Time-resolved Fluorescence Measurements Research Articles

Donor-Acceptor-Appended Triarylboron Lewis Acids: Ratiometric or Time-Resolved Turn-On Fluorescence Response toward Fluoride Binding.

Triarylboron Lewis acid compounds (CzmBoT (1c) and DPAmBoT (2c)), in which carbazole (Cz) or diphenylamine (DPA) donors are linked with a triazine acceptor in the ortho position of the phenylene ring are prepared and characterized. The treatment of 1c and 2c with the fluoride anion produces the corresponding fluoride adducts [1cF]- and [2cF]- as a tetraethylammonium salt. An X-ray diffraction study of [1cF]- reveals a twisted conformation between the Cz and phenylene rings. The Cz-containing 1c shows a ratiometric fluorescence change upon fluoride binding, while the DPA-containing 2c exhibits a turn-on fluorescence response in tetrahydrofuran. In particular, both fluoride adducts exhibit thermally activated delayed fluorescence (TADF) properties with microsecond-range lifetimes. Electrochemical and theoretical analysis suggests that the intramolecular charge-transfer transition from the donor to a conjugated acceptor fragment is switched to the donor to a triazine transition after fluoride binding. Theoretical analysis further demonstrates the twisted structure, effective highest occupied molecular orbital-lowest unoccupied molecular orbital separation, and the small energy splitting between the excited singlet and triplet states for the fluoride adducts, with all supporting the observed TADF. The time-resolved fluorescence measurements of 2c in the presence of both fluoride and a competitive fluorescent dye (Coumarin 6) effectively eliminate the short-lived fluorescence of a dye, retaining long-lived fluorescence signals originating only from [2cF]-.

Supramolecular interaction of sanguinarine dye with sulfobutylether-β-cyclodextrin: modulation of the photophysical properties and antibacterial activity.

The noncovalent host–guest interaction of sanguinarine (SGR), a benzophenanthridine alkaloid, with a nontoxic, water soluble sulfobutylether-β-cyclodextrin (SBE7βCD, commercially available as Captisol) macrocyclic host has been investigated using ground-state optical absorption, and steady-state and time-resolved fluorescence measurements. The pH-dependent changes in the absorbance of the dye at 327 nm showed a pKa value of 7.5, which has been shifted to 8.1 in the presence of SBE7βCD. The changes in the pKa values, absorption and fluorescence spectra, and fluorescence lifetime values of these two forms of SG with SBE7βCD indicate complex formation between them. The cationic form shows 3 times higher interaction towards SEB7βCD (K = 1.2 × 104 M−1) as compared to the neutral form (K = 3.9 × 103 M−1) which leads to a moderate upward pKa shift (pKa values of SGR shifted by more than 0.6 units). The subsequent fluorescence “turn off” was demonstrated to be responsive to chemical stimuli, such as metal ions (Ca2+ ions). Upon addition of Ca2+ ions, nearly quantitative dissociation of the complex was established to regenerate the free dye and result in fluorescence “turn on”. Apart from improving the stability under ambient light conditions, the upward pKa shift of SGR in the presence of SBE7βCD results in increasing the antibacterial activity of the SBE7βCD:SGR complex compared to that of the free dye towards four pathogenic micro-organisms at the physiological pH range. This work further compares SGR interaction with parent β-cyclodextrin.

Electron Transfer in a Naphthalene Diimide System Studied by Single-Molecule Delayed Fluorescence

Electron transfer (ET) is a key chemical reaction in nature and has been extensively studied in bulk systems, but remains challenging to investigate at the single-molecule level. A previously reported naphthalene diimide (NDI)-based system (Higginbotham et al., Chem. Commun. 2013, 49, 5061–5063) displays delayed fluorescence with good quantum yield (~0.5) and long-lived (nanoseconds) prompt and delayed fluorescence lifetimes, providing an opportunity to interrogate the underlying ET processes in single molecules. Time-resolved single-molecule fluorescence measurements enabled forward and reverse ET rate constants to be calculated for 45 individual molecules embedded in poly(methylmethacrylate) (PMMA) film. Interpretation of the results within the framework of Marcus–Hush theory for ET demonstrates that variation in both the electronic coupling and the driving force for ET is occurring from molecule to molecule within the PMMA film and over time for individual molecules.

Spectroscopic insight into the interaction of dopamine with spherical gold nanoparticles

Dopamine (DA) is a monoamine neurotransmitter of phenethylamine and catecholamine families, which is present in the central nervous system (CNS) and its periphery. Since DA is associated with several functions in the brain and body (motivational salience, reward, motor control, paracrine messenger, etc.), any imbalance in the DA level can trigger several neurodegenerative and other diseases. On the other hand, the spherical gold nanoparticles (AuNPs) can be used for drug delivery in several parts of the body. In addition, AuNPs also have the potentiality to penetrate through the blood-brain barrier and interact with the central nervous system without causing any toxicity. In view of many applications, it is important to look into the interaction between DA and AuNPs for a potential drug delivery model in DA related diseases. Here, we have used the steady-state and time-resolved fluorescence spectroscopic tools to investigate the binding interaction of DA with AuNPs. The nature of the quenching mechanism was confirmed through both steady-state and time-resolved fluorescence measurements. The binding constants along with the number of binding sites were estimated from the steady-state fluorescence measurements. The distance between DA and AuNPs was calculated using Förster’s theory to verify the possibility of fluorescence resonance energy transfer (FRET) from DA to AuNPs.

A terpyridine luminophore: Synthesis, photophysics and selective metal ion–Mediated hydrogelation

Terpyridine ligands are the fascinating materials for metal co-ordination. A new fluorescent 4-(3,4-dimethoxyphenyl) terpyridine ligand (DMPTP) was synthesized. We have explored the solvent dependent photophysical characteristics and the metal ion mediated hydrogel formation of DMPTP. The optical behavior of DMPTP is studied by UV–Visible, steady state and time resolved fluorescence measurements in different solvents. DMPTP exhibits structured emission in non-polar hexane solution and broad emission in polar DMSO solution. In addition, DMPTP holds a large solvent dependent fluorescence shift. These results indicate DMPTP undergoes intramolecular charge transfer (ICT) in the excited state. The fluorescence lifetimes also corroborated the ICT behavior of DMPTP. Moreover, DMPTP produces hydrogel upon formation of complex with Cu2+ and Zn2+ in 1:1 ratio. The xerogels of Zn2+ and Cu2+complex exhibit fibrous and flake like morphology, respectively. Interestingly, the hydrogel obtained from Zn2+ complex is highly fluorescent and the one obtained from Cu2+ complex is non-fluorescent.

Photo-Cross-Linkable Polymer Inks for Solution-Based OLED Fabrication

We introduce a catalyst-free, highly efficient, ambient temperature Diels-Alder reaction employing o-methylbenzaldehyde derivatives as photocaged dienes as an ideal approach for forming three-dimensional insoluble networks for inkjet printing of OLED emissive layer. Herein, poly(methyl methacrylate) based polymers containing 4-(9H-carbazol-9-yl)-2-(3′-hydroxy-[1,1′-biphenyl]-3-yl)isoindoline-1,3-dione as a blue-green (λmax = 495-500 nm) thermally activated delayed fluorescence (TADF) emitter and a photochemically active maleimide/o-methylbenzaldehyde cross-linker couple were synthesized and their photo-cross-linking behavior was studied. Time resolved fluorescence measurements confirm that the TADF properties are maintained upon integration in a polymer network and HOMO/LUMO levels of the emitter species remain unchanged by the photo-cross-linking at 365 nm of the polymer chains. The network formation of the fluorescent films is evidenced by solvent resistance tests and monitored by Fourier transform infrared (FT-IR) spectroscopy as well as time of flight secondary ion mass spectroscopy (ToF-SIMS), showing the consumption of maleimide and o-methylbenzaldehyde groups with increasing irradiation time. The surface roughness is investigated via atomic force microscopy (AFM) and found to be unchanged by a solvent wash after the cross-linking. Furthermore, confirmation that the polymer solution can be printed on an inkjet-printer and subsequently photo-cross-linked for multilayer OLED device fabrication is obtained. © 2019 American Chemical Society.

Investigation of the binding interactions between dimethyl phthalate and its metabolite with bovine serum albumin by multispectroscopy.

To understand the behavior of dimethyl phthalate (DMP) and realize the effect of DMP and its metabolite, monomethyl phthalate (MMP), on the conformation changes of bovine serum albumin (BSA), the interaction mechanisms of DMP and MMP with BSA were investigated by multispectroscopy and molecular docking. The results of the fluorescence quenching experiment showed that the fluorescence quenching of BSA by DMP and MMP was due to the formation of a complex though static quenching, which was also confirmed by time-resolved fluorescence measurements. The binding constants were 8.06 × 104 M-1 and 4.74 × 104 M-1 for DMP-BSA and MMP-BSA, respectively, and the number of binding sites were 1.20 (DMP) and 1.18 (MMP). The thermodynamic parameters showed different binding forces for DMP and MMP with BSA. The binding of DMP to BSA was driven mainly by hydrophobic interactions and hydrogen bonding, and MMP bound to BSA by van der Waals forces and hydrogen bonding, which were in accordance with the results from the molecular docking. The conformation and structural alterations of BSA upon DMP or MMP binding were studied by UV-vis spectroscopy, circular dichroism spectroscopy and synchronous fluorescence spectroscopy. The presence of metabolite did not change the quenching mechanism but decreased the binding affinity of DMP toward BSA as well as shortened the binding distance, which may be attributed to the competition between DMP and MMP for binding to BSA. This study revealed the combined effects of DMP and its metabolite on BSA at the molecular level.

Aggregation-Related Nonphotochemical Quenching in the Photosynthetic Membrane.

The photosynthetic apparatus of plants is a robust self-adjustable molecular system, able to function efficiently under varying environmental conditions. Under strong sunlight, it switches into photoprotective mode to avoid overexcitation by safely dissipating the excess absorbed light energy via nonphotochemical quenching (NPQ). Unfortunately, heterogeneous organization and simultaneous occurrence of multiple processes within the thylakoid membrane impede the study of natural NPQ under in vivo conditions; thus, usually artificially prepared antennae have been studied instead. However, it has never been shown directly that the origin of fluorescence quenching observed in these artificial systems underlies natural NPQ. Here we report the time-resolved fluorescence measurements of the dark-adapted and preilluminated-to induce NPQ-intact chloroplasts, performed over a broad temperature range. We show that their spectral response matches that observed in the LHCII aggregates, thus demonstrating explicitly for the first time that the latter in vitro system preserves essential properties of natural photoprotection.

Understanding the excitation wavelength dependent spectral shift and large exciton binding energy of tungsten disulfide quantum dots and its interaction with single-walled carbon nanotubes

Herein, we investigate the origin of excitation wavelength dependent spectral features and high fluorescence quantum yield in fluorescent 2D tungsten disulfide (WS2) quantum dots (QDs) of average size 2.4 nm. The as-prepared WS2 QDs possess high optical bandgap and reasonably high fluorescence quantum yield ~15.4% in the green region without any functionalization. The broad photoluminescence (PL) spectrum consists of multiple peaks owing to emissions from excitonic transitions and surface defect-related transitions. The excitation wavelength-dependent spectral redshift and narrowing of line shape in the PL peak are analyzed carefully, and it is attributed to the selective excitation/recombination of carriers from different energy levels. The temperature-dependent PL analysis yields an exciton binding energy of ~301 meV in the QDs. Furthermore, we study the interaction between fluorescent WS2 QDs and single-walled carbon nanotubes (SWCNTs) and explore the mechanism of systematic quenching of PL of QDs by SWCNTs. The nature of the Stern–Volmer plot is found to be linear, and the time-resolved fluorescence measurements reveal that the quenching follows primarily the static behavior. Our study further reveals that defect sites in SWCNTs primarily act as the binding sites for WS2 QDs and form non-fluorescent complexes for effective quenching of the PL. The strong interaction between the WS2 QDs and the SWCNTs is evidenced from the spectral shift in the X-ray photoelectron spectroscopy and Raman peaks. Our study reveals the origin of excitation wavelength dependent PL emission from WS2 QDs and the nature of the interaction between WS2 QDs and SWCNTs, which are important for their applications in biomedical imaging and sensing, such as surface-enhanced Raman scattering, etc.

CuO Nanoparticle-Protein Bioconjugate: Characterization of CuO Nanoparticles for the Study of the Interaction and Dynamic of Energy Transfer with Bovine Serum Albumin

Here, we aim to disclose the role of two different ranges concentration of copper oxide nanoparticles (CuO NPs) for the adsorption of BSA to CuO NP surfaces and the kinetics of the energy transfer between CuO NPs and BSA molecule by using different spectroscopy, time-resolved fluorescence measurements, and DLS study. The grown ~ 20.31 nm CuO NPs showed 3.60-eV band gap and 1.026-eV Urbach energy. The XRD pattern showed that the unit cell of the synthesized CuO nano-crystal is monoclinic phase. The photoluminescence spectrum of pure CuO NPs showed a high quantum yield of the blue emission. A small red shift of the absorption peak of BSA is determined because of binding with CuO NPs. The calculated value of an apparent association constant (Kapp) in the CuO NPs–BSA bioconjugate was found to be 6.51 × 103 M−1 and 2.16 × 103 M−1 for the small concentration range and large concentration range, respectively. The total change in energy transfer efficiency (ΔEeff) at room temperature is 22% and 5.6% for the use large and small concentration, respectively; at body temperature, this change is 13.6% and 6.6%, respectively. The BSA quenching is a mixed type in lower temperature in the low-concentration range and fully dynamic in the high-concentration range. The nature of interaction is exothermic, electrostatic, and hydrophobic. The fluorescence lifetimes of pure BSA decreased from 4.94 to 1.04 ns upon adsorption onto CuO NPs, corresponding to Eeff of 79.35%. The use of large concentration leads to aggregation rather than individual corona formation under the small concentration of CuO NPs.

Singlet Energy Transfer in Anthracene-Porphyrin Complexes: Mechanism, Geometry, and Implications for Intramolecular Photon Upconversion.

In this work we show that the mechanism for singlet excitation energy transfer (SET) in coordination complexes changes upon changing a single atom. SET is governed by two different mechanisms; Förster resonance energy transfer (FRET) based on Coulombic, through-space interactions, or Dexter energy transfer relying on exchange, through-bond interactions. On the basis of time-resolved fluorescence and transient absorption measurements, we conduct a mechanistic study of SET from a set of photoexcited anthracene donors to axially coordinated porphyrin acceptors, revealing the effect of coordination geometry and a very profound effect of the porphyrin central metal atom. We found that FRET is the dominating mechanism of SET for complexes with zinc-octaethylporphyrin (ZnOEP) as the acceptor, while Dexter energy transfer is the dominating mechanism of SET in a corresponding ruthenium complex (RuOEP). In addition, by analyzing the coordination geometry of the complexes and its temperature dependence, the binding angle potential energy of axially coordinated porphyrin complexes could be estimated. The results of this study are of fundamental importance and are discussed with respect to the consequences for developing intramolecular triplet-triplet annihilation photon upconversion in coordination complexes.

Dynamics at the non-ionic micelle/water interface: Impact of linkage substitution.

The impact of atom substitution on the glycoside linkage bridging the head and the tail parts in a nonionic surfactant molecule on aqueous dynamics of the resultant micellar solutions has been explored, employing time-resolved fluorescence and dielectric relaxation (DR) measurements. We have utilized n-octyl-β-D-glucopyranoside (OG) and n-octyl-β-D-thioglucopyranoside (OTG) as nonionic surfactants where the oxygen atom in the glucopyranoside unit is substituted by a sulfur atom. The substitution impact is immediately reflected in the dynamic light scattering measurements of aqueous solutions where the estimated size of the OTG micelles is found to be approximately four times larger than the OG micelles. Steady state spectral features obtained by using a fluorescent probe solute, coumarin 153 (C153), in these micellar solutions are quite similar and indicate locations of the solute at the micelle/water interface for both the surfactants. Interestingly, significant differences in the rotational and solvation dynamics of C153 in these two micellar solutions have been registered. The corresponding DR measurements do not indicate any signature of relaxation typical of bound water. The absence of bound water is further supported by the differential scanning calorimetric measurements. However, the typical slow solvation time scale for aqueous micellar solutions has been observed for these surfactants. Fluctuations in the solute-interface interaction energy due to the solute motion has been argued to be the origin for this slow solvation component as DR measurements do not indicate the presence of qualitatively similar relaxation time scale in the medium.

Interaction and Dynamics in a Fully Biodegradable Glucose-Containing Naturally Abundant Deep Eutectic Solvent: Temperature-Dependent Time-Resolved Fluorescence Measurements.

A new room-temperature deep eutectic solvent (DES) composed of glucose, urea, and water has been prepared and its relaxation dynamics explored via temperature-dependent time-resolved fluorescence measurements employing hydrophilic and hydrophobic solute probes. Differential scanning calorimetry measurements indicate a glass transition temperature (Tg) of ∼236 K. Measured viscosity coefficients (η) vary from ∼600 to ∼100 cP in the temperature range 318 ≤ T/K ≤ 343 and exhibit Arrhenius-type temperature dependence with an activation energy of ∼65 kJ mol-1. Interestingly, this DES forms a stable liquid at ∼300 K but is too viscous to be accurately measured by us below 318 K. Temperature-dependent dynamic fluorescence anisotropy measurements using hydrophobic and hydrophilic solutes of similar sizes reveal bi-exponential kinetics and Arrhenius-type temperature dependence for solute rotation times (⟨τr⟩) but with significantly decreased activation energies, ∼31 kJ mol-1 (hydrophobic) and ∼21 kJ mol-1 (hydrophilic). Deviation from hydrodynamics is further reflected in the strong fractional viscosity dependence of ⟨τr⟩: ⟨τr⟩ ∝ (η/T)p with p ≈ 0.3-0.5, indicating pronounced temporal heterogeneity in the relaxation dynamics. Dynamic fluorescence Stokes shift measurements (temporal resolution ∼85 ps) produce dynamic shifts of ∼500-700 cm-1, bi-exponential solvation energy relaxation with time constants in the range ∼0.2 ns and ∼4 ns, and estimated missing amplitudes of ∼65-75%. Impact of the density difference between a nonpolar solvent and this DES on the estimated missing amplitudes is explored via measuring the temperature-dependent densities and refractive indices of this DES. Lifetime measurements suggest considerable temperature dependence for the hydrophobic solute but no such dependence for the hydrophilic one. Excitation energy dependence of fluorescence emission of various solutes with widely different lifetimes indicates mild spatial heterogeneity for this DES.

Synthesis and characterization of manganese ferrite nanoparticles, and its interaction with bovine serum albumin: A spectroscopic and molecular docking approach

Magnetic nanoparticles are attractive for biomedical applications as they can be used for magnetic targeting of drugs, as contrast agents for MRI, and as externally-activated therapeutic agents (e.g. in magnetic hyperthermia). The understanding of biological interactions of the magnetic particles is mandatory for their successful clinical use, as these interactions give insights into blood circulation and biodistribution of the nanoparticles. In this work, we have synthesized manganese ferrite nanoparticles and investigated their interactions with bovine serum albumin (BSA) using various spectroscopic techniques. The nanoparticles were synthesized facilely through precipitation and were characterized by TEM, SEM, XRD, VSM, and DLS. The particles quenched the fluorescence of BSA statically indicating complex formation, established by Stern-Volmer equation and time-resolved fluorescence measurements. The binding constant is determined using UV–visible absorption and fluorescence spectroscopies. The structural integrity of BSA is not lost upon binding, as seen by circular dichroism data, indicating biocompatibility and potential for biomedical use. Molecular docking studies were used to identify the main amino acid residues within BSA involved in the interaction with the nanoparticles.

Impact of Acyl Chain Mismatch on the Formation and Properties of Sphingomyelin-Cholesterol Domains

Lateral segregation and the formation of lateral domains are well-known phenomena in ternary lipid bilayers composed of an unsaturated (low gel-to-liquid phase transition temperature (Tm)) phospholipid, a saturated (high-Tm) phospholipid, and cholesterol. The formation of lateral domains has been shown to be influenced by differences in phospholipid acyl chain unsaturation and length. Recently, we also showed that differential interactions of cholesterol with low- and high-Tm phospholipids in the bilayer can facilitate phospholipid segregation. Now, we have investigated phospholipid-cholesterol interactions and their role in lateral segregation in ternary bilayers composed of different unsaturated phosphatidylcholines (PCs) with varying acyl chain lengths, N-palmitoyl-D-erythro-sphingomyelin (PSM), and cholesterol. Using deuterium NMR spectroscopy, we determined how PSM was influenced by the acyl chain composition in surrounding PC environments and correlated this with the affinity of cholestatrienol (a fluorescent cholesterol analog) for PSM in the different PC environments. Results from a combination of time-resolved fluorescence measurements of trans-parinaric acid and Förster resonance energy transfer experiments showed that the relative affinity of cholesterol for phospholipids determined the degree to which the sterol promoted domain formation. From Förster resonance energy transfer, deuterium NMR, and differential scanning calorimetry results, it was clear that cholesterol also influenced both the thermostability of the domains and the degree of order in and outside the PSM-rich domains. The results of this study have shown that the affinity of cholesterol for both low-Tm and high-Tm phospholipids and the effects of low- and high-Tm phospholipids on each other influence both lateral structure and domain properties in complex bilayers. We envision that similar effects also contribute to lateral heterogeneity in even more complex biological membranes.

Formation of a PSI-PSII megacomplex containing LHCSR and PsbS in the moss Physcomitrella patens.

Mosses are one of the earliest land plants that diverged from fresh-water green algae. They are considered to have acquired a higher capacity for thermal energy dissipation to cope with dynamically changing solar irradiance by utilizing both the "algal-type" light-harvesting complex stress-related (LHCSR)-dependent and the "plant-type" PsbS-dependent mechanisms. It is hypothesized that the formation of photosystem (PS) I and II megacomplex is another mechanism to protect photosynthetic machinery from strong irradiance. Herein, we describe the analysis of the PSI-PSII megacomplex from the model moss, Physcomitrella patens, which was resolved using large-pore clear-native polyacrylamide gel electrophoresis (lpCN-PAGE). The similarity in the migration distance of the Physcomitrella PSI-PSII megacomplex to the Arabidopsis megacomplex shown during lpCN-PAGE suggested that the Physcomitrella PSI-PSII and Arabidopsis megacomplexes have similar structures. Time-resolved chlorophyll fluorescence measurements show that excitation energy was rapidly and efficiently transferred from PSII to PSI, providing evidence of an ordered association of the two photosystems. We also found that LHCSR and PsbS co-migrated with the Physcomitrella PSI-PSII megacomplex. The megacomplex showed pH-dependent chlorophyll fluorescence quenching, which may have been induced by LHCSR and/or PsbS proteins with the collaboration of zeaxanthin. We discuss the mechanism that regulates the energy distribution balance between two photosystems in Physcomitrella.

Systematically controlled fluorescence resonance energy transfer from cadmium telluride quantum dots to rhodamine 101 dye: steady-state versus time-resolved measurements

This article highlights the studies on precise measurements of energy transfer efficiency in a steady-state method by adopting sample preparation methods. Steady-state and time-resolved fluorescence measurements were employed to evaluate the cadmium telluride (CdTe) quantum dot (QD) photoluminescence quenching induced by the proximal rhodamine 101 dye (Rh 101) molecule via fluorescence resonance energy transfer (FRET). Further, the evaluation of steady-state measurements in comparison with time-resolved measurements was carried out. The FRET parameters, such as spectral overlap, Forster distance, and intermolecular distance for each donor–acceptor pair, were determined and variation of these parameters with respect to the size of the QD is discussed. Further, efforts to understand the optical and carrier relaxation dynamics of CdTe QDs are made using steady-state and time-resolved fluorescence data.

Chemical vapor deposition all-inorganic CsPbI2Br perovskite nanofibers for photodetector

Chemical vapor deposition method can prepare perovskite films with controllable morphology and adjustable photoelectric properties. In this work, we have prepared CsPbI2Br films at 400 °C–700 °C using chemical vapor deposition method. The grain size of perovskite film increases with the deposition temperature increment because higher temperature provides greater driving force for nucleation expansion. CsPbI2Br film deposited at 600 °C shows nanofiber structure with polycrystalline property accompanied by local single crystal structure. Steady-state and time-resolved fluorescence measurement explored the charge carrier recombination process of perovskite films. The photodetector based on CsPbI2Br film deposited at 600 °C shows excellent photoelectric response and switching ratio characteristics. This is ascribed to low defect density and fast charge transport along 1D fiber channels.

Benzimidazole and benzothiazole fluorophores with large Stokes shift and intense sky-blue emission in aggregation as Al3+ and Pb2+ sensors

New fluorophores based on 2-(2′-hydroxyphenyl)benzimidazole (HBZ) and 2-(2′-hydroxyphenyl)benzothiazole (HBT) for metal ion sensing were designed and synthesized using a simple method. The photophysical behaviour of these fluorophores were investigated in various solvents using UV–visible and fluorescence spectra. All the heterocycles showed strong excited state intramolecular proton transfer (ESIPT) characteristics with remarkably large Stokes shift (190–252 nm). Spatial charge distribution in the frontier molecular orbitals also demonstrated the ESIPT mechanism through intramolecular charge transfer. Time resolved fluorescence measurements for these heterocycles showed two long-life decay mechanisms which may be attributed to excited state enol and keto emission. These intense sky-blue emitters also exhibited aggregation induced blue shifted emissions due to restriction of intramolecular rotation processes. Fluorescence sensing studies for metal ions revealed the good selectivity of these fluorophores towards Al3+ and Pb2+. Theoretical computations performed using density functional theory methods showed two possible geometric configurations for Al3+ binding.

Self-assembly of porphyrin on graphene oxide in aqueous medium: fabrication, characterization, and photocatalytic studies.

We herein report the supramolecular self-assembly of a water soluble porphyrin, namely, 5,10,15,20-tetrakis(1-methyl-4-pyridinio)porphyrin-tetra(p-toluenesulfonate) (TMPyP), on the surface of graphene oxide (GO). The fabricated GO nanosheet and GO@TMPyP hybrid material composite have been characterized by using various spectroscopic and analytical techniques, e.g., scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), and Fourier transform infrared (FT-IR) spectroscopy. The steady state absorption measurements of the GO@TMPyP self-assembly showed a significant red shift (∼20 nm) compared to those of the control TMPyP in water. The steady state fluorescence measurements showed a significant fluorescence quenching of the singlet excited state of TMPyP in the presence of GO. These findings suggest the electron transfer reaction from TMPyP to GO. The time resolved fluorescence measurements showed a considerable decrease in the lifetime of the singlet state of TMPyP in the presence of GO, from which the rate and efficiency of the electron transfers from TMPyP to GO were determined to be 1.93 × 109 s-1 and 91%, respectively. The transient absorption measurements showed a considerable quenching of the triplet excited state of TMPyP in the self-assembly. All these findings confirm the occurrence of efficient electronic interactions between TMPyP and GO in both the ground and excited states. In addition, the fabricated GO@TMPyP showed high photocatalytic activity for the degradation of methylene blue (MB) and methyl orange (MO) mixed dye pollutants in water under visible light irradiation.