How Penetrable Are Thioalkyl Self-Assembled Monolayers?

The depth of penetration of photolytically generated, gas-phase O(P-3) atoms into thioalkyl self-assembled monolayers (SAMs) has been investigated. Custom-synthesized, site-selectively deuterated SAMs were prepared on Au substrates and characterized by scanning tunneling microscopy (STM). Relative yields of gas-phase OD were detected by laser-induced fluorescence (LIF) Reaction was suppressed at the terminal CD3 by the higher abstraction barriers for primary D atoms, yielding only 16 +/- 3 % of the total OD. The C-2 (first secondary) site is the individually most reactive (42 +/- 5%). The remaining significant contribution (42 +/- 4%) from positions as deep as C-3-C-6 is a considerable surprise when compared with previous related experiments using higher-energy O+ ion projectiles and detecting OH- products. The apparent greater penetrability of the SAM layer found here may have prior theoretical support. Furthermore, we show that NO2 damages the surfaces but that C-12 SAMs are considerably more resistant than C-6 SAMs.