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.

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.

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.

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.

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.

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.

Solvent Effects on Spin-Orbit Charge-Transfer Intersystem Crossing in Aryl-Substituted Boron-dipyrromethene Donor-Acceptor Dyads.

In heavy-atom-free organic molecules, the rate of triplet generation through charge recombination, as dictated by the El-Sayed rule, can be enhanced by 101-102 times compared with the rate of spontaneous spin flipping between π-π* orbitals. This mechanism is known as the spin-orbit charge-transfer intersystem crossing (SOCT-ISC). Within the framework of the SOCT-ISC mechanism, facilitating the generation of charge-separated (CS) states and suppressing the spin-allowed direct charge recombination to the ground state are pivotal for maximizing the efficiency of generating localized triplet states. Herein, a series of orthogonal aryl-substituted boron-dipyrromethene dyads were studied by time-resolved spectroscopy to unravel the multichannel competitive relationships in the SOCT-ISC mechanism. The energy level of the electron donor and the stabilization of the solvent effect to the charge-transfer state are reflected in the Gibbs free energy changes of the electron transfer and recombination reactions, leading to significantly different triplet quantum yields. Additionally, solvation-induced electronic coupling changes in excited states lead to the fact that the spin-allowed charge recombination rate cannot be well simply predicted by the Marcus inverted region but has to consider the specific excited-state dynamics in optimizing the proportion of triplet generation channels based on charge recombination.

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.

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.

Imidazole functionalized magnesium phthalocyanine photosensitizer: modified photophysics, singlet oxygen generation and photooxidation mechanism.

Magnesium phthalocyanine (MgPc) was covalently attached by four imidazole units to form a novel photosensitizer (PS). The photophysical processes within the dyad PS were explored by steady state and time-resolved fluorescence as well as laser flash photolysis. Although the imidazole units caused a 50% decrease in fluorescence quantum yield and a remarkable shortening of fluorescence lifetime of the MgPc moiety, the triplet yield (Φ(T)) is higher and the triplet lifetime becomes longer. The transient absorption bands for MgPc(•-) were observed, indicating the occurrence of intramolecular photoinduced electron transfer (PET) from imidazole subunits to the lowest excited singlet state (S(1)) of the MgPc moiety. The kinetic and thermodynamic analysis also supports the involvement of PET in S(1) deactivation. The quantum efficiency of photosensitized oxidation of diphenylisobenzofuran (DPBF) by the PS is 0.52. This value is much higher than Φ(T) (0.26), since DPBF is photo-oxidized not only by singlet oxygen (type II reaction, 54%) but also by superoxide anion radical (type I reaction, 46%). The result suggests that the mechanism of photosensitized oxidation could be changed upon the conjugation of a PS to biological molecules, so that the importance of type I reaction is enhanced.

Dual fluorescence of graphene oxide: a time-resolved study.

The fluorescence properties of graphene oxide (GO) was studied by recording the fluorescence lifetime, fluorescence emission, and excitation spectra, as well as UV-visible and near-IR absorption spectra. For the first time, we showed that a blue band (ca. 440 nm) and a long wavelength (LW) band (ca. 700 nm) are coexistent, which can be recorded simultaneously by controlling concentration, excitation wavelength, and pH values. Two bands are closely related by the protonation or deprotonation of GO. The blue band is favored by low GO concentration, short excitation wavelength, and high pH value, while the LW band is favored by low pH and long excitation wavelength. To reveal the nature of the dual emission of GO, the fluorescence lifetimes under various conditions were also measured. The blue band contains three emitting components; one of them has a lifetime as long as 10 ns, and its emitting intensity is fairly sensitive to pH, showing the potential for applications in sensing H(+) and fluorescence lifetime imaging. Combining the results under various conditions, we conclude that the electronic transition for this component is very likely due to n-π* transition. The LW band contains two main emitting components (0.2 and 2.1 ns) that also appear in the blue band as minor contributors; the related emission is assigned to π-π* transition. In summary, GO emission is of broadband (300-1250 nm), long-lived, pH sensitive, and excitation wavelength dependent. This makes it easily tailored for versatile applications.

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.

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.

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.

Fluorescence properties of phenol-modified zinc phthalocyanine that tuned by photoinduced intra-molecular electron transfer and pH values.

Tetra[α-(4-hydroxyphenoxy)] zinc phthalocyanine, ZnPc(α-OPhOH)(4), was synthesized and its photophysics was found to be sharply pH dependent. Dual fluorescence emission around 700 nm was observed when it is dissolved in basic solution. The fluorescence of the phthalocyanine can be sharply switched off at pH 9.1 due to the intramolecular photoinduced electron transfer (PET) in ZnPc(α-OPhONa)(4), formed by the deprotonation of ZnPc(α-OPhOH)(4). The photophysics of both ZnPc(α-OPhOH)(4) and ZnPc(α-OPhONa)(4) were studied in detail by UV-vis absorption, steady state and time-resolved fluorescence and transient absorption (TA) to reveal the fluorescence quenching mechanism. Intra-molecular PET in ZnPc(α-OPhONa)(4) from the donor, PhONa subunits, to the acceptor, ZnPc moiety, was characterized by the much smaller fluorescence quantum yield (0.003) and lifetime (<0.20 ns). PET was further evidenced by the occurrence of charge separation state (CSS) in TA spectra, i.e. the bands due to anion radical of ZnPc and phenol radical. The lifetime of the charge separation state is ca. 3 ns, the efficiency of PET is ca. 99% and the rate constant of PET is 2.3 × 10(10) s(-1).

Morpholine-phthalocyanine (donor-acceptor) construct: photoinduced intramolecular electron transfer and triplet formation from its charge separation state.

Silicon phthalocyanine (SiPc) with two axially attached morpholine (MP) units was prepared, and its photophysics was studied by laser flash photolysis, steady state and time-resolved fluorescence methods. Both the fluorescence efficiency and lifetime of SiPc moiety were remarkably quenched, because of the efficient intramolecular photoinduced electron transfer (PET) from morpholine donors to SiPc moiety. The generated charge separation state (CSS), SiPc(•-)-MP(•+), which was observed by transient absorption spectra, showed a lifetime of 4.8 ns. The triplet quantum yield of SiPc unit in the supra-molecule is unexpectedly high, and the predominant spectral signal in microsecond-scale is triplet-triplet (T(1)-T(n)) absorption. This high triplet yield is due to the charge recombination of CSS that generates T(1) in 32% efficiency: SiPc(•-)-MP(•+) → (3)SiPc-MP. The T(1) formation process occurred efficiently because the CSS SiPc(•-)-MP(•+) has a higher energy (1.65 eV) than that of the triplet state (3)SiPc-MP (1.0 eV). Emission from the CSS was also observed: SiPc(•-)-MP(•+) → SiPc-MP + hν'.

The effect of phenyl substitution on the fluorescence characteristics of fluorescein derivatives via intramolecular photoinduced electron transfer.

UV-vis absorption, steady state fluorescence emission, time-correlated single photon counting and laser flash photolysis methods were employed to examine the excited state properties of fluorescein derivatives to understand the mechanism that controls their fluorescence efficiency. The fluorescein derivatives contain amino, t-butyl, carboxyl or nitro on their phenyl moieties, respectively. These substituents are not directly connected to the fluorophore but still showed a very remarkable effect on the fluorescence properties. Compared to fluorescein, the introduction of nitro, a strong electron withdrawing group, or amino, a strong electron donating group, caused a substantial quenching of both the fluorescence quantum yield and lifetime. The presence of a t-butyl or carboxyl, on the other hand, caused a smaller decrease. The mechanism for the substituent effect is due to the involvement of an additional de-excitation process, i.e. intramolecular photoinduced electron transfer (PET). The thermodynamics and kinetics of PET were analyzed. Depending on the nature of the substituent, the xanthenic ring acts as an electron acceptor (or donor), while the phenyl moiety is the corresponding electron donor (or acceptor) in PET. The rate constant of PET for the amino case is larger than 4.79 x 10(9) s(-1), while for nitro substitution it is 0.67 x 10(9) s(-1). Both values are much larger than the radiation rate constant of 0.20 x 10(9) s(-1), meaning that PET plays important roles in the deactivation of S(1) for the two dyes. The charge transfer state generated by PET was observed by laser flash photolysis.

Charge transfer photophysics of tetra(alpha-amino) zinc phthalocyanine.

The absorption, fluorescence, and transient absorption spectra of Tetra(alpha-amino) zinc phthalocyanine, ZnPc(alpha-NH2)4, have been measured in polar solvents and compared with that of ZnPc(alpha-R)4 (R = H, NO2, OCH(CH3)2). While the latter three showed the typical photophysics of phthalocyanines, ZnPc(alpha-NH2)4 exhibits distinct spectral properties, a very low fluorescence quantum yield and a relatively long fluorescence lifetime. These observations are explained by the substantial charge transfer characters in the absorption and fluorescence spectra of ZnPc(alpha-NH2)4. NMR indicates that intramolecular H-bonding makes atoms in NH2 actually coplanar with other elements of ZnPc(alpha-NH2)4. The local excited state is non emissive and the weak emission is assigned to its charge transfer state. The transient absorption bands from laser flash photolysis located at 630 nm, 645 nm is assigned to the mono-charge transfer state, while that at 545 nm is assigned to the di-charge transfer state.

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.

3-(2-Nitro-phen-oxy)phthalonitrile.

In the title compound, C(14)H(7)N(3)O(3), the dihedral angle between the two arene units is 62.57 (12)°.