Ligand Recruitment and
Spin Transitions in the Solid-State
Photochemistry of Fe<sup>(III)</sup>TPPCl
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Abstract
We report evidence for the formation of long-lived photoproducts
following excitation of iron(III) tetraphenylporphyrin chloride (Fe<sup>(III)</sup>TPPCl) in a 1:1 glass of toluene and CH<sub>2</sub>Cl<sub>2</sub> at 77 K. The formation of these photoproducts is dependent
on solvent environment and temperature, appearing only in the presence
of toluene. No long-lived product is observed in neat CH<sub>2</sub>Cl<sub>2</sub> solvent. A 2-photon absorption model is proposed to
account for the power-dependent photoproduct populations. The products
are formed in a mixture of spin states of the central iron(III) metal
atom. Metastable six-coordinate high-spin and low-spin complexes and
a five-coordinate high-spin complex of iron(III) tetraphenylporphyrin
are assigned using structure-sensitive vibrations in the resonance
Raman spectrum. These species appear in conjunction with resonantly
enhanced toluene solvent vibrations, indicating that the Fe<sup>(III)</sup> compound formed following photoexcitation recruits a toluene ligand
from the surrounding environment. Low-temperature transient absorption
(TA) measurements are used to explain the dependence of product formation
on excitation frequency in this photochemical model. The six-coordinate
photoproduct is initially formed in the high-spin Fe<sup>(III)</sup> state, but population relaxes into both high-spin and low-spin state
at 77 K. This is the first demonstration of coupling between the optical
and magnetic properties of an iron-centered porphyrin molecule