Evidence for π-π interactions in the S1 state of zinc porphyrin dimers revealed by picosecond time-resolved resonance Raman spectroscopy

The S-1 states of Zn(II)-porphyrin dimers have been investigated with picosecond time-resolved resonance Raman spectroscopy. The transient absorption and Raman spectra of porphyrin dimers, in which two Zn(II)-porphyrins are covalently linked at the ortho or meta positions of phenylene spacers, are compared with those of their component monomer unit. Although the Q-band of the ortho dimer was definitely different from those of the meta dimer and reference monomer, the ground-state Raman spectra of the ortho and meta dimers are nearly the same as that of the monomer, suggesting that the porphyrin pi-pi interactions strongly appear in the excited stare but little in the ground state in the ortho dimer. Several characteristic Raman bands were observed for the S-1 excited state at 2 ps after photoexcitation. The monomer S-1 state gave the marker bands at slightly lower frequencies (by 3-4 cm(-1)) than the corresponding ground state, and they showed no frequency shifts with time between 2 and 300 ps. On the contrary, in the case of the ortho dimer S-1 state, two characteristic bands (nu(2), nu(4)) appeared at frequencies significantly lower (by 10-13 cm(-1)) than the corresponding ground-state bands, and in addition, the frequency of the nu(4) band exhibited an upshift around 10-20 ps following photoexcitation. This frequency shift of the ortho dimer was appreciably perturbed by steric hindrance between the two porphyrins groups when bulky tert-butyl groups at the para position were incorporated. The behaviors of transient Raman bands of the meta dimer appeared intermediate between the monomer and the ortho dimer. These observations give the first clear evidence for the presence of pi-pi interactions in the S-1 excited state of porphyrin dimers with phenylene spacer and the occurrence of geometric relaxation toward the monomer-type structure in several tens of picoseconds.