Exfoliated semiconducting MoS and WS were covalently functionalized with 1,2-dithiolane-modified carbon nanodots (CNDs). The newly synthesized CND-MoS and CND-WS hybrids were characterized by spectroscopic, thermal, and electron microscopy imaging methods. Based on electronic absorption and fluorescence emission spectroscopy, modulation of the optoelectronic properties of TMDs by interfacing with CNDs was accomplished. Electrochemical studies revealed facile oxidation of MoS over WS in the examined hybrids, suggesting it to be better electron donor. Excited state events, investigated by femtosecond transient absorption spectroscopic studies, revealed ultrafast energy transfer from photoexcited CNDs to both MoS and WS. Interestingly, upon MoS photoexcitation, charge transfer from an exciton dissociation path of MoS to CNDs, within CND-MoS, was observed. However, such a process in CND-WS was found to be absent due to energetic reasons. The present study highlights the importance of TMD-derived donor-acceptor hybrids in light energy harvesting and optoelectronic applications. Furthermore, the fundamental information obtained from the current results will benefit design strategies and impact the development of additional TMD-based hybrid materials to efficiently manage and perform in electron-transfer processes.
A new set of donor–acceptor (D–A) conjugates capable of undergoing ultrafast electron transfer were synthesized using 4,4‐difluoro‐4‐bora‐3a,4a‐diaza‐s‐indacene (BODIPY)‐substituted phenothiazine, SM1–SM3, by a Pd‐catalyzed Sonogashira cross‐coupling reaction and a [2+2] cycloaddition–electrocyclic ring‐opening reaction. The incorporation of 1,1,4,4‐tetracyanobuta‐1,3‐diene (TCBD) and cyclohexa‐2,5‐diene‐1,4‐diylidene‐expanded TCBD (abbreviated as DCNQ=dicyanodiquinodimethane) in BODIPY‐substituted phenothiazine resulted in significant perturbation of the optical and electronic properties. The absorption spectrum of both SM2 and SM3 showed red shifted absorption as compared to SM1. Additionally, both SM2 and SM3 exhibited a distinct intramolecular charge‐transfer (ICT) transition in the near‐IR region more so for SM3. The electrochemical study revealed multi‐redox processes due to the presence of redox‐active phenothiazine, BODIPY, TCBD or DCNQ entities. Using data from spectral, electrochemical and computational studies, an energy‐level diagram was established to witness excited‐state electron‐transfer events. Finally, evidence of electron transfer and their kinetic information was secured from studies involving a femtosecond transient absorption technique. The time constants for excited‐state electron‐transfer events in the case of SM2 and SM3 were less than 5 ps revealing ultrafast processes.
The functionalization of MoS2 is of paramount importance for tailoring its properties towards optoelectronic applications and unlocking its full potential. Zinc phthalocyanine (ZnPc) carrying an 1,2‐dithiolane oxide linker was used to functionalize MoS2 at defect sites located at the edges. The structure of ZnPc‐MoS2 was fully assessed by complementary spectroscopic, thermal, and microscopy imaging techniques. An energy‐level diagram visualizing different photochemical events in ZnPc‐MoS2 was established and revealed a bidirectional electron transfer leading to a charge separated state ZnPc.+‐MoS2.−. Markedly, evidence of the charge transfer in the hybrid material was demonstrated using fluorescence spectroelectrochemistry. Systematic studies performed by femtosecond transient absorption revealed the involvement of excitons generated in MoS2 in promoting the charge transfer, while the transfer was also possible when ZnPc was excited, signifying their potential in light‐energy‐harvesting devices.
Usage of multimodular donor–acceptor
systems capable of
revealing tunable ground- and excited-state properties is gaining
momentous interest for applications in light energy harvesting and
optoelectronics. Here, we demonstrate conversion of a large-bandgap
donor–acceptor–donor (D–A–D) type system,
(triphenylamine–benzothiadizole–triphenylamine, TPA–BTD–TPA)
into low-bandgap, unsymmetrical, D–A′–A–D
and D–A′–A–A″–D type donor–acceptor
systems by the insertion of tetracyanobutadiene (A′)
or dicyanoquinodimethane (A″) by [2 + 2] cycloaddition–retro-electrocyclization
reactions. Because of the existence of strong charge transfer in the
ground and excited states, these low-bandgap unsymmetrical donor–acceptor
chromophores exhibit strong electronic absorption covering the visible
and near-IR regions. Electrochemical, spectroelectrochemical, and
computational studies are performed to evaluate their redox potentials
and spectral characterization of oxidized/reduced species as well
as to realize their electronic structures. Finally, the occurrence
of ultrafast charge separation in these conjugates has been established
from femtosecond transient absorption covering the visible–near-IR
regions in polar and nonpolar solventsproperties relevant
toward their optoelectronic applications.
The effect of acceptor strength on excited-state charge transfer (CT) and charge separation (CS) in central phenothiazine (PTZ)-derived symmetric 1 (PTZ-(TCBD-TPA) 2 ) and asymmetric 2 (PTZ-(TCBD/DCNQ-TPA) 2 ) push−pull conjugates, in which triphenylamine (TPA) acts as end capping and 1,1,4,4− tetracyanobuta−1,3−diene (TCBD) and cyclohexa−2,5−diene−1,4−ylidene−expanded TCBD (DCNQ) act as electron acceptor units, is reported. Due to strong push−pull effects, intramolecular CT was observed in the ground state, extending the absorption into the near-infrared region. Electrochemical, spectroelectrochemical, and computational studies coupled with energy-level calculations predicted both 1 and 2 to be efficient candidates for ultrafast CT. Subsequent femtosecond transient absorption studies along with global target analysis, performed in both polar and nonpolar solvents, confirmed such processes in which the CS was efficient in asymmetric 2, having both TCBD and DCNQ acceptors in polar benzonitrile, while in toluene, only CT was witnessed. This work highlights the significance of the number and strength of electron acceptor entities and the role of solvent polarity in multimodular push−pull systems to achieve ultrafast CS.
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