Abstract
9,9-dioctyl-9H-fluorene (9DOF) is one of the most important molecule structures with unique photophysical properties. We investigated the Time-Dependent Density Functional Theory (TD-DFT), optical, morphological, ellipsometry, and laser characteristics of an oligomer 9,9,9′,9′,9″,9″-hexakis(octyl)-2,7′,2′,7″-trifluorene (9HOTF) with 9DOF as the core structure. The molecules were subjected to toluene cavitation for the TD-DFT studies, which showed that the high oscillator strength (ε) of the oligomer is 2.714 at 350 nm. The absorption spectra of the oligomer films were redshifted to different degrees compared to the solution, indicating the formation of a self-ordered β-phase. Oligomer films showed that β-phase aggregation increases with time and heating–cooling cycles; and influences the absorption and emission spectra. The computational studies of optical properties of 9HOTF were studied using Gaussian 06 software. Variable-angle spectroscopic ellipsometry (VASE) showed an excellent fitting for an extensive range of wavelengths, indicating that the calculated refractive index of the oligomer is very reliable. Optical parameters of the oligomer, such as bandgap, refractive index, and extinction coefficient, were also calculated. The oligomer produced intense amplified spontaneous emission (ASE) at 420 nm (fresh film) and 412 ± 2 nm (film after a day), attributed to the emission S0-1 vibration band with a full-width half-maximum (FWHM) of approximately 7 nm, when pumped using 355 nm, 5 ns Nd: YAG laser. Using the calculated refractive index, the distributed feedback (DFB) Bragg length (Λeff) was estimated to be 550 nm for n = 4. Holographic reflection grating with Λeff = 500 nm and 555 nm were selected, and 9HOTF was dropped and spin-coated to form optically pumped DFB lasers. The FWHM of the designed lasers were 1.0 ± 0.2 nm and 1.0 ± 0.2 nm, centered at 429 nm and 464 nm.