posted on 2023-01-03, 17:57authored byArjun Halder, David C. Bain, Julia Oktawiec, Matthew A. Addicoat, Stavrini Tsangari, José J. Fuentes-Rivera, Tristan A. Pitt, Andrew J. Musser, Phillip J. Milner
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
metal–organic 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
(TCPE4–) 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.