Dependence of Spurious Charge-Transfer Excited States on Orbital Exchange in TDDFT: Large Molecules and Clusters

Charge-transfer excited states in the donor/acceptor interface from large-scale GW calculations

Predicting the charge-transfer (CT) excited states across the donor/acceptor (D/A) interface is essential for understanding the charge photogeneration process in an organic solar cell. Here, we present a fragment-based GW implementation that can be applied to a D/A interface structure and thus enables accurate determination of the CT states. The implementation is based on the fragmentation approximation of the polarization function and the combined GW and Coulomb-hole plus screened exchange approximations for self-energies. The fragment-based GW is demonstrated by application to the pentacene/C₆₀ interface structure containing more than 2000 atoms. The CT excitation energies were estimated from the quasiparticle energies and electron-hole screened Coulomb interactions; the computed energies are in reasonable agreement with experimental estimates from the external quantum efficiency measurements. We highlight the impact of the induced polarization effects on the electron-hole energetics. The proposed fragment-based GW method offers a first-principles tool to compute the quasiparticle energies and electronic excitation energies of organic materials.

Photophysicochemical properties and photodynamic therapy activity of chloroindium(III) tetraarylporphyrins and their gold nanoparticle conjugates

Novel chloroindium(III) complexes of tetra(4-methylthiophenyl)porphyrin (2a) and tetra-2-thienylporphyrin (2b) dyes have been synthesized and characterized. The main goal of the project was to identify fully symmetric porphyrin dyes with Q-band regions that lie partially in the therapeutic window that are suitable for use in photodynamic therapy (PDT). 2a and 2b were found to have fluorescence quantum yield values [Formula: see text] 0.01 and moderately high singlet oxygen quantum yields (0.54−0.73) due to heavy atom effects associated with the sulfur and indium atoms. The dark toxicity and PDT activity against epithelial breast cancer cells (MCF-7) were investigated over a dose range of 3.0−40 [Formula: see text]g [Formula: see text] mL[Formula: see text]. The in vitro dark cytotoxicity of 2a is significantly lower than that of 2b at [Formula: see text] 40 [Formula: see text]g [Formula: see text] mL[Formula: see text]. 2a was conjugated with gold nanoparticles (AuNPs) to form a nanoconjugate (2a-AuNPs), which exhibited a higher singlet oxygen quantum yield ([Formula: see text] value and PDT activity than was observed for 2a alone. The results suggest that the AuNPs nanoconjugates of readily synthesized fully symmetric porphyrin dyes are potentially suitable for PDT applications, if meso-aryl substituents that provide scope for nanoparticle conjugation can be introduced that shift the Q bands into the therapeutic window.

Excitation-Dependent Multiple Fluorescence of a Substituted 2-(2'-Hydroxyphenyl)benzoxazole

Excitation-dependent multiple fluorescence of a 2-(2'-hydroxyphenyl)benzoxazole (HBO) derivative (1) is described. Compound 1 contains the structure of a charge-transfer (CT) 4-hydroxyphenylvinylenebipy fluorophore and an excited-state intramolecular proton transfer capable (ESIPT-capable) HBO component that intersect at the hydroxyphenyl moiety. Therefore, both CT and ESIPT pathways, while spatially mostly separated, are available to the excited state of 1. The ESIPT process offers two emissive isomeric structures (enol and keto) of 1 in the excited state, while the susceptibility of 1 to a base adds another option to tune the composite emission color. In addition to the ground-state acid-base equilibrium that can be harnessed for the control of emission color by excitation energy, compound 1 exhibits excitation-dependent emission that is attributed to solvent-affected ground-state structural changes. Therefore, depending on the medium and excitation wavelength, the emission from the enol, keto, and anion forms could occur simultaneously, which are in the color ranges of blue, green, and orange/red, respectively. A composite color of white with CIE coordinates of (0.33, 0.33) can be materialized through judicious choices of medium and excitation wavelength.

Photophysical and enhanced nonlinear optical response in asymmetric benzothiazole substituted phthalocyanine covalently linked to semiconductor quantum dots

The synthesis of asymmetric benzothiazole substituted phthalocyanines (complexes 3 to 5) and their covalent attachment to glutathione (GSH) functionalized quantum dots (QDs) are reported in this work. Additionally, their photophysical and nonlinear optical properties were investigated. A decrease in the fluorescence quantum yield with corresponding increase in the triplet quantum yield was observed when the complexes were covalently linked to glutathione (GSH) functionalized cadmium telluride (CdTe) quantum dots. Reverse saturable absorption was found to be predominantly dominated by excited state absorption. The observed limiting threshold values range from 0.29-0.75 J/cm².

Exciton localization in excited-state dynamics of a tetracene trimer: a surface hopping LC-TDDFTB study

Excitons in the molecular aggregates of chromophores are key participants in important processes such as photosynthesis or the functioning of organic photovoltaic devices. Therefore, the exploration of exciton dynamics is crucial. Here we report on exciton localization during excited-state dynamics of the recently synthesized tetracene trimer [Liu et al., Org. Lett., 2017, 19, 580]. We employ the surface hopping approach to nonadiabatic molecular dynamics in conjunction with the long-range corrected time-dependent density functional tight binding (LC-TDDFTB) method [Humeniuk and Mitrić, Comput. Phys. Commun., 2017, 221, 174]. Utilizing a set of descriptors based on the transition density matrix, we perform comprehensive analysis of exciton dynamics. The obtained results reveal an ultrafast exciton localization to a single tetracene unit of the trimer during excited-state dynamics, along with exciton transfer between units.

Theoretical studies on the structural and spectral properties of two specific C54 isomers and the chlorinated species C54Cl8

X-ray photoelectron (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectra as well as the ground-state electronic/geometrical structures of ^#540C₅₄ captured in experiment and the most controversial isomer ^#369C₅₄ (C_2v- and C_s-symmetry, respectively) have been calculated at the density functional theory (DFT) level. After chlorination, significant changes were observed in the electronic structure and X-ray spectra. Both XPS and NEXAFS spectra showed strong isomer dependence. The results indicated that the “fingerprints” in the X-ray spectra afforded an effective way to identify the fullerene isomers mentioned above. Ultraviolet-visible (UV-Vis) absorption spectroscopy of C₅₄Cl₈ was also simulated at the time-dependent (TD) DFT level, and the simulated UV-Vis spectrum was in accordance with the experimental result. The results of this study can provide valuable information for further experimental and theoretical studies of new fullerenes and their derivatives through X-ray and ultraviolet spectroscopy. The study of newly synthesized fullerene isomers and their derivatives using X-ray and UV-Vis spectra offers valuable information for further experimental and theoretical exploration.

Chloride derivatives of fullerenes containing TSFP, TDFP and DFP carbon sites were characterized by spectral simulation.

Effect of auxiliary group for p-type organic dyes in NiO-based dye-sensitized solar cells: The first principal study

Auxiliary acceptor groups play a crucial role in D-A-π-A structured organic dyes. In this paper, we designed three D-A-π-A structured organic molecules based on the prototype dye QT-1, named ME18-ME20, and further investigated their electronic and optical properties with density functional theory (DFT) and time-dependent DFT (TDDFT). The calculated results indicate that the scope and intensity of dyes' absorption spectra have some outstanding changes by inserting auxiliary groups. ME20 has not only 152nm redshifts to long wave orientation, but also 78% increased oscillator strength compared to QT-1, and its absorption spectrum broadens region even up to 1400nm. Then, we studied the reason that the effect of the introduced different auxiliary acceptor groups in these dyes through their ground states geometries and energy levels, electron transfer and recombination rate.

Can TDDFT Describe Excited Electronic States of Naphthol Photoacids? A Closer Look with EOM-CCSD

The ¹L_b and ¹L_a excited states of naphthols are characterized by using time-dependent density functional theory (TDDFT), configuration interaction with singles (CIS), and equation-of-motion coupled cluster singles and doubles (EOM-CCSD) methods. TDDFT fails dramatically at predicting the energy and ordering of the ¹L_a and ¹L_b excited states as observed experimentally, while EOM-CCSD accurately predicts the excited states as characterized by natural transition orbital analysis. The limitations of TDDFT are attributed to the absence of correlation from doubly excited configurations as well as the inconsistent description of excited electronic states of naphthol photoacids revealed by excitation analysis based on the one-electron transition density matrix.

The photophysicochemical behavior of symmetric and asymmetric zinc phthalocyanines, surface assembled onto gold nanotriangles

The synthesis of a novel asymmetric ZnPc, (4-(4-(benzo[d]thiazol-2-yl)phenoxy)-2,10,17-tris(4-(2-carboxyethyl)phenoxy)phthalocyaninatol)zinc(ii), complex3, is reported.

The Bethe–Salpeter equation in chemistry: relations with TD-DFT, applications and challenges

We review the Bethe–Salpeter formalism and analyze its performances for the calculation of the excited state properties of molecular systems.

Photo-induced resonance energy transfer and nonlinear optical response in ball-type phthalocyanine conjugated to semiconductor and graphene quantum dots

Ball-type zinc and gallium phthalocyanines were covalently linked to graphene and semiconductor quantum dots resulting in enhanced triplet parameters and nonlinear optical performance.

Why does 4-biphenyl carbonyl azide have ultra-short lived excited states? An ultrafast UV-vis spectroscopic and computational study

A new model is presented for ultrashort-lived excited states of carbonyl azides, their solvent dependence and other experimental observations.

Mechanistic Analysis of Fluorescence Quenching of Reduced Nicotinamide Adenine Dinucleotide by Oxamate in Lactate Dehydrogenase Ternary Complexes

Fluorescence of Reduced Nicotinamide Adenine Dinucleotide (NADH) is extensively employed in studies of oxidoreductases. A substantial amount of static and kinetic work has focused on the binding of pyruvate or substrate mimic oxamate to the binary complex of lactate dehydrogenase (LDH)-NADH where substantial fluorescence quenching is typically observed. However, the quenching mechanism is not well understood limiting structural interpretation. Based on time-dependent density functional theory (TDDFT) computations with cam-B3LYP functional in conjunction with the analysis of previous experimental results, we propose that bound oxamate acts as an electron acceptor in the quenching of fluorescence of NADH in the ternary complex, where a charge transfer (CT) state characterized by excitation from the highest occupied molecular orbital (HOMO) of the nicotinamide moiety of NADH to the lowest unoccupied molecular orbital (LUMO) of oxamate exists close to the locally excited (LE) state involving only the nicotinamide moiety. Efficient quenching in the encounter complex like in pig heart LDH requires that oxamate forms a salt bridge with Arg-171 and hydrogen bonds with His-195, Thr-246 and Asn-140. Further structural rearrangement and loop closure, which also brings about another hydrogen bond between oxamate and Arg-109, will increase the rate of fluorescence quenching as well.

UV-Vis Action Spectroscopy Reveals a Conformational Collapse in Hydrogen-Rich Dinucleotide Cation Radicals

We report the generation of deoxyriboadenosine dinucleotide cation radicals by gas-phase electron transfer to dinucleotide dications and their noncovalent complexes with crown ether ligands. Stable dinucleotide cation radicals of a novel hydrogen-rich type were generated and characterized by tandem mass spectrometry and UV-vis photodissociation (UVPD) action spectroscopy. Electron structure theory analysis indicated that upon electron attachment the dinucleotide dications underwent a conformational collapse followed by intramolecular proton migrations between the nucleobases to give species whose calculated UV-vis absorption spectra matched the UVPD action spectra. Hydrogen-rich cation radicals generated from chimeric riboadenosine 5'-diesters gave UVPD action spectra that pointed to novel zwitterionic structures consisting of aromatic π-electron anion radicals intercalated between stacked positively charged adenine rings. Analogies with DNA ionization are discussed.

Attosecond Charge Migration with TDDFT: Accurate Dynamics from a Well-Defined Initial State

We investigate the ability of time-dependent density functional theory (TDDFT) to capture attosecond valence electron dynamics resulting from sudden X-ray ionization of a core electron. In this special case the initial state can be constructed unambiguously, allowing for a simple test of the accuracy of the dynamics. The response following nitrogen K-edge ionization in nitrosobenzene shows excellent agreement with fourth-order algebraic diagrammatic construction (ADC(4)) results, suggesting that a properly chosen initial state allows TDDFT to adequately capture attosecond charge migration. Visualizing hole motion using an electron localization picture (ELF), we provide an intuitive chemical interpretation of the charge migration as a superposition of Lewis dot resonance structures.

Evaluating the electronic structure of formal LnII ions in LnII(C5H4SiMe3)31- using XANES spectroscopy and DFT calculations

The isolation of [K(2.2.2-cryptand)][Ln(C5H4SiMe3)3], formally containing LnII, for all lanthanides (excluding Pm) was surprising given that +2 oxidation states are typically regarded as inaccessible for most 4f-elements. Herein, X-ray absorption near-edge spectroscopy (XANES), ground-state density functional theory (DFT), and transition dipole moment calculations are used to investigate the possibility that Ln(C5H4SiMe3)31- (Ln = Pr, Nd, Sm, Gd, Tb, Dy, Y, Ho, Er, Tm, Yb and Lu) compounds represented molecular LnII complexes. Results from the ground-state DFT calculations were supported by additional calculations that utilized complete-active-space multi-configuration approach with second-order perturbation theoretical correction (CASPT2). Through comparisons with standards, Ln(C5H4SiMe3)31- (Ln = Sm, Tm, Yb, Lu, Y) are determined to contain 4f6 5d0 (SmII), 4f13 5d0 (TmII), 4f14 5d0 (YbII), 4f14 5d1 (LuII), and 4d1 (YII) electronic configurations. Additionally, our results suggest that Ln(C5H4SiMe3)31- (Ln = Pr, Nd, Gd, Tb, Dy, Ho, and Er) also contain LnII ions, but with 4f n 5d1 configurations (not 4f n+1 5d0). In these 4f n 5d1 complexes, the C3h-symmetric ligand environment provides a highly shielded 5d-orbital of a' symmetry that made the 4f n 5d1 electronic configurations lower in energy than the more typical 4f n+1 5d0 configuration.

Characterization of the Photochemical Properties of 5-Benzyluracil via Time-Dependent Density Functional Theory

We present a detailed study of the excited state properties of 5-benzyluracil (5BU) in the gas phase and in implicit solvent using different electronic structure approaches ranging from time-dependent density functional theory in the linear response regime (LR-TDDFT) to a set of different wave-function-based methods for excited states, namely perturbed coupled cluster (CC2), algebraic diagrammatic construction method to second order (ADC(2)), and perturbed configuration interaction (CIS(D)). 5BU has been used to investigate DNA base-amino acid interactions. In particular, it served as a model of protein-DNA photoinduced cross-linking. While LR-TDDFT is computationally the most efficient first-principles approach for static and dynamic simulations of this bichromophoric system, its accuracy is difficult to assess due to the presence of excited states with charge transfer character. In this work, the performance of different exchange correlation functionals is compared against accurate benchmarks obtained either from high level wave-function-based methods or directly from experimental absorption spectra. Our investigation shows that accurate results for the excitation energies can be obtained using the hybrid meta-GGA functional M06. In view of dynamical studies of the relaxation of 5BU after photoexcitation, we also show that the PBE functional, while failing in the Franck-Condon region, provides qualitatively good results for the characterisation of a possible photocyclization path.

Ultrafast electron and hole transfer dynamics of a solar cell dye containing hole acceptors on mesoporous TiO2 and Al2O3

The stability of dye cations against recombination with conduction band electrons in mesoporous TiO₂ electrodes is a key property for improving light harvesting in dye-sensitised solar cells. Using ultrafast transient broadband absorption spectroscopy, we monitor efficient intramolecular hole transfer in the solar cell dye E6 having two peripheral triarylamine acceptors. After photoexcitation, two hole transfer mechanisms are identified: a concerted mechanism for electron injection and hole transfer (2.4 ps) and a sequential mechanism with time constants of 3.9 ps and 30 ps. This way the dye retards unwanted recombination with a TiO₂ conduction band electron by quickly moving the hole further away from the surface. Contact of the E6/TiO₂ surface with the solvent acetonitrile has almost no influence on the electron injection and hole transfer kinetics. Fast hole transfer (2.8 ps) is also observed on a "non-injecting" Al₂O₃ surface generating a radical cation-radical anion species with a lifetime of 530 ps. The findings confirm the good intramolecular hole transfer properties of this dye on both thin films. In contrast, intramolecular hole transfer does not occur in the mid-polar organic solvent methyl acetate. This is confirmed by TDDFT calculations suggesting a polarity-induced reduction of the driving force for hole transfer. In methyl acetate, only the relaxation of the initially photoexcited core chromophore is observed including solvent relaxation processes of the electronically excited state S₁/ICT.

Development of a Novel Index for Analysis of Electronically Excited States

Concerning the major factors in the context of excited states analyses, namely charge centroids of the orbitals involved in the excitations, the distance between orbital centroids, and overlap integrals, a new metric-the Ω index-is proposed to assign the character and optical properties of electronically excited states. Using several molecules from different classes and also a well-studied standard database for time-dependent density functional theory (TD-DFT) studies as benchmark criteria, accountability of the developed index is numerically assessed for local, charge transfer, and Rydberg excitations. It is shown that the nature of excited states can be discriminated using the Ω index, where its superior performance for those situations in which the previous descriptors were not helpful is also unveiled. Relationships are also examined between the Ω index and optical properties of some molecules under study in the framework of the sum-over-state approach. It is observed that there are correlations between the proposed index and computed hyperpolarizabilities based on the sum-over-state scheme. These findings offer the possibility of estimating excited-state properties of large systems from simple descriptors without explicitly performing calculations of high-order response functions.

4-Bis(4-aminophenoxy)phenoxy derivitized phthalocyanine conjugated to metallic nanoparticles: searching for enhanced optical limiting materials

Amino derivative phthalocyanine linked to metallic nanoparticles showed enhanced photophysical and nonlinear optical response.

DFT and TD-DFT calculation of new thienopyrazine-based small molecules for organic solar cells

BACKGROUND

Novel six organic donor-π-acceptor molecules (D-π-A) used for Bulk Heterojunction organic solar cells (BHJ), based on thienopyrazine were studied by density functional theory (DFT) and time-dependent DFT (TD-DFT) approaches, to shed light on how the π-conjugation order influence the performance of the solar cells. The electron acceptor group was 2-cyanoacrylic for all compounds, whereas the electron donor unit was varied and the influence was investigated.

METHODS

The TD-DFT method, combined with a hybrid exchange-correlation functional using the Coulomb-attenuating method (CAM-B3LYP) in conjunction with a polarizable continuum model of salvation (PCM) together with a 6-31G(d,p) basis set, was used to predict the excitation energies, the absorption and the emission spectra of all molecules.

RESULTS

The trend of the calculated HOMO-LUMO gaps nicely compares with the spectral data. In addition, the estimated values of the open-circuit photovoltage (V_oc) for these compounds were presented in two cases/PC₆₀BM and/PC₇₁BM.

CONCLUSION

The study of structural, electronics and optical properties for these compounds could help to design more efficient functional photovoltaic organic materials.

Efficiency improvement of new Tetrathienoacene-based dyes by enhancing donor, acceptor and bridge units, a theoretical study

A series of metal free Tetrathienoacene-based (TTA-based) organic dyes are designed and investigated as sensitizers for application in dye sensitized solar cells (DSSCs). Density function theory and time dependent density function theory calculations were performed on these dyes at vacuum and orthodichlorobenzene as the solvent. Effects of changing π-conjugation bridges and different functional groups in acceptor and donor units were investigated. UV-Vis absorption spectra were simulated to show the wavelength shifting and absorption properties. Inserting nitro and acyl chloride functional groups in acceptor and NH2 in donor units leads to the reduction of HOMO-LUMO gap by lowering the lowest unoccupied molecular orbital (LUMO) energy level and raising the highest occupied molecular orbital (HOMO) energy level and the increase in effective parameters in DSSC' efficiency. The results show that changing spacer units from thiophene to furan has a great effect on electronic structure and absorption spectra. Investigation of the electron distributions of frontier orbitals shows the HOMO and LUMO localization in donor and acceptor, respectively. Some key parameters that were studied here include light harvesting efficiency, free energy of electron injection and open circuit photo-voltage.

Truncated Transition Densities for Analysis of (Nonlinear) Optical Properties of carbo-Chromophores

The optical properties of several quadrupolar carbo-benzene derivatives are investigated at various levels of calculation (TDDFT and CASPT2) and analyzed using a new theoretical tool here disclosed: The "visualization" of the transition dipole moment from the transition density truncated to the main monoexcitations involved in the electronic transition (TTD). The experimental or calculated one-photon UV-visible absorption spectra of the carbo-benzene derivatives fit with the Gouterman model originally proposed for porphyrins, where the first four excited states involve linear combinations of monoexcitations of the same four frontier molecular orbitals. The relative intensities of the absorption bands are analyzed from the transition dipole moments calculated from the TTDs and an analogy between porphyrins and carbo-benzenes is argued. The two-photon absorption (TPA) cross section related to the third-order nonlinear optical response is calculated for each two-photon-allowed excited state |f⟩ from the contribution of all possible intermediate excited states |i⟩ using the "sum-over-state" (SOS) scheme. The quadrupolar carbo-benzene derivatives fit into the three-level model, as their TPA cross section exhibits a dominant contribution of one of the intermediate excited states. The origin of TPA efficiency (enhancement) upon carbo-merisation of the C-C link to the para-substituents is discussed from the excitation energies of the intermediate and final excited states and from the two corresponding transition dipole moments (μ0i and μif). The latter may be calculated from the TTDs.

Interference between Coulombic and CT-mediated couplings in molecular aggregates: H- to J-aggregate transformation in perylene-based π-stacks

The spectroscopic differences between J and H-aggregates are traditionally attributed to the spatial dependence of the Coulombic coupling, as originally proposed by Kasha. However, in tightly packed molecular aggregates wave functions on neighboring molecules overlap, leading to an additional charge transfer (CT) mediated exciton coupling with a vastly different spatial dependence. The latter is governed by the nodal patterns of the molecular LUMOs and HOMOs from which the electron (te) and hole (th) transfer integrals derive. The sign of the CT-mediated coupling depends on the sign of the product teth and is therefore highly sensitive to small (sub-Angstrom) transverse displacements or slips. Given that Coulombic and CT-mediated couplings exist simultaneously in tightly packed molecular systems, the interference between the two must be considered when defining J and H-aggregates. Generally, such π-stacked aggregates do not abide by the traditional classification scheme of Kasha: for example, even when the Coulomb coupling is strong the presence of a similarly strong but destructively interfering CT-mediated coupling results in "null-aggregates" which spectroscopically resemble uncoupled molecules. Based on a Frenkel/CT Holstein Hamiltonian that takes into account both sources of electronic coupling as well as intramolecular vibrations, vibronic spectral signatures are developed for integrated Frenkel/CT systems in both the perturbative and resonance regimes. In the perturbative regime, the sign of the lowest exciton band curvature, which rigorously defines J and H-aggregation, is directly tracked by the ratio of the first two vibronic peak intensities. Even in the resonance regime, the vibronic ratio remains a useful tool to evaluate the J or H nature of the system. The theory developed is applied to the reversible H to J-aggregate transformations recently observed in several perylene bisimide systems.

Communication: Exciton analysis in time-dependent density functional theory: How functionals shape excited-state characters

Excited-state descriptors based on the one-particle transition density matrix referring to the exciton picture have been implemented for time-dependent density functional theory. State characters such as local, extended ππ(∗), Rydberg, or charge transfer can be intuitively classified by simple comparison of these descriptors. Strong effects of the choice of the exchange-correlation kernel on the physical nature of excited states can be found and decomposed in detail leading to a new perspective on functional performance and the design of new functionals.

Charge localization in a diamine cation provides a test of energy functionals and self-interaction correction

Density functional theory (DFT) is widely applied in calculations of molecules and materials. Yet, it suffers from a well-known over-emphasis on charge delocalization arising from self-interaction error that destabilizes localized states. Here, using the symmetric diamine N,N'-dimethylpiperazine as a model, we have experimentally determined the relative energy of a state with positive charge localized on one of the two nitrogen atoms, and a state with positive charge delocalized over both nitrogen atoms. The charge-localized state was found to be 0.33 (0.04) eV higher in energy than the charge-delocalized state. This provides an important test of theoretical approaches to electronic structure calculations. Calculations with all DFT functionals commonly used today, including hybrid functionals with exact exchange, fail to predict a stable charge-localized state. However, the application of an explicit self-interaction correction to a semi-local functional identifies both states and gives relative energy in excellent agreement with both experiment and CCSD(T) calculations.

"Rate-limited effect" of reverse intersystem crossing process: the key for tuning thermally activated delayed fluorescence lifetime and efficiency roll-off of organic light emitting diodes

The rate constant of reverse intersystem crossing was found to be the “rate-limited step” in thermally activated delayed fluorescence lifetime governing.

Issues concerning excited state lifetime (τ_TADF) tuning of thermally activated delayed fluorescence (TADF) materials are critical for organic light emitting diode (OLED) applications and other specific fields. For TADF-OLEDs, employing emitters with a short τ_TADF gives rise to suppressed singlet–triplet annihilation (STA) and triplet–triplet annihilation (TTA), leading to reduced efficiency roll-off at practical relevant brightness (100 and 1000 cd m^–2 for display and illumination applications, respectively). Through molecular design, exciton dynamic process rate constants including fluorescence (k_F), intersystem crossing (k_ISC), internal conversion (k_IC) and reverse intersystem crossing (k_RISC) are selectively altered, affording four representative TADF emitters. Based on lifetime and quantum yield measurements, k_F, k_ISC, k_IC and k_RISC are calculated for four emitters and their interrelationship matches corrected time-dependent density functional theory simulation. Among them, even with a small k_F, low photoluminescence quantum efficiency (Φ) and large k_ISC, molecules with a small singlet–triplet splitting energy (ΔE_ST) and lowest charge transfer triplet excited state (³CT) eventuate in shortening the τ_TADF. Herein, k_RISC, which is inversely proportional to ΔE_ST, turns out to be the rate-limited factor in tuning the τ_TADF (“rate limited effect” of the RISC process). As revealed by flexible potential surface scanning, PyCN–ACR exhibited a moderate k_F, reduced k_IC and enlarged k_RISC, resulting in a short τ_TADF and a moderate Φ with orange-red emission. OLEDs containing PyCN–ACR as the emitting guest achieved orange-red TADF-OLEDs with an emission peak at 590 nm and the best external quantum efficiencies (EQEs) of 12.4%/9.9%/5.1% at practical luminances of 100/1000/10 000 cd m^–2.

Exciton Splitting of Adsorbed and Free 4-Nitroazobenzene Dimers: A Quantum Chemical Study

Molecular photoswitches such as azobenzenes, which undergo photochemical trans ↔ cis isomerizations, are often mounted for possible applications on a surface and/or surrounded by other switches, for example, in self-assembled monolayers. This may suppress the isomerization cross section due to possible steric reasons, or, as recently speculated, by exciton coupling to neighboring switches, leading to ultrafast electronic quenching (Gahl et al., J. Am. Chem. Soc. 2010, 132, 1831). The presence of exciton coupling has been anticipated from a blue shift of the optical absorption band, compared to molecules in solution. From the theory side the need arises to properly analyze and quantify the change of absorption spectra of interacting and adsorbed switches. In particular, suitable methods should be identified, and effects of intermolecule and molecule-surface interactions on spectra should be disentangled. In this paper by means of time-dependent Hartree-Fock (TD-HF), various flavors of time-dependent density functional theory (TD-DFT), and the correlated wave function based coupled-cluster (CC2) method we investigated the 4-nitroazobenzene molecule as an example: The low-lying singlet excited states in the isolated trans monomer and dimer as well as their composites with a silicon pentamantane nanocluster, which serves also as a crude model for a silicon surface, were determined. As most important results we found that (i) HF, CC2, range-separated density functionals, or global hybrids with large amount of exact exchange are able to describe exciton (Davydov) splitting properly, while hybrids with small amount of exact exchange fail producing spurious charge transfer. (ii) The exciton splitting in a free dimer would lead to a blue shift of the absorption signal; however, this effect is almost nullified or even overcompensated by the shift arising from van der Waals interactions between the two molecules. (iii) Adsorption on the Si "surface" leads to a further, strong red shift for the present system. (iv) At a next-nearest neighbor distance (of ∼3.6 Å), the exciton splitting is ∼0.3 eV, with or without "surface", suggesting a rapid quenching of the molecular π → π* excitation. At larger distances, exciton splitting decreases rapidly.

Photoexcitation and relaxation kinetics of molecular systems in solution: towards a complete in silico model

A theoretical–computational method is proposed for modelling the complete kinetics – from photo-excitation to relaxation – of a chromophore in solution.

Effects of the acceptor unit in dyes with acceptor–bridge–donor architecture on the electron photo-injection mechanism and aggregation in DSSCs

The photo-injection mechanism is influenced by the acceptor motif of dyes in dye-sensitized solar cells (DSSCs).

MCD spectroscopy and TD-DFT calculations of magnesium tetra-(15-crown-5-oxanthreno)-phthalocyanine

An analysis of the MCD spectroscopy and TD-DFT calculations of magnesium tetra-(15-crown-5-oxanthreno)-phthalocyanine is reported. This study provides a reassessment of an earlier study on the nature of the bands in UV-visible absorption spectra of magnesium and zinc tetra-(15-crown-5-oxanthreno)-phthalocyanine that was based on an analysis of TD-DFT calculations for a series of model complexes with the B3LYP functional. A detailed analysis of MCD spectral data and TD-DFT calculations with the CAM-B3LYP functional for the complete Mg(II) complex provides an additional insight into the optical properties and electronic structures of tetra-(15-crown-5-oxanthreno)-phthalocyanines. Thus, the bands in the Q-band region are reassigned as being due exclusively to the Q transition of Gouterman’s 4-orbital model, since intense pseudo-[Formula: see text] terms are observed in the MCD spectrum in a spectral region that had previously been assigned as charge transfer bands.