Recently, organic fluorescent molecules harnessing the excited-state intramolecular proton transfer (ESIPT) process are drawing great attention due to their unique photophysical properties which facilitate novel optoelectronic applications. After a brief introduction to the ESIPT process and related photo-physical properties, molecular design strategies towards tailored emission are discussed in relation to their theoretical aspects. Subsequently, recent studies on advanced ESIPT molecules and their optoelectronic applications are surveyed, particularly focusing on chemical sensors, fluorescence imaging, proton transfer lasers, and organic light-emitting diodes (OLEDs).

Advanced Organic Optoelectronic Materials: Harnessing Excited-State Intramolecular Proton Transfer (ESIPT) Process

Charge and proton transfer reactions in the excited states of organic dyes can be coupled in many different ways. Despite the complementarity of charges, they can occur on different time scales and in different directions of the molecular framework. In certain cases, excited-state equilibrium can be established between the charge-transfer and proton-transfer species. The interplay of these reactions can be modulated and even reversed by variations in dye molecular structures and changes of the surrounding media. With knowledge of the mechanisms of these processes, desired rates and directions can be achieved, and thus the multiple emission spectral features can be harnessed. These features have found versatile applications in a number of cutting-edge technological areas, particularly in fluorescence sensing and imaging.

Excited-state proton coupled charge transfer modulated by molecular structure and media polarization

One of the most important drawbacks of classical and new advanced functional materials for applications outdoors, or in environments with high UV irradiation, is the light induced damage that reduces drastically their effective operation lifetime or durability. This makes protecting light sensitive materials against UV irradiation a nowadays important technological demand in almost every industrial field. This tutorial review incorporates the main aspects of UV damage to materials and describes the recently developed highly effective thin UV-protective coatings, based on UV-absorber molecules entrapped in a Sol–Gel derived ormosil matrix.

Preventing UV-light damage of light sensitive materials using a highly protective UV-absorbing coating

The excited-state intramolecular proton transfer (ESIPT) of a series of water-soluble 2-(2‘-hydroxyphenyl)benzazole derivatives has been studied under physiological conditions using absorbance and steady-state emission spectroscopy. At neutral pH in the presence of 0.1 M ionic background, the fluorescence properties of these derivatives differ substantially compared to previously reported data in nonaqueous solvents. The ESIPT process is disrupted, presumably due to intermolecular hydrogen bonding with surrounding water molecules combined with increased stabilization of the trans-rotamer, which cannot undergo the ESIPT process. The emission spectrum of the benzimidazole derivative depends significantly on the solvent polarity, as revealed by titrations with Zn(II) in methanol, ethanol, and under physiological conditions. Inhibition of ESIPT via metal coordination shows a significant wavelength shift together with a substantial ratio increase by a factor of 13.7. Titration of the benzoxazole derivative wit...

Excited State Intramolecular Proton Transfer and Metal Ion Complexation of 2-(2‘-Hydroxyphenyl)benzazoles in Aqueous Solution

A novel class of 2-(2‘-hydroxyphenyl)benzothiazole-based (HBT-based) excited-state intramolecular proton-transfer (ESIPT) compounds, N,N‘-di[3-Hydroxy-4-(2‘-benzothiazole)phenyl]isophthalic amide (DHIA) and N,N‘-di[3-Hydroxy-4-(2‘-benzothiazole)phenyl]5-tert-butyl-isophthalic amide (DHBIA) has been feasibly synthesized and the properties of their nanoparticles in THF/H2O mixed solvent were investigated. Both compounds were found to exhibit aggregation-induced emission enhancement (AIEE) due to restricted intramolecular motion and easier intramolecular proton transfer in solid state. On identical experimental conditions, the emission of DHBIA aggregates increased more remarkably than that of DHIA. Different aggregation forms of these two organic compounds, due to the steric hindrance of a single tert-butyl group, could be responsible for the notably different degrees of the fluorescence enhancement. Their aggregation modes were investigated on the basis of time-dependent absorption, scanning electron micro...

Aggregation-induced emission enhancement of 2-(2'-hydroxyphenyl)benzothiazole-based excited-state intramolecular proton-transfer compounds.

Excited-state proton transfer in aqueous and ethanolic solutions of 2-(2‘-hydroxyphenyl)benzimidazole (HBI) was investigated by means of UV−vis absorption and fluorescence spectroscopy. The behavior of HBI in water differed from its behavior in ethanol, and in both solvents fluorescence behavior depended on acidity. In both neutral water and neutral ethanol, ground-state HBI exhibits conformational equilibrium between a cis-enol form with an intramolecular hydrogen bond and a trans-enol form that is hydrogen-bonded to the solvent; the ground-state keto tautomer is also present in water but was not detected in ethanol. The excited cis-enol conformer always undergoes ultrafast intramolecular proton transfer to afford the excited keto tautomer. The excited trans-enol form fluoresces in both solvents, and in water it also loses its hydroxyl proton to the solvent, leaving the excited anion. In both acidic aqueous solution and acidic ethanol, excited protonated HBI loses its hydroxyl proton to give the excited ...

Photoinduced Inter- and Intramolecular Proton Transfer in Aqueous and Ethanolic Solutions of 2-(2‘-Hydroxyphenyl)benzimidazole: Evidence for Tautomeric and Conformational Equilibria in the Ground State†

and M. Carmen Ríos Rodríguez

Papers

Photoinduced Inter- and Intramolecular Proton Transfer in Aqueous and Ethanolic Solutions of 2-(2‘-Hydroxyphenyl)benzimidazole: Evidence for Tautomeric and Conformational Equilibria in the Ground State†