Phosphorescence Tuning through Heavy Atom Placement in Unsymmetrical Difluoroboron β‐Diketonate Materials

• Published in: Chemistry : a European journal 2018-02, Vol.24 (8), p.1859-1869
• Main Authors: Liu, Tiandong, Zhang, Guoqing, Evans, Ruffin E., Trindle, Carl O., Altun, Zikri, DeRosa, Christopher A., Wang, Fang, Zhuang, Meng, Fraser, Cassandra L.
• Format: Article
• Language: English
• Subjects: boron complexes; charge transfer; heavy atom; oxygen sensing; phosphorescence
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201703513

Difluoroboron β‐diketonates (BF2bdks) show both fluorescence (F) and room‐temperature phosphorescence (RTP) when confined to a rigid matrix, such as poly(lactic acid). These materials have been utilized as optical oxygen sensors (e.g., in tumors, wounds, and cells). Spectral features include charge transfer (CT) from the major aromatic donor to the dioxaborine acceptor. A series of naphthyl–phenyl dyes (BF2nbm) (1–6) were prepared to test heavy‐atom placement effects. The BF2nbm dye (1) was substituted with Br on naphthyl (2), phenyl (3), or both rings (4) to tailor the fluorescence/phosphorescence ratio and RTP lifetime—important features for designing O2 sensing dyes by means of the heavy atom effect. Computational studies identify the naphthyl ring as the major donor. Thus, Br substitution on the naphthyl ring produced greater effects on the optical properties, such as increased RTP intensity and decreased RTP lifetime compared to phenyl substitution. However, for electron‐donating piperidyl‐phenyl dyes (5), the phenyl aromatic is the major donor. As a result, Br substitution on the naphthyl ring (6) did not alter the optical properties significantly. Experimental data and computational modeling show the importance of Br position. The S1 and T1 states are described by two singly occupied MOs (SOMOs). When both of these SOMOs have substantial amplitude on the heavy atom, passage from S1 to T1 and emission from T1 to S0 are both favored. This shortens the excited‐state lifetimes and enhances phosphorescence. Unsymmetrical difluoroboron β‐diketonate materials were designed to probe the influence of bromide substitution on the relative fluorescence and phosphorescence intensities of these oxygen‐sensing materials. Based on experimental results and theoretical calculations, heavy‐atom placement on phenyl, naphthyl or both arene donors has a pivotal effect on optical properties, with important implications for oxygen sensing applications.