Photochemical Grafting of n-Alkenes onto Carbon Surfaces:  the Role of Photoelectron Ejection

Abstract

The grafting of molecular layers to carbon-based materials provides a way to combine the high chemical and thermal stability of these materials with surface properties such as chemical recognition or reactivity. The functionalization of surfaces with ultraviolet light has emerged as a way to modify difficult-to-functionalize materials, such as diamond. We have performed a combined experimental and computational investigation of the photochemical reaction of terminal alkenes with hydrogen-terminated carbon surfaces. 1-Alkenes carrying various terminal functional groups (-NHCOCF3, -NHCOO(tert-butyl), -COOCH3, -CH3) were grafted from the neat liquids using 254 nm light. These layers were characterized using X-ray Photoelectron Spectroscopy and Infrared Reflectance Absorption Spectroscopy. Pronounced differences in reactivity were observed between the molecules: trifluoroacetamide-terminated alkenes grafted the fastest and yielded self-terminating layers after approximately 4 h. Ultraviolet photoelectron spectroscopy and photocurrent measurements show that the grafting reaction involves photoemission of electrons into the liquid. Density functional calculations show that the reactivities of the four molecules are correlated with their electron affinities, with the trifluoroacetamide group acting as the best electron acceptor and having the highest reactivity. Our results demonstrate that photoejection of electrons from the solid into the acceptor levels of the alkenes initiates the functionalization reaction and controls the overall rate. Finally, marginally reactive n-alkenes were induced to react and form dense monolayers by seeding the carbon surface with small amounts of a good electron acceptor, such as the trifluoroacetamide moiety. This study provides important new mechanistic insights into the use of ultraviolet light to initiate grafting of alkenes onto surfaces.

Surface functionalization of thin-film diamond for highly stable and selective biological interfaces

“…Nucleophilic attack by the electron-rich alkene group then grafts the molecules to the diamond surface. Experimental and computational results show that reactivity of different alkenes correlates with the electron affinity of the "R" group (24,28,38).…”

“…While the surface chemistry of carbon is relatively unexplored, in recent work we have shown that photochemical grafting of organic alkenes can provide an extremely robust way to link functional organic molecules to surfaces of carbon, including diamond (22)(23)(24)(25)(26), amorphous carbon (23,27,28), glassy carbon (29), and carbon nanofibers (30,31). Carbon surfaces functionalized in this manner have shown excellent stability, even at elevated temperatures (32).…”

Molecular and biomolecular interfaces to metal oxide semiconductors

“…TFAAD is used because it has excellent grafting properties, because it can be easily detected via x-ray photoelectron spectroscopy (XPS), and because it is a precursor to amine-modified surfaces that are useful for subsequent modification with DNA or proteins. [1] In recent work on carbon surfaces we found that TFAAD is also an exceptionally good molecule to use because its chemical structure facilitates initiation of the grafting reaction [23,24]; a more detailed discussion of the mechanism on metal oxides and use of other molecules will be reported separately. Here, we report on (bio)molecular layers formed using TFAAD.…”

ToF‐SIMS characterisation of methane‐ and hydrogen‐plasma‐modified graphite using principal component analysis

“…It is the hydrogen on the surface that enables the later photochemical grafting of functional molecules, [10] thus the ERDA results suggest that the hydrogen plasma treatment is the superior modification method in this respect. However, the STM and SIMS PCA results showed that the hydrogen plasma treatment causes greater disruption and etching of the surface, which in turn is likely to have detrimental implications (on factors such as electron transduction and transport from sensing molecules to the substrate) for the success of electrochemical sensor applications.…”