Spectroscopic and Theoretical Studies of Optically Active Porphyrin Dimers: A System Uninterpretable by Exciton Coupling Theory

• Published in: Chemphyschem 2006-06, Vol.7 (6), p.1235-1240
• Main Authors: Muranaka, Atsuya, Asano, Yoshiaki, Tsuda, Akihiko, Osuka, Atsuhiro, Kobayashi, Nagao
• Format: Article
• Language: English
• Subjects: Ab initio calculations; Atomic and molecular physics; Calculations and mathematical techniques in atomic and molecular physics (excluding electron correlation calculations); circular dichroism; Density-functional theory; electronic structure; Electronic structure of atoms, molecules and their ions: theory; Exact sciences and technology; helical structures; Molecular properties and interactions with photons; Optical activity and dichroism; magnetooptical and electrooptical spectra; Optical activity, optical rotation; circular dichroism; Physics; porphyrins
• ISSN: 1439-4235; 1439-7641
• DOI: 10.1002/cphc.200500611

The electronic excited states of a meso–meso β–β doubly linked bis‐porphyrin are comprehensively investigated by measuring its circular dichroism (CD) and magnetic circular dichroism (MCD) spectra. The observed spectroscopic properties are rationalized by DFT calculations. The frontier molecular orbitals (MOs) are constructed by the linear combinations of the constituent monomers′ four MOs. Comparison of a theoretical CD spectrum based on time‐dependent DFT (TDDFT) with the experimental spectra resulted in the assignment of the helical conformation of the dimer. This assignment is contrary to the previous assignment based on the point‐dipole approximation (exciton coupling theory). Coupled chiral complexes: The helical configuration of a meso–meso β–β doubly linked bis‐porphyrin molecule is reassigned as right‐handed (see picture) by inspection of the experimental spectroscopic properties in conjunction with time‐dependent DFT calculations. This assignment is contrary to that previously reported on the basis of the exciton chirality method.