Caishun Zhang

Bodipy Derivatives as Organic Triplet Photosensitizers for Aerobic Photoorganocatalytic Oxidative Coupling of Amines and Photooxidation of Dihydroxylnaphthalenes

We used iodo-Bodipy derivatives that show strong absorption of visible light and long-lived triplet excited states as organic catalysts for photoredox catalytic organic reactions. Conventionally most of the photocatalysts are based on the off-the-shelf compounds, usually showing weak absorption in the visible region and short triplet excited state lifetimes. Herein, the organic catalysts are used for two photocatalyzed reactions mediated by singlet oxygen ((1)O2), that is, the aerobic oxidative coupling of amines and the photooxidation of dihydroxylnaphthalenes, which is coupled to the subsequent addition of amines to the naphthoquinones, via C-H functionalization of 1,4-naphthoquinone, to produce N-aryl-2-amino-1,4-naphthoquinones (one-pot reaction), which are anticancer and antibiotic reagents. The photoreactions were substantially accelerated with these new iodo-Bodipy organic photocatalysts compared to that catalyzed with the conventional Ru(II)/Ir(III) complexes, which show weak absorption in the visible region and short-lived triplet excited states. Our results will inspire the design and application of new organic triplet photosensitizers that show strong absorption of visible light and long-lived triplet excited state and the application of these catalysts in photoredox catalytic organic reactions.

Molecular Engineering of Simple Phenothiazine-Based Dyes To Modulate Dye Aggregation, Charge Recombination, and Dye Regeneration in Highly Efficient Dye-Sensitized Solar Cells

A series of simple phenothiazine-based dyes, namely, TP, EP, TTP, ETP, and EEP have been developed, in which the thiophene (T), ethylenedioxythiophene (E), their dimers, and mixtures are present to modulate dye aggregation, charge recombination, and dye regeneration for highly efficient dye-sensitized solar cell (DSSC) applications. Devices sensitized by the dyes TP and TTP display high power conversion efficiencies (PCEs) of 8.07 (Jsc = 15.2 mA cm(-2), Voc =0.783 V, fill factor (FF) = 0.679) and 7.87 % (Jsc = 16.1 mA cm(-2), Voc = 0.717 V, FF = 0.681), respectively; these were measured under simulated AM 1.5 sunlight in conjunction with the I(-)/I3(-) redox couple. By replacing the T group with the E unit, EP-based DSSCs had a slightly lower PCE of 7.98 % with a higher short-circuit photocurrent (Jsc) of 16.7 mA cm(-2). The dye ETP, with a mixture of E and T, had an even lower PCE of 5.62 %. Specifically, the cell based on the dye EEP, with a dimer of E, had inferior Jsc and Voc values and corresponded to the lowest PCE of 2.24 %. The results indicate that the photovoltaic performance can be finely modulated through structural engineering of the dyes. The selection of T analogues as donors can not only modulate light absorption and energy levels, but also have an impact on dye aggregation and interfacial charge recombination of electrons at the interface of titania, electrolytes, and/or oxidized dye molecules; this was demonstrated through DFT calculations, electrochemical impedance analysis, and transient photovoltage studies.

Effects of various π-conjugated spacers in thiadiazole[3,4-c]pyridine-cored panchromatic organic dyes for dye-sensitized solar cells

A series of new metal-free panchromatic organic photosensitizers based on a strong electron-deficient thiadiazole[3,4-c]pyridine core has been prepared and applied in dye-sensitized solar cells. The incorporation of the auxiliary thiadiazole[3,4-c]pyridine unit can effectively adjust the HOMO and LUMO energy levels, to design small band-gap photosensitizers with panchromatic absorption. The impacts of various π-conjugated spacers on the absorption properties, electrochemical properties and photovoltaic performances have been investigated systematically. The sensitizer Y3 with a benzene unit adjacent to the anchoring cyanoacrylic group produces a higher photocurrent and photovoltage in cell performance, as compared to Y1 and Y2 with thiophene and n-hexylthiophene unit adjacent to the anchoring group, respectively. Further structural optimization in Y4 with a n-hexylthiophene π-conjugated spacer inserted between the donor and thiadiazole[3,4-c]pyridine core results in the best photovoltaic performance. For comparison, the sensitizer Y5 with thiophene instead of n-hexylthiophene in the molecule exhibits the most inferior performance; this demonstrates that the long alkyl chains can effectively improve the cell performance by suppressing the dye aggregation on TiO2 film, enhancing electron injection efficiency, and retarding charge recombination by shielding the surface of TiO2 from I3− ions. The overall conversion efficiency of liquid–electrolyte DSSC based on Y4 shows the highest efficiency of 6.30% with a short-circuit photocurrent density (Jsc) of 12.54 mA cm−2, an open-circuit photovoltage (Voc) of 0.749 V, and a fill factor (FF) of 0.671, under standard global AM 1.5 solar light condition. Density functional theory calculations and electrochemical impedance spectroscopy analysis of these sensitizers provide further insight into the molecular geometry and the impact of the different π-conjugated spacers on the photophysical and photovoltaic performance.

Dual phosphorescent dinuclear transition metal complexes, and their application as triplet photosensitizers for TTA upconversion and photodynamic therapy

Two novel homo Ru(II) and Ir(III) complexes (Ru-2 and Ir-2), containing a bridging boron-dipyrromethene (BODIPY) chromophore were synthesised. The BODIPY moiety was covalently attached to the coordinated bipyridine (bpy) or phenylpyridine (ppy) via two acetylene linkers to produce bimetallic-complexes, which were employed as triplet photosensitizers. Both Ru-2 and Ir-2 absorb strongly in the visible region (λabs = 570 nm, e = 113 317 dm−3 mol−1 cm−1 for Ru-2 and λabs = 567 nm, e = 105 713 dm−3 mol−1 cm−1 for Ir-2). Due to a strong intraligand feature, and a small contribution from the metal, to the triplet state, the triplet-state lifetimes are particularly long for both complexes (1316.0 μs for Ru-2, 630.7 μs for Ir-2). High upconversion quantum yields were found (19.1% for Ru-2 and 25.5% for Ir-2). The intermolecular triplet energy transfer between the metal centres were studied using nanosecond time-resolved transient absorption spectroscopy (ΦTTET = 94% Ru-2 and ΦTTET = 86% Ir-2). Knowing the desirable photophysical properties of the complexes, both were then tested for their application in photodynamic therapy (PDT).

Thiol-activated triplet-triplet annihilation upconversion: study of the different quenching effect of electron acceptor on the singlet and triplet excited states of Bodipy.

Thiol-activated triplet-triplet annihilation (TTA) upconversion was studied with two different approaches, i.e., with 2,4-dinitrobenzenenesulfonyl (DNBS)-caged diiodoBodipy triplet photosensitizers (perylene as the triplet acceptor/emitter of the upconversion) and DNBS-caged Bodipy fluorophore as the triplet acceptor/emitter (PdTPTBP as the triplet photosensitizer, TPTBP = tetraphenyltetrabenzoporphyrin). The photophysical processes were studied with steady-state UV-vis absorption spectroscopy, fluorescence spectroscopy, electrochemical characterization, nanosecond transient absorption spectroscopy, and DFT/TDDFT computations. DNBS-caged triplet photosensitizer shows a shorter triplet state lifetime (24.7 μs) than the uncaged triplet photosensitizer (86.0 μs), and the quenching effect is due to photoinduced electron transfer (PET). TTA upconversion was enhanced upon cleavage of the DNBS moiety by thiols. On the other hand, the DNBS-caged Bodipy shows no fluorescence, but the uncaged fluorophore shows strong fluorescence; thus, TTA upconversion is able to be enhanced with the uncaged fluorophore as the triplet energy acceptor/emitter. The results indicate that the DNBS moiety exerts a significant quenching effect on the singlet excited state of Bodipy, but the quenching on the triplet excited state is much weaker. Calculation of the Gibbs free energy changes of the photoinduced electron transfer indicates that the singlet state gives a larger driving force for the PET process than the triplet state.

Co-sensitization of 3D bulky phenothiazine-cored photosensitizers with planar squaraine dyes for efficient dye-sensitized solar cells

A series of new phenothiazine-cored 3D bulky organic sensitizers TP1–TP4 have been prepared and employed in dye-sensitized solar cells (DSSCs). The 3D bulky configuration of these molecules can effectively retard the charge recombination at the TiO2/electrolyte interface. Amongst the four dyes, the co-adsorbent-free DSSC based on the dye TP3 exhibited the best conversion efficiency (η) of 8.00%. Subsequently, the photosensitizer TP3 with strong UV-visible absorption and excellent performance in adsorbent-free DSSCs was co-sensitized with a near-infrared (NIR) absorbing squaraine dye YR6 to realize a UV-visible-NIR light-harvesting capability, which can effectively suppress the dye aggregation of YR6 with a planar structure and retard the charge recombination in the as prepared DSSC. Upon optimization, the co-sensitized DSSCs exhibited remarkable overall efficiency enhancements of 33% and 356% as compared with the devices based on TP3 and YR6 alone, respectively, and a high efficiency up to 9.84% was achieved at the TP3/YR6 molar ratio of 25 : 1.

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.

Preparation of ketocoumarins as heavy atom-free triplet photosensitizers for triplet–triplet annihilation upconversion

A series of ketocoumarin compounds were prepared as heavy atom-free triplet photosensitizers. The photophysical properties of the compounds were studied with steady state and time-resolved spectroscopy. The compounds show absorption in the visible spectral region (molar absorption coefficients are up to ε = 136,000 M(-1) cm(-1) at 448 nm) and long-lived triplet excited states (τT = 199.7 μs) upon visible light photoexcitation. The compounds were used as triplet photosensitizers for singlet oxygen ((1)O2)-mediated photooxidation of 1,5-dihydroxylnaphthalene (DHN) to produce juglone. (1)O2 quantum yields of these compounds were determined in the range of 0.28-0.48. The ketocoumarins were also used as triplet photosensitizers for triplet-triplet annihilation (TTA) upconversion, and upconversion quantum yields up to 11.3% were observed. The results are useful for preparation of heavy atom-free triplet photosensitizers and for their application in photocatalysis and TTA upconversion.

Triplet excited state of diiodoBOPHY derivatives: preparation, study of photophysical properties and application in triplet–triplet annihilation upconversion

A pyrrole-BF2-based chromophore bis(difluoroboron)1,2-bis((pyrrol-2-yl)methylene) hydrazine (BOPHY) was used for the first time for the preparation of new triplet photosensitizers. The UV-vis absorption spectra show that the absorption bands of the photosensitizers cover the 400–700 nm range. Nanosecond time-resolved transient absorption spectra show that the triplet excited state of the compounds was populated upon photoexcitation and the compound diiodoBOPHY (C-2) is with a long triplet excited state lifetime of 177.2 μs. The triplet state energy level of C-2 was demonstrated to be higher than the traditional BODIPY chromophores. DFT/TDDFT calculations were carried out for rationalization of the photophysical properties of the compounds. C-2 was used in triplet–triplet annihilation upconversion, using 9,10-diphenylanthracene (DPA) as the triplet energy acceptor. The TTA upconversion quantum yield is 2.8%. With the styryl substituents on the BOPHY core, the sensitizers demonstrated lower triplet state energy levels and shorter triplet state lifetimes. Using these triplet photosensitizers, photooxidation of 1,3-diphenylisobenzofuran (DPBF) by singlet oxygen (1O2) photosensitization was also carried out.