Structural and Electronic Properties of D-π-A Coumarin Dyes: A DFT/TD-DFT Assisted Design of Low-Molecular-Weight, Long-Wavelength-Emissive Fluorescent Dyes.

This study employs density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations to systematically explore the structural and electronic properties of designed D-π-A coumarin dyes, aiming to develop low-molecular-weight fluorescent dyes with long-wavelength emission. The results emphasize the importance of substituent choice and positioning in determining dye spectral profiles. Specifically, placing an electron-withdrawing (EW) substituent at the 4-position of the coumarin ring results in longer emission wavelengths compared to substitution at the 3-position, while disubstitution at both the 3- and 4-positions provides a balance between emission wavelength and intensity. Introducing a julolidine group at the 7-position leads to red-shifted emissions and larger Stokes shifts, although this comes at the cost of reduced fluorescence intensity relative to N, N-diethylamino-substituted analogues. Notably, combining -COOH or -CN groups at the 3, 4-positions with either N, N-diethylamino or julolidine group at the 7-position offers a promising approach for tailored fluorescent dyes for optoelectronics and biological imaging. This work lays a theoretical foundation for guiding future synthesis and application of advanced fluorescent materials.