We investigate herein the excited state dynamics and symmetry breaking processes in three benzothiazole-derived two-photon absorbing chromophores by femtosecond fluorescence and transient absorption (fs-TA) spectroscopies in solvents of various polarity. The chromophores feature a quasi-quadrupolar D-π-A-π-D architecture comprised of an electron-withdrawing benzothiazole core and lateral triphenylamine donors (Qbtz-H), while the acceptor strength of the central unit is enforced by attached cyano groups (Qbtz-CN) and the electron-donating strength of the arylamine moieties by introduction of peripheral methoxy groups (Qbtz′-CN). Steady state spectroscopy reveals positive solvatochromism, which is mostly pronounced for Qbtz′-CN. Femtosecond spectroscopy of Qbtz-H reveals the coexistence of the Franck–Condon (FC) state and states populated after symmetry breaking (SB) in low-polarity solvents such as toluene and tetrahydrofuran, while the SB state becomes favorable in polar acetonitrile. For the other two molecules possessing a stronger electron-accepting unit and thus more polar excited state, SB takes place even in low-polarity solvents, as shown by fs-TA spectroscopy. Global fitting of the fs-TA spectra together with investigation of the evolution associated spectra (EAS) reveals the existence of an initial FC state in Qbtz-H, in all studied solvents, which relaxes toward Intermediate Charge Transfer (I-CT) and SB states. On the other hand, for Qbtz-CN and Qbtz′-CN in more polar solvents, the FC state undergoes ultrafast relaxation toward symmetry-broken charge transfer (SB-CT) states which in turn show very fast recombination to the ground state. Our measurements confirm that the extent of symmetry breaking is larger for D-π-A-π-D systems with the stronger acceptor core and increases further by increasing electron-donating strength of triarylamine moieties, giving rise to symmetry breaking in these nonionic quadrupolar molecules with ethynylene (triple bond) π-spacers also in less polar solvents.

Direct iodination of benzothiazoles under strong oxidative/acidic conditions leads to a mixture of iodinated heteroarenes with 1-2 major components, which are easily separable and which structures depend on the I2 equivalents used. Among unexpected but dominant products were identified 4,7-diiodobenzothiazoles with a rare substitution pattern for SEAr reactions of this scaffold. These were employed in the synthesis of 4,7-bis(triarylamine-ethynyl)benzothiazoles  a new class of highly efficient quasiquadrupolar fluorophores displaying large two-photon absorption cross-sections (540−1374 GM) in the near-infrared region.

Copper(II)-catalyzed C-H/C-H coupling of dipolar 2-H-benzothiazoles end-capped with triphenylamine moieties affords highly fluorescent 2,2'-bibenzothiazoles with D-π-A-π-D architecture displaying large two-photon absorption (TPA) crosssections (5431252 GM) in the near-infrared region. The notably higher TPA performance as compared to quadrupolar π-systems with a widely-used 2,2'-bipyridine core, along with the ease of the synthesis and chelating N^N ability makes the title biheteroaryl platform an attractive building block for a large scope of functional dyes exploiting nonlinear optical phenomena.

An alternative strategy for the mechanochemical defluorinative acylation of ortho-hydroxyarylenaminones has been developed to synthesise 3-acylchromones utilizing CF 3 -compounds via activation of the CÀ F bound of the trifluoromethyl group in the presence of ytterbia (Yb 2 O 3 ). The current protocol tolerated a wide range of coupling substrates to access a library of diversely substituted 3-acylchromones under the mechanochemically induced domino cyclisation mode. This is the first report for the deflourinative transformation of the inert CF 3 group to the corresponding carbonyl functionality under the mechanochemical conditions.