Effect of Facial Encumbrance on Excimer Formation and Charge Resonance Stabilization in Model Bichromophoric Assemblies

Excimer
formation and charge resonance stabilization in covalently
linked bichromophoric systems with flexible spacers are important
processes relevant to biochemistry and functional materials. Requiring
a π-stacked cofacial arrangement of a pair of aromatic molecules
at a van der Waals contact, the underlying geometrical reorganization
that accompanies these events continues to be debated. Here we use
a variety of methods including two-color resonant two-photon ionization
spectroscopy (2CR2PI), ion yield measurements, hole-burning spectroscopy
(HB), and laser-induced fluorescence (LIF) excitation and emission
spectroscopy to compare the gas-phase spectroscopy and dynamics of
the van der Waals dimers of fluorene, 9-methyl­fluorene (MF),
and 9,9′-dimethyl­fluorene (F1). The goal of this work
is to probe the influence of methyl substitution on the dynamics of
excimer formation and charge resonance (CR) stabilization. The fluorene
dimer, (F)2, displays lifetime broadened electronic spectra
and the dominance of excimer emission, consistent with a rapid (picoseconds)
formation of a π-stacked excimer upon electronic excitation.
Ion yield measurements of (F)2 reveal a lowering of the
ionization potential (IP) by some 0.38 eV relative to the monomer,
reflecting significant CR stabilization. These trends are mirrored
in the 9-methylfluorene dimer, (MF)2, as one face of the
π-system remains open. In contrast, the electronic spectrum
of the dimethyl-substituted dimer, (F1)2, shows narrow
features representing a single band system, and analysis of the torsional
structure in dispersed fluorescence spectra identifies this as emission
from the locally excited state of a tilted (non-π-stacked) dimer,
with no evidence of excimeric emission. The structure of this dimer
reflects the increased importance of C–H/π interactions
in the dimethyl-substituted system, as increased steric constraints
block a cofacial approach. The IP of (F1)2 shows CR stabilization
which is roughly 1/2 of that in π-stacked (F)2 dimer.
Extensive theoretical calculations support these findings and show
the importance of sandwich-type configurations for excitonic delocalization
and CR stabilization.