We have carried out a femtosecond transient absorption spectroscopic study on nickel(II) porphyrins in various solvents in order to obtain detailed information on vibrational relaxation processes occurring in the initial stage after photoexcitation to the highly excited states. We found the decay process of time constant of approximately 1 ps corresponding to the intramolecular vibrational relaxation process for Ni(II)TPP and Ni(II)OEP in toluene. In addition to this process, the intermolecular vibrational relaxation process with 10−20 ps lifetime was also observed for Ni(II)OEP in toluene, although its contribution to the overall decay process is relatively weak probably due to the weak solute/solvent interaction. In coordinating solvents such as pyridine and piperidine, we observed the intramolecular vibrational relaxation processes before complete population of the bottleneck excited metal 1|0,d z 2 〉 or 3|0,3(d,d)〉 state. In this case, it is likely that the intermolecular vibrational relaxation process associated with photodissociation/photoassociation processes depending on the selective excitation of four- and six-coordinate species is accompanied by the intramolecular vibrational relaxation due to the strong solute/solvent interaction. These processes are also believed to be responsible for the excess energy dissipation of highly excited nickel(II) porphyrins into the surrounding solvent molecules.

To elucidate the interaction mechanism of water-soluble copper(II) tetrakis(4-N-methylpyridyl)porphyrin (CuII(TMpy-P4)) with synthetic polynucleotides such as poly(dA-dT)2 and poly(dG-dC)2, the exciplex formation dynamics of photoexcited CuII(TMpy-P4) with added polynucleotides have been investigated by using femtosecond transient absorption as well as transient Raman spectroscopic methods. The nanosecond transient Raman spectra of CuII(TMpy-P4) mixed with poly(dA-dT)2 clearly demonstrate the exciplex formation between photoexcited CuII(TMpy-P4) and poly(dA-dT)2. On the other hand, the exciplex formation of photoexcited CuII(TMpy-P4) with poly(dG-dC)2 is not so manifest as compared with that mixed with poly(dA-dT)2. The transient absorption of CuII(TMpy-P4) mixed with poly(dG-dC)2 exhibits a rise component of 1.3 ps in addition to the very slow decay component (τ ∼ 22 ns). This observation is quite different from that of CuII(TMpy-P4) in aqueous solution or CuII(TMpy-P4) mixed with poly(dA-dT)2, because the exciplex formed in poly(dG-dC)2, if any, is so short-lived that it returns quickly back to four-coordinate CuII(TMpy-P4) intercalated at poly(dG-dC)2, which blocks the axial coordination by water molecules and consequently gives rise to a long lifetime (τ ∼ 22 ns).

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KIMbPPV was sensitized by ethyl viologen which is well known as a good electron acceptor, to achieve efficient ionization of the excitons created by photoexcitation of PPV. PPV/ethyl-viologen composites were successfully prepared from PPV-precursor and ethyl-viologen because both compounds could be soluble in polar solvents such as methanol, water etc. We observed dramatic decrease of photoluminescence efficiency in the PPV/ethyl-viologen composite film as compared with PPV film. This is attributed to photoinduced electron transfer in PPV/ethyl-viologen composites and the efficient ionization of the excitons was occurred in this composite film. Time-resolved photoluminescence of composite films was also studied to obtain the information of this photoinduced electron transfer dynamics.

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