Timothy Quainoo - Academia.edu (original) (raw)
Uploads
Papers by Timothy Quainoo
Nano Letters, Oct 10, 2022
Relationships between chemical structure and conductivity in ordered polymers (OPs) are difficult... more Relationships between chemical structure and conductivity in ordered polymers (OPs) are difficult to probe using bulk samples. We propose that conductance measurements of appropriate molecular-scale models can reveal trends in electronic coupling(s) between repeat units that may help inform OP design. Here we apply the scanning tunneling microscope-based break junction (STM-BJ) method to study transport through single-molecules comprising OPrelevant imine, imidazole, diazaborole, and boronate ester dynamic covalent chemical bridges. Notably, solution-stable boron-based compounds hydrolyze in situ unless measured under a rigorously inert glovebox atmosphere. We find that junction conductance correlates with the electronegativity difference between bridge atoms, and corroborative first-principles calculations further reveal a different nodal structure in the transmission eigenchannels of boronate ester junctions. This work reaffirms expectations that highly polarized bridge motifs represent poor choices for the construction of OPs with high through-bond conductivity and underscores the utility of glovebox STM-BJ instrumentation for studies of air-sensitive materials.
Dynamic covalent chemistry (DCC) plays a critical role in the preparation of extended polymeric m... more Dynamic covalent chemistry (DCC) plays a critical role in the preparation of extended polymeric materials such as covalent-organic frameworks (COFs). Using DCC, the formation of targeted equilibrium, rather than kinetic, products are driven by the error-correcting capabilities of the reversible bond forming reactions involved. As work to develop conductive COFs (and metal-organic frameworks, MOFs) intensifies, it is of increasing interest to characterize the electronic transparency of bridge motifs formed from different DCC reactions. Here we apply the scanning tunneling microscope-based break junction (STM-BJ) method to measure the conductance of atomically-precise molecular junctions comprising imine, imidazole, diazaborole, and boronic ester bridge groups. Their comparison is facilitated through utilization of a glovebox-based STM-BJ setup operating under an inert atmosphere that avoids the apparent hydrolysis of boronic ester-containing compounds when these are studied in air. We find that junction transport generally increases as the difference in electronegativity (Δχ) between bridge group atoms decreases, and that conductance decays most rapidly with distance for compounds comprising boronic esters. Our experimental results are supported by first-principles calculations that reveal a different nodal structure in the transmission eigenchannel in boronic ester-containing systems compared to the other molecules. Taken together, our work reaffirms expectations that highly polarized bridge motifs represent poor choices for the preparation of extended materials with high through-bond electronic conductivity. We propose that such molecular-scale transport studies of "framework fragments" can provide new insights into the intrinsic properties of bulk COF and MOF systems that may be exploited in the design of improved materials.
International Journal of Chemical and Biochemical Sciences, 2021
The study focused on using locally produced palm kernel oil, palm ethanol and CaO catalyst from g... more The study focused on using locally produced palm kernel oil, palm ethanol and CaO catalyst from guinea fowl eggshells for biodiesel production. The physicochemical properties such as density (0.8936±0.00008165 g/ml), viscosity (83.32±0.193cP/P), refractive index (1.4540±0.00036), pH (5.89±0.084 mol/L H +), acid value (13.9±0.3 mgKOH/g), saponification value (283.305±23.24 mgKOH/g) and iodine value (20.7±5.77 mgI 2 /g) of the crude oil was determined. The transesterification process was carried out by optimizing reaction conditions using the One-Factor (OFAC) at a time method. The biodiesel produced from optimum conditions had density (0.877±0.001g/ml), iodine value (13.97±1.037 mgI 2 /g) and cetane index (76.215±0.531). The results also gave a kinematic viscosity (7.87±0.531 mm²/s), pour point (8.667±0.943°C), acid value (0.92±0.145 mgKOH/g) and saponification value (161.75±11.017 mgKOH/g).
Journal of Materials Research
Metal–organic frameworks (MOFs) have received significant attention thanks to their promising fea... more Metal–organic frameworks (MOFs) have received significant attention thanks to their promising features in the storage and separation of guest molecules. MOFs without open metal sites are emerging as they are often less susceptible to poisoning compared to those with open metal sites. However, a complete understanding of the binding and gas separation mechanisms in such materials is still missing. In this work, we perform a comparative study of two classes of vanadium-based MOFs without open metal sites: MFM-300-V$$^\text{(III)}$$ (III) and MFM-300-V$$^\text{(IV)}$$ (IV) , as well as MIL-47-V$$^\text{(III)}$$ (III) and MIL-47-V$$^\text{(IV)}$$ (IV) . We employ first-principles van der Waals density functional theory to find the optimal binding conformations and binding energies of a series of small hydrocarbons within the pores of the aforementioned MOFs. Our study provides insight into the host–guest interactions in such MOFs without open metal sites, especially the role played by t...
Nano Letters, Oct 10, 2022
Relationships between chemical structure and conductivity in ordered polymers (OPs) are difficult... more Relationships between chemical structure and conductivity in ordered polymers (OPs) are difficult to probe using bulk samples. We propose that conductance measurements of appropriate molecular-scale models can reveal trends in electronic coupling(s) between repeat units that may help inform OP design. Here we apply the scanning tunneling microscope-based break junction (STM-BJ) method to study transport through single-molecules comprising OPrelevant imine, imidazole, diazaborole, and boronate ester dynamic covalent chemical bridges. Notably, solution-stable boron-based compounds hydrolyze in situ unless measured under a rigorously inert glovebox atmosphere. We find that junction conductance correlates with the electronegativity difference between bridge atoms, and corroborative first-principles calculations further reveal a different nodal structure in the transmission eigenchannels of boronate ester junctions. This work reaffirms expectations that highly polarized bridge motifs represent poor choices for the construction of OPs with high through-bond conductivity and underscores the utility of glovebox STM-BJ instrumentation for studies of air-sensitive materials.
Dynamic covalent chemistry (DCC) plays a critical role in the preparation of extended polymeric m... more Dynamic covalent chemistry (DCC) plays a critical role in the preparation of extended polymeric materials such as covalent-organic frameworks (COFs). Using DCC, the formation of targeted equilibrium, rather than kinetic, products are driven by the error-correcting capabilities of the reversible bond forming reactions involved. As work to develop conductive COFs (and metal-organic frameworks, MOFs) intensifies, it is of increasing interest to characterize the electronic transparency of bridge motifs formed from different DCC reactions. Here we apply the scanning tunneling microscope-based break junction (STM-BJ) method to measure the conductance of atomically-precise molecular junctions comprising imine, imidazole, diazaborole, and boronic ester bridge groups. Their comparison is facilitated through utilization of a glovebox-based STM-BJ setup operating under an inert atmosphere that avoids the apparent hydrolysis of boronic ester-containing compounds when these are studied in air. We find that junction transport generally increases as the difference in electronegativity (Δχ) between bridge group atoms decreases, and that conductance decays most rapidly with distance for compounds comprising boronic esters. Our experimental results are supported by first-principles calculations that reveal a different nodal structure in the transmission eigenchannel in boronic ester-containing systems compared to the other molecules. Taken together, our work reaffirms expectations that highly polarized bridge motifs represent poor choices for the preparation of extended materials with high through-bond electronic conductivity. We propose that such molecular-scale transport studies of "framework fragments" can provide new insights into the intrinsic properties of bulk COF and MOF systems that may be exploited in the design of improved materials.
International Journal of Chemical and Biochemical Sciences, 2021
The study focused on using locally produced palm kernel oil, palm ethanol and CaO catalyst from g... more The study focused on using locally produced palm kernel oil, palm ethanol and CaO catalyst from guinea fowl eggshells for biodiesel production. The physicochemical properties such as density (0.8936±0.00008165 g/ml), viscosity (83.32±0.193cP/P), refractive index (1.4540±0.00036), pH (5.89±0.084 mol/L H +), acid value (13.9±0.3 mgKOH/g), saponification value (283.305±23.24 mgKOH/g) and iodine value (20.7±5.77 mgI 2 /g) of the crude oil was determined. The transesterification process was carried out by optimizing reaction conditions using the One-Factor (OFAC) at a time method. The biodiesel produced from optimum conditions had density (0.877±0.001g/ml), iodine value (13.97±1.037 mgI 2 /g) and cetane index (76.215±0.531). The results also gave a kinematic viscosity (7.87±0.531 mm²/s), pour point (8.667±0.943°C), acid value (0.92±0.145 mgKOH/g) and saponification value (161.75±11.017 mgKOH/g).
Journal of Materials Research
Metal–organic frameworks (MOFs) have received significant attention thanks to their promising fea... more Metal–organic frameworks (MOFs) have received significant attention thanks to their promising features in the storage and separation of guest molecules. MOFs without open metal sites are emerging as they are often less susceptible to poisoning compared to those with open metal sites. However, a complete understanding of the binding and gas separation mechanisms in such materials is still missing. In this work, we perform a comparative study of two classes of vanadium-based MOFs without open metal sites: MFM-300-V$$^\text{(III)}$$ (III) and MFM-300-V$$^\text{(IV)}$$ (IV) , as well as MIL-47-V$$^\text{(III)}$$ (III) and MIL-47-V$$^\text{(IV)}$$ (IV) . We employ first-principles van der Waals density functional theory to find the optimal binding conformations and binding energies of a series of small hydrocarbons within the pores of the aforementioned MOFs. Our study provides insight into the host–guest interactions in such MOFs without open metal sites, especially the role played by t...