Excited-state intramolecular proton transfer on 2-(2′-hydroxy-4′-R-phenyl)benzothiazole nanoparticles and fluorescence wavelength depending on substituent and temperature

Abstract

The fluorescence emission properties of 2-(2'-hydroxy-4'-R-phenyl)benzothiazole (HBT-R) nanoparticles with different substituents (R = -COOH, -H, -CH(3), -OH, and -OCH(3)) were investigated using spectroscopic and theoretical methods. HBT-Rs displayed dual enol and keto (excited-state intramolecular proton transfer (ESIPT)) emissions in nonpolar solvents. The spectral change of their ESIPT emissions was affected differently by the electron donating (or withdrawing) power of the substituents; a bathochromic shift for the electron donating group and a hypsochromic shift in electron withdrawing group. In addition, the changes in energy levels calculated by the ab initio method were consistent with the spectral shifts of HBT-R in solution. We prepared aggregated HBT-R nanoparticles using a simple reprecipitation process in tetrahydrofuran-water solvents. The ESIPT emission of aggregated HBT-R nanoparticles was strongly enhanced (over 45 times) compared to those of monomer HBT-Rs in toluene, as markedly shifted ESIPT emissions are observed at longer wavelength without any quenching by self-absorption. Aggregated HBT-R nanoparticles showed longer lifetimes than those of monomer molecules. The temperature effect on the aqueous dispersion of the aggregated HBT-R nanoparticles was also explored. It shows a fluorescent ratiometric change in a range of temperature from 7 to 65 degrees C. A mechanism of a temperature-dependent equilibrium between the nanoparticles and the solvated enols is proposed for the emission color change.

Excited‐State Proton Transfer Can Tune the Color of Protein Fluorescent Markers

“…Conformers with green fluorescencea rising from keto emission were not observedi nt his test case, but we know that they exist, as shown by the spectroscopy of HBT in non-biological environments. [16,18] The results demonstrate how ESIPT tuning may enableP BT derivatives to serve as fluorescentp robeso fm utations in TK, as any modifications in the protein active site should result in ad ifferent conformational distribution and, therefore, ad ifferent emission spectrum.…”

“…[15] In the physical chemistry community,t he HBT isomer 2-(2'hydroxyphenyl)benzothiazole is well knownf or its strong Stokes shift caused by excited-state intramolecular proton transfer (ESIPT). [3,5,[16][17][18] HBT undergoes ESIPT in the gas phase and in aprotic solution ( Figure 1). Upon photoexcitation, the first singlet excited state (S 1 )o ft he enol form is populated.…”

“…species. [17,18] ESIPT can also be followed by other relaxation processes affecting fluorescencei ntensities:i ntersystem crossing (ISC) with formation of the keto tautomer in the triplet ground state (T 1 )o ri somerization of the cis-t ot he trans-keto form, which will proceed through aS 1 /S 0 crossing region and may thus lead to internal conversion (IC). [17] This scenario opens the possibility of creating two-color fluorescentmarkers.…”

π–π Interactions: Influence on Molecular Packing and Solid‐State Emission of ESIPT and non‐ESIPT Motifs

“…TPA is nonplanar, and therefore it can prevent interaction or close stacking . The BZT unit helps to achieve solid‐state emission by reducing self‐absorption . Based on this idea, we have synthesized D–π‐A‐type chromophores I – III (Figure ), and systematically examined the relationship between the solid‐state fluorescence quantum efficiency and molecular packing.…”

Towards NIR‐Active Hydroxybenzazole (HBX)‐Based ESIPT Motifs: A Recent Research Trend

“…HBX mainly have electron‐donating or electron‐withdrawing substituent directly attached at A, B, C, and F site (see Figure ) . Most of the HBX derivatives with electron‐donating or electron‐withdrawing substituents on either side of the rings emit in blue to the yellow region some of them are listed in Figure .…”