π-Extended Octupolar Cyclized Indole Tetramer and Trimer Derivatives with Second- and Third-Order Nonlinear Optical Properties

Abstract

Several aromatic-fused octupolar tetraindole (TTI) and triindole (TI) derivatives have been synthesized via POCl3-promoted cyclopolymerization of the aromatic-fused oxindoles. As a result, the terminal phenyls of TTI and TI are fused with the phenyl group, indenyl group, and benzothienyl group separately in the π-extended target compounds TTIp and TIp, TTIid and TIid, and TTIbt and TIbt (including pairs of syn- and anti-isomers). Single-crystal X-ray diffraction disclosed that the tetramer syn-TTIbt keeps an S 4-symmetric architecture and crystallizes in the P42/n space group. With the extension of the π-conjugation, the absorption and emission bands of the above tetramers and trimers are remarkably red-shifted. The second-order nonlinear optical (NLO) coefficients (β value) of syn-TTIbt and anti-TTIbt have been calculated to be 1.5 and 2.1 times that of their parent compound TTI, respectively. In the domain of the third-order NLO, the two-photon absorption sections (δ value) of syn-TIbt and anti-TIbt have been measured to be 37 and 102 times that of TI, respectively. These results indicate that the π-extension strategy here is successful, and the anti-isomers have better NLO properties and better π-conjugation than their corresponding syn-isomers.

Significant Enhancement of Reverse Saturable Absorption Ability via Pt(II) Incorporation into the Conjugated Backbone of Benzo[1,2-b:4,5-b′]dithiophene-Based Donor–Acceptor Copolymers

“…Optical limiting (OL) materials based on nonlinear optical (NLO) responses, which show high transmittance for weak laser pulses and low transmittance for intense ones, have attracted increasing attention due to their potential application in laser protection. − In the past few decades, abundant excellent NLO materials have been elaborately designed and explored. ,− Representatively, the carbon-based materials (e.g., fullerene, graphene, carbon nanotubes) and organic materials (e.g., porphyrins, phthalocyanines, and related macrocycles), ,,,− , attributing to the extended π-conjugation, exhibit remarkable NLO performance. The main NLO mechanisms of carbon-based materials originate from the nonlinear scattering of the microbubbles and/or microplasmas, which are formed in their suspensions upon laser pulses. ,,, The dominant mechanism for organic NLO materials, on the other hand, is considered to originate from reverse saturable absorption (RSA), which mainly depends on the relative absorption section of the ground state and excited state and, more specifically, on the chemical structures as well as the molecular energy level of materials. ,,− , Therefore, the rational design of the π-conjugated molecular structure with suitable energy levels toward the functional wavelength of laser pulses is one of the effective strategies to explore outstanding organic NLO materials. ,,− Except for the construction of a π-conjugated backbone, (1) introducing an electron donor (D) and/or acceptor (A) into the conjugated backbone provides straightforward access toward remarkable NLO materials, attributing to the efficient intramolecular charge transfer (ICT) process from D to A (Figure a). − For example, Jia et al constructed D–A covalent organic frameworks with ultralow band gaps to improve the NLO property efficiently . (2) Incorporating a metallic atom into the π-conjugated backbone is another approach to facilitate the NLO response through the heavy-atom effect, which could lead to intersystem crossing (ISC) from the singlet excited state to the triplet excited state, attributing to the spin–orbital coupling (SOC; Figure b). ,−…”