Physical mechanisms of photoinduced charge transfer in neutral and charged donor–acceptor systems

In this paper, we provide visualization methods to reveal the physical mechanisms of photoinduced charge transfer in neutral and charged donor–acceptor systems. These visualization methods use the charge density difference and transition density matrix, which can promote deeper understanding of photoinduced charge transfer in donor–acceptor systems.

In this paper, we provide visualization methods to reveal the physical mechanisms of photoinduced charge transfer in neutral and charged donor–acceptor systems.     

Photophysical properties and fluorosolvatochromism of D–π–A thiophene based derivatives

Solvation-dependent photophysical properties of two push–pull thiophene-based compounds with donor–π–acceptor (D–π–A) structures were investigated using absorption, fluorescence emission and time resolved spectroscopy, and supported by different solvation models. Intramolecular charge transfer characteristics of the structurally similar 2-fluoro-4-(5-(4-methoxyphenyl)thiophen-2-yl)benzonitrile (MOT) and 4-(5-(4-(dimethylamino)phenyl)thiophen-2-yl)-2-fluorobenzonitrile (DMAT) were investigated. Significant enhancement of intramolecular charge transfer strength has been observed through molecular structure modification of the electron donating group from a methoxy to dimethylamine group. Ground state absorption spectra show a small red shift of about 10 nm and 18 nm while the fluorescence emission spectra show a large red shift of about 66 nm and 162 nm on changing from the nonpolar cyclohexane to the aprotic polar DMSO for MOT and DMAT, respectively. Dipole moment change from the ground state to the charge transfer excited state is calculated to be 6.6 D in MOT and 9.0 D in DMAT. The fluorescence quantum yield, fluorescence lifetime and the derived radiative and non-radiative rate constants were found to be better correlated to the emission energy rather than any of the solvent properties. Three multi-parametric relationships were used in the interpretation of the specific versus non-specific solute–solvent interactions, namely, Kamlet–Taft, Catalán and Laurence et al. models. The findings of these approaches are used to extract useful information about different aspects of solvent effects on the photophysical properties of the two studied compounds. Kamlet–Taft solvatochromic model indicates that non-specific interactions are dominant in controlling the photophysical properties. Catalán''s solvent dipolarity/polarizability parameter is found to play a significant role in solvatochromic behaviour which is also designated by the Laurence model.

Solvation-dependent photophysical properties of two push–pull thiophene-based compounds with donor–π–acceptor (D–π–A) structures were investigated using absorption, fluorescence emission and time resolved spectroscopy, and supported by different solvation models.     

Theoretical investigation of fused N-methyl-dithieno-pyrrole derivatives in the context of acceptor–donor–acceptor approach

In this work we have theoretically investigated the optoelectronic properties of a series of acceptor–donor–acceptor type molecules by employing density functional theory formalism. We have used 1,1-dicyano-methylene-3-indanone as the acceptor unit and a fused N-methyl-dithieno-pyrrole as the donor unit. We have calculated the values of dihedral angle, inter-ring bond length, bond length alteration parameters, HOMO–LUMO gap, ionization potential, electron affinity, partial density of states, reorganization energies for holes and electrons, charge transfer rate for holes and electrons of the seven types of compounds designed via molecular engineering. Calculated IP and EA values manifest that PBDB-C2 shows excellent charge transportation compared to others. Absorption spectra of the designed compounds have been studied using the time-dependent density functional theory method. From the calculation of reorganization energy it is confirmed that our designed molecules behave more likely as donor materials. Our calculated results also reveal that compounds with electron donating substituents at the acceptor units show higher value of λ_max. Absorption spectra of donor/acceptor blends show similar trends with the isolated compounds. Observed lower exciton binding energy values for all the compounds indicate facile charge carrier separation at the donor/acceptor interface. Moreover, the negative values of Gibb''s free energy change also indicate the ease of exciton dissociation of all the designed compounds. The photovoltaic characteristics of the studied compounds infer that all the designed compounds have the potential to become suitable candidate for the fabrication of organic semiconductors. However, PBDB-C2 and PBDB-C4 with the highest PCE of 18.25% can become the best candidate for application in photovoltaics.

In this work we have theoretically investigated the optoelectronic properties of a series of acceptor–donor–acceptor type molecules by employing density functional theory formalism.     

Insights into ultrafast charge-pair dynamics in P3HT:PCBM devices under the influence of static electric fields

Polymer-fullerene blends based on poly(3-hexylthiophene-2,5-diyl) (P3HT) and phenyl-C₆₁-butyric-acid methyl ester (PCBM) have been extensively studied as promising bulk heterojunction materials for organic semiconductor devices with improved performance. In these donor–acceptor systems where the bulk morphology plays a crucial role, the generation and subsequent decay mechanisms of photoexcitation species are still not completely understood. In this work, we use femtosecond transient absorption spectroscopy to investigate P3HT:PCBM diodes under the influence of applied static electric fields in comparison to P3HT:PCBM thin films. At the same time, we try to present a detailed overview about work already done on these donor–acceptor systems. The excited state dynamics obtained at 638 nm from P3HT:PCBM thin films are found to be similar to those observed earlier in neat P3HT films, while those obtained in the P3HT:PCBM devices are affected by field-induced exciton dissociation, resulting not only in comparatively slower decay dynamics, but also in bimolecular deactivation processes. External electric fields are expected to enhance charge generation in the investigated P3HT:PCBM devices by dissociating excitons and loosely bound intermediate species like polaron pairs (PPs) and charge transfer (CT) excitons, which can already dissociate only due to the intrinsic fields at the donor–acceptor interfaces. Our results clearly establish the formation of PP-like transient species different from CT excitons in the P3HT:PCBM devices as a result of a field-induced diffusion-controlled exciton dissociation process. We find that the loosely bound transient species formed in this way also are reduced in part via a bimolecular annihilation process resulting in charge loss in typical donor–acceptor P3HT:PCBM bulk heterojunction semiconductor devices, which is a rather interesting finding important for a better understanding of the performance of these devices.

Electric field effects in P3HT:PCBM solar cell result in polaron-pair-like secondary photoexcitation species showing slower and bimolecular decay characteristics.     

Impact of tunable 2-(1H-indol-3-yl)acetonitrile based fluorophores towards optical,thermal and electroluminescence properties

Herein, we have synthesized 4,5-diphenyl-1H-imidazole and 2-(1H-indol-3-yl)acetonitrile based donor–π–acceptor fluorophores and studied their optical, thermal, electroluminescence properties. Both the fluorophores exhibit high fluorescence quantum yield (Φ_f = <0.6) and good thermal stability (T_d10 = <300 °C), and could be excellent candidates for OLED applications. Moreover, the ground and excited state properties of the compounds were analysed in various solvents with different polarities. The geometric and electronic structures of the fluorophores in the ground and excited states have been studied using density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods. The absorption of BIPIAN and BITIAN in various solvents corresponds to S₀ → S₁ transitions and the most intense bands with respect to the higher oscillator strengths are mainly contributed by HOMO → LUMO transition. Significantly, the vacuum deposited non-doped OLED device was fabricated using BITIAN as an emitter, and the device shows electroluminescence (EL) at 564 nm, maximum current efficiency (CE) 0.687 cd A⁻¹ and a maximum external quantum efficiency (EQE) of 0.24%.

Herein, we have synthesized 4,5-diphenyl-1H-imidazole and 2-(1H-indol-3-yl)acetonitrile based donor–π–acceptor fluorophores and studied their optical, thermal, electroluminescence properties.     

Excited-state intramolecular proton transfer with and without the assistance of vibronic-transition-induced skeletal deformation in phenol–quinoline

The excited-state intramolecular proton transfer (ESIPT) reaction of two phenol–quinoline molecules (namely PQ-1 and PQ-2) were investigated using time-dependent density functional theory. The five-(six-) membered-ring carbocycle between the phenol and quinolone moieties in PQ-1 (PQ-2) actually causes a relatively loose (tight) hydrogen bond, which results in a small-barrier (barrier-less) on an excited-state potential energy surface with a slow (fast) ESIPT process with (without) involving the skeletal deformation motion up to the electronic excitation. The skeletal deformation motion that is induced from the largest vibronic excitation with low frequency can assist in decreasing the donor–acceptor distance and lowering the reaction barrier in the excited-state potential energy surface, and thus effectively enhance the ESIPT reaction for PQ-1. The Franck–Condon simulation indicated that the low-frequency mode with vibronic excitation 0 → 1′ is an original source of the skeletal deformation vibration. The present simulation presents physical insights for phenol–quinoline molecules in which relatively tight or loose hydrogen bonds can influence the ESIPT reaction process with and without the assistance of the skeletal deformation motion.

Skeletal deformation motion is demonstrated from the specific vibronic excitation of phenol–quinoline molecules.     

Synthesis and charge transfer characteristics of a ruthenium–acetylide complex

A novel ruthenium–acetylide complex was synthesised and characterised in solid state and solution. Thin films of the complex were evaporated on silver and gold foils in ultra high vacuum in order to probe the electronic properties with photoemission spectroscopy. The charge transfer characteristics of the complex with the strong acceptor F₆TCNNQ were investigated by UV-vis absorption in solution as well as at an interface with photoemission spectroscopy. A new excitation in the former optical gap of the pristine materials was probed in solution. Moreover, it was possible to identify the oxidised complex as well as the reduced acceptor by X-ray photoemission spectroscopy. In particular, our data reveal that oxidation of the complex mainly occurs at the Ru centre. The charge transfer can be characterised as localised and mainly ionic although signs of a reaction of the acceptors aminogroups with the ruthenium–acetylide complex were found.

A newly synthesised ruthenium–acetylide complex is characterised by various methods and its charge transfer to the acceptor F₆TCNNQ is studied by optical and photoelectron spectroscopy.     

Thermal dewetting tunes surface enhanced resonance Raman scattering (SERRS) performance

Surface Enhanced Raman Spectroscopy (SERS) belongs to the techniques of ultra-sensitive chemical analysis and involves both identification and quantification of molecular species. Despite the fact that theoretically derived enhancement factors imply that even single molecules may be identified, which in some cases has indeed been experimentally observed, the application of this specific technique as an analytical tool is still an open field of research due to the need for reproducible, stable and simple to prepare SERS active substrates. The current work attempts to contribute to the already established knowledge on the substrates of metallic nanostructured films by a systematic study on the optimal conditions required for the detection of a specifically selected (model) material, the antitumor drug mitoxantrone (MTX). Au thin film deposition on Si substrates, by sputtering followed by solid state thermal dewetting is a facile and reproducible way to prepare Au nanoparticles with the desired particle size distribution. This offers control over their optical – plasmon resonance – properties that can be efficiently tailored to the prerequisites of the resonance Raman conditions, associated to the species under inspection, which is a supplement to the overall enhancement scattering factor. Furthermore, this work attempts to confirm the quantification capabilities of SERS, via the aforementioned substrates, in view of extending SERS applications to food safety, biosensors etc.

Simple, reproducible and low-cost SERS substrates for ultra-sensitive chemical analysis/quantification offered by thermal dewetting of thin metallic films.     

First principles study of electronic and nonlinear optical properties of A–D–π–A and D–A–D–π–A configured compounds containing novel quinoline–carbazole derivatives

Materials with nonlinear optical (NLO) properties have significant applications in different fields, including nuclear science, biophysics, medicine, chemical dynamics, solid physics, materials science and surface interface applications. Quinoline and carbazole, owing to their electron-deficient and electron-rich character respectively, play a role in charge transfer applications in optoelectronics. Therefore, an attempt has been made herein to explore quinoline–carbazole based novel materials with highly nonlinear optical properties. Structural tailoring has been made at the donor and acceptor units of two recently synthesized quinoline–carbazole molecules (Q1, Q2) and acceptor–donor–π–acceptor (A–D–π–A) and donor–acceptor–donor–π–acceptor (D–A–D–π–A) type novel molecules Q1D1–Q1D3 and Q2D2–Q2D3 have been quantum chemically designed, respectively. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) computations are performed to process the impact of acceptor and donor units on photophysical, electronic and NLO properties of selected molecules. The λ_max values (321 and 319 nm) for Q1 and Q2 in DSMO were in good agreement with the experimental values (326 and 323 nm). The largest shift in absorption maximum is displayed by Q1D2 (436 nm). The designed compounds (Q1D3–Q2D3) express absorption spectra with an increased border and with a reduced band gap compared to the parent compounds (Q1 and Q2). Natural bond orbital (NBO) investigations showed that the extended hyper conjugation and strong intramolecular interaction play significant roles in stabilising these systems. All molecules expressed significant NLO responses. A large value of β_tot was elevated in Q1D2 (23 885.90 a.u.). This theoretical framework reveals the NLO response properties of novel quinoline–carbazole derivatives that can be significant for their use in advanced applications.

Materials with nonlinear optical properties have significant applications in nuclear science, biophysics, medicine, chemical dynamics, solid physics & materials science. We show how π bridges, donors & acceptors can be reconfigured to improve optical properties.     

Blue organic light-emitting diodes with hybridized local and charge-transfer excited state realizing high external quantum efficiency

Donor–spacer–acceptor (D–π–A) materials CAPI and CCAPI, with hybridized local and charge transfer (HLCT) emissive states, have been synthesized. The twisting D–π–A architecture promotes the partial separation of HOMO and LUMO, leading to an enhanced % CT component, and the anthracene moiety in CAPI and CCAPI increases the conjugation length, leading to an enhanced % LE component. The non-doped device with CCAPI^b shows the blue emission (450 nm) with maximum current efficiency (η_c), power efficiency (η_p), and external quantum efficiency (η_ex) of 16.83 cd A⁻¹, 15.32 lm W⁻¹, and 12.0%, respectively, as well as exciton utilization efficiency (EUE) of 95% with a luminance of 32 546 cd m⁻² and a roll-off efficiency of 0.53%. The new design strategy has great potential for developing high-performance blue electroluminescent materials.

Donor–spacer–acceptor (D–π–A) materials CAPI and CCAPI, with hybridized local and charge transfer (HLCT) emissive states, have been synthesized.     

Stimuli-responsive fluorescence switching of cyanostilbene derivatives: ultrasensitive water,acidochromism and mechanochromism

A novel donor–π–acceptor structure stimuli-responsive fluorescent material of (Z)-2-(4′-(diphenylamino)-[1,1′-biphenyl]-4-yl)-3-(pyridin-2-yl)acrylonitrile (oN-TPA) was designed and synthesized, with the cyano-group and pyridine as the acceptors (A) and triphenylamine as the donor (D). oN-TPA exhibits an obvious solvatochromic effect and the excited state is confirmed to be a hybridized local and charge-transfer (HLCT) state that simultaneously possesses the locally-excited (LE) state and charge transfer (CT) state characters. The LE state ensures relatively high fluorescence efficiency while the CT state provides multi-stimuli responsive fluorescence behaviors because it is easily tuned by the surrounding environment. Firstly, oN-TPA exhibits “on–off–on” fluorescence properties in the mixture of water/tetrahydrofuran (THF) with the increasing water content. For the “on–off” part, a good linear relationship between fluorescence intensity and water fraction is achieved, which is ascribed to the synergistic effect of protons in water and intramolecular charge-transfer (ICT) effect depending on solvent polarity. The “off–on” part demonstrates the aggregation-induced enhanced emission (AIEE) character of oN-TPA. Secondly, oN-TPA can be used as a protonic acid sensor to detect trifluoroacetic acid (TFA) in solvent and HCl vapour in the solid state due to the binding of the proton to the pyridine group. Finally, oN-TPA presents remarkable and reversible mechanochromic fluorescence switching between 552 nm and 642 nm (90 nm red-shift) during the pressurizing–depressurizing process. This work not only comprehensively demonstrates the stimuli-responsive fluorescence behaviors of oN-TPA, but also provides a D–π–A structure fluorescent material possessing potential applications in detection and sensing with remarkable fluorescence changes.

A donor–acceptor dye exhibits high luminescence efficiency and high-contrast piezochromism with HLCT properties. The doped films show the ratiometric photo-luminescence peak shift under high pressure, interestingly, making them piezo-sensors.     

Efficient solid-state emission and reversible mechanofluorochromism of a tetraphenylethene-pyrene-based β-diketonate boron complex

A new twisted donor–acceptor (D–A) dye (BF₂-TP) that was composed of tetraphenylethene and pyrene connected with a β-diketonate boron moiety has been synthesized and characterized. Such a dye showed unique intramolecular charge transfer (ICT) features, which were evidenced by spectral analysis and theoretical calculations. More importantly, BF₂-TP solid samples exhibited an obvious mechanofluorochromic (MFC) behavior. Upon grinding with a spatula, the as-prepared powder sample illustrated a remarkable red shift of 62 nm, with considerable color contrast from yellow (562 nm) to orange red (624 nm). Its fluorescence color can be reversibly switched by repeating both the grinding–fuming and grinding–annealing processes. The mechanochromism is attributed to the phase transformation between amorphous and crystalline states. The results obtained would be helpful for designing novel MFC materials.

A new twisted dye (BF₂-TP) was synthesized, it possesses distinct mechanofluorochromism with large spectral shift of 62 nm.     

The structural transition of bimetallic Ag–Au from core/shell to alloy and SERS application

It is well-known that Ag–Au bimetallic nanoplates have attracted significant research interest due to their unique plasmonic properties and surface-enhanced Raman scattering (SERS). In recent years, there have been many studies on the fabrication of bimetallic nanostructures. However, controlling the shape, size, and structure of bimetallic nanostructures still has many challenges. In this work, we present the results of the synthesis of silver nanoplates (Ag NPls), and Ag–Au bimetallic core/shell and alloy nanostructures, using seed-mediated growth under green LED excitation and a gold salt (HAuCl₄) as a precursor of gold. The results show that the optical properties and crystal structure strongly depend on the amount of added gold salt. Interestingly, when the amount of gold(x) in the sample was less than 0.6 μmol (x < 0.6 μmol), the structural nature of Ag–Au was core/shell, in contrast x > 0.6 μmol gave the alloy structure. The morphology of the obtained nanostructures was investigated using the field emission scanning electron microscopy (FESEM) technique. The UV–Vis extinction spectra of Ag–Au nanostructures showed localized surface plasmon resonance (LSPR) bands in the spectral range of 402–627 nm which changed from two peaks to one peak as the amount of gold increased. Ag–Au core/shell and alloy nanostructures were utilized as surface enhanced Raman scattering (SERS) substrates to detect methylene blue (MB) (10⁻⁷ M concentration). Our experimental observations indicated that the highest enhancement factor (EF) of about 1.2 × 10⁷ was obtained with Ag–Au alloy. Our detailed investigations revealed that the Ag–Au alloy exhibited significant EF compared to pure metal Ag and Ag–Au core/shell nanostructures. Moreover, the analysis of the data revealed a linear dependence between the logarithm of concentration (log C) and the logarithm of SERS signal intensity (log I) in the range of 10⁻⁷–10⁻⁴ M with a correlation coefficient (R²) of 0.994. This research helps us understand better the SERS mechanism and the application of Raman spectroscopy on a bimetallic surface.

It is well-known that Ag–Au bimetallic nanoplates have attracted significant research interest due to their unique plasmonic properties and surface-enhanced Raman scattering (SERS).     

The role of π-linkers and electron acceptors in tuning the nonlinear optical properties of BODIPY-based zwitterionic molecules

Intramolecular charge transfer process can play a key role in developing strong nonlinear optical (NLO) response in a molecule for technological application. Herein, two series of boron dipyrromethene (BODIPY)-based push–pull systems have been designed with zwitterionic donor–acceptor groups, and their NLO properties have been evaluated using a density functional theory-based approach. Different π-conjugated linkers and electron acceptor groups were used to understand their roles in tuning the NLO properties. The molecules were analyzed through HOMO–LUMO gaps, frontier molecular orbitals, polarizabilities, hyperpolarizabilities, Δr indices, transition dipole moment densities, ionization potentials, electron affinities and reorganization energies for holes and electrons. These observations correlated well with the computed absorption spectra of the molecules. It is found that with the introduction of different π-linkers in the molecule, planarity is maintained and the HOMO–LUMO gap is systematically decreased, which leads to a large NLO response. It was noted that the electronic absorption wavelength maxima were found in the near-infrared region (934–1650 nm). The results show that compared to the pyridinium acceptor group, the imidazolium acceptor group in the BODIPY systems amplifies the NLO response to a larger extent. It is also observed that the BODIPY-based dye with an imidazolium acceptor and thienothiophene π-linker shows the highest first hyperpolarizability value of 3194 × 10⁻³⁰ esu. Furthermore, the charge transfer occurs in the z-direction, as the z-component of the first hyperpolarizability is the dominant factor in this system. Here, the designed molecules show a characteristic reorganisation energy value, which is a deciding factor in the rate of hole/electron transport for favourable intermolecular coupling. As a whole, this theoretical work highlights that π-conjugated linkers and electron acceptor groups can be used judiciously to design new molecular systems for optoelectronic applications.

BODIPY-based zwitterionic molecules with pyridinium and imidazolium electron acceptors and thienothiophene π-linkers reveal significant first hyperpolarizability.     

Raman photostability of off-resonant gap-enhanced Raman tags

Surface-enhanced Raman scattering (SERS) nanoprobes show promising potential for biosensing and bioimaging applications due to advantageous features of ultrahigh sensitivity and specificity. However, very limited research has been reported on the SERS photostability of nanoprobes upon continuous laser irradiation, which is critical for high-speed and time-lapse microscopy. The core–shell off-resonant gap-enhanced Raman tags (GERTs) with built-in Raman reporters, excited at near-infrared (NIR) region but with a plasmon resonance at visible region, allow decoupling the plasmon resonance behaviors with the SERS performance and therefore show ultrahigh Raman photostability during continuous laser irradiation. In this work, we have synthesized five types of off-resonant GERTs with different embedded Raman reporters, numbers of shell layer, or nanoparticle shapes. Via thorough examination of time-resolved SERS trajectories and quantitative analysis of photobleaching behaviors, we have demonstrated that double metallic-shell GERTs embedded with 1,4-benzenedithiol molecules show the best photostability performance, to the best of our knowledge, among all SERS nanoprobes reported before, with a photobleaching time constant up to 4.8 × 10⁵ under a laser power density of 4.7 × 10⁵ W cm⁻². Numerical calculations additionally support that the local plasmonic heating effect in fact can be greatly minimized using the off-resonance strategy. Moreover, double-shell BDT-GERTs are highly potential for high-speed and high-resolution Raman-based cell bioimaging.

Off-resonant gap-enhanced Raman tags (GERTs) show ultrahigh Raman enhancement and photostabilities and therefore can be used as ideal highly photostable nanoprobes for high-speed and high-resolution Raman bioimaging.

The surface-enhanced Raman scattering (SERS) effect strongly boosts the Raman signal of reporter molecules adsorbed on the surface of metallic plasmonic nanoparticles with the intense electromagnetic field enhancement.^1–7 With the unique fingerprint spectral feature, SERS nanoprobes, namely, metallic nanoparticles together with molecules as Raman reporters, have been extensively investigated for the biomedical applications including biosensing and bioimaging similar to the fluorescent nanoprobes.^8–14 In contrast to fluorophores, SERS nanoprobes exhibit a much larger multiplexing capability due to the narrow spectral linewidth. In addition, SERS nanoprobes show better stability than fluorophores since fluorophores easily suffer the photobleaching issue caused by modification of covalent bonds or non-specific reactions between the fluorophores and surrounding molecules upon singlet state-triplet state transition,^15,16 which is especially problematic in time-lapse microscopy.¹⁷ Typically the photobleaching in SERS nanoprobes does not follow this process and is much less problematic than that in fluorophores. It can be further minimized by decreasing the laser power and prolonging the laser exposure time. However, the photobleaching is still not favorable for high-contrast SERS-based bioimaging, which recently shows great potential for intraoperative precise identification of tumor margins and microscopic tumor invasion^18–21 and inevitably requires high-speed and a number of imaging cycles.Recently a new type of SERS nanoprobes, namely, gap-enhanced Raman tags (GERTs), have been reported to show excellent SERS enhancement,^7,22,23 which is favorable for high-speed SERS imaging.^13,22,24 GERTs are composed of plasmonic Au core–shell nanomatryoshka structures^25–27 with a uniform and nanometer-sized interior gap between the metallic core and the shell in addition to an external mesoporous silica layer if needed.²² Such nanoprobes show strong near-infrared (NIR) Raman enhancement due to the combined near-field electromagnetic and chemical enhancement in the subnanometer core–shell junction geometry while they only present one localized surface plasmon resonance (LSPR) in the visible range in the far-field spectrum.²⁷ Therefore GERTs with the built-in nanogap geometry allow decoupling the LSPR spectrum with the SERS performance. This off-resonance NIR excitation strategy is able to minimize the excitation laser induced photo-thermal effect to GERTs, leading to their ultrahigh SERS photostability during 30 min continuous cell and tumor SERS imaging without being photobleached.²² The off-resonant NIR GERTs as imaging probes are also favorable for generating minimal photothermal damage to the biological samples during the imaging process, as demonstrated by monitoring the changes in mitochondrial membrane potential of cancer cells during imaging.²⁸ The core–shell structure of GERTs additionally offers a variety of embedded Raman reporters and the numbers of shell layer,²⁹ but it remains a question and a challenge to understand how these factors of nanoprobe composition and morphology affect their SERS photostability.In this work, we synthesized five types of off-resonant GERTs either with different embedded Raman reporters (including 1,4-benzenedithiol (BDT), 4,4′-biphenyldithiol (BPDT), 4,4′-terphenyldithiol (TPDT), and 4-nitrobenzenethiol (NBT)), numbers of shell layer, or nanoparticle (NP) shapes. We have compared their particle morphologies, optical properties, and SERS photostability under continuous laser irradiation. Careful examination of time-resolved SERS trajectories and quantitative analysis of photobleaching behaviors indicate that double metallic-shell GERTs embedded with BDT molecules show the best photostability performance to the best of our knowledge. Numerical calculations are additionally performed to estimate the local laser-induced lattice temperature change of GERTs at on-resonance and off-resonance conditions. Further investigations of Raman-based cell imaging have demonstrated that those double-shell GERTs are great nanoprobes for high-speed and high-resolution Raman bioimaging.     

Highly efficient core–shell Ag@carbon dot modified TiO2 nanofibers for photocatalytic degradation of organic pollutants and their SERS monitoring

In the present study, a novel hybrid nanomaterial composed of core–shell structured Ag@carbon dot (CD) modified TiO₂ nanofibers (NFs) was successfully fabricated via a simple two-step strategy for the first time. Herein, the Ag@CDs–TiO₂ NFs are demonstrated to be an efficient SERS substrate. The strong LSPR-induced electromagnetic enhancement (EM) by Ag@CDs NPs and efficient charge transfer (CT) effect between Ag@CDs and TiO₂ NFs synergistically contribute to the excellent SERS enhancement. In addition, the Ag@CDs–TiO₂ NFs exhibit enhanced photocatalytic activity regarding the organic pollutant degradation under visible light irradiation because of the enhanced light absorption and improved separation of photo-generated electron–hole pairs. Thus, this new nanocomposite can be used as a sensitive SERS substrate for determining the catalytic activity and reaction kinetics during the photodegradation of methylene blue (MB). Compared with UV-vis spectroscopy, the SERS technique enables more accurate monitoring of the changes of adsorption molecules and actual catalytic process on the surface of the catalyst. These results are significant for the development of metal or semiconductor-based catalysts for ensuring optoelectronic, energy and environmental applications.

The possible mechanism of enhanced photocatalytic performance of Ag@CDs–TiO₂ hybrid NFs.     

Tuning the charge transfer and optoelectronic properties of tetrathiafulvalene based organic dye-sensitized solar cells: a theoretical approach

Here, we have designed a series of dyes following the donor–π–acceptor (D–π–A) architecture by incorporating tetrathiafulvalene (TTF) as the donor unit and phthalazine (PTZ), diketopyrrolopyrrole (DPP) and quinoxaline (QNX) as the acceptor units, along with the thiophene unit as a π-bridge. The designed dyes have been designated as TTF-PTZ, TTF-DPP and TTF-QNX respectively. We have used cyanoacrylic acid as the anchoring group for the dyes TTF-PTZ and TTF-DPP, while for the third dye, TTF-QNX, we used a carboxylic group. The structural, electronic and photochemical properties of the designed dyes are investigated under the regime of density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods. In this regard, the dihedral angle, energies of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), energy difference between the HOMO and LUMO (Δ_H–L values), partial density of states (PDOS), ground state oxidation potential (GSOP), excited state oxidation potential (ESOP), ionization potential (IP), electron affinity (EA), molecular electrostatic potential surface (MEPS) analysis, reorganization energy (λ), electronic coupling matrix element (V), charge transfer rate (k_CT), hopping mobility (μ_hop), absorption spectra, exciton binding energy (EBE) and electron density difference (EDD) of the designed dyes are calculated. This study reveals that the dyes TTF-DPP-4 and TTF-DPP-6′ exhibit the lowest Δ_H–L values. The study also reveals that the attachment of the –NH₂ group at the donor unit and the –NO₂ and –CF₃ groups at the acceptor units lower the Δ_H–L values of all of the designed dyes. We have also observed that the GSOP of all the designed dyes lie below the redox potential of the I⁻/I₃⁻ electrolyte couple. However, the ESOP of the TTF-PTZ and TTF-QNX groups of dyes, along with the most of the dyes belonging to the TTF-DPP group, lie above the conduction band of the TiO₂ semiconducting surface. Moreover, the total reorganization energy (λ_tot) values are low for the TTF-DPP and TTF-QNX groups of dyes, which confirm the better electron–hole separation efficiency in these groups of dyes. Furthermore, the absorption properties of the designed dyes indicate that the TTF-DPP groups of dyes possess the maximum absorption wavelength (λ_max) values and attachment of the –CH₃ group at the donor part increases the electron density of the dyes, which in turn results into the maximum red-shift. Therefore, the study reveals that the designed dyes are likely to exhibit facile charge transport. Moreover, the electronic properties of the dye–TiO₂ clusters strengthen the performance of the dyes compared to those of the isolated dyes. Hence, our study provides good recommendations for the further design of dyes to enhance the performance of dye-sensitized solar cells (DSSCs).

Here, we have designed a series of dyes following the donor–π–acceptor (D–π–A) architecture by incorporating TTF as the donor unit and PTZ, DPP and QNX as the acceptor units, along with the thiophene unit as a π-bridge.     

Fabrication of Ag@Au (core@shell) nanorods as a SERS substrate by the oblique angle deposition process and sputtering technology

Gold (Au) and silver (Ag) are the main materials exhibiting strong Surface-Enhanced Raman Scattering (SERS) effects. The Ag nano-rods (AgNRs) and Au nano-rods (AuNRs) SERS substrates prepared using the technology of the oblique angle deposition (OAD) process have received considerable attention in recent years because of their rapid preparation process and good repeatability. However, AgNR substrates are unstable due to the low chemical stability of Ag. To overcome these limitations, an Ag@Au core–shell nano-rod (NR) array SERS substrate was fabricated using the OAD process and sputtering technology. Moreover, simulation analysis was performed using finite-difference time-domain calculations to evaluate the enhancement mechanism of the Ag@Au NR array substrate. Based on the simulation results and actual process conditions, the Ag@Au core–shell NR array substrate with the Au shell thickness of 20 nm was studied. To characterize the substrate''s SERS performance, 1,2-bis(4-pyridyl)ethylene (BPE) was used as the Raman probe. The limit of detection of BPE could reach 10⁻¹² M. The Ag@Au NR array substrate demonstrated uniformity with an acceptable relative standard deviation. Despite the strong oxidation of the hydrogen peroxide (H₂O₂) solution, the Ag@Au NR array substrate maintains good chemical stability and SERS performance. And long-term stability of the Ag@Au NR substrate was observed over 8 months of storage time. Our results show the successful preparation of a highly sensitive, repeatable and stable substrate. Furthermore, this substrate proves great potential in the field of biochemical sensing.

A highly sensitive, repeatable and stable Ag@Au core–shell nano-rod array SERS substrate was successfully prepared using the OAD process and sputtering technology which proves great potential in the field of biochemical sensing.     

The role of the donor group and electron-accepting substitutions inserted in π-linkers in tuning the optoelectronic properties of D–π–A dye-sensitized solar cells: a DFT/TDDFT study

The design of low-cost and high-efficiency sensitizers is one of the most important factors in the expansion of dye-sensitized solar cells (DSSCs). To obtain effective sensitizer dyes for applications in dye-sensitized solar cells, a series of metal-free organic dyes with the D–π–A–A arrangement and with different donor and acceptor groups have been designed by using computational methodologies based on density functional theory (DFT) and time-dependent density functional theory (TD-DFT). We have designed JK-POZ(1–3) and JK-PTZ(1–3) D–π–A–A organic dyes by modifying the donor and π-linker units of the JK-201 reference dye. Computational calculations of the structural, photochemical properties and electrochemical properties, as well as the key parameters related to the short-circuit current density and open-circuit voltage, including light-harvesting efficiency (LHE), singlet excited state lifetime (τ), reorganization energies (λ_total), electronic injection-free energy (ΔG^inject) and regeneration driving forces (ΔG^reg) of dyes were calculated and analyzed. Moreover, charge transfer parameters, such as the amount of charge transfer (q^CT), the charge transfer distance (D^CT), and dipole moment changes (μ^CT), were investigated. The results show that ΔG^reg, λ_max, λ_total and τ of JK-POZ-3 and JK-PTZ-3 dyes are superior to those of JK-201, indicating that novel JK-POZ-3 and JK-PTZ-3 dyes could be promising candidates for improving the efficiency of the DSSCs devices.

A series of metal-free organic dyes with the D–π–A–A arrangement and with different donor and acceptor groups have been designed theoretically.     

Synthesis and evaluation of the SERS effect of Fe3O4–Ag Janus composite materials for separable,highly sensitive substrates

Fe₃O₄–Ag Janus composites were synthesized using a two-step solvothermal method. The optimal growth process was determined by investigating the relationship between the particle morphologies and reaction time. Magnetic and Raman spectroscopic measurements showed that the as-synthesized Janus composites have both good magnetic response and significant surface-enhanced Raman scattering (SERS) effects, as well as reproducibility. The calculated Raman enhancement factor reached an unprecedented magnitude of 10⁹ compared with the values of other Fe₃O₄–Ag compounds. Furthermore, the SERS effect was exhibited even at a concentration of probe molecules as low as 10⁻¹³ M. This demonstrates that the as-synthesized Fe₃O₄–Ag Janus composite particles have promise for application as separable, highly sensitive SERS substrates.

Fe₃O₄–Ag Janus composites were synthesized using a two-step solvothermal method.