Porphyrinic molecular devices: towards nanoscaled processes

Abstract

The structural, coordinative, photochemical and electrochemical properties of the porphyrin macrocycle that make them the functional element of choice in ubiquitous biological systems, e.g., chlorophyll, cytochrome P450 and hemoglobin, also contribute to making porphyrins and metalloporphyrins desirable in a "bottom-up" approach to the construction of nanosized devices. This paper highlights some recent advances in the construction of supramolecular assemblies based on the porphyrin macrocycle that display optically readable functions as a result of photonic or chemical stimuli.

Keywords: electron transfer; logic operations; molecular devices; molecular rotors; porphyrins; self-assembly.

Figure 1.

Comparison of two approaches for the preparation of devices at the nanometer level. The dimensions of the porphyrin macrocycle (nm) and its ability to be redox- and photo-active make it an ideal candidate for nanodevice fabrication.

Figure 2.

Demonstration of a molecular logic gate operation using the simple macrocyle tetraphenylporphyrin (TPP). Addition of acid, base, or both to a solution of TPP produces spectral changes, which can be detected by either transmission (difference output) or emission (borrow output). Interpretation of their conjoined use in Boolean algebra terms yields an example of a molecular half-subtractor.[9]

Figure 3.

Assembly of multichromophoric constructs of nanoscale dimensions (a) the stoichiometry of the dyad assembly is phase dependant while the efficiency of the transduction, leading in one instant to a useful molecular battery, is component driven (b) the efficient self-assembly of seven components yields a molecular cube. Each face of the cube is constructed from a porphyrin macrocycle.

Figure 4.

Schematic representation of tin(IV) porphyrin-phenolate complex that demonstrates rotor function. The rotation of the planar naphthalene diimde portion of the ligand (represented by red block) relative to the coordinated phenolate (green block) influences the fluorescent output of the tin(IV) porphyrin.

Figure 5.

(a) The macrocyclic structure of the porphyrin can be envisaged as a clock face. The four meso- (corresponding to clock numbers 3, 6, 9 and 12) and eight β–pyrrolic positions (remaining clock numbers) may be readily functionalized. (b) Schematic demonstration of molecular control using metalloporphyrins to function as a stopwatch. Addition of silver ions provides the stimulus to block the free rotation of the axial ligands effectively “stopping” the stopwatch, which, upon addition of a further bromide salt, can reverse the complexation to “start” the stopwatch again.

Figure 6.

Schematic structure of “intelligent” porphyrins and their patterning on a graphite surface. (a) STM representation of free base porphyrins indicates face-on-face tilt packing along the surface. (b) Metallation introduces an axial ligand approximately orthogonal to the porphyrin macrocycle that patterns the surface in a planar, 2D layering. (L = pyridine).