A theoretical study of electronic effects in alkyne‐linked reactive intermediates: (nitrenoethynyl)methylenes, (nitrenoethynyl)silylenes,and (nitrenoethynyl)germylenes

Acetylene‐linked reactive intermediates of (nitrenoethynyl)‐X‐methylenes, (nitrenoethynyl)‐X‐silylenes, and (nitrenoethynyl)‐X‐germylenes are almost experimentally unreachable (X–M–C≡C–N; X=H ( 1 ), CN ( 2 ), OH ( 3 ), NH₂ ( 4 ), NO₂ ( 5 ), and CHO ( 6 ); M=C, Si, and Ge). The effects of the electron‐donating and electron withdrawing groups were compared and contrasted at seven levels of theory. All singlet species as ground states with one local open‐shell singlet carbene subunit (π¹π¹) and another local open‐shell singlet nitrene subunit (π¹π¹) were found to be more stable than their corresponding triplets including one local open‐shell singlet carbene (δ¹π¹) (or one local closed‐shell singlet carbene [δ²π⁰]) and another local triplet nitrene subunit (π¹π¹) with 45.94–77.996 kcal/mol singlet–triplet energy gap (ΔE_s‐t). Their relative silylenes and germylenes made reduction of ΔE_s‐t, so the triplet ground states were found for species 3 _Si , 4 _Si , 5 _Si , 2 _Ge , 3 _Ge , 4 _{Ge ,} and 5 _Ge . All the singlet silylenes/germylenes formed by one local closed‐shell singlet silylenes/germylenes (δ²π⁰) and one local closed‐shell singlet nitrene subunit (π²π⁰). Also, one local closed‐shell singlet silylene/germylene subunit (δ²π⁰) and one local triplet nitrene subunit (π¹π¹) were observed for triplet silylenes/germylenes. The singlet and triplet species 3 _Si , 4 _Si , 3 _Ge , and 4 _Ge , due to their electrophilic (Si₄/Ge₄) and nucleophilic (X₅) centers, could be identified as intermediates in chemical reactions.     

Novel disilyleno- and digermylenocarbenes and SiSi containing cyclopropenylidenes at theoretical levels

Following our quest for silylenes and germylenes, we have reached at W shaped novel triplet disilyleno- and digermylenocarbenes

through optimization of 2,3-di(X)disilacyclopropenylidenes and 2,3-di(X)digermacyclopropenylidenes, respectively, at five ab initio and DFT levels (M = Si, Ge; X = H, F, Cl, Br). Also, we have met the challenge of confining SiSi double bonds in singlet cyclopropenylidene ground states by optimizing singlet 2,3-di(X)disilacyclopropenylidenes, at the same levels of theory. Results are compared and contrasted to the corresponding singlet and triplet 2,3-di(X)cyclopropenylidenes.     

Novel excited quintet state in porphyrin: bis(quinoline-TEMPO)-yttrium-tetraphenylporphine complex

New mono- and bis[4-(3-hydroxy-2-methyl-4-quinolinoyloxy)-2,2,6,6-tetramethylpiperidin-1-oxyl](meso-tetraphenylporphyrinato)yttrium(III) complexes have been synthesized, and the properties of the excited states generated by photoexcitation of porphyrin were studied by time-resolved (TR) and pulsed two-dimensional electron paramagnetic resonance (EPR) spectroscopy. A TR-EPR spectrum was observed in the quartet (S=3/2) or quintet (S=2) states generated from interactions of one or two radicals with the photoexcited triplet state of the porphyrin. The zero-field splitting D values of these states were analyzed in terms of those of the triplet and the radical-triplet pair. The spin states of the excited states were definitely assigned by measuring the mutation frequencies with pulsed EPR.     

Ab initio calculation of the NH(3sigma-)-NH(3sigma-) interaction potentials in the quintet, triplet, and singlet states

We present the ab initio potential-energy surfaces of the NH-NH complex that correlate with two NH molecules in their 3sigma- electronic ground state. Three distinct potential-energy surfaces, split by exchange interactions, correspond to the coupling of the S(A) = 1 and S(B) = 1 electronic spins of the monomers to dimer states with S = 0, 1, and 2. Exploratory calculations on the quintet (S = 2), triplet (S = 1), and singlet (S = 0) states and their exchange splittings were performed with the valence bond self-consistent-field method that explicitly accounts for the nonorthogonality of the orbitals on different monomers. The potential surface of the quintet state, which can be described by a single Slater determinant reference function, was calculated at the coupled cluster level with single and double excitations and noniterative treatment of the triples. The triplet and singlet states require multiconfiguration reference wave functions and the exchange splittings between the three potential surfaces were calculated with the complete active space self-consistent-field method supplemented with perturbative configuration interaction calculations of second and third orders. Full potential-energy surfaces were computed as a function of the four intermolecular Jacobi coordinates, with an aug-cc-pVTZ basis on the N and H atoms and bond functions at the midpoint of the intermolecular vector R. An analytical representation of these potentials was given by expanding their dependence on the molecular orientations in coupled spherical harmonics, and representing the dependence of the expansion coefficients on the intermolecular distance R by the reproducing kernel Hilbert space method. The quintet surface has a van der Waals minimum of depth D(e) = 675 cm(-1) at R(e) = 6.6a0 for a linear geometry with the two NH electric dipoles aligned. The singlet and triplet surfaces show similar, slightly deeper, van der Waals wells, but when R is decreased the weakly bound NH dimer with S = 0 and S = 1 converts into the chemically bound N2H2 diimide (also called diazene) molecule with only a small energy barrier to overcome.     

Silabenzene through divalent precursors at theoretical levels

Abstract  Based on geometries and relative energies, three different mechanisms are proposed for the rearrangements of five isomers of silacyclohexadienylidenes to silabenzene at B3LYP and MP2 levels: (1) [1,2]-hydrogen migration through a planar transition state, (2) [1,4]-hydrogen migration through a boat transition state, and (3) zip-zap mechanism, comprised of three successive [1,2]-hydrogen migrations. The above results are compared and contrasted to rearrangements of the corresponding cyclohexadienylidenes to benzene. Graphical abstract   Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Silabenzene through divalent precursors at theoretical levels

Abstract  

Based on geometries and relative energies, three different mechanisms are proposed for the rearrangements of five isomers of silacyclohexadienylidenes to silabenzene at B3LYP and MP2 levels: (1) [1,2]-hydrogen migration through a planar transition state, (2) [1,4]-hydrogen migration through a boat transition state, and (3) zip-zap mechanism, comprised of three successive [1,2]-hydrogen migrations. The above results are compared and contrasted to rearrangements of the corresponding cyclohexadienylidenes to benzene.     

Electron spin polarization generated by transport of singlet and quintet multiexcitons to spin-correlated triplet pairs during singlet fissions

Singlet fission (SF) is expected to exceed the Shockley–Queisser theoretical limit of efficiency of organic solar cells. Transport of spin-entanglement in the triplet–triplet pair state via one singlet exciton is a promising phenomenon for several energy conversion applications including quantum information science. However, direct observation of electron spin polarization by transport of entangled spin-states has not been presented. In this study, time-resolved electron paramagnetic resonance has been utilized to observe the transportation of singlet and quintet characters generating correlated triplet–triplet (T + T) exciton-pair states by probing the electron spin polarization (ESP) generated in thin films of 6,13-bis(triisopropylsilylethynyl)pentacene. We have clearly demonstrated that the ESP detected at the resonance field positions of individual triplet excitons is dependent on the morphology and on the detection delay time after laser flash to cause SF. ESP was clearly explained by quantum superposition of singlet–triplet–quintet wavefunctions via picosecond triplet-exciton dissociation as the electron spin polarization transfer from strongly exchange-coupled singlet and quintet TT states to weakly-coupled spin-correlated triplet pair states. Although the coherent superposition of spin eigenstates was not directly detected, the present interpretation of the spin correlation of the separated T + T exciton pair may pave new avenues not only for elucidating the vibronic role in the de-coupling between two excitons but also for scalable quantum information processing using quick T + T dissociation via one-photon excitation.

Singlet fission (SF) is expected to exceed the Shockley–Queisser theoretical limit of efficiency of organic solar cells.     

Luminescence and triplet—triplet absorption spectra of rhodium (III) porphyrins

Photophysical studies of four rhodium(III) porphyrins [RhTPPS (H₂O)₃^3?, RhTPP (Cl) (L), RhOEP (Cl) (L) and RhMesoPMEI(CI) (L)] show that these porphyrins are characterised by a moderate phosphorescence (φ ? 10^?2) and a very weak fluorescence (φ ? 5 × 10^?4) in solution at room temperature. TPP derivatives also have moderately intense triplet—triplet absorption extending to 900 nm.     

Molecular weight dependence of triplet—triplet processes in poly(2-vinylnaphthalene)

Delayed fluorescence and phosphorescence spectra and decay curve measurements at 77 K are reported for samples of poly(2-vinylnaphthalene) with molecular weights determined by viscometry to be in the range 15 600 to 505 000. The intensity of delayed fluorescence relative to phosphorrescence was found to increase with molecular weight up to the highest molecular weight sample of solution polymerized material (MW ≈ 100 000) with a leveling off for the higher molecular weight bulk polymers. It is argued that these results imply a triplet exciton migration distance of up to 700 chromophore units. Decay curves imply that most mobile triplet excitons are trapped within approximately 0.3 s. The excitation dependence of the delayed emissions imply that unimolecular processes dominate the kinetics of triplet state chromophores.     

Pi topology and spin alignment in unique photoexcited triplet and quintet states arising from four unpaired electrons of an organic spin system

Syntheses, electronic structures in the ground state, unique photoexcited states, and spin alignment are reported for novel biradical 1, which was designed as an ideal model compound to investigate photoinduced spin alignment in the excited state. Electron spin resonance (ESR), time-resolved ESR (TRESR), and laser-excitation pulsed ESR experiments were carried out. The magnetic properties were examined with a SQUID magnetometer. In the electronic ground state, two radical moieties interact very weakly (almost no interaction) with each other through the closed-shell diphenylanthracene spin coupler. On photoirradiation, a novel lowest photoexcited state with the intermediate spin (S = 1) arising from four unpaired electrons with low-lying quintet (S = 2) photoexcited state was detected. The unique triplet state has an interesting electronic structure, the D value of which is reduced by antiferromagnetic spin alignment between two radical spins through the excited triplet spin coupler. The general theoretical predictions of the spin alignment and the reduction of the fine-structure splitting of the triplet bis(radical) systems are presented. The fine-structure splitting of the unique photoexcited triplet state of 1, as well as the existence of the low-lying quintet state, is interpreted well on the basis of theoretical predictions. Details of the spin alignment in the photoexcited states are discussed.     

Quintet bis(carbene) from triplet diarylcarbenes introduced at the 1,8-position of anthracene

[9-(10-phenyl)anthryl](4-bromo-2,6-dimethylphenyl)diazomethane was found to be stable enough to survive Sonogashira coupling reaction conditions and was converted to [9-(10-phenyl)anthryl](4-trimethylsilylethynyl-2,6-dimethylphenyl)diazomethane, which was reacted with 1,8-diiodoanthracene to give bis(diazo) compound. Bis(carbene) generated by irradiation of the bis(diazo) compound generated a fairly persistent S = 2 quintet state. [structure: see text]     

Experimental and theoretical study of triplet energy transfer in rigid polymer films

With the judicious selection of triplet energy donor (D) and acceptor (A) pairs, a laser flash photolysis procedure has provided a sensitive method for the study of triplet energy transfer in rigid polymer films. By monitoring changes in triplet-triplet (T-T) absorptions the kinetics of triplet energy transfer were evaluated at short time scales, and overall energy-transfer quantum yields were also obtained. Combinations of xanthone- or thioxanthone-type donors and polyphenyl acceptors were particularly suited to these measurements because the former have high intersystem-crossing quantum yields and the latter have very high extinction coefficients for T-T absorption. For exothermic transfer most of the energy transfer that occurred within the lifetime of triplet D ( (3)D) took place in less than a few microseconds after (3)D formation in poly(methyl methacrylate), and triplet A yields were limited largely by the number of A molecules in near contact with (3)D. The kinetics of triplet energy transfer were modeled using a modified Perrin-type statistical arrangement of D/A separations with allowance for excluded volume in combination with a Dexter-type formula for the distance-dependent exchange energy-transfer rate constant. Experimental observations were best explained by constraining D/A separations to reflect the dimensions of intervening molecules of the medium. Rate constants, k 0, for exothermic energy transfer from (3)D to A molecules in physical contact are approximately 10 (11) s (-1) and very similar to triplet energy-transfer rate constants determined from solution encounters. Energy-transfer rate constants, k( r), fall off as approximately exp(-2 r/ 0.85), where r is the separation distance between D and A centers in angstroms. Exchange energy transfer is not restricted to (3)D and A in physical contact, but at 相似文献   

Generation and characterization of triplet anthryl(aryl)carbenes

The title carbenes where aryl groups are phenyl, 2,4, 6-trimethylphenyl, and octahydro-1,4:5,8-di(ethano)anthryl were generated for the first time by irradiation of the corresponding diazo precursors and fully characterized by ESR in a rigid matrix at low temperature. It has been demonstrated that anthryl groups can act as excellent reservoirs for the unpaired electrons as well as relatively effective kinetic protectors for carbene.     

N-(2-methoxycarbonyl-2-yloethyl) (3-nitrenophenyl)aminyl: EPR observation of a quintet hetero-spin molecule

[reaction: see text]. The EPR observation of a ground-state quintet hetero-spin molecule, N-(2-methoxycarbonyl-2-yloethyl) (3-nitreno-phenyl)aminyl 3, in a MTHF matrix at cryogenic temperatures is reported. The molecule has two different kinds of triplet sites, and the ZFS parameters of the quintet are /D/hc/ = 0.170 cm(-1) and /E/hc/ = 0.004 cm(-1). As a result of the nature of the 1,3-localized biradical, the quintet 3 was thermally labile over 5 K.     

Effect of semi-empirical parameters on the triplet energy levels and triplet–triplet transition in benzene

Triplet energy levels and triplet–triplet transition in benzene have been calculated semi-empirically by considering CI up to and including doubly excited configurations and using various values of the core resonance and electron repulsion integrals. The usual method of calibrating β (core resonance integral) from one of the observed transitions in the semi-empirical methods is critically examined with respect to the triplet levels of this molecule. Semi-empirical parameters are found to have a pronounced effect on the energy of the triplets and the triplet–triplet transitions; but the intensities of such transitions are quite insensitive to the choice of such parameters. Numerical results show several ³B_1u → ³E_2g transitions of various intensities. Out of these, the transitions which correspond energetically to the observed triplet–triplet bands are found to have low intensity. Some strong triplet–triplet bands are predicted in the far ultra violet region.     

On the electronic structure of MoO: Spin-polarized density functional calculations of spectroscopic properties of low-lying quintet,triplet, and septet states

Density functional theory (DFT ) calculations for the ground state and four excited quintet, two septet, and two triplet states of the molybdenum oxide molecule are reported. Equilibrium geometries and other spectroscopic constants are determined for these states and compared both with recent spectroscopic measurements and other theoretical calculations, where available. Experimental assignments of the ⁵II ground state and excited ⁵Σ⁺, ⁵Σ^?, ⁵Δ, and B^{' 5}II states are confirmed; also candidates for low-lying triplet³ Δ and ³Σ^? and septet ⁷II and ⁷Σ⁺ are presented. Theoretical calculations for ⁵Σ^?, B^{' 5}II, and ³Σ^? states are reported for the first time. The results are in many cases in better agreement with experiment than are other calculations, already at the simplest level of approximation within DFT , which confirms that this method is a useful tool for investigation of transition-metal compounds. © 1994 John Wiley & Sons, Inc.     

Redox chemistry of the triplet complex (PNP)Co(I)

Reaction of PNPCo, where PNP is (tBu2PCH2SiMe2)2N-, with the persistent radical galvinoxyl, G, gives PNPCoIIG, a nonplanar S = 3/2 species. Reaction with PhCH2Cl or with 0.5 mol I2 gives PNPCoX (X = Cl or I, respectively), but additional I2, seeking CoIII, gives instead oxidation at phosphorus: (tBu2P(I)CH2SiMe2NSiMe2CH2PtBu2)CoI2. Hydrogen-atom transfer reagents fail to give PNPCoH, but H2 gives instead PNPCo(H)2, a result rationalized thermodynamically based on DFT calculations. Multiple equiv of PhSiH3 give a product of Co(V), where N/SiPh and P/Si bonds have formed. N2CH(SiMe3) gives a 1:1 adduct of PNPCo, whose metric parameters suggest partial oxidation above CoI; N2CHPh gives a 1:1 adduct but with very different spectroscopic features. PhN3 reacts fast, via several intermediates detected below 0 degrees C, to finally release N2 and form a CoI product where one phosphorus has been oxidized, PN(P=NPh)Co. Whereas PNPCo(N3) resists loss of N2 on heating, one electron oxidation gives a rapid loss of N2, and the remaining nitride nitrogen is quickly incorporated into the chelate ligand, giving [tBu2PCH2SiMe2NSiMe2NP(tBu2)=CH2Co]. O2 or PhI=O generally gives products where one or both phosphorus centers are converted to its oxide, bonded to cobalt.