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 atom-free organic triplet PSs is discussed, with which ISC becomes predictable, C60 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, triplet-triplet annihilation upconversion and luminescent oxygen sensing are briefly introduced.

Triplet photosensitizers: from molecular design to applications

New benzoimidazoles (bi) have been synthesized. These compounds readily undergo cyclometalation with iridium trichloride, and bis-cyclometalated iridium complexes, (bi)2Ir(acac) (bi = cyclometalated benzoimidazole; acac = acetylacetonate), can be isolated. One of the complexes, (fbi)2Ir(acac) (fbi = 2-(9,9-diethyl-9H-fluoren-2-yl-1H-benzoimidazole), was also characterized by single-crystal X-ray structural determination. Some of the complexes, (bi)2Ir(acac), are highly phosphorescent at ambient condition. Light-emitting devices using these complexes as dopants were fabricated, and the emission colors range from green to red. Some green- and yellow-emitting devices exhibit very high efficiencies.

Highly Phosphorescent Bis-Cyclometalated Iridium Complexes Containing Benzoimidazole-Based Ligands

Photodynamic therapy (PDT), a therapeutic mode involving light triggering, has been recognized as an attractive oncotherapy treatment. However, nonnegligible challenges remain for its further clinical use, including finite tumor suppression, poor tumor targeting, and limited therapeutic depth. The photosensitizer (PS), being the most important element of PDT, plays a decisive role in PDT treatment. This review summarizes recent progress made in the development of PSs for overcoming the above challenges. This progress has included PSs developed to display enhanced tolerance of the tumor microenvironment, improved tumor-specific selectivity, and feasibility of use in deep tissue. Based on their molecular photophysical properties and design directions, the PSs are classified by parent structures, which are discussed in detail from the molecular design to application. Finally, a brief summary of current strategies for designing PSs and future perspectives are also presented. We expect the information provided in this review to spur the further design of PSs and the clinical development of PDT-mediated cancer treatments.

Recent progress in photosensitizers for overcoming the challenges of photodynamic therapy: from molecular design to application.

Artificial light-harvesting antennae: electronic energy transfer by way of molecular funnels

Improving the photophysical properties of copper(I) bis(phenanthroline) complexes

The synthesis and photophysical properties of two Ru(II) diimine complexes bearing one (dyad) and three (tetrad) pyrenyl units, respectively, are presented. The pyrene chromophore in each metal complex is tethered through a single C−C bond in the 5-position of 1,10-phenanthroline (py-phen). Both Ru(II) complexes display increased absorption cross sections near 340 nm largely due to the presence of the pyrenyl chromophore(s). Excitation from 300 to 540 nm results exclusively in the observation of metal-to-ligand charge transfer (MLCT) based emission that is exceptionally long lived, 23.7 μs and 148 μs in deaerated CH3CN, respectively. This luminescence was analyzed using steady-state and time-resolved techniques at room temperature and 77 K. The tetrad complex, [Ru(py-phen)3]2+, displays a dynamic self-quenching reaction at room temperature in dilute CH3CN solutions that is well modeled by a Stern−Volmer expression. The excited-state processes occurring between the MLCT core and the pyrenyl units were furt...

Jason Bialecki

Papers

Excited State Processes in Ruthenium(II)/Pyrenyl Complexes Displaying Extended Lifetimes

Ruthenium(II) complex with a notably longexcited state lifetime