Oxygen sensing properties of room temperature phosphorescent halogenated hexahydroxanthene derivatives

A series of halogenated hexahydroxanthene (XAN) derivatives, XAN-F, XAN-Cl, XAN-Br, XAN-I, and XAN-5F, were synthesized to investigate their room-temperature phosphorescence (RTP) and optical oxygen-sensing performance. Single-crystal X-ray diffraction revealed that heavier halogens (Br and I) induce nearly orthogonal dihedral angles between the xanthene and phenyl rings (84.87° for XAN-Br and 86.18° for XAN-I), enhancing molecular rigidity and spin–orbit coupling (SOC). This structural configuration promotes efficient intersystem crossing (ISC) and suppresses nonradiative decay, enabling pronounced RTP. In contrast, XAN-Ph without halogen substitution displayed fluorescence, while derivatives bearing lighter halogens (F, Cl, and 5F) exhibited only low-temperature phosphorescence. Thin films of XAN-Br and XAN-I containing 1 wt% Zeonex displayed strong RTP with lifetimes up to 4.48 ms. They also showed exceptional ISC quantum yields (93.02 % for XAN-Br and 93.51 % for XAN-I) and large vacuum-to-air photoluminescence intensity ratios (14.67 and 15.33, respectively). These features translated to excellent oxygen sensitivity, with Stern-Volmer quenching constants (K sv ) of 6.08 × 10 −5 and 1.11 × 10 −4 ppm −1 for XAN-Br and XAN-I, respectively. This high oxygen sensitivity arises from efficient triplet–triplet energy transfer from the XAN's triplet excited state to ground-state triplet oxygen, leading to nonradiative quenching. With their metal-free composition, efficient RTP, excellent oxygen responsiveness, and good solution-processability into thin films, XAN-Br and XAN-I represent promising candidates for use in optical oxygen sensors for biomedical diagnostics, food preservation, and environmental monitoring. • Six halogenated xanthene derivatives are designed for optical oxygen sensing. • XAN-Br and XAN-I films exhibit RTP and strong sensitivity to oxygen. • High Stern–Volmer constant (K sv ) up to 1.11 × 10 −4 ppm −1 is achieved. • Emission color shifts from blue/green to white to enable visual detection. • Simple, metal-free synthesis supports low-cost oxygen sensor development.