Intramolecular energy relaxation and competing electron transfer in porphyrin-acceptor supermolecule systems

Fluorescence dynamics has been studied for some porphyrin derivatives to elucidate the relation between the intramolecular energy relaxation and the electron-transfer (ET) reaction over a wide spectral range by using fs-fluorescence up-conversion technique. Zn-tetraphenylporphyrin (ZnTPP) in ethanol solution has shown fluorescence dynamics over the whole spectral range from 430 to 620 nm when excited to S₂ state, which reflects the intramolecular energy-relaxation dynamics. In the short wavelength region from 430 to 500 nm, the fluorescence has shown a single-exponential decay with a time constant of approx. 2.3 ps, whereas a single-exponential rise with the same time constant of approx. 2.3 ps has been observed at 600 nm. In the intermediate spectral region, fluorescence rise and decay showed more or less complex behavior. These results have been qualitatively interpreted by considering two relaxation paths, an immediately relaxing one as a main relaxation channel and a slower one in which molecules are once trapped in higher vibrational states of S₁. For Zn-diphenylporphyrin (ZnDPP)-electron acceptor supermolecule systems, we have confirmed that the rate of ET from S₂ state decreases with increasing of the free-energy gap for ET. This is a first unambiguous observation of the inverted effect for the charge-separation process. Furthermore, ZnDPP attached to naphthylimide has shown multiexponential S₁ fluorescence decay, which implies the influence of the intramolecular relaxation on the ET reaction.