Intramolecular Charge Separation Research Articles

Symmetry-Breaking Charge Separation in Molecular Constructs for Efficient Light Energy Conversion

The generation of electron–hole radical pair at the active layer of organic photovoltaics through symmetry-breaking charge separation (SB-CS) has a crucial role in enhancing open-circuit voltage (Voc) and thereby increasing power conversion efficiency. Since the SB-CS materials achieve intramolecular charge separation with a negligible energetic driving force and decelerated charge recombination (CR) rate, SB-CS has been subjected to extensive experimental and theoretical studies. This Focus Review assesses the fundamentals of photosynthetic reaction centers, especially the “special pair”, and discusses how covalent control over the geometric arrangement, surrounding dielectric medium, and substitutions on multichromophoric perylenediimide architecture affects the energy landscape of SB-CS and CR. We systematically summarize the kinetically favored undesirable radiative and non-radiative deactivation channels of SB-CS and CR processes on diverse chromophoric arrangements. Here, we suggest new rational design principles to fine-tune the electron-transfer dynamics at the molecular level to improve the performance of light–energy conversion devices.

Stable room temperature ferroelectricity in hydrogen-bonded supramolecular assemblies of ambipolar π-systems.

This article reports H-bonding driven supramolecular polymerization of naphthalimide (A)–thiophene (D)–naphthalimide (A) (ADnA, n = 1–4) conjugated ambipolar π-systems and its remarkable impact on room temperature ferroelectricity. Electrochemical studies confirm the ambipolar nature of these ADnA molecules with the HOMO–LUMO gap varying between 2.05 and 2.29 eV. Electron density mapping from ESP calculations reveals intra-molecular charge separation as typically observed in ambipolar systems. In the aggregated state, AD1A and AD2A exhibit bathochromically shifted absorption bands while AD3A and AD4A show typical H-aggregation with a hypsochromic shift. Polarization vs. electric field (P–E) measurements reveal stable room temperature ferroelectricity for these supramolecular assemblies, most prominent for the AD2A system, with a Curie temperature (Tc) ≈ 361 K and saturation polarization (Ps) of ∼2 μC cm−2 at a rather low coercive field of ∼2 kV cm−1. Control molecules, lacking either the ambipolar chromophore or the amide functionality, do not show any ferroelectricity, vindicating the present molecular and supramolecular design. Computational studies enable structural optimization of the stacked oligomer(s) of AD2A molecules and reveal a significant increase in the macro-dipole moment (in the range of 10–12 Debye) going from the monomer to the oligomer(s), which provides the rationale for the origin of ferroelectricity in these supramolecular polymers.

Mapping 13C hyperfine couplings and exchange interactions in short-lived charge separated states of rigid donor-bridge-acceptor dyads.

A detailed experimental study on reversible photo-induced intramolecular charge separation is presented based on nuclear magnetic resonance detection of chemically induced dynamic nuclear polarization. From variation of such polarization with the external magnetic field, the coupling constants of isotropic and anisotropic hyperfine interactions at individual 13C sites are measured in the short-lived charge separated state of dyad molecules composed of donor-bridge-acceptor parts. The objects of study were rigid donor-bridge-acceptor dyads, consisting of triarylamine as a donor, naphthalene diimide as an acceptor, and a meta-conjugated diethynylbenzene fragment as a bridge. By systematic variation of side groups in the bridging moiety, their influence on the electron withdrawing strength is traced. In combination with similar data for the 1H positions obtained previously for the same compounds [I. Zhukov et al., J. Chem. Phys. 152, 014203 (2020)], our results provide a reliable basis for the determination of the spin density distribution in the charge separated state of such dyads.

Bridge-Length- and Solvent-Dependent Charge Separation and Recombination Processes in Donor-Bridge-Acceptor Molecules.

The photoinduced intramolecular charge separation (CS) and charge recombination (CR) phenomena in a series of donor-bridge-acceptor (D-B-A) molecules are intensively investigated as a means of understanding electron transport through the π-B. Pyrene (Pyr) and triarylamine (TAA) moieties connected via phenylene Bs of various lengths are studied because their CS and CR behaviors can be readily monitored in real time by femtosecond transient absorption (fs-TA) spectroscopy. By combining the steady-state and fs-TA spectroscopic measurements in a variety of solvents together with chemical calculations, the parameters that govern the CS behaviors of these dyads were obtained, such as the solvent effects on free energy and the B-length-dependent electronic coupling (VDA) between D and A. We observed the sharp switch of the CS behavior with the increase of the solvent polarity and B-linker lengths. Furthermore, in the case of the shortest distance between D and A when the electron coupling is sufficiently large, we observed that the CS phenomenon occurs even in low-polar solvents. Upon increasing the length of B, CS occurs only in strong polar solvents. The distance-dependent decay constant of the CS rate is determined as ∼0.53 Å-1, indicating that CS is governed by superexchange tunneling interactions. The CS rate constants are also approximately estimated using Marcus electron transfer theory, and the results imply that the VDA value is the key factor dominating the CS rate, while the facile rotation of the phenylene B is important for modulating the lifetime of the charge-separated state in these D-B-A dyads. These results shed light on the practical strategy for obtaining a high CS efficiency with a long-lived CS state in TAA-B-Pyr derivatives.

A Multifunctional Tyrosine‐Immobilized PAH Molecule as a Universal Cathode Interlayer Enables High‐Efficiency Inverted Polymer Solar Cells

AbstractAmong the various biological components in the photosynthesis reaction, tyrosine (Tyr) is considered crucial for facilitation of charge/proton transfer. However, the practical realization of this capability in recent artificial solar‐energy conversion systems is limited by the required alignment of Tyr molecules with both host materials and themselves. In this work, Tyr−perylene diimide−Tyr (PDI‐Y) is designed for implementation of a molecular‐based artificial photosystem II, where PDI and Tyr act as an antenna and internal mediator, respectively. The experimental and theoretical investigations prove that the presence of covalently immobilized Tyr can enhance the intramolecular charge separation, leading to a significant reduction of the exciton recombination probability. To explore the feasibility of using Tyr‐immobilized PDI in practical applications, PDI‐Y is applied as a universal cathode interlayer in an inverted polymer solar cell (iPSC). The result demonstrates potential multifunctional roles of PDI‐Y as a sub‐photosensitizer and electron transport layer, leading to a superior photovoltaic performance in both fullerene (PC71BM) and nonfullerene (Y6) based iPSCs. Consequently, the simple yet efficient Tyr‐immobilization approach is expected to offer tremendous opportunities for the advancement of electronic and optoelectronic applications.

Intramolecular Photoinduced Charge Transfer and Recombination Dynamics in Vinylarene Terminated Organosilanes.

We report on charge-transfer dynamics of newly designed acceptor-donor-acceptor organosilanes, with a specific focus on how donor-acceptor combination and local chemical environment can be used to control the lifetime for intramolecular charge-separation between silane electron donors and organic acceptors. In this work linear oligosilanes were capped with arene-vinyl end groups of variable electron-accepting strength: weak (diester vinyl), intermediate (ester,cyano vinyl), and strong (dicyanovinyl). Ultrafast transient absorption spectroscopy was used to characterize their structure-dependent charge-transfer and recombination behaviors. All structures exhibit similar photoinduced ultrafast spectral dynamics that we ascribe to relaxation of the nascent charge-separated excited state followed by a return to the ground state via charge recombination. We find that relaxation of the nascent "hot" charge-separated excited state scales with the strength of dipole-dipole interactions between solvent molecules and the polar arene-vinyl acceptor. Furthermore, electron-accepting strength governs whether electronic coupling dictates charge recombination rate: weak acceptors produce charge-separated states that exhibit relatively large electronic coupling for back-electron transfer (approaching the adiabatic limit) that result in fast recombination, whereas the strong and moderate-strength acceptors support more stable charge-separated states with weaker coupling and longer lifetimes. We find that recombination rates increase substantially for structures with weak and moderate-strength acceptors in cyclohexane (i.e., negligible solvent reorganization energy), which we attribute to an increased electronic coupling in a nonpolar solvent environment where charge pairs are weakly screened. In contrast, for structures with strong electron acceptors, the very low reorganization energy of cyclohexane places back-electron transfer even further into the Marcus inverted regime, with a resultant increase in charge-separation lifetime. Together these results provide critical insights on how to tune photoinduced charge-transfer behavior in organic-inorganic hybrids that have potential material applications in molecular electronics and optoelectronics.

(Invited) Optical Control of the Membrane Potential of Living Cells with Donor-Acceptor-Linked Molecules

In living animal cells, there exists a difference in the electrical charge on the two extreme sides of the plasma membrane, that is, a slight excess of positive and negative ions outside and inside a cell, respectively. Such a charge difference produces a membrane potential (V m) across the plasma membrane. The V m (< 0) is controlled by various ion channels, and, especially, in neuronal cells, it is drastically changed for neurotransmission. In response to some input, the ion channels of neuronal cells get quickly activated such that the abovementioned difference is cancelled (∆V m > 0, depolarization). This electrical signal propagates along the neuronal axons like a wave through coordinated activation and deactivation of the ion channels. Therefore, the control of membrane potential and ion transport across the plasma membrane using a light source is an attractive strategy that can allow targeted, fast control of precisely defined events in the plasma membrane. In this symposium, we have demonstrated a novel general strategy for the control of membrane potential and ion transport by using donor-acceptor-linked molecules (D-A molecules) capable of intramolecular long-lived charge separation, nanocarriers for solubilization, membrane targeting of D-A molecules, and light (Figure 1).For the preparation of the D-A molecule, porphyrin (H2P) derivatives with ferrocene (Fc) and fullerene (C60) were synthesized to create a triadic arrangement (Fc-H2P-C60). We had previously reported that, under light irradiation, Fc-H2P-C60 yields the ferrocenium cation (Fc+)-H2P-C60 radical anion (C60 •-) with a long lifetime of ~0.01 ms [1]. For its solubilization in the physiological media and safer plasma membrane delivery, we focused on a lipoprotein in our body considering its amphiphilic property. High-density lipoprotein (HDL) can be reconstituted with recombinant lipid-binding proteins and phospholipids. We had previously reported that HDL genetically and chemically engineered with cytophilic peptides and fusogenic lipids is capable of high plasma membrane binding without significant cytotoxicity [2]. For this purpose, Fc-H2P-C60 was solubilized with one of our engineered HDL nanoparticles (cpHDL) and added to a cell culture medium of rat pheochromocytoma cells (PC12). The cell membrane delivery of Fc-H2P-C60 was suggested by confocal analysis and freeze-fracture electron microscopy. Subsequent light irradiation of PC12 cells increased the membrane potential (depolarization), which was not observed for the reference molecule (H2P) that was incapable of charge separation [3]. In addition, the inhibition of potassium ion flow across the membrane was found to be responsible for this depolarization (Figure 1). Our following study with 3 D-A molecules, including Fc-H2P-C60, revealed that the degree of the depolarization was positively correlated with the charge separation yield [4]. In this symposium, I will discuss more about the potential of applying our D-A molecules/carriers system in the biomedical fields.Figure 1. Schematic illustration of optical control of membrane potential and cation flow using a D-A molecule (Fc-H2P-C60)[1] Imahori, H. et al., J. Am. Chem. Soc. 2001, 123, 2607-2617.[2] Murakami, T., Biotechncol. J. 2012, 7, 762-767.; Kim, H. et al. Biochim. Biophys. Acta Biomembr. 2019, 1861, 183008.[3] Numata, T. et al., J. Am. Chem. Soc. 2012, 134, 6092-6095.[4] Takano, Y. et al., Chem. Sci. 2016, 7, 3331-3337. Figure 1

Unveiling Zwitterionization of Glycine in the Microhydration Limit.

Charge separation under solvation stress conditions is a fundamental process that comes in many forms in doped water clusters. Yet, the mechanism of intramolecular charge separation, where constraints due to the molecular structure might be intricately tied to restricted solvation structures, remains largely unexplored. Microhydrated amino acids are such paradigmatic molecules. Ab initio simulations are carried out at 300 K in the frameworks of metadynamics sampling and thermodynamic integration to map the thermal mechanisms of zwitterionization using Gly(H2O)n with n = 4 and 10. In both cases, a similar water-mediated proton transfer chain mechanism is observed; yet, detailed analyses of thermodynamics and kinetics demonstrate that the charge-separated zwitterion is the preferred species only for n = 10 mainly due to kinetic stabilization. Structural analyses disclose that bifurcated H-bonded water bridges, connecting the cationic and anionic sites in the fluctuating microhydration network at room temperature, are enhanced in the transition-state ensemble exclusively for n = 10 and become overwhelmingly abundant in the stable zwitterion. The findings offer potential insights into charge separation under solvation stress conditions beyond the present example.

Long-range light-modulated charge transport across the molecular heterostructure doped protein biopolymers.

Biological electron transfer (ET) across proteins is ubiquitous, such as the notable photosynthesis example, where light-induced charge separation takes place within the reaction center, followed by sequential ET via intramolecular cofactors within the protein. Far from biology, carbon dots (C-Dots) with their unique optoelectronic properties can be considered as game-changers for next-generation advanced technologies. Here, we use C-Dots for making heterostructure (HS) configurations by conjugating them to a natural ET mediator, the hemin molecule, thus making an electron donor–acceptor system. We show by transient absorption and emission spectroscopy that the rapid intramolecular charge separation happens following light excitation, which can be ascribed to an ultrafast electron and hole transfer (HT) from the C-Dot donor to the hemin acceptor. Upon integrating the HS into a protein matrix, we show that this HT within the HS configuration is 3.3 times faster compared to the same process in solution, indicating the active role of the protein in supporting the rapid light-induced long-range intermolecular charge separation. We further use impedance, electrochemical, and transient photocurrent measurements to show that the light-induced transient charge separation results in an enhanced ET and HT efficiency across the protein biopolymer. The charge conduction across our protein biopolymers, reaching nearly 0.01 S cm−1, along with the simplicity and low-cost of their formation promotes their use in a variety of optoelectronic devices, such as artificial photosynthesis, photo-responsive protonic–electronic transistors, and photodetectors.

Benzoyl isothiocyanate as a precursor to design of ultrathin and high-crystalline g-C3N4-based donor–acceptor conjugated copolymers for superior photocatalytic H2 production

Graphitic carbon nitride (g-C3N4) has been demonstrated as ideal photocatalyst for photocatalytic H2 evolution, but simultaneously improves the photocatalytic activity and the apparent quantum efficiency (AQE) remains a big challenge. Recently, ultrathin and donor–acceptor (D-A) conjugated structure have attracted considerable attention in photocatalytic H2 evolution, which are capable to favorable provide more reactive sites and forward intramolecular charge separation. Inspired by these advantages, benzoyl isothiocyanate (BI) as a precursor is used to design the ultrathin g-C3N4 (UCN)-BIx D-A conjugated copolymers by copolymerization with urea, which can favor forward intramolecular charge separation so as to achieve ultrahigh photocatalytic activity and AQE in photocatalytic H2 evolution. The experimental results and density functional theory (DFT) calculations reveal that the obtained UCN-BIx D-A conjugated copolymers not only represent effective suppression of charge reverses recombination and but also broaden the range of light absorption. Additionally, UCN-BIx D-A conjugated copolymers has higher crystallinity and more negative conduction band (CB) position relative to the pure g-C3N4. And femtosecond transient absorption (fs-TA) spectroscopy also indicates that UCN-BI400 D-A conjugated copolymer shows the forward transfer ability of charge carriers is enhanced effectively. Marvelously, the best photocatalytic H2 evolution activity over UCN-BI400 D-A conjugated copolymer (5442.74 μmol g-1h−1) has driven by visible light exhibits an 11.96-fold enhancement relative to pure g-C3N4. Most noteworthy, the AQE of 23.7% and 7.0% is achieved at 420 nm and 450 nm that far exceeds majority of the previously reported g-C3N4-based D-A conjugated structures and UCN nanosheets. This contribution extends a novel design concept of effectively combining the ultrathin and D-A conjugated structures to simultaneously improving the activity and AQE of photocatalytic H2 production.

The Efficiency of Photoinduced Intramolecular Charge Separation from the Second Excited State: What Factors Can Control It?

The efficiency of photoinduced charge separation (CS) in electron donor-acceptor compounds is commonly limited due to fast deactivation processes, such as the excited-state internal conversion and ultrafast hot reverse electron transfer to the acceptor, charge recombination (CR). A traditional way to avoid undesired energy losses due to CR is to put the reverse electron transfer into the Marcus inverted region, thus effectively suppressing it. This method, however, is not generally applicable when considering CS from the second locally excited state because the driving force of CR to the first excited state is small, and thus charge recombination is ultrafast and efficient. In this paper, we study the kinetic features of CS/CR from the second locally excited state of the donor using a semiclassical stochastic model of electron transfer. Particular attention is paid to the CS efficiency as well as the influence of the polar environment and intramolecular high-frequency vibrational modes on the kinetics of the charge-separated state. The influence of a number of model parameters on the CS yield and the energy efficiency has been analyzed using the results of numerical simulations. Several simple practical recipes for creating molecular compounds with high CS yields have been suggested. Simulations have also revealed a strong and non-monotonous (double-humped) dependence of both the yield and energy efficiency of CS on the driving force.

Vibrational decoherence induced by ultrafast intramolecular charge separation of an asymmetric bianthryl derivative.

Coherent wavepacket oscillation accompanying the ultrafast photoexcited intramolecular charge separation (CS) of 9,9'-bianthryl (BA) and 10-cyano-9,9'-bianthryl (CBA) in a room temperature ionic liquid, N,N-diethyl-N-methyl-N-(methoxyethyl)ammonium tetrafluoroborate (DemeBF4), was investigated by femtosecond time-resolved transient absorption spectroscopy. The frequency of the coherent oscillation observed for CBA in nonpolar n-hexane solution (Hex) was 377 cm-1, while this oscillation was undetectable in DemeBF4. For BA in DemeBF4, coherent oscillation with a frequency of 394 cm-1 was observed, which is similar to that for CBA in Hex. CS of CBA occurs in the ultrashort time range of ≤100 fs, while that of BA occurs in a few picosecond range [E. Takeuchi et al., J. Phys. Chem. C 120, 14502-14512 (2016)]. Hence, the oscillation of CBA in Hex and that of BA in DemeBF4 are assigned to the molecular vibration in the locally excited state, while this oscillation dephases instantaneously for CBA in DemeBF4 due to the ultrafast CS and no oscillation was generated in the CS state. This result suggests that the CS reaction is not mediated by a specific intramolecular vibration in the CS state but occurs incoherently through higher levels of multiple vibrational modes.

Quantum yield and energy efficiency of photoinduced intramolecular charge separation.

Kinetics of photoinduced intramolecular charge separation (CS) and the ensuing ultrafast charge recombination (CR) in electron-donor-acceptor dyads are studied numerically, taking into account the excitation of charge-transfer active intramolecular vibrations and multiple relaxation time scales of the surrounding polar solvent. Both energetic and dynamic properties of intramolecular and solvent reorganization are considered, and their influence on the CS/CR kinetics and quantum yield of ultrafast CS is explored. Particular attention is paid to the energy efficiency of CS, as one of the most important parameters indicating the promise of using a molecular compound as a basis for emerging optoelectronic devices. The CS quantum yield and the energy efficiency of CS are shown to depend differently on the key model parameters. Necessary conditions for the highly efficient CS are evaluated using analytic formulae for the electron transfer rates and derived from numerical simulation data. The reasons why low-exergonic CS taking place in the Marcus normal region can be much slower than CR in the deep inverted region are discussed.

A Synthetic Chlorophyll Dimer Appending Fullerene: Effect of Chlorophyll Pairing on (Photo)redox Properties.

Accurately mimicking structure and function of natural chlorophyll (Chl) assemblies is very challenging. Herein, we report the synthesis of a fullerene-appended Chl dimer being capable of intramolecular photoinduced charge separation (CS) with a unique structure reminiscent of reaction centers (RCs) in phototrophs. Structural analyses revealed that the Chl dimer adopts a bird-like structure in which two Chl components overlapped partially with one of the four pyrrole rings in a Chl ring similar to in a Chl pair in the natural RC complexes. A comparative study including voltammetry and spectrometric analyses using the Chl dimer and its corresponding monomer with and without a fullerene moiety was performed to gain insight into the effect of Chl pairing on (photo)redox properties. Our results suggest that the present dimer motif that closely resemble the Chl pair in natural RCs lead to more facile oxidation and lower energy of the CS state of the Chl dimer than those of the Chl monomer, resulting in its photoredox behavior different from that of the monomer Chl.

Exploring the simultaneous biothiols-differentiating detecting feature of a BODIPY chemosensor with DFT/TDDFT

The simultaneous biothiols-differentiating fluorescence detecting feature of a boron dipyrromethene (BODIPY) derivative (Probe1) has been investigated through DFT/TDDFT calculations. Two decay pathways are revealed for the fluorescence sensing process, and the probe's ability to simultaneously and fluorescently differentiate cysteine (Cys), homocysteine (Hcy), sulfureted hydrogen (H2S), glutathione (GSH) is associated with the different excited-state properties of Probe1 and the biothiols-induced reaction products: the S1 state of Probe1 and the GSH/H2S-induced reaction product NBD-S-GSH/NBD-S is non-emissive, with complete/obvious charge separation; whereas, the S1 state of Cys/Hcy-induced reaction product NBD-S-Cys/NBD-S-Hcy as well as a common product P1 from all biothiols-induced reactions is a bright emissive state. The energetic and charge features of the excited states suggest a donor photoinduced electron transfer mechanism for the fluorescence quenching. It is also found that the intramolecular charge separation character of S1 state accounts for some of spectral behaviors, such as the signal intensity changes and the spectral shift, which can also be utilized in the simultaneously differentiating sensing of biothiols.

Molecular engineering of polymeric carbon nitride for highly efficient photocatalytic oxytetracycline degradation and H2O2 production

Developing highly efficient photocatalysts for refractory pollutants degradation and hydrogen peroxide (H2O2) production is an ideal strategy to tackle environmental pollution and energy crisis. Herein, we synthesize a novel 2-hydroxy-4,6-dimethylpyrimidine (HDMP) grafted polymeric carbon nitride (CN) photocatalyst (ACN) by a facile in situ keto-enol cyclization method for addressing these issues. The photogenerated electrons and holes are migrated to HDMP and heptazine moiety, respectively. And the average decay lifetime of photogenerated charges of CN and ACN-10 is increased form (222 ± 23) to (289 ± 38) ps. Benefiting from the controllable electronic migration and accelerated intramolecular charge separation, the ACN photocatalyst exhibits a superior visible-light-driven photocatalytic activity for oxytetracycline degradation and H2O2 production. This work makes an insight into the intramolecular charge dynamics in CN, and presents a promising approach to the oxytetracycline degradation and H2O2 production.

Electron-Withdrawing Boron Dipyrromethene Dyes As Visible Light Absorber/Sensitizers on Semiconductor Oxide Surfaces.

The synthesis, characterization, and electrochemical and photophysical properties of the phosphonate-derivatized carbazole (CBZ) and boron dipyrromethene (BODIPY) chromophores in the dyes, BODIPY(CBZ)2PO3H2 (8) and BODIPY(Tol)2PO3H2 (7), are described. The oxide-bound dyes have been explored as light absorbers in dye-sensitized photoelectrosynthesis cell (DSPEC) applications. The BODIPY-CBZ phosphonate ester (6) features a broad, intense UV-visible absorption spectrum with absorptions at 297 and 650 nm that arise from mixed transitions at the CBZ and BODIPY units. Electrochemical measurements on BODIPY(CBZ)2Br (4) in 0.1 M [nBu4N][PF6] in dichloromethane, vs normal hydrogen electrode (NHE), reveal reversible oxidations at 1.19 and 1.41 V and a reversible reduction at -0.59 V. On indium tin oxide (ITO) and TiO2, a reversible one-electron oxidation appears for 7 at 0.86 and 0.90 V vs NHE in dichloromethane, respectively, which demonstrates the redox stability on metal oxide surfaces. The results of nanosecond transient absorption measurements on SnO2/TiO2 electrodes provide direct evidence for excited-state electron injection into the conduction band of TiO2 following 590 nm excitation. A longer lifetime for 8+ compared to 7+ is consistent with extensive intramolecular charge separation between the CBZ and BODIPY units on the surface. Photoelectrochemical studies on 8 on a SnO2/TiO2 photoanode resulted in sustained photocurrents with current maxima of ∼200 μA/cm2 with hydroquinone added as a reductant under 1 sun (AM1.5 100 mW·cm-2) illumination at pH 4.5 in 0.1 M acetate buffer and 0.4 M LiClO4. On mixed SnO2/TiO2 electrode surfaces, with the added catalyst [Ru(Mebimpy)((4,4'-(OH)2PO-CH2)2bpy)(OH2)]2+ and chromophores 7 and 8, addition of 0.1 M benzyl alcohol resulted in sustained photocurrents of 12 and 35 μA/cm2, consistent with oxidation to benzaldehyde.

Effect of nano-confinement on the structure and properties of water clusters: An ab initio study

An ab initio investigation on water clusters confined to armchair carbon nanotubes (CNT) with varying diameters has been performed using the density functional theory-based calculations. Different parameters have been investigated including structure, hydrogen bonding pattern and vibrational spectra of water-CNT complexes. Our results reveal that one-dimensional water chain parallel to CNT axis is formed in narrow nanotubes CNT(4,4) and CNT(5,5), whereas in CNT(6,6), zigzag structure is observed. An increase in the CNT diameter results in more symmetric structures similar to the gas phase. The vibrational analysis shows a redshift in stretching frequency of the hydrogen bond assisted O–H in CNT(6,6) due to the reduction in O—O separation whereas a significant blue shift in stretching frequency mode is observed in highly confined CNT(4,4) and CNT(5,5). It implies that the hydrogen bond strength between water molecules is strongest in CNT(6,6). It is also observed that water cluster tends to be near CNT wall due to H···π interaction between water molecule and the π-electron cloud of CNT. An inverse relation between the electronic charge transfer (from CNT to water) and the diameter is also established. This study demonstrates that the degree of confinement is extremely important in deciding the properties of confined water molecules. The structure and hydrogen bonding properties of the water clusters under nano-confined regime is investigated and the results reveal that the intramolecular charge separation for each water molecule increases under confinement and irrespective of the number of water molecules, the highest tube-water interaction energy is achieved in CNT(5,5) (diameter~7 A). The important finding is that the degree of confinement (diameter of carbon nanotube) is extremely important in deciding the properties of confined water molecules.

The influence of driving force on intramolecular electron transfer: A theoretical study of subphthalocyanine‐AzaBODIPY‐C60 supramolecular triad

AbstractThe driving force of electron transfer is one of important factors for initializing inter‐ and intramolecular charge separation. In this work, the main goal is to understand how driving force determines electron transfer pathway in subphthalocyanine‐AzaBODIPY‐C60 supramolecular triad. Experimental observations have suggested that there are only two intramolecular charge transfer states (subPC+‐AzaBODIPY−‐C60 and subPC+‐AzaBODIPY‐C60−) after photon absorption, where subPC is the donor. Through the calculations by using tuned long range corrected density functionals with polarizable continuum model, we find two more new intramolecular charge transfer states: subPC−‐AzaBODIPY+‐C60 and subPC‐AzaBODIPY+‐C60−, where AzaBODIPY is the donor. We compare the HOMO/LUMO energy of subPC, AzaBODIPY, and C60 monomers to their corresponding orbital energy in the triad. The results indicate that the driving force (HOMO/LUMO energy offsets) is not enough for electron transfer from AzaBODIPY to subPC or C60, which can explain why subPC−‐AzaBODIPY+‐C60 and subPC‐AzaBODIPY+‐C60− intramolecular charge transfer states cannot be observed in the experiment. In addition, this work may provide a simple and practical method to find the intramolecular charge transport pathway of a supramolecule.

2,4,6-Tri(4-pyridyl)-1,3,5-triazine: Photoinduced Charge Separation and Photochromism in the Crystalline State.

2,4,6-Tri(4-pyridyl)-1,3,5-triazine (tpt) is a widely used ligand for functional coordination compounds. In this work, tpt has shown unprecedented photochromism in the crystalline state. Experimental and theoretical data has revealed that the photocoloration of tpt very likely originates from intramolecular charge separation and the formation of a triplet diradical product. This finding demonstrates a new simple, neutral photochromic molecule and endows the tpt molecule and related compounds with potential optical applications.