Mustafa Hayvali

BODIPY triads triplet photosensitizers enhanced with intramolecular resonance energy transfer (RET): broadband visible light absorption and application in photooxidation

Resonance energy transfer (RET) was used to enhance the light absorption in triad triplet photosensitizers to access strong and broadband absorption in visible region (from 450–750 nm). This strategy was demonstrated by preparation of (BODIPY)2-diiodo-aza-BODIPY triad (B-2) and (carbazole-styryl BODIPY)2-diiodo-aza-BODIPY triad (B-3), in which the energy donor (BODIPY or styryl-BODIPY) and the energy acceptor (aza-BODIPY, also as the spin converter) parts were connected by click chemistry. Both the energy donors and the energy acceptors show strong absorption in the visible spectral region, but at different wavelengths, therefore the triads show broadband absorption in visible spectra region, e.g. the two major absorption bands of B-3 are located at 593 nm and 683 nm, with e up to 220000 M−1 cm−1 and 81000 M−1 cm−1, respectively. For comparison, a reference compound with only diiodo-aza-BODIPY as the light-harvesting unit was prepared (B-1), which shows only one major absorption band in visible spectral region. Fluorescence studies indicated intramolecular energy transfer for these BODIPY hybrids, a conclusion which is supported by the femtosecond time-resolved transient absorption spectroscopy. Nanosecond transient absorption spectra show that triplet excited states of the dyad and the triad are localized on the iodo-aza-BODIPY part. The compounds were used as triplet photosensitizers for singlet oxygen (1O2) mediated photooxidation of 1,5-dihydroxylnaphthalene and the photosensitizing ability of the new triplet photosensitizers are more efficient than the mono-chromophore based triplet photosensitizers. The molecular design rationale of these RET-enhanced multi-chromophore triplet photosensitizer is useful for development of efficient triplet photosensitizers and for their applications in photocatalysis, photodynamic therapy, photovoltaics and upconversion.

DiiodoBodipy-Perylenebisimide Dyad/Triad: Preparation and Study of the Intramolecular and Intermolecular Electron/Energy Transfer

2,6-diiodoBodipy-perylenebisimide (PBI) dyad and triad were prepared, with the iodoBodipy moiety as the singlet/triplet energy donor and the PBI moiety as the singlet/triplet energy acceptor. IodoBodipy undergoes intersystem crossing (ISC), but PBI is devoid of ISC, and a competition of intramolecular resonance energy transfer (RET) with ISC of the diiodoBodipy moiety is established. The photophysical properties of the compounds were studied with steady-state and femtosecond/nanosecond transient absorption and emission spectroscopy. RET and photoinduced electron transfer (PET) were confirmed. The production of the triplet state is high for the iodinated dyad and the triad (singlet oxygen quantum yield ΦΔ = 80%). The Gibbs free energy changes of the electron transfer (ΔGCS) and the energy level of the charge transfer state (CTS) were analyzed. With nanosecond transient absorption spectroscopy, we confirmed that the triplet state is localized on the PBI moiety in the iodinated dyad and the triad. An exceptionally long lived triplet excited state was observed (τT = 150 μs) for PBI. With the uniodinated reference dyad and triad, we demonstrated that the triplet state localized on the PBI moiety in the iodinated dyad and triad is not produced by charge recombination. These information are useful for the design and study of the fundamental photochemistry of multichromophore organic triplet photosensitizers.

Resonance energy transfer-enhanced rhodamine–styryl Bodipy dyad triplet photosensitizers

Organic triplet photosensitizers (R-1 and R-2) enhanced with the resonance energy transfer (RET) effect were prepared. Rhodamine was used as an intramolecular energy donor, and iodo-styryl-Bodipy was used as intramolecular energy acceptor/spin converter. Both the energy donor and energy acceptor in R-1 and R-2 give strong absorption in the visible region but at different wavelengths (e.g. for R-1, e = 120 000 M−1 cm−1 at 557 nm for the energy donor and e = 73 300 M−1 cm−1 at 639 nm for the energy acceptor). As a result, the photosensitizers show broadband absorption in the visible spectral region. In comparison, conventional triplet photosensitizers contain only one visible light-harvesting chromophore; thus, there is usually only one major absorption band in the visible spectral region. Using steady state and time-resolved spectroscopy, we demonstrated that photoexcitation in the energy donor was followed by intramolecular singlet energy transfer, and then via intersystem crossing (ISC) of the energy acceptor (spin converter), triplet excited states localized on the iodo-styryl-Bodipy were produced, which was confirmed by nanosecond time-resolved transient difference absorption spectroscopy. The organic dyad triplet photosensitizers were used for photoredox catalytic organic reactions to prepare pyrrolo[2,1-a]isoquinoline, and we found that the photocatalytic capability was improved with the RET effect. The dyads were also used as fluorescent stains for LLC cancer cells. Photodynamic effect was observed with the same cells, which were killed on photoirradiation with 635 nm red-emitting LED after incubation with the triplet photosensitizers. Therefore, these photosensitizers can be potentially developed as dual functional theranostic reagents. Using the molecular structural protocol reported herein, organic triplet photosensitizers with strong broadband absorption in the visible spectral region and predictable ISC can be easily designed. These results are useful for the study of organic triplet photosensitizers in the area of organic photochemistry/photophysics, photoredox catalytic organic reactions and photodynamic therapy (PDT).

Near-IR Broadband-Absorbing trans-Bisphosphine Pt(II) Bisacetylide Complexes: Preparation and Study of the Photophysics

Broadband near-IR absorbing trans-bis(trialkylphosphine) Pt(II) bisacetylide binuclear complex (Pt-1) was prepared with boron-dipyrromethene (Bodipy) and styrylBodipy acetylide ligands. Pt-1 shows strong absorption bands at 731 and 503 nm. Singlet energy transfer (EnT) and efficient intersystem crossing of the central coordinated Bodipy ligand were proposed to be responsible for the efficient funneling of the excitation energy to the triplet-state manifold. Reference complexes containing only a single Bodipy ligand were prepared for comparison (with styrylBodipy ligand Pt-0 or Bodipy ligand Pt-2). The molecular structures were confirmed by single-crystal X-ray diffraction. The photophysical properties were studied with steady-state and time-resolved transient absorption spectroscopies, electrochemical characterization, and density functional theory/time-dependent density functional theory calculations. Dual fluorescence was observed for Pt-1. Singlet EnT in Pt-1 was proposed based on the fluorescence quenching/excitation spectra, and femtosecond transient absorption spectra (energy transfer rate constant kEnT = 2.2 × 10(10) s(-1)). With nanosecond transient absorption spectra, intramolecular triplet-state energy transfer in Pt-1 was proved. Gibbs free energy changes of charge separation indicate that the photoinduced intramolecular electron transfer in Pt-1 is thermodynamically prohibited. Intermolecular triplet transfer between Pt-2 and L-1 was studied with nanosecond transient absorption spectra; the EnT rate and energy transfer efficiency were determined as 3.6 × 10(4) s(-1) and 94.5%, respectively. The singlet oxygen ((1)O2) photosensitizing of Pt-1 was improved as compared to the complexes containing only a single visible-light-absorbing chromophore.

trans-Bis(alkylphosphine) platinum(II)-alkynyl complexes showing broadband visible light absorption and long-lived triplet excited states

Heteroleptic trans-bis(alkylphosphine) platinum(II) bisacetylide complexes Pt-1 and Pt-2 were prepared to achieve broadband absorption of visible light. Two different ethynylBodipy ligands, 2-ethynylBodipy and 2,6-diethynylBodipy or 8-(4′-ethynylphenyl)Bodipy, were used in each complex. Each Bodipy ligand gives strong absorption in the visible spectral region, but at different wavelengths, thus broadband absorption in the visible spectral region was achieved for the Pt(II) complexes (e is up to 1.85 × 105 M−1 cm−1 in the region of 450–700 nm). Singlet energy transfer from the peripheral coordinated Bodipy to the central coordinated Bodipy (with 2,6-diethynyl substitution) was confirmed by steady state absorption/luminescence spectroscopy, fluorescence excitation spectroscopy and nanosecond/femtosecond ultrafast time-resolved transient absorption spectroscopy. Long-lived triplet excited states were observed for both complexes (τT = 63.13 μs for Pt-1 and τT = 94.18 μs for Pt-2). Nanosecond time-resolved transient absorption spectroscopy indicated that the triplet excited state of Pt-1 is distributed on both Bodipy units. For Pt-2, however, the T1 state is confined to the central coordinated Bodipy ligand. These complexes show high singlet oxygen (1O2) quantum yields (ΦΔ = 76.0%). With nanosecond pulsed laser excitation, delayed fluorescence was observed for the complexes (τDF = 43.8 μs for Pt-1 and τDF = 111.0 μs for Pt-2), which is rarely reported for transition metal complexes. The complexes were used as efficient multi-wavelength excitable triplet photosensitizers for triplet–triplet annihilation upconversion.

Radical-Enhanced Intersystem Crossing in New Bodipy Derivatives and Application for Efficient Triplet–Triplet Annihilation Upconversion

A long-lived triplet excited state of the well-known fluorophore boron dipyrromethene (Bodipy) was observed for the first time via efficient radical-enhanced intersystem crossing (EISC). The triplet state has been obtained in two dyads in which the Bodipy unit is linked to a nitroxide radical, 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO), with two different length spacers. The photophysical properties were studied with steady-state and time-resolved transient optical spectroscopies and electron spin resonance (cw-ESR and TR-ESR). The fluorescence of Bodipy units is significantly quenched in the dyads, and the spin-polarized TEMPO signals were observed with TR-ESR, generated by a radical triplet pair mechanism. Efficient EISC (ΦT = 80%) was observed for the dyad with a shorter linker, and the triplet state lifetime of the Bodipy chromophore is exceptionally long (62 μs). The EISC takes 250 ps. Poor ISC was observed for the dyad with a longer linker. The efficient ISC and long-lived triplet excited state in this flexible system are in stark contrast to the previously studied rigid EISC systems. The EISC effect was employed for the first time to perform triplet-triplet annihilation (TTA) upconversion (quantum yield ΦUC = 6.7%).

The Synthesis of Some New Mono and bis(Crown Ether)s and Their Sodium Complexes. Tautomerism in o‐Hydroxybenzo‐15‐crown‐5 Schiff Bases as Studied by UV‐VIS Spectrophotometry

Abstract New mono‐ and bis(crown ether) ligands have been synthesized by the condensation of the appropriate formylbenzo‐15‐crown‐5 [(2) and (4)] with ethanolamine and 1,2‐bis[(4‐aminophenoxy)methyl]benzene (6), respectively. The diamine (6), was prepared by the reaction of 1,2‐bis(bromomethyl)benzene and p‐nitrophenol in the presence of NaOH forming the dinitro compound (5) which is subsequently reduced to compound (6) using hydrazine hydrate and Pd/C. Homonuclear sodium complexes of the mono‐ and bis(crown ether) ligands have been prepared with sodium perchlorate. The UV‐VIS spectra of o‐hydroxycrown ether Schiff bases (7), (8), (7a), and (8a) have been studied in various solvents. The results indicate that bands observed at higher values than 400 nm to be the result of the keto‐amine form of the ligands and complexes. All of the crown ether ligands and their complexes have been characterized by elemental analyses, IR, UV, and NMR spectra.

Phosphorus-nitrogen compounds Part 17: The synthesis, spectral and electrochemical investigations of porphyrino-phosphazenes

The reactions of unsymmetrical porphyrins (1 and 2) with Ni(OAc)2·4H2O in boiling DMF produce porphyrin complexes (3 and 4). From the reactions of free porphyrin ligands 1 and 2 with hexachlorocyclotriphosphazatriene, N3P3Cl6, the new free porphyrino-phosphazene derivatives (5 and 6) are obtained. On the other hand, the reactions of N3P3Cl6 with porphyrin complexes (3 and 4) afford the new porphyrino-phosphazene complexes (7 and 8). In the literature there are a few examples of the porphyrino-phosphazene architectures. The structural investigations of all the compounds have been made by elemental analyses, MS, FTIR, 1H NMR, 31P NMR and UV-visible techniques. The cyclic voltammograms (CVs) are examined in acetonitrile (MeCN) containing 0.1 M tetrabutylammonium-tetrafluoroborate (TBATFB) to investigate the surface attachment properties at the glassy carbon electrode (GCE) and the influence of the presence of metal cations in the porphyrin ring.

Titrations in non-aqueous media: potentiometric investigation of the basicity of meso-porphyrins in nitrobenzene solvent

The basicity of the meso-porphyrins, namely meso-tetraphenylporphyrin (I), meso-tetrakis(4-methylphenyl)-porphyrin (II), meso-tetrakis(4-methoxyphenyl)porphyrin (III), meso-tetrakis(4-aminophenyl)porphyrin (IV), meso-tetrakis(4-chlorophenyl)porpyrin (V) and meso-tetrakis(4-nitrophenyl)porphyrin (IV), was investigated potentiometrically in nitrobenzene solvent. This investigation showed that these compounds are basic, rather than acidic. Whereas they cannot be titrated even with tetrabutylammonium hydroxide, they can readily be titrated with perchloric acid to give well shaped and stoichiometric end-points. In addition, they undergo two proton reactions per porphyrin molecule. However, IV shows a second end-point corresponding to a six-proton reaction per porphyrin molecule. Half-neutralization potentials (measures of their basicity) of these compounds are I = 321, II = 270, III = 255, IV = 155, V = 395 and VI = 480 mV. These values clearly indicate that, if para-hydrogen with respect to the bond found to be between the phenyl group and porphyrin core of meso-tetraphenylprophyrin (I) is replaced with basifying methyl, methoxy and amino groups, the basicity of I increases (270, 255 and 155 mV, respectively); if the same hydrogen atom is replaced with acidifying chlorine and nitro groups, the basicity of I decreases (395 and 480 mV, respectively). These observations show that the nitrogen atoms at the centre of the prophyrin molecules are strongly influenced by changes even at the perifery of the molecules, which is a good indication that prophyrin molecules are flat.

Titrations in non-aqueous media: potentiometric investigation of symmetrical and unsymmetrical tetra-aryl porphyrins with 4-nitrophenyl and 4-aminophenyl substituents in nitrobenzene solvent

The basicity of the symmetrical and unsymmetrical tetraphenylporphyrins, namely 5,10,15,20-tetraphenylporphyrin (I) (references), 5-(4-nitrophenyl)-10,15,20-triphenylporphyrin (II), a mixture of 5,10-bis(4-nitrophenyl)-15,20-diphenylporphyrin and 5,15-bis(4-nitrophenyl)-10,20-diphenylporphyrin (III), 5,10,15-tris(4-nitrophenyl)-20-phenylporphyrin (IV), 5,10,15,20-tetrakis(4-nitrophenyl)porphyrin (V), 5-(4-aminophenyl)-10,15,20-triphenylporphyrin (VI), a mixture of 5,10-bis(4-aminophenyl)-15,20-diphenylporphyrin and 5,15-bis(4-aminophenyl)-10,20-diphenylporphyrin (VII), 5,10,15-tris(4-aminophenyl)-20-phenylporphyrin (VIII) and 5,10,15,20-tetrakis(4-aminophenyl)porphyrin (IX), was investigated potentiometrically in nitrobenzene solvent. This investigation showed that these compounds are basic rather than acidic. Although they can not be titrated even with tetrabuthylammonium hydroxide, they can easily be titrated with perchloric acid to give well shaped and stoichiometric end-points. In addition they all undergo two proton reactions per porphyrin molecule. However, compounds VI, VII, VIII and IX each shows a second end-point to give three, four, five and six proton reactions, respectively, per porphyrin molecule. Half neutralization potentials (measures of their basicity) of these compounds are: I=368, II=409, III=432, IV=461, V=520, VI=340, VII=302, VIII=238 and IX=225 mV versus Ag/AgCl in methanol. These potentials clearly indicate that, if para-hydrogen with respect to the porphyrin core of tetraphenylporphyrin (I) is replaced with an acidifying nitro group (II, III, IV and V) the basicity of I decreases. This decrease is approximately proportional to the number of nitro groups. Each nitro group decreases the half neutralization potential by about 35 mV. On the other hand, if para-hydrogen indicated above is replaced with a basifying amino group (VI, VII, VIII and IX) the basicity increases. This increase is also approximately proportional to the number of amino groups. Each amino group increases the half neutralization potential by about 36.7 mV. The values 35 and 36.7 mV indicate that in nitrobenzene solvent the electron releasing power of an amino group to the porphyrin system is a little stronger than the electron withdrawing power of a nitro group from the porphyrin system. All these observations reveal that the nitrogen atoms at the core of the porphyrin molecules are strongly influenced by changes at the periphery of the molecules, which is a very good indication that the substituted phenyl groups and the cores of the porphyrins are nearly in the same plane.