Modulating the Favored Emissive State of Functionalized Phenoxazines in both Solution and the Solid-State

In this contribution, a set of five differently substituted phenoxazines (POA) derived from a central terephthalonitrile core is investigated. Depending on incorporated electron-donating groups (-NH2, -NMe2, and -OMe) or electron-withdrawing groups (-NO2 and -CN), distinctive luminescent behavior is observed, with preferred emission either in solution, in the solid-state, or in both. An in-depth assessment of the photophysical properties revealed a subtle influence of the substituents on the absorption and emission wavelengths, which correlate with the Hammett parameters, yet a strong sensitivity to the surrounding environment. Hence, solvatochromic response and aggregation studies were conducted to identify the optimal conditions for efficient emission. Absolute photoluminescence quantum yield values of up to 0.47 in tetrachloromethane and 0.33 in the solid state were determined, underscoring the impressive photophysical characteristics of the presented compounds. X-ray diffractometric analyses in combination with Hirshfeld surface evaluation were employed to elucidate the packing effects and their possible relation to the photophysical performance. Finally, density functional theory calculations provided a detailed understanding of the occurring electronic transitions and spatial localization of the natural transition orbitals.