Ultrafast Intramolecular Charge Transfer Dynamics and Nonlinear Optical Properties of Phenothiazine-Based Push–pull Zinc Porphyrin

Comprehensive investigations of the ultrafast photophysical properties and charge injection dynamics (adsorbed on mesoporous TiO2) of phenothiazine-functionalized push-pull Zinc porphyrin (LG6) utilizing a benzothiadiazole (BTD) as an acceptor unit is presented herewith. This includes density functional theory (DFT) calculations, fluorescence measurements, time-dependent fluorescence measurements, and femtosecond transient absorption (fs-TA) spectroscopy measurements. The charge injection dynamics of LG6 were compared with another porphyrin, LG5, with a similar molecular structure but thiophene as the acceptor unit instead of BTD. The multicomponent electron injection analysis from different excited electronic energy levels reveals ultrafast electron injection times of 434 fs and 336 fs for LG6 and LG5, respectively, revealing the impact of acceptor units on charge injection times. Further, femtosecond nonlinear optical (NLO) properties were characterized using the femtosecond Z-scan measurements with 70 fs, 800 nm pulses. The analysis of Z-scan open aperture data has been performed by solving three-level rate equations incorporating excited-state lifetimes calculated from the target/ global analysis of fs-TA data set. The estimated NLO parameters and ultrafast NLO response times of LG6 indicated the strong potential for optical limiting and optical signal processing applications. The second hyperpolarizability was estimated to be gamma = 2.5 x 10-31esu, which was found to be in close agreement with the theoretically calculated value of 0.4 x 10-31e.s.u.