The
goal of harnessing the theoretical potential of singlet fission
(SF), a process in which one singlet excited state is split into two
triplet excited states, has become a central challenge in solar energy
research. Covalently linked dimers provide crucial models for understanding
the role of chromophore arrangement and coupling in SF. Sensitizers
can be integrated into these systems to expand the absorption bandwidth
through which SF can be accessed. Here, we define the role of the
sensitizer-chromophore geometry in a sensitized SF model system. To
this end, two conjugates have been synthesized consisting of a pentacene
dimer (SF motif) connected via a rigid alkynyl bridge to a subphthalocyanine
(the sensitizer motif) in either an axial or a peripheral arrangement.
Steady-state and time-resolved photophysical measurements are used
to confirm that both conjugates operate as per design, displaying
near unity energy transfer efficiencies and high triplet quantum yields
from SF. Decisively, energy transfer between the subphthalocyanine
and pentacene dimer occurs ca. 26 times faster in the peripheral conjugate,
even though the two chromophores are ca. 3 Å farther apart than
in the axial conjugate. Following a theoretical evaluation of the
dipolar coupling, V
dip
2, and
the orientation factor, κ2, of both the axial (V
dip
2 = 140 cm–2; κ2 = 0.08) and the peripheral (V
dip
2 = 724 cm–2; κ2 = 1.46) arrangements, we establish that this rate acceleration
is due to a more favorable (nearly co-planar) relative orientation
of the transition dipole moments of the subphthalocyanine and pentacenes
in the peripheral constellation.
The unique singlet-fission (SF) properties of a previously studied naphthalene-linked perylene monoimide (PMI-N-PMI) motivated the synthesis of a phenylene spaced perylene monoimide (PMI-P-PMI) and their corresponding phenylene- and napthylene-spaced perylene-diimides...
Certain chemical groups give rise to characteristic excited-state deactivation mechanisms. Here, we target the role of a protonated N–N chemical group in the excited-state deactivation of protonated indazole by comparison to its isomer that lacks this group, protonated benzimidazole. Gas-phase protonated indazole and protonated benzimidazole ions are investigated at room temperature using picosecond laser pump–probe photodissociation experiments in a linear ion-trap. Excited state lifetimes are measured across a range of pump energies (4.0–5.4 eV). The 1ππ* lifetimes of protonated indazole range from 390 ± 70 ps using 4.0 eV pump energy to ≤18 ps using 4.6 eV pump energy. The 1ππ* lifetimes of protonated benzimidazole are systematically longer, ranging from 3700 ± 1100 ps at 4.6 eV pump energy to 400 ± 200 ps at 5.4 eV. Based on these experimental results and accompanying quantum chemical calculations and potential energy surfaces, the shorter lifetimes of protonated indazole are attributed to πσ* state mediated elongation of the protonated N–N bond.
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