Photo-modulated wide-spectrum chromism in Eu3+ and Eu3+/Tb3+ photochromic coordination polymer gels: application in decoding secret information

Photo-switching emission of photochromic materials has paramount importance in the field of optoelectronics. Here, we report synthesis and characterization of a dithienylethene (DTE) based photochromic low molecular weight gelator (LMWG) and self-assembly with lanthanide (Eu³⁺ and Tb³⁺) ions to form a photochromic coordination polymer gel (pcCPG). Based on DTE ring opening and closing, the TPY-DTE gel shuttles from pale-yellow coloured TPY-DTE-O to dark blue coloured TPY-DTE-C and vice versa upon irradiating with UV and visible light, respectively, and both the photoisomers show distinct optical properties. Furthermore, integration of Eu³⁺ and Tb³⁺ lanthanides with TPY-DTE resulted in red and green emissive Eu-pcCPG (Q.Y. = 18.7% for the open state) and Tb-pcCPG (Q.Y. = 23.4% for the open state), respectively. The photoisomers of Eu-pcCPG exhibit photo-switchable spherical to fibrous reversible morphology transformation. Importantly, an excellent spectral overlap of the Eu³⁺ centred emission and absorption of DTE in the closed form offered photo-switchable emission properties in Eu-pcCPG based on pcFRET (energy transfer efficiency >94%). Further, owing to the high processability and photo-switchable emission, the Eu-pcCPG has been utilized as invisible security ink for protecting confidential information. Interestingly, mixed Eu³⁺/Tb³⁺ pcCPG exhibited photo-modulated multi-spectrum chromism reversibly where the colour changes from yellow, blue, and red to green and vice versa under suitable light irradiation.

A lanthanide based photochromic coordination polymer gel (pcCPG) material has been developed which showed photomodulated colour change based on pcFRET and has the potential to be employed for decoding secret information.