RESUMO
Fluorescent solvatochromic dyes and molecular rotors increase their popularity as fluorogenic probes for background-free detection of biomolecules in cellulo in no-wash conditions. Here, we introduce a push-pull boron-containing (dioxaborine) dye that presents unique spectroscopic behavior combining solvatochromism and molecular rotor properties. Indeed, in organic solvents, it shows strong red shifts in the absorption and fluorescence spectra upon increase in solvent polarity, typical for push-pull dyes. On the other hand, in polar solvents, where it probably undergoes Twisted Intramolecular Charge Transfer (TICT), the dye displays strong dependence of its quantum yield on solvent viscosity, in accordance to Förster-Hoffmann equation. In comparison to solvatochromic and molecular rotor dyes, dioxaborine derivative shows exceptional extinction coefficient (120,000 M-1 cm-1), high fluorescence quantum yields and red/far-red operating spectral range. It also displays much higher photostability in apolar media as compared to Nile Red, a fluorogenic dye of similar color. Its reactive carboxy derivative has been successfully grafted to carbetocin, a ligand of the oxytocin G protein-coupled receptor. This conjugate exhibits >1000-fold turn on between apolar 1,4-dioxane and water. It targets specifically the oxytocin receptor at the cell surface, which enables receptor imaging with excellent signal-to-background ratio (>130). We believe that presented push-pull dioxaborine dye opens a new page in the development of fluorogenic probes for bioimaging applications.
RESUMO
We present an experimental and theoretical investigation of the linear and nonlinear optical properties of a series of acceptor-pi-acceptor symmetrical anionic polymethine dyes with diethylamino-coumarin-dioxaborine terminal groups and different conjugation lengths. Two-photon absorption (2PA) cross sections (delta(2PA)) are enhanced with an increase of pi-conjugation length in the investigated series of dyes. 2PA spectra for all dyes consist of two well-separated bands. The first band, located within the telecommunications window, occurs upon two-photon excitation into the vibrational levels of the main S(0) --> S(1) transition, reaching a large delta(2PA) = 2200 GM (1 GM = 1 x 10(-50) cm(4) s/photon) at 1600 nm for the longest conjugated dye. The position of the second, and strongest, 2PA band for all anionic molecules corresponds to the second-excited final state, which is confirmed by quantum-chemical calculations and excitation anisotropy measurements. Large delta(2PA) values up to 17,000 GM at 1100 nm are explained by the combination of the large ground- and excited-state transition dipole moments. The three shortest dyes show good photochemical stability and surprisingly large fluorescence quantum yields of approximately 0.90, approximately 0.66, and approximately 0.18 at the red to near-IR region of approximately 640, approximately 730, and approximately 840 nm, respectively. The excited-state absorption spectra for all samples are also studied and exhibit intense bands throughout the visible wavelength region with peak cross section close to 5 x 10(-16) cm(2) with a corresponding red shift with increasing conjugation lengths.
