Interplay between photoinduced charge and energy transfer in manganese doped perovskite quantum dots.

A comprehensive study on the photo-excited relaxation dynamics in semiconducting perovskite quantum dots (PQDs) is pivotal in realizing their extensive potential for optoelectronics applications. Among different competing photoinduced relaxation kinetics, energy transfer and charge transfer (CT) in PQDs need special attention, as they often influence the device efficacy, particularly with the donor-acceptor hybrid architecture. In this work, we explore a detailed investigation into photoinduced CT dynamics in mixed halide undoped CsPb(Br/Cl)3 and Mn2+ doped CsPb(Br/Cl)3 PQDs with a quinone molecule, p-benzoquinone (BQ). The energy level alignment of undoped PQDs with BQ allows an efficient CT, whereas Mn2+ doping reduces the CT efficiency, experiencing a competition between energy transfer from host to dopant and CT to BQ. The conductive atomic force microscopy measurements unveil a direct correlation with the spectroscopic studies by showing a significant improvement in the conductance of undoped PQDs in the presence of BQ, while an inappreciable change is observed for doped PQDs. A much-reduced transition voltage and barrier height in the presence of BQ further validate faster CT for undoped PQD than the doped one. Furthermore, Mn2+ doping in PQDs is observed to enhance their stability, showing better air and thermal stability compared to their undoped counterparts. These results reveal that doping strategy can regulate the CT dynamics in these PQDs and increase their stability, which will be beneficial for the development of desired optoelectronic devices with long-term stability.

Modulation of carrier conduction in CsPbBr3 perovskite quantum dots with band-aligned electron and hole acceptors.

The lead halide perovskites have emerged as promising materials with intriguing photo-physical properties and have immense potential for photovoltaic applications. A comprehensive study on the kinetics of charge carrier (electron/hole) generation and transfer across the interface is key to realizing their future scope for efficient device engineering. Herein, we investigate the interfacial charge transfer (CT) dynamics in cesium lead halide (CsPbBr3) perovskite quantum dots (PQDs) with energetically favorable electron acceptors, anthraquinone (AQ) and p-benzoquinone (BQ), and hole acceptors such as pyrene and 4-(dimethylamino)pyridine (DMAP). With various steady-state and time-resolved spectroscopic and microscopic measurements, a faster electron transfer rate is estimated for CsPbBr3 PQDs with BQ compared to that of AQ, while a superior hole transfer for DMAP is divulged compared to pyrene. In concurrence with the spectroscopic measurements, conducting atomic force microscopic studies across the electrode-PQD-electrode junction reveals an increment in the conductance of the PQD in the presence of both the electron and hole acceptors. The variation of the density of states calculation in the presence of the hole acceptors offers strong support and validation for faster CT efficiency. The above findings suggest that a careful selection of simple yet efficient molecular arrangements can facilitate rapid carrier transfer, which can be designed as auxiliary layers for smooth CT and help in the engineering of cost-effective photovoltaic devices.

"Flexible hinge" dynamics in mismatched DNA revealed by fluorescence correlation spectroscopy.

Altered unwinding/bending fluctuations at DNA lesion sites are implicated as plausible mechanisms for damage sensing by DNA-repair proteins. These dynamics are expected to occur on similar timescales as one-dimensional (1D) diffusion of proteins on DNA if effective in stalling these proteins as they scan DNA. We examined the flexibility and dynamics of DNA oligomers containing 3 base pair (bp) mismatched sites specifically recognized in vitro by nucleotide excision repair protein Rad4 (yeast ortholog of mammalian XPC). A previous Forster resonance energy transfer (FRET) study mapped DNA conformational distributions with cytosine analog FRET pair primarily sensitive to DNA twisting/unwinding deformations (Chakraborty et al. Nucleic Acids Res. 46: 1240-1255 (2018)). These studies revealed B-DNA conformations for nonspecific (matched) constructs but significant unwinding for mismatched constructs specifically recognized by Rad4, even in the absence of Rad4. The timescales of these unwinding fluctuations, however, remained elusive. Here, we labeled DNA with Atto550/Atto647N FRET dyes suitable for fluorescence correlation spectroscopy (FCS). With these probes, we detected higher FRET in specific, mismatched DNA compared with matched DNA, reaffirming unwinding/bending deformations in mismatched DNA. FCS unveiled the dynamics of these spontaneous deformations at ~ 300 µs with no fluctuations detected for matched DNA within the ~ 600 ns-10 ms FCS time window. These studies are the first to visualize anomalous unwinding/bending fluctuations in mismatched DNA on timescales that overlap with the < 500 µs "stepping" times of repair proteins on DNA. Such "flexible hinge" dynamics at lesion sites could arrest a diffusing protein to facilitate damage interrogation and recognition.

Evidence for a bind-then-bend mechanism for architectural DNA binding protein yNhp6A.

The yeast Nhp6A protein (yNhp6A) is a member of the eukaryotic HMGB family of chromatin factors that enhance apparent DNA flexibility. yNhp6A binds DNA nonspecifically with nM affinity, sharply bending DNA by >60°. It is not known whether the protein binds to unbent DNA and then deforms it, or if bent DNA conformations are 'captured' by protein binding. The former mechanism would be supported by discovery of conditions where unbent DNA is bound by yNhp6A. Here, we employed an array of conformational probes (FRET, fluorescence anisotropy, and circular dichroism) to reveal solution conditions in which an 18-base-pair DNA oligomer indeed remains bound to yNhp6A while unbent. In 100 mM NaCl, yNhp6A-bound DNA unbends as the temperature is raised, with no significant dissociation of the complex detected up to ∼45°C. In 200 mM NaCl, DNA unbending in the intact yNhp6A complex is again detected up to ∼35°C. Microseconds-resolved laser temperature-jump perturbation of the yNhp6a-DNA complex revealed relaxation kinetics that yielded unimolecular DNA bending/unbending rates on timescales of 500 µs-1 ms. These data provide the first direct observation of bending/unbending dynamics of DNA in complex with yNhp6A, suggesting a bind-then-bend mechanism for this protein.

Unraveling the Relevance of Electron and Hole Transfer in Lead Halide Perovskite Nanocrystals on Current Conduction.

Optimization of perovskite-based optoelectronic performance demands prudent engineering in the device architecture with facile transport of generated charge carriers. Herein, we explore the charge transfer (CT) kinetics in perovskite nanocrystals (PNCs), CsPbBr3, with two redox-active quinones, menadione (MD) and anthraquinone (AQ), and its alteration in halide exchanged CsPbCl3. With a series of spectroscopic and microscopic measurements, we infer that both electron and hole transfer (ET-HT) prevail in CsPbCl3 with quinones, resulting in a faster CT, while ET predominates for CsPbBr3. Furthermore, current-sensing atomic force microscopy measurements demonstrate that the conductance across a metal-PNC-metal nanojunction is improved in the presence of quinones. The contributions of ET and HT to current conduction across PNCs are well supported and validated by theoretical calculations of the density of states. These outcomes convey a new perspective on the relevance of ET and HT in the optimal current conduction and optoelectronic device engineering of perovskites.

Deciphering the Relevance of Quantum Confinement in the Optoelectronics of CsPbBr3 Perovskite Nanostructures.

Perovskites (PVKs) have emerged as an exciting class of semiconducting materials owing to their magnificent photophysical properties and been used in solar cells, light-emitting diodes, photodetectors, etc. The growth of multidimensional nanostructures has revealed many exciting alterations in their optoelectronic properties compared to those of their bulk counterparts. In this work, we have spotlighted the influence of quantum confinement in CsPbBr3 PVKs like the quantum dot (PQD), nanoplatelet (PNPL), and nanorod (PNR) on their charge transfer (CT) dynamics with 1,4-naphthoquinone (NPQ). The energy band alignment facilitates the transfer of both electrons and holes in the PNPL to NPQ, enhancing its CT rate, while only electron transfer in the PQD and PNR diminishes CT. The tunneling current across a metal-nanostructure-metal junction for the PNPL is observed to be higher than others. The higher exciton binding energy in the PNPL results in efficient charge transport by enhancing the mobility of the excited-state carrier and its lifetime compared to those of the PNR and PQD.

Influence of 2'-deoxy sugar moiety on excited-state protonation equilibrium of adenine and adenosine with acridine inside SDS micelles: a time-resolved study with quantum chemical calculations.

The protonation dynamics of the DNA base adenine (Ade) and its nucleoside 2'-deoxyadenosine (d-Ade) are investigated by monitoring the deprotonation kinetics of an N-heterocyclic DNA intercalator, acridine (Acr), in the confined environment of sodium dodecyl sulfate (SDS) micelles. Protonation of acridine (AcrH(+)) occurs at the hydrophilic interface and this species remains in dynamic equilibrium with its deprotonated counterpart (Acr) inside the hydrophobic core of SDS micelles. Quenching of the fluorescence of AcrH(+)* at 478 nm is observed after addition of Ade and d-Ade with Stern-Volmer constant (K(SV)) 298 and 75 M(-1), respectively, with a concomitant increment in Acr* at 425 nm. Time-resolved fluorescence studies reveal quenching in the lifetime of AcrH(+)*. The relative amplitude of AcrH(+)* decreases from 0.97 to 0.51 and 0.97 to 0.89 with equimolar addition of Ade and d-Ade, respectively. These observations are explained by excited-state proton transfer (ESPT) from AcrH(+)* to the bases. The reduced K(SV) value and negligible change in the relative amplitudes of AcrH(+)* with d-Ade infer that ESPT is hindered substantially by the presence of a 2'-deoxy sugar unit. Transient time-resolved absorption spectra of Acr reflect that Ade reduces the absorbance of (3)AcrH(+)*; however, d-Ade keeps it unaltered for more than a time delay of 2 µs. The optimized geometries calculated by quantum chemical methods reflect deprotonation of AcrH(+)* with protonation at the N1 position of Ade, while it remains protonated with d-Ade. The hindered ESPT between AcrH(+)* and d-Ade singles out the significance of the 2'-deoxy sugar moiety in controlling the deprotonation kinetics.

Förster Resonance Energy Transfer Assisted Enhancement in Optoelectronic Properties of Metal Halide Perovskite Nanocrystals.

Regulated excited state energy and charge transfer play a pivotal role in nanoscale semiconductor device performance for efficient energy harvesting and optoelectronic applications. Herein, we report the influence of Förster resonance energy transfer (FRET) on the excited-state dynamics and charge transport properties of metal halide perovskite nanocrystals (PNCs), CsPbBr3, and its anion-exchanged counterpart CsPbCl3 with CdSe/ZnS quantum dots (QDs). We report a drop in the FRET efficiency from ∼85% (CsPbBr3) to ∼5% (CsPbCl3) with QDs, inviting significant alteration in their charge transport properties. Using two-probe measurements we report substantial enhancement in the current for the blend structure of PNCs with QDs, originating from the reduced trap sites, compared to that of the pristine PNCs. The FRET-based upshot in the conduction mechanism with features of negative differential resistance and negligible hysteresis for CsPbBr3 PNCs can add new directions to high performance-based photovoltaics and optoelectronics.

Tunable Conductance of MoS2 and WS2 Quantum Dots by Electron Transfer with Redox-Active Quinone.

Due to their uniqueness in tunable photophysics, transition metal dichalcogenide (TMD) based quantum dots (QDs) have emerged as the next-generation quantum materials for technology-based semiconductor applications. This demands frontline research on the rational synthesis of the TMD QDs with controlled shape, size, nature of charge migration at the interface, and their easy integration in optoelectronic devices. In this article, with a controlled solution-processed synthesis of MoS2 and WS2 QDs, we demonstrate the disparity in their structural, optical, and electrical characteristics in bulk and confinement. With a series of steady-state and time-resolved spectroscopic measurements in different media, we explore the uncommon photophysics of MoS2 and WS2 QDs such as excitation-dependent photoluminescence and assess their excited state charge transfer kinetics with a redox-active biomolecule, menadione (MQ). In comparison to the homogeneous aqueous medium, photoinduced charge transfer between the QDs and MQ becomes more plausible in encapsulated cetyltrimethylammonium bromide (CTAB) micelles. Current sensing atomic force microscopy (CS-AFM) measurements at a single molecular level reveal that the facilitated charge transfer of QDs with MQ strongly correlates with an enhancement in their charge transport behavior. An increase in charge transport further depends on the density of states of the QDs directing a change in Schottky emission to Fowler-Nordheim (FN) type of tunneling across the metal-QD-metal junction. The selective response of the TMD QDs while in proximity to external molecules can be used to design advanced optoelectronic devices and applications involving rectifiers and tunnel diodes for future quantum technology.

Photophysical behavior of acridine with amines within the micellar microenvironment of SDS: a time-resolved fluorescence and laser flash photolysis study.

The photophysical behavior of acridine (Acr) shows a facilitated water assisted protonation equilibrium between its deprotonated (Acr* ∼ 3.4 ns) and protonated forms (AcrH(+)* ∼ 33 ns) within a confined environment of sodium dodecyl sulphate (SDS) micelles above the critical micellar concentration of 8 mM. The acidic interface of the micelles is capable of protonating Acr whereas deprotonated Acr is partitioned into the hydrophobic core. The time-resolved-area-normalized-emission spectra confirm the presence of both Acr* and AcrH(+)*, while time-resolved-emission spectra depict time evolution between them. Quenching of AcrH(+)* with triethylamine (TEA) results in a linear Stern-Volmer (S-V) plot, whereas non-linearity arises with N,N-dimethylaniline (DMA). Both steady-state and time-resolved quenching results with TEA are explained on the basis of excited state proton transfer (ESPT), however the reasons behind the quenching of excited Acr with DMA are proposed as ESPT followed by a photoinduced electron transfer. Partitioning of DMA at the interface makes it accessible for both Acr* and AcrH(+)* in hydrophobic and hydrophilic regions of micelles respectively. The rate of electron transfer at the interface is found to be slower compared to that in the hydrophobic core. Characterization of transient intermediates formed during ESPT and PET between Acr and amines by laser-flash photolysis also supports the observation obtained during fluorescence studies. The mode of interactions between Acr and amines inside micelles is controlled by the localization of the proton/electron donors and acceptors in different hydrophobic or hydrophilic regions of such nano-confined environments.

Influence of heterogeneity of confined water on photophysical behavior of acridine with amines: a time-resolved fluorescence and laser flash photolysis study.

The photophysical behavior of acridine (Acr) shows facilitated water-assisted protonation equilibrium between its deprotonted (Acr* ∼ 10 ns) and protonated forms (AcrH(+*) ∼ 28 ns) within confined region of ordered water molecules inside AOT/H(2)O/n-heptane reverse micelles (RMs). The time-resolved-area-normalized-emission spectra confirm both Acr* and AcrH(+*), while time-resolved-emission spectra depict time evolution between them. Quenching of AcrH(+*) with N,N-dimethylaniline (DMA) is a purely diffusion-controlled bimolecular quenching with linear Stern-Volmer (S-V) plot, while nonlinearity arises with triethylamine (TEA) that forms ground state complex with AcrH(+) (AcrH(+)··H(2)O··TEA) indicating both static and dynamic quenching. Transient intermediates, DMA(•+) and AcrH(•) infer photoinduced electron transfer from DMA to Acr, while those from AcrH(+)··H(2)O··TEA complex suggest water mediated excited-state proton transfer (ESPT) between AcrH(+) and TEA. The ESPT becomes faster in larger RMs due to enhanced mobility of hydronium ions in AcrH(+)··H(2)O··TEA, which reduces in smaller RMs as water becomes much more constrained owing to stronger complexation by excess confinement.

Metal nanoparticle alters adenine induced charge transfer kinetics of vitamin K3 in magnetic field.

In this article, we highlight the alterations in the photoinduced electron transfer (ET) and hydrogen atom transfer (HAT) pathways between an anti-tumor drug vitamin-K3 (MQ) and a nucleobase adenine (ADN) in the presence of gold (Au) and iron (Fe) nanoparticles (NPs). Inside the confined micellar media, with laser flash photolysis corroborated with an external magnetic field (MF), we have detected the transient geminate radicals of MQ and ADN, photo-generated through ET and HAT. We observe that the presence of AuNP on the MQ-ADN complex (AuMQ-ADN) assists HAT by limiting the ET channel, on the other hand, FeNP on the MQ-ADN complex (FeMQ-ADN) mostly favors a facile PET. We hypothesize that through selective interactions of the ADN molecules with AuNP and MQ molecules with FeNP, a preferential HAT and PET process is eased. The enhanced HAT and PET have been confirmed by the escape yields of radical intermediates by time-resolved transient absorption spectroscopy in the presence of MF.

Micellar control over tautomerization and photo-induced electron transfer of Lumichrome in the presence of aliphatic and aromatic amines: a transient absorption study.

Lumichrome (Lc), a molecule consisting of a trinuclear alloxazine moiety is our present subject of interest. This molecule is subjected to tautomerization in the presence of pyridine, acetic acid, etc, through the formation of an eight-membered ring. In our present contribution, we have attempted to analyze the influence of the presence of an aliphatic amine, triethylamine (TEA) and an aromatic amine, N,N-dimethylaniline (DMA) in the double proton transfer step of the tautomerization as well as the photo-induced electron transfer (PET) from those amines to Lc. We have studied these phenomena within micelles, anionic and neutral, to observe the effect of confinement. Through our experiments, it could be stated that along with tautomerization and proton transfer, there is also evidence of PET in triplet excited state.

Prototropic interactions of pyrimidine nucleic acid bases with acridine: a spectroscopic investigation.

In this article, we have investigated the interactions of three pyrimidine nucleic acid bases, cytosine (C), thymine (T), and uracil (U) with acridine (Acr), an N-heterocyclic DNA intercalator, through the changes in photophysics of Acr inside SDS micelles. Fluorescence of AcrH(+)* at 478 nm and its lifetime are quenched on addition of C, T, and U, while a concomitant increment of Acr* is observed only with C. However, the relative amplitude of Acr* increases with a simultaneous decrease in AcrH(+)* only with C. The fluorescence quenching of AcrH(+)* is explained by photoinduced electron transfer (PET), while changes in the relative contributions of Acr* and AcrH(+)* with C are due to associated excited-state proton transfer (ESPT). The rate of electron transfer (kET) is maximum for T, followed by U and C. The associated ESPT from AcrH(+)* is the reason behind the reduced efficiency of PET with C. The lack of proton transfer with T and U as well as the higher kET for T compared to U are explained by keto-enol tautomerization and subtle changes in the structure and geometry of the pyrimidine bases.