Porphyrinic chromophore, excited states lifetime

Let us first consider the situation when porphyrin chromophores are excited. In this case, only electron transfers are possible within the triads, and also within the model dyads (not represented) used as reference compounds. Upon excitation of the iridium chromophore in the UV, an ET process to both porphyrins occurs. In these systems, it is interesting to note that the nature of the solvent used for the photophysical studies has a strong impact on the photoinduced processes and on the lifetimes of the CS states generated. These data are summarized in Figure 13.61 in the form of energy diagrams. 

Moore, Gust, and coworkers synthesized the quinone-porphyrin-carotenoid (Figme 5) triad molecule. Upon excitation of the porphyrin moiety, initial charge separation occurred between porphyrin and quinone. Hole shift from porphyrin to carotenoid formed the final charge-separated state, that is, quinone radical anion and carotene radical cation, with a lifetime of 170 ns. These processes were confirmed by means of the picosecond and nanosecond laser flash photolysis. Their covalent bonding system was extended to tetrad and pentad using similar chromophores. 

The ultimate single molecule detection limit is set by the probability with which a typical chromophore can emit a photon within a sub-picosecond time window. Even for the best chromophores with large transition dipoles and correspondingly high radiative rates this probability is low. The instantaneous brightness of a fluorophore is determined by the coefficient of spontaneous emission and hence by the extinction coefficient of the molecule. For an excellent molecular emitter with a radiative lifetime of 1 ns, such as for example the S2 state of porphyrins (smax = 600 000), the probability of emission within the initial 100 fs following excitation is only approximately 0.1%. Therefore, the molecule must be excited approximately 1000-times in order for one photon to be emitted with the specified 100 fs time window. Naturally, more than one photon must be collected in order to determine the dynamics of the system of interest and the collection efficiency of even the best microscope is far from 100%. 

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