Coumarin Dyes as Donors and Methyl viologen and TiO2 as Acceptors

Letter pubs.acs.org/Langmuir

Photoinduced Electron Transfer Across a Molecular Wall: Coumarin Dyes as Donors and Methyl viologen and TiO2 as Acceptors Mintu Porel,† Agnieszka Klimczak,‡ Marina Freitag,‡ Elena Galoppini,*,‡ and V. Ramamurthy*,† †

Department of Chemistry, University of Miami, Coral Gables, Florida 33124, United States Chemistry Department, Rutgers University, Newark, New Jersey 07102, United States

‡

S Supporting Information *

ABSTRACT: Coumarins C-153, C-480, and C-1 formed 1:2 (guest:host) complexes with a watersoluble cavitand having eight carboxylic acid groups (OA) in aqueous borate buffer solution. The complexes were photoexcited in the presence of electron acceptors (methyl viologen, MV2+, or TiO2) to probe the possibility of electron transfer between a donor and an acceptor physically separated by a molecular wall. In solution at basic pH, the dication MV2+ was associated to the exterior of the complex C-153@OA2, as suggested by diffusion constants (∼1.2 × 10−6 cm2/s) determined by DOSY NMR. The fluorescence of C-153@OA2 was quenched in the presence of increasing amounts of MV2+ and Stern−Volmer plots of Io/I and τo/τ vs [MV2+] indicated that the quenching was static. As per FT-IR-ATR spectra, the capsule C-153@OA2 was bound to TiO2 nanoparticle films. Selective excitation (λexc = 420) of the above bound complex resulted in fluorescence quenching. When adsorbed on insulating ZrO2 nanoparticle films, excitation of the complex resulted in a broad fluorescence spectrum centered at 500 nm and consistent with C-153 being within the lipophilic capsule interior. Consistent with the above results, colloidal TiO2 quenched the emission while colloidal ZrO2 did not.

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Scheme 1. Structures of Host and Guest Moleculesa

INTRODUCTION Photoinduced electron transfer plays an important role in a number of fundamental chemical and biological transformations, and commercial applications including capture and storage of solar energy.1−4 Although a large number of organic dyes are easily synthesized and commercially available, their use as electron donors/acceptors is hampered by their tendency to aggregate and photodegrade upon exposure to light. Encapsulating dyes in a host molecule to overcome such problems as well as to slow the back electron transfer process has been investigated by several groups.5−9 This Letter details the results of one such approach yet to be fully explored. The quenching of fluorescence of azulene enclosed within a hemicarcerand by TiO2 nanoparticles in aqueous solution was established by Piotrowiak and co-workers several years ago.6 The closed nature and smaller internal cavity volume (∼200 Å3) of the hemicarcerand limited the guest molecules that could be incarcerated in this host. In this study we have used a cavitand host, known as octa acid (Scheme 1) that forms a capsule (cavity volume ∼500 Å3)10 by assembling two molecules of the host and one or two molecules of the guest.11 A secondary host, cucurbit[7]uril (CB7), was used to selectively complex 4,4′-dimethyl viologen chloride (MV2+: Scheme 1) that was used as an electron acceptor. We demonstrate below that photoinduced electron transfer can occur between donor (coumarin dyes) and acceptor molecules (4,4′-dimethyl viologen chloride) despite physical separation by the molecular wall of the host. The host we have employed to separate the donor and acceptor molecules is the water-soluble octa acid, a carcerand (OA, Scheme 1), that is © 2012 American Chemical Society

a

The counter anion of MV2+ is Cl−.

substituted with COOH groups at the periphery.12 We believed the acid groups would help anchor OA on a TiO2 surface and, in a basic medium, hold the positively charged MV2+close to the exterior walls of OA capsule through electrostatic interactions with the COO− groups. Prompted by the established use as sensitizers in dye-sensitized solar cells, we chose three coumarins shown in Scheme 1 as electron donors.13−17 Coumarins tend to aggregate limiting their Received: January 4, 2012 Revised: January 31, 2012 Published: February 3, 2012 3355

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applicability as sensitizers.13,15 Furthermore, although electron injection into semiconductor surfaces occurs at subpicosecond time scale, a donor dye with a long excited state lifetime was preferred.18−20 Coumarins C-1, C-153, and C-480 with S1 lifetime in the range 4 to 7.5 ns were therefore appropriate test probes for the encapsulation approach described here. Based on the reduction potential and the excited state (S1) energy of MV2+, oxidation potentials, and the excited state (S1) energies of coumarins, we believed that the fluorescence would be quenched by MV2+ via electron transfer and not by singlet− singlet energy transfer (oxidation potential of C-153, 0.89 V; C1, 1.09 V; and C-480, 0.72 V; and reduction potential of MV2+, −0.69 V; S1 energies of the three coumarins >2.7 eV; S1 energy of MV2+ ∼4 eV).21 In solution, the S1 excited state of C-1 has been established to be quenched by MV2+ by an electron transfer process at diffusion controlled rate.21 Thus the selected coumarin dyes are ideally suited to test the possibility of photoinduced electron transfer across a molecular wall.

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RESULTS AND DISCUSSION Although all three coumarins can be selectively photoexcited in the presence of MV2+, only C-153 can be selectively excited in presence of TiO2, as the absorption spectrum of C-153 (λ max = 420 nm) is not obscured by the fundamental absorption of the semiconductor (108 s−1). To explore whether relocating the acceptor away from the

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

Figure 2. Fluorescence titration spectra of C-153@OA2 with MV2+; Inset: Stern−Volmer plot of steady state and time-resolved quenching experiment of C-153@OA2 with MV2+; λex = 420 nm; [C-153] = 1.5 × 10−5 M, [OA] = 1 × 10−4 M, and [MV2+] = 1.5 × 10−5 M to 7.5 × 10−5 M in 10 mM sodium tetraborate buffer.

exterior wall of the capsule would have any effect on the quenching process we introduced a second host cucurbit[7]uril (CB7, Scheme 1) that is known to encapsulate MV2+ in aqueous solutions.23 Upon addition of CB7, the fluorescence of C-153@OA2 was fully recovered (Figure S8 in SI) suggesting that for efficient electron transfer the acceptor must be next to the capsule that holds the donor. From the above studies we concluded that photoinduced electron transfer between an encapsulated donor and a free acceptor separated by a single molecular wall was possible in aqueous solution at pH ∼9. This process was inhibited when both the donor and acceptor molecules were included within organic hosts. Examination if C-153 incarcerated within a OA capsule would transfer an electron to TiO2 requires careful consideration of capsule stability and energy levels of TiO2. First, it is known that COOH rather than COO− adsorbs well on TiO2 surface.24−26 Second, it has been demonstrated that potential determining ions, as well as addition of acids or bases, result in a dramatic shift of the TiO2 conduction band edge,27,28 a Nernstian shift of ∼59 mV/pH, thereby influencing the 3356

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Figure 3. Fluorescence spectra of (a) C-153@OA2 and (b) C-153 at different pH; λex = 420 nm; [C-153] = 1.5 × 10−5 M, [OA] = 1 × 10−4 M in 10 mM sodium tetraborate buffer.

injection process. In general, acidic conditions lower the energy of the Ecb (energy level of the conduction band) thus favoring injection from a photoreductant dye, whereas strongly basic conditions have the opposite effect, and may inhibit charge injection of a dye with an excited state close to the Ecb. These observations necessitated neutral or acidic rather than basic conditions. However, OA is more readily soluble at basic pH. To test the pH dependent stability of the capsule we recorded the emission spectra of C-153@OA2 in water at various pH (Figure 3a). We used the known polarity dependent emission maximum of C-153 to determine its location (inside or outside) relative to the capsule.29,30 The emission maximum at 483 nm at pH 9, similar to the value observed in benzene, and consistent with C-153 being within the lipophilic capsule interior shifted to 535 nm, closer to that in water (547 nm) in pH 6 suggesting that the capsule had dissociated under acidic conditions. The observed shift in the emission maximum for C153 was inferred to result from the pH dependent stability of the complex as the emission maximum of C-153 alone (in the absence of OA) was independent of the pH (Figure 3b). We concluded that the capsule dissociated below pH ∼ 7 based on the emission spectra recorded under various pH conditions, and carried out all experiments with TiO2 at pH 7. Electron transfer from C-153@OA2 to TiO2 was probed with two types of nanostructured TiO2: TiO2 films and a colloidal aqueous suspension of TiO2. As a control we used nanostructured ZrO2 that (a) has a wider band gap than TiO2, (Ebg ZrO2 ∼ 5.0 eV compared to Ebg TiO2 ∼ 3.2 eV) precluding electron injection from the photoexcited C-153@OA2 and (b) has the morphology resembling closely that of TiO2 films. FTIR-ATR spectra of TiO2 and ZrO2 films that were immersed into a 1 mM aqueous solution of C-153@OA2 at pH ∼ 7 showed a band at 1707 cm−1 due to the ν(CO) stretch of unbound COOH groups, broad, intense bands in the 1500− 1650 region assigned to the ν(O...C...O) stretch of bound carboxylate groups, a band at 1298 cm−1 due to the v(C−O) stretch of the carboxylic acid and the broad O−H stretch above 3000 cm−1 (Figure 4). Comparison of these spectra with that of OA as a powder (Figure S9 in SI) indicated that C-153@OA2 was bound to TiO2 and ZrO2 films. The emission spectra of C153@OA2 bound to TiO2 and ZrO2 films (λexc = 420 nm), shown in Figure 5a, indicated that fluorescence emission of C153 was fully quenched on TiO2, while when bound to ZrO2 films only a broad fluorescence spectrum (Figure 5a) resulted. Interestingly, the emission λmax on ZrO2 corresponded to that observed for C-153@OA2 in solution, suggesting that C-153 is encapsulated and shielded from a polar environment. Control experiments indicated that C-153 in the absence of the host OA does not bind or physisorb to the nanostructured semi-

Figure 4. FT-IR-ATR spectra of C-153@OA2 on (a) TiO2 and (b) ZrO2 film.

Figure 5. Fluorescence spectra of (a) C-153@OA2 on TiO2 and ZrO2 film, λex = 420 nm; (b) titration of C-153@OA2 with TiO2 solution; λex = 440 nm; [C-153] = 1.5 × 10−5 M, [OA] = 1 × 10−4 M in water.

conducting films. Although we recognize that further confirmation is needed to probe that electron transfer had occurred, this preliminary observation is important in the context of establishing the value of OA type cavitands in problems related to charge separation and dye shielding from heterogeneous environment of a semiconductor surface. Since on TiO2 films the fluorescence of C-153@OA2 was completely quenched, we wanted to explore the possibility of controlling the quenching by adjusting the concentration of TiO2. For this purpose we employed aqueous colloidal TiO2 as the quencher. Thus the studies were conducted in solution than 3357

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Notes

in films. The emission spectra of C-153@OA2 were recorded in the presence of increasing concentrations of TiO2 (Figure 5b). As expected, the fluorescence intensity decreased with increasing concentration of TiO2, and, in a control experiment, colloidal ZrO2 had no effect on the emission intensity (Figure S10). As in the case of MV2+ we concluded that the quenching must be static in nature, as the decrease in emission was not accompanied by decrease in excited state lifetime (Figure S11). The above observations indicate that emission quenching in TiO2 film must be the result of physisorption of C-153@OA2 complex on the TiO2 surface.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS V.R. is grateful to the National Science Foundation, USA for generous financial support for the project (CHE-0848017). E.G. and M.F. thank the American Chemical Society Petroleum Research Fund (Grant #46663-AC10) for support, and to the Office of Basic Energy Sciences of the U.S. Department of Energy (Grant DE-FG02-01ER15256) for the FT-IR-ATR instrumentation.

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CONCLUSIONS We have shown in this Letter that electron transfer between an acceptor molecule and a dye molecule (incarcerated within a host). The details of the mechanism of the electron transfer await further investigation. The OA host offers additional advantages over other previously used molecular hosts including cyclodextrins, cucurbiturils, and hemicarcerands.5 Including larger guest molecules within hemicarcerand and cavitands like cyclodextrins and cucurbiturils could be challenging. Furthermore, OA with multiple COOH groups can interact with the TiO2 surface more strongly than cyclodextrins and cucurbiturils having OH and CO functionalities, respectively. The approach presented here could eventually eliminate the need to synthetically modify the dyes with anchor groups (typically COOH or P(O)(OH)2). Furthermore, the use of dyes encapsulated in a host molecule would avoid dye-aggregation and possibly degradation. The advantage of a dye included in a capsule with respect to a free dye and a dye with anchoring groups is illustrated in Figure 6. The general applicability of this approach would

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Figure 6. Schematic illustration of chromophore (a) aggregated, (b) directly bound through a linker-anchor group and (c) chromophore@ host bound though the host to metal oxide semiconductor surfaces, C: chromophore, H: host, and A: anchoring group.

depend on knowledge of the dynamics of forward and backward electron transfer processes. We are currently working toward realizing this goal.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental procedures; emission spectra of C-1 and C-480 in presence of MV2+; emisison spectra of C-153, C-1 and C-480 in presence of MV2+ and CB7; time-resolved spectra of C-153, C1 and C-480 encapsulated within OA in presence of MV2+, DOSY data, and FT-IR-ATR spectra of OA solid. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E. G. [email protected]; V. R. [email protected]. 3358

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