Isocyanonaphthol Derivatives: Excited-State Proton Transfer and Solvatochromic Properties

Abstract

Fluorescent probes that exhibit solvatochromic or excited-state proton-transfer (ESPT) properties are essential tools for the study of complex biological or chemical systems. Herein, the synthesis and characterization of a novel fluorophore that reveals both features, 5-isocyanonaphthalene-1-ol (ICOL), are reported. Various solvatochromic methods, such as Lippert–Mataga and Bilot–Kawski, together with time-dependent density functional theory (TD-DFT) and time-resolved emission spectroscopy (TRES), were applied to gain insights into its excited-state behavior. To make comparisons, the octyloxy derivative of ICOL, 5-isocyano-1-(octyloxy)naphthalene (ICON), was also prepared. We found that internal charge transfer (ICT) takes place between the isocyano and –OH groups of ICOL, and we determined the values of the dipole moments for the ground and excited states of both ICOL and ICON. Furthermore, in the emission spectra of ICOL, a second band at higher wavelengths (green emission) in solvents of higher polarities (dual emission), in addition to the band present at lower wavelengths (blue emission), were observed. The extent of this dual emission increases in the order of 2-propanol < methanol < N,N-dimethylformamide (DMF) < dimethyl sulfoxide (DMSO). The presence of the dual fluorescence of ICOL in these solvents can be ascribed to ESPT. For ICOL, we also determined ground- and excited-state pKa values of 8.4 ± 0.3 and 0.9 ± 0.7, respectively, which indicates a considerable increase in acidity upon excitation. The TRES experiments showed that the excited-state lifetimes of the ICOL and ICON spanned from 10.1 ns to 5.0 ns and from 5.7 ns to 3.8 ns, respectively. In addition, we demonstrated that ICOL can be used as an effective indicator of not only the critical micelle concentration (cmc) of ionic (sodium lauryl sulfate (SLS)) and nonionic surfactants (Tween 80), but also other micellar parameters, such as partition coefficients, as well as to map the microenvironments in the cavities of biomacromolecules (e.g., BSA). It is also pointed out that fluorescence quenching by pyridine can effectively be utilized for the determination of the fractions of ICOL molecules that reside at the water–micelle interface and in the interior spaces of micelles.

Application of meso-CF3-Fluorophore BODIPY with Phenyl and Pyrazolyl Substituents for Lifetime Visualization of Lysosomes

“…The same experiments, carried out in pure DMSO, showed slow bleaching of BODIPY 1 , resulting in a visible color change from blue to virtually colorless and the disappearance of the red fluorescence. Probably, this happened due to the specific solute–solvent interaction between fluorophore and DMSO [ 58 ]. The exact mode of action of DMSO (and/or H 2 O) and an acid was not studied in this work.…”

Solvatochromic and Acid–Base Molecular Probes in Surfactant Micelles: Comparison of Molecular Dynamics Simulation with the Experiment

“…Solvatochromic dyes have been widely used for studying colloidal systems since long ago [117], and such works are ongoing [1,[118][119][120][121]. Among other solvatochromic indicators, the pyridinium N-phenolate betaine dyes belong to the most powerful ones [21].…”

A Polarity-Sensitive Far-Red Fluorescent Probe for Glucose Sensing through Skin

“…Although red and NIR light show improved tissue depth penetration and decreased melanin absorption when compared to lower wavelengths, reduced transmittance has still been observed for both darker and thicker skin samples and must be further investigated in terms of an implantable biosensor [ 12 ]. Fluorescent probes within the far-red and NIR range can be designed to provide intensity-based detection by leveraging their response to their microenvironment such as the polarity, acidity/basicity, or fluidity [ 13 , 14 ]. The change in a molecule’s absorption and/or emission due to dissolution in solvents of varying polarity is called solvatochromism and has been researched widely in the field of fluorescent probes [ 13 , 15 , 16 , 17 , 18 , 19 ].…”

“…Fluorescent probes within the far-red and NIR range can be designed to provide intensity-based detection by leveraging their response to their microenvironment such as the polarity, acidity/basicity, or fluidity [ 13 , 14 ]. The change in a molecule’s absorption and/or emission due to dissolution in solvents of varying polarity is called solvatochromism and has been researched widely in the field of fluorescent probes [ 13 , 15 , 16 , 17 , 18 , 19 ].…”