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We present a mechanistic study of pH-sensitive photoluminescence (PL) in two deliberately designed systems of carbon dots (CDs), which are relatively poor and rich in carboxyl groups anchored on their surfaces, denoted CDs-COOH(p) and CDs-COOH(r), respectively. The underlying PL mechanisms for the two contrasting CD systems are revealed to be different. As for CDs-COOH(p), the pH response of PL exhibits an asymmetric volcano-shaped pattern featuring dynamic and static quenching under acidic and alkaline conditions, dominated by the effects of hydrogen bonding and non-emissive ground-state complex, respectively. As for CDs-COOH(r), however, the pH response exhibits an interesting sigmoid-shaped pattern featuring PL quenching under acidic conditions but PL enhancement under alkaline conditions, both of which become more pronounced with increasing photoexcitation energy, exhibiting a nearly symmetric trumpet-shaped pattern. Such patterns of PL response to acidity/alkalinity and photoexcitation energy can be understood in terms of the prominent effect of excited-state proton transfer that is coupled to the surface emissive centers of the carboxyl group and can be effectively modulated via pH-regulated protonation/deprotonation. Our comparative analyses of the pH-regulated surface-sensitive PL quenching/enhancement behaviors in the two CD systems allow for elucidating the different surface-state-controlled PL mechanisms, highlighting the specific role of carboxyl groups in the pH-sensitive PL of CDs. The mechanistic insights gleaned from this work would be useful for CDs-based applications such as luminescence, sensing, and bioimaging. [ABSTRACT FROM AUTHOR]

Singlet fission (SF) is a spin-allowed exciton multiplication process, in which a photogenerated singlet separates efficiently into two free triplets. Herein, we report an experimental study on the solution-phase intermolecular SF (xSF) in a prototype radical dianion system of PTCDA2−, which is produced from its neutral precursor PTCDA (i.e., perylenetetracarboxylic dianhydride) via a two-step consecutive photoinduced electron transfer mechanism. Our ultrafast spectroscopic results enable a comprehensive mapping of the elementary steps involved in the solution-phase xSF process of photoexcited PTCDA2−. Along the cascading xSF pathways, the three intermediates including excimer 1(S1S0), spin-correlated triplet pair 1(T1T1), and spatially separated triplet pair 1(T1·S0·T1) have been identified, with their formation/relaxation time constants being determined. This work demonstrates that the solution-phase xSF materials can be extended to charged radical systems and that the three-step model usually adopted to describe the crystalline-phase xSF can also be valid in describing solution-phase xSF. [ABSTRACT FROM AUTHOR]

Organic-cation engineering has recently proven effective in flexibly regulating two-dimensional hybrid organic–inorganic perovskites (2D HOIPs) to achieve a diversity of newly emerging applications. There have been many mechanistic studies based on the structural tunability of organic cations; nevertheless, those with an emphasis on the effect solely caused by the organic cations remain lacking. To this end, here we deliberately design a set of 2D HOIPs in which the inorganic layers are kept nearly intact upon cation modification, i.e., the precursor phenethylammonium lead iodide and its four derivatives with the phenyl group's para-position H being replaced by CH3, F, Cl, and Br. By means of femtosecond time-resolved transient absorption spectroscopy and temperature-dependent/time-resolved photoluminescence spectroscopy, we interrogate the subtle impact of cation modification on phonon dynamics, coherent phonon modes, phonon-dressed exciton dynamics, and excitonic emissions. A concerted trend for phonon lifetimes and exciton relaxation lifetimes regulated by cation modification is revealed, evidencing the existence of strong exciton–phonon coupling in this 2D HOIP system. The observed mass effect can be ascribed to the change in moment of inertia of organic cations. In addition, we observe an interesting interplay of exciton kinetics pertinent to population transfers between two emissive states, likely linked to the subtle variation in crystal symmetry induced by cation modification. The mechanistic insights gained from this work would be of value for the 2D HOIPs-based applications. [ABSTRACT FROM AUTHOR]

The integration of confined exciton and localized surface plasmon in a hybrid nanostructure has recently stimulated extensive interests. The mechanistic insights into the elusive exciton–plasmon interplay at the nanoscale are of both fundamental and applicable values. Herein, by taking a hybrid WSe2/CuFeS2 system as a prototype, in which the excitonic semiconductor WSe2 nanosheets are interfaced with the plasmonic semiconductor CuFeS2 nanocrystals to form a heterostructure, we design and perform an ultrafast dynamics study to glean information in this regard. Specifically, the band-alignment relationship between the two components enables the contrasting case studies in which the excitonic excited states of WSe2 are pre-selected to be on-/off-resonant with the plasmon band of CuFeS2. As revealed by the joint observations from steady-state absorption and photoexcitation-dependent/temperature-dependent femtosecond time-resolved transient absorption (fs-TA) spectroscopy, an effective energy transfer process occurs in this exciton–plasmon system where the energy donor (acceptor) is the excitonic WSe2 (plasmonic CuFeS2) and its efficiency is modulated by the exciton–plasmon coupling strength. Furthermore, as inferred from the temperature-dependent fs-TA analysis, the opening of such an energy-transfer channel turns out to take place during the early phase of plasmon decay (∼1 ps). In addition, the activation energy of energy transfer for a specific exciton-state-selected case is estimated (∼200 meV). This work provides a dynamics perspective to the plasmon semiconductor-involved exciton–plasmon interplay that features excited-state selectivity of exciton band and, hence, would be of guiding value for rational design and optimization of relevant applications based on exciton–plasmon manipulation. [ABSTRACT FROM AUTHOR]

Inspired by its great success in the photovoltaic field, methylammonium lead iodide perovskite (MAPbI3) has recently been actively explored as photocatalysts in H2 evolution reactions. However, the practical application of MAPbI3 photocatalysts remains hampered by the intrinsically fast trapping and recombination of photogenerated charges. Herein, we propose a novel strategy of regulating the distribution of defective areas to promote charge‐transfer dynamics of MAPbI3 photocatalysts. By deliberately designing and synthesizing the MAPbI3 photocatalysts featuring a unique continuation of defective areas, we demonstrate that such a feature enables retardation of charge trapping and recombination via lengthening the charge‐transfer distance. As an outcome, such MAPbI3 photocatalysts turn out to achieve an impressive photocatalytic H2 evolution rate as high as 0.64 mmol ⋅ g−1 ⋅ h−1, one order of magnitude higher than that of the conventional MAPbI3 photocatalysts. This work establishes a new paradigm for controlling charge‐transfer dynamics in photocatalysis. [ABSTRACT FROM AUTHOR]

Photoinduced intermolecular proton-transfer processes from N,N-dimethyl-3-arylpropan-1-ammonium chloride salts (ArCl, with aryl as 1-pyrenyl, 9-anthryl, and 2-naphtyl) to a solvent molecule have been investigated by steady-state and dynamic spectroscopic methods. The intermolecular proton-transfers are coupled either to the formation of an exciplex or to a solvent-separated ion pair in what we have termed a ‘proton-coupled charge-transfer reaction’. A range of solvents has been observed to mediate both the ground-state conformations of the ArCl and the extent of electron transfer. Unlike typical photoacids, in which through-bond interactions control photoacidity, through-space charge-transfer interactions are responsible in the excited singlet states of the ArCl. Transient absorption experiments reveal a range of electronic comportments in the excited-states of the ArCl. Excited-state pKa values of −3.4, 1.3, and −3.3 in THF were calculated using a Förster-like approach for the 1-pyrenyl, 9-anthryl, and 2-naphthyl salts, respectively. The observed rate of proton-transfer was found to be independent of the thermodynamic driving force and the short-term reversibility of these reactions has been approximated. The data suggest how other systems may be designed to facilitate this novel process. [ABSTRACT FROM AUTHOR]