Triplet exciton formation due to interaction between singlet excitons in polydiacetylene

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Abstract
The time‐resolved spectroscopy in polydiacetylene single crystals (PDA‐MADF {poly‐1‐[3‐(methylamino)phenyl]‐4‐[3,5‐bis(trifluoromethyl)phenyl]‐1, 3‐butadiyne}) on femtosecond and picosecond time scales was performed. The time dependence of the photoinduced reflectance change in PDA‐MADF exhibits three components with lifetimes ∼200 fs, 1–2 ps, and ≫200 ps. These components are assigned to the free 1Bu excitons, self‐trapped excitons, and triplet excitons. From the excitation intensity dependence of the reflectance change, a significant role of bimolecular interactions between singlet excitons for the formation of triplet excitons is clarified. A model of the mechanism of triplet exciton formation after the creation of singlet excitons is proposed.