Lowest Lying Singlet Excited States Research Articles

Thermally activated delayed fluorescence tetradentate ligand-containing gold(III) complexes with preferential molecular orientation and their application in organic light-emitting devices.

A new class of thermally activated delayed fluorescence (TADF) pyridine-/pyrazine-containing tetradentate C^C^N^N gold(III) complexes have been designed and synthesized. Displaying photoluminescence quantum yields (PLQYs) of up to 0.77 in solid-state thin films, these complexes showed at-least a six-fold increase in the radiative decay rate constant (kr) in toluene upon increasing temperature from 210 to 360 K. Using variable-temperature (VT) ultrafast transient absorption (TA) spectroscopy, the reverse intersystem crossing (RISC) processes were directly observed and the activation parameters were determined, in line with the results of the Boltzmann two-level model fittings, in which the energy separation values between the lowest-lying singlet excited state (S1) and the lowest-lying triplet excited state (T1), ΔE(S1-T1), of these complexes were estimated to be in the range of 0.16-0.18 eV. Through strategic modification of the position of the electron-donating -tBu substituent in the cyclometalating ligand, the permanent dipole moments (PDMs) of these tetradentate gold(III) emitters could be manipulated to enhance their horizontal alignment in the emitting layer of organic light-emitting devices (OLEDs). Consequently, the resulting vacuum-deposited OLEDs demonstrated a 30% increase in the theoretical out-coupling efficiency (ηout), as well as promising electroluminescence (EL) performance with maximum external quantum efficiencies (EQEs) of up to 15.7%.

Nature of ultrafast dynamics in the lowest-lying singlet excited state of [Ru(bpy)3]2.

This work presents mechanisms to rationalize the nature of ultrafast photochemical and photophysical processes on the first singlet metal-ligand charge transfer state (1MLCT1) of the [Ru(bpy)3]2+ complex. The 1MLCT1 state is the lowest-lying singlet excited state and the most important intermediate in the early evolution of photoexcited [Ru(bpy)3]2+*. The results obtained from simple but interpretable theoretical models show that the 1MLCT1 state can be very quickly formed via both direct photo-excitation and internal conversions and then can efficiently relax to its equilibrium geometry in ca. 5 fs. The interligand electron transfer (ILET) on the potential energy surface of the 1MLCT1 state is also extremely fast, with a rate constant of ca. 1.38 × 1013 s-1. The ultrafast ILET implies that the excited electron can dynamically delocalize over the three bpy ligands, despite the fact that the excited electron may be localized on either one of the three ligands at the equilibrium geometries of the three symmetric equivalent minima. Since rapid ILET essentially suggests delocalization, the long-standing controversy in inorganic photophysics-whether the excited electron is localized or delocalized-may therefore be calmed down to some extent.

Experimental and theoretical studies on charge transfer complex formed by TCTA and TPBi

The studies on charge transfer (CT) complex formed by 4,4,4″-tris(N-carbazolyl)-triphenylamine (TCTA) and 1,3,5-tris(N-phenylbenzimidazol-2-yl) benzene (TPBi) were carried out by experiment and simulation calculations. Experimentally, exciplex-forming in TCTA:TPBi mixture is observed and electroplex is formed for TCTA/TPBi bi-layer structure. The density functional theory via DMol3 package was performed to explore the geometrical structures, stability, and electronic structures of the complexes. The calculation results demonstrate that geometrically stable structure of the complex is assumed to enable electroplex-forming because the molecular planes of donor and acceptor are separated by 5.5 Å. However, the formation of exciplex requires a small intermolecular separation of below 4 Å, which is relatively unstable for TCTA−TPBi complex and has to be driven by an external force. The requisition can be fulfilled in TCTA:TPBi mixture due to a reduction of intermolecular interval caused by tight molecular packing and strong intermolecular interaction deriving from mixing. The emission of exciplex species is thereof observed in TCTA:TPBi mixture and analogue electroplex in TCTA/TPBi bi-layer system. The calculated frontier molecular orbitals of the complexes exhibit the lowest unoccupied molecular orbital (LUMO) is positioned on TPBi and the highest occupied molecular orbital (HOMO) on TCTA, inferring that the emission of the CT complexes is originated from electron cross transition between the LUMO of TPBi and HOMO of TCTA. Meanwhile, the greater overlap of the molecular orbitals and larger oscillator strength of lowest-lying singlet excited states manifest that the CT complex in TCTA:TPBi mixture can be more efficiently produced, resulting in better electroluminescent performance, compared to TCTA/TPBi system.

Vibrationally resolved two-photon electronic spectra including vibrational pre-excitation: Theory and application to VIPER spectroscopy with two-photon excitation.

Following up on our previous work on vibrationally resolved electronic absorption spectra including the effect of vibrational pre-excitation [von Cosel et al., J. Chem. Phys. 147, 164116 (2017)], we present a combined theoretical and experimental study of two-photon-induced vibronic transitions in polyatomic molecules that are probed in the VIbrationally Promoted Electronic Resonance experiment using two-photon excitation (2P-VIPER). In order to compute vibronic spectra, we employ time-independent and time-dependent methods based on the evaluation of Franck-Condon overlap integrals and Fourier transformations of time-domain correlation functions, respectively. The time-independent approach uses a generalized version of the FCclasses method, while the time-dependent approach relies on the analytical evaluation of Gaussian moments within the harmonic approximation, including Duschinsky rotation effects. For the Coumarin 6 dye, two-dimensional 2P-VIPER experiments involving excitation to the lowest-lying singlet excited state (S1) are presented and compared with corresponding one-photon VIPER spectra. In both cases, coumarin ring modes and a CO stretch mode show VIPER activity, albeit with different relative intensities. Selective pre-excitation of these modes leads to a pronounced redshift of the low-frequency edge of the electronic absorption spectrum, which is a prerequisite for the VIPER experiment. Theoretical analysis underscores the role of interference between Franck-Condon and Herzberg-Teller effects in the two-photon experiment, which is at the root of the observed intensity distribution.

Solution Structure and Ultrafast Vibrational Relaxation of the PtPOP Complex Revealed by ΔSCF-QM/MM Direct Dynamics Simulations

Recent ultrafast experiments have unveiled the time scales of vibrational cooling and decoherence upon photoexcitation of the diplatinum complex [Pt2(P2O5H2)4]4– in solvents. Here, we contribute to the understanding of the structure and dynamics of the lowest lying singlet excited state of the model photocatalyst by performing potential energy surface calculations and Born–Oppenheimer molecular dynamics simulations in the gas phase and in water. Solvent effects were treated using a multiscale quantum mechanics/molecular mechanics approach. Fast sampling was achieved with a modified version of delta self-consistent field implemented in the grid-based projector-augmented wave density functional theory code. The known structural parameters and the PESs of the first singlet and triplet excited states are correctly reproduced. Besides, the simulations deliver clear evidence that pseudorotation of the ligands in the excited state leads to symmetry lowering of the Pt2P8 core. Coherence decay of Pt–Pt stretching ...

Tuning the optical and electronic properties of perylene diimides through transversal core extension

Herein, we present a systematic study under the density functional theory on a series of perylene diimides (PDIs) to unravel the effect that transversal π-extension of the perylene core has on the optical and electronic properties. An unexpected increase in the HOMO–LUMO gap is predicted upon increasing the size of the polycyclic core going from the simplest PDI to the coronene-based CDI, whereas a systematic reduction in the gap is calculated upon further increase of the π-core in the D(X)CDI derivatives (X = B, N, A and T refer to benzene, naphthalene, anthracene and tetracene, respectively). This behaviour is explained in terms of orbital topology, geometry parameters and accumulated charges, evidencing an electronically isolated coronene moiety in CDI, whereas the π-extension for D(X)CDI disrupts the coronene conjugation pattern and creates electronically defined acene units. A hypsochromic shift of the lowest-lying singlet excited state is predicted going from PDI to CDI, in good accord with the experimental evidence. The low-lying experimental bands of CDI are unequivocally assigned to the vibrational structure of the first singlet electronic transition. Otherwise, moving to higher extended D(X)CDI derivatives, the expected bathochromic shift of the first singlet excited state was found along with an increase in the absorption intensity. Finally, appealing charge-transport properties are demonstrated for the family of perylene diimides. A gradual decrease in both hole and electron reorganization energies is calculated upon transversal core extension but for CDI. A hole reorganization energy of only 0.05 eV is calculated for the most π-extended DTCDI derivative, which competes with the best hole-transporting materials reported so far. These derivatives can therefore be viewed as appealing ambipolar systems, and especially as hole-transporters, to be exploited in next-generation photovoltaics.

Ultraviolet relaxation dynamics of aniline, N, N-dimethylaniline and 3,5-dimethylaniline at 250 nm.

Time-resolved photoelectron imaging was used to investigate the electronic relaxation dynamics of gas-phase aniline, N, N-dimethylaniline, and 3,5-dimethylaniline following ultraviolet excitation at 250 nm. Our analysis was supported by ab initio coupled-cluster calculations evaluating excited state energies and (in aniline) the evolution of a range of excited state physical properties as a function of N-H bond extension. Due to a lack of consistency between several earlier studies undertaken in aniline, the specific aim of this present work was to gain new insight into the previously proposed non-adiabatic coupling interaction between the two lowest lying singlet excited states S1(ππ(∗)) and S2(3s/πσ(∗)). The methyl-substituted systems N, N-dimethylaniline and 3,5-dimethylaniline were included in order to obtain more detailed dynamical information about the key internal molecular coordinates that drive the S1(ππ(∗))/S2(3s/πσ(∗)) coupling mechanism. Our findings suggest that in all three systems, both electronic states are directly populated during the initial excitation, with the S2(3s/πσ(∗)) state then potentially decaying via either direct dissociation along the N-X stretching coordinate (X = H or CH3) or internal conversion to the S1(ππ(∗)) state. In aniline and N, N-dimethylaniline, both pathways most likely compete in the depletion of S2(3s/πσ(∗)) state population. However, in 3,5-dimethylaniline, only the direct dissociation mechanism appears to be active. This is rationalized in terms of changes in the relative rates of the two decay pathways upon methylation of the aromatic ring system.

Computational Investigations about the Ground and Excited States’Properties of Trans-4-N,N-Dimethylamino-4’-Nitro-Stilbene (DNS) and Trans-4-N,N-Dimethyl-Amino-4’-Cyanostilbene (DCS) Derivatives

The ground and lowest-lying singlet excited state geometries and dipole moments of trans-4-N,N-dimethylamino- 4’-nitro-stilbene (DNS) derivative and the trans-4-N,N-dimethyl-amino-4’-cyanostilbene (DCS) derivative are calculated at the B3LYP level of theory for the first time in this work. The vertical excitation energies and the fluorescence emission energies are obtained for the two species. The calculated results are compared with available experimental results and show good consistency. The molecular orbital analyses in these molecules have been performed. The comparison of the atomic charge distributions in the ground and excited states of both the DNS derivative and DCS derivative implies the intra molecular charge transfer (ICT) process during the excitation period.

A quantum Monte Carlo study of zinc-porphyrin: Vertical excitation between the singlet ground state and the lowest-lying singlet excited state

We have calculated the vertical excitation energy between the singlet ground state (11Ag) and the lowest-lying singlet excited state (11Eu) of Zn-porphyrin employing the fixed-node diffusion Monte Carlo technique. The determinantal parts of trial wave functions were constructed using results from Configuration Interaction and time-dependent Density Functional Theory.

ChemInform Abstract: Subpicosecond Fluorescence Depolarization Studies of Tryptophan and Tryptophanyl Residues of Proteins

Fluorescence depolarization of the exposed tryptophanyl residue in the peptide melittin and the buried tryptophanyl residue in the protein azurin (from the bacteria Pseudomonas aeruginosa) was measured with subpicosecond resolution at various excitation and emission wavelengths. A short time component was found in both the fluorescence decay and fluorescence anisotropy decay for the tryptophanyl residue in melittin with time constants of 1.4±0.2 and 3.8±2 ps, respectively. The short time components originate from internal conversion between the two lowest lying singlet excited states

Dehydroindigo, the Forgotten Indigo and Its Contribution to the Color of Maya Blue

A comprehensive investigation of the electronic spectral and photophysical properties of the oxidized form of indigo, dehydroindigo (DHI), has been carried out in solution at 293 K. It is shown that dehydroindigo readily converts into its neutral keto form, the blue indigo, in a process which depends on the solvent and water content of the medium. DHI was investigated in toluene, in benzene, and in methanol and it was found that both the oxidized and the keto indigo forms are present in solution. In marked contrast to what has been found for keto-indigo, where the internal conversion channel dominates >99% of the excited state deactivation, or with the fully reduced leuco-indigo, where fluorescence, internal conversion, and singlet-to-triplet intersystem crossing coexist, in the case of DHI in toluene and benzene, the dominant excited state deactivation channel involves the triplet state. Triplet state yields (phi(T)) of 70-80%, with negligible fluorescence (< or = 0.01%) are observed in these solvents. In methanol the phi(T) value decreases to approximately 15%, with an increase of the fluorescence quantum yield to 2%, which makes these processes competitive with the S(1) --> S(0) internal conversion deactivation process. The data are experimentally compatible with the existence of a lowest lying singlet excited state of n,pi* origin in toluene and of pi,pi* origin in methanol. A time-resolved investigation in the picosecond time domain suggests that the emission of DHI involves three interconnected species (involving rotational isomerism), with relative contributions depending on the emission wavelength. DFT calculations (B3LYP 6-31G** level) were performed in order to characterize the electronic ground (S(0)) and excited singlet (S(1)) and triplet (T(1)) states of DHI. The HOMO-LUMO transition was found to accompany an n --> pi* transition of the oxygen nonbonding orbitals to the central CC and adjacent C-N bonds. Calculations also revealed that in S(0) the two indole-like moieties deviate from planarity from ca. 20 degrees, whereas in S(1) and T(1) the predicted structure is basically planar; a gradual decrease of the carbon-carbon central bond distance is seen in the order S(0), S(1), T(1). An additional study on the blue pigment Maya Blue was made, and the comparison between the solid-state spectra of indigo, DHI, and Maya Blue suggests that, in line with recent investigations, DHI is present together with indigo in Maya Blue. These results are relevant to the discussion of the involvement of dehydroindigo in the palette of colors of the ancient Maya Blue pigment.

Photophysical Characterization of Natural cis-Carotenoids¶

By means of steady-state fluorescence spectroscopy we explore the photophysics of two lowest lying singlet excited states in two natural 15-cis-carotenoids, namely phytoene and phytofluene, possessing three and five conjugated double bonds (N), respectively. The results are interpreted in relation to the photophysics of all-trans-carotenoids with varying N. The fluorescence of phytofluene is more Stokes-shifted relative to that of phytoene, and is ascribed to the forbidden S1→ S0 transition, with its first excited singlet state (S1) lying 3340 cm−1 below the dipole allowed second excited singlet state (S2), at 77 K. For phytoene the S2 and S1 potential surfaces are closer in energy, probably giving rise to the mixed S2 and S1 fluorescence characteristics. The origin of phytoene fluorescence is discussed and is suggested to be due to the S1→ S0 transition; with the S1 state located 1100 cm−1 below S2 at 77 K. The dependence of the fluorescence quantum yield on temperature and viscosity shows that large amplitude molecular motions are involved in the radiationless relaxation process of phytoene. The transition dipole moment of absorption and emission are parallel in phytoene and nonparallel in phytofluene.

Predictions of the Geometries and Fluorescence Emission Energies of Oxyluciferins

The complete active space self-consistent field (CASSCF) method and multiconfigurational second-order perturbation theory (CASPT2) have been used to study the structures and spectra of oxyluciferins (OxyLH2). The ground and lowest-lying singlet excited states geometries have been optimized using CASSCF. CASPT2 has been used to predict relaxed emission energies. The focus is on the lowest-lying singlet excited states of the anionic keto and enol forms of OxyLH2(-1) at the optimized excited-state geometries. The planar keto and enol forms of OxyLH2(-1) are minima on both the S0 and the S1 potential energy surfaces. The twisted keto and enol forms of OxyLH2(-1) are transition states on the S0 and S1 potential energy surfaces. The S1 --> S0 fluorescence emission energies are in the range of 54.2-58.4 kcal/mol for the anionic planar keto forms of OxyLH2, and in the range of 55.7-63.2 kcal/mol for the anionic enol forms of OxyLH2. S0 and S1 potential energy surfaces and thus are not implicated in the emission spectra in the gas phase.

The ground and two lowest-lying singlet excited electronic states of copper hydroxide (CuOH)

Various ab initio methods, including self-consistent field (SCF), configuration interaction, coupled cluster (CC), and complete-active-space SCF (CASSCF), have been employed to study the electronic structure of copper hydroxide (CuOH). Geometries, total energies, dipole moments, harmonic vibrational frequencies, and zero-point vibrational energies are reported for the linear 1Sigma+ and 1Pi stationary points, and for the bent ground-state X 1A', and excited-states 2 1A' and 1 1A". Six different basis sets have been used in the study, Wachters/DZP being the smallest and QZVPP being the largest. The ground- and excited-state bending modes present imaginary frequencies for the linear stationary points, indicating that bent structures are more favorable. The effects of relativity for CuOH are important and have been considered using the Douglas-Kroll approach with cc-pVTZ/cc-pVTZ_DK and cc-pVQZ/cc-pVQZ_DK basis sets. The bent ground and two lowest-lying singlet excited states of the CuOH molecule are indeed energetically more stable than the corresponding linear structures. The optimized geometrical parameters for the X 1A' and 1 1A" states agree fairly well with available experimental values. However, the 2 1A' structure and rotational constants are in poor agreement with experiment, and we suggest that the latter are in error. The predicted adiabatic excitation energies are also inconsistent with the experimental values of 45.5 kcal mol(-1) for the 2 1A' state and 52.6 kcal mol(-1) for the 1 1A" state. The theoretical CC and CASSCF methods show lower adiabatic excitation energies for the 1 1A" state (53.1 kcal mol(-1)) than those for the corresponding 2 1A' state (57.6 kcal mol(-1)), suggesting that the 1 1A" state might be the first singlet excited state while the 2 1A' state might be the second singlet excited state.

Predissociation of HCN/DCN in Two Lowest-Lying Singlet Excited States: Effect of Fermi Resonances on Spectra and Dynamics

The three-dimensional potential energy surfaces of the 11A‘‘ and 21A‘ states and the corresponding transition dipole surfaces from the ground electronic (11A‘) state of HCN have been obtained from high-level ab initio calculations. Using these surfaces, we investigate the predissociation dynamics of HCN and DCN with a recursive quantum mechanical method. Whereas the perpendicular transition to the 11A‘‘ state leads to narrow predissociative resonances, the absorption spectrum of the parallel 21A‘ ← 11A‘ transition is dominated by relatively broad peaks associated with rapid fragmentation. Detailed analysis suggests that the 11A‘‘ spectrum of DCN is affected by a 1:2 Fermi resonance between the C−N stretching and bending modes. Both theoretical and experimental evidence suggests that the so-called β band should be reassigned to the 11A‘‘ absorption. In the 21A‘ state, a strong isotopic effect is found in the fragment internal state distribution and attributed to a Fermi resonance between the H−C and C−N vibrational modes.

Half-projected Hartree-Fock calculations for the spectroscopic parameters, frequencies, and intensities of the torsional mode of the lowest lying singlet excited states of hydrogen peroxide

Quantum chemical calculations have been performed in half-projected Hartree–Fock (HPHF) and CIS approaches for estimating the energy levels of the lowest two excited states of hydrogen peroxide. Geometry optimization was performed for the respective states at the equilibrium configuration with an elaborate quadrupole-zeta basis set augmented with polarization and diffuse functions. The model of the nonrigid rotor in an anharmonic potential was used for calculating the frequencies and intensities of the torsional mode, and the spectroscopic parameters were evaluated with a view to understand more closely the vibrational structure associated with the experimental ultraviolet (UV) spectrum of this molecule. While the spectroscopic parameters are new, reasonably good agreement is achieved for the frequencies of the torsional mode with the existing theoretical and experimental data for the 11B state. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 75: 631–636, 1999

ChemInform Abstract: POTENTIAL ENERGY AND DIPOLE MOMENT SURFACES FOR SIMULTANEOUS TORSION AND PYRAMIDALIZATION OF ETHYLENE IN ITS LOWEST‐LYING SINGLET EXCITED STATES: A CI STUDY OF THE SUDDEN POLARIZATION EFFECT

Chemischer InformationsdienstVolume 11, Issue 50 Physical Organic Chemistry ChemInform Abstract: POTENTIAL ENERGY AND DIPOLE MOMENT SURFACES FOR SIMULTANEOUS TORSION AND PYRAMIDALIZATION OF ETHYLENE IN ITS LOWEST-LYING SINGLET EXCITED STATES: A CI STUDY OF THE SUDDEN POLARIZATION EFFECT R. J. BUENKER, R. J. BUENKERSearch for more papers by this authorV. BONACIC-KOUTECKY, V. BONACIC-KOUTECKYSearch for more papers by this authorL. POGLIANI, L. POGLIANISearch for more papers by this author R. J. BUENKER, R. J. BUENKERSearch for more papers by this authorV. BONACIC-KOUTECKY, V. BONACIC-KOUTECKYSearch for more papers by this authorL. POGLIANI, L. POGLIANISearch for more papers by this author First published: December 16, 1980 https://doi.org/10.1002/chin.198050057Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume11, Issue50December 16, 1980 RelatedInformation