Origin of Intramolecular Low‐Threshold Amplified Spontaneous Emission

Amorphous thin films from solution‐processable semiconductors are key materials for low‐cost and large area optoelectronics. Design rules toward novel amorphous compounds with outstanding light emission and light amplification properties require understanding of the intimate relation between chemical and electronic structure. Here, a series of compounds with tunable electronic transition characters of the lowest excited state, from local excited (LE), hybrid local charge‐transfer (HLCT) to charge‐transfer (CT) character is delicately designed. By deploying a combination of computational calculations and femtosecond‐transient absorption experiments, it is shown that pure LE states strongly coupled to high wavenumber vibrational modes favor to form a few dominant discrete vibrational levels and are essential for optical gain, whereas HLCT or CT states are preferably coupled to low frequency vibrational modes and form a large number of consecutive vibrational levels which lead to broad excited‐state absorption overwhelming stimulated emission. The results provide guidelines for the rational design of efficient organic laser materials.