Impact of 4,4′-Functionalization on the Electronic and Optical Properties of Biphenyl and 9,9′-Dimethylfluorene

Understanding how functional groups modulate the electronic and optical properties of small organic molecules is essential for designing better devices such as solar cells and luminescence sensors. As such, we performed DFT and TDDFT calculations on three previously studied molecules—benzene, biphenyl, and 9,9′-dimethylfluorene—functionalized with electron-donating groups (amino, methoxy) and electron-withdrawing groups (chloro, cyano, nitro). The results showed that functionalization with amino, methoxy, chloro, and cyano groups generates coherent states, with amino and cyano having the most coherence strength that is reflected in the UV-Vis absorption spectra. In contrast, functionalization with the nitro group introduces charge transfer character into the molecules, resulting in distinct UV-Vis and fluorescence spectral features compared to the other functional groups. For monomers, all functional groups caused redshifts and decreased intensities compared to benzene. In dimers, electron-donating groups produced more pronounced redshifts and lower intensities than electron-withdrawing groups.