Intramolecular Energy Transfer Research Articles

Boosting Visible Light-induced Photocyclization Quantum Yield of a Diarylethene-perylenebisimide Dyad by Introducing a Ketone Spacer Group

Abstract Recently, we found a unique visible light-induced photocyclization reaction in a diarylethene-perylenebisimide dyad based on multiplicity conversion via intramolecular energy transfer. Here we report on the remarkable enhancement of photocyclization quantum yield under visible light irradiation by changing the spacer unit from an ester to a ketone group. This simple modification lifts the restrictions of solvent choice and oxygen quenching.

Fullerene-Perylenediimide (C60-PDI) Based Systems: An Overview and Synthesis of a Versatile Platform for Their Anchor Engineering.

An overview of the different covalent bonding synthetic strategies of two electron acceptors leading to fullerene-perylenediimide (C60-PDI)-based systems, essentially dyads and triads, is presented, as well as their more important applications. To go further in the development of such electron and photoactive assemblies, an original aromatic platform 5-benzyloxy-3-formylbenzoic acid was synthesized to graft both the PDI dye and the fullerene C60. This new C60-PDI dyad exhibits a free anchoring phenolic function that could be used to attach a third electro- and photoactive unit to study cascade electron and/or energy transfer processes or to obtain unprecedented side-chain polymers in which the C60-PDI dyads are attached as pendant moieties onto the main polymer chain. This C60-PDI dyad was fully characterized, and cyclic voltammetry showed the concomitant reduction process onto both C60 and PDI moieties at identical potential. A quasi-quantitative quenching of fluorescence was demonstrated in this C60-PDI dyad, and an intramolecular energy transfer was suggested between these two units. After deprotection of the benzyloxy group, the free hydroxyl functional group of the platform was used as an anchor to reach a new side-chain methyl methacrylate-based polymer in which the PDI-C60 dyad units are located as pendants of the main polymer chain. Such polymer which associates two complementary acceptors could find interesting applications in optoelectronics and in particular in organic solar cells.

Microscopic Mechanism on the Heat Conduction of Organic Liquids: A Molecular Dynamics Study

The research on energy conversion and transportation of fuels at a microscopic level is of great significance to the development of industry. As a new alternative fuel, alcohols are widely used in industry and daily life, so it is necessary to investigate the thermophysical properties of them. In this work, seven species of pure liquid alcohols were performed to investigate the microscopic mechanisms of thermal energy transfer by non-equilibrium molecular dynamic (NEMD) method. Firstly, the thermal conductivity of alcohols was calculated and was found to be consistent with the experimental data. Then, the influence of temperature on energy transfer is investigated, the results show that the contribution of convection energy transfer increases and both the inter- and intramolecular terms decrease with the increase of temperature. Finally, the influence of molecular length on energy transfer was investigated at the same temperature, and it is concluded that the contribution of the convective term decreases and the interactive term increases to the total heat flux with increasing the length of the chain. It is worth mentioning that the contribution of intramolecular energy transfer gradually becomes a dominant part of the total energy transfer as the linear chain molecule increases to a certain length and the number of carbon atoms at the intersection point of inter- and intramolecular energy transfer is similar to the turning point of thermal conductivity.

Molecular Tailoring Based on Forster Resonance Energy Transfer for Initiating Two-Photon Theranostics with Amplified Reactive Oxygen Species.

The fabrication of multifunctional photosensitizers (PSs) with abundant Type I/II ROS for efficient theranostics in the "therapeutic window" (700-900 nm) is an appealing yet significantly challenging task. We herein report a molecular tailoring strategy based on intramolecular two-photon Forster Resonance Energy Transfer (TP-FRET) to obtain a novel theranostic agent (Lyso-FRET), featuring the amplified advantage of energy donor (NH) and acceptor (COOH), because of the reuse of fluorescence energy with high efficiency of FRET (∼83%). Importantly, under the excitation by the near-infrared (840 nm) window, Lyso-FRET can not only penetrate the deeper tissue with a higher resolution for fluorescence imaging due to the nonlinear optical (NLO) nature, but also generate more Type I (superoxide anion) and Type II (singlet oxygen) reactive oxygen species for hypoxic PDT. Moreover, Lyso-FRET targeting lysosomes further promotes the effect of treatment. The experiments in vitro and in vivo also verify that the developed TP-FRET PS is conducive to treating deep hypoxic tumors. This strategy provides new and significant insights into the design and fabrication of advanced multifunctional PSs.

Charge Separation/Recombination, Intersystem Crossing, and Unusually Slow Intramolecular Triplet-Triplet Energy Transfer in Naphthalenediimide-Anthracene Compact Energy Donor-Acceptor Dyads.

Three anthracene (An)-naphthalenediimide (NDI) compact electron donor-acceptor dyads were prepared. Femtosecond transient absorption (fs-TA) spectra show fast charge separation (ca. 0.9-1.7 ps) and relatively slow charge recombination (ca. 8-565 ps) upon photoexcitation; moreover, the 3An state was observed for 9-An-NDI, whereas the final state is 3NDI for both 9-An-Ph-NDI and 2-An-Ph-NDI, which have an intervening phenyl linker between the An and NDI units. Nanosecond transient absorption (ns-TA) spectra indicate that the lowest triplet state of all the dyads is 3An, with triplet lifetimes of 139-354 μs. An unusually slow intramolecular triplet-triplet energy transfer (TTET) was observed for 9-An-Ph-NDI and 2-An-Ph-NDI (32-85 ns). Time-resolved electron paramagnetic resonance (TREPR) spectroscopy confirms that the intersystem crossing (ISC) mechanism is spin orbit charge transfer ISC (SOCT-ISC) for all the dyads; for 9-An-NDI, only the 3An state was observed, while for the other two dyads, both 3NDI and 3An states were observed, with their relative population changing with increasing delay time, which supports TTET.

A pyrene-derived ratiometric fluorescent probe for pH monitoring in cells and zebrafish based on monomer-excimer emission

Acidity in biological microenvironments plays a crucial role in physiology and pathology, so that pH monitoring can provide insights into the mechanisms underlying pH-related processes. Many pH ratiometric fluorescent probes have been developed based on intramolecular charge transfer (ICT) and fluorescence resonance energy transfer (FRET) principles. In contrast, those based on intermolecular excimer are scarce. The present work rationally designed and readily synthesized a pyrene-derived ratiometric fluorescent probe (Pyrene-PA) for pH detection using the conversion of monomer-excimer emission. In an acidic aqueous medium, Pyrene-PA exists in a cationic monomer with an emission centered at 463 nm. As pH increases, Pyrene-PA converts to neutral molecules and exhibits reduced dispersibility, generating an intense excimer emission around 630 nm. Based on the above mechanism, Pyrene-PA displays a sensitive, selective, photostable, and reversible response to pH changes without interference from coexisting species such as anions, cations, thiols, and reactive oxygen species. The ratio of fluorescence intensity at 630 nm and 463 nm increases linearly with pH values from 4.0 to 7.0, and the pKa value is 5.6, suitable for pH quantification in acidic to neutral environments. Moreover, Pyrene-PA with low cytotoxicity and good cell penetrability enables monitoring pH changes in cells and zebrafish.

Apoptosis Detection in Retinal Ganglion Cells Using Quantitative Changes in Multichannel Fluorescence Colocalization.

KcapTR488 is a dual-fluorophore peptide sensor for the real-time reporting of programmed cell death by fluorescence imaging. KcapTR488 contains a nuclear localization sequence (NLS) conjugated with Texas Red, a caspase-cleavable sequence (DEVD), and a C-terminus conjugated to Alexa Fluor 488 (AF488). The synthesis and preliminary evaluation in cellulo of KcapTR488 for monitoring cell death by fluorescence imaging has been previously reported, but its utility in vivo has yet to be tested or validated. Herein, in vitro solution experiments verified the intramolecular fluorescence resonance energy transfer (FRET) between the two fluorophores and enabled a quantitative analysis of enzyme rates and selectivity. The sensor delivery kinetics in live rat models were quantified by ex vivo fluorescence microscopy. Studies in healthy control retinas demonstrated that KcapTR488 concentrated in the nucleus of retinal ganglion cells (RGC), with a strong colocalization of red and green fluorescence signals producing robust FRET signals, indicating an intact reporter. By contrast, using an acute but mild NMDA-induced retinal injury model, dual-color confocal ex vivo microscopy of cleaved KcapTR488 identified sensor activation as early as 2 h after injection. Quantitative changes in fluorescence colocalization were superior to changes in FRET for monitoring injury progression. Longitudinal monitoring revealed that the NLS-Texas Red fragment of the cleaved sensor moved out of the cell body, down the axon, and exited the retina, consistent with anterograde axonal transport. Thus, KcapTR488 may be a powerful tool to study RGC death pathways in live preclinical models of glaucoma.

Enhancing the Luminescence of Eu(III) Complexes with the Ruthenocene Organometallic Unit as Ancillary Ligand.

Five novel Eu(III)-β-diketonate complexes containing ruthenocene ancillary ligands (1,1'-bis(diphenylphosphoryl)ruthenocene─RcBPO) were synthesized and characterized. The coordination compounds presented the general formula [Eu(β-dik)3(RcBPO)], where β-dik stands for 2-thenoyltrifluoroacetonate (tta), 3-benzoyl-1,1,1-trifluoroacetone (btf), 2-dibenzoylmethanate (dbm), 2-acetyl-1,3-indandionate (aind), and 2-benzoyl-1,3-indandionate (bind), and RcBPO stands for 1,1'-bis(diphenylphosphoryl)ruthenocene. The [Eu(aind)3(RcBPO)] complex crystallizes in a monoclinic Cc non-centrosymmetric space group with the europium site environment, assuming a bicapped trigonal prism coordination polyhedron with the symmetry point group close to C2v. Photoluminescent properties for the solid-state samples were described in terms of excitation, emission, lifetime decay curves, and intrinsic and overall quantum yields. The replacement of the two coordinated H2O molecules by the RcBPO ancillary ligand leads to great enhancements of the overall quantum yields (QEuL), with the minimum increment by a factor of 5 for the case of [Eu(btf)3(RcBPO)] and the maximum enhancement of 270 times for the case of the [Eu(dbm)3(RcBPO)] complex. In addition, theoretical calculations were carried out to model the spectroscopic properties of the investigated compounds. To obtain theoretical Judd-Ofelt parameters (Ωλ, λ = 2, 4, and 6) and intramolecular energy transfer rates, the JOYSpectra web platform was employed using the structure obtained from density functional theory calculations. Hence, a rate equation model provided theoretical overall quantum yields, which are in great agreement with measured data.

Complex-as-Ligand Strategy as a Tool for the Design of a Binuclear Nonsymmetrical Chromium(III) Assembly: Near-Infrared Double Emission and Intramolecular Energy Transfer

Complex-as-Ligand Strategy as a Tool for the Design of a Binuclear Nonsymmetrical Chromium(III) Assembly: Near-Infrared Double Emission and Intramolecular Energy Transfer

All-Visible (>500 nm)-Light-Induced Diarylethene Photochromism Based on Multiplicity Conversion via Intramolecular Energy Transfer.

Photoswitching molecules that reversibly switch upon visible-light irradiation are some of the most attractive targets for biological and imaging applications. In this study, we found a diarylethene (DAE) derivative having a covalently attached perylenebisimide (PBI) unit (DAE-PBI dyad) underwent an unexpected cyclization reaction upon irradiation with green (500-550 nm) light, where the DAE unit has no absorbance. The photoreactivity was enhanced in solvents containing heavy atoms and in the presence of oxygen. As inferred from the solvent dependence and the calculated excited-state energies of DAE and PBI units, it was suggested that the probable mechanism for this unique visible-light-induced cyclization reaction is multiplicity conversion based on intramolecular energy transfer from the excited singlet state of the PBI unit to the triplet state of DAE units (i.e., DAE-1[PBI]* → 3[DAE]*-PBI). Such a unique photoreaction mechanism with the assistance of oxygen will pave the way for new molecular design for the development of visible-light switching molecules.

Visible-light-controlled ternary chiroptical switches with high-performance circularly polarized luminescence for advanced optical information storage and anti-counterfeiting materials

Photoswitching chiroptical materials with alternate emission colors are urgently demanded due to their promising prospects in the next-generation intelligent materials. Herein, we developed an integrated strategy to design the ternary chiroptical switches through the introduction of phosphorescent platinum(II) and photochromic spiropyran as triplet-sensitizer and photo-regulator building-blocks, respectively, which were bridged by the chiral gene, diaminocyclohexane. This novel tactic synergistically realized the photo-controllable characteristics of triplet-sensitized photochromism, and intramolecular energy and chirality transfer processes regulated by all-visible light. Especially, the phototriggered chirality transformation was effectively achieved from diaminocyclohexane to the open merocyanine moiety via Pt(II) sensitized center, associated with gradually emission color changes from green to red as a result of triplet-singlet exciton transfer from the cyclometalated ligand to the merocyanine moiety. Thereby, the established photo-controlled CPL switches were demonstrated to develop quaternary molecular logic gate, all-visible-light photorewritable patterning and anti-counterfeiting applications with advantage of unique visible-light photochromism including high-performance photostability and robust photo-fatigue resistance.

Self-Assembled Nanodot Actuator with Changeable Fluorescence by π-π Stacking Force Based on a Four-Armed Foldable Phthalocyanine Molecule and Its Supersensitive Molecular Recognition.

Designing intelligent molecules and smart nanomaterials as molecular machines is becoming increasingly important in the nanoscience fields. Herein, we report a nanodot actuator with changeable fluorescence by π-π stacking force based on a four-armed foldable phthalocyanine molecule. The assembled nanodot possessed a three-dimensional molecular space structure and multiple supramolecular interactions. The arms of the nanodot could fold and open intelligently in response to environmental molecular stimuli such as natural plant mimosa, which could lead to multiple variable fluorescence emissions. The nanodot was highly sensitive to the biomolecule thyroxine at the molecular level. The accurate molecular recognition and the changeable fluorescence conversion of the nanodot were attributed to multiple supramolecular interactions, including photoinduced electron transfer (PET), intramolecular fluorescence resonance energy transfer (FRET), and π-π stacking of the nanodots, resulting in an intelligent "nanodot machine with folding arms". The self-assembled nanodot actuators with changeable fluorescence have potential applications in advanced intelligent material fields.

Photoswitchable AIE photosensitizer for reversible control of singlet oxygen generation in specific bacterial discrimination and photocontrolled photodynamic killing of bacteria

The photosensitizer system that allows for reversible noninvasive control over singlet oxygen generation (1O2) is in great demand for precise photodynamic therapy (PDT) aimed at reducing non-specific damage to the normal cells or tissues. However, they often suffer from fluorescence quenching and insufficient 1O2 generation due to aggregation-caused quenching (ACQ) phenomenon in a hydrophilic and physiological environment, severely limiting their applications in both imaging and PDT therapy. Herein, a novel positively charged photoswitchable photosensitizer with aggregation-induced emission (AIE) has been rationally designed and successfully developed, in which pyridinium-tetraphenylethylene (Py-TPE) photosensitizing fragment and dithienylethene (DTE) switch unit were covalently linked through a flexible C4 chain enabling efficient intramolecular energy transfer between both functional entities, thus allowing for reversible control of 1O2 generation. Preliminary evaluation of the photophysical properties revealed excellent AIE properties and photoswitching behavior in various solvents. Furthermore, the nanoparticles (NPs) fabricated by the amphiphilic Pluronic F127 exhibited excellent photochromic performance in the aqueous medium. Significantly, 1O2 generation in the closed NPs was suppressed by the competitive energy transfer, while the generation of 1O2 in the open NPs was activated upon light irradiation, demonstrating its reversible control over 1O2 generation under irradiation with 420 nm light. As expected, it has been proved to perform well in specific bacterial discrimination and photocontrolled antimicrobial PDT against Staphylococcus aureus (S. aureus) in vitro.

Synthesis, photophysical properties and theoretical evaluation of alkyl-linked porphyrin – BODIPY dyads

Novel porphyrin-BODIPY Dyads linked by alkyl chains of different lengths as molecular conjugates were prepared and their intramolecular energy transfer properties have been investigated. Steady-state absorption and emission spectra have shown the successful intramolecular energy transfer (FRET) from BODIPY to porphyrin units approved with the calculated Förster radii and FRET efficiencies. Alkyl chain shortening has revealed an explicit increase in energy transfer as it could be expected. Further, DFT and TD-DFT studies for the synthesized dyads were carried out and frontier molecular orbital energies were investigated at the HSEH1PBE/6-31+G** theory level. Strongly electron-withdrawing C6F5- substituents on the porphyrin periphery extended the HOMO-LUMO gap in contrast to the 4-CF3C6H4- substituent from ca. 2.37 eV to ca. 2.50 eV. Open-circuit voltage values (Voc), reorganization energies (λe and λh), ionization potentials, and adiabatic electron affinities were calculated to evaluate the optical and electronic behaviors of Dyads. Calculated Voc values exceeding 1.00 V, and low reorganization energies of the Dyads indicate that they are promising structures for developing new photovoltaics.

Complex-as-Ligand Strategy as a Tool for the Design of a Binuclear Nonsymmetrical Chromium(III) Assembly: Near-Infrared Double Emission and Intramolecular Energy Transfer.

The chromium(III) polypyridyl complexes are appealing for their long-lived near-infrared (NIR) emission reaching the millisecond range and for the strong circularly polarized luminescence of their isolated enantiomers. However, harnessing those properties in functional polynuclear CrIII devices remains mainly inaccessible because of the lack of synthetic methods for their design and functionalization. Even the preparation and investigation of most basic nonsymmetrical CrIII dyads exhibiting directional intramolecular intermetallic energy transfer remain unexplored. Taking advantage of the inertness of heteroleptic chromium(III) polypyridyl building blocks, we herein adapt the "complex-as-ligand" strategy, largely used with precious 4d and 5d metals, for the preparation of a binuclear nonsymmetrical CrIII complex (3d metal). The resulting [(phen)2Cr(L)Cr(tpy)]6+ dyad shows dual long-lived NIR emission and a directional intermetallic energy transfer that is controlled by the specific arrangements of the different coordination spheres. This strategy opens a route for building predetermined polynuclear assemblies with this earth-abundant metal.

Near-infrared aza-BODIPYs bearing tetraphenylethylene: Synthesis, photophysical studies, and cell imaging application

We provide a simple method to synthesize tetraphenylethylene-modified aza-BODIPY dyes (AZB-TPE1 and AZB-TPE2) with aggregation-enhanced emission. Single-crystal X-ray method, electrochemical experiments, and TD-DFT calculations were used to understand more about their electronic and photophysical properties. The intramolecular energy transfer between the TPE donor and the aza-BODIPY acceptor was observed from both dyes, with 84–97% energy transfer efficiencies and large pseudo-Stokes shifts of 395 – 406 nm. Aggregation studies in THF/water solution showed increased fluorescence in the aggregated form. X-ray crystal structure of AZB-TPE2 suggested the reduction of intermolecular pi-pi interactions between the structures. Finally, these NIR-emitting particles with superior water dispersion and biocompatibility demonstrated amazing features in bioimaging applications.

Intramolecular energy transfer dyes as temperature- and polarity-sensitive fluorescence probes

New temperature - and polarity-sensitive dye probes were synthesized, which demonstrate intramolecular energy transfer from excited violet pyrene chromophore to green dansyl chromophore. Fluorescence of these dye probes is highly sensitive to environment polarity changes. The intramolecular energy transfer efficiencies of these dye probes increase from 55.0% in nonpolar solvent environment to 95.3% in polar solvent environment. The emitting-light colors of these dye probes change from blue in nonpolar solvent environment to yellow in polar solvent environment. These dye probes demonstrate highly temperature-sensitive fluorescence changes due to intramolecular energy transfer ON/OFF under environment cooling/heating condition. Fluorescence intensity of these dye probes significantly changes with environment temperature. Dynamic Diels-Alder covalent bonds enable dye exchange and conversion to achieve structurally optimized emitting-lighting probes.

Ultrafast vibrational dynamics of the free OD at the air/water interface: Negligible isotopic dilution effect but large isotope substitution effect.

Vibrational relaxation dynamics of the OH stretch of water at the air/water interface has been a subject of intensive research, facilitated by recent developments in ultrafast interface-selective nonlinear spectroscopy. However, a reliable determination of the vibrational relaxation dynamics in the OD stretch region at the air/D2O interface has not been yet achieved. Here, we report a study of the vibrational relaxation of the free OD carried out by time-resolved heterodyne-detected vibrational sum frequency generation spectroscopy. The results obtained with the aid of singular value decomposition analysis indicate that the vibrational relaxation (T1) time of the free OD at the air/D2O interface and air/isotopically diluted water (HOD-H2O) interfaces show no detectable isotopic dilution effect within the experimental error, as in the case of the free OH in the OH stretch region. Thus, it is concluded that the relaxation of the excited free OH/OD predominantly proceeds with their reorientation, negating a major contribution of the intramolecular energy transfer. It is also shown that the T1 time of the free OD is substantially longer than that of the free OH, further supporting the reorientation relaxation mechanism. The large difference in the T1 time between the free OD and the free OH (factor of ∼2) may indicate the nuclear quantum effect on the diffusive reorientation of the free OD/OH because this difference is significantly larger than the value expected for a classical rotational motion.

Organo-soluble dendritic zinc phthalocyanine: photoluminescence and fluorescence properties

In this study, the spectroscopic measurements of the second generation dendritic zinc phthalocyanine (phthalocyanine dendrimer with tris) 2 were made and its potential to be used in optoelectronic applications was investigated. To examine the intramolecular energy transfer properties, fluorescence quantum yields were calculated and the contribution of first (phthalocyanine dendrimer with triethyl methanetricarboxylate) 1 and second 2 generation dendrimers to this energy transfer was investigated. The photoluminescence properties and optical band gap of the second generation zinc phthalocyanine 2 were investigated depending on the temperature. In addition, the quantum efficiency of singlet oxygen was calculated to evaluate the usability of this compound in photodynamic therapy. Also, aggregation behavior of compound 2 was examined in different solvents (DMF, DMSO and THF) and different concentrations in THF. To the best of our knowledge, this is the first study to examine the photoluminescence properties and temperature dependent changes of dendritic phthalocyanines.

Solution structures and ultrafast vibrational energy dissipation dynamics in cyclotetramethylene tetranitramine.

Steady-state and time-resolved infrared (IR) studies of cyclotetramethylene tetranitramine (HMX) were carried out, using the asymmetric nitro-stretch as probe, to investigate its solution structures and vibrational energy transfer processes in pure dimethyl sulfoxide (DMSO) and in a DMSO/water mixture. A linear IR spectrum in the nitro-stretching mode region shows two major bands and one minor band in DMSO but changes to the two major bands mainly picture when adding water as an antisolvent of HMX, suggesting a transition from well-solvated and less perfect β-conformation to a less-solvated and close-to-perfect β-conformation. The latter bears a similar asymmetric nitro-stretch vibration profile to the β-polymorph in the crystal form. Density functional theory computations of the nitro-stretching vibrations suggest that HMX in DMSO may be in a NO2 group rotated β-conformation. Two-dimensional IR cross-peak intensity reveals intramolecular energy transfer between the axial and equatorial nitro-groups in the β-HMX on the ps time scale, which is slightly faster in the mixed solvent case. The importance of water as an antisolvent in influencing the equilibrium solvation structure, as well as the vibrational and orientational relaxation dynamics of HMX, is discussed.