The generation of high-yielding long-lived triplet excited states in solid state materials is a crucial aspect in the improvement of optoelectronics, triplet-triplet annihilation upconversion (TTA-UC) materials and gas sensing. However, the precise control of triplet sensitizer orientation to investigate the traits that control triplet exciton formation and longevity in these solid-state materials is difficult. Synthetically tunable crystalline networks known as metal-organic frameworks, have been receiving growing interest as substrates to systematically probe orientationally-dependent chromophore photophysics in the solid state. In this work, the triplet excitonic states of three Zr-node frameworks sharing the same porphyrin linker (tetracarboxyphenylporphyrin, TCPP), are photophysically characterized with ns-transient absorption (TA) spectroscopy. The key difference among the three frameworks is the orientation of the chromophores in relation to one another: PCN-221 (90°), PCN-222 (60°), and a new in-house MOF, VPI-110 (0°, stacked). By varying the angle between adjacent TCPP units, we have determined key design features that promote the existence of long-lived triplet excitons in porphyrinic MOFs.

Probing triplet exciton dynamics in Zr-porphyrin MOFs as a function of MOF topology

ACS Spring 2022

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