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.

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

The response of the spin state to in situ variation of the coordination number (CISSS) is a promising and viable approach to smart sensor materials, yet it suffers to date from insensitive detection. Herein, we present the synthetic access to a family of planar nickel(II) complexes, whose CISSS is sensitively followed by means of fluorescence detection. For this purpose, nickel(II) complexes with four phenazine-based Schiff base-like ligands were synthesized and characterized through solution-phase spectroscopy (NMR and UV-vis), solid-state structure analysis (single-crystal XRD), and extended theoretical modeling. All of them reveal CISSS in solution through axial ligating a range of N- and O-donors. CISSS correlates nicely with the basicity of the axial ligand and the substitution-dependent acidity of the nickel(II) coordination site. Remarkably, three out of the four nickel(II) complexes are fluorescent in noncoordinating solvents but are fluorescence-silent in the presence of axial ligands such as pyridine. As these complexes are rare examples of fluorescent nickel(II) complexes, the photophysical properties with a coordination number of 4 were studied in detail, including temperature-dependent lifetime and quantum yield determinations. Most importantly, fluorescence quenching upon adding axial ligands allows a "black or white", i.e. digital, sensoring of spin state alternation. Our studies of fluorescence-detected CISSS (FD-CISSS) revealed that absorption-based CISSS and FD-CISSS are super proportional with respect to the pyridine concentration: FD-CISSS features a higher sensitivity. Overall, our findings indicate a favored ligation of these nickel(II) complexes in the excited state in comparison to the ground state.

A Fluorescence-Detected Coordination-Induced Spin State Switch.

Bilayer spin crossover (SCO)@polymer nanocomposites show robust and controllable actuation cycles upon an electrical stimulus. The anisotropic shape of the embedded particles as well as the mechanical coupling between the SCO particles and the matrix can substantially intensify the work output of the actuators.

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Colossal expansion and fast motion in spin-crossover@polymer actuators.

The flexibility of long chain substituents influences spin-crossover in isomorphous lipid bilayer crystals.

Mechanochemical Processing of Highly Conducting Organic/Inorganic Composites Exhibiting Spin Crossover–Induced Memory Effect in Their Transport Properties

Spin crossover polymer composites, polymers and related soft materials

Two benchmark Fe(II)-triazole spin-crossover (SCO) compounds, [Fe(Htrz)2trz]BF4 (1) and [Fe(NH2trz)3]SO4 (2), have been investigated using variable temperature and pressure powder X-ray diffraction, dynamical mechanical analysis, and differential scanning calorimetry techniques. We assessed the transformation strain, the compressibility, and their anisotropy in both compounds. The measured bulk moduli are relatively high (20 and 13 GPa in 1 and 2, respectively), but against expectations, the Fe–triazole chain direction is the most compressible crystal axis in 1. We revealed considerable elastic softening and associated mechanical damping at the SCO in both compounds, which we interpreted, using the Landau theory, as a consequence of the coupling between the spin state and the lattice strain. The magnitudes of the elastic potential energy (∼15%), stored elastic work (<2–3%), and frictional work (<3–4%) with respect to the latent heat of the spin transition (26.8 and 19.5 kJ/mol in 1 and 2, respectively) have been also assessed. The knowledge of these properties is important both for a better understanding of the SCO behavior of this important family of complexes and for guiding their integration into mechanical transducers and other technological applications. 

Mario Piedrahita Bello

Papers

Spin crossover polymer composites, polymers and related soft materials

Elastic Properties of the Iron(II)–Triazole Spin Crossover Complexes [Fe(Htrz)2trz]BF4 and [Fe(NH2trz)3]SO4