Electron Transfer Dynamics from CsPbBr3 Nanocrystals to Au144 Clusters.

Lead halide perovskite nanocrystals have received significant attention as an absorber material for designing efficient optoelectronic devices. The fundamental understanding of the hot carrier (HC) dynamics as well as its extraction in hybrid systems is essential to further boost the performance of solar cells. Herein, we have explored the electron transfer dynamics in the CsPbBr3-Au144 cluster hybrid using ultrafast transient absorption spectroscopy. Our analysis reveals faster HC cooling time (from 515 to 334 fs) and a significant drop in HC temperature from 1055 to 860 K in hybrid, suggesting the hot electron transfer from CsPbBr3 nanocrystals to the Au nanoclusters (NCs). Eventually, we observe a much faster hot electron transfer compared to the band-edge electron transfer, and 45% hot-electron transfer efficiency was achieved at 0.64 eV, above band-edge photoexcitation. Furthermore, the significant enhancement of the photocurrent to the dark current ratio in this hybrid system confirms the charge separation via the electron transfer from CsPbBr3 nanocrystals to Au144 NCs. These findings on HC dynamics could be beneficial for optoelectronic devices.

Modulating the Carrier Relaxation Dynamics in Heterovalently (Bi3+) Doped CsPbBr3 Nanocrystals.

Manipulation of intrinsic carrier relaxation is crucial for designing efficient lead halide perovskite nanocrystal (NC) based optoelectronic devices. The influence of heterovalent Bi3+ doping on the ultrafast carrier dynamics and hot carrier (HC) cooling relaxation of CsPbBr3 NCs has been studied using femtosecond transient absorption spectroscopy and first-principles calculations. The initial HC temperature and LO phonon decay time point to a faster HC relaxation rate in the Bi3+-doped CsPbBr3 NCs. The first-principles calculations disclose the acceleration of carrier relaxation in Bi3+-doped CsPbBr3 NCs due to the appearance of localized bands (antitrap states) within the conduction band. The higher Born effective charges (Z*) and higher soft energetic optical phonon density of states cause higher electron-phonon scattering rates in the Bi-doped CsPbBr3 system, which is responsible for the faster HC cooling rate in doped systems.

Unraveling the Effect of Single Atom Doping on the Carrier Relaxation Dynamics of MAg24n- Nanoclusters.

Precisely doped metal nanoclusters (NCs) are currently emerging nanomaterials for their unique photophysical properties. Here, we report the influence of single atom doping on the excited state relaxation dynamics of a series of MAg24(2,4-Me2PhS)18n- NCs where M is Ag, Au, Pd, and Pt. The NCs with a group 11 metal (Ag and Au) as central atoms exhibit dual emission at NIR and visible range, whereas it shows only NIR emission for group 10 metal (Pd and Pt) doped NCs. Global target analyses of transient absorption (TA) data reveal the three-state relaxation, i.e., initially excited state (Sn), ligand-centered charge transfer (CT) state (SL), and metal-centered lowest excited state (S1). Apart from the HOMO-LUMO (H-L) energy gap, the electron affinity of the central metal atom and rigidity of the NC structural framework influence the relaxation processes of the NCs. The extensive study into the relaxation dynamics will bestow the single atomic level modulation of photophysical properties.

Deciphering the Relaxation Mechanism of Red-Emitting Carbon Dots Using Ultrafast Spectroscopy and Global Target Analysis.

Red-emitting carbon dots (C-dots) have tremendous potential for bioimaging and optoelectronic applications. Here, we investigated the structural modification of red-emitting C-dots due to boron doping and their ultrafast relaxation dynamics. It is evident from the X-ray photoelectron spectroscopy study that the relative percentage of pyrridinic nitrogen is increased at the expense of amino nitrogen and graphitic nitrogen in B-doped C-dots. Transient absorption spectroscopy and global target analysis reveal the formation of an additional excited-state level that takes away a significant amount of the excited-state population after boron doping. This new excited state slows the initial relaxation process toward the emissive state from 317 to 750 fs and increases the overall lifetime from 1.03 to 1.45 ns in B-doped C-dots.

Implications of relaxation dynamics of collapsed conjugated polymeric nanoparticles for light-harvesting applications.

Conjugated polymer-based nanostructures have been explored extensively from energy harvesting to healthcare applications due to their unique photophysical properties. This perspective includes the mechanism of the formation of polymer nanoparticles from linear chain polymers by utilizing experimental and theoretical studies. Conjugated polymer nanoparticles lead to changes in excitonic absorption bands, photoluminescence (PL) bands, and relaxation kinetics due to the inter-chain interactions between the chromophoric sub-units and the formation of the low-lying delocalized collective state. Here, we have illustrated the current understanding of the ultrafast relaxation dynamics of π-conjugated polymer-based nanostructured materials using global and target analysis. We have shown the impacts of the photoinduced carrier dynamics of polymer nanoparticles on the energy and charge transfer processes. Polymer nanoparticles found promising applications in bio-imaging, photothermal and photodynamic therapeutic agents, photocatalysis, and lasing materials. Finally, we have given the future perspectives of luminescent polymer nanoparticles.

The Impact of Aggregation of Quaterthiophenes on the Excited State Dynamics.

Oligothiophenes and their aggregates play a dominant role in optoelectronic and light-harvesting applications. Here, we controlled the degree of aggregation of 2,2':5',2″:5'',2‴-quaterthiophene (QTH) to shed light on the impact of the aggregation on the excited state dynamics. QTH aggregation realized the control over the Intersystem Crossing (ISC) rates and, in turn, the formation of triplet excited states via the simple addition of water to QTH solutions in THF. From global target analysis, the time scale was 345.5 ps for ISC for QTHs in THF, but it was 2.33 ns in the case of QTH solutions featuring 70% water. Notably, the excitonic coupling between closely packed QTHs occurred predominantly in the aggregates formed in the presence of large water concentrations. Relaxation dynamics of the resulting QTH-aggregates differed substantially from QTH solutions at lower water content. For example, QTH-aggregates lacked any triplet excited states, and the unusual emission occurs from lower excitonic states from these predominantly H-aggregates.

Implementation of in silico methods to predict common epitopes for vaccine development against Chikungunya and Mayaro viruses.

Being a Positive sense RNA virus the recent reemergence of Chikungunya and Mayaro virus has taken the concern of the leading scientific communities of the world. Though the outbreak of Mayaro virus is limited to Neotropical region only, Chikungunya is already identified in over 60 countries around the world. Besides, the lack of a strong protective treatment, misdiagnosis issue and co-circulation of both the viruses calls for a new strategy which could potentially prevent these infections from spreading. In this study, we therefore, identified the peptide based vaccine candidates e.g. epitopes for B cell and T cell from Chikungunya virus which also showed to be homologous to the Mayaro virus through immuno-informatics and computational approaches. Final epitopes identified from the most antigenic structural polyprotein of both the viruses were 5 for CD8+ T cell Epitopes (209KPGDSGRPI217, 219TGTMGHFIL227, 239ALSVVTWNK247, 98KPGRRERMC106 and 100GRRERMCMK108), 2 epitopes for CD4+ T cell (105MCMKIENDCIFEVKH119 and 502DRTLLSQQSGNVKIT516) and a single epitope for B cell (504GGRFTIPTGAGKPGDSGRPI518). Analysis of our predicted epitopes for population coverage showed prominent population coverage (92.43%) around the world. Finally, molecular docking simulation of the foreseen T cell epitopes with respondent HLA alleles secured good HLA-epitope interaction. This study was directed towards the discovery of potential antigenic epitopes which can open up a new skyline to design novel vaccines for combating both of the diseases at the same time.

Global and target analysis of relaxation processes of the collapsed state of P3HT polymer nanoparticles.

Organic-inorganic heterostructure materials have received significant research interest for designing light harvesting devices because of their efficient charge separation. Here, we design organic and inorganic nano-heterostructures using conjugated polymer nanoparticles (PNPs) [poly(3-hexylthiophene-2,5-diyl), P3HT] and Au nanoparticles. We investigate the carrier relaxation processes of this heterostructure at different time scales by ultrafast transient absorption spectroscopy. The lifetime of the singlet state (S1) of the pristine polymer shortens from 480.7 ps to 2.8 ps due to the formation of nanoparticles, and the formation of a delocalized collective state (CLS) is obtained in polymer nanoparticles whose lifetime is found to be 384.6 ps. The hot and ultrafast electron transfers occur from P3HT polymer nanoparticles to Au nanoparticles and the time constants are 253 fs and 37.7 ps, respectively, which are responsible for the efficient charge separation in such heterostructures. Such a fundamental study of relaxation processes of organic-inorganic nano heterostructures is very significant for designing light harvesting systems.

Investigation of Morphology-Controlled Ultrafast Relaxation Processes of Aggregated Porphyrin.

Here, we have synthesized rod and flake shaped morphology of porphyrin aggregates from 5, 10, 15, 20-tetra (4-n-octyloxyphenyl) porphyrin (4-opTPP) molecule which are evident from scanning electron microscopy (SEM). The formation of J-type aggregation is evident from steady state and time-resolved fluorescence spectroscopic studies. Ultrafast transient absorption spectroscopic studies reveal that the excited state lifetime is controlled by the morphology and the time constant for S1→S0 relaxation changes from 3.05 ps to 744 ps with changing the shape from rod to flake, respectively. In spite of similar exciton coupling energy in both the aggregates, the flake shaped aggregates undergo a faster exciton relaxation process and the non-radiative relaxation channels are found to depend on the shape of aggregates. The fundamental understanding of morphology controlled ultrafast relaxation processes of aggregated porphyrin is important for designing efficient light harvesting devices.

Ultrafast Relaxation Processes of Conjugated Polymer Nanoparticles in the Presence of Au Nanoparticles.

Considering the importance of conjugated polymer nanoparticles, major emphasis has been given for designing and understanding the energy transfer and charge transfer processes of organic-inorganic hybrids for light harvesting applications. In the present study, we have designed an aqueous solution-based light harvesting system using conjugated polymer nanoparticles (poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene], MEH-PPV) and Au nanoparticles. The change in photo-induced processes in the presence of metal nanoparticles are studied by steady-state absorption, time-resolved emission, time-resolved fluorescence up-conversion, ultrafast anisotropy and femtosecond transient absorption spectroscopy. Global and target analysis of transient absorption data validate the creation of a collective delocalized state in polymer nanoparticles, and the time scale for excitation energy funnelling from S1 state to low lying collective delocalized state (CLs ) is 18 ps. Then, the electron transfer from the CLs state to Au NP occurs with a time constant of 150 ps. The 815 ps long lived charge transfer (CT) state signifies the charge transfer from the CLs state of polymer nanoparticles to Au NP. Such basic understanding of relaxation processes in hybrid systems is very important for designing inorganic-organic hybrid light-harvesting systems.

Engineering the Excited-State Dynamics of 3-Aminoquinoline by Chemical Modification and Temperature Variation.

The role of the amino group in the excited-state dynamics of 3-aminoquinoline (3AQ) has been investigated by comparison with its synthetic derivative 3-(piperidin-1-yl)quinoline (3PQ). The absence of amino hydrogen atoms in 3PQ eliminates, to a large extent, the complexity of the excited-state processes observed in 3AQ. The polarity of the medium is found to be the most important determinant in the nonradiative rate constants of 3PQ, unlike in 3AQ where hydrogen bonding plays the most significant role. The nonradiative rate constants decrease with increase in micropolarity. This trend is opposite to what is usually observed with dipolar states. Temperature dependence of the fluorescence spectra and lifetime has been studied to understand this unexpected observation. An unusual redshift in the emission of 3AQ and 3PQ is observed in nonpolar media at low temperatures. This is surprising, as a process involving a barrier is expected to be hindered at low temperatures and be manifested in a blueshift of the spectra, due to the predominance of the locally excited (LE) state. Moreover, the variation of emission maxima of 3AQ with temperature is sigmoidal in nature, indicating the involvement of two distinct states. The counterintuitive observation of the predominance of the state with comparatively lower emission energy, at low temperatures, establishes the following: the photophysics in 3AQ is dominated by a LE state at room temperature in nonpolar media. This state is associated with rapid flip-flop of the amino group, which provides an efficient nonradiative channel of deactivation. At low temperatures, this flip-flop is hindered and the molecule can undergo intramolecular charge transfer (ICT), whereby the lower energy state is populated. In the case of 3PQ, the ICT state is the only one present, owing to the tertiary amino group.