Ultrafast Relaxation Dynamics of Photoexcited Zinc-Porphyrin: Electronic-Vibrational Coupling

Cyclic tetrapyrroles are the active core of compounds with crucial roles in living systems, such as hemoglobin and chlorophyll, and in technology as photocatalysts and light absorbers for solar energy conversion. Zinc-tetraphenylporphyrin (Zn-TPP) is a prototypical cyclic tetrapyrrole that has been intensely studied in past decades. Because of its importance for photochemical processes the optical properties are of particular interest, and, accordingly, numerous studies have focused on light absorption and excited-state dynamics of Zn-TPP. Relaxation after photoexcitation in the Soret band involves internal conversion that is preceded by an ultrafast process. This relaxation process has been observed by several groups. Hitherto, it has not been established if it involves a higher lying ”dark” state or vibrational relaxation in the excited S₂ state. Here we combine high time resolution electronic and vibrational spectroscopy to show that this process constitutes vibrational relaxation in the anharmonic S₂ potential

Visualization 1: Ultraviolet femtosecond Kerr-gated wide-field fluorescence microscopy

Movie showing the transient evolution of a single ZnO nanowire excited at 11.5 mJ/cm2 over a period of 3 ps with a frame rate of 15 frames per second. Originally published in Optics Letters on 01 June 2016 (ol-41-11-2462

Heterogeneous Electron-Transfer Dynamics through Dipole-Bridge Groups

Heterogeneous electron transfer (HET) between photoexcited molecules and colloidal TiO₂ has been investigated for a set of zinc porphyrin chromophores attached to the semiconductor by linkers that allow the level alignment to be changed by 200 meV through reorientation of the dipole moment. These unique dye molecules were studied by femtosecond transient absorption spectroscopy in solution and adsorbed on a TiO₂ colloidal film in a vacuum. In solution, energy transfer from the excited chromophore to the dipole group was identified as a slow relaxation pathway competing with S₂–S₁ internal conversion. On the film, heterogeneous electron transfer was found to occur in 80 fs, much faster than all intramolecular pathways. Despite a difference of 200 meV in level alignment of the excited state with respect to the semiconductor conduction band, identical electron-transfer times were measured for different linkers. The measurements were compared to a quantum-mechanical model that accounts for electronic–vibronic coupling and a finite bandwidth for the acceptor states. We conclude that HET occurs into a distribution of transition states that differ from regular surface states or bridge-mediated states