Fine-Tuning of β‑Substitution to Modulate the Lowest Triplet Excited States: A Bioinspired Approach to Design Phosphorescent Metalloporphyrinoids

Learning
nature’s approach to modulate photophysical properties
of NIR porphyrinoids by fine-tuning β-substituents including
the number and position, in a manner similar to naturally occurring
chlorophylls, has the potential to circumvent the disadvantages of
traditional “extended π-conjugation” strategy
such as stability, molecular size, solubility, and undesirable π–π
stacking. Here we show that such subtle structural changes in Pt­(II)
or Pd­(II) cis/trans-porphodilactones (termed by cis/trans-Pt/Pd) influence photophysical properties
of the lowest triplet excited states including phosphorescence, Stokes
shifts, and even photosensitization ability in triplet–triplet
annihilation reactions with rubrene. Prominently, the overall upconversion
capability (η, η = ε·ΦUC)
of Pd or Pt trans-complex is 104 times higher than that
of cis-analogue. Nanosecond time-resolved infrared (TR-IR) spectroscopy
experiments showed larger frequency shift of ν­(CO) bands
(ca. 10 cm–1) of cis-complexes than those of trans-complexes
in the triplet excited states. These spectral features, combining
with TD-DFT calculations, suggest the strong electronic coupling between
the lactone moieties and the main porphyrin chromophores and thus
the importance of precisely positioning β-substituents by mimicking
chlorophylls, as an alternative to “extended π-conjugation”,
in designing NIR active porphyrinoids.