Study of organized and biological systems using an ultrafast laser

Dodecahedron CsPbBr3 Perovskite Nanocrystals Enable Facile Harvesting of Hot Electrons and Holes

This Letter reports the facile harvesting of hot carriers (HCs) in a composite of 12-faceted dodecahedron CsPbBr₃ nanocrystal (NC) and a scavenger molecule. We recorded ∼3.3 × 10¹¹ s^-1 HC cooling rate in NC when excited with ∼1.4 times the band gap energy (E_g), increasing to >3 × 10¹² s^-1 in the presence of scavengers at high concentration due to the HC extractions. Since the observed intrinsic charge transfer rate (∼1.7 × 10¹² s^-1) in our NC-scavenger complex is about an order of magnitude higher than the HC cooling rate (∼3.3 × 10¹¹ s^-1), carriers are harvested before their cooling. Further, a fluorescence correlation spectroscopy study reveals NC tends to form a quasi-stable complex with a scavenger molecule, ensuring charge transfer completed (τ_ct ≈ 0.6 ps) much before the complex breaks apart (>600 μs). The overall results of our study highlight the promise shown by 12-faceted NCs and their implications in modern applications, including hot carrier solar cells.

Ultrafast Excited State Dynamics of Spatially Confined Organic Molecules

This Feature Article highlights the role of spatial confinement in controlling the fundamental behavior of molecules. Select examples illustrate the value of using space as a tool to control and understand excited-state dynamics through a combination of ultrafast spectroscopy and conventional steady-state methods. Molecules of interest were confined within a closed molecular capsule, derived from a cavitand known as octa acid (OA), whose internal void space is sufficient to accommodate molecules as long as tetracene and as wide as pyrene. The free space, i.e., the space that is left following the occupation of the guest within the host, is shown to play a significant role in altering the behavior of guest molecules in the excited state. The results reported here suggest that in addition to weak interactions that are commonly emphasized in supramolecular chemistry, the extent of empty space (i.e., the remaining void space within the capsule) is important in controlling the excited-state behavior of confined molecules on ultrafast time scales. For example, the role of free space in controlling the excited-state dynamics of guest molecules is highlighted by probing the cis-trans isomerization of stilbenes and azobenzenes within the OA capsule. Isomerization of both types of molecule are slowed when they are confined within a small space, with encapsulated azobenzenes taking a different reaction pathway compared to that in solution upon excitation to S₂. In addition to steric constraints, confinement of reactive molecules in a small space helps to override the need for diffusion to bring the reactants together, thus enabling the measurement of processes that occur faster than the time scale for diffusion. The advantages of reducing free space and confining reactive molecules are illustrated by recording unprecedented excimer emission from anthracene and by measuring ultrafast electron transfer rates across the organic molecular wall. By monitoring the translational motion of anthracene pairs in a restricted space, it has been possible to document the pathway undertaken by excited anthracene from inception to the formation of the excimer on the excited-state surface. Similarly, ultrafast electron transfer experiments pursued here have established that the process is not hindered by a molecular wall. Apparently, the electron can cross the OA capsule wall provided the donor and acceptor are in close proximity. Measurements on the ultrafast time scale provide crucial insights for each of the examples presented here, emphasizing the value of both "space" and "time" in controlling and understanding the dynamics of excited molecules.

Subpicosecond Hot Hole Transfer in a Graphene Quantum Dot Composite with High Efficiency

Extraction of hot carriers is of prime importance because of its potential to overcome the energy loss that limits the efficiency of an optoelectronic device. Employing a femtosecond upconversion setup, herein we report a few picoseconds carrier cooling time of colloidal graphene quantum dots (GQDs) is at least an order of magnitude slower compared to that in its bulk form. A slower carrier cooling time of GQDs compared to that of the other semiconductor quantum dots and their bulk materials is indeed a coveted property of GQDs that would allow one easy harvesting of high energy species employing a suitable molecular system as shown in this study. A subpicosecond hot hole transfer time scale has been achieved in a GQD-molecular system composite with high transfer efficiency. Our finding suggests a dramatic enhancement of the efficiency of GQD based optoelectronic devices can possibly be a reality.

TDDFT study of the polarity controlled ion-pair separation in an excited-state proton transfer reaction

6-Hydroxyquinoline (6HQ) is an ideal photoacid system for exploring excited-state proton transfer (ESPT) reactions. We have previously (Mahata et al. (2002)) shown that the ESPT reaction between 6HQ and trimethylamine (TMA) leads to an "unusual" emission in the 440-450 nm range, containing two decay components (∼5 ns and ∼12 ns). The observed results suggest the presence of a contact ion-pair and a solvent separated ion-pair. In this work, density functional theory (DFT) and time-dependent density functional theory (TD-DFT) have been employed to study the nature of the contact ion-pair formed between 6HQ and TMA and to determine why the decay component ∼12 ns is absent in a non-polar solvent. Calculations of the hydrogen-bonded complexes formed between 6HQ and TMA and its ESPT reaction product, namely 6HQ-TMA and 6HQ-TMA-PT, respectively, have been carried out, both in the electronic ground and excited states. Moreover, by using the CPCM model, different dielectric constants have been introduced into the calculations. On increasing the dielectric constant, the hydrogen bond in 6HQ-TMA-PT becomes weaker and the hydrogen bond length becomes larger; this effectively facilitates the proton transfer reaction and formation of separated ion-pair. Thus, the separation and diffusion of the contact ion-pair can be controlled by changing the polarity of the surroundings.

Reactive dynamics in confined liquids: ultrafast torsional dynamics of auramine O in nanoconfined water in aerosol OT reverse micelles

The effects of confinement on the ultrafast torsional reaction of auramine O in aqueous solution are investigated through ultrafast fluorescence up-conversion with 50 fs time resolution. The aqueous solution is confined in nanoscale water droplets by an ionic surfactant. The torsional motion is orders of magnitude slower in the confined droplets than in bulk aqueous solution. The dynamics become faster with increasing radius of the nanodroplet but never reach the bulk value, even when the radius is as large as 10 nm. Time-dependent fluorescence spectra were constructed and subsequently analyzed using a one-dimensional generalized Smoluchowski equation. An accurate description of the data was achieved using a time-dependent diffusion coefficient. This is suggested to arise because the medium friction reflects dynamics on a broad range of time scales spanning the reaction dynamics. The friction recovered suggests strongly hindered motion in the confined droplet and can be qualitatively related to solvation dynamics measured in AOT, consistent with auramine O torsional dynamics being accompanied by intramolecular charge redistribution.

Chemical and biological caging effects on the relaxation of a proton-transfer dye

We report studies of the interaction between a proton-transfer dye (1'-hydroxy,2'-acetonaphthone, HAN), with the human serum albumin (HSA) protein and a beta-cyclodextrin derivative (DM-beta-CD) in neutral water solutions. We used steady-state and picosecond time-resolved emission spectroscopy to follow the structural changes of HAN due to the hydrophobicity and confinement effect of these nanocavities. Upon encapsulation, the fluorescence intensity of the 1:1 inclusion complex in both cavities increases, and the emission lifetimes become longer. For the DM-beta-CD complexes, we obtained 430 and 920 ps, whereas for the HSA complexes we obtained 630 ps and 2 ns. Picosecond anisotropy measurements show strong confinement due to protein docking. The rotational time for the CD complex is 660 ps, whereas for the protein complex we find 6 ns. The process of energy transfer from the excited triptophan 214 (Trp214) of HSA to the trapped HAN occurs with high efficiency (71%), and the calculated distance between both chromophores is 17 A. We believe that the results are important for a better understanding of the processes occurring in inclusion complexes such as those in nanopharmacodynamics.

Intramolecular charge transfer reaction, polarity, and dielectric relaxation in AOT/water/heptane reverse micelles: pool size dependence

Intramolecular charge transfer (ICT) reaction in a newly synthesized molecule, of 4-(1-morpholenyl) benzonitrile (M6C), in AOT/water/heptane reverse micelles at different pool sizes has been studied by using steady-state and time-resolved fluorescence emission spectroscopy. The pool size dependences of the reaction equilibrium constant and reaction rate have been explained in terms of the average polarity of the confined solvent pools estimated from the fluorescence emission Stokes shift of a nonreactive probe, coumarin 153, dissolved in these microemulsions. The complex permittivity measurements in the frequency range 0.01<or=nu/GHz<or=2 for these microemulsions at different pool sizes (0<or=w0<or=40) and AOT concentrations (0.1<or=c/M<or=0.5) at 298.15 K have also been performed. At sufficient water content, a large dispersion with a relaxation time of approximately 600 ps has been observed at approximately 300 MHz and attributed to the average reorientation of water molecules residing in the close vicinity of the polar interface of the AOT headgroup and n-heptane. The reorientation of these interfacial water molecules is probably responsible for the nanosecond component observed in numerous polar solvation dynamics experiments in these reverse micelles. Subsequently, the estimated polarity and the measured reorientational time scale have been used to explain the dramatic slowing down of the ICT reaction rate and its dependence on pool size in these confined environments.

Dynamic solvation in phosphonium ionic liquids: comparison of bulk and micellar systems and considerations for the construction of the solvation correlation function, C(t)

Dynamic solvation of the dye coumarin 153 is studied in a phosphonium ionic liquid: hexadecyltributylphosphonium bromide, [(C4)3C16P+][Br-]. It forms micelles in water, and the bulk also exists as a liquid under our experimental conditions. This system permits a comparison with an imidazolium ionic liquid studied earlier, which also formed micelles in water (J. Phys. Chem. A 2006, 110, 10725-10730). We conclude that our analysis of the comparable situation in a phosphonium liquid is not as definitive as we had proposed earlier, i.e., that the majority of the early-time solvation arises from the organic cation. Part of the difficulty in performing this analysis is most likely due to the amount of water that is associated with the micelle. In the course of this work, we have focused on the calculation of the solvation correlation function, C(t), and investigated how it depends upon the methods with which the "zero-time" spectrum is constructed.