Comparative studies of multi-photon induced emission by pyridine-based small molecular probes in biological media: Selective binding of bioactive molecules and in vitro imaging

A new class of organic molecular probes (1-3) based on a 1,3-disubstituted diethynylbenzene core has been developed. Both 1 and 2 (with the esters of 1 replaced by diethylamides in 2) show good linear and three-photon induced photophysical properties with two-photon absorption cross-sections (185-210 cm⁴sphoton^-1molecule^-1) that are suitable for biological applications in live specimens. The propeller π-conjugated systems of 3 (a C₃ analogue of 1) shows threefold enhancement for the two-photon absorption cross-section (650 cm⁴sphoton ^-1molecule^-1). Solvatochromism was observed in the fluorescence spectra of all these molecular probes; in acidic medium (pH = 4-5) their fluorescence emissions are slightly blueshifted with a threefold enhancement in intensity relative to those observed under basic conditions (pH = 10-11). In the fluorometric titration study against a variety of bioactive small molecules, only 2 shows strong binding affinity (log K_B > 7) towards citrates and bicarbonates with approximately 30 nm redshift. The in vitro emission spectra of 2 obtained show the same emission upon addition of anions to the solution. The results of these studies could provide new molecular-design strategies for two-photo absorption (TPA) chromophores and new materials for two/multi-photon imaging in vitro. A new class of organic molecular probes has been developed, and the cell-permeable properties were improved dramatically by introducing diethylamide moieties. Only 2 shows strong emission enhancement uponbinding with citrates and bicarbonates. The emission spectra of 2 obtained in vitro show the same emission with citrate and hydrogencarbonate.