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Invited Feature Article pubs.acs.org/Langmuir

Photon Upconversion and Molecular Solar Energy Storage by Maximizing the Potential of Molecular Self-Assembly Nobuo Kimizuka,*,† Nobuhiro Yanai,†,‡ and Masa-aki Morikawa† †

Department of Chemistry and Biochemistry, Graduate School of Engineering, Center for Molecular Systems (CMS), Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan ‡ PRESTO, JST, Honcho 4-1-8, Kawaguchi, Saitama 332-0012, Japan ABSTRACT: The self-assembly of functional molecules into ordered molecular assemblies and the fulfillment of potentials unique to their nanotomesoscopic structures have been one of the central challenges in chemistry. This Feature Article provides an overview of recent progress in the field of molecular self-assembly with the focus on the triplet−triplet annihilation-based photon upconversion (TTA-UC) and supramolecular storage of photon energy. On the basis of the integration of molecular selfassembly and photon energy harvesting, triplet energy migration-based TTAUC has been achieved in varied molecular systems. Interestingly, some molecular self-assemblies dispersed in solution or organogels revealed oxygen barrier properties, which allowed TTA-UC even under aerated conditions. The elements of molecular self-assembly were also introduced to the field of molecular solar thermal fuel, where reversible photoliquefaction of ionic crystals to ionic liquids was found to double the molecular storage capacity with the simultaneous pursuit of switching ionic conductivity. A future prospect in terms of innovating molecular self-assembly toward molecular systems chemistry is also discussed. orientation and spatial organization of dye molecules.10,11 Meanwhile, after the development of synthetic bilayer membranes12 and subsequent extensive studies by Kunitake et al.,13 the self-assembly of designed molecules has come to be recognized as a field of chemistry.14−17 The stream latterly merged with supramolecular chemistry,18 and the self-assembly of photofunctional molecules has been a flourishing area.19−30 Although modern chemistry offers the advantage of designing a variety of molecular units and their self-assembly, biological photon-harvesting machinery in terms of its complex and sophisticated architecture with prominent performance is still far beyond synthetic systems. In this light, it is essential to embody the archetypal characteristics of photosynthetic molecular systems that lay the basis of efficient photon energy harvesting and storage. From this perspective, we focused our attention especially on the adaptive and high-density accommodation of supramolecular pigments in photosynthetic membranes as the essence of efficient biological photosynthetic systems. The principles of excited singlet energy migration and the conversion of the harvested photon energy encouraged us to develop supramolecular photon upconversion systems from a wide range of condensed molecular materials and self-assemblies. In

1. INTRODUCTION Being confronted with the ever-growing demand for sustainable energy, efficient conversion and storage of solar energy have been the greatest challenges of our age. Recent progress in the field of photovoltaics1−4 that generate electricity and photocatalysis5−8 to produce chemical fuels has benefitted from the developments in colloid and interface chemistry as well as materials science, playing indispensable roles in the design and synthesis of photofunctional nanomaterials and their operation as a system. These solar energy technologies are primitively inspired by biological photosynthesis systems that convert light energy into successively more stable forms of energy storage. For example, in the photosynthetic supramolecular machinery of purple bacteria, hundreds of pigment molecules cooperate through quantum coherence to achieve remarkable light-harvesting efficiency.9 The pigments display a hierarchical organization and controlled molecular alignment in the light-harvesting complexes that allow the delocalization of electronic excitation among strongly interacting groups coherently following light absorption, thus efficiently transferring excitation energy to the reaction center. These light-harvesting systems are formed by ingeniously employing multiple noncovalent interactions and their controlled self-assembly in biomembranes. Inspired by sophisticated photosynthetic nanoarchitectures, chemists have long endeavored to develop corresponding artificial molecular assemblies.10−30 In the early stages, Kuhn et al. took full advantage of the potential of surface monolayers and Langmuir−Blodgett techniques to control the molecular © XXXX American Chemical Society

Special Issue: Tribute to Toyoki Kunitake, Pioneer in Molecular Assembly Received: September 12, 2016 Revised: October 18, 2016 Published: October 19, 2016 A

DOI: 10.1021/acs.langmuir.6b03363 Langmuir XXXX, XXX, XXX−XXX

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give an excited triplet state of the donor (T1, Figure 1). The triplet energy of the donor is then transferred to the acceptor (emitter) by triplet−triplet energy transfer (TTET). This process repeats, and when two acceptor molecules in the longlived triplet excited states diffuse and collide in their lifetime, a higher-energy singlet state S1 and a ground state S0 are formed by TTA. This upconversion process requires the condition that the sum of the energy of two triplet acceptors (2 × E(T1)) is greater than or equal to the energy of S1. The resultant excited singlet state emits the delayed upconverted fluorescence with an antiStokes shift with respect to the excitation wavelength of the donor. It is important to note that the triplet energy transfer and TTA processes occur via electron exchange (Dexter energy transfer), which requires an overlap of wave functions between donors and acceptors (Figure 2). This process has a steep

addition, a supramolecular solar thermal energy storage system was developed that can storage photon energy with a capacity exceeding that of a single molecule. This article introduces our approaches to evolving the chemistry of self-assembly toward molecular systems chemistry, which compliments earlier reviews.31−34

2. TRIPLET ENERGY MIGRATION IN MOLECULAR ASSEMBLIES AND PHOTON UPCONVERSION Photovoltaic cells cannot absorb photons with energies below their band gaps, and their performance is severely restricted by the Shockley−Queisser limit,35 i.e., intrinsic thermodynamic limit by intrinsic losses such as band edge thermalization, radiative recombination, and the inability to absorb below-bandgap photons. One of the possibility to reduce these transmission losses and to overcome the limit is upconverting the transmitted lower-energy photons from the solar spectrum to higher-energy photons that are usable by the solar cell. Photon upconversion (UC) thus offer a possibility to harvest the unabsorbed lowerenergy photons, and triplet−triplet annihilation-based photon upconversion (TTA-UC, Figure 1) has been considered to be a

Figure 1. Schematic energy-level diagram of TTA-based upconversion for a model donor/acceptor pair. Solid arrows indicate transitions. The process involves the population of the singlet excited state of a donor (S1) upon absorption of incident light, which is followed by intersystem crossing (ISC) to the triplet excited state (T1). When the triplet donor encounters the ground-state acceptor, triplet−triplet energy transfer (TTET) from the donor to an acceptor yields an acceptor triplet state. When two acceptors in their triplet state undergo triplet−triplet annihilation (TTA), one of the acceptors is excited to its excited singlet state while the other acceptor is relaxed to its ground state. The photon emitted from the acceptor singlet state (UC emission, turquoise blue arrow) has a higher energy than that of the initially absorbed photons (green arrow).

Figure 2. Schematic representation of nonradiative electron-exchange energy transfer processes. (a) Dexter energy-transfer mechanism and (b) triplet−triplet annihilation (TTA) mechanism. D is the energy donor, A is the energy acceptor, and * denotes an excited state. S0 is the ground state, S1 is the first singlet excited state, and T1 is the triplet excited state.

exponential dependence on the distance that typically occurs within 10 Å.47 This is in remarkable contrast to the singlet energy transfer based on the dipole−dipole coupling mechanism (fluorescence resonance energy transfer, FRET), which shows a sixth-power dependence and occurs over long distances on the order of 10 to 100 Å.47 We note that the intensity of the upconverted singlet fluorescence displays a quadratic incident light power dependence because TTA requires two photons to produce two sensitized triplet acceptor molecules.37,38 The quantum yield (ΦUC) of TTA-UC is expressed as the product of efficiencies of all of the involved photophysical steps

promising methodology compared to other UC mechanisms such as second-harmonic generation or two-photon absorption techniques that require high excitation intensities (MW cm−2 ≈ GW cm−2).36−41 TTA-UC exploits the large oscillator strength of singlet−singlet transitions to absorb and emit light with a high quantum yield and triple excited states that are long-lived with a typical natural lifetime up to the range of milliseconds. Because of these benefits, it does not require the simultaneous absorption of two photons and can work at power excitation as low as solar irradiance (100 mW cm−2, under AM1.5 conditions, for the whole solar spectrum).36−41 These features are advantageous for practical applications that range from sunlight-powered renewable energy production technologies including photovoltaics42,43 and photocatalysis44,45 to bioimaging.46 The TTA-UC processes start with the long-wavelength selective excitation of the triplet sensitizer (donor) to the singlet state (S1), which is followed by intersystem crossing (ISC) to

ΦUC =

⎛1⎞ ⎜ ⎟f Φ Φ Φ Φ ⎝ 2 ⎠ ISC ET TTA FL

(1)

1

where prefactor /2 is set because two low-energy photons are required to generate one photon with higher energy. Accordingly, the maximum quantum yield (Φ) for TTA-UC is 0.5. The other parameters are the efficiencies of the donor ISC (ΦISC), the donor/acceptor triplet energy transfer (ΦET), the acceptor TTA (ΦTTA), and the fluorescence (ΦFL). f represents the probability to obtain a singlet excited state after the B

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acceptor triplets [∼9.1 Å for 9,10-diphenylanthracene (DPA)52], and τT is the lifetime of the acceptor triplet. To use sunlight as an excitation source, Ith should be lower than the solar irradiance. As is apparent from eqs 1 and 2, efficient donor-toacceptor triplet energy transfer is essential not only for attaining the high UC quantum yield but also for lowering the Ith value. The lifetime of excited triplets is also important, and common TTA-based UC acceptors show long millisecond-scale lifetimes. Note that a high DT value is essentially required to realize TTAUC with low Ith. To date, most effective TTA-UC systems have been achieved in molecularly dispersed organic media because they allow fast diffusion of excited molecules (Figure 3a). The triplet state of the

annihilation of two acceptor triplets. To achieve nearly quantitative ΦISC, usually triplet sensitizers that exhibit a heavy atom effect, i.e., transition-metal-containing donors, are employed.36−40 The annihilation is mediated by the formation of a collisional complex (T−T) of singlet (S), triplet (T), or quintet (Q) multiplicity that originates from the tensor product of the initial spin states, and the weighted statistical probability of formation of these states is 1:3:5, respectively. Although a factor of f = 1/9 is expected as the fraction of generated singlets on purely statistical grounds, this potential fraction is enlarged to 1/4 because the quintet states are energetically unattainable or they dissociate back to two triplets.38,48 Schmidt et al. have shown that the f parameter depends on the energy-level structure of the annihilating molecules and it is possible to overcome the spin statistical limit for TTA-UC.48,49 Meinardi and Monguzzi et al. have achieved a nearly unity conversion efficiency (f ≈ 1) for the donor−acceptor pair of palladium(II) meso-tetraphenyltetrabenzporphyrin and perylene, which satisfies the condition that the energy of the T2 level, E(T2) ≈ 4.0 eV, is considerably larger than twice E(T1) ≈ 1.51 eV.50 These parameters are determined by intrinsic photophysical properties of the donor and acceptor molecules (f, ΦISC, ΦFL) as well as extrinsic conditions such as concentration, solvent viscosity, temperature, and excitation intensity (ΦET, ΦTTA). To increase the yields of TTA-UC, it is imperative to find optimum conditions for a given donor− acceptor pair. Although the optical density of materials at a wavelength of excitation should be sufficiently high, the probability of back Förster energy transfer from the acceptor to the donor increases at higher donor concentration; consequently, the concentration of the donor should be kept lower than that of the acceptor. At low concentrations, molecular diffusion of chromophores during their triplet lifetimes plays an essential role, and the experimental conditions for TTA-UC need to be carefully optimized, including the selection of the donor− acceptor pair. An important point to be stressed here is that molecular oxygen acts as a powerful quencher for triplets. Consequently, traditional organic bimolecular TTA-UC systems work efficiently only in oxygen-free solutions (O2 concentration below 1 ppm).51 It is therefore a dream come true that high TTAUC efficiency is attainable with reasonably weak excitation light intensity such as sunlight, in contact with ambient air. A useful figure of merit of TTA-UC is given by a threshold excitation intensity Ith, at which the spontaneous decay rate of the excited acceptor triplets equals the TTA rate (ΦTTA = 0.5).38,52 The upconverted emission intensity through sensitized TTA shows a quadratic dependence on the incident light power in the weak excitation region as expected for the bimolecular annihilation process. In this regime, TTA is inefficient because of the prevailing nonradiative decay of the acceptor triplets. Meanwhile, by increasing the incident power to Ith, TTA provides the main deactivation channel for the acceptor triplet. Consequently, at the incident light power density of Ith, the incident power dependence changes from quadratic to linear, and above that the upconversion efficiency ΦUC is maximized.38,43,52 The threshold Ith is expressed as a function of the system’s fundamental parameters using the equation Ith = (α ΦET8πDTa0)−1(τT)−2

Figure 3. Schematic representation of the triplet−triplet annihilationbased photon upconversion (TTA-UC) process. D and A represent donor and acceptor molecules, respectively. (a) Conventional molecular diffusion-based TTA-UC. The triplet state of the donor, formed by intersystem crossing (ISC) from the photoexcited (green arrow) singlet state, diffuses and collides with the acceptor, resulting in donor-toacceptor TTET. When the acceptor excited triplet diffuses and collides with the other acceptor triplet, annihilation occurs to form a higherenergy excited singlet, which consequently produces upconverted delayed fluorescence (blue arrow). (b) Proposed supramolecular TTAUC. Acceptor molecules self-assemble with a regular array of acceptor chromophores. Donor molecules are coassembled with (preferably) controlled spatial molecular orientation. Upon photoexcitation of the donor and succeeding ISC, efficient TTET occurs to the neighboring acceptor, and the excited triplet state of the acceptor migrates in the selfassembled chromophore arrays. Eventually, two excited states collide and TTA occurs to give an excited acceptor, which gives higher-energy UC emission (blue arrow).

donor, formed by intersystem crossing (ISC) from the photoexcited (green arrow) singlet state, diffuses and collides with the acceptor, resulting in donor-to-acceptor TTET. Two acceptor excited triplets again diffuse, collide, and annihilate to form a higher-energy excited singlet, which consequently produces upconverted delayed fluorescence. However, the use of volatile organic solvents, the solubility limit of aromatic chromophores, and the overwhelming deactivation of excited triplets by dissolved oxygen severely limits their real world applications. As a way of approaching this problem, recent reports on TTA-UC under aerated conditions employ specific solid polymers or viscous liquids as matrixes to block oxygen.44,53 However, such thick matrixes inevitably restrict the diffusion of

(2)

where α is the absorption coefficient at the excitation wavelength, ΦET is the donor-to-acceptor TTET efficiency, DT is the diffusion constant of the acceptor triplet (in the three-dimensional diffusion system), a0 is the annihilation distance between C

DOI: 10.1021/acs.langmuir.6b03363 Langmuir XXXX, XXX, XXX−XXX

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Figure 4. Chemical structures of liquid acceptor 1 and donor 2. (a) Photoluminescence spectra of the doped liquid (2/1 = 0.01 mol %) with different incident power intensities. Inset: photographs of the 2-doped liquid upon being exposed to white light (up) and a 532 nm green laser (bottom). (b) Schematic illustration of donor-to-acceptor TTET and energy migration among the acceptor liquid molecules. Adopted with permission from ref 66. Copyright 2013 American Chemical Society.

2.1. TTA-UC in Condensed π-Liquid Acceptor Systems. The modification of aromatic chromophores with branched alkyl chains gives nonvolatile room-temperature π liquids,62−64 as widely known for 2-ethylhexyl-p-methoxycinnate, which has been utilized for sunscreen over several decades.65 Nakanishi et al. reported a series of DPA-derived π liquids with high fluorescence quantum yields.63 Although singlet fluorescence characteristics of these amorphous π liquids have been investigated, their application to triplet energy transfer, migration, and TTA has been unprecedented. We started with these condensed liquid materials to get an insight into the importance of molecular alignment in the triplet energy migration-based TTA-UC. A π liquid with DPA chromophore 1 and Pt(II) porphyrin derivative 2 with branched alkyl chains were synthesized as a liquid acceptor and donor, respectively (Figure 4). The branched chains were introduced to 2 because PtOEP was not molecularly dissolved in 1 as observed by polarized optical microscopy.66 π-liquid 1 had a fluorescence lifetime (τ ≈ 7.6 ns) and quantum yield (ΦFL ≈ 0.84) comparable to those observed for the dilute chloroform solution, indicating the absence of strong electronic interactions among DPA chromophores in the neat liquid state. This is ascribed to the bulky, branched alkyl chains surrounding the DPA chromophore. Meanwhile, 1 containing 0.1 mol % 2 exhibited blue UC emission upon excitation of 2 with a 532 nm green laser even in air (inset in Figure 4b). Figure 4b shows steady-state luminescence spectra of 2-doped liquid 1 obtained at varied incident laser power (2/1 = 0.01 mol %). Note that the phosphorescence of the donor at 660 nm was completely quenched regardless of the excitation power, indicating the efficient triplet−triplet energy transfer from 2 to surrounding acceptor liquid 1. Moreover, the UC emission of the 2-doped acceptor liquid at 433 nm showed a decay on the millisecond

excited triplet molecules, which necessitates the use of highpower incident light to ensure effective concentration of the excited triplet states. Therefore, it is absolutely imperative to develop molecular systems that show oxygen blocking ability while allowing efficient TTA-UC under low-power excitation. On the basis of the backgrounds in studying exciton coupling and singlet energy migration among ordered chromophore selfassemblies20,21,54−57 and further encouraged by the observation of phosphorescence from a Pt-porphyrin wrapped in coordination nanoparticles in aerated water,58 we started to integrate the concept of molecular self-assembly and TTA-UC by harnessing triplet energy migration (TEM) in varied molecular systems (Figure 3b).59 It is essential to regularly self-assemble acceptor chromophores so that the excited triplet energy efficiently migrates among the preorganized acceptor arrays via efficient electron exchange between the adjacent chromophores. Moreover, the donor molecules also need to be preorganized in the vicinity (