Light-driven C-H activation mediated by 2D transition metal dichalcogenides

C-H bond activation enables the facile synthesis of new chemicals. While C-H activation in short-chain alkanes has been widely investigated, it remains largely unexplored for long-chain organic molecules. Here, we report light-driven C-H activation in complex organic materials mediated by 2D transition metal dichalcogenides (TMDCs) and the resultant solid-state synthesis of luminescent carbon dots in a spatially-resolved fashion. We unravel the efficient H adsorption and a lowered energy barrier of C-C coupling mediated by 2D TMDCs to promote C-H activation. Our results shed light on 2D materials for C-H activation in organic compounds for applications in organic chemistry, environmental remediation, and photonic materials.


Main text
The emergence of C-H bond activation has provided revolutionary opportunities in organic chemistry, materials science, and biomedical engineering 1 .Specifically, the activation and functionalization of the ubiquitous C-H bonds enable new synthetic routes for functional molecules in a more straightforward and atom-economical way [2][3][4][5] .Since C-H bonds are thermodynamically strong and kinetically inert 6 , many catalysts have been developed for C-H activation, including transition metals (e.g., palladium 7 , cobalt 8 , and gold 9,10 ), zeolites 11,12 , and metal-organic frameworks 13,14 .
While intensive research efforts have been focused on C-H bonds in short-chain alkanes (e.g., methane and ethane) 15,16 and aromatic compounds 17 , C-H activation in long-chain organic molecules is rarely reported.Yet, the derivation of C-H bonds in these complex molecules has significant potential in synthesizing functional organic complexes and transforming environmental pollutants (e.g., fossilresource-derived hydrocarbons) into more valuable chemicals 18,19 .
Herein, we report the light-driven C-H activation in long-chain molecules mediated by twodimensional (2D) transition metal dichalcogenides (TMDCs).This TMDC-mediated C-H activation in organic molecules enables optical synthesis and patterning of luminescent carbon dots on solid substrates.
As a first example, we achieve the light-driven transformation of cetyltrimethylammonium chloride (CTAC, C19H42ClN), a long-chain quaternary ammonium surfactant 20 , into luminescent carbon dots (CDs) on WSe2 monolayers.By coupling experiments with density functional theory (DFT) calculations, we unravel the role of Se vacancies and oxidized states of WSe2 in promoting the H adsorption.We further show that 2D TMDCs can facilitate the C-C coupling with a lowered energy barrier to catalyze C-H activation in complex organic molecules.This type of light-driven reaction mediated by 2D materials can be generalized to other long-chain organic compounds for the broader impacts on organic synthesis, chemical degradation, and photonics.A typical experimental configuration is presented in Fig. 1a.A thin layer of solid CTAC is coated on a monolayer WSe2 grown by chemical vapor deposition (CVD).The monolayer feature of WSe2 is confirmed by the strong photoluminescence (PL) peak at ~ 750 nm (Fig. 2b, blue curve).Under the irradiation of a low-power continuous-wave laser (~ 0.2-5 mW), CTAC molecules undergo WSe2mediated C-H bond activation and the subsequent C=C bond formation (Fig. 1b).CTAC contains long carbon chains and quaternary ammonium cations, which has been commonly used as surfactants for chemical synthesis and fabric softeners 21 .Here, we choose CTAC as a first example due to its clean carbon-chain structure, solid form under ambient conditions, and wide existence in nanomaterials systems.This light-driven reaction can also be applied to other organic compounds.
The laser irradiation on hybrid CTAC/Wse2 thin films leads to the emergence of bright luminescence from CDs (Fig. 2a).The evidence of CDs formation and materials characterizations are presented in Fig. 3.The optically generated CDs show pronounced broadband PL emission centered at ~ 600 nm under the excitation of a 532 nm laser (Fig. 2b, red curve).Additionally, the PL peak from WSe2 exhibits a clear redshift from ~750 nm to ~780 nm, resulting from the charge transfer between the CDs and WSe2 22,23 .
Due to the negligible light absorption of CTAC and monolayer WSe2 (Supplementary Fig. 1), we preclude the contribution of photothermal effects.Instead, this light-driven reaction is ascribed to the WSe2catalyzed C-H activation, which will be discussed later.
The photochemical reaction rate for the synthesis of CDs can be tuned by two orders of magnitude by controlling the laser power (Fig. 2c and Supplementary Movie 1).Under low-power laser irradiation, the emission of synthesized CDs remains stable for more than 20 min (Supplementary Fig. 2).Besides WSe2, we also demonstrate the light-driven C-H activation and generation of CDs from CTAC on CVD-grown WS2 and MoS2 monolayers (Fig. 2d,e).Similar orangish PL emission from CDs can be directly visualized in optical imaging (Inset in Fig. 2d).The PL spectra of MoS2/WS2 + CDs also showed similar features, including a broadband emission from CDs centered at ~ 600 nm and a redshifted peak from MoS2/WS2.
In addition, under the 660 nm laser excitation, the PL spectra from the WSe2/WS2 + CDs are distinct from those under the 532 nm excitation (Fig. 2f).This excitation wavelength-dependent PL emission is a characteristic feature of CDs 24,25 .The light-driven, 2D TMDC-mediated synthesis of CDs is confirmed by multiple characterization techniques.The Raman spectrum shows a D band at ~1380 cm -1 and a G band at ~1600 cm -1 (Fig. 3a), which are signatures of CDs 26 .The scanning electron microscope (SEM) images also reveal the existence of CD nanoparticles at the laser-irradiated areas (Fig. 3b,c).The as-synthesized CDs have a size distribution of 5-15 nm, as shown in the transmission electron microscope (TEM) images (Fig. 3e,f).The selected-area electron diffraction pattern exhibits bright diffraction spots and amorphous rings (Inset in Fig. 3f), indicating a semi-crystalline structure of CDs.The chemical composition of CDs is further examined by a near-field nanoscale Fourier transform infrared spectroscopy (nano-FTIR).Compared to the pristine CTAC film, the nano-FTIR spectrum of CDs presents a prominent absorption band at ~ 1660 cm -1 (Fig. 3d), which is assigned to the vibrations of C=C bonds in CDs 27 .Next, we discuss the underlying mechanisms of the light-driven C-H bond activation medicated by 2D materials.C-H activation requires a sufficiently negative hydrogen adsorption free energy 28 , however, pristine 2D TMDCs usually cannot meet this prerequisite since they are known to be facile hydrogen evolution materials 29 .To identify the potential active sites in our study that drive the C-H bond activation, we first measured the X-ray photoelectron spectroscopy spectra of the monolayer WSe2.The results indicate the existence of prevalent Se vacancies and O adsorption on the CVD-grown WSe2 surfaces (Supplementary Fig. 3) 30,31 .To analyze the role of Se vacancies and O substitution on WSe2, we calculated the projected density of states (PDOS) of local W-sites using DFT calculations (Fig. 4a and Supplementary Fig. 4).With the increasing number of Se vacancies, there is an obvious shift of the peak toward the Fermi level (Fig. 4b).The calculated average energies of the d-electrons (i.e., the d-band center) of the sites with Se vacancies are also closer to the Fermi level compared to a pristine WSe2.According to the d-band center theory 32 , a surface site with a d-band center closer to the Fermi level corresponds to a significantly stronger H adsorption capacity 33 , which facilitates the C-H bond activation due to the stronger driving force to "pull" a H down to the surface 34 .Similar conclusions can be found on a WSe2 surface with oxygen substitution at Se sites (Fig. 4c).Meanwhile, the existence of adsorbed oxygen and the subsequently formed hydroxyl can act as the promoters to expedite C-H activation due to a facile O/HO-promoted mechanism [35][36][37][38] .To verify the theoretical hypothesis, we conducted control experiments on mechanically exfoliated WSe2 flakes with fewer surface defects 39 , and the results show that a much higher optical power is required for this reaction to occur (Supplementary Fig. 5).These theoretical analyses and experiments indicate that the Se vacancy and O substitution in WSe2 can both lead to a more facile C-H activation capacity due to either higher reactivity of a defected surface or an O-promotion effect.
For long carbon chains, the C-H activation is followed by the formation of C=C bonds 40 .We further investigate the capability of 2D TMDCs to drive the C=C formation.We analyze the C-C coupling on material surfaces (Fig. 4d), where two carbon atoms are bonded together.We compare the calculated kinetic energy barriers of this process for WSe2 and other common catalyst surfaces for C-H activation (Supplementary Fig. 6), including gold (Au) and palladium (Pd).The energy barrier of C-C coupling on WSe2 surfaces is calculated to be 0.29 eV (Fig. 4e), which is significantly lower than Au (0.57 eV) and Pd (1.29 eV).These results indicate that while metal catalysts (e.g., Pd and Au) are suitable for C-H activation in short-chain molecules, they cannot be generalized to long carbon chains due to the high activation energy of C-C coupling to form C=C bonds.This energy barrier is further reduced to 0.23 eV on WSe2 surfaces with Se vacancies (Fig. 4e and Supplementary Fig. 7).These results demonstrate the potential of 2D WSe2 as promising catalysts to drive the C-H activation of long-chain molecules and facilitate the subsequent C=C formation.In summary, we discover the 2D-TMDC-mediated C-H activation in long-chain organic molecules under light illumination.Our experimental characterizations coupled with theoretical calculations reveal the role of defects and oxidized states on TMDCs in the promotion of H adsorption and C-H activation reactions.Moreover, we find that the energy barrier of C-C coupling mediated by 2D TMDCs is much lower than the commonly used metal catalysts for C-H activation of short-chain alkanes, highlighting its promising performance of C-H activation for complex molecules.This light-controlled site-specific C-H activation also enables optical printing of luminescent carbon dots on solid substrates and provides an approach towards data encryption and information technology 41 .
By controlling the thickness of CTAC layer, laser power, and irradiation time, we can write CDs by laser scanning without changing the morphology of the film (Supplementary Fig. 8).Thus, the embedded patterns remain hidden under white light illumination and can be read out by fluorescence, Raman, or PL imaging (Supplementary Fig. 9).Besides CTAC, this strategy is general to other long-chain molecules, such as octyltrimethylammonium chloride and polyvinyl alcohol (Supplementary Fig. 10).We envision that the 2D-TMDC-mediated light-driven C-H activation in complex organic molecules will enable new applications in chemical synthesis, photonics, degradation of organic pollutants, and plastic recycling.

Fig. 1 .
Fig. 1.General concept of light-driven C-H activation in long-chain molecules mediated by 2D materials.a, Schematic showing the light-driven transformation of CTAC on an atomic layer of WSe2 into luminescent CDs.b,

Fig. 2 .
Fig. 2. Optical characterizations of 2D-mediated C-H activation and CD synthesis.a, Optical images showing the CTAC on the WSe2 sample under a 532 nm laser irradiation at t = 0 s and t = 10 s.The laser power is 2.5 mW.The yellowish PL emission comes from the optically synthesized CDs.b, The PL spectra of WSe2 and WSe2 + CDs hybrids.c, Time-resolved PL intensity of CDs at 600 nm from the CTAC on WSe2 sample under a 532 nm laser irradiation with different optical power.d,e, The PL spectra of (d) WS2 and WS2 + CDs hybrids and (e) MoS2 and MoS2 + CDs hybrids under the excitation of a 532 nm laser.Inset in (d): optical image showing the PL emission from the WS2 + CDs sample.f, The PL spectra of WSe2/WS2 + CDs samples excited by a 660 nm laser.

Fig. 3 .
Fig. 3. Materials characterizations of optically synthesized CDs.a, Raman spectra of WSe2 and WSe2 + CDs hybrids.b,c, SEM images of the synthesized CDs.d, Near-field nano-FTIR spectra of the CDs and pristine CTAC films.e,f, High-resolution TEM images of the synthesized CDs.Inset in (f) shows the selected area electron diffraction (SAED) pattern of the CDs.

Fig. 4 .
Fig. 4. First-principles calculations to provide insights into the light-driven C-H activation mediated by 2D materials.a, Optimized structures considered for DFT calculations.Pristine WSe2 and WSe2-x with Se vacancies or O substitutions are considered.b,c, PDOS of the d-electrons of local W-sites (red triangles in c) at pristine WSe2 and WSe2-x with Se vacancies (b) or O substitutions (c).The calculated d-band centers are marked with vertical lines.The Fermi levels are shifted to zero.d, The process of C-C coupling considered for DFT calculations on the WSe2 surface.e, Comparison of the kinetic barriers of C-C coupling on the WSe2 and other surfaces.