) to backward electron transfer rate (deactivation), which is about ~56 ps.

Figure 8. (a) Kinetics plot obtained from transient absorption spectroscopy at 600 nm (b) fluorescence transient obtained from up-conversion study at 400 nm exciting the sample at 360 nm. Concentration of MV2+ used is 10 mM. The black and blue lines represent the fitting lines.


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To confirm that the electron donor is excited Az, fluorescence decay of Az was monitored. As in our experiments there is an electron acceptor within the quenching sphere (Perrin formulation condition), the excited donor will always find a quencher molecule in its vicinity and the fluorescence decay shall reflect that of unquenched molecules outside the sphere. Samples were excited at 360 nm and the fluorescence decay collected at 400 nm. However due to the ultrafast nature of photo eT between Az and MV2+, time correlated single photon count measurement (IRF = 120 ps) was inadequate to observe the deactivation of Az fluorescence through eT. However we were able to record an ultrafast decay of Az fluorescence with femtosecond fluorescence up-conversion method (IRF = 200 fs). The data obtained is represented in Figure 8b and the time constants obtained from a 3 exponential fitting are tabulated in Table 1. It can be seen that the observed ultrafast time component (~2 ps) is similar to the MV+• formation time component measured through TA study. The inference from the similarity between the ultrafast components obtained from two different studies is that there is excited state electron transfer from Az to MV2+ through the molecular wall of OA with a time constant of ~4 ps. Similar measurements with GAz yielded the time components for the forward eT and the return eT to be 3.6 ps and 36.9 ps respectively (Table 1).

Table 1 Time constants for eT between azulene and guaiazulene and 4,4’-dimethylviologen dichloride as measured by fs time resolved experiments

Technique

τ1 (ps)

τ2 (ps)

τ3 (ps)

Az2@OA2 + MV2+ Transient absorption Fluorescence up-conversion

4.0 (rise)

55.7 (decay)

1000 (fixed)

2.0 (decay)

39.5 (decay)

2000 (fixed)

GAz@OA2 + MV2+ Transient absorption

3.6 (rise)

36.9 (decay)


4000 (fixed)

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Discussion This study is aimed at (a) ascertaining the occurrence of host-guest complexation between OA and azulenes, the electron donor molecules, (b) establishing the feasibility of eT and measuring the eT time constants between a donor molecule within the water-soluble OA and an acceptor molecule present in the aqueous exterior and (c) probing the possibility of eT from an encapsulated donor molecule to TiO2 surface. Long distance eT through covalent single and double bonds has been extensively investigated. However, examples of eT between a donor and an acceptor screened off by a well-defined molecular wall are scarce in the literature. Thus the examples presented here on eT across the walls of a closed capsule add valuable knowledge concerning this type of eT. In the absence of information from NMR regarding the structure of guests within OA capsule MD simulations were carried out in explicit water for 100 ns with OPLS force-field parameters on the GROMACS software package.51-53 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. For both Az and GAz several simulations were performed by arranging them in different orientations with random velocity distribution (Figures S14 and 15 in SI). Root mean square deviations (RMSDs) of the entire trajectory indicated formation of equilibrium host-guest assembly. As shown in Figure 9 and in Figures S14 and 15 in SI, the all atom simulation results suggest that regardless of initial configurations of guests, the equilibrium configurations and orientations are similar and both Az and GAz molecules are held within the closed OA capsule. Since GAz molecule is too large to be enclosed vertically it is held at the wider median of the capsule and protected from water by two OA molecules. On the other hand, two smaller Az molecules are accommodated within OA capsule near the median with a perpendicular stacked orientation. Both molecules are insoluble in water and can only be solubilized in water in presence of OA. These capsules are dynamic and partially open and close ~ 5 µs70 and fully disassemble and assemble in ~2.7 s.28 Thus in the time scale of excited state lifetime the capsule can only partially open-close (‘breathe’) (Figure 10). Based on these we surmise that the eT between the excited encapsulated donor and free acceptor through the walls and there is no direct orbital overlap between them.


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Figure 9. Most representative structures of (a) GAz@OA2 and (b) Az2@OA2 obtained from 100 ns MD simulations. Water molecules are not shown for clarity. OA capsules are represented by sticks; GAz and Az are represented by spheres.

Figure 10. Closed, fully open and partially open OA-guest complex. The time scales for fully opening and partially opening of the complex are different ( ~2.7 s and ~ 5 µs respectively).

As already discussed Az and GAz form 2:2 and 2:1 (host to guest) complexes respectively with OA. 1H-NMR spectra (Figure S3 in SI) and DOSY data of the host-guest complexes and the acceptors suggested that MV2+ and Py+ are electrostatically held at the


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exterior of the capsule. Restoration of the fluorescence by the addition of the second host CB-7 (Figure 4 and Figures S7 and S9 in SI) suggested the need for the electron acceptor to be very close to the capsule for eT to occur. Based on the structure of the donor@)OA2 complex, the location of the acceptor and the lifetime of the capsule we believe that the eT occurs through the wall of the capsule without any direct orbital contact between the excited donor and acceptor molecules. Quenching of Az S2 fluorescence and recording of MV radical cation transient spectrum (Figure 5 and Figure S10 in SI) upon excitation of Az and GAz confirmed the occurrence of eT from excited azulenes to MV2+ chloride. Comparative studies done with ZrO2 which has larger band gap than TiO2 and its failure to quench Az fluorescence lends support for the proposal that eT is responsible for fluorescence quenching by TiO2. Among the various donor-acceptor pairs mentioned above, we elaborate the eT between OA encapsulated Az and free MV2+ as an illustrative example. Based on oxidation potential of Az (0.71 eV), reduction potential of MV2+ (-0.45 eV) and S2 energy of Az (3.47 eV) both the forward and the return eT’s are expected to be exergonic. Though ∆G values indicate the possibility of eT, the observed high rate is unexpected. The rise time of the development of the transient spectrum measured to be ~ 4 ps corresponds to eT rate of 2.5 x 1011 M-1 s-1. A similar rate (5.0 x 1011 M-1 s-1) was obtained from the fluorescence up conversion experiment (Table 1). Interestingly, the return eT rate constant slower than the forward one is higher than diffusion rate. These higher rates of both forward and return eT than diffusion is consistent with the donor and acceptor molecules being adjacent to each other. The question that rises then is how the eT can be so fast despite a molecular layer (of OA) separating the two species and in what way the host wall facilitates eT. At this stage we do not fully comprehend the role of OA in enabling the eT, excepting for the fact that it brings the donor and acceptor molecules closer. A point to note in this context is that OA itself with several electron rich C-O-C bridges can act as an electron donor (oxidation potential: ~1.5 eV).71 However, in the presence of much better donor namely excited Az, OA is not expected to donate or accept electrons directly. Results observed here bears resemblance to hemicarcerand encapsulated duraquinone quenching the triplet Zn-substituted Cytochrome C (Zn-CC)21 in water than to the amine quenching the hemicarcerand encapsulated triplet biacetyl in methylene chloride.19 In the


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former the rate of eT from encapsulated acceptor (duraquinone) was higher than when free in solution, whereas in the latter case it was slower for the encapsulated donor. The enhanced rate in the protein system was attributed to pre-association of the donor and acceptor through electrostatic interaction. Similarly, the enhanced rate of eT in our system could be attributed partially to pre-association. Three additional factors are likely to be important in the eT process: (a) thermodynamics (b) electronic exchange coupling and (c) reorganization energy. As indicated above based on oxidation and reduction potentials and excitation energy eT between excited Az and methylviologen is exergonic. Since the absorption spectrum of Az is nearly identical both within OA and in solution, the S2 energy is expected to be the same as well. The reduction potential of methyl viologen in water is expected to be the same in OA’s presence or absence. However, the oxidation potential of Az is likely to be different as within the established non-polar and dry interior environment of the OA capsule it would be expected to be higher than in the polar solvent. Based on such reasoning the difference in the exothermicity of the reaction for the encapsulated and free donor-acceptor pair is unlikely to be significant enough to increase the rate by an order of magnitude. Stabilization of the product radical ions by the solvent could lower the activation barrier and increase the rate of eT (in the normal Marcus region). In the absence of solvent molecules around the encapsulated Az, the water molecules outside the capsule will have no role in the stabilization of Az radical cation. On the other hand the electron rich oxygen bridges of the cavity’s interior could solvate and stabilize the Az radical cation. Thus the role of the OA host in the reorganization process cannot be ignored. The most important factor that would influence the rate of eT is the orbital overlap that defines the electronic exchange matrix element. Donor - acceptor orbital overlap is expected to be different between two free molecules and an encapsulated and a free molecule. Encapsulation certainly would lower the direct orbital overlap between donor-acceptor pairs to slow the rate of eT. The observed higher eT rate points to the capsular wall not being inert and to likely participate in the eT process.

Conclusions In this presentation we have established that Az and GAz form 2:2 and 2:1 closed capsule with OA. Upon excitation to the second excited state these molecules transfer


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electrons to the acceptors present outside the capsule. Acceptors such as methylviologen and pyridinium salts remain electrostatically attached to the OA capsule. Thus the eT occurs between encapsulated azulenes and acceptors bound to the capsule exterior. The time constant for the forward transfer is in the range of 2-5 ps and the reverse in the range of 40 ps. The fast rate of eT is partially attributable to the proximity of the donor and acceptor molecules also highlights the important role played by the capsular wall in the process. Theoretical understanding of the process is lacking at this stage. Our studies also demonstrated that eT can occur from azulenes to TiO2 nanoparticles/films. The above examples have established that inclusion of organic molecules within a capsule does not prevent them from participating in eT process. The ability to generate and confine radical ions in such systems should open up novel opportunities in controlling their chemical behavior. This approach offers endless prospects to explore the reactivity of organic radical ions in confined spaces. The possibility of eT from encapsulated donor molecules to TiO2 surface portends the utility of a combined supramolecular- and surface chemistry approach to solving the dye aggregation and degradation problems. This approach could also in the long run eliminate the need to synthetically modify visible light absorbing dye molecules for adsorption onto TiO2 surface. With such prospects the results emphasize the need for a fundamental understanding of eT between two molecules separated by a well defined molecular wall.

Acknowledgement VR (CHE-1411458) and EG (CHE-1213669) thank the National Science Foundation for financial support. PS thanks Science and Engineering Research Board, Department of Science and Technology, Government of India and Indian Institute of Technology Kanpur, India for financial support.

Supporting Information 1

H NMR titration spectra of Az with OA

1

H NMR titration spectra of GAz with OA

1

H NMR spectra of Az2@OA2 + MV2+ + CB-7


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Ultrafast Electron Transfer from Upper Excited State of Encapsulated

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