Systematic Computational Designing of Efficient Phenalene and Pyrene Based Derivatives to Tune the Optical and Nonlinear Optical Response.

Nonlinear optical (NLO) materials are among the smartest materials of the present era and are employed to modulate the phase and frequency of the laser. The present study presents a quantum chemical framework for tailoring nitrogen/boron-doped (N/B) derivatives of phenalene and pyrene through the terminal and central core modifications to check their NLO properties. The derivatives of these compounds have been designed by introducing various π-conjugated connectors and N/B heteroatoms in the phenalene and pyrene rings. Density functional theory (DFT) methods are used to optimize the ground state molecular geometries of designed compounds, represented as Phe-1 to Phe-4 (phenalene derivatives) and Pyr-1 to Pyr-4 (pyrene derivatives) at the M06-2X/6-311G* level of theory. A systematic impact of structural modulations is established by comparing first linear polarizability (α) and second hyperpolarizability (γ). For linear polarizability, Phe-4 has shown the highest value for α_iso and α_aniso values, which are 114.7 × 10^-24 esu and 197.9 × 10^-24 esu, respectively. For the second hyperpolarizability (γ), among the designed compounds, Pyr-4 has achieved the highest γ amplitude of 1929.9 × 10^-36 esu owing to its unique molecular structural design and the presence of strong donor-acceptor groups. The origin of higher γ amplitudes is attributed to its lower energy electronic transition and higher oscillator strength. Further analysis of electronic parameters, such as electron density difference (EDD) maps, the density of states (DOS), electrostatic potentials, transition density matrix (TDM) analysis, and frontier molecular orbitals (FMOs) analysis, demonstrated the more effective intramolecular charge transfer (ICT), resulting in a good NLO response. The compounds were also analyzed for their potential in photovoltaic applications based on open circuit voltage values between 2.144 eV to 0.395 eV and light harvesting efficiency ranging from 0.486 eV to 0.989 eV. These findings suggest that the designed compounds can be suitable NLO materials and possess significant potential for photovoltaic applications.