Impact of Tuning the Structure of Dibromo‑9 H ‑fluorene-9-ylidene Acceptor-Based Dyes on the Optical and Electronic Properties for Photonic Applications

Three new fluorene-9-ylidene (F-9-Y)-based donor–acceptor dyes with varying donor strength and linker length were synthesized and studied to understand their photophysical properties for potential optoelectronic applications. The dyes incorporate dimethylaniline (DMA), triphenylamine (TPA), and bis(diethylaniline) (B-DEA) as donor groups, with B-DEA having an extended allyl linker. Spectroscopic techniques (UV–vis, emission, Raman) and DFT calculations (B3LYP/6-31G(d)) were used to probe their electronic structure and charge transfer behavior. B-DEA exhibited broader absorption (360–600 nm) due to an additional intramolecular charge transfer transition (CT2), although CT2 showed a lower charge transfer character than CT1 in DMA and TPA. Stokes shifts, dipole moment changes, and excited-state geometries confirmed this. The extent of the conjugation and orbital alignment had a more pronounced effect on the charge transfer efficiency than the donor. Due to the nonradiative decay pathways, low quantum yield and short lifetimes were recorded for all dyes. These findings highlight how structural variations influence charge transfer properties and suggest promising applications in organic solar cells.