Tailoring the acceptor moiety of novel thiophene-based chromophores: Conjoined experimental and theoretical study on the nonlinear optical properties

Abstract

In this paper, we present a series of thiophene derivatives (ThD) with a schematic structure of D-π-A, in which we investigated the impact of the acceptor moiety design on nonlinear optical (NLO) properties. All molecules contain benzothiophene as an electron-donating group covalently bonded to the thiophene, which plays a role of a π-conjugated bridge. Ten different acceptor moieties were attached to the main core using the Knoevenagel reaction scheme to achieve novel rod-like derivatives. The UV-VIS absorption analysis supported by DFT calculations revealed that molecules enclosing prolonged conjugation length and nitro or dicyano groups as acceptors display the most profound intramolecular charge transfer (ICT) in the THF solution. Furthermore, samples were prepared as guest-host systems, where each ThD was embedded in a poly(methyl methacrylate) matrix and deposited on the glass substrate. By the means of second and third harmonic generation (SHG/THG), along with the optical Kerr effect (OKE) we established the most significant nonlinear optical parameters. The compounds with highly conjugated structures have shown SHG response of the 10−11 m/V order, which is a noteworthy result for purely organic systems. Moreover, the time of trans-cis-trans isomerization was recorded in the range of tens of microseconds, which may be competitive with commercially available azobenzenes. Presented results indicate that designed thiophene derivatives are promising candidates for optoelectronic and photonic applications, such as all-optical switchers or nonlinear microscopy. We clearly demonstrate that by applying small changes in the design of the acceptor moiety, it is possible to efficiently control and tailor the nonlinear response.