Study of excited-state dynamics of D-π-A thiophene derivatives: Enhanced excited-state absorption and refraction

Two D-π-A-type thiophene derivatives Br-TH and 3-TH with different donor units are designed and synthesized to investigate the effect of intra-molecular charge transfer (ICT) on their excited state absorption (ESA) and excited state refraction (ESR). The photodynamic evolution of these compounds in different polar solvents is analyzed by transient absorption spectra (TAS), and the electronic relaxation process from local excited-state (LES) to charge transfer state (CTS) is observed. Z-scan experiments indicate that ESA and ESR of these molecules can be significantly enhanced by extending the π-conjugated length. Phase object pump–probe (POPP) experiments further describe the photophysical mechanisms of CTS-based ESA and ESR of 3-TH. ESA cross section and ESR volume are obtained by establishing the equivalent five-energy level with corresponding rate equations. Results indicate that ICT performance can be effectively modulated by extending the π-conjugated length of these thiophene derivatives, and further improve their ESA and ESR properties. Furthermore, nanosecond optical limiting (OL) experiments indicate that compound 3-TH has excellent OL performance, which has potential application prospects in protecting sensitive devices or human eyes.