Comprehensive Experimental and DFT-Based Insights into the Structural and Optical Investigations of Nanocrystalline Coomassie Brilliant Blue R-250 Thin Films

This study presents a comprehensive investigation into the structural, optical, and electronic properties of nanocrystalline Coomassie Brilliant Blue R-250 (CBBR), combining experimental measurements with theoretical calculations. X-ray diffraction (XRD) confirmed a polycrystalline cubic phase in the CBBR powder with an average crystallite size of 60.3 nm and minimal lattice strain. In contrast, thin films exhibited an amorphous structure with significantly smaller crystallite sizes (∼2.54 nm) and elevated microstrain, attributed to rapid film formation. UV–Vis spectroscopy revealed both direct (3.58 eV and 4.81 eV) and indirect (2.19 eV and 3.10 eV) optical transitions, while an Urbach energy of 0.59 eV indicated moderate structural disorder. Dielectric analysis showed interband transitions with oscillator and dispersion energies of 7.626 eV and 25.6 eV, respectively, and a refractive index near 2.4, accompanied by a high extinction coefficient. Density Functional Theory (DFT) calculations using the B3LYP/6–311++G(d,p) level yielded a HOMO–LUMO energy gap of 2.624 eV, confirming semiconducting behavior. Additionally, the molecule displayed a strong third-order nonlinear optical (NLO) response, with hyperpolarizability values far exceeding those of standard NLO materials such as urea. The combination of these properties demonstrates the potential of CBBR thin films in advanced optoelectronic devices, UV photodetectors, and optical limiting technologies.