| Abstract
| - Recently we have developed an unsymmetrical azobenzene disulfide with a short alkyl side chain,4-hexyl-4‘-(12-(dodecyldithio)-dodecyloxy)azobenzene (C6AzSSC12), aiming of a high efficiency in photoisomerization in SAMs on planar gold surfaces (Tamada, K.; et al. Langmuir2002, 18, 5239). In thispaper, we introduce an additional modification on the molecule to improve the thermal endurance for thephotoreaction by attachment of a methyl group to the azobenzene ring, thus avoiding dye aggregationsterically. A “methyl-derivatized” azobenzenethiol (C6Az(Me)SH) SAM revealed a significant improvementin their photoreactivity compared with an unmodified azobenzenethiol (C6AzSH) SAM due to the stericeffect of the methyl group. A “methyl-derivatized” unsymmetrical azobenzene disulfide (C6Az(Me)SSC12)SAM exhibited quite a similar photoresponse to that of C6AzSSC12 SAM before heat treatment owing tothe free volume given by the unsymmetrical structure; however, only the C6Az(Me)SSC12 SAM couldretain high photoreactivity in phase-segregated domains formed by annealing, unlike the C6AzSSC12SAM. The C6Az(Me)SH and C6Az(Me)SSC12 SAMs exhibited a reaction kinetics different from that ofC6AzSSC12 SAM due to a different quantum yield and the molecular tilt angle of the azobenzene unit.The C6Az(Me)SH SAM exhibited a typical character of “congested surface”, where the reaction rate fromcis to trans was faster, while that from trans to cis was slower compared with those of C6Az(Me)SSC12SAM.
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