Ligand and solvent effects on the absorption spectra of CdS magic-sized clusters.

The absorption spectra of congenetic wurtzite (WZ) and zincblende (ZB) CdS magic-sized clusters are investigated. We demonstrate that the exciton peak positions can be tuned by up to 500 meV by varying the strong coupling between X-type ligands and the semiconductor cores, while the addition of L-type ligands primarily affects cluster midgap states. When Z-type ligands are displaced by L-type ligands, red shifts in the absorption spectra are observed, despite the fact there is a small decrease in cluster size. Density functional theory calculations are used to explain these findings and they reveal the importance of Cd and S dangling bonds on the midgap states during the Z- to L-type ligand exchange process. Overall, ZB CdS clusters show higher chemical stability than WZ clusters but their optical properties exhibit greater sensitivity to the solvent. Conversely, WZ CdS clusters are not stable in a Lewis base-rich environment, resulting in various changes in their spectra. Our findings enable researchers to select capping ligands that modulate the optical properties of semiconductor clusters while maintaining precise control over their solvent interactions.

The quantum chemical solvation of indole: accounting for strong solute-solvent interactions using implicit/explicit models.

In this paper, we investigate the efficacy of different quantum chemical solvent modelling methods of indole in both water and methylcyclohexane solutions. The goal is to show that one can yield good photophysical properties in strongly coupled solute-solvent systems using standard DFT methods. We use standard and linearly-corrected Polarisable Continuum Models (PCM), as well as explicit solvation models, and compare the different model parameters, including the choice of density functional, basis set, and number of explicit solvent molecules. We demonstrate that implicit models overestimate energies and oscillator strengths. In particular, for indole-water, no level inversion is observed, suggesting a dielectric medium on its own is insufficient. In contrast, energies are seen to converge fairly rapidly with respect to cluster size towards experimentally measured properties in the explicit models. We find that the use of B3LYP with a diffuse basis set can adequately represent the photophysics of the system with a cluster size of between 9-12 explicit water molecules. Sampling of configurations from a molecular dynamics simulation suggests that the single point results are suitably representative of the solvated ensemble. For indole-water, we show that solvent reorganisation plays a significant role in stabilisation of the excited state energies. It is hoped that the findings and observations of this study will aid in the choice of solvation model parameters in future studies.

Efficient enumeration of bosonic configurations with applications to the calculation of non-radiative rates.

This work presents algorithms for the efficient enumeration of configuration spaces following Boltzmann-like statistics, with example applications to the calculation of non-radiative rates, and an open-source implementation. Configuration spaces are found in several areas of physics, particularly wherever there are energy levels that possess variable occupations. In bosonic systems, where there are no upper limits on the occupation of each level, enumeration of all possible configurations is an exceptionally hard problem. We look at the case where the levels need to be filled to satisfy an energy criterion, for example, a target excitation energy, which is a type of knapsack problem as found in combinatorics. We present analyses of the density of configuration spaces in arbitrary dimensions and how particular forms of kernel can be used to envelope the important regions. In this way, we arrive at three new algorithms for enumeration of such spaces that are several orders of magnitude more efficient than the naive brute force approach. Finally, we show how these can be applied to the particular case of internal conversion rates in a selection of molecules and discuss how a stochastic approach can, in principle, reduce the computational complexity to polynomial time.

A computational exploration of aggregation-induced excitonic quenching mechanisms for perylene diimide chromophores.

Perylene diimide (PDI) derivatives are widely used materials for luminescent solar concentrator (LSC) applications due to their attractive optical and electronic properties. In this work, we study aggregation-induced exciton quenching pathways in four PDI derivatives with increasing steric bulk, which were previously synthesized. We combine molecular dynamics and quantum chemical methods to simulate the aggregation behavior of chromophores at low concentration and compute their excited state properties. We found that PDIs with small steric bulk are prone to aggregate in a solid state matrix, while those with large steric volume displayed greater tendencies to isolate themselves. We find that for the aggregation class of PDI dimers, the optically accessible excitations are in close energetic proximity to triplet charge transfer (CT) states, thus facilitating inter-system crossing and reducing overall LSC performance. While direct singlet fission pathways appear endothermic, evidence is found for the facilitation of a singlet fission pathway via intermediate CT states. Conversely, the insulation class of PDI does not suffer from aggregation-induced photoluminescence quenching at the concentrations studied here and therefore display high photon output. These findings should aid in the choice of PDI derivatives for various solar applications and suggest further avenues for functionalization and study.

Interexcited State Photophysics I: Benchmarking Density Functionals for Computing Nonadiabatic Couplings and Internal Conversion Rate Constants.

We present the first benchmarking study of nonadiabatic matrix coupling elements (NACMEs) calculated using different density functionals. Using the S1 → S0 transition in perylene solvated in toluene as a case study, we calculate the photophysical properties and corresponding rate constants for a variety of density functionals from each rung of Jacob's ladder. The singlet photoluminescence quantum yield (sPLQY) is taken as a measure of accuracy, measured experimentally here as 0.955. Important quantum chemical parameters such as geometries, absorption, emission, and adiabatic energies, NACMEs, Hessians, and transition dipole moments were calculated for each density functional basis set combination (data set) using density functional theory based multireference configuration interaction (DFT/MRCI) and compared to experiment where possible. We were able to derive simple relations between the TDDFT and DFT/MRCI photophysical properties; with semiempirical damping factors of ∼0.843 ± 0.017 and ∼0.954 ± 0.064 for TDDFT transition dipole moments and energies to DFT/MRCI level approximations, respectively. NACMEs were dominated by out-of-plane derivative components belonging to the center-most ring atoms with weaker contributions from perturbations along the transverse and longitudinal axes. Calculated theoretical spectra compared well to both experiment and literature, with fluorescence lifetimes between 7.1 and 12.5 ns, agreeing within a factor of 2 with experiment. Internal conversion (IC) rates were then calculated and were found to vary wildly between 106-1016 s-1 compared with an experimental rate of the order 107 s-1. Following further testing by mixing data sets, we found a strong dependence on the method used to obtain the Hessian. The 5 characterized data sets ranked in order of most promising are PBE0/def2-TZVP, ωB97XD/def2-TZVP, HCTH407/TZVP, PBE/TZVP, and PBE/def2-TZVP.

Quantifying the Relaxation Dynamics of Higher Electronic Excited States in Perylene.

Gating logical operations through high-lying electronic excited states presents opportunities for developing ultrafast, subnanometer computational devices. A lack of molecular systems with sufficiently long-lived higher excited states has hindered practical realization of such devices, but recent studies have reported intriguing photophysics from high-lying excited states of perylene. In this work, we use femtosecond spectroscopy supported by quantum chemical calculations to identify and quantify the relaxation dynamics of monomeric perylene's higher electronic excited states. The 21B2u state is accessed through single-photon absorption at 250 nm, while the optically dark 21Ag state is excited via the 11B3u state. Population of either state results in subpicosecond relaxation to the 11B3u state, and we quantify 21Ag and 21B2u state lifetimes of 340 and 530 fs, respectively. These lifetimes are significantly longer than the singlet fission time constant from the perylene 21B2u state, suggesting that the higher electronic states of perylene may be useful for gating logical operations.

The dominant nature of Herzberg-Teller terms in the photophysical description of naphthalene compared to anthracene and tetracene.

The first order and second order corrected photoluminescence quantum yields are computed and compared to experiment for naphthalene in this manuscript discussing negative results. Results for anthracene and tetracene are recalled from previous work (Manian et al. in J Chem Phys 155:054108, 2021), and the results for all three polyacenes are juxtaposed to each other. While at the Franck-Condon point, each of the three noted polyacenes were found to possess a quantum yield near unity. Following the consideration of Herzberg-Teller effects, quantum yields stabilised for anthracene and tetracene to 0.19 and 0.08, respectively. Conversely, the second order corrected quantum yield for naphthalene was found to be 0.91. Analysis of this result showed that while the predicted non-radiative pathways correlate well with what should be expected, the approximation used to calculate second order corrected fluorescence, which yielded very positive results for many other molecular systems, here is unable to account for strong second order contributions, resulting in a grossly overestimated rate of fluorescence. However, substitution of an experimental radiative rate results in a quantum yield of 0.33. This work extols the importance of Herzberg-Teller terms in photophysical descriptions of chromophores, and highlights those cases in which a treatment beyond the above approximation is required.

Exciton Dynamics of a Diketo-Pyrrolopyrrole Core for All Low-Lying Electronic Excited States Using Density Functional Theory-Based Methods.

Ab initio treatments of interexcited state internal conversion (IC) are more often than not missing from exciton dynamic descriptions, because of their inherent complexity. Here, we define "interexcited state IC" as a same-spin nonradiative transition between states i and j, where i ≠ j ≠ 0. Competing directly with multiexciton processes such as singlet fission or triplet photoupconversion, inclusion of this mechanism in the narrative of molecular photophysics would allow for strategic synthesis of chromophores for more efficient photon-harvesting applications. Herein, we present a robust formalism which can model these rates using density functional theory (DFT)-based methods within the Franck-Condon and Herzberg-Teller regime. Using an unsubstituted diketo-pyrrolopyrrole (DPP) core as a case study, we illustrate the exciton dynamics along the first four excited states for both singlet and triplet manifolds, showing ultrafast same-spin transfer mechanisms due to all excited states, excluding the first triplet level, being in close energetic proximity (within 0.8 eV of each other). The resulting electron same-spin rates outcompete the electron spin-flipping intersystem crossing (ISC) rates, with excitons firmly obeying Kasha's rule as they cascade down from the high-lying excited states toward the lower states. Furthermore, we calculated that only the first singlet excited state displayed a reasonable probability of triplet exciton generation, of ∼40%, with a near-zero chance of the exciton reverting to the singlet manifold once the electron-hole pair are of parallel spin.

A first principles examination of phosphorescence.

This paper explores phosphorescence from a first principles standpoint, and examines the intricacies involved in calculating the spin-forbidden T 1 → S 0 transition dipole moment, to highlight that the mechanism is not as complicated to compute as it seems. Using gas phase acridine as a case study, we break down the formalism required to compute the phosphorescent spectra within both the Franck-Condon and Herzberg-Teller regimes by coupling the first triplet excited state up to the S 4 and T 4 states. Despite the first singlet excited state appearing as an L b state and not of nπ* character, the second order corrected rate constant was found to be 0.402 s-1, comparing well with experimental phosphorescent lifetimes of acridine derivatives. In showing only certain states are required to accurately describe the matrix elements as well as how to find these states, our calculations suggest that the nπ* state only weakly couples to the T 1 state. This suggest its importance hinges on its ability to quench fluorescence and exalt non-radiative mechanisms rather than its contribution to the transition dipole moment. A followup investigation into the T 1 → S 0 transition dipole moment's growth as a function of its coupling to other electronic states highlights that terms dominating the matrix element arise entirely from the inclusion of states with strong spin-orbit coupling terms. This means that while the expansion of the transition dipole moment can extend to include an infinite number of electronic states, only certain states need to be included.

Enhancement of DNAzymatic activity using iterative in silico maturation.

DNAzyme-based (catalytic nucleic acid) biosensing technology is recognised as a valuable biosensing tool in diagnostic medicine and seen as a cheaper, more stable alternative to antibodies or enzymes. However, like enzyme discovery, no method exists to predict DNAzyme sequences that result in high catalytic activity using computer software (in silico). In this work, iterative in silico maturation and in vitro evaluation were applied to a DNAzyme oligodeoxynucleotide (ODN) sequence to elucidate novel synthetic sequences with enhanced DNAzyme activity. An already well-known model DNAzyme, the G-quadruplex/hemin complex, was iterated over eight generations to elucidate synthetic sequences that were up to five times faster than the original parent sequence. By combining molecular dynamics simulations, we found that the POD-mimicking activities were largely affected by docking modes and the tightness of locking between complexes. Ultimately, the theoretical models showed significant sequence-dependencies.