Photoswitching of dipole moments, charge-transfer and spectroscopic properties

The photoswitching of the dipole moment, absorption spectra and oxidation/reduction potentials of donor–acceptor-substituted dihydropyrene–[2,2]metacyclophanediene photochromic systems has been studied theoretically. Two types of basic structures can be formulated for the closed-ring dihydropyrene photoisomers: a conjugated betaine dye type and a merocyanine type structure, respectively. The thermodynamically less stable [2,2]metacyclophanediene forms have significantly larger dipole moments (>40 D) and longer wavelength absorption (CT transition in NIR region) in relation to the closed-ring dihydropyrene structures (<20 D and absorption in visible region). The reason for the strong changing of these properties is that the [2,2]metacyclophanediene fragment acts as a π-electron conjugation barrier which gives rise to a charge separation of the donor and acceptor substituents, respectively.