Design Strategies for Ratiometric Chemosensors: Modulation of Excitation Energy Transfer at the Energy Donor Site

Design and synthesis of photostable triphenylamine based neutral AIE nano luminogens: specific and long-term tracking of mitochondria in cells

With the increasing dependence on fluorescence bioimaging, luminogens with aggregation-induced emission (AIE) properties have gained significant attention due to their excellent photostabilization, minimal photobleaching, high reliability, and superior biocompatibility. Since mitochondria are crucial subcellular organelles in eukaryotic cells with important biological functions, organelle-specific AIE emitters with distinct functions have been highly sought after, but with limited success using simple synthetic methods. Here, we describe a strategy for synthesizing two triphenylamine (TPA) based acrylonitriles, tethered to different donor groups, TPA and phenothiazine (PTZ), respectively, with superior AIE properties using Suzuki coupling. We conducted a systematic and detailed experimental analysis of the structural characteristics of both AIE luminogens, which exhibited excellent photostability, a large Stokes shift, and bright solid-state emission. A cell viability study carried out with F1 and F2 dyes revealed that both luminogens exhibited excellent biocompatibility. Based on fluorescence experiments, F2 displayed excellent AIE characteristics, permeability, biocompatibility, and photostability compared to rhodamine 123, allowing it to selectively stain and track mitochondria in cancer cells over an extended period of time. The Pearson correlation coefficient of F2 and rhodamine 123 was estimated to have an r-value of 0.99. Our findings are expected to provide insight into the synthesis of an extensive archive of AIE-based acrylonitriles with fascinating properties for mitochondrial staining.

A highly selective and sensitive Zn2+ fluorescent sensor based on zinc finger-like peptide and its application in cell imaging

Developing new chemosensors for detection of Zn²⁺ has attracted great attentions because of the important roles of Zn²⁺ in biological systems, and it will produce toxic effects with an excessive intake of zinc ion. Metalloproteins are often used as an effective template for the design and development of peptide-based fluorescent sensors. In this study, we designed a new and simple ratiometric fluorescent sensor for Zn²⁺, which was based on a zinc finger-like peptide and labeled with a dansyl group, i.e., Dansyl-His-Gln-Arg-Thr-His-Trp-NH₂ (D-P6), by using solid phase peptide synthesis (SPPS). The dimeric peptide has a high affinity for Zn²⁺ overothermetalions, as indicated by spectroscopic studies, as well as molecular modeling. Remarkably, the sensor exhibited a highly selective and sensitive ratiometric fluorescent response to Zn²⁺ by fluorescent resonance energy transfer effect between tryptophan residue and fluorophore dansyl group, with a very low detection limit of 33 nM in aqueous solution. Furthermore, the sensor displayed a very low biotoxicity, which allows successful detection of Zn²⁺ in living HeLa cells. We believe that the new sensor may have potential applications in biological science.

Torsion-Induced Nonradiative Relaxation of the Singlet Excited State of meso-Thienyl Bodipy and Charge Separation, Charge Recombination-Induced Intersystem Crossing in Its Compact Electron Donor/Acceptor Dyads

We prepared a series of meso-thienyl boron-dipyrromethene (Bodipy) derivatives to investigate the spin-orbit charge transfer intersystem crossing (SOCT-ISC). The photophysical properties of the compounds were studied by steady-state and femtosecond/nanosecond transient absorption spectroscopy, as well as density functional theory (DFT) computations. Different from the meso-phenyl Bodipy analogues, the meso-thienyl Bodipy are weakly fluorescent. Based on femtosecond transient absorption and DFT computations, we propose that the torsion of the thienyl group and the distortion of the Bodipy core (19.7 ps) in the S₁ state lead to a conical intersection on the potential energy surface as an efficient nonradiative decay channel (408 ps), which is responsible for the observed weak fluorescence as compared to the meso-phenyl analogue. The increased fluorescence quantum yield (from 5.5 to 14.5%) in viscous solvents supports this hypothesis. With the electron donor 4'-hydroxylphenyl moiety attached to the meso-thienyl unit, the fast charge separation (CS, 15.3 ps) and charge recombination (CR, 238 ps) processes outcompete the torsion-induced nonradiative decay and induce fast ISC through the SOCT-ISC mechanism. The triplet quantum yield of the electron donor/acceptor dyad is highly dependent on solvent polarity (Φ_T = 1.9-45%), which supports the SOCT-ISC mechanism, and the triplet-state lifetime is up to 247.3 μs. Using the electron donor-acceptor dyad showing SOCT-ISC as a triplet photosensitizer, efficient triplet-triplet annihilation (TTA) upconversion was observed with a quantum yield of up to 6.0%.

Effect of the Molecular Conformation on Excitation Energy Transfer in Conformationally Constrained Boryl-BODIPY Dyads

We studied the dual emission characteristics of a series of boryl-BODIPYs (1-6) comprised of triarylborane (TAB) as an energy donor and BODIPY as an energy acceptor. The molecular conformations of dyads 1-6 were systematically tuned by judiciously changing the spacer that bridged the boryl and BODIPY moieties. Frontier molecular orbitals (FMOs) are localized in 3, 4, and 6 with a twisted molecular conformation. In contrast, FMOs are significantly delocalized in 1, 2, and 5 with the least-twisted molecular conformation. Dyads 1-6 showed dual emission features when they were excited at the TAB-dominated absorption band. However, the ratio between the two emission bands in 1-6 significantly varied depending on the molecular conformations. Systematic photoluminescence (PL) studies (both steady-state and time-resolved PL) together with computational, crystal structure, and anion binding studies established that the frustrated excited-state energy transfer from borane to BODIPY is the cause of the dual emission features in these molecular dyads. These studies also revealed that the energy transfer from borane to BODIPY can be elegantly tuned by modulating the dihedral angle between these two moieties.

Ultrafast Electron/Energy Transfer and Intersystem Crossing Mechanisms in BODIPY-Porphyrin Compounds

Meso-substituted borondipyrromethene (BODIPY)-porphyrin compounds that include free base porphyrin with two different numbers of BODIPY groups (BDP-TTP and 3BDP-TTP) were designed and synthesized to analyze intramolecular energy transfer mechanisms of meso-substituted BODIPY-porphyrin dyads and the effect of the different numbers of BODIPY groups connected to free-base porphyrin on the energy transfer mechanism. Absorption spectra of BODIPY-porphyrin conjugates showed wide absorption features in the visible region, and that is highly valuable to increase light-harvesting efficiency. Fluorescence spectra of the studied compounds proved that BODIPY emission intensity decreased upon the photoexcitation of the BODIPY core, due to the energy transfer from BODIPY unit to porphyrin. In addition, ultrafast pump-probe spectroscopy measurements indicated that the energy transfer of the 3BDP-TTP compound (about 3 ps) is faster than the BDP-TTP compound (about 22 ps). Since the BODIPY core directly binds to the porphyrin unit, rapid energy transfer was seen for both compounds. Thus, the energy transfer rate increased with an increasing number of BODIPY moiety connected to free-base porphyrin.

Synthesis and DSSC application of BODIPY decorated triazole bridged and benzene nucleus cored conjugated dendrimers

Conjugated dendrimers decorated with 5,5-difluoro-10-(4-(prop-2-ynyloxy)phenyl)-5H-dipyrrolo[1,2-c:1',2'-f][1,3,2]diazaborinin-4-ium-5-uide, usually known as boron dipyrromethene (BODIPY), have been synthesized and their application as photosensitizer in dye sensitized solar cells (DSSCs) has been evaluated. Third generation triazole bridged BODIPY dendrimers show higher light energy harvesting efficiency of 2.5% better than the first and second generation dendrimers, when used as a dye material in solar cells. The current intensity increases with an increase in the generation of the dendrimer as revealed by cyclic voltammetry. Fluorescence decay analysis shows that the relaxation times τ ₁ and τ ₂ increase as the dendrimer generation increases, however τ ₂ for the third generation dendrimer decreases because of fluorescence quenching due to molecular crowding.

A highly selective ratiometric fluorescent probe based on naphthalimide for detection and imaging of CYP1A1 in living cells and zebrafish

Real-time monitoring of the cytochrome P450 1A1 (CYP1A1) activity in complex biological systems via a practical tool is highly sought after because of its significant role in the metabolism and bioactivation of various xenobiotics. Herein, according to slight differences in the 3D structure and substrate preference between CYP1A1 and its homologous CYP1A2, a series of novel ratiometric fluorescent probes were designed and synthesized using 1,8-naphthalimide because of its trait of naked-eye visualization and ratiometric fluorescence to achieve the detection of CYP1A1 in biological samples. Among these probes, NEiPN showed good water solubility, highly isoform selectivity and great sensitivity (LOD = 0.04874 nM) for CYP1A1 under simulated physiological conditions, which makes it favorable for monitoring CYP1A1 in vivo. Remarkably, NEiPN exhibited excellent reproducibility when it was used to detect the CYP1A1 content in human liver microsomes, which indicated that it has a great potential for quantifying the CYP1A1 content in real biological samples. Furthermore, NEiPN showed relatively low cytotoxicity and has been successfully applied in biological imaging in living cells and zebrafish. These findings indicate that NEiPN is capable of real-time monitoring of the activity of endogenous CYP1A1, which could provide support for CYP1A1-associated pathological processes.

A novel rhodamine-based optical probe for mercury(II) ion in aqueous medium: A nanomolar detection, wide pH range and real water sample application

In present work, we designed and synthesized new chemosensor RPy, containing the rhodamine and 2,6-pyridinedicarboxaldehyde functionality, for the selective detection of mercury (II) (Hg²⁺) ion in aqueous DMSO solvents. The RPy acts as "turn ON" probe for Hg²⁺ ion with high selectivity and sensitivity over the series of other competing metal ions based on colorimetric and fluorimetric techniques. Due to the incorporation of two rhodamine moieties enhance the chelation sites for mercury binding, which reflects in the lowering of the detection limit up to 26 nM. The Job plot method confirms the 1:2 stoichiometric interactions between the RPy and Hg²⁺ ion. The formation of the chelation complex between RPy and Hg²⁺ ion with spirolactam ring opening was thoroughly investigated by absorption, emission, ¹H NMR, and mass analysis. The detection of Hg²⁺ ion by RPy is retained at broad pH range 4-9. Further, the probe RPy is successfully explored to measure the contamination of Hg²⁺ ion in the real water samples using spike and recovery method.

A simple D–π–A system of phenanthroimidazole-π-fluorenone for highly efficient non-doped bipolar AIE luminogens: synthesis, and molecular optical, thermal and electrochemical properties

We report the synthesis of phenanthroimidazole based dyes incorporating fluorenone using Suzuki coupling and their aggregation induced emission characterisation.

Synthesis, photophysical and electrochemical properties of pyridine, pyrazine and triazine-based (D-π-)2A fluorescent dyes

The donor-acceptor-π-conjugated (D-π-)₂A fluorescent dyes OUY-2, OUK-2 and OUJ-2 with two (diphenylamino)carbazole thiophene units as D (electron-donating group)-π (π-conjugated bridge) moiety and a pyridine, pyrazine or triazine ring as electron-withdrawing group (electron-accepting group, A) have been designed and synthesized. The photophysical and electrochemical properties of the three dyes were investigated by photoabsorption and fluorescence spectroscopy, Lippert-Mataga plots, cyclic voltammetry and density functional theory calculations. The photoabsorption maximum (λ_max,abs) and the fluorescence maximum (λ_max,fl) for the intramolecular charge-transfer characteristic band of the (D-π-)₂A fluorescent dyes show bathochromic shifts in the order of OUY-2 < OUK-2 < OUJ-2. Moreover, the photoabsorption bands of the (D-π-)₂A fluorescent dyes are nearly independent of solvent polarity, while the fluorescence bands showed bathochromic shifts with increasing solvent polarity (i.e., positive fluorescence solvatochromism). The Lippert-Mataga plots for OUY-2, OUK-2 and OUJ-2 indicate that the Δμ (= μ_e - μ_g) value, which is the difference in the dipole moment of the dye between the excited (μ_e) and the ground (μ_g) states, increases in the order of OUY-2 < OUK-2 < OUJ-2. Therefore, the fact explains our findings that OUJ-2 shows large bathochromic shifts of the fluorescence maxima in polar solvents, as well as the Stokes shift values of OUJ-2 in polar solvents are much larger than those in nonpolar solvents. The cyclic voltammetry of OUY-2, OUK-2 and OUJ-2 demonstrated that there is little difference in the HOMO energy level among the three dyes, but the LUMO energy levels decrease in the order of OUY-2 > OUK-2 > OUJ-2. Consequently, this work reveals that for the (D-π-)₂A fluorescent dyes OUY-2, OUK-2 and OUJ-2 the bathochromic shifts of λ_max,abs and λ_max,fl and the lowering of the LUMO energy level are dependent on the electron-withdrawing ability of the azine ring, which increases in the order of OUY-2 < OUK-2 < OUJ-2.

Unveiling the Photophysical Properties of Boron Heptaaryldipyrromethene Derivatives

Increased interest has been devoted to the discovery of multifunctional materials with desirable properties, as continuous performance enhancement of various devices mainly depends on high-performance materials. Now, density functional theory has become a powerful tool to design new materials and rationalize experimental observations. In this work, we explored the photophysical properties origin of chiral boron heptaaryldipyrromethene (heptaaryl-BODIPY), which has charming optoelectronic properties. At the same time, we designed the other five compounds on the basis of heptaaryl-BODIPY. The simulated electronic absorption and emission spectra of heptaaryl-BODIPY are in agreement with experimental ones, allowing us to reliably assign its electronic transition property. The designed compound 6 shows remarkably large first hyperpolarizability value up to 82.78×10^-30  esu. For this kind of compounds, their NLO response values associate with not only position but also electronic nature of substituent groups. Moreover, electron reorganization energies of compounds 1-4 are comparable to tris(8-hydroxyquinolinato)aluminium(III) which is a typical electron transport material. Intriguingly, the studied compounds are the excellent fluorescent probe materials from the standpoint of large Stokes shift and high emission efficiency. Our work enables an opportunity for understanding the relationship between microelectronic structure and macroscopic performance of BODIPY derivatives.

Rational Design of a Green-Light-Mediated Unimolecular Platform for Fast Switchable Acidic Sensing

A controllable sensing ability strongly connects to complex and precise events in diagnosis and treatment. However, imposing visible light into the molecular-scale mediation of sensing processes is restricted by the lack of structural relevance. To address this critical challenge, we present the rational design, synthesis, and in vitro studies of a novel cyanostyryl-modified azulene system for green-light-mediated fast switchable acidic sensing. The advantageous features of the design include a highly efficient green-light-driven Z/E-isomerization (a quantum yield up to 61.3%) for fast erasing chromatic and luminescent expressions and a superior compatibility with control of ratiometric protonation. Significantly, these merits of the design enable the development of a microfluidic system to perform a green-light-mediated reusable sensing function toward a gastric acid analyte in a miniaturized platform. The results may provide new insights for building future integrated green materials.

Functional phenylethynylene side arm poly(arylene ethynylene) conjugated polymers: optical and electrochemical behavior for enrichment of electronic applications

The new type of poly(phenyl ethynylene) side arm conjugated polymers were synthesised with effective photophysical and electrochemical properties.

A3-Type star stilbene and cyanostilbene molecules: synthesis, fluorescence properties and bio-imaging application

In this article, we report star-like stilbene and cyanostilbene molecules exhibiting strong fluorescence, ICT and AIE properties. Star cyanostilbene was found to be an excellent fluorophore for bio-imaging application.

BODIPY-Bacteriochlorin Energy Transfer Arrays: Toward Near-IR Emitters with Broadly Tunable, Multiple Absorption Bands

A series of energy transfer arrays, comprising a near-IR absorbing and emitting bacteriochlorin, and BODIPY derivatives with different absorption bands in the visible region (503-668 nm) have been synthesized. Absorption band of BODIPY was tuned by installation of 0, 1, or 2 styryl substituents [2-(2,4,6-trimethoxyphenyl)ethenyl], which leads to derivatives with absorption maxima at 503, 587, and 668 nm, respectively. Efficient energy transfer (>0.90) is observed for each dyad, which is manifested by nearly exclusive emission from bacteriochlorin moiety upon BODIPY excitation. Fluorescence quantum yield of each dyad in nonpolar solvent (toluene) is comparable with that observed for corresponding bacteriochlorin monomer, and is significantly reduced in solvent of high dielectric constants (DMF), most likely by photoinduced electron transfer. Given the availability of diverse BODIPY derivatives, with absorption between 500-700 nm, BODIPY-bacteriochlorin arrays should allow for construction of near-IR emitting agents with multiple and broadly tunable absorption bands. Solvent-dielectric constant dependence of Φf in dyads gives an opportunity to construct environmentally sensitive fluorophores and probes.

Synthesis and optical and electrochemical properties of julolidine-structured pyrido[3,4-b]indole dye

The julolidine-structured pyrido[3,4-b]indole dye ET-1 possesses the ability to act as a calorimetric and fluorescent sensor for Brønsted and Lewis acids.

Development of a unique family of two-photon full-color-tunable fluorescent materials for imaging in live subcellular organelles, cells, and tissues

We outline the rational design, synthesis, optical property studies, and biological imaging studies of a unique family of two-photon full-color-tunable functional fluorescent materials.

Sequential Block Copolymer Self-Assemblies Controlled by Metal-Ligand Stoichiometry

While numerous efforts have been devoted to developing easy-to-use probes based on block copolymers for detecting analytes due to their advantages in the fields of self-assembly and sensing, a progressive response on block copolymers in response to a continuing chemical event is not readily achievable. Herein, we report the self-assembly of a 4-piperazinyl-1,8-naphthalimide based functional block copolymer (PS-b-PN), whose self-assembly and photophysics can be controlled by the stoichiometry-dependent metal-ligand interaction upon the side chain. The work takes advantages of (1) stoichiometry-controlled coordination-structural transformation of the piperazinyl moiety on PS-b-PN toward Fe(3+) ions, thereby resulting in a shrinkage-expansion conversion of the self-assembled nanostructures in solution as well as in thin film, and (2) stoichiometry-controlled competition between photoinduced electron transfer and spin-orbital coupling process upon naphthalimide fluorophore leading to a boost-decline emission change of the system. Except Fe(3+) ions, such a stoichiometry-dependent returnable property cannot be observed in the presence of other transition ions. The strategy for realizing the dual-channel sequential response on the basis of the progressively alterable nanomorphologies and emissions might provide deeper insights for the further development of advanced polymeric sensors.

Single near-infrared fluorescent probe with high- and low-sensitivity sites for sensing different concentration ranges of biological thiols with distinct modes of fluorescence signals

We describe a unique approach for the development of an interesting type of the fluorescent probes, which can show different modes of fluorescence signals to distinct concentration ranges of a target of interest. The key points for the design of the new type of the fluorescent probes include the judicious selection of the dye platforms and the corresponding high- and low-sensitivity sites. It is known that the normal concentrations of biological thiols have significant biological functions. However, up- or down-regulated concentrations of thiols may induce several diseases. Therefore, it is highly important to monitor the changes of thiol concentrations in living systems. Based on the proposed strategy, we engineer the novel NIR fluorescent probe, CHMC-thiol, which remarkably can display a turn-on signal to the low concentration range of thiols and a ratiometric response to the high concentration range of thiols for the first time. We anticipate that the intriguing strategy formulated herein will be widely useful for the development of concentration range-dependent fluorescent probes.

Enhancement of two photon absorption properties and intersystem crossing by charge transfer in pentaaryl boron-dipyrromethene (BODIPY) derivatives

Indication of charge transfer and an intersystem crossing mechanism for the B4 compound by using ultrafast pump probe spectroscopy.

A perylene bisimide derivative with pyrene and cholesterol as modifying structures: synthesis and fluorescence behavior

Intramolecular singlet–singlet excitation energy transfer from pyrene to perylene bisimide via Dexter mechanism and aggregate emission of perylene bisimide.

Synthesis of energy transfer cassettes via click and Suzuki–Miyaura cross coupling reactions

Novel cassettes capable of energy transfer involving simple synthetic methodsviz., click reaction at C-8 position and palladium catalyzed Suzuki–Miyaura cross coupling at C-6 position of imidazo[1,2-a]pyrazine, have been represented.

A Fluorescent Hypochlorite Probe Built on 1,10-Phenanthroline Scaffold and its Ion Recognition Features

In this study, the synthesis of 7-((Hydroxyimino)methyl)-1,10-phenanthroline-4-carbaldehyde oxime (1) in two steps starting from 4,7-dimethyl-1,10-phenanthroline (2) is reported. It is found that compound 1 can be used as a fluorogenic probe for the detection of hypochlorite ion in aqueous solution. NMR and mass spectral analysis indicate that probe 1 undergoes a chemical transformation through its oxime units upon treatment with hypochlorite, which results in a remarkable enhancement of the emission intensity. Also, metal ion recognition properties of probe 1 is investigated. It is noted that compound 1 is responsive to Zn(2+), Cd(2+), Ni(2+) and Cu(2+) metal ions, which reduced the emission intensity under identical conditions. Graphical Abstract The design, synthesis and properties of a new fluorescent hypochlorite probe is described. It is found that probe 1 immediately undergoes an oxidation reaction with NaClO through its oxime units in 0.1 M Na2CO3-NaHCO3 buffer containing DMF (pH = 9.0, 30:1 v/v) at room temperature, which resulted in a remarkable enhancement of the emission intensity. It is noteworthy that this novel probe 1 is highly selective to hypochlorite ion when compared to some other ROS and anions. On the other hand, probe 1 also induces turn-off fluorogenic responses to metal ions such as Zn(2+), Cd(2+), Ni(2+) and Cu(2+) ions under identical conditions.

Sequential oligodiacetylene formation for progressive luminescent color conversion via co-micellar strategy

This work takes advantage of a diphenyl-diacetylene-based co-micellar nano-ensemble that can undergo a sequential photocrosslinking to form a corresponding trimeric oligodiacetylene and a dimeric oligodiacetylene.

Design and construction of organic systems that can enhance internal quantum efficiency to fully make use of radiative decay during the excitation process is highly desirable for the development of new-generation emissive materials. In practice, to employ those materials with an effective tuning of triplet-state involved multicolor luminescence is quite a considerable issue. This work takes advantage of a diphenyl-diacetylene-based co-micellar nano-ensemble that can undergo a sequential photocrosslinking to form a corresponding trimeric oligodiacetylene and a dimeric oligodiacetylene. Their emissive bands individually cover the visible-light spectral region and such a controllable dual band characteristic can result in a progressive luminescent color conversion from light-green to white and finally to purplish blue. In addition, the oligodiacetylenes possess an ultrafast intersystem crossing characteristic with a small S₁–T₁ energy gap. The leading thermally activated delayed fluorescence and phosphorescence can be employed to achieve long-life broadband emissions without chromatic aberration in this single organic system, towards the creation of emitting materials with high quantum efficiency. The synthesis of the sub-components is simple and the co-micellar strategy is extremely straightforward. Such a material design and approach may find potential uses in the microfabrication of novel organic light-emitting diodes and sophisticated controllable optoelectronic devices.

From Dark to Light to Fluorescence Resonance Energy Transfer (FRET): Polarity-Sensitive Aggregation-Induced Emission (AIE)-Active Tetraphenylethene-Fused BODIPY Dyes with a Very Large Pseudo-Stokes Shift

The work presented herein is devoted to the fabrication of large Stokes shift dyes in both organic and aqueous media by combining dark resonance energy transfer (DRET) and fluorescence resonance energy transfer (FRET) in one donor-acceptor system. In this respect, a series of donor-acceptor architectures of 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) dyes substituted by one, two, or three tetraphenylethene (TPE) luminogens were designed and synthesised. The photophysical properties of these three chromophore systems were studied to provide insight into the nature of donor-acceptor interactions in both THF and aqueous media. Because the generation of emissive TPE donor(s) is strongly polarity dependent, due to its aggregation-induced emission (AIE) feature, one might expect the formation of appreciable fluorescence emission intensity with a very large pseudo-Stokes shift in aqueous media when considering FRET process. Interestingly, similar results were also recorded in THF for the chromophore systems, although the TPE fragment(s) of the dyes are non-emissive. The explanation for this photophysical behaviour lies in the DRET. This is the first report on combining two energy-transfer processes, namely, FRET and DRET, in one polarity-sensitive donor-acceptor pair system. The accuracy of the dark-emissive donor property of the TPE luminogen is also presented for the first time as a new feature for AIE phenomena.

Molecular Structure-Intersystem Crossing Relationship of Heavy-Atom-Free BODIPY Triplet Photosensitizers

A thiophene-fused BODIPY chromophore displays a large triplet-state quantum yield (ΦT = 63.7%). In contrast, when the two thienyl moieties are not fused into the BODIPY core, intersystem crossing (ISC) becomes inefficient and ΦT remains low (ΦT = 6.1%). First-principles calculations including spin-orbit coupling (SOC) were performed to quantify the ISC. We found larger SOC and smaller singlet-triplet energy gaps for the thiophene-fused BODIPY derivative. Our results are useful for studies of the photochemistry of organic chromophores.

Engineering a FRET strategy to achieve a ratiometric two-photon fluorescence response with a large emission shift and its application to fluorescence imaging

Two-photon excitation (TPE) probe-based fluorescence imaging has become one of the most attractive diagnostic techniques to investigate biomolecules and biological events in live cells and tissues. At the current stage most of the TPE-based sensing is reflected by fluorescence intensity changes. Nevertheless the mere altering of intensity could be facilely affected by ambient conditions. On the other hand, TPE probes based on an intramolecular charge transfer (ICT) strategy could solve this problem to some extent with a morphology change-induced emission shift. However their applications are yet constrained due to the inherent limitation of ICT, e.g. the high degree of overlap of two emissions bands and shifts of the TPE maxima. To achieve the desired TPE-based sensing and to circumvent the problems stated above, we adapted a Förster resonance energy transfer (FRET) strategy to develop small molecule ratiometric TPE fluorescent probes. Our FRET-based ratiometric TPE fluorescent probe displays a remarkable emission shift (up to 125 nm) with two well-resolved emission bands. Hence the ratio of these two emission bands could enable the measurement of fluorescence changes more accurately, thus further improving imaging in live cells and deep tissues. To the best of our knowledge, the current reported probe has the largest emission shift among all the small molecule ratiometric TPE fluorescent probes while the maximum TPE wavelength remains unchanged. This work has provided a FRET approach to fabricate novel small molecule ratiometric TPE fluorescent probes that improve imaging in deep tissues.

Picosecond fluorescence dynamics of tryptophan and 5-fluorotryptophan in monellin: slow water-protein relaxation unmasked

Time dependent fluorescence Stokes (emission wavelength) shifts (TDFSS) from tryptophan (Trp) following sub-picosecond excitation are increasingly used to investigate protein dynamics, most recently enabling active research interest into water dynamics near the surface of proteins. Unlike many fluorescence probes, both the efficiency and the wavelength of Trp fluorescence in proteins are highly sensitive to microenvironment, and Stokes shifts can be dominated by the well-known heterogeneous nature of protein structure, leading to what we call pseudo-TDFSS: shifts that arise from differential decay rates of subpopulations. Here we emphasize a novel, general method that obviates pseudo-TDFSS by replacing Trp by 5-fluorotryptophan (5Ftrp), a fluorescent analogue with higher ionization potential and greatly suppressed electron-transfer quenching. 5FTrp slows and suppresses pseudo-TDFSS, thereby providing a clearer view of genuine relaxation caused by solvent and protein response. This procedure is applied to the sweet-tasting protein monellin which has uniquely been the subject of ultrafast studies in two different laboratories (Peon, J.; et al. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 10964; Xu, J.; et al. J. Am. Chem. Soc. 2006, 128, 1214) that led to disparate interpretations of a 20 ps transient. They differed because of the pseudo-TDFSS present. The current study exploiting special properties of 5FTrp strongly supports the conclusion that both lifetime heterogeneity-based TDFSS and environment relaxation-based TDFSS are present in monellin and 5FTrp-monellin. The original experiments on monellin were most likely dominated by pseudo-TDFSS, whereas, in the present investigation of 5FTrp-monellin, the TDFSS is dominated by relaxation and any residual pseudo-TDFSS is overwhelmed and/or slowed to irrelevance.