A computational study for optical and nonlinear optical properties of distinctive V-shaped cyclopenta dithiophene derivatives

We systematically study the optical and nonlinear optical (NLO) properties of various cyclopentadithiophene (CPDT) derivatives using quantum chemical calculations. The designed derivatives are classified into three classes and each class have three types of derivatives as a, b and c. For example, compounds 1a, 2a, and 3a are formed by substituting nitrophenyl at the terminal positions of 2,6-diphenyl-4 H-cyclopenta[2,1-b:3,4-b′]dithiophene at the terminal positions to design V-shaped (Λ-shaped) compounds and similar pattered were adopted for other classes. The dipole moment (µ), linear isotropic and anisotropic polarizability along with second-order and third-order NLO polarizability are computed by applying density functional theory (DFT) methods at PBE1PBE/6-311G** level of theory. Among all the derivatives, 3a and 1a shows the maximum linear isotropic (α_iso) and anisotropic (α_aniso) polarizability of 120.4 × 10⁻²⁴ and 161.4 × 10⁻²⁴ esu respectively. The noteworthy effect of incorporating different electron donor and acceptor moieties to the overall NLO polarizability is observed. The largest average second-order NLO polarizability value is observed 92.59 × 10⁻³⁰ esu for system 1a. The β_// amplitude of 1a is ~ 1.5 to 11 times more as compared to other derivatives. Additionally, the nonlinear anisotropy of β_// amplitudes is also seen significantly larger ranging from 4.96 to 129.63 which is due to the V-shaped structure of designed compounds. While on the other hand, some compounds shows the third-order NLO polarizability amplitudes which are ranging in thousands including 4107 × 10⁻³⁶, 5020 × 10⁻³⁶, 5857 × 10⁻³⁶, 6412 × 10⁻³⁶, 6700 × 10⁻³⁶ and 9057 × 10⁻³⁶ esu for compounds 1b, 2b, 3a, 2a, 3b and 1a, respectively. Strikingly, the β_// and <γ> amplitudes of compound 1a are ~ 14 and ~ 1242 times greater than that of standard para-nitroaniline (prototype NLO molecule), respectively, as computed at same PBE1PBE/6-311G** method. The TD-DFT calculations are also performed to see origin of larger NLO polarizabilities. The molecular electrostatic potential maps and frontier molecular orbitals are drawn to get further insight into the structure-NLO property relationship of newly designed CPDT derivatives. Hence, we considered that the current investigation will interest the researchers to further explore the optical and NLO applications of these CPDT derivatives.