New Bichromophoric Triplet Photosensitizer Designs and Their Application in Triplet-Triplet Annihilation Upconversion

As indispensable molecular components, photosensitizers play a crucial role in determining the quantum efficiency of triplet-triplet annihilation upconversion (TTA UC). This emergent technology has attracted great attention in recent years for realizing large anti-Stokes shifts with noncoherent excitation sources. In a typical TTA UC, low-energy photons are first harvested by the photosensitizers, which upon intersystem crossing (ISC) undergo triplet-triplet energy transfer (TTET) to emitters (i.e., annihilators). Following the bimolecular TTA among the emitters, high-energy photons are given off by the singlet excited state of the emitters. Apparently, the efficiencies of photon absorption, ISC, and TTET are all dependent on the sensitizers. With a Dexter-type ET mechanism requiring collisional interactions, a long triplet lifetime of the energy donor (photosensitizer) is evidently favorable for enhancing the efficiency of TTET. This progress report summarizes the recent developments of photosensitizers used for TTA UC, many of which feature a bichromophoric molecular scaffold. Among the various consequences and functions entailed by such bichromophoric designs, the extended triplet lifetime is a particularly advantageous property for TTA UC. Additionally, these new potent photosensitizers with long triplet lifetimes are also useful for other applications such as singlet oxygen sensitization and oxygen sensing.

作为不可或缺的分子组分，光敏剂在决定三重态 - 三重态湮灭上转换（TTA UC）的量子效率方面起着至关重要的作用。近年来，这种新兴技术因利用非相干激发源实现大的反斯托克斯位移而备受关注。在典型的TTA UC中，低能光子首先被光敏剂吸收，光敏剂经过系间窜越（ISC）后发生三重态 - 三重态能量转移（TTET）至发射体（即湮灭剂）。在发射体之间发生双分子TTA之后，发射体的单重态激发态发射出高能光子。显然，光子吸收、ISC和TTET的效率都取决于敏化剂。由于德克斯特型电子转移机制需要碰撞相互作用，能量供体（光敏剂）较长的三重态寿命显然有利于提高TTET的效率。本进展报告总结了用于TTA UC的光敏剂的最新发展情况，其中许多光敏剂具有双发色团分子支架。在这种双发色团设计所带来的各种结果和功能中，延长的三重态寿命是TTA UC的一个特别有利的特性。此外，这些具有长三重态寿命的新型高效光敏剂也可用于其他应用，如单线态氧敏化和氧传感。