Effect of Solvent Polarizability on the Forbidden 1.DELTA.g .fwdarw. 3.SIGMA.g- Transition in Molecular Oxygen: A Fourier Transform Near-Infrared Luminescence Study

Stimulated Raman generation of aqueous singlet oxygen without photosensitizers

Singlet oxygen is traditionally produced via photosensitizer molecules such as methylene blue, which function as catalysts. Here we investigate stimulated Raman generation of singlet oxygen from dissolved oxygen in both water (H₂O) and heavy water (D₂O) using nanosecond-pulsed visible blue laser light in the 400-440 nm spectral region without singlet oxygen photosensitizers. We report an oxygen-dependent Stokes peak in the red spectrum (600-670 nm) that is identical when produced in H₂O and D₂O. These red Stokes photons are not detected when an oxygen quencher is present. Temporal photodepletion of the uric acid absorbance peak at 294 nm is consistent with singlet oxygen generation. We postulate that a two-photon stimulated Raman process produces singlet oxygen from O₂ dissolved within the solvents. We note that the energy difference between input and output photons of 0.97 eV is precisely the energy needed to excite O₂ to its singlet state.

X3Σg- → b1Σg+ Absorption Spectra of Molecular Oxygen in Liquid Organic Solvents at Atmospheric Pressure

Spectra and absorption coefficients of the forbidden 765 nm X³Σ_g^- → b¹Σ_g⁺ transition of molecular oxygen dissolved in organic solvents at atmospheric pressure were recorded over a 5 m path length using a liquid waveguide capillary cell. The results show that it is possible to investigate this weak near-infrared absorption transition in a common liquid hydrocarbon solvent without the need for a potentially dangerous high oxygen pressure. Proof-of-principle data from benzene, toluene, chlorobenzene, bromobenzene, and iodobenzene reveal a pronounced heavy atom effect on this spin-forbidden transition. For example, the absorption coefficient at the band maximum in iodobenzene, (28.9 ± 3.3) × 10^-3 M^-1 cm^-1, is approximately 21 times larger than that in benzene, (1.4 ± 0.1) × 10^-3 M^-1 cm^-1. These absorption measurements corroborate results obtained from O₂(X³Σ_g^-) → O₂(b¹Σ_g⁺) excitation spectra of O₂(a¹Δ_g) → O₂(X³Σ_g^-) phosphorescence, which depended on data from a plethora of convoluted experiments. Spectroscopic studies of molecular oxygen in liquid solvents can help evaluate aspects of the seminal Strickler-Berg approach to treat the effect of solvent on Einstein's A and B coefficients for radiative transitions. In particular, our present results are a key step toward using the O₂(X³Σ_g^-) → O₂(b¹Σ_g⁺) transition to evaluate the speculated limiting condition of applying the Strickler-Berg treatment to a highly forbidden process. This latter issue is but one example of how an arguably simple homonuclear diatomic molecule continues to aid the scientific community by providing fundamental physical insight.

Salt-mediated, plasmonic field-field/field-lattice coupling-enhanced NIR-II photodynamic therapy using core-gap-shell gold nanopeanuts

Plasmonic field-field coupling-induced enhancement of the optical properties of dye molecules in the nanogaps among metal nanoparticle clusters and thin films has attracted significant attention especially in disease-related theranostic applications. However, it is very challenging to synthesize plasmonic core-gap-shell nanostructures with a well-controlled nanogap, uniform shape, and distances to maximize the plasmonic field-field coupling between the core and the shell. Herein, we synthesized Au@gap@AuAg nanopeanut-shaped core-gap-shell nanostructures (Au NPN) and tuned their optical absorption from near-infrared region-I (NIR-I) to near-infrared region-II (NIR-II) by filling their nanogap with a high dielectric NaCl_(aq) aqueous solution, which led to a dramatic redshift in the plasmonic absorption band by 320 nm from 660 to 980 nm and a 12.6-fold increase (at 1064 nm) in the extinction coefficient in the NIR region (1000-1300 nm). Upon filling the nanogap with NaCl_(aq) aqueous solution, the Au NPN6.5(NaCl) (i.e., ∼6.5 nm nanogap)-mediated NIR-II photodynamic therapy effect was dramatically enhanced, resulting in a much longer average lifespan of >55 days for the mice bearing a murine colon tumor and treated with Au NPN6.5(NaCl) plus 1064 nm light irradiation compared to the mice treated with Au NPN6.5 + 1064 nm light irradiation (without nanogap filled with dielectric NaCl_(aq), 40 d) and the doxorubicin-treated group (23 d). This study demonstrates a simple but effective method to tune and maximize the plasmonic field-field coupling between the metal shell and metal core of core-gap-shell nanostructures, the plasmonic field-lattice interactions, and biomedical applications for the treatment of tumors. Overall, our work presents a new way to enhance/maximize the plasmonic field-field and field-lattice coupling, and thus the performance/sensitivities in nanogap-based bioimaging, sensing, and theranostic nanomaterials and devices.

Significant differences between solid state and solution photochemistry and photophysics of mesogenic organometallic gold complexes

Five new gold complexes 4-ROC6H4C≡CAuN≡CC6H4-4-OR′ (R/R′ = CH3/C9H19 (C1N9 ), C15H31/C9H19 (C15N9 ), C6H13/C15H31 (C6N15 ), C9H19/C15H31 (C9N15 ), C12H25/C15H31 (C12N15 )) were synthesized and characterized (1H and 13C NMR, IR, Raman spectroscopy, and high resolution mass spectrometry). Their organized smectic phases were investigated by TGA, DSC, powder XRD, and polarized light optical microscopy, and the solids are found to have crystalline and amorphous domains. No evidence for Au•••Au interactions was observed. The steady state and time-resolved absorption and emission properties at 298 and 77 K were examined, and surprisingly, the excited lifetime of the triplet excited state in the solid state is extremely short-lived (<100 ps) in comparison with the microsecond time scale recorded for the solution and at 77 K. The photosensitization of 1O2 was observed in solution but not in the solid state. The nature of the singlet (ligand-to-ligand charge transfer) and triplet (ethynyl/intraligand ππ*) excited states were assessed using DFT and TD-DFT computations. The thermal and UV-photochemical formation of gold nanoparticles were performed in solution (slow) and in the solid state (faster). The thermally generated nanoparticles are found to be larger (2–20 nm; TEM) and exhibit well-defined shapes, whereas the photochemically generated ones are smaller (1–10 nm) and show ill-defined shapes.

O2 solvation cavity in voids of ionic liquids studied by the solvatochromic red shift of O2(1Δg) phosphorescence

Phosphorescence spectroscopy of singlet oxygen [=O₂(¹Δ_g)] was applied to study the solvation properties of small solute molecule, O₂, in ionic liquids. Unlike conventional molecular solvents, the spectral red shift of the O₂(¹Δ_g) phosphorescence in ionic liquids from the gas phase was found to depend not only on the refractive index of solvents but also on the vdW volume of anions. This unusual spectral shift of the O₂(¹Δ_g) luminescence is interpreted by considering the size of solvation cavities in voids, which is estimated by analyzing the free volume in ionic liquids. These results suggest the potential of the O₂(¹Δ_g) phosphorescence spectral shift measurement in the study of molecular-scale voids in ionic liquids.

Singlet oxygen luminescence detector based on low-cost InGaAs avalanche photodiode

Molecular oxygen excited to singlet state (Singlet oxygen, ¹O₂) becomes highly reactive and cytotoxic chemical. ¹O₂ is commonly generated by photoexcitation of dyes (photosensitizers), including the photodynamic therapy and diagnostics of cancer. However, the formation of singlet oxygen is often unwanted for various light-sensitive compounds, e.g. it causes the photobleaching of fluorescent probes. In either case, during a development of new photosensitive chemicals and drugs there is a need to evaluate the amount of ¹O₂ formed during photoexcitation. The direct approach in measuring the amount of singlet oxygen is based on the detection of its luminescence at 1270 nm. However, this luminescence is usually weak, which implies the use of highly sensitive single-photon detectors. Thus the existing instruments are commonly complicated and expensive. Here we suggest an approach and report a device to measure the ¹O₂ luminescence using low-cost InGaAs avalanche photodiode and simple electronics. The measurements can be performed in stationary (not time-resolved) mode in organic solvents such as tetrachloromethane (CCl₄), ethanol and DMSO. In particular, we performed spectral-resolved measurements of the singlet oxygen luminescence in CCl₄ with the device and demonstrated high complementarity to literature data. The simple setup allows to evaluate the efficiency (or speed) of singlet oxygen generation and hence facilitates the development and characterization of new photosensitizers and other photosensitive chemicals.

Solvent and Heavy-Atom Effects on the O2(X3Σg-) → O2(b1Σg+) Absorption Transition

The effect of 16 liquid solvents on both the spectrum and molar absorption coefficient of the X³Σ_g^- → b¹Σ_g⁺ transition in molecular oxygen has been examined. The ability to monitor this weak transition using air or oxygen saturated samples at atmospheric pressure was facilitated by the rapid and efficient O₂(b¹Σ_g⁺) → O₂(a¹Δ_g) transition, which allowed the use of O₂(a¹Δ_g) phosphorescence as a sensitive probe of O₂(b¹Σ_g⁺) production. The results of these O₂(a¹Δ_g) phosphorescence experiments are consistent with the results of independent experiments in which the O₂(a¹Δ_g) thus produced was "trapped" via a chemical reaction. The data recorded were used to calculate rate constants for the O₂(b¹Σ_g⁺) → O₂(X³Σ_g^-) radiative transition, a parameter that is otherwise difficult to directly obtain from such a wide range of solvents using O₂(b¹Σ_g⁺) → O₂(X³Σ_g^-) phosphorescence. The data show that the response of the O₂(b¹Σ_g⁺) → O₂(X³Σ_g^-) radiative transition to solvent is not the same as that of the O₂(b¹Σ_g⁺) → O₂(a¹Δ_g) and O₂(a¹Δ_g) → O₂(X³Σ_g^-) radiative transitions, both of which have been extensively examined over the years. However, our data are consistent with a theoretical model proposed by Minaev for the effect of solvent on radiative transitions in oxygen and, as such, arguably provide one of the final chapters in describing a system that has challenged the scientific community for years.

Reactive oxygen species: Reactions and detection from photosynthetic tissues

Reactive oxygen species (ROS) have long been recognized as compounds with dual roles. They cause cellular damage by reacting with biomolecules but they also function as agents of cellular signaling. Several different oxygen-containing compounds are classified as ROS because they react, at least with certain partners, more rapidly than ground-state molecular oxygen or because they are known to have biological effects. The present review describes the typical reactions of the most important ROS. The reactions are the basis for both the detection methods and for prediction of reactions between ROS and biomolecules. Chemical and physical methods used for detection, visualization and quantification of ROS from plants, algae and cyanobacteria will be reviewed. The main focus will be on photosynthetic tissues, and limitations of the methods will be discussed.

Evaluation of photodynamic activity of C60/2-hydroxypropyl-β-cyclodextrin nanoparticles

The objective of this study is to evaluate the ability of C(60)/2-hydroxypropyl-β-cyclodextrin (HP-β-CyD) naonparticles to generate reactive oxygen species (ROS) and to induce cell toxicity by the photoirradiation. C(60) nanoparticles were prepared by cogrinding with HP-β-CyD for 3 h at 4°C under reduced pressure. The photodynamic activity of C(60)/HP-β-CyD nanoparticles was evaluated by spectroscopic methods, including the electron spin resonance spin-trapping method, and by the cell viability test using Hela cells. C(60)/HP-β-CyD nanoparticles efficiently generated not only superoxide anion radical (O(2)(·-)) and hydroxyl radical (·OH), but also singlet oxygen ((1)O(2)) through photoirradiation. The ROS generation was enhanced by decreasing the mean particle diameter of C(60) nanoparticles, and the particle size smaller than 90 nm showed a high generation of ·OH and (1)O(2). In addition, HP-β-CyD enhanced the generation of (1)O(2), compared with polyvinylpyrrolidone (an effective solubillizer for C(60)), due to partial disposition of C(60) in the hydrophobic CyD cavity. Furthermore, C(60) /HP-β-CyD nanoparticles showed cell toxicity after the light irradiation, but no toxicity was observed without the light irradiation. Therefore, HP-β-CyD is useful for the preparation of stable C(60) nanoparticles with high ROS generation ability, and C(60)/HP-β-CyD nanoparticles are a promising photosensitizer for photodynamic therapy.

Singlet oxygen: there is indeed something new under the sun

Singlet oxygen, O(2)(a(1)Delta(g)), the lowest excited electronic state of molecular oxygen, has been known to the scientific community for approximately 80 years. It has a characteristic chemistry that sets it apart from the triplet ground state of molecular oxygen, O(2)(X(3)Sigma), and is important in fields that range from atmospheric chemistry and materials science to biology and medicine. For such a "mature citizen", singlet oxygen nevertheless remains at the cutting-edge of modern science. In this critical review, recent work on singlet oxygen is summarized, focusing primarily on systems that involve light. It is clear that there is indeed still something new under the sun (243 references).

Singlet oxygen in a cell: spatially dependent lifetimes and quenching rate constants

Singlet molecular oxygen, O(2)(a(1)Delta(g)), can be created in a single cell from ground-state oxygen, O(2)(X(3)Sigma(g)(-)), upon focused laser irradiation of an intracellular sensitizer. This cytotoxic species can subsequently be detected by its 1270 nm phosphorescence (a(1)Delta(g) --> X(3)Sigma(g)(-)) with subcellular spatial resolution. The singlet oxygen lifetime determines its diffusion distance and hence the intracellular volume element in which singlet-oxygen-initiated perturbation of the cell occurs. In this study, the time-resolved phosphorescence of singlet oxygen produced by the sensitizers chlorin (Chl) and 5,10,15,20-tetrakis(N-methyl-4-pyridyl)-21H,23H-porphine (TMPyP) was monitored. These molecules localize in different domains of a living cell. The data indicate that (i) the singlet oxygen lifetime and (ii) the rate constant for singlet oxygen quenching by added NaN(3) depend on whether Chl or TMPyP was the photosensitizer. These observations likely reflect differences in the chemical and physical constituency of a given subcellular domain (e.g., spatially dependent oxygen and NaN(3) diffusion coefficients), thereby providing evidence that singlet oxygen responds to the inherent heterogeneity of a cell. Thus, despite a relatively long intracellular lifetime, singlet oxygen does not diffuse a great distance from its site of production. This is a consequence of an apparent intracellular viscosity that is comparatively large.

Singlet oxygen phosphorescence photosensitized in nano-aggregates of C60 buckminsterfullerene is insensitive to solvent and quenchers and strongly red-shifted indicating highly polarizable interior

We investigated the strongly red-shifted singlet oxygen (1O2) phosphorescence spectra in an aqueous preparation of C60 buckminsterfullerene. The ~10 nm red shift was associated with H2O dispersions of C60 nanoaggregates (C60)n that can photosensitize 1O2 in their polarizable cores. In contrast to 1O2 produced by the water-soluble C60-(γ-cyclodextrin)2 complex, 1O2 trapped inside (C60)n was short-lived (~2-3 μs), insensitive to solvent and 1O2 quenchers, and did not induce photocytotoxicity. To our knowledge, 1O2 spectrum from (C60)n is the most red-shifted 1O2 spectrum recorded to date and it may be used to probe the inner polarizability of carbon (nano)aggregates.

Synthesis and photophysical properties of silicon phthalocyanines with axial siloxy ligands bearing alkylamine termini

Eleven silicon phthalocyanines which can be grouped into two homologous series [SiPc[OSi(CH3)2(CH2)(n)N(CH3)2]2, n = 1-6 (series 1), and SiPc[OSi(CH3)2(CH2)3N((CH2)(n)H)2]2, n = 1-6 (series 2)] as well as an analogous phthalocyanine, SiPc[OSi(CH3)2(CH2)3NH2]2, were synthesized. The ground state absorption spectra, the triplet state dynamics, and singlet oxygen quantum yields of 10 of these phthalocyanines were measured. All compounds displayed similar ground state absorption spectral properties in dimethylformamide solution with single Q band maxima at 668 +/- 2 nm and B band maxima at 352 +/- 1 nm. Photoexcitation of all compounds in the B bands generated the optical absorptions of the triplet states which decayed with lifetimes in the hundreds of microseconds region. Oxygen quenching bimolecular rate constants near 2 x 10(9) M(-1) s(-1) were measured, indicating that energy transfer to oxygen was exergonic. Singlet oxygen quantum yields, phi(delta), were measured, and those phthalocyanines in which the axial ligands are terminated by dimethylamine residues at the end of alkyl chains having four or more methylene links exhibited yields near > or = 0.35. Others gave singlet oxygen quantum yields near 0.2, and still others showed singlet oxygen yields of <0.1. The reduced singlet oxygen yields are probably caused by a charge transfer quenching of the 1pi,pi* state of the phthalocyanine by interaction with the lone pair electrons on the nitrogen atoms of the amine termini. In some cases, these can approach and interact with the electronically excited pi-framework, owing to diffusive motions of the flexible oligo-methylene tether.

Theoretical and experimental analysis of the luminescence signal of singlet oxygen for different photosensitizers

After the generation by different photosensitizers, the direct detection of singlet oxygen is performed by measuring its luminescence at 1270 nm. Using an infrared sensitive photomultiplier, the complete rise and decay time of singlet oxygen luminescence is measured at different concentrations of a photosensitizer, quencher, or oxygen. This allows the extraction of important information about the photosensitized generation of singlet oxygen and its decay, in particular at different oxygen concentrations. Based on theoretical considerations all important relaxation rates and rate constants were determined for the triplet T(1) states of the photosensitizers and for singlet oxygen. In particular, depending on the oxygen or quencher concentration, the rise or the decay time of the luminescence signal exhibit different meanings regarding the lifetime of singlet oxygen or triplet T(1)-state. To compare with theory, singlet oxygen was generated by nine different photosensitizers dissolved in either H2O, D2O or EtOD. When using H2O as solvent, the decaying part of the luminescence signal is frequently not the lifetime of singlet oxygen, in particular at low oxygen concentration. Since cells show low oxygen concentrations, this must have an impact when looking at singlet oxygen detection in vitro or in vivo.

Photosensitized Generation of Singlet Oxygen

This work gives an overview of what is currently known about the mechanisms of the photosensitized production of singlet oxygen. Quenching of pi pi* excited triplet states by O2 proceeds via internal conversion of excited encounter complexes and exciplexes of sensitizer and O2. Both deactivation channels lead with different efficiencies to singlet oxygen generation. The balance between the deactivation channels depends on the triplet-state energy and oxidation potential of the sensitizer, and on the solvent polarity. A model has been developed that reproduces rate constants and efficiencies of the competing processes quantitatively. Sensitization by excited singlet states is much more complex and hence only qualitative rules could be elaborated, despite serious efforts of many groups. However, the most important deactivation paths of fluorescence quenching by O2 are again directed by excess energies and charge-transfer interactions similar to triplet-state quenching by O2. Finally, two recent developments in photosensitization of singlet oxygen are reviewed: Two-photon sensitizers with particular application potential for photodynamic therapy and fluorescence imaging of biological samples and singlet oxygen sensitization by nanocrystalline porous silicon, a material with very different photophysics compared to molecular sensitizers.

Overview of Theoretical and Computational Methods Applied to the Oxygen–Organic Molecule Photosystem

The challenges of using modern theoretical and computational tools to model the unique features of the oxygen-organic molecule photosystem are discussed from a historical and pedagogical perspective. This review is written for the novice, but the problems formulated should stimulate the expert.

Two-Photon Singlet Oxygen Microscopy: The Challenges of Working with Single Cells

A microscope is described in which singlet molecular oxygen, O2(a1deltag), is produced in a femtoliter focal volume via a nonlinear two-photon photosensitized process, and the 1270 nm phosphorescence from this population of O2(a1deltag) is detected in a photon counting experiment. Although two-photon excitation of a sensitizer is less efficient than excitation by a one-photon process, nonlinear excitation has several distinct advantages with respect to the spatial resolution accessible. Pertinent aspects of this two-photon O2(a1deltag) microscope were characterized using bulk solutions of photosensitizers. These data were compared to those obtained from a single biological cell upon linear one-photon excitation of a sensitizer incorporated in the cell. On the basis of the results obtained, we outline the challenges of using nonlinear optical techniques to create O2(aldeltag) at the single cell level and to then optically detect the O2(aldeltag) thus produced in a time-resolved experiment.

Optical detection of singlet oxygen from single cells

The lowest excited electronic state of molecular oxygen, singlet molecular oxygen, O(2)(a (1)Delta(g)), is a reactive species involved in many chemical and biological processes. To better understand the roles played by singlet oxygen in biological systems, particularly at the sub-cellular level, optical tools have been developed to create and directly detect this transient state in time- and spatially-resolved experiments from single cells. Data obtained indicate that, contrary to common perception, this reactive species can be quite long-lived in a cell and, as such, can diffuse over appreciable distances including across the cell membrane into the extracellular environment. On one hand, these results demonstrate that the behavior of singlet oxygen in an intact cell can be significantly different from that inferred from model bulk studies. More generally, these results provide a new perspective for mechanistic studies of intra- and inter-cellular signaling and events that ultimately lead to photo-induced cell death.

Direct optical detection of singlet oxygen from a single cell

Singlet oxygen has been detected in single nerve cells by its weak 1270 nm phosphorescence (a1deltag --> X3sigmag-) upon irradiation of a photosensitizer incorporated in the cell. Thus, one can now consider the application of direct optical imaging techniques to mechanistic studies of singlet oxygen at the single-cell level.

Response calculations of electronic and vibrational transitions in molecular oxygen induced by interaction with noble gases

The Einstein coefficient for the singlet oxygen emission a1Deltag-->X3Sigmag- at lambda=1270 nm and b1Sigmag+-->X3Sigmag- emission at lambda=750 nm were calculated by quadratic response (QR) multiconfiguration self-consisted field (MCSCF) method for a number of collision complexes O2+M, where M=He, Ne, Ar. Interaction with He clusters was studied in order to simulate cooperative effect of the environment on the oxygen emission. Calculations of the dipole transition moment for the Noxon band, b1Sigmag+-a1Deltag, by linear response (LR) MCSCF method were also performed for a number of collision complexes. Spin-orbit coupling (SOC) between the b1Sigmag+ and X3Sigmag- (MS=0) states does not change much upon collisions, thus the a-X transition borrows intensity mostly from the collision-induced Noxon band b-a. The a-X intensity borrowing from the Schumann-Runge transition is negligible. The calculations show that the b-a and a-X transition probabilities are enhanced approximately by 10(5) and 10(3) times by O2+M collisions. An order of magnitude differences occur for both transitions for noble gases with large difference in polarizability. A strong cooperative effect is obtained when few He atoms perturb the oxygen molecule. Depending on mutual orientation of the partners it can be a complete quenching of the a-->X emission or strong non-additive enhancement. Collision-induced infrared vibrational transitions in a number of molecular oxygen excited states were studied and shown to be state selective.