Steady-state Emission Spectra Research Articles

Excited Charge Separation in a π-Interacting Phenothiazine-Zinc Porphyrin-Fullerene Donor-Acceptor Conjugate.

We have designed, synthesized, and characterized a donor-acceptor triad, SPS-PPY-C60, that consists of a π-interacting phenothiazine-linked porphyrin as a donor and sensitizer and fullerene as an acceptor to seek charge separation upon photoexcitation. The optical absorption spectrum revealed red-shifted Soret and Q-bands of porphyrin due to charge transfer-type interactions involving the two ethynyl bridges carrying electron-rich and electron-poor substituents. The redox properties suggested that the phenothiazine-porphyrin part of the molecule is easier to oxidize and the fullerene part is easier to reduce. DFT calculations supported the redox properties wherein the electron density of the highest molecular orbital (HOMO) was distributed over the donor phenothiazine-porphyrin entity while the lowest unoccupied molecular orbital (LUMO) was distributed over the fullerene acceptor. TD-DFT studies suggested the involvement of both the S2 and S1 states in the charge transfer process. The steady-state emission spectrum, when excited either at porphyrin Soret or visible band absorption maxima, revealed quenched emission both in nonpolar and polar solvents, suggesting the occurrence of excited state events. Finally, femtosecond transient absorption spectral studies were performed to witness the charge separation by utilizing solvents of different polarities. The transient data was further analyzed by GloTarAn by fitting the data with appropriate models to describe photochemical events. From this, the average lifetime of the charge-separated state calculated was found to be 169 ps in benzonitrile, 319 ps in dichlorobenzene, 1.7 ns in toluene for Soret band excitation, and ∼320 ps for Q-band excitation in benzonitrile.

Green synthesis of fluorescent carbon dots from ascorbic acid and their application in sensing and biological imaging

A simple, highly efficient, and environmentally friendly one-step method has been developed to synthesize fluorescent carbon dots (CDs) with the assistance of hydrogen peroxide (H2O2). Here H2O2 acts as a green accelerator to promote the preparation of CDs, while temperature plays a critical role in controlling the entire synthesis process. The prepared CDs demonstrate monodispersion, with an average diameter of ∼6.8 nm, superior stability, and excitation-dependent photoluminescent (PL) behavior. The PL mechanism of the CDs was investigated using time-resolved PL decay, revealing that the PL originates from both the defect state and the intrinsic state of the CDs. Furthermore, the CDs show significant promise as pH sensors and versatile nanothermometry devices, owing to their remarkable sensitivity to both pH and temperature, as demonstrated by the pronounced dependence of their steady-state fluorescence emission spectra. It’s worth noting that temperature and pH sensors present significant reversibility, sensitivity, and linearity. More importantly, the CDs display low cytotoxicity, good biocompatibility, and the ability to enter human breast cancer Bcap-37 cells, making them highly suitable candidates for cellular imaging and labeling.

Pt-S Bond-Enabled Temperature-Dependent Phosphorescence in S-Heteroaryl Tetradentate Pt(S^C^N^O) Complexes.

This study presents the rare examples of S-heteroaryl tetradentate Pt(S^C^N^O) luminescent complexes (PtSZ and PtSZtBu) containing a Pt-S bond. The presence of the Pt-S bond allows the novel Pt(S^C^N^O) complexes to exhibit temperature-dependent phosphorescent emission behavior. The PtSZtBu exhibits dual-emission phenomena and biexponential transient decay spectra above 250 K, indicating the presence of two minimal excited states in the potential energy surface (PES) of the T1 state. Through complementary experimental and computational studies, we have identified changes in orbital composition between Pt(dxy)-S(px) and Pt(dyz)-S(pz) in excited states with increasing temperature. This results in two energy minima, enabling the excited states to decay selectively and radiatively at different temperatures. Consequently, this leads to remarkable steady-state and transient emission spectra changes. Our work not only provides valuable insights for the development of novel Pt-S bond-based tetradentate Pt(II) complexes but also enhances our understanding of the distinctive properties governed by the Pt-S bond.

Environmentally Driven Symmetry Breaking Quenches Dual Fluorescence in Proflavine.

Nonadiabatic couplings between several electronic excited states are ubiquitous in many organic chromophores and can significantly influence optical properties. A recent experimental study demonstrated that the proflavine molecule exhibits surprising dual fluorescence in the gas phase, which is suppressed in polar solvent environments. Here, we uncover the origin of this phenomenon by parametrizing a linear-vibronic coupling Hamiltonian from spectral densities of system-bath coupling constructed along molecular dynamics trajectories, fully accounting for interactions with the condensed-phase environment. The finite-temperature absorption, steady-state emission, and time-resolved emission spectra are then computed using powerful, numerically exact tensor network approaches. We find that the dual fluorescence in vacuum is driven by a single well-defined coupling mode but is quenched in solution due to dynamic solvent-driven symmetry breaking that mixes the two low-lying electronic states. We expect the computational framework developed here to be widely applicable to the study of non-Condon effects in complex condensed-phase environments.

Refolding SDS-mediated partially-unfolded proteins: A distinguishing role of tetraalkylammonium ionic liquids via mixed micellar aggregation

The current study reports a new strategy for ionic liquid (ILs) assisted refolding of sodium dodecyl sulfate (SDS)-induced partially unfolded protein via mixed micellar aggregate formation between SDS and the ionic liquid. This approach can indeed be utilized to keep many essential and sensitive proteins as active and prolong their shelf life. For SDS-induced partially unfolded proteins, the ionic liquid has been identified to function as a refolding agent. The phenomenon has been observed by us for the first time and we have thoroughly investigated the same utilizing a variety of spectroscopic, microscopic approaches, and attempted to rationalize from computational techniques as well. The steady-state fluorescence emission spectra and time-resolved fluorescence decay show that both the intrinsic emission intensity and the average lifetime, respectively, of the surfactant-induced partially-unfolded protein increase in the presence of ionic liquid, indicating a possible reversal of unfolding (refolding) of the protein (up to ∼95 % revival of luminescence intensity and average lifetime with respect to the native protein). The CD and FTIR results also confirmed that the α-helical content of the pre-unfolded protein reverts substantially and is very close to that of the native protein. To understand the refolding process of SDS-induced partially-unfolded protein, we have further studied the structural recovery of urea-induced pre-denatured protein utilizing the same ionic liquid. Interestingly, no such refolding was observed in this case. From the experimental results, it appears that ionic liquid is involved in scavenging the unfolding surfactant monomers by forming mixed-micellar aggregates where the ion-ion attachment is likely to have impact on the said process.

Excited-State Dynamics of Carbazole and tert-Butyl-Carbazole in Thin Films

Thin films of carbazole (Cz) derivatives are frequently used in organic electronics, such as organic light-emitting diodes (OLEDs). Because of the proximity of the Cz units, the excited-state relaxation in such films is complicated, as intermolecular pathways, such as singlet–singlet annihilation (SSA), kinetically compete with the emission. Here, we provide an investigation of two benchmark systems employing neat carbazole and 3,6-di-tert-butylcarbazole (t-Bu-Cz) films and also their thin film blends with poly(methyl methacrylate) (PMMA). These are investigated by a combination of atomic force microscopy (AFM), femtosecond and nanosecond transient absorption spectroscopy (fs-TA and ns-TA) and time-resolved fluorescence. Excitonic J-aggregate-type features are observed in the steady-state absorption and emission spectra of the neat films. The S1 state shows a broad excited-state absorption (ESA) spanning the entire UV–Vis–NIR range. At high S1 exciton number densities of about 4 × 1018 cm−3, bimolecular diffusive S1–S1 annihilation is found to be the dominant SSA process in the neat films with a rate constant in the range of 1–2 × 10−8 cm3 s−1. SSA produces highly vibrationally excited molecules in the electronic ground state (S0*), which cool down slowly by heat transfer to the quartz substrate. The results provide relevant photophysical insight for a better microscopic understanding of carbazole relaxation in thin-film environments.

Excitation energy transfer and vibronic relaxation through light-harvesting dendrimer building blocks: A nonadiabatic perspective.

The light-harvesting excitonic properties of poly(phenylene ethynylene) (PPE) extended dendrimers (tree-like π-conjugated macromolecules) involve a directional cascade of local excitation energy transfer (EET) processes occurring from the "leaves" (shortest branches) to the "trunk" (longest branch), which can be viewed from a vibronic perspective as a sequence of internal conversions occurring among a connected graph of nonadiabatically coupled locally excited electronic states via conical intersections. The smallest PPE building block that is able to exhibit EET, the asymmetrically meta-substituted PPE oligomer with one acetylenic bond on one side and two parallel ones on the other side (hence, 2-ring and 3-ring para-substituted pseudo-fragments), is a prototype and the focus of the present work. From linear-response time-dependent density functional theory electronic-structure calculations of the molecule as regards its first two nonadiabatically coupled, optically active, singlet excited states, we built a (1 + 2)-state-8-dimensional vibronic-coupling Hamiltonian model for running subsequent multiconfiguration time-dependent Hartree wavepacket relaxations and propagations, yielding both steady-state absorption and emission spectra as well as real-time dynamics. The EET process from the shortest branch to the longest one occurs quite efficiently (about 80% quantum yield) within the first 25fs after light excitation and is mediated vibrationally through acetylenic and quinoidal bond-stretching modes together with a particular role given to the central-ring anti-quinoidal rock-bending mode. Electronic and vibrational energy relaxations, together with redistributions of quantum populations and coherences, are interpreted herein through the lens of a nonadiabatic perspective, showing some interesting segregation among the foremost photoactive degrees of freedom as regards spectroscopy and reactivity.

A Quantitative Approach to Characterize the Surface Modification on Nanoparticles Based on Localized Dielectric Environments.

Nanoparticles (NPs) are utilized for the functionalization of composite materials and nanofluids. Although oxide NPs (e.g., silica (SiO2)) exhibit less dispersibility in organic solvents or polymers due to their hydrophilic surface, the surface modification using silane coupling agents can improve their dispersibility in media with low dielectric constants. Herein, SiO2 NPs were functionalized using octyltriethoxysilane (OTES, C8) and dodecyltriethoxysilane (DTES, C12), wherein the degrees of surface modification of SiO2@C8 and SiO2@C12 were quantitatively evaluated based on the ratio of modifier to surface silanol group (θ) and the volume fraction of organic modifier to total particle volume (ϕR). The variations of surface properties were revealed by analyzing the Hansen solubility parameters (HSP). Particularly, the surface modification using OTES or DTES significantly affected the polarity (δP) of NPs. The local dielectric environments of surface-modified SiO2 NPs were characterized using a solvatochromic dye, Laurdan. By analyzing the peak position of the steady-state emission spectrum of Laurdan in a NP suspension, the apparent dielectric environments surrounding NPs (εapp) were obtained. A good correlation between ϕR and εapp was observed, indicating that ϕR is a reliable quantity for understanding the properties of surface-modified NPs. Furthermore, the generalized polarization (GP) of NPs was investigated. The surface-modified SiO2 NPs with higher ϕR (≥0.15) exhibited GP > 0, suggesting that the modifiers are well-organized on the surface of NPs. The localized dielectric environment surrounding NPs could be predicted by analyzing the volume fraction of nonpolar moieties derived from modifiers. Alternatively, εapp and GP can be utilized for understanding the properties of inorganic-organic hybrid NPs.

Intramolecular charge transfer character of D-π-A chromophores with modulating the subunits sequence of π-bridge

Two D-B-A compounds (TRD and TTRD) were designed with the different bridges (B1 and B2) consisting of T and TT moieties. They were synthesized and investigated by UV–Vis spectra, steady state emission spectra, X-ray diffraction, electrochemical measurement, the photovoltaic performances and theoretical calculation. The large Stokes shift suggested the formation of an intramolecular charge transfer (ICT) state in the excited state. TTRD based device showed high short-circuit current density (JSC) and fill factor (FF). It was essential to understanding the underlying mechanism caused by the subtle structural difference of the two spacers. The results showed that the B2 bridge extending the electron increment region has shown a significant improvement for excitation delocalization, which played a crucial role in improving effective dissociation of the ICT excitons, leading to the increase of photogenerated charge carriers.

Preference for excited state imine-amine isomerisation over enol-keto isomerisation: spectroscopic exploration and quantum chemical calculations.

In this article, the photophysical properties of ethyl-3-(benzo[D]thiazol-2-yl)-5-chloro-2-hydroxybenzoate (EBTCH) and ethyl-3-(benzo[D]oxazol-2-yl)-5-chloro-2-hydroxybenzoate (EBOCH) have been explored spectroscopically along with quantum chemical calculations. From a structural viewpoint, both molecules have two proton acceptor sites (thiazole/oxazole N atom and ester O atom) and a common proton donor site (phenolic -OH) connected by a six-membered H-bonding ring capable of both imine-amine and enol-keto photoisomerisation. Steady state absorption and emission spectra and time-resolved fluorescence characteristics of EBTCH and EBOCH and a comparison with the spectral data of controlled compounds 2-(benzo[D]thiazol-2-yl)-4-chlorophenol (BTCP), 2-(benzo[D]oxazol-2-yl)-4-chlorophenol (BOCP) and ethyl 5-chloro-2-hydroxybenzoate (ECHB) support the preference for imine-amine isomerisation over enol-keto isomerization in the excited state. The computed structural parameters for the ground and excited states for both the molecules from the density functional theory (DFT) calculations are found to be in favour of the imine-amine isomerization process. Theoretical potential energy curves along the proton transfer coordinates of EBTCH and EBOCH support the existence of both isomers in the ground state (S0). However, the formation of a tautomeric form involving imine-amine isomerisation obtained through asymmetric double-well potential is favoured over the enol-keto isomerisation in the excited state (S1) due to lower barrier energy.

Steady-State and Time-Resolved Fluorescence Study of Selected Tryptophan-Containing Peptides in an AOT Reverse Micelle Environment.

The aim of this study was to demonstrate the utility of time-resolved fluorescence spectroscopy in the detection of subtle changes in the local microenvironment of a tryptophan chromophore in a confined and crowded medium of AOT reverse micelles, which mimic biological membranes and cell compartmentalization. For this purpose, fluorescence properties of L-tryptophan and several newly synthesized tryptophan-containing peptides in buffer and in an AOT reverse micelle medium were determined. It was shown that insertion of tryptophan and its short di- and tripeptides inside micelles led to evident changes in both the steady-state emission spectra and in fluorescence decay kinetics. The observed differences in spectral characteristics, such as a blue shift in the emission maxima, changes in the average fluorescence lifetime, and the appearance of environmental-dependent fluorescent species, showed the utility of time-resolved fluorescence spectroscopy as a sensitive tool for detecting subtle conformational modifications in tryptophan and its peptides induced by changes in polarity, viscosity, and specific interactions between chromophores and water molecules/polar groups/ions that occur inside reverse micelles.

Synthesis, spectroscopic characterizations, photophysical properties and DFT studies of a novel iridium(III) complex containing 2-(2-butoxy-4-fluorophenyl)pyridine ligand

The iridium(III) complex, Ir(bFppy)2(pyim) (bFppy = 2-(2-butoxy-4-fluorophenyl)pyridine and pyim = 2-(1H-Imidazol-2-yl)pyridine) was excellently prepared between 2-(1H-Imidazol-2-yl)pyridine and chloro-bridged iridium(III) dimer via reflux reaction. Butoxy functional group and fluorine atom were added to the phenyl ring of 2-phenylpyridine at positions 2 and 4 respectively, as cyclometalating ligand. The Ir(bFppy)2(pyim) complex was characterized by spectroscopic methods: FTIR, NMR, LCMS and UV–Vis absorption. The presence of two sharp peaks at 1256 cm−1 and 1129 cm−1 due to ν(CO) was revealed by IR analysis. The bands appear in the range of 1594–1567 cm−1 owing to the CC and CN aromatic ring stretching vibrations of phenyl and pyridine. The 1H NMR spectrum showed signals in the range of δ 5.50–8.60 ppm corresponding to phenylpyridine and pyridylimidazole protons. In contrast, the proton signals in the range of δ 0.80–4.50 ppm, assignable to butyl substituent. The UV–Vis spectrum displayed weaker and broader bands (333 nm and 376 nm) in the visible region due to the spin-forbidden 3MLCT transitions. The steady-state emission spectrum of the Ir(bFppy)2(pyim) complex in air-equilibrated methylene chloride solution at 298 K exhibited almost blue emission with a λmax at 466 nm. To support the experimental spectroscopic data, DFT and TD-DFT measurements were conducted using B3LYP in combination with the LanL2DZ basis set. The chemical shifts, vibrational modes and maximum absorptions of the Ir(bFppy)2(pyim) complex were relatively well reproduced.

Phosphine-Oxide-Balanced Intra- and Interchain Through-Space Charge Transfer in Thermally Activated Delayed Fluorescence Polymers: Beyond 30% External Quantum Efficiency.

Through-space charge transfer (TSCT) is crucial for developing highly efficient thermally activated delayed fluorescence polymers. The balance of intra- and interchain TSCT can markedly improve performance, but it is still a big challenge. In this work, an effective strategy for "intra- and interchain TSCT balance" is demonstrated by way of a series of non-conjugated copolymers containing a 9,9-dimethylacridine donor and triazine-phosphine oxide (PO)-based acceptors. Steady-state and transient emission spectra indicate that compared to the corresponding blends, the copolymers can indeed achieve balanced intra- and interchain TSCT by accurately optimizing the inductive and steric effects of the acceptors. The DPOT acceptor with the strongest electron-withdrawing ability and the second bigger steric hindrance endows its copolymers with state-of-the-art photoluminescence and electroluminescence quantum efficiencies beyond 95% and 32%, respectively. This demonstrates that, compared to other congeners, the synergistic inductive and steric effects effectively enhance TSCT in DPOT-based copolymers for radiation, and suppress singlet and triplet quenching. The record-high efficiencies of its devices make this kind of copolymers hold the potential for low-cost, large-scale, and high-efficiency applications.

Photophysics of 7-hydroxycoumarin 3-carboxylic acid in micelles

7-Hydroxy coumarin-3-carboxylic acid (7-HCCA) is a photoacid that exists in three distinct forms: neutral, anionic and di-anionic. It falls under the category of hydroxy aromatic and carboxy aromatic molecules. This molecule exhibits strong fluorescence and is commonly employed in the detection of hydroxyl radicals. In our study, we utilised absorption and fluorescence spectroscopy to investigate the photophysical behaviour of 7-HCCA in different micellar systems. Through our experiments, we observed that as we varied the pH within the ranges of 2–3, 4–6, 7–10, the three aforementioned forms of 7-HCCA are emerged. Specifically, in cationic surfactants, the dye displayed an equilibrium between two forms of the 7-HCCA in the ground state at any pH. Furthermore, our steady-state excitation studies also proved the presence of an equilibrium between two forms of the 7-HCCA in all cationic systems. By examining the steady-state emission spectra, we also observed the presence of two distinct species in the excited state. When cationic surfactants were introduced, we observed a decrease in the fluorescence intensity and quantum yield compared to that observed in the water medium. It was evident that the photophysical properties of 7-HCCA were modulated with variations of the chain length and head group of the surfactants. In summary, our investigation shed light on the photophysical characteristics of 7-HCCA in different micellar systems with varying the pH. By employing the absorption and fluorescence spectroscopy, we successfully identified the coexistence of multiple forms of 7-HCCA and observed how the cationic surfactants influenced its fluorescence emission properties, ultimately contributing to a deeper understanding of its behaviour in these systems.

Energy dissipation efficiency in the CP43 assembly intermediate complex of photosystem II

Photosystem II in oxygenic organisms is a large membrane bound rapidly turning over pigment protein complex. During its biogenesis, multiple assembly intermediates are formed, including the CP43-preassembly complex (pCP43). To understand the energy transfer dynamics in pCP43, we first engineered a His-tagged version of the CP43 in a CP47-less strain of the cyanobacterium Synechocystis 6803. Isolated pCP43 from this engineered strain was subjected to advanced spectroscopic analysis to evaluate its excitation energy dissipation characteristics. These included measurements of steady-state absorption and fluorescence emission spectra for which correlation was tested with Stepanov relation. Comparison of fluorescence excitation and absorptance spectra determined that efficiency of energy transfer from β-carotene to chlorophyll a is 39 %. Time-resolved fluorescence images of pCP43-bound Chl a were recorded on streak camera, and fluorescence decay dynamics were evaluated with global fitting. These demonstrated that the decay kinetics strongly depends on temperature and buffer used to disperse the protein sample and fluorescence decay lifetime was estimated in 3.2–5.7 ns time range, depending on conditions. The pCP43 complex was also investigated with femtosecond and nanosecond time-resolved absorption spectroscopy upon excitation of Chl a and β-carotene to reveal pathways of singlet excitation relaxation/decay, Chl a triplet dynamics and Chl a → β-carotene triplet state sensitization process. The latter demonstrated that Chl a triplet in the pCP43 complex is not efficiently quenched by carotenoids. Finally, detailed kinetic analysis of the rise of the population of β-carotene triplets determined that the time constant of the carotenoid triplet sensitization is 40 ns.

On the Nature of Stationary and Time-Resolved Fluorescence Spectroscopy of Collagen Powder from Bovine Achilles Tendon.

This paper presents a more systematic study of steady-state and time-resolved autofluorescence spectroscopy of collagen isolated from bovine Achilles tendon. In steady-state fluorescence measurements, the excitation and emission spectra of collagen powder, recorded at different fluorescence excitation and detection wavelengths, were compared with the fluorescence excitation and emission spectra of the amino acids phenylalanine, tyrosine, and tryptophan, as well as with similar spectra for 13 autofluorescent collagen cross-links, which have been identified and described in the literature so far. In time-resolved studies, fluorescence was excited by the pulsed light of different wavelengths, and for each excitation wavelength, fluorescence decay was recorded for several detection wavelengths. Data analysis allowed recovery of the fluorescence decay times for each experimental excitation detection event. The obtained information on the decay times of the measured fluorescent signals was discussed, taking into account the available literature data from similar studies of isolated collagen and collagen-rich tissues. Based on the obtained results, it was found that the shape and position of the measured fluorescence excitation and emission spectra of collagen strongly depend on the emission and excitation wavelengths selected in the measurements. From the recorded excitation and emission bands of collagen, it can be concluded with high probability that there are additional, so far unidentified, collagen cross-links, which can be excited at longer excitation wavelengths. In addition, the collagen excitation spectra were measured at longer emission wavelengths at which the collagen cross-links emit fluorescent light. In addition to the emission spectra obtained for excitation in the deep-UV region, the results of time-resolved fluorescence studies with excitation in the deep-UV region and detection at longer wavelengths suggest that fluorescence excitation energy transfer processes occur from the amino acids to the collagen cross-links, and also between the cross-links themselves.

Photogrammetry of Ultrafast Excited-State Intramolecular Proton Transfer Pathways in the Fungal Pigment Draconin Red.

Proton transfer processes of organic molecules are key to charge transport and photoprotection in biological systems. Among them, excited-state intramolecular proton transfer (ESIPT) reactions are characterized by quick and efficient charge transfer within a molecule, resulting in ultrafast proton motions. The ESIPT-facilitated interconversion between two tautomers (PS and PA) comprising the tree fungal pigment Draconin Red in solution was investigated using a combination of targeted femtosecond transient absorption (fs-TA) and excited-state femtosecond stimulated Raman spectroscopy (ES-FSRS) measurements. Transient intensity (population and polarizability) and frequency (structural and cooling) dynamics of -COH rocking and -C=C, -C=O stretching modes following directed stimulation of each tautomer elucidate the excitation-dependent relaxation pathways, particularly the bidirectional ESIPT progression out of the Franck-Condon region to the lower-lying excited state, of the intrinsically heterogeneous chromophore in dichloromethane solvent. A characteristic overall excited-state PS-to-PA transition on the picosecond timescale leads to a unique "W"-shaped excited-state Raman intensity pattern due to dynamic resonance enhancement with the Raman pump-probe pulse pair. The ability to utilize quantum mechanics calculations in conjunction with steady-state electronic absorption and emission spectra to induce disparate excited-state populations in an inhomogeneous mixture of similar tautomers has broad implications for the modeling of potential energy surfaces and delineation of reaction mechanisms in naturally occurring chromophores. Such fundamental insights afforded by in-depth analysis of ultrafast spectroscopic datasets are also beneficial for future development of sustainable materials and optoelectronics.

Nature of Linear Spectral Properties and Fast Relaxations in the Excited States and Two-Photon Absorption Efficiency of 3-Thiazolyl and 3-Phenyltiazolyl Coumarin Derivatives.

Coumarin-based fluorescent agents play an important role in the manifold fundamental scientific and technological areas and need to be carefully studied. In this research, linear photophysics, photochemistry, fast vibronic relaxations, and two-photon absorption (2PA) of the coumarin derivatives, methyl 4-[2-(7-methoxy-2-oxo-chromen-3-yl)thiazol-4-yl]butanoate (1) and methyl 4-[4-[2-(7-methoxy-2-oxo-chromen-3-yl)thiazol-4-yl]phenoxy]butanoate (2), were comprehensively analyzed using stationary and time-resolved spectroscopic techniques, along with quantum-chemical calculations. The steady-state one-photon absorption, fluorescence emission, and excitation anisotropy spectra, as well as 3D fluorescence maps of 3-hetarylcoumarins 1 and 2 were obtained at room temperature in solvents of different polarities. The nature of relatively large Stokes shifts (∼4000-6000 cm-1), specific solvatochromic behavior, weak electronic π → π* transitions, and adherence to Kasha's rule were revealed. The photochemical stability of 1 and 2 was explored quantitatively, and values of photodecomposition quantum yields, on the order of ∼10-4, were determined. A femtosecond transient absorption pump-probe technique was used for the investigation of fast vibronic relaxation and excited-state absorption processes in 1 and 2, while the possibility of efficient optical gain was shown for 1 in acetonitrile. The degenerate 2PA spectra of 1 and 2 were measured by an open aperture z-scan method, and the maximum 2PA cross-sections of ∼300 GM were obtained. The electronic nature of the hetaryl coumarins was analyzed by quantum-chemical calculations using DFT/TD-DFT level of theory and was found to be in good agreement with experimental data.

Exploring the structural, photophysical and optoelectronic properties of a diaryl heptanoid curcumin derivative and identification as a SARS-CoV-2 inhibitor

Developing modifiable natural products those having antiviral activities against SARS-CoV-2 is a key research area which is popular in current scenario of COVID pandemic. A diaryl heptanoid curcumin and its derivatives are already presenting promising candidates for anti-viral drug development. We have synthesized single crystals of a dimethylamino derivative of natural curcumin and structural characterization was done by single crystal XRD analysis. Using steady-state absorption and emission spectra and guided by complimentary ab initio calculations, we unraveled the solvent effects on the photophysical properties of the dimethyl amino curcumin derivative. Chemical reactivity of the compound has investigated using frontier molecular orbitals and molecular electrostatic potential surface. High stability of the curcumin derivative in water environment has evaluated by Radial Distributions Functions (RDF) calculated via Molecular Dynamics (MD) simulations. The inhibitory activity of the title compound was evaluated by in silico methods and the stability of the protein-ligand complexes were studied using Molecular Dynamics simulations and MM-PBSA analysis. With this detailed study, we hope to motivate scientific community to develop new curcumin derivatives against SARS-CoV-2 virus.

Supramolecular Tuning of Exciton Transport in Pi-Peptide Assemblies

We demonstrate the use of sequence-dependent interactions between peptide-functionalized pi-conjugated pigments to tune exciton transport behavior in their supramolecular assemblies in aqueous solutions. Peptide sequences attached to a perylene diimide (PDI) core were selected based on their ability to foster a wide range of excitonic coupling strengths within organized assemblies, as reflected by experimentally measured steady-state absorption and emission spectra. Photoresponses of organized assemblies of weakly interacting PDIs closely resemble those observed for weakly assembled chromophores. In contrast, organized assemblies that support strong intermolecular coupling exhibit significant excitonic delocalization and transport, as observed through distinct transient signatures of fluence-dependent singlet–singlet annihilation and a short-lived biexcitonic state at high initial exciton densities. A one-dimensional (1D) diffusion model appropriately accounts for exciton–exciton encounters in assemblies, with effective exciton diffusion constants that scale with interchromophore coupling strength, ranging between 0 and 7 sites2/ps (0–1 nm2/ps or LD ranging from 0 to 40 nm). Variations in effective diffusion constant arise from peptide-tuned variations in interchromophore alignment and wavefunction overlap, with mixed Frenkel-CT exciton coupling identified as the key interaction that facilitates site-to-site exciton diffusion within PDI stacks. This work demonstrates the potential to use simple peptide sequence variation as a tool for rational engineering of exciton transport and other excited-state behaviors in supramolecular materials.