Investigation of third-order nonlinear optical properties in two novel ethyne-linked chromophores: Effect of donor-acceptor alternation

Two novel compounds of the alternating donor (D)/acceptor (A) system were designed and synthesized. In BSB (A-π-D-π-A) and SBS (D-π-A-π-D), thiophene, 2,1,3-benzothiadiazole (BTD) and ethylene act as donor, acceptor groups and π-bridge moiety, respectively. Z-scan experiments at 532 nm with various pulse durations show a transition from saturable absorption to reverse saturable absorption and self-defocusing effects in both compounds. The different alternating modes of D-A units lead to an order of magnitude increment in the second-order hyperpolarizability γ of SBS (4.4 × 10⁻²⁸ esu) compared to BSB. Ultrafast transient absorption spectroscopy was carried out to analyze the enhancement of second-order hyperpolarizability. Results indicate that this enhancement originates from excited state absorption of the long-lived intramolecular charge transfer state. Using quantum chemical calculations, the stronger symmetrical intramolecular charge transfer in SBS can be demonstrated by the difference between the ground- and excited-state dipole moments Δμ_ge. It is shown intuitively in the S₀–S₁ hole-electron analysis. All of the experimental and theoretical results suggest that changing the D-A alternation in organic molecules can effectively modulate the third-order nonlinear optical behavior, which may serve as a guide for future molecular design.