Heavy-atom free sensitizers for triplet-triplet annihilation upconversion systems

A photon upconversion system, comprising:

• a sensitizer having a structure of Formula (I)
• wherein:
• X and X′ are each independently selected from S, Se, and Te,
• L is (CRR′)n, wherein:
• R at each occurrence is independently H, halogen, C1-10 alkyl, or C1-10 haloalkyl,
• R′ at each occurrence is absent, or independently H, halogen, C1-10 alkyl, or C1-10 haloalkyl, and
• n is an integer selected from 0, 1, 2, or 3;
• L′ is absent, or (CR)m-(CRR′)p, wherein
• R at each occurrence is independently H, halogen, C1-10 alkyl, or C1-10 haloalkyl,
• R′ at each occurrence is absent, or independently H, halogen, C1-10 alkyl, or C1-10 haloalkyl, and
• m is 1;
• and
• p is an integer selected from 0, 1, or 2;
• and
• Ar and Ar′ are each independently selected from aryl and heteroaryl, wherein said aryl and heteroaryl are each optionally substituted with 1, 2, 3, or 4 substituents independently selected from amino, C1-10 alkyl, aryl, halo, OH, wherein each of said amino, C1-10 alkyl, and aryl is optionally substituted with 1, 2, 3, or 4 substituents independently selected from amino, halo, C1-10 alkyl, C1-6 haloalkyl, and OH;
• wherein one of Ar and Ar′ is positively charged;
• and
• an emitter,
• wherein the sensitizer absorbs an energy from an incident radiation, and
• the emitter accepts the energy from the sensitizer via triplet-triplet energy transfer, and emits at a lower wavelength than the incident radiation via a triplet-triplet annihilation process.