Benzothiazoles with Tunable Electron-Withdrawing Strength and Reverse Polarity: A Route to Triphenylamine-Based Chromophores with Enhanced Two-Photon Absorption

A series of dipolar and octupolar triphenylamine-derived dyes containing a benzothiazole positioned in the matched or mismatched fashion have been designed and synthesized via palladium-catalyzed Sonogashira cross-coupling reactions. Linear and nonlinear optical properties of the designed molecules were tuned by an additional electron-withdrawing group (EWG) and by changing the relative positions of the donor and acceptor substituents on the heterocyclic ring. This allowed us to examine the effect of positional isomerism and extend the structure property relationships useful in the engineering of novel heteroaromatic-based systems with enhanced two-photon absorption (TPA). The TPA cross-sections wavelength (em), (delta(TPA)) in the target compounds dramatically increased with the branching of core and with the strength of the auxiliary acceptor. In addition, a change from the commonly polarity in push-pull benzothiazoles to a reverse one has been revealed as a particularly useful strategy (regioisorneric control) for enhancing TPA cross-sections and shifting the absorption and emission maxima to longer Wavelengths. The maximum TPA cross-sections of the star-shaped three-branched triphenylamines are similar to 500-2300 GM in the near infrared region (740-810 nm); the moleclar weight normalized delta(TPA)/MW values of the best performing dyes within the seties (2.0-2.4 GM.g(-1).mol) are comparable to those of the most efficient TPA chromophores reported to date. The large TPA cross-sections combined with high emission qunatum yields and large Stokes shifts make these compounds excellent candidates for various TPA applications, including two-photon fluorescence microscopy.