Platinum–Acetylide Complexes as Nonfullerene Acceptors in Organic Photovoltaic Systems

Three distinct n-type semiconductors were derived from a platinum-trialkyl phosphine complex; to lower their LUMO levels, various indene derivatives were incorporated using thiophene (PtTIC (1)), thieno[3,2-b]thiophene (PtT2IC (2)), and 4H-cyclopenta[2,1-b:3,4-b′]dithiophene (PtCDTIC (3)) as the acetylide donor units. Single-crystal X-ray diffractometry analysis revealed translinear platinum–acetylide complexation in all cases. The strong (═O···S) interactions between the oxygen atoms of the indene acceptor units and the sulfur atoms of the thiophene-derived donor units induced a highly planar orientation among the heterocyclic ligands, featuring π–π interactions between the planes. Platinum complexes 1, 2, and 3 exhibited strong absorption in the 500–800 nm range, resulting from efficient intramolecular charge transfer transitions from the central platinum-containing donor unit to the terminal acceptors, as well as unique emission in the near-infrared region owing to the heavy-atom effect. The ultraviolet photoelectron spectroscopy results indicated that the LUMO levels were comparable to those of typical nonfullerene acceptors (NFAs). The n-type semiconductors comprising 1, 2, and 3 as NFAs exhibited photoelectric conversion properties in the corresponding organic photovoltaics. The highest conversion efficiency (3.0%) was attained by complex 3.