Molecular photodiode

J.R. Bolton In solution most photochemical electron transfer reactions occur from the triplet state because in the collision complex there is a spin inhibition for back electron transfer to the ground state of the dye. Electron transfer from the singlet excited state probably occurs in such systems but the back electron transfer is too effective to allow separation of the electron transfer products from the solvent cage. In our linked compound, the quinone cannot get as close to the porphyrin as in a collision complex, yet it is still close enough for electron transfer to occur from the excited singlet state of the porphyrin Now the back electron transfer is inhibited by the distance and molecular structure between the two ends. Our future work will focus on how to design the linking structure to obtain the most favourable operation as a molecular "photodiode . 

The highly oriented molecules in thin organic films such as Langmuir-Blodgett (LB) films and self-assembled monolayers (SAM) [1] are essential for some molecular functions. Non linear optical and opto-electronic properties are two of the most important and interesting functions of these molecular assemblies. In the past more than thirteen years, simulation of the primary process of photosynthesis using such molecular assemblies and its application to molecular photodiodes [2,3] have been one of the main subjects of our laboratory. 

Figure 2. Compartmentation of the LB films for molecular photodiodes by (a) micro processing with an AFM tip and by (b) making use of phase separation of the mixed monolayer of HC-FC amphiphiles. 

The demonstration of molecular electronics has been really difficult, as can be seen for the Aviram-Ratner rectifier. The only molecular device that has been demonstrated experimentally is the molecular photodiode, by Fujihira et al. [5-7]. In the course of time, the term molecular electronics has acquired a broader definition, such as that of being the field in which organic molecular materials perform an active function in the processing and its transmission and storage [8]. 

M. Fujihira, H. Yamada, Molecular Photodiodes Consisting of Unidirectionally Oriented Amphipathic Acceptor Sensitized Donor Triads , Thin Solid Films, 160, 125 (1988)... 

These materials are effectively molecular photodiodes. It is highly possible that future applications of molecular electronic devices will arise through this overlap of technologies. 

Ward D, Scott G, Keane Z, Halas N, Natelson D (2008) Electronic and optical properties of electromigrated molecular junctions. J Phys Condens Matter 20 374118 Yasutomi S, Morita T, Imanishi Y, Kimura S (2004) A molecular photodiode system that can switch photocurrent direction. Science 304 1944... 

Fujihira et al. fabricated the accep-tor/sensitizer/donor (A/S/D) type LB multilayer system as a molecular photodiode based on Kuhn s idea of the light-driven electron pump [32] on a gold optically transparent electrode (AuOTE) [33]. The structure and function of the molecular photodiode of A/S/D are shown in Fig. 18(a), where hydrophilic parts and hydrophobic units are indicated by circles and squares, respectively. Three functional compounds tend to orient regularly in the heterogeneous LB films due to their am-phiphihc properties. 

Fig. 18 Molecular photodiode with heterogeneous A/S/D LB film on an electrode (a) structure, (b) energy diagram, and (c) structural formulae of A, S, and D amphiphiles.

Therefore, to design a better molecular photodiode the distance and AG dependence should be kept in mind. In Fig. 18(b), the energy diagram of the A/S/D molecular photodiode is depicted as a function of distance across the LB film. If the forward processes indicated by arrows with solid lines are accelerated, and the backward processes with dashed lines are retarded by setting the distances and the energy levels appropriately, the photoinduced vectorial flow of electrons can be achieved, that is, the acceleration by setting AG° equal to X is assumed for the forward electron transfer processes (ii) and (iii), while the retardation, as a consequence of the inverted region, is assumed for the back-electron transfer processes (iv) and (v). Once an electron-hole pair is separated successfully, the recombination of... 

LB Assemblies of Synthetic Molecular Photodiodes Another approach to shortening the distances between the functional moieties is the use of synthetic molecular... 

Fig. 20 Structural forrDulae of D, S, and A components of polyimide heterogeneous LB film molecular photodiode.

To confirm that the triad absolutely acts as a molecular photodiode in the mono-layer, the photocurrent of the AuOTE modified with the triad monolayer with opposite directional orientation (Fig. 24) should be examined. For the purpose of the study, a new type of photoelectrochem-ical cell (Fig. 24 c) equippmg such an LB... 

---