Electron Injection and Energy-Transfer Properties ... - ACS Publications

Article pubs.acs.org/JPCC

Electron Injection and Energy-Transfer Properties of Spiropyran− Cyclodextrin Complexes Coated onto Metal Oxide Nanoparticles: Toward Photochromic Light Harvesting Viktoras Dryza* and Evan J. Bieske School of Chemistry, The University of Melbourne, Parkville, Victoria, Australia 3010 S Supporting Information *

ABSTRACT: The photochromic and spectroscopic properties of spiropyran−γ-cyclodextrin inclusion complexes coated onto the surface of zirconia and titania nanoparticles (NPs) are examined. After ultraviolet (UV) irradiation, the colorless spiropyran (SP) isomer contained within the cyclodextrin (CD) is converted to the colored merocyanine (MC) isomer, which absorbs light in the 400−600 nm region. A significant increase in the excited-state lifetime for MC-CD on zirconia NPs (τMC = 1.32 ns) is observed compared to MC in solution (τMC = 0.21 ns), indicating that MC → SP photoisomerisation is hindered by the surrounding CD. Excitation of MC-CD on titania NPs results in electron injection into the titania conduction band with a quantum yield of 0.70. Excitation of MC-CD on zirconia NPs when a squaraine-based dye sensitizer is also attached to the NP surface results in energy transfer to the dye sensitizer with a quantum yield of 0.65. The prolonged excited-state lifetime of the encapsulated MC-CD is responsible for the high electron injection and energy-transfer quantum yields. The SP-CD/MC-CD system has potential applications for photochromic, light-harvesting electrodes in dye-sensitized solar cells. photochromic molecule acting as a FRET acceptor,5,6,17,18 to our knowledge it has been used as a FRET donor only once before (within an intramolecular-based scheme).19 In the current work, we have coated the surface of zirconia and titania nanoparticles (NPs) with SP-CD complexes and examined their spectroscopic properties before and after ultraviolet (UV) irradiation. We demonstrate that CD encapsulation substantially increases the excited-state lifetime of the MC isomer on zirconia NPs. This enables excited MCCD to efficiently inject electrons into the metal oxide conduction band when attached to titania NPs. Furthermore, excited MC-CD functions as an efficient intermolecular FRET donor on zirconia NPs when a squaraine-based dye sensitizer is also attached to the NP surface. Overall, the present study demonstrates that the performance of functional devices could be enhanced by increasing the excited-state lifetimes of their photochromic components through molecular encapsulation. One possible application for these types of systems may lie in creating photochromic dyesensitized solar cells (DSSCs).20−23 The operative lightharvesting properties of the SP-CD/MC-CD complexes coated onto metal oxide NPs make them ideal candidates as either photochromic dye sensititzers or energy relay dyes for DSSCs.

1. INTRODUCTION Photochromic molecules have been proposed as components in future molecular technologies, including molecular data storage, molecular motors, drug delivery, and high-resolution imaging.1−5 Photochromic molecules, such as spiropyrans and diarylethenes, normally have two isomeric forms: one that is colorless and another that is colored, with interconversion between the two forms driven by light.1,2 Currently, the most common application of photochromic molecules is in sunglasses and windows that vary their opacity according to the intensity of incident sunlight. Spiropyrans are an interesting class of photochromic molecules because the colorless and colored isomers have quite different structures. The colorless “spiropyran” isomer (SP) has a compact nonplanar structure, whereas the colored “merocyanine” isomer (MC) has an extended planar structure.1,5 Because of this structural difference, placing spiropyrans in a rigid environment slows SP ↔ MC interconversion.6−9 One such example involves incorporating a spiropyran within the cavity of a cyclodextrin (CD) capsule.10−16 Hindering the MC → SP photoisomerisation should also extend the MC excited-state lifetime, enhancing the efficiency of photophysical processes involving the excited chromophore. Potential processes include electron transfer or Fö rster resonance energy transfer (FRET) to an accepting species. Although, numerous cases exist for the colored isomer of a © XXXX American Chemical Society

Received: May 27, 2015

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DOI: 10.1021/acs.jpcc.5b05032 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

2. EXPERIMENTAL AND COMPUTATIONAL APPROACH A detailed description of the experimental and computational approach is given in the Supporting Information, section S1. Briefly, the SP-CD complexes were prepared by the precipitation method.10,12,13 SP-CD coated NPs were prepared by mixing a SP-CD dispersion solution with either zirconia or titania NPs. Dye-sensitized NPs were prepared by mixing a SQ2 dye solution with zirconia NPs. SP-CD coated, dyesensitized NPs were prepared by mixing a SP-CD dispersion solution with the dye-sensitized NPs. To examine the samples, several drops of a NP solution were placed on a sample holder, followed by air drying. Absorption spectra of the samples were recorded using diffuse reflectance absorption spectroscopy. The fluorescence experiments were performed by inserting the sample into a vacuum chamber containing 1 Torr of nitrogen buffer gas. The samples were excited with the output of a picosecond laser (532 nm, τ ∼ 12 ps). For dispersed emission experiments, the emitted light was passed through a 532 nm long pass filter and sent to a spectrometer. For the time-correlated single photon counting (TCSPC) experiments, the emitted light was passed through a 610−630 nm band-pass filter and sent to a cooled photomultiplier tube connected to a TCSPC card. UVirradiated samples were exposed to 385 nm light from a light emitting diode (LED) . The fluorescence decay data were fitted to a stretched exponential function, convoluted with the instrument response function (IRF), to account for the multitude of slightly different MC-CD binding environments that give rise to overall decay curves that are nonexponential: I(t ) = I0 exp[−(t /τc)β ]

Figure 1. Molecules used in this study: spiropyran isomer (SP), merocyanine isomer (MC), γ-cyclodextrin (CD), and SQ2 dye sensitizer.

and emission spectra of the spiropyran in ethanol, before and after exposure to UV light from a LED (λ = 385 nm).

(1)

where I0 is the initial intensity, τc the characteristic lifetime, and β the dispersion parameter. The stretched exponential function has been used to model the excited-state decays of dyesensitized surfaces,24,25 with β = 1 corresponding to a singleexponential decay and values of β < 1 accounting for a distribution of first-order decays. Fitted parameters were used to determine the average lifetime, τ:

τ=

τc ⎛ 1 ⎞ Γ⎜ ⎟ β ⎝β⎠

⎛1⎞ Γ⎜ ⎟ = ⎝β⎠

∫0

∞

(2)

x1/ β − 1 e−x dx (3)

The properties of the MC and SP isomers were explored theoretically using density functional theory (DFT) within the Gaussian 09 program [CAM-B3LYP/6-311+G(2d,p)].26,27 The lowest-energy geometries of the gas-phase SP and MC isomers were located and subject to a vibrational frequency analysis, with their excited electronic states examined using timedependent density functional theory (TD-DFT) .

Figure 2. Absorption and emission spectra of SP (before UV irradiation) and MC (after UV irradiation) (top panel) and SQ2 dye (bottom panel) in ethanol.

3. RESULTS AND DISCUSSION 3.1. Solution-Phase Spectroscopic Properties. The structures of the colorless SP isomer and colored MC isomer of the photochromic compound used in this study are shown in Figure 1.5,28 SP can be switched to MC using UV light, with the reverse isomerization process driven by visible light. MC can also thermally relax back to SP. Figure 2 shows the absorption

Before UV irradiation, only SP is present, with the spectrum displaying a single absorption band with a maximum at 340 nm. TD-DFT calculations [CAM-B3LYP/6-311+G(2d,p), Table 1] predict that this absorption is due to the S2 ← S0 transition of SP: vertical excitation wavelength (λvert) = 289 nm and oscillator strength ( f) = 0.18. The S1 ← S0 transition of SP has λvert = 306 nm, but a f that is essentially zero. After UV B


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Metal oxide NPs are chosen as the substrate because they act as useful models for the porous titania (TiO2) electrode within DSSCs. Here, we initially use zirconia (ZrO2) as it has surface properties similar to those of titania, yet does not have electronic interactions with the SP-CD and MC-CD excited states, a consequence of its large band gap (∼5.5 eV) and high conduction band edge.33−36 The zirconia data provide a benchmark for the intrinsic MC-CD excited-state dynamics, so that electron injection and FRET can be clearly identified in more complex arrangements. Because SP-CD displays limited solubility in the sensitizing solution, the prepared samples can be considered a mixture of SP-CD and zirconia NP powders. However, previous studies demonstrate that CDs and dye−CD complexes bind strongly to the titania surface through their multiple hydroxyl groups,37−41 with computational calculations on the binding of catechol (two hydroxyl groups) to titania predicting a reasonably strong physisorption interaction (∼0.3−1.0 eV), with the binding strength substantially increasing if the hydroxyl groups are deprotonated.42,43 The amount of SP-CD applied to coat the zirconia NPs is estimated to give a coverage of 0.4 complexes/ nm2 (see Supporting Information, section S1), similar to that found for monolayer coverage of bare γ-CD (∼0.3 molecules/ nm2).39 Therefore, it is possible that the sonication of SP-CD and zirconia NPs in solution, before being deposited on a glass slide and dried for analysis, causes the dispersed SP-CD powder to form monolayers or multilayer clusters bound to the NP surface. The absorption and emission spectra for the SP-CD coated zirconia NP sample, recorded before and after UV irradiation (labeled SP-CD-Zr and MC-CD-Zr, respectively), are shown in Figure 3. Before UV irradiation, only the absorption band of SP-CD is present, with a maximum at 350 nm. After UV

Table 1. TD-DFT Vertical Excitation Energies and Oscillator Strengths for the Electronic Absorptions of the Gas-Phase SP and MC Isomers [CAM-B3LYP/6-311+G(2d,p)] SP S1 ← S2 ← S3 ← S4 ← MC S1 ← S2 ← S3 ← S4 ← S5 ←

λvert (nm)

f

S0 S0 S0 S0

306 289 281 272

0.00 0.18 0.01 0.09

S0 S0 S0 S0 S0

444 393 318 310 295

0.87 0.00 0.22 0.00 0.34

irradiation, both SP and MC will be present, leading to a new absorption band with a maximum at 540 nm appearing in the spectrum due to the S1 ← S0 transition of MC. The TD-DFT calculations predict λvert = 444 nm and f = 0.87 for this excitation. Although UV irradiation depletes the amount of SP in the solution, the absorption band at 340 nm increases in intensity and becomes slightly broader. This is due to SP and MC having electronic absorptions that overlap in this region (MC: S3 ← S0 λvert = 318 nm and f = 0.22, S5 ← S0 λvert = 295 nm and f = 0.34). The TD-DFT results are consistent with those reported for similar spiropyrans.29 Excitation of the MC optical absorption band at 532 nm yields an emission band centered at 635 nm, assigned to the S0 ← S1 transition. The SQ2 squaraine dye is commonly used as a sensitizer in DSSCs, with its structure containing a carboxylic acid group for binding to metal oxide surfaces (Figure 1).30 SQ2 displays narrow absorption and emission bands in ethanol, with maxima at 655 and 670 nm, respectively (Figure 2). The SQ2 dye’s absorption band overlaps the emission band of MC, meaning that the MC−SQ2 combination should function as a FRET donor−acceptor pair, i.e., nonradiative electronic energy transfer occurs from MC to SQ2 via long-range dipole−dipole coupling.31 3.2. Spiropyran−Cyclodextrin Spectroscopic Properties. Previous studies have shown that a spiropyran molecule can be inserted into a γ-CD (Figure 1) to form a stable inclusion complex.10−14 According to Zhang et al., SP-CD complexes made via the precipitation method used in this study have 1:1 guest:host stoichiometries.12 This procedure relies on the hydrophobic SP being attracted into the hydrophobic CD cavity when added to ethanol:water solution. SP has suitable dimensions (W × L ≈ 6.5 × 13 Å) for favorable noncovalent bonding interactions with the CD cavity (ϕ × L ≈ 7.5−8.3 × 7.9 Å),11 although some portion of the SP molecule will protrude from the cavity. Encapsulation of SP by CD gives rise to several beneficial properties. First, in contrast to pure SP, SP-CD remains highly photochromic in its crystalline state, allowing functional layers to be formed on surfaces.10−13,32 Second, reversible SP ↔ MC photoswitching and thermal MC → SP decay occur much more slowly within the CD than in solution.10−13 This suggests that the MC excited-state lifetime should also be increased within the CD. These attributes make the SP-CD/MC-CD system ideal for demonstrating photochromic electron injection and FRET on the surface of metal oxide NPs.

Figure 3. Absorption (top panel) and emission (bottom panel) spectra of SP-CD-Zr (before UV irradiation) and MC-CD-Zr (after UV irradiation). The top spectrum also shows the difference between the MC-CD-Zr and SP-CD-Zr absorption spectra. C


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The Journal of Physical Chemistry C irradiation, the optical absorption band of MC-CD appears, spanning the 400−600 nm range. The presence of various MC···CD, MC−CD···MC−CD/SP−CD, and MC−CD···ZrO2 interactions broadens both the MC-CD optical and UV absorption bands, compared solution. The difference between the absorption spectra before and after UV irradiation (Figure 3), which reflects the overall change in absorption caused by SP-CD depletion and MC-CD growth, suggests the MC-CD optical band is broadened to shorter wavelength compared to MC in solution and has a maximum at ∼500 nm. It is important to remember that not all of the SP-CD is converted into MC-CD in the sample labeled MC-CD-Zr. Measuring the photoisomerisation conversion efficiency is not possible via the absorption spectra because the UV bands of MC-CD and SP-CD overlap. However, using the TD-DFT oscillator strengths for the UV and visible absorptions of SP and MC, respectively, in combination with the absorption spectra, we estimate ∼10% of SP-CD is converted to MC-CD after UV irradiation. Excitation of SP-CD-Zr at 532 nm does not produce any significant emission. However, when MC-CDs generated by UV irradiation are excited at this wavelength they produce a strong emission band with a maximum at 630 nm. To check the ability of the SP-CD/MC-CD system to reversibly modulate the MC-CD emission, we alternated between 532 nm and UV irradiation to switch between SP-CD and MC-CD populations, respectively (see Supporting Information, section S2). The experiments show that the SP-CD ↔ MC-CD switching cycle is reversible and can be repeatedly switched multiple times, although there is evidence of fatigue. To investigate how the excited S1 state lifetime (τMC) of the MC chromophore is influenced by its surrounding environment, we have recorded time-resolved fluorescence decay curves for MC in ethanol and MC-CD-Zr, shown in Figure 4.

Table 2. Photophysical Parameters for MC (Ethanol), MCCD-Zr, MC-CD-Ti, and MC-CD-SQ2-Zr Derived from the Time-Resolved Fluorescence Decay Curves MC (ethanol) MC-CD-Zr MC-CD-Ti MC-CD-SQ2-Zr a

β

τc (ns)

τMC (ns)a

1.00 0.70 0.50 0.50

0.21 1.04 0.20 0.23

0.21 1.32 0.40 0.46

Φinjb

ΦFRETb

0.70 0.65

Estimated error, ±0.05 ns. bEstimated error, ±0.05.

Kinashi et al. in toluene (τ MC = 0.07 and 0.22 ns, respectively).44,45 For MC-CD-Zr, the fluorescence decay is substantially slower, with τMC = 1.32 ns. Because an identical τMC is measured for MC-CD powder, we can conclude that it is the CD, rather than the zirconia surface, that causes this effect. This increase in τMC is proposed to be due to the CD cavity restricting the conformational freedom of MC, sterically hindering MC → SP photoisomerisation and slowing internal conversion from S1 to S0. Overall, these effects will decrease kNR. Using the measured τMC value for MC in ethanol and the fluorescence quantum yield (ΦR) of 0.016 reported by Zhang et al.,8 we are able to derive kR and kNR. By then assuming that kR is constant between MC in ethanol and MC-CD-Zr, after accounting for changes in the host medium’s refractive index, we can then use the MC-CD-Zr τMC value to estimate its kNR and ΦR (see Supporting Information, section S3). Following this procedure, we arrive at kNR = 4.7 ns−1 and 0.6 ns−1 for MC in ethanol and MC-CD-Zr, respectively. This means that kNR decreases by ∼87% upon MC being trapped within the CD, most likely primarily due to a decrease in kisom. For MC-CD-Zr, ΦR = 0.17 is estimated. Significantly, because the excited state of the MC chromophore in MC-CD-Zr is no longer rapidly deactivated, photophysical processes that can be initiated from this state (e.g., electron injection and FRET) should become more efficient. 3.3. Spiropyran−Cyclodextrin Electron Injection Properties. To investigate the electron injection properties of the SP-CD/MC-CD system, we prepared titania NPs coated with the SP-CD complexes. It is expected that both excited SP-CD and MC-CD will inject electrons into the titania conduction band, as DFT calculations predict no energetic constraints (see Supporting Information, section S4). UV irradiation of the titania-based sample (labeled MC-CD-Ti) induces spectral features similar to those described above for the zirconia-based sample (see Supporting Information, section S5). That is, absorption in the 400−600 nm region increases and the 630 nm emission band is activated. This demonstrates that SP-CD → MC-CD photoisomerisation occurs on the titania surface, which is possible only if the SP-CD electron injection quantum yield (Φinj) is less than unity. Time-resolved fluorescence can be used to probe the presence of electron injection. However, our fluorescence setup is limited to probing MC-CD, not SP-CD. The presence of the electron injection decay channel (kinj) on the titania surface will reduce the excited-state τ MC of the MC chromophore compared to that on the insulating zirconia surface: 1 1 = + k inj τMCTiO τMCZrO (5)

Figure 4. Time-resolved fluorescence decay curves for MC in ethanol and MC-CD-Zr.

After UV irradiation, the samples are excited using 532 nm picosecond laser pulses, with the emission between 610−630 nm monitored. Parameters derived from analysis of the decays are given in Table 2. The decay of the MC excited state (kMC) is due to radiative (kR) and nonradiative (kNR) decay, with the latter caused by internal conversion to the MC ground state (kIC) and photoisomerisation to the SP isomer (kisom): 1 kMC = kR + kNR = kR + kIC + k isom = τMC (4) In ethanol, the MC fluorescence decay is fast (τMC = 0.21 ns), in agreement with values reported by Wohl et al. and

2

D

2


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The Journal of Physical Chemistry C From the τMC values, we can also estimate Φinj: τMCTiO 2 Φinj = 1 − τMCZrO 2

(6)

This approach assumes that kR, kIC, and kisom are constant between the two metal oxide surfaces; studies have found this to be a reasonable assumption.34,35 The time-resolved fluorescence decay curve for MC-CD-Ti after excitation at 532 nm is shown in Figure 5. Parameters

Figure 5. Time-resolved fluorescence decay curves for MC-CD-Zr and MC-CD-Ti.

derived from analysis of the decays are given in Table 2. The fluorescence decay of MC-CD-Ti is considerably faster than that of MC-CD-Zr, with τMC = 0.40 and 1.32 ns, respectively. Therefore, excited MC-CD undergoes electron injection into the titania conduction band, with an estimated Φinj = 0.70. This Φinj value is slightly less than those measured for typical DSSC dyes (Φinj ≈ 0.9),35,46 which is probably due to a combination of two factors. First, the electron transfer may be subdued by the surrounding CD container, an effect observed previously for dye−CD complexes on titania.37,38 Second, because SP-CD/ MC-CD multilayers are believed to be present on the surface, the recorded decay may have contributions from a proportion of MC-CD complexes with reduced electron injection rates that are not directly adjacent to the titania surface. The photophysical scheme for this system is shown in Figure 6. The fact that SP-CD → MC-CD photoisomerisation and MC-CD electron injection occur on titania suggest that the SPCD/MC-CD system could be used as a photochromic sensitizer for DSSCs. In this arrangement the DSSC would be transparent at night because only SP-CD is present, yet upon exposure to UV-containing sunlight, a steady-state population of the colored MC-CD isomer would be generated, absorbing visible light, darkening the DSSC, and injecting electrons into the titania electrode to generate electricity. Such photochromic DSSCs could be exploited as smart windows for buildings,47 offering integrated shading and energy production that is responsive to the weather conditions. Whereas electrochromic materials have previously been combined with DSSCs to achieve a similar function, the darkening agent does not contribute to the light-harvesting process.48 Zhang et al. and Wu et al. have previously used photochromic dye sensitizers in DSSCs: azobenzene- and diarylethene-based dyes, respectively.22,23 However, as photoisomerisation detrimentally competes with electron injection,49 reducing the rate of the former process via molecular encapsulation, as achieved here for MC-CD, should enhance Φinj and lead to a superior DSSC performance.

Figure 6. Scheme illustrating the photophysical processes occurring within SP-CD-Ti and MC-CD-Ti.

One issue that will arise from SP-CD injecting electrons is the time taken to reach a steady-state population of MC-CD will increase, affecting the system’s responsiveness to the sunlight. Furthermore, if Φinj is much greater for SP-CD than for MC-CD, this would lead to a decrease in the relative concentration of steady-state MC-CD, diminishing the intended photochromic color-switching effect. A possible route to overcome this issue may be to use the SP-CD/MCCD system as a photochromic DSSC energy relay dye, exploiting it as a FRET donor instead of an electron injector. 3.4. Spiropyran−Cyclodextrin FRET Properties. To implement a photochromic FRET donor scheme, we placed SP-CD complexes and SQ2 dye sensitizers on a zirconia NP surface to create a FRET donor−acceptor pair. The SQ2sensitized zirconia NPs were made first and then coated with SP-CD. The SQ2 dye surface coverage is estimated to be 0.03 molecules/nm2 (see Supporting Information, section S1), well below that typically found at complete dye coverage (∼0.3−2 molecules/nm2).50,51 A low dye coverage is used to provide space for the SP-CD complexes on the surface and also limit the formation of SQ2 dye aggregates. The absorption and emission spectra for the SP-CD coated, SQ2-sensitized, zirconia NP sample, recorded before and after UV irradiation (labeled SP-CD-SQ2-Zr and MC-CD-SQ2-Zr, respectively), are shown in Figure 7. Before UV irradiation, the absorption band of the SQ2 dye dominates the spectrum, with a maximum at 650 nm. The SP-CD absorption band is also evident at 350 nm. After UV irradiation, the absorption across the 400−600 nm range increases markedly because of the generated MC-CD. Before UV irradiation, excitation at 532 nm E


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Parameters derived from analysis of the decays are given in Table 2. The decay is recorded by monitoring the 610−630 nm emission region to focus only on the MC-CD emission. If FRET occurs within MC-CD-SQ2-Zr, the τMC of the MC chromophore “donor” will be reduced compared to MC-CD-Zr because of the available FRET decay channel (kFRET) introduced by the SQ2 “acceptor”: 1 1 = + kFRET τMC + SQ2 τMC

(7)

The fluorescence decay of MC-CD-SQ2-Zr is considerably faster than that of MC-CD-Zr, with τMC = 0.46 and 1.32 ns, respectively. This confirms that UV-formed MC-CD complexes undergo FRET of their excitation energy to the SQ2 dye. The function used to describe a donor’s fluorescence decay when surrounded by multiple acceptors will depend on the dimensional arrangement of the acceptors (e.g., 2-D, 3-D) and their concentration.31 However, this function relies on the donor’s fluorescence decay in the absence of acceptors being a single exponential, which is not true for our samples, with the acceptors’ dimensional distribution in our samples also not well-defined. Therefore, a stretched exponential function is used instead to fit the fluorescence decays because it accounts for the samples containing a distribution of first-order decays and employs a limited number of variable parameters, leading to a reliable and systematic approach for analyzing the data. Furthermore, the derived average lifetimes are proportional to the integrated areas under the fluorescence decay curves (once deconvoluted for the IRF), allowing the τMC values to be used to estimate the FRET quantum yield (ΦFRET): τMC + SQ2 ΦFRET = 1 − τMC (8)

Figure 7. Absorption (top panel) and emission (bottom panel) spectra of SP-CD-SQ2-Zr (before UV irradiation) and MC-CD-SQ2-Zr (after UV irradiation) .

produces a narrow emission band at 685 nm, originating from the SQ2 dye, which still can weakly absorb light at this excitation wavelength (see Supporting Information, section S6). Following UV irradiation, the intensity of the SQ2 emission band significantly increases. A new feature on the short wavelength side of the SQ2 emission band also appears (550− 650 nm), which is associated with emission from MC-CD. However, the increase in the SQ2 emission after UV irradiation of the sample is much greater than that contributed from the overlapping MC-CD emission band. After accounting for the MC-CD’s contribution to the SQ2 emission band’s intensity, we find that the SQ2 dye’s emission is approximately doubled after UV irradiation. This enhancement is proposed to be a result of energy transfer (radiative and FRET) from the UVformed MC-CD to SQ2 dye. To explore the nature of the MC-CD to SQ2 energy transfer, we recorded the time-resolved fluorescence decay curve for MC-CD-SQ2-Zr after excitation at 532 nm (Figure 8).

Using the τMC values determined for MC-CD-Zr and MCCD-SQ2-Zr, ΦFRET = 0.65 is estimated. The FRET efficiency can also be quantified by the Förster distance (R0), which is the donor−acceptor separation at which ΦFRET = 0.50. A theoretical calculation of R0 is possible using the parameters that influence FRET for a donor−acceptor pair: the donor’s ΦR; the degree of overlap between the donor’s emission and acceptor’s absorption spectra, i.e., overlap integral (J); the relative alignment of the donor and acceptor’s transition dipole moments, i.e., orientation factor (κ2); and refractive index of the host medium (n):31 R 0 = 0.0211 × (n−4 ΦR J κ 2)1/6

(9)

Using the estimated ΦR for MC-CD, spectroscopic data, and κ2 = 0.476 (randomly distributed, but static donor−acceptors),31 we estimate R0 = 5.4 nm for the MC-CD+SQ2 pair (see Supporting Information, section S6). The reasonably large R0 is primarily a result of the substantial overlap between the MCCD emission and SQ2-Zr absorption bands and the extremely large extinction coefficient of the SQ2 dye (3.19 × 105 M−1 cm−1).30 It is clear from the results described above that UV-induced SP-CD → MC-CD photoisomersation generates an energy relay dye for capturing 400−600 nm light and funnelling the energy via FRET to the SQ2 dye attached to the zirconia NP. The photophysical scheme for this system is shown in Figure 9. To our knowledge this is the first time the colored form of a photochromic molecule has been used as an intermolecular FRET donor. Using an encapsulated MC chromophore as a

Figure 8. Time-resolved fluorescence decay curves of MC-CD-Zr and MC-CD-SQ2-Zr. F


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zirconia NPs, excitation of MC-CD results in FRET to the SQ2 dye sensitizer with a ΦFRET of 0.65. Overall, we have demonstrated that photophysical processes initiated by a colored photochromic isomer’s excited state (e.g., electron injection and FRET) are enhanced by using molecular capsules. We propose that this type of molecular architecture on metal oxide surfaces should be suitable for creating photochromic DSSCs for smart window applications.

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ASSOCIATED CONTENT

S Supporting Information *

Additional information regarding the experimental methods, photophysical parameter calculations, and DFT geometries. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b05032.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +613 8344 8163. Fax: +613 9347 5180. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported under the Australian Research Council’s Discovery Project funding scheme (Project DP120100100). V.D. acknowledges an Australian Renewable Energy Agency Postdoctoral Fellowship (6-F004) and support from the University of Melbourne’s Early Career Researcher Grant Scheme. The picosecond laser was kindly provided by K. P. Ghiggino and T. A. Smith.

Figure 9. Scheme illustrating the photophysical processes occurring within SP-CD-SQ2-Zr and MC-CD-SQ2-Zr.

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FRET donor opens up the possibility of initiating and modulating photoinduced molecular processes within a FRET acceptor species through a combination of UV and visible light. It has been shown that DSSCs can be constructed with visible-light absorbing energy relay dyes that undergo FRET to near-IR dye-sensitized electrodes.36,52−58 The purpose of the energy relay dye, either doped in the electrolyte or linked to the surface or dye sensitizer, is to funnel its absorbed energy to the dye sensitizer, which is the primary electron-injecting component. We have demonstrated a high ΦFRET is achieved between MC-CDs and squaraine dye sensitizers on a zirconia surface. Therefore, it may be possible to use MC-CD as a UVactivated DSSC energy relay dye by employing a molecular chain to tether MC-CD to the titania electrode or injection dye, such that it is located away from the surface to prevent electron injection, yet still close enough to facilitate FRET to the injection dye.

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4. CONCLUSIONS Photochromic SP-CD complexes are prepared and coated onto the surface of zirconia and titania NPs. UV irradiation of the samples initiates SP-CD → MC-CD conversion, resulting in a new absorption band in the 400−600 nm region and activation of a 630 nm emission band. Excitation of MC-CD on zirconia reveals that encapsulation of MC within the CD cavity significantly increases its excited-state lifetime, compared to free MC in solution (τMC = 1.32 and 0.21 ns, respectively). When the SP-CD complexes are coated on titania NPs, excitation of MC-CD results in electron injection with a Φinj of 0.70. When the SP-CD complexes are coated on dye-sensitized G


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