Singlet Vertical Excitation Energies Research Articles

Excited-State Properties of Some Thermally Activated Delayed Fluorescence Emitters: Quest for an Accurate and Reliable Computational Method.

Thermally activated delayed fluorescence (TADF) finds application in organic light-emitting diodes. The molecules exhibiting TADF are characterized by small singlet-triplet energy gaps that help reverse intersystem crossing. Recently, ionization potential (IP)-tuned range-separated (RS) density functionals have been well accepted for studying excited-state properties. In the present work, two efficient descriptor-based tuning schemes [electron localization function (ELF) and Sol] of RS density functionals have been used to accurately reproduce the excited-state properties of TADF emitters by performing a single self-consistent field calculation. The lowest singlet vertical excitation energies (EVA(S1)) and the vertical singlet-triplet energy gaps (ΔEVST) are computed with ELF-, Sol-, and IP-tuned RS functionals (LC-BLYP, ωB97, ωB97X, and ωB97XD). Encouraging mean absolute deviations from the experimental values with ELF*-, Sol*-, and IP-tuned functionals are observed. Consistent performance of the non-empirical tuned functionals is noted in different solvent dielectrics. In addition to these, fractional occupation calculations have shown that our tuned functionals almost satisfy the energy linearity curve. Thus, ELF*- and Sol*-tuned functionals are promising and reliable alternatives in computing the excited-state properties. Considering the small experimental singlet-triplet gap, we recommend ELF* to calculate EVA(S1) and Sol* to calculate ΔEVST.

Comparing molecular and electronic properties of perylene tetracarboxylic diimides decorated at 1,6- and 1,7-bay-positions using DFT/TDDFT method

Geometrical structures and singlet vertical excitations of perylene tetracarboxylic diimides decorated at 1,6- and 1,7-bay positions investigated using density functional theory and its time dependent formalism. Such compounds are attracting much attention in the field of organic functional materials science. While 1,7-PDI regioisomers are synthetically accessible in pure form, it is still a challenging task to obtain 1,6-regioisomers in pure form. Ground state equilibrium geometries were optimized using the B3LYP functional with triple-ζ quality TZVP basis set. Introduction of either bromine or amino substituent groups at 1,6(7)-positions distorts the planarity of perylene core. Core twisting is larger for the derivatives substituted at 1,6-positions. For computing singlet vertical excitation energies, global hybrids B3LYP, PBE0 and range-separated hybrid CAM-B3LYP density functionals tested. B3LYP computed the most accurate excitation energies against experimental absorption maximum (λmax), with a mean absolute deviation (MAD) of 0.050 eV when solvent effects included. N-alkyl substituents at the bay positions shift the absorption maxima towards the ∼700 nm region. Red shifting is more efficient with 1,7-substitution, while 1,6-amino substituted PDIs display an additional absorption peak resulting from HOMO-1 to LUMO electronic transition appearing as extension of lowest energy absorption band and covers a broader region of visible spectrum. Such electronic transition is hardly detectable for 1,7-regioisomers. Calculations reveal that amino substituted 1,6-regioisomers display lower-lying HOMOs compared to the respective 1,7-regioisomers. This observation is in line with the NBO analysis, suggesting higher contribution of side groups to HOMO levels in 1,7-regioisomers. Plotted frontier molecular orbital isodensity surfaces reveal that electron density distribution is affected markedly from the position of amino substituents on PDI core.

Electronically excited states and photochemical reaction mechanisms of β-glucose

Carbohydrates are important molecular components of living matter. While spectroscopic and computational studies have been performed on carbohydrates in the electronic ground state, the lack of a chromophore complicates the elucidation of the excited-state properties and the photochemistry of this class of compounds. Herein, we report on the first computational investigation of the singlet photochemistry of β-glucose. It is shown that low-lying singlet excited states are of nσ* nature. Our computations of the singlet vertical excitation energies predict absorption from 6.0 eV onward. Owing to a dense manifold of weakly-absorbing states, a sizable and broad absorption in the ultraviolet-C range arises. We have explored two types of photochemical reaction mechanisms: hydrogen-detachment processes for each of the five O-H groups and a C-O ring-opening process. Both types of reactions are driven by repulsive nσ* states that are readily accessible from the Franck-Condon region and lead to conical intersections in a barrierless fashion. We have optimized the geometries of the conical intersections involved in these photochemical processes and found that these intersections are located around 5.0 eV for the O-H hydrogen-detachment reactions and around 4.0 eV for the C-O ring-opening reaction. The energies of all conical intersections are well below the computed absorption edge. The calculations were performed using linear-response methods for the computation of the vertical excitation energies and multiconfigurational methods for the optimization of conical intersections and the computation of energy profiles.

Nature of Low-Lying Excited States in H-Aggregated Perylene Bisimide Dyes: Results of TD-LRC-DFT and the Mixed Exciton Model

We characterize the nature of the low-lying excited states in H-aggregated perylene bisimide (PBI) dyes in terms of the recently proposed long-range-corrected density functional theory (LRC-DFT) and the mixed intramolecular Frenkel exciton (FE) and the intermolecular charge-transfer exciton (CTE) model. The singlet vertical excitation energies of H-aggregated PBI dimers and multimers are theoretically evaluated by considering the aggregation details. It is found that the dimer absorption follows closely the spectra of PBI aggregates in solution and demonstrates strong mixing of two kinds of excitons. The experimentally observed low-energy absorption peak near 2.3 eV should not be a pure CT state but a mixed excitonic state. To model the electronic excitations of H-aggregated PBI multichromophoric oligomers, we adopt a mixed FE-CTE model Hamiltonian, whose microscopic parameters are determined from LRC-DFT/TD-LRC-DFT calculations performed on an individual chromophore or a chromophore pair. By involving four to five PBI chromophores, we have succeeded in reproducing the experimental trends of optical properties in PBI stacking, such as the large blue shift of the absorption band, the mixed FE-CTE nature of the low-energy absorption bands, and the large Stokes shift of emission spectra. The long-range correction on DFT exchange-correlation energy functionals is crucial to correctly evaluate the photophysical characteristics of pi-pi stacks.

Can phthalocyanines and their substituted α-para-(methoxy)phenyl derivatives act as photosensitizers in photodynamic therapy? A TD-DFT study

A time-dependent density functional theory study (TD-DFT) is presented regarding the substituent effects on the Q-bands of two classes of non-planar phthalocyanines: the alpha-octaphenyl and p-alpha-octamethoxyphenyl substituted compounds, in their free-base and zinc complex forms. Singlet vertical excitation energies, computed at the PBE0/SVP//BP86/SVP level of theory also including bulk solvent effects (COSMO model), resulted within 0.1 eV of experiment. The experimental red-shift for the Q-band, going from the phenylated to the methoxyphenylated case, was well-reproduced theoretically and in the latter case it was found to depend mainly on the nature of the substituents and partly on structure distortion effects. The energetic gap between the singlet ground and first triplet excited state was calculated in solvent to be 1.28 eV for the free-base phthalocyanine (H2Pc) and 1.45 eV for the unsubstituted zinc complex (ZnPc) and lower than 0.98 eV for all the other compounds, which is the energetic lower limit for a molecule to act as photosensitiser in photodynamic therapy according to a Type II reaction mechanism. As a consequence, since this property-requirement for drugs used in photodynamic therapy is not fulfilled by the investigated near-infrared photosensitizers, they cannot be proposed as candidates for their use in this medical treatment.

Theoretical Characterization of a Tridentate Photochromic Pt(II) Complex Using Density Functional Theory Methods.

Density functional theory methods have been used to characterize a tridentate photochromic Pt(II) complex [Pt(AAA)Cl], its acetonitrile complex [Pt(AAA)Cl·CH3CN], and the transition state in the complexation reaction. B3LYP/6-31G* (effective core potential for Pt) optimized geometries of Pt(AAA)Cl and Pt(AAA)Cl·CH3CN are found to be in reasonably good agreement with most of the applicable parameters for the available experimental crystal structures of Pt(AAA)Cl and a Pt(AAA)Cl-triphenylphoshine complex, with the exception of one of the dihedral angles, the deviation of which is determined to be due to a steric cis versus trans effect. Vibrational frequencies are calculated for Pt(AAA)Cl and cis-Pt(AAA)Cl·CH3CN, and the predicted shift in the benzaldehyde carbonyl frequency is found to be in line with that observed experimentally. Singlet vertical excitation energies are calculated for Pt(AAA)Cl and cis-Pt(AAA)Cl·CH3CN using time-dependent density-functional theory and are found to be in good agreement with the experimental transition energies, although for cis-Pt(AAA)Cl·CH3CN, the calculations suggest a reassignment of the experimental S1 and S2 transitions. Single point energies are calculated at the B3LYP/6-311+G(2d,2p) level (effective core potential for Pt) and the calculations predict the complexation reaction (dark reaction) to be exothermic and, after a correction to the entropy, to be exoergic at 298 K and to proceed with a reasonable activation energy. Based on singlet and triplet vertical excitation energies, it is speculated that the photoreaction occurs via an intersystem crossing from S1 to T1 for cis-Pt(AAA)Cl·CH3CN followed by an adiabatic reaction along the T1 surface and then nonradiative intersystem crossing to the S0 state of Pt(AAA)Cl.

Photophysical Processes Involved within the Anisole−Thioindoxyl Dyad System

The photophysical properties and the nature of the photoinduced electron transfer (PET) reactions within a synthesized anisole (A)-thioindoxyl (T) dyad system (24MBTO) have been studied by electrochemical, steady-state, and time-resolved spectroscopic techniques. Computations on the dyad were performed both in gas phase as well as solvent environment by TD-DFT method with B3LYP density function. The geometry optimization calculation of 24MBTO was done by 6-311G(d,p) basis function set implemented in the Gaussian package. The theoretical values of singlet vertical excitation energies were found to correlate well with the experimentally observed ones. The electrochemical measurements indicate the possibility of occurrence of PET reactions within 24MBTO between the linked redox centers A and T. Both steady-state and time-resolved spectroscopic measurements on the novel synthesized 24MBTO dyad demonstrate the formations of the two types of isomeric species: Z- and E- forms, resulted from the charge separation reactions. From the detailed studies it reveals that the present thioaurone may behave as a versatile photoswitchable system. It has been hinted that the loss process (charge recombination) within 24MBTO could possibly be prevented by incorporating it within the hydrophobic cavity of beta-cyclodextrin (betaCD).

Kohn–Sham calculations using hybrid exchange-correlation functionals with asymptotically corrected potentials

The theory is presented for asymptotically correcting the potentials of hybrid exchange-correlation functionals, i.e., those that include a fraction of orbital exchange. The Kohn–Sham equations involve a multiplicative potential due to the continuum part of the hybrid functional and a nonmultiplicative term due to the orbital exchange. In asymptotic regions the multiplicative σ-spin potential is corrected to take the form (CX−1)/r+εHOMO,σ+Iσ, where CX is the fraction of orbital exchange; εHOMO,σ is the σ-spin self-consistent highest occupied Kohn–Sham eigenvalue; and Iσ is an approximate ionization energy. For the hydrogen atom, the asymptotic correction leads to a potential that closely resembles the exact potential; the eigenvalue spectrum is intermediate between the Schrödinger and Hartree–Fock eigenvalues, reflecting the presence of orbital exchange. Kohn–Sham orbitals and eigenvalues determined from this procedure have been used to calculate singlet vertical excitation energies for CO, N2, H2CO, C2H4, and C6H6. The correction significantly improves excitation energies to Rydberg states, with mean absolute errors below 0.2 eV. However, despite including orbital exchange, the results do not represent an improvement over the results obtained by asymptotically correcting a recently developed GGA functional. The asymptotic correction is also shown to reduce static isotropic polarizabilities.