Triplet photosensitizers: from molecular design to applications

Abstract

Triplet photosensitizers (PSs) are compounds that can be efficiently excited to the triplet excited state which subsequently act as catalysts in photochemical reactions. The name is originally derived from compounds that were used to transfer the triplet energy to other compounds that have only a small intrinsic triplet state yield.Triplet PSs are not only used for triplet energy transfer, but also for photocatalytic organic reactions, photodynamic therapy (PDT), photoinduced hydrogen production from water and triplet-triplet annihilation (TTA) upconversion. A good PS should exhibit strong absorption of the excitation light, a high yield of intersystem crossing (ISC) for efficient production of the triplet state, and a long triplet lifetime to allow for the reaction with a reactant molecule. Most transition metal complexes show efficient ISC, but small molar absorption coefficients in the visible spectral region and short-lived triplet excited states, which make them unsuitable as triplet PSs. One obstacle to the development of new triplet PSs is the difficulty in predicting the ISC of chromophores, especially of organic compounds without any heavy atoms. This review article summarizes some molecular design rationales for triplet PSs, based on the molecular structural factors that facilitate ISC. The design of transition metal complexes with large molar absorption coefficients in the visible spectral region and long-lived triplet excited states is presented. A new method of using a spin converter to construct heavy atomfree organic triplet PSs is discussed, with which ISC becomes predictable, C 60 being an example. To enhance the performance of triplet PSs, energy funneling based triplet PSs are proposed, which show broadband absorption in the visible region. Applications of triplet PSs in photocatalytic organic reactions, hydrogen production, triplettriplet annihilation upconversion and luminescent oxygen sensing are briefly introduced.

Recent advance of photochromic diarylethenes-containing supramolecular systems

“…By selectively exciting the triplet photosensitizers in the presence of acceptor, the triplet excited states of the photosensitizers are populated, and the triplet energy migrates to the triplet acceptor from the photosensitizer by the triplet−triplet energy-transfer (TTET) process. Then the TTA between the triplet acceptors produces the singlet excited state, and the upconverted fluorescence can be observed [49]. When the open form isomers of the dithienylethene derivatives were added to the TTA UC system, TTA UC emission will not be perturbed.…”

A study of acridine and acridinium-substituted bis(terpyridine)zinc(ii) and ruthenium(ii) complexes as photosensitizers for O2 (1Δg) generation

“…17 Nevertheless "classic" triplet sensitizers and their application remain an active research area in recent years and the progress in this field has been summarized only recently. 18 Advances in this field are on the one hand a "side-effect" due to the need for phosphorescent dyes in materials chemistry but also due to the need for biocompatible (water-soluble, non-toxic, etc.) far-red absorbing dyes 18 for photodynamic therapy 19 (PDT).…”

“…18 Advances in this field are on the one hand a "side-effect" due to the need for phosphorescent dyes in materials chemistry but also due to the need for biocompatible (water-soluble, non-toxic, etc.) far-red absorbing dyes 18 for photodynamic therapy 19 (PDT).…”

“…This was achieved by manipulation of the terpyridine ligand itself, 40,41 but also by attachment of multiple chromophores with a matching triplet level. 42 The choice of a suitable chromophore for such purposes is a priori difficult and certain limitations apply as summarized by Zhao et al, 18 i.e., one or several chromophores tethered to the metal-coordination side with an excited singlet state higher in energy than the singlet state of interest may serve as an additional photon absorber ("antenna effect"). However, if we focus on the T 1 as state of interest and consider the chromophore triplet state higher in energy but the metal ion as main cause for the intersystem crossing (ISC) then no photophysical benefit is gained in terms of triplet photophysics.…”

Photoluminescent nanoplatforms in biomedical applications

“…First, PS molecules are activated using Vis or UV light, which hinders their use for the treatment of deep-seated tumors. Since it is a localized treatment, reaching the appropriate light dosimetry is still problematic, particularly in solid and bulky tumors; and finally, there is a lack of sufficient selectivity for the tumor tissue, regarding the absence of targeting moieties accompanied by hydrophobic character that limits their bioavailability [163].…”