Stimulated Emission Enhancement Using Shaped Pulses

Near-infrared electroluminescence beyond 940 nm in Pt(N^C^N)X complexes: influencing aggregation with the ancillary ligand X

We present a study of aggregate excited states formed by complexes of the type Pt(N^C^N)X, where N^C^N represents a tridentate cyclometallating ligand, and X = SCN or I. These materials display near-infrared (NIR) photoluminescence in film and electroluminescence in NIR OLEDs with λ max EL = 720-944 nm. We demonstrate that the use of X = SCN or I modulates aggregate formation compared to the parent complexes where X = Cl. While the identity of the monodentate ligand affects the energy of Pt-Pt excimers in solution in only a subtle way, it strongly influences aggregation in film. Detailed calculations on aggregates of different sizes support the experimental conclusions from steady-state and time-resolved luminescence studies at variable temperatures. The use of X = I appears to limit aggregation to the formation of dimers, while X = SCN promotes the formation of larger aggregates, such as tetramers and pentamers, leading in turn to NIR photo- and electroluminescence > 850 nm. A possible explanation for the contrasting influence of the monodentate ligands is the lesser steric hindrance associated with the SCN group compared to the bulkier I ligand. By exploiting the propensity of the SCN complexes to form extended aggregates, we have prepared an NIR-emitting OLED that shows very long wavelength electroluminescence, with λ max EL = 944 nm and a maximum EQE = 0.3 ± 0.1%. Such data appear to be unprecedented for a device relying on a Pt(ii) complex aggregate as the emitter.

Control of the excited-to-ionized atoms ratio in a dense gas in the wake of an intense femtosecond laser pulse

We demonstrate the sensitivity of the plasma composition in the filament wake channel in a dense gas to the temporal shape of the driving femtosecond laser pulse. During the pulse, the electrons released via strong-field ionization and driven by oscillating laser field are actively engaged in collisional processes with neutral neighbor atoms, including inverse Bremsstrahlung, impact ionization, and collisional excitation. By the end of the pulse, these collisional processes produce considerable numbers of additional free electrons (or ionized atoms) and excited atoms, and these contents of the filament wake channel determine its subsequent evolution dynamics. Addressing the case of high-pressure argon gas and using a kinetic model of these competing collisional processes, we explore the sensitivity of the resulting excited-to-ionized atoms number density ratio to the envelope shape of the driving laser pulse. By considering several families of pulses, we show that asymmetric pulse envelopes skewed toward the earlier time allow for efficient control of the ratio of excited atoms to ionized atoms. The pulse-shape control of the plasma composition in the immediate wake of the laser pulse projects into control of the wake channel evolution and of the associated transient electronic and optical nonlinearities.

Intracellular Dynamic Assembly of Deep-Red Emitting Supramolecular Nanostructures Based on the Pt…Pt Metallophilic Interaction

Many drug delivery systems end up in the lysosome because they are built from covalent or kinetically inert supramolecular bonds. To reach other organelles, nanoparticles hence need to either be made from a kinetically labile interaction that allows re-assembly of the nanoparticles inside the cell following endocytic uptake, or, be taken up by a mechanism that short-circuits the classical endocytosis pathway. In this work, the intracellular fate of nanorods that self-assemble via the Pt…Pt interaction of cyclometalated platinum(II) compounds, is studied. These deep-red emissive nanostructures (638 nm excitation, ≈700 nm emission) are stabilized by proteins in cell medium. Once in contact with cancer cells, they cross the cell membrane via dynamin- and clathrin-dependent endocytosis. However, time-dependent confocal colocalization and cellular electron microscopy demonstrate that they directly move to mitochondria without passing by the lysosomes. Altogether, this study suggests that Pt…Pt interaction is strong enough to generate emissive, aggregated nanoparticles inside cells, but labile enough to allow these nanostructures to reach the mitochondria without being trapped in the lysosomes. These findings open new venues to the development of bioimaging nanoplatforms based on the Pt…Pt interaction.

Milliradian precision ultrafast pulse control for spectral phase metrology

A pulse-shaper-based method for spectral phase measurement and compression with milliradian precision is proposed and tested experimentally. Measurements of chirp and third-order dispersion are performed and compared to theoretical predictions. The single-digit milliradian accuracy is benchmarked by a group velocity dispersion measurement of fused silica.

Quantum coherent control of H3 + formation in strong fields

Quantum coherent control (QCC) has been successfully demonstrated experimentally and theoretically for two- and three-photon optical excitation of atoms and molecules. Here, we explore QCC using spectral phase functions with a single spectral phase step for controlling the yield of H₃ ⁺ from methanol under strong laser field excitation. We observe a significant and systematic enhanced production of H₃ ⁺ when a negative 34 π phase step is applied near the low energy region of the laser spectrum and when a positive 34 π phase step is applied near the high energy region of the laser spectrum. In some cases, most notably the HCO⁺ fragment, we found the enhancement exceeded the yield measured for transform limited pulses. The observation of enhanced yield is surprising and far from the QCC prediction of yield suppression. The observed QCC enhancement implies an underlying strong field process responsible for polyatomic fragmentation controllable by easy to reproduce shaped pulses.

Time-resolved signatures across the intramolecular response in substituted cyanine dyes

The optically populated excited state wave packet propagates along multidimensional intramolecular coordinates soon after photoexcitation. This action occurs alongside an intermolecular response from the surrounding solvent. Disentangling the multidimensional convoluted signal enables the possibility to separate and understand the initial intramolecular relaxation pathways over the excited state potential energy surface. Here we track the initial excited state dynamics by measuring the fluorescence yield from the first excited state as a function of time delay between two color femtosecond pulses for several cyanine dyes having different substituents. We find that when the high frequency pulse precedes the low frequency one and for timescales up to 200 fs, the excited state population can be depleted through stimulated emission with efficiency that is dependent on the molecular electronic structure. A similar observation at even shorter times was made by scanning the chirp (frequencies ordering) of a femtosecond pulse. The changes in depletion reflect the rate at which the nuclear coordinates of the excited state leave the Franck-Condon (FC) region and progress towards achieving equilibrium. Through functional group substitution, we explore these dynamic changes as a function of dipolar change following photoexcitation. Density functional theory calculations were performed to provide greater insight into the experimental spectroscopic observations. Complete active space (CAS) self-consistent field and CAS second order perturbation theory calculated potential energy surfaces tracking twisting and pyramidalization confirm that the steeper potential at the FC region leads to the observation of faster wave packet dynamics.