arXiv subjects

Ala Aldin M. H. M. Darghouth

Publications and source records attributed to Ala Aldin M. H. M. Darghouth.

5 recordsLinked to original sources

arXiv2024

Test of the Orbital-Based LI3 Index as a Predictor of the Height of the 3MLCT to 3MC Transition-State Barrier for [Ru(N N)3]2+ Polypyridine Complexes in CH3CN

Ruthenium(II) polypyridine compounds often have a relatively long lived triplet metalligand charge transfer (3MLCT) state, making these complexes useful as chromophores for photoactivated electron transfer in photomolecular devices (PMDs). As different PMDs typically require different ligands and as the luminescence lifetime of the 3MLCT is sensitive to the structure of the ligand, it is important to understand this state and what types of photoprocesses can lead to its quenching. Recent work has increasingly emphasized that there are likely multiple competing pathways involved which should be explored in order to fully comprehend the 3MLCT state. However the lowest barrier that needs to be crossed to pass over to the nonluminescent triplet metal-centered (3MC) state has been repeatedly found to be a trans dissociation of the complex, at least in the simpler cases studied. This is the fourth in a series of articles investigating the possibility of an orbital based luminescence index (LI3, because it was the most successful of three) for predicting luminescence lifetimes. In an earlier study of bidentate (N N) ligands, we showed that the gas-phase 3MLCT to 3MC mechanism proceeded via an initial charge transfer to a single N N ligand which moves symmetrically away from the central ruthenium atom, followed by a bifurcation pathway to one of two 3MC enantiomers. The actual transition state barrier was quite small and independent, to within the limits of our calculations, to the choice of ligand studied. Here we investigate the same reaction in acetonitrile, CH3CN, solution and find that the mechanism differs from that in the gas phase in that the reaction passes directly via a trans mechanism. This has implications for the interpretation of LI3 via the Bell-Evans Polanyi principle.

physics.chem-ph↗

arXiv2023

Test of the Orbital-Based LI3 Index as a Predictor of the Height of the $^3$MLCT $\rightarrow$ $^3$MC Transition-State Barrier for Gas-Phase [Ru(N$^\wedge$N)$_3$]$^{2+}$ Polypyridine Complexes

Luminescence lifetimes of Ruthenium (II) polypyridine compounds is thought to be controlled by the barrier to conversion of triplet metal-ligand charge transfer ($^{3}$MLCT) state to a non-luminescent triplet metal-centered ($^{3}$MC) state. This work builds on earlier work [J. Photochem. Photobiol. A 276, 8 (2014)] and [J. Photochem. Photobiol. A 348, 305 (2017)] that derived several orbital-based luminescence indices of which the third (LI3) was based upon frontier-molecular-orbital-like ideas and correlated linearly with values of $E_{ave}$. $E_{ave}$ is a large underestimate of the true $^3$MLCT$\rightarrow$ $^3$MC TS barrier height in the case of the tris bipyridine ruthenium(II) cation {[Ru(bpy)$_3]^{2+}$}, but accurate TS barrier heights are difficult to obtain experimentally, it was judged useful to verify the ideas used to derive the LI3 index by calculating the energetics of the gas-phase $^{3}$MLCT $\rightarrow$ $^{3}$MC reaction for four complexes $\{$[Ru(N$^\wedge$N)$_3$]$^{2+}$ with N$^\wedge$N = bpy ({6}), 4,4'-dm-bpy ({70}), 4,4'-dph-bpy ({73}), and 4,4'-DTB-bpy ({74}) $\}$ using the same density functional and basis sets used in calculating LI3. We examine the trans dissociation mechanism in detail at the B3LYP/6-31G+LANLDZ(Ru) level and uncover a two-part mechanism. In the first part, the electron is transferred to a single ligand rather than symmetrically to all three ligands. It is the two Ru-N bonds to this ligand which are equally elongated in the transition state. The intrinsic reaction coordinate then continues down a ridge in hyperspace and bifurcates into one of two symmetry-equivalent $^3$MC structures with elongated trans bonds. Interestingly, no significant difference is found for the TS barriers for the four complexes treated here. Instead, LI3 is linearly correlated with the energy difference $Δ$ E = E($^{3}$MLCT) - E($^{3}$MC).

physics.chem-ph↗

arXiv2021

Effect of Varying the TD-lc-DFTB Range-Separation Parameter on Charge and Energy Transfer in a Model Pentacene/Buckminsterfullerene Heterojunction

Density-functional tight binding (DFTB) has become a popular form of approximate density-functional theory (DFT) based upon a minimal valence basis set and neglect of all but two center integrals. We report the results of our tests of a recent long-range correction (lc) for time-dependent (TD) lc-DFTB by carrying out TD-lc-DFTB fewest switches surface hopping (FSSH) calculations of energy and charge transfer times using the relatively new DFTBaby program. An advantage of this method is the ability to run enough trajectories to get meaningful ensemble averages. Our interest in the present work is less in determining exact energy and charge transfer rates than in understanding how the results of these calculations vary with the value of the range-separation parameter (Rlc = 1/μ) for a model organic solar cell heterojunction consisting of a van der Waals complex P/F made up of single pentacene (P) molecule together with a single buckminsterfullerene (F) molecule. The default value of Rlc = 3.03 a0 is found to be much too small as neither energy nor charge transfer is observed until Rlc ~ 10 a0. Tests at a single geometry show that best agreement with high-quality ab-initio spectra is obtained in the limit of no lc (i.e., very large Rlc.) A plot of energy and charge transfer rates as a function of Rlc is provided which suggests that a value of Rlc ~ 15 a0 yields the typical literature charge transfer time of about 100 fs. However, energy and charge transfer times become as high as ~ 300 fs for Rlc ~ 25 a0. A closer examination of the charge transfer process P*/F to P+/F- shows that the initial electron transfer is accompanied by a partial delocalization of the P hole onto F which then relocalizes back onto P, consistent with a polaron-like picture in which the nuclei relax to stabilize the resultant redistribution of charges.

physics.chem-ph↗

arXiv2018

Davydov-Type Excitonic Effects on the Absorption Spectra of Parallel-Stacked and Herringbone Aggregates of Pentacene: Time-Dependent Density-Functional Theory and Time-Dependent Density-Functional Tight Binding

Exciton formation leads to J-bands in solid pentacene. Describing these exciton bands represents a challenge for both time-dependent (TD) density-functional theory (DFT) and for its semiempirical analogue, namely for TD density-functional tight binding (DFTB) for three reasons (i) solid pentacene and pentacene aggregates are bound only by van der Waals forces which are notoriously difficult to describe with DFT and DFTB, (ii) the proper description of the long-range coupling between molecules, needed to describe Davydov splitting, is not easy to include in TD-DFT with traditional functionals and in TD-DFTB, and (iii) mixing may occur between local and charge transfer excitons, which may, in turn, require special functionals. We assess how far TD-DFT and TD-DFTB have progressed towards a correct description of this type of exciton by including both a dispersion correction for the ground state and a range-separated hybrid functional for the excited state. Analytic results for parallel-stacked ethylene are derived which go beyond Kasha's exciton model in that we are able to make a clear distinction between charge transfer and energy transfer excitons. This is further confirmed when it is shown that range-separated hybrids have a markedly greater effect on charge-transfer excitons than on energy-transfer excitons in the case of parallel-stacked pentacenes. TD-DFT calculations with the CAM-B3LYP functional and TD-lc-DFT calculations lead to negligeable excitonic corrections for the herringbone crystal structure, possibly because of an overcorrection of charge-transfer effects. In this case, TD-DFT calculations with the B3LYP functional or TD-DFTB calculations parameterized to B3LYP give the best results for excitonic corrections for the herringbone crystal structure as judged from comparison with experimental spectra and with Bethe-Salpeter equation calculations from the literature.

physics.chem-ph↗

arXiv2017

Density-Functional Theory Study of the Optoelectronic Properties of π-Conjugated Copolymers for Organic Light-Emitting Diodes

Novel low-band-gap copolymer oligomers are proposed on the basis of density functional theory (DFT) quantum chemical calculations of photophysical properties. These molecules have an electron donor-accepter (D-A) architecture involving poly(3-hexylthiophene-2,5-diyl) (P3HT) as D units and furan, aniline, or hydroquinone as A units. Structural parameters, electronic properties, highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gaps and molecular orbital densities are predicted. The charge transfer process between the D unit and the A unit one is supported by analyzing the optical absorption spectra of the compounds and the localization of the HOMO and LUMO.

physics.chem-ph↗