Enhanced Triplet Sensitizing Ability of an Iridium Complex by Intramolecular Energy-Transfer Mechanism

Abstract

The photodynamic properties involving both intra- and intermolecular triplet energy transfers (ET) of a bichromophoric photosensitizer having a tris-cyclometalated Ir(III) tethered with a pyrene derivative are studied. Due to the triplet energy gap of the two chromophores, a reversible intramolecular triplet ET equilibrium is quickly established upon photoexcitation, with the triplet exciton mainly residing on the acceptor side in the photostationary state. By virtue of the very small decay rate of triplet pyrene, a considerably extended triplet lifetime (2 ms) is observed. Next, the intermolecular triplet-triplet ET properties are investigated. Using steady-state and time-resolved spectroscopy, the ET rate constants from the Ir complex and pyrene unit in the sensitizer to an external triplet acceptor (unattached, free pyrene derivative) in solution are found to be around 10 s and 10 M s, respectively. In spite of a lower ET rate constant, the tethered pyrene serves as the main intermolecular ET channel because of the large, favorable intramolecular ET equilibrium ( K ∼ 10). Importantly, this cascade ET process, from Ir complex to linked pyrene, and then to free pyrene, offers an overall improved ET efficiency than a direct ET from Ir complex to free pyrene, by virtue of the much smaller spontaneous decay rate compared to that of the metal complex. Finally, the more efficient ET ability is demonstrated experimentally by applying the molecule as sensitizer in a triplet-triplet annihilation upconversion. The bichromophoric sensitizer achieved upconverted emission intensity 5 times higher than a monochromophoric Ir-complex analogue.

An Activatable Triplet Sensitizer Based on Triplet Electron Transfer and Its Application for Triplet–Triplet Annihilation Upconversion

“…This is consistent with the increased TTA-UC yield for ZnPor–S–S–ZnPor when adding RSH. The elongated triplet lifetime will enhance the TTA-UC efficiency according to the previous reports. , However, the triplet lifetime of ZnPor–C–C–ZnPor is not influenced by the addition of RSH. All of these results suggest that the faster decay of the triplet state in ZnPor–S–S–ZnPor is brought by the disulfide bond.…”

“…Moreover, the lifetime of the triplet state of ZnPor–S–S–ZnPor is only 7.38 μs, which is much shorter than those of ZnPor–C–C–ZnPor and ZnTPP. According to the previous report, , the shortened triplet lifetime of the sensitizer should lead to a decrease in the triplet energy transfer efficiency from the sensitizer to the acceptor and thus a reduction in the TTA-UC yield. The decrease in the triplet energy transfer efficiency of ZnPor–S–S–ZnPor can be proved by the smaller K SV value (2.04 × 10 4 ) with respect to those of ZnPor–C–C–ZnPor (2.80 × 10 5 ) and ZnTPP (2.96 × 10 5 ) (Figure S5).…”

“…25 previous reports. 37,38 However, the triplet lifetime of ZnPor− C−C−ZnPor is not influenced by the addition of RSH. All of these results suggest that the faster decay of the triplet state in ZnPor−S−S−ZnPor is brought by the disulfide bond.…”

A new near-infrared phosphorescent iridium(iii) complex conjugated to a xanthene dye for mitochondria-targeted photodynamic therapy

“…Research work has reported that the long-lived 3 ILexcited state of metal-based PSs can be achieved by π-conjugated ligands, such as an xanthene dye. [49] Thus, two luminescent Ir(III) complexes of the type [Ir(C^N) 2 (N^N)][PF 6 ] (Scheme 2) [HC^N= btpq; N^N= XE-P and bpy] were designed and synthesized to investigate the effect of the production of the xanthene dye on singlet oxygen ( 1 O 2 ).…”

Long-Lived Triplet Excited-State Bichromophoric Iridium Photocatalysts for Controlled Photo-Mediated Atom-Transfer Radical Polymerization

“…The “energy reservoir” tactic is a universal method to prolong the triplet lifetime of photosensitizers by enabling a reversible intramolecular triplet-triplet energy transfer (TTET). − According to this tactic, long-lived bichromophoric iridium complexes have been designed and successfully applied as highly efficient photosensitizers for oxygen sensing and triplet-triplet annihilation (TTA) upconversion in our previous studies. − This approach involves coupling an organic chromophore (energy acceptor, A) to a metal complex (energy donor, D) through a linker. In accordance with our previous results, for the molecules with a large D-A energy gap (Δ E ), the back-energy transfer from the energy acceptor to donor is much more impeded because of the uphill energy gap.…”