Electronic spectra and ionization potentials of a stable class of closed shell polycyclic aromatic hydrocarbon cations

Accurate Electronic and Optical Properties of Organic Doublet Radicals Using Machine Learned Range-Separated Functionals

Luminescent organic semiconducting doublet-spin radicals are unique and emergent optical materials because their fluorescent quantum yields (Φfl) are not compromised by the spin-flipping intersystem crossing (ISC) into a dark high-spin state. The multiconfigurational nature of these radicals challenges their electronic structure calculations in the framework of single-reference density functional theory (DFT) and introduces room for method improvement. In the present study, we extended our earlier development of ML-ωPBE [J. Phys. Chem. Lett., 2021, 12, 9516-9524], a range-separated hybrid (RSH) exchange-correlation (XC) functional constructed using the stacked ensemble machine learning (SEML) algorithm, from closed-shell organic semiconducting molecules to doublet-spin organic semiconducting radicals. We assessed its performance for a new test set of 64 doublet-spin radicals from five categories while placing all previously compiled 3926 closed-shell molecules in the new training set. Interestingly, ML-ωPBE agrees with the nonempirical OT-ωPBE functional regarding the prediction of the molecule-dependent range-separation parameter (ω), with a small mean absolute error (MAE) of 0.0197 a0-1, but saves the computational cost by 2.46 orders of magnitude. This result demonstrates an outstanding domain adaptation capacity of ML-ωPBE for diverse organic semiconducting species. To further assess the predictive power of ML-ωPBE in experimental observables, we also applied it to evaluate absorption and fluorescence energies (Eabs and Efl) using linear-response time-dependent DFT (TDDFT), and we compared its behavior with nine popular XC functionals. For most radicals, ML-ωPBE reproduces experimental measurements of Eabs and Efl with small MAEs of 0.299 and 0.254 eV, only marginally different from those of OT-ωPBE. Our work illustrates a successful extension of the SEML framework from closed-shell molecules to doublet-spin radicals and will open the venue for calculating optical properties for organic semiconductors using single-reference TDDFT.

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.

Towards a comprehensive electronic database of polycyclic aromatic hydrocarbons and its application in constraining the identities of possible carriers of the diffuse interstellar bands

A theoretical approach is developed to pre-select individual polycyclic aromatic hydrocarbons (PAHs) as possible carriers of the diffuse interstellar bands (DIBs). In this approach, a computer program is used to enumerate all PAH molecules with up to a specific number of fused benzene rings. Fast quantum chemical calculations are then employed to calculate the electronic transition energies, oscillator strengths, and rotational constants of these molecules. An electronic database of all PAHs with up to any specific number of benzene rings can be constructed this way. Comparison of the electronic transition energies, oscillator strengths, and rotational band contours of all PAHs in the database with astronomical spectra allows one to constrain the identities of individual PAHs as possible carriers of some of the intense narrow DIBs. Using the current database containing up to 10 benzene rings we have pre-selected 8 closed-shell PAHs as possible carriers of the famous lambda6614 DIB.

Push-pull bithienyl chromophore with an unusual transverse path of conjugation

The synthesis, structure, and electronic properties of a novel cross-conjugated 10H-bisthienodithiocin-10-dicyanoethylene are reported. The X-ray single-crystal structure of the compound reveals a nonplanar conformation. The FT-IR and FT-Raman spectra of the compound show a great resemblance, which is a spectroscopic observation common to many push-pull systems. The UV-vis spectrum in CHCl3 displays a strong absorption at 370 nm accompanied by a shoulder at 430 nm so that the optical gap is 2.88 eV. On the other hand, the electrochemical gap amounts to 2.38 V. DFT and TDDFT quantum chemical calculations, at the B3LYP/6-31G** level, have been also performed to (i) determine the minimum-energy molecular structure, (ii) gain knowledge about the equilibrium atomic charges distribution, the topologies, and absolute energies of the frontier molecular orbitals around the gap and about the molecular vibrations which give rise to the most outstanding Raman bands experimentally evidenced, and (iii) to analyze the nature of the vertical one-electron excitations associated to the strongest UV-vis absorptions.

Characterizing the Dimerizations of Phenalenyl Radicals by ab Initio Calculations and Spectroscopy: σ-Bond Formation versus Resonance π-Stabilization

Electronic-structure calculations for the self-association of phenalenyl radical (P*) predict the formation of dimeric species (sigma-P2) in which both moieties are connected by a sigma-bond with rP-P approximately 1.59 A and bond dissociation enthalpy of DeltaH(D) approximately 16 kcal mol(-1). Such an unusually weak sigma-bond is related to the loss of aromatic stabilization energy of approximately 34 kcal mol(-1) per phenalenyl moiety, largely owing to rehybridization. Ab initio calculations also reveal that the corresponding (one-electron) bond between phenalenyl radical and its closed-shell cation in sigma-P2+* is unstable relative to dissociation. Time-dependent DFT computations indicate the absence of any (strongly allowed) electronic transition in the visible region of the absorption spectrum of phenalenyl sigma-dimer. Such theoretical predictions are supported by experimental (ESR and UV-NIR) spectroscopic studies, in which the availability of a series of sterically hindered phenalenyl radicals allows definitive separations of the sigma-dimerization process from interference by pi-dimerization. As such, the thermodynamic parameters (determined from the temperature dependence of the ESR signals) with DeltaH(D) = 14 kcal mol(-1) and DeltaS(D) = 52 e.u. can be assigned to the formation of the colorless sigma-dimer. Similar results are obtained for all phenalenyl derivatives (provided their substitution patterns allow sigma-bond formation) to confirm the energetic preference of sigma-dimerization over pi-dimerization.

Synthesis and Characterization of a Novel Terthiophene-Based Quinodimethane Bearing a 3,4-Ethylenedioxythiophene Central Unit

The synthesis and a combined spectroscopic and density functional theoretical characterization of a 3',4'-ethylenedioxy-5,5' '-bis(dicyanomethylene)-5,5' '-dihydro-2,2':5',2' '-terthiophene analogue of 7,7,8,8-tetracyanoquinodimethane (TCNQ) are presented. Electrochemical data show that this novel trimer can be both reversibly reduced and oxidized at relatively low potentials. Quantum-chemical calculations show that the compound exhibits a quinoidal structure in its ground electronic state and that a certain degree of intramolecular charge transfer takes place from the central terthienyl moiety toward both =C(CN)2 end-caps. Therefore, the amphoteric redox behavior of this novel material can be related to the coexistence of an electron-impoverished terthienyl core endowed by two electron-enriched =C(CN)2 substituents. The UV-vis spectrum is dominated by the appearance of a strong absorption near 660 nm, attributable to the highest occupied molecular orbital (HOMO) --> lowest unoccupied molecular orbital (LUMO) pi-pi electronic transition of the terthienyl spine on the basis of time-dependent density functional theory (DFT) computations. The DFT calculations performed on the minimum-energy molecular geometry about the equilibrium atomic charge distribution, topologies, and energies of the frontier orbitals around the gap and about the Raman-active vibrations associated with the strongest Raman features are also consistent with a rather effective pi-electron conjugation and the partial degree of intramolecular charge transfer mentioned above. Our study reveals this novel heteroquinoid trimer could act as a promising candidate in organic field-effect transistor (OFET) applications.