  Long range resonance energy transfer (RET) between a donor and an acceptor
molecule is increasingly being used in many areas of biological and material
science. The phenomenon is used to monitor the in vivo separation between
different (bio) polymers/units of (bio) polymers and hence the dynamics of
various biomolecular processes. Because of the sensitivity of the rate on to
the distance between the donor (D) and the acceptor (A), the technique is
popularly termed as "spectroscopic ruler". In this work we examine the distance
and orientation dependence of RET in three different systems: (i) between a
conjugated polymer and a fluorescent dye, (ii) between a nanometal particle
(NMP) and a fluorescent dye and (iii) between two NMP. We show that in all the
three cases, the rate of RET follows a distance dependence of d^(- sigma) where
exponent sigma approaches 6 at large d (F"orster type dependence) but has a
value varying from 3-4 at short to intermediate distance. At short separation,
the amplitude of rate is considerably smaller than predicted by the F"orster
theory.
