DFT and Experimental Investigation of the Optoelectronic Properties of TFB Polymer for Lasing Application

The current work studies the electronic and optical properties of a conjugated polymer Poly(9,9-dioctylfluorene-alt-N-(4-sec-butylphenyl)-diphenylamine) (TFB) after dissolving in tetrahydrofuran (THF) in different concentrations ranging from 2.5 to 0.6 mg/mL. It shows an absorption maxima at 390 nm, however, an increase in concentration (2.5 mg/ml) gives an additional band at 482 nm. This additional band suggests the presence of ground-state aggregation. On the other hand, its fluorescence spectrum at different concentrations ranging from 30 to 1.88 mg/mL showed two distinctive bands around 433 and 451 nm and a tail at 480 nm. The experimental optical bandgaps of TFB are evaluated to be 2.95-2.83 eV depending on its concentration. Its computed electronic structures (n = 1 to 4) are also explored by using the density functional theory (DFT). The observed fluorescence emission of TFB can be represented by the superposition of three modified Franck-Condon progressions. The transition energy (E₀) is red-shifted for all Franck-Condon Fits due to increasing the concentration of TFB. The calculated HOMO-LUMO bandgap and absorption spectra were compared with the experimental result. The amplified spontaneous emission (ASE) characteristics of TFB in THF at and under various pump power excitations were studied. The results show the ASE peaked at 443 nm with an FWHM of approximetely11 nm, and the threshold energy was 2 mJ.