Polymorphism and Amplified Spontaneous Emission in a Dicyano‐Distyrylbenzene Derivative with Multiple Trifluoromethyl Substituents: Intermolecular Interactions in Play

Abstract

Intermolecular interactions are decisive in controlling optoelectronic properties of molecular materials. Polymorphism in one and the same material gives a unique opportunity to gain a direct insight into the intermolecular contributions. Here, stimulated emission properties in two polymorphs of a dicyanodistyrylbenzene derivative with multiple trifluoromethyl groups in the terminal rings are investigated, and are correlated to the intermolecular arrangement and resulting excitonic coupling. The polymorphs have distinctly different emission behavior with one emitting in the blue, while the other emits in the green. Only the blue polymorph exhibits light amplification with an apparent threshold of spectral narrowing about 100 μJ cm−2. The green polymorph fails to exhibit any gain even at two orders of magnitude higher excitation fluence. Pump–probe studies indicate a more extended excited state absorption toward the emission region in the green polymorph compared to the blue, allowing net gain only on a subpicosecond (ps) time scale, too short for the observation of amplified spontaneous emission. This is attributed to intermolecular breathing/shearing modes coupled to the electronic transition due to strong π–π overlap in the green polymorph. These observations emphasize the significance of intermolecular interactions in controlling the optical properties as well as light amplification processes in organic conjugated materials.