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·Φ_UC) 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.