Effect of Mixture Composition on the Photophysics of Indoline Dyes in Imidazolium Ionic Liquid-Molecular Solvent Mixtures: A Femtosecond Transient Absorption Study.

We conducted a study on the photophysics of three indoline dyes, D102, D149, and D205, in binary mixtures of ionic liquids (IL) and polar aprotic molecular solvents (MS). Specifically, we examined the behavior of these dyes in IL-MS mixtures containing four different imidazolium-based ILs and three different polar aprotic MSs. Our investigation involved several techniques, including stationary absorption and emission measurements, as well as femtosecond transient absorption (TA) spectroscopy. Through our analysis, we discovered a peculiar behavior of several photophysical properties at low IL mole fractions (0 < XIL < 0.2). Indeed, in this range of mixture composition, the absorption maximum wavelength decreases noticeably, while the emission maximum wavelength and the Stokes shift, expressed in wavenumbers, reach a maximum. while a minimum occurs in the relative quantum yield and the excited state lifetime. These results indicate that the solvation of dye undergoes a large change in this range of mixture composition. We found that, at high ionic liquid content, the excited relaxation times are correlated with the high viscosity, while at low content, it is the polarity of the solvent that influences the behavior of the excited relaxation times. At a mixture composition of around 0.10, the behavior of the photophysical properties of the studied IL-MS mixtures indicates a crossover between situations where the solvation is dominated by that of ions and that dominated by the solvent.

Realization of nitroaromatic chromophores with intense two-photon brightness.

Strong fluorescence is a general feature of dipyrrolonaphthyridinediones bearing two nitrophenyl substituents. Methyl groups simultaneously being weakly electron-donating and inducing steric hindrance appear to be a key structural parameter that allows for significant emission enhancement, whereas Et2N groups cause fluorescence quenching. The magnitude of two-photon absorption increases if 4-nitrophenyl substituents are present while the contribution of Et2N groups is detrimental.

Study on the Origin and Evolution of Femtosecond Laser-Induced Surface Structures: LIPSS, Quasi-Periodic Grooves, and Aperiodic Micro-Ridges.

We investigate the evolution mechanisms of the laser-induced periodic surface structures (LIPSS) and quasi-periodic grooves that are formed on the surface of monocrystalline silicon (mono-Si) when exposed to femtosecond laser radiation of different pulse duration, state of polarization, and fluence. The conditions required for producing LIPSS-free complex micro-ridge patterns are elaborated. The LIPSS evolution mechanism is explained in terms of scattering/interference-based phenomena. To establish the basis for our interpretation, single femtosecond pulses of different pulse durations are irradiated on mono-Si. The absence/appearance of LIPSS rudiments is explained in the context of spectral bandwidth and the associated effects on the intensity of the central wavelength. Shorter fs pulses of a wider bandwidth are employed to induce LIPSS-free micro-ridge patterns. It is demonstrated that the resultant micro-ridge patterns depend on the laser fluence distribution and can be manipulated through laser polarization. The curved morphology of LIPSS rudiments and the evolution mechanism of low- and high-spatial frequency LIPSS, i.e., LSFL and HSFL, are discussed. Finally, it is demonstrated that the consolidated quasi-periodic grooves result from HSFL welding together groups of LSFL. Although our findings are based on fs laser interaction with mono-Si, the results can also be applied to many other materials.

Enhancing Anticorrosion Resistance of Aluminum Alloys Using Femtosecond Laser-Based Surface Structuring and Coating.

We report a robust two-step method for developing adherent and anticorrosive molybdenum (Mo)-based coatings over an aluminum (Al) 6061 alloy substrate using a femtosecond (fs) laser. The fs laser nanostructuring of Al 6061 alloy in air gives rise to regular arrays of microgrooves exhibiting superhydrophilic surface properties. The microstructured surface is further coated with an Mo layer using the fs-pulsed laser deposition (fs-PLD) technique. The combination of the two femtosecond laser surface treatments (microstructuring followed by coating) enabled the development of a highly corrosion-resistant surface, with a corrosion current of magnitude less than that of the pristine, the only structured, and the annealed alloy samples. The underlying mechanism is attributed to the laser-assisted formation of highly rough hierarchical oxide structures on the Al 6061 surface along with post heat treatment, which passivates the surface and provide the necessary platform for firm adhesion for Mo coating. Our results reveal that the corrosive nature of the Al-based alloys can be controlled and improved using a combined approach of femtosecond laser-based surface structuring and coating.

A Two-Step Femtosecond Laser-Based Deposition of Robust Corrosion-Resistant Molybdenum Oxide Coating.

A two-step femtosecond-pulsed laser deposition (fs-PLD) process is reported for the rapid development of uniform, poreless, crack-free, and well-adhering amorphous coatings of source materials with a high melting point. The first step comprises a high-rate raw deposition of the source material via fs-PLD, followed by a second step of scanning the raw sample with fs laser pulses of optimized fluence and scan parameters. The technique is applied to develop substoichiometric molybdenum oxide (MoOx, x < 3) coatings on mild steel. The thickness of the layer was ~4.25 µm with roughness around 0.27 µm. Comprehensive surface characterization reveals highly uniform and relatively moderate roughness coatings, implying the potential of these films as robust corrosion-resistant coats. Corrosion measurements in an aqueous NaCl environment revealed that the coated mild steel samples possess an average corrosion inhibition efficiency of around 95% relative to polished mild steel.

Potent strategy towards strongly emissive nitroaromatics through a weakly electron-deficient core.

Nitroaromatics seldom fluoresce. The importance of electron-deficient (n-type) conjugates, however, has inspired a number of strategies for suppressing the emission-quenching effects of the strongly electron-withdrawing nitro group. Here, we demonstrate how such strategies yield fluorescent nitroaryl derivatives of dipyrrolonaphthyridinedione (DPND). Nitro groups near the DPND core quench its fluorescence. Conversely, nitro groups placed farther from the core allow some of the highest fluorescence quantum yields ever recorded for nitroaromatics. This strategy of preventing the known processes that compete with photoemission, however, leads to the emergence of unprecedented alternative mechanisms for fluorescence quenching, involving transitions to dark nπ* singlet states and aborted photochemistry. Forming nπ* triplet states from ππ* singlets is a classical pathway for fluorescence quenching. In nitro-DPNDs, however, these ππ* and nπ* excited states are both singlets, and they are common for nitroaryl conjugates. Understanding the excited-state dynamics of such nitroaromatics is crucial for designing strongly fluorescent electron-deficient conjugates.

Dynamics in the BMIM PF6/acetonitrile mixtures observed by femtosecond optical Kerr effect and molecular dynamics simulations.

We have performed the measurements of the optical Kerr effect signal time evolution up to 4 ns for a mixture of 1-alkyl-3-methyl-imidazolium hexafluorophosphate (BMIM PF6) ionic liquid and acetonitrile in the whole mole fractions range. The long delay line in our experimental setup allowed us to capture the complete reorientational dynamics of the ionic liquid. We have analysed the optical Kerr effect signal in the time and frequency domains with help of molecular dynamics simulations. In our approximation of the slow picosecond dynamics with a multi-exponential decay, we distinguish three relaxation times. The highest two are assigned to the reorientation of the cation and acetonitrile molecules that are in the vicinity of the imidazolium ring. The third one is recognized as originating from cation rotations and reorientation of acetonitrile molecules in the bulk or in the vicinity of the aliphatic chains of the cation. With help of the simulation we interpret the intermolecular band in the reduced spectral density, obtained from Kerr signal, as follows: its low-frequency side results from oscillations of one of the components in the cage formed by its neighbors, while the high-frequency side is attributed to the librations of the cation and acetonitrile molecule as well as the intermolecular oscillations of system components involved in specific interactions. We use this assignment and concentration dependence of the spectra obtained from velocity and angular velocity correlations to explain the mole fraction dependence of Kerr reduced spectral density.

Temperature-Dependent Ultrafast Solvation Response and Solute Diffusion in Acetamide-Urea Deep Eutectic Solvent.

In the present paper, we have studied the temperature dependence of translational diffusion and solvation dynamics of a dissolved dipolar dye in the nonionic acetamide-urea deep eutectic solvent (DES), to characterize the viscosity coupling of the measured relaxation times and verify the dynamical heterogeneity aspect of this medium. Three different time-resolved experimental techniques have been employed for this purpose: fluorescence correlation spectroscopy, transient absorption (TA) spectroscopy, and optical Kerr effect (OKE) spectroscopy. The first method provides the proof that the translational diffusion time of a solute in acetamide-urea DES [fCH3CONH2 + (1 - f)CO(NH2)2, f = 0.6] exhibits a fractional viscosity dependence, with exponent 0.758, which, when compared with the viscosity-diffusion relationship for the same solute in common molecular solvents, suggests moderate deviation from the Stokes-Einstein relation. Stokes shift dynamics of a solvatochromic dye, 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran in this DES, followed via femtosecond TA measurements, have been found to be triexponential in nature and dominated by a ∼100 fs component. The other two components, which contribute to a total dynamic Stokes shift magnitude of ∼2500 cm-1, are characterized by time constants in the ∼5 and ∼50 ps regimes. Subsequent comparison with the femtosecond OKE measurements suggests that the relatively slower picosecond solvation components originate from the rapid reorientation of the solvent molecules, while the subpicosecond solvation response arises from the participation of the collective low-frequency solvent modes (such as intermolecular vibrations and librations). We find that the rotational diffusion lifetimes also exhibit fractional power dependence on medium viscosity and thus deviate from the Stokes-Einstein-Debye pprediction. All of these results therefore suggest that the nonionic acetamide-urea DES is a moderately heterogeneous medium.

Building a 22-ring nanographene by combining in-solution and on-surface syntheses.

A nanographene formed by the fusion of 22 benzene rings has been prepared by combining an in-solution Pd-catalyzed cycloaddition reaction and on-surface Au-promoted cyclodehydrogenation. The structure and electronic properties of the resulting three-fold symmetric C66H24 molecule have been characterized by scanning probe microscopy with atomic resolution and corroborated by theoretical modelling.

How photon pump fluence changes the charge carrier relaxation mechanism in an organic-inorganic hybrid lead triiodide perovskite.

This study explores the excitation wavelength and fluence dependence of processes occurring in formamidinium lead triiodide (FAPbI3) film using time-resolved transient absorption and terahertz spectroscopies. The results indicate that second-order processes are responsible for charge carrier recombination at low fluences of the absorbed photons (below 8.4 × 1012 ph per cm2). An increase in fluence leads to the appearance and successive reduction of the time component assigned to the Auger recombination of free charge carriers (240-120 ps). Simultaneously, the bimolecular recombination time decreases from ∼1400 to ∼700 ps. Further increasing the pump fluence produces an exciton population that recombines in 6 ps. The comparison of two characteristic bleaching bands located at 480 and 775 nm provides evidence for the validity of the two valence bands model. Excitation with higher fluences results in a marked difference in the probed dynamics at these bands, reflecting the action of two excited states at the conduction band. Our results demonstrate that a single model cannot be applied in characterizing the perovskite absorber transitions at all pump fluences. These findings are relevant in understanding their operating mechanism under specific experimental conditions, which should differ for perovskite based solar cells, lasing media or photon detectors.

Unraveling Charge Carriers Generation, Diffusion, and Recombination in Formamidinium Lead Triiodide Perovskite Polycrystalline Thin Film.

We report on studies of the formamidinium lead triiodide (FAPbI3) perovskite film using time-resolved terahertz (THz) spectroscopy (TRTS) and flash photolysis to explore charge carriers generation, migration, and recombination. The TRTS results show that upon femtosecond excitation above the absorption edge, the initial high photoconductivity (∼75 cm(2) V(-1) s(-1)) remains constant at least up to 8 ns, which corresponds to a diffusion length of 25 µm. Pumping below the absorption edge results in a mobility of 40 cm(2) V(-1) s(-1) suggesting lower mobility of charge carriers located at the bottom of the conduction band or shallow sub-bandgap states. Furthermore, analysis of the THz kinetics reveals rising components of <1 and 20 ps, reflecting dissociation of excitons having different binding energies. Flash photolysis experiments indicate that trapped charge carriers persist for milliseconds.

Mechanism of Charge Transfer and Recombination Dynamics in Organo Metal Halide Perovskites and Organic Electrodes, PCBM, and Spiro-OMeTAD: Role of Dark Carriers.

Despite the unprecedented interest in organic-inorganic metal halide perovskite solar cells, quantitative information on the charge transfer dynamics into selective electrodes is still lacking. In this paper, we report the time scales and mechanisms of electron and hole injection and recombination dynamics at organic PCBM and Spiro-OMeTAD electrode interfaces. On the one hand, hole transfer is complete on the subpicosecond time scale in MAPbI3/Spiro-OMeTAD, and its recombination rate is similar to that in neat MAPbI3. This was found to be due to a high concentration of dark charges, i.e., holes brought about by unintentional p-type doping of MAPbI3. Hence, the total concentration of holes in the perovskite is hardly affected by optical excitation, which manifested as similar decay kinetics. On the other hand, the decay of the photoinduced conductivity in MAPbI3/PCBM is on the time scale of hundreds of picoseconds to several nanoseconds, due to electron injection into PCBM and electron-hole recombination at the interface occurring at similar rates. These results highlight the importance of understanding the role of dark carriers in deconvoluting the complex photophysical processes in these materials. Moreover, optimizing the preparation processes wherein undesired doping is minimized could prompt the use of organic molecules as a more viable electrode substitute for perovskite solar cell devices.

Direct monitoring of ultrafast electron and hole dynamics in perovskite solar cells.

Organic-inorganic hybrid perovskite solar cells have emerged as cost effective efficient light-to-electricity conversion devices. Unravelling the time scale and the mechanisms that govern the charge carrier dynamics is of paramount importance for a clear understanding and further optimization of the perovskite based devices. For the classical FTO/bulk titania blocking layer/mesoporous titania/perovskite/Spiro-OMeTAD (FTO/TPS) cell, further detailed and systematic studies of the ultrafast events related to exciton generation, electron and hole transfer, ultrafast relaxation are still needed. We characterize the initial ultrafast processes attributed to the exciton-perovskite lattice interactions influenced by charge transfer to the electron and hole transporters that precede the exciton diffusion into free charge carriers occurring in the sensitizer. Time-resolved transient absorption studies of the FTO/perovskite and FTO/TPS samples under excitation at different wavelengths and at low fluence 2 (µJ cm(-2)) indicate the sub-picosecond electron and hole injection into titania and Spiro-OMeTAD, respectively. Furthermore, the power-dependent femtosecond transient absorption measurements support the ultrafast charge transfer and show strong Auger-type multiparticle interactions at early times. We reveal that the decays of the internal trap states are the same for both films, while those at surfaces differ. The contribution of the former in the recombination is small, thus increasing the survival probability of the charges in the excited perovskite.