Structure–Property Relationships of ITIC-Based Acceptor–Donor–Acceptor Type Molecules as Novel Organic Nonlinear Optical Materials

Abstract

Organic π-conjugated molecules incorporating with an electron donor (D) and/or acceptor (A) have been extensively applied in nonlinear optical (NLO) fields. The efficient electron transfers and delocalization in the molecular backbone can boost the NLO behavior effectively. Thus, the rational constructions of D−A type π-conjugated structures are one of the key approaches to facilitate the development of organic NLO materials and their applications. Because of the unique structure characteristics, ITIC-based A−D−A type molecules are promising NLO materials. However, there is no report about ITIC-based molecules as NLO materials so far; furthermore, their structure−property relationships remain unclear. Herein, five ITIC-based A− D−A type molecules (1−5) incorporating different electron-rich/-poor groups in the electron-donating core or electronwithdrawing groups were designed and synthesized. Meanwhile, D−A type compound 6 with the "half-structure" of 1 was synthesized for comparison. The relationships of the structure and NLO property were studied by the theoretical calculations and Zscan experiments. Expectedly, these molecules exhibit an NLO response originating from reverse saturable absorption (RSA) to a nanosecond pulsed laser at 532 nm, which is attributed to the strong intramolecular D−A interaction and planar π-conjugated backbones of molecules. The A−D−A type compound 1 exhibits an excellent NLO property compared to compound 6. Compounds with smaller band gaps exhibit more excellent NLO properties. The structurally tunable nonlinear extinction coefficients of A−D−A type compounds increase from 70 to 162 cm GW −1 at the representative input laser energy of 10 μJ. The length of the alkyl chain bearing on the side of the conjugated backbone has a slight effect on their NLO behavior although it has a significant influence on the performances of organic field-effect transistors and organic solar cells. Because of the RSA behaviors, these compounds can be applied as optical limiting materials and show laser limiting thresholds varying from 12.3 to 3.6 J cm −2 . This work exploits a novel application of ITIC-based A−D−A type molecules in organic NLO fields and, more importantly, affords a prototype of structure− property relationships as a guideline for the rational development toward outstanding materials.