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Prior studies of halobenzene−ammonia complexes have shown that the nature of the cationic intermediate (i.e., Wheland-type vs ion-radical) may play a key role in determining the reaction products. To probe this link, we report here the reaction dynamics of the chlorobenzene-ammonia 1:1 complex (PhCl···NH3) using product ion imaging following two-color resonant two-photon ionization. A threshold value of 8.863 ± 0.008 eV was determined for the appearance of protonated aniline, which accompanies Cl atom loss and is the dominant product channel at energies near threshold. Scanning down to energies close to threshold, we find no evidence for a roaming halogen radical mechanism leading to HCl products, which was evidenced in the related bromobenzene−ammonia complex, and proceeded through an ion-radical intermediate structure. Here, supporting calculations indicate that both types of intermediates are present, but the Wheland-type structure is significantly more stable. Addressing a key question of earlier work, analysis of the PhCl···NH3 potential energy surface (PES) in the reactant region establishes a complicated entrance channel pathway linking the in-plane σ-type complex to the Wheland intermediate (iWH) on the [PhCl···NH3]+• cationic surface. This pathway involves stepwise transition of the weakly bound ammonia from the initial in-plane σ-type complex to an ortho Wheland intermediate, followed by rearrangement to the ipso position. Finally, given that fluorine has been shown to stabilize aromatic ions, we hypothesized that fluorine substitution might alter the structure of the intermediate, favoring the ion-radical intermediate. To test this hypothesis, as an illustrative example the PES of the meta-PhClF-NH3 system on the cationic surface was computed. Confirming our hypothesis, these calculations show an inversion in stability for the Wheland-type and ion−radical complex intermediates, with the latter preferred energetically at the examined level of theory. 

A single photoexcited electron−hole pair within a polar semiconductor nanocrystal (SNC) alters the charge screening and shielding within it. Perturbations of the crystal lattice and of the valence and conduction bands result, and the quantum-confinement states in a SNC shift uniquely with a dependence on the states occupied by the carriers. This shifting is termed quantum-state renormalization (QSR). This Perspective highlights QSR in semiconductor quantum wires and dots identified in time-resolved transient absorption and two-dimensional electronic spectroscopy experiments. Beyond the interest in understanding the principles of QSR and energy-coupling mechanisms, we pose the contributions of QSR in time-resolved spectroscopy data must be accounted for to accurately identify the time scales for intraband relaxation of the carriers within SNCs. 

Ion time-of-flight velocity-map imaging was used to measure the kinetic-energy distributions of the I2 ion-pair fragments formed after photoexcitation of Ar⋯I2 complexes to intermolecular vibrational levels bound within the Ar + I2 (E, vE = 0–2) potential energy surfaces. The kinetic-energy distributions of the I2 products indicate that complexes in the Ar⋯I2 (E, vE) levels preferentially dissociate into I2 in the D and β ion-pair states with no change in I2 vibrational excitation. The energetics of the levels prepared suggest that there is a non-adiabatic coupling of the initially prepared levels with the continuum of states lying above the Ar + I2 (D, vD = vE) and Ar + I2 (β, vβ = vE) dissociation limits. The angular anisotropies of the I2 product signals collected for many of the Ar⋯I2 (E, vE) levels have maxima parallel to the laser polarization axis. This contradicts expectations for the prompt dissociation of complexes with T-shaped geometries, which would result in images with maxima perpendicular to the polarization axis. These anisotropies suggest that there is a perturbation of the transition moment in these clusters or there are additional intermolecular interactions, likely those sampled while traversing above the attractive wells of the lower-energy potentials during dissociation. I2 (D′, vD′) products are also identified when preparing several of the low-lying levels localized in the T-shaped well of the Ar + I2 (E, vE = 0–2) potentials, and they are formed in multiple νD′ vibrational levels spanning energy ranges up to 500 cm−1. 

Two-color, two-photon laser-induced fluorescence experiments were performed to probe the intermolecular interactions within the Ar + I2(E, vE = 0–3) potential energy surfaces. Spectra were recorded using the lowest-energy T-shaped level and an excited intermolecular vibrational level with bending excitation within the Ar + I2(B, vB = 23) potential as intermediate levels to guide the spectral assignments. Progressions of intermolecular stretching and bending levels bound within the Ar + I2(E, vE) potentials were identified, and their vibrational frequencies were determined. The harmonic frequency and anharmonic constant for the bending vibrational mode were determined to be ωe(b) ∼ 34.8 cm−1 and ωeχe(b) ∼ 0.3 cm−1. The frequency and anharmonic constant for the stretching mode were found to be the same as reported previously [V.V. Baturo, et al. Chem. Phys. Lett. 647 (2016) 161], ωe(s) = 37.2(1.1) cm−1 and ωeχe(s) = 1.8(2) cm−1. 

The magnitude and temporal evolution of the quantum-state renormalization (QSR), or the energetic shifting of the quantum-confinement states caused by photoexcitation and changes in electron screening, were probed in transient absorption (TA) spectroscopy measurements of colloidal semiconductor nanoparticles. Experiments were performed on high- and lower-quality wurtzite CdTe quantum wires (QWs) with photoluminescence quantum yields of 8.8% and ∼0.2% using low-excitation fluences. The QSR shifts the spectral features to lower energies in both samples, with larger shifts measured in the high-quality QWs. The TA spectral features measured for both samples shift uniquely with time after photoexcitation, illustrating dynamic QSR that depends on the quantum-confinement states and on the states occupied by carriers. The higher fraction of carriers that reach the band-edge states in the high-quality QWs results in larger renormalization, with the energies of the band-edge states approaching the Stokes shift of the steady-state photoluminescence feature below the band-edge absorption energy. The intraband relaxation dynamics of charge carriers photoexcited in semiconductor nanoparticles was also characterized after accounting for contributions from QSR in the TA data. The intraband relaxation to the band-edge states was slower in the high-quality QWs than in the lower-quality QWs, likely due to the reduced number of trap states accessible. The contrasting relaxation time scales provide definitive evidence for a dependence of the photoluminescence efficiency on excitation energy. These studies reveal the complicated interplay between the energetics and relaxation mechanisms of carriers within semiconductor nanoparticles, even those with the same dimensionality. 

Ligand-exchange reactions of wurtzite CdSe quantum platelets (QPs) and quantum belts (QBs) with methyl viologen (MV2+) and the derivative ligands MV2+(CH2)nNH2 (n = 2, 4, or 6) are investigated. The QP and QB photoluminescence is quenched after partial ligand exchange. Spectroscopic and compositional data establish that this initial ligand substitution occurs on the thin QP and QB edges. The MV2+(CH2)nNH2 ligands are shown to be more-efficient photoluminescence quenchers than the parent MV2+ ion. The ligands on the thin, nonpolar, long-edge facets quench the photoluminescence via the trapping of excitons. Transient absorption experiments indicate the excitons dissociate, and electron transfer to the MV2+(CH2)nNH2 ligands only occurs at the polar, short-edge facets of the wurtzite CdSe QPs and QBs. Electron transfer to the MV2+(CH2)nNH2 ligands occurs within 100 fs when exciting at the band edge and on longer time scales, due to intraband relaxation, when exciting at higher energies. 

The vibrational predissociation dynamics of H2/D2···I35Cl(B,v′=3) complexes containing both para- and ortho-hydrogen prepared in different intermolecular vibrational levels were investigated. The Δv = −1 I35Cl(B,v = 2,j) rotational product-state distributions measured for excitation to the lowest-energy T-shaped levels of these complexes are mostly bimodal. The rotational distributions measured for excitation of the H2···I35Cl(B,v′=3) complexes are colder than those of the D2···I35Cl(B,v′=3) complexes, and there are only slight differences between those measured for the para- and ortho-hydrogen containing complexes. Excitation of the delocalized bending levels results in slightly colder rotational product-state distributions. The distributions suggest the dynamics result from more than impulsive dissociation off of the inner repulsive wall of the lower-energy H2/D2 + I35Cl(B,v = 2) potential surfaces of the products. The depths of these potentials and the energies available to these products also contribute to the dynamics. The formation of the Δv = −2, I35Cl(B,v = 1) product channel was only identified for excitation of levels within the ortho(j = 0)-D2 + I35Cl(B,v′=3) potential. The formation of this channel occurs via I35Cl(B,v′=3) vibrational to D2 rotational energy transfer forming the ortho(j = 2)-D2 + I35Cl(B,v = 1,j) products. 

Wurtzite CdSe quantum belts with L-type n-octylamine, L-type ammonia, or Z-type Cd(oleate)2 ligands are exchanged for several metal-dithiocarbamate ligands [M(S2CNR1R2)2]: Cd(S2CNPhMe)2, Cd(S2CNEt2)2, Zn(S2CNPhMe)2, and Zn(S2CNEt2)2. Successful ligand exchange with all M(S2CNR1R2)2 compounds occurs from {CdSe[Cd(oleate)2]0.19} quantum belts (QBs), which induce similar spectral shifts in the absorption spectra of the ligand-exchanged QBs. Spectroscopic data, experimentally determined lattice strains, and ligand exchanges with [Na][Et2NCS2] and [NH4][MePhNCS2] establish that the [M(S2CNR1R2)2] ligands bind as bound-ion-paired X-type ligands with (S2CNR1R2)− groups ligated directly to the QB surfaces and [M(S2CNR1R2)]+ groups serving as the charge-balancing ion-paired countercations. The X-type dithiocarbamate ligands do not impart any special electronic effects to the CdSe QBs.