No abstract

Optical spectra of two families of symmetrical polymethine dyes, bearing a positive and a negative charge, are analyzed based on an essential-state model recently developed for quadrupolar dyes. The model accounts for molecular vibrations and polar solvation and reproduces the anomalous evolution with solvent polarity of experimental absorption band shapes. Fluorescence and excited-state absorption spectra are well-described within the same model, which also quantitatively reproduces the recent observation of an intense two-photon absorption toward the (two-photon forbidden) lowest excited state. An extensive analysis of optical spectra demonstrates that the essential-state model developed for quadrupolar dyes also applies to polymethine dyes and that long polymethine dyes offer the first experimental example of class III quadrupolar chromophores.

The photophysical properties of two pairs of squaraine molecules where two oxygen atoms in a squaraine bridge are replaced with sulfur atoms are investigated. This substitution leads to an inversion of the lowest singlet π−π* electronic transition by an n−π* transition, effectively reducing the energy difference between singlet and triplet states and significantly increasing the intersystem crossing efficiency without the use of “heavy atoms”. Experimental results, in agreement with quantum chemical calculations, show near-unity values for triplet quantum yield and singlet oxygen generation quantum yield for sulfur-containing squaraines, which are potentially useful for two-photon photodynamic therapy.

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.