Ping‐Pong Energy Transfer in Covalently Linked Porphyrin‐MoS2 Architectures

Abstract Molybdenum disulfide nanosheets covalently modified with porphyrin were prepared and fully characterized. Neither the porphyrin absorption nor its fluorescence was notably affected by covalent linkage to MoS2. The use of transient absorption spectroscopy showed that a complex ping‐pong energy‐transfer mechanism, namely from the porphyrin to MoS2 and back to the porphyrin, operated. This study reveals the potential of transition‐metal dichalcogenides in photosensitization processes.

Abstract: Molybdenum disulfide nanosheets covalently modified with porphyrin were prepared and fully characterized. Neither the porphyrin absorption nor its fluorescence was notably affected by covalent linkage to MoS 2 . The use of transient absorption spectroscopy showed that a complex pingpong energy-transfer mechanism, namely from the porphyrin to MoS 2 and back to the porphyrin, operated. This study reveals the potential of transition-metal dichalcogenides in photosensitization processes.
Molybdenum disulfide (MoS 2 ) is a typical example of a layered transition-metal dichalcogenide. In general, the structure of such dichalcogenides is analogous to that of graphene, with an atomic layer of a transition metal sandwiched between two layers of chalcogen atoms. More specifically, each Mo atom is bound to six S atoms and forms a three-atom-thick monolayer. [1] Exfoliated MoS 2 materials have received enormous attention in recent years because of their extraordinary optoelectronic and electrocatalytic properties, especially in the area of energy applications and catalysis. [2,3] Diverse exfoliation strategies have been developed for MoS 2 . Most notable is the employment of a) strong intercalants such as Li + , which result in phase transfer to an octahedral metallic 1T polytype, [4] and b) nonoxidative Brønsted acids such as chlorosulfonic acid, which offer the benefit of retaining the 2H semiconducting phase. [5] Additional efforts have been placed on developing strategies for chemically modifying transition-metal dichalcogenides. [6] The aim has been to fully harness the properties of, for example, MoS 2 . In fact, incorporation of molecular dopants results in fine-tuning of the electronic as well as optical properties of MoS 2 which, in turn, enables broadening the spectrum of its applications. In MoS 2 , the S atoms in the basal plane are, however, rather inert as a result of saturation. Furthermore, Mo atoms are placed between S layers and, thereby have challenging chemical reactivity. However, the covalent functionalization of MoS 2 has been accomplished. [7] For example, the basal plane of the 1T-MoS 2 polytype has been reacted with organoiodides, [8] while reaction with diazonium salts has been reported for the basal plane modification of 1T [9] as well as 2H-MoS 2 . [10] Herein we adopt the recently developed method based on the addition of 1,2-dithiolanes to S-vacant sites located at the edges of exfoliated 2H-MoS 2 nanosheets. [11] A true advantage of this approach is that diverse 1,2-dithiolanes are easily prepared and they react with exfoliated MoS 2 to yield interesting hybrids. Pyrene and phthalocyanines have, for example, been coupled with MoS 2 . [11] Alternatively, ammonium moieties have been grafted onto MoS 2 and WS 2 , which facilitated the electrostatic association of carbon dots [12] and anionic porphyrins. [13] In the resulting ensembles, excited-state electronic interactions between the components were responsible for quenching the fluorescence from the pyrene, carbon dots, or porphyrin. The major drawbacks of electrostatically associated ensembles are their lack of stability and processability in organic solvents. They also have moderate binding strengths, especially when compared to the robust and strong bonding in covalently linked conjugates. Therefore, it is not only imperative, but also timely, to explore covalent modification of MoS 2 with photoactive porphyrins.
Herein, we report on the modification of the edges of exfoliated MoS 2 with a 1,2-dithiolane derivative 1 featuring a porphyrin (H 2 P). The newly prepared H 2 P-MoS 2 2 was comprehensively characterized by spectroscopic, thermal, and microscopy means. We also gathered insight into the electronic interactions between the porphyrin and MoS 2 on the femto-to nanosecond timescales upon photoexcitation.
Evidence for the success in the covalent modification of MoS 2 with H 2 P came from vibrational spectroscopy. Attenuated total reflectance infrared (ATR-IR) measurements on H 2 P-MoS 2 2 (Figure 2 a) revealed the presence of a) C À H stretching vibrations corresponding to the alkyl chain, which connects H 2 P with MoS 2 , at 2848 and 2950 cm À1 , b) an amide carbonyl vibration at 1660 cm À1 , and c) an aromatic C=C bending at 1590 cm À1 . We further validated the origin of the IR bands related to H 2 P within the hybrid by performing a reference experiment in which a mixture of tetraphenylporphyrin and exfoliated MoS 2 was processed under the same experimental conditions as those for obtaining 2. The presence of physisorbed tetraphenylporphyrin was not detected in the UV/Vis and emission spectra of the reference material, and no contribution in the mass loss detected by TGA was observed ( Figures S2-S4).
Additional structural information regarding 2 came from Raman spectroscopy. Excitation at l = 514 nm showed the existence of the C=C and C=N bonds of H 2 P in the 1000-1600 cm À1 region ( Figure S5). Characteristic modes corresponding to MoS 2 were observed in the lower wavenumber region, especially upon excitation at l = 633 nm. These modes become visible as a result of coupling with the A1 excitonic transition, which in turn produces resonance Raman enhancement of the first and second order vibrational modes. Exfoliated semiconducting MoS 2 gave rise to bands corresponding to 2LA(M) at 447 cm À1 , A 1g at 406 cm À1 , and E 1 2g at 382 cm À1 (Figure 2 b). The frequency difference between A 1g  and E 1 2g was 24 cm À1 , from which we conclude the existence of few-layered MoS 2 . [14,15] The 2LA(M) vibration relates to the S-vacancies. [16] By comparing the Raman spectrum of exfoliated MoS 2 with that of 2, the intensity ratio of A 1g compared to 2LA(M) was found to be considerably higher for 2, that is, 3.6 versus 1.9 for exfoliated MoS 2 . Complementary Raman mapping assays revealed an increased I A 1g /I 2LA(M) intensity ratio for 2 compared to that of exfoliated MoS 2 (Figure 2 c,d). This observation is in sharp contrast to the recorded I A 1g /I 2LA(M) maps for the reference material prepared by physisorption ( Figure S6). Overall, this is indicative of a "healing" of the S-vacancies by the 1,2-dithiolane species in 2.
[9] Likewise, the absence of features at 150, 225, and 325 cm À1 , indicative of the metallic polytype, highlights the semiconducting nature of MoS 2 in 2.
Thermogravimetric analysis (TGA) provided information related to the degree of MoS 2 functionalization in 2. Exfoliated MoS 2 was found to be thermally stable under N 2 from 100 to 800 8C. Notably, the mass loss observed in the modified MoS 2 is directly related to the thermal decomposition of the organic addends. Hence, the mass loss of 9 % for 2 ( Figure 2 e), which occurred up to 520 8C, is ascribed to the porphyrin. Based on this mass loss, a loading of one H 2 P moiety per 54 units of MoS 2 was calculated. This is in good agreement with values reported for similar functionalizations of MoS 2 with pyrene and zinc phthalocyanine. [11] Spatially resolved electron energy loss spectroscopy (SR-EELS) using a scanning transmission electron microscope (STEM) enables the morphology and chemical composition of complex hybrid nanostructures to be investigated at the local scale. [17] Thus, we performed SR-EELS STEM analyses of 2. Figure 3 shows an EEL spectrum line (SPLI) of a functionalized MoS 2 flake (see also Figure S7). Figure 3 a corresponds to the high angular annular dark field (HAADF) STEM image of the flake and the SPLI was acquired along the highlighted green line. Typical EEL spectra are displayed in Figure 3 b. Each of them corresponds to the sum of four EEL spectra extracted from the two areas marked in the SPLI (red (i) and green (ii) lines, respectively). S-L 2,3 , Mo-M 4,5 , C-K, and Mo-M 2,3 edges are observed in both spectra. The energyloss near-edge structure (ELNES) investigations of the C-K edge are displayed in the right panel of Figure S7. Two signatures at 291.2 and 293.4 eV can be observed in the s* region (ca. 290-325 eV) for 2. Such signals are not discerned in the spectra of other carbon-containing materials employed as references. Some differences can also be observed in the main contribution to the p* signal at % 284-290 eV. Here, the C-K edge is dominated by an aromatic C contribution (ca. 285.5 eV [17b,c, 18] ), which is less pronounced in the case of H 2 P. Furthermore, another feature at about 287.2 eV is clearly seen in the spectra of 2. This signal can be attributed to pyrrolic (CÀN) contributions of H 2 P. [18c] Importantly, the N-K edge cannot be observed in H 2 P because the Mo-M 2,3 edge is in the same energy range. For all these reasons, the features highlighted in the C-K edge of 2 correspond to a porphyrin contribution. Furthermore, we conclude from the EELS analyses that H 2 P is homogeneously distributed across the surface of the MoS 2 flakes in 2 ( Figure S8).
Next, focusing on the optical properties of 2, the UV/Vis spectrum was recorded in DMF and compared with that of 1. In particular, the UV/Vis spectrum of 1 showed a sharp absorption at l = 418 nm that corresponds to the Soret band, accompanied by weaker Q-band absorptions at l = 516, 550, 596, and 648 nm (Figure 4, left). Spectroscopic fingerprints at l = 401, 494 (C-exciton), 626 (B-exciton), and 681 nm (Aexciton) are discernable in the UV/Vis spectrum of exfoliated MoS 2 (Figure 4, left). The UV/Vis spectrum of 2 is best described as a superimposition of the individual spectra of 1 and exfoliated MoS 2 . The spectrum is, for example, dominated by the characteristic Soret-band absorption of H 2 P at l = 418 nm in addition to the MoS 2 -related transition at l = 679 nm. From this similarity, we deduce that the ground-state lacks appreciable electronic interactions.
Excited-state electronic interactions could also be used to probe interactions between the H 2 P and MoS 2 in 2. Specifically, strong fluorescence, centered at l = 651 and 715 nm (Figure 4, right), was found upon excitation of 1 at the Soretband absorption at l = 420 nm. A fluorescent lifetime of 10.3 ns ( Figure S9) was determined by time-correlated single photon counting (TCSPC) measurements on 1 with DMF as the solvent. Photoexcitation of 2 at l = 420 nm, however, resulted in no significant quenching of the porphyrin-centered fluorescence relative to that of 1 (Figure 4, right). For 2, the fluorescence maxima were observed at l = 649 and 714 nm, and the fluorescence lifetime was 10.3 ns ( Figure S9).
As a complement to the aforementioned method, we probed 1, exfoliated MoS 2 , and 2 by means of femtosecond transient absorption spectroscopy (fsTAS) using an excitation wavelength of 420 nm ( Figure 5, see also Figures S10 and S11). For 1, bleaching of the ground state at l = 517 and 653 nm is observed early in the experiments. In addition, positive absorptions evolved at l = 442, 538, 574, 623, and 690 nm ( Figure S10). Global analysis revealed the presence of three exponential decays on the fsTAS timescale. These three decays are followed by a much slower decay, whose dynamics are outside the timescale of our experiments. Therefore, a target model was applied to fit the transient absorption data (Figure 6, right). This involves an initial population of a second singlet excited state [S 2 (H 2 P)], consistent with excitation into the Soret-band absorption, which internally converts into the first singlet excited state [S 1 (H 2 P)] within 16 ps. The latter undergoes intersystem crossing within 10 ns to the triplet state [T 1 (H 2 P)], which, in turn, decays back to the ground state on the microsecond timescale. Additionally, aggregates of 1 are present in the sample and they were found to decay to the ground state within 142 ps. These dynamics resemble very closely those already reported for a variety of porphyrins. [19,20] For exfoliated MoS 2 , ground-state bleaching evolved at l = 476, 623, and 683 nm together with positive absorptions at l = 591, 655, and 743 nm ( Figure S11). Global analysis of the data revealed five exponential decays after the absorption of light. The first three decays, which take place within 2.7, 7.3, and 250 ps, involve biexciton [be(MoS 2 )] and trion [tr(MoS 2 )] formation as well as the decay of these many-body particles into single excitons [se(MoS 2 )]. Single excitons then diffuse within 1.8 ns across the layers in the z direction [de(MoS 2 )], before they recombine to recover the ground state within 21.2 ns (Figure 6, left). These dynamics resemble those already reported for exfoliated MoS 2 . [21] For 2, excited states of both components are observed to participate in the decay cascade ( Figure 5). Selective excitation into the Soret-band absorption of 2 affords a photoinduced absorption at l = 450 nm and ground-state bleaching of the bands at l = 617 and 679 nm. Please note that, while the first absorption belongs to the porphyrin, the last two are fingerprints of MoS 2 . Considering that porphyrin absorptions in 2 are around 20 times stronger than those of MoS 2 at the excitation wavelength (Figure 4, left), the similar intensities of the differential features at l = 450 and 679 nm imply an ultrafast energy transfer (< 1 ps) from S 2 , which is populated upon excitation at l = 420 nm, to MoS 2 . However, since the energy transfer is faster than the time resolution of our instrument, we are unable to measure the population of the S 2 (H 2 P). Global analysis showed four exponential decays on the fsTAS timescale and an additional decay, which is,   however, outside of our detection range. Our interpretation is based on a kinetic target model, which is depicted in the middle panel of Figure 6. This considers an initial excited state population governed by a porphyrin-centered S 2 , with a minor contribution from a MoS 2 -centered excited state. A nonquantitative ultrafast energy transfer from H 2 P to MoS 2 then occurs, which results in comparable populations of the H 2 P-centered (70 %) and MoS 2 -centered (30 %) excited states. Afterwards, these three species, which govern the deactivation of exfoliated MoS 2 and which result in the formation of single excitons, are discernible. The times for the interconversions are 2.5, 7.5, and 204 ps. Once populated, the single exciton of MoS 2 is subject to a second energy transfer. This time, however, single excitons are transferred from the MoS 2 to the S 1 state of H 2 P. This energy transfer takes 1.75 ns. From here, the porphyrin decay proceeds through intersystem crossing to the corresponding triplet excited state within 14 ns, and recovery of the ground state on the microsecond timescale. This mechanism is also based on the energetics of 2. The S 1 state of 1, with its fluorescence at 1.73 eV (l = 715 nm) and the MoS 2 fluorescence at 1.82 eV, [22] render the second energy transfer a downhill process. This ping-pong energytransfer model enables us to rationalize the nearly identical steady-state photophysical characteristics of 1 and 2 upon photoexcitation.
In summary, we accomplished the covalent grafting of porphyrin 1 onto exfoliated MoS 2 to afford novel H 2 P-MoS 2 2, which was fully characterized. Thorough photophysical investigations based on steady-state and time-resolved measurements corroborate that the decay of the photoexcited porphyrin involves a ping-pong energy transfer to and from MoS 2 . Our findings suggest that transition-metal dichalcogenides have great potential in photosensitization. Such hybrid materials may be useful in energy-conversion applications.