Photochromic materials refer to a class of materials that can change color after being excited by a light source. The exploration of such materials has aroused a surge of interest in the field of materials science, mainly due to their potential applications in optical data storage and optoelectronic devices. During photochromic process, several isomerizations occur, including trans-cis conversion, cleavage of C-O bond and electrocyclization. Although photochromic families with excellent properties have been developed, studies were mainly confined to that of the organic system in their early development. A systematic investigation of the design of photochromic ligands and their transition metal complexes has not been reported until the last few years. The design and synthesis of novel structures and high-performance coordinated complexes are expected to facilitate the rapid development of the future of optoelectronics.
Energy transfer paths
In metal-sensitized photochromic complexes, there exist two significant energy transfer paths: first one is metal centre to high-energy isomer of photochromic core; The second is metal centre to low-energy isomer. Setting the most classic diarylethene as an example, upon UV light irradiation, absorption band shift form UV to visible region along with isomerization from high-energy open isomer to low-energy closed isomer. The first energy transfer path, which is from triplet metal-to-ligand charge transfer (3MLCT) state to triplet intraligand (3IL) state of open isomer, directly promotes sensitized photochromism. After cyclization, energy level of 3IL state goes down, resulting in the second energy transfer path (from 3MLCT to 3IL state of closed isomer). According to rule of energy transfer, the second path is more efficient, which may lead to luminescence quenching. Thus, through switching between these two paths can both energy manipulation and metal-assisted photochromism be achieved.
Among all the photochromic complexes, the lanthanide complexes are the most representative class. It has unique characteristics, such as narrow emission band, which enable them to show clear emission of color with high color purity, and wide variation of emission color from the visible to near-IR range depending on the central metal ion. Because the emission of the lanthanide (III) ions mainly comes from the electric dipole (ED) transitions in 4f orbitals, the wavelength of the emission lines of their complexes is insensitive to the nature of the ligand and coordination structure. Lanthanide complexes have been widely used as photochromic materials. For example, Nakai and co-workers  have demonstrated that the photoresponsive gadolinium (III) complex is a useful compound to construct a unique luminescence photochromic system. Upon irradiation (λirr< 405 nm), blue emission of gadolinium (III) complex changes to intense red emission that reverts to the original blue emission after O2 exposure. In addition to gadolinium complexes, luminescence modulation using various lanthanide complexes such as neodymium, europium and dysprosium with photochromic units has also been extensively investigated in recent years.
Figure 1. Unique luminescence photochromism of gadolinium (III) complex (3.3×10-6 M) in THF at room temperature (λex=365 nm).
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- Nakai. H.; et al. Reversible switching of the luminescence of a photoresponsive gadolinium (III) complex. Angew. Chem. 2013, 125: 8884-8887.