Investigation of interporphyrin charge resonance of dihedral angle controlled porphyrin dimers by resonance Raman spectroscopy and MO approaches

Strongly interacting porphyrin dimers as a form of cofacial or side-to-side linkage have been ideally suited for investigation on the structure-electronic property relationship in porphyrin dimers. The relative orientation and interchromophore distance determined by the dihedral angle between the two porphyrin units are key factors in controlling the interchromophoric interaction. In the present work, directly linked porphyrin dimers with a systematic change in the dihedral angle between the two porphyrin planes have been investigated by resonance Raman (RR) and emission spectroscopies. Extremely strong molecular orbital interactions between the porphyrin units by decreasing the dihedral angle give rise to strongly enhanced interporphyrin charge-resonance transitions. With a decrease of the dihedral angle, unique RR spectra of the porphyrin dimers were also observed depending on the excitation wavelengths of 406.7, 457.9, and 488.0 nm. With the aid of supermolecular MO approaches it was found that the charge-resonance band at ca. 450 nm involves a_1u^A→(1/√2)(e_gx+e_gy) ^B transition. Upon excitation at this transition, the Raman modes involving ν(C_αC_β) and ν(C_βC_β) motions such as ν₃₈, ν₁₁, and ν₃ are predominantly enhanced. The MO calculation also revealed a considerable mixing between the charge-resonance transition and the excitonic transition with a decrease of the dihedral angle. The fast component in the fluorescence decay (ca. 500 ps) was found to increase in its contribution as the dihedral angle decreases, which was tentatively interpreted in terms of the vibronic mixing of the Q-band mixed with the charge-resonance transition.