Functional materials: making the world go round

Some of the most pressing of today's global challenges are climate change, the implementation of renewable energy sources, protection of the environment, controlling the dramatic population increase, health and ageing issues, antibacterial resistance and ensuring food and water safety, as well as pushing forward information technology and digitalization.¹ To successfully tackle these challenges with modern solutions, materials developed with a specific function and with a dedicated application in mind are key. Society needs materials with special catalytic, electrical, optical and magnetic properties that define their functions in order to shape core modern technologies and applications. Examples are manifold, ranging from magnetic materials with applications in data storage² to electronic materials for information processing,³ from adaptive materials responsive to external stimuli useful in, e.g., drug delivery or sensing applications,⁴ to supramolecular materials for efficient antibacterial therapy,⁵ or to materials useful for light-harvesting, energy conversion or storage.^6–9 The term “functional materials” covers all kinds of material classes ranging from small molecules to polymers and molecular crystals to nanoparticles, including ceramics, metals, organic and inorganic molecules. In addition, let’s not forget hybrid materials and composites, which combine two or more functionalities at once or generate new synergistic and cooperative effects involving processes at interfaces.

Physical chemists and computer scientists all over the world are striving to understand the complex relationship between the structure and the physical and chemical properties of emergent complex functional materials. Only this will allow us to answer questions like: how is it possible that Cu films with larger pores catalyze other products from CO₂ than those with smaller pores? How can nanoparticle shape affect catalytic activity? How can we harvest solar radiation over a wide spectral range with minimal thermal losses? How should we design flexible batteries? How can the properties of interfaces be controlled and switched? Which materials are best suited to effective medicinal therapy? Sophisticated characterization methods and theoretical tools are necessary to answer these questions, ideally under in operando conditions. This opens up a vast playground, in which chemists, physicists, computer scientists and materials scientists alike are needed side by side. The complexity of this field unites expertise from complementary fields comprising synthesis, theory, spectroscopy and device engineering in a multidisciplinary fashion. This themed issue is a colorful kaleidoscopic view of current research in physical chemistry and presents a collection of recent work in the field of functional materials. This is only a small sample of how functional materials can be approached and of their immense, versatile applicability (Fig. 1). The selected papers revolve around energy conversion and light harvesting, report on stimuli-responsive materials and tell about fascinating electrochemical applications of novel materials synthesized with elaborate strategies.

Fig. 1 Functional materials in health and life sciences, for energy conversion and storage, for environment and sustainability, advancing information technology – to name just a few global challenges to be solved in the future. Source: Leibniz-IPHT.

Energy conversion technologies are critical for harnessing natural resources and transforming them into energy carriers that sustain human life. Efficient light harvesting over a broad spectral range is thus a central issue, which has been tackled by realizing multichromophoric systems capable of funneling excitation energy to a central reactive unit mimicking natural photosynthesis.¹⁰ At the basis of developing efficient antenna systems is an understanding of the properties of the excited states involved after excitation. An example of the necessity of interaction between experiment and theory in this field of research is the joint experimental–theoretical contribution by Lambert and Mitrić (DOI: 10.1039/C9CP00908F). The work addresses the question of whether energy transfer in a small molecule donor–acceptor–donor trimer, a pyrene–BODIPY–pyrene dye conjugate, takes place in the coherent regime or via Förster resonance energy transfer (FRET). Thus, the paper presents an intriguing and in-depth model study related to recent advances in understanding coherent energy transfer processes in much larger biological multichromophoric systems. Using a combination of ultrafast time-resolved transient absorption and (time- and polarization-resolved) fluorescence spectroscopy and surface-hopping calculations that allow the electronic transition density to be followed, the authors detail the excited state relaxation from an initially delocalized excitation involving all three molecular sub-units to the central BODIPY acceptor within only 200 fs.

This is only one example of the power of modern theoretical approaches. A large array of theoretical methods is available nowadays to model functional materials, ranging from electronic structure methods to molecular dynamics simulations and combinations thereof. Applying similar reaction dynamics methodologies, the group around Corminboeuf (DOI: 10.1039/C9CP00691E) employed adiabatic and non-adiabatic molecular dynamics simulations to investigate the fluorescence quenching mechanism in three archetypal aggregation-induced emission systems, both in the gas phase and in solution. Such compounds are exciting for constructing sensing materials, as they are non-fluorescent in solution but emissive in the solid state. The simulations emphasize the role of conical intersection accessibility to explain the fluorescence quenching in solution. In their contribution (DOI: 10.1039/C9CP00335E), Martynow and coworkers employed time-dependent density functional theory (TD-DFT) to calculate the electronic excited states of a series of photocatalysts of the type [(tbbpy)₂M1(tpphz)M2X₂]²⁺ (M1 = Ru, Os; M2 = Pd, Pt; X = Cl, I) (tbbpy = 4,4′-di-tert-butyl-2,2′-bipyridine, tpphz = tetrapyrido[3,2-a:2′,3′-c:3′′,2′′-h:2′′′,3′′′-j]phenazine) to explore the relationships between structure and charge-separation and electron recombination processes. The fine-tuning of these three component systems, which include a photosensitizer, a bridging ligand and a catalytic center, is very important to develop functional materials aimed at solar energy conversion. A similar theoretical goal was pursued by Shi and coworkers (DOI: 10.1039/C9CP00635D), who employed mono-configurational methods, such as TD-DFT and an algebraic diagrammatic construction of the polarizator propagator to the second-order (ADC(2)), and also more involved multiconfigurational methods to investigate the excitation and emission energies of stacked polycyclic aromatic hydrocarbons (PAHs). PAHs are the building blocks of graphene quantum dots and carbon dots, which hold promise as luminescent carbon nanomaterials. Particularly important in this work is understanding excimer formation using charge transfer numbers derived from transition density matrices; this allows an educated quantification of Frenkel excitons and charge separated states. PAHs were also investigated by Casanova and coworkers (DOI: 10.1039/C9CP00641A) by employing DFT and correlated wave function calculations on small triangular graphene nanofragments. Their research aimed to characterize the relative stability of ground and electronic states with different spin multiplicities, paying particular attention to describing correctly the radical and poly-radical character of some of the systems. They showed how substitution affects the distribution of unpaired electrons, which in turn can be used to tune the electronic and magnetic properties of these materials.

An important field of functional systems comprises stimuli responsive materials. Stimuli-responsive changes in material properties are of the highest interest for, e.g., applications in molecular and organic electronics, active recognition systems, drug delivery or sensor fabrication.⁴ The external stimulus can be, e.g., light, temperature, pH or the presence of a certain molecule or ion. Zharnikov and colleagues (DOI: 10.1039/C9CP00255C) report on azobenzene-substituted alkanethiolate (AT) self-assembled monolayers on Au(111) surfaces for light-induced work function variation, which can be exploited in responsive organic field effect transistors. Azobenzene is known for its photochromic behavior, based on trans–cis isomerization induced by UV light, which can be reversed either by exposure to visible light or thermally. This isomerization is accompanied by a change in overall molecular dipole moment, which is reflected in a change in the work function which is observed using Kelvin probe measurements. The impact of steric constraints in the SAMs and the change in dipole moment modulated by the introduction of electron donating/withdrawing groups is investigated in this work. Bigall and coworkers (DOI: 10.1039/C9CP00281B) report in their contribution on stimuli-responsive photoelectrochemical properties of nanoplatelet cryoaerogels with the potential for application as photoelectrodes in photoelectrochemical sensing. The advantage of cryogels compared to mono- or multilayers of similar nanostructures is their high porosity, and hence high surface area to volume ratio, and the high charge carrier mobility in these nanoparticle assemblies. The photoelectrochemical properties of the cryoaerogels were characterized using linear sweep voltammetry (LSV) and intensity modulated photocurrent spectroscopy (IMPS). IMPS was shown to be a viable tool to gain insight into charge carrier dynamics in nanoparticle assemblies. In a proof-of-principle experiment the photoresponse to ferricyanide ions was studied.

Electrochemical methods are routinely applied in many labs around the globe for material characterization. Electrochemistry as an analytical tool for the investigation of the electronic structure of nanocrystals is introduced by Scheele et al. (DOI: 10.1039/C9CP00301K). The importance of combining electrochemical (cyclic voltammetry (CV), differential pulse voltammetry (DPV) and electrochemical gating) and spectroscopic techniques (UV/Vis-spectroelectrochemistry and potential modulated absorption spectroscopy (EMAS)) to characterize nanocrystals functionalized with organic π-systems is pointed out and the challenges of every method are discussed. This approach is illustrated for the case of thin films of CdSe nanocrystals crosslinked by phthalocyanines with different metal centers. Applying DPV it is revealed that phthalocyanines attached to the nanocrystal surface induce new states within the band gap. The number and energetic position of these states depends on the metal center in the phthalocyanine. For a system containing Co-phthalocyanines, results from investigating the in-gap states near the conduction band edge using EMAS indicate hybridization of the electronic levels of the nanocrystal by coupling with a molecular orbital of the organic ligand. Interactions between nanocrystals and surface ligands allow modification of the optoelectronic properties, e.g., the electronic couplings in nanocrystal films are important for applications in LEDs through impacting film conductivities. Detailed knowledge about the electronic structure at the organic/inorganic interface allows for rational design of novel systems with tailored properties. Lately, CV has also been used to study the kinetic performance of carbon felt as an electrode in vanadium redox flow batteries. In a more qualitative performance assessment, peak-to-peak separations and peak heights are compared for different materials. Meanwhile, for quantitative analysis, the Randles–Ševčík relation is applied, despite the fact that it has been derived for a one-step electron transfer reaction occurring at a planar electrode in semi-infinite diffusion space. Tichter et al. [DOI: 10.1039/C9CP00548J] propose an approach that allows precise fitting of cyclic voltammetry data and can be used for simultaneous investigations of the kinetic and geometric properties of carbon felt electrodes. In their paper, the authors present an algorithm based on the use of a modified Talbot contour for inverse Laplace transformation, providing the mass transfer functions required for the calculation of CV responses in external cylindrical finite diffusion space. VO²⁺ oxidation served as a model reaction and was investigated at pristine and electrochemically aged commercial felt electrodes. With their approach, the authors showed that electrochemical aging predominantly affected the kinetics of the electron transfer reaction, whereas the internal electrode surfaces and pore size distribution remained constant. The estimated pore size distributions were in excellent agreement with porosimetry measurements making this strategy a promising approach for the “online” determination of electrode porosity and electrode kinetics.

Electrochemical concepts with sufficient capacity and highly efficient charge and discharge characteristics are of high importance for short (supercapacitors) and medium/long-term energy storage (batteries and fuel cells). In the manuscript by Wittscher et al. [DOI: 10.1039/C9CP00483A] the impact of carbonate solvents on the self-discharge, thermal stability and performance retention of high voltage electrochemical double layer capacitors is discussed. The authors focus mostly on electrochemical studies of these non-conventional alternative electrolytes; a detailed mechanistic study will be published in the future. Specific emphasis is placed on the self-discharge mechanism, which significantly impacts the material's final function in the dedicated application, but temperature effects are also investigated. A combination of thermogravimetry (TGA), infrared spectroscopy (IR) and gas chromatography and mass spectrometry (GC-MS) was used to learn more about the decomposition behaviour of the novel solvents and their stability.

Nanosized noble metal structures have many applications, due to their electrical and optical properties. The group of Eychmüller (DOI: 10.1039/C9CP00680J) investigated Ag nanowire networks on glass with potential applications as a transparent electrode in solar cells, LEDs or touch screens. The Ag nanowires were produced in a polyol process using poly(N-vinylpyrrolidone) (PVP) with varying molar mass as the capping agent controlling the elongated growth. With a dependence on the temperature and time Ag nanowires with varying length and diameter were generated and, subsequently, the electrodes were fabricated. A systematic dependence of the conductivity on the molar mass of PVP, which forms a shell around the nanowires, was revealed showing inhibitory effects with increasing chain length. The Ag nanowire electrodes were tested in organic solar cell model devices and showed comparable performance to the solar cells prepared with indium tin oxide (ITO) electrodes with comparable sheet resistance. The plasmonic properties of noble metal nanoparticles are exploited, e.g., for field enhancement in surface-enhanced Raman spectroscopy (SERS). To realize SERS on a single particle level, particles with rough surfaces and controllable size distribution are necessary, which offers tunability for a broader range of bioapplications. Jeong and coworkers report on the generation of uniform gold bumpy nanocubes (BNCs) with controllable size (DOI: 10.1039/C9CP00138G). These particles showed 15 times stronger SERS than normal cubic Au nanoparticles.

The present collection allows only a sneak peek of the huge variety of research in the field of functional materials. In this issue we want to highlight current work demonstrating the joint efforts of colleagues from materials chemistry, physical chemistry and theory to evolve the field and to promote new developments. We would like to thank all the authors for their valuable contributions to this themed issue and to welcome our colleagues taking part in the 118th General Assembly of the German Bunsen Society for Physical Chemistry in Jena.

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