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

Abstract

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 10(4) 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.

Gadolinium(III) Porpholactones as Efficient and Robust Singlet Oxygen Photosensitizers

“…Notably, Gd‐4 showed an extinction coefficient ratio of Q y (0,0) band and Soret band ( ɛ Q(0,0) / ɛ Soret ) up to 0.56. Such redshifted absorption with increasing intensity, along with the replacement of β‐oxazolone moieties and the orientation of β‐dioxazolone moieties, is consistent with our previously reported Zn, Pt, and Pd analogs, and suggests the capability for effective excitation in the deep‐red to NIR region.…”

“…Upon excitation at approximately 425 nm (B band maxima) in degassed CH 2 Cl 2 at room temperature, Gd‐1 – 4 exhibited vibrationally structured emission bands at about 700–1300 nm (Figure b). According to previous spectroscopic studies on Gd III porphyrinates and Pt II or Pd II ‐based porpholactonates, such low‐energy emission bands for Gd‐1 – 4 are tentatively assigned as the triplet states of intraligand (IL, π→π*) transitions. Simultaneously, emission at 580–700 nm, originating from the singlet states of 1 – 4 is strongly reduced (by ca.…”

“…The redox properties of the Gd III complexes were measured by cyclic voltammograms (CV) (Table S2 and Figure S6 in the Supporting Information) in acetonitrile (CH 3 CN) solutions in the presence of 0.1 m tetra‐ n ‐butylammonium hexafluorophosphate (NBu 4 PF 6 ) at room temperature. In general, Gd‐1 – 4 displayed redox couples that are tentatively assigned as the reduction and oxidation of porphyrinoid ligands with reference to the electrochemical studies on the related Zn, Pd, and Pt complexes . Importantly, the first oxidation potentials (+1.08 to +1.14 V) versus Ag/AgCl electrode for Gd‐1 – 4 were insensitive to the extent and orientation of β‐oxazolone moiety replacement, whereas the first reduction potentials (−1.19 to −0.63 V) were highly related and the ease of reduction follows the trend Gd‐4 > Gd‐3 > Gd‐2 > Gd‐1 .…”

“…Porpholactones, in which one or two pyrrole units of porphyrin are replaced by β‐oxazolone moieties, are representative ligands to mimic natural tetrapyrrole pigments such as chlorin, bacteriochlorin, and iso ‐bacteriochlorin, through modulation of π‐conjugation systems, in a manner similar to nature's approach. Previous studies demonstrated similar coordination ability of porpholactones and their derivatives to porphyrins, which facilitates the design of metal‐based pH sensors, air‐pressure sensitive painting, and photosensitizers for triplet–triplet annihilation (TTA) . Recently, we also reported the capability of porpholactones to efficiently generate singlet oxygen and their tendency to preferentially accumulate in tumor cells .…”

Pd–Porphyrin Oligomers Sensitized for Green‐to‐Blue Photon Upconversion: The More the Better?

“…[17][18][19][20][21][22] The Qb and absorptions of porphyrin complexes locate in the green region and the lifetimes of their triplet states can last tens to hundreds of microseconds, which benefit the TTA-UC process.F urthermore, porphyrinoids with strong and broad absorption have also been appliedt oi mprovet he TTA-UC performance. [23][24][25][26] Developing and improving TTA-UC systemsb ased on porphyrin complexes with broad absorption bands of long wavelength and intense absorption coefficients is an attractive focus. Researchers have developed as eries of meso-meso-linked porphyrin complexes, which exhibit broad and enhanced absorption bands arisingf rom the excitonic interactions between constituent porphyrin units.…”