Enhanced Reverse Saturable Absorption in Substituted Twistacenes from Visible to Near-Infrared: Modulation of Terminal Twisted ᴨ-Conjugated Units

Four substituted twistacenes decorated with different twisted ᴨ-conjugated units are synthesized to investigate their structure-property relationship. In order to improve the nonlinear optical (NLO) response of these twistacenes, phenyl, naphthyl, anthryl and pyrenyl are selected as the twisted units to gradually increase the population of ᴨ-electron. Broad-band excited state absorption (ESA) and reverse saturable absorption (RSA) of these compounds show significant enhancement in femtosecond transient absorption spectra (TAS) and multiwavelength Z-scan experiments, respectively. The photophysical mechanism exhibits a typical feature of two-photon absorption (TPA) induced ESA (TPA-ESA) on the singlet state. With the analysis of electron transition calculated by time-dependent density functional theory (TD-DFT), it can be inferred that TPA and ESA of these twistacenes are modulated by intramolecular charge transfer (ICT) and local excitation (LE), respectively. The TPA cross section of these twsitacenes at...