3,9-Disubstituted Bis[1]benzothieno[3,2-b;2',3'-e][1,4]thiazines with Low Oxidation Potentials and Enhanced Emission

Dibrominated bis[1]benzothieno[3,2-b;2',3'-e][1,4]thiazines (BBTT) are efficiently synthesized and applied in Suzuki and Buchwald-Hartwig cross-coupling reactions to give access to 3,9-disubstituted BBTT derivatives with extended π-conjugation and enhanced electronic properties. For instance, 3,9-di(hetero)aryl substituted BBTT derivatives surpass their parent congeners phenothiazines with lower oxidation potentials and pronounced yellow to orange-red fluorescence (Φ_f ≈ 30-45%). In addition, 3,9-bis(di(hetero)arylamino substituted BBTT possess very high lying HOMO energy (E_HOMO = -4.46 to -4.83 eV), a favorable property of hole transport molecules. A representative X-ray structure analysis reveals that the central BBTT core in these extended π-systems is essentially planarized. Upon protonation of a 3,9-bis(di(hetero)arylamino) substituted BBTT, the absorption color shifts from yellow to deep blue with a concomitant loss of the emission. The optical properties of these novel BBTT derivatives can be plausibly rationalized by time-dependent density functional theory (TD(DFT)) calculations and correlation between experimentally determined oxidation potentials and σ_p parameters as well as between photophysical data and the specific substituent parameter σ_p^- by establishing electronic structure-property relationships.