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

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.