Photophysics of Untethered ZnTPP–Fullerene Complexes in Solution

Molecular Tweezer Based on Perylene and Crown Ether for Selective Recognition of Fullerenes

A molecular tweezer trans-di(perylene-3-ylmethanaminobenzo)-18-crown-6 (DP-18C6) incorporating two perylene subunits in a single crown ether core was designed and synthesized as a host for fullerenes. Through the cooperative effect of the perylene subunits and the crown ether moiety, DP-18C6 can efficiently recognize fullerenes including C₆₀, C₇₀, and C₇₆. ¹H NMR titration and fluorescence titration experiments demonstrated that DP-18C6 can effectively grasp the fullerene molecule to form a 1:1 host-guest complex. Density functional theory calculations revealed the presence of intermolecular π-π interactions between the perylene subunits of DP-18C6 and the fullerene molecule. More importantly, DP-18C6 exhibited remarkably high binding selectivity for higher fullerenes over C₆₀, revealing potential application for the separation of fullerenes by means of host-guest interactions.

Reduced Recombination and Capacitor-like Charge Buildup in an Organic Heterojunction

Organic photovoltaic (OPV) efficiencies continue to rise, raising their prospects for solar energy conversion. However, researchers have long considered how to suppress the loss of free carriers by recombination-poor diffusion and significant Coulombic attraction can cause electrons and holes to encounter each other at interfaces close to where they were photogenerated. Using femtosecond transient spectroscopies, we report the nanosecond grow-in of a large transient Stark effect, caused by nanoscale electric fields of ∼487 kV/cm between photogenerated free carriers in the device active layer. We find that particular morphologies of the active layer lead to an energetic cascade for charge carriers, suppressing pathways to recombination, which is ∼2000 times less than predicted by Langevin theory. This in turn leads to the buildup of electric charge in donor and acceptor domains-away from the interface-resistant to bimolecular recombination. Interestingly, this signal is only experimentally obvious in thick films due to the different scaling of electroabsorption and photoinduced absorption signals in transient absorption spectroscopy. Rather than inhibiting device performance, we show that devices up to 600 nm thick maintain efficiencies of >8% because domains can afford much higher carrier densities. These observations suggest that with particular nanoscale morphologies the bulk heterojunction can go beyond its established role in charge photogeneration and can act as a capacitor, where adjacent free charges are held away from the interface and can be protected from bimolecular recombination.

Molecular-scale engineering of the charge-transfer excited states in non-covalently bound Zn–porphyrin and carbon fullerene based donor–acceptor complex

Tailoring charge-transfer through selective pyrrole ring hydrogenation in a novel Zn–porphyrin and PCBM based donor–acceptor complex has been investigated using quantum chemical computations.

Explication of bovine serum albumin binding with naphthyl hydroxamic acids using a multispectroscopic and molecular docking approach along with its antioxidant activity

In the present investigation, the protein-binding properties of naphthyl-based hydroxamic acids (HAs), N-1-naphthyllaurohydroxamic acid (1) and N-1-naphthyl-p-methylbenzohydroxamic acid (2) were studied using bovine serum albumin (BSA) and UV-visible spectroscopy, fluorescence spectroscopy, diffuse reflectance spectroscopy-Fourier transform infrared (DRS-FTIR), circular dichroism (CD), and cyclic voltammetry along with computational approaches, i.e. molecular docking. Alteration in the antioxidant activities of compound 1 and compound 2 during interaction with BSA was also studied. From the fluorescence studies, thermodynamic parameters such as Gibb's free energy (ΔG), entropy change (ΔS) and enthalpy change (ΔH) were calculated at five different temperatures (viz., 298, 303, 308, 313 or 318 K) for the HAs-BSA interaction. The results suggested that the binding process was enthalpy driven with dominating hydrogen bonds and van der Waals' interactions for both compounds. Warfarin (WF) and ibuprofen (IB) were used for competitive site-specific marker binding interaction and revealed that compound 1 and compound 2 were located in subdomain IIA (Sudlow's site I) on the BSA molecule. Conclusions based on above-applied techniques signify that various non-covalent forces were involved during the HAs-BSA interaction. Therefore the resulted HAs-BSA interaction manifested its effect in transportation, distribution and metabolism for the drug in the blood circulation system, therefore establishing HAs as a drug-like molecule.

Personal reflections on 50 years of funding via the Canadian academic research grant system

The year 2019 marks the 50th anniversary of the author’s first application for federal grant funds to support his independent academic research program at the University of Saskatchewan. Scientific and funding milestones are highlighted; a few personal observations are included.

Hierarchical Hybrid Nanostructures Constructed by Fullerene and Molecular Tweezer

For the construction of well-defined hierarchical superstructures of pristine [60]fullerene (C₆₀) arrays, pyrene-based molecular tweezers (PT) were used as host molecules for catching and arranging C₆₀ guest molecules. The formation of host-guest complexes was systematically studied in solution as well as in the solid state. Two-dimensional proton nuclear magnetic resonance spectroscopic studies revealed that PT-host and C₆₀-guest complexes were closely related to the molecular self-assembly of PT. Ultraviolet and fluorescence spectroscopic titrations indicated the formation of stable 1:1 and 2:1 (PT/C₆₀) complexes. From the nonlinear curve-fitting analysis, equilibrium constants for the 1:1 (log K₁) and 2:1 (log K₂) complexes were estimated to be 4.96 and 5.01, respectively. X-ray diffraction results combined with transmission electron microscopy observations clearly exhibited the construction of well-defined layered superstructures of the PT-host and C₆₀-guest complexes. From electron mobility measurements, it was demonstrated that the well-defined hierarchical hybrid nanostructure incorporating a C₆₀ array exhibited a high electron mobility of 1.7 × 10^-2 cm² V^-1 s^-1. This study can provide a guideline for the hierarchical hybrid nanostructures of host-guest complex and its applications.

A quercetin-based flavanoid (rutin) reverses amyloid fibrillation in β-lactoglobulin at pH 2.0 and 358 K

β-lactoglobulin (BLG) is a well characterized milk protein and a model for folding and aggregation studies. Rutin is a quercetin based-flavanoid and a famous dietary supplement. It is a potential protector from coronary heart disease, cancers, and inflammatory bowel disease. In this study, amyloid fibrillation is reported in BLG at pH 2.0 and temperature 358 K. It is inhibited to some extent by rutin with a rate of 99.3 h^-1 M^-1. Amyloid fibrillation started taking place after 10 h of incubation and completed near 40 h at a rate of 16.6 × 10^-3 h^-1, with a plateau during 40-108 h. Disruption of tertiary structure of BLG and increased solvent accessibility of hydrophobic core seem to trigger intermolecular assembly. Increase in 7% β-sheet structure at the cost of 10% α-helical structures and the electron micrograph of BLG fibrils at 108 h further support the formation of amyloid. Although it could not block amyloidosis completely, and even the time required to reach plateau remains the same, a decrease of growth rate from 16.6 × 10^-3 to 13.5 × 10^-3 h^-1 was observed in the presence of 30.0 μM rutin. Rutin seems to block solvent accessibility of the hydrophobic core of BLG. A decrease in the fibril population was observed in electron micrographs, with the increase in rutin concentration. All evidences indicate reversal of fibrillation in BLG in the presence of rutin.

Prospects for efficient solar energy upconversion using metalloporphyrins as dual absorber-upconverters

The novel potential use of selected metalloporphyrins as dual absorber-upconverters in solar photovoltaics is discussed. Additional efficiencies are available if use can be made of the porphyrin's short-lived S2 state, which is formed directly by excitation in the strong Soret transition in the blue-violet and also by absorption in the Q bands followed by rapid intersystem crossing and upconversion by triplet-triplet annihilation. The main challenge in realizing a working photovoltaic based on such a protocol is that energy must be extracted from the S2 state of the porphyrin within its picosecond lifetime. The structure-property relationships that may be used to select metalloporphyrins with the longest possible intrinsic lifetimes are outlined. The prospects for energy extraction from S2via ultrafast electron transfer or ultrafast resonant electronic energy transfer within a solid structure designed to maximize efficiency are discussed. Both MOF and pendant porphyrin polymer structures offer reasonable possibilities.

Determinants of the efficiency of photon upconversion by triplet-triplet annihilation in the solid state: zinc porphyrin derivatives in PVA

Spectroscopic, photophysical and computational studies designed to expose and explain the differences in the efficiencies of non-coherent photon upconversion (NCPU) by triplet-triplet annihilation (TTA) have been carried out for a new series of alkyl-substituted diphenyl and tetraphenyl zinc porphyrins, both in fluid solution and in solid films. Systematic variations in the alkyl-substitution of the phenyl groups in both the di- and tetraphenyl porphyrins introduces small, but well-understood changes in their spectroscopic and photophysical properties and in their TTA efficiencies. In degassed toluene solution TTA occurs for all derivatives and produces the fluorescent S₂ product states in all cases. In PVA matrices, however, none of the di-phenylporphyrins exhibit measurable NCPU whereas all the tetraphenyl-substituted compounds remain upconversion-active. In PVA the NCPU efficiencies of the zinc tetraphenylporphyrins vary significantly with their steric characteristics; the most sterically crowded tetraphenyl derivative exhibits the greatest efficiency. DFT-D computations have been undertaken and help reveal the sources of these differences.

Electronic energy pooling in organic systems: a cross-disciplinary tutorial review

Electronic energy pooling via excited state (exciton) annihilation, primarily in organic systems, is reviewed in tutorial form. Cross-disciplinary terminologies and references are used and reference is made to the historical origins of the phenomena. Applications in organic photovoltaic and electroluminescent devices are addressed. Particular attention is paid to the kinetics of the processes involved; a standard format for all systems is developed. Within the organic materials framework, all triplet–triplet, triplet–singlet, and singlet–singlet annihilation processes are discussed. Examples from gas, liquid, and solid phase systems, including both homo- and hetero-species interactions, are employed. Particular attention is given to triplet–triplet annihilation processes in which product states other than the lowest excited singlet state are formed.

Excited state dynamics of organic semi-conducting materials

Time-resolved absorption and emission spectroscopy has been applied to investigate the dynamics of excited state processes in oligomer models for semi-conducting organic materials. Following the photo-excitation of a pentamer oligomer that is a model for the conjugated polymer MEH-PPV, an ultrafast component of a few picoseconds is observed for the decay of the initially formed transient species. Variable temperature absorption and emission spectra combined with X-ray crystallography and calculations support the assignment of this rapid relaxation process to an excited state conformational rearrangement from non-planar to more planar molecular configurations. The implications of the results for the overall photophysics of conjugated polymers are considered.

Corrole–ferrocene and corrole–anthraquinone dyads: synthesis, spectroscopy and photochemistry

Two different donor–acceptor systems based on corrole–ferrocene (Cor–Fc) and corrole–anthraquinone (Cor–AQ) have been designed and synthesized. Excited state properties of these dyads indicates intramolecular photoinduced electron transfer (PET) take place in these dyads and the electron-transfer rates (k_ET) was found to be ∼10¹¹s⁻¹. The charge separation (CS) and charge recombination (CR) are found to be identical.

TD-DFT calculations of the excited states of metalloporphyrins relevant to organic solar photovoltaic cells

The molecular orbital energies and symmetries, electronic state energies and symmetries, and orbital compositions and oscillator strengths for one-photon radiative transitions up to an energy of 4 eV have been calculated by DFT and TD-DFT methods for 15 d0 and d10 metalloporphyrins. Data for both singlet and triplet excited states are reported and used to identify potential candidates for use as photon upconverters by homomolecular triplet–triplet annihilation.

Photophysics of soret-excited tin(IV) porphyrins in solution

The photophysics of low-chlorin tin(IV) tetraphenylporphyrin dihydroxide, a core building block for axially substituted supramolecular tin porphyrin constructs, has been studied in a variety of hydrogen-bonding, nonpolar, and aprotic polar solvents using steady-state, nanosecond, and femtosecond time-resolved emission, and femtosecond time-resolved absorption methods. In hydrogen-bonding solvents the metalloporphyrin exists as solvated monomers, and its Soret-excited S2 state in these solvents exhibits the expected linear energy gap law relationship with first-order population decay times in the 0.8 to 1.7 ps range. Evidence is presented that this metalloporphyrin aggregates in other solvents at the concentrations typically used for these ultrafast measurements and yields species-averaged time-resolved data. Cw laser excitation in the Q-band under deaerated conditions produces weak S2-S0 fluorescence (photon upconversion) as a result of triplet-triplet annihilation.