Enhancement of ultrafast nonlinear absorption by different substituted positions in pyrene-containing twistacenes isomers

End-substituted planar and twisted isomers (C1 and C2) of pyrene-containing twistacenes were designed and synthesized to investigate the modulation of ectopic substitution on nonlinear optical (NLO) properties. Results of ultrafast transient absorption spectra reveal that C1 exhibits broad-band excited state absorption (ESA), meanwhile C2 displays an additional electronic delocalized state due to charge transfer. The twisted structure caused by large steric hindrance in C2 is thought to be the cause of enhancement according to the density functional theory calculation. Moreover, the evolution of ESA regarding the transition from local excited (LE) to intramolecular charge transfer (ICT) state is observed in the transient spectra. The performance of reverse saturable absorption at 480 nm, 5 ns, is improved by ESA originating from ICT in C2. The extension of π-conjugated system results in a significant enhancement of the two-photon absorption (TPA) cross-section of C1/toluene from 235 to 737 GM at 532 nm. With different substitution positions of terminal group in isomers, TPA and ESA of these chromophores can be dramatically modulated. This work may provide an idea for the optimization of nonlinear functional isomeric materials.