Assessing the Location of Ionic and Molecular Solutes in a Molecularly Heterogeneous and Nonionic Deep Eutectic Solvent

Copyright © 2020 American Chemical Society. Deep eutectic solvents (DES) are emerging sustainable designer solvents viewed as greener and better alternatives to ionic liquids. Nonionic DESs possess unique properties such as viscosity and hydrophobicity that make them desirable in microextraction applications such as oil-spill remediation. This work builds upon a nonionic DES, NMA-LA DES, previously designed by our group. The NMA-LA DES presents a rich nanoscopic morphology that could be used to allocate solutes of different polarities. In this work, the possibility of solvating different solutes within the nanoscopically heterogeneous molecular structure of the NMA-LA DES is investigated using ionic and molecular solutes. In particular, the localized vibrational transitions in these solutes are used as reporters of the DES molecular structure via vibrational spectroscopy. The FTIR and 2DIR data suggest that the ionic solute is confined in a polar and continuous domain formed by NMA, clearly sensing the direct effect of the change in NMA concentration. In the case of the molecular nonionic and polar solute, the data indicates that the solute resides in the interface between the polar and nonpolar domains. Finally, the results for the nonpolar and nonionic solute (W(CO)6) are unexpected and less conclusive. Contrary to its polarity, the data suggest that the W(CO)6 resides within the NMA polar domain of the DES, probably by inducing a domain restructuring in the solvent. However, the data are not conclusive enough to discard the possibility that the restructuring comprises not only the polar domain but also the interface. Overall, our results demonstrate that the NMA-LA DES has nanoscopic domains with affinity to particular molecular properties, such as polarity. Thus, the presented results have a direct implication to separation science

Assembling Artificial Photosynthetic Models in Water Using β-Cyclodextrin-Conjugated Phthalocyanines as Building Blocks

Article describes how two water-soluble zinc(II) phthalocyanines substituted with two or four permethylated β-cyclodextrin (β-CD) moieties at the α positions have been utilized as building blocks for the construction of artificial photosynthetic models in water. The hydrophilic and bulky β-CD moieties not only can increase the water solubility of the phthalocyanine core and prevent its stacking in water but can also bind with a tetrasulfonated zinc(II) porphyrin (ZnTPPS) and/or sodium 2-anthraquinonesulfonate (AQ) in water through host–guest interactions

Nanomolecular singlet oxygen photosensitizers based on hemiquinonoid-resorcinarenes, the fuchsonarenes

Singlet oxygen sensitization involving a class of hemiquinonoid-substituted resorcinarenes prepared from the corresponding 3,5-di-t-butyl-4-hydroxyphenyl-substituted resorcinarenes is reported. Based on variation in the molecular structures, quantum yields comparable with that of the well-known photosensitizing compound meso-tetraphenylporphyrin were obtained for the octabenzyloxy-substituted double hemiquinonoid resorcinarene reported herein. The following classes of compounds were studied: benzyloxy-substituted resorcinarenes, acetyloxy-substituted resorcinarenes and acetyloxy-substituted pyrogallarenes. Single crystal X-ray crystallographic analyses revealed structural variations in the compounds with conformation (i.e., rctt, rccc, rcct) having some influence on the identity of hemiquinonoid product available. Multiplicity of hemiquinonoid group affects singlet oxygen quantum yield with those doubly substituted being more active than those containing a single hemiquinone. Compounds reported here lacking hemiquinonoid groups are inactive as photosensitizers. The term ‘fuchsonarene’ (fuchson + arene of resorcinarene) is proposed for use to classify the compounds

Panchromatic light capture and efficient excitation transfer leading to near-IR emission of BODIPY oligomers

All-BODIPY-based (BODIPY=boron-dipyrromethene) donor–acceptor systems capable of wide-band absorbance leading to efficient energy transfer in the near-IR region are reported. A covalently linked 3-pyrrolyl BODIPY–BODIPY dimer building block bearing an ethynyl group at the meso-aryl position is synthesized and coupled with three different monomeric BODIPY/pyrrolyl BODIPY building blocks with a bromo/iodo group under Pd0 coupling conditions to obtain three covalently linked 3-pyrrolyl-BODIPY-based donor–acceptor oligomers in 19–29 % yield. The oligomers are characterized in detail by 1D and 2D NMR spectroscopy, high-resolution mass spectrometry, and optical spectroscopy. Due to the presence of different functionalized BODIPY derivatives in the oligomers, panchromatic light capture (300–725 nm) is witnessed. Fluorescence studies reveal singlet–singlet energy transfer from BODIPY monomer to BODIPY dimer leading to emission in the 700–800 nm range. Theoretical modeling according to the Förster mechanism predicts ultrafast energy transfer due to good spectral overlap of the donor and acceptor entities. Femtosecond transient absorption studies confirm this to be the case and thus show the relevance of the currently developed all-BODIPY-based energy-funneling supramolecular sytems with near-IR emission to solar-energy harvesting applications

Bottom-up approach to assess the molecular structure of aqueous poly(N‑isopropylacrylamide) at room temperature via infrared spectroscopy

© 2020 American Chemical Society The structure of poly(N-isopropylacrylamide) (PNIPAM) in solution is still an unresolved topic. Here, the PNIPAM structure in water was investigated using a bottom-up approach, involving the monomer, dimer, and trimer, and a combination of infrared (IR) spectroscopies as well as molecular dynamics simulations. The experiments show that the monomer and oligomers exhibit a broad and asymmetric amide I band with two underlying transitions, while PNIPAM presents the same major transitions and a minor one. Analysis of the 2D IR spectra and theoretical modeling of the amide I band indicates that the two transitions of the monomer do not have the same molecular origin as the oligomers and the polymer. In the monomer, the two bands originate from the ultrafast rotation of its ethyl group, which leads to different solvation structures for the various rotational conformers. In the case of the oligomers, the asymmetry and splitting of the amide I band is caused by the vibrational coupling among adjacent amide side chains. Moreover, it is deduced from the simulations that the oligomers have three distinct backbone conformations for neighboring amides. In particular, two of the backbone conformations have a closed and compact structure, while in the third, the backbone is open and elongated. The bottom-up approach allowed us to infer that such backbone conformations exist in PNIPAM as well. Consequently, the two major amide I transitions of the polymer are also assigned to split amide I transitions resulting from the vibrationally coupled nearest-neighboring amides. In contrast, the additional minor transition observed in PNIPAM is assigned to unsolvated amide units of the polymer. The proposed molecular model successfully describes that PNIPAM amide I band changes with temperature in terms of its molecular structure. This new model strongly suggests that PNIPAM does not have a completely random backbone structure, but has distinct backbone conformers between neighboring amides

Femtosecond Transient Absorption Study of Supramolecularly Assembled Metal Tetrapyrrole–TiO₂ Thin Films

Photoexcited electron injection and back electron transfer dynamics of metal tetrapyrrole bound to TiO₂ nanoparticle surfaces via the metal–ligand axial coordination approach have been investigated using femtosecond pump–probe transient spectroscopic technique. The employed metal tetrapyrroles include zinc and magnesium metalated <i>meso</i>-tetraarylporphyrins having halogen substituents on the peripheral aryl groups, perfluorinated zinc phthalocyanine derivatives, and zinc naphthalocyanine. The employed metal tetrapyrroles covered absorption at different portions of the visible and near-IR region of the spectrum with excited-state reduction potentials ranging between −0.61 eV and −1.34 V, that is, having energy higher than the TiO₂ conduction band edge (−0.57 V vs NHE). Two linkers, pyridine and phenylimidazole, have been employed to visualize electronic coupling between the dye and metal oxide surface for optimal electron injection and back electron transfer dynamics. In agreement with the previously reported photocurrent generation of dye-sensitized solar cells constructed using this self-assembly approach (<i>J. Am. Chem. Soc</i>. <b>2009</b>, <i>131</i>, 14646), spectral evidence for electron injection from the excited metal tetrapyrrole to the TiO₂ nanoparticle in the form of a tetrapyrrole radical cation has been obtained. The time profile of the π-cation radical of tetrapyrroles revealed the occurrence of ultrafast electron injection (time constant = 470–700 fs), while the back electron transfer processes were found to be complicated due to the intricate environment of the metallotetrapyrrole–TiO₂ interface

Chloring e6 Sensitized Photovoltaic Cells: Effect of Co-Adsorbents on Cell Performance, Charge Transfer Resistance, and Charge Recombination Dynamics

This article investigates the effect of dye-aggregation-preventing co-adsorbents, cholic acid and deoxycholic acid, on the performance of dye-sensitized solar cells constructed using a metal-free sensitizer, chlorin e6 adsorbed onto TiO₂ surface

Directly Connected AzaBODIPY–BODIPY Dyad: Synthesis, Crystal Structure, and Ground- and Excited-State Interactions

Directly connected, strongly interacting sensitizer donor–acceptor dyads mimic light-induced photochemical events of photosynthesis. Here, we devised a dyad composed of BF₂-chelated dipyrromethene (BODIPY) directly linked to BF₂-chelated tetraarylazadipyrromethene (azaBODIPY) through the β-pyrrole position of azaBODIPY. Structural integrity of the dyad was arrived from two-dimensional NMR spectral studies, while single-crystal X-ray structure of the dyad provided the relative orientation of the two macrocycles to be ∼62°. Because of direct linking of the two entities, ultrafast energy transfer from the ¹BODIPY* to azaBODIPY was witnessed. A good agreement between the theoretically estimated Förster energy transfer rate and experimentally determined rate was observed, and this rate was found to be higher than that reported for BODIPY–azaBODIPY analogues connected with spacer units. In agreement with the free-energy calculations, the product of energy transfer, ¹azaBODIPY* revealed additional photochemical events such as electron transfer leading to the creation of BODIPY^•+–azaBODIPY^•– radical ion pair, more so in polar benzonitrile than in nonpolar toluene, as evidenced by femtosecond transient spectroscopic studies. Additionally, the spectral, electrochemical, and photochemical studies of the precursor compound azaBODIPY–dipyrromethane also revealed occurrence of excited-state events. In this case, electron transfer from the ¹azaBODIPY* to dipyrromethane (DPM) yielded DPM^•+–azaBODIPY^•– charge-separated state. The study described here stresses the role of close association of the donor and acceptor entities to promote ultrafast photochemical events, applicable of building fast-response optoelectronic and energy-harvesting devices

Directly Connected AzaBODIPY–BODIPY Dyad: Synthesis, Crystal Structure, and Ground- and Excited-State Interactions

Directly connected, strongly interacting sensitizer donor–acceptor dyads mimic light-induced photochemical events of photosynthesis. Here, we devised a dyad composed of BF₂-chelated dipyrromethene (BODIPY) directly linked to BF₂-chelated tetraarylazadipyrromethene (azaBODIPY) through the β-pyrrole position of azaBODIPY. Structural integrity of the dyad was arrived from two-dimensional NMR spectral studies, while single-crystal X-ray structure of the dyad provided the relative orientation of the two macrocycles to be ∼62°. Because of direct linking of the two entities, ultrafast energy transfer from the ¹BODIPY* to azaBODIPY was witnessed. A good agreement between the theoretically estimated Förster energy transfer rate and experimentally determined rate was observed, and this rate was found to be higher than that reported for BODIPY–azaBODIPY analogues connected with spacer units. In agreement with the free-energy calculations, the product of energy transfer, ¹azaBODIPY* revealed additional photochemical events such as electron transfer leading to the creation of BODIPY^•+–azaBODIPY^•– radical ion pair, more so in polar benzonitrile than in nonpolar toluene, as evidenced by femtosecond transient spectroscopic studies. Additionally, the spectral, electrochemical, and photochemical studies of the precursor compound azaBODIPY–dipyrromethane also revealed occurrence of excited-state events. In this case, electron transfer from the ¹azaBODIPY* to dipyrromethane (DPM) yielded DPM^•+–azaBODIPY^•– charge-separated state. The study described here stresses the role of close association of the donor and acceptor entities to promote ultrafast photochemical events, applicable of building fast-response optoelectronic and energy-harvesting devices

Vectorial Charge Separation and Selective Triplet-State Formation during Charge Recombination in a Pyrrolyl-Bridged BODIPY–Fullerene Dyad

A donor–acceptor dyad composed of BF₂-chelated dipyrromethene (BDP or BODIPY) and fullerene connected with a pyrrole ring spacer, <b>1</b>, has been newly synthesized and characterized. Because of α-carbon substitution and extended conjugation offered by the pyrrole ring, the optical absorbance and emission spectra of BDP macrocycle were found to be red-shifted significantly. Electrochemical studies provided information on the redox potentials while computational studies performed at the B3LYP/6-31G* level yielded an optimized geometry of the dyad that was close to that reported earlier for a BDP-C₆₀ dyad covalently connected through the central boron atom, <b>2</b>. The HOMO of the dyad was found to be on the BDP macrocycle, extended over the pyrrole bridging group, a property that is expected to facilitate electronic communication between the BDP and fullerene entities. The established energy level diagram using spectral, redox, and optimized structural results predicted possibility of photoinduced electron transfer in both benzonitrile and toluene, representing polar and nonpolar solvents. However, such energy diagram suggested different routes for the charge recombination processes, that is, direct relaxation of the radical ion-pair in polar solvent while populating the triplet level of the sensitizer (³BDP* or ³C₆₀*) in nonpolar solvent. Proof for charge separation and solvent dependent charge recombination processes were established from studies involving femto- and nanosecond pump–probe spectroscopy. The measured rate of charge separation, <i>k</i>_CS, for <b>1</b> was higher in both solvents compared to the earlier reported values for <b>2</b> due to electronically well-communicating pyrrole spacer. The charge recombination in toluene populated ³BDP* as an intermediate step while in benzonitrile it yielded directly ground state of the dyad. The present findings reveal the significance of pyrrole spacer between the donor and acceptor to facilitate charge separation and solvent polarity dependent charge recombination processes