RESUMEN
Bis(dioxaborine) dyes of the A-π-A format (A: acceptor, π: conjugated bridge) were prepared and photophysically characterized. The best performing dyes feature (a)â visible-light absorption (>400â nm), (b)â high molar absorption coefficients (up to 70000 m-1 â cm-1 ), (c)â Stokes shifts in the range of ca. 2500-5800â cm-1 , and (d)â strong fluorescence emission with quantum yields of up to 0.74. This yields very bright-emitting dyes for one-photon excitation. However, the most intriguing feature of the dyes is their strong two-photon absorption. This was achieved by means of increased π-conjugation in the phenylene or phenylene-thiophene bridges through the variation of the conjugation length and rigidity. This provided two-photon absorption cross sections of up to 2800â GM (1 Goeppert-Mayer (GM)=10-50 â cm4 s photon-1 ). Considering the mentioned high fluorescence quantum yields, exceptionally bright-emitting A-π-A two-photon absorbing dyes with low molecular mass are obtained. Time-dependent density-functional theory calculations corroborated the experimental results.