Room-Temperature Orange-Red Phosphorescence by Way of Intermolecular Charge Transfer in Single-Component Phenoxazine–Quinoline Conjugates and Chemical Sensing

Achieving red phosphorescence from purely organic system is a challenging feat due to the predominant thermal nonradiative decay pathways of the excited electrons. Here, we design single-component charge-transfer (CT) complexes based on phenoxazine–quinoline conjugates (PQ1–PQ3), in which the phenoxazine ring is covalently attached to the quinolinyl fragment via a C–N bond. These conjugates in concentration-dependent absorption studies show a new low-energy CT absorption band along with the parent π–π* band with binding constants of up to 10² M^–1 due to self-association via intermolecular CT (I₂CT). Steady-state emission and phosphorescence decay transient measurements of all of the conjugates in solutions, thin films, and crystals reveal the signature of I₂CT that leads to orange-red phosphorescence (ORP) at ambient conditions. Theoretical calculations show the existence of dimer with stronger I₂CT characteristics and reduced energy gap between the lowest singlet (S₁) and triplet (T₁) states (ΔE_ST = 0.05–0.14 eV), which is in line with emission measurements. These conjugates are used for solid-state dichloromethane vapor sensors, and PQ3 can be transformed into oxygen sensor. These studies give an insight into the ORP properties and provide a rational strategy for the design of single-component self-CT complexes with ORP feature at ambient conditions.