Theoretical study on photoisomerization effect with a reversible nonlinear optical switch for dithiazolylarylene

Area of Science:

• Computational Chemistry
• Materials Science
• Photochemistry

Background:

• Photoisomerization is a crucial phenomenon in molecular switches and photochromic materials.
• Understanding the electronic and structural changes during photoisomerization is key to designing advanced materials.
• Dithiazolylarylene derivatives show potential for photoresponsive applications.

Purpose of the Study:

• To investigate the photoisomerization effect in dithiazolylarylene derivatives using computational methods.
• To elucidate the factors influencing the photoisomerization quantum yield and mechanism.
• To analyze the changes in optical and nonlinear optical properties upon photoisomerization.

Main Methods:

• Density Functional Theory (DFT) and Time-Dependent DFT (TDDFT) calculations were employed.
• Calculations included UV-Vis absorption spectra, Electron Circular Dichroism (ECD) spectra, and Potential Energy Surfaces (PES).
• Second-order nonlinear optical (NLO) responses were evaluated for both open and closed isomers.

Main Results:

• Weak S···N interactions and CH···N hydrogen bonds stabilize the open isomer, leading to a high photoisomerization quantum yield.
• A significant bathochromic shift from 320 nm (open isomer) to 647 nm (closed isomer) in UV-Vis spectra confirms the photochromic behavior.
• The closed isomer exhibits distinct Cotton effects in ECD spectra, dominated by intramolecular charge transfer (ICT) and local excitation (LE).
• Photoisomerization from open to closed isomer is efficient in the photoexcited state but thermodynamically forbidden.
• The closed isomer shows a nearly six-fold increase in second-order nonlinear optical response compared to the open isomer.

Conclusions:

• The study successfully explains the photoisomerization mechanism and properties of dithiazolylarylene.
• The dramatic changes in NLO properties upon photoirradiation highlight potential applications in photochromic materials and reversible NLO switches.
• Computational methods provide valuable insights for designing novel photoresponsive molecular systems.