A PHOTOPHYSICAL INVESTIGATION OF NICKEL TETRAPYRROLE MACROCYCLES

A PHOTOPHYSICAL INVESTIGATION OF NICKEL TETRAPYRROLE MACROCYCLES (pdf)

ANDREY V. ZAMYATIN, Del Mar Photonics,Inc., 4119 Twilight Ridge, San Diego, CA 92130;
MICHAEL A.J. RODGERS, Center for Photochemical Sciences, Department of Chemistry, Bowling Green
State University, Bowling Green, Ohio, 43403.

The porphyrins are an important class of chromophores with high stability and ef cient light absorption ability in the visible and
near-infrared regions of the optical spectrum. Porphyrins have a set of highly delocalized 􀀀 -orbitals situated on the carbon-nitrogen
framework. When such a ligand is coordinated to a centrally-located metal atom of the transition series, having incompletely lled
d-orbitals (Ni, Co, Fe, Cu, etc.), the 􀀀 - and d-manifolds can interact. As a result the excited state deactivation dynamics becomes very
dependent on the electronic nature of the central metal. This work has two major parts. The rst part focuses on the investigation of dual
excited states of some Ni(II) porphyrins. Dual excited states are molecular entities that have absorbed two successive photons to generate
molecular states with electronic excitation in two distinct regions of the metalloporphyrin. The dual excited states were studied by means
of femtosecond two-pump-one-probe transient absorption spectroscopy. Two successive pulses of 400 nm and 550 nm were used for
excitation. Based on the kinetic and spectral information of the observed transients we propose a mechanism for the Ni(II) porphyrin dual
excited state deactivation involving formation of an intramolecular charge transfer state. The second part concerns the photophysical
studies of Ni(II) meso-tetraphenyl tetrabenzoporphyrin (NiTPTBP). The excited states metal-ligand dynamics of Ni(II)TPTBP have
been investigated using time-resolved absorption spectroscopy in different solvents. The DFT computations revealed the presence of
two ligand-to-metal charge transfer states located close in energy to the (d,d) state. These LMCT states are responsible for the lifetime
dependence on solvent polarity. Upon photoexcitation two competing processes depopulate the porphyrin lowest singlet excited state
S

. One involves the charge transfer states manifold and another involves the metal excited state manifold

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