  Dispersions of rodlike colloidal particles exhibit a plethora of liquid
crystalline states, including nematic, smectic A, smectic B, and columnar
phases. This phase behavior can be explained by presuming the predominance of
hard-core volume exclusion between the particles. We show here how the
self-organization of rodlike colloids can be controlled by introducing a weak
and highly localized directional attractive interaction between one of the ends
of the particles. This has been performed by functionalizing the tips of
filamentous viruses by means of regioselectively grafting fluorescent dyes onto
them, resulting in a hydrophobic patch whose attraction can be tuned by varying
the number of bound dye molecules. We show, in agreement with our computer
simulations, that increasing the single tip attraction stabilizes the smectic
phase at the expense of the nematic phase, leaving all other liquid crystalline
phases invariant. For a sufficiently strong tip attraction, the nematic state
may be suppressed completely to get a direct isotropic liquid-to-smectic phase
transition. Our findings provide insights into the rational design of building
blocks for functional structures formed at low densities.
