Extreme Fluorescence Sensitivity of Some Aniline Derivatives to Aqueous and Nonaqueous Environments: Mechanistic Study and Its Implication as a Fluorescent Probe

A novel aggregation-induced enhanced emission aromatic molecule: 2-aminophenylboronic acid dimer

Aggregation-induced enhanced emission (AIEE) molecules have significant applications in optoelectronics, biomedical probes and chemical sensors, and large amounts of AIEE molecules have been reported since the concept of AIEE was proposed. Most aromatic AIEE molecules have complex structures consisting of multiple aromatic rings and/or polycyclic skeletons. In this study, we find that 2-aminophenylboronic acid (2-APBA) with a simple structure is highly emissive in the solid state. Further studies reveal that 2-APBA exists in a dimeric form, and the 2-APBA dimer is a novel AIEE molecule. The underlying AIEE mechanism is that the 2-APBA dimeric units aggregate through intermolecular interactions to produce highly ordered molecular packing without the presence of π–π stacking interactions that would lead to aggregation-caused quenching. Furthermore, the 2-APBA dimer aggregates could reversibly transform into its non-fluorescent monomer form driven by new kinds of dynamic covalent B–N and B–O bonds, illustrating its good potential in molecular recognition, nanogating, chemo/bio-sensing and controlled drug release.

The 2-APBA dimer tending to aggregate into a highly ordered structure is discovered to be AIEE active. Through alternate treatment with CO₂ and N₂, 2-APBA can switch between monomer and dimer aggregates driven by dynamic covalent B–N and B–O bonds.

A promising strategy for improved solubilization of ionic drugs simply by electrostatic pushing

Addition of soluble salts to the SDS-bound phenosafranin induces electrostatic pushing of the probe within the micellar interior from its initial location at the micelle–water interface.

A Reduction-Based Sensor for Acrolein Conjugates with the Inexpensive Nitrobenzene as an Alternative to Monoclonal Antibody

Acrolein, a highly toxic α, β-unsaturated aldehyde, has been a longstanding key biomarker associated with a range of disorders related to oxidative stresses. One of the most promising methods for detecting acrolein involves the use of antibodies that can recognize the acrolein-lysine conjugate, 3-formyl-3, 4-dehydropiperidines (FDP), within oxidatively stressed cells and tissues from various disease states. We have uncovered here that FDP could reduce nitroarenes in high yields at 100 °C in the presence of excess CaCl2 as a Lewis acid promoter. This unique transformation allowed for the development of a de novo method for detecting levels of FDPs generated from proteins in urine or blood serum samples. Thus we successfully converted a non-fluorescent and inexpensive 4-nitrophthalonitrile probe to the corresponding fluorescent aniline, thereby constituting the concept of fluorescent switching. Its sensitivity level (0.84 nmol/mL) is more than that of ELISA assays (3.13 nmol/mL) and is already equally reliable and reproducible at this early stage of development. More importantly, this method is cost effective and simple to operate, requiring only mixing of samples with a kit solution. Our method thus possesses potential as a future alternative to the more costly and operatively encumbered conventional antibody-based methods.

Revising Intramolecular Photoinduced Electron Transfer (PET) from First-Principles

Photoinduced electron transfer (PET) plays relevant roles in many areas of chemistry, including charge separation processes in photovoltaics, natural and artificial photosynthesis, and photoluminescence sensors and switches. As in many other photochemical scenarios, the structural and energetic factors play relevant roles in determining the rates and efficiencies of PET and its competitive photodeactivation processes. Particularly, in the field of fluorescent sensors and switches, intramolecular PET is believed (in many cases without compelling experimental proof) to be responsible of the quench of fluorescence. There is an increasing experimental interest in fluorophore's molecular design and on achieving optimal excitation/emission spectra, excitation coefficients, and fluorescence quantum yields (importantly for bioimaging purposes), but less efforts are devoted to fundamental mechanistic studies. In this Account, I revise the origins of the fluorescence quenching in some of these systems with state-of-the-art quantum chemical tools. These studies go beyond the common strategy of analyzing frontier orbital energy diagrams and performing PET thermodynamics calculations. Instead, the potential energy surfaces (PESs) of the lowest-lying excited states are explored with time-dependent density functional theory (TD-DFT) and complete active space self-consistent field (CASSCF) calculations and the radiative and nonradiative decay rates from the involved excited states are computed from first-principles using a thermal vibration correlation function formalism. With such a strategy, this work reveals the real origins of the fluorescence quenching, herein entitled as dark-state quenching. Dark states (those that do not absorb or emit light) are often elusive to experiments and thus, computational investigations can provide novel insights into the actual photodeactivation mechanisms. The success of the dark-state quenching mechanism is demonstrated for a wide variety of fluorescent probes, including proton, cation and anion targets. Furthermore, this mechanism provides a general picture of the fluorescence quenching which englobes intramolecular charge-transfer (ICT), ratiometric quenching, and those radiationless mechanisms believed to be originated by PET. Finally, this Account provides for the first time a computational protocol to quantitatively estimate this phenomenon and provides the ingredients for the optimal design of fluorescent probes from first principles.

Competitive binding of nile red between lipids and β-cyclodextrin

Fluorometric measurements are exploited to explore the binding interactions of nile red (NR) with anionic lipid dimyristoyl-L-α-phosphatidylglycerol (DMPG), zwitterionic lipid dimyristoyl-L-α-phosphatidylcholine (DMPC) as well as neutral cyclic oligosaccharide β-cyclodextrin (β-CD) solutions. The binding constants are found to be quite high and comparable (within a factor of five). Series of spectral techniques like steady state fluorescence and fluorescence anisotropy study, micropolarity study, quenching study and time resolved experiments reveal that the addition of β-CD to the probe-lipid complexes results in weakening of the lipid-probe interaction and formation of probe-β-CD inclusion complexes leading to the removal of some of the probe (NR) molecules from the lipid environments. The extent of removal of NR is, however, more from DMPG than DMPC lipid. The phenomena are explained from the concept of competitive binding of the probe between the lipids and β-CD. Since lipids are the principal constituents of the cell walls, the work might make a foundation for the possible removal of excess of molecules like nile red adsorbed on the cell walls.

Modeling of multi-exciton transient absorption spectra of protochlorophyllide aggregates in aqueous solution

Protochlorophyllide (Pchlide) is a natural porphyrin, a precursor of chlorophyll, synthesized by plants for its photosynthetic apparatus. The pigment spontaneously forms aggregates when dissolved in neat water solution. We present here calculations of the transient absorption spectra and its comprising components (ground-state bleach, stimulated emission, and excited-state absorption) for a strongly excitonically coupled linear chain of four Pchlide chromophores, using exciton theory with phenomenological Gaussian line shapes and without energetic disorder. A refined multiexciton model that includes static disorder is applied to fit the experimental power-dependent transient absorption spectra of aqueous protochlorophyllide and the kinetics for delay times up to 20 ps after photoexcitation. We show that population up to the 4-exciton manifold is sufficient to explain the pronounced saturation of the bleaching and the shape changes in the instantaneous, t = 0.2 ps transient spectra when the pulse energy is increased from 10 to 430 nJ per pulse. The decay of the multiexciton manifold is relatively slow and is preceded by a spectroscopically distinct process. We suggest that the exciton states in the Pchlide aggregates are mixed with charge-transfer states (CTS) and that the population and repopulation of the CTS coupled to the exciton states explains the relatively slow decay of the multiexciton manifold. The relevance of our results to the optical properties and dynamics of natural photosynthetic complexes and the possible physical origin of CTS formation are discussed.

Preparation of high-quality organic semiconductor nanoparticle films by solvent-evaporation-induced self-assembly

Organic semiconductor nanoparticles are expected to be used in organic optical and electronic devices due to their unique optical and electrical properties. However, no method has been reported for the preparation of high-quality organic nanoparticle films without remaining additives and being capable of dealing with binary nanoparticle blends. We developed a simple approach to fabricate high-quality organic semiconductor nanoparticle films from their aqueous solutions by solvent-evaporation-induced self-assembly. Only volatile solvents are employed in the nanoparticle solutions, so the self-assembled nanoparticle films are free of additives. Moreover, this method is also suitable for fabricating thin films containing binary nanoparticles. Therefore, it paves the way for potential applications of organic semiconductor nanoparticles in nanoscale optical and electronic devices.

Excited-state proton transfer to solvent from phenol and cyanophenols in water

The excited-state proton transfer (ESPT) to solvent from phenol (PhOH) and cyanophenols (CNOHs) in water was studied by means of time-resolved fluorescence and photoacoustic spectroscopy. A characteristic property of PhOH and CNOHs is that the fluorescence quantum yields of the deprotonated forms are remarkably small (< or = 10(-3)) and the lifetimes are extremely short (< or = 30 ps). Time-resolved fluorescence measurements for PhOH, CNOHs, and their methoxy analogues at 298 K indicate that o- and m-cyanophenols (o- and m-CNOH) undergo rapid ESPT to the solvent water with rate constants of 6.6 x 10(10) and 2.6 x 10(10) s(-1), respectively, whereas the fluorescence properties of PhOH and p-CNOH does not exhibit clear evidence of the ESPT reaction. Photoacoustic measurements show that photoexcitation of o- and m-CNOH in water results in negative volume changes, supporting the occurrence of ESPT to produce a geminate ion pair. In contrast, the volume contractions for the PhOH and p-CNOH solutions are negligibly small, which indicates that, in these compounds, the yields of solvent-separated ion pairs resulting from the ESPT are very small. The volume change per absorbed Einstein (DeltaV(r)) for o-CNOH is obtained to be -5.0 mL Einstein(-1), which is much smaller than the estimated volume contraction per photoconverted mole (DeltaV(R)). This suggests that the geminate recombination between the ejected proton and the cyanophenolate anion occurs after rapid deactivation of the excited ion pair. In the temperature range between 275 and 323 K, the proton dissociation rates of o- and m-CNOH in H(2)O and D(2)O are slower than the solvent relaxation rates evaluated from the Debye dielectric relaxation time, indicating that the overall rate constant is determined mainly by the proton motion along the reaction coordinate.

Efficient singlet-state deactivation of cyano-substituted indolines in protic solvents via CN--HO hydrogen bonds

The photophysical properties of indoline (I) and three of its derivatives, namely, N-methylindoline (MI), 5-cyanoindoline (CI), and 5-cyano-N-methylindoline (CMI), are studied in H-donating solvents of varying polarity. Based on measurements of fluorescence yield and lifetime, and of triplet yield and hydrated-electron formation, two distinct mechanisms of solvent-induced fluorescence quenching are evidenced. The first mechanism involves the cyano substituent and leads to an increase in the rate constant of internal conversion of one order of magnitude in ethanolic solution and of more than two orders of magnitude in water, as compared to solutions in n-hexane or acetonitrile. A similar trend had previously been observed in the case of 4-N,N-dimethylaminobenzonitrile (DMABN). The second mechanism reduces the fluorescence lifetimes of the non-cyanated derivatives in aqueous solution by one order of magnitude and is related to the formation of hydrated electrons. Neither of these mechanisms is influenced by methylation at the ring nitrogen. Quantum chemical calculations are performed on the ground and excited states of the hydrogen-bonded complexes between protic solvents and MI as well as CMI. Stable hydrogen-bonded configurations involving the CN substituent and a solvent OH group are found; these configurations are stable both in the ground and the first excited singlet states, whereas the corresponding complex at the ring amino nitrogen is stable in the ground state only. The CN--HO configuration is therefore a prime candidate for a mechanistic explanation of the observed quenching by the first mechanism. These findings may have useful applications for the design of fluorescence probes for water in biological systems.

Photophysics of a Cationic Biological Photosensitizer in Anionic Micellar Environments: Combined Effect of Polarity and Rigidity

A steady-state and time-resolved photophysical study of a cationic phenazinium dye, phenosafranin (PSF), has been investigated in well-characterized biomimetic micellar nanocavities formed by anionic surfactants of varying chain lengths, namely, sodium decyl sulfate (S(10)S), sodium dodecyl sulfate (S(12)S), and sodium tetradecyl sulfate (S(14)S). In all these micellar environments, the charge transfer fluorescence of PSF shows a large hypsochromic shift along with an enhancement in the fluorescence quantum yield as compared to that in aqueous medium. A reduction in the nonradiative deactivation rate within the hydrophobic interior of micelles led to an increase in the fluorescence yield and lifetime. The present work shows the degree of accessibility of the fluorophore toward the ionic quencher in the presence of surfactants of different surfactant chain lengths. The fluorometric and fluorescence quenching studies suggest that the fluorophore resides at the micelle-water interfacial region. The enhancements in the fluorescence anisotropy and rotational relaxation time of the probe in all the micellar environments from the pure aqueous solution suggest that the fluorophore binds in motionally restricted regions introduced by the micelles. Polarity and viscosity of the microenvironments around the probe in the micellar systems have been determined. The work has paid proper attention to the hydrophobic effect of the surfactant chain length on photophysical observations.

Effect of cyclodextrin nanocavity confinement on the photophysics of a beta-carboline analogue: a spectroscopic study

Interaction of a beta-carboline based biologically active molecule, 3-acetyl-4-oxo-6,7-dihydro-12H indolo-[2,3-a] quinolizine (AODIQ), with alpha-, beta-, and gamma-cyclodextrins (CDs) in aqueous solution has been studied using steady state and time-resolved fluorescence and steady-state fluorescence anisotropy techniques. Polarity dependent intramolecular charge transfer (ICT) process is responsible for the remarkable sensitivity of this biological fluorophore to the CD environments. Upon encapsulation, the CT fluorescence exhibits hypsochromic shift along with enhancements in the fluorescence yield, fluorescence anisotropy (r), and fluorescence lifetime. The reduction in the nonradiative deactivation rate of the fluorophore within the CD nanocavities leads to an increase in both fluorescence yield and lifetime. Among the three CDs, gamma-CD shows the most spectacular confinement effect. The results establish the formation of 1:1 AODIQ:CD inclusion complexes in alpha- and beta-CDs. In aqueous gamma-CD solutions, however, depending on the concentration of the gamma-CD, formation of both 1:1 and 1:2 complexes have been revealed. Hydrodynamic radii of the 1:1 and 1:2 probe-gamma-CD supramolecular complexes have also been determined.