Kinetic and Thermodynamic Control in Porphyrin and Phthalocyanine Self-Assembled Monolayers

Abstract

Porphyrins and phthalocyanines are ubiquitous in modern science and technology. Their stability, redox properties, and photoresponse make them candidates for numerous applications. Many of these applications rely on thin films, and these are critically dependent on the first monolayer. In this article, we focus on noncovalently bound self-assembled monolayers of porphyrins and phthalocyanines at the solution-solid interface with special emphasis on the kinetic and thermodynamic processes that define the films and their reaction chemistry. We first discuss the difference between film-formation kinetics and desorption kinetics from fully formed films. We then present evidence that many of these monolayers are controlled by adsorption kinetics and are not in thermodynamic equilibrium. Measurement of the solution-solid interface desorption energy by scanning tunneling microscopy is discussed, and data is presented for cobalt, nickel, and free base octaethylporphyrin. The activation energy for the desorption of these compounds into phenyloctane is about half of the computed desorption energy in vacuum, and this is discussed in terms of the role of the solvent. Preexponential factors are very low compared to desorption into vacuum, and this is attributed to a reduction in the entropy of activation due to the participation of solvent in the transition state. An example of the use of relative desorption kinetics to create a new binary surface structure is given. It is suggested that this is a synthesis route that may have been missed because of the large difference in solution concentrations required to drive binary film formation. Attention then turns to the axial reaction chemistry of metalloporphyrins and metallophthalocyanines supported on conducting surfaces. We show several examples of chemistry unique to the supported complexes: cases where the metal binds ligands more readily and cases where the substrate induces ligand loss. Understanding this new axial coordination chemistry is of great importance in catalysis, sensing, and the growth of 3D materials from a self-assembled template.

Controlling the Two-Dimensional Self-Assembly of Functionalized Porphyrins via Adenine–Thymine Quartet Formation

“…self-assembly of porphyrins functionalized with G and C nucleobase groups followed by the synthesis of asymmetrically functionalized porphyrins 82 containing different combinations of nucleobase groups within a single tecton. Using STM to perform quantitative studies of the varying roles of kinetics and thermodynamics in the self-assembly of these systems [83][84][85][86]…”

Probing functional self-assembled molecular architectures with solution/solid scanning tunnelling microscopy

“…Of particular concern is the difficulty associated with unravelling the competition between kinetic and thermodynamic factors, 23 and in discerning whether an observed structure represents a thermodynamic minimum. 24,25 However, in recent years, careful experimentation has led to new insight into this matter. For example, the Gibbs free energy change associated with solution/solid self-assembly has been understood by considering a Born-Haber cycle adapted for the molecule/substrate/solvent system.…”

Investigation of catalytic reactions on electrode surface by scanning tunneling microscopy

“…除Pt基催化剂外, 过渡金属卟啉与酞菁类分子也 是非常重要的一类电催化剂 [40,41] . 在不同的外界条件 下, 它们吸附于基底表面所形成的组装结构也截然不 同 [42] , 而电催化剂的电催化活性又与其组装结构存在 联系. 因此, 观察催化剂分子在表面形成的不同组装 结构并研究其催化活性就变得尤为重要.…”