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Perylenebisimide (PBI)–phenothiazine (PTZ) and PBI–diphenylamine (DPA) dyads were prepared, with the PTZ or DPA as the electron donor and the 6-subsituted PBI unit as the electron acceptor. The dyads were designed in such a way that electronic coupling (magnitude is the matrix elements, VDA and VDA*) between the electron donor and acceptor is controlled by conformation restriction. The effects of the electronic coupling on UV–Vis absorption and emission were studied. More significant charge-transfer (CT) absorption and CT fluorescence emission bands were observed for dyads with smaller dihedral angles between the electron donor and acceptor, thus stronger electronic coupling (VDA = 3290 cm–1 and VDA* = 4317 cm–1) was observed than those with larger dihedral angle, showing weaker coupling effect (VDA = 1210 cm–1 and VDA* = 2770 cm–1). Triplet state production was observed only for PBI–DPA but not for PBI–PTZ dyads. With an intermolecular triplet photosensitizing method, the triplet state of PBI–PTZ dyads w...

We report a new triplet–triplet annihilation photon up-conversion (TTA-UC) system using an epitaxial Zn-perylene surface-supported metal–organic framework (SURMOF) grown on metal oxide surface as “emitter”, and a platinum octaethylporphyrin (PtOEP) as “sensitizer” in [Co(bpy)3]2+/3+ acetonitrile solution. It has been demonstrated that the photocurrent can be significantly enhanced relative to epitaxial Zn-perylene SURMOF due to the TTA-UC mechanism. This initial result holds promising applications toward SURMOF-based solar energy conversion devices.

A simple and ultrasensitive surface-enhanced Raman scattering (SERS) assay for heparin detection has been demonstrated based on an in situ hot spot assembly method using N,N'-bis[3,3'-(trimethylammonium)propylamine]-3,4,9,10-perylenediimide (bis-quaternized perylene dye, abbreviated as BQPER). In this assay, a BQPER probe with two positive charges is employed as both Raman reporter and electrostatic bridge to cause the aggregation of citrate-capped Ag nanoparticles (Ag NPs), resulting in a strong SERS signal due to the formation of numerous hot spots in the gaps of the aggregates. The SERS intensity of BQPER decreases in the presence of heparin, because BQPER binds primarily to heparin due to its high anionic charge to mass ratio compared to that of Ag NPs. Accordingly, a novel method for the detection of heparin has been developed based on SERS of the unbound BQPER probe. Two obvious advantages of the proposed heparin assay over other reported methods are: its much wider linear concentration range (10-5-10-10 g mL-1) and lower limit of detection (0.08 ng mL-1). Besides, our proposed heparin assay has been successfully applied to the detection of heparin in serum samples, indicating its great potential in the diagnosis of heparin-related diseases. This work provides a new Raman probe for the SERS analysis of heparin and opens a different perspective for SERS-based transducer design in charged analytes.

Triplet photosensitizers showing strong absorption in the red/deep red spectral region, high intersystem crossing, a long-lived triplet state and a high triplet state energy level are crucial for triplet–triplet annihilation upconversion. Herein we selected two tetrasulfonyl-substituted phthalocyanine (Pc) Pt(II) and Pd(II) complexes (Pd-Pc and Pt-Pc) likely to meet the above criteria. The complexes showed prolonged triplet state lifetimes (15.9 μs and 3.03 μs) and high triplet state energy levels (1.5 eV) as compared to a Pc complex bearing electron donating groups (triplet lifetimes

The spin-orbit charge transfer intersystem crossing (SOCT-ISC) of perylenemonoimide-phenothiazine compact dyads was shown to depend on the molecular geometry and the vector dipole orientations of the two chromophores, and the dyads show high triplet state quantum yields (57%).

Novel mono- and di-nuclear Ru(ii) and Ir(iii) complexes, bearing a modified carbazole moiety are synthesised. In comparison to their mononuclear analogues, the homonuclear diatomic complexes (RuCRu and IrCIr), in which the carbazole containing-ligand functions as a bridge, display increased absorbance in the visible region, and give rise to higher singlet oxygen quantum yields.

Three new isomeric naphthalimide conjugated porphyrins, ZnT(p-NI)PP, ZnT(m-NI)PP and ZnT(o-NI)PP are developed for photocatalytic H2 production. The para-substituted isomer, ZnT(p-NI)PP delivered a much higher H2 production rate (ηH2) of 973 μmol g-1 h-1 compared to that of meta- and ortho-substituted isomers, ZnT(m-NI)PP (597 μmol g-1 h-1) and ZnT(o-NI)PP (54 μmol g-1 h-1), respectively. The ZnTPP produced ηH2 of 7 μmol g-1 h-1, which is 200 times lower than the ZnT(p-NI)PP. The highest ηH2 of ZnT(p-NI)PP among the isomers could be attributed to the most efficient intramolecular energy transfer from the naphthalimide to the porphyrin ring which further enhanced the electron transfer from the photo-excited porphyrin moiety to the Pt co-catalyst.

In this review, recent progress in heavy atom-free triplet photosensitizers was summarized. The general approaches include attaining S1/Tn states sharing similar energy levels or proper molecular geometry to satisfy the angular momentum reservation in intersystem crossing (ISC). ISC via the higher singlet excited state (Sn, n > 1) → Tm (m > 1), which is a rarely reported phenomenon, was also discussed. The ISC of some Bodipy dimers was proposed to be via the 'doubly excited state', but recent studies show that the ISC mechanism of these Bodipy dimers is charge separation/recombination. These new findings in the study of triplet photosensitizers are useful for photovoltaics, photodynamic therapy and photocatalysis, as well as in fundamental photochemistry studies.

Os(ii) complexes are particularly interesting for triplet-triplet annihilation (TTA) upconversion, due to the strong direct S0→ T1 photoexcitation, as in this way, energy loss is minimized and large anti-Stokes shift can be achieved for TTA upconversion. However, Os(bpy)3 has an intrinsic short T1 state lifetime (56 ns), which is detrimental for the intermolecular triplet-triplet energy transfer (TTET), one of the crucial steps in TTA upconversion. In order to prolong the triplet state lifetime, we prepared an Os(ii) tris(bpy) complex with a Bodipy moiety attached, so that an extended T1 state lifetime is achieved by excited state electronic configuration mixing or triplet state equilibrium between the coordination center-localized state (3MLCT state) and Bodipy ligand-localized state (3IL state). With steady-state and time-resolved transient absorption/emission spectroscopy, we proved that the 3MLCT is slightly above the 3IL state (by 0.05 eV), and the triplet state lifetime was prolonged by 31-fold (from 56 ns to 1.73 μs). The TTA upconversion quantum yield was increased by 4-fold as compared to that of the unsubstituted Os(ii) complex.

Thionated naphthalenediimide (NDI) derivatives with 2,6-dibromo and diaminoalkyl substituents are prepared by treating of 1,4,5,8-naphthalenetetracarboxylic dianhydride with Lawesson's reagent. Red shifted absorption was obtained for all thionated derivates as compared to the respective oxo-compounds. Thionated NDI derivative with 2,6-diamino substituents shows strong absorption at 674 nm. Nanosecond time-resolved transient absorption spectroscopy confirmed the population of triplet excited state of the thionated compounds upon photoexcitation and the triplet state quantum yield was determined as Φ T = 58%. Thionated derivatives were used for singlet oxygen ( 1 O 2 ) photosensitizing, the compound shows high singlet oxygen quantum yield ( Φ Δ = 56%) with excitation at 675 nm. With femtosecond transient absorption spectroscopy, the intersystem crossing (ISC) of the thionated NDI was found to take 32 ps. Based on TD-DFT computations, the ISC of these heavy atom-free NDI is attributed to the S 1 → T 2 /T 3 transition, which is in 1 (π-π)* → 3 (n-π)* feature, which obeys with El-Sayed rule. Moreover, the S 1 /T 3 state energy gap is 0.02 eV, much smaller than that of S 1 /T 1 energy gap (0.86 eV). These results are useful for the design of heavy-atom-free triplet photosensitizers.

Two triads (BDP-SQ and Styryl-BDP-SQ) were prepared with Bodipy, styrylBodipy and Squaraine (SQ) units. SQ shows unexpected efficient intersystem crossing (ISC. ΦT = 50%), which is attributed to S1→T1 transition. In the two triads, the Forster Resonance Energy Transfer (FRET) direction, as well as the spatial localization of the T1 state, was judiciously tuned. The cascade photophysical properties of the triads were studied with steady-state and time-resolved optical spectroscopies, as well as with electrochemical characterization and theoretical computations. We show that triplet state was produced in triad BDP-SQ upon photoexcitation, but in Styryl-BDP-SQ the fast FRET and the charge separation (CS) processes compete with the ISC of the SQ unit, and no triplet state was formed upon photoexcitation. The singlet energy transfer kinetics were found to be 1.6 and 0.6 ps, respectively and are solvent polarity dependent. Charge transfer was confirmed with ultrafast transient absorption spectroscopy.

To achieve activatable triplet-triplet-annihilation (TTA) upconversion, we linked a diiodoBodipy triplet photosensitizing unit and perylene triplet energy acceptor/annihilation/emitter using a disulfide bond (dyad BP-1), which can be selectively cleaved by thiols. For comparison, a reference dyad featuring a shorter and more chemically robust 1,2,3-triazole linker between the two components was also prepared (dyad BP-2). The photophysical properties of these compounds have been studied using steady-state and time-resolved transient spectroscopies, forward singlet energy transfer and backward triplet energy transfer (ping-pong energy transfer) were observed. For BP-1, the rate for forward intramolecular Forster Resonance Energy Transfer from perylene to diiodoBodipy is kFRET = 1.9 x 10^8 s^-1, while the backward triplet-triplet-energy-transfer (TTET) process from diiodoBodipy to perylene was slightly slower, with kTTET = 3.7x10^7 s^-1. For BP-2, faster energy transfer kinetics were determined (kFRET = 3.1 ...

A series of diphenylphosphoryl-substituted bis-cyclometalated Ir(III) complexes ( POFIrpic , 4Me-POFIrpic , 5Me-POFIrpic , 5F-POFIrpic and 5CF 3 -POFIrpic ) with a picolinic acid as the ancillary ligand have been synthesized and characterized. The influence of substituents (CH 3 , F, CF 3 ) at the pyridyl moiety of the cyclometalating ligand POFdFppy (2-(2′,4′-difluoro-3′-diphenylphosphoryl)phenylpyridine) on the photophysical and electrochemical properties of these Ir(III) complexes has been investigated. These Ir(III) complexes exhibit intense emission bands at 455–492 nm and high phosphorescence quantum yields (Φ PL ) in a range of 54–64% in CH 2 Cl 2 . The phosphorescence oxygen sensing properties of these complexes were studied in solution and in polymer film, and a fast response time and excellent operational stability were observed. Two-site model fitting indicates that 5Me-POFIrpic is the most sensitive oxygen sensing material among the complexes, with a quenching constant K s v a p p of 0.00793 Torr −1 . Photodegradation tests demonstrate that introduction of a CF 3 group into the cyclometalating ligand improves the photostability of the corresponding Ir(III) complex.

Bodipy-anthracene dyads with two chromophores assuming orthogonal geometry to enhance the spin-orbital charge-transfer intersystem crossing (SOCT-ISC) were prepared. The photosensitizers show strong absorption of visible light, efficient triplet-state formation (quantum yield 90%), and a long-lived triplet state (85 μs). The dipole moment orientation exerts significant effect on the ISC efficiency. It is also the first time that photosensitizers based on SOCT-ISC were used for triplet-triplet annihilation upconversion. The upconversion quantum yield is up to 15.8%.

Two different chromophores of pyrene and naphthalimide were introduced as acetylide ligands into diimine N^N heteroleptic Pt(II) bisacetylide complex (Pt-NI-Py) to broaden the absorption bands and study the intramolecular energy transfer process. Steady state and transient spectroscopies were used for study of the photophysical properties of the complexes. The results show that the lowest triplet state of Pt-NI-Py (lifetime = 34 microseconds) is localized on the pyrene part, although the singlet state energy level of naphthalimide unit is lower. Intramolecular energy transfer processes were observed. Moreover, Pt-NI-Py shows enhanced visible light harvesting property compared with bis(pyrenylacetylide) complex (Pt-Py) and relatively lower triplet state energy level compared to bis(ethynylnaphthalimide) complex (Pt-NI). The results demonstrated that the relative singlet excited state energy levels of the different chromophores do not agree with the relative triplet state energy levels of the same chromophores. The efficient triplet state generation property (singlet oxygen quantumy yield = 0.8) gives Pt-NI-Py high TTA upconversion efficiency (34%).

The photophysical processes of intramolecular “ping-pong” energy transfers in the iodinated reference dyad BDP-I2-Py, as well as the uniodinated dyad BDP-Py and triad BDP-2Py, were studied. For BDP-I2-Py, a forward Forster resonance energy transfer (FRET) from the perylene (Py) unit to the diiodoBDP unit (τ = 7 ps) and a backward triplet energy transfer (TTET, τ = 3 ns) from the diiodoBDP unit to the Py unit were observed. For the BDP-Py and BDP-2Py systems, a FRET (τ = 5–8 ps) and a photoinduced electron transfer (PET) (τ = 1–1.5 ns) were observed in acetonitrile. The uniodinated dyad and triad were used as the triplet energy acceptor and emitter for a triplet-triplet annihilation (TTA) upconversion with palladium tetraphenyltetrabenzoporphyrin as the triplet photosensitizer. A maximum upconversion quantum yield of 12.6% was observed. Given that the dyad (BDP-Py) contains one BDP unit and one Py unit while the triad (BDP-2Py) contains two Py units and one BDP unit and based on the results from steady-sta...

Fluorescent nitrogen-doped carbon dots (N-CDs) are synthesized by a facile one-pot solvothermal approach from Taixi anthracite in the presence of dimethyl formamide (DMF). Benefiting from superior dispersion of light components in DMF and easy exfoliation of smaller sp2-carbon structures from the anthracite, the as-prepared N-CDs show a strong photoluminescence (PL) with a high quantum yield (QY) of 47.0% and production yield of 25.6 wt%. Besides, the N-CDs also exhibit a singlet oxygen (1O2) generation behavior with a 1O2 generation QY of 19.0%. Moreover, the as-obtained N-CDs emit blue PL under the excitation wavelength of 365 nm, and can be easily dispersed in polymethyl methacrylate (PMMA), retaining the inherent advantages of the N-CDs and the PMMA matrix. The possible mechanism involved in the formation of N-CDs is proposed and discussed in terms of the macromolecular structure of the anthracite and the solvothermal approach adopted in the study.

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 i...

A stable 3D TbIII -based metal-organic framework [Tb(BPDC)2 ]⋅(CH3 )2 NH2 (DUT-101) was synthesized, and it is the first efficient dual-channel luminescence sensor for aqueous UO22+ ions. DUT-101 contains an anionic three-dimensional framework and protonated dimethylamine molecules embedded within the channels. The intense green emission of DUT-101 could be highly selectively and sensitively quenched by UO22+ ions even in the presence of other competing metal ions. A possible sensing mechanism was proposed based on both suppression of luminescence resonance energy transfer and enhancement of intermolecular electron transfer. Furthermore, visual green fluorescent test papers based on DUT-101 were fabricated and could be used to discriminate UO22+ ions among various metal ions.

A N^N Pt(II) complex, Pt-1, with two heteroleptic ligands was prepared, which is a rarely reported molecular structure. The two different acetylide ligands, i.e., boron-dipyyromethane (BDP) and naphthalenediimide (NDI) chromophores, show strong absorption in the visible region. The photophysical properties of the complex were investigated by using steady-state and femtosecond/nanosecond time-resolved optical spectroscopies, as well as electrochemical characterization. Upon selective photoexcitation of the coordinated BDP acetylide ligand at 503 nm, the Forster-resonance energy transfer (FRET, kFRET = 1.2 × 1011 s–1) process from the BDP to NDI ligand was observed, which leads to the population of the singlet excited state of the latter. After that, intersystem crossing (ISC) process occurs (kISC = 3.3 × 109 s–1), which generates the triplet excited state of the NDI ligand (τ = 28.1 μs). The overall excited-state dynamics are fairly similar in both nonpolar toluene and polar benzonitrile, indicating that p...

Photoirradiation generated radical from hexaphenyl-biimidazole (HPBI) was used for reversible switching of triplet-triplet annihilation (TTA) upconversion, based on quenching of the photosensitizer triplet state by radical-triplet pair mechanism. Upon 365 nm irradiation, the TTA upconversion in a system composed by a boron-dipyrromethene (BODIPY) derivative and perylene, was completely switched off due to quenching of triplet state of photosensitizer by photogenerated radical from HPBI. The upconversion was recovered after leaving the samples in darkness, due to regeneration of HPBI Dimer. The photophysical process involved in the photochromism and photoswitching of TTA upconversion were studied with steady-state UV–vis absorption spectroscopy, nanosecond transient absorption spectroscopy and EPR spectroscopy.

A series of anthracene derivatives with 9,10-substituents were prepared as triplet acceptors/emitters for triplet-triplet-annihilation (TTA) upconversion. Different linkages of C C single bonds and C C triple bonds were used to tune the singlet and triplet state energy levels, which may enhance the TTA upconversion. The study of the photophysical properties of the compounds indicates that the C C linker does not alter the T 1 state energy level substantially, whereas the C C linker significantly reduced the T 1 state energy levels. With nanosecond transient absorption spectroscopy, the intermolecular triplet-triplet-energy-transfer (TTET) process was studied. The lack of the upconversion for some anthracene derivatives was attributed to the inappropriate T 1 energy levels thus the lack of TTET. On the other hand, different upconversion quantum yields were observed for some acceptors, although the TTET processes are similar. This result is due to the different TTA. These studies will be useful for future development of the TTA upconversion and for study of the triplet state properties of organic chromophores.

Fluorinated Pt(II)-porphyrins (F1–F8) were synthesized, characterized; photophysical and electrochemical studies were conducted. The electronic absorption spectra of the complexes exhibited an intense Soret band (392–402 nm) and two hypsochromically shifted visible (Q) bands (508–510 and 538–541 nm). They also exhibited two emission bands (701–721 nm and 651–658 nm), corresponding to the T → S0 transition. The cyclic voltammetric studies of the porphyrins revealed the order of electron deficiency is as follows; F2 > F7 ≈ F8 > F5 > F4 > F3. The crystals of F3, F4, and F7 were structurally characterized and the crystal packing was mainly controlled by F⋯H, O⋯H, O⋯O, C⋯H close contacts. The oxygen sensing studies of the synthesized Pt(II)-porphyrins demonstrate that the highest sensitivity was observed for F8 compared to other complexes. The modified Stern–Volmer or two-site model provides the highest weighted quenching constant (KSV) of 0.068 Torr−1 for F8, which is twofold of the reference compound F1.

Inspired by nature, improving photosensitization represents a vital direction for the development of artificial photosynthesis. The sensitization ability of photosensitizers (PSs) reflects in their electron-transfer ability, which highly depends on their excited-state lifetime and redox potential. Herein, for the first time, we put forward a facile strategy to improve sensitizing ability via finely tuning the excited state of Ru(II)-PSs (Ru-1–Ru-4) for efficient CO2 reduction. Remarkably, [Ru(Phen)2(3-pyrenylPhen)]2+ (Ru-3) exhibits the best sensitizing ability among Ru-1–Ru-4, over 17 times higher than that of typical Ru(Phen)32+. It can efficiently sensitize a dinuclear cobalt catalyst for CO2-to-CO conversion with a maximum turnover number of 66 480. Systematic investigations demonstrate that its long-lived excited state and suitable redox driving force greatly contributed to this superior sensitizing ability. This work provides a new insight into dramatically boosting photocatalytic CO2 reduction via improving photosensitization.

Triplet-triplet annihilation upconversion (TTA UC) was intensively investigated for developing efficient photosensitizers and emitters. But an emission wavelength tunable TTA UC system with only one emitter was rarely reported. A novel hetero-bichromophore dyad, HB-An, showing solvatochromic emission and high fluorescence quantum yields in weakly polar solvents (such as n-hexane, dichloromethane (DCM), and so on) was used as triplet energy acceptor/emitter for polarity tuned TTA UC. A high TTA UC quantum yield up to 25% was achieved and the UC emission wavelengths can be fine-tuned from cyan to yellow by changing the media polarity. This information will be useful for constructing efficient emitting-light-tunable TTA UC system.

The intersystem crossing (ISC) and the triplet states in two representative BODIPY orthogonal dimers were studied with time-resolved electron paramagnetic resonance (TREPR) spectroscopy. The electron spin polarization (ESP) of the triplet state of the dimers, accessed with spin-orbit charge-transfer ISC, is different from that of the monomer (spin-orbit coupling-induced ISC). The TREPR spectra show that the triplet state initially formed by charge recombination is localized on either of two subunits, with different preference and ESP patterns. On the basis of the relative orientation of the respective zero field splitting principal axes, the Tx state on one subunit and the Tz state on another subunit in the dimer are overpopulated. The balance between the two triplet states is confirmed by the temperature dependency of the population ratio. No quintet state was detected with TREPR down to 20 K; thus, the recently proposed singlet fission ISC mechanism is excluded.

The emission properties of a series of flavin (FL) decorated Ru (II) polyimine complexes were investigated by extensive time-dependent (TD) density functional theory (DFT) and DFT based calculations. We attributed the moderate emission properties of FL decorated Ru(II) polyimine complex (Ru-1), such as triplet lifetime and luminescence quantum yield, to the dominant fast nonradiative decay due to the small adiabatic energy gap between the ground state and the lowest lying triplet state (Δ E

Two novel porphyrin-core systems were prepared by Sonogashira cross-coupling of the terminal alkyne groups of meso-tetra(4-ethynylphenyl)porphyrin-Zn(ii) (P-1) with halogenated Ru(ii)- or Ir(iii)-phenanthroline complexes. The resulting compounds (P-Ru and P-Ir) were spectroscopically characterised and their photophysical properties were investigated (λem 625, 665 nm; τT 339.6 μs (P-Ru) and λem 530, 612, 664 nm; τT 396.6 μs (P-Ir)). Nanosecond time-resolved transient absorption studies were used to explore the 3MLCT nature of the triplet excited states, and the singlet oxygen quantum yields were determined (ΦΔ 44.8 (P-Ru), 33.2 (P-Ir)%). The subcellular uptake of P-Ru and P-Ir and their application as photosensitisers (PS) in photodynamic therapy (PDT) were explored due to their solution photophysics and absence of dark toxicity. Upon irradiation (λexc = 620-630 nm; 10 min; 33 J cm-2), both P-Ru and P-Ir killed 90% of SKBR-3 cells at 1 μM. Notably P-Ru induced a 77% decrease in cell viability at only 0.25 μM.

Aggregation-induced emission (AIE) molecules show all kinds of application in biological research, chemical sensing, and medical study. However, most of the reported molecules are based on the performance of the single molecular entity. In this paper, a molecular system for real-time sensing through combination of dynamic covalent chemistry and aggregation-induced emission was rationally designed and tested. The aggregated particles exhibit different fluorescence emission colors upon the addition of various kinds of chemical reagents. The LC-MS analysis reveals that the breakage, formation, and exchange of the disulfide bonds in the molecular system occur spontaneously upon different reagents (base/acid and cysteine), which leads to a change in the proportion of different components in the system accordingly. Meanwhile, the fluorescence emission of the AIE system exhibits blue/red shift accompanied by intensity changes. Moreover, the particle size of the aggregated molecules gradually increased with the change of the chemical environment, which could be the result of the nucleus growing through intermolecular hydrogen bonding among molecular components. Thus, the chemical environment change results in the interactions of molecules, which further leads to the variation of dynamic fluorescence emission and morphology. The result represents a promising future for a dynamic AIE molecular system in the bioimaging and sensing study.

The spin state transition from low spin to high spin upon substrate addition is one of the key steps in cytochrome P450 catalysis. External perturbations such as pH and hydrogen bonding can also trigger the spin state transition of hemes through deprotonated histidine (e.g. Cytochrome c). In this work, we report the isolated 2-methylimidazole Cobalt(II) [Co(TPP)(2-MeHIm)] and [Co(TTP)(2-MeHIm)], and the corresponding 2-methylimidazolate derivatives where the N−H proton of axial 2-MeHIm is removed. Interestingly, various spectroscopies including EPR and XAFS determine a high-spin state (S = 3/2) for the imidazolate derivatives, in contrast to the low-spin state (S = 1/2) of all known imidazole analogs. DFT assisted stereoelectronic investigations are applied to understand the metal-ligand interactions, which suggest that the dramatically displaced metal center allowing a promotion eg(dπ) → b1g(\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$d_{x^2 - y^2}$$\end{document}dx2-y2) is crucial for the occurrence of the spin state transition., Studying the electronic structures and spin transitions of synthetic heme analogs is crucial to advancing our understanding of heme enzyme mechanisms. Here the authors show that a Co(II) porphyrin complex undergoes an unexpected spin state transition upon deprotonation of its axial imidazole ligand.

We attached different electron donors of phenyl, anthryl, and alkylamino moieties, to electron acceptor naphthalenediimide (NDI) to construct compact electron donor/acceptor dyads. The purpose is to study the effect of electron coupling (the magnitude is the matrix element, VDA) on the photophysical properties of UV-vis absorption, fluorescence emission, especially spin-orbit charge transfer intersystem crossing. We found that the magnitude of VDA depends on the electron donating strength of the aryl moieties ( VDA = 0.22-0.55 eV), as well as the molecular conformation, based on steady state and time-resolved transient absorption spectroscopies. We also found that electron coupling does not show the add-up (or synergetic) effect. Solvent polarity-dependent intersystem crossing (ISC) was observed for the dyad/triads and singlet oxygen quantum yield decreases in polar solvents. Femtosecond transient absorption results indicate that the charge separation (CS) for 9-An-NDI-NH occurs on time scale of 0.83 ps (in toluene) or 0.71 ps (in acetonitrile). The charge recombination (CR) process (50 ps in toluene) produces triplet state with ΦISC = 19%. The triplet state lifetime is up to 22 μs. This result indicates that orthogonal geometry for a compact electron donor/acceptor does not lead to efficient ISC via CR. Other factors such as the energy gap between the CS state and triplet state also determine the ISC efficiency.

Using naphthalimide (NI), complexes (Pt-PhNI and Pt-PhMeNI) based on the N^N platinum(II) bis(phenylacetylide) coordination framework were prepared, in which there are two close-lying triplet states, i.e., the metal-to-ligand-charge-transfer (

The theoretical investigation of electronically excited stated intermolecular hydrogen bonding dynamics of the 2D luminescent polypyrene covalent organic framework and methanol molecule (PPy-COF-MeOH) was performed using the density functional theory (DFT) and time-dependent (TD-DFT) method. The strengthening of Hydrogen bonds C-H---O-H and B-O---H-O upon photoexcitation was confirmed via comparison of geometric structures, electronic transition energies, 1H-NMR, binding energies, UV-Vis and infrared spectra in S0 and S1 states. Frontier molecular orbitals (MOs) analysis, electronic configuration, Mulliken charge analysis; and the charge density variation in hydrogen bonding proximity demonstrated that the strengthened hydrogen bonds facilitate the nonradiative path which may consequently proceed the luminescence quenching. Hence, the molecular material property prediction package (MOMAP) programme verified the fluorescence quenching because PPy-COF-MeOH complex showed a lower fluorescent rate constant compared to isolated PPy-COF fragment. The S1-T1 energy gap analysis also revealed the possibility of the Intersystem crossing (ISC). Above results significantly highlighted the role of the hydrogen bonding dynamics on luminescence property of the PPy-COF.

Spin-orbit charge transfer-induced intersystem crossing (SOCT-ISC) is of particular interest for preparation of heavy atom-free triplet photosensitizers. Up to now, examples for SOCT-ISC dyads are limited and electron donor/acceptor SOCT-ISC dyads showing strong visible-light harvesting are rare. Herein, we studied the photophysics of a series of Bodipy-anthracene (BDP-An) compact dyads, especially the triplet state electron spin dynamics with the time-resolved electron paramagnetic resonance (TREPR) spectroscopy. The electronic coupling matrix elements (VDA*) between the 1CT (charge transfer) state and 1LE (locally excited) state are in the range 773–1545 cm–1. For one dyad, we observed three triplet states simultaneously with TREPR, that is triplet states confined on the anthracene (3An) and the Bodipy (3BDP) moieties as well as a 3CT state. Based on the electron spin polarization of these three triplet states and the optical experiments, the SOCT-ISC mechanism is confirmed and the radical pair-ISC mech...

Photo-induced charge separation (CS) and charge recombination (CR) are crucial for molecular systems used in artificial photosynthesis, photovoltaics, and photocatalysis, and these processes are also important for fundamental photochemical studies. This review article focuses on the recent developments in CS/CR, long-lived charge transfer (CT) state formation, and CR-induced intersystem crossing (ISC) in organic molecular systems, ranging from discussions on the molecular structural features dictating the kinetics and thermodynamics of CS/CR and electron spin transport, as well as the spectroscopies used for the characterization of the CS/CR and the CR-induced ISC. Emphases are put on the molecular design rationales, from the point of view of organic chemists, to achieve fast and efficient CS but slow CR, as such to access the long-lived CT state. The roles of the electron spin in controlling the CS, CR and formation of the long-lived CT state are discussed with exemplars. Time-resolved electron paramagnetic resonance (TREPR) spectroscopy for characterization of the charge and electron spin transport in the CS, CR and ISC processes is introduced.

A BODIPY-functionalized ethoxypillar[5]arene (EtP5-BDP) and a guest compound G containing both of the cyano and triazole moieties were synthesized. The a-PET process between ethoxypillar[5]arene (EtP5) moiety and BODIPY moiety in EtP5-BDP was predicted by DFT/TDDFT calculations before experiment research. It was easier to occur for the a-PET process in polar solvents (e.g. acetone and CH3CN). The fluorescence of BODIPY moiety in EtP5-BDP was quenched to varying degrees in toluene, CHCl3, acetone and CH3CN. Due to the stronger interactions between EtP5 and G in toluene and CHCl3, when guest G was added, the fluorescence emissions of BODIPY moiety in EtP5-BDP in these two solvents were boosted but they still not recovered to the original emission level of BODIPY. Therefore, the electron transfer between EtP5 moiety and BODIPY moiety could be weakened but not inhibited completely by the host−guest complexation of EtP5-BDP and G. The ΔGCS was −0.01, −0.23, −0.40 and −0.42 eV for the PET process from EtP5 moiety to BODIPY moiety in toluene, CHCl3, acetone and CH3CN, respectively. The ECS of EtP5-BDP was lower than S1 energy state of BODIPY in various solvents. The charge transfer efficiencies (ΦCT) from EtP5 moiety to BODIPY moiety in toluene, CHCl3, acetone and CH3CN were 21%, 46%, 64% and 68%, respectively. All the results proved the a-PET process from EtP5 moiety to BODIPY moiety in EtP5-BDP.

Two bianthryl dyads, with the two units connected at either the 2- or 9- positions of the anthryl moiety, were studied to establish the relationship between orientation of the anthryl moieties and the electronic coupling and intersystem crossing (ISC) efficiency. The anthryl moieties in the two dyads adopt a close-to-orthogonal geometry, with dihedral angles of 90° (9,9′-bianthryl) and 106° (2,9′-bianthryl) at the ground state, respectively. The charge transfer (CT) emission efficiency and the fluorescence lifetimes are clearly dependent on the electronic coupling between the two anthryls, and stronger coupling lead to higher fluorescence quantum yields (34% vs. 9%) and shorter luminescence lifetimes (13.9 ns vs. 38.6 ns). The bianthryl with more orthogonal geometry shows higher singlet oxygen quantum yields ΦΔ (9,9′-bianthryl, ΦΔ = 53%) than 2,9′-bianthryl (ΦΔ = 32%). Moreover, highly solvent polarity-dependent fluorescence emission and ΦΔ were observed for the dyads (ΦΔ = 22–53%), which is different from the trend of the monomer anthracene, thus we propose the spin-orbit charge transfer ISC (SOCT-ISC) is responsible for the triplet state productions of the dyads. Interestingly, we found that inducing a heavy atom (Br) does not increase the ISC yield of anthracene. ISC in bianthryls was also confirmed with nanosecond transient absorption spectroscopy, the featured T1→Tn absorption at ca. 433 nm was observed, and the triplet state lifetime are long (9,9′-bianthryl, τT = 353 μs; 2,9′-bianthryl, τT = 493 μs, in acetonitrile).

We prepared a series of compact Bodipy-anthryl electron donor/acceptor triads and dyads by attaching anthryl moieties at the 3-,5-positions of the Bodipy core, with a novel conformation restriction approach, to study the spin-orbit charge transfer intersystem crossing (SOCT-ISC). The conformation restrictions are imposed by the BF2 unit of Bodipy without invoking the previously reported method with 1,7-dimethyl or 1,3-dimethyl groups. Our new approach shows a few advantages, including the stronger electron accepting ability of the methyl-free Bodipy core (reduction potential anodically shifted by +0.3 V vs the methylated Bodipy), red-shifted absorption (by 21 nm), and longer triplet state lifetime (372 µs vs 126 µs). The effects of the different mutual orientations of the electron donor and acceptor on ultraviolet-visible absorption, fluorescence, triplet state quantum yields, and lifetimes were studied. Triads with orthogonal geometries show higher singlet oxygen quantum yields (ΦΔ = 37%) than those with more coplanar geometries. Since the non-radiative decay for the S1 state is significant in the parent Bodipy chromophore (ΦF = 6.0%), we propose that in dyads/triads, the charge separation and recombination-induced ISC outcompete the non-radiative decay to the ground state, which is new in the study of SOCT-ISC. Density functional theory computation indicated a shallow torsion potential energy curve as compared to the meso-anthryl-Bodipy dyad analog, which may contribute a low triplet state quantum yield of the new dyads/triads. Triplet-triplet annihilation upconversion was performed with the electron donor/acceptor dyads as the triplet photosensitizer, with an upconversion quantum yield of 12.3%.

PtII complexes have attracted much attention due to their readily derivatizable molecular structures and efficient production of triplet excited states and have been extensively used in many areas. Conventional PtII complexes usually exhibit small molar absorption coefficients in the visible spectral region, and the triplet excited state lifetimes are short. However, for applications such as photocatalysis, triplet–triplet annihilation (TTA) upconversion, nonlinear optics, etc., visible-light-harvesting PtII bis(acetylide) complexes with long-lived triplet excited state are cucial. We summarize strategies directed towards achieving these goals, through the attachment of appropriate strong visible-light-harvesting organic chromophores to the ligand to switch the S0→1MLCT (metal-to-ligand charge transfer) transition to the S0→1IL (intraligand) transition (π–π*) and to switch the lowest triplet excited state from the 3MLCT to the 3IL state. We also summarize the properties of PtII bis(acetylide) complexes that show broadband absorption and reverse saturate absorption for application in nonlinear optics. Singlet and triplet energy transfer and localization of the triplet excited state in multiple-chromophore complexes are also discussed.

Tissue penetration depth is a major challenge in practical photodynamic therapy (PDT). A biocompatible and highly effective near infrared (NIR)-light-absorbing carbazole-substituted BODIPY (Car-BDP) molecule is reported as a class of imaging-guidable deep-tissue activatable photosensitizers for PDT. Car-BDP possesses an intense, broad NIR absorption band (600–800 nm) with a remarkably high singlet oxygen quantum yield (ΦΔ = 67%). After being encapsulated with biodegradable PLA–PEG-FA polymers, Car-BDP can form uniform and small organic nanoparticles that are water-soluble and tumor-targetable. Rather than using laser light, such nanoparticles offer an unprecedented deep-tissue, tumor targeting photodynamic therapeutic effect by using an exceptionally low-power-density and cost-effective lamp light (12 mW cm–2). In addition, these nanoparticles can be simultaneously traced in vivo due to their excellent NIR fluorescence. This study signals a major step forward in photodynamic therapy by developing a new cl...

“Chemistry-on-the-complex” synthetic methods have allowed the selective addition of 1-ethynylpyrene appendages to the 3-, 5-, 3,8- and 5,6-positions of IrIII-coordinated 1,10-phenanthroline via Sonogashira cross-coupling. The resulting suite of complexes has given rise to the first rationalization of their absorption and emission properties as a function of the number and position of the pyrene moieties. Strong absorption in the visible region (e.g. 3,8-substituted Ir-3: λabs=481 nm, ϵ=52 400 m−1 cm−1) and long-lived triplet excited states (e.g. 5-substituted Ir-2: τT=367.7 μs) were observed for the complexes in deaerated CH2Cl2. On testing the series as triplet sensitizers for triplet–triplet annihilation upconversion, those IrIII complexes bearing pyrenyl appendages at the 3- and 3,8-positions (Ir-1, Ir-3) were found to give optimal upconversion quantum yields (30.2 % and 31.6 % respectively).

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 (ΦF =17-34 %), and negligible singlet oxygen generation. In polar solvents, enhancements of the singlet-oxygen quantum yield (ΦΔ =19-27 %, λex =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.