Theoretical insights into the optoelectronic properties of (1,1′-biphenyl-1-H-imidazol-2-yl)methanone derivatives: DFT, CDFT and TD-DFT analysis

Abstract

In this study, the compounds (4′-diphenylamino)-[1,1′-biphenyl]-4)(4-(4′-(diphenylamino)-[1,1′-biphenyl]-4)-1-methyl-1H-imidazol-2-yl)methanone (L1-Methyl), (4′-diphenylamino)-[1,1′-biphenyl]-4)(4-(4′-(diphenylamino)-[1,1′-biphenyl]-4)-1-octyl-1H-imidazol-2-yl)methanone (L1-Octyl), (4’-9H-carbazol-9-yl)-[1,1′-biphenyl]-4)(4-(4’-9H-carbazol-9-yl)-[1,1′-biphenyl]-4)-1-methyl-1H-imidazol-2-yl)methanone (L2-Methyl), (4’-9H-carbazol-9-yl)-[1,1′-biphenyl]-4)(4-(4’-9H-carbazol-9-yl)-[1,1′-biphenyl]-4)-1-octyl-1H-imidazol-2-yl)methanone (L2-Octyl) were evaluated for optoelectronic properties in the gas phase, using Density Functional Theory (DFT), Conceptual Density Functional Theory (CDFT) and Time-Dependent Density Functional Theory (TD-DFT) calculations. Through TD-DFT calculations, various charge transfer (CT) parameters, such as the disparity between the overall extent of electron-hole spatial extension and the distance of CT (t_index), overlap parameter (S_r), electron-delocalization index (EDI), hole-delocalization index (HDI), binding energies, excitation energies, transition density matrices (TDM), transition dipole moment densities (TDMD), ghost hunter indices, and charge transfer spectra (CTS), were employed to investigate the electron excitation processes in five different excited states, revealing a comprehensive description of these excited state parameters. Notably, local excitations were observed consistently throughout all excited states in the compounds, which is desirable for luminescence. Moreover, a correlation between electron-hole overlap and the length of the alkyl chain was also observed, which was consistent with various other parameters such as C_ele, C_hole, RMSD of electron, RMSD of hole, reorganization energies (λ_e and λ_h), electronic coupling (t_e and t_h), ionization potential (IP), electron affinity (EA), hardness (η), electronegativity (χ), electron donating powers (ω⁻) and electron accepting powers (ω⁺), localized orbital locator π-over plane (LOLIPOP), radiative lifetimes (τ), molar emission coefficients (ε_em), and molar absorption coefficients (ε_Abs). Through CDFT calculations, a potential radical attack on the carbonyl moiety of each compound was identified, explained using the Intrinsic Bond Strength Index (IBSI^IGMH) derived from the Hirshfeld partition. This descriptor can aid in enhancing the lifespan of OLED devices. Covalent character was observed across all the compounds, which could contribute to their stability and performance in optoelectronic applications.