Review article
Calix[4]arene-based molecular photosensitizers for sustainable hydrogen production and other solar applications

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This review collects the most representative literature reports on the use of calix[4]arene-based molecules as components, namely photosensitizer dyes, in solar devices, including photovoltaics (dye-sensitized solar cells, DSSC), hydrogen photocatalytic and photoelectrochemical generation from water and sunlight, and CO2 photoreduction. Calix[4]arenes are versatile and easily obtainable scaffolds to be integrated with solar device molecular components either as electron-donor groups in π-conjugated dyes or as host-guest moieties to favor intermolecular interactions. Their beneficial role has been exploited to enhance photovoltaic, hydrogen production, and CO2 reduction performance, paving the way to a new class of molecular active components for next generation solar devices.

Introduction

In the urgent search for sustainable energy production, sunlight is a very promising energy source that can be exploited to produce both electricity and the so-called ‘solar fuels’ using different devices for different applications [1, 2, 3, 4]. In particular, photovoltaic panels can provide electricity, while photocatalysis (PC) and photoelectrochemical cells (PEC) are valuable techniques to directly produce, under sunlight irradiation, ‘green’ hydrogen from water via water splitting or carbon-based fuels and chemicals via CO2 reduction [5, ∗∗6, 7, 8]. A mandatory requirement for these devices is their ability to absorb as many photons as possible in order to trigger the subsequent catalyzed processes. Therefore, a material able to efficiently absorb solar radiation, in particular, across the visible range, is compulsory. A straightforward solution to efficiently extend the absorption to the visible range is to use a semiconductor (SC) sensitized by a proper molecular sensitizer. Typically, TiO2 is the most common choice as SC, and a variety of organic and organometallic molecular sensitizers have been proposed in the last years, offering wide visible absorption and potentially low-cost for the final device [9]. Such approach is used in Dye-Sensitized Solar Cells (DSSC) [10], dye-sensitized PC (DS-PC) [11,12] and dye-sensitized PEC (DS-PEC) [13,14]. Metal-free organic dyes have attracted much attention thanks to their easily tailorable structure that allows a better fitting of the solar spectrum, low toxicity, absence of expensive rare elements, and easy synthesis with high batch-to-batch reproducibility and potential for scale-up. One of the most common approaches is the use of donor–acceptor molecules (D–π–A), composed by a donor electron-rich part (D) connected by a π-conjugated bridge to an acceptor electron-poor moiety (A), which also embeds an anchoring group needed to covalently bind the dye to SC surface. Multibranched or multifunctional structures can be prepared to increase light-harvesting properties and/or strengthen bonding to the TiO2 surface. Very interestingly, in the last years, calix[4]arene-based sensitizers have been investigated for solar devices. Calix[4]arenes are versatile phenol-formaldehyde macrocycles easily obtainable in high yields from inexpensive starting materials and easily functionalizable on the phenol OH groups and on the aromatic positions para-to the OHs. By introducing on the OH groups alkyl groups larger than ethyl, the calix[4]arene scaffold can be blocked in one of four different conformations (cone, partial cone, 1,2- and 1,3-alternate) identified by the reciprocal orientation of the aromatic rings [15]. In the cone structure, the four rings are sin oriented and slightly tilted outward, defining a truncated cone shape endowed with a permanent cavity. Thanks to the combination of the aromatic cavity with the possibility of attaching additional binding sites on both rims, calix[4]arenes have been extensively exploited in the field of Supramolecular Chemistry as receptors for cations, anions, neutral molecules, or as building blocks for the preparation of functional self-assembled architectures [16]. However, despite their success, their scope is hardly limited to the field of molecular recognition. Indeed, the ease of introduction of up to eight moieties in well-defined positions and orientations makes the calixarene macrocycle an ideal molecular scaffold to obtain complex systems with specific properties [17]. In particular, the functionalization of the calix[4]arene with multiple copies of the same or different dyes allowed the synthesis of multichromophoric systems for the study of energy/charge transfer phenomena [18, 19, 20], fluorescent sensors [21], OLEDs [22] and fluorescent smart materials [23], as well as metal-free organic sensitizers in solar technologies [11,24]. For this last purpose, calix[4]arene-based molecules offer several advantages. First of all, the calix[4]arene cone conformation could hinder detrimental dye aggregation, which leads to excited state self-quenching, and thus, lower photocurrents and amounts of generated products. Typically, two or more light-harvesting molecular units are connected to the calix[4]arene cone, thus offering higher molar extinction coefficients for the multidye integrated system without decreasing dye loading, and usually a more efficient electron transfer from the dye to the SC due to preferred geometry with respect to the SC surface. Each sensitizing unit has its own anchoring group, so a stronger bond to the surface is expected. Finally, calix[4]arenes are highly photostable and thermostable that is crucial for DS-PC, DS-PEC, and DSSC. Calix[4]arene derivatives are also being deeply investigated in conjunction with titanium-oxo clusters, with good results in their stabilization (this type of cluster is easily hydrolyzed) and in widening their absorption spectrum to the visible region [25]. In 2015, Both M.J. Blesa et al. and C.-Y. Su et al. first reported different calix[4]arene-based molecules for energy application. The former used it as a scaffold [26] and the latter as the donor part of a multibranched D–π–A dye [27]. In 2016, a monolayer of calix[4]arenes derivatives was also successfully exploited to suppress charge recombination and minimize back electron transfer between N-719 dye and the CB of TiO2 [28].

In this paper, the use of calix[4]arene-based molecules as photosensitizers for DSSC, DS-PC, and DS-PEC will be reviewed, focusing attention on the most interesting results reported in the literature in the last four years. More precisely, we will distinguish two types of calix[4]arene-based sensitizers: (a) calix[4]arene-embedded structures, where the calix[4]arene moiety is part of the D–π–A framework, and (b) calix[4]arene-scaffolded structures, where the calix[4]arene moiety acts as an external (covalently bonded) scaffold of D–π–A units. The calix[4]arene-based systems herein described are depicted in Figure 1.

Section snippets

Applications in electricity photogeneration: Dye-sensitized solar cells (DSSC)

In the last four years, Prof. Blesa's group published other three sets of sensitizers where calix[4]arenes were used as scaffolds [29, 30, ∗∗31]. All these dyes were based on p-tert-butyl-calix[4]arene that should hinder aggregation between D–π–A portions of the dye. In the first work, the investigated sensitizers consisted of a donor group based on triphenylamine (TPA) and/or 4H-pyranylidene (P) moieties, a thiophene ring as a heteroaromatic π-conjugated spacer, and a cyanoacetic acid as

Applications in ‘green’ hydrogen photogeneration from water and sunlight: DS-PC and DS-PEC

Besides DSSCs, much attention has been devoted in the last years to organic sensitizers for photocatalytic applications. It must be underlined that, when presenting photocatalytic H2 production results, the direct comparison of H2 production rates does not provide a reliable assessment about the performance of various photocatalysts, considering that the experimental conditions adopted in different works can significantly differ (in terms of reactor geometry, light power, amount of catalyst,

Conclusions

In conclusion, calix[4]arene-based molecular dyes are a promising class of photosensitizers for solar applications, in particular DSSC, hydrogen photogeneration, and CO2 reduction. The calix[4]arene scaffold introduces additional relevant features such as unconventional donor groups in donor–acceptor dyes and host properties to be exploited for ancillary functions. In this review, we have seen that the introduction of the calix[4]arene is useful to induce beneficial effects for improved

Funding sources

N.M., C.L.B., and A.A. wish to acknowledge the Ministry of research and university (MIUR, grant ‘Dipartimenti di Eccellenza – 2017’ ‘Materials for Energy’, the national PRIN project ‘Unlocking Sustainable Technologies Through Nature-inspired Solvents’ (NATUREChem), grant number: 2017 A5HXFC_002) and University of Milano-Bicocca (grant Fondo di Ateneo- Quota Competitiva 2017 and 2019) for financial support. L.B. wishes to acknowledge the Italian Ministry of Instruction, University and Research

Authors contributions

Chiara Liliana Boldrini: Writing - Review & Editing, Norberto Manfredi: Writing - Review & Editing, Tiziano Montini: Writing - Review & Editing, Laura Baldini: Conceptualization, Writing - Review & Editing, Alessandro Abbotto: Conceptualization, Writing - Review & Editing, Paolo Fornasiero: Conceptualization, Writing - Review & Editing

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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