Application of visible-light photosensitization to form alkyl-radical-derived thin films on gold

Abstract

Visible-light irradiation of phthalimide esters in the presence of the photosensitizer [Ru(bpy)₃]²⁺ and the stoichiometric reducing agent benzyl nicotinamide results in the formation of alkyl radicals under mild conditions. This approach to radical generation has proven useful for the synthesis of small organic molecules. Herein, we demonstrate for the first time the visible-light photosensitized deposition of robust alkyl thin films on Au surfaces using phthalimide esters as the alkyl radical precursors. In particular, we combine visible-light photosensitization with particle lithography to produce nanostructured thin films, the thickness of which can be measured easily using AFM cursor profiles. Analysis with AFM demonstrated that the films are robust and resistant to mechanical force while contact angle goniometry suggests a multilayered and disordered film structure. Analysis with IRRAS, XPS, and TOF SIMS provides further insights.

Keywords: TOF SIMS; atomic force microscopy; organic thin film; particle lithography; photosensitization.

Scheme 1

Proposed visible-light-promoted thin film deposition on Au surface.

Scheme 2

Phthalimide esters used for the deposition of thin films.

Figure 1

Nanopores within a film of Au–Me on Au(111) produced using immersion particle lithography and the radical precursor Phth–Me. (a) Topography image acquired in air; (b) zoom-in view of hexagonally packed nanopores; (c) cursor profile across a single nanopore in (b); (d) corresponding phase image of (b).

Figure 2

Attempted nanoshaving of the Au–Me Film on Au (111). (a) Topography image of nanopores within the film; (b) topography image acquired after nanoshaving; the film was not displaced.

Figure 3

Au–Me thin film prepared in the absence of Ru(bpy)₃Cl₂ was not robust. (a) Topography image of nanopores within the film; (b) topograph of a nanoshaved rectangular area; (c) cursor profile of an individual nanopore in (a); (d) cursor profile for the nanoshaved area in (b).

Figure 4

Film of Au–NHBoc prepared on Au(111) with nanopores using particle lithography. (a) Topograph of nanopores; (b) zoom-in topography view of nanopores; (c) cursor profile for the line in (b); (d) corresponding phase image of (b).

Figure 5

Au–NHBoc film after storage in ambient conditions for six months. (a) Topography image; (b) zoom-in view; (c) cursor profile for the line in (b); (d) corresponding phase image of (b).

Figure 6

IRRAS Spectrum of Au–Me.

Figure 7

IRRAS Spectrum of Au–NHBoc.

Figure 8

Au 4f, C 1s and O 1s photoelectron spectra of the deposited Au–Me layer.

Figure 9

a) Positive ion mass spectrum m/z 40–75 and b) negative ion mass spectrum m/z 190–290 of the deposited layer (Au–Me).

Scheme 3

A mechanistic hypothesis for multilayer formation, crosslinking, and alkene formation.