Tuning NIR Absorption and Emission of Diphenyl-Dihydrophenazine-Based Merocyanines with Ultra Narrow Band Gap.

Five merocyanine derivatives with a 5,10-diphenyl-dihydrophenazine donor and various indanone-and indandione based-acceptors with one or two dicyanovinylene groups were prepared by Knoevenagel condensation for tuning absorption and fluorescence in the near-infrared region. Molecular conformation, bond length alternation, and molecular packing in the solid state were studied by X-ray diffraction of single crystals in combination with density functional theory (DFT) calculations. By enhancing electron-accepting ability, a considerable decrease of lowest unoccupied molecular orbitals (LUMO) energy by 1.01 eV and retained highest occupied molecular orbitals (HOMO) energy within 0.13 eV were estimated by cyclic and rotating disc electrode voltammetry, relating semi-quantitatively to DFT prediction. Optical properties in solutions with various polarity, neat amorphous films, and crystalline powder states were studied. The absorption maxima of the neat films evolved from 545 nm to 931 nm. An ultranarrow optical band gap of DPPZ-IDD (1.09 eV) was found from the onset of thin film absorption and well agreed with the electrochemical gap of 0.93 eV. Detectable fluorescence in the NIR region was observed in the film and polycrystalline powder states.