Design of zinc porphyrin-perylene diimide donor-bridge-acceptor chromophores for large second-order nonlinear optical response: A theoretical exploration

The electronic structures and second-order nonlinear optical (NLO) properties of a series of zinc porphyrin (ZnP)-perylene diimide (PDI) donor-bridge-acceptor (D-π-A) molecules have been investigated using density functional theory (DFT) and time-dependent DFT (TDDFT). The results show that these compounds possess excellent second-order NLO properties and large static first hyperpolarizabilities (β₀) values on the order of 10³ – 10⁴ esu⁻³⁰. A DFT benchmark calculation of β₀ value was first performed, confirming the optimally tuned range-separated functionals (LC-BLYP* and ωB97X*) can produce similar magnitudes for β₀ as Møller–Plesset second-order perturbation (MP2) calculations. The magnitudes of β₀ values were studied as a function of different types of bridges: oligo-p-phenylenevinylene (OPV), carbon-bridged oligo-p-phenylenevinylene (COPV), oligo-thienylenevinylene (OTV), carbon-bridged oligo-thienylenevinylene (COTV), and also the length of π-bridges. The calculated β₀ values were found to be inversely related to the bond length alternation (BLA) values of various π-bridges. Next, we found that the static β₀ value of designed ZnP-COTV₁-PDI molecule can be significantly enhanced by finely tuning the dihedral angles between the π-bridge and ZnP donor or PDI acceptor. The calculated β₀ value is also sensitive to the substitution positions of PDI, that is, ortho-(α-), bay-(β-), nitrogen-(N-). A two-level model was proven to be useful for the qualitative description of the β₀ values and further the substantial oscillator strength and the difference in transition dipole moments between the ground and excited state mainly contribute to the large β₀ values. The presented work will be beneficial for potential applications of the ZnP-PDI-based chromophores in the optoelectronic devices and high-performance NLO materials.