  Here we report the formation of fluorescent H aggregates of a cyanine dye
Oxa18 in ultrathin film and its effect on energy transfer between Oxa18 and
sRhb. Ideally H aggregate do not fluoresce. However, due to imperfect stacking
of Oxa18 molecules in the aggregates, fluorescence occurred from Oxa18 H
aggregates. This H aggregated band has substantial effect on the enhancement of
energy transfer from Oxa18 to sRhb both in solution and in ultrathin film.
