Singlet-triplet Energy Separation Research Articles

Delayed Luminescence in 2-Methyl-5-(penta(9-carbazolyl)phenyl)-1,3,4-oxadiazole Derivatives.

2,5-Diphenyl-1,3,4-oxadiazole has been widely used as an acceptor portion of donor-acceptor fluorophores that exhibit thermally activated delayed fluorescence (TADF), but analogous 2-alkyl-5-phenyl-1,3,4-oxadiazoles have been much less widely investigated. Here the properties of carbazole-substituted 2-methyl-5-phenyl-1,3,4-oxadiazoles are compared to those of their 2,5-diphenyl analogues. The fluorescence of each of the former compounds is blue-shifted by ca. 50-100 meV relative to that in the latter, while similar estimated values of the singlet-triplet energy separation (ΔEST) are maintained. In particular, 2-methyl-5-(penta(9-carbazolyl)phenyl)-1,3,4-oxadiazole and 2-methyl-5-(penta(3,6-di-tert-butyl-9-carbazolyl)phenyl)-1,3,4-oxadiazole exhibit solution fluorescence maxima of 466 and 485 nm and estimated ΔEST values of 0.12 and 0.03 eV, respectively. In both cases the reverse intersystem crossing (RISC) rates inferred from their solution fluorescence behavior are over twice those of the corresponding 2-phenyl derivatives. Organic light-emitting diodes (OLEDs) in which the 2-methyl derivatives are used as emitters yield external quantum efficiency (EQE) values of up to 23%. OLEDs with 2-methyl-5-(penta(9-carbazolyl)phenyl)-1,3,4-oxadiazole and 2-methyl-5-(penta(3,6-di-tert-butyl-9-carbazolyl)phenyl)-1,3,4-oxadiazole emitters show reduced efficiency rolloff at high current densities relative to their 2-phenyl counterparts, the latter exhibiting an EQE of 16% at 1000 cd m-2.

Unravelling the Potential of Ylides in Stabilizing Low-Valent Group 13 Compounds: Theoretical Predictions of Stable, Five-Membered Group 13 (Aluminum and Gallium) Carbenoids Capable of Small-Molecule Activation.

Computational investigations provide evidence toward the remarkable ability of strongly electron-donating ylidic functionalities in stabilizing singlet group 13 carbenoids with promising ligand properties. All of the proposed carbenoids are found to be considerably nucleophilic and possess significant singlet-triplet energy separation values. The calculated activation barriers and reaction free energies obtained for the cleavage of different enthalpically strong bonds by these carbenoids are found to be either comparable to or lower than those of the experimentally evaluated aluminum and gallium carbenoids, thereby indicating their potential in small-molecule activation.

Remote Substituent Effects on the Structures and Stabilities of P═E π-Stabilized Diphosphatetrylenes (R2P)2E (E = Ge, Sn).

A rare P-E π interaction between the lone pair of a planar P center and the vacant p orbital at the Ge or Sn center provides efficient stabilization for P-substituted tetrylenes (R2P)2E (E = Ge, Sn) and enables isolation of the first example of a compound with a crystallographically authenticated P═Sn bond. Subtle changes in the electronic properties of the bulky aryl substituents in these compounds change the preference for planar versus pyramidal P centers in the solid state; however, variable-temperature NMR spectroscopy indicates that in solution these species are subject to a dynamic equilibrium, which interconverts the planar and pyramidal P centers. Consistent with this, density functional theory studies suggest that there is only a small energy difference between the planar and pyramidal forms of these compounds and reveal a small singlet-triplet energy separation, suggesting potentially interesting reactivities.

Singlet–triplet separation induced by the Pseudo Jahn–Teller effect examples: The CX2 and NX2+ (X = H, Cl) molecules

A theoretical investigation with high-level multireference methods is performed to reveal the Jahn–Teller origin of the triplet and singlet bent structures for CH2, CCl2, NH2+, and NCl2+ molecules and rationalize the relative energy ordering of the lowest singlet and triplet states. At the highest-symmetry linear configurations of these triatomic systems, the Pseudo Jahn–Teller effect (PJTE) between the ground state 3Σg- and excited state 3Πu upon the bending mode, denoted as (3Σg-+3Πu)⊗πu, is the reason of producing triplet 3B1 bent structures, and the combined PJTE of (1Δg+1Πu)⊗πu and Renner–Teller effect (RTE) of 1Δg state result in the formations of singlet 1A1 and 1B1 minima. The PJTE of “Δ–Π” plays a dominant role for the instability of degenerate Delta state which is subject to RTE. The spin-crossover induced by PJTE is responsible for the reversed order of singlet and triplet ground states in CH2/NH2+ and CCl2/NCl2+. The different singlet–triplet energy separations of those molecules can be rationalized from different Jahn–Teller vibronic coupling intensity at linear configurations.

White Phosphorus Degradation with a NacNac Aluminum Carbene Analogue: The Biradical Reaction Mechanism

The title compound Al-NacNac is isolobal to the imidazol-2-ylidene (NHC); the latter is considered as a nucleophilic carbene. However, the title compound is different from a typical carbene, as aluminum is a heavier group 13 element with a predominant inert s orbital. Its singlet ground state is a poor Lewis donor (acceptor) toward white phosphorus, but its corresponding lowest energy triplet state forms a strong Al-P bond with (opened) white phosphorus. The reaction of Al-NacNac with white phosphorus proceeds in two steps: after the addition of a first carbene analogue, a second one is added, resulting in a transient biradicaloid species. This undergoes facile subsequent rearrangement, and a final ring closure reaction leads to the observed product with a bicyclobutane moiety. It is determined by intramolecular bond formation of two phosphorus centered radicals. Finally, a structure with a large singlet-triplet energy separation is formed. An analogy to the noninnocent ligand character as well as the exciplex view of the monoadduct of white phosphorus with the Al-NacNac system is drawn.

Electron number dependence of spin triplet–singlet relaxation time

In a GaAs single quantum dot, the relaxation time T1 between spin triplet and singlet states has been measured for the last few even electron numbers. The singlet–triplet energy separation EST is tuned as a control parameter for the comparison of T1 between different electron numbers. T1 steadily decreases with increasing electron numbers from 2-electrons to 6-electrons. This implies an enhancement of the spin–orbit coupling strength due to multi-electron interaction in a quantum dot.

On the acceptor properties of a Al-Nacnac carbene analogue, a density functional investigation

The Al-Nacnac compound is isolobal to the imidazol-2-ylidene (NHC) and forms donor complexes with phosphines and NHCs. These structural types were studied by density functional calculations. They reveal a strong biradical character, since donor addition to the Al-Nacnac reduces the singlet stability of the free carbene-analogue. This facilitates further biradical reactions, such as a subsequent hydrogen shift to the final corresponding tetravalent aluminium product. The latter is determined by maximum singlet stability, i.e. a large singlet-triplet energy separation. Relevance to the analogues Wittig ylids is drawn; these are constituted formally by addition of a phosphine to a carbene, and are well-known as stable synthons in organic chemistry.

A density functional theory study on the stability and ligand properties of the different substituted phenyl carbenes

In regard to the worldwide interest in synthesis and application of stable carbenes, DFT calculations (B3LYP/6-311++G**//B3LYP/6-31+G* levels) are employed to reach at a series of phenyl carbenes. The singlet-triplet energy separations (ΔES-T), HOMO–LUMO energy gaps (ΔEHOMO-LUMO), as well as philicity indices (N and ω) and basicity of these carbenes are compared and contrasted with the synthesized N-heterocyclic carbenes. The investigations reveal that F, Cl, Br, NMe2 and PMe2 stabilize singlet states more than their corresponding triplet states. The reactivity of the species is discussed in terms of nucleophilicity, electrophilicity and proton affinity. This detailed study offers new insights into the chemistry of these classes of carbenes. Key words: Stability, phenyl carbenes, nucleophilicity and electrophilicity, proton affinity, DFT.

Monoheteroatom substituted six-membered carbenes: A computational survey of stability and reactivity

1-(X)-3,3-dimethyl-2-cyclohexanylidenes, where X = amino, oxy, phosphino, and thio, are compared and contrasted to probe the effect of monoheteroatom substitution on the stability, multiplicity and reactivity of six-membered saturated and unsaturated cyclic carbenes. The stabilizing effects of substituents are discussed through geometrical parameters, and AIM analysis. The strong π-donor/σ-acceptor amino group exerts singlet–triplet energy separation (Δ E S–T) of 38.4 kcal/mol while more electronegative oxy group induces a Δ E S–T of 23.0 kcal/mol. Aminoalkylcarbenes rebuff dimerization while oxyalkylcarbenes show a high dimerizing affinity. The reactivity of species is discussed in terms of nucleophilicity and electrophilicity indices as well as proton affinities. It seems that our six-membered carbenes are more nucleophilic than common five-membered N-heterocyclic carbenes.

Effects of α‐cyclopropyl on heterocyclic carbenes stability at DFT

α‐Cyclopropyl stability impacts on singlet and triplet heterocyclic carbenes with acyclic, cyclic, and cyclic‐unsaturated structures are compared and contrasted to di‐t‐butyl as well as t‐butylcyclopropylcarbenes through appropriate isodesmic reactions at B3LYP/AUG‐cc‐pVTZ level. Substitution of one of the t‐butyl groups of di‐t‐butylcarbene with a cyclopropyl alters the ground state multiplicity from triplet to singlet with a singlet–triplet energy separation (ΔEs–t) of 7.2 kcal/mol. Additional heteroatom substitution increases ΔEs–t values for the resulting α‐heteroatom cyclopropylcarbenes in the following order: amino > oxy > thio > phophino. α‐Cyclopropyl group stabilizes singlet states of all our carbenes two to three times more than their corresponding triplet states. The ΔEs–t values of all the carbenes are increased through cyclization, while the introduction of unsaturation in the rings causes small and rather random changes. To probe the kinetic stability of the species, we calculated the transition states for the opening of cyclopropyl through 1,2‐C shift. Interestingly, the 4.1 kcal/mol energy barrier in cyclopropylcarbene is significantly increased in the presence of heteroatoms to 31.2 kcal/mol for aminocyclopropylcarbene. The reactivity of the species is discussed in terms of nucleophilicity and electrophilicity issues showing our carbenes, especially acyclic ones, more nucleophilic than the common N‐heterocyclic carbenes. Copyright © 2010 John Wiley & Sons, Ltd.

Substituted six-membered ring carbenes: the effects of amino and cyclopropyl groups through DFT calculations

DFT calculations are employed to compare and contrast six-membered ring carbenes including 1,3-dimethyltetrahydropyrimidin-2-ylidene ( 1a), 1-methyl-3-cyclopropyltetrahydropyridine-2-ylidene ( 2a), and 1,3-dicyclopropylcyclohexane-2-ylidene ( 3a) as well as their unsaturated analogues 1b, 2b, 3b, and 2c. The amino groups exert singlet–triplet energy separation (∆ E s−t) of 60.9 kcal/mol to 1a while cyclopropyls induce a ∆ E s−t of 14.8 kcal/mol to 3a. The simultaneous presence of amino and cyclopropyl in 2a leads to a ∆ E s−t of 43.3 kcal/mol. Unsaturation slightly increases the ∆ E s−t of 1a and 3a but not that of 2a. Our thermodynamic, kinetic, and reactivity results are compared with those of synthetic five-membered ring N-heterocyclic carbenes.

Multireference Correlation Consistent Composite Approach [MR-ccCA]: Toward Accurate Prediction of the Energetics of Excited and Transition State Chemistry†

The correlation consistent Composite Approach, ccCA [ Deyonker , N. J. ; Cundari , T. R. ; Wilson , A. K. J. Chem. Phys. 2006 , 124 , 114104 ] has been demonstrated to predict accurate thermochemical properties of chemical species that can be described by a single configurational reference state, and at reduced computational cost, as compared with ab initio methods such as CCSD(T) used in combination with large basis sets. We have developed three variants of a multireference equivalent of this successful theoretical model. The method, called the multireference correlation consistent composite approach (MR-ccCA), is designed to predict the thermochemical properties of reactive intermediates, excited state species, and transition states to within chemical accuracy (e.g., 1 kcal/mol for enthalpies of formation) of reliable experimental values. In this study, we have demonstrated the utility of MR-ccCA: (1) in the determination of the adiabatic singlet-triplet energy separations and enthalpies of formation for the ground states for a set of diradicals and unsaturated compounds, and (2) in the prediction of energetic barriers to internal rotation, in ethylene and its heavier congener, disilene. Additionally, we have utilized MR-ccCA to predict the enthalpies of formation of the low-lying excited states of all the species considered. MR-ccCA is shown to give quantitative results without reliance upon empirically derived parameters, making it suitable for application to study novel chemical systems with significant nondynamical correlation effects.

Dispersion-corrected energy barriers for silylene addition to white phosphorus, a density functional investigation into substituent effects

A density functional investigation into differently substituted silylenes with respect to the first step in the addition to white phosphorus is presented. The investigations include dispersion corrections in the density functional treatment. They become sizable for the transition state geometries for the silylenes as they become increasingly substituted by bulky groups. Hence, dispersion corrections are essential for a quantum chemical treatment of real molecules using density functional theory. The different singlet–triplet energy separations of differently substituted silylenes were also investigated and compared with calculated activation barriers for the first step in the addition reaction.

Fast Reactions of Hydroxycarbenes: Tunneling Effect versus Bimolecular Processes

Different uni- and bimolecular reactions of hydroxymethylene, an important intermediate in the photochemistry of formaldehyde, as well as its halogenated derivatives (XCOH, X = H, F, Cl, Br), have been considered using high-level CCSD(T)/CBS quantum chemical methods. The Wentzel-Kramers-Brillouin (WKB) and Eckart approximations were applied to estimate the tunneling rate constant of isomerization of trans-HCOH to H(2)CO, and the WKB procedure was found to perform better in this case. In agreement with recent calculations and experimental observations [Schreiner et al., Nature 2008, 453, 906], the half-life of HCOH at the low temperature limit in the absence of bimolecular processes was found to be very long (approximately 2.1 h). The corresponding half-life at room temperature was also noticeable (approximately 35 min). Bimolecular reactions of trans-hydroxymethylene with parent formaldehyde yield primarily more thermodynamically favorable glycolaldehyde via the specific mechanism involving 5-center transition state. The most preferable reaction of cis-hydroxymethylene with formaldehyde yields carbon monoxide and methanol. Due to very low activation barriers, both processes occur with nearly a collision rate. If the concentration of HCOH (and its halogenated analogues XCOH as well) is high enough, the bimolecular reactions of this species with itself become important, and H(2)CO (or X(H)CO) is then formed with a collision rate. The singlet-triplet energy separation of trans-HCOH is confirmed to be approximately -25 kcal/mol.

Novel α-spirocyclic (alkyl)(amino)carbenes at the theoretical crossroad of flexibility and rigidity

Spirocyclic (alkyl)(amino)carbenes (CAACs) with reasonable conformations of cyclopropyl, cyclobutyl, cyclopentyl, and cyclohexyl substituents in the position α to the carbenic center are investigated at B3LYP/AUG-cc-pVTZ//B3LYP/6-31+G*. The singlet–triplet energy separations (∆E s−t), HOMO–LUMO energy gaps (∆E HOMO−LUMO), hydrogenation energies (∆E H), heats of methylene formation (∆E) as well as philicity indices (N and ω) of these carbenes are compared and contrasted. The highest ∆E s−t is encountered for the cyclopropyl substituted CAAC (54.2 kcal/mol), while the other ones lay in a narrow range of 48.2–51.8 kcal/mol. Assignment of the barriers for ring inversions is carried out in order to probe the “flexible steric bulk” of cyclic substituents, showing negligible differences with those of the unsubstituted rings. The calculated N and ω indices indicate that in contrast to the popular belief, CAACs appear less electrophilic than N-heteorcyclic carbenes (NHCs).

Conceptual density functional theory based intrinsic radical stabilities: Application to substituted silylenes and p-benzynes

Spin-polarized Conceptual DFT descriptors, such as the sum of the spin potentials and the spin-philicity index, are used to characterize the vertical and adiabatic singlet–triplet energy separation of silylenes and p-benzynes. In addition, an (approximately) intrinsic stability scale is constructed for both systems, using a model linking bond dissociation enthalpies to chemical concepts such as the electrophilicity index, Pauling’s electronegativity and the stability of the individual radical fragments, used with success in a recent contribution for radical systems. The new stability scales are compared to other scales in the literature and are correlated with the singlet–triplet gaps and the electrophilicity index values for the examined compounds, investigating the relationship between reactivity and stability.

Triplet germylenes with separable minima at ab initio and DFT levels

Six ground state triplet germylenes, with HGeX and GeX 2 formulae, are reached at QCISD(T)/6-311++G** and B3LYP/6-311++G** levels of theory (X = Li, Na, and K). Their corresponding less stable electronic isomers appears as singlet germylene minima. Scanning Ф as a function of energy for every triplet species gives two energy minima, separated by significant energy barriers. The same scanning gives merely one minimum for most singlet species, at the potential energy hyper-surface. Thus, only the open-shell (σ 1π 1) triplet states exhibit bond-angle isomerism, since no such isomers could be found for the (σ 2π 0) singlet states except for singlet GeLi 2 which shows a possible covalent bonding interaction between the two Li atoms. All triplet X Ge X species appear more linear than their corresponding triplet H Ge X. As the electro-positivity of X increases, the Ge X bond distances in all species lengthen, while their singlet–triplet energy separations (Δ E s–t) decrease, with the lowest Δ E s–t given for H Ge K. The Δ E LUMO–HOMO for singlet HGeX and GeX 2 species appear inversely proportional to the size of X, showing a trend of K > Na > Li. Interestingly, for both singlet HGeX and GeX 2 species, linear correlations are found between the Δ E LUMO–HOMO and their corresponding Δ E s–t,X.

3,5,7,9-Substituted Hexaazaacridines: Toward Structures with Nearly Degenerate Singlet−Triplet Energy Separations

Toward the goal of preparing stable, neutral open-shell systems, we synthesized a novel series of p-phenyl-substituted 3,5,7,9-hexaazaacridine and 3,5,7,9-hexaazaanthracene derivatives. The effects of substitution on the molecular electronic properties were probed both experimentally and computationally [B3LYP/6-311G(d,p)//B3LYP/6-31G(d,p)]. While the experimentally prepared structures already have small (20-25 kcal/mol) singlet-triplet energy gaps, systems with even smaller (<9 kcal/mol) singlet-triplet energy separations can be realized through systematic variation of the substituent numbers, types, and patterns. Hexaazaanthracenes show generally smaller singlet-triplet energy gaps than hexaazaacridines. Nitrogen-bonded sigma- and pi-acceptor substituents that cause positive inductive and mesomeric effects as well as carbon-bonded sigma-donor substituents make substituted hexaazaanthracenes promising candidates for purely organic high-spin systems.

Halogen switching of azacarbenes C2NH ground states at ab initio and DFT levels

AbstractRelative stabilities and singlet–triplet energy differences are calculated for 24 C2NX azacarbenes (where X is H, F, Cl, and Br). Three skeletal arrangements are employed including azacyclopropenylidene, [(imino)methylene]carbene, and cyanocarbene. Halogens appear to alternate the electronic ground states of C2NH azacarbenes, from triplet to singlet states, at MP3/6‐311++G**, B1LYP/6‐311++G**, B3LYP/6‐311++G**, MP2/6‐311++G**, MP4(SDTQ)/6‐311++G**, QCISD(T)/6‐311++G**, CCSD(T)/6‐311++G**, CCSD(T)/cc‐pVTZ, G1, and G2 levels of theory. The aromatic characters of singlet cyclic azacyclopropenylidenes are measured using GIAO–NICS calculations. Linear correlations are found between the B3LYP/6‐311++G** calculated LUMO–HOMO energy gaps (ΔEHOMO ‐ LUMO) of the singlet carbenes versus their corresponding singlet–triplet energy separations (ΔE). Electrophilic characters are found for all singlet azacarbenes in their addition reactions to alkenes with the highest electrophilicity being exhibited for X = F. © 2008 Wiley Periodicals, Inc. Heteroatom Chem 19:377–388, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20442

Influence of Solvent on Carbene Intersystem Crossing Rates

The influence of coordinating solvents on singlet-to-triplet carbene intersystem crossing (ISC) rates has been studied with diphenylcarbene (DPC) and para-biphenyltrifluoromethylcarbene (BpCCF 3) by using ultrafast time-resolved spectroscopy. DPC has a triplet ground state in all of the solvents considered, and the concentration of singlet carbene at equilibrium is too small to be measured. It is found that the lifetime of (1)DPC is extended in acetonitrile, benzene, tetrahydrofuran, dichloromethane, and halobenzene solvents relative to cyclohexane. The solvent effect does not well correlate with bulk measures of solvent polarity. The singlet-triplet energy separation of BpCCF 3 is close to zero. The data demonstrates that BpCCF 3 has a triplet ground state in benzene, fluorobenzene, and hexafluorobenzene. Halogenated solvents are found to dramatically retard the rate of ISC in (1)BpCCF 3. We postulate that the empty p orbital of a singlet carbene coordinates with a nonbonding pair of electrons of a halogen atom of the solvent to form a pseudoylide solvent complex, stabilize the singlet carbene, and decrease the singlet-triplet (S-T) energy gap. The "golden rule" of radiationless transitions posits that the smaller the energy gap between the two states, the faster their rate of interconversion. To explain the apparent violation of the golden rule of radiationless transitions for the carbene ISC processes monitored in this study, we propose that the significantly different specific solvation of the singlet and triplet carbenes imposes a Franck-Condon-like factor on the ISC process. Those solvents that most solvate the singlet carbene will also have the greatest structural difference between singlet carbene-solvent complex and their triplet spin isomer-solvent complex, the smallest S-T gap, and the slowest ISC rate. Alternatively, one can propose that a highly solvated singlet carbene must desolvate prior to ISC, and that this requirement decelerates the radiationless transition.