Nonradiative Relaxation Pathways Research Articles

The carbonyl-lock mechanism underlying non-aromatic fluorescence in biological matter

Challenging the basis of our chemical intuition, recent experimental evidence reveals the presence of a new type of intrinsic fluorescence in biomolecules that exists even in the absence of aromatic or electronically conjugated chemical compounds. The origin of this phenomenon has remained elusive so far. In the present study, we identify a mechanism underlying this new type of fluorescence in different biological aggregates. By employing non-adiabatic ab initio molecular dynamics simulations combined with a data-driven approach, we characterize the typical ultrafast non-radiative relaxation pathways active in non-fluorescent peptides. We show that the key vibrational mode for the non-radiative decay towards the ground state is the carbonyl elongation. Non-aromatic fluorescence appears to emerge from blocking this mode with strong local interactions such as hydrogen bonds. While we cannot rule out the existence of alternative non-aromatic fluorescence mechanisms in other systems, we demonstrate that this carbonyl-lock mechanism for trapping the excited state leads to the fluorescence yield increase observed experimentally, and set the stage for design principles to realize novel non-invasive biocompatible probes with applications in bioimaging, sensing, and biophotonics.

When do tripdoublet states fluoresce? A theoretical study of copper(II) porphyrin.

Open-shell molecules rarely fluoresce, due to their typically faster non-radiative relaxation rates compared to closed-shell ones. Even rarer is the fluorescence from states that have two more unpaired electrons than the open-shell ground state, since they involve excitations from closed-shell orbitals to vacant-shell orbitals, which are typically higher in energy compared to excitations from or out of open-shell orbitals. States that are dominated by the former type of excitations are known as tripdoublet states when they can be described as a triplet excitation antiferromagnetically coupled to a doublet state, and their description by unrestricted single-reference methods (e.g., U-TDDFT) is notoriously inaccurate due to large spin contamination. In this work, we applied our spin-adapted TDDFT method, X-TDDFT, and the efficient and accurate static-dynamic-static second order perturbation theory (SDSPT2), to the study of the excited states as well as their relaxation pathways of copper(II) porphyrin; previous experimental works suggested that the photoluminescence of some substituted copper(II) porphyrins originate from a tripdoublet state, formed by a triplet ligand π → π* excitation antiferromagnetically coupled with the unpaired d electron. Our results demonstrated favorable agreement between the X-TDDFT, SDSPT2 and experimental excitation energies, and revealed noticeable improvements of X-TDDFT compared to U-TDDFT, not only for vertical excitation energies but also for adiabatic energy differences. These suggest that X-TDDFT is a reliable tool for the study of tripdoublet state fluorescence. Intriguingly, we showed that the aforementioned tripdoublet state is only slightly above the lowest doublet excited state and lies only slightly higher than the lowest quartet state, which suggests that the tripdoublet of copper(II) porphyrin is long-lived enough to fluoresce due to a lack of efficient non-radiative relaxation pathways; an explanation for this unusual state ordering is given. Indeed, thermal vibration correlation function (TVCF)-based calculations of internal conversion, intersystem crossing, and radiative transition rates confirm that copper(II) porphyrin emits thermally activated delayed fluorescence (TADF) and a small amount of phosphorescence at low temperature (83K), in accordance with experiment. The present contribution is concluded by a few possible approaches of designing new molecules that fluoresce from tripdoublet states.

Solid State Phosphorescence Enhancement of PtII-Based Emitters via Combination of π-Hole(Isocyano Group)⋅⋅⋅ dz2[PtII] and I···Cl Halogen-Bonding Interactions

The PtII isocyanide complex [Pt(ppy)Cl(CNC6H4-C≡C-Ph)] (1, Hppy=2-phenylpyridine) was co-crystallized with 1,4-diiodotetrafluorobenzene (1,4-DITFB), yielding 1·½(1,4-DITFB) adduct. The I···Cl halogen-bonding and π-π-stacking interactions combined with the rare π-hole(isocyano group)⋅⋅⋅dz2[PtII] interactions were identified via analysis of X-ray diffraction data of the co-crystals. These two types of structure-determining interactions supplemented each other, and the system of I⋯Cl and π-hole(isocyano group)⋅⋅⋅dz2[PtII] contacts achieved a 1D extended ladder-type architecture. The density functional theory calculations, employing a set of computational tools, verified the role of I⋯Cl and π-hole(isocyano group)⋅⋅⋅dz2[PtII] noncovalent bonds in the spectrum of noncovalent forces. The solid-state photophysical study revealed an amplification of luminescence intensity in the co-crystals, which is attributed to the suppression of the nonradiative relaxation pathways due to an increase in the rigidity of the chromophore center.

Microemulsion-Based Fabrication of Organic Nanocrystals for Persistent Phosphorescence Bioimaging

Persistent phosphorescence eliminates the need for light excitation and thus avoids the issue of autofluorescence, holding great promise in various fields. To achieve bright phosphorescence emission, many methods were developed to confine most organic persistent phosphorescent materials in a rigid environment aiming to quench nonradiative relaxation pathways; among them, the nanocrystal method is a good candidate which can rigidify the molecules as well as offer the desirable water dispersity and size for further bioapplications. However, the current efficient preparation of phosphorescent nanocrystals still remains challenging. This study reports an efficient microemulsion-based method for the fabrication of phosphorescent nanocrystals with a desired diameter in the nanosize range (below 200 nm), a high phosphorescence quantum yield of about 6.5%, and ultralong persistent phosphorescence to the timescale after the removal of the external light source, making it easy for the commercial imaging system to detect, which is suitable for practical bioimaging. Furthermore, the synthesized nanocrystals with biocompatibility were utilized directly for in vitro imaging, showing effective cellular uptake and bright phosphorescence emission, which provides a reasonable strategy to develop a phosphorescent nanocrystal for application.

Ultra-narrow-band luminescence with spectral width down to 32 nm in bismuth-activated phosphate phosphors with superior thermal stability

Bismuth-activated phosphors have become popular as a replacement for phosphors based on intensively studied lanthanide ions because of their low spectral reabsorption and sustainable utilization. However, it is of a great challenge to yield narrow-band emission from Bi3+ since its ns2-nsp electron transition lacks the shielding of outer electron orbitals. Herein, we report a new ultra-narrow band Bi3+-activated blue-violet phosphor Ca6BaP4O17 (Ca6BaP4O17:Bi3+). Impressively, its emission band width is down to ∼32 nm, which is less than that of all the reported Bi3+-activated ones. Experimental and theoretical calculation results corroborate that such unusual narrow-band emission originates from Bi3+ ions occupying two cation sites, Ca1 and Ca2, in the Ca6BaP4O17 compound. At 423 K, Ca6BaP4O17:0.5%Bi3+ shows a robust thermal quenching resistance with an integrated emission intensity loss of merely 1%. This is attributable to rare structural advantages of the host such as a high Debye temperature (Θ = 632 K) and large bandgap (∼6.1 eV), which could minimize thermally non-radiative relaxation pathways. We fabricate a blue-violet LED prototype by using the resulting phosphors and ultraviolet LED chip, and demonstrate its potential application in porrets growth. These findings provide in-depth insights the on site occupancy of Bi3+ ions and may simulate more research studies on Bi3+-activated phosphors.

Ultrafast Charge Injection in Silver-Modified Graphitic Carbon Nitride.

Graphitic carbon nitride (gCN) is a promising organic platform for driving light-activated charge-transfer reactions in a number of valuable photocatalytic cycles. A primary limitation of gCN as a photocatalyst is its short excited-state lifetime, which is mediated by a high density of trap and defect sites that result in rapid excited-state decay and low photocatalytic efficiency. To enhance the catalytic activity, gCN is often functionalized with a metal co-catalyst; however, the mechanism by which metal co-catalysts enhance the reactivity has not been clearly established. In this work, the excited-state dynamics of gCN and silver-modified gCN are compared using ultrafast transient absorption and time-resolved photoluminescence spectroscopies. In silver-modified gCN, an ultrafast spectral shift in the silver plasmon resonance provides direct spectral evidence of electron transfer from gCN to the silver nanoparticles. The electron-transfer rate is competitive with other non-radiative relaxation pathways, with electron-transfer yields approaching 50%, thus providing an effective strategy for mitigating losses associated with defects and trap sites.

Photoluminescence of the Au38(SR)26 nanocluster comprises three radiative processes

Photoluminescence of ultrasmall, atomically precise gold nanoclusters constitutes an area of significant interest in recent years for both fundamental research and biological applications. However, the exploration of near-infrared photoluminescence of gold nanoclusters is still in its infancy due to the limitations of synthetic methods and characterization techniques. Herein, the photoluminescence properties of an Au38(PET)26 (PET = 2-phenylethanethiolate) nanocluster are investigated in detail. The Au38(PET)26 exhibits an emission peak at 865 nm, which is revealed to be a mix of fluorescence, thermally activated delayed fluorescence, and phosphorescence via the combined analyses of time-resolved and temperature-dependent photoluminescence measurements. The quantum yield of Au38(PET)26 is determined to be 1.8% at room temperature under ambient conditions, which increases to above 90% by suppressing the non-radiative relaxation pathway at a cryogenic temperature (80 K). Overall, the results of this work discover the coexistence of three radiative processes in thiolate-protected Au nanoclusters and will pave the way for understanding the intriguing photoluminescence properties of gold nanoclusters in future studies.

Does Cr3+ Occupy Tetrahedral Sites and Luminesce in Oxides? A First-Principles Exploration.

The Cr3+ activators have been adopted to produce desired near-infrared broadband emission via ligand field engineering by choosing hosts with appropriate sites. First-principles calculations help to analyze the site, valence, and luminescent mechanism of the activators. Our calculations on Mg2Al4Si5O18:Cr elucidate that the activators are dominated by Cr3+ at tetrahedral Al and octahedral Mg sites, while the experimentally reported near-infrared emission previously assigned to tetrahedral sites is actually produced by Cr3+ at the octahedral site. Meanwhile, our results show that the emission energies of Cr3+ activators at octahedral sites can be well predicted. Moreover, further calculations show that the quenching of the 4T2 → 4T1 transition of Cr3+ at a tetrahedral site is general due to nonradiative relaxation pathways mediated by sublevels split off from the 4T1 multiplet states by intrinsic or Jahn-Teller distortions. Our work shows that the sophisticated first-principles calculations put together here can be effective in exploring Cr3+ and potentially more general activators in crystals, which benefit the design and optimization of luminescent materials.

Determination of fluorescence quantum yields and decay times of NADH and FAD in water–alcohol mixtures: The analysis of radiative and nonradiative relaxation pathways

Combined studies on fluorescence quantum yield of coenzymes NADH and FAD in water–methanol, water–ethanol and water–propylene glycol mixtures and on time-resolved fluorescence of these molecules under excitation at 450 and 355 nm by means of time-correlated single photon counting (TCSPC) method have been carried out. A novel model that allowed to separate the contributions from several excited state relaxation mechanisms to fluorescence dynamics in the pico- and nanosecond time domains and to fluorescence quantum yield was developed and used for analysis of experimental data. The results indicated the existence of at least two different nonradiative relaxation mechanisms in NADH and FAD: a relatively slow nanosecond and a much faster picosecond ones. The analysis suggested that the increase of NADH fluorescence quantum yield with all alcohols concentration occurred mainly due to relatively slow nanosecond non-radiative and radiative relaxation mechanisms related with the change of conformation distributions, while the picosecond fluorescence quenching was found to have practically the same and conformation-insensitive value in water–monohydric alcohol mixtures. Similar conclusion was made for NADH in water–propylene glycol solutions where however a moderate decrease of the picosecond quenching rate was determined at high propylene glycol concentrations that was attributed to a slowdown of picosecond quenching under high viscosity conditions. In the case of FAD the analysis suggested that the dramatic rise of fluorescence quantum yield with alcohol concentration was mostly due to the conformation-sensitive picosecond quenching. This conclusion supports the known mechanism of the picosecond fluorescence quenching in FAD through electron transfer reaction in the π-stacked conformation between isoalloxazine and adenine moieties. Radiative decay times in NADH and FAD were estimated from experimental data and found to be about 10 ns and 20–30 ns, respectively.

Tailoring the Electron-Phonon Interaction in Au25(SR)18 Nanoclusters via Ligand Engineering and Insight into Luminescence.

Understanding the electron-phonon interaction in Au nanoclusters (NCs) is essential for enhancing and tuning their photoluminescence (PL) properties. Among all the methods, ligand engineering is the most straightforward and facile one to design Au NCs with the desired PL properties. However, a systematic understanding of the ligand effects toward electron-phonon interactions in Au NCs is still missing. Herein, we synthesized four Au25(SR)18- NCs protected by different -SR ligands and carefully examined their temperature-dependent band-gap renormalization behavior. Data analysis by a Bose-Einstein two-oscillator model revealed a suppression of high-frequency optical phonons in aromatic-ligand-protected Au25 NCs. Meanwhile, a low-frequency breathing mode and a quadrupolar mode are attributed as the main contributors to the phonon-assisted nonradiative relaxation pathway in aromatic-ligand-protected Au25 NCs, which is in contrast with non-aromatic-ligand-protected Au25 NCs, in which tangential and radial modes play the key roles. The PL measurements of the four Au25 NCs showed that the suppression of optical phonons led to higher quantum yields in aromatic-ligand-protected Au25 NCs. Cryogenic PL measurements provide insights into the nonradiative energy relaxation, which should be further investigated for a full understanding of the PL mechanism in Au25 NCs.

Polarity-Dependent Twisted Intramolecular Charge Transfer in Diethylamino Coumarin Revealed by Ultrafast Spectroscopy

Twisting intramolecular charge transfer (TICT) is a common nonradiative relaxation pathway for a molecule with a flexible substituent, effectively reducing the fluorescence quantum yield (FQY) by swift twisting motions. In this work, we investigate coumarin 481 (C481) that contains a diethylamino group in solution by femtosecond transient absorption (fs-TA), femtosecond stimulated Raman spectroscopy (FSRS), and theoretical calculations, aided by coumarin 153 with conformational locking of the alkyl arms as a control sample. In different solvents with decreasing polarity, the transition energy barrier between the fluorescent state and TICT state increases, leading to an increase of the FQY. Correlating the fluorescence decay time constant with solvent polarity and viscosity parameters, the multivariable linear regression analysis indicates that the chromophore’s nonradiative relaxation pathway is affected by both hydrogen (H)-bond donating and accepting capabilities as well as dipolarity of the solvent. Results from the ground- and excited-state FSRS shed important light on structural dynamics of C481 undergoing prompt light-induced intramolecular charge transfer from the diethylamino group toward –C=O and –CF3 groups, while the excited-state C=O stretch marker band tracks initial solvation and vibrational cooling dynamics in aprotic and protic solvents (regardless of polarity) as well as H-bonding dynamics in the fluorescent state for C481 in high-polarity protic solvents like methanol. The uncovered mechanistic insights into the molecular origin for the fluorogenicity of C481 as an environment-polarity sensor substantiate the generality of ultrafast TICT state formation of flexible molecules in solution, and the site-dependent substituent(s) as an effective route to modulate the fluorescence properties for such compact, engineerable, and versatile chemosensors.

First-Principles Study on Optoelectronic Properties of Fe-Doped Montmorillonite Clay.

A theoretical investigation is conducted to describe optoelectronic properties of Fe-doped montmorillonite nanoclay under spin states of low spin (LS), intermediate spin (IS), and high spin (HS). Ground state electronic properties are studied using spin-polarized density functional theory calculations. The nonradiative and radiative relaxation channels of charge carriers are studied by computing nonadiabatic couplings (NACs) using an "on-the-fly" approach from adiabatic molecular dynamics trajectories. The NACs are further processed using a reduced density matrix approach with the Redfield formalism. The computational results are presented for electronic density of states, absorption spectra, charge carrier dynamics, and photoluminescence (PL) by comparing various spin multiplicities. Results on spin α and spin β components are independent and quite different because of the partial occupation of Fe 3d states. Overall, HS is the most stable with the largest Fe-O distances. One finds different nonradiative relaxation pathways in space and on the time scale for electrons and holes. The Redfield PL reveals obvious Fe 3d-3d transitions for LS and IS.

Ultrafast Collective Excited-State Dynamics of a Virus-Supported Fluorophore Antenna.

Radiation brightening was recently observed in a multifluorophore-conjugated brome mosaic virus (BMV) particle at room temperature under pulsed excitation. On the basis of its nonlinear dependence on the number of chromophores, the origins of the phenomenon were attributed to a collective relaxation. However, the mechanism remains unknown. We present ultrafast transient absorption and fluorescence spectroscopic studies which shed new light on the collective nature of the relaxation dynamics in such radiation-brightened, multifluorophore particles. Our findings indicate that the emission dynamics is consistent with a superradiance mechanism. The ratio between the rates of competing radiative and nonradiative relaxation pathways depends on the number of chromophores per virus. The findings suggest that small icosahedral virus shells provide a unique biological scaffold for developing nonclassical, deep subwavelength light sources and may open new avenues for the development of photonic probes for medical imaging applications.

Determination of Fluorescence Quantum Yields and Decay Times of Nadh and Fad in Water-Alcohol Mixtures: The Analysis of Radiative and Nonradiative Relaxation Pathways

Determination of Fluorescence Quantum Yields and Decay Times of Nadh and Fad in Water-Alcohol Mixtures: The Analysis of Radiative and Nonradiative Relaxation Pathways

Solvent Effects on Ultrafast Photochemical Pathways.

Photochemical reactions are increasingly being used for chemical and materials synthesis, for example, in photoredox catalysis, and generally involve photoexcitation of molecular chromophores dissolved in a liquid solvent. The choice of solvent influences the outcomes of the photochemistry because solute-solvent interactions modify the energies of and crossings between electronic states of the chromophores, and they affect the evolving structures of the photoexcited molecules. Ultrafast laser spectroscopy methods with femtosecond to picosecond time resolution can resolve the dynamics of these photoexcited molecules as they undergo structural and electronic changes, relax back to the ground state, dissipate their excess internal energy to the surrounding solvent, or undergo photochemical reactions. In this Account, we illustrate how experimental studies using ultrafast lasers can reveal the influences that different solvents or cosolutes exert on the photoinduced nonadiabatic dynamics of internal conversion and intersystem crossing in nonradiative relaxation pathways. Although the environment surrounding a solute molecule is rapidly changing, with fluctuations in the coordination to neighboring solvent molecules occurring on femtosecond or picosecond time scales, we show that it is possible to photoexcite selectively only those molecular chromophores transiently experiencing specific solute-solvent interactions such as intermolecular hydrogen bonding.The effects of different solvation environments on the photodynamics are illustrated using four selected examples of photochemical processes in which the solvent has a marked effect on the outcomes. We first consider two aromatic carbonyl compounds, benzophenone and acetophenone, which are known to undergo fast intersystem crossing to populate the first excited triplet state on time scales of a few picoseconds. We show that the nonadiabatic excited-state dynamics are modified by transient hydrogen bonding of the carbonyl group to a protic solvent or by coordination to a metal cation cosolute. We then examine how different solvents modify the competition between two alternative relaxation pathways in a photoexcited UVA-sunscreen molecule, diethylamino hydroxybenzoyl hexyl benzoate (DHHB). This relaxation back to the ground electronic state is an essential part of the effective operation of the sunscreen compound, but the dynamics are sensitive to the surrounding environment. Finally, we consider how solvents of different polarity affect the energies and lifetimes of excited states with locally excited or charge-transfer character in heterocyclic organic compounds used as excited-state electron donors for photoredox catalysis. With these and other examples, we seek to develop a molecular level understanding of how the choice of solution environment might be used to control the outcomes of photochemical reactions.

Strategic Engineering of Sub‐5 nm Dyes@CDs Nanoassemblies Platform for Super Resolution Imaging

AbstractThe design and construction of fluorescence probes with small size, high quantum yield, photostability, and solubility in an aqueous solution to achieve biomarker have become a focus of interest in super‐resolution bioimaging application. Herein, four kinds of GFP‐like supramolecular nanoassemblies (SNAs) are constructed in a very facile manner by the host–guest interaction between isopercolic acid derivatives (IADs) and cyclodextrin (CD). 1H NMR, Fourier transform infrared spectroscopy (FT‐IR) and single crystal structure well prove the formation of SNAs. Moreover, compared to the initial state, the fluorescence quantum yield of the SNAs is dramatically improved through suppressing nonradiative relaxation pathways and shielding quenchers. The bimolecular exciton–exciton annihilation (bmEA) process under the high pumping fluence has been suppressed effectively. Both the improved gain and the reduced loss facilitate the buildup of stimulated emission and enable the high stimulated emission efficiency of these SNAs. Sub‐5 nm IADs@β‐CDs‐mediated STED imaging exhibits excellent stimulated emission depletion (STED) performance: high photostability, low saturation intensity (≈0.50 MW cm−2), and high resolution (30 nm) under PSTED‐775 nm of 109 MW cm−2. This supramolecular strategy avoids the tedious molecular synthesis process and paves a new pathway for insights into building up novel high‐performance STED probes.

Photostable Ruthenium(II) Isocyanoborato Luminophores and Their Use in Energy Transfer and Photoredox Catalysis.

Ruthenium(II) polypyridine complexes are among the most popular sensitizers in photocatalysis, but they face some severe limitations concerning accessible excited-state energies and photostability that could hamper future applications. In this study, the borylation of heteroleptic ruthenium(II) cyanide complexes with α-diimine ancillary ligands is identified as a useful concept to elevate the energies of photoactive metal-to-ligand charge-transfer (MLCT) states and to obtain unusually photorobust compounds suitable for thermodynamically challenging energy transfer catalysis as well as oxidative and reductive photoredox catalysis. B(C6F5)3 groups attached to the CN– ligands stabilize the metal-based t2g-like orbitals by ∼0.8 eV, leading to high 3MLCT energies (up to 2.50 eV) that are more typical for cyclometalated iridium(III) complexes. Through variation of their α-diimine ligands, nonradiative excited-state relaxation pathways involving higher-lying metal-centered states can be controlled, and their luminescence quantum yields and MLCT lifetimes can be optimized. These combined properties make the respective isocyanoborato complexes amenable to photochemical reactions for which common ruthenium(II)-based sensitizers are unsuited, due to a lack of sufficient triplet energy or excited-state redox power. Specifically, this includes photoisomerization reactions, sensitization of nickel-catalyzed cross-couplings, pinacol couplings, and oxidative decarboxylative C–C couplings. Our work is relevant in the greater context of tailoring photoactive coordination compounds to current challenges in synthetic photochemistry and solar energy conversion.

Elucidating Halogen‐Assisted Self‐Assembly Enhanced Mechanochromic Aggregation‐Induced Emission

AbstractWe present a successful strategy to obtain a self‐assembled material stabilized by halogen bonding with enhanced mechanochromic aggregation‐induced emission (AIE). The meta‐bromophenyl‐substituted tetraphenylethene fluorophore (m‐BrTBE) exhibits an emission which is more red‐shifted compared to the phenyl‐ring‐substituted tetraphenylethene fluorophore (TBE) in film. Remarkably, m‐BrTBE molecules self‐assemble into uniform ball‐like aggregates with high photoluminescence quantum yield up to 85.3 %, which indicates that the molecular conformation and intermolecular interactions are different from those in the crystalline state. Based on single‐crystal analysis, scanning tunneling microscopic observations, and theoretical calculations, the unusual self‐assembly enhanced AIE behavior is attributed to the important effect of meta‐bromide substituents which not only form intermolecular Br⋅⋅⋅π halogen bonding and H⋅⋅⋅Br hydrogen bonding interactions to block nonradiative relaxation pathways effectively but also promote radiative processes.

Interexciton nonradiative relaxation pathways in the peridinin-chlorophyll protein

Excitation energy transfer can be unusually efficient and structurally robust when mediated by molecular excitons, which arise in a photosynthetic light-harvesting protein when delocalized excitations of the electronic chromophores are created upon absorption of light. Here we report results from two-dimensional electronic spectroscopy and electronic structure calculations, revealing that excitation energy is transferred between the peridinin and chlorophyll excitons in the peridinin-chlorophyll protein from marine dinoflagellates over a delocalized, two-step pathway. Upon absorption of light by the peridinins in the strong, mid-visible absorption band, excitation energy is trapped by the chlorophylls in less than 50 fs at room temperature. The overall process is slowed only by a few fs upon replacement of chlorophyll a with chlorophyll b. The key step in the pathway transfers excitation from higher-energy peridinin excitons with more extensive delocalization to the lowest-energy peridinin exciton, which is delocalized only over peridinin 614 and the neighboring chlorophyll. A two-step pathway transfers energy from peridinin excitons to chlorophylls Excitation energy is trapped by chlorophyll acceptors in less than 50 fs Delocalization collapses as excitation progresses irreversibly down the pathway Transfer rates are robust when chlorophyll a is replaced with chlorophyll b Tilluck et al. reveal how excitation energy transfer in the peridinin-chlorophyll protein from marine dinoflagellates involves quantum coherent mixing of the excited states of the clustered chromophores in each protein domain. The results suggest that solar photons might be captured efficiently in photocatalytic devices and solar cells by delocalized nanomaterials.

Self-Propelled and Near-Infrared-Phototaxic Photosynthetic Bacteria as Photothermal Agents for Hypoxia-Targeted Cancer Therapy.

Hypoxia can increase the resistance of tumor cells to radiotherapy and chemotherapy. However, the dense extracellular matrix, high interstitial fluid pressure, and irregular blood supply often serve as physical barriers to inhibit penetration of drugs or nanodrugs across tumor blood microvessels into hypoxic regions. Therefore, it is of great significance and highly desirable to improve the efficiency of hypoxia-targeted therapy. In this work, living photosynthetic bacteria (PSB) are utilized as hypoxia-targeted carriers for hypoxic tumor therapy due to their near-infrared (NIR) chemotaxis and their physiological characteristics as facultative aerobes. More interestingly, we discovered that PSB can serve as a kind of photothermal agent to generate heat through nonradiative relaxation pathways due to their strong photoabsorption in the NIR region. Therefore, PSB integrate the properties of hypoxia targeting and photothermal therapeutic agents in an "all-in-one" manner, and no postmodification is needed to achieve hypoxia-targeted cancer therapy. Moreover, as natural bacteria, noncytotoxic PSB were found to enhance immune response that induced the infiltration of cytotoxicity T lymphocyte. Our results indicate PSB specifically accumulate in hypoxic tumor regions, and they show a high efficiency in the elimination of cancer cells. This proof of concept may provide a smart therapeutic system in the field of hypoxia-targeted photothermal therapeutic platforms.