Photoinduced biological processes are complex and can often be reduced to a series of sequential events after the absorption of the photon. Each of these steps, if separable, can be compared with the evolution of a much simpler system, mimicking its essential characteristics, a biomimetic system. This stems from the very local properties of the initial event where a small reaction centre has been excited, spatially limited within the biomolecule. Gas phase conditions provide the unique way to study these model systems.
The aim of gas phase investigations in biomimetic molecules is to study the first step of this series of events, before excitation diffuses over the biomolecule system. Only time resolved studies are can resolve the initial evolution, often occurring within the femtosecond time scale. By the study of smaller model systems, we can investigate with the unprecedented detail and power of the experimental gas phase methods such processes that can be directly transposed to the related, parent biomolecule. Besides, only these “small systems” in the gas phase are amenable to quantum chemistry calculations and modelisations, they provide thus a basis for interpretation of the evolution in the more complex, real systems.
In this view, the electronic relaxation in metalloproteins containing as an active centre a metalloporphyrin can modelled by simple metalloporphyrins (tetraphenyl Porphyrins , TPP or octaethyl porphyrins, OEP). For photoexcited processes, intramolecular relaxation (electronic) is essential to characterise, since it will compete strongly with intermolecular energy transfers reactions or phototherapy. Therefore the chromophore (the metalloporphyrin), in which the relaxation occurs should be studied independently of the protein.
A systematic study of the ultrafast decay of metalloporphyrins containing various transition metals with partially filled 3d shells and zinc (3d filled), after excitation in the second excited state of the system (Soret band) has been undertaken. Both time of flight mass spectrometry and velocity map imaging have been used for detection. A general biexponential decay with a short time constant τ1~100 fs is observed for transition metal porphyrins, followed by a τ2~1 ps time decay. This evolution is interpreted as a porphyrin to metal charge transfer, τ1, followed by a back transfer, τ2 then leading to an excited state (d,d*) localised on the metal. These conclusions stem from the different behaviour of zinc and transition metal porphyrins. The porphyrin-to-metal charge transfer model is chosen to describe the relaxation mechanism, based upon the fact that transition metalloporphyrins can accept electrons on the metal site, in difference with zinc porphyrins.