Metalated porphyrin-napthalimide based donor-acceptor systems with long-lived triplet states and effective three-photon absorption

Abstract

• Excited state electron dynamics of novel Metalated porphyrins-napthalimide molecules was investigated by femtosecond transient absorption spectroscopy (fs-TAS). • The rate constants of excited states are estimated using the global analysis of the obtained fs-TAS data. • Long-lived triplet state (lifetime ∼10 μ s for PN-Zn) formed via intersystem crossing. • Open aperture z-scan at non-resonant excitation of 800 nm revealed strong three-photon absorption by these molecules. • The experimental second hyperpolarizability was estimated to be ∼ 10 -31 esu and verified by DFT calculations. Porphyrin applications are primarily related to electronic transitions across a broad spectrum. In this context, understanding the excited state electron dynamics is necessary. Herein, we report the ultrafast photophysical characterization of one free base and three transition metallated porphyrin - napthalimide based donor - acceptor systems ( PN-Fb , PN-Ni , PN-Cu , and PN-Zn ) in dichloromethane (DCM) solution using transient absorption spectroscopy (TAS) in the spectral range of 430-780 nm by the Soret band excitation at 400 nm. The different rate constants were extracted using a target model analysis of the collected transient absorption spectra and established a consistent photophysical model of molecular excited state population relaxations. The photophysical model consisted of the following various processes: (a) internal conversion in the range of 215-400 fs, (b) vibrational relaxation in the range of 1.35-62 ps, and (c) singlet state relaxation times in the range of 0.17-2.06 ns, and finally (d) 0.03-10 μs was attributed to the triplet state lifetimes. We have also measured the nonlinear optical properties of these molecules by the open and closed aperture Z-scan studies, which was carried out by 800 nm, 70 fs photoexcitation. The theoretical fittings to the experimental data resulted in an effective three-photon absorption coefficient and positive nonlinear refractive index. Further, from the Z-scan data, the second-order hyperpolarizability of the molecules was calculated, and the values were 10 - 31 esu. Additionally, complete optimization of individual structures and construction of these PN systems was achieved using density functional theory (DFT) calculations and the obtained results were found to be consistent with the experimental observations.