Current Physical Chemistry - Volume 1, Issue 3, 2011

Light is an essential ingredient of life as poetically expressed by W. Ostwald [1]: “Life is a water mill: the effect produced by the falling water is achieved by the rays of the sun. Without the sun the wheel of life cannot be kept going”. Light is indeed at the basis of life not only because plants produce oxygen and carbohydrates through a rather complex process known as photosynthesis, but also because it sustains our body through the synthesis of vitamins, is responsible for the object's colours perception, provides work, leisure, psychological equilibrium, and information, affects our alertness, comfort and emotional state, brings architecture, art, and fashion to life. Beside these essential aspects, light is, and will even more be in the future, of great importance for the progress of mankind; exploitation of its properties and its interactions with matter plays, indeed, a fundamental role for the development of science and technology. The interaction of light with matter is a natural phenomenon as old as the world, but photochemistry as a science it is rather young. We only need to go back one century to find the early pioneers of photochemistry. Among them, an important role was played by Giacomo Ciamician, professor of chemistry at the University of Bologna from 1889 to 1922. It is sufficient to read his most famous paper “The Photochemistry of the Future”[2] to understand that he had a lot of original ideas, starting from that of using solar energy in the place of fossil fuels: “Up until now the development of civilization has been based on coal, which is fossil solar energy. It would be much more convenient to use present solar energy, the solar energy that arrives every day on the Earth.” In the same paper there are several other interesting suggestions about the photochemistry of the future, e.g.: “Processes caused by ultraviolet radiations might eventually take place by sunlight, provided that suitable sensitizers were discovered. … It is conceivable that we might make photo-electrical batteries. … By using suitable catalyzers, it should be possible to transform the mixture of water and carbon dioxide into oxygen and methane or cause other endo-energetic processes. …Photochromic substances might be used very effectively; …the dress of a lady, so prepared, would change its colour according to the intensity of light…”. Although most of Ciamician's predictions have not yet been achieved, in the last decade the progress concerning the interaction between light and matter has been outstanding. Light is made of photons which are at the same time energy quanta and information bits. All the natural phenomena related to the interaction between light and matter and the great number of applications of photochemistry in science and technology can ultimately be traced back to these two aspects of light. Living examples of this double-faced nature of light are provided by the two most important photochemical processes taking place in the biological world: photosynthesis and vision. Understanding the interaction between light and molecules, together with the progress in chemical synthesis, has led to the point where one can conceive artificial photochemical molecular devices and machines capable of using light as an energy supply (to sustain energy-expensive functions and/or to induce extensive conformational changes) or as an input signal (to be processed and/or stored). These systems, the construction and the working mechanisms of which are based on the concepts of supramolecular chemistry [3] and photochemistry [4], are fundamental for the development of nanotechnology. Such a new approach to technology is capable of exploiting the “plenty of room at the bottom” [5] offered by the nanometer world of molecules..........

Supramolecular self-assembly of organic materials is an interesting alternative for the bottom-up design of organic devices. By using desired supramolecular interactions such as hydrogen bonding, π-π stacking, electrostatic interactions, and other nonconvalent interactions, scientists have begun taking control of the nanoscale ordering of the active layer. Synthetic and supramolecular strategies enable us to construct functional molecular architectures for devices. This review describes current progress in organic light-emitting (OLED) and photovoltaic (OPV) devices in which the molecular components possess designed supramolecular interactions - as opposed to non-specific cohesive forces - used to instill or modify functionality.

Semiconductor quantum dots are inorganic nanoparticles which, because of their unique size-dependent electronic properties, are of high potential interest for the construction of functional nanodevices. Photoinduced electronand energy-transfer processes between quantum dots and surface-bound molecular species offer versatile strategies to implement functionalities such as luminescence sensing and switching. In this article we discuss the general principles underlying the rational design of this kind of multicomponent species. Successively, we illustrate a few prominent examples, taken from the recent literature, of luminescent chemosensors and switches based on quantum dots derivatized with redox- or photo-active molecular species.

Among the large number of molecular subunits used for dendrimer chemistry, C60 has proven to be a versatile building block and fullerene-functionalized dendrimers, i.e. fullerodendrimers, have generated significant research activities in recent years. In particular, the peculiar physical properties of fullerene derivatives make fullerodendrimers attractive candidates for a variety of interesting features in supramolecular chemistry and materials science. In this review, selected examples of photoactive fullerene-containing dendrimers are presented to highlight the specific features resulting from the dendritic structures.

Electrochromic materials (ECMs) change their colors when an appropriate voltage is applied. We have discussed various conceptual and practical electrochromic materials beyond the range of classical conducting polymers and inorganic compounds. The advantages and drawbacks of examples have also been discussed in detail. These materials provide a new way to realize low-power consumption multicolor display. For future consideration, waterprocessable electrochromic materials may be popularized worldwide in the next decade.

Diffraction prevents the spatial resolution of fluorescent species co-localized within areas of nanoscaled dimensions. As a result, conventional fluorescence microscopes cannot resolve structural features at the molecular level. Time, however, can be exploited to distinguish fluorophores within the same subdiffraction area, if their fluorescence can be switched independently, and reconstruct sequentially their spatial distribution. In this context, photochromic transformations can be invoked to switch fluorescence under optical control. Indeed, fluorescent and photochromic components can be integrated within the same molecular construct or operated within a common supramolecular matrix to produce photoswitchable fluorescent assemblies. In the resulting systems, electronic communication between the fluorescent and photochromic components can be designed to occur in the excited or ground state in order to photodeactivate or photoactivate fluorescence respectively. In the first instance, intercomponent electron or energy transfer processes can be engineered to quench fluorescence after the photochromic transformation. In the second instance, bathochromic shifts can be imposed on the fluorescent component with the photochromic transformation to permit its excitation and activate fluorescence. Both mechanisms can be exploited to overcome diffraction, on the basis of patterned illumination or stochastic localization respectively, and permit the reconstruction of images with resolution down to the nanometer level. Thus, fluorophore-photochrome constructs might well evolve into valuable molecular probes for the investigation of biological samples with nanoscaled resolution.

Acenes, especially rubrene and pentacene are bricks-and-mortar of modern organic electronics, while smaller members of the family (anthracene derivatives) are common components of luminescent sensors and molecular logic gates. Recent development in semiconductor-based molecular scale devices has turned our attention towards acenes as modifiers of semiconducting surfaces. This review presents some recent achievements in the field of surface-modified titanium dioxide and provides basic tools and data towards understanding acene-titanium dioxide system at molecular level. A review of experimental data is supplemented with some DFT calculations of geometries, electronic structures and spectral properties of acene derivatives equipped with anchoring groups suitable for TiO2 modification. Surprisingly, higher acenes cannot support optical electron transfer with TiO2 surfaces on excitation of the lowest electronic transitions, as it was observed in the case of catechol and naphthalenediol. These systems, however can be still involved in photoinduced electron transfer, which altogether makes them useful surface dopants of wide band gap semiconductors.

The construction and operation of photo-driven molecular machines, capable of performing linear or rotary motion at the molecular scale in response to external light stimuli, continue to be a hot topic in the field of nano-science and nanotechnology. A current big challenge in this field is to make these intriguing molecules perform useful tasks. Besides a discussion of novel means used to control linear and rotary motion in solution phases among such lightresponsive molecular systems, this minireview focuses on selected recent examples of light-driven molecular machines that can perform linear or rotary motion on surfaces and have been harnessed to generate changes in surface properties. This minireview also highlights recent advances on photo-controllable nanovalves based on mesoporous silica nanoparticles. The first example to demonstrate the collective unidirectional rotary motion of an ensemble of molecular motors that can rotate microscale object is highlighted, followed by a brief conclusion and outlook.