Porphyrins as Molecular Electronic Components of Functional Devices

Abstract

The proposal that molecules can perform electronic functions in devices such as diodes, rectifiers, wires, capacitors, or serve as functional materials for electronic or magnetic memory, has stimulated intense research across physics, chemistry, and engineering for over 35 years. Because biology uses porphyrins and metalloporphyrins as catalysts, small molecule transporters, electrical conduits, and energy transducers in photosynthesis, porphyrins are an obvious class of molecules to investigate for molecular electronic functions. Of the numerous kinds of molecules under investigation for molecular electronics applications, porphyrins and their related macrocycles are of particular interest because they are robust and their electronic properties can be tuned by chelation of a metal ion and substitution on the macrocycle. The other porphyrinoids have equally variable and adjustable photophysical properties, thus photonic applications are potentiated. At least in the near term, realistic architectures for molecular electronics will require self-organization or nanoprinting on surfaces. This review concentrates on self-organized porphyrinoids as components of working electronic devices on electronically active substrates with particular emphasis on the effect of surface, molecular design, molecular orientation and matrix on the detailed electronic properties of single molecules.

Figure 1

(A) Structure of the porphyrin macrocycles with a cadre of common meso (5,10,15,20) aryl derivatives. Note the meso alkane compounds are also readily accessible synthetically. (B) Much of the supramolecular chemistry of porphyrins uses less symmetric compounds, e.g. those used in the formation of SAMs on surfaces, rely on a mixed aldehyde synthesis wherein two aryl aldehydes are mixed with pyrrole to form a ‘combinatorial’ library of six compounds. The chromatographic separation of the compounds and isomers yields compounds that can be used to study molecular topologies, surface binding geometries, self-assembly into discrete arrays, or self-organization into films. Many of these compounds can also be made by more direct routes.

Figure 2

The phthalocyanine macrocycle and numbering scheme. The isoindole positions near the ring are often referred to as α (i.e. 1,4,8,11,15,18,22,25) and those away from the ring β. Substitution at the β positions is more typical. For substituted phthalocyanines with an odd number of substituents on the isoindoles there are usually positional isomers, and these isomers are usually not specifically enumerated. PcF₁₆ where all positions bear a fluorine and the Cu(II) complex, are commonly used in photovoltaic applications.

Figure 3

Parts of a porphyrin molecule for surface attachment include the reactive surface moiety, a tether, and the linking group to the porphyrin. Metal ions and exocyclic moieties modulate electronic properties.

Figure 4

Large planar macrocycles such as porphyrins and phthalocyanines tend to adsorb cofacially on surfaces such as highly ordered pyrolytic graphite (HOPG), Au(III), NaCl, and other atomically smooth substrates. Here Ni(TPP) and PcF₁₆ are deposited on HOPG as well as a 2:1 mixture. Reproduced from ref. [117] with permission of the copyright holders.

Figure 5

Assemblies of Ni(II) and Fe(II) phthalocyanine on Au(111). While both systems contain a central metal atom with the same valance, the Fe d⁶ system has greater orbital density near the Fermi level leading to an observed increased tunneling probability than that of the Ni d⁸ containing species. Reproduced from ref. [112] with permission of the copyright holders. These authors also examined d⁷ and d⁹ phthalocyanines [111].

Figure 6

Formation of supramolecular arrays of halogenated aryl porphyrins allows covalent bond assembly of 1- and 2- dimensional structures on a gold surface upon heating. In this case the formation of the 1-dimensional tapes is shown. The arrow indicates overlapping of two linear chains rather than a covalent bond. Reproduced from ref. [126] with permission of the copyright holders.

Figure 7

A 3×3 array of nanografted islands of a zinc porphyrinthiol (top) patterned in a background matrix of dodecanethiol. The feature size illustrated here is ca. 20 nm (FWHM) as determined from the topographic image (left). The porphyrins are found to be protruding above the dodecanethiol matrix by ca. 0.6 nm. The friction image (right) more clearly shows the patterned array. The scalebars in the lower right are 50 nm. (Batteas and coworkers, unpublished results).

Figure 8

Perrine et al. demonstrated cofacial deposition of porphyrins on Au surfaces with thiols directly attached to the porphyrin. Reproduced from ref [140] with permission of the copyright holders.

Figure 9

Top: Electrical properties of directly linked porphyrin wires across Au nanoelectrodes with spacing of less than 5 nm were prepared using an electromigration-induced break-junction technique Reproduced from ref. [141] with permission of the copyright holders. Bottom: an acetylene-linked porphyrin construct has been studied to look at distance dependence on the conductivity. Reproduced from ref. [36] with permission of the copyright holders.

Figure 10

(A) The dynamics of a surface attached trimer is one of the factors that determine the surface density, where SAG is the surface attachment group. (B) The heteroleptic Por-Eu-Pc-Eu-Pc serves as a multi-bit information storage molecule. Reproduced from ref [155] with permission of the copyright holders.

Figure 11

I–V spectra (averaged from 50 curves each) for the (A) dodecanethiol matrix, (B) small (~2 nm) porphyrin domains and (C, D) large (>6 nm) porphyrin domains. Reproduced from ref. [137] with permission of the copyright holders.

Figure 12

The conductance of tetraphenylboride (TPhB⁻) through a lipid bilayer saturates with increasing concentration because of space charge limits (●). An implicit equation describes the space charge limited current, solid line: ρ_TPhB− = 0.602C β exp(−qV_TPhB) where 0.602 converts units of concentration mol liter⁻¹, to ions per nm³, C is the concentration of the lipophilic ion, β is the partition coefficient, q is the molecular charge, and V is the potential of the ion inside the membrane based on electrostatic calculations. Cancelation of the space charge of the lipophilic anion by photo-formation of a porphyrin cation and self-assembly into an ion chain can increase the conductance by >20-fold (○), were the dashed line is the calculated non-space charge limited conductance. This photogated device is an early example of an all-organic molecular electronic. Reproduced from ref. [26] with permission of the copyright holders.

Figure 13

Conductive polycrystalline wires of Marks and coworkers reported in the late 1970s. Redrawn from ref. [–179].