Tailoring the nonlinear absorption of fluorescent dyes by substitution at a boron center

The tuning of the spectroscopic signatures of boron-carrying fluorescent dyes is achieved by subtle chemical modifications. In more detail, we propose a new series of compounds incorporating up to three electron-donating moieties around the central accepting core, using various positions for the donating moieties, including the central boron atom. For all dyes, a thorough experimental and computational investigation of the absorption and emission properties is presented, with specific emphasis on two-photon absorption. Our key finding is that the two-photon absorption cross section, a property vital for bioimaging applications, can be tuned to a large extent (eightfold increase) by changing the topology of the molecule and using an optimal substitution pattern, while mainly conserving the position of the absorption/emission band and fluorescence quantum yield. In addition, these dyes combine significant values of two-photon absorption cross sections (exceeding 500 GM) to significant fluorescence quantum yields - a beneficial feature for several applications J. M. L. is grateful for the financial support from the Spanish MICIN PGC2018-098212-B-C22 and the Catalan DIUE 2017SGR39