ABSTRACT
1,5-(Alkadiynyl)anthracenes self-assemble single component and multicomponent monolayers at the solution-HOPG interface. An alkadiynyl chain's kinked shape constrains the molecular structures with which it can close-pack. This affords rudimentary molecular recognition that has been used to direct self-assembly of 1-D patterned, multicomponent monolayers. The unit cell building blocks of single- and multicomponent alkadiynylanthracene monolayers repeat with high fidelity for 100s of nanometers along the side chain direction. Unit cell repeat fidelity along the orthogonal, anthracene column direction of the monolayer depends on diyne location within the side chain; even-position diyne side chains produce high fidelity of unit cell repeats and wider domain widths along the anthracene columns, whereas odd-position diyne side chains produce more frequent domain interfaces that disrupt the anthracene columns. Alkadiynylanthracene monolayers may be viewed as stacks of 1-D molecular tapes. 1-D tape molecular composition, sequence, and intratape side chain alignment are dictated by shape complementarity of the kinked alkadiynyl side chains. Stacking alignments of adjacent 1-D tapes are controlled by shape matching of tape peripheries and determine repeat fidelity along the anthracene columns. Tapes stacked with a constant intertape alignment comprise crystalline domains that repeat along the anthracene columns. The 1-D tapes formed by anthracenes with odd-position diynes have triangle wave peripheries that close-pack in multiple stacking alignments. This reduces unit cell repeat fidelity and decreases the widths of crystalline domains along the anthracene columns. Even-position diyne side chains form 1-D tapes with trapezoid wave peripheries that close-pack in only one stacking alignment. This generates higher stacking fidelity, larger domain widths, and fewer domain interfaces along the anthracene columns of even-position diyne monolayers. Even- and odd-position diyne monolayers exhibit comparable densities of interfaces between enantiotopic domains and between domains aligned along different graphite symmetry axes. These interfaces likely arise through collisions of independently nucleated/growing domains and persist for lack of kinetically competent pathways that interconvert or merge the domains.
ABSTRACT
The self-assembly of multi-component monolayers with designed patterns requires molecular recognition among components. Dipolar interactions have been found to influence morphologies of self-assembled monolayers and can affect molecular recognition functions. Ketone groups have large dipole moments (2.6 D) and are easily incorporated into molecules. The potential of ketone groups for dipolar patterning has been evaluated through synthesis of two 1,5-disubstituted anthracenes bearing mono-ketone side chains, STM characterization of monolayers self-assembled from their single and two component solutions and molecular mechanics simulations to determine their self-assembly energetics. The results reveal that (i) anthracenes bearing self-repulsive mono-ketone side chains assemble in an atypical monolayer morphology that establishes dipolar attraction, instead of repulsion, between ketones in adjacent side chains; (ii) pairs of anthracene molecules whose self-repulsive ketone side chains are dipolar complementary spontaneously assemble compositionally patterned monolayers, in which the two components segregate into neighboring, single component columns, driven by side chain dipolar interactions; (iii) compositionally patterned monolayers also assemble from dipolar complementary anthracene pairs that employ different dipolar groups (ketones or CF2 groups) in their side chains; (iv) the ketone group, with its larger dipole moment and size, provides comparable driving force for patterned monolayer formation to that of the smaller dipole, and smaller size, CF2 group.
