Effect of the Media on the Quantum Yield of Singlet Oxygen (O2(1Δg)) Production by 9H‐Fluoren‐9‐one: Solvents and Solvent Mixtures

We have investigated the effect of a series of 18 solvents and mixtures of solvents on the production of singlet molecular oxygen (O₂(¹Δ_g), denoted as ¹O₂) by 9H‐fluoren‐9‐one (FLU). The normalized empirical parameter E derived from E_T(30) has been chosen as a measure of solvent polarity using Reichardt's betaine dyes. Quantum yields of ¹O₂ production (Φ_Δ) decrease with increasing solvent polarity and protic character as a consequence of the decrease of the quantum yield of intersystem crossing (Φ_ISC). Values of Φ_Δ of unity have been found in alkanes. In nonprotic solvents of increasing polarity, Φ_ISC and, therefore, Φ_Δ decrease due to solvent‐induced changes in the energy levels of singlet and triplet excited states of FLU. This compound is a poor ¹O₂ sensitizer in protic solvents, because hydrogen bonding considerably increases the rate of internal conversion from the singlet excited state, thus diminishing Φ_Δ to values much lower than those in nonprotic solvents of similar polarity. In mixtures of cyclohexane and alcohols, preferential solvation of FLU by the protic solvent leads to a fast decrease of Φ_Δ upon addition of increasing amounts of the latter.     

Unraveling the Efficiency of Thioxanthone Based Triplet Sensitizers: A Detailed Theoretical Study

Photochemical activation by triplet photosensitizers is highly expedient for a green focus society. In this work, we have theoretically probed excited state characteristics of thioxanthone and its derivatives for their triplet harvesting efficiency using density functional theory (DFT) and time-dependent density functional theory (TDDFT). Absorption and triplet energies corroborate well with the available experimental data. Our results predict that both the S₁ and T₁ states are π-π^* in nature, which renders a high oscillator strength for S₀ to S₁ transition. Major triplet exciton conversion occurs through intersystem crossing (ISC) channel between the S₁ (¹π-π^*) and high energy ³n- π^* state. Apart from that, there is both radiative and non-radiative channel from S₁ to S₀, which competes with the ISC channel and reduces the triplet harvesting efficiency. For thioxanthones with −OMe (Me=Methyl) or −F substitution at 2 or 2’ positions, the ISC channel is not energetically feasible, causing sluggish intersystem crossing quantum yield (Φ_ISC). For unsubstituted thioxanthone and for isopropyl substitution at 2’ position, the S₁-T₁ gap is slightly positive ( ), rendering a lower triplet harvesting efficiency. For systems with −OMe or −F substitution at 3 or 3’ position of thioxanthone, because of buried π state and high energy π^* state, the S₁-³nπ^* gap becomes negative. This leads to a high Φ_ISC (>0.9), which is key to being an effective photocatalyst.     

Time-Resolved Study of the Sensitized Formation and Decay of 1O2(1δg) in the Presence of Cyclodextrins*

The effect of β-cyclodextrin and β-hydroxypropyl-cyclodextrin on some properties of the aromatic ketone 1-H-phenalen-1-one and its sulfonate derivative 1-H-phen-alen-1-one-2-sulfonic acid was measured in aqueous solution. From the changes in the UV-visible range of the absorption spectra, the association equilibrium constants for the formation of inclusion complexes were determined. Because these ketones are very efficient sensitizers for the generation of singlet oxygen, time-resolved infrared luminescence was used to measure the lifetime of singlet oxygen in D₂O. Cyclodextrins are weak deactivating agents of singlet oxygen; the upper limits for the bimolecular deactivation constants are 2 times 10⁵M^?1s^?1 and 1 times 10⁵M^?1s^?1 for β-cyclodextrin and β-hydroxypropyl-cyclodextrin, respectively. Besides, they do not affect noticeably the extent of formation of singlet oxygen; this result is explained in terms of relocation of the sensitizer (exit from the cyclodextrin cavity) in the triplet excited state.     

Singlet-Oxygen (1Δg) Production by Ruthenium(II) complexes containing polyazaheterocyclic ligands in methanol and in water

In the context of our studies on ruthenium(II) complexes containing polyazaheterocyclic ligands, we have determined the rate constants of quenching by molecular oxygen (k_q) of the metal-to-ligand charge-transfer-excited state of a series of homoleptic [RuL₃] complexes (where L stands for 2,2′-bipyridine (bpy), 1,10-phenanthroline (phen), 2,2′-bipyrazine (bpz), 4,7-diphenyl-1,10-phenanthroline (dip), diphenyl-1,10-phenanthroline-4,7-disulfonate (dpds), and 1, 10-phenanthroline-5-octadecanamide (poda)) in H₂O and in MeOH. These compounds are singlet-oxygen (O₂(¹Δ_g)) sensitizers, and quantum yields of singlet-oxygen production (Ψ_Δ) in both solvents are also reported. Values of k_q and Ψ_Δ depend on the nature of the ligand L and on the solvent, Ψ_Δ values showing a large range of variation (0.2 to 1.0). In MeOH, the only pathway for quenching of the excited [RuL₃] complexes by molecular oxygen is energy transfer: the fraction of quenched excited states yielding singlet oxygen (?) is unity for all compounds in the series investigated. Changing from MeOH to H₂O has several remarkable effects: higher k_q and lower Ψ_Δ values are observed; ? drops to ca. 0.5 except for [Ru(bpz)₃]²⁺. In fact, [Ru(bpz)₃]²⁺ is by far the weakest reductant in the series and behaves differently from the other complexes, with lowest k_q and Ψ_Δ values and a ? equal to 1 in both solvents. Results are interpreted on the basis of the role played by charge-transfer interactions between the sensitizer excited state and molecular oxygen in the quenching mechanism. Ru^II Complexes based on the 4,7-diphenyl-1, 10-phenanthroline (dip) ligand are very efficient and stable singlet-oxygen sensitizers with Ψ_Δ values close to unity in air-saturated MeOH.     

Rational Design of Emissive NIR‐Absorbing Chromophores: RhIII Porphyrin‐Aza‐BODIPY Conjugates with Orthogonal Metal–Carbon Bonds

The facile synthesis of Group 9 Rh^III porphyrin‐aza‐BODIPY conjugates that are linked through an orthogonal Rh?C(aryl) bond is reported. The conjugates combine the advantages of the near‐IR (NIR) absorption and intense fluorescence of aza‐BODIPY dyes with the long‐lived triplet states of transition metal rhodium porphyrins. Only one emission peak centered at about 720 nm is observed, irrespective of the excitation wavelength, demonstrating that the conjugates act as unique molecules rather than as dyads. The generation of a locally excited (LE) state with intramolecular charge‐transfer (ICT) character has been demonstrated by solvatochromic effects in the photophysical properties, singlet oxygen quantum yields in polar solvents, and by the results of density functional theory (DFT) calculations. In nonpolar solvents, the Rh^III conjugates exhibit strong aza‐BODIPY‐centered fluorescence at around 720 nm (Φ_F=17–34 %), and negligible singlet oxygen generation. In polar solvents, enhancements of the singlet‐oxygen quantum yield (Φ_Δ=19–27 %, λ_ex=690 nm) have been observed. Nanosecond pulsed time‐resolved absorption spectroscopy confirms that relatively long‐lived triplet excited states are formed. The synthetic methodology outlined herein provides a useful strategy for the assembly of functional materials that are highly desirable for a wide range of applications in material science and biomedical fields.     

IS SINGLET OXYGEN FORMATION DOMINANT IN TRIPLET QUENCHING PROCESSES? MEASUREMENT OF THE QUANTUM YIELD OF SINGLET OXYGEN FORMATION BY THE TIME-RESOLVED THERMAL LENS METHOD

The quantum yields of singlet oxygen formation (Ø_Δ) by the quenching of triplet states of organic sensitizers are measured at various concentrations of the sensitizers by using the time-resolved thermal lens method. Above a certain concentration, Ø_Δ is independent of the sensitizer concentration. Below the threshold, Ø_Δ gradually decreases as the concentration of the sensitizer decreases. The extrapolation of Ø_Δ to zero concentration indicates that singlet oxygen formation is not necessarily dominant in the quenching process even for the ³ππ* state in benzene.     

Synthesis, photophysical and photochemical properties of aryloxy tetra-substituted gallium and indium phthalocyanine derivatives

The synthesis, photophysical and photochemical properties of the tetra-substituted aryloxy gallium(III) and indium(III) phthalocyanines are reported for the first time. General trends are described for photodegradation, singlet oxygen, fluorescence, and triplet quantum yields and triplet lifetimes of these compounds. The introduction of phenoxy and tert-butylphenoxy substituents on the ring resulted in lowering of fluorescence quantum yields and lifetimes, and triplet quantum yields, and an increase of k_IC, k_ISC, and k_F. Photoreduction of the complexes was observed during laser flash photolysis. The singlet oxygen quantum yields (Φ_Δ), which give an indication of the potential of the complexes as photosensitizers in applications where singlet oxygen is required (Type II mechanism) ranged from 0.41 to 0.91. Thus, these complexes show potential as Type II photosensitizers.     

Radical-Enhanced Intersystem Crossing in a Bay-Substituted Perylene Bisimide−TEMPO Dyad and the Electron Spin Polarization Dynamics upon Photoexcitation**

A 4-amino-2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO) radical was attached to the bay position of perylene-3,4 : 9,10-bis(dicarboximide) (perylenebisimide, PBI) to study the radical-enhanced intersystem crossing (REISC) and electron spin dynamics of the photo-induced high-spin states. The dyads give strong visible light absorption (ϵ=27000 M⁻¹ cm⁻¹at 607 nm). Attaching a TEMPO radical to the PBI unit transforms the otherwise non-radiative decay of S₁ state (fluorescence quantum yield: Φ_F=2.9 %) of PBI unit to ISC (singlet oxygen quantum yield: Φ_Δ=31.8 %, Φ_F=1.6 %). Moreover, the REISC is more efficient as compared to the heavy atom effect-induced ISC (Φ_Δ=17.8 % for 1,8-dibromoPBI). For the dyad, ISC takes 245 ps and triplet state lifetime is 1.5 μs, much shorter than the native PBI (τ_T=126.6 μs). X- and Q-band time-resolved electron paramagnetic resonance spectroscopy shows that the exchange interaction in the photoexcited radical-chromophore dyad is larger than the triplet zero-field splitting (ZFS) and the difference of Zeeman energies of the radical and chromophore. The inversion of electron spin polarization from emissive to absorptive was observed and attributed to the initial completion of the quartet state population and the subsequent depopulation processes induced by the zero-field splitting.     

Ru催化N2与H2反应合成氨的两态反应机理

采用密度泛函理论(DFT)UB3LYP方法对Ru在单重态、三重态及五重态势能面上催化N_2与H_2反应合成氨的两态反应机理进行理论研究,发现该反应为典型的两态反应。计算得到最低能量交叉点(MECP)处自旋-轨道耦合常数(H_(soc))及双程系间窜越几率(P~(ISC)),MECP1:H_(soc)=508.34 cm~(-1),P_2~(ISC)=0.85,MECP9:H_(soc)=269.21 cm~(-1),P_2~(ISC)=0.27。运用能量跨度模型(energetic span model)确定Ru催化合成氨反应的转化频率(TOF)决速过渡态(TDTS)为~3TS2-3,TOF决速中间体(TDI)为~3IM9。     

Effect of the Media on the Quantum Yield of Singlet Oxygen (O2(1Δg)) Production by 9H‐Fluoren‐9‐one: Microheterogeneous Systems

The quantum yield (Φ_Δ) of singlet oxygen (O₂(¹Δ_g) production by 9H‐fluoren‐9‐one (FLU) is very sensitive to the nature of the solvent (0.02 in a highly polar and protic solvent, such as MeOH, to 1.0 in apolar solvents). This high sensitivity has been used for probing the interaction of FLU with micellar media and microemulsions based on anionic (sodium dodecyl sulfate, SDS; bis‐(2‐ethylhexyl)sodium sulfosuccinate, AOT), cationic (cetyltrimethylammonium chloride, CTAC) and nonionic (Triton X‐100, TX) surfactants. Values of Φ_Δ of FLU vary in a wide range (0.05–1.0) in both microheterogeneous media and neat solvent, and provide information on the microenvironment of FLU, i.e., on its localization within organized media. In ionic and nonionic micellar media, as well as in four‐component microemulsions, FLU is, to various extents, exposed to solvation by the polar and protic components of the microheterogeneous systems (water and/or butan‐1‐ol) in the micellar interfacial region (Φ_Δ=0.05–0.30). In contrast, in AOT reverse micelles (consisting of AOT as surfactant, cyclohexane as hydrophobic component, and water), FLU is located in the hydrophobic continuous pseudophase, and is totally separated from the micellar water pools (Φ_Δ≈1.0).     

Long-lived Triplet Excited State of Perylenemonoimide (PMI) in a Diimine Pt(II) Bis-acetylide Complex: Preparation and Study of the Photophysical Property with Transient Spectra

We prepared a N^N Pt(II) bisacetylide complex that has strong absorption of visible light (molar absorption coefficients ϵ=6.7×10⁴ M⁻¹ cm⁻¹ at 570 nm), and the singlet oxygen quantum yield (Φ_Δ) is up to 78 %. Femtosecond transient absorption spectra show the intersystem crossing (ISC) of the complex takes 81.8 ps, nanosecond transient absorption spectra show the triplet excited state lifetime is 7.6 μs. Density functional theory (DFT) computation demonstrated that the S₁ and T₁ states are mainly localized on the perylenemonoimide (PMI) ligands, although the involvement of the Pt(II) centre is noticeable. The complex was used as triplet photosensitizer to generate delayed fluorescence with perylenebisimide (PBI) as the triplet state energy acceptor and emitter, via the intermolecular triplet-triplet energy transfer (TTET) and triplet-triplet annihilation (TTA), the delayed fluorescence lifetime is up to 52.5 μs under the experimental conditions.     

Aza‐BODIPY Derivatives: Enhanced Quantum Yields of Triplet Excited States and the Generation of Singlet Oxygen and their Role as Facile Sustainable Photooxygenation Catalysts

A new series of aza‐BODIPY derivatives ( 4 a – 4 c , 5 a , c , and 6 b , c ) were synthesized and their excited‐state properties, such as their triplet excited state and the yield of singlet‐oxygen generation, were tuned by substituting with heavy atoms, such as bromine and iodine. The effect of substitution has been studied in detail by varying the position of halogenation. The core‐substituted dyes showed high yields of the triplet excited state and high efficiencies of singlet‐oxygen generation when compared to the peripheral‐substituted systems. The dye 6 c , which was substituted with six iodine atoms on the core and peripheral phenyl ring, showed the highest quantum yields of the triplet excited state (Φ_T=0.86) and of the efficiency of singlet‐oxygen generation (Φ_Δ=0.80). Interestingly, these dyes were highly efficient as photooxygenation catalysts under artificial light, as well as under normal sunlight conditions. The uniqueness of these aza‐BODIPY systems is that they are stable under irradiation conditions, possess strong red‐light absorption (620–680 nm), exhibit high yields of singlet‐oxygen generation, and act as efficient and sustainable catalysts for photooxygenation reactions.     

Photophysical Behavior of Angelicins and Thioangelicins: Semiempirical Calculations and Experimental Study

The decay processes of the lowest excited singlet and triplet states of five methylated angelicins (4,6,4′-trimethyl-angelicin, MA, and four methylated thioangelicins, MTA; see Scheme 1) were investigated in live solvents by stationary and pulsed fluorometric and flash photolytic techniques. In particular, the solvent effects on absorption, fluorescence, quantum yields of fluorescence (φ_F) and triplet formation (φ_T), lifetimes of fluorescence (τ_F) and the triplet state (τ_T) and the quantum yields of singlet oxygen production (φ_Δ) were investigated. Semiempirical (ZINDO/S-CI) calculations were carried out to obtain information (transition probabilities and nature) on the lowest excited singlet and triplet states. The quantum mechanical calculations and the solvent effect on the photophysical properties showed that the lowest excited singlet state (S¹) is a partially allowed π,π* state, while the close-lying S₂ state is n,π* in nature. The efficiencies of fluorescence, S₁→T₁ intersystem crossing (ISC) and S₁→ S₀ internal conversion (IC) strongly depend on the energy gap between S₁, and S₂ and are explained in terms of the so-called proximity effect. In fact, for MA in cyclohexane, only the S₁→ S₀ internal conversion is operative, while in acetonitrile and ethanol, where the n.π* state is shifted to higher energy, the efficiencies of fluorescence and ISC increase significantly. The energy gap between S₁ and S₂ increases in MTA, where the furanic oxygen is replaced by a sulfur atom. Consequently, the solvent effect on the photophysical parameters of MTA is less marked than for MA; e.g. fluorescence and triplet-triplet absorption are also detectable in the nonpolar cyclohexane. The lowest excited singlet state of molecular oxygen O₂(¹D_g) was produced efficiently in polar solvents by energy transfer from the T₁ state of MA and MTA.     

Ab initio calculations on urea

Multiconfiguration wave functions constructed from contracted Gaussian-lobe functions have been found for the ground and valence-excited states of urea. ICSCF molecular orbitals of the excited states were used as the parent configurations for the CI calculations except for the ¹A₁(π → π*) state. The ¹A₁(π → π*) state used as its parent configuration an orthogonal linear combination of natural orbitals obtained from the second root of a three-configuration SCF calculation. The lowest excited states are predicted to be the n π → π* and π → π* triplet states. The lowest singlet state is predicted to be the n π → π* state with an energy in good agreement with the one known UV band at 7.2 eV. The π → π* singlet state is predicted to be about 1.9 eV higher, contrary to several previous assignments which assumed the lowest band was a π → π* amide resonance band. The predicted ionization energy of 9.0 eV makes this and higher states autoionizing.     

EXCITED STATES OF SIX-MEMBERED N-HETEROCYCLES. FLUORESCENCE, PHOSPHORESCENCE AND ACID—BASE EQUILIBRIA OF ELECTRONICALLY EXCITED PHENAZINE

Abstract— An account of a systematic study of the acid-base equilibria of phenazine in the two lowest excited (π,π) states is presented. Pure electronic levels of the free base and of both its protonated forms have been located by spectroscopic methods. Fluorescence, phosphorescence and corresponding absorption spectra have been measured. The O-O energies of the free base, of the singly-protonated species and of the doubly protonated form in the lowest triplet state (³L_α(π, π)) are: 15, 475 cm^-1, 14, 175 cm^-1 and about 9300cm^-1, respectively. This last value has been estimated from the experimentally determined S-T splitting in the other two forms. Corresponding energies of the lowest singlet state (^IL_α(π,π)) are: 23,500 cm^-1, 21,250cm^-1 and 17,300 cm^-1. The fluorescence of the free base has been found in polar as well as in non-polar solvents and has been checked by the fluorescence excitation spectrum. Fluorescence quantum yields for the free base have been measured: 8.6 times 10^-4 and 3.0 × 10^-5 in ethanol and hexane solutions, respectively. Emission in ethanol has been ascribed to (π,π), that in hexane —to (π, π). fluorescence. The changes of pK_α's under excitation, calculated from the Forster's cycle, are equal: δpK_a¹=+2.8±0.3; δpK_a¹¹?+10±1.5 in the lowest (π, π) triplet state and δpK_a¹=+4.8±0.5; δpK_a¹¹=+8.4 ± 0.5 in the lowest (π,π) singlet state. The δpK_a¹¹ in the triplet state is at least as high as that in the ¹L_a(π, π) state. P P P calculations of the electronic levels and of the molecular diagrams have been performed. The energies obtained exceed experimental values by not more than 0.5 eV. An increase of the net charge on nitrogen δp under excitation has been found to be +50, +70 and +19 per cent in the ¹L_a, ¹L_b and ³L_a states, respectively. A good correlation has been found between δpK_a¹ and δp in both excited states, which have been studied experimentally.     

Highly Efficient Triplet Photosensitizers: A Systematic Approach to the Application of IrIII Complexes containing Extended Phenanthrolines

A series of Ir^III complexes, based on 1,10‐phenanthroline featuring aryl acetylene chromophores, were prepared and investigated as triplet photosensitizers. The complexes were synthesized by Sonogashira cross‐coupling reactions using a “chemistry‐on‐the‐complex” method. The absorption properties and luminescence lifetimes were successfully tuned by controlling the number and type of light‐harvesting group. Intense UV/Vis absorption was observed for the Ir^III complexes with two light‐harvesting groups at the 3‐ and 8‐positions of the phenanthroline. The asymmetric Ir^III complex (with a triphenylamine (TPA) and a pyrene moiety attached) exhibited the longest lifetime. Red emission was observed for all the complexes in deaerated solutions at room temperature. Their emission at low temperature (77 K) and nanosecond time‐resolved transient difference absorption spectra revealed the origin of their triplet excited states. The singlet‐oxygen (¹O₂) sensitization and triplet‐triplet annihilation (TTA)‐based upconversion were explored. Highly efficient TTA upconversion (Φ_UC=28.1 %) and ¹O₂ sensitization (Φ_Δ=97.0 %) were achieved for the asymmetric Ir^III complex, which showed intense absorption in the visible region (λ_abs=482 nm, ?=50900 m ^?1 cm^?1) and had a long‐lived triplet excited state (53.3 μs at RT).     

Intersystem Crossing and Electron Spin Selectivity in Anthracene-Naphthalimide Compact Electron Donor-Acceptor Dyads Showing Different Geometry and Electronic Coupling Magnitudes

Anthracene-naphthalimide (An-NI) compact electron donor-acceptor dyads were prepared, in which the orientation and distance between the two subunits were varied by direct connection or with intervening phenyl linker. Efficient intersystem crossing (ISC) and long triplet state lifetime (Φ_Δ=92 %, τ_T=438 μs) were observed for the directly connected dyads showing a perpendicular geometry (81°). This efficient spin-orbit charge transfer ISC (SOCT-ISC) takes 376 fs, inhibits the direct charge recombination (CR) to ground state (¹CT→S₀, takes 3.04 ns). Interestingly, efficient SOCT-ISC for dyads with intervening phenyl linker (Φ_Δ=40 % in DCM) was also observed, although the electron donor and acceptor adopt almost coplanar geometry (dihedral angle: 15°). Time-resolved electron paramagnetic resonance (TREPR) spectroscopy shows that the electron spin polarization of the triplet state, i. e. the electron spin selectivity of ISC, is highly dependent on the dihedral angle and the linker. For the dyads showing weaker coupling between the donor and acceptors, the charge separation and the intramolecular triplet energy transfer are inhibited at 80 K (frozen solution), because both the ³An and ³NI states were observed and the ESP are same as compared to the native anthracene and naphthalimide, which unravel their origin. The dyads were used as triplet photosensitizers for triplet−triplet annihilation upconversion (TTA UC). High UC quantum yield (Φ_UC=12.9 %) as well as a large anti-Stokes shift (0.72 eV) was attained by excitation into the CT absorption band.     

A spectroscopic investigation of aggregations in chlorophyll solutions

Abstract— Absorption and emission spectra are reported for polar and non-polar solutions of chlorophyll a and chlorophyll b. These spectra can be interpreted in terms of the formation of chlorophyll dimers and more highly aggregated forms. The phosphorescence spectra of polar and non-polar solutions of chlorophyll a are identical and are associated with emission from a π-π-* triplet state.     

Photophysics and photochromism of Δ2,2' -BI-(2H-1,4-benzothiazine)

Δ^2,2' -bi-(2H-1,4-benzothiazine) shows striking photochromism and thermochromism. The color change is thermally and photochemically reversible. The photochromism can be ascribed to the cis-trans isomerization about the central double bond, the yellow trans isomer being more stable. The quantum yield of formation ø_CP of the cis-benzothiazine from trans in dioxan was found to be 0.035 at 355 nm excitation. Laser flash photolysis of the trans-benzothiazine revealed that the excited singlet state of π, π^* character is the originating state for the photochromism since there is no experimental evidence of a triplet intermediate.