Substituent Positioning Controls the Trade-Off between Charge Transfer and ESIPT Kinetics in Disubstituted HBT...

Area of Science:

• Computational Chemistry
• Photophysics
• Organic Electronics

Background:

• 2-(2'-hydroxyphenyl)-benzothiazole (HBT) derivatives are important fluorophores.
• Understanding structure-property relationships is key for designing new materials.

Purpose of the Study:

• Investigate how substituent positioning affects photophysical properties of HBT derivatives.
• Elucidate the interplay between intramolecular charge transfer (CT) and excited-state intramolecular proton transfer (ESIPT).
• Establish a design paradigm for tuning emission color and ESIPT behavior.

Main Methods:

• Density Functional Theory (DFT) and Time-Dependent DFT (TD-DFT) calculations.
• Rigorous computational analysis including geometry optimization, excitation/emission energies, and potential energy curves.
• Investigation of substituent effects on CT and ESIPT pathways.

Main Results:

• Discovered an inverse relationship between emission wavelength and ESIPT barrier based on substituent topology.
• Meta-EWG/para-EDG configurations maximize CT, leading to longer emission wavelengths and higher ESIPT barriers (>4.5 kcal/mol).
• Ortho-EWG/para-EDG isomers maintain LE character with ultralow ESIPT barriers (<0.7 kcal/mol) but blue-shifted emission.

Conclusions:

• Substituent topology offers independent control over spectral tuning and ESIPT energy landscape in HBT fluorophores.
• This provides a generalizable strategy for designing HBT-based fluorophores with tailored properties.
• Enables rational engineering for applications in bioimaging and optoelectronics.