4D Printing for Biomedical Applications

4D (bio-)printing endows 3D printed (bio-)materials with multiple functionalities and dynamic properties. 4D printed materials have been recently used in biomedical engineering for the design and fabrication of biomedical devices, such as stents, occluders, microneedles, smart 3D-cell engineered microenvironments, drug delivery systems, wound closures, and implantable medical devices. However, the success of 4D printing relies on the rational design of 4D printed objects, the selection of smart materials, and the availability of appropriate types of external (multi-)stimuli. Here, this work first highlights the different types of smart materials, external stimuli, and design strategies used in 4D (bio-)printing. Then, it presents a critical review of the biomedical applications of 4D printing and discusses the future directions of biomedical research in this exciting area, including in vivo tissue regeneration studies, the implementation of multiple materials with reversible shape memory behaviors, the creation of fast shape-transformation responses, the ability to operate at the microscale, untethered activation and control, and the application of (machine learning-based) modeling approaches to predict the structure-property and design-shape transformation relationships of 4D (bio)printed constructs.

Luminescent Platinum(II) Complexes with Stimuli-Responsive Flexible Lewis Pair Ligands: Spectroscopic and Computational Studies

A series of new luminescent bimetallic platinum(II) complexes with stimuli-responsive flexible Lewis pair (FlexLP) ligands are described. The FlexLP ligands consist of a dimesitylboron Lewis acid and diphenylphosphine oxide Lewis base which are in equilibrium between the unbound open form and the Lewis adduct, controlled by the hydrogen bond donating strength of the solvent. Spectroscopic techniques and density functional theory (DFT) calculations were used to interpret the photophysics of the platinum(II) complexes. All complexes exhibit tunable absorption in the region of 300-500 nm and green to orange photoluminescence, depending on the ratio of weak (THF) to strong (MeOH) hydrogen bond donating solvent employed. Spectroscopic and computational data shows that phosphine and peripheral acetylide ligands on the platinum(II) centers have limited influence on the emission energy, indicating the emission originates from the FlexLP-dominated fluorescence. Using time-resolved transient absorption spectroscopy it is shown that the complexes undergo intersystem crossing (ISC) to the triplet excited state upon photoexcitation, and the ISC efficiency is affected by the peripheral acetylide ligands. The triplet excited state lifetime can also be manipulated by the state of the FlexLP ligand, with the closed form complexes having longer lifetimes than the open form complexes.

Design of Bis(1,10-phenanthroline) Copper(I)-Based Mechanochromic Indicators

In the growing field of single-molecule mechanochromism, the potential of transition metal complexes is yet to be examined. In this work, we have synthesized a series of [Cu(phen)2]+ complexes: bis-Cu(I)-phenanthroline, bis-Cu(I)-phenanthroline-2-amine, and bis-Cu(I)-phenanthroline-2-acetamide. After that, we characterized the complexes by UV-vis spectroscopy and employed density functional theory (DFT) calculations to investigate the changes in UV-vis upon mechanical pulling via force calculations. The results of our examination of time-dependent (TD)-DFT-calculated UV-vis suggests that the bis-Cu(I)-phenanthroline-2-acetamide complex is predicted to have an observable shift of the metal-to-ligand charge transfer band upon pulling from 0 to 0.6 nN in the visible region. We have demonstrated the ability to synthesize and characterize bis-Cu(I)-phenanthroline-2-acetamide. In addition, the TD-DFT calculations predict an observable shift in the visible region of the UV-vis spectrum. This indicates that transition metal complexes are feasible candidates as mechanophores and are worthy of further exploration as to their potential role in a new subclass of mechanochromic indicators.

Tuning the luminescence of transition metal complexes with acyclic diaminocarbene ligands

Organometallics featuring acyclic diaminocarbene ligands have recently emerged as powerful emitters for use in electroluminescent technologies.

A Mechanochromic and Vapochromic Luminescent Cuprous Complex Based on a Switchable Intramolecular π···π Interaction

An in-depth study on a stimuli-responsive tetranuclear cuprous luminescent complex is reported and gives new insights into the origin and possible use of the observed stimuli-responsive luminescence. Its crystalline polymorphs with two different shapes are obtained by using different crystallization solvents and show distinct emissions, with one being blue emissive and the other being yellow emissive. Upon grinding, only the blue-emitting polymorph has a marked change in the emission color from blue to yellow, and its ground sample exhibits a yellow emission similar to that of the yellow-emitting polymorph. Interestingly, the yellow-emitting polymorph after exposure to acetone vapor can emit a blue emission and display luminescence mechanochromism similar to that of the blue-emitting polymorph. Single-crystal structural analyses of the two different polymorphs reveal the relationship between the mechanochromic luminescence and the geometrical configuration of the {Cu(μ-dppm)₂Cu} unit and intramolecular "pyridyl/phenyl" π···π interactions, supported as well by their PXRD, FT-IR, TGA, and PL studies in various states and by TD-DFT analyses. The results demonstrate the different roles of switchable intramolecular π···π interactions and the geometrical configuration of the {Cu(μ-dppm)₂Cu} unit in this stimuli-responsive luminescence and potential applications of such stimuli-responsive luminescence in optical sensing and anticounterfeiting encryption technologies and deepen the understanding of such stimuli-responsive luminescence originating from switchable intramolecular π···π interactions. In addition, it is clearly suggested that the rational utilization of switchable intramolecular π···π interactions is a feasible route for developing stimuli-responsive intelligent luminescent materials and devices.

Study of a phosphorescent cationic iridium(III) complex displaying a blue-shift in crystals

We report the synthesis and the characterization of a new cationic iridium(III) complex featuring two 1-(p-methoxyphenyl)-5-methoxybenzimidazole cyclometallating ligands and a dimethylbipyridine ancillary ligand. The complex has been fully characterized by 1D and 2D NMR (¹H, ¹³C, ¹⁹F and ³¹P), elemental analysis and high-resolution mass spectrometry (HRMS). The photoluminescence studies performed in a solution, on amorphous powder and on crystals revealed an unexpected behavior. Indeed, the emission spectra observed in both solution (CH₂Cl₂) and amorphous powder samples are centered at around 580 nm, whereas in crystals the emission displays a large hypsochromic shift of ∼800 cm^-1 (λ_em = 558 nm). X-ray diffraction experiments, photophysical studies and DFT calculations allow for rationalizing the hypsochromic shift.

Luminescent monomeric and dimeric Ru(ii) acyclic carbene complexes as selective sensors for NH3/amine vapor and humidity

A new class of luminescent bis(bipyridyl) Ru(ii) pyridyl acyclic carbene complexes with environmentally-sensitive dimerization equilibrium have been developed. Owing to the involvement of the orbitals of the diaminocarbene ligand in the emissive excited state, the phosphorescence properties of these complexes are strongly affected by H-bonding interactions with various H-bonding donor/acceptor molecules. With the remarkable differences in the emission properties of the monomer, dimer, and H-bonded amine adducts together with the change of the dimerization equilibrium, these complexes can be used as luminescent gas sensors for humidity, ammonia, and amine vapors. With the responses to amines and humidity and the corresponding change in the luminescence properties, a proof-of-principle for binary optical data storage with a reversible concealment process has been described.

Mechanical Modulation of the Solid-State Luminescence of Tricarbonyl Rhenium(I) Complexes through the Interplay between Two Triplet Excited States

Mechanoresponsive luminescence (MRL) materials promise smart devices for sensing, optoelectronics and security. We present here the first report on the MRL activity of two Re^I complexes, opening up new opportunities for applications in these fields. Both complexes exhibit marked solid-state luminescence enhancement (SLE). Furthermore, the pristine microcrystalline powders emit in the yellow-green region, and grinding led to an amorphous phase with concomitant emission redshift and shrinking of the photoluminescence (PL) quantum yields and lifetimes. Quantum chemical calculations revealed the existence of two low-lying triplet excited states with very similar energy levels, that is, ³ IL and ³ MLCT, having, respectively, almost pure intraligand (IL) and metal-to-ligand charge-transfer (MLCT) character. Transition between these states could be promoted by rotation around the pyridyltriazole-phenylbenzoxazole bond. In the microcrystals, in which rotations are hindered, the ³ IL state induces the prominent PL emission at short wavelengths. Upon grinding, rotation is facilitated and the transition to the ³ MLCT state results in a larger proportion of long-wavelength PL. FTIR and variable-temperature PL spectroscopy showed that the opening of the vibrational modes favours non-radiative deactivation of the triplet states in the amorphous phase. In solution, PL only arises from the ³ MLCT state. The same mechanism accounts for the spectroscopic differences observed when passing from crystals to amorphous powders, and then to solutions, thereby clarifying the link between SLE and MRL for these complexes.

PdII- and PtII-mediated coupling of aryl isocyanides with N-heterocyclic thiones

A Pd^II- and Pt^II-mediated reaction of aryl isocyanides with N-heterocyclic thiones results in a previously undescribed type of regioselectivity for ambident nucleophile addition to coordinated isocyanides.