Spin-orbit charge-transfer intersystem crossing in heavy-atom-free orthogonal covalent boron-dipyrromethene heterodimers.

Boron-dipyrromethene (BODIPY) derivatives are prospective organic-based triplet photosensitizers. Since the triplet generation yield of the parent BODIPY is low, heavy atoms are widely used to improve the triplet yield. However, the dimerization of BODIPYs can also significantly improve their ability to produce triplets. Through a comparative study of the triplet formation dynamics of two heavy-atom-free orthogonal covalent BODIPY heterodimers that differ in their dihedral angles, we have demonstrated that the mechanism of spin-orbit charge-transfer intersystem crossing (SOCT-ISC) promotes the triplet generation of BODIPY heterodimers in solution. Different from the general understanding of SOCT-ISC, the heterodimer with a smaller dihedral angle and low structural rigidity showed better triplet generation due to (a) the stronger inter-chromophoric interaction in the heterodimer, which promoted the formation of a solvent-stabilized charge-transfer (CT) state, (b) the more favorable energy level alignment with sizeable spin-orbit coupling strength, and (c) the balance between the stabilized singlet CT state and limited direct charge recombination to the ground state in a weakly polar solvent. The complete spectral characterization of the triplet formation dynamics clarified the SOCT-ISC mechanism and important factors affecting the triplet generation in BODIPY heterodimers.

Ultrafast Charge Separation Driven by Torsional Motion in Orthogonal Boron Dipyrromethene Dimer.

In this work, the photoinduced charge separation (CS) via symmetry breaking in an orthogonal meso-ß-linked boron dipyrromethene (BODIPY) dimer was investigated by polarized transient absorption spectroscopy. The time constant about 0.76 ps of the CS reaction determined in dimethyl sulfoxide is much faster than the solvation dynamics. The observed transient anisotropy of the BODIPY anion band implies that both hole and electron transfers occur with similar probabilities. The bidirectional charge transfer processes suggest that the locally excited state is weakly coupled to the polar solvent, and the solvation coupled excited-state structural relaxation within the BODIPY monomeric unit is rather limited. In combination with the electronic excitation analysis based on time-dependent density-functional theory calculations, we deduced that the CS in the orthogonal BODIPY dimer is enabled via the torsional motion associated with covalently connected BODIPY units, promoting the electronic coupling, and irrelevant to the dynamic solvent relaxation.

Photophyical and photosensitizing properties of BODIPYs substantially changed by alkyl- and phenyl-amino groups on meso carbon.

meso-RNH (R = C3H7, C4H9, PhCH2, H, and Ph) substituted BODIPY compounds have been prepared to examine their photophysical properties and photosensitizing abilities. We have measured the UV-vis absorption, steady state and time resolved fluorescence, excited triplet state formation using laser flash photolysis, singlet oxygen generation ability using chemical trapping method. The results show that the presence of meso-RNH leads to large blue shift of absorption and emission wavelength, remarkable decrease in fluorescence quantum yield and lifetime values, and significant increase in singlet oxygen formation quantum yield. Quantum chemical calculation also reveals the photoinduced charge transfer (PCT) mechanism. We conclude that property changes are due to: 1) S0 and S1 geometry, 2) ground state structural isomerization, and 3) intramolecular PCT. These results and mechanisms are helpful for designing new functional materials.

Evaluation of fertilization capability of frozen-thawed completely immotile spermatozoa collected from a white bengal tiger after interspecific ICSI with bovine oocytes.

The objective of this study was to evaluate the fertilization capability of White Bengal Tiger frozen-thawed completely immotile spermatozoa after interspecific intracytoplasmic sperm injection (ICSI) with bovine oocytes. The fertilization status of presumptive zygotes was assessed 18 h after ICSI by immunofluorescence staining and confocal microscopy. The fertilization rate was 34.8% (8/23), as confirmed by the extrusion of two polar bodies, or male and female pronuclei formation. For unfertilized oocytes (65.2%, 15/23), one activated oocyte had an activated spermatozoon but most were unactivated oocytes with unactivated spermatozoa (1/15, 6.7% vs 10/15, 66.7%, respectively, p < 0.05). These results showed that White Bengal Tiger frozen-thawed completely immotile spermatozoa retained the capacity to fertilize bovine oocytes after interspecific ICSI. This is the first report of in vitro produced zygotes using tiger immotile sperm with bovine oocytes by interspecific ICSI technique, which provides an efficient and feasible method for preservation and utilization of endangered feline animals.

Halogenated BODIPY photosensitizers: Photophysical processes for generation of excited triplet state, excited singlet state and singlet oxygen.

We have systematically examined the formation of singlet oxygen O2(1Δg), the excited triplet state (T1), and excited singlet state (S1) for halogenated BODIPY photosensitizers (halogen = Cl, Br, and I) in eight solvents to understand how halogen atoms and solvent affect these properties. The phosphorescence spectra and lifetimes of singlet oxygen generated by these halogenated BODIPYs have been measured by steady state/time resolved NIR emission, while the formation quantum yield of singlet oxygen (ΦΔ) has been determined by chemical method using diphenylisobenzofuran (DPBF) as the trapping agent. The formation quantum yield ΦΔ of singlet oxygen can be as high as 0.96 for iodinated BODIPY and 0.71 for brominated BODIPY. The triplet state T1 absorption spectra of brominated and iodinated BODIPYs have been recorded by laser flash photolysis method, in which T1 shows high formation efficiency and long lifetime. The formation and decay of excited singlet state S1 of four BODIPYs have been measured by ground state (S0) absorption and steady state/time resolved fluorescence. The results show that larger halogen atoms on BODIPY core lead to smaller fluorescence quantum yield, shorter fluorescence lifetime and higher singlet oxygen formation quantum yield due to heavy atom effect that promotes the formation of triplet state. On the other hand, higher solvent polarity causes lower singlet oxygen formation quantum yield, smaller fluorescence quantum yield, and shorter fluorescence lifetime. This solvent effect is explained by the presence of photoinduced charge transfer (ICT) process from halogen atoms to BODIPY. The ICT efficiency has been estimated and the results are agreed with ICT theory. ICT process in halogenated BODIPYs has never been revealed in literature. HOMO/LUMO obtained from DFT calculation also supports the presence of ICT. The involvement of ICT in the photosensitizing process of halogenated BODIPYs provides new insights for designing BODIPY photosensitizers for photodynamic therapy of tumor.

The excited state behavior of Group IA alkaline-metal phthalocyanines revealed by photoluminescence and singlet oxygen formation.

Group IA alkaline-metal phthalocyanine (Pc) complexes (Li2Pc, Na2Pc, K2Pc, Rb2Pc, Cs2Pc) have been synthesized and purified to study their excited state behavior. Their UV-Vis electronic absorption spectra, fluorescence emission and excitation spectra, fluorescence quantum yields and lifetimes, as well as singlet oxygen formation quantum yields have been measured in DMF. These photophysical properties are compared with that of H2Pc. The fluorescence and singlet oxygen formation properties reveal that alkaline metal Pcs show weak heavy atom effect. The results also reveal that alkaline-metal Pcs (Na2Pc, K2Pc, Rb2Pc, and Cs2Pc) show very different excited singlet state (S1) behavior from that of Li2PC and H2Pc. While S1 decay of Li2PC is mainly via intersystem crossing and fluorescence, the S1 decay of M2Pc (M = Na, K, Rb, Cs) is mainly metal ion dissociation: M2Pc(S1) â†’ 2 M+ + [Pc]2-(S1), in addition to the intersystem crossing.

Organo metal halide perovskites effectively photosensitize the production of singlet oxygen (1Δg).

Using the steady state and time resolved NIR emission and specific chemical trapping techniques, we show for the first time that metal halide perovskite quantum dots can effectively generate singlet oxygen with a quantum yield of up to 0.34, the highest among nano semiconductor/nano metal singlet oxygen photosensitizers. The mechanism is concluded to be due to energy transfer from triplet excitons to molecular oxygen.

Methylated Unsymmetric BODIPY Compounds: Synthesis, High Fluorescence Quantum Yield and Long Fluorescence Time.

We show that unsymmetric BODIPY compounds with one, two, and three methyl groups can be synthesized easily and efficiently by the unsymmetric reaction method. Their steady state and time-resolved fluorescence properties are examined in solvents of different polarity. These compounds show high fluorescence quantum yields (0.87 to 1.0), long fluorescence lifetimes (5.89 to 7.40 ns), and small Stokes shift (199 to 443 cm-1). The methyl substitution exhibits influence on the UV-Vis absorption and fluorescence properties, such as the blue shift in emission and absorption spectra. It is the number rather than the position of methyls that play major roles. Except for 3 M-BDP, the increase in the number of methyls on BODIPY core leads to the increase in both fluorescence quantum yield and radiative rate constant, but causes the decrease in fluorescence lifetime. H-bonding solvents increase both the fluorescence lifetime and quantum yields. The methylated BODIPYs show the ability to generate singlet oxygen (1Δg) which is evidenced by near-IR luminescence and DPBF chemical trapping techniques. The formation quantum yield of singlet oxygen (1Δg) for the compounds is up to 0.15 ± 0.05.

Attaching naphthalene derivatives onto BODIPY for generating excited triplet state and singlet oxygen: Tuning PET-based photosensitizer by electron donors.

meso-Naphthalene substituted BODIPY compounds were prepared in a facile one pot reaction. The naphthalene functionalization of BODIPY leads up to a 5-fold increase in the formation efficiency of excited triplet state and singlet oxygen in polar solvents. Steady state and time resolved fluorescence, laser flash photolysis, and quantum chemistry methods were used to reveal the mechanism. All measured data and quantum chemical results suggest that these systems can be viewed as electron donor-acceptor (D-A) pair (BODIPY acts as the acceptor), photoinduced charge transfer (PCT) or photoinduced electron transfer (PET) occurs upon photo excitation (D-A+hν→Dδ+-Aδ-, 0<δ≤1), and the charge recombination induced the formation of triplet state (Dδ+-Aδ-→D-A (T1). These novel PCT- or PET-based photosensitizers (PSs) show different features from traditional PSs, such as the strong tunability by facile structural modification and good selectivity upon medium polarity. The new character for this type of PSs can lead to important applications in organic oxygenation reactions and photodynamic therapy of tumors.

PET-based bisBODIPY photosensitizers for highly efficient excited triplet state and singlet oxygen generation: tuning photosensitizing ability by dihedral angles.

Herein, four covalent BODIPY heterodimers that differ by dihedral angles were shown to be highly efficient excited triplet state (T1) photosensitizers (PSs) for singlet oxygen formation with a quantum yield (ΦΔ) of up to 0.94 as compared to their respective monomers, which had only negligible ΦΔ of ca. 0.060. More interestingly, these PSs generate T1via charge recombination mechanism rather than traditional inter-system crossing. The photosensitizing ability of dimers is easily tuned by either the dihedral angle (between the two linked BODIPYs) or solvent polarity. Laser flash photolysis, time-resolved and steady state fluorescence, quantum chemical calculation, as well as thermodynamic analysis were employed to study the associated photophysical process to reveal the T1 formation mechanism: photo-induced electron transfer (PET) followed by charge recombination. Due to its heavy-atom-free nature, polarity selectivity, high efficiency, and easy tunability, this PET-based PS and its mechanism are very useful in developing new PS for photodynamic therapy of tumors, photobiology, and organic photochemistry.

Photoinduced Electron Transfer-based Halogen-free Photosensitizers: Covalent meso-Aryl (Phenyl, Naphthyl, Anthryl, and Pyrenyl) as Electron Donors to Effectively Induce the Formation of the Excited Triplet State and Singlet Oxygen for BODIPY Compounds.

Pristine BODIPY compounds have negligible efficiency to generate the excited triplet state and singlet oxygen. In this report, we show that attaching a good electron donor to the BODIPY core can lead to singlet oxygen formation with up to 58 % quantum efficiency. For this purpose, BODIPYs with meso-aryl groups (phenyl, naphthyl, anthryl, and pyrenyl) were synthesized and characterized. The fluorescence, excited triplet state, and singlet oxygen formation properties for these compounds were measured in various solvents by UV/Vis absorption, steady-state and time-resolved fluorescence methods, as well as laser flash photolysis technique. In particular, the presence of anthryl and pyrenyl showed substantial enhancement on the singlet oxygen formation ability of BODIPY with up to 58 % and 34 % quantum efficiency, respectively, owing to their stronger electron-donating ability. Upon the increase in singlet oxygen formation, the fluorescence quantum yield and lifetime values of the aryl-BODIPY showed a concomitant decrease. The increase in solvent polarity enhances the singlet oxygen generation but decreases the fluorescence quantum yield. The results are explained by the presence of intramolecular photoinduced electron transfer from the aryl moiety to BODIPY core. This method of promoting T1 formation is very different from the traditional heavy atom effect by I, Br, or transition metal atoms. This type of novel photosensitizers may find important applications in organic oxygenation reactions and photodynamic therapy of tumors.

The Fluorescence Properties of Three Rhodamine Dye Analogues: Acridine Red, Pyronin Y and Pyronin B.

The fluorescence spectra, fluorescence quantum yield, and fluorescence lifetime of Acridine Red (AR), Pyronin Y (PYY), and Pyronin B (PYB) in aqueous and organic solvents were measured by steady state fluorescence, time-correlated single photon counting, and electronic absorption methods. The rate constants of radiation and non radiation process (kf and kic) were calculated to elucidate the structural effect on the fluorescence mechanism. The data for each compound are compared with that of the corresponding rhodamine dye. AR showed significant longer lifetime and higher quantum yield than PYY and PYB, because the alkyls on N atom enhance the internal conversion (IC), the longer the alkyl the faster the IC. However, the structural variation does not alter the rate constant of radiation process (kf) but does change kic significantly. The phenyl in Rhodamine B or Rhodamine 6G shows only a slight effect on the fluorescence properties. Ethanol is the solvent in which all five compounds exhibit longest lifetime and highest fluorescence quantum yield.

Synthesis and Photophysics of BF2 -Rigidified Partially Closed Chain Bromotetrapyrroles: Near Infrared Emitters and Photosensitizers.

We report the synthesis, crystallographic, optical, and triplet and singlet oxygen generation properties of a series of BF2 -rigidified partially closed chain bromotetrapyrroles as near infrared emitters and photosensitizers. These novel dyes were efficiently synthesized from a nucleophilic substitution reaction between pyrroles and the 3,5-bromo-substituents on the tetra- and hexabromoBODIPYs and absorb in the near-infrared region (681-754 nm) with high molar extinction coefficients. Their fluorescent emission (708-818 nm) and singlet oxygen generation properties are significantly affected by alkyl substitutions on the two uncoordinated pyrrole units of these dyes and the polarity of solvents. Among them, dyes 4 ca and 4 da show good singlet oxygen generation efficiency and good NIR fluorescence emission (fluorescence quantum yields of 0.14-0.43 in different solvents studied).

Intracellular modulation of excited-state dynamics in a chromophore dyad: differential enhancement of photocytotoxicity targeting cancer cells.

The photosensitized generation of reactive oxygen species, and particularly of singlet oxygen [O2 (a(1) Δg )], is the essence of photodynamic action exploited in photodynamic therapy. The ability to switch singlet oxygen generation on/off would be highly valuable, especially when it is linked to a cancer-related cellular parameter. Building on recent findings related to intersystem crossing efficiency, we designed a dimeric BODIPY dye with reduced symmetry, which is ineffective as a photosensitizer unless it is activated by a reaction with intracellular glutathione (GSH). The reaction alters the properties of both the ground and excited states, consequently enabling the efficient generation of singlet oxygen. Remarkably, the designed photosensitizer can discriminate between different concentrations of GSH in normal and cancer cells and thus remains inefficient as a photosensitizer inside a normal cell while being transformed into a lethal singlet oxygen source in cancer cells. This is the first demonstration of such a difference in the intracellular activity of a photosensitizer.

Preparation and investigation of CaZr4(PO4)6:Dy(3+) single-phase full-color phosphor.

A novel single-phase full-color phosphor CaZr4(PO4)6:Dy(3+) has been synthesized by a high-temperature solid-state reaction. X-ray powder diffraction (XRD) analysis and FT-IR spectra confirmed the phase formation of CaZr4(PO4)6:Dy(3+) materials. The photoluminescence excitation and emission spectra, the concentration dependence of the emission intensity, decay curves, ultraviolet-visible absorption spectroscopy and Commission International de I(')Eclairage (CIE) of the phosphor were investigated. The results showed that the phosphor could be efficiently excited by the near ultraviolet (NUV) light region from 340 to 440 nm, and it exhibited blue (487 nm) and yellow (577 nm) emission corresponding to (4)F9/2→(6)H15/2 transitions and (4)F9/2→(6)H13/2 transitions, respectively. The luminescence intensity of Ca(1-x)Zr4(PO4)6:xDy(3+) phosphor firstly increased and then decreased with increasing Dy(3+) concentration, and reached the maximum at x=0.04. The band gap energy of CaZr4(PO4)6 and Ca0.96Zr4(PO4)6:0.04Dy(3+) are about 4.184 eV from the diffuse reflection spectrum. The decay time was also determined for various concentrations of Dy(3+) in CaZr4(PO4)6. The calculated color coordinates lies in the blue white region. Therefore, these obtained results suggest that the prepared phosphors exhibit great potential for use as single-phase full-color phosphor for near ultraviolet white light emitting diodes (NUV WLEDs).

Spectroscopic insights on selfassembly and excited state interactions between rhodamine and phthalocyanine molecules.

The absorption and fluorescence spectra as well as fluorescence lifetimes of tetrasulfonated zinc phthalocyanine ZnPc(SO3Na)4 were measured in the absence and presence of four rhodamine dyes, Rhodamine B (RB), Ethyl rhodamine B (ERB), Rhodamine 6G (R6G), Rhodamine 110 (R110), and Pyronine B (PYB). The ground state complexes of phthalocyanine-(Rhodamine)2 were observed which exhibit new absorption bands. The binding constants are all very large (0.86×10(5)-0.22×10(8) M(-1)), suggesting rhodamine-phthalocyanine pairs are very good combinations for efficient selfassembly. Both the fluorescence intensity and the lifetime values of ZnPc(SO3Na)4 were decreased by the presence of rhodamines. The structural effect of rhodamines on selfassembly is significant. The ground state binding and dynamic quenching capability is PYB>R6G>ERB>RB>R110. The dynamic fluorescence quenching is due to the photoinduced electron transfer (PET). The PET rate constant is very large and in the order of 10(13) M(-1) s(-1), much greater than kf and kic (in the order of 10(8) M(-1) s(-1)), which means that the PET efficiency is almost 100%. Therefore the non-covalent Pc-rhodamine is a very good pair of donor/acceptor for potential efficient solar energy conversion.

Spectroscopic insights on imidazole substituted phthalocyanine photosensitizers: fluorescence properties, triplet state and singlet oxygen generation.

Imidazole substituted metal phthalocyanine (Pc) complexes were synthesized. UV-vis absorption, steady state and time-resolved fluorescence, as well as laser flash photolysis were used to measure the photophysical and photosensitizing properties. All the imidazole-phthalocyanine conjugates show high ΦT (quantum yield of excited triplet formation), high ΦΔ (singlet oxygen formation yield, >0.50) and good fluorescence properties (quantum yield Φf>0.20 and lifetime τf>3.0 ns). Compared to the unsubstituted Pc, both α- and ß-imidazole substitutions result in the remarkable decrease in Φf and τf, but the α-substitution is stronger. The imidazole substitution, on the other hand, causes the increase of ΦT, τT, and ΦΔ values. Magnesium phthalocyanine (MgPc) is more susceptible to the substitution than zinc phthalocyanine (ZnPc). The mechanism responsible for the result is suggested based on the involvement of intramolecular photoinduced electron transfer. The high ΦΔ and appropriate fluorescence properties make the Pcs good candidate for PDT photosensitizers.

Fluorescence properties of twenty fluorescein derivatives: lifetime, quantum yield, absorption and emission spectra.

The fluorescence lifetime (τf), emission quantum yield (Φf), absorption and emission spectral data of 20 fluorescein derivatives were measured under the same conditions by using time-correlated single photon counting, steady state fluorescence and absorption methods to get comparable data. Based on the results, the factors and mechanism that control the fluorescence properties of the fluorescein dyes are discussed. Both Φf and τf are remarkably dependent on the substitution on either xanthene or phenyl rings, but their ratio (Φf/τf), i.e. rate constant of radiation process, is a constant value (0.20 × 10(9) s(-1)). The rate constant of nonradiation process, on the other hand, is varied with both the structure and the solvent used.

Photosensitizer that selectively generates singlet oxygen in nonpolar environments: photophysical mechanism and efficiency for a covalent BODIPY dimer.

Photosensitizers that selectively generate singlet oxygen in non polar/polar microenvironments are highly desirable for photodynamic therapy of tumor but not yet reported. BODIPY (boron-dipyrromethene complexes) covalent dimer 1 is such a photosensitizer that forms singlet oxygen only in hexane, cyclohexane, and toluene significantly but not in polar solvents. Its corresponding monomer is not photoactive in any solvents for forming singlet oxygen. To reveal the mechanism, we measured the excited triplet-, singlet-, and ground-state properties as well as singlet oxygen generation capability with laser flash photolysis, fluorescence spectroscopy, time-correlated single photon counting, and absorption spectroscopy in various solvents. The striking difference is due to the fact that the excited dimer (excimer) undergoes very fast intramolecular charge transfer (ICT) that makes intersystem crossing noncompetitive in polar solvents, while ICT is negligible in nonpolar solvents.

Noncovalent binding of xanthene and phthalocyanine dyes with graphene sheets: the effect of the molecular structure revealed by a photophysical study.

The fluorescence and absorption properties of several xanthene and phthalocyanine dyes were measured in the presence and absence of chemically derived graphene (CDG) sheets. The interaction of pyronine Y (PYY) with graphene sheets was compared with that of rhodamine 6G (R6G) to reveal the effect of the molecular structure. Although the presence of the perpendicular benzene moiety in a R6G or phthalocyanine molecule does cause the difficulty for forming dye-CDG complex and make CDG less efficient in quenching the fluorescence intensity and shortening the fluorescence lifetime, it does not affect the band position of charge transfer absorption, suggesting that no molecular shape change occurred in a dye molecule caused by the interaction with CDG sheets. The spectroscopic and thermodynamic data indicated that the dye-CDG binding is of charge transfer nature, while the dynamic fluorescence quenching is due to photoinduced energy and electron transfer.