Theoretical evidence of low-threshold amplified spontaneous emission in organic emitters: Transition density and intramolecular vibrational mode analysis

Organic gain materials are highly attractive for lasing due to their chemical tunability and large stimulated emission cross sections. In previous reports, the radiative decay rate k_r was considered as an important factor to determine outstanding amplified spontaneous emission (ASE) performance of organic molecules. In this study, we use quantum mechanics to reveal the influential factors on photophysical properties of organic emitters, and give insight into the effect of k_r on ASE performance. Based on the theoretical analysis of three molecules with similar structure, calculated results show that large k_r derives from enhanced transition density ρ between the electronic wave functions of the ground-state and the lowest excited singlet state as well as a handful of low-frequency torsional modes with small Huang-Rhys factor S, further, k_r values are calculated depending on molecular vibration terms. In addition, through the analysis of non-radiative decay rate k_nr considering vibration terms (vibronic coupling constants), the comparison of k_r and k_nr shows that the radiative decay process is promoted in the three molecules. The two aspects are desired for outstanding ASE performance. Our work shows that the roles of transition density and vibrational modes are crucial to clarify the photophysical properties, which govern the ASE performance in organic light emitters.