Influences on the Dynamics and Stability of Self-Assembly: Solvent, Substrate, and Concentration

Abstract

The formation dynamics and stability of CoOEP at the solution/Au(111) interface are captured in situ using scanning tunneling microscopy (STM) in a dynamic solution flow cell at room temperature. The intermediate steps of self-assembly of CoOEP into an ordered monolayer were captured, and fractional coverage as a function of time was measured to extract characteristic parameters of the self-assembly process. Adlayer structure and formation under various solvents are compared to previous studies conducted on HOPG. The choice of substrate is found to have a dramatic influence on adlayer structure and stability. It was found that the CoOEP adlayer assembly on HOPG is an equilibrium process, and the monolayer can be readily formed within minutes of contact with solution (above a solvent-dependent threshold solution concentration); the dissolution of the formed adlayer is feasible, though the rate of dissolution is solvent-dependent. The assembly of an adlayer on Au(111) is kinetically driven, monolayer formation occurs within minutes, and dissolution is very slowonly minimal island dissolution was achieved after hours of pure solvent flow. Solvent incorporation of 1,2,4-trichlorobenzene (TCB) was observed to form a CoOEP pseudorectangular adlayer structure (REC) on both HOPG and Au(111), though a solvent-free pseudohexagonal structure (HEX) occurred at much higher concentrations of CoOEP on HOPG than on Au(111). This is likely due to the fact that CoOEP binds more strongly to Au(111) than HOPG, which promotes the REC to HEX transition on Au(111) at lower concentrations. Solvent incorporation of toluene (Tol) into a CoOEP adlayer on Au(111) was observed, but it did not incorporate into the adlayer on HOPG. There is a significant increase in the Arrhenius desorption rate factor (∼350) of toluene on HOPG relative to Au that is likely a driving factor for Tol coadsorption on Au. A very short-lived decane incorporated adlayer was also observed. The transformation on Au(111) from REC to HEX structure under 1 μM CoOEP in Tol occurred within ∼10 min, while under a solution of 470 μM CoOEP in TCB the transformation required ∼102 min. This variance is primarily due to the relative residence times of the solvent molecule on the Au(111) surface, where Tol has an estimated desorption rate 500 times greater than TCB. The unit cells of the CoOEP adlayer are also substrate-dependent. The commensurate TCB-incorporated REC structure on Au(111) contains two CoOEP but only one CoOEP on HOPG. Thus, the adlayer formation of CoOEP on Au(111) was more significantly affected by solvent than for adsorption on HOPG.

Selective Self-Assembly and Modification of Herringbone Reconstructions at a Solid–Liquid Interface of Au(111)

“…Small bright features (∼4 Å in size) were observed between pairs of TMPB molecules (denoted as α; green circle in Figure b) as well as at the cell vertices between six molecules (denoted as β; blue circle in Figure b). We attribute these to co-adsorbed solvent molecules, and a simple geometrical consideration suggests that the TCB molecules adopt a vertical configuration, which is stabilized by the interactions with the Au substrate and the TMPB molecules (Figure S5 of the Supporting Information). − …”

Computational Study of Solvent Incorporation into a Porphyrin Monolayer

“…The STM image of the TCB-CoOEP pseudorectangular (REC) polymorph on Au(111) as obtained by Gurdumov et al is shown in Figure . Also shown in Figure are the DFT optimized lowest energy structures found both for the persistent TCB pseudopolymorph and for the TOL pseudopolymorph.…”

“…The REC and HEX cell dimensions were chosen to match the unit cell dimensions given by Gurdumov et al for the REC and HEX cells. 25 a r = 10a 1 + 11a 2 , b r = −3a 1 + 3a 2 , and a h = a r with b h = 5a 2 , for the REC and HEX cells, respectively, where a 1 and a 2 are the Au(111) atomic lattice vectors and are separated by 120°. Cells had at least 1.8 nm of vacuum space above the highest point on the monolayer surface.…”

“…Recently, Gurdumov and co-workers published experimental studies of the effects of solvent on the structure and kinetics of cobalt octaethylporphyrin (CoOEP) self-assembled monolayers (SAMS) on HOPG and on Au(111) . They found that, on HOPG, a CoOEP pseudohexagonal (HEX) structure containing two CoOEPs per unit cell was formed from toluene (TOL), phenyloctane (PhO), and decane (DEC) solution.…”