Excited-state dynamics of poly(para-phenylene)-type ladder polymers at high photoexcitation density

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Abstract
Ultrafast dynamics in methyl-substituted poly(para-phenylene)-type ladder polymer films was studied by femtosecond pump-probe spectroscopy in a wide range of excitation densities, up to values typical for the regime of narrow band emission. The singlet excitons relaxation dynamics, monitored at the stimulated emission peak (2.53 eV), showed an intensity-dependent ultrafast decay component, which occurs on a subpicosecond time scale, ascribed to the onset of an amplified spontaneous emission process. In addition, we attributed the photoinduced absorption bands at 1.48 and 1.9 eV to singlet excitons and polarons, respectively. At high excitation density an additional absorption band becomes evident at 2.63 eV. Based on anisotropy decay kinetics we assigned it to short-lived interchain species.