Pd and Pt Complexes of Benzo-Fused Dipyrrins: Synthesis, Structure, Electrochemical, and Optical Properties.

Benzo-fused dipyrrins are π-extended analogs of conventional dipyrrins, which exhibit bathochromically shifted absorption and possess the synthetic capability to bind various metal ions. We aimed to investigate the synthetic potential of benzo-fused dipyrrins in the complexation with transition metals. Two new complexes with Pd2+ and Pt2+ were synthesized and characterized. X-ray crystallography reveals that both complexes exhibit a zigzag geometry with square planar coordination of the central metal. The Pd2+ complex possesses a very weak fluorescence at 665 nm, while the Pt2+ complex is completely nonemissive. Transient absorption spectroscopy confirmed triplet excited state formation for both complexes; however, they are short-lived and no phosphorescence was observed even at 77K. DFT calculations support the experimental observation, revealing the existence of the low-lying ligand-metal charge-transfer (LMCT) triplet state acting as an energy sink.

Energy Funneling in a Noninteger Two-Dimensional Perovskite.

Energy funneling is a phenomenon that has been exploited in optoelectronic devices based on low-dimensional materials to improve their performance. Here, we introduce a new class of two-dimensional semiconductor, characterized by multiple regions of varying thickness in a single confined nanostructure with homogeneous composition. This "noninteger 2D semiconductor" was prepared via the structural transformation of two-octahedron-layer-thick (n = 2) 2D cesium lead bromide perovskite nanosheets; it consisted of a central n = 2 region surrounded by edge-lying n = 3 regions, as imaged by electron microscopy. Thicker noninteger 2D CsPbBr3 nanostructures were obtained as well. These noninteger 2D perovskites formed a laterally coupled quantum well band alignment with virtually no strain at the interface and no dielectric barrier, across which unprecedented intramaterial funneling of the photoexcitation energy was observed from the thin to the thick regions using time-resolved absorption and photoluminescence spectroscopy.

Red-Colored Circularly Polarized Luminescence from a Benzo-Fused BODIPY-BINOL Complex.

Red emission with sharp bandwidth and high quantum yield is a desired characteristic for organic chromophores in optoelectronic, spintronic, and biomedical applications. Here, we observe circularly polarized luminescence (CPL) with these characteristics from a benzo-fused BODIPY-BINOL complex (1). Using time-resolved optical spectroscopy, electrochemistry, and density functional theory calculations, we showed that the emissive excited state of 1 does not have a charge-transfer (CT) character, unlike that of the regular BODIPY counterpart (2). The rigidity and the lack of CT character make this class of molecules an appealing platform for CPL-active molecules in the red spectral region, with ample room for improvement in the dissymmetry factor and brightness.

Anti-Arrhenius behavior of electron transfer reactions in molecular dimers.

Rates of chemical reactions typically accelerate as the temperature rises, following the Arrhenius law. However, electron transfer reactions may exhibit weak temperature dependence or counterintuitive behavior, known as anti-Arrhenius behavior, wherein reaction rates decrease as temperature increases. Solvent reorganization energy and torsion-induced changes in electronic couplings could contribute to this unusual behavior, but how each contributes to the overall temperature dependence is unclear. One can decelerate the charge recombination process in photogenerated radical pairs or charge-separated states by harnessing this often-overlooked phenomenon. This means that we could achieve long-lived radical pairs without relying on conventional cooling. Using a series of homo molecular dimers, we showed that the degree of torsional hindrance dictates temperature-dependent torsion-induced changes in electronic coupling and, therefore, charge recombination rates. The overall temperature dependence is controlled by how changes in electronic coupling and the temperature-dependent solvent reorganization energy contribute to the rates of charge recombination. Our findings pave the way for rationally designing molecules that exhibit anti-Arrhenius behavior to slow down charge recombination, opening possibilities for applications in energy-related and quantum information technologies.

Type-I CdS/ZnS Core/Shell Quantum Dot-Gold Heterostructural Nanocrystals for Enhanced Photocatalytic Hydrogen Generation.

Developing Type-I core/shell quantum dots is of great importance toward fabricating stable and sustainable photocatalysts. However, the application of Type-I systems has been limited due to the strongly confined photogenerated charges by the energy barrier originating from the wide-bandgap shell material. In this project, we found that through the decoration of Au satellite-type domains on the surface of Type-I CdS/ZnS core/shell quantum dots, such an energy barrier can be effectively overcome and an over 400-fold enhancement of photocatalytic H2 evolution rate was achieved compared to bare CdS/ZnS quantum dots. Transient absorption spectroscopic studies indicated that the charges can be effectively extracted and subsequently transferred to surrounding molecular substrates in a subpicosecond time scale in such hybrid nanocrystals. Based on density functional theory calculations, the ultrafast charge separation rates were ascribed to the formation of intermediate Au2S layer at the semiconductor-metal interface, which can successfully offset the energy confinement introduced by the ZnS shell. Our findings not only provide insightful understandings on charge carrier dynamics in semiconductor-metal heterostructural materials but also pave the way for the future design of quantum dot-based hybrid photocatalytic systems.

Quantum Dot-Organic Molecule Conjugates as Hosts for Photogenerated Spin Qubit Pairs.

The inherent spin polarization present in photogenerated spin-correlated radical pairs makes them promising candidates for quantum computing and quantum sensing applications. The spin states of these systems can be probed and manipulated with microwave pulses using electron paramagnetic resonance spectrometers. However, to date, there are no reports on magnetic resonance-based spin measurements of photogenerated spin-correlated radical pairs hosted on quantum dots. In the current work, we prepare dye molecule-inorganic quantum dot conjugates and show that they can produce photogenerated spin-polarized states. The dye molecule, D131, is chosen for its ability to undergo efficient charge separation, and the nanoparticle materials, ZnO quantum dots, are chosen for their promising spin properties. Transient and steady state optical spectroscopy performed on ZnO quantum dot-D131 conjugates shows that reversible photogenerated charge separation is occurring. Transient and pulsed electron paramagnetic resonance experiments are then performed on the photogenerated radical pair, which demonstrate that (1) the radical pair is polarized at moderate temperatures and well modeled by existing theories and (2) the spin states can be accessed and manipulated with microwave pulses. This work opens the door to a new class of promising qubit materials that can be photogenerated in polarized states and hosted by highly tailorable inorganic nanoparticles.

Anionic ligand-induced chirality in perovskite nanoplatelets.

Perovskite materials passivated by chiral ligands have recently shown unique chiroptical activity with promising optoelectronic applications. However, the ligands have been limited to chiral amines. Here, chiral phosphate molecules have been exploited to synthesize CsPbBr3 nanoplatelets. The nanoplatelets showed a distinct circular dichroism signal and maintained their chiroptical properties after purification with anti-solvent.

Acceleration of Nonradiative Charge Recombination Reactions at Larger Distances in Kinked Donor-Bridge-Acceptor Molecules.

Photoinduced electron transfer in donor-bridge-acceptor (D-B-A) molecular systems can occur via tunneling over long distances (rDA) of well over 10 Å. We commonly observe decreasing rates of electron transfer with increasing distances, a result of a decrease in the electronic coupling of the donor and acceptor moiety. In the study of D-B-A molecules with Ru(bpy)32+ as a bridge/core, Kuss-Petermann and Wenger observed the opposite trend (J. Am. Chem. Soc.2016, 138, 1349); a maximum rate constant of electron transfer was observed at an intermediate electron transfer distance. Within the high-temperature limit of the classical Marcus equation, their observation was qualitatively explained by a sharp distance dependence of outer sphere (or solvent) reorganization energy, as predicted by Sutin and co-workers (J. Am. Chem. Soc.1984, 106, 6858), and almost distance-independent electronic couplings. Here, we report another example of such an underexplored behavior with three kinked D-B-A systems of rDA ∼ 10-19 Å, showing increasing rates of nonradiative charge recombination with increasing rDA. The three D-B-A systems are based on boron dipyrromethene and triphenylamine as electron acceptor and donor groups, respectively, with aryl bridges where the donor and acceptor moieties are connected at meso-positions. These D-B-A molecules exhibit radiative electron transfer reactions (or charge-transfer emission), which enables us to experimentally determine the solvent reorganization energy and the electronic couplings. The analysis of charge-transfer emission that explicitly considers electron-vibration coupling, in conjunction with the temperature-dependent analysis and computational method, revealed that the solvent reorganization energy indeed increases with distance, and at the same time, the electronic coupling decreases with distance expectedly. Therefore, under the right conditions for solvent reorganization energy and electronic coupling values, our results show that we can observe the acceleration of electron transfer reactions with increasing distance, even when we have the expected distance dependence of electronic coupling. This work indicates that the acceleration of electron transfer with increasing distance may be achieved with a fine-tuning of molecular design.

Chemical redox-induced chiroptical switching of supramolecular assemblies of viologens.

A chiral supramolecular assembly exhibiting redox-induced changes in its chiroptical properties was prepared using viologen-modified glutamide (G-V2+) derivatives. Achiral viologen moieties in the G-V2+ assembly were chirally orientated by glutamide groups, affording a unique orange-colored solution, with a visible absorption band at around 470 nm, having electronic circular dichroism (CD) signals (molar ellipticity [θ] = 0.58 × 105 deg cm2 dmol-1: absorption dissymmetry factors (g) = 5.2 × 10-3 at 512 nm). The G-V2+ could be reduced to its cation radical (G-V+˙) but retains its chiral assembly. After chemical reduction, the color change from orange to blueish violet, indicating an absorption band at approximately 560 nm, and the sign change of the CD signal from positive to negative ([θ] = -0.36 × 105 deg cm2 dmol-1; g = -2.9 × 10-3 at 580 nm) were observed in water. Subsequent oxidation re-introduces the G-V2+ chiroptical behavior before reduction.

meso-Antracenyl-BODIPY Dyad as a New Photocatalyst in Atom-Transfer Radical Addition Reactions.

We demonstrate that because of the efficient generation of triplet excited state under UV or visible-light irradiation, meso-antracenyl-BODIPY donor-acceptor dyad can catalyze atom-transfer radical addition (ATRA) reactions between bromomalonate and alkenes. This finding paves the way for the design and application of the new type of heavy atom-free organic chromophores for photocatalysis.

IR linewidth and intensity amplifications of nitrile vibrations report nuclear-electronic couplings and associated structural heterogeneity in radical anions.

Conjugated molecular chains have the potential to act as "molecular wires" that can be employed in a variety of technologies, including catalysis, molecular electronics, and quantum information technologies. Their successful application relies on a detailed understanding of the factors governing the electronic energy landscape and the dynamics of electrons in such molecules. We can gain insights into the energetics and dynamics of charges in conjugated molecules using time-resolved infrared (TRIR) detection combined with pulse radiolysis. Nitrile ν(C[triple bond, length as m-dash]N) bands can act as IR probes for charges, based on IR frequency shifts, because of their exquisite sensitivity to the degree of electron delocalization and induced electric field. Here, we show that the IR intensity and linewidth can also provide unique and complementary information on the nature of charges. Quantifications of IR intensity and linewidth in a series of nitrile-functionalized oligophenylenes reveal that the C[triple bond, length as m-dash]N vibration is coupled to the nuclear and electronic structural changes, which become more prominent when an excess charge is present. We synthesized a new series of ladder-type oligophenylenes that possess planar aromatic structures, as revealed by X-ray crystallography. Using these, we demonstrate that C[triple bond, length as m-dash]N vibrations can report charge fluctuations associated with nuclear movements, namely those driven by motions of flexible dihedral angles. This happens only when a charge has room to fluctuate in space.

Structural and Photophysical Characterization of All Five Constitutional Isomers of the Octaethyl-ß,ß'-dioxo-bacterio- and -isobacteriochlorin Series.

It is well-known that treatment of ß-octaethylporphyrin with H2 O2 /conc. H2 SO4 converts it to a ß-oxochlorin as well as all five constitutional isomers of the corresponding ß,ß'-dioxo-derivatives: two bacteriochlorin-type isomers (ß-oxo groups at opposite pyrrolic building blocks) and three isobacteriochlorin-type isomers (ß-oxo-groups at adjacent pyrrolic building blocks). By virtue of the presence of the strongly electronically coupled ß-oxo auxochromes, none of the chromophores are archetypical chlorins, bacteriochlorins, or isobacteriochlorins. Here the authors present, inter alia, the single crystal X-ray structures of all free-base diketone isomers and a comparative description of their UV-vis absorption spectra in neutral and acidic solutions, and fluorescence emission and singlet oxygen photosensitization properties, Magnetic Circular Dichroism (MCD) spectra, and singlet excited state lifetimes. DFT computations uncover underlying tautomeric equilibria and electronic interactions controlling their electronic properties, adding to the understanding of porphyrinoids carrying ß-oxo functionalities. This comparative study lays the basis for their further study and utilization.

Magnetic Control of Recombination Fluorescence and Tunability by Modulation of Radical Pair Energies in Rigid Donor-Bridge-Acceptor Systems.

Magnetic control of molecular emission holds the promise of developing new magneto-optical technologies. Spin dynamics of radical pairs can serve as a basis of control of chemical reactions by weak magnetic fields (<1 T) orders of magnitude smaller than the thermal energy kBT at room temperature. Here we demonstrate control of recombination fluorescence, produced by charge recombination of photogenerated radical pairs, by weak magnetic fields in rigid donor-bridge-acceptor molecules excited with visible light. We can tune the field response range by chemically modulating the energies of the radical pairs affecting exchange interactions. Our results present a new strategy for designing magneto-optical probes for imaging and other molecular spin technology applications.

Micelles Embedded in Multiphasic Protein Hydrogel Enable Efficient and Air-Tolerant Triplet Fusion Upconversion with Heavy-Atom and Spin-Orbit Charge-Transfer Sensitizers.

The applications of triplet-triplet annihilation-based photon upconversion (TTA-UC) in solar devices have been limited by the challenges in designing a TTA-UC system that is efficient under aerobic conditions. Efficient TTA-UC under aerobic conditions is typically accomplished by using soft matter or solid-state media, which succeed at protecting the triplet excited states of upconverters (sensitizer and annihilator) from quenching by molecular oxygen but fail at preserving their mobility, thus limiting the TTA-UC efficiency (ΦUC). We showcase a protein/lipid hydrogel that succeeded in doing both of the above due to its unique multiphasic design, with a high ΦUC of 19.0 ± 0.7% using a palladium octaethylporphyrin sensitizer. This hydrogel was made via an industrially compatible method using low-cost and eco-friendly materials: bovine serum albumin (BSA), sodium dodecyl sulfate (SDS), and water. A dense BSA network provided oxygen protection while the encapsulation of upconverters within a micellar SDS environment preserved upconverter mobility, ensuring near-unity triplet energy transfer efficiency. In addition to heavy atom-containing sensitizers, several completely organic, spin-orbit charge-transfer intersystem crossing (SOCT-ISC) Bodipy-based sensitizers were also studied; one of which achieved a ΦUC of 3.5 ± 0.2%, the only reported SOCT-ISC-sensitized ΦUC in soft matter to date. These high efficiencies showed that our multiphasic design was an excellent platform for air-tolerant TTA-UC and that it can be easily adapted to a variety of upconverters.

Turning on solid-state phosphorescence of platinum acetylides with aromatic stacking.

Neat solids that phosphoresce under ambient conditions are rare due to aggregation-caused quenching. This communication describes a platinum acetylide (PtPE) that phosphoresces as a solid due to programmed aromatic stacking interactions of pendant groups that prevent intermolecular aggregation.

Vibrational Spectroscopy Reveals Effects of Electron Push-Pull and Solvent Polarity on Electron Delocalization in Radical Anions of Donor-Acceptor Compounds.

The nature of excess electrons has been studied in donor-acceptor (D-A) compounds based on substituted triarylamines and a nitrile-functionalized fluorene by changing the substituents on the triarylamines and also the solvent polarity. We observed that both electron push-pull capability at the distant location in the amine donor unit and solvation in solvents of varying polarity significantly affect the nitrile ν(C≡N) vibrations of the fluorene acceptor unit in radical anions of these D-A compounds. Quantum calculations show that the push-pull capability translates the position of an excess electron while keeping its width relatively constant. On the other hand, solvation changes both, making an electron more compact in polar media. The current study points to the idea that solvation plays a more significant role in controlling the nature of excess electrons, while synthetic modification that influences electron push-pull capability enables further tuning.

Intramolecular Long-Range Charge-Transfer Emission in Donor-Bridge-Acceptor Systems.

Charge recombination to the electronic ground state typically occurs nonradiatively. We report a rational design of donor-bridge-acceptor molecules that exhibit charge-transfer (CT) emission through conjugated bridges over distances of up to 24 Å. The emission is enhanced by intensity borrowing and extends into the near-IR region. Efficient charge recombination to the initial excited state results in recombination fluorescence. We have established the identity of CT emission by solvent dependence, sensitivity to temperature, femtosecond transient absorption spectroscopy, and unique emission polarization patterns. Large excited-state electronic couplings and small energy gaps enable the observation of intramolecular long-range CT emission over the unprecedented long distance. These results open new possibilities of using intramolecular long-range CT emission in molecular electronic and biomedical imaging probe applications.

Nitrile Vibration Reports Induced Electric Field and Delocalization of Electron in the Charge-Transfer State of Aryl Nitriles.

An electric field is created upon photoinduced charge separation in electron donor-acceptor (D-A) molecules. The photophysics of a prototypical D-A molecule, 4-(dimethylamino)-benzonitrile (DMABN), has been under extensive investigation for decades. Here, by using the framework of the vibrational Stark effect (VSE), we show that the nitrile vibration quantifies a significant induced electric field in the intramolecular charge-transfer state of DMABN. We further demonstrate that such a phenomenon can be observed in a structurally similar aryl nitrile and that the VSE depends on solvent polarity due to dielectric screening. Our current work shows how the superb sensitivity of the nitrile vibration can be used to identify the nature of electron delocalization and quantify the induced electric field in photoinduced charge transfer processes.

Probing Intermolecular Electron Delocalization in Dimer Radical Anions by Vibrational Spectroscopy.

Delocalization of charges is one of the factors controlling charge transport in conjugated molecules. It is considered to play an important role in the performance of a wide range of molecular technologies, including organic solar cells and organic electronics. Dimerization reactions are well-suited as a model to investigate intermolecular spatial delocalization of charges. While dimerization reactions of radical cations are well investigated, studies on radical anions are still scarce. Upon dimerization of radical anions with neutral counterparts, an electron is considered to delocalize over the two molecules. Here, by using time-resolved infrared (TRIR) detection coupled with pulse radiolysis, we show that radical anions of 4-n-hexyl-4'-cyanobiphenyl (6CB) undergo such dimerization reactions, with an electron equally delocalized over the two molecules. We have recently demonstrated that nitrile ν(C≡N) vibrations respond to the degree of electron localization of nitrile-substituted anions: we can quantify the changes in the electronic charges from the neutral to the anion states in the nitriles by monitoring the ν(C≡N) IR shifts. In the first part of this article, we show that the sensitivity of the ν(C≡N) IR shifts does not depend on solvent polarity. In the second part, we describe how probing the shifts of the nitrile IR vibrational band unambiguously confirms the formation of dimer radical anions, with Kdim = 3 × 104 M-1. IR findings are corroborated by electronic absorption spectroscopy and electronic structure calculations. We find that the presence of a hexyl chain and the formation of π-π interactions are both crucial for dimerization of radical anions of 6CB with neutral 6CB. The present study provides clear evidence of spatial delocalization of electrons over two molecular fragments.

Gold Tris(carboxyphenyl)corroles as Multifunctional Materials: Room Temperature Near-IR Phosphorescence and Applications to Photodynamic Therapy and Dye-Sensitized Solar Cells.

Two amphiphilic corroles-5,10,15-tris(3-carboxyphenyl)corrole (H3[mTCPC]) and 5,10,15-tris(4-carboxyphenyl)corrole (H3[pTCPC])-and their gold complexes have been synthesized, and their photophysical properties and photovoltaic behavior have been investigated. Like other nonpolar gold corroles, Au[mTCPC] and Au[pTCPC] were both found to exhibit room temperature phosphorescence in deoxygenated solutions with quantum yields of ∼0.3% and triplet lifetimes of ∼75 µs. Both compounds exhibited significant activity as dyes in photodynamic therapy experiments and in dye-sensitized solar cells. Upon irradiation at 435 nm, both Au corroles exhibited significant phototoxicity against AY27 rat bladder cancer cells while the free-base corroles proved inactive. Dye-sensitized solar cells constructed using the free bases H3[mTCPC] and H3[pTCPC] exhibited low efficiencies (≪1%), well under that obtained with 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin, H2[pTCPP] (1.9%, cf. N719 9.5%). Likewise, Au[pTCPC] proved inefficient, with an efficiency of ∼0.2%. By contrast, Au[mTCPC] proved remarkably effective, exhibiting an open-circuit voltage (Voc) of 0.56 V, a short-circuit current of 8.7 mA cm(-2), a fill factor of 0.72, and an efficiency of 3.5%.