Energy‐Level Alignment and Electronic Structure of Dual‐State Luminogens Thin Films

Abstract

Organic dual‐state emitters show high quantum yields of luminescence in both solution and the aggregated state. Alkyl side chains are frequently used to engineer solid‐state structures and prominent examples are naphthalimide‐functionalized cyanostilbene derivatives (NICS‐X), where H‐aggregation takes place for ethoxyl substitution (NICS‐E), while methoxyl and butoxyl substitution (NICS‐M and NICS‐B) lead to the quasi‐isolated Q‐type structure. While this takes place for powder samples, vacuum‐sublimed thin films are used, and it is shown by photoluminescence (PL) measurements that H‐aggregation takes place for all three NICS derivatives. In contrast, the energy‐level alignment of NICS‐X films on graphite exhibits disparities as shown by photoelectron spectroscopy: pronounced disorder in NICS‐B films leads to energy‐level bending, while the energy levels of NICS‐M and NICS‐E films remain flat. In such a way, it is demonstrated that side‐chain engineering of luminogens affects the short‐range order (responsible for the PL) and the long‐range order (responsible for the energy‐level alignment) in different ways. Furthermore, the importance of a substrate (thin films vs powder) on the solid‐state aggregation is highlighted.