Directional charge separation in isolated organic semiconductor crystalline nanowires

Noble metal-free SERS: mechanisms and applications

Surface-enhanced Raman scattering (SERS) is a very important tool in vibrational spectroscopy. The coupling of nanomaterials induces local surface plasmon resonance (LSPR), which contributes greatly to SERS. Due to its remarkable sensitivity in trace detection, SERS has gained prominence in the fields of catalysis, biosensors, drug tracking, and optoelectronic devices. SERS activity is believed to be closely related to the LSPR and charge transfer (CT) of the material. Noble metal nanostructures have been commonly used as SERS-active substrates due to their strong local electric fields and relatively mature preparation, application, and enhancement mechanisms. In recent years, SERS research based on semiconductor materials has attracted significant attention because semiconductor materials have advantages such as repeatable preparation, simple pretreatment, stable SERS spectra and superior biocompatibility, stability, and reproducibility. Semiconductor-based SERS has the potential to enrich SERS theory and applications. Thus, the development of semiconductor materials will introduce a new epoch for SERS-based research. In this review, we outline the two main kinds of semiconductor SERS-active substrates: inorganic and organic semiconductor SERS-active substrates. We also provide an overview of the SERS mechanism for different kinds of materials and SERS-based applications.

Dual Time-Scale Proton Transfer and High-Energy, Long-Lived Excitons Unveiled by Broadband Ultrafast Time-Resolved Fluorescence in Adenine-Uracil RNA Duplexes

In contrast to the immense amount of research on electronically excited DNA, surprisingly little has been done about the excited states of RNA. Herein, we demonstrate an ultrafast broadband time-resolved fluorescence and fluorescence anisotropy study to probe directly the intrinsic fluorescence and overall dynamics of the fluorescence from a homopolymeric adenine·uracil RNA duplex adopting the A-form structure. The results unveiled complex deactivation through distinctive multichannels mediated by states of varied energy, a character of charge transfer, and a lifetime from sub-picosecond to nanoseconds. In particular, we observed an unprecedented kinetic isotopic effect and participation of unusual proton transfer from states in two discrete energies and time domains. We also identified a high-energy nanosecond emission that we attributed to its fluorescence anisotropy to long-lived weakly emissive excitons not reported in DNA. These distinguishing features originate from the stacking, pairing, and local hydration environment specific to the A-form conformation of the adenine·uracil double helix.

Synthesis and Electrochemical and Photophysical Properties of Azaterrylene Derivatives

A series of terrylene derivatives, such as monoazaterrylene (MATerry), 1,6-diazaterrylene (DiATerry) and pristine terrylene (Terry), were synthesized by changing the number of nitrogen atoms at the bay region (1 and 6 positions of the Terry core). The electrochemical measurements suggested that the first one-electron reduction and oxidation potentials became positively shifted with increasing numbers of nitrogen atoms. This agreed with the energies of the corresponding highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) states estimated by DFT methods. In contrast, the HOMO-LUMO gaps approximately remained constant. This trend is quite similar to the spectroscopic behaviors observed by absorption and fluorescence spectra. The solvent polarity-dependent spectroscopic trends of DiATerry suggested the intramolecular charge-transfer (ICT) characters. The evaluation of the excited-state dynamics in various solvents indicated the electronic configurational changes of the excited states relative to the ground state via the ICT. This was supported by the Lippert-Mataga plots. Finally, the reversible protonation and deprotonation processes were also observed.

Fluorescence Anisotropy Reloaded-Emerging Polarization Microscopy Methods for Assessing Chromophores' Organization and Excitation Energy Transfer in Single Molecules, Particles, Films, and Beyond

Fluorescence polarization is widely used to assess the orientation/rotation of molecules, and the excitation energy transfer between closely located chromophores. Emerging since the 1990s, single molecule fluorescence spectroscopy and imaging stimulate the application of light polarization for studying molecular organization and energy transfer beyond ensemble averaging. Here, traditional fluorescence polarization and linear dichroism methods used for bulk samples are compared with techniques specially developed for, or inspired by, single molecule fluorescence spectroscopy. Techniques for assessing energy transfer in anisotropic samples, where the traditional fluorescence anisotropy framework is not readily applicable, are discussed in depth. It is shown that the concept of a polarization portrait and the single funnel approximation can lay the foundation for alternative energy transfer metrics. Examples ranging from fundamental studies of photoactive materials (conjugated polymers, light-harvesting aggregates, and perovskite semiconductors) to Förster resonant energy transfer (FRET)-based biomedical imaging are presented. Furthermore, novel uses of light polarization for super-resolution optical imaging are mentioned as well as strategies for avoiding artifacts in polarization microscopy.

Mesomorphic Behavior in Silver(I) N-(4-Pyridyl) Benzamide with Aromatic π⁻π Stacking Counterions

Organic semiconductor materials composed of π–π stacking aromatic compounds have been under intense investigation for their potential uses in flexible electronics and other advanced technologies. Herein we report a new family of seven π–π stacking compounds of silver(I) bis-N-(4-pyridyl) benzamide with varying counterions, namely [Ag(NPBA)2]X, where NPBA is N-(4-pyridyl) benzamine, X = NO₃⁻ (1), ClO₄⁻ (2), CF₃SO₃⁻ (3), PF₆⁻ (4), BF₄⁻ (5), CH₃PhSO₃⁻ (6), and PhSO₃⁻ (7), which form extended π−π stacking networks in one-dimensional (1D), 2D and 3D directions in the crystalline solid-state via the phenyl moiety, with average inter-ring distances of 3.823 Å. Interestingly, the counterions that contain π–π stacking-capable groups, such as in 6 and 7, can induce the formation of mesomorphic phases at 130 °C in dimethylformamide (DMF), and can generate highly branched networks at the mesoscale. Atomic force microscopy studies showed that 2D interconnected fibers form right after nucleation, and they extend from ~30 nm in diameter grow to reach the micron scale, which suggests that it may be possible to stop the process in order to obtain nanofibers. Differential scanning calorimetry studies showed no remarkable thermal behavior in the complexes in the solid state, which suggests that the mesomorphic phases originate from the mechanisms that occur in the DMF solution at high temperatures. An all-electron level simulation of the band gaps using NRLMOL (Naval Research Laboratory Molecular Research Library) on the crystals gave 3.25 eV for (1), 3.68 eV for (2), 1.48 eV for (3), 5.08 eV for (4), 1.53 eV for (5), and 3.55 eV for (6). Mesomorphic behavior in materials containing π–π stacking aromatic interactions that also exhibit low-band gap properties may pave the way to a new generation of highly branched organic semiconductors.

Expanded Theory of H- and J-Molecular Aggregates: The Effects of Vibronic Coupling and Intermolecular Charge Transfer

The electronic excited states of molecular aggregates and their photophysical signatures have long fascinated spectroscopists and theoreticians alike since the advent of Frenkel exciton theory almost 90 years ago. The influence of molecular packing on basic optical probes like absorption and photoluminescence was originally worked out by Kasha for aggregates dominated by Coulombic intermolecular interactions, eventually leading to the classification of J- and H-aggregates. This review outlines advances made in understanding the relationship between aggregate structure and photophysics when vibronic coupling and intermolecular charge transfer are incorporated. An assortment of packing geometries is considered from the humble molecular dimer to more exotic structures including linear and bent aggregates, two-dimensional herringbone and "HJ" aggregates, and chiral aggregates. The interplay between long-range Coulomb coupling and short-range charge-transfer-mediated coupling strongly depends on the aggregate architecture leading to a wide array of photophysical behaviors.

Interface Engineering via Photopolymerization-Induced Phase Separation for Flexible UV-Responsive Phototransistors

Interface engineering has been recognized to be substantially critical for achieving efficient charge separation, charge carrier transport, and enhanced device performance in emerging optoelectronics. Nevertheless, precise control of the interface structure using current techniques remains a formidable challenge. Herein, we demonstrate a facile and versatile protocol wherein in situ thiol-ene click photopolymerization-induced phase separation is implemented for constructing heterojunction semiconductor interfaces. This approach generates continuous mountainlike heterojunction interfaces that favor efficient exciton dissociation at the interface while providing a continuous conductive area for hole transport above the interface. This facile low-temperature paradigm presents good adaptability to both rigid and flexible substrates, offering high-performance UV-responsive phototransistors with a normalized detectivity up to 6.3 × 10¹⁴ cm Hz^1/2 W^-1 (also called jones). Control experiments based on ex situ photopolymerization and in situ thermal polymerization are also implemented to demonstrate the superiority of this novel paradigm.

Triisopropylsilylethynyl-substituted indenofluorenes: carbonyl versus dicyanovinylene functionalization in one-dimensional molecular crystals and solution-processed n-channel OFETs

Carbonyl vs. dicyanovinylene functionalization in indenofluorenes leads to substantial differences in solid-state packings and results in significant variations (×1000) in electron mobilities.

Disentangling "Bright" and "Dark" Interactions in Ordered Assemblies of Organic Semiconductors

We report on spatially correlated wavelength-resolved photoluminescence and Kelvin probe force microscopy to probe ground state charge-transfer coupling and its correlation with pi-stacking order in nanoscale assemblies of a small molecule n-type organic semiconductor, tetraazaterrylene (TAT). We find a distinct upshift in surface potential contrast (SPC) corresponding to a decrease in work function in TAT in the transition from disordered spun-cast films to ordered crystalline nanowire assemblies, accompanied by a nanowire size dependence in the SPC shift suggesting that the shift depends on both ground state charge transfer interaction and a size (volume)-dependent intrinsic doping associated with the nitrogen substitutions. For the smallest nanowires studied (surface height ≈ 10-15 nm), the SPC shift with respect to disordered films is +110 meV, in close agreement with recent theoretical calculations. These results illustrate how "dark" (ground-state) interactions in organic semiconductors can be distinguished from "bright" (excited-state) exciton coupling typically assessed by spectral measurements alone.

Nanostructured organic semiconductor films for molecular detection with surface-enhanced Raman spectroscopy

π-Conjugated organic semiconductors have been explored in several optoelectronic devices, yet their use in molecular detection as surface-enhanced Raman spectroscopy (SERS)-active platforms is unknown. Herein, we demonstrate that SERS-active, superhydrophobic and ivy-like nanostructured films of a molecular semiconductor, α,ω-diperfluorohexylquaterthiophene (DFH-4T), can be easily fabricated by vapour deposition. DFH-4T films without any additional plasmonic layer exhibit unprecedented Raman signal enhancements up to 3.4 × 10³ for the probe molecule methylene blue. The combination of quantum mechanical computations, comparative experiments with a fluorocarbon-free α,ω-dihexylquaterthiophene (DH-4T), and thin-film microstructural analysis demonstrates the fundamental roles of the π-conjugated core fluorocarbon substitution and the unique DFH-4T film morphology governing the SERS response. Furthermore, Raman signal enhancements up to ∼10¹⁰ and sub-zeptomole (<10^-21 mole) analyte detection were accomplished by coating the DFH-4T films with a thin gold layer. Our results offer important guidance for the molecular design of SERS-active organic semiconductors and easily fabricable SERS platforms for ultrasensitive trace analysis.

Poly[2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene] Oligomer Single-Crystal Nanowires from Supercritical Solution and Their Anisotropic Exciton Dynamics

Supercritical fluids, exhibiting a combination of liquid-like solvation power and gas-like diffusivity, are a relatively unexplored medium for processing and crystallization of oligomer and polymeric semiconductors whose optoelectronic properties critically depend on the microstructure. Here we report oligomer crystallization from the polymer organic semiconductor, poly[2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT) in supercritical hexane, yielding needle-like single crystals up to several microns in length. We characterize the crystals' photophysical properties by time- and polarization-resolved photoluminescence (TPRPL) spectroscopy. These techniques reveal two-dimensional interchromophore coupling facilitated by the high degree of π-stacking order within the crystal. Furthermore, the crystals obtained from supercritical fluid were found to be similar photophysically as the crystallites found in solution-cast thin films and distinct from solution-grown crystals that exhibited spectroscopic signatures indicative of different packing geometries.

Organic Optoelectronic Materials: Mechanisms and Applications

Organic (opto)electronic materials have received considerable attention due to their applications in thin-film-transistors, light-emitting diodes, solar cells, sensors, photorefractive devices, and many others. The technological promises include low cost of these materials and the possibility of their room-temperature deposition from solution on large-area and/or flexible substrates. The article reviews the current understanding of the physical mechanisms that determine the (opto)electronic properties of high-performance organic materials. The focus of the review is on photoinduced processes and on electronic properties important for optoelectronic applications relying on charge carrier photogeneration. Additionally, it highlights the capabilities of various experimental techniques for characterization of these materials, summarizes top-of-the-line device performance, and outlines recent trends in the further development of the field. The properties of materials based both on small molecules and on conjugated polymers are considered, and their applications in organic solar cells, photodetectors, and photorefractive devices are discussed.