The degree of charge transfer in ground and charge-separated states revealed by ultrafast visible pump/mid-IR probe spectroscopy.

We demonstrate a new femtosecond visible pump/mid-IR probe spectroscopic approach to assess directly the ground- and excited-state degrees of charge transfer (CT) in donor-spacer-acceptor (D-Sp-A) structures. Two classes of (porphinato)zinc(II) (PZn)-based D-Sp-A compounds with either quinonyl (Q) or N-(N'-octyl)pyromellitic diimide (PI) electron acceptors were interrogated. Carbonyl antisymmetric stretching mode frequency domain transient-IR spectra of these species were recorded and analyzed for the Q/PI moieties.

Interrogating conformationally dependent electron-transfer dynamics via ultrafast visible pump/IR probe spectroscopy.

We demonstrate for the first time the utility of time-resolved visible pump/mid-infrared (IR) probe spectroscopy to interrogate directly, and provide unique infomation regarding, conformationally dependent photoinduced ET dynamics and the subsequent structural evolution of the resulting charge-separated (CS) state. Exemplary polarized visible pump/IR probe experiments involving N-[5-(10,20-diphenylporphinato)zinc(II)]-N'(octyl)pyromellitic diimide (PZn-PI) and [5-[4'-(N-(N'-octyl)pyromellitic diimide)phenyl)ethynyl]-10,20-diphenylporphinato]zinc(II) (PZn()PI) show that it is possible to assess the mean PZn-to-PI interplanar torsional angle of electronically excited structural conformers that undergo ET within the sub-ps time domain for both of these donor-spacer-acceptor (D-Sp-A) systems. Further transient specroscopic experiments carried out on PZn()PI determine how this angle evolves with time.

Synthesis, electronic structure, and electron transfer dynamics of (Aryl)ethynyl-bridged donor-acceptor systems.

The ET dynamics of a series of donor-spacer-acceptor (D-Sp-A) systems featuring (porphinato)zinc(II), (aryl)ethynyl bridge, and arene diimide units were investigated by pump-probe transient absorption spectroscopy. Analysis of these data within the context of the Marcus-Levich-Jortner equation suggests that the pi-conjugated (aryl)ethynyl bridge plays an active role in the charge recombination (CR) reactions of these species by augmenting the extent of (porphinato)zinc(II) cation radical electronic delocalization; this increase in cation radical size decreases the reorganization energy associated with the CR reaction and thereby attenuates the extent to which the magnitudes of the CR rate constants are solvent dependent. The symmetries of porphyrin-localized HOMO and HOMO-1, the energy gap between these two orbitals, and D-A distance appear to play key roles in determining whether the (aryl)ethynyl bridge simply mediates electronic superexchange or functions as an integral component of the D and A units.

Synthesis, excited-state dynamics, and reactivity of a directly-linked pyromellitimide-(porphinato)zinc(II) complex.

N-[5-(10,20-Diphenylporphinato)zinc(II)]-N'-(octyl)pyromellitic diimide (PZn-PI), a meso-pyromellitimide-substituted (porphinato)zinc(II) compound, has been fabricated from the reaction of (5-amino-10,20-diphenylporphinato)zinc(II) with pyromellitic dianhydride in the presence of octylamine. Interrogation of the photoinduced charge separation (CS) and thermal charge recombination (CR) electron-transfer (ET) dynamics for PZn-PI in CH(2)Cl(2) via pump-probe transient absorption spectroscopic methods shows that tau(CS) and tau(CR) are 770 and 5200 fs, respectively. These ET dynamics differ from those elucidated previously for closely related 5-quinonyl-substituted (porphinato)metal compounds, and derive from the fact that the low-lying excited states for PZn-PI are porphyrin-localized, possessing little charge-transfer character.