Control of Electron Flow Direction in Photoexcited Cycloplatinated Complex Containing Conjugated Polymer-Single-Walled Carbon Nanotube Hybrids

Exploring Solvent Effects on the Proton Transfer Processes of Selected Benzoxazole Derivatives by Femtosecond Time-Resolved Fluorescence and Transient Absorption Spectroscopies

Excited-state intramolecular proton transfer (ESIPT) is of great importance due to the large Stokes shift emission that can be observed in some ESIPT molecules. Although steady-state spectroscopies have been employed to study the properties of some ESIPT molecules, their excited-state dynamics have not been examined directly with time-resolved spectroscopy methods yet for a number of systems. Here, an in-depth investigation of the solvent effects on the excited-state dynamics of two prototypical ESIPT molecules, 2-(2'-hydroxyphenyl)-benzoxazole (HBO) and 2-(2'-hydroxynaphthalenyl)-benzoxazole (NAP), have been accomplished by using femtosecond time-resolved fluorescence and transient absorption spectroscopies. Solvent effects affect the excited-state dynamics of HBO more significantly than that of NAP. Particularly in the presence of water, the photodynamics pathways of HBO are changed, while only small changes can be found in NAP. An ultrafast ESIPT process that occurs within our instrumental response is observed for HBO, and this is followed by an isomerization process in ACN solution. However, in aqueous solution, the obtained syn-keto* after ESIPT can be solvated by water in about 3.0 ps, and the isomerization process is totally inhibited for HBO. The mechanism of NAP is different from HBO and is determined to be a two-step excited-state proton transfer process. Upon photoexcitation, NAP is deprotonated first in the excited state to generate the anion*, which can transfer to the syn-keto* form followed by an isomerization process.

Interfacial Engineering of Hybrid Polydopamine/Polypyrrole Nanosheets with Narrow Band Gaps for Fluorescence Sensing of MicroRNA

Nanoquencher-based biosensors have emerged as powerful tools for the detection of tumor markers, where challenges in efficiently docking the π-electron interaction interface toward nucleic acid probes containing π-electron-rich units of bases and fluorescent dyes still remain. Herein, we present hybrid polydopamine/polypyrrole nanosheets (PDA-PPy-NS) with π electron coupling and ultranarrow band gap (0.29 eV) by interfacial engineering of polymer hybrids at the nanoscale. PDA-PPy-NS were first prepared through oxidant-induced polymerization of pyrrole on PDA nanosheets. By utilizing fluorescent-dye-labeled single-stranded DNA as a probe, the hybrid nanoquencher showed ultrahigh fluorescence quenching ability, i.e., a Cy5-ssDNA/nanoquencher mass ratio of 36.9 under the complete quenching condition, which is comparable to that of graphene oxide. It was demonstrated that the energy level coupling of nanosheets and nucleic acid dye (Cy5) was the key factor contributing to the efficient photoinduced electron transfer (PET). Subsequently, the nanoquencher/DNA probe was proved to possess superior sensitivity and selectivity for efficient and reliable detection of miRNA-21 with a detection limit of 23.1 pM. Our work proves that the π-electron-rich biosensor interface can significantly enhance the PET efficiency, providing a theoretical basis for developing novel high-performance sensors.

Computational study of the water-driven graphene wrinkle life-cycle towards applications in flexible electronics

The ubiquitous presence of wrinkles in two-dimensional materials alters their properties significantly. It is observed that during the growth process of graphene, water molecules, sourced from ambient humidity or transferred method used, can get diffused in between graphene and the substrate. The water diffusion causes/assists wrinkle formation in graphene, which influences its properties. The diffused water eventually dries, altering the geometrical parameters and properties of wrinkled graphene nanoribbons. Our study reveals that the initially distributed wrinkles tend to coalesce to form a localized wrinkle whose configuration depends on the initial wrinkle geometry and the quantity of the diffused water. The movement of the localized wrinkle is categorized into three modes—bending, buckling, and sliding. The sliding mode is characterized in terms of velocity as a function of diffused water quantity. Direct bandgap increases linearly with the initial angle except the highest angle considered (21°), which can be attributed to the electron tunneling effect observed in the orbital analysis. The system becomes stable with an increase in the initial angle of wrinkle as observed from the potential energy plots extracted from MD trajectories and confirmed with the DOS plot. The maximum stress generated is less than the plastic limit of the graphene.

Electron-Phonon Scattering Is Much Weaker in Carbon Nanotubes than in Graphene Nanoribbons

Carbon nanotubes (CNTs) and graphene nanoribbons (GNRs) are lower-dimensional derivatives of graphene. Similar to graphene, they exhibit high charge mobilities; however, in contrast to graphene, they are semiconducting and thus are suitable for electronics, optics, solar energy devices, and other applications. Charge carrier mobilities, energies, and lifetimes are governed by scattering with phonons, and we demonstrate, using ab initio nonadiabatic molecular dynamics, that charge-phonon scattering is much stronger in GNRs. Focusing on a GNR and a CNT of similar size and electronic properties, we show that the difference arises because of the significantly higher stiffness of the CNT. The GNR undergoes large-scale undulating motions at ambient conditions. Such thermal geometry distortions localize wave functions, accelerate both elastic and inelastic charge-phonon scattering, and increase the rates of energy and carrier losses. Even though, formally, both CNTs and GNRs are quantum confined derivatives of graphene, charge-phonon scattering differs significantly between them. Showing good agreement with time-resolved photoconductivity and photoluminescence measurements, the study demonstrates that GNRs are quite similar to molecules, such as conjugated polymers, while CNTs exhibit extended features attributed to bulk materials. The state-of-the-art simulations alter the traditional view of graphene nanostructures and demonstrate that the performance can be tuned not only by size and composition but also by stiffness and response to thermal excitation.

Time-Resolved Spectroscopic Study of N,N-Di(4-bromo)nitrenium Ions in Acidic Aqueous Solution

Nitrenium ions are common reactive intermediates with high activities towards some biological nucleophiles. In this paper, we employed femtosecond transient absorption (fs-TA) and nanosecond transient absorption (ns-TA) as well as nanosecond time-resolved resonance Raman (ns-TR³) spectroscopy and density function theory (DFT) calculations to study the spectroscopic properties of the N(4,4′–dibromodiphenylamino)–2,4,6–trimethylpyridinium BF₄⁻ salt (1) in an acidic aqueous solution. Efficient cleavage of the N–N bond (4 ps) to form the N,N–di(4–bromophenyl)nitrenium ion (DN) was also observed in the acidic aqueous solution. As a result, the dication intermediate 4 appears more likely to be produced after abstracting a proton for the nitrenium ion DN in the acid solution first, followed by an electron abstraction to form the radical cation intermediate 3. These new and more extensive time-resolved spectroscopic data will be useful to help to develop an improved understanding of the identity, nature, and properties of nitrenium ions involved in reactions under acidic aqueous conditions.