10.1002/anie.201804337
Angewandte Chemie International Edition
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excited state of MeF2, where in the displaced conformation the
exciton is localized on a single fluorene (Figure 3).
In this study, we have used a set of novel torsionomers to
examine the contrasting requirements for exciton and hole
stabilization in -stacked assemblies. Exciton stabilization
requires a perfect sandwich-like arrangement, as evidenced by
the presence of strong excimeric-like emission only in CF2 and
MeF2. In contrast, cationic charge can be delocalized even for
structures where the path to cofaciality is sterically hindered, i.e.
tBuF2, as judged by the 160 mV hole stabilization and the
presence of a near-IR transition in its cation radical spectrum.
These findings underscore important design principles for next
generation optoelectronic materials.
The electronic spectrum of tBuF2+• shows a much less
intense intervalence band that is shifted to longer wavelength
(2400 nm), suggesting a reduced electronic coupling and/or
increased reorganization energy in tBuF2+• as compared to those
in CF2+• and MeF2+•. DFT calculations showed that two
energetically similar (ΔG = 3.5 kcal/mol) conformations of tBuF2+•
may exist. The sandwich-like arrangement displays a significant
puckering of the fluorene moieties due the steric hindrance from
bulky tBu groups, yet the spin/charge remains delocalized over
both fluorenes. In the displaced tBuF2+•, the spin/charge is
largely localized on a single fluorene, due to the significant
displacement and therefore reduced orbital overlap/electronic
coupling.
Acknowledgements
Additional insights come from gas-phase spectra of the
three torsionomers, obtained via laser induced fluorescence
(LIF) spectroscopy using a heated supersonic nozzle; details are
provided in the Supporting Information. As shown in Figure 5,
the differences in these spectra are striking. The spectrum of
MeF2 is similar to that of F2 in displaying a broad and largely
unresolved profile (Figure 5A); the breadth is explained in the
terms of an underlying progression in the torsional motion which
is broadened by the fast dynamics and strong modulation of the
electronic coupling (Figure 1A). The underlying progression is
clearly seen in the spectrum of tBuF2 (Figure 5B), absent any
spectral broadening. On the other hand, the spectrum of CF2
(Figure 5C) is both sharp and distinct, and strikingly red-shifted.
Here, the torsional libration is quenched and the Franck-Condon
active vibration corresponds to a "breathing" type motion of the
two subunits (see Figure S14 in Supporting Information). The
dramatic consequences of steric hindrance in the tBuF2
We thank the NSF (CHE-1508677) and NIH (R01-HL112639-04)
for financial support. The calculations were performed on the
high-performance computing cluster Père at Marquette
University and XSEDE.
Keywords: -stacking • charge delocalization • exciton
delocalization
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