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A simplified version of the frontier orbital model for a noncovalent dimer is used to derive guidelines for dimer geometries that maximize the square of the electronic matrix element for singlet fission. The use of the guidelines requires only the knowledge of the highest occupied and lowest unoccupied orbital of the monomer and the overlaps of the atomic orbitals on partner A with those on partner B.
In search for a qualitative understanding of the effects of molecular packing on singlet fission (SF) rate, a simplified version of the frontier orbital model is described and illustrated on a pair of tetracene molecules. To identify all favorable physically accessible pair geometries, all significant local maxima of the square of the electronic matrix element for SF have been located within the six-dimensional space of possible arrangements of two rigid bodies, using a grid of over 4.7 × 108 pair geometries. Those at which the molecules interpenetrate were excluded using a hard-sphere model. The effects of intermolecular interaction on the SF energy balance and thus its rate constant kSF were approximated using Marcus theory at each of the maxima using the same simplified version of the frontier orbital model. Starting at these local maxima, the pair geometries were optimized for maximum kSF and the 21 best are reported along with their computed Davydov splitting and triplet biexciton binding energies. The optimal pair structures at the resulting maxima follow qualitative rules published previously and further elaborated here.
Crystal
structures, singlet fission (SF) rate constants, and other
photophysical properties are reported for three fluorinated derivatives
of 1,3-diphenylisobenzofuran and compared with those of the two crystal
forms of the parent. The results place constraints on the notion that
the effects of molecular packing on SF rates could be studied separately
from effects of chromophore structural changes by examining groups
of chromophores related by weakly perturbing substitution if their
crystal structures are different. The results further provide experimental
evidence that dimer-based models of SF are not sufficiently general
and that trimer- and possibly even higher oligomer-based or many-body
models need to be formulated.
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