  Engineering the position of the lowest triplet state (T1) relative to the
first excited singlet state (S1) is of great importance in improving the
efficiencies of organic light emitting diodes and organic photovoltaic cells.
We have carried out model exact calculations of substituted polyene chains to
understand the factors that affect the energy gap between S1 and T1. The
factors studied are backbone dimerisation, different donor-acceptor
substitutions and twisted geometry. The largest system studied is an eighteen
carbon polyene which spans a Hilbert space of about 991 million. We show that
for reverse intersystem crossing (RISC) process, the best system involves
substituting all carbon sites on one half of the polyene with donors and the
other half with acceptors.
