Observation of strong electron-phonon interaction in polymeric diluted organic semiconductor

Incorporating organic semiconductors in insulating polymer films, creating the so-called diluted organic semiconductors, has been successfully used in different optoelectronic devices, including organic light-emitting diodes. However, the fundamental understanding of their excited state dynamics and spin physics remains elusive. Here, we present the temperature-dependent photoluminescence (PL) and transient absorption study of a model fluorescent organic dilute semiconductor where organic fluorophore pyrene is incorporated in the insulating Poly-methyl methacrylate (PMMA) polymeric backbone as pendant unit (py-PMMA). We observe that by lowering the temperature from 300 K to 140 K, the monomer fluorescence emission from py-PMMA gets blue-shifted, and their intensity enhances, which is attributed to less charge carrier scattering lattice sites at low temperature. Interestingly, we observe additional higher energy peaks at 368 nm (similar to 3.36 eV) below 70 K in fluorescence emission, explained by the exciton-phonon coupling. The exciton-phonon interaction constant (gamma(ph)) is determined as 168 mu eV/K, higher than other reported systems. The transient absorption measurement reveals the hot band relaxation (tau(1) = 1.9 ps) and exciton- acoustic phonon interaction (tau(2) = 27.2 ps) dominating in the excited state. Our study establishes that the electron-phonon interaction can play significant role in determining the optoelectmnic properties of the fluorescent dilute organic semiconductor.