2 research outputs found
Activated Singlet Exciton Fission in a Semiconducting Polymer
Singlet exciton fission is a spin-allowed
process to generate two
triplet excitons from a single absorbed photon. This phenomenon offers
great potential in organic photovoltaics, but the mechanism remains
poorly understood. Most reports to date have addressed intermolecular
fission within small-molecular crystals. However, through appropriate
chemical design chromophores capable of intramolecular fission can
also be produced. Here we directly observe sub-100 fs activated singlet
fission in a semiconducting polyĀ(thienylenevinylene). We demonstrate
that fission proceeds directly from the initial 1B<sub>u</sub> exciton,
contrary to current models that involve the lower-lying 2A<sub>g</sub> exciton. In solution, the generated triplet pairs rapidly recombine
and decay through the 2A<sub>g</sub> state. In films, exciton diffusion
breaks this symmetry and we observe long-lived triplets which form
charge-transfer states in photovoltaic blends
Enhancing Dynamic Spectral Diffusion in MetalāOrganic Frameworks through Defect Engineering
The crystal packing of organic chromophores has a profound
impact
on their photophysical properties. Molecular crystal engineering is
generally incapable of producing precisely spaced arrays of molecules
for use in photovoltaics, light-emitting diodes, and sensors. A promising
alternative strategy is the incorporation of chromophores into crystalline
metalāorganic frameworks (MOFs), leading to matrix coordination-induced
emission (MCIE) upon confinement. However, it remains unclear how
the precise arrangement of chromophores and defects dictates photophysical
properties in these systems, limiting the rational design of well-defined
photoluminescent materials. Herein, we report new, robust Zr-based
MOFs constructed from the linker tetrakis(4-carboxyphenyl)ethylene
(TCPE4ā) that exhibit an unexpected structural transition
in combination with a prominent shift from green to blue photoluminescence
(PL) as a function of the amount of acid modulator (benzoic, formic,
or acetic acid) used during synthesis. Time-resolved PL (TRPL) measurements
provide full spectral information and reveal that the observed hypsochromic
shift arises due to a higher concentration of linker substitution
defects at higher modulator concentrations, leading to broader excitation
transfer-induced spectral diffusion. Spectral diffusion of this type
has not been reported in a MOF to date, and its observation provides
structural information that is otherwise unobtainable using traditional
crystallographic techniques. Our findings suggest that defects have
a profound impact on the photophysical properties of MOFs and that
their presence can be readily tuned to modify energy transfer processes
within these materials