Comprehensive investigation of the ultrafast excited state dynamics and nonlinear optical properties of pyrene...

The intramolecular charge transfer (ICT) process is an essential phenomenon in nonlinear optical (NLO) materials. Tuning the ICT process in a molecule can be used to modify its linear or NLO properties. Over the years, ICT processes have been explored in organic molecules exhibiting "push-pull" characteristics, where donor-acceptor moieties are connected directly or via a π bridge. In this study, we synthesized three D-π-A pyrene derivatives, namely TA-4, TA-7, and TA-8, with varying push-pull characteristics to examine the effect of ICT on excited state absorption (ESA) and NLO properties using fs transient absorption spectroscopy (TAS) and Z-scan experiments, respectively. TAS showed the superposition of locally excited (LE) state based ESA and ICT-based ESA, effectively broadening the ESA at a longer time delay influencing their NLO properties. All three derivatives demonstrated a higher degree of ICT, and their relaxation dynamics were influenced by solvent polarity. The Z scan study indicates an enhancement in two-photon absorption value, and a variation in non-linear refractive index (n₂) at higher laser intensity, showing their potential for applications in nonlinear optical devices.