Spectral sensitization of semiconductors

The examples discussed show that coordination compounds, and in particular metal polypyridine complexes, are versatile building blocks for supramolecular systems of photochemical interest. In such supramolecular systems, thermodynamic and kinetic control of intercomponent energy and electron transfer can be achieved by careful choice of molecular components and connectors. By appropriate supramolecular design, simple but interesting light-induced functions can be obtained. 

Taking the imaginative, though somewhat futuristic approach out-lined in Section 1.4, some of the polynuclear complexes discussed in this article can be viewed as very simple photochemical molecular devices. Examples are the Ru(II)-Cr(III) chromophore-luminophore systems of Section 4, which perform the function of spectral sensitization (Fig 7a). The coupling of the systems for photoinduced electron transfer and charge shift described in Section 3 could lead to triads for photoinduced charge separation (Fig 7b). The trichromophoric systems described in section 5.2 can be viewed as very simple examples of the antenna effect (Fig. 7a), while the longer chain-like systems of Section 5.2 could be considered as molecular optical fibers suitable for remote photosensitization (Fig. 7a) and other related functions. The system described in Section 5.3, on the other hand, couples antenna effect and photoinduced electron transfer into an antenna-sensitizer function. 

Although the development of antenna-sensitizer devices for the spectral sensitization of semiconductors might have some practical impact, it would clearly be unrealistic to consider most of the work described in this article of direct relevance to photocatalysis. It seems likely, however, that the need for increasing efficiency and selectivity will lead towards the development of complex photocatalytic systems of supramolecular nature. Thus, further fundamental studies on ways to control and direct photoinduced energy and electron transfer processes in supramolecular systems seem to be worthwhile. 

Finally, it seems appropriate to point out a hidden aspect of this work. Contrary to what happens in organic chemistry, where a wealth of well-established, general synthetic methods is available, the synthesis of a specific inorganic supramolecular system from molecular components is generally not trivial. Therefore, synthesis, though not dealt with to any appreciable extent in this article, has actually been a major part of the work. As an outcome of this synthetic activity, some progress has been made towards the development of rational synthetic methods in coordination chemistry, by which polynuclear complexes of desired composition and structure can be constructed from simple molecular components. 

Acknowledgment. The work reviewed in this article has largely been a team work. The authors wish to express their appreciation to the other members of the team, Claudio Chiorboli and Maria Anita Rampi, for their essential contribution. A portion of this article was written while one of us (F.S) was visiting professor at the Universite de Paris-Sud, Orsay, Laboratoire de Physico-Chimie des Rayonnements. He is very pleased to acknowledge the hospitality and discussions of Dr. E. Amouyal and his colleagues there. 

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Diaz AF, Logan JA(1980)Electroactive polyanihne films. JElectroanalChem 111 111-114 Noufi R, Nozik AJ, White J, Warren LF (1982) Enhanced stability of photoelectrodes with electrogenerated polyanUine films. J Electrochem Soc 129 2261-2265 Noufi R, Tench D, Warren LE (1981) Protection of semiconductor photoanodes with photoelectrochemicaUy generated polypyrrole films. J Electrochem Soc 128 2596-2599 Jaeger CD, Fan FRF, Bard AJ (1980) Semiconductor electrodes. 26. Spectral sensitization of semiconductors with phthalocyanine. J Am Chem Soc 102 2592-2598 Gerischer H (1977) On the stability of semiconductor electrodes against photodecomposition. J Electroanal Chem 82 133-143... 

Spectral Sensitization of Semiconductor by Mixture of J-aggregated Cyanine Dyes... 

The original concept of spectral sensitization of semiconductors involves simple molecular species as sensitizers. In this section, we will discuss some of the reasons why the use of a supramolecular (instead of a simple molecular) species as sensitizer could enhance the performance of photoelectrochemical cells for light energy conversion. The key concepts at the roots of this possibility are those of photoinduced charge separation and antenna effects. 

Systems that present spectral sensitization, very important when the light absorption properties of a potentially photoactive (generally, luminescent) species does not permit efficient excitation in the desired wavelength range. This kind of phenomenon is crucial for many applications in different fields, such as, for example, the spectral sensitization of semiconductor electrodes in solar energy conversion. 

Jaeger CD, Fan F-RF, Bard AJ (1980) Semiconductor electrodes. 26. Spectral sensitization of semiconductors with phthalocyanine. J Am Chem Soc 102 2592-2598... 

Some of the early studies of spectral sensitization of semiconductors have been on ZnO and CdS. The first report was published by Putzeiko and Terenin in 1949 when they reported sensitization of pressed ZnO powder by adsorbed Rhodamine B, Eosin, Erythrosin and by Cyanine dyes [12]. Hauffe and Gerischer coworkers made some pioneering studies in the seventies and these have been extended by many others [13-18]. Most of these early studies focussed on organic dyes of interest to photographic industry (e.g. Xanthenes such as Rhodamine B or Eosin or Cyanines). 

Kohtani S, Kudo A, Sakata T (1993) Spectral sensitization of a TiOa semiconductor electrode by CdS microcrystals and its photoelectrochemical properties. Chem Phys Lett 206 166-170... 

The photoelectrochemical activity inherent in thin films of aggregated cyanine dyes permits them to act as the spectral sensitizers of wide bandgap semiconductors [69]. It is seen from Fig. 4.14 that the photoelectrochemical behaviour of semiconductor/dye film heterojunctions fabricated by deposition of 200 nm-thick films of cyanine dyes on the surface of TiC>2 and WO3 electrodes, bears close similarity to that of semiconductor electrodes sensitized by the adsorption of dye aggregates. Thus, both anodic and cathodic photocurrents can be generated under actinic illumination, the efficiency of the photoanodic and photocathodic processes and the potential at which photocurrent changes its direction being dependent on dye and semiconductor substrate [69]. 

The use of low bandgap polymers (ER < 1.8 eV) to extend the spectral sensitivity of bulk heterojunction solar cells is a real solution to this problem. These polymers can either substitute one of the two components in the bulk hetero junction (if their transport properties match) or they can be mixed into the blend. Such a three-component layer, comprising semiconductors with different bandgaps in a single layer, can be visualized as a variation of a tandem cell in which only the current and not the voltage can be added up. 

Much attention has been devoted to the development of optimal photo sensitizers of semiconductor electrodes [36, 43]. Ruthenium(II) polypyridine complexes are especially well suited for this purpose. They are strong light absorbers in the visible spectral region and bpy or tpy ligands can be easily derivatized with anchoring groups. Moreover, localization of the excited electron on the ligand which is attached to the semiconductor surface facilitates the electron injection. 

Watanabe T., Fnjishima A., Tatsuoki 0. and Honda K. (1976), pH dependence of spectral sensitization at semiconductor electrodes . Bull. Chem. Soc. Jpn. 49, 8-11. 

A. J. (1980) Semiconductor electrodes 30. Spectral sensitization of the semiconductors titanium oxide (n-Ti02) and tungsten oxide (n-W03) with metal phthalocyanines. J. Am. Chem. Soc., 102, 5137-5142. 

Potential application of these complexes as photosensitizers for light energy harvesting relies heavily on demanding synthetic schemes. The CN-bridged trinuclear complex 1 (Fig. 8) has been found to be an excellent photosensitizer for spectral sensitization of the wide band gap semiconductor Ti02 [75]. Among the possible MLCT excited states associated... 

Recent work on polynuclear metal complexes [3], i.e., supramolecular systems containing covalently linked transition metal complexes as molecular components, is described in this article. Such systems have been designed in order to study intercomponent processes relevant to photoinduced charge separation and antenna functions. The possibility to use supramolecular anterma systems in the spectral sensitization of wide-bandgap semiconductors is also discussed in some detail. 

Figure 7. Schematic representation of an antenna-sensitizer molecular device for the spectral sensitizatirai of semiconductors.

Electron Level Position. One essential condition of spectral sensitization by electron transfer is that the LUMO of the dye be positioned above the bottom of the conduction band, eg, > —3.23 eV in AgBr or > —4.25 eV in ZnO (108). To provide the desired frontier level position respectively to the valence and conduction bands of the semiconductor, it is necessary to use a polymethine with suitable electron-donor abiHty (Pq. Increasing the parameter (Pq leads to the frontier level shift up, and vice versa. Chain lengthening is known to be accompanied by a decrease of LUMO energy and hence by a decrease of sensitization properties. As a result, it is necessary to use dyes with high electron-donor abiHty for sensitization in the near-ir. The desired value of (Pq can be provided by end groups with the needed topological index Oq or suitable substituents (112). 

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