The Effect of Terminal Methoxyl Substitution on Photoabsorption and Charge Transfer Characteristics of Diketo-Pyrrolo-Pyrrole Derivatives.

The Diketo-Pyrrolo-Pyrrole (DPP) chromophore has emerged as a promising building block for nonlinear optical material owing to its extended π-conjugation and superior electron transport properties. This work establishes a systematic structure-property relationship by investigating how terminal substituents modulate two-photon absorption (TPA) and intramolecular charge transfer (ICT) characteristics in DPP-based D-π-A-π-D structural dyes through density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations combined with visualization analysis. Significantly, our results demonstrate that terminal electron donors with enhanced electron donating abilities substantially facilitate the ICT characteristics as well as the TPA response through orbital energy level modulation. DPP-2 with strong electron donor triphenylamine as terminal groups, gains four times enhancement compared with DPP-1. Further methoxyl substituted at the terminal of triphenylamine groups, red-shifts TPA peak wavelength approximately 36 nm. Notably, terminal methoxyl substituents at the end of triphenylamine induce unique electron cloud redistribution, which not only strengthens the ICT effect but also activates unexpected aggregation-induced emission (AIE) characteristics. These findings are systematically validated through the simulated transition density matrix (TDM) and charge density difference (CDD) diagrams as well as the vibrational frequency calculations. The revealed mechanism may provide molecular-level insights for design and synthesize of DPP derivatives with integrated TPA enhancement and AIE functionality, offering a rational design strategy for developing advanced photonics materials in bioimaging and optoelectronic applications.