Impact of Molecular Arrangement and Torsional Motion on the Fluorescence of Salophen and Its Metal Complexes

Salophen is a weakly emissive molecule with a flexible structure. The decrease in the flexibility of the molecule, which can be achieved by chemical or physical means, causes a significant increase in the emissivity and fluorescence lifetime. This phenomenon has been observed upon incorporation of salophen in the solid polymer matrix of poly­(methyl methacrylate) (PMMA). The enhancement in emission is even more prominent in the pure solid form of salophen. An enhancement of emission is also observed in the case of the zinc complex of salophen, SalZn, which is inherently more emissive than free salophen in solution. However, the enhancement in emission is greater in the PMMA matrix for the complex than in its solid form. Interestingly, a quenching of fluorescence is observed in the crystals of the aluminum complex of salophen (SalAl⁺), which is strongly emissive in solution phase. These apparently conflicting trends have been rationalized in the light of the molecular arrangement of salophen and its complexes in a solid matrix and in the pure solid forms. In the case of salophen, torsional motion provides major nonradiative channels of depopulation of its excited state in solution. These channels are blocked in the rigid environment provided of the polymer matrix and of the crystal, giving rise to aggregation induced enhancement of emission (AIEE). In the case of SalAl⁺, the torsional motion is restricted anyway due to complexation. The X-ray crystal structure indicates the possibility of π–π interaction between the planar ligands of two neighboring complex molecules, which could lead to aggregation-caused quenching (ACQ). This provides a justification for the lower emissivity of SalZn, as compared to SalAl⁺. SalZn is likely to exist as a dimer, in which intramolecular π–π interaction is possible. Thus, the emissivity of salophen and its complexes is found to be governed by interplay of torsional motion and intermolecular interaction. Experiments have been performed at liquid nitrogen temperature, whereby conformational motion is arrested, but additional intermolecular interactions are not brought in. Maximal fluorescence of each of the three species studied is observed in this condition

The Prospect of Salophen in Fluorescence Lifetime Sensing of Al³⁺

We have assessed the potential of salophen, a tetradentate Schiff base, in fluorescence sensing of Al³⁺ ions. While performing this investigation, we have noticed conflicting literature reports on the fluorescence spectral maximum and quantum yield of salophen. So, the compound has been purified by repeated crystallization. Fluorescence studies have been performed on samples in which the absorption and excitation spectra are completely superimposable. The purified compound exhibits a feeble fluorescence at 545 nm, associated with an ultrafast fluorescence decay. This is rationalized by excited state proton transfer and torsional motions within the molecule, which provide efficient nonradiative channels of deactivation of its excited state. The fluorescence quantum yield increases upon complexation of salophen with Zn²⁺ as well as Al³⁺. The increase is significantly more upon complexation with Al³⁺. However, fluorescence maxima are similar for the two complexes. This indicates that fluorescence intensity may not be a good parameter for Al³⁺ sensing by salophen, in the presence of a large excess of Zn²⁺. This problem can be circumvented if fluorescence lifetime is used as the sensing parameter, as the lifetime of the Al³⁺ complex is in the nanosecond time regime while that of the Zn²⁺ complex is in tens of picoseconds. The significant difference in the fluorescence quantum yield and lifetime between the two complexes is explained as follows: the Al³⁺ complex is monomeric, but the Zn²⁺ complex is dimeric. Quantum chemical calculations indicate a higher density of states near the locally excited state for the dimeric complex. This may lead to more efficient nonradiative pathways

Water Rearrangements upon Disorder-to-Order Amyloid Transition

Water plays a critical role in governing the intricate balance between chain-chain and chain-solvent interactions during protein folding, misfolding, and aggregation. Previous studies have indicated the presence of different types of water in folded (globular) proteins. In this work, using femtosecond and picosecond time-resolved fluorescence measurements, we have characterized the solvation dynamics from ultrafast to ultraslow time scale both in the monomeric state and in the amyloid state of an intrinsically disordered protein, namely κ-casein. Monomeric κ-casein adopts a compact disordered state under physiological conditions and is capable of spontaneously aggregating into highly ordered β-rich amyloid fibrils. Our results indicate that the mobility of “biological water” (type I) gets restrained as a result of conformational sequestration during amyloid formation. Additionally, a significant decrease in the bulk water component with a concomitant increase in the ultraslow component revealed the ordering of trapped interstitial water (type II) upon disorder-to-order amyloid transition. Our results provide an experimental underpinning of significant water rearrangements associated with both chain desolvation and water confinement upon amyloid formation

Pseudohalide (SCN^–)‑Doped MAPbI₃ Perovskites: A Few Surprises

Pseudohalide thiocyanate anion (SCN^–) has been used as a dopant in a methylammonium lead tri-iodide (MAPbI₃) framework, aiming for its use as an absorber layer for photovoltaic applications. The substitution of SCN^– pseudohalide anion, as verified using Fourier transform infrared (FT-IR) spectroscopy, results in a comprehensive effect on the optical properties of the original material. Photoluminescence measurements at room temperature reveal a significant enhancement in the emission quantum yield of MAPbI_3–<i>x</i>(SCN)_<i>x</i> as compared to MAPbI₃, suggestive of suppression of nonradiative channels. This increased intensity is attributed to a highly edge specific emission from MAPbI_3–<i>x</i>(SCN)_<i>x</i> microcrystals as revealed by photoluminescence microscopy. Fluoresence lifetime imaging measurements further established contrasting carrier recombination dynamics for grain boundaries and the bulk of the doped material. Spatially resolved emission spectroscopy on individual microcrystals of MAPbI_3–<i>x</i>(SCN)_<i>x</i> reveals that the optical bandgap and density of states at various (local) nanodomains are also nonuniform. Surprisingly, several (local) emissive regions within MAPbI_3–<i>x</i>(SCN)_<i>x</i> microcrystals are found to be optically unstable under photoirradiation, and display unambiguous temporal intermittency in emission (blinking), which is extremely unusual and intriguing. We find diverse blinking behaviors for the undoped MAPbI₃ crystals as well, which leads us to speculate that blinking may be a common phenomenon for most hybrid perovskite materials

Pseudohalide (SCN^–)‑Doped MAPbI₃ Perovskites: A Few Surprises

Pseudohalide thiocyanate anion (SCN^–) has been used as a dopant in a methylammonium lead tri-iodide (MAPbI₃) framework, aiming for its use as an absorber layer for photovoltaic applications. The substitution of SCN^– pseudohalide anion, as verified using Fourier transform infrared (FT-IR) spectroscopy, results in a comprehensive effect on the optical properties of the original material. Photoluminescence measurements at room temperature reveal a significant enhancement in the emission quantum yield of MAPbI_3–<i>x</i>(SCN)_<i>x</i> as compared to MAPbI₃, suggestive of suppression of nonradiative channels. This increased intensity is attributed to a highly edge specific emission from MAPbI_3–<i>x</i>(SCN)_<i>x</i> microcrystals as revealed by photoluminescence microscopy. Fluoresence lifetime imaging measurements further established contrasting carrier recombination dynamics for grain boundaries and the bulk of the doped material. Spatially resolved emission spectroscopy on individual microcrystals of MAPbI_3–<i>x</i>(SCN)_<i>x</i> reveals that the optical bandgap and density of states at various (local) nanodomains are also nonuniform. Surprisingly, several (local) emissive regions within MAPbI_3–<i>x</i>(SCN)_<i>x</i> microcrystals are found to be optically unstable under photoirradiation, and display unambiguous temporal intermittency in emission (blinking), which is extremely unusual and intriguing. We find diverse blinking behaviors for the undoped MAPbI₃ crystals as well, which leads us to speculate that blinking may be a common phenomenon for most hybrid perovskite materials

Pseudohalide (SCN^–)‑Doped MAPbI₃ Perovskites: A Few Surprises

Pseudohalide thiocyanate anion (SCN^–) has been used as a dopant in a methylammonium lead tri-iodide (MAPbI₃) framework, aiming for its use as an absorber layer for photovoltaic applications. The substitution of SCN^– pseudohalide anion, as verified using Fourier transform infrared (FT-IR) spectroscopy, results in a comprehensive effect on the optical properties of the original material. Photoluminescence measurements at room temperature reveal a significant enhancement in the emission quantum yield of MAPbI_3–<i>x</i>(SCN)_<i>x</i> as compared to MAPbI₃, suggestive of suppression of nonradiative channels. This increased intensity is attributed to a highly edge specific emission from MAPbI_3–<i>x</i>(SCN)_<i>x</i> microcrystals as revealed by photoluminescence microscopy. Fluoresence lifetime imaging measurements further established contrasting carrier recombination dynamics for grain boundaries and the bulk of the doped material. Spatially resolved emission spectroscopy on individual microcrystals of MAPbI_3–<i>x</i>(SCN)_<i>x</i> reveals that the optical bandgap and density of states at various (local) nanodomains are also nonuniform. Surprisingly, several (local) emissive regions within MAPbI_3–<i>x</i>(SCN)_<i>x</i> microcrystals are found to be optically unstable under photoirradiation, and display unambiguous temporal intermittency in emission (blinking), which is extremely unusual and intriguing. We find diverse blinking behaviors for the undoped MAPbI₃ crystals as well, which leads us to speculate that blinking may be a common phenomenon for most hybrid perovskite materials