Herein, we have fabricated self-assembled semiconducting organic nanomaterials with various morphologies (1Dfiber, 2D-flakes, and 2D-nanosheets) made of small conjugated oligomer 2,2′:5′,2″:5″,2‴-quaterthiophene (α-QTH) by a simple solution-based coprecipitation method. By simply varying the good-solvent-to-bad-solvent ratio, we can critically tune the selfassembly process and eventually can control the intermolecular interactions of the constituent molecules in these self-assembled nanostructures. Different types of self-assembled nanostructures have been utilized for photocatalytic solar H 2 production. The H 2 production efficiencies directly depend on the morphology of selfassembledselfassembled nanomaterials as well as intermolecular interactions of QTH molecules. The overall photocatalytic properties are further correlated with the ongoing photophysical properties by means of detailed steady-state and time-resolved fluorescence spectroscopy and dimer-based time dependent-density functional theory (TD-DFT) calculations. Furthermore, femtosecond transient absorption spectroscopy has been utilized to explore the detailed photoinduced exciton dynamics by global analysis of spectrally resolved pump−probe traces. In addition to that, the overall photocatalytic activities are further supported by an in-depth electrochemical study. Finally, a boost in photocatalytic H 2 production has been observed by using 4-methylbenzyl alcohol (4-MBA) as a specific hole scavenger for the completion of the redox cycle. Therefore, the present system can be utilized for simultaneous solar H 2 production and the specific organic transformation through a green and cost-efficient approach.
Herein, we have synthesized three different types of carbon dots, i.e., (a) CDs from citric acid, (b) nitrogen atom functionalized CDs from citric acid and ammonia, and (c) phosphorus atom functionalized CDs from citric acid and sodium dihydrogen orthophosphate through a simple bottom-up carbonization technique. Detailed morphological and elemental features are investigated by high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy study, and it is further correlated to the ongoing photophysical properties. To investigate the specific role of the heteroatom functionalities on the room temperature phosphorescence, we have incorporated all these three types of CDs in a boric acid matrix to diminish the flexibility of surface functional groups and decrease the nonradiative relaxation processes. Various heteroatom functionalities play a very specific role to tune the afterglow properties by altering the energy gap (ΔE ST ) between the lowest excited singlet (S 1 ) and triplet state (T 1 ) and the spin−orbit coupling constant which eventually control the radiative recombination from the triplet state. Finally, a switchable fluorescence and room temperature phosphorescence have been observed depending on the specific heteroatom functionalities. A detailed temperature-dependent study has been performed to investigate the tunability between prompt fluorescence and phosphorescence properties. This is further correlated to the conversion of phosphorescence with thermally activated delayed fluorescence (TADF). Computational studies based on time-dependent density functional theory (TD-DFT) have been performed by using optimized model systems in connection to the elemental study, which nicely support our experimental findings.
Structure−property relationships of different conformers of an organic D−A−D triad are explored to rationalize the structural motif toward photoluminescence activity. In a recent experiment (Chem. Sci. 2017, 8, 2677−2686, Takeda and coworkers revealed that the PTZ-DBPHZ-PTZ (D−A−D) triad exhibited multicolor luminescence properties and thermally activated delayed fluorescence (TADF) emission. We computationally studied the photophysical properties of the conformers of that D−A−D triad to provide a detailed description of the luminescence activity. Our analysis confirms that the twisting of the axial phenothiazine (PTZ) unit to an equatorial position altered the nature of the S 1 state from local to a charge transfer state and was responsible for the large red shift in emission (S 1 ) energy. Calculated fluorescence and intersystem crossing (ISC) rate constants suggest that the prompt fluorescence is turned on for axial−axial conformers while it is turned off for others. Fast reverse intersystem crossing (RISC) from triplet CT to the S 1 state ( 3 CT 1 → 1 CT 1 ), close spacing and effective crossing between 3 LE 1A , 3 CT 1 and 1 CT 1 states cause efficient harvesting of triplet excitons to S 1 state, thus enabling TADF emission for equatorial−equatorial conformer.
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