Search

Various coordination complexes have been the subject of experimental or theoretical studies in recent decades because of their fascinating photophysical properties. In this work a combined experimental and computational approach applied to investigate the optical properties of monocationic Ir(III) complexes. In result, an interpretative machine learning-based Quantitative Structure-Activity Relationship (QSAR) model was successfully developed, which can reliably predict the emission wavelength of the Ir(III) complexes and provides foundations for theoretical evaluation of the optical properties of Ir(III) complexes. A hypothesis was proposed to mechanistically explain the differences in emission wavelengths between structurally different individual Ir(III) complexes. To the best of our knowledge, this is the first attempt to develop predictive machine learning (QSAR) model for the optical properties of Ir(III) complexes. The efficacy of the developed model was demonstrated by high R2 values for the training and test sets of 0.84 and 0.87, respectively, and by performing the validation using y-scrambling techniques. A notable relationship between the N-N distance in the diimine ligands of the Ir(III) complexes and emission wavelengths was revealed. This combined experimental and computational approach shows a great potential for rational design of new Ir(III) complexes with desired optical properties. Moreover, the developed methodology could be extended to other octahedral transition-metal complexes.

The absorption and fluorescence spectra of 14 In(III) dipyrrin-based complexes are studied using time-dependent density functional theory (TDDFT). Calculations confirm that both heteroatom substitution of oxygen (N2O2-type) by nitrogen (N4-type) in dipyrrin ligand and functionalization at the meso-position by aromatic rings with strong electron-withdrawing (EW) substituents or extended π-conjugation are efficient tools in extending the fluorescence spectra of In(III) complexes to the near-infrared (NIR) region of 750-960 nm and in red-shifting the lowest absorption band to 560-630 nm. For all complexes, the emissive singlet state has π-π* character with a small addition of intraligand charge transfer (ILCT) contributing from the meso-aryl substituents to the dipyrrin ligand. Stronger EW nitro group on the meso-phenyl or meso-aryl group with extended π-conjugation induces red-shifted electronic absorption and fluorescence. More tetrahedral geometry of the complexes with N4-type ligands leads to less intensive but more red-shifted fluorescence to NIR, compared to the corresponding complexes with N2O2-type ligands that have a more planar geometry.

Using non-adiabatic dynamics and Redfield theory, we predicted the optical spectra, radiative and nonradiative decay rates, and photoluminescence quantum yields (PLQYs) for In(III) dipyrrin-based complexes (i) with electron-withdrawing (EW) or electron-donating (ED) substituents on the

Cadmium chalcogenide quantum dots (QDs) passivated by thiol-based ligands exhibit several advantages in their applications in lighting, sensing, and imaging technologies. However, their emission is...

We study the impact of the chemical composition on phonon-mediated exciton relaxation in the core/shell quantum dots (QDs), with 1 nm core made of PbX and the monolayer shell made of CdX, where X = S and Se. For this, time-domain nonadiabatic molecular dynamics (NAMD) based on density functional theory (DFT) and surface hopping techniques are applied. Simulations reveal twice faster energy relaxation in PbS/CdS than PbSe/CdSe because of dominant couplings to higher-energy optical phonons in structures with sulfur anions. For both QDs, the long-living intermediate states associated with the core-shell interface govern the dynamics. Therefore, a simple exponential model is not appropriate, and the four-state irreversible kinetic model is suggested instead, predicting 0.9 and 0.5 ps relaxation rates in PbSe/CdSe and PbS/CdS QDs, respectively. Thus, 2 nm PdSe/CdSe QDs with a single monolayer shell exhibit the phonon-mediated relaxation time sufficient for carrier multiplications to outpace energy dissipation and benefit the solar conversion efficiency.

The thermal decomposition and ignition of high-performance high explosives occur via a mechanism where the solid phase sublimes and the parent molecules decompose rapidly in the gas phase to form unstable and charged intermediates. These intermediates continue to react and form the final products to release energy and do work. We have observed that the presence of electromagnetic energy significantly reduces the ignition temperature of a common high explosive, and data suggest that this occurs via electromagnetic interactions with the charged gas-phase intermediates. Here, we modified the thermal decomposition kinetic expressions for octogen (High Melt eXplosive, HMX) to couple the effects of an incident microwave (MW) field. This modified kinetic model is used to investigate our previous experimental work which showed that the surface temperature at ignition of HMX powder is reduced by the MW field. The Fridman–Macheret α-model is a common approach in plasma chemistry and was incorporated into the Henson/Smilowitz HMX kinetics; this effectively reduces the activation energy ( Ea) by vibronically excited charged reactive intermediates. A modified kinetic model was implemented into the COMSOL Multiphysics Software. The thermal time to ignition was validated; as a result, plasma formation reduced the surface temperature by ∼23 °C compared to thermal ignition. With a validated kinetic model that can simulate both pure thermal ignition and mixed thermal/plasma ignition, we are able to simulate our previous experimental work showing that plasma ignition reduces the surface temperature at ignition compared to thermal initiation.

The synthesis and photophysics (UV-vis absorption, emission, and transient absorption) of four neutral heteroleptic cyclometalated iridium(III) complexes (Ir-1-Ir-4) incorporating thiophene/selenophene-diketopyrrolopyrrole (DPP)-substituted N-heterocyclic carbene (NHC) ancillary ligands are reported. The effects of thiophene versus selenophene substitution on DPP and bis- versus monoiridium(III) complexation on the photophysics of these complexes were systematically investigated via spectroscopic techniques and density functional theory calculations. All complexes exhibited strong vibronically resolved absorption in the regions of 500-700 nm and fluorescence at 600-770 nm, and both are predominantly originated from the DPP-NHC ligand. Complexation induced a pronounced red shift of this low-energy absorption band and the fluorescence band with respect to their corresponding ligands due to the improved planarity and extended π-conjugation in the DPP-NHC ligand. Replacing the thiophene units by selenophenes and/or biscomplexation led to the red-shifted absorption and fluorescence spectra, accompanied by the reduced fluorescence lifetime and quantum yield and enhanced population of the triplet excited states, as reflected by the stronger triplet excited-state absorption and singlet oxygen generation.

The absorption and fluorescence spectra of 14 In(III) dipyrrin-based complexes are studied using time-dependent density functional theory (TDDFT). Calculations confirm that both heteroatom substitution of oxygen (N

Ten biscyclometalated monocationic Ir(III) complexes were synthesized and studied to elucidate the effects of extending π-conjugation of the diimine ligand (N^N = 2,2'-bipyridine in Ir1, 2-(pyridin-2-yl)quinoline in Ir2, 2-(pyridin-2-yl)[6,7]benzoquinoline in Ir3, 2-(pyridin-2-yl)-[7,8]benzoquinoline in Ir4, phenanthroline in Ir5, benzo[ f][1,10]phenanthroline in Ir6, naphtho[2,3- f][1,10]phenanthroline in Ir7, 2,2'-bisquinoline in Ir8, 3,3'-biisoquinoline in Ir9, and 1,1'-biisoquinoline in Ir10) via benzannulation at 2,2'-bipyridine on the excited-state properties and reverse saturable absorption (RSA) of these complexes. Either a bathochromic or a hypsochromic shift of the charge-transfer absorption band and emission spectrum was observed depending on the benzannulation site at the 2,2'-bipyridine ligand. Benzannulation at the 3,4-/3',4'-position or 5,6-/5',6'-position of 2,2'-bipyridine ligand or at the 6,7-position of the quinoline ring on the N^N ligand caused red-shifted charge-transfer absorption band and emission band for complexes Ir2, Ir8, Ir10 vs Ir1 and Ir3 vs Ir2, while benzannulation at the 4,5-/4',5'-position of 2,2'-bipyridine ligand or at the 7,8-position of the quinoline ring on the N^N ligand induced a blue shift of the charge-transfer absorption and emission bands for complex Ir9 vs Ir1 and Ir4 vs Ir2. However, benzannulation at the 2,2',3,3'-position of 2,2'-bipyridine or 5,6-position of phenanthroline ligand had no impact on the energy of the charge-transfer absorption band and emission band of complexes Ir5-Ir7 compared with those of Ir1. The observed phenomenon was explained by the frontier molecular orbital (FMO) symmetry analysis. Site-dependent benzannulation also impacted the spectral feature and intensity of the triplet transient absorption spectra and lifetimes drastically. Consequently, the RSA strength of these complexes varied with a trend of Ir7 > Ir5 ≈ Ir6 ≈ Ir1 > Ir3 > Ir2 > Ir10 > Ir4 > Ir8 > Ir9 at 532 nm for 4.1 ns laser pulses.

Phosphorescence is commonly utilized for applications including light-emitting diodes and photovoltaics. Machine learning (ML) approaches trained on ab initio datasets of singlet–triplet energy gaps may expedite the discovery of phosphorescent compounds with the desired emission energies. However, we show that standard ML approaches for modeling potential energy surfaces inaccurately predict singlet–triplet energy gaps due to the failure to account for spatial localities of spin transitions. To solve this, we introduce localization layers in a neural network model that weight atomic contributions to the energy gap, thereby allowing the model to isolate the most determinative chemical environments. Trained on the singlet–triplet energy gaps of organic molecules, we apply our method to an out-of-sample test set of large phosphorescent compounds and demonstrate the substantial improvement that localization layers have on predicting their phosphorescence energies. Remarkably, the inferred localization weights have a strong relationship with the ab initio spin density of the singlet–triplet transition, and thus infer localities of the molecule that determine the spin transition, despite the fact that no direct electronic information was provided during training. The use of localization layers is expected to improve the modeling of many localized, non-extensive phenomena and could be implemented in any atom-centered neural network model., We address phosphorescence, a localized phenomenon, by building localization layers into a DNN model of singlet–triplet energy gaps. These layers improve model performance and simultaneously infer the location of spin excitations within molecules.

Bulk-scale syntheses of sp2 nanocarbon have typically been generated by extensive chemical oxidation to yield graphite oxide from graphite, followed by a reductive step. Materials generated via harsh random processes lose desirable physical characteristics. Loss of sp2 conjugation inhibits long-range electronic transport and the potential for electronic band manipulation. Here, we present a nanopatterned holey graphene material electronically hybridized with metal-containing nanoparticles. Oxidative plasma etching of highly ordered pyrolytic graphite via previously developed covalent organic framework (COF)-5-templated patterning yields bulk-scale materials for electrocatalytic applications and fundamental investigations into band structure engineering of nanocomposites. We establish a broad ability (Ag, Au, Cu, and Ni) to grow metal-containing nanoparticles in patterned holes in a metal precursor-dependent manner without a reducing agent. Graphene nanoparticle compounds (GNCs) show metal-contingent changes in the valence band structure. Density functional theory investigations reveal preferences for uncharged metal states, metal contributions to the valence band, and embedding of nanoparticles over surface incorporation. Ni-GNCs show activity for oxygen evolution reaction in alkaline media (1 M KOH). Electrocatalytic activity exceeds 10,000 mA/mg of Ni, shows stability for 2 h of continuous operation, and is kinetically consistent via a Tafel slope with Ni(OH)2-based catalysis.

A series of monocationic iridium(III) complexes, [Ir(C^N)2(pqu)]+PF6– [pqu = 2-(pyridin-2-yl)quinoline, C^N = 2-phenylquinoline (1), 3-phenylisoquinoline (2), 1-phenylisoquinoline (3), benzo[h]quinoline (4), 2-(pyridin-2-yl)naphthalene (5), 1-(pyridin-2-yl)naphthalene (6), 2-(phenanthren-9-yl)pyridine (7), 2-phenylbenzo[g]quinoline (8), 2-(naphthalen-2-yl)quinoline (9), and 2-(naphthalen-2-yl)benzo[g]quinoline (10)], were synthesized in this work. These complexes bear C^N ligands with varied degrees of π conjugation and sites of benzannulation, allowing for elucidation of the effects of the benzannulation site at the C^N ligand on the photophysics of the complexes. Ultraviolet–visible (UV–vis) absorption and emission of the complexes were systematically investigated via spectroscopic techniques and time-dependent density functional theory calculations. Their triplet excited-state absorption and reverse saturable absorption (RSA) were studied by nanosecond transient absorption (TA) spectroscopy and nonline...

Isomerization of molecular systems is ubiquitous in chemistry and biology, and is also important for many applications. Atomistic simulations can help determine the tunable parameters influencing this process. In this paper, we use the Nonadiabatic EXcited state Molecular Dynamics (NEXMD) software to study the photoisomerization of a representative molecule, 4-styrylquinoline (SQ). trans-SQ transforms into dihydrobenzophenanthridine (DHBP) upon irradiation with laser light, with the cis conformer acting as an intermediate. We study how varying three different external stimuli (i.e., apolar versus polar solvent, low versus high photoexcitation energy, and vacuum versus a constant temperature thermostat) affects the trans-to- cis photoisomerization of SQ. Our results show that polarization effects due to implicit solvation and the thermostat play a crucial role in the isomerization process, whereas photoexcitation energy plays a lesser role on the outcome and efficiency. We also show that NEXMD captures the correct energy profile between the ground and first singlet excited state, showing that there are two distinct reaction pathways to the final stable product that vary by the number of photons absorbed, in agreement with experiment. Ultimately, NEXMD proves to be an effective tool for investigating excited state single molecule dynamics subject to various environments and initial conditions.

Nonadiabatic excited-state molecular dynamics (NEXMD) has been used to study photodecomposition in a class of recently synthesized bicyclic conjugated energetic materials (CEMs) composed of fused tetrazole and tetrazine derivatives with increasing oxygen substitutions. Modification by oxygen functionalization has already been demonstrated to increase the two-photon absorption intensity in the target CEMs while simultaneously improving oxygen balance. Photodecomposition mechanisms in materials that undergo nonlinear absorption could be used to achieve controlled, direct optical initiation. Here, we use NEXMD simulations to model the nonradiative relaxation and photodecomposition in CEMs following photoexcitation by a simulated Nd:YAG laser pulse. Excess electronic energy is quickly converted into vibrational energy on a sub-100 fs time scale resulting in bond dissociation. We find that, for the studied tetrazine derivatives, the bicyclic framework is an important structural feature that enhances the photochemical quantum yield and the high atomic oxygen content increases the relaxation lifetime and opens additional photodissociation pathways targeting the oxygen-substituted sites. The presented analysis scheme based on bond orders in the swarm of NEXMD trajectories is a useful tool for determining photochemical reactions.

The synthesis, photophysics, reverse saturable absorption, and photodynamic therapeutic effect of six cationic biscyclometalated Ir(III) complexes (1–6) with extended π-conjugation on the diimine ligand and/or the cyclometalating ligands are reported in this paper. All complexes possess ligand-localized 1π,π* absorption bands below 400 nm and charge-transfer absorption bands above 400 nm. They are all emissive in the 500–800 nm range in deoxygenated solutions at room temperature. All complexes exhibit strong and broad triplet excited-state absorption at 430–800 nm, and thus strong reverse saturable absorption for ns laser pulses at 532 nm. Complexes 1–4 are strong reverse saturable absorbers at 532 nm, while complex 6 could be a good candidate as a broadband reverse saturable absorber at 500–850 nm. The degree of π-conjugation of the diimine ligand mainly influences the 1π,π* transitions in their UV-vis absorption spectra, while the degree of π-conjugation of the cyclometalating ligand primarily affects the nature and energies of the lowest singlet and emitting triplet excited states. However, the lowest-energy triplet excited states for complexes 3–6 that contain the same benzo[i]dipyrido[3,2-a:2′,3′-c]phenazine (dppn) diimine ligand but different cyclometalating ligands remain the same as the dppn ligand-localized 3π,π* state, which gives rise to the long-lived, strong excited-state absorption in the visible to the near-IR region. All of the complexes exhibit a photodynamic therapeutic effect upon visible or red light activation, with complex 6 possessing the largest phototherapeutic index reported to date (>400) for an Ir(III) complex. Interactions with biological targets such as DNA suggest that a novel mechanism of action may be at play for the photosensitizing effect. These Ir(III) complexes also produce strong intracellular luminescence that highlights their potential as theranostic agents.

The synthesis and photophysical properties (UV−vis absorption, emission, and transient absorption) of four cationic Ir(III) complexes (C^N)2Ir(R-quqo)+ (HC^N = 1-phenylisoquinoline (piq) and 1,2-diphenylpyreno[4,5-d]imidazole (dppi), quqo = 2-(2-quinolinyl)quinoxaline, R = H or fluorenyl) are reported. The UV–vis absorption and emission were simulated by time-dependent density functional theory (TDDFT). Influences of extending π-conjugation of the C^N ligand and the diimine ligand on the singlet and triplet excited-state absorption and lifetimes of these complexes were explored. All complexes exhibited intense ligand-localized 1π,π transitions, broad and structureless metal-to-ligand charge transfer (1MLCT) / ligand-to-ligand charge transfer (1LLCT) transitions, and very weak spin-forbidden 3MLCT/3LLCT/3π,π transitions in their UV–vis absorption spectra. The two complexes that bear fluorenyl-substituted quqo ligands (Ir-3 and Ir-4) also possessed a broad intraligand charge transfer (1ILCT) / 1π,π band at 430–550 nm. The predominant 3ILCT/3π,π characters of the triplet excited states of Ir-3 and Ir-4 improved their phosphorescent emission quantum yields and prolonged their triplet lifetimes compared to the weaker and short-lived emission of Ir-1 and Ir-2. In contrast to the very weak nanosecond transient absorption (TA) of Ir-1 and Ir-2, Ir-3 and Ir-4 possessed much stronger TA signals at 520−800 nm upon nanosecond laser excitation. These complexes exhibited moderate to strong reverse saturable absorption (RSA) at 532 nm for ns laser pulses, with the RSA trend following Ir-1 > Ir-2 ≈ Ir-3 > Ir-4. Considering the long triplet excited-state lifetimes and broadband TA, complexes Ir-3 and Ir-4 could be potential broadband RSA materials.

A series of cationic dinuclear iridium(III) complexes (Ir1 - Ir5) bearing terpyridine-capped fluorenyl bridging ligand and different polypyridyl or cyclometalating terminal tridentate ligands were synthesized, characterized, and evaluated for their photophysical and photobiological activities. The influence of the bridging and terminal ligands on the photophysical properties of the complexes was investigated by UV-vis absorption, emission, and transient absorption spectroscopy, and simulated by TDDFT calculations. All of the complexes displayed strong bridging-ligand localized visible (1)π,π* absorption and red- or near-infrared (NIR) phosphorescence as well as broad triplet excited-state absorption across both visible and NIR wavelengths. These triplet states were assigned as predominantly (3)π,π* for Ir1 (τ = 3.1 μs) and Ir4 (τ = 48 μs), and predominantly (3)CT (charge transfer) for Ir2, Ir3 and Ir5 (τ = 1.7–2.7 μs). Complexes Ir1 – Ir5 acted as in vitro photodynamic therapy (PDT) agents toward human SK-MEL-28 melanoma cells when activated with visible light, with submicromolar photocytotoxicity and phototherapeutic indices (PIs) ranging from 20 to almost 300. The in vitro PDT effects with visible light did not correlate with singlet oxygen ((1)O(2)) quantum yields or DNA photocleaving capacity probed under cell-free conditions. All of the Ir(III) complexes phosphoresced brightly when associated with compromised cells (with or without a light treatment) and exhibited photoactivated cellular uptake, highlighting the theranostic potential of this new class of Ir(III) complex photosensitizers.

Extending π-conjugation of the diimine ligand (N^N ligand) via benzannulation is a common way to tune the absorption and emission energies of cationic iridium(III) complexes. However, it can cause either a red- or blue-shift of the absorption and emission bands depending on the site of benzannulation. To understand the mechanism of changes in optical transitions upon benzannulation on the diimine ligand, a series of new cationic iridium(III) complexes [Ir(dppi)2(N^N)]PF6 (1–6) (where dppi =1,2-diphenylpyreno[4,5-d]imidazole; N^N = 2-(pyridin-2-yl)quinoline (1), 2-(pyridin-2-yl)[7,8]benzoquinoline (2), 2,2′-bisquinoline (3), 2-(quinolin-2-yl)[7,8]benzoquinoline (4), 2-(pyridin-2-yl)[6,7]benzoquinoline (5), 2-(quinolin-2-yl)[6,7]benzoquinoline (6)) containing diimine ligand with varied degrees of π-conjugation via benzannulation at different sites of the 2-(pyridin-2-yl)quinoline ligand were synthesized. Experimental results and density functional theory (DFT) calculations revealed that benzannulation at th...

To understand the effects of the terminal substituent at the diimine ligand on the photophysics of heteroleptic cationic Ir(III) complexes and to obtain Ir(III) complexes with extended ground-state absorption to the near-IR region while retaining the long-lived and broadly absorbing triplet excited state, we synthesized three heteroleptic cationic iridium(III) complexes bearing cyclometalating 1-phenylisoquinoline (C^N) ligands and substituted 6,6′-bis(7-R-fluoren-2-yl)-2,2′-biquinoline (N^N) ligand (R = H, NO2, or NPh2). The photophysics of these complexes was systematically investigated via spectroscopic methods and time-dependent density functional theory. All complexes possess strong ligand-localized 1π,π* transitions mixed with ligand-to-ligand charge transfer (1LLCT)/metal-to-ligand charge transfer (1MLCT) transitions below 400 nm, and a broad and featureless absorption band above 400 nm that arises from the N^N ligand-localized 1π,π*/1ILCT (intraligand charge transfer) transitions as well as the ve...

These are the slides for all of the 2016 T Division lightning talks. There are 350 pages worth of slides from different presentations, all of which cover different topics within the theoretical division at Los Alamos National Laboratory (LANL).

The synthesis and photophysics of four platinum(II) bipyridine (bpy) bisacetylide complexes with different degrees of π-conjugation and an electron-donating diphenylamino (NPh2) or electron-withdrawing benzothiazolyl (BTZ) terminal substituent on the acetylide ligands are reported. The UV-vis absorption spectra of these complexes are composed of intense ligand-localized 1π,π* transitions at 330–430 nm and broad, moderately strong ligand-to-ligand charge transfer/metal-to-ligand charge transfer (1LLCT/1MLCT) transitions at 430–530 nm. All complexes are phosphorescent in solutions at room temperature and exhibit very broad and moderately strong triplet excited-state absorption in the visible to the NIR spectral region (425–800 nm). It is found that extending the π-conjugation of the acetylide ligands via adding one or two more ethynylfluorenyl unit(s) to the acetylide ligand does not change the energies of the 1π,π* and 1LLCT/1MLCT transitions pronouncedly except for increasing the molar extinction coefficients of the 1π,π* transitions. The emitting triplet excited states of the four complexes are the 3MLCT/3LLCT states and have the same energy. However, the complex that contains the tris(ethynylfluorenyl) units and the terminal NPh2 substituent on the acetylide ligand exhibits longer triplet lifetimes than the corresponding complex that has the bis(ethynylfluorenyl) units. The transient absorption band maxima of the complexes with tris(ethynylfluorenyl) units are slightly red-shifted in comparison to those of their respective counterparts with bis(ethynylfluorenyl) units. The nature of the terminal substituent does not influence the parentage and energies of the lowest singlet and triplet excited states. However, the triplet excited-state lifetimes of the complexes with the NPh2 terminal substituent on the bis(ethynylfluorenyl) or tris(ethynylfluorenyl) ligands are much longer than that of their counterpart with monofluorenylacetylide ligands; while the triplet lifetimes of the complexes containing the BTZ terminal substituent are similar to their counterpart with monofluorenylacetylide ligands. All complexes exhibit strong reverse saturable absorption (RSA) at 532 nm for nanosecond laser pulses.