Dark and bright exciton formation, thermalization, and photoluminescence in monolayer transition metal dichalcogenides

The remarkably strong Coulomb interaction in atomically thin transition metal dichalcogenides (TMDs) results in an extraordinarily rich many-particle physics including the formation of tightly bound excitons. Besides optically accessible bright excitonic states, these materials also exhibit a variety of dark excitons. Since they can even lie below the bright states, they have a strong influence on the exciton dynamics, lifetimes, and photoluminescence. While very recently, the presence of dark excitonic states has been experimentally demonstrated, the origin of these states, their formation, and dynamics have not been revealed yet. Here, we present a microscopic study shedding light on time- and energy-resolved formation and thermalization of bright and dark intra- and intervalley excitons as well as their impact on the photoluminescence in different TMD materials. We demonstrate that intervalley dark excitons, so far widely overlooked in current literature, play a crucial role in tungsten-based TMDs giving rise to an enhanced photoluminescence and reduced exciton lifetimes at elevated temperatures

Microsolvation and sp²-stereoinversion of monomeric α-(2, 6-di-tert-butylphenyl)vinyllithium as measured by NMR

The beta-unsubstituted title compound dissolves in THF as a uniformly trisolvated monomer, whereas it forms exclusively disolvated monomers in tert-butyl methyl ether, Et2O, TMEDA, or toluene with TMEDA (1.4 equiv). This was established at low temperatures through the observation of separated NMR signals for free and lithium-coordinated ligands and/or through the patterns and magnitudes of C-13, Li-6 NMR coupling constants. An aggregated form was observed only with Et2O (2 equiv) in toluene as the solvent. The olefinic geminal interproton coupling constants of the H2C= part can be used as a secondary criterion to differentiate between these differently solvated ground-states (3, 2, or <2 coordinated ligands per Li). Due to a kinetic trisolvation privilege of THF, the cis/trans sp(2)-stereoinversion rates could be measured through analyses of H-1 NMR line broadening and coalescence only in THF as the solvent: The pseudomonomolecular (because THF-catalyzed),ionic mechanism is initialized by a C-Li bond heterolysis with the transient immobilization of one additional THF ligand, followed by stereoinversion of the quasi-sp(2)-hybridized carbanionic center in cooperation with a "conducted tour" migration of Li+(THF)(4) along the alpha-aryl group within the solvent-separated ion pair

Carbenoid-mediated nucleophilic "hydrolysis" of 2-(dichloromethylidene)-1, 1, 3, 3-tetramethylindane with DMSO participation, affording access to one-sidedly overcrowded ketone and bromoalkene descendants

2-(Dichloromethylidene)-1, 1, 3, 3-tetramethylindane was "hydrolyzed" by solid KOH in DMSO as the solvent at >= 100 degrees C through an initial chlorine particle transfer to give a Cl, K-carbenoid. This short-lived intermediate disclosed its occurrence through a reversible proton transfer which competed with an oxygen transfer from DMSO that created dimethyl sulfide. The presumably resultant transitory ketene incorporated KOH to afford the potassium salt of 1, 1, 3, 3-tetramethylindan-2-carboxylic acid (the product of a formal hydrolysis). The lithium salt of this key acid is able to acylate aryllithium compounds, furnishing one-sidedly overcrowded ketones along with the corresponding tertiary alcohols. The latter side-products (ca. 10%) were formed against a substantially increasing repulsive resistance, as testified through the diminished rotational mobility of their aryl groups. As a less troublesome further side-product, the dianion of the above key acid was recognized through carboxylation which afforded 1, 1, 3, 3-tetramethyl-indan-2, 2-dicarboxylic acid. Brominative deoxygenation of the ketones furnished two one-sidedly overcrowded bromoalkenes. Some presently relevant properties of the above Cl, K-carbenoid are provided in Supporting Information File 1

Nucleofugal behavior of a beta-shielded alpha-cyanovinyl carbanion

Sterically well-shielded against unsolicited Michael addition and polymerization reactions, alpha-metalated alpha-(1,1,3,3-tetramethylindan-2-ylidene) acetonitriles added reversibly to three small aldehydes and two bulky ketones at room temperature. Experimental conditions were determined for transfer of the nucleofugal title carbanion unit between different carbonyl compounds. These readily occurring retro-additions via C-C(O) bond fission may also be used to generate different metal derivatives of the nucleofugal anions as equilibrium components. Fluoride-catalyzed, metal-free desilylation admitted carbonyl addition but blocked the retro-addition

Unusual traits of cis and trans-2,3-dibromo-1,1-dimethylindane on the way from 1,1-dimethylindene to 2-bromo-, 3-bromo-, and 2,3-dibromo-1,1-dimethylindene

Do not rely on the widely accepted rule that vicinal, sp(3)-positioned protons in cyclopentene moieties should always have more positive (3)J NMR coupling constants for the cis than for the trans arrangement: Unrecognized exceptions might misguide one to wrong stereochemical assignments and thence to erroneous mechanistic conclusions. We show here that two structurally innocent-looking 2,3-dibromo-1,1-dimethylindanes violate the rule by means of their values of (3)J(cis) = 6.1 Hz and (3)J(trans) = 8.4 Hz. The stereo-selective formation of the trans diastereomer from 1,1-dimethylindene was improved with the tribromide anion (Br-3(-)) as the brominating agent in place of elemental bromine;the ensuing, regiospecific HBr elimination afforded 3-bromo-1,1-dimethylindene. The addition of elemental bromine to the latter compound, followed by thermal HBr elimination, furnished 2,3-dibromo-1,1-dimethylindene, whose Br/Li interchange reaction, precipitation, and subsequent protolysis yielded only 2-bromo-1,1-dimethylindene

Unusual traits of cis and trans-2,3-dibromo-1,1-dimethylindane on the way from 1,1-dimethylindene to 2-bromo-, 3-bromo-, and 2,3-dibromo-1,1-dimethylindene

Do not rely on the widely accepted rule that vicinal, sp(3)-positioned protons in cyclopentene moieties should always have more positive (3)J NMR coupling constants for the cis than for the trans arrangement: Unrecognized exceptions might misguide one to wrong stereochemical assignments and thence to erroneous mechanistic conclusions. We show here that two structurally innocent-looking 2,3-dibromo-1,1-dimethylindanes violate the rule by means of their values of (3)J(cis) = 6.1 Hz and (3)J(trans) = 8.4 Hz. The stereo-selective formation of the trans diastereomer from 1,1-dimethylindene was improved with the tribromide anion (Br-3(-)) as the brominating agent in place of elemental bromine;the ensuing, regiospecific HBr elimination afforded 3-bromo-1,1-dimethylindene. The addition of elemental bromine to the latter compound, followed by thermal HBr elimination, furnished 2,3-dibromo-1,1-dimethylindene, whose Br/Li interchange reaction, precipitation, and subsequent protolysis yielded only 2-bromo-1,1-dimethylindene

Phonon Sidebands in Transition Metal Dichalcogenides

Excitons dominate the optical properties of monolayer transition metal dichalcogenides (TMDs). Besides optically accessible bright exciton states, TMDs exhibit also a multitude of optically forbidden dark excitons. Here, we show that efficient exciton-phonon scattering couples bright and dark states and gives rise to an asymmetric excitonic line shape. The observed asymmetry can be traced back to phonon-induced sidebands that are accompanied by a polaron redshift. We present a joint theory-experiment study investigating the microscopic origin of these sidebands in different TMD materials taking into account intra- and intervalley scattering channels opened by optical and acoustic phonons. The gained insights contribute to a better understanding of the optical fingerprint of these technologically promising nanomaterials

Ultrafast dynamics in monolayer TMDCs: the interplay of dark excitons, phonons and intervalley Coulomb exchange

Understanding the ultrafast coupling and relaxation mechanisms between valleys in transition metal dichalcogenide semiconductors is of crucial interest for future valleytronic devices. Recent ultrafast pump-probe experiments showed an unintuitive significant bleaching at the excitonic $B$ transition after optical excitation of the energetically lower excitonic $A$ transition. Here, we present a possible microscopic explanation for this surprising effect. It is based on the joint action of exchange coupling and phonon-mediated thermalization into dark exciton states and does not involve a population of the B exciton. Our work demonstrates how intra- and intervalley coupling on a femtosecond timescale governs the optical valley response of 2D semiconductors