Comparative study of photochromic ferrocene-conjugated dimethyldihydropyrene derivatives

The photochemical properties and the mixed-valence state of bis(ferrocenylethynyl)benzodimethyldihydropyrene (1) and other benzodimethyldihydropyrene (BzDHP) derivatives were investigated to understand the reversible photoswitching in the electronic communication of 1. Absorption spectra of 1 were characterized by UV/Vis spectroscopy and calculated by using time-dependent density functional theory (TD-DFT), and the d orbitals of the ferrocene (Fc) moieties were shown to contribute to the occupied valence orbitals that were responsible for the photochromic behavior. 1 exhibited reversible photoisomerization in THF; however, photochromic behavior was not observed in dichloromethane. Analysis of redox potentials showed that the mixed-valence state of 1 was more stable in dichloromethane than in THF. This is consistent with the observation that chemical oxidation led to an intervalence charge-transfer (IVCT) band between the Fc moieties in the mixed-valence state of 1 in dichloromethane, whereas such a band was not observed for one-electron-oxidized 1 in THF. Bis(pentamethylferrocenylethynyl)benzodimethyldihydropyrene (2) did not show photochromic behavior even in THF. The mixed-valence state of 2 was much less stable than that of 1 in dichloromethane, and no obvious IVCT band was observed for one-electron-oxidized 2 in dichloromethane. The difference in the redox contribution of Fc and pentamethylferrocene (Me5Fc) to BzDHP played an important role for these redox and photochromic behaviors; this was supported by analysis of valence orbital energies from DFT calculations. Designing molecules that connect redox centers through the use of a photochromic linker with a redox potential close to that of the redox centers could constitute a useful approach for the production of photochromic redox-active metal complexes with strong electronic communication.