  Structurally well-defined TIPS-acetylene substituted tetracene (TIPS BT1')
and pentacene (TIPS BP1') dimers utilizing a [2.2.1] bicyclic norbornyl bridge
have been studied -primarily using time-resolved spectroscopic methods - to
uncover mechanistic details about primary steps in singlet fission leading to
formation of the biexcitonic 1TT state as well as decay pathways to the ground
state. For TIPS BP1' in room temperature toluene, 1TT formation is rapid and
complete, occurring in 4.4 ps. Decay to the ground state in 100 ns is the
primary loss pathway for 1TT in this system. For TIPS BT1', the 1TT is also
observed to form rapidly (with a time constant of 5 ps) but in this case it
occurs in concert with establishment of an excited state equilibrium (K~1) with
the singlet exciton state S1 at an energy of 2.3 eV above the ground state. The
equilibrated states survive for 36 ns and are lost to ground state through both
radiative and non-radiative pathways via the S1 and non-radiative pathways via
the 1TT. The rapidity of 1TT formation in TIPS BT1' is at first glance
surprising. However, our analysis suggests that the few-parameter rate constant
expression of Marcus theory explains both individual and comparative findings
in the set of systems, thus establishing benchmarks for diabatic coupling and
reorganization energy needed for efficient 1TT formation. Finally, a comparison
of TIPS BT1' with previous results obtained for a close constitutional isomer
(TIPS-BT1) differing in the placement of TIPS-acetylene side groups suggests
that the magnitude of exchange interaction in the correlated triplet manifold
plays a critical role dictating 1TT yield in the tetracenic systems.
