Size Isn't Everything: Geometric Tuning in Polycyclic Aromatic Hydrocarbons and Its Implications for Carbon Nanodots

Recent developments in light-emitting carbon nanodots and molecular organic semiconductors have seen renewed interest in the properties of polycyclic aromatic hydrocarbons (PAHs) as a family. The networks of delocalized π electrons in sp2-hybridized carbon grant PAHs light-emissive properties right across the visible spectrum. However, the mechanistic understanding of their emission energy has been limited due to the ground state-focused methods of determination. This computational chemistry work, therefore, seeks to validate existing rules and elucidate new features and characteristics of PAHs that influence their emissions. Predictions based on (time-dependent) density functional theory account for the full 3-dimensional electronic structure of ground and excited states and reveal that twisting and near-degeneracies strongly influence emission spectra and may therefore be used to tune the color of PAHs and, hence, carbon nanodots. We particularly note that the influence of twisting goes beyond torsional destabilization of the ground-state and geometric relaxation of the excited state, with a third contribution associated with the electric transition dipole. Symmetries and peri-condensation may also have an effect, but this could not be statistically confirmed. In pursuing this goal, we demonstrate that with minimal changes to molecular size, the entire visible spectrum may be spanned by geometric modification alone; we have also provided a first estimate of emission energy for 35 molecules currently lacking published emission spectra as well as clear guidelines for when more sophisticated computational techniques are required to predict the properties of PAHs accurately.

Cavity Ring-Down Spectroscopy of Anthracene, 9-Methylanthracene, and 2-Methylanthracene in Supersonic Expansion

The S₀-S₁ absorption spectra of anthracene (C₁₄H₁₀), 9-methylanthracene (C₁₅H₁₂), and 2-methylanthracene (C₁₅H₁₂) are measured in the ultraviolet region between 330 and 375 nm (26,666 to 30,303 cm^-1) with cavity ring-down spectroscopy in supersonic free-jet expansions of argon. The associated vibronic band systems and their spectroscopic assignments are discussed and compared to previous studies performed using fluorescence excitation and dispersed fluorescence techniques. Density functional theory (DFT) calculations were carried out to study the structures and evaluate the vibrational transitions of the ground and excited states. Time-dependent DFT calculations of the first electronic excited states and Franck-Condon factor calculations were carried out to assist in the assignment of the experimentally measured vibronic bands. The vibronic spectra obtained in absorption agree well with fluorescence excitation spectra in terms of peak positions but exhibit different relative band intensities. We find a very good match between experimentally obtained vibronic line positions and the peak positions of the quantum chemically calculated Franck-Condon excitation lines.

The COMPAS Project: A Computational Database of Polycyclic Aromatic Systems. Phase 1: cata-Condensed Polybenzenoid Hydrocarbons

Chemical databases are an essential tool for data-driven investigation of structure-property relationships and for the design of novel functional compounds. We introduce the first phase of the COMPAS Project─a COMputational database of Polycyclic Aromatic Systems. In this phase, we developed two data sets containing the optimized ground-state structures and a selection of molecular properties of ∼34k and ∼9k cata-condensed polybenzenoid hydrocarbons (at the GFN2-xTB and B3LYP-D3BJ/def2-SVP levels, respectively) and placed them in the public domain. Herein, we describe the process of the data set generation, detail the information available within the data sets, and show the fundamental features of the generated data. We analyze the correlation between the two types of computations as well as the structure-property relationships of the calculated species. The data and insights gained from them can inform rational design of novel functional aromatic molecules for use in, e.g., organic electronics, and can provide a basis for additional data-driven machine- and deep-learning studies in chemistry.

Simulation of vibrationally resolved absorption spectra of neutral and cationic polyaromatic hydrocarbons

The identification of the carriers of the absorption features associated with the diffuse interstellar bands (DIBs) is a long-standing problem in astronomical spectroscopy. Computational simulations can contribute to the assignment of the carriers of DIBs since variations in molecular structure and charge state can be studied more readily than through experimental measurements. Polyaromatic hydrocarbons have been proposed as potential carriers of these bands, and it is shown that simulations based upon density functional theory and time-dependent density functional theory calculations can describe the vibrational structure observed in experiment for neutral and cationic naphthalene and pyrene. The vibrational structure arises from a small number of vibrational modes involving in-plane atomic motions, and the Franck–Condon–Herzberg–Teller approximation improves the predicted spectra in comparison with the Franck–Condon approximation. The study also highlights the challenges for the calculations to enable the assignment in the absence of experimental data, namely prediction of the energy separation between the different electronic states to a sufficient level of accuracy and performing vibrational analysis for higher-lying electronic states.

Functional and Basis Set Dependence for Time-Dependent Density Functional Theory Trajectory Surface Hopping Molecular Dynamics: Cis-Azobenzene Photoisomerization

Within three functionals (TD-B3LYP, TD-BHandHLYP, and TD-CAM-B3LYP) in combination with four basis sets (3-21g, 6-31g, 6-31g(d), and cc-pvdz), global switching (GS) trajectory surface hopping molecular dynamics has been performed for cis-to-trans azobenzene photoisomerization up to the S₁ (nπ*) excitation. Although all the combinations show artificial double-cone structure of conical intersection between ground and first excited states, simulated quantum yields and lifetimes are in good agreement with one another; 0.6 (±5%) and 40.5 fs (±10%) by TD-B3LYP, 0.5 (±10%) and 35.5 fs (±4%) by TD-BHandHLYP, and 0.44 (±9%) and 35.2 fs (±10%) by TD-CAM-B3LYP. By analyzing distributions of excited-state population decays, hopping spots, and typical trajectories with performance of 12 functional/basis set combinations, it has been concluded that functional dependence for given basis set is slightly more sensitive than basis set dependence for given functional. The present GS on-the-fly time-dependent density functional theory (TDDFT) trajectory surface hopping simulation can provide practical benchmark guidelines for conical intersection driven excited-state molecular dynamics simulation involving in large complex system within ordinary TDDFT framework. © 2019 Wiley Periodicals, Inc.

Correlated Electronic States of a Few Polycyclic Aromatic Hydrocarbons: A Computational Study

In recent years, polycyclic aromatic hydrocarbons (PAHs) have been studied for their electronic properties as they are viewed as nanodots of graphene. They have also been of interest as functional molecules for applications such as light-emitting diodes and solar cells. Since the last few years, varying structural and chemical properties corresponding to the size and geometry of these molecules have been studied both theoretically and experimentally. Here, we carry out a systematic study of the electronic states of several PAHs using the Pariser-Parr-Pople model, which incorporates long-range electron correlations. In all of the molecules studied by us, we find that the 2A state is below the 1B state and hence none of them will be fluorescent in the gaseous phase. The singlet-triplet gap is more than half of the singlet-singlet gap in all cases, and hence, none of these PAHs can be candidates for improved solar cell efficiencies in a singlet fission. We discuss in detail the properties of the electronic states, which include bond orders and spin densities (in triplets) of these systems.

A novel pyrene-based selective colorimetric and ratiometric turn-on sensing for copper

Detection of copper attracts important in most environmental and biological systems. In this study, a simple probe BisPyTSC containing bis-pyrene core was synthesized, and cation binding and sensing properties were studied using colorimetric and fluorometric detection. The research indicated that the specific ligand affinity for Cu²⁺ ions results in drastic color and spectral changes. According to the data obtained, while the peak intensity increases at 376 nm, the peak intensity decreased at 280 nm in the absorption spectrum of BisPyTSC and an increase in fluorescence intensity of BisPyTSC was observed in the presence of Cu²⁺ ions. The binding ratio of BisPyTSC to Cu²⁺ was found to be 1:1 according to Job's plot experiments. The binding constant was calculated using the Benesi-Hildebrand equation and found to be 3.26 × 10⁴ M^-1. Based on these concentration dependent fluorescence changes, the limit of detection (LOD) value was calculated to be 14.5 μM for Cu²⁺, which is the range of copper that should be in the blood (11.8-23.6 μM). As a result of all these studies, we can understand that BisPyTSC is a good selective candidate turn-on sensor that can be used for Cu²⁺ detection.

Atomic hydrogen interactions with gas-phase coronene cations: hydrogenation versus fragmentation

Sequential hydrogenation of polycyclic aromatic hydrocarbon (PAH) cations drives a gradual transition from a planar to a puckered geometry and from an aromatic to an aliphatic electronic structure. The resulting H-induced weakening of the molecular structure together with the exothermic nature of the consecutive H-attachment processes can lead to substantial molecular fragmentation. We have studied H attachment to gas-phase coronene cations in a radiofrequency ion trap using tandem mass spectrometry. With increasing hydrogenation, C2Hi loss and multifragmentation are identified as main de-excitation channels. To understand the dependence of both channels on H-exposure time, we have simulated the molecular stability and fragmentation channels of hydrogenated PAHs using a molecular dynamics approach employing potential energies determined by a density functional based tight binding method. As the coronene fragmentation patterns depend on the balance between energy deposition by H-attachment and the extent of cooling in between subsequent attachment processes, we investigate several scenarios for the energy distribution of hydrogenated PAHs. Good agreement between experiment and simulation is reached, when realistic energy distributions are considered.

Spectroscopy of prospective interstellar ions and radicals isolated in para-hydrogen matrices

The p-H₂ matrix-isolation technique coupled with photolysis in situ or electron bombardment produces protonated or hydrogenated species important in astrochemistry.

Low-lying, Rydberg states of polycyclic aromatic hydrocarbons (PAHs) and cyclic alkanes

TD-DFT calculations of low-lying, Rydberg states of a series of polycyclic hydrocarbons and cyclic alkanes are presented.

Synthesis, in vitro β-glucuronidase inhibitory activity and in silico studies of novel (E)-4-Aryl-2-(2-(pyren-1-ylmethylene)hydrazinyl)thiazoles

Current research is based on the synthesis of novel (E)-4-aryl-2-(2-(pyren-1-ylmethylene)hydrazinyl)thiazole derivatives (3-15) by adopting two steps route. First step was the condensation between the pyrene-1-carbaldehyde (1) with the thiosemicarbazide to afford pyrene-1-thiosemicarbazone intermediate (2). While in second step, cyclization between the intermediate (2) and phenacyl bromide derivatives or 2-bromo ethyl acetate was carried out. Synthetic derivatives were structurally characterized by spectroscopic techniques such as EI-MS, ¹H NMR and ¹³C NMR. Stereochemistry of the iminic double bond was confirmed by NOESY analysis. All pure compounds 2-15 were subjected for in vitro β-glucuronidase inhibitory activity. All molecules were exhibited excellent inhibition in the range of IC₅₀=3.10±0.10-40.10±0.90μM and found to be even more potent than the standard d-saccharic acid 1,4-lactone (IC₅₀=48.38±1.05μM). Molecular docking studies were carried out to verify the structure-activity relationship. A good correlation was perceived between the docking study and biological evaluation of active compounds.

Theoretical investigations of absorption and fluorescence spectra of protonated pyrene

The equilibrium geometry and 75 vibrational normal-mode frequencies of the ground and first excited states of protonated pyrene isomers were calculated and characterized in the adiabatic representation by using the complete active space self-consistent field (CASSCF) method. Electronic absorption spectra of solid neon matrixes in the wavelength range 495-415 nm were determined by Maier et al. and they were analyzed using time-dependent density functional theory calculations (TDDFT). CASSCF calculations and absorption and emission spectra simulations by one-photon excitation equations were used to optimize the excited and ground state structures of protonated pyrene isomers. The absorption band was attributed to the S0 → S1 electronic transition in 1H-Py(+), and a band origin was used at 20580.96 cm(-1). The displaced harmonic oscillator approximation and Franck-Condon approximation were used to simulate the absorption spectrum of the (1) (1)A' ← X[combining tilde](1)A' transition of 1H-Py(+), and the main vibronic transitions were assigned for the first ππ* state. It shows that the vibronic structures were dominated by one of the eight active totally symmetric modes, with ν15 being the most crucial. This indicates that the electronic transition of the S1((1)A') state calculated in the adiabatic representation effectively includes a contribution from the adiabatic vibronic coupling through Franck-Condon factors perturbed by harmonic oscillators. The present method can adequately reproduce experimental absorption and fluorescence spectra of a gas phase.

An optical spectrum of a large isolated gas-phase PAH cation: C78H26

A gas-phase optical spectrum of a large polycyclic aromatic hydrocarbon (PAH) cation - C78H26+- in the 410-610 nm range is presented. This large all-benzenoid PAH should be large enough to be stable with respect to photodissociation in the harsh conditions prevailing in the interstellar medium (ISM). The spectrum is obtained via multi-photon dissociation (MPD) spectroscopy of cationic C78H26 stored in the Fourier Transform Ion Cyclotron Resonance (FT-ICR) cell using the radiation from a mid-band optical parametric oscillator (OPO) laser. The experimental spectrum shows two main absorption peaks at 431 nm and 516 nm, in good agreement with a theoretical spectrum computed via time-dependent density functional theory (TD-DFT). DFT calculations indicate that the equilibrium geometry, with the absolute minimum energy, is of lowered, nonplanar C2 symmetry instead of the more symmetric planar D2h symmetry that is usually the minimum for similar PAHs of smaller size. This kind of slightly broken symmetry could produce some of the fine structure observed in some diffuse interstellar bands (DIBs). It can also favor the folding of C78H26+ fragments and ultimately the formation of fullerenes. This study opens up the possibility to identify the most promising candidates for DIBs amongst large cationic PAHs.

Protonated thiophene-based oligomers as formed within zeolites: understanding their electron delocalization and aromaticity

In an earlier work, protonated thiophene-based oligomers were identified inside ZSM-5 zeolites.

Cationic Methylene-Pyrene Isomers and Isomerization Pathways: Finite Temperature Theoretical Studies

This paper provides spectral characterizations of the two isomers of the 1-methylenepyrene cation, namely, the 1-pyrenemethylium and a pyrene-like isomer owing a tropylium cycle. Both are possible photodissociation products of the 1-methylpyrene cation and were proposed as potential contributors to the diffuse interstellar bands. In that respect, vibrational and electronic spectra are computed for the optimized structures at the density functional theory (DFT) and time-dependent (TD-)DFT levels. Finite temperature effects on these spectra are estimated from molecular dynamics simulations within the density functional-based tight-binding (DFTB) and TD-DFTB frameworks, these methods being first benchmarked against DFT and TD-DFT calculations. The computed spectra allow discrimination of the two isomers. When the temperature increases, bands are observed to redshift and merge. The isomerization mechanism is investigated with the metadynamics technique, a biased dynamics scheme allowing to probe reaction mechanisms with high energy barriers by investigating the free energy surface at various temperatures. Four pathways with similar barrier heights (3.5-4 eV) are found, showing that the interconversion process would only occur in interstellar clouds under photoactivation. The present study opens the way to simulations on larger methyl- and methylenePAHs of astrophysical interest and their experimental investigation.

Electronic Spectroscopy of [FePAH](+) Complexes in the Region of the Diffuse Interstellar Bands: Multireference Wave Function Studies on [FeC6H6](+)

The low-energy states and electronic spectrum in the near-infrared-visible region of [FeC6H6](+) are studied by theoretical approaches. An exhaustive exploration of the potential energy surface of [FeC6H6](+) is performed using the density functional theory method. The ground state is found to be a (4)A1 state. The structures of the lowest energy states ((4)A2 and (4)A1) are used to perform multireference wave function calculations by means of the multistate complete active space with perturbation at the second order method. Contrary to the density functional theory results ((4)A1 ground state), multireference perturbative calculations show that the (4)A2 state is the ground state. The vertical electronic spectrum is computed and compared with the astronomical diffuse interstellar bands, a set of near-infrared-visible bands detected on the extinction curve in our and other galaxies. Many transitions are found in this domain, corresponding to d → d, d → 4s, or d → π* excitations, but few are allowed and, if they are, their oscillation strengths are small. Even though some band positions could match some of the observed bands, the relative intensities do not fit, making the contribution of the [Fe-C6H6](+) complexes to the diffuse interstellar bands questionable. This work, however, lays the foundation for the studies of polycyclic aromatic hydrocarbons (PAHs) complexed to Fe cations that are more likely to possess d → π* and π → π* transitions in the diffuse interstellar bands domain. PAH ligands indeed possess a larger number of π and π* orbitals, respectively, higher and lower in energy than those of C6H6, which are expected to lead to lower energy d → π* and π → π* transitions in [FePAH](+) than in [FeC6H6](+) complexes.

Gas-phase electronic transitions of C₁₇H₁₂N⁺ at 15 K

The electronic spectrum of C17H12N(+), phenanthrene with a side chain, was measured in the gas phase at a vibrational and rotational temperature of ∼15 K in an ion trap using a resonant multiphoton dissociation technique. The C17H12N(+) structure was produced in a chemical ionization source and identified by a comparison with theoretical calculations of stable structures and excitation energies. The (3), (2), (1) (1)A ← X (1)A electronic transitions of this nitrogen-containing aromatic species with 30 atoms have origin band maxima at 23,586 ± 1 cm(-1), 16,120 ± 50 cm(-1), and 14,519 ± 30 cm(-1). Distinct vibrational structure in the (3) (1)A state is observed, and assignments are made. Astronomical aspects are considered.

The substituent effect on benzene dications

It was shown that the substituent effect in the dicationic benzene ring, which is a 4n π-electron system, can be numerically quantified and then further interpreted.

Infrared Spectra of Protonated Pyrene and Its Neutral Counterpart in Solid para-Hydrogen

Protonated polycyclic aromatic hydrocarbons (H(+)PAHs) have been reported to have infrared (IR) bands at wavenumbers near those of unidentified infrared (UIR) emission bands from interstellar objects. We produced 1-C16H11(+) and 1-C16H11 upon electron bombardment during matrix deposition of p-H2 containing pyrene (C16H10) in a small proportion. Intensities of absorption features of 1-C16H11(+) decreased after the matrix was maintained in darkness or irradiated with light at 365 nm, whereas those of 1-C16H11 increased. The observed line wavenumbers and relative intensities of 1-C16H11(+) and 1-C16H11 agree satisfactorily with the scaled vibrational wavenumbers and IR intensities predicted with the B3PW91/6-311++G(2d,2p) method. Our method, being relatively clean with negligible fragmentation, is applicable to larger H(+)PAH; it has the advantages of producing excellent IR spectra covering a broad spectral range with narrow lines and accurate intensities, so that structural identification among various isomers is feasible.