Thermochromic effects in a Jahn-Teller active CuCl^4- ₆ layered hybrid system

The hybrid material copper (II) tetrachloro-bis(phenyl ethyl ammonium) (C6H5CH2CH2NH3)2CuCl4, or PEACuCl, has been investigated by temperature- dependent spectroscopic absorption experiments. The absorption bands observed in the near-infrared region (1.3-1.9 eV) generally exhibit redshifts with increasing temperature. The temperature-induced energy shifts of the spectral components are shown to be consistently related to temperature-induced Cu-Cl bond length changes. Additionally, the thermochromic color change is caused by a charge transfer band edge redshifting (in the visible region 2.0-2.8 eV) with increasing temperature. By comparison with similar Cu-based systems, it is suggested that this shift is caused by broadening and strengthening of the band.