Hot carrier dynamics in metalated porphyrin–naphthalimide thin films

Abstract

This study employs femtosecond transient absorption spectroscopy to investigate the rapid dynamics of excited state carriers in three metalated porphyrin–naphthalimide (PN) molecules and one free-base molecule. The dynamics of electron injection, from PN to mesoporous titania (TiO₂), in PN adsorbed TiO₂ films (Ti-PN), were carefully investigated and compared to PN adsorbed ZrO₂ films (Zr-PN). In addition, we examined the self-assembled PN films and found that, in their self-assembled state, these molecules exhibited a longer relaxation time than Zr-PN monomeric films, where the charge injection channel was insignificant. The ground-state bleach band in the Ti-PN films gradually shifted to longer wavelengths, indicating the occurrence of the Stark effect. Faster electron injection was observed for the metalated PN systems and the electron injection times from the various excited states to the conduction band of TiO₂ (CB-TiO₂) were obtained from the target model analysis of the transient absorption spectra data matrix. In these metal–organic complexes, hot electron injection from PN to CB-TiO₂ occurred on a time scale of <360 fs. Importantly, Cu(II)-based PN complexes exhibited faster injection and longer recombination times. The injection times have been estimated to result from a locally excited state at ≈280 fs, a hot singlet excited state at 4.95 ps, and a vibrationally relaxed singlet excited state at 97.88 ps. The critical photophysical and charge injection processes seen here provide the potential for exploring the underlying factors involved and how they correlate with photocatalytic performance.