Bis(dioxaborine) Dyes with Variable π-Bridges: Towards Two-Photon Absorbing Fluorophores with Very High Brightness. 
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) visible-light absorption (>400 nm), (b) high molar absorption coefficients (up to 70000 m-1 cm-1 ), (c) Stokes shifts in the range of ca.  2500-5800 cm-1 , and (d) 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 2800 GM (1 Goeppert-Mayer (GM)=10-50 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.