  We are currently undertaking a large survey for redshifted atomic and
molecular absorption ... only one clear and one tentative detection were
obtained: HI absorption at z = 0.097 in PKS 1555-140 and OH absorption at z
=0.126 in PKS 2300-189, respectively... In order to determine why no clear
molecular absorption was detected in any of the 13 sources searched, we
investigate the properties of the five redshifted systems currently known to
exhibit OH absorption. In four of these, molecules were first detected via
millimetre-wave transitions and the flat radio spectra indicate compact
background continuum sources, which may suggest a high degree of coverage of
the background source by the molecular clouds in the absorber. Furthermore, for
these systems we find a relationship between the molecular line strength and
red optical--near infrared (V-K) colours, thus supporting the notion that the
reddening of these sources is due to dust, which provides an environment
conducive to the formation of molecules. Upon comparison with the V-K colours
of our sample, this relationship suggests that, presuming the reddening occurs
at the host galaxy redshift at least in some of the targets, many of our
observations still fall short of the sensitivityrequired to detect OH
absorption, although a confirmation of the ``detection'' of OH in 2300-189
could contravene this.
